- Email: [email protected]
Jaw movement-related primary somatosensory cortical area in the rat
Neuroscience 284 (2015) 55–64
JAW MOVEMENT-RELATED PRIMARY SOMATOSENSORY CORTICAL AREA IN THE RAT K. UCHINO, a,b K. HIGASHIYAMA, a T. KATO, a T. HAQUE, a F. SATO, a A. TOMITA, a K. TSUTSUMI, a M. MORITANI, a K. YAMAMURA c AND A. YOSHIDA a*
the rostral S1 to the trigeminal premotoneuron pools, especially to the dorVjuxt. Ó 2014 IBRO. Published by Elsevier Ltd. All rights reserved.
a Department of Oral Anatomy and Neurobiology, Osaka University Graduate School of Dentistry, Suita, Osaka 565-0871, Japan
Key words: Primary somatosensory cortex, SI, Intracortical microstimulation, Corticofugal, Trigeminal motoneuron.
b
Department of Acupuncture, Takarazuka University of Medical and Health Care, Takarazuka, Hyogo 666-0162, Japan c Division of Oral Physiology, Niigata University Graduate School of Medical and Dental Sciences, Niigata 951-8514, Japan
INTRODUCTION Ferrier (1876) provided the first evidence that electrical stimulation of the cerebral cortex evokes jaw movements in the dog. Since then, intracortical microstimulation techniques were widely used to identify jaw movement-related cortical areas in several species (monkey, Vogt and Vogt, 1919, 1926; Walker and Green, 1938; cat, Iwata et al., 1985, 1990; rabbit, Lund et al., 1984; guinea pig, Iriki et al., 1987; rat, Donoghue and Wise, 1982; Neafsey et al., 1986). Based on earlier anatomical studies, direct projections from the cerebral cortex to the cranial nerve motor nuclei including the trigeminal motor nucleus (Vmo) are rare in various species (human, Kuypers, 1958a; Schoen, 1969; monkey, Kuypers, 1958b, 1960; Kuypers and Lawrence, 1967; cat, Rossi and Brodal, 1956; Walberg, 1957; rat, Valverde, 1962; Ohta et al., 1989). Therefore, the motor information from the jaw movement-related cortical areas to jaw-closing and jawopening motoneurons in the Vmo is considered to be dior multi-synaptically conveyed thorough the jaw-closing premotoneurons (interneurons projecting directly to the jaw-closing component of the Vmo) and jaw-opening premotoneurons (those to the jaw-opening component), respectively (Olsson and Landgren, 1980; Lund et al., 1984; Olsson et al., 1986a). Our previous morphological study in the rat (Yoshida et al., 2009) has revealed that the intertrigeminal region (reticular formation between the Vmo and the trigeminal principal nucleus) mainly includes jaw-closing premotoneurons, whereas the reticular formation medial to the Vmo mainly contains jaw-opening premotoneurons. By contrast, the dorsal part of the trigeminal oral subnucleus (dorVo) and the dorsal part of the juxtatrigeminal region (dorVjuxt; lateral reticular formation medial to the trigeminal spinal nucleus) contain both the jaw-closing and jaw-opening premotoneurons. This morphological study (Yoshida et al., 2009) has also shown that the rostral part of the lateral agranular cortex (Agl; homologous to the primary motor cortex of the primate; see Neafsey et al., 1986) sends direct projections to the
Abstract—It has anatomically been revealed that the rostral part of the rat primary somatosensory cortex (S1) directly projects to the dorsal part of the trigeminal oral subnucleus (dorVo) and the dorsal part of juxtatrigeminal region (dorVjuxt), and that the dorVo and dorVjuxt contain premotoneurons projecting directly to the jaw-opening or jaw-closing motoneurons in the trigeminal motor nucleus (Vmo). However, little is known about how the rostral S1 regulates jaw movements in relation to its corticofugal projections. To address this issue, we performed intracortical microstimulation of the rat rostral S1 by monitoring jaw movements and electromyographic (EMG) activities. We for the first time found that low-frequency long-train stimulation of the rostral S1 induced single sustained opening of the jaw with elevated EMG activities of the anterior digastric muscles (jawopener). The effective sites for the low-frequency long-train stimulation overlapped the S1 sites where traditional highfrequency short-train stimulation was effective to induce single twitch-like jaw movement. We also found that the effective sites for the two kinds of train stimuli were included in the rostral S1 area, which has previously been identified to send direct projections to the dorVo or the dorVjuxt. Specifically, the most effective stimulation sites for the two kinds of train stimuli were located in the rostralmost part of S1 which has been reported to emanate strong direct projections to the dorVjuxt but less to the dorVo. Therefore, the present study suggests that the rat rostral S1, especially its rostralmost part, plays an important role in controlling jaw movements by activation of direct descending projections from
*Corresponding author. Address: Department of Oral Anatomy and Neurobiology, Osaka University Graduate School of Dentistry, 1-8 Yamadaoka, Suita, Osaka 565-0871, Japan. Tel: +81-6-6879-2877; fax: +81-6-6879-2880. E-mail address: [email protected] (A. Yoshida). The first two authors equally contribute to this study. Abbreviations: Agl, lateral agranular cortex; c-Dig, contralateral Dig; c-Mas, contralateral Mas; Dig, digastric; dorVo, dorsal part of the trigeminal oral subnucleus; dorVjuxt, dorsal part of juxtatrigeminal region; EMG, electromyography; HRP, horseradish peroxidase; i-Dig, ipsilateral Dig; i-Mas, ipsilateral Mas; Mas, masseter; S1, primary somatosensory cortex; vjuxt-S1, S1 area projecting to the dorVjuxt; Vmo, trigeminal motor nucleus; vo-S1, S1 area projecting to the dorVo. http://dx.doi.org/10.1016/j.neuroscience.2014.09.072 0306-4522/Ó 2014 IBRO. Published by Elsevier Ltd. All rights reserved. 55
56
K. Uchino et al. / Neuroscience 284 (2015) 55–64
intertrigeminal region, dorVo, and dorVjuxt; the rostral Agl seems to involve the A-area termed by Sasamoto et al. (1990). On the other hand, earlier electrophysiological studies in the rat have demonstrated that traditional high-frequency short-train (or single) stimulation of the rostral Agl elicits twitch-like jaw-closing or jawopening with short-latency electromyographic (EMG) activities, whereas low-frequency long-train stimulation of it induces rhythmical jaw movements (Neafsey et al., 1986; Sasamoto et al., 1990; Satoh et al., 2007). Therefore, these earlier studies suggest that the EMG activities of jaw-closing and jaw-opening muscles and the jaw movements are regulated by the activation of corticofugal projections from the rostral Agl to the premotoneuron areas (intertrigeminal region, dorVo, and dorVjuxt). In addition, it has been demonstrated that low-frequency long-train stimulation of the rat insular cortex, which seems to involve P-area termed by Sasamoto et al. (1990), also induces rhythmical jaw movements (Sasamoto et al., 1990; Zhang and Sasamoto, 1990; Inoue et al., 1992; Satoh et al., 2007; Maeda et al., 2014). Yet, anatomical evidence is currently lacking whether the jaw movement-related insular cortex gives off direct projections to the jaw-closing or jaw-opening premotoneurons. With regard to the rat primary somatosensory cortex (S1), our morphological study (Yoshida et al., 2009) has revealed that the rostral S1 gives off direct projections principally to the dorVo and dorVjuxt, whereas electrophysiological studies have shown that traditional highfrequency short-train (or single) stimulation of the rostral S1 elicits short-latency twitch-like jaw-closing or -opening (Sapienza et al., 1981; Neafsey et al., 1986; Avivi-Arber et al., 2010). These data suggest that the twitch-like jaw movements may be controlled by the direct projections of the rostral S1 to the premotoneuron areas (i.e., dorVo and dorVjuxt). Furthermore, it has been demonstrated that low-frequency long-train stimulation of the S1 induces jaw movements with elevated EMG activities of jaw-closing or jaw-opening muscles in other species (monkey, Huang et al., 1989; Hatanaka et al., 2005; rabbit, Liu et al., 1993; Masuda et al., 2002; guinea pig, Isogai et al., 2012). However, no studies have reported whether jaw movements can be elicited by the low-frequency long-train stimulation of the S1 in the rat. Thus, the present study in the rat was designed to test (1) the location of the S1 where the lowfrequency long-train stimuli can evoke jaw movements with EMG activities of jaw-closing or jaw-opening muscles, (2) whether the effective sites for the lowfrequency long-train stimulation match the effective sites for the high-frequency short-train stimulation, and (3) whether the effective sites for the two kinds of train stimuli match the rostral S1 which has already been revealed to directly project to the dorVo and dorVjuxt (Yoshida et al., 2009). To test this issue, we performed intracortical microstimulation methods (lowfrequency long-train and high-frequency short-train stimuli) in the rat rostral S1 by monitoring jaw movements and EMG activities.
EXPERIMENTAL PROCEDURES Animals and surgical procedures Eleven male Wistar rats in the body weight range 250– 350 g were used in the present study. All experimental procedures were approved by the Osaka University Graduate School of Dentistry Intramural Animal Care and Use Committee in accordance with the guidelines of NIH, USA. All efforts were made to minimize the number of animals used. For general anesthesia and sedative, a combination of ketamine hydrochloride (60 mg/kg, Daiichisankyo, Tokyo, Japan) and xylazine hydrochloride (5 mg/kg, Intervet, Ibaragi, Japan) was injected into the femoral muscle; their supplementary doses were given if necessary to keep the animals under a stable anesthesia level that neither corneal reflex nor spontaneous eye movements were detected. All wound margins were anesthetized using small, local injections of lidocaine hydrochloride (AstraZeneca, Osaka, Japan). Rectal temperature was maintained between 36 °C and 38 °C with a heating pad. An electrocardiogram was continuously monitored. To record EMG activities, the anterior belly of the digastric (Dig) muscles (jaw-opener) and the masseter (Mas) muscles (jaw-closer) on the right side (contralateral to the stimulation side; contralateral Dig (c-Dig) and contralateral Mas (c-Mas) muscles) and on the left side (ipsilateral to the stimulation side; ipsilateral Dig (i-Dig) and ipsilateral Mas (i-Mas) muscles) were exposed, and then pairs of polyurethane-coated stainless-steel wire electrodes (0.12-mm diameter, 1-mm exposed tip length, 1.5-mm inter-electrode distance) were inserted into the muscles. A magnet system for monitoring jaw movement trajectories in the anteroposterior and dorsoventral directions in small animals, originally developed by the Division of Oral Physiology, Niigata University Graduate School of Medical and Dental Science (Yamada et al., 1988; Kobayashi et al., 2002; Inoue et al., 2004), was applied to this study. A neodymium magnet (3-mm diameter) was attached with dental resin to the labial faces of crowns of bilateral lower incisors to face the magnet down. Then, the head of animal was fixed to a stereotaxic apparatus. A pair of magnetic sensors were positioned (18-mm inter-sensor distance in a mediolateral direction) on the platform of the stereotaxic apparatus beneath the magnet. The left parietal bone was partly removed to insert a metal electrode obliquely (60° from the vertical in the coronal plane) into the rostral S1 by reference to the stereotaxic coordinates of the rostral S1 demonstrated by Paxinos and Watson (1998), Yoshida et al. (2009) and Tomita et al. (2012). Then, the exposed dura was partly cut, and a monopolar glass-insulated Elgiloy electrode (0.05-mm tip exposure, 1–2 MX impedance at 1 kHz) was inserted obliquely (60° from the vertical in the coronal plane) into the rostral S1. Intracortical microstimulation Electrical stimulation was performed at every 0.3 mm of depth in each penetration track (the 0.4-mm surface
57
K. Uchino et al. / Neuroscience 284 (2015) 55–64
areas were excluded). The electrode penetration tracks were separated by 0.5 mm in anteroposterior and mediolateral directions. Two kinds of electrical train stimuli were applied at each stimulation site: lowfrequency long-train stimulation (450 cathodal squarewave pulses of 0.5-ms duration at 30 Hz, total 15-s stimulation, 20–80-lA for stimulus intensity) and highfrequency short-train stimuli (three pulses of 0.1-ms duration at 500 Hz, 30–80 lA). Amplified EMG activities and jaw movements were stored on a personal computer (sampling rate, 2 kHz for EMG activity; 3 kHz for jaw movements). Offline analysis was performed with computer assistance (PowerLab 8/30, ADInstruments, Sydney, Australia). At the end of the stimulation experiment, 7% horseradish peroxidase (HRP, Toyobo, Osaka, Japan), which was dissolved in saline and filled in a glass microelectrode (tip diameter 5–10 lm), was injected iontophoretically (positive pulses, 2 lA, 300 ms, 2 Hz, 20 s) into some stimulation sites to mark their positions. Additional two reference points (e.g., a point beneath the bregma) were also marked by HRP injections to confirm their stereotaxic coordinates. Histology and data analysis Soon after HRP injections, anesthesia level was deepened with an injection of sodium pentobarbital (i.p., 100 mg/kg, Dainippon Sumitomo Pharma, Osaka, Japan), and the animals were perfused through the ascending aorta with 0.02 M phosphate-buffered saline (pH 7.4) followed by a fixative containing 4% paraformaldehyde in 0.1 M phosphate buffer (pH 7.4). The brain was removed and placed in 25% sucrose in 0.02 M phosphate buffer for a
few days, and its serially ordered coronal sections (60lm thickness) were made on a freezing microtome. For detection of HRP, the sections were processed with the CoCl2-intensified diaminobenzidine method described by Adams (1977). Then, the sections were mounted on gelatin-coated slides, and dried. They were counterstained with Neutral Red or Thionin, dehydrated in graded alcohols, cleared in xylene, and coverslipped. The electrode penetration tracks, HRP injection sites, and cytoarchitecture of the brain were observed under a light microscope (Olympus BH-2 or BX 50, Japan), and drawn by using a camera lucida attached to the microscope. The atlas of the rat brain by Paxinos and Watson (1998) was used as a reference to delineate brain structures. Electrical stimulation sites which were found only in the cytoarchitectonically defined S1 were adopted in the present study (Fig. 1). To determine the stereotaxic coordinates of the stimulation sites, we referred to locations of the electrode penetration tracks as well as the reference points and some stimulation sites which were marked with HRP. Dorsolateral views of the stimulation sites (Figs. 1, 3, 5, 7 and 8) were obtained by reflecting these sites to the cortical surface in a perpendicular direction on the camera lucida drawings as representatively shown in Fig. 2.
