Ultrastructural identification of trigeminal nerve terminals in the pterygopalatine ganglion of rats: an anterograde tracing and immunohistochemical study

The Anti-CGRP Antibody Fremanezumab Lowers CGRP Release from Rat Dura Mater and Meningeal Blood Flow

Monoclonal antibodies directed against the neuropeptide calcitonin gene-related peptide (CGRP) belong to a new generation of therapeutics that are effective in the prevention of migraine. CGRP, a potent vasodilator, is strongly implicated in the pathophysiology of migraine, but its role remains to be fully elucidated. The hemisected rat head preparation and laser Doppler flowmetry were used to examine the effects on CGRP release from the dura mater and meningeal blood flow of the subcutaneously injected anti-CGRP monoclonal antibody fremanezumab at 30 mg/kg, when compared to an isotype control antibody. Some rats were administered glycerol trinitrate (GTN) intraperitoneally to produce a migraine-like sensitized state. When compared to the control antibody, the fremanezumab injection was followed by reduced basal and capsaicin-evoked CGRP release from day 3 up to 30 days. The difference was enhanced after 4 h of GTN application. The samples from the female rats showed a higher CGRP release compared to that of the males. The increases in meningeal blood flow induced by acrolein (100 µM) and capsaicin (100 nM) were reduced 13–20 days after the fremanezumab injection, and the direct vasoconstrictor effect of high capsaicin (10 µM) was intensified. In conclusion, fremanezumab lowers the CGRP release and lasts up to four weeks, thereby lowering the CGRP-dependent meningeal blood flow. The antibody may not only prevent the released CGRP from binding but may also influence the CGRP release stimulated by noxious agents relevant for the generation of migraine pain.

Schwann cell endosome CGRP signals elicit periorbital mechanical allodynia in mice

Efficacy of monoclonal antibodies against calcitonin gene-related peptide (CGRP) or its receptor (calcitonin receptor-like receptor/receptor activity modifying protein-1, CLR/RAMP1) implicates peripherally-released CGRP in migraine pain. However, the site and mechanism of CGRP-evoked peripheral pain remain unclear. By cell-selective RAMP1 gene deletion, we reveal that CGRP released from mouse cutaneous trigeminal fibers targets CLR/RAMP1 on surrounding Schwann cells to evoke periorbital mechanical allodynia. CLR/RAMP1 activation in human and mouse Schwann cells generates long-lasting signals from endosomes that evoke cAMP-dependent formation of NO. NO, by gating Schwann cell transient receptor potential ankyrin 1 (TRPA1), releases ROS, which in a feed-forward manner sustain allodynia via nociceptor TRPA1. When encapsulated into nanoparticles that release cargo in acidified endosomes, a CLR/RAMP1 antagonist provides superior inhibition of CGRP signaling and allodynia in mice. Our data suggest that the CGRP-mediated neuronal/Schwann cell pathway mediates allodynia associated with neurogenic inflammation, contributing to the algesic action of CGRP in mice.

An Immunohistochemical Study of Cocaine- and Amphetamine-Regulated Transcript (Cart) Expression in the Pterygopalatine Ganglion of the Pig

Although, a great effort has been made to understand the synthesis, regulation, processing and function of cocaine- and amphetamine-regulated transcript (CART) peptide at the central level, its peripheral function(s) are still obscure. Moreover, scarce studies describing the presence of CART in peripheral autonomic ganglia are mainly limited to laboratory rodents. Thus, the aim of the present study was to immunohistochemically investigate the expression of CART in Hu C/D-positive neurons of the porcine pterygopalatine ganglion (PPG). The distribution pattern of CART-IR nerve elements in PPG has been also assessed. The co-localization of CART with substance P (SP), galanin or somatostatin was studied by means of double immunohistochemical stainings. The presence of Hu C/D-positive/CART-positive neurons was detected both in the left and right PPG (4.7±1.2% and 5.2% ± 1.4%, respectively). The CARTimmunoreactive (IR) neurons were categorized as either middle (ca. 80%) or small (ca. 20%) in size. Moderate numbers of CART-IR boutons were also detected between CART-negative ganglionic neurons. CART-IR basket-like formations around PPG neurons were regularly observed. Virtually all CART-IR neurons additionally co-stored VIP, whereas none of the CART-expressing cells showed the presence of galanin, SP or somatostatin. CART-IR basket-like formations exclusively encircled VIP-IR PPG neurons. Thus, CART-IR nerve cells seem to constitute a relatively small homologous population of the porcine PPG neurons with largely unknown functions. Further functional studies aiming at whether CART-IR neurons could serve as interneurons are necessary.

