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Contralateral projections of barosensitive neurons of the nucleus tractus solitarii
ELSEVIER
Neuroscience Letters 219 (1996) 37-40
NEUROSCI[NC[ I[1T[BS
Contralateral projections of barosensitive neurons of the nucleus tractus solitarii Kazuhisa
Ezure*,
Ikuko Tanaka
Department of Neurobiology, Tokyo Metropolitan Institutefor Neuroscience, 2-6 Musashidai, Fuchu, Tokyo 183, Japan
Received 13 September 1996; revised version received 11 October 1996; accepted 11 October 1996
Abstract
Axonal projections of single barosensitive neurons of the nucleus tractus solitarii (NTS) which fired in synchrony with heartbeat, were studied in Nembutal-anesthetized, paralyzed, and artificially ventilated cats. The majority of them were orthodromically activated by electrical stimulation of the ipsilateral cervical vagal nerve. Antidromic mapping by electrical stimulation of the medulla could identify the axonal projections in 14 of the 25 barosensitive NTS neurons examined. Their stem axons crossed the midline to the contralateral side and ascended rostrally. The contralateral axons of some neurons issued collaterals in the rostral ventrolateral medulla (RVLM) medial to and ventromedial to the facial nucleus. The contralateral axons of the other neurons ascended toward the pons without any sign of issuing collaterals within the medulla. Keywords: Baroreceptor; Nucleus tractus solitarii; Cardiac pulse; Axonal projection; Antidromic mapping; Rostral ventrolateral medulla; Cat medulla
The baroreceptor reflex and its pathways have been investigated extensively with various anatomical and electrophysiological techniques [12]. The afferents from baroreceptors in the carotid sinus and the aortic arch project to the nucleus tractus solitarii (NTS) and synapse on the second-order relay neurons. Neuroanatomical tracing studies have provided substantial evidence that NTS neurons project to the medullary cardiovascular areas, such as the rostral and the caudal ventrolateral medulla ( R V L M and C V L M ) [1,11]. However, an uncertainty remains as to whether the revealed projections originate from baroreceptor relay neurons or not, since in the same NTS area there are neurons which receive inputs from receptors different from baroreceptors. In fact, there is a consensus that the projections from the NTS to the R V L M are of chemoreceptor-related functions rather than baroreceptor-related functions [3,12]. At present, the wealth of experimental evidence suggests that the projections from the NTS to the C V L M and then to the R V L M form the basic pathways of the baroreceptor reflex [ 1,3,7,12,13]. However, direct * Corresponding author. Tel.: +81 423 253881, ext. 4222; Fax: +81 423 218678; e-mail: [email protected]
evidence that these projections originate in NTS neurons which receive baroreceptor inputs is still not available. Thus, the present study aimed to find medullary projections, by electrical antidromic mapping, of single NTS neurons identified as baroreceptor relay neurons: the positive data so far obtained are their projections to the R V L M rather than the CVLM. Experiments were conducted on 14 adult cats anesthetized with sodium pentobarbitone (Nembutal, 40 mg/kg; i.p.). Eight of them were from the same population used in our previous study [6]. Almost all the experimental procedures, such as surgery, care of animals, electrodes, vagal nerve stimulation, neural recording, antidromic stimulation, or histological reconstruction of the brainstem, were the same as those described previously [6]. Extracellular recordings were made from neurons in the vicinity of the solitary tract around the level of the obex. W e explored barosensitive NTS neurons which fired in synchrony with heartbeat (Fig. 1A1), and this report is based on twenty-five such neurons. The barosensitive NTS neurons showed their maximum firing around the rising phase of the blood pressure monitored in the femoral artery as shown in Fig. 1A1,A2. Electrical stimulation of
0304-3940/96/$12.00 © 1996 Elsevier Science Ireland Ltd. All rights reserved PII S0304-3940(96) 13169-4
38
K. Ezure, 1. Tanaka / Neuroscience Letters 219 (1996) 37 40
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Fig. 1. (A l) Extracellular recording (upper trace) from a barosensitive NTS neuron and blood pressure from the femoral artery (lower trace). (A2) Spike histogram (upper trace) made by averaging ten consecutive cycles (from (Al)) aligned (vertical line) at the rising slope of the blood pressure (lower trace). (BI) 5 superimposed traces (neuron shown in (A1)) of the responses to vagal nerve stimulation (arrowhead). (B2) Distribution of orthodromic latencies (n = 19). Distribution of 15 barosensitive NTS neurons are projected onto horizontal (C 1) and transverse (C2) planes: each plane in (C2) corresponds to levels shown in (CI). Filled circles: neurons with medullary axonal branches. Open circles, neurons without medullary branches (shown in Fig. 3). Filled triangle, a neuron whose contralateral projection only was confirmed. Cross, a neuron whose projection could not be found in spite of extensive mapping. CU, Cuneate nucleus; DX, dorsal motor nucleus of the vagus; GR, gracile nucleus; S, solitary tract; 12N, hypoglossaI nucleus.
