Neonatal capsaicin treatment (NCT) alters the metabolic activity of the rat somatosensory cortex in response to mechanical deflection of the mystacial vibrissae

DEVELOPMENTAL BRAIN RESEARCH

ELSEVIER

Developmental Brain Research 87 (1995) 62-68

Research report

Neonatal capsaicin treatment (NCT) alters the metabolic activity of the rat somatosensory cortex in response to mechanical deflection of the mystacial vibrissae Chi-Cheng Wu a,*, Manuel F. Gonzalez b a Department of Neurosciences, School of Medicine, University of California at San Diego, La Jolla, CA 92093-0608, USA b Department of Psychology, UniL~ersityof California at San Diego, La Jolla, CA 92093-0109, USA

Accepted 14 March 1995

Abstract

Capsaicin, a selective neurotoxin of unmyelinated C-fibers, was administered to neonatal rat pups at birth. Following a recovery period of 10 days, pups were injected with 2-deoxy-glucose (2DG) and subjected to repetitive mechanical stimulation to the left whiskerpad. Their brains were then harvested for autoradiography. The observed changes in 2DG uptake in the somatosensory cortex of capsaicin-treated rats were compared to vehicle-treated rats. The cross-sectional area and density of 2DG uptake by the primary and the secondary somatosensory cortex (SSI and SSII, respectively) were measured. Capsaicin-treated rats significantly exhibited a reduction in area of activation and a decrease of 2DG uptake in both structures. The present data indicates that neonatal capsaicin affects the functional activity of the rat somatosensory cortex. It is suggested that unmyelinated sensory afferents play a role in the development of the rat somatosensory system. Keywords: Neurotoxicity; Metabolism; Barrel cortex; Vibrissae stimulation

1. Introduction

Capsaicin (8-methyl-N-vanillyl-6-nonenamide), the pungent ingredient in red peppers of the genus C a p s i c u m , exerts a selective neurotoxic effect predominantly on small caliber, unmyelinated sensory C-fibers that are responsive to nociceptive stimuli [12,19,32]. A large body of literature has demonstrated that systemic administration of capsaicin at birth is capable of producing approximately a 95% depletion of C-fibers, leading to long-lasting insensitivity to noxious stimuli [19-23,31,54]. An important feature of capsaicin is its site-specific degeneration in the brain. The toxicity of intracisternal [18], intrathecal [25], and intraventricular [9,36] capsaicin treatment is restricted to the same sites damaged by systemic capsaicin [20]. Capsaicin-induced degeneration is confined to the spinal cord and caudal portions of the brainstem trigeminal complex (BTC), which receive unmyelinated C-fiber afferent projections [20-24,32].

* Corresponding author. Fax: (1) (619) 534-6602. 0165-3806/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0165-3806(95)00056-9

Although unmyelinated fibers comprise approximately one third of the innervation of rat vibrissae follicle, no evidence of characteristic long-latency or high-threshold responses of unmyelinated fibers has been reported in rat's vibrissal afferents [16,17,55]. However, it has been recently suggested that neonatal capsaicin depletes unmyelinated C-fibers and thus alters the functional activity of the rat somatosensory system. Studies have demonstrated that neonatal capsaicin treatment (NCT) results in the disruption of the one-to-one relationship of the vibrissal receptive field to its corresponding vibrissae in rat somatosensory cortex (SSI) [33,49]. Following NCT, the SSI is no longer dominated by neurons with a mechanoreceptive field for a single vibrissa, but instead the mechanoreceptive fields of the SSI encompass several whiskers. In the rat somatosensory system, the BTC is the first sensory relay of vibrissal inputs from the face to the central somatosensory structures [50]. The development of the somatosensory system takes place in a sequential fashion beginning at the periphery and then progressing centrally [2,26]. Damage to the vibrissae or the brainstem during the first week of life leads to an alteration of the

63

c.-c. Wu, M.F. Gonzalez / Developmental Brain Research 87 (1995) 62-68

whisker somatotopy in the SSI [26,46], as well as in subcortical structures [2,7]. Due to the neurotoxic effect of neonatal capsaicin on the BTC, the aim of the present study was to examine whether NCT affects metabolic activity of the central somatosensory cortex using the 2DG metabolic mapping technique. Preliminary reports of this study have been presented in abstract form [52].