RESULTS Intracortical microstimulation sites Our previous studies have revealed that the rat rostral S1 gives off direct descending projections to the dorVo and dorVjuxt that contain both the jaw-closing and jaw-opening premotoneurons (Chang et al., 2009;
Fig. 1. Location of electrical stimulation sites (blue shaded area) represented on the dorsolateral view of the rat left cerebral hemisphere with a 1-mm grid. Note that, in this study, electrical stimulation sites which were cytoarchitectonically defined only in the primary somatosensory cortex (S1) were adopted. As presented in our previous study (Yoshida et al., 2009), the distribution area of cortical neurons projecting to the dorsal part of trigeminal oral subnucleus is delineated by a green line, and the area in the S1 is referred to as the S1 area projecting to the dorVo (vo-S1); the distribution area of cortical neurons projecting to the dorsal part of juxtatrigeminal region is delineated by an orange line, and the area in the S1 is referred to as the S1 area projecting to the dorVjuxt (vjuxt-S1). The green and orange lines are also applied to Figs. 3, 5, 7 and 8. The homunculus map of the rat body, adapted from Neafsey et al. (1986) and Yoshida et al. (2009), is superimposed on the S1. Heavy black lines indicate cytoarchitectonic borders. The frontal pole is shown at the left, and the midline at the top; numbers (+5, 5) indicate rostral and caudal distance in millimeters from bregma (B). Agl, lateral agranular cortex; Agm, medial agranular cortex; RhF, rhinal fissure; S2, secondary somatosensory cortex.
58
K. Uchino et al. / Neuroscience 284 (2015) 55–64
Fig. 2. A photomicrograph of a coronal section showing electrical stimulation sites in and around the S1 (A) and its corresponding camera lucida drawing (B). Rostrocaudal level of the section was estimated approximately around 2.6 mm rostral to bregma. (B) Each electrode penetration track is indicated by a long blue line. Each stimulation site is represented by a short blue (red, or pink) line crossing the long blue line. Note that a total of 30 stimulation sites were located in the S1 in this section. A short red line indicates a site where both low-frequency long-train stimuli and highfrequency short-train stimuli evoked jaw movements. Two short pink lines indicate sites where only the low-frequency long-train stimuli evoked jaw movements. A yellow line representatively indicates the direction perpendicular to the cortical surface, which is used to reflect a stimulation site (marked with a short pink line) to the reflected point (marked with a pink dot) on the cortical surface. Green lines indicate borders of cortical layers. cc, corpus callosum; Cl, claustrum; Ins, insular cortex. Scale bar = 1 mm in (B) (also applies to A).
Yoshida et al., 2009); the S1 areas projecting to the dorVo and to the dorVjuxt were referred to as the S1 area projecting to the dorVo (vo-S1) and the S1 area projecting to the dorVjuxt (vjuxt-S1), respectively. Yoshida et al. (2009) have also shown that the rat vo-S1 occupies approximately the rostral one-third of the S1, whereas the vjuxtS1 overlaps the rostral half of the vo-S1 (e.g., Fig. 1). Based on the knowledge, we have targeted the rostral S1 for the intracortical microstimulation experiments in this study. Our histological reconstruction of stimulation sites (Fig. 2; see also the Experimental procedures) revealed that a total of 911 stimulation sites were scattered roughly in the rostral half of the S1 in 11 rats; they completely covered not only the vjuxt-S1 but also the vo-S1 (Fig. 1). Effective stimulation sites for jaw movements At each stimulation site, we applied both the lowfrequency long-train stimuli and high-frequency shorttrain stimuli (up to 80 lA for both stimuli). At 74 of the 911 stimulation sites, the low-frequency long-train stimuli or the high-frequency short-train stimuli evoked jaw movements with EMG activities of Dig muscles (jaw-opener) (Fig. 3); the 74 effective stimulation sites were obtained in eight of the 11 rats. Importantly, most of the effective stimulation sites (72/74) were distributed within the vjuxt-S1 (the rostral half of the vo-S1). At 28 of the 74 stimulation sites, both kinds of train stimuli evoked jaw movements; most of the 28 stimulation sites were located in the rostralmost part of vjuxt-S1 (the rostralmost part of vo-S1) in five rats. Jaw movements and EMG activities were also evoked exclusively by lowfrequency long-train stimuli at 38 sites in eight rats and exclusively by high-frequency short-train stimuli at eight sites in three rats.
Of the 74 effective stimulation sites, 38 sites were located in layer V of the S1, 32 sites in layer VI (e.g., Fig. 2), and the remaining four sites in the deep part of layer IV; these data are in agreement with earlier studies (McGuinness et al., 1980; Sapienza et al., 1981; Asanuma, 1989; Neafsey et al., 1986; Aldes, 1988). Both layers V and VI contained all of three types of effective stimulation sites: (1) the site responding exclusively to the low-frequency long-train stimuli, (2) the site responding exclusively to the high-frequency short-train stimuli, and (3) the site responding to both the stimuli. Jaw movements and EMG activities evoked by lowfrequency long-train stimuli Jaw movements were evoked by low-frequency long-train stimuli at total 66 sites as described above; the patterns of trajectories of the jaw and EMG activities were very similar across the stimulation sites. Fig. 4 shows an example of the jaw movements and EMG activities; in this case, the stimulus intensity was 70 lA, and the stimulation site was located in the rostralmost part of vjuxt-S1 (the rostralmost part of vo-S1) (Figs. 3, 5, 7 and 8). Single jaw-opening was induced and it continued during the stimulation period (Fig. 4C). In the late stimulation period, rhythmical but very small jaw movements appeared. During the sustained jaw-opening period, elevated EMG activities were observed both in the c-Dig and i-Dig muscles, but not in the c-Mas and iMas muscles (jaw-closer) (Fig. 4D–G). The EMG activities in the c-Dig muscle were slightly higher than in the i-Dig muscle (Fig. 4E, G). No activities were recorded in the c-Mas and i-Mas muscles at up to 80-lA intensity; at only four other sites out of the 66 sites, very small EMG activities were induced both in
59
K. Uchino et al. / Neuroscience 284 (2015) 55–64
Fig. 3. Distribution of 74 S1 sites where electrical stimulation with low-frequency long-train stimuli (up to 80 lA) or high-frequency shorttrain stimuli (up to 80 lA) evoked jaw movements and electromyographic (EMG) activities of the anterior digastric (Dig) muscles. Jaw movements evoked only by the low-frequency long-train stimuli were observed at 38 sites (pink filled circles), whereas those only by the high-frequency short-train stimuli were observed at eight sites (blue filled circles). The stimulation sites effective for both the train stimuli were observed at 28 sites (red filled circles). The stimulation sites are represented on the dorsolateral view of the rostral part of rat left S1 with a 1-mm grid. An arrow indicates the stimulation site where the low-frequency long-train stimulation evoked jaw movement shown in Fig. 4, as also presented in Figs. 5, 7 and 8. An arrow head indicates the stimulation site where the high-frequency short-train stimulation evoked jaw movement shown in Fig. 6, as also presented in Figs. 5, 7 and 8.
the c-Mas and i-Mas muscles by 80-lA intensity. When the stimulus intensity was lowed to 20 lA, single sustained jaw-opening was still observed, while the rhythmical small jaw movements (the late component) disappeared and the jaw position returned to the resting position before the offset of the stimuli. At 12 out of the 66 sites, the single sustained jawopening could be induced by the lower stimulus intensity (20 lA) (Fig. 5). Importantly, these 12 sites were located only in the rostralmost vjuxt-S1 (the rostralmost vo-S1). Jaw movements and EMG activities evoked by highfrequency short-train stimuli As described above, jaw movements were evoked by highfrequency short-train stimuli (up to 80 lA) at 36 sites (Figs. 3, 7 and 8); the trajectories of the jaw and EMG activities again showed very similar patterns across the stimulation sites. Fig. 6 shows an example of the jaw movements and EMG activities; in this case, the stimulus intensity was 60 lA, and the stimulation site was located in the rostralmost part of vjuxt-S1 (the rostralmost part of vo-S1) (Figs. 3, 5, 7 and 8). Single twitch-like jaw movement was induced (Fig. 6C). The EMG activities in
the c-Dig muscle were also higher than in the i-Dig muscle (Fig. 6E, G). The latencies of EMG activities of cDig muscle and i-Dig muscle were 11.1 ms and 14.5 ms, respectively; the averaged latency (±SD) of EMG activities of c-Dig muscle, which was obtained by 80-lA stimulus intensity at the 36 sites, was 13.1 (±1.8) ms. No activities were recorded in the c-Mas and i-Mas muscles at the maximum stimulus intensity (80 lA); at only two other sites out of the 36 sites, very small EMG activities were induced both in the c-Mas and i-Mas muscles by 80-lA intensity. When the stimulus intensity was lowered to 30 lA, EMG activities were recorded only in the c-Dig muscle, but not in the i-Dig muscle. At four out of the 36 sites, single twitch-like jaw movement and EMG activities were evoked by highfrequency short-train stimuli with lower stimulus intensity (30 lA); these four sites were located in the rostralmost part of vjuxt-S1 (the rostralmost part of vo-S1) (Fig. 7). Using the high-frequency short-train stimuli with 80-lA intensity, at 23 out of 36 stimulation sites EMG activities were evoked in the bilateral Dig muscles and at 13 sites they were evoked only in the c-Dig muscle, but at no sites they were induced only in the i-Dig muscle. Importantly, the 23 stimulation sites were located in the rostralmost vjuxt-S1 (the rostralmost vo-S1) (Fig. 8).