5-HT(1D) receptor immunoreactivity in the sphenopalatine ganglion: implications for the efficacy of triptans in the treatment of autonomic signs associated with cluster headache

OBJECTIVE

To determine if 5-HT(1D) receptors are located in the sphenopalatine ganglion.

BACKGROUND

While the 5-HT(1D) receptor has been described in sensory and sympathetic ganglia in the head, it was not known whether they were also located in parasympathetic ganglia.

METHODS

We used retrograde labeling combined with immunohistochemistry to examine 5-HT(1D) receptor immunoreactivity in rat sphenopalatine ganglion neurons that project to the lacrimal gland, nasal mucosa, cerebral vasculature, and trigeminal ganglion.

RESULTS

We found 5-HT(1D) receptor immunoreactivity in nerve terminals around postganglionic cell bodies within the sphenopalatine ganglion. All 5-HT(1D) -immunoreactive terminals were also immunoreactive for calcitonin gene-related peptide but not vesicular acetylcholine transporter, suggesting that they were sensory and not preganglionic parasympathetic fibers. Our retrograde labeling studies showed that approximately 30% of sphenopalatine ganglion neurons innervating the lacrimal gland, 23% innervating the nasal mucosa, and 39% innervating the trigeminal ganglion were in apparent contact with 5-HT(1D) receptor containing nerve terminals.

CONCLUSION

These data suggest that 5-HT(1D) receptors within primary afferent neurons that innervate the sphenopalatine ganglion are in a position to modulate the excitability of postganglionic parasympathetic neurons that innervate the lacrimal gland and nasal mucosa, as well as the trigeminal ganglion. This has implications for triptan (5-HT(1D) receptor agonist) actions on parasympathetic symptoms in cluster headache.

Structure of peripheral synapses: autonomic ganglia

Final motor neurons in sympathetic and parasympathetic ganglia receive synaptic inputs from preganglionic neurons. Quantitative ultrastructural analyses have shown that the spatial distribution of these synapses is mostly sparse and random. Typically, only about 1%-2% of the neuronal surface is covered with synapses, with the rest of the neuronal surface being closely enclosed by Schwann cell processes. The number of synaptic inputs is correlated with the dendritic complexity of the target neuron, and the total number of synaptic contacts is related to the surface area of the post-synaptic neuron. Overall, most neurons receive fewer than 150 synaptic contacts, with individual preganglionic inputs providing between 10 and 50 synaptic contacts. This variation is probably one determinant of synaptic strength in autonomic ganglia. Many neurons in prevertebral sympathetic ganglia receive additional convergent synaptic inputs from intestinofugal neurons located in the enteric plexuses. The neurons support these additional inputs via larger dendritic arborisations together with a higher overall synaptic density. There is considerable neurochemical heterogeneity in presynaptic boutons. Some synapses apparently lack most of the proteins normally required for fast transmitter release and probably do not take part in conventional ganglionic transmission. Furthermore, most preganglionic boutons in the ganglionic neuropil do not form direct synaptic contacts with any neurons. Nevertheless, these boutons may well contribute to slow transmission processes that need not require conventional synaptic structures.

Expression of immunoreactivity to neurokinin-1 receptor by subsets of cranial parasympathetic neurons: correlation with neuropeptides, nitric oxide synthase, and pathways

We examined the patterns of coexistence of immunoreactivity to the neurokinin-1 (NK(1)) tachykinin receptor, nitric oxide synthase, and neuropeptides in the sphenopalatine and otic ganglia of guinea pigs using a combination of multiple-labeling immunohistochemistry and pathway tracing in vitro. Most neurons had immunoreactivity to vasoactive intestinal peptide (85-96%) and neuropeptide Y (60%). Subpopulations of vasoactive intestinal peptide-immunoreactive neurons also had immunoreactivity to nitric oxide synthase (37-48%) or enkephalin (25-35%), but these formed mutually exclusive populations. Almost all neurons expressing NK(1) receptor immunoreactivity contained immunoreactivity to enkephalin, vasoactive intestinal peptide, and neuropeptide Y, but not nitric oxide synthase. Using a combination of retrograde axonal tracing and axonal crushing, we found that most neurons with immunoreactivity to nitric oxide synthase projected along the nasopalatine and ethmoidal nerves to the nasal mucosa. In contrast, most neurons with immunoreactivity to enkephalin followed the zygomatic nerve to the facial skin and lacrimal gland. Based on their peptide content, we conclude that the neurons with immunoreactivity to enkephalin and NK(1) receptor projected selectively to the skin. In both the sphenopalatine and the otic ganglia, about half of the neurons with NK(1) receptor immunoreactivity were surrounded by varicose nerve fibers with substance P immunoreactivity. Many of these fibers are likely to have originated in the trigeminal ganglion. Taken together, these observations establish a strong anatomical basis for a range of interactions between trigeminal and cranial parasympathetic pathways that may underlie pathophysiological conditions such as trigeminal neuralgia.