Table 2 Comparison of activated cross-sectional area a of SSI and SSII in capsaicin (CAP)- and vehicle (VEH)-treated rats following left vibrissal stimulation SSI SSII

CAP (n = 10)

VEH (n = 7)

Reduction (%)

0.009 0.003

0.025 0.014

64.00 * * 78.57 * *

Changes in the cross-sectional area (mm2) were determined using t-tests ( * * P < 0.001). a

2. Materials and methods S p r a g u e - D a w l e y rat pups served as subjects. At birth, the subjects were anesthetized by hypothermia and then given a single injection of capsaicin (CAP; n = 10) (Sigma Chemical Co., 50 m g / k g , i.p.) or capsaicin vehicle (VEH; n = 7) (10% alcohol: 10% Tween 80). Respiratory assistance was given to avoid respiratory arrest that can occur shortly after administration of capsaicin. After recovery, the pups were returned to their dams. The body weight of the subjects was recorded before the stimulation procedure. Following a 10-day recovery period, the subjects were given i.p. injections of (14C) 2-deoxy-glucose (8 / x C i / k g ) and taped to a heating pad set at 3 5 ° C so that their bodies and limbs were immobilized but their heads were left unrestrained. Care was taken not to immobilize any of their vibrissae. Immediately after the administration of 2DG, the vibrissae of the left whisker pad were stroked at a rate of about 4 - 6 sweeps per second for 30 min using a hand-driven pendulum [11]. Care was taken to deflect as many of the whiskers from rows A through E [50] as possible and to minimize contact with the face of the subject or other facial hair. The decision to apply the experimental condition at PND 10 was based on a previous study showing that the onset of adult-like metabolic activity in SSI begins at this age [53]. After vibrissal stimulation the subjects were anesthetized with a xylazine-ketamine mixture and sacrificed. The brains were then extracted, frozen in 2-methylbutane

Table 1 Body weight gain in postnatal rats treated with CAP or VEH a PND 10 b PND 3 c PND 5 PND 7

CAP

VEH

25.25 -+3.67 (10) 9.02-+ 1.40 (5) 14.10_+ 1.07 (9) 18.60 _+1.53 (4)

24.34 -+2.87 (7) 9.17 _+1.71 (3) 12.78_+1.89 (6) 19.42 _+0.46 (5)

a Data are expressed as the mean_+S.D. Differencesbetween groups were compared using Student's t-test for unpaired data. b Neonates were treated with capsaicin 50 mg/kg or vehicle (Tween 80/alcohol/normal saline; 10:10:80 v/v) as described in the text. Subjects were studied at 10 days of age. All weights are in gram (g), mean+ S.D. The number in the parentheses represents the number of the subjects tested. c To ascertainNCT-inducedchanges in body weight gain within survival period, the body weight of the three differentage groups, i.e., PNDs 3, 5, and 7, was also measured.

cooled to - 3 0 ° C, coated in embedding medium and sectioned on a cryostat. Coronal sections, 20 /xm thick, were taken of the entire SSI and SSII, mounted on glass coverslips, and placed on a slide warming table for rapid dehydration. The coverslips were glued to cardboard with rubber cement and placed in contact with a sheet of Kodak SB5 single-emulsion X-ray film inside standard X-ray cassettes. Calibrated (14C) methyl methacrylate standards were autoradiographed with each brain. After the film was exposed for 7 to 10 days it was developed, fixed, washed, and dried. Once the autoradiographs were obtained, key sections of subject material were stained with Cresyl violet for histological identification of anatomical structures (not shown here). These procedures have been used and are detailed elsewhere [11]. Autoradiographs were initially screened using a light table. The autoradiographs were analyzed using a MCID computer based image analysis system. In order to ascertain NCT-induced changes in the SSI and SSII, two measures were taken: the cross-sectional area of activation (mm 2) and uptake density values. For each subject, the two measures were averaged across serial sections where the optical density values were averaged for both the right and left sides of the SSI and SSII. Optical density ratios (ODR) were then computed by dividing the averaged optical density value of the structure by the optical density value of the white matter. The numerical data was statistically analyzed using two-way A N O V A tests (Left and

Table 3 Optical density ratios (ODRs) for 2DG uptake in SSI and SSII of capsaicin (CAP)- and vehicle (VEH)-treated rats following left vibrissal stimulationa CAP (n = 10)

VEH (n = 7)

L

L

R

% of L-R R

Difference

SSI 1.336+0.03 1.583_+0.03 1.316+0.02 1.802+0.05 49.17% * SSII 1.292_+0.03 1.338_+0.03 1.247_+0.02 1.530_+0.03 83.74% * Mean optical density ratios (ODRs) for the left (L) and the right (R) sides of SSI and SSII are listed along with S.D. for each value; the relative ratios of L-R differencesin ODRs of SSI and SSII between CAP and VEH are also listed. Two way ANOVAs showed that L-R differences of the structures in both CAP and VEH were statisticallysignificant ( * * P < 0.001). T-testswere used to measure the relative ratios of L-R differences between CAP and VEH-treated rats and were also significant (* P < 0.05). a

C.-C. Wu, M.F. Gonzalez / Developmental Brain Research 87 (1995) 62-68

64

Right Side × CAP and VEH Treatment) and unpaired ttests (see Tables).