DISCUSSION Low-frequency long-train stimulation of S1 In the present study, we provided the first evidence that low-frequency long-train stimulation of the rostral part of rat S1 evoked single sustained jaw-opening with elevated EMG activities of Dig muscles. By contrast, lowfrequency long-train stimulation of the face S1 or the deep part of the cortical masticatory area (possibly part of the S1 or the secondary somatosensory cortex) evokes the vertical type of rhythmical jaw movements with alternating EMG activities of the c-Dig and c-Mas muscles in the monkey (Huang et al., 1989; Hatanaka et al., 2005), and that of the S1 induces natural chewinglike rhythmical jaw movements in the rabbit (Liu et al., 1993; Masuda et al., 2002) and the guinea pig (Isogai et al., 2012). These differences in jaw movement patterns among animals may indicate species differences of the S1 function for jaw movements. High-frequency short-train stimulation of S1 In this study, high-frequency short-train stimulation of the rostral S1 evoked single twitch-like jaw movement and EMG activities in the Dig muscle (especially c-Dig muscle) but rarely in the Mas muscle. These data were in agreement with earlier rat studies by Adachi et al. (2008) and Avivi-Arber et al. (2010). Importantly, they used general anesthesia (ketamine hydrochloride) and electrical stimulation methods (monopolar stimulation and 60-lA intensity), which were also used in the present study. However, they did not examine the most effective stimulation site of the S1 for evoking jaw movements, and the relation of the effective stimulation site of the S1 to the S1 sites giving off descending projections.
60
K. Uchino et al. / Neuroscience 284 (2015) 55–64
Fig. 4. Single sustained jaw-opening and Dig EMG activities induced by the low-frequency long-train stimulation of the rostral S1. The stimulation site in this case is indicated by an arrow in Figs. 3, 5, 7 and 8. (A) The stimulus intensity was 70 lA. Stimulus pulses were delivered from time 0 to 15 s. (B) Horizontal jaw position. (C) Vertical jaw position. Small but rhythmic jaw movements were seen in the late stimulation period. (D–G) EMG activities of the contralateral masseter (c-Mas) muscle (D), contralateral Dig (c-Dig) muscle (E), ipsilateral Mas (i-Mas) muscle (F) and ipsilateral Dig (i-Dig) muscle (G). Note that the high-frequency short-train stimulation of this stimulation site also evoked single twitch-like jaw movement and EMG activities similar to those shown in Fig. 6.
et al., 1989; Hatanaka et al., 2005). In our rat study, however, the high-frequency short-train stimulation induced single twitch-like jaw movement at 28 (42.4%) out of the 66 sites where the low-frequency long-train stimulation evoked single sustained jaw-opening. These differences may also indicate species differences in the functional significance of the S1 for the jaw movements. Significance of neural pathways from S1 to trigeminal premotoneuron areas
Fig. 5. Distribution of 66 S1 sites where the low-frequency long-train stimuli evoked single sustained jaw-opening and Dig EMG activities. Red filled circles indicate the sites where the stimulation evoked single sustained jaw-opening at the lower stimulus intensity (20 lA). Open circles indicate the sites where the stimulation evoked single sustained jaw-opening at the higher stimulus intensity (80 lA), but not at the lower stimulus intensity (20 lA).
In the monkey, high-frequency short-train stimulation of the S1, in which low-frequency long-train stimulation evokes the vertical type of rhythmical jaw movements, does not induce short-latency muscle twitch (Huang
In the present study, we found that the effective sites for the two kinds of stimulation were mainly located in the vjuxt-S1 (the rostral half of the vo-S1). Neurons in the vjuxt-S1 and vo-S1 make direct contacts with jawopening and jaw-closing premotoneurons (Chang et al., 2009; Yoshida et al., 2009). These present and previous findings strongly suggest that the activation of neural pathways from the S1 to the jaw-opening and jaw-closing motoneurons via the premotoneurons in the dorVjuxt and dorVo is involved in the control of jaw movements. However, cortico-cortical connections from the S1 to the Agl (primary motor cortex) have been demonstrated to exist in the rat (Akers and Killackey, 1978; Donoghue and Parham, 1983). These data raise a possibility that single sustained jaw movements by vjuxt-S1 (vo-S1) stimulation were evoked via the neural connections from the S1 to the Agl. If this is the case, the properties of jaw movements and EMG activities induced by S1 stimulation should be the same or similar to those by Agl stimulation. However, the movement patterns evoked by S1 stimulation in this study were different from the patterns by the low-frequency long-train stimulation of the Agl (rhythmical jaw movements with alternating EMG activities of jaw-opening
K. Uchino et al. / Neuroscience 284 (2015) 55–64
61
Fig. 6. Single twitch-like jaw movement and Dig EMG activities induced by high-frequency short-train stimulation of the S1. Stimulation site in this case is indicated by an arrow head in Figs. 3, 5, 7 and 8. The stimulus intensity was 60 lA. Note that low-frequency long-train stimulation of this site also evoked single sustained jaw-opening and EMG activities similar to those shown in Fig. 4. (A) Three stimulus pulses were delivered from time 0. Other conventions are as in Fig. 4.
Fig. 7. Distribution of 36 S1 sites where the high-frequency shorttrain stimuli evoked single twitch-like jaw movement and Dig EMG activities. Blue filled circles indicate the sites where the stimulation evoked single twitch-like jaw movement at the lower stimulus intensity (30 lA). Open circles indicate where the stimulation evoked single twitch-like jaw movement at the higher stimulus intensity (80 lA), but not at the lower stimulus intensity (30 lA).
and -closing muscles; see Sasamoto et al., 1990; Satoh et al., 2007). In the monkey, different types of jaw movements can also be evoked by low-frequency long-train and high-frequency short-train stimulation protocols,
Fig. 8. Distribution of 38 S1 sites where the high-frequency shorttrain stimuli evoked single twitch-like jaw movement. Blue filled circles indicate the sites where the stimulation with 80 lA stimulus intensity evoked the bilateral Dig EMG activities. Open circles indicate the sites where the stimulation with 80-lA stimulus intensity induced EMG activities only from the c-Dig muscle.
depending on stimulation sites: the face S1, the deep part of the cortical masticatory area, and the orofacial primary motor cortex (Huang et al., 1989; Hatanaka et al., 2005). Another explanation to exclude the possibility of the S1 to Agl pathway is that, if the single sustained jaw
62
K. Uchino et al. / Neuroscience 284 (2015) 55–64
movements by S1 stimulation were evoked via the Agl, the latency of c-Dig EMG activation by S1 stimulation should be longer than the one by Agl stimulation. However the mean latency by S1 stimulation (13.1 ± 1.8 ms) in the present study was not longer, but shorter than the one by Agl stimulation (14.2 ms) reported in the paper of Avivi-Arber et al. (2010). Taken together, it seems plausible that the jaw movements and EMG activities evoked by S1 stimulation were induced by direct activation of neural pathways from the vjuxt-S1 (vo-S1) to the premotoneuron areas (the dorVjuxt and dorVo). The effective stimulation sites for the two kinds of stimulation were distributed in more than half of the vjuxtS1 but in less than half of the vo-S1, although the most effective sites were located in the rostralmost vjuxt-S1 (the rostralmost vo-S1). The rostralmost vjuxt-S1 (the rostralmost vo-S1) contains the largest numbers of neurons projecting to the dorVjuxt but not to the dorVo (Yoshida et al., 2009); the largest numbers of neurons projecting to the dorVo are located in the rostral part of vo-S1 excluding its rostralmost part. These findings may further indicate that the activation of neural pathways to the dorVjuxt rather than to the dorVo is more important to induce jaw movements. Our idea may be supported by the previous work that the descending axons from the rat rostral S1 make more contacts with jaw-closing and jaw-opening premotoneurons in the dorVjuxt than in the dorVo (Chang et al., 2009). In the present study, jaw movements could not be induced by both types of train stimulation (at up to 80 lA intensity) of the caudodorsal part of the vjuxt-S1 or the caudal half of the vo-S1 where there are only smaller number of neurons projecting to the dorVjuxt or dorVo (Yoshida et al., 2009). Higher intensity more than 80 lA may be needed to induce the jaw movements. At all of the 74 effective stimulation sites, higher EMG activities were recorded from the c-Dig muscle rather than from the i-Dig muscle by the two kinds of stimulation of the rostral S1. Although it may be difficult to simply compare the magnitudes of EMG from different sources (i.e., c-Dig muscle vs. i-Dig muscle) due to the variability of resistance and locations of the EMG electrodes, we prefer to think that there was a contralateral predominance between the c-Dig and i-Dig muscle activation. Our idea may be supported by the previous tract-tracing studies that the direct projections from the rostral S1 to the dorVjuxt and dorVo are contralaterally predominant, and the direct projections from the dorVjuxt and dorVo to the jaw-opening component of the Vmo are ipsilaterally predominant (Li et al., 1995; Chang et al., 2009; Yoshida et al., 2009). The low-frequency long-train stimulation of the rostral part of vjuxt-S1 (of vo-S1) did not induce Mas muscle activities in our study, whereas the stimulation of the rostral part of rat Agl activates the jaw-closing muscle and thereby induces rhythmical and obvious jaw movements with alternative jaw-opening and jaw-closing muscle activities (Neafsey et al., 1986; Sasamoto et al., 1990; Zhang and Sasamoto, 1990; Satoh et al., 2007). This difference may be attributable to the difference in their corticofugal projections. Yoshida et al. (2009) have
demonstrated that both the vjuxt-S1 (part of vo-S1) and the rostral Agl project to the dorVjuxt and dorVo, but only the rostral Agl further projects to the intertrigeminal region. The intertrigeminal region mainly contains jawclosing premotoneurons (Yoshida et al., 2009), which fire rhythmically during cortically induced fictive mastication (Olsson et al., 1986a,b; Donga et al., 1990; Donga and Lund, 1991; Inoue et al., 1994; Westberg et al., 1998). In this respect, the Agl activation can facilitate the Mas muscle (via jaw-closing premotoneurons) more easily than the vjuxt-S1 (vo-S1) activation. We should also mention that the dorVjuxt and dorVo contain not only jaw-opening premotoneurons but also jaw-closing premotoneurons, and both the premotoneurons receive strong direct projections from the rostral part of vjuxt-S1 (of vo-S1) (Chang et al., 2009; Yoshida et al., 2009). In the present study, however, both types of stimulation evoked EMG activities in the Dig muscle but not in the Mas muscle. A possible explanation may be that the rostral part of vjuxt-S1 (of vo-S1) gives off axons making contacts with excitatory jaw-opening premotoneurons and inhibitory jaw-closing premotoneurons in the dorVjuxt and dorVo, which in turn project respectively to the trigeminal jaw-opening and jawclosing motoneurons. Because the dorVjuxt and dorVo are considered to contain both excitatory and inhibitory jaw-opening and jaw-closing premotoneurons (Turman and Chandler, 1994; Li et al., 1996; Shigenaga et al., 2000; Gemba-Nishimura et al., 2010). Further studies are needed to clarify the cortical inhibitory pathways to the trigeminal motoneurons.