the ipsilateral vagal nerve orthodromically excited these neurons (Fig. 1B 1). When stimulation was applied around the rising phase of the blood pressure, the orthodromic latencies were shorter; whereas, when stimulation was applied in other phases, the latencies were longer or spikes were not elicited. Orthodromic latencies could be measured in 19 neurons and their shortest latencies ranged from 3.4 to 5.8 ms: the mean was 4.8 _+ 0.57 ms (±SD). No clear responses could be observed in the six remaining neurons. The locations of the barosensitive NTS neurons whose axonal projections could be identified (see below) were marked as shown in Fig. 1C1,C2. They were distributed in close vicinity to, and largely dorsomedial to, the solitary tract (Fig. 1C2) at the level between about 1 mm caudal to and 1 m m rostral to the obex (Fig. 1CI). The 25 barosensitive NTS neurons were tested for their projections to the brainstem by antidromic mapping. Fourteen neurons could be antidromically activated with stimulation of the medullary reticular areas, whereas the other eleven neurons could not. All of the 14 neurons were antidromically activated from the contralateral medulla (Figs 2 and 3). Their antidromic latencies increased gradually from caudal to rostral areas, indicating that all of these contralateral axons ran in a rostral direction. In five neu-
rons, antidromic responses with several distinct latencies were evoked from adjacent tracks or even within a track (Fig. 2); among such tracks there were often tracks in which no responses could be evoked (Fig. 2). These observations could be considered as the evidence for axonal arborizations. In three of these five neurons, the presence of axons running toward the pons was also suggested (e.g. Fig. 2). On the other hand, in eight neurons, the axons ascended toward the pons without any sign of axonal arborizations within the medulla (Fig. 3). The contralateral axon of the one remaining neuron could not be traced sufficiently. In ten of the 14 neurons with contralateral projections and in three of the 11 neurons whose projections could not be found, stimulation of the ipsilateral medulla could not evoke antidromic responses (see Fig. 2A2 and 3A). In the five neurons mentioned above, tile axonal arborizations were found consistently in the area medial to and ventromedial to the facial nucleus (Fig. 2AI,B,C). This area is a part of the nucleus paragigantocellularis lateralis (PGL) [8,11], corresponding to the facial division and the rostral part of the retrofacial division of the P G L [2]. In two of the five neurons, their axonal branches extended to the area ventral to the facial nucleus which was close to the retrotrapezoid nucleus [2](Fig. 2B,C). In these five neurons, the longest antidromic latencies from the level of the caudal pole of the facial nucleus were 2 . 6 - 8 . 6 ms, and the shortest latencies suggestive of their stem axons were 2 . 1 2.8 ms. Approximate conduction velocities of the stem axons, calculated from roughly estimated distance of about 8 mm, ranged from 2.9 to 3.8 m/s. In the eight neurons whose axons ran toward the ports without suggestions of collaterals, the antidromic latencies from the level of the facial nucleus were 1.5-3.4 ms (Fig. 3). Their approximate conduction velocities, calculated from the assumed distance of about 9 mm, ranged from 2.6 to 6.0 m/s.