3. Results

3.1. The effect of neonatal capsaicin treatment on body weight gain The data of overall body weight gain of VEH- and CAP-treated rats showed no significant difference following the 10-day survival period ( P > 0.05, Table 1). Additionally, the overall body weight of three different age groups of rat pups (PNDs 3, 5, and 7) receiving CAP and VEH treatments showed no significant difference ( P > 0.05, Table 1). It is worth noting that a few cases of CAp-treated rat pups showed relative increases in body weight, but this was not significant when compared to VEH-treated rats.

VEH

fisl

The present results were consistent with previous autoradiographic studies [11,53] that showed that repetitive stimulation of the left vibrissae for 30 min generated statistically significant 2DG uptake in the contralateral side of the somatosensory cortex compared to the non-stimulated side ( P < 0.001, Table 2). More importantly, NCT produced decreases in the metabolic activity of SSI in response to mechanical deflection of the mystacial vibrissae ( P < 0.05, Table 2). The average cross-sectional area of activation and the ODR of SSI and SSII were examined. Compared to the control, significant decreases in 2DG uptake by SSI and SSII of CAP-treated rats were observed (see Fig. 1). In SSI, there was a 64.00% reduction in the average crosssectional area of activation and a 78.57% reduction in SSII ( P < 0.001, see Table 2). There was also a 49.17% reduction of ODR in SSI and an 83.74% in SSII ( P < 0.05, see Table 3). Note that there was a near complete absence of uptake observed in SSII of the CAP-treated group (see Fig. 1).

4. Discussion

liilt,

~11

-

CAP

3.2. NCT-induced metabolic changes in primary and secondary somatosensory cortex

,•SI

-~II

Given the plastic nature of the newborn rat central nervous system (CNS), degeneration of unmyelinated primary afferent neurons has a critical impact on the anatomical and functional development of the primary sensory afferent nerves and second- and higher-order neurons of the CNS [12,23,31]. Neonatal capsaicin treatment (NCT) produces irreversible degeneration of primary sensory fibers and inhibition of both axoplasmic transport of organelle-specific enzymes and retrograde transport o f nerve growth factor in sensory nerves [30,34]. These events probably occur when axonal connections are being established and subsequently result in the blockade of the conductance velocity of primary sensory afferents (see for review [12]). It has been shown that NCT disrupts the organization of the sensory pathways in the spinal cord, caudal brainstem trigeminal complex (BTC) [20-24,31,40], spinothalamic tract [39], and the organization of the somatotopic map in the SSI [49]. 4.1. The rat vibrissae somatosensory system

Fig. 1. Coronal [14C]2DG autoradiographs of the primary (SSI) and secondary (SSII) somatosensory cortex of PND 10 rats that received neonatal treatment with CAP or VEH. Calibration bar = 6 mm.

Primary afferents conveying vibrissae information from the face enter the BTC and project to four divisions: the spinal trigeminal nuclei pars caudalis (Sp5C), interpolaris (Sp5I), oralis (Sp50), and pars principalis (Pr5). All of these nuclei receive anatomically segregated inputs from the vibrissae [50]. Vibrissal inputs arriving in the BTC are directed toward the ventro-posterior medial nucleus of the