CONCLUSIONS The present study using intracortical microstimulation methods has for the first time revealed that lowfrequency long-train stimulation of the rat rostral S1 can induce single sustained opening of the jaw with elevated EMG activities of the Dig muscle (jaw-opener) predominantly on the contralateral side. The effective sites for the low-frequency long-train stimulation overlap the S1 sites where traditional high-frequency short-train stimulation is effective to induce single twitch-like jaw movement. The effective sites for the two kinds of train stimuli are included in the rostral S1 area, which has previously been identified to send direct projections to the dorVo or the dorVjuxt that contains jaw-opening and closing premotoneurons. Specifically, the most effective stimulation sites for the two kinds of train stimuli are located in the rostralmost part of S1, which has been reported to send strong direct projections to the dorVjuxt but less to the dorVo. Therefore, the present findings suggest that the rat rostral S1, especially its rostralmost part, plays an important role in controlling jaw movements by activation of direct descending projections from the rostral S1 to the trigeminal premotoneuron pools, especially to the dorVjuxt. Acknowledgments—We wish to thank Dr. Yoshihisa Tachibana for his helpful advice. We are also indebted to Drs. Ayaka Oka, Rieko Takeda, Takashi Fujio and Haruka Ohara for their technical
K. Uchino et al. / Neuroscience 284 (2015) 55–64 help. This work was supported by Ministry of Education, Culture, Sports, Science, and Technology of Japan (Grants 23592699 and 26293391 to A.Y.).
REFERENCES Adachi K, Murray GM, Lee JC, Sessle BJ (2008) Noxious lingual stimulation influences the excitability of the face primary motor cerebral cortex (face MI) in the rat. J Neurophysiol 100:1234–1244. Adams JC (1977) Technical considerations on the use of HRP as a neuronal marker. Neuroscience 2:141–145. Akers RM, Killackey HP (1978) Organization of corticocortical connections in the parietal cortex of the rat. J Comp Neurol 181:513–538. Aldes LD (1988) Thalamic connectivity of rat somatic motor cortex. Brain Res Bull 20:333–348. Asanuma H (1989) The motor cortex. New York: Raven Press. Avivi-Arber L, Lee JC, Sessle BJ (2010) Effects of incisor extraction on jaw and tongue motor representations within face sensorimotor cortex of adult rats. J Comp Neurol 518:1030–1045. Chang Z, Haque T, Iida C, Seki S, Sato F, Kato T, Uchino K, Ono T, Nakamura M, Bae Y-C, Yoshida A (2009) Distribution of premotoneurons for jaw-closing and jaw-opening motor nucleus receiving contacts from axon terminals of primary somatosensory cortical neurons in rats. Brain Res 1275:43–53. Donga R, Lund JP (1991) Discharge patterns of trigeminal commissural last-order interneurons during fictive mastication in the rabbit. J Neurophysiol 66:1564–1578. Donga R, Lund JP, Veilleux D (1990) An electrophysiological study of trigeminal commissural interneurons in the anaesthetized rabbit. Brain Res 515:351–354. Donoghue JP, Parham C (1983) Afferent connections of the lateral agranular field of the rat motor cortex. J Comp Neurol 217:390–404. Donoghue JP, Wise SP (1982) The motor cortex of the rat: cytoarchitecture and microstimulation mapping. J Comp Neurol 212:76–88. Ferrier D (1876) The functions of the brain. New York: G.P. Putnam’s Sons. Gemba-Nishimura A, Inoue T, Nakamura S, Nakayama K, Mochizuki A, Shintani S, Yoshimura S (2010) Properties of synaptic transmission from the reticular formation dorsal to the facial nucleus to trigeminal motoneurons during early postnatal development in rats. Neuroscience 166:1008–1022. Hatanaka N, Tokuno H, Nambu A, Inoue T, Takada M (2005) Input– output organization of jaw movement-related areas in monkey frontal cortex. J Comp Neurol 492:401–425. Huang CS, Hiraba H, Murray GM, Sessle BJ (1989) Topographical distribution and functional properties of cortically induced rhythmical jaw movements in the monkey (Macaca fascicularis). J Neurophysiol 61:635–650. Inoue M, Ariyasinghe S, Yamamura K, Harasawa Y, Yamada Y (2004) Extrinsic tongue and suprahyoid muscle activities during mastication in freely feeding rabbits. Brain Res 1021:173–182. Inoue T, Chandler SH, Goldberg LJ (1994) Neuropharmacological mechanisms underlying rhythmical discharge in trigeminal interneurons during fictive mastication. J Neurophysiol 71:2061–2073. Inoue T, Masuda Y, Nagashima T, Yoshikawa K, Morimoto T (1992) Properties of rhythmically active reticular neurons around the trigeminal motor nucleus during fictive mastication in the rat. Neurosci Res 14:275–294. Iriki A, Nozaki S, Nakamura Y (1987) Reorganization of sensorimotor cortex corresponding to conversion of food ingestive behavior from sucking to chewing during development of guinea pigs. J Physiol Soc Jpn 49:432. Isogai F, Kato T, Fujimoto M, Toi S, Oka A, Adachi T, Maeda Y, Morimoto T, Yoshida A, Masuda Y (2012) Cortical area inducing
63
chewing-like rhythmical jaw movements and its connections with thalamic nuclei in guinea pigs. Neurosci Res 74:239–247. Iwata K, Itoga H, Ikukawa A, Hanashima N, Sumino R (1985) Movements of the jaw and orofacial regions evoked by stimulation of two different cortical areas in cats. Brain Res 359:332–337. Iwata K, Muramatsu H, Tsuboi Y, Sumino R (1990) Input–output relationships in the jaw and orofacial motor zones of the cat cerebral cortex. Brain Res 507:337–340. Kobayashi M, Masuda Y, Fujimoto Y, Matsuya T, Yamamura K, Yamada Y, Maeda N, Morimoto T (2002) Electrophysiological analysis of rhythmic jaw movements in the freely moving mouse. Physiol Behav 75:377–385. Kuypers HGJM (1958a) Corticobulbar connexions to the pons and lower brain-stem in man. An anatomical study. Brain 81: 364–388. Kuypers HGJM (1958b) Some projections from the peri-central cortex to the pons and lower brain stem in monkey and chimpanzee. J Comp Neurol 110:221–255. Kuypers HGJM (1960) Central cortical projections to motor and somato-sensory cell groups. An experimental study in rhesus monkey. Brain 83:161–184. Kuypers HGJM, Lawrence DG (1967) Cortical projections to the red nucleus and the brain stem in the rhesus monkey. Brain Res 4:151–188. Li YQ, Takada M, Kaneko T, Mizuno N (1995) Premotor neurons for trigeminal motor nucleus neurons innervating the jaw-closing and jaw-opening muscles: differential distribution in the lower brainstem of the rat. J Comp Neurol 356:563–579. Li YQ, Takada M, Kaneko T, Mizuno N (1996) GABAergic and glycinergic neurons projecting to the trigeminal motor nucleus: a double labeling study in the rat. J Comp Neurol 373:498–510. Liu ZJ, Masuda Y, Inoue T, Fuchihata H, Sumida A, Takada K, Morimoto T (1993) Coordination of cortically induced rhythmic jaw and tongue movements in the rabbit. J Neurophysiol 69:569–584. Lund JP, Sasamoto K, Murakami T, Olsson KA (1984) Analysis of rhythmical jaw movements produced by electrical stimulation of motor-sensory cortex of rabbits. J Neurophysiol 52:1014–1029. Maeda N, Kobashi M, Mitoh Y, Fujita M, Minagi S, Matsuo R (2014) Differential involvement of two cortical masticatory areas in submandibular salivary secretion in rats. Brain Res 1543:200–208. Masuda Y, Tachibana Y, Inoue T, Iwata K, Morimoto T (2002) Influence of oro-facial sensory input on the output of the cortical masticatory area in the anesthetized rabbit. Exp Brain Res 146:501–510. McGuinness E, Sivertsen D, Allman JM (1980) Organization of the face representation in macaque motor cortex. J Comp Neurol 193:591–608. Neafsey EJ, Bold EL, Haas G, Hurley-Gius KM, Quirk G, Sievert CF, Terreberry RR (1986) The organization of the rat motor cortex: a microstimulation mapping study. Brain Res 396:77–96. Ohta M, Ishizuka S, Saeki K (1989) Corticotrigeminal motor pathway in the rat – II. Anterio- and retrograde HRP labeling. Comp Biochem Physiol 94:405–414. Olsson KA, Landgren S (1980) Facilitation and inhibition of jaw reflexes evoked by electrical stimulation of the cat’s cerebral cortex. Exp Brain Res 39:149–164. Olsson KA, Landgren S, Westberg KG (1986a) Location of, and peripheral convergence on, the interneuron in the disynaptic path from the coronal gyrus of the cerebral cortex to the trigeminal motoneurons in the cat. Exp Brain Res 65:83–97. Olsson KA, Sasamoto K, Lund JP (1986b) Modulation of transmission in rostral trigeminal sensory nuclei during chewing. J Neurophysiol 55:56–75. Paxinos G, Watson C (1998) The rat brain in stereotaxic coordinates. 4th ed. Sydney: Academic Press. Rossi GF, Brodal A (1956) Corticofugal fibres to the brain-stem reticular formation: an experimental study in the cat. J Anat 90:42–62. Sapienza S, Talbi B, Jacquemin J, Albe-Fessard D (1981) Relationship between input and output of cells in motor and
64
K. Uchino et al. / Neuroscience 284 (2015) 55–64
somatosensory cortices of the chronic awake rat. A study using glass micropipettes. Exp Brain Res 43:47–56. Sasamoto K, Zhang G, Iwasaki M (1990) Two types of rhythmical jaw movements evoked by stimulation of the rat cortex. Jpn J Oral Biol 32:57–68. Satoh Y, Ishizuka K, Murakami T (2007) Changes in cortically induced rhythmic jaw movements after lesioning of the red nucleus in rats. Brain Res 1165:60–70. Schoen (1969) The corticofugal projection on the brain stem and spinal cord in man. Psychiatr Neurol Neurochir 72:121–128. Shigenaga Y, Hirose Y, Yoshida A, Fukami H, Honma S, Bae YC (2000) Quantitative ultrastructure of physiologically identified premotor neuron terminals in the trigeminal motor nucleus in the cat. J Comp Neurol 426:13–30. Tomita A, Kato T, Sato F, Haque T, Oka A, Yamamoto M, Ono T, Bae Y-C, Maeda Y, Sessle BJ, Yoshida A (2012) Somatotopic direct projections from orofacial areas of primary somatosensory cortex to pons and medulla, especially to trigeminal sensory nuclear complex, in rats. Neuroscience 200:166–185. Turman Jr J, Chandler SH (1994) Immunohistochemical evidence for GABA and glycine-containing trigeminal premotoneurons in the guinea pig. Synapse 18:7–20. Valverde F (1962) Reticular formation of the albino rat’s brain stem cytoarchitecture and corticofugal connections. J Comp Neurol 119:25–53. Vogt O, Vogt C (1919) Allgemeinere Ergebnisse unserer Hirnforschung. J Psychol Neurol 25:277–462.
Vogt O, Vogt C (1926) Die vergleichend-architktonische und vergleichend-reizphysiologische Felderung der Grosshirnrinde unter besonderer Beru¨cksichtigung der menschlichen. Naturwissenschaften 14:1190–1194. Walberg F (1957) Corticofugal fibers to the nuclei of the dorsal columns. An experimental study in the cat. Brain 80:273–287. Walker AE, Green HD (1938) Electrical excitability of the motor face area: a comparative study in primates. J Neurophysiol 1:152–165. Westberg K, Clavelou P, Sandstro¨m G, Lund JP (1998) Evidence that trigeminal brainstem interneurons form subpopulations to produce different forms of mastication in the rabbit. J Neurosci 18:6466–6479. Yamada Y, Haraguchi N, Oi K, Sasaki M (1988) Two-dimensional jaw tracking and EMG recording system implanted in the freely moving rabbit. J Neurosci Methods 23:257–261. Yoshida A, Taki I, Chang Z, Iida C, Haque T, Tomita A, Seki S, Yamamoto S, Masuda Y, Moritani M, Shigenaga Y (2009) Corticofugal projections to the trigeminal motoneurons innervating antagonistic jaw muscles in rats as demonstrated by anterograde and retrograde tract tracing. J Comp Neurol 514:368–386. Zhang G, Sasamoto K (1990) Projections of two separate cortical areas for rhythmical jaw movements in the rat. Brain Res Bull 24:221–230.