This study has revealed axonal trajectories and projections of a certain population of barosensitive NTS neurons. The majority of the present barosensitive NTS neurons could be orthodromically activated by single-shock stimulation of the cervical vagal nerve which contains the aortic baroreceptor afferents. The latencies are consistent with the idea that they are second-order relay neurons which receive myelinated afferents from the aortic baroreceptors [5]. However, the possibility cannot be denied and remains to be studied that these neurons receive afferents from the carotid sinus and/or atrial baroreceptors [4]. The fact that the latencies of the orthodromic spikes fluctuated in synchrony with the blood pressure, suggests that synchronous afferent volleys are necessary to activate these barosensitive NTS neurons, or that these neurons are inhibited during their inactive phase. All of the antidromic responses we could evoke in the 14 barosensitive NTS neurons were from the contralateral side, and none from the ipsilateral side. Firm conclusions
39
K. Ezure, 1. Tanaka / Neuroscience Letters 219 (1996) 37-40
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Fig. 2. Examples of antidromic mapping from three barosensitive NTS neurons. A horizontal projection (A2) and transverse sections (A 1) for one neuron (filled square in (A2)). Tracks of stimulation are shown by vertical lines and thresholds for activation by filled circles (either 100 or 200/~A). Tracks in which no antidromic activation was found are shown by crosses (A2). Representative latencies are given by numbers (ms). For two neurons (B,C) only transverse sections at the level of the caudal pole of the facial nucleus are shown. (A3) Collision test performed in plane d (five traces are superimposed). Stimulus (arrowhead) was applied 8 ms (upper trace) and 7 ms (lower trace) after the spontaneous spikes. The stimulus in the lower trace elicited no spikes because of collision. AMB, Nucleus ambiguus; IO, inferior olivary nucleus; RA, nucleus retroambigualis; RFN, retrofacial nucleus; TN, trapezoid nucleus; 5ST, spinal trigeminal tract; 7N, facial nucleus. a b o u t the i p s i l a t e r a l p r o j e c t i o n c a n n o t b e m a d e s i n c e the p r e s e n t s t i m u l a t i o n o f the ipsilateral m e d u l l a w a s n e v e r t h o r o u g h (see Fig. 2 A 2 a n d 3A). N e v e r t h e l e s s , w e b e l i e v e that the i p s i l a t e r a l p r o j e c t i o n s are, i f any, m i n o r , b e c a u s e this s t u d y h a d e n o u g h c h a n c e o f d e t e c t i n g i p s i l a t e r a l proj e c t i o n s if s u c h p r o j e c t i o n s are a b u n d a n t . In fact, p r e d o m i nantly ipsilateral projections of NTS neurons which r e c e i v e a f f e r e n t s f r o m the p u l m o n a r y stretch r e c e p t o r s
h a v e b e e n d e m o n s t r a t e d in the c o m p a n i o n e x p e r i m e n t s [6] c o n d u c t e d in p a r a l l e l w i t h the p r e s e n t e x p e r i m e n t s . T h e a x o n a l a r b o r i z a t i o n s f r o m the p r e s e n t b a r o s e n s i t i v e N T S n e u r o n s w e r e f o u n d in the facial a n d r e t r o f a c i a l (its rostral part) d i v i s i o n s o f the P G L . T h i s area, lying m e d i a l l y to a n d v e n t r o m e d i a l l y to the facial a n d retrofacial nuclei, c o i n c i d e s well with the area in w h i c h a n t e r o g r a d e l y l a b e l e d t e r m i n a l s f r o m t h e N T S were f o u n d [11]. A l t h o u g h
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Fig. 3. Antidromic mapping of a barosensitive NTS neuron which has an ascending axon toward the pons without suggestions of axonal branches within the medulla (A). Current intensity of the stimulation was fixed at 100 /zA. Extensive stimulation of the shaded areas (without strict histological reconstruction) could not antidromically activate this neuron. Axonal trajectories of eight such neurons, including a neuron shown in (A), are schematically drawn based on the data of antidromic mapping (B). Antidromic latencies at the rostral ends of mapping for each neuron are given by numbers (ms).
