Enteric origin of bombesin immunoreactive fibres in the rat coeliac-superior mesenteric ganglion

Bombesin-like immunoreactivity occurs in a dense network of nerve fibres in the rat coeliac-superior mesenteric ganglion. No bombesin immunoreactive cell bodies have been observed in this ganglion. An origin outside the ganglion was therefore assumed for the bombesin immunoreactive fibres. Ligation of the mesenteric nerve, between the ganglion and the gut resulted in an accumulation of bombesin immunoreactive material on the distal side, i.e. towards the gut. Transection of the nerve caused a depletion of bombesin immunoreactive fibres in the ganglion. These results suggest that bombesin immunoreactive fibres in the coeliac-superior mesenteric ganglion of the rat originate in neurons in the gastrointestinal wall. Application of colchicine to the stomach wall caused an accumulation of bombesin immunoreactive material in myenteric neurons.     

The participation of the sympathetic innervation of the gastrointestinal tract in disease states

Abstract  Knowledge of neural circuits, neurotransmitters and receptors involved in the sympathetic regulation of gastrointestinal (GI) function is well established. However, it is only recently that the interaction of sympathetic neurons, and of sympathetic transmitters, with the GI immune system and with gut flora has begun to be explored. Changes in the behaviour of sympathetic nerves when gut function is compromised, for example in ileus and in inflammation, have been observed, but the roles of the sympathetic innervation in these and other pathologies are not adequately understood. In this article, we first review the principal roles of the sympathetic innervation of the GI tract in controlling motility, fluid exchange and gut blood flow in healthy individuals. We then discuss the evidence that there are important interactions of sympathetic transmitters with the gut immune system and enteric glia, and evidence that inflammation has substantial effects on sympathetic neurons. These reciprocal interactions contribute to pathological changes in ways that are not yet clarified. Finally, we focus on inflammation, diabetes and postoperative ileus as conditions in which there is sympathetic involvement in compromised gut function.     

Transmitter diversity in ganglion cells of the guinea pig gallbladder: an immunohistochemical study.

Several neurotransmitters have been reported to exist in the ganglionated plexus of the guinea pig gallbladder. These include substance P, neuropeptide Y (NPY), calcitonin gene-related peptide, vasoactive intestinal peptide (VIP), acetylcholine, norepinephrine, serotonin, and dopamine. To determine which neuropeptides are intrinsic to gallbladder ganglia, we performed immunohistochemistry on colchicine-treated preparations. In separate, single-labeled preparations, a majority of neurons contained substance P-, NPY-, or somatostatin-like immunoreactivity. In double-labeled preparations, a large majority of the neurons that contained substance P-like immunoreactivity also contained NPY-like immunoreactivity and somatostatin-like immunoreactivity. Immunoreactivity for VIP was present in a small percentage of the gallbladder neurons which did not contain substance P-like immunoreactivity. Additional experiments were done to test for the presence of other compounds, known to exist in the neurons of the gut. Although immunoreactivity was found in control preparations of small intestine, the ganglionated plexus of the gallbladder lacked immunoreactivity for galanin, dynorphin, enkephalin, gastrin-releasing peptide, or gamma-aminobutyric acid. We conclude that ganglia of the guinea pig gallbladder contain at least two populations of neurons, based on transmitter phenotype. One of these populations appears to contain substance P, NPY, and somatostatin. Another population, which represents a small contingent of the total population of neurons, contains VIP.     

Heterogeneity of ganglia of the guinea pig myenteric plexus: an in vitro study of the origin of terminals within single ganglia using a covalently bound fluorescent retrograde tracer