Intrinsic neurons in the duck choroid are contacted by CGRP-immunoreactive nerve fibres: evidence for a local pre-central reflex arc in the eye

Intrinsic choroidal neurons represent peripherally displaced autonomic nerve cells supposed to work as a local integrative network similar to the enteric nervous system, to control choroidal vasculature and stromal smooth muscle. A typical feature of such intramural neuronal networks is the innervation by primary afferent collaterals expressing peptides, e.g. CGRP. The present study was aimed at determining primary afferent contacts on nitrergic intrinsic choroidal neurons (ICN) in the duck eye. In addition, a sympathetic innervation of ICN was assessed. Choroids were immunohistochemically processed for the following markers: neuronal nitric oxide synthase (nNOS), galanin (GAL), calcitonin gene-related peptide (CGRP), and tyrosine hydroxylase (TH). For evaluation, fluorescence as well as confocal laser scanning microscopy were used. For electron microscopy, immunoperoxidase staining for CGRP in combination with NADPH-diaphorase histochemistry was applied. ICN immunoreactive for nNOS or GAL spread over the entire choroid, although they were concentrated in an equatorial zone passing obliquely from naso-cranial to temporo-caudal. About 40% of ICN showed close relationships with CGRP-immunoreactive nerve fibres, originating most likely in the trigeminal ganglion, as seen in the fluorescence and confocal laserscanning microscope. These appositions could be ultrastructurally defined as both synapses and close contacts without synaptic specialization. Some ICN endowed with CGRP-positive fibres also received TH-immunoreactive boutons. CGRP-immunoreactive profiles were also detected in close relationship to choroidal non-vascular smooth muscle cells and collagen fibres connected to them. In many instances, they were intercalated between smooth muscle cells and processes of ICN forming triads. These results suggest that ICN, similar to other intramural autonomic systems integrate signals from trigeminal primary afferent collaterals. The 'sensory' terminals of these primary afferents may be located in the anterior eye segment but also within the smooth muscle stroma of the choroid itself. Thus, ocular homeostasis may be regulated via intraocular pre-central reflexes which are probably subject to sympathetic modulation.

Calbindin-D28k in cerebrovascular extrinsic innervation system of the rat

Calbindin-D28k, one of the calcium-binding proteins, belongs to the EF hand family and is commonly found in neurons. It serves as a representative neuronal marker for neuroanatomical investigations. The authors' knowledge of its precise function, however, is yet very limited. In this study, we examined the existence of nerve fibers with calbindin-D28k immunoreactivity in the cerebral blood vessels and ganglia that innervate the cerebral blood vessels in the rat. Numerous nerve fibers with calbindin-D28k immunoreactivity were observed on the walls of the major extracerebral arteries forming the circle of Willis and its branches. Calbindin-D28k immunoreactivity was seen in many neurons of the trigeminal, dorsal root and jugular ganglia. A small number of neurons showed calbindin-D28k immunoreactivity in the otic and superior cervical ganglia. Calbindin-D28k immunoreactivity was not detected in the sphenopalatine or internal carotid ganglia. Pericellular basket-like formations of nerve terminals with calbindin-D28k immunoreactivity were observed in the sphenopalatine, otic, internal carotid and superior cervical ganglia. The present study demonstrated calbindin-D28k immunoreactivity in the cerebrovascular nerve fibers as well as in their origins--the cranial ganglia. These findings are significant in understanding the calcium-mediated mechanism of the neural control of the cerebral blood vessels.