(Accepted 2 September 2014) (Available online 5 October 2014)
JAW MOVEMENT-RELATED PRIMARY SOMATOSENSORY CORTICAL AREA IN THE RAT K. UCHINO, a,b K. HIGASHIYAMA, a T. KATO, a T. HAQUE, a F. SATO, a A. TOMITA, a K. TSUTSUMI, a M. MORITANI, a K. YAMAMURA c AND A. YOSHIDA a*
the rostral S1 to the trigeminal premotoneuron pools, especially to the dorVjuxt. Ó 2014 IBRO. Published by Elsevier Ltd. All rights reserved.
a Department of Oral Anatomy and Neurobiology, Osaka University Graduate School of Dentistry, Suita, Osaka 565-0871, Japan
Key words: Primary somatosensory cortex, SI, Intracortical microstimulation, Corticofugal, Trigeminal motoneuron.
b
Department of Acupuncture, Takarazuka University of Medical and Health Care, Takarazuka, Hyogo 666-0162, Japan c Division of Oral Physiology, Niigata University Graduate School of Medical and Dental Sciences, Niigata 951-8514, Japan
INTRODUCTION Ferrier (1876) provided the first evidence that electrical stimulation of the cerebral cortex evokes jaw movements in the dog. Since then, intracortical microstimulation techniques were widely used to identify jaw movement-related cortical areas in several species (monkey, Vogt and Vogt, 1919, 1926; Walker and Green, 1938; cat, Iwata et al., 1985, 1990; rabbit, Lund et al., 1984; guinea pig, Iriki et al., 1987; rat, Donoghue and Wise, 1982; Neafsey et al., 1986). Based on earlier anatomical studies, direct projections from the cerebral cortex to the cranial nerve motor nuclei including the trigeminal motor nucleus (Vmo) are rare in various species (human, Kuypers, 1958a; Schoen, 1969; monkey, Kuypers, 1958b, 1960; Kuypers and Lawrence, 1967; cat, Rossi and Brodal, 1956; Walberg, 1957; rat, Valverde, 1962; Ohta et al., 1989). Therefore, the motor information from the jaw movement-related cortical areas to jaw-closing and jawopening motoneurons in the Vmo is considered to be dior multi-synaptically conveyed thorough the jaw-closing premotoneurons (interneurons projecting directly to the jaw-closing component of the Vmo) and jaw-opening premotoneurons (those to the jaw-opening component), respectively (Olsson and Landgren, 1980; Lund et al., 1984; Olsson et al., 1986a). Our previous morphological study in the rat (Yoshida et al., 2009) has revealed that the intertrigeminal region (reticular formation between the Vmo and the trigeminal principal nucleus) mainly includes jaw-closing premotoneurons, whereas the reticular formation medial to the Vmo mainly contains jaw-opening premotoneurons. By contrast, the dorsal part of the trigeminal oral subnucleus (dorVo) and the dorsal part of the juxtatrigeminal region (dorVjuxt; lateral reticular formation medial to the trigeminal spinal nucleus) contain both the jaw-closing and jaw-opening premotoneurons. This morphological study (Yoshida et al., 2009) has also shown that the rostral part of the lateral agranular cortex (Agl; homologous to the primary motor cortex of the primate; see Neafsey et al., 1986) sends direct projections to the
Abstract—It has anatomically been revealed that the rostral part of the rat primary somatosensory cortex (S1) directly projects to the dorsal part of the trigeminal oral subnucleus (dorVo) and the dorsal part of juxtatrigeminal region (dorVjuxt), and that the dorVo and dorVjuxt contain premotoneurons projecting directly to the jaw-opening or jaw-closing motoneurons in the trigeminal motor nucleus (Vmo). However, little is known about how the rostral S1 regulates jaw movements in relation to its corticofugal projections. To address this issue, we performed intracortical microstimulation of the rat rostral S1 by monitoring jaw movements and electromyographic (EMG) activities. We for the first time found that low-frequency long-train stimulation of the rostral S1 induced single sustained opening of the jaw with elevated EMG activities of the anterior digastric muscles (jawopener). The effective sites for the low-frequency long-train stimulation overlapped the S1 sites where traditional highfrequency short-train stimulation was effective to induce single twitch-like jaw movement. We also found that the effective sites for the two kinds of train stimuli were included in the rostral S1 area, which has previously been identified to send direct projections to the dorVo or the dorVjuxt. Specifically, the most effective stimulation sites for the two kinds of train stimuli were located in the rostralmost part of S1 which has been reported to emanate strong direct projections to the dorVjuxt but less to the dorVo. Therefore, the present study suggests that the rat rostral S1, especially its rostralmost part, plays an important role in controlling jaw movements by activation of direct descending projections from
*Corresponding author. Address: Department of Oral Anatomy and Neurobiology, Osaka University Graduate School of Dentistry, 1-8 Yamadaoka, Suita, Osaka 565-0871, Japan. Tel: +81-6-6879-2877; fax: +81-6-6879-2880. E-mail address: [email protected] (A. Yoshida). The first two authors equally contribute to this study. Abbreviations: Agl, lateral agranular cortex; c-Dig, contralateral Dig; c-Mas, contralateral Mas; Dig, digastric; dorVo, dorsal part of the trigeminal oral subnucleus; dorVjuxt, dorsal part of juxtatrigeminal region; EMG, electromyography; HRP, horseradish peroxidase; i-Dig, ipsilateral Dig; i-Mas, ipsilateral Mas; Mas, masseter; S1, primary somatosensory cortex; vjuxt-S1, S1 area projecting to the dorVjuxt; Vmo, trigeminal motor nucleus; vo-S1, S1 area projecting to the dorVo. http://dx.doi.org/10.1016/j.neuroscience.2014.09.072 0306-4522/Ó 2014 IBRO. Published by Elsevier Ltd. All rights reserved. 55
56
K. Uchino et al. / Neuroscience 284 (2015) 55–64
intertrigeminal region, dorVo, and dorVjuxt; the rostral Agl seems to involve the A-area termed by Sasamoto et al. (1990). On the other hand, earlier electrophysiological studies in the rat have demonstrated that traditional high-frequency short-train (or single) stimulation of the rostral Agl elicits twitch-like jaw-closing or jawopening with short-latency electromyographic (EMG) activities, whereas low-frequency long-train stimulation of it induces rhythmical jaw movements (Neafsey et al., 1986; Sasamoto et al., 1990; Satoh et al., 2007). Therefore, these earlier studies suggest that the EMG activities of jaw-closing and jaw-opening muscles and the jaw movements are regulated by the activation of corticofugal projections from the rostral Agl to the premotoneuron areas (intertrigeminal region, dorVo, and dorVjuxt). In addition, it has been demonstrated that low-frequency long-train stimulation of the rat insular cortex, which seems to involve P-area termed by Sasamoto et al. (1990), also induces rhythmical jaw movements (Sasamoto et al., 1990; Zhang and Sasamoto, 1990; Inoue et al., 1992; Satoh et al., 2007; Maeda et al., 2014). Yet, anatomical evidence is currently lacking whether the jaw movement-related insular cortex gives off direct projections to the jaw-closing or jaw-opening premotoneurons. With regard to the rat primary somatosensory cortex (S1), our morphological study (Yoshida et al., 2009) has revealed that the rostral S1 gives off direct projections principally to the dorVo and dorVjuxt, whereas electrophysiological studies have shown that traditional highfrequency short-train (or single) stimulation of the rostral S1 elicits short-latency twitch-like jaw-closing or -opening (Sapienza et al., 1981; Neafsey et al., 1986; Avivi-Arber et al., 2010). These data suggest that the twitch-like jaw movements may be controlled by the direct projections of the rostral S1 to the premotoneuron areas (i.e., dorVo and dorVjuxt). Furthermore, it has been demonstrated that low-frequency long-train stimulation of the S1 induces jaw movements with elevated EMG activities of jaw-closing or jaw-opening muscles in other species (monkey, Huang et al., 1989; Hatanaka et al., 2005; rabbit, Liu et al., 1993; Masuda et al., 2002; guinea pig, Isogai et al., 2012). However, no studies have reported whether jaw movements can be elicited by the low-frequency long-train stimulation of the S1 in the rat. Thus, the present study in the rat was designed to test (1) the location of the S1 where the lowfrequency long-train stimuli can evoke jaw movements with EMG activities of jaw-closing or jaw-opening muscles, (2) whether the effective sites for the lowfrequency long-train stimulation match the effective sites for the high-frequency short-train stimulation, and (3) whether the effective sites for the two kinds of train stimuli match the rostral S1 which has already been revealed to directly project to the dorVo and dorVjuxt (Yoshida et al., 2009). To test this issue, we performed intracortical microstimulation methods (lowfrequency long-train and high-frequency short-train stimuli) in the rat rostral S1 by monitoring jaw movements and EMG activities.
EXPERIMENTAL PROCEDURES Animals and surgical procedures Eleven male Wistar rats in the body weight range 250– 350 g were used in the present study. All experimental procedures were approved by the Osaka University Graduate School of Dentistry Intramural Animal Care and Use Committee in accordance with the guidelines of NIH, USA. All efforts were made to minimize the number of animals used. For general anesthesia and sedative, a combination of ketamine hydrochloride (60 mg/kg, Daiichisankyo, Tokyo, Japan) and xylazine hydrochloride (5 mg/kg, Intervet, Ibaragi, Japan) was injected into the femoral muscle; their supplementary doses were given if necessary to keep the animals under a stable anesthesia level that neither corneal reflex nor spontaneous eye movements were detected. All wound margins were anesthetized using small, local injections of lidocaine hydrochloride (AstraZeneca, Osaka, Japan). Rectal temperature was maintained between 36 °C and 38 °C with a heating pad. An electrocardiogram was continuously monitored. To record EMG activities, the anterior belly of the digastric (Dig) muscles (jaw-opener) and the masseter (Mas) muscles (jaw-closer) on the right side (contralateral to the stimulation side; contralateral Dig (c-Dig) and contralateral Mas (c-Mas) muscles) and on the left side (ipsilateral to the stimulation side; ipsilateral Dig (i-Dig) and ipsilateral Mas (i-Mas) muscles) were exposed, and then pairs of polyurethane-coated stainless-steel wire electrodes (0.12-mm diameter, 1-mm exposed tip length, 1.5-mm inter-electrode distance) were inserted into the muscles. A magnet system for monitoring jaw movement trajectories in the anteroposterior and dorsoventral directions in small animals, originally developed by the Division of Oral Physiology, Niigata University Graduate School of Medical and Dental Science (Yamada et al., 1988; Kobayashi et al., 2002; Inoue et al., 2004), was applied to this study. A neodymium magnet (3-mm diameter) was attached with dental resin to the labial faces of crowns of bilateral lower incisors to face the magnet down. Then, the head of animal was fixed to a stereotaxic apparatus. A pair of magnetic sensors were positioned (18-mm inter-sensor distance in a mediolateral direction) on the platform of the stereotaxic apparatus beneath the magnet. The left parietal bone was partly removed to insert a metal electrode obliquely (60° from the vertical in the coronal plane) into the rostral S1 by reference to the stereotaxic coordinates of the rostral S1 demonstrated by Paxinos and Watson (1998), Yoshida et al. (2009) and Tomita et al. (2012). Then, the exposed dura was partly cut, and a monopolar glass-insulated Elgiloy electrode (0.05-mm tip exposure, 1–2 MX impedance at 1 kHz) was inserted obliquely (60° from the vertical in the coronal plane) into the rostral S1. Intracortical microstimulation Electrical stimulation was performed at every 0.3 mm of depth in each penetration track (the 0.4-mm surface
57
K. Uchino et al. / Neuroscience 284 (2015) 55–64
areas were excluded). The electrode penetration tracks were separated by 0.5 mm in anteroposterior and mediolateral directions. Two kinds of electrical train stimuli were applied at each stimulation site: lowfrequency long-train stimulation (450 cathodal squarewave pulses of 0.5-ms duration at 30 Hz, total 15-s stimulation, 20–80-lA for stimulus intensity) and highfrequency short-train stimuli (three pulses of 0.1-ms duration at 500 Hz, 30–80 lA). Amplified EMG activities and jaw movements were stored on a personal computer (sampling rate, 2 kHz for EMG activity; 3 kHz for jaw movements). Offline analysis was performed with computer assistance (PowerLab 8/30, ADInstruments, Sydney, Australia). At the end of the stimulation experiment, 7% horseradish peroxidase (HRP, Toyobo, Osaka, Japan), which was dissolved in saline and filled in a glass microelectrode (tip diameter 5–10 lm), was injected iontophoretically (positive pulses, 2 lA, 300 ms, 2 Hz, 20 s) into some stimulation sites to mark their positions. Additional two reference points (e.g., a point beneath the bregma) were also marked by HRP injections to confirm their stereotaxic coordinates. Histology and data analysis Soon after HRP injections, anesthesia level was deepened with an injection of sodium pentobarbital (i.p., 100 mg/kg, Dainippon Sumitomo Pharma, Osaka, Japan), and the animals were perfused through the ascending aorta with 0.02 M phosphate-buffered saline (pH 7.4) followed by a fixative containing 4% paraformaldehyde in 0.1 M phosphate buffer (pH 7.4). The brain was removed and placed in 25% sucrose in 0.02 M phosphate buffer for a
few days, and its serially ordered coronal sections (60lm thickness) were made on a freezing microtome. For detection of HRP, the sections were processed with the CoCl2-intensified diaminobenzidine method described by Adams (1977). Then, the sections were mounted on gelatin-coated slides, and dried. They were counterstained with Neutral Red or Thionin, dehydrated in graded alcohols, cleared in xylene, and coverslipped. The electrode penetration tracks, HRP injection sites, and cytoarchitecture of the brain were observed under a light microscope (Olympus BH-2 or BX 50, Japan), and drawn by using a camera lucida attached to the microscope. The atlas of the rat brain by Paxinos and Watson (1998) was used as a reference to delineate brain structures. Electrical stimulation sites which were found only in the cytoarchitectonically defined S1 were adopted in the present study (Fig. 1). To determine the stereotaxic coordinates of the stimulation sites, we referred to locations of the electrode penetration tracks as well as the reference points and some stimulation sites which were marked with HRP. Dorsolateral views of the stimulation sites (Figs. 1, 3, 5, 7 and 8) were obtained by reflecting these sites to the cortical surface in a perpendicular direction on the camera lucida drawings as representatively shown in Fig. 2.