Experiments were done to test the hypothesis that individual ganglia of the myenteric plexus of the guinea pig small intestine are heterogeneous with respect to the location of the neurons that provide terminals to them. The myenteric plexus, attached to the longitudinal layer of smooth muscle, was maintained in vitro. Individual ganglia were injected with a variety of potential retrograde tracers by pressure microejection from the tip (20-micron diameter) of a glass micropipette. The fluorescent dye 4-acetoamido, 4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS) was found to be an effective tracer, labeling neuronal perikarya, evidently by retrograde transport. SITS has previously been shown not to cross plasma membranes, but to be covalently bound to the outer surface of that membrane, and to be taken up by nerve terminals to be retrogradely transported to label neuronal cell bodies. SITS fluorescence was found in about 12% of the neurons within the ganglion into which it was injected and also in approximately ten times more neurons in discretely located distant ganglia. No labeling of neurons was found when SITS was injected into the bath or into the smooth muscle below the myenteric plexus. Damage to neural connectives obstructed the labeling of neurons in ganglia distal to the injection site. Individual SITS-injected myenteric ganglia were found to vary greatly in the ratios of intraganglionic SITS-labeled neurons to the total number of neurons within the injected ganglion. The ratios of the number of intraganglionic SITS-labeled neurons to SITS-labeled neurons in distant ganglia projecting to the injected ganglion from elsewhere in the myenteric plexus also varied greatly. More strikingly, individual ganglia differed over a wide range with respect to whether the neurons in distant ganglia that provided them with terminals were situated in the oral, anal, or circumferential direction. Although the majority of projections were found to be from orally located ganglia, individual ganglia were observed that received predominantly or exclusively anal or oral projections. Others received mixtures of terminals from ganglia that were anal, oral, or circumferential. This anatomical heterogeneity in the location of afferent inputs to individual myenteric ganglia is probably reflected in a functional heterogeneity as well and will have to be taken into account in further studies of the physiology of the myenteric plexus. Individual ganglia of the plexus can no longer be taken as anatomically and functionally equivalent to one another.     

Effects of the probiotic yeast Saccharomyces boulardii on the neurochemistry of myenteric neurones in pig jejunum

We studied the effects of food supplementation with Saccharomyces boulardii (S. boulardii; synonym S. cerevisiae HANSEN CBS 5926; 1 g per day for 9 days) on the presence and co-localization patterns of neuronal markers in myenteric neurones of the pig jejunum. The pan neuronal marker Hu revealed no change in the number of neuronal cell bodies per ganglion (37 +/- 7 in control vs 34 +/- 9 in the S. boulardii group). Ranked by size the following cell populations were identified: choline acetyltransferase (ChAT), calbindin-28k (CALB), substance P (SP), neurofilament 160 kD (NF-160), vasoactive intestinal polypeptide (VIP), nitric oxide synthase (NOS), calcitonin gene-related peptide (CGRP), calretinin (CALRET). We found a significant decrease in the number of CALB myenteric neurones in animals which received S. boulardii supplemented diet. None of the other neuronal markers revealed any difference between controls and S. boulardii treated animals. The study reports transmitter-localization patterns in the myenteric plexus of the pig jejunum and provides evidence that changes in the neurochemistry of enteric neurones occur with S. boulardii supplemented diet. Although only CALB expression was altered and the functional significance of this finding remains unknown, our study identified a possible new effector level of probiotics in the gut.     

Structure of neurons and ganglia of the guinea pig gallbladder: light and electron microscopic studies.

This study was undertaken to examine the morphological features of cells within ganglia of the guinea pig gallbladder, and to examine the ultrastructure of the ganglionated plexus. Gallbladder neurons are large, with a relatively simple form, having only one or two major processes. Neurobiotin often filled axons to their varicose arbors on smooth muscle in close proximity to the interganglionic connectives. With the exception of connective tissue clefts that sometimes penetrated into them, ganglia were devoid of intercellular spaces, capillaries, or connective tissue elements such as collagen and basal laminae. However, ganglia were surrounded by a single, continuous basal lamina that was enclosed within a fibroblast and collagen capsule. Within ganglia, neurons were insulated by the processes of cells that resembled the astrocyte-like glia of enteric ganglia. Although few classical synapses were observed, numerous sites of direct apposition were identified between vesicle-rich profiles and processes of gallbladder neurons. Direct appositions between vesicle-rich profiles and the ganglion-limiting basal laminae were also observed. Vesiculated profiles contained small clear vesicles and large dense-core vesicles. Within interganglionic connectives, axons were unmyelinated and were isolated from one another by processes of glia that resembled Schwann cells. As was seen in the ganglia, direct appositions between vesicle-rich profiles and the connective-limiting basal laminae were observed. The results of this study demonstrate that gallbladder ganglia are similar, ultrastructurally, to enteric ganglia in the CNS-like composition of the neuropil. However, the greater degree of glial investment, lesser degree of innervation, and simpler neurons indicated differences from the enteric nervous system that may be functionally significant.     