40
K. Ezure, I. Tanaka / Neuroscience Letters 219 (1996) 37-40
the N T S p r o j e c t i o n s to the R V L M are n o t n e c e s s a r i l y a s c r i b e d to b a r o r e c e p t o r - r e l a t e d f u n c t i o n s [3,12], t h e present s t u d y h a s s h o w n that at least s o m e o f t h e s e p r o j e c t i o n s are b a r o r e c e p t o r - r e l a t e d . It s h o u l d b e n o t e d that the p r e s e n t a x o n a l p r o j e c t i o n s are r a t h e r c o n f i n e d to t h e rostral part o f the R V L M , a n d are rostral to the b u l k o f the s u b r e t r o f a c i a l n u c l e u s [10] in w h i c h b u l b o s p i n a l s y m p a t h o e x c i t a t o r y n e u r o n s are c o n c e n t r a t e d [9]. T h e r e f o r e , the p r e s e n t barosensitive NTS neurons may not make synaptic contacts w i t h t h e s e s y m p a t h o e x c i t a t o r y n e u r o n s , a l t h o u g h the c o n tacts are p o s s i b l e w i t h s i m i l a r n e u r o n s w h i c h are also scatt e r e d in t h e r e g i o n v e n t r a l to the facial n u c l e u s [10]. S y n a p t i c c o n t a c t s w i t h n e u r o n s o f the r e t r o t r a p e z o i d nucleus, whose respiratory functions have been reported [2], are also p o s s i b l e . O n t h e o t h e r h a n d , a n y p r o j e c t i o n s to the area in a n d a r o u n d the n u c l e i a m b i g u u s a n d r e t r o a m bigualis, w h i c h c o r r e s p o n d s to the C V L M o r p e r i a m b i g u a l area [8], h a v e n o t b e e n d e m o n s t r a t e d . It s e e m s r e a s o n a b l e to t h i n k t h a t s u c h p r o j e c t i o n s are m a d e b y o t h e r t y p e s o f b a r o s e n s i t i v e N T S n e u r o n s [12] w h i c h we d i d n o t r e c o r d f r o m . F u r t h e r m o r e , a l t h o u g h the a x o n s w h i c h r u n t o w a r d the p o n s c o u l d p r o j e c t to s t r u c t u r e s in the p o n s , m i d b r a i n , h y p o t h a l a m u s a n d f o r e b r a i n , n o f u r t h e r i n f o r m a t i o n is a v a i l a b l e . C l e a r l y , f u r t h e r studies are n e e d e d to d i s c l o s e m o r e p r e c i s e e f f e r e n t p r o j e c t i o n s o f the p r e s e n t b a r o s e n s i tive N T S n e u r o n s a n d t h e f u n c t i o n a l s i g n i f i c a n c e o f the observed axonal projections. [1] Aicher, S.A., Kurucz, O.S., Reis, D.J. and Milner, T.A., Nucleus tractus solitarius efferent terminals synapse on neurons in the caudal ventrolateral medulla that project to the rostral ventrolateral medulla, Brain Res., 693 (1995) 51-63. [2l Connelly, C.A., Ellenberger, H.H. and Feldman, J.L., Respiratory activity in retrotrapezoid nucleus in cat, Am. J. Physiol., 258 (1990) L33-L44.
[3] Dampney, R.A.L., Functional organization of central pathways regulating the cardiovascular system, Physiol. Rev., 74 (1994) 323-364. [4] Donoghue, S., Felder, R.B., Jordan, D. and Spyer, K.M., The central projections of carotid baroreceptors and chemoreceptors in the cat: a neurophysiological study, J. Physiol. (London), 347 (1984) 397-409. [5] Donoghue, S., Garcia, M., Jordan, D. and Spyer, K.M., Identification and brain-stem projections of aortic baroreceptor afferent neurones in nodose ganglia of cats and rabbits, J. Physiol. (London), 322 (1982) 337-352. [6] Ezure, K. and Tanaka, I., Pump neurons of the nucleus of the solitary tract project widely to the medulla, Neurosci. Lett., 215 (1996) 123-126. [7] Jeske, I., Reis, D.J. and Milner, T.A., Neurons in the barosensory area of the caudal ventrolateral medulla project monosynaptically on to sympatfioexcitatory bulbospinal neurons in the rostral ventrolateral medulla, Neuroscience, 65 (1995) 343-353. [8] Guyenet, P.G., Role of the ventral medulla oblongata in blood pressure regulation. In A.D. Lowey and K.M. Spyer (Eds.), Central Regulation of Autonomic Functions, Oxford University Press, Oxford, 1990, pp. 145-167. [9] McAllen, R.M., Central respiratory modulation of subretrofacial bulbospinal neurones in the cat, J. Physiol. (London), 388 (1987) 533-545. [10] Polen, J.W., Halliday, G.M., McAllen, R.M., Coleman, M.J. and Dampney, R.A.L., Rostrocaudal differences in morphology and neurotransmitter content of cells in the subretrofacial vasomotor nucleus, J. Autonomic Nerv. Syst., 38 (1992) 117-138. [11] Ross, C.A., Ruggiero, D.A. and Reis, D.J., Projections from the nucleus tractus solitarii to the rostral ventrolateral medulla, J. Comp. Neurol., 242 (1985) 511-534. [12] Sun, M.-K., Central neural organization and control of sympathetic nervous system in mammals, Prog. Neurobiol., 47 (1995) 157-233. [13] Terui, N., Masuda, N., Saeki, Y. and Kumada, M., Activity of barosensitive neurons in the caudal ventrolateral medulla that send axonal projections to the rostral ventrolateral medulla in rabbits, Neurosci. Lett., 118 (1990) 211-214.