An electrophysiological study of pterygopalatine ganglion neurons in the rabbit

A preparation was developed to investigate the synaptic connections of the pterygopalatine ganglion (P.P.G.) neurons of the rabbit. Many neurons received synaptic inputs from more than one preganglionic fiber in the vidian nerve (preganglionic nerve) with a wide range of conduction velocities. It is assumed that P.P.G. neurons integrate synaptic inputs from the higher centers. In some neurons. nicotinic fast excitatory postsynaptic potentials (e.p.s.p.s) were evoked in response to stimulation of one posterior nasal nerve, and also an antidromic action potential occurred in response to stimulation of the other posterior nasal nerve. Fast e.p.s.p.s were recorded from a ganglion neuron in response to the cooling stimulation of the nasal mucosa. These results revealed that the ganglionic reflex are mediated through the nasal afferent fibers exists in the P.P.G. Moreover, the appearance of slow inhibitory postsynaptic potentials (slow i.p.s.p.s) and slow excitatory postsynaptic potentials (slow e.p.s.p.s) in response to repetitive stimuli of the vidian nerve may influence the synaptic transmissions of P.P.G. neurons. The P.P.G. plays a significant role as a complicated key point of signal transmissions from both the periphery and higher centers.

Innervation of the lumbar facet joints. Origins and functions

STUDY DESIGN

The levels of dorsal root ganglia and paravertebral sympathetic ganglia innervating the lumbar facet joint were investigated in rats using the retrograde transport method. The pathways and functions of the nerve fibers supplying the lumbar facet joint were determined immunohistochemically.

OBJECTIVES

To study lumbar facet pain in relation to its innervation.

SUMMARY OF BACKGROUND DATA

The lumbar facet joints have been reported to be innervated segmentally. Little is known, however, about the origins and functions of the nerve fibers.

METHODS

Cholera toxin B subunit, a neural tracer, was placed in the L5-L6 facet joint, and the bilateral dorsal root ganglia and paravertebral sympathetic ganglia were examined immunohistochemically. The serial sections of lumbar vertebrae of newborn rats and the sections of the facet joint capsules, dorsal root ganglia, and paravertebral sympathetic ganglia of adult rats were investigated immunohistochemically. The pathways of the nerve fibers supplying the facet joint were reconstituted.

RESULTS

Labeled neurons existed in ipsilateral dorsal root ganglia from L1 to L5 and in paravertebral sympathetic ganglia from T12 to L6. The dorsal ramus of the spinal nerve and rami communicantes were connected to each other by calcitonin gene-related peptide immunoreactive fibers and dopamine beta-hydroxylase immunoreactive fibers.

CONCLUSIONS

The L5-L6 facet joint was innervated by ipsilateral dorsal root ganglia and paravertebral sympathetic ganglia, segmentally and nonsegmentally. Some of the sensory fibers from the facet joint may pass through the paravertebral sympathetic trunk, reaching L1 and/or L2 dorsal root ganglia. Inguinal and/or anterior thigh pain with lower lumbar facet joint lesions may be explained as referred pain.

CGRP-immunoreactive nerve fibers projecting to lumbar facet joints through the paravertebral sympathetic trunk in rats

We previously reported that the L5-6 facet joint is innervated from DRGs from L1 to L5 and the paravertebral sympathetic ganglia from T12 to L6 in rats. In the present study, to determine the sensory pathway from L5-6 facet joint, we placed the fluorescent carbocyanine dye, DiI, in the L5-6 facet joint, and examined the paravertebral sympathetic trunks and ganglia bilaterally. We found some DiI-labeled nerve fibers exhibiting calcitonin gene-related peptide (CGRP)-immunoreactivity, and some DiI-labeled neurons surrounded by CGRP-immunoreactive varicose fibers in the ganglia. The results suggest that a sensory pathway from the L5-6 facet joint to L1 and/or L2 DRGs is present in the paravertebral sympathetic trunk, and that sensory nerve fibers may connect with sympathetic postganglionic neurons projecting to lumbar facet joints.

Differential distribution of neuronal markers and neuropeptides in the human lacrimal gland

BACKGROUND

The present study was undertaken in an attempt to broaden the spectrum of known neuronal markers and neuropeptides in the main lacrimal gland of the human by light-microscopic immunohistochemistry.

METHODS

Using antisera against the neuronal markers protein gene product (PGP) and S-100 protein (S-100), the distribution of nerve fibers in the human main lacrimal gland was studied. Vasoactive intestinal polypeptide (VIP), calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY) and tyrosine hydroxylase (TH) were identified by their specific antisera.