Comparison of extrinsic efferent innervation of guinea pig distal colon and rectum

The extrinsic efferent innervation of the distal colon and rectum of the guinea pig was compared, by using retrograde tracing combined with immunohistochemistry. Application of the carbocyanine tracer DiI to the rectum filled significantly greater numbers of extrinsic neurons than similar injections into the distal colon. Approximately three-fourths of all filled neurons from either location were either sympathetic or parasympathetic; the rest were spinal sensory neurons. Nerve cell bodies in sympathetic prevertebral ganglia labelled from the two regions were similar in number. Both regions were innervated by sympathetic neurons in paravertebral ganglia; however, the rectum received much more input from this source than the colon. The rectum received significantly more input from pelvic ganglia than the colon. The rectum also received direct innervation from two groups of neurons in the spinal cord. Neurons located in the spinal parasympathetic nucleus in segment S2 and S3 were labelled by DiI injected into the rectal wall. Similar numbers of neurons, located in intermediolateral cell column and dorsal commissural nucleus of lumbar segments, also projected directly to rectum, but not colon. The great majority (>80%) of retrogradely labelled nerve cell bodies in sympathetic ganglia were immunoreactive for tyrosine hydroxylase. In pelvic ganglia, retrogradely labelled neurons contained choline acetyltransferase and/or nitric oxide synthase or tyrosine hydroxylase. Although the rectum and colon in this species are continuous and macroscopically indistinguishable, they have significantly different patterns of extrinsic efferent innervation, presumably reflecting their different functions.     

Characterization of circular muscle motor neurons of the duodenum and distal colon in the Australian brush-tailed possum.

The motor innervation of the duodenum and distal colon remains uncharacterized within the same species. Our aim was to compare the projections and neurochemical properties of duodenal and distal colon circular muscle motor neurons. Circular muscle motor neurons were retrogradely traced by using a neural tracer in vitro, processed for choline acetyltransferase (ChAT) and nitric oxide synthase (NOS) immunoreactivity and then visualized by using indirect immunofluorescence. A mean of 372 +/- 64 and 156 +/- 23 neurons (mean +/- SEM) were tracer-labeled within the duodenum and colon, respectively. The ChAT+/NOS- neurons comprised 57.6 +/- 6.6% and 39.6 +/- 4.4% of all labeled cells in the duodenum and colon, respectively, and projected mainly in the oral direction. Of all labeled cells, the ChAT-/NOS+ neurons comprised 8.5 +/- 2.3% in the duodenum and 46.6 +/- 5.0% in the distal colon and projected mainly in the anal direction. Of the remainder, 20.6 +/- 5.0% and 8.2 +/- 2.4% were ChAT+/NOS+ and 13.2 +/- 0.9% and 5.6 +/- 1.4% were ChAT-/NOS- in the duodenum and distal colon, respectively. Within both regions, the distribution of the ChAT+/NOS- and ChAT-/NOS+ neurons are consistent with the ascending excitatory and descending inhibitory reflexes. The proportion of ChAT-/NOS+ neurons is greater within the colon in comparison with the duodenum. A considerable proportion of duodenal motor neurons were ChAT+/NOS+ and ChAT-/NOS-. These two classes may underlie nonperistaltic motor patterns, which predominate within the duodenum. These findings demonstrate regional differences in the innervation of intestinal circular muscle.     

Effects of testosterone on the electrical properties and nicotinic transmission of the major pelvic and coeliac ganglion neurones

The effects of testosterone on the electrical properties and nicotinic activation of prevertebral ganglion neurones were investigated in vitro on the male rat major pelvic ganglion and rabbit coeliac ganglion. The electrical activity of the neurones was recorded using intracellular recording techniques. Nicotinic activation was triggered for neurones of the major pelvic ganglion by stimulating the hypogastric, pelvic and cavernous nerves and for coeliac neurones by stimulating the splanchnic nerves. Testosterone modified the resting membrane potential of neurones in the major pelvic ganglion by triggering a slow depolarization, and was without significant effect on the resting membrane potential of coeliac ganglion neurones. In neurones of the major pelvic and coeliac ganglia, testosterone had no significant effect on the firing pattern, on the characteristics of the action potential (firing threshold, duration, overshoot) and on the after-hyperpolarization (amplitude and duration). Testosterone affected, in opposite ways, the nicotinic activation of neurones of the two prevertebral ganglia. In the major pelvic ganglion, testosterone triggered an increase in the amplitude of excitatory postsynaptic potentials induced by stimulation of the hypogastric, pelvic and cavernous nerves with a single pulse, revealing a facilitation of nicotinic activation. On coeliac ganglion neurones, testosterone elicited a decrease in the amplitude of excitatory postsynaptic potentials induced by stimulation of the splanchnic nerves, indicating an inhibition of nicotinic activation. Our study shows that testosterone acts differently on neurones of prevertebral ganglia involved in the nervous control of different functions, its facilitatory action being exerted on neurones of the major pelvic ganglion which is particularly involved in the control of the urogenital tract. Our study reinforces the concept, derived from neuroanatomical and pharmacological studies, of the major pelvic ganglion as a major peripheral target for testosterone.     