RESULTS Intracortical microstimulation sites Our previous studies have revealed that the rat rostral S1 gives off direct descending projections to the dorVo and dorVjuxt that contain both the jaw-closing and jaw-opening premotoneurons (Chang et al., 2009;
Fig. 1. Location of electrical stimulation sites (blue shaded area) represented on the dorsolateral view of the rat left cerebral hemisphere with a 1-mm grid. Note that, in this study, electrical stimulation sites which were cytoarchitectonically defined only in the primary somatosensory cortex (S1) were adopted. As presented in our previous study (Yoshida et al., 2009), the distribution area of cortical neurons projecting to the dorsal part of trigeminal oral subnucleus is delineated by a green line, and the area in the S1 is referred to as the S1 area projecting to the dorVo (vo-S1); the distribution area of cortical neurons projecting to the dorsal part of juxtatrigeminal region is delineated by an orange line, and the area in the S1 is referred to as the S1 area projecting to the dorVjuxt (vjuxt-S1). The green and orange lines are also applied to Figs. 3, 5, 7 and 8. The homunculus map of the rat body, adapted from Neafsey et al. (1986) and Yoshida et al. (2009), is superimposed on the S1. Heavy black lines indicate cytoarchitectonic borders. The frontal pole is shown at the left, and the midline at the top; numbers (+5, 5) indicate rostral and caudal distance in millimeters from bregma (B). Agl, lateral agranular cortex; Agm, medial agranular cortex; RhF, rhinal fissure; S2, secondary somatosensory cortex.
58
K. Uchino et al. / Neuroscience 284 (2015) 55–64
Fig. 2. A photomicrograph of a coronal section showing electrical stimulation sites in and around the S1 (A) and its corresponding camera lucida drawing (B). Rostrocaudal level of the section was estimated approximately around 2.6 mm rostral to bregma. (B) Each electrode penetration track is indicated by a long blue line. Each stimulation site is represented by a short blue (red, or pink) line crossing the long blue line. Note that a total of 30 stimulation sites were located in the S1 in this section. A short red line indicates a site where both low-frequency long-train stimuli and highfrequency short-train stimuli evoked jaw movements. Two short pink lines indicate sites where only the low-frequency long-train stimuli evoked jaw movements. A yellow line representatively indicates the direction perpendicular to the cortical surface, which is used to reflect a stimulation site (marked with a short pink line) to the reflected point (marked with a pink dot) on the cortical surface. Green lines indicate borders of cortical layers. cc, corpus callosum; Cl, claustrum; Ins, insular cortex. Scale bar = 1 mm in (B) (also applies to A).
Yoshida et al., 2009); the S1 areas projecting to the dorVo and to the dorVjuxt were referred to as the S1 area projecting to the dorVo (vo-S1) and the S1 area projecting to the dorVjuxt (vjuxt-S1), respectively. Yoshida et al. (2009) have also shown that the rat vo-S1 occupies approximately the rostral one-third of the S1, whereas the vjuxtS1 overlaps the rostral half of the vo-S1 (e.g., Fig. 1). Based on the knowledge, we have targeted the rostral S1 for the intracortical microstimulation experiments in this study. Our histological reconstruction of stimulation sites (Fig. 2; see also the Experimental procedures) revealed that a total of 911 stimulation sites were scattered roughly in the rostral half of the S1 in 11 rats; they completely covered not only the vjuxt-S1 but also the vo-S1 (Fig. 1). Effective stimulation sites for jaw movements At each stimulation site, we applied both the lowfrequency long-train stimuli and high-frequency shorttrain stimuli (up to 80 lA for both stimuli). At 74 of the 911 stimulation sites, the low-frequency long-train stimuli or the high-frequency short-train stimuli evoked jaw movements with EMG activities of Dig muscles (jaw-opener) (Fig. 3); the 74 effective stimulation sites were obtained in eight of the 11 rats. Importantly, most of the effective stimulation sites (72/74) were distributed within the vjuxt-S1 (the rostral half of the vo-S1). At 28 of the 74 stimulation sites, both kinds of train stimuli evoked jaw movements; most of the 28 stimulation sites were located in the rostralmost part of vjuxt-S1 (the rostralmost part of vo-S1) in five rats. Jaw movements and EMG activities were also evoked exclusively by lowfrequency long-train stimuli at 38 sites in eight rats and exclusively by high-frequency short-train stimuli at eight sites in three rats.
Of the 74 effective stimulation sites, 38 sites were located in layer V of the S1, 32 sites in layer VI (e.g., Fig. 2), and the remaining four sites in the deep part of layer IV; these data are in agreement with earlier studies (McGuinness et al., 1980; Sapienza et al., 1981; Asanuma, 1989; Neafsey et al., 1986; Aldes, 1988). Both layers V and VI contained all of three types of effective stimulation sites: (1) the site responding exclusively to the low-frequency long-train stimuli, (2) the site responding exclusively to the high-frequency short-train stimuli, and (3) the site responding to both the stimuli. Jaw movements and EMG activities evoked by lowfrequency long-train stimuli Jaw movements were evoked by low-frequency long-train stimuli at total 66 sites as described above; the patterns of trajectories of the jaw and EMG activities were very similar across the stimulation sites. Fig. 4 shows an example of the jaw movements and EMG activities; in this case, the stimulus intensity was 70 lA, and the stimulation site was located in the rostralmost part of vjuxt-S1 (the rostralmost part of vo-S1) (Figs. 3, 5, 7 and 8). Single jaw-opening was induced and it continued during the stimulation period (Fig. 4C). In the late stimulation period, rhythmical but very small jaw movements appeared. During the sustained jaw-opening period, elevated EMG activities were observed both in the c-Dig and i-Dig muscles, but not in the c-Mas and iMas muscles (jaw-closer) (Fig. 4D–G). The EMG activities in the c-Dig muscle were slightly higher than in the i-Dig muscle (Fig. 4E, G). No activities were recorded in the c-Mas and i-Mas muscles at up to 80-lA intensity; at only four other sites out of the 66 sites, very small EMG activities were induced both in
59
K. Uchino et al. / Neuroscience 284 (2015) 55–64
Fig. 3. Distribution of 74 S1 sites where electrical stimulation with low-frequency long-train stimuli (up to 80 lA) or high-frequency shorttrain stimuli (up to 80 lA) evoked jaw movements and electromyographic (EMG) activities of the anterior digastric (Dig) muscles. Jaw movements evoked only by the low-frequency long-train stimuli were observed at 38 sites (pink filled circles), whereas those only by the high-frequency short-train stimuli were observed at eight sites (blue filled circles). The stimulation sites effective for both the train stimuli were observed at 28 sites (red filled circles). The stimulation sites are represented on the dorsolateral view of the rostral part of rat left S1 with a 1-mm grid. An arrow indicates the stimulation site where the low-frequency long-train stimulation evoked jaw movement shown in Fig. 4, as also presented in Figs. 5, 7 and 8. An arrow head indicates the stimulation site where the high-frequency short-train stimulation evoked jaw movement shown in Fig. 6, as also presented in Figs. 5, 7 and 8.
the c-Mas and i-Mas muscles by 80-lA intensity. When the stimulus intensity was lowed to 20 lA, single sustained jaw-opening was still observed, while the rhythmical small jaw movements (the late component) disappeared and the jaw position returned to the resting position before the offset of the stimuli. At 12 out of the 66 sites, the single sustained jawopening could be induced by the lower stimulus intensity (20 lA) (Fig. 5). Importantly, these 12 sites were located only in the rostralmost vjuxt-S1 (the rostralmost vo-S1). Jaw movements and EMG activities evoked by highfrequency short-train stimuli As described above, jaw movements were evoked by highfrequency short-train stimuli (up to 80 lA) at 36 sites (Figs. 3, 7 and 8); the trajectories of the jaw and EMG activities again showed very similar patterns across the stimulation sites. Fig. 6 shows an example of the jaw movements and EMG activities; in this case, the stimulus intensity was 60 lA, and the stimulation site was located in the rostralmost part of vjuxt-S1 (the rostralmost part of vo-S1) (Figs. 3, 5, 7 and 8). Single twitch-like jaw movement was induced (Fig. 6C). The EMG activities in
the c-Dig muscle were also higher than in the i-Dig muscle (Fig. 6E, G). The latencies of EMG activities of cDig muscle and i-Dig muscle were 11.1 ms and 14.5 ms, respectively; the averaged latency (±SD) of EMG activities of c-Dig muscle, which was obtained by 80-lA stimulus intensity at the 36 sites, was 13.1 (±1.8) ms. No activities were recorded in the c-Mas and i-Mas muscles at the maximum stimulus intensity (80 lA); at only two other sites out of the 36 sites, very small EMG activities were induced both in the c-Mas and i-Mas muscles by 80-lA intensity. When the stimulus intensity was lowered to 30 lA, EMG activities were recorded only in the c-Dig muscle, but not in the i-Dig muscle. At four out of the 36 sites, single twitch-like jaw movement and EMG activities were evoked by highfrequency short-train stimuli with lower stimulus intensity (30 lA); these four sites were located in the rostralmost part of vjuxt-S1 (the rostralmost part of vo-S1) (Fig. 7). Using the high-frequency short-train stimuli with 80-lA intensity, at 23 out of 36 stimulation sites EMG activities were evoked in the bilateral Dig muscles and at 13 sites they were evoked only in the c-Dig muscle, but at no sites they were induced only in the i-Dig muscle. Importantly, the 23 stimulation sites were located in the rostralmost vjuxt-S1 (the rostralmost vo-S1) (Fig. 8).