Neuroscience Letters 219 (1996) 37-40
NEUROSCI[NC[ I[1T[BS
Contralateral projections of barosensitive neurons of the nucleus tractus solitarii Kazuhisa
Ezure*,
Ikuko Tanaka
Department of Neurobiology, Tokyo Metropolitan Institutefor Neuroscience, 2-6 Musashidai, Fuchu, Tokyo 183, Japan
Received 13 September 1996; revised version received 11 October 1996; accepted 11 October 1996
Abstract
Axonal projections of single barosensitive neurons of the nucleus tractus solitarii (NTS) which fired in synchrony with heartbeat, were studied in Nembutal-anesthetized, paralyzed, and artificially ventilated cats. The majority of them were orthodromically activated by electrical stimulation of the ipsilateral cervical vagal nerve. Antidromic mapping by electrical stimulation of the medulla could identify the axonal projections in 14 of the 25 barosensitive NTS neurons examined. Their stem axons crossed the midline to the contralateral side and ascended rostrally. The contralateral axons of some neurons issued collaterals in the rostral ventrolateral medulla (RVLM) medial to and ventromedial to the facial nucleus. The contralateral axons of the other neurons ascended toward the pons without any sign of issuing collaterals within the medulla. Keywords: Baroreceptor; Nucleus tractus solitarii; Cardiac pulse; Axonal projection; Antidromic mapping; Rostral ventrolateral medulla; Cat medulla
The baroreceptor reflex and its pathways have been investigated extensively with various anatomical and electrophysiological techniques [12]. The afferents from baroreceptors in the carotid sinus and the aortic arch project to the nucleus tractus solitarii (NTS) and synapse on the second-order relay neurons. Neuroanatomical tracing studies have provided substantial evidence that NTS neurons project to the medullary cardiovascular areas, such as the rostral and the caudal ventrolateral medulla ( R V L M and C V L M ) [1,11]. However, an uncertainty remains as to whether the revealed projections originate from baroreceptor relay neurons or not, since in the same NTS area there are neurons which receive inputs from receptors different from baroreceptors. In fact, there is a consensus that the projections from the NTS to the R V L M are of chemoreceptor-related functions rather than baroreceptor-related functions [3,12]. At present, the wealth of experimental evidence suggests that the projections from the NTS to the C V L M and then to the R V L M form the basic pathways of the baroreceptor reflex [ 1,3,7,12,13]. However, direct * Corresponding author. Tel.: +81 423 253881, ext. 4222; Fax: +81 423 218678; e-mail: [email protected]
evidence that these projections originate in NTS neurons which receive baroreceptor inputs is still not available. Thus, the present study aimed to find medullary projections, by electrical antidromic mapping, of single NTS neurons identified as baroreceptor relay neurons: the positive data so far obtained are their projections to the R V L M rather than the CVLM. Experiments were conducted on 14 adult cats anesthetized with sodium pentobarbitone (Nembutal, 40 mg/kg; i.p.). Eight of them were from the same population used in our previous study [6]. Almost all the experimental procedures, such as surgery, care of animals, electrodes, vagal nerve stimulation, neural recording, antidromic stimulation, or histological reconstruction of the brainstem, were the same as those described previously [6]. Extracellular recordings were made from neurons in the vicinity of the solitary tract around the level of the obex. W e explored barosensitive NTS neurons which fired in synchrony with heartbeat (Fig. 1A1), and this report is based on twenty-five such neurons. The barosensitive NTS neurons showed their maximum firing around the rising phase of the blood pressure monitored in the femoral artery as shown in Fig. 1A1,A2. Electrical stimulation of
0304-3940/96/$12.00 © 1996 Elsevier Science Ireland Ltd. All rights reserved PII S0304-3940(96) 13169-4
38
K. Ezure, 1. Tanaka / Neuroscience Letters 219 (1996) 37 40
l,lll .
l iii .
.
.
.
.
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.