Presynaptic GABA-A receptors prevent depression of nicotinic transmission in rabbit coeliac ganglion neurones

We investigated the involvement of GABA-A receptors in the modulation of the nicotinic transmission of central origin in isolated rabbit coeliac ganglia. Our study was performed in vitro and the electrical activity of the ganglionic neurones was recorded using intracellular recording techniques. During iterative stimulation of the splanchnic nerves, the synaptic action potential probability decreased gradually, indicating a depression of the nicotinic activation. Pharmacological agents acting at GABA-A receptors modulated the action potential probability during the train of pulses. Muscimol (a GABA-A receptor agonist), diazepam (a benzodiazepine site agonist) and 1-[2-[[(diphenylmethylene)imino]oxy]ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride (a GABA uptake blocker) increased this probability. Conversely, gabazine or bicuculline (two GABA-A receptor antagonists), picrotoxin (a picrotoxin site agonist) and flumazenil (a benzodiazepine site antagonist) reduced it. These results demonstrate that endogenous GABA, released during the train of pulses, facilitates the central nicotinic activation of the ganglionic neurones by acting on GABA-A receptors. Muscimol also reduced the amplitude ratio of excitatory postsynaptic potentials triggered during the paired-pulse protocol without any change in postsynaptic properties. This result is consistent with a presynaptic action of GABA-A receptors. Our study shows that presynaptic GABA-A receptors facilitate the central nicotinic activation of prevertebral ganglionic neurones and thus play a novel role in the integrative properties of the sympathetic prevertebral ganglia.     

Immunohistochemical identification of neurons in ganglia of the guinea pig sphincter of oddi

The sphincter of Oddi is a smooth muscle sphincter that regulates the flow of bile into the duodenum. To identify potential chemical coding in sphincter of Oddi neurons, immunohistochemistry and histochemistry were employed to assay for putative neurotransmitters and related synthetic enzymes in wholemount preparations, with and without colchicine treatment. Immunoreactivities for enkephalin-endorphin (ENK-END), substance P (SP), nitric oxide synthase, vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), and calcitonin generelated peptide (CGRP) were demonstrated within the ganglionated plexus. Roughly half of the neurons in the sphincter of Oddi expressed immunoreactivity for both SP and ENK-END, but not for nitric oxide synthase. About 25% of the neurons expressed nitric oxide synthase immunoreactivity as well as NADPH-diaphorase activity. This contingent of neurons was made up of two subgroups: one that expressed immunoreactivity for VIP, the other for NPY. Neurons that expressed CGRP immunoreactivity were sparse in sphincter of Oddi ganglia; however, many axons immunoreactive for both CGRP and SP were present in the ganglionated plexus. The CGRP/SP fibers are probably visceral afferents that may influence ganglionic output through axon reflex circuits. These results, along with studies of the actions of these neuroactive compounds on sphincter tone, support the view that ganglia of the sphincter of Oddi are largely comprised of excitatory (SP/ENK-END-immunoreactive) and inhibitory (nitric oxide synthase/VIP- or NPY-immunoreactive) neurons, and that sphincter of Oddi tone is controlled by the regulation of the outputs of these two groups of cells. © 1995 Wiley-Liss, Inc.     

Separate origins for the dynorphin and enkephalin immunoreactive fibers in the inferior mesenteric ganglion of the guinea pig

By different denervation procedures the origin of dynorphin-(1-17) and enkephalin immunoreactive fibers in the guinea pig inferior mesenteric ganglion was investigated. It was found that the dynorphin-(1-17)-positive fibers reached the ganglion predominantly via the colonic nerves and to a lesser extent via the hypogastric and intermesenteric nerves whereas the enkephalin-positive fibers reached the ganglion via the lumbar splanchnic nerves. These findings show that the dynorphin-(1-17) and enkephalin systems are separate in this ganglion.     