C.-C. Wu, M.F. Gonzalez /Developmental Brain Research 87 (1995) 62-68

thalamus, and then toward the SSI (barrel cortex). Sp5C and Pr5 are the two major substrates that relay tactile sensory input toward the central somatosensory structures [15]. Structure-function relationships in the subnuclei of the BTC have been systematically studied using electrophysiological methods [13,14,16]. In particular, it has been documented that there is a distinct boundary of the somatotopic arrangement between the localized facial (e.g. vibrissa) mechanoreceptive and the nociceptive fields in Sp5C (see details in Ref. [13]). Deeper laminae of Sp5C (III and IV) are mainly recipients of myelinated, mechanoreceptive afferent projections and predominantly project to Pr5. Superficial laminae (I and II) of Sp5C receive extensive unmyelinated C-fiber afferent projections [10,14,44,54]. Pr5 receives projections from primary sensory neurons and relays them to the dorsal thalamus, which in turn, projects to the SSI [50]. In the periphery of the rat, it has been demonstrated that one third of follicle innervation is unmyelinated and the function of these fibers remains obscure [47]. A number of electrophysiological studies investigating sensory activity from vibrissae follicles has provided little evidence of long-latency responses that are characteristic of unmyelinated nerve fibers [16,17,47,55]. Moreover, extensive anatomical studies have revealed that the critical period in which peripheral lesions disrupt the development of the somatotopic representations in the SSI occurs in the first few postnatal days (see for review [50]). It has been noted that removal of whisker follicles by PND 5 causes the absence of barrel-like representations in the SSI [26]. Similarly, early lesions in the periphery as well as in the subnuclei of the BTC produce abnormality in functional and anatomical properties of vibrissa neurons in the thalamus and the barrel cortex [38,41,46,51]. This indicates that at an early age the trigemino-lemniscal pathways are important in the establishment of the central somatotopic representations [51]. 4.2. The effects of NCT on the nutrition of the rat In the present study, functional activity of the SSI following NCT was assessed at PND 10, the age when normal cortical responses first exhibit adult-like characteristics [53]. Due to the neurotoxic effect of capsaicin on the physiology of the nervous system, decreased cortical metabolic activity observed in the present study suggested the following question: does NCT affect the nutritional intake of the rat, which subsequently may influence the functional development of the somatosensory system? To address this question, we compared body weight gain of both groups and found no significant difference, despite the phenomenon in which a few of the CAP-treated rats obtained slightly heavier body weight than that of the control group. However, this phenomenon did not show statistically significant differences (see Results). Similarly, no significant difference in body weight gain was found at PNDs 3, 5, and 7, indicating the normal nutritional intake

65

of CAP-treated rat pups within the survival period. The observed absence of body weight change in NCT-treated rats corresponds to previous studies [5,29,43], in which systemic NCT attenuates the level of cholecystokinin (CCK), a peripheral and central mediator of short-term satiety. This is a plausible explanation for the normal body weight gain of CAP-treated rats in which central regulation of body weight compensates for the loss of satiety signals. Accordingly, it seems to suggest that NCT-induced neuronal changes, not nutritional changes act as the main cause for functional alteration of the rat somatosensory cortex. 4.3. Age-related changes following NCT Previous anatomical studies agree with the view that the neurotoxicity of capsaicin is limited to small diameter primary sensory neurons in the spinal cord and caudal hindbrain (see for review [12]). Treatment with capsaicin causes more degeneration in rat pups than in adult rats [9,23,37]. A recent report by Ritter and Dinh [37] has suggested that the toxicity of capsaicin varies during development in the rat, indicating that systemic capsaicin treatment may cause more CNS degeneration in rat pups than in adult rats. However, Ritter and Dinh did not provide data of NCT-induced degeneration of cortical areas in 10-day-old rats [35,37]. In their anatomical data, most areas damaged by systemic capsaicin within the brain of rat pups were not vibrissa-related, except for Sp5C. While aware that further information of the effect of NCT on neonatal central somatosensory structures is needed, we feel confident that Ritter and Dinh's evidence of degeneration of Sp5C is informative enough to support the rationale of the present study that NCT-induced abnormality in vibrissa neurons of the BTC, if any, may account for metabolic alterations in the somatosensory cortex. As previously suggested, neonatal capsaicin treatment produces a permanent functional deficit in rats [12]. In contrast, administration of capsaicin at PND 14 or older does not produce a major degeneration of nerve fibers [21,23]. While evidence of NCT-induced adverse nutritional effect on the rat was not found (see Results), it is expected that NCT-induced functional changes observed in the 10-day-old rat would also be observed at later ages. Accordingly, NCT-induced 2DG uptake decreases observed in the neonate (up to PND 10) may be taken as an indicator of the development of permanent functional a n d / o r anatomical alteration in the adult rat. 4.4. The effects of NCT on the anatomical and functional properties of the somatosensory system Reports suggest that the development of vibrissal somatotopy is regarded as an overlay on a pre-existing topographic order [50]. Thus, it is likely that vulnerability to NCT in the subnuclei of the BTC potentially affects