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.=,,
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,/~ (~
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Fig. 1. (A l) Extracellular recording (upper trace) from a barosensitive NTS neuron and blood pressure from the femoral artery (lower trace). (A2) Spike histogram (upper trace) made by averaging ten consecutive cycles (from (Al)) aligned (vertical line) at the rising slope of the blood pressure (lower trace). (BI) 5 superimposed traces (neuron shown in (A1)) of the responses to vagal nerve stimulation (arrowhead). (B2) Distribution of orthodromic latencies (n = 19). Distribution of 15 barosensitive NTS neurons are projected onto horizontal (C 1) and transverse (C2) planes: each plane in (C2) corresponds to levels shown in (CI). Filled circles: neurons with medullary axonal branches. Open circles, neurons without medullary branches (shown in Fig. 3). Filled triangle, a neuron whose contralateral projection only was confirmed. Cross, a neuron whose projection could not be found in spite of extensive mapping. CU, Cuneate nucleus; DX, dorsal motor nucleus of the vagus; GR, gracile nucleus; S, solitary tract; 12N, hypoglossaI nucleus.
the ipsilateral vagal nerve orthodromically excited these neurons (Fig. 1B 1). When stimulation was applied around the rising phase of the blood pressure, the orthodromic latencies were shorter; whereas, when stimulation was applied in other phases, the latencies were longer or spikes were not elicited. Orthodromic latencies could be measured in 19 neurons and their shortest latencies ranged from 3.4 to 5.8 ms: the mean was 4.8 _+ 0.57 ms (±SD). No clear responses could be observed in the six remaining neurons. The locations of the barosensitive NTS neurons whose axonal projections could be identified (see below) were marked as shown in Fig. 1C1,C2. They were distributed in close vicinity to, and largely dorsomedial to, the solitary tract (Fig. 1C2) at the level between about 1 mm caudal to and 1 m m rostral to the obex (Fig. 1CI). The 25 barosensitive NTS neurons were tested for their projections to the brainstem by antidromic mapping. Fourteen neurons could be antidromically activated with stimulation of the medullary reticular areas, whereas the other eleven neurons could not. All of the 14 neurons were antidromically activated from the contralateral medulla (Figs 2 and 3). Their antidromic latencies increased gradually from caudal to rostral areas, indicating that all of these contralateral axons ran in a rostral direction. In five neu-
rons, antidromic responses with several distinct latencies were evoked from adjacent tracks or even within a track (Fig. 2); among such tracks there were often tracks in which no responses could be evoked (Fig. 2). These observations could be considered as the evidence for axonal arborizations. In three of these five neurons, the presence of axons running toward the pons was also suggested (e.g. Fig. 2). On the other hand, in eight neurons, the axons ascended toward the pons without any sign of axonal arborizations within the medulla (Fig. 3). The contralateral axon of the one remaining neuron could not be traced sufficiently. In ten of the 14 neurons with contralateral projections and in three of the 11 neurons whose projections could not be found, stimulation of the ipsilateral medulla could not evoke antidromic responses (see Fig. 2A2 and 3A). In the five neurons mentioned above, tile axonal arborizations were found consistently in the area medial to and ventromedial to the facial nucleus (Fig. 2AI,B,C). This area is a part of the nucleus paragigantocellularis lateralis (PGL) [8,11], corresponding to the facial division and the rostral part of the retrofacial division of the P G L [2]. In two of the five neurons, their axonal branches extended to the area ventral to the facial nucleus which was close to the retrotrapezoid nucleus [2](Fig. 2B,C). In these five neurons, the longest antidromic latencies from the level of the caudal pole of the facial nucleus were 2 . 6 - 8 . 6 ms, and the shortest latencies suggestive of their stem axons were 2 . 1 2.8 ms. Approximate conduction velocities of the stem axons, calculated from roughly estimated distance of about 8 mm, ranged from 2.9 to 3.8 m/s. In the eight neurons whose axons ran toward the ports without suggestions of collaterals, the antidromic latencies from the level of the facial nucleus were 1.5-3.4 ms (Fig. 3). Their approximate conduction velocities, calculated from the assumed distance of about 9 mm, ranged from 2.6 to 6.0 m/s.