Evidence of substance P immunoreactive neurons in dorsal root ganglia and vagal ganglia projecting to the guinea pig pylorus

The origin of extrinsic substance P fibers in the guinea pig pyloric wall was investigated by combining retrograde axonal tracing and indirect immunofluorescence techniques. After injection of Fast Blue into the pyloric wall labeled cells were found in the T7-T9 dorsal root ganglia and the nodose and jugular ganglia. About 60% of the labeled cells in the dorsal root ganglia contained substance P-like immunoreactivity. After local application of colchicine, a few substance P positive cells were observed in the nodose and jugular ganglia, some of which also contained Fast Blue.     

Correlation of electrophysiology, neurochemistry and axonal projections of guinea-pig sphincter of Oddi neurones

Sphincter of Oddi (SO) ganglia are comprised of two main types of neurones based either on their electrical or neurochemical properties. This study investigated whether any correlation exists between the electrical and neurochemical properties of these cells. SO neurones were characterized electrically as either Tonic or Phasic cells, labelled with neurobiotin, fixed, and processed for β-nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-DA) staining and choline acetyltransferase immunoreactivity to identify whether electrically characterized neurones were nitrergic or cholinergic. A total of 119 cells were analysed in this manner; 45% of cells were Tonic and 37% were Phasic. An equivalent number of Tonic (58.1%, 18/31) and Phasic cells (60%, 21/35) were choline acetyltransferase (ChAT) positive. Three of 34 Phasic cells were NADPH-DA positive, whereas 11/33 Tonic cells were NADPH-DA positive. In none of the preparations was ChAT immunoreactivity and NADPH-DA reactivity ever observed in the same neurone. Calretinin immunoreactivity was present in a subpopulation of both Tonic and Phasic neurones. No correlation was observed between the direction of axon projections and the electrophysiological or neurochemical properties of the cell. These results suggest that there is a lack of correlation between the electrical properties and the neurochemical content of SO neurones. Various explanations for these findings are discussed.     

Intraocular projections from the pterygopalatine ganglion in the cat.

The intraocular projection of the cat pterygopalatine (sphenopalatine) ganglion was examined by using retrograde axoplasmic transport techniques in order to investigate the possibility of the involvement of the facial nerve in ocular parasympathetic innervation. Following an injection of horseradish peroxidase (HRP) or wheat germ agglutinin-HRP into the eye, retrogradely labeled cells were observed in the ipsilateral pterygopalatine ganglion, principally in the caudal part. By dissection of silver-impregnated, acetylcholinesterase- and cholinesterase-stained orbital preparations, it was determined that two different nerve pathways link the pterygopalatine ganglion and the eye. One took a retrograde course to join the retro-orbital plexus and then traveled forward accompanying the ciliary artery, the long ciliary nerve, the short ciliary nerve, and/or the optic nerve sheath. The other entered the orbit directly, fused with the ethmoidal nerve or the infratrochlear nerve in a retrograde fashion, and then turned forward along the long ciliary nerve to enter the eye. All these nerves arose from the caudal part of the ganglion. These results are discussed in relation to recent biochemical and histochemical data demonstrating the involvement of the facial nerve in the control of ocular blood flow and intraocular pressure.     

Somal size and location within the ganglia for electrophysiologically identified myenteric neurons of the guinea pig ileum

The main goal of the present study was to examine the possibility of electrophysiologically identifying the excitable enteric S and AH neurons by use of one single criterion. Intracellular recordings were made from 189 cells of 64 ganglia in isolated preparations of the myenteric plexus of the guinea pig distal ileum. The recordings were made under visual control of the cells by using Hoffman Modulation Contrast optics at high magnification (600 X). From photomicrographs, the soma size and the location within the ganglion of the individual (unstained) cells were determined. The cells were classified into three types according to their electrical excitability and the shape of the action potential. Excitable cells were classified as AH cells (n = 84) if the action potential showed a shoulder on the falling phase, otherwise as S cells (n = 56). Cells in which no action potential could be evoked by current injection were classified as nonspiking (NS) cells (n = 49). The three classes of cells showed significant differences with respect to membrane potential, input resistance and fast synaptic input. The AH cells had significantly larger somata (P < 0.01) than the S cells. The NS cells were significantly smaller than the AH and S cells (P < 0.01). AH and S cells were found to be randomly located in the ganglia, whereas the NS cells clustered (P < 0.008) in close proximity to the onsets of internodal strands. We conclude that the shoulder of the action potential canbe used as a single criterion to distinguish “on line” S and AH neurons unequivocally.     