66

C. -C. Wu, M.F. Gonzalez / Developmental Brain Research 87 (1995) 62-68

functional a n d / o r anatomical properties of vibrissa neurons in central somatosensory system structures. Following NCT, evidence of degenerating terminals of unmyelinated C-fiber afferents has been found in Sp5C and some portions of Sp50, but not in other subnuclei [20]. A recent anatomical report demonstrates that depletion of unmyelinated C-fibers following NCT induces sprouting of mechanoreceptive fiber afferents from the deeper laminae of Sp5C into the superficial laminae where unmyelinated C-fibers normally terminate [40]. Similar phenomenon has been observed elsewhere [31]. Given prior indications that deeper laminae of Sp5C are mainly mechanoreceptive and predominantly project to Pr5, which contributes to projection patterns in the central somatosensory structures [50], capsaicin-induced dendritic orientation of laminae III/IV neurons of Sp5C would probably alter their projection pattern to Pr5, thereby affecting the vibrissal processing of the central somatosensory structures. In our preliminary data, NCT led to alterations in metabolic activity of the vibrissae thalamus and all BTC subnuclei [52]. This evidence of capsaicin-induced abnormality in vibrissae anatomy makes it plausible to infer that the neurotoxicity occurring in the BTC is a major cause of functional alterations of the central somatosensory structures. Moreover, the present metabolic observations parallel findings of electrophysiological studies that have demonstrated the critical effect of NCT on the functional activity of the somatosensory cortex. Striking evidence provided by Wall and his colleagues indicates that systemic treatment of capsaicin at birth causes an increase in the receptive field of the SSI while its somatotopic representation remains unchanged [33,49]. Another important electrophysiological study demonstrated that loss of unmyelinated afferents by systemic NCT produces a strong decrease in primary afferent depolarization of myelinated afferents [48]. In their study, the ability of NCT-treated rats to generate primary afferent depolarization, despite the presence of myelinated afferents, is greatly depressed. This finding indicates that the primary afferent depolarization of the myelinated (vibrissae) afferents may be dependent on the integrity of the unmyelinated C-fibers. If true, the evidence of NCT-derived reduction in unmyelinated fibers of the vibrissa follicles could, in part, account for the functional abnormality in the maturing SSI [47]. Moreover, a recent report has demonstrated that neonatal capsaicin administered into the mystacial pad of postnatal rat pups heavily impairs mystacial somatosensory potentials [8]. Similarly, systemic capsaicin induces changes in somatosensory cortical responses evoked by mechanical stimulation of the vibrissae [45]. Taken together, the importance of functional a n d / o r anatomical integrity of myelinated and unmyelinated afferents in the course of somatosensory system development is substantial. In addition, the observed decrease in the metabolic activity in the SSII reflects an intracortical impact on functional activity in the SSI, in that the SSII is organized

roughly as a mirror-image of the somatotopic representation of the SSI [3,4,27].

4.5. Possible mechanisms underlying NCT-induced changes in metabolic activity of the SSI While the mechanism underlying the disorganized receptive field of the SSI induced by NCT remains unknown, one possible mechanism could be the reduction in inhibitory activity of somatosensory thalamic-recipient a n d / o r local-circuit interneurons of the barrel cortex. In the rat somatosensory system, GABAergic neurons are present throughout the barrels but are most prominent in the sides of the barrels and the septum in the SSI [6,28]. A recent immunohistochemical study suggests that a neonatal whisker trimming procedure induces a specific loss of GABAergic neurons in layer IV of the SSI [1]. Electrophysiologically, neonatal whisker trimming induces pronounced abnormalities in both the excitatory and the inhibitory components of the barrel receptive fields [42]. It seems likely that there are relative reductions in the inhibitory afferent projections of the thalamocortical relays, which lead to an increase in the responsiveness of single barrel receptive fields of the SSI to several whiskers [49]. The consequences of NCT and the whisker trimming procedure appear to affect both the functional and the anatomical development of the central somatosensory structures [50]. Accordingly, the reduction in the inhibitory activity of the barrelfields is likely to produce an overall decrease in the metabolic activity of the SSI following NCT.

5. Conclusion Changes in the metabolic activity of the somatosensory cortex reflect the secondary effect of NCT derived from changes in subcortical structures. The present finding indicates an important role of unmyelinated C-fiber nociceptive afferents in the establishment and the maintenance of primary vibrissa-related responsiveness mediated by myelinated sensory afferents. We conclude that neural processes underlying NCT-induced functional changes in the somatosensory cortex of the developing rat may be implicated in the development of functional and/or anatomical alterations observed in adult animals. Extension of the present research is needed to investigate the metabolic activity of the BTC and the vibrissal thalamus in order to outline a complete picture of capsaicin-induced changes in the rat somatosensory system (also see [52]).