This study has revealed axonal trajectories and projections of a certain population of barosensitive NTS neurons. The majority of the present barosensitive NTS neurons could be orthodromically activated by single-shock stimulation of the cervical vagal nerve which contains the aortic baroreceptor afferents. The latencies are consistent with the idea that they are second-order relay neurons which receive myelinated afferents from the aortic baroreceptors [5]. However, the possibility cannot be denied and remains to be studied that these neurons receive afferents from the carotid sinus and/or atrial baroreceptors [4]. The fact that the latencies of the orthodromic spikes fluctuated in synchrony with the blood pressure, suggests that synchronous afferent volleys are necessary to activate these barosensitive NTS neurons, or that these neurons are inhibited during their inactive phase. All of the antidromic responses we could evoke in the 14 barosensitive NTS neurons were from the contralateral side, and none from the ipsilateral side. Firm conclusions
39
K. Ezure, 1. Tanaka / Neuroscience Letters 219 (1996) 37-40
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26
Fig. 2. Examples of antidromic mapping from three barosensitive NTS neurons. A horizontal projection (A2) and transverse sections (A 1) for one neuron (filled square in (A2)). Tracks of stimulation are shown by vertical lines and thresholds for activation by filled circles (either 100 or 200/~A). Tracks in which no antidromic activation was found are shown by crosses (A2). Representative latencies are given by numbers (ms). For two neurons (B,C) only transverse sections at the level of the caudal pole of the facial nucleus are shown. (A3) Collision test performed in plane d (five traces are superimposed). Stimulus (arrowhead) was applied 8 ms (upper trace) and 7 ms (lower trace) after the spontaneous spikes. The stimulus in the lower trace elicited no spikes because of collision. AMB, Nucleus ambiguus; IO, inferior olivary nucleus; RA, nucleus retroambigualis; RFN, retrofacial nucleus; TN, trapezoid nucleus; 5ST, spinal trigeminal tract; 7N, facial nucleus. a b o u t the i p s i l a t e r a l p r o j e c t i o n c a n n o t b e m a d e s i n c e the p r e s e n t s t i m u l a t i o n o f the ipsilateral m e d u l l a w a s n e v e r t h o r o u g h (see Fig. 2 A 2 a n d 3A). N e v e r t h e l e s s , w e b e l i e v e that the i p s i l a t e r a l p r o j e c t i o n s are, i f any, m i n o r , b e c a u s e this s t u d y h a d e n o u g h c h a n c e o f d e t e c t i n g i p s i l a t e r a l proj e c t i o n s if s u c h p r o j e c t i o n s are a b u n d a n t . In fact, p r e d o m i nantly ipsilateral projections of NTS neurons which r e c e i v e a f f e r e n t s f r o m the p u l m o n a r y stretch r e c e p t o r s
h a v e b e e n d e m o n s t r a t e d in the c o m p a n i o n e x p e r i m e n t s [6] c o n d u c t e d in p a r a l l e l w i t h the p r e s e n t e x p e r i m e n t s . T h e a x o n a l a r b o r i z a t i o n s f r o m the p r e s e n t b a r o s e n s i t i v e N T S n e u r o n s w e r e f o u n d in the facial a n d r e t r o f a c i a l (its rostral part) d i v i s i o n s o f the P G L . T h i s area, lying m e d i a l l y to a n d v e n t r o m e d i a l l y to the facial a n d retrofacial nuclei, c o i n c i d e s well with the area in w h i c h a n t e r o g r a d e l y l a b e l e d t e r m i n a l s f r o m t h e N T S were f o u n d [11]. A l t h o u g h
i
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i
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AMB : --
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Fig. 3. Antidromic mapping of a barosensitive NTS neuron which has an ascending axon toward the pons without suggestions of axonal branches within the medulla (A). Current intensity of the stimulation was fixed at 100 /zA. Extensive stimulation of the shaded areas (without strict histological reconstruction) could not antidromically activate this neuron. Axonal trajectories of eight such neurons, including a neuron shown in (A), are schematically drawn based on the data of antidromic mapping (B). Antidromic latencies at the rostral ends of mapping for each neuron are given by numbers (ms).