RESULTS

The nerve fibers are distributed throughout the interstice between the glandular tubules. Associations were also found between nerve fibers and both the interlobular ductal system and blood vessels (mainly arterioles). Within the glandular lobules isolated groups of secretory cells stained positive for S-100 protein. Nerve fibers situated in the glandular interstice between the tubules showed predominantly positive immunoreactions for the neuropeptide VIP, while only very few fibers stained positive for CGRP, NPY and the catecholamine marker TH. Nerve fibers associated with interlobular blood vessels were mainly CGRP and NPY positive and stained only very rarely for VIP. The epithelia of interlobular ducts and excretory ducts were associated with CGRP-immunoreactive nerve fibers.

CONCLUSION

The neuropeptides identified in the lacrimal gland indicate the complexity with which a variety of biologic signals regulate and modulate the lacrimal gland.

Immunohistochemical localization of neuropeptides in the human ciliary ganglion

In human ciliary ganglia, 18% of neurons were in contact with substance P (SP) and 12% with calcitonin gene-related peptide (CGRP) like-immunoreactive (LI) varicose axons. CGRP was colocalized with SP. Numerous SP-LI and CGRP-LI non-varicose nerve fibers were found between the ganglion cells and in nerve trunks that entered the ganglia. Axons immunoreactive for neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP), tyrosine hydroxylase (TH) or dopamine-beta-hydroxylase (DBH) never contacted neuronal cell bodies. Perikarya of ciliary neurons neither stained for any of the neuropeptides nor for DBH. 23% of ciliary perikarya were TH-immunoreactive. These observations suggest an innervation of human ciliary ganglion neurons by peptidergic primary afferent collaterals presumably of trigeminal origin.

Peripheral neural circuits regulating IOP? A review of its anatomical backbone

The peripheral nervous system is classically separated into a somatic division containing both afferent and efferent pathways and an autonomic division composed of efferents only. The somatic afferent division is divided in A- and B-neurons. The B-neurons are supposed to be autonomic afferents as part of a reflex system involved in homeostasis. Recent data obtained by neuronal tracing and immunohistochemical experiments concerning the eye related peripheral nervous system endorse the existence of these peripheral reflex systems. Somatic afferents of trigeminal origin synaptically innervate parasympathetic neurons in the pterygopalatine ganglion. This probably represents a pathway mediating autonomically regulated ocular activity in response to sensory stimulation. In addition, it has been hypothesized that trigeminal sensory nerve fibres have an efferent function in response to noxious stimuli e.g. the ocular injury response. Sympathetic nerve fibres originating in the superior cervical ganglion course through the trigeminal and pterygopalatine ganglion without forming direct synaptic contacts. These fibres, however, contain clusters of vesicles suggesting some kind of interneural communication. Parasympathetic nerve fibres of pterygopalatine origin course through the ciliary ganglion. These nerve fibre terminals also contain clusters of vesicles without direct synaptic contacts. Experimental data concerning the distribution of neuropeptides revealed a more detailed knowledge of the anterior eye segment innervation. These experimental data are subject to some debate. The pros and cons of different techniques are discussed. Neural circuits regulating IOP have long been postulated. The possible role of peripheral reflex systems in the regulation of IOP is discussed.

Sensory and autonomic innervation of the rat eyelid: neuronal origins and peptide phenotypes