Acknowledgements We thank Dr. Morton P. Printz for making available to us the image analysis program for data analysis and Dr.

c.-c. Wu, M.F. Gonzalez / Developmental Brain Research 87 (1995) 62-68 J a m e s W . H u f o r h i s a d v i c e in t h e d e v e l o p m e n t o f t h i s p r o j e c t . W e a r e a l s o g r a t e f u l to S t e p h e n C. C r u z , K e v i n Cox,

Mark

A. Williams,

and

Robert

Duncan

for their

[18]

editorial and technical assistance. [19]

References [20] [1] Beaulieu, C. and Micheva, K., Experience-dependent regulation of GABA neurons in the rat barrel cortex, Soc. Neurosci. Abstr., 257 (1993) 616. [2] Belford, G.R. and Killackey, H.P., The development of vibrissae representation in subcortical trigeminal centers of the neonatal rat, J. Comp. Neurol., 188 (1979) 63-74. [3] Carvell, G.E. and Simons, D.J., Somatotopic organization of the second somatosensory area (SII) in the cerebral cortex of the mouse, Somatosens. Res., 3 (1986) 213-237. [4] Carvell, G.E. and Simons, D.J., Thalamic and corticocortical connections of the second somatic sensory area of the mouse, J. Comp. Neurol., 265 (1987) 409-427. [5] Castonguay, T.W. and Bellinger, L.L., Capsaicin and its effects upon meal patterns, and glucagon and epinephrine suppression of food intake, Physiol. Behav., 40 (1987) 337-342. [6] Chmielwska, J., Stewart, M.G., Bourne, R.C. and Hfimori, J., ),Aminobutyric acid immunoreactivity in mouse barrel field: a light microscopical study, Brain Res., 368 (1986) 371-374. [7] Durham, D. and Woolsey, T.A., Effects of neonatal whisker lesions on mouse central trigeminal pathways, J. Comp. Neurol., 223 (1984) 424-447. [8] Feh6r, O., Antal, A., Toldi, J. and Wolff, J.R., Deep sensibility of the mystacial pad in the rat and its cortical representation, Acta Physiol. Hung., 81 (1993) 121-135. [9] Gamse, R., Leeman, S.E., Holzer, R. and Lembeck, F., Differential effects of capsaicin on the content of somatostatin, substance P, and neurotensin in the nervous system of the rat, Naunyn-Schmiedebergs Arch. Pharmakol., 317 (1981) 140-148. [10] Gobel, S., Hockfield, S. and Ruda, M.A., Anatomical similarities between medullary and spinal dorsal horns. In Y. Kawamura and R. Dubner (Eds.), Oral-Facial Sensory and Motor Functions, Quintessence, Tokyo, 1981, pp. 211-223. [11] Gonzalez, M.F. and Sharp, F.R., Vibrissae tactile stimulation: (14C) 2-deoxyglucose uptake in rat brainstem, thalamus, and cortex, J. Comp. Neurol., 231 (1985) 457-472. [12] Holzer, P., Capsaicin: cellular targets, mechanisms of action, and selectivity for thin sensory axons, Pharmacol. Rev., 43 (1991) 143-201. [13] Hu, J.W., Response properties of nociceptive and non-nociceptive neurons in the rat's trigeminal subnucleus caudalis (medullary dorsal horn) related to cutaneous and deep craniofacial afferent stimulation and modulation by diffuse noxious inhibitory controls, Pain, 41 (1990) 331-345. [14] Hu, J.W., Dostrovsky, J.O. and Sessle, B.J., Functional properties of neurons in cat trigeminal subnucleus caudalis (medullary dorsal horn): I. Responses to oral-facial noxious and nonnoxious stimuli and projections to thalamus and subnucleus oralis, J. Neurophysiol., 45 (1981) 173-192. [15] Jacquin, M.F., Chiaia, N.L., Klein, B.G. and Rhoades, R.W., Intersubnuclear connections within the rat trigeminal brainstem complex, Somatosens. Mot. Res., 7 (1990) 391-398. [16] Jacquin, M.F., Renehan, W.E., Mooney, R.D. and Rhoades, R.W., Structure-function relationships in rat medullary and cervical dorsal horn: I. Trigeminal primary afferents, J. Neurophysiol., 55 (1986) 1153-1180. [17] Jacquin, M.F., Renehan, W.E., Klein, B.G., Mooney, R.D. and