Guinea pig pancreatic ganglia: projections, transmitter content, and the type-specific localization of monoamine oxidase.

The ganglionated plexus of the guinea pig pancreas was investigated by using histochemical, immunocytochemical, and tract-tracing methods in order to determine whether pancreatic ganglia are analogous to the ganglia of the enteric nervous system (ENS). Three lines of evidence suggest that the ganglia of the pancreas appear to be interconnected with one another, as are enteric ganglia. First, microinjections of the retrograde tracer Fluoro-Gold into individual pancreatic ganglia labeled the perikarya of neurons in distant pancreatic ganglia, whereas no labeling of neurons was observed if injections were placed in the connective tissue adjacent to pancreatic ganglia. Second, when the intercalating dye DiI was microinjected into single pancreatic ganglia in fixed tissues, DiI-labeled terminals were found in additional pancreatic ganglia. Finally, microinjections of the beta subunit of cholera toxin into individual pancreatic ganglia yielded similar results. The ganglionated plexus of the pancreas also expresses a diversity of transmitter content and cell type-specific localization of monoamine oxidase (MAO) that is analogous to the ENS. In common with guinea pig enteric ganglia, pancreatic ganglia contain highly varicose 5-hydroxytryptamine (5-HT)-immunoreactive axons and intrinsic neuropeptide Y (NPY)- and substance P (SP)-immunoreactive neurons. The vast majority, but not all, of SP-immunoreactive fibers in the pancreatic parenchyma also contain calcitonin gene-related peptide (CGRP) immunoreactivity. MAO-B was the primary type of MAO found in the intrinsic elements of the pancreas where it was located in neurons and fibers in the pancreatic parenchyma. In common with serotoninergic enteric neurons, MAO-B immunoreactivity was not found at the LM level in pancreatic serotoninergic neurites. In contrast, NPY- and tyrosine hydroxylase (TH)-immunoreactive perivascular axons were found to contain abundant MAO-A, but no MAO-B immunoreactivity. It is concluded that MAO-B immunoreactivity is characteristic of a portion of the intrinsic innervation of the pancreas, whereas MAO-A immunoreactivity is a marker for the extrinsic sympathetic innervation of the pancreas. Because of its receipt of a direct neural innervation from myenteric ganglia of the bowel (Kirchgessner and Gershon, '90: J. Neurosci 10:1626-1642), similar connections, transmitter content and localization of type-specific MAO, the ganglionated plexus of the pancreas should be regarded as an extension or subset of the ENS.     

Both oral and caudal parts of the spinal trigeminal nucleus project to the somatosensory thalamus in the rat

Recent evidence has been accumulated that not only spinal trigeminal nucleus caudalis (Sp5C) neurons but also spinal trigeminal nucleus oralis (Sp5O) neurons respond to noxious stimuli. It is unknown, however, whether Sp5O neurons project to supratrigeminal structures implicated in the sensory processing of orofacial nociceptive information. This study used retrograde tracing with Fluorogold in rats to investigate and compare the projections from the Sp5O and Sp5C to two major thalamic nuclei that relay ascending somatosensory information to the primary somatic sensory cortex: the ventroposteromedial thalamic nucleus (VPM) and the posterior thalamic nuclear group (Po). Results not only confirmed the existence of contralateral projections from the Sp5C to the VPM and Po, with retrogradely labelled neurons displaying a specific distribution in laminae I, III and V, they also showed consistent and similar numbers of retrogradely labelled cell bodies in the contralateral Sp5O. In addition, a topographic distribution of VPM projections from Sp5C and Sp5O was found: neurons in the dorsomedial parts of Sp5O and Sp5C projected to the medial VPM, neurons in the ventrolateral Sp5O and Sp5C projected to the lateral VPM, and neurons in intermediate parts of Sp5O and Sp5C projected to the intermediate VPM. All together, these data suggest that not only the Sp5C, but also the Sp5O relay somatosensory orofacial information from the brainstem to the thalamus. Furthermore, trigemino-VPM pathways conserve the somatotopic distribution of primary afferents found in each subnucleus. These results thus improve our understanding of trigeminal somatosensory processing and help to direct future electrophysiological investigations.