40
K. Ezure, I. Tanaka / Neuroscience Letters 219 (1996) 37-40
the N T S p r o j e c t i o n s to the R V L M are n o t n e c e s s a r i l y a s c r i b e d to b a r o r e c e p t o r - r e l a t e d f u n c t i o n s [3,12], t h e present s t u d y h a s s h o w n that at least s o m e o f t h e s e p r o j e c t i o n s are b a r o r e c e p t o r - r e l a t e d . It s h o u l d b e n o t e d that the p r e s e n t a x o n a l p r o j e c t i o n s are r a t h e r c o n f i n e d to t h e rostral part o f the R V L M , a n d are rostral to the b u l k o f the s u b r e t r o f a c i a l n u c l e u s [10] in w h i c h b u l b o s p i n a l s y m p a t h o e x c i t a t o r y n e u r o n s are c o n c e n t r a t e d [9]. T h e r e f o r e , the p r e s e n t barosensitive NTS neurons may not make synaptic contacts w i t h t h e s e s y m p a t h o e x c i t a t o r y n e u r o n s , a l t h o u g h the c o n tacts are p o s s i b l e w i t h s i m i l a r n e u r o n s w h i c h are also scatt e r e d in t h e r e g i o n v e n t r a l to the facial n u c l e u s [10]. S y n a p t i c c o n t a c t s w i t h n e u r o n s o f the r e t r o t r a p e z o i d nucleus, whose respiratory functions have been reported [2], are also p o s s i b l e . O n t h e o t h e r h a n d , a n y p r o j e c t i o n s to the area in a n d a r o u n d the n u c l e i a m b i g u u s a n d r e t r o a m bigualis, w h i c h c o r r e s p o n d s to the C V L M o r p e r i a m b i g u a l area [8], h a v e n o t b e e n d e m o n s t r a t e d . It s e e m s r e a s o n a b l e to t h i n k t h a t s u c h p r o j e c t i o n s are m a d e b y o t h e r t y p e s o f b a r o s e n s i t i v e N T S n e u r o n s [12] w h i c h we d i d n o t r e c o r d f r o m . F u r t h e r m o r e , a l t h o u g h the a x o n s w h i c h r u n t o w a r d the p o n s c o u l d p r o j e c t to s t r u c t u r e s in the p o n s , m i d b r a i n , h y p o t h a l a m u s a n d f o r e b r a i n , n o f u r t h e r i n f o r m a t i o n is a v a i l a b l e . C l e a r l y , f u r t h e r studies are n e e d e d to d i s c l o s e m o r e p r e c i s e e f f e r e n t p r o j e c t i o n s o f the p r e s e n t b a r o s e n s i tive N T S n e u r o n s a n d t h e f u n c t i o n a l s i g n i f i c a n c e o f the observed axonal projections. [1] Aicher, S.A., Kurucz, O.S., Reis, D.J. and Milner, T.A., Nucleus tractus solitarius efferent terminals synapse on neurons in the caudal ventrolateral medulla that project to the rostral ventrolateral medulla, Brain Res., 693 (1995) 51-63. [2l Connelly, C.A., Ellenberger, H.H. and Feldman, J.L., Respiratory activity in retrotrapezoid nucleus in cat, Am. J. Physiol., 258 (1990) L33-L44.
[3] Dampney, R.A.L., Functional organization of central pathways regulating the cardiovascular system, Physiol. Rev., 74 (1994) 323-364. [4] Donoghue, S., Felder, R.B., Jordan, D. and Spyer, K.M., The central projections of carotid baroreceptors and chemoreceptors in the cat: a neurophysiological study, J. Physiol. (London), 347 (1984) 397-409. [5] Donoghue, S., Garcia, M., Jordan, D. and Spyer, K.M., Identification and brain-stem projections of aortic baroreceptor afferent neurones in nodose ganglia of cats and rabbits, J. Physiol. (London), 322 (1982) 337-352. [6] Ezure, K. and Tanaka, I., Pump neurons of the nucleus of the solitary tract project widely to the medulla, Neurosci. Lett., 215 (1996) 123-126. [7] Jeske, I., Reis, D.J. and Milner, T.A., Neurons in the barosensory area of the caudal ventrolateral medulla project monosynaptically on to sympatfioexcitatory bulbospinal neurons in the rostral ventrolateral medulla, Neuroscience, 65 (1995) 343-353. [8] Guyenet, P.G., Role of the ventral medulla oblongata in blood pressure regulation. In A.D. Lowey and K.M. Spyer (Eds.), Central Regulation of Autonomic Functions, Oxford University Press, Oxford, 1990, pp. 145-167. [9] McAllen, R.M., Central respiratory modulation of subretrofacial bulbospinal neurones in the cat, J. Physiol. (London), 388 (1987) 533-545. [10] Polen, J.W., Halliday, G.M., McAllen, R.M., Coleman, M.J. and Dampney, R.A.L., Rostrocaudal differences in morphology and neurotransmitter content of cells in the subretrofacial vasomotor nucleus, J. Autonomic Nerv. Syst., 38 (1992) 117-138. [11] Ross, C.A., Ruggiero, D.A. and Reis, D.J., Projections from the nucleus tractus solitarii to the rostral ventrolateral medulla, J. Comp. Neurol., 242 (1985) 511-534. [12] Sun, M.-K., Central neural organization and control of sympathetic nervous system in mammals, Prog. Neurobiol., 47 (1995) 157-233. [13] Terui, N., Masuda, N., Saeki, Y. and Kumada, M., Activity of barosensitive neurons in the caudal ventrolateral medulla that send axonal projections to the rostral ventrolateral medulla in rabbits, Neurosci. Lett., 118 (1990) 211-214.