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29] [30]

[31]

[32]

[33]

[34]

[35]

[36]

67

Rhoades, R.W., Functional consequences of neonatal infraorbital nerve section in rat trigeminal ganglion, J. Neurosci., 6 (1986) 3706-3720. Jancsr, G., Intracisternal capsaicin: selective degeneration of chemosensitive primary sensory afferents in the adult rat, Neurosci. Leg., 27 (1981) 41-45. Jancs6, G., Pathobiologlcal reactions of C-fiber primary sensory neurones to peripheral nerve injury, Exp. Physiol., 77 (1992) 405431. Jancs6, G. and Kirfily, E., Distribution of chemosensitive primary sensory afferents in the central nervous system, J. Comp. Neurol., 190 (1980) 781-792. Jancs6, G. and Kirfily, E., Sensory neurotoxins: chemically induced selective destruction of primary sensory neurons, Brain Res., 210 (1981) 83-89. Jancsr, G., Kirfily, E. and Jancsr-G~bor, A., Pharmacologically induced selective degeneration of chemosensitive primary sensory neurons, Nature, 270 (1977) 741-743. Jancsr, G., Ferencsik, M., Such, G., Kir~ly, E., Nagy, A. and Bujdos6, M., Morphological effects of capsaicin and its analogues in newborn and adult mammals. In R. H~kanson and F. Sundler (Eds.), Tachykinin Antagonists, Elsevier, Amsterdam, 1985, pp. 35-44. Jancs6, G., Hlffelt, T., Lundberg, J.M., Kir~ly, E., Halsz, N., Nilsson, G., Terenius, L., Rehfeld, J., Steinbusch, H., Verhofstad, A., Elde, R., Said, S. and Brown, M., lmmunohistochemical studies on the effect of capsaicin on spinal and medullary peptide and monoamine neurons using antisera to substance P, gastrin/CCK, somatostatin, VIP, enkephalin, neurotensin and 5-hydroxytryptamine, .L Neurocytol., 10 (1981) 963-980. Jhamandas, K., Yaksh, T.L., Harty, G., Szolcs~nyi, J. and Go, V.L.W., Action of intrathecal capsaicin and its structural analogues on the content and release of spinal substance P: selective action and relationship to analgesia, Brain Res., 306 (1984) 215-225. Killackey, H.P. and Belford, G., The formation of afferent patterns in the somatosensory cortex of the neonatal rat, J. Comp. Neurol., 183 (1979) 285-301. Koralek, K.A., Olaverria, J. and Killackey, H.P., Areal and laminar organization of corticocortical projections in rat somatosensory cortex, J. Comp. Neurol., 299 (1990) 133-150. Lin, C.-S., Lu, S.M. and Schmechel, D., Glutamic acid decarboxylase immunoreactivity in layer IV of barrel cortex of rat and mouse, J. Neurosci., 5 (1985) 1934-1939. MacLean, D.B., Abrogation of peripheral cholecystokinin-satiety in the capsaicin treated rat, ReguL Pept., 11 (1985) 321-333. McDougal, D.B., Yuan, M.J.C., Dargar, R.V. and Johnson, E.M., Neonatal capsaicin and guanethidine and axonally transported organelle-specific enzymes in sciatic nerve and in sympathetic and dorsal root ganglia, J. Neurosci., 3 (1983) 124-132. Nagy, J.I. and Hunt, S.P., The termination of primary afferents within the rat dorsal horn: evidence for rearrangement following capsaicin treatment, J. Comp. Neurol., 218 (1983) 145-158. Nagy, J.I., Vincent, S.R., Staines, W.M.A., Fibiger, H.C., Reisine, T.D. and Yamamura, H.I., Neurotoxic action of capsaicin on spinal substance P neurons, Brain Res., 186 (1980) 435-444. Nussbaumer, J.C. and Wall, P.D., Expansion of receptive fields in the mouse cortical barrelfield after administration of capsaicin to neonates or local application on the infraorbital nerve in adults, Brain Res., 360 (1985) 1-9. Otten, U., Lorez, H.P. and Businger, F., Nerve growth factor antagonizes the neurotoxic action of capsaicin on primary sensory neurones, Nature, 301 (1983) 515-517. Ritter, S. and Dinh, T.T., Capsaicin-induced neuronal degeneration in the brain and retina of preweanling rats, J. Comp. Neurol., 296 (1990) 447-461. Ritter, S. and Dinh, T.T., Capsaicin: a selective probe for studying specific neuronal populations in brain and retina. In P. Michael