DISCUSSION Low-frequency long-train stimulation of S1 In the present study, we provided the first evidence that low-frequency long-train stimulation of the rostral part of rat S1 evoked single sustained jaw-opening with elevated EMG activities of Dig muscles. By contrast, lowfrequency long-train stimulation of the face S1 or the deep part of the cortical masticatory area (possibly part of the S1 or the secondary somatosensory cortex) evokes the vertical type of rhythmical jaw movements with alternating EMG activities of the c-Dig and c-Mas muscles in the monkey (Huang et al., 1989; Hatanaka et al., 2005), and that of the S1 induces natural chewinglike rhythmical jaw movements in the rabbit (Liu et al., 1993; Masuda et al., 2002) and the guinea pig (Isogai et al., 2012). These differences in jaw movement patterns among animals may indicate species differences of the S1 function for jaw movements. High-frequency short-train stimulation of S1 In this study, high-frequency short-train stimulation of the rostral S1 evoked single twitch-like jaw movement and EMG activities in the Dig muscle (especially c-Dig muscle) but rarely in the Mas muscle. These data were in agreement with earlier rat studies by Adachi et al. (2008) and Avivi-Arber et al. (2010). Importantly, they used general anesthesia (ketamine hydrochloride) and electrical stimulation methods (monopolar stimulation and 60-lA intensity), which were also used in the present study. However, they did not examine the most effective stimulation site of the S1 for evoking jaw movements, and the relation of the effective stimulation site of the S1 to the S1 sites giving off descending projections.
60
K. Uchino et al. / Neuroscience 284 (2015) 55–64
Fig. 4. Single sustained jaw-opening and Dig EMG activities induced by the low-frequency long-train stimulation of the rostral S1. The stimulation site in this case is indicated by an arrow in Figs. 3, 5, 7 and 8. (A) The stimulus intensity was 70 lA. Stimulus pulses were delivered from time 0 to 15 s. (B) Horizontal jaw position. (C) Vertical jaw position. Small but rhythmic jaw movements were seen in the late stimulation period. (D–G) EMG activities of the contralateral masseter (c-Mas) muscle (D), contralateral Dig (c-Dig) muscle (E), ipsilateral Mas (i-Mas) muscle (F) and ipsilateral Dig (i-Dig) muscle (G). Note that the high-frequency short-train stimulation of this stimulation site also evoked single twitch-like jaw movement and EMG activities similar to those shown in Fig. 6.
et al., 1989; Hatanaka et al., 2005). In our rat study, however, the high-frequency short-train stimulation induced single twitch-like jaw movement at 28 (42.4%) out of the 66 sites where the low-frequency long-train stimulation evoked single sustained jaw-opening. These differences may also indicate species differences in the functional significance of the S1 for the jaw movements. Significance of neural pathways from S1 to trigeminal premotoneuron areas
Fig. 5. Distribution of 66 S1 sites where the low-frequency long-train stimuli evoked single sustained jaw-opening and Dig EMG activities. Red filled circles indicate the sites where the stimulation evoked single sustained jaw-opening at the lower stimulus intensity (20 lA). Open circles indicate the sites where the stimulation evoked single sustained jaw-opening at the higher stimulus intensity (80 lA), but not at the lower stimulus intensity (20 lA).
In the monkey, high-frequency short-train stimulation of the S1, in which low-frequency long-train stimulation evokes the vertical type of rhythmical jaw movements, does not induce short-latency muscle twitch (Huang
In the present study, we found that the effective sites for the two kinds of stimulation were mainly located in the vjuxt-S1 (the rostral half of the vo-S1). Neurons in the vjuxt-S1 and vo-S1 make direct contacts with jawopening and jaw-closing premotoneurons (Chang et al., 2009; Yoshida et al., 2009). These present and previous findings strongly suggest that the activation of neural pathways from the S1 to the jaw-opening and jaw-closing motoneurons via the premotoneurons in the dorVjuxt and dorVo is involved in the control of jaw movements. However, cortico-cortical connections from the S1 to the Agl (primary motor cortex) have been demonstrated to exist in the rat (Akers and Killackey, 1978; Donoghue and Parham, 1983). These data raise a possibility that single sustained jaw movements by vjuxt-S1 (vo-S1) stimulation were evoked via the neural connections from the S1 to the Agl. If this is the case, the properties of jaw movements and EMG activities induced by S1 stimulation should be the same or similar to those by Agl stimulation. However, the movement patterns evoked by S1 stimulation in this study were different from the patterns by the low-frequency long-train stimulation of the Agl (rhythmical jaw movements with alternating EMG activities of jaw-opening
K. Uchino et al. / Neuroscience 284 (2015) 55–64
61
Fig. 6. Single twitch-like jaw movement and Dig EMG activities induced by high-frequency short-train stimulation of the S1. Stimulation site in this case is indicated by an arrow head in Figs. 3, 5, 7 and 8. The stimulus intensity was 60 lA. Note that low-frequency long-train stimulation of this site also evoked single sustained jaw-opening and EMG activities similar to those shown in Fig. 4. (A) Three stimulus pulses were delivered from time 0. Other conventions are as in Fig. 4.
Fig. 7. Distribution of 36 S1 sites where the high-frequency shorttrain stimuli evoked single twitch-like jaw movement and Dig EMG activities. Blue filled circles indicate the sites where the stimulation evoked single twitch-like jaw movement at the lower stimulus intensity (30 lA). Open circles indicate where the stimulation evoked single twitch-like jaw movement at the higher stimulus intensity (80 lA), but not at the lower stimulus intensity (30 lA).
and -closing muscles; see Sasamoto et al., 1990; Satoh et al., 2007). In the monkey, different types of jaw movements can also be evoked by low-frequency long-train and high-frequency short-train stimulation protocols,
Fig. 8. Distribution of 38 S1 sites where the high-frequency shorttrain stimuli evoked single twitch-like jaw movement. Blue filled circles indicate the sites where the stimulation with 80 lA stimulus intensity evoked the bilateral Dig EMG activities. Open circles indicate the sites where the stimulation with 80-lA stimulus intensity induced EMG activities only from the c-Dig muscle.
depending on stimulation sites: the face S1, the deep part of the cortical masticatory area, and the orofacial primary motor cortex (Huang et al., 1989; Hatanaka et al., 2005). Another explanation to exclude the possibility of the S1 to Agl pathway is that, if the single sustained jaw
62
K. Uchino et al. / Neuroscience 284 (2015) 55–64
movements by S1 stimulation were evoked via the Agl, the latency of c-Dig EMG activation by S1 stimulation should be longer than the one by Agl stimulation. However the mean latency by S1 stimulation (13.1 ± 1.8 ms) in the present study was not longer, but shorter than the one by Agl stimulation (14.2 ms) reported in the paper of Avivi-Arber et al. (2010). Taken together, it seems plausible that the jaw movements and EMG activities evoked by S1 stimulation were induced by direct activation of neural pathways from the vjuxt-S1 (vo-S1) to the premotoneuron areas (the dorVjuxt and dorVo). The effective stimulation sites for the two kinds of stimulation were distributed in more than half of the vjuxtS1 but in less than half of the vo-S1, although the most effective sites were located in the rostralmost vjuxt-S1 (the rostralmost vo-S1). The rostralmost vjuxt-S1 (the rostralmost vo-S1) contains the largest numbers of neurons projecting to the dorVjuxt but not to the dorVo (Yoshida et al., 2009); the largest numbers of neurons projecting to the dorVo are located in the rostral part of vo-S1 excluding its rostralmost part. These findings may further indicate that the activation of neural pathways to the dorVjuxt rather than to the dorVo is more important to induce jaw movements. Our idea may be supported by the previous work that the descending axons from the rat rostral S1 make more contacts with jaw-closing and jaw-opening premotoneurons in the dorVjuxt than in the dorVo (Chang et al., 2009). In the present study, jaw movements could not be induced by both types of train stimulation (at up to 80 lA intensity) of the caudodorsal part of the vjuxt-S1 or the caudal half of the vo-S1 where there are only smaller number of neurons projecting to the dorVjuxt or dorVo (Yoshida et al., 2009). Higher intensity more than 80 lA may be needed to induce the jaw movements. At all of the 74 effective stimulation sites, higher EMG activities were recorded from the c-Dig muscle rather than from the i-Dig muscle by the two kinds of stimulation of the rostral S1. Although it may be difficult to simply compare the magnitudes of EMG from different sources (i.e., c-Dig muscle vs. i-Dig muscle) due to the variability of resistance and locations of the EMG electrodes, we prefer to think that there was a contralateral predominance between the c-Dig and i-Dig muscle activation. Our idea may be supported by the previous tract-tracing studies that the direct projections from the rostral S1 to the dorVjuxt and dorVo are contralaterally predominant, and the direct projections from the dorVjuxt and dorVo to the jaw-opening component of the Vmo are ipsilaterally predominant (Li et al., 1995; Chang et al., 2009; Yoshida et al., 2009). The low-frequency long-train stimulation of the rostral part of vjuxt-S1 (of vo-S1) did not induce Mas muscle activities in our study, whereas the stimulation of the rostral part of rat Agl activates the jaw-closing muscle and thereby induces rhythmical and obvious jaw movements with alternative jaw-opening and jaw-closing muscle activities (Neafsey et al., 1986; Sasamoto et al., 1990; Zhang and Sasamoto, 1990; Satoh et al., 2007). This difference may be attributable to the difference in their corticofugal projections. Yoshida et al. (2009) have
demonstrated that both the vjuxt-S1 (part of vo-S1) and the rostral Agl project to the dorVjuxt and dorVo, but only the rostral Agl further projects to the intertrigeminal region. The intertrigeminal region mainly contains jawclosing premotoneurons (Yoshida et al., 2009), which fire rhythmically during cortically induced fictive mastication (Olsson et al., 1986a,b; Donga et al., 1990; Donga and Lund, 1991; Inoue et al., 1994; Westberg et al., 1998). In this respect, the Agl activation can facilitate the Mas muscle (via jaw-closing premotoneurons) more easily than the vjuxt-S1 (vo-S1) activation. We should also mention that the dorVjuxt and dorVo contain not only jaw-opening premotoneurons but also jaw-closing premotoneurons, and both the premotoneurons receive strong direct projections from the rostral part of vjuxt-S1 (of vo-S1) (Chang et al., 2009; Yoshida et al., 2009). In the present study, however, both types of stimulation evoked EMG activities in the Dig muscle but not in the Mas muscle. A possible explanation may be that the rostral part of vjuxt-S1 (of vo-S1) gives off axons making contacts with excitatory jaw-opening premotoneurons and inhibitory jaw-closing premotoneurons in the dorVjuxt and dorVo, which in turn project respectively to the trigeminal jaw-opening and jawclosing motoneurons. Because the dorVjuxt and dorVo are considered to contain both excitatory and inhibitory jaw-opening and jaw-closing premotoneurons (Turman and Chandler, 1994; Li et al., 1996; Shigenaga et al., 2000; Gemba-Nishimura et al., 2010). Further studies are needed to clarify the cortical inhibitory pathways to the trigeminal motoneurons.