Neuronal origins, peptide phenotypes and target distributions were determined for sensory and autonomic nerves projecting to the eyelid. The retrograde tracer, Fluoro-Ruby, was injected into the superior tarsal muscle and meibomian gland of Sprague-Dawley rats. Labelled neurons were observed within the pterygopalatine (31 +/- 6 of a total of 8238 +/- 1610 ganglion neurons), trigeminal (173 +/- 43 of 62,082 +/- 5869) and superior cervical ganglia (184 +/- 35 of 21,900 +/- 1741). Immunostaining revealed vasoactive intestinal polypeptide immunoreactivity (VIP-ir) in nearly all Fluoro-Ruby-labelled pterygopalatine ganglion neurons (86 +/- 5%) but only rarely in trigeminal (0.3 +/- 0.3%) or superior cervical (1.4 +/- 1.4%) ganglion neurons. Calcitonin gene-related peptide (CGRP)-ir was not observed in pterygopalatine or superior cervical ganglion somata, but was present in 24 +/- 4% of trigeminal neurons. Bright dopamine beta-hydroxylase (DBH) immunofluorescence was observed in the majority of eyelid-projecting neurons within the superior cervical ganglia (65 +/- 5%) and lighter staining was detected in pterygopalatine neurons (63 +/- 3%), but no DBH-ir was observed in trigeminal neurons. Examination of eyelid sections revealed dense VIP-ir innervation of meibomian gland acini and vasculature and modest distribution within tarsal muscle. CGRP-ir fibers surrounded ductal and vascular elements of the meibomian gland and the perimeter of tarsal muscle. DBH-ir fibers were associated with meibomian gland blood vessels and acini, and were more densely distributed within tarsal muscle. This study provides evidence for prominent meibomian gland innervation by parasympathetic pterygopalatine ganglion VIP-ir neurons, with more restricted innervation by sensory trigeminal CGRP-ir and sympathetic neurons. Tarsal muscle receives abundant sympathetic innervation, as well as moderate parasympathetic and sensory CGRP-ir projections. The eyelid contains substantial non-CGRP-ir sensory innervation, the targets of which remain undetermined. The distribution of identified autonomic and sensory fibers is consistent with the idea that meibomian gland function, as well as that of the tarsal muscle, is regulated by peripheral innervation.

Sympathetic innervation of the rat's eye and peripheral ganglia: an electron microscopic autoradiographic tracing study

The sympathetic innervation of the rat anterior eye segment and related peripheral ganglia was studied using light and electron microscopic autoradiography after anterograde tracing with 3H-leucine from the superior cervical ganglion. In the trigeminal and pterygopalatine ganglia, unmyelinated nerve fibers were labeled. Some fibers contained accumulations of small vesicles. Close apposition of these labeled sympathetic fibers with other unmyelinated fibers was common, and was also observed at sites where accumulations of vesicles were found. In the iris, ciliary body and trabeculum, numerous fibers and vesicle-containing varicosities were labeled, which all had a similar morphology. No labeling was found in the cornea. Sympathetic fibers traversing the trigeminal and pterygopalatine ganglion closely appose other unmyelinated fibers and contain clusters of vesicles without forming classical synaptic contacts. However, non-synaptic information transfer needs further investigation. The anterior eye segment, except for the cornea, is densely innervated by sympathetic nerve terminals.

Light- and electron-microscopic study of synaptic connections in the paracervical ganglion of the female rat: special reference to calcitonin gene-related peptide-, galanin- and tachykinin (substance P and neurokinin A)-immunoreactive nerve fibers and terminals

Nerve fibers and varicosities in the pelvic paracervical ganglia (PG) are immunoreactive for the neuropeptides calcitonin gene-related peptide, galanin, and the tachykinins substance P and neurokinin A. Many of these fibers and varicosities are capsaicin-sensitive, originate in dorsal root ganglia and, thus, are considered to be primary afferent fibers. Numerous immunoreactive varicosities are pericellular to principal neurons in the PG. The present study examines the ultrastructure of calcitonin gene-related peptide-, galanin-, substance P-, and neurokinin A-immunoreactive nerve fibers and varicosities in the ganglia to determine their relationships to principal neurons and their synaptic connectivity. Paracervical ganglia of female rats were processed for light-microscopic immunohistochemistry using antisera against synapsin I, as a nerve terminal marker, and microtubule-associated protein-2 to define soma and dendrites. The rationale for performing this co-immunohistochemical analysis was to reveal the relationship between nerve endings and principal neurons. Synapsin I endings were predominantly axosomatic with fewer being axodendritic. Other ganglia were processed for electron-microscopic immunohistochemistry using both standard immunogold and peroxidase-anti-peroxidase procedures. Unmyelinated fibers and varicosities immunoreactive for calcitonin gene-related peptide, galanin, and the tachykinins were routinely observed in the interstitium between neuron somas. Numerous immunoreactive axon profiles were present in small groups that were ensheathed by Schwann cells. Immunoreactive fibers and varicosities were also observed within the satellite-cell sheath of the neuron soma and often intimately associated with the membrane of the soma, somal protrusions, or with the proximal part of a dendrite. Membrane specializations, indicative of synaptic contacts, between the fibers and the principal neurons were observed. It is suggested that these peptide-immunoreactive sensory fibers and varicosities are involved in regulation of activity in the PG.