68

[37] [38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

C. -C. Wu, M.F. Gonzalez / Developmental Brain Research 87 (1995) 62-68 Conn (Ed.), Methods in Neurosciences, Vol. 8, Neurotoxins, Academic Press, Orlando, FL, 1992, pp. 118-136. Ritter, S. and Dinh, T.T., Age-related changes in capsaicin-induced degeneration in rat brain, Jr. Comp. Neurol., 318 (1992) 103-116. Rhoades, R.W., Belford, G.R. and Killackey, H.P., Receptive-field properties of rat ventral posterior medial neurons before and after selective kainic acid lesions of the trigeminal brainstem complex, J. Neurophysiol., 57 (1987) 1577-1600. Saporta, S., Loss of spinothalamic tract neurons following neonatal treatment of rats with the neurotoxin capsaicin, Somatosens. Res., 4 (1986) 153-173. Shortland, P., Molander, C., Woolf, C.J. and Fitzgerald, M., Neonatal capsaicin treatment induces invasion of the substantia gelatinosa by the terminal arborization of hair follicle afferents in the rat dorsal horn, J. Comp. Neurol., 296 (1990) 23-31. Simons, D.J., Durham, D. and Woolsey, T.A., Functional organization of mouse and rat Sml barrel cortex following damage on different postnatal days, Somatosens. Res., 1 (1984) 207-245. Simons, D.J., Land, P.W. and Suter, K., Neonatal whisker trimming reduces surround inhibition in adult barrel cortex, Soc. Neurosci. Abstr., 29 (1993) 46. South, E.H. and Ritter, R.C., Capsaicin application to central or peripheral vagal fibers attenuates CCK satiety, Peptides, 9 (1988) 601-612. Sugiura, Y., Terui, N. and Hosoya, Y., Difference in distribution of central terminals between visceral and somatic unmyelinated (C) primary afferent fibers, J. Neurophysiol., 62 (1989) 834-840. Toldi, J., Joo, F. and Wolfe, J.-R., Capsaicin differentially influences somatosensory cortical responses evoked by peripheral electrical or mechanical stimulation, Neuroscience, 49 (1992) 135-139.

[46] Van der Loos, H. and Woolsey, T.A., Somatosensory cortex: structural alterations following early injury to sense organs, Science, 179 (1973) 395-398. [47] Waite, P.M.E. and Li, L., Unmyelinated innervation of sinus hair follicles in rats, Anat. EmbryoL, 188 (1993) 457-465. [48] Wall, P.D., The effect of peripheral nerve lesions and of neonatal capsaicin in the rat on primary afferent depolarization, J. Physiol., 329 (1982) 21-35. [49] Wall, P.D., Fitzgerald, M., Nussbaumer, J.C., Van der Loos, H. and Devor, M., Somatotopic maps are disorganized in adult rodents treated neonatally with capsaicin, Nature, 295 (1982) 691-693. [50] Woolsey, T.A., Peripheral alterations and somatosensory development. In J.R. Coleman (Ed.), Development of Sensory Systems in Mammals, John Wiley and Sons, New York, 1990, pp. 461-516. [51] Woolsey, T.A., Anderson, J.R., Wann, J.R. and Stanfield, B.B., Effects of early vibrissae damage on neurons in the ventrobasal (VB) thalamus of the mouse, J. Comp. Neurol., 184 (1979) 363-380. [52] Wu, C.C. and Gonzalez, M.F., Alterations in metabolic activity induced by neonatal capsaicin in the developing rodent somatosensory system, Soc. Neurosci. Abstr., 29 (1993) 46. [53] Wu, C.C. and Gonzalez, M.F., Functional development of the vibrissae somatosensory system of the rat: a (14C) 2-deoxyglucose metabolic mapping study, J. Comp. Neurol., in press. [54] Yokota, T., Neural mechanisms of trigeminal pain. In H.L. Fields, R. Dubner and R. Cervero (Eds.), Advances in Pain Research and Therapy, Raven, New York, 1985, pp. 221-232. [55] Zucker, E. and Welker, W.I., Coding of somatic sensory input by vibrissae neurons in the rat's trigeminal ganglion, Brain Res., 12 (1969) 138-156.