CONCLUSIONS The present study using intracortical microstimulation methods has for the first time revealed that lowfrequency long-train stimulation of the rat rostral S1 can induce single sustained opening of the jaw with elevated EMG activities of the Dig muscle (jaw-opener) predominantly on the contralateral side. The effective sites for the low-frequency long-train stimulation overlap the S1 sites where traditional high-frequency short-train stimulation is effective to induce single twitch-like jaw movement. The effective sites for the two kinds of train stimuli are included in the rostral S1 area, which has previously been identified to send direct projections to the dorVo or the dorVjuxt that contains jaw-opening and closing premotoneurons. Specifically, the most effective stimulation sites for the two kinds of train stimuli are located in the rostralmost part of S1, which has been reported to send strong direct projections to the dorVjuxt but less to the dorVo. Therefore, the present findings suggest that the rat rostral S1, especially its rostralmost part, plays an important role in controlling jaw movements by activation of direct descending projections from the rostral S1 to the trigeminal premotoneuron pools, especially to the dorVjuxt. Acknowledgments—We wish to thank Dr. Yoshihisa Tachibana for his helpful advice. We are also indebted to Drs. Ayaka Oka, Rieko Takeda, Takashi Fujio and Haruka Ohara for their technical
K. Uchino et al. / Neuroscience 284 (2015) 55–64 help. This work was supported by Ministry of Education, Culture, Sports, Science, and Technology of Japan (Grants 23592699 and 26293391 to A.Y.).
REFERENCES Adachi K, Murray GM, Lee JC, Sessle BJ (2008) Noxious lingual stimulation influences the excitability of the face primary motor cerebral cortex (face MI) in the rat. J Neurophysiol 100:1234–1244. Adams JC (1977) Technical considerations on the use of HRP as a neuronal marker. Neuroscience 2:141–145. Akers RM, Killackey HP (1978) Organization of corticocortical connections in the parietal cortex of the rat. J Comp Neurol 181:513–538. Aldes LD (1988) Thalamic connectivity of rat somatic motor cortex. Brain Res Bull 20:333–348. Asanuma H (1989) The motor cortex. New York: Raven Press. Avivi-Arber L, Lee JC, Sessle BJ (2010) Effects of incisor extraction on jaw and tongue motor representations within face sensorimotor cortex of adult rats. J Comp Neurol 518:1030–1045. Chang Z, Haque T, Iida C, Seki S, Sato F, Kato T, Uchino K, Ono T, Nakamura M, Bae Y-C, Yoshida A (2009) Distribution of premotoneurons for jaw-closing and jaw-opening motor nucleus receiving contacts from axon terminals of primary somatosensory cortical neurons in rats. Brain Res 1275:43–53. Donga R, Lund JP (1991) Discharge patterns of trigeminal commissural last-order interneurons during fictive mastication in the rabbit. J Neurophysiol 66:1564–1578. Donga R, Lund JP, Veilleux D (1990) An electrophysiological study of trigeminal commissural interneurons in the anaesthetized rabbit. Brain Res 515:351–354. Donoghue JP, Parham C (1983) Afferent connections of the lateral agranular field of the rat motor cortex. J Comp Neurol 217:390–404. Donoghue JP, Wise SP (1982) The motor cortex of the rat: cytoarchitecture and microstimulation mapping. J Comp Neurol 212:76–88. Ferrier D (1876) The functions of the brain. New York: G.P. Putnam’s Sons. Gemba-Nishimura A, Inoue T, Nakamura S, Nakayama K, Mochizuki A, Shintani S, Yoshimura S (2010) Properties of synaptic transmission from the reticular formation dorsal to the facial nucleus to trigeminal motoneurons during early postnatal development in rats. Neuroscience 166:1008–1022. Hatanaka N, Tokuno H, Nambu A, Inoue T, Takada M (2005) Input– output organization of jaw movement-related areas in monkey frontal cortex. J Comp Neurol 492:401–425. Huang CS, Hiraba H, Murray GM, Sessle BJ (1989) Topographical distribution and functional properties of cortically induced rhythmical jaw movements in the monkey (Macaca fascicularis). J Neurophysiol 61:635–650. Inoue M, Ariyasinghe S, Yamamura K, Harasawa Y, Yamada Y (2004) Extrinsic tongue and suprahyoid muscle activities during mastication in freely feeding rabbits. Brain Res 1021:173–182. Inoue T, Chandler SH, Goldberg LJ (1994) Neuropharmacological mechanisms underlying rhythmical discharge in trigeminal interneurons during fictive mastication. J Neurophysiol 71:2061–2073. Inoue T, Masuda Y, Nagashima T, Yoshikawa K, Morimoto T (1992) Properties of rhythmically active reticular neurons around the trigeminal motor nucleus during fictive mastication in the rat. Neurosci Res 14:275–294. Iriki A, Nozaki S, Nakamura Y (1987) Reorganization of sensorimotor cortex corresponding to conversion of food ingestive behavior from sucking to chewing during development of guinea pigs. J Physiol Soc Jpn 49:432. Isogai F, Kato T, Fujimoto M, Toi S, Oka A, Adachi T, Maeda Y, Morimoto T, Yoshida A, Masuda Y (2012) Cortical area inducing
63
chewing-like rhythmical jaw movements and its connections with thalamic nuclei in guinea pigs. Neurosci Res 74:239–247. Iwata K, Itoga H, Ikukawa A, Hanashima N, Sumino R (1985) Movements of the jaw and orofacial regions evoked by stimulation of two different cortical areas in cats. Brain Res 359:332–337. Iwata K, Muramatsu H, Tsuboi Y, Sumino R (1990) Input–output relationships in the jaw and orofacial motor zones of the cat cerebral cortex. Brain Res 507:337–340. Kobayashi M, Masuda Y, Fujimoto Y, Matsuya T, Yamamura K, Yamada Y, Maeda N, Morimoto T (2002) Electrophysiological analysis of rhythmic jaw movements in the freely moving mouse. Physiol Behav 75:377–385. Kuypers HGJM (1958a) Corticobulbar connexions to the pons and lower brain-stem in man. An anatomical study. Brain 81: 364–388. Kuypers HGJM (1958b) Some projections from the peri-central cortex to the pons and lower brain stem in monkey and chimpanzee. J Comp Neurol 110:221–255. Kuypers HGJM (1960) Central cortical projections to motor and somato-sensory cell groups. An experimental study in rhesus monkey. Brain 83:161–184. Kuypers HGJM, Lawrence DG (1967) Cortical projections to the red nucleus and the brain stem in the rhesus monkey. Brain Res 4:151–188. Li YQ, Takada M, Kaneko T, Mizuno N (1995) Premotor neurons for trigeminal motor nucleus neurons innervating the jaw-closing and jaw-opening muscles: differential distribution in the lower brainstem of the rat. J Comp Neurol 356:563–579. Li YQ, Takada M, Kaneko T, Mizuno N (1996) GABAergic and glycinergic neurons projecting to the trigeminal motor nucleus: a double labeling study in the rat. J Comp Neurol 373:498–510. Liu ZJ, Masuda Y, Inoue T, Fuchihata H, Sumida A, Takada K, Morimoto T (1993) Coordination of cortically induced rhythmic jaw and tongue movements in the rabbit. J Neurophysiol 69:569–584. Lund JP, Sasamoto K, Murakami T, Olsson KA (1984) Analysis of rhythmical jaw movements produced by electrical stimulation of motor-sensory cortex of rabbits. J Neurophysiol 52:1014–1029. Maeda N, Kobashi M, Mitoh Y, Fujita M, Minagi S, Matsuo R (2014) Differential involvement of two cortical masticatory areas in submandibular salivary secretion in rats. Brain Res 1543:200–208. Masuda Y, Tachibana Y, Inoue T, Iwata K, Morimoto T (2002) Influence of oro-facial sensory input on the output of the cortical masticatory area in the anesthetized rabbit. Exp Brain Res 146:501–510. McGuinness E, Sivertsen D, Allman JM (1980) Organization of the face representation in macaque motor cortex. J Comp Neurol 193:591–608. Neafsey EJ, Bold EL, Haas G, Hurley-Gius KM, Quirk G, Sievert CF, Terreberry RR (1986) The organization of the rat motor cortex: a microstimulation mapping study. Brain Res 396:77–96. Ohta M, Ishizuka S, Saeki K (1989) Corticotrigeminal motor pathway in the rat – II. Anterio- and retrograde HRP labeling. Comp Biochem Physiol 94:405–414. Olsson KA, Landgren S (1980) Facilitation and inhibition of jaw reflexes evoked by electrical stimulation of the cat’s cerebral cortex. Exp Brain Res 39:149–164. Olsson KA, Landgren S, Westberg KG (1986a) Location of, and peripheral convergence on, the interneuron in the disynaptic path from the coronal gyrus of the cerebral cortex to the trigeminal motoneurons in the cat. Exp Brain Res 65:83–97. Olsson KA, Sasamoto K, Lund JP (1986b) Modulation of transmission in rostral trigeminal sensory nuclei during chewing. J Neurophysiol 55:56–75. Paxinos G, Watson C (1998) The rat brain in stereotaxic coordinates. 4th ed. Sydney: Academic Press. Rossi GF, Brodal A (1956) Corticofugal fibres to the brain-stem reticular formation: an experimental study in the cat. J Anat 90:42–62. Sapienza S, Talbi B, Jacquemin J, Albe-Fessard D (1981) Relationship between input and output of cells in motor and
64
K. Uchino et al. / Neuroscience 284 (2015) 55–64
somatosensory cortices of the chronic awake rat. A study using glass micropipettes. Exp Brain Res 43:47–56. Sasamoto K, Zhang G, Iwasaki M (1990) Two types of rhythmical jaw movements evoked by stimulation of the rat cortex. Jpn J Oral Biol 32:57–68. Satoh Y, Ishizuka K, Murakami T (2007) Changes in cortically induced rhythmic jaw movements after lesioning of the red nucleus in rats. Brain Res 1165:60–70. Schoen (1969) The corticofugal projection on the brain stem and spinal cord in man. Psychiatr Neurol Neurochir 72:121–128. Shigenaga Y, Hirose Y, Yoshida A, Fukami H, Honma S, Bae YC (2000) Quantitative ultrastructure of physiologically identified premotor neuron terminals in the trigeminal motor nucleus in the cat. J Comp Neurol 426:13–30. Tomita A, Kato T, Sato F, Haque T, Oka A, Yamamoto M, Ono T, Bae Y-C, Maeda Y, Sessle BJ, Yoshida A (2012) Somatotopic direct projections from orofacial areas of primary somatosensory cortex to pons and medulla, especially to trigeminal sensory nuclear complex, in rats. Neuroscience 200:166–185. Turman Jr J, Chandler SH (1994) Immunohistochemical evidence for GABA and glycine-containing trigeminal premotoneurons in the guinea pig. Synapse 18:7–20. Valverde F (1962) Reticular formation of the albino rat’s brain stem cytoarchitecture and corticofugal connections. J Comp Neurol 119:25–53. Vogt O, Vogt C (1919) Allgemeinere Ergebnisse unserer Hirnforschung. J Psychol Neurol 25:277–462.
Vogt O, Vogt C (1926) Die vergleichend-architktonische und vergleichend-reizphysiologische Felderung der Grosshirnrinde unter besonderer Beru¨cksichtigung der menschlichen. Naturwissenschaften 14:1190–1194. Walberg F (1957) Corticofugal fibers to the nuclei of the dorsal columns. An experimental study in the cat. Brain 80:273–287. Walker AE, Green HD (1938) Electrical excitability of the motor face area: a comparative study in primates. J Neurophysiol 1:152–165. Westberg K, Clavelou P, Sandstro¨m G, Lund JP (1998) Evidence that trigeminal brainstem interneurons form subpopulations to produce different forms of mastication in the rabbit. J Neurosci 18:6466–6479. Yamada Y, Haraguchi N, Oi K, Sasaki M (1988) Two-dimensional jaw tracking and EMG recording system implanted in the freely moving rabbit. J Neurosci Methods 23:257–261. Yoshida A, Taki I, Chang Z, Iida C, Haque T, Tomita A, Seki S, Yamamoto S, Masuda Y, Moritani M, Shigenaga Y (2009) Corticofugal projections to the trigeminal motoneurons innervating antagonistic jaw muscles in rats as demonstrated by anterograde and retrograde tract tracing. J Comp Neurol 514:368–386. Zhang G, Sasamoto K (1990) Projections of two separate cortical areas for rhythmical jaw movements in the rat. Brain Res Bull 24:221–230.
(Accepted 2 September 2014) (Available online 5 October 2014)