Convergent responses of Barrington's nucleus neurons to pelvic visceral stimuli in the rat: a juxtacellular labelling study

Barrington's nucleus impacts on bladder and distal colon function and relays pelvic visceral information to the forebrain. This study investigated processing of information from the bladder and the distal colon by Barrington's nucleus in the rat. The responses of individual Barrington's nucleus neurons to bladder and/or colon distention were characterized using extracellular recording and the recorded neurons were identified using juxtacellular labelling. Most neurons within Barrington's nucleus (79%) were activated by bladder distention, consistent with its role as a pontine micturition centre. Although no neurons were selectively responsive to colon distention, the majority of bladder-responsive neurons (73%) were also activated by colon distention. In a second study, Barrington's nucleus neurons were characterized with respect to their response to colon distention and their immunoreactivity for the stress-related neuropeptide corticotropin-releasing factor (CRF). Of 30 labelled neurons in the central part of Barrington's nucleus, 53% were activated by colon distention and 63% of these were CRF-ir. This is the first report demonstrating that Barrington's nucleus neurons are responsive to colon distention. The results provide evidence for convergence of information from the bladder and the colon onto individual Barrington's nucleus neurons. Taken with evidence that many Barrington's nucleus neurons are synaptically linked to the bladder and colon, the present study suggests a role for these neurons in coordinating peripheral parasympathetic and central responses to both viscera and implicate CRF as a neurotransmitter in this function. Dysfunctions in this circuit may underlie the coexistence of colon and bladder symptoms observed in functional bowel disorders.     

Novel role for the pontine micturition center, Barrington's nucleus: evidence for coordination of colonic and forebrain activity

This report provides evidence for a novel role of Barrington's nucleus, considered the pontine micturition center, in regulation of colonic function. Barrington's activation elicited increases in colonic intraluminal pressure that were eliminated by scopolamine and intrathecal lidocaine, suggesting an impact of Barrington's neurons on colonic activity via projections to lumbosacral parasympathetic neurons. Consistent with this, Barrington's neurons were transsynaptically labeled from the distal colon by pseudorabies virus and several of these were also retrogradely labeled from the locus coeruleus, which projects extensively to the forebrain. Thus, Barrington's nucleus is strategically positioned to coordinate colonic and forebrain activity. Dysfunctions within this divergent system may underlie the frequent comorbidity of colonic and psychiatric symptoms.     

Evidence for divergent projections to the brain noradrenergic system and the spinal parasympathetic system from Barrington's nucleus

The present study was designed to determine whether Barrington's nucleus, which lies ventromedial to the locus coeruleus (LC) and projects to the sacral parasympathetic nucleus, is a source of afferent projections to the LC. Restricted injections of the anterograde tracer, biocytin, into Barrington's nucleus labeled varicose fibers that extended from the injection site into the LC. Consistent with this, injections of the retrograde tracers, wheatgerm agglutinin conjugated to horseradish peroxidase coupled to gold particles (WGA-Au-HRP) or fluorescein-conjugated latex beads, into the LC labeled numerous (approximately 10%) Barrington's neurons that were also retrogradely labeled by Fluoro-Gold (FG) injections in the spinal cord. Retrograde tracing from the LC combined with corticotropin-releasing hormone (CRH) immunohistochemistry revealed that at least one third of the retrogradely labeled neurons in Barrington's nucleus were CRH-immunoreactive (CRH-IR). Finally, in triple labeling studies, CRH-Barrington's neurons were consistently observed that were retrogradely labeled from both the LC and spinal cord. These findings implicate Barrington's nucleus as an LC afferent and a source of CRH-IR fibers in the LC. Additionally, the results suggest that some Barrington's neurons diverge to innervate both the spinal cord and the LC. This divergent innervation may serve to coregulate the sacral parasympathetic nervous system and brain noradrenergic system, thus providing a mechanism for coordinating pelvic visceral functions with forebrain activity.     

Central representation of bladder and colon revealed by dual transsynaptic tracing in the rat: substrates for pelvic visceral coordination

The neurocircuitry underlying regulation of bladder and distal colon function by Barrington's nucleus (the pontine micturition centre) was investigated in rats by identifying neurons which were transsynaptically labelled from these viscera, with pseudorabies virus (PRV) or genetically modified forms of PRV [PRV-beta-galactosidase (PRV-beta-Gal) and PRV-green fluorescent protein (PRV-GFP)]. PRV injection into the bladder or the colon of separate rats suggested an overlap in the distribution of bladder- and colon-related neurons in Barrington's nucleus, as well as a topographical arrangement whereby dorsal neurons were bladder-related and ventral neurons were colon-related. In rats injected with PRV-beta-Gal into one viscera and PRV-GFP into another, neurons in the major pelvic ganglion and lumbosacral spinal cord were primarily single-labelled at relatively early survival times. With longer survival times many double-labelled neurons (>70%) appeared in Barrington's nucleus, suggesting that individual Barrington's nucleus neurons are synaptically linked to preganglionic parasympathetic neurons which independently innervate the colon or the bladder. In addition to these dual-labelled neurons, Barrington's nucleus neurons which were single-labelled from either viscera were observed and these exhibited a viscerotopic organization consistent with the single-labelling studies. Together, these findings suggest the existence of three neuronal populations in Barrington's nucleus, one which is synaptically linked to both the bladder and the colon and the other two populations which are specifically linked to either viscera. These anatomical substrates may underlie the central coordination of bladder and colon function and play a role in disorders characterized by a coexistence of bladder and colonic symptoms.     

Locus coeruleus neurons and sympathetic nerves: Activation by visceral afferents

Previously brain norepinephrine (NE) neurons in the locus coeruleus (LC) have been shown to respond profoundly to external, environmental stimuli and are thought to be involved in behavioral functions such as vigilance, alarm and anxiety reactions to novel and, especially, threatening stimuli. Here we have used electrophysiological techniques to show that distension of the urinary bladder, the distal colon, rectum or the stomach causes pronounced activation responses of brain NE-LC neurons of the rat essentially without concomitant responses in splanchnic, sympathetic nerve activity (NE-SNA), thus indicating the non-noxious character of these internal stimuli. Our findings directly implicate the LC in micturition and, probably, defecation and we suggest that a high NE-LC activity may facilitate these phasic, vegetative events. In addition, the results implicate the LC as a pivotal system by which autonomic or visceral functions can affect behavior and, conversely, by which environmental stress can affect autonomic functions, for example in the opiate withdrawal syndrome.     

Colonic mechanoreceptor inputs to rat lumbo-sacral dorsal horn neurones: distribution, thresholds and chemosensory modulation.

In order to better understand the central processing of visceral sensory information, we studied the responses of lumbo-sacral dorsal horn (L4-S1) neurones to colonic stimuli in anaesthetized rats. Twenty-four neurones responded to distal colonic distension with a 2.5-cm balloon; six of these were tested with proximal colonic distension, to which none responded. All neurones tested responded to somatic non-noxious inputs (tail movement). Responses to colonic distension were excitatory (n=22) or inhibitory (n=2). Sixteen neurones responded at a threshold of 20 mmHg or less, five at 20-40 mmHg, and three at 40-80 mmHg. Three of 10 neurones tested showed increased responses to colonic distension after intraluminal perfusion with bile. Bile itself did not evoke a response. We conclude that lumbo-sacral spinal neurones selectively receive mechanosensory inputs from the distal colon. Neurones respond at thresholds within and above the physiological range. Dorsal horn neurones receiving colonic mechanosensory inputs are not directly modulated by chemosensory inputs, but their responsiveness to distension may be augmented.     

Proximal colon distension induces Fos expression in the brain and inhibits gastric emptying through capsaicin-sensitive pathways in conscious rats

We assessed brain nuclei activated during noxious mechanical distension of the proximal colon in conscious rats, using Fos as a marker of neuronal activation, and functional reflex changes in gastric emptying associated to colon distension. The role of capsaicin-sensitive afferents in Fos and gastric responses to distension was also investigated. Compared with sham distension, isovolumetric phasic distension of the proximal colon (10 ml, 30 s on/off for 10 min) increased significantly Fos expression 1 h after distension in selective brain areas, most prominently, the paraventricular and supraoptic nuclei of the hypothalamus (13-fold and 80-fold, respectively), the locus coeruleus-Barrington's nucleus complex (2-fold), area postrema (7-fold) and the nucleus tractus solitarius (4-fold). Increased Fos expression was also observed in the cingulate cortex, posterior paraventricular nucleus of the thalamus, periaqueductal gray and ventrolateral medulla. Distension of the proximal colon significantly inhibited gastric emptying by 82% and 34%, as measured 30 and 60 min after the distension respectively, compared with control. Pretreatment with systemic capsaicin prevented both the brain increase in Fos expression and the inhibition of gastric emptying induced by the colon distension. These results show that visceral pain arising from the proximal colon activates a complex neuronal network that includes specific brain nuclei involved in the integration of autonomic, neuroendocrine and behavioral responses to pain and an inhibitory motor reflex in other gut areas (delayed gastric emptying). Capsaicin-sensitive afferent pathways are involved in mediating brain neuronal activation and functional changes associated with noxious visceral stimulation.     

Transneuronal labeling from the rat distal colon: anatomic evidence for regulation of distal colon function by a pontine corticotropin-releasing factor system

Neural circuits that are positioned to regulate rat distal colon function were identified by immunohistochemical detection of pseudorabies virus (PRV) and corticotropin-releasing factor (CRF). The distribution of PRV-immunoreactive neurons was examined in spinal cord and brain at increasing times (72-118 hours) after distal colon injection. At 72-80 hours, PRV-labeling was confined to the spinal cord, in the parasympathetic preganglionic column in the lumbosacral spinal cord and in the intermediolateral column of the thoracic spinal cord. At longer survival times (88 hours), PRV-immunolabeled neurons in the lumbosacral spinal cord were also distributed in superficial layers of the dorsal horn, the dorsal commissure, and around the central canal. Trans-synaptic labeling was identified in the medullary raphe nuclei, parapyramidal region, A5, Barrington's nucleus, A7, and the dorsal cap of the paraventricular nucleus of the hypothalamus after longer survival times (88-91 hours). Substantial labeling of the locus coeruleus, periaqueductal gray and forebrain regions occurred at later survival times (> or = 96 hours). In dual-labeled sections, CRF terminal labeling surrounded PRV-labeled neurons in the parasympathetic preganglionic column of the lumbosacral spinal cord. Additionally, many neurons in Barrington's nucleus, but not other CRF-containing nuclei, were double labeled for CRF and PRV. These results, taken with previous studies, support a convergence in transneuronal labeling from different pelvic viscera that may be related to coordination of overall pelvic visceral functions. Importantly, they provide an anatomic substrate for an impact of CRF from Barrington's nucleus in normal and pathophysiological functions of the distal colon.     

Activation of locus coeruleus neurons by nucleus paragigantocellularis or noxious sensory stimulation is mediated by intracoerulear excitatory amino acid neurotransmission

The nucleus paragigantocellularis (PGi), located in the rostral ventrolateral medulla, is one of two major afferents to the nucleus locus coeruleus (LC). Electrical stimulation of PGi exerts a robust, predominantly excitatory influence on LC neurons that is blocked by intracerebroventricular (i.c.v.) administration of the broad spectrum excitatory amino acid (EAA) antagonists kynurenic acid (KYN) or gamma-D-glutamylglycine (DGG), but not by the selective N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-7-phosphonoheptanoate (AP7). I.c.v. injection of KYN or DGG also blocked activation of LC neurons evoked by noxious somatosensory stimuli. These results indicate that activation of LC neurons by PGi and noxious stimuli may be mediated by an EAA acting at a non-NMDA receptor in LC. In the present study, microiontophoretic techniques were used to determine the sensitivity of LC neurons in vivo to the selective EAA receptor agonists kainate (KA), NMDA and quisqualate (QUIS). Microinfusion and microiontophoresis were also used to determine whether direct application of KYN, the preferential non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3 dione (CNQX) or the selective NMDA receptor antagonist 2-amino-5-phosphonovalerate (AP5) onto LC neurons blocked excitation elicited by stimulation of PGi or the sciatic nerve. The results demonstrated that individual LC neurons were robustly activated by direct application of KA, NMDA and QUIS. Iontophoretically applied KYN reduced or completely antagonized responses evoked by all 3 agonists. In contrast, iontophoretically applied AP5 strongly attenuated NMDA-evoked excitation, while KA-and QUIS-evoked responses were not affected by this agent. Furthermore, direct application of KYN or the specific non-NMDA receptor antagonist, CNQX, onto LC neurons substantially attenuated or completely blocked synaptic activation produced by PGi or sciatic nerve stimulation in nearly every LC neuron tested. Microinfusion of the selective NMDA receptor antagonist AP5 had no effect on sciatic nerve-evoked responses. These results confirm our hypothesis that activation of LC neurons from PGi is mediated by an EAA operating primarily at a non-NMDA receptor subtype on LC neurons. Furthermore, these findings provide additional support for the hypothesis that this pathway mediates at least some sensory-evoked responses of LC neurons.     

Ultrastructure of endomorphin-1 immunoreactivity in the rat dorsal pontine tegmentum: evidence for preferential targeting of peptidergic neurons in Barrington's nucleus rather than catecholaminergic neurons in the peri-locus coeruleus

Endomorphins are opioid tetrapeptides that have high affinity and selectivity for mu-opioid receptors (muORs). Light microscopic studies have shown that endomorphin-1 (EM-1) -containing fibers are distributed within the brainstem dorsal pontine tegmentum. Here, immunoelectron microscopy was conducted in the rat brainstem to identify potential cellular interactions between EM-1 and tyrosine hydroxylase (TH) -labeled cellular profiles in the locus coeruleus (LC) and peri-LC, an area known to contain extensive noradrenergic dendrites of LC neurons. Furthermore, sections through the rostral dorsal pons, from colchicine-treated rats, were processed for EM-1 and corticotropin releasing factor (CRF), a neuropeptide known to be present in neurons of Barrington's nucleus. EM-1 immunoreactivity was identified in unmyelinated axons, axon terminals, and occasionally in cellular profiles resembling glial processes. Within axon terminals, peroxidase labeling for EM-1 was enriched in large dense core vesicles. In sections processed for EM-1 and TH, approximately 10% of EM-1-containing axon terminals (n=269) targeted dendrites that exhibited immunogold-silver labeling for TH. In contrast, approximately 30% of EM-1-labeled axon terminals analyzed (n = 180) targeted CRF-containing somata and dendrites in Barrington's nucleus. Taken together, these data indicate that the modulation of nociceptive and autonomic function as well as stress and arousal responses attributed to EM-1 in the central nervous system may arise, in part, from direct actions on catecholaminergic neurons in the peri-LC. However, the increased frequency with which EM-1 axon terminals form synapses with CRF-containing profiles in Barrington's nucleus suggests a novel role for EM-1 in the modulation of functions associated with Barrington's nucleus neurons such as micturition control and pelvic visceral function.     

Functional characterization of preganglionic neurons projecting in the lumbar splanchnic nerves: neurons regulating motility

Lumbar preganglionic neurons, which projected in the lumbar splanchnic nerves and were probably involved in regulating motility of colon and pelvic organs (motility-regulating, MR neurons), were analyzed for their discharge patterns. The responses of the neurons to the following stimuli were tested: stimulation of arterial baro- and chemoreceptors and of afferents from the urinary bladder, colon, mucosal skin of the anus and perianal hairy skin. The following findings were made: a total of 131 preganglionic neurons were classified as MR neurons; these reacted to natural stimulation of at least one of the afferent inputs from the urinary bladder, colon and anal and perianal skin. The ongoing activity of these neurons did not correlate with the cardiac cycle or the cycle of the artificial ventilation. Most of them did not respond to an increase of blood pressure produced by i.v. injection of adrenaline or noradrenaline; some showed a weak depression or weak excitation which, in the time course, was untypical for visceral vasoconstrictor neurons. Stimulation of arterial chemoreceptors either did not influence MR neurons or produced only a secondary response owing to contraction of the urinary bladder. Ninety-seven preganglionic MR neurons could be subclassified: MR1 neurons were excited by distension and contraction of the urinary bladder and/or inhibited by distension and contraction of the colon (n = 61), a few were excited from both organs (n = 4); MR2 neurons were inhibited by distension and contraction of the urinary bladder and/or excited by distension and contraction of the colon (n = 32). Ninety-five out of 121 MR neurons (78.5%) were excited, 10 (8%) were inhibited and 16 (13%) not influenced by mechanical shearing stimuli applied to the mucosal skin of the anus. Most neurons which were excited by anal stimulation were not influenced by mechanical stimulation of the perianal (perigenital) skin. Twenty-eight per cent of the MR neurons (18 out of 64) were excited or inhibited upon stimulation of perianal skin. A few of these (7 out of 64 neurons, 11%) were involved in reflex responses which were different from those elicited from anal skin. At present no further consistent subclassification of MR1 and MR2 neurons appears possible on the basis of the excitatory and inhibitory anal and perianal reflexes. The results show that the population of visceral preganglionic neurons, which are probably involved in regulation of motility of colon and pelvic organs, is not homogeneous and probably consists of several subpopulations.     

Fos expression in rat pontine tegmental neurons following activation of the medial preoptic area

Fos immunohistochemistry was used to map the distribution of pontine neurons excited by activation of the medial preoptic area (MPO). Although we have previously shown that Barrington's nucleus receives a very dense focal input from the MPO, electrical stimulation of the preoptic area unexpectedly induced very little Fos expression in Barrington's neurons. These results suggest that the MPO→Barrington's projection utilizes a transmitter(s) that does not involve transduction of the Fos protein; alternatively, MPO afferents to Barrington's nucleus may be inhibitory in nature. As Barrington's nucleus plays a critical role in micturition, MPO projections to Barrington's nucleus may regulate voiding reflexes during sexual behavior. Interestingly, while the locus coeruleus (LC) proper receives only a sparse projection from the MPO, extensive Fos expression was present in LC. The finding of Fos immunoreactive LC neurons suggests that the excitatory influence of MPO may regulate LC neuronal activity and NE release during reproductive behaviors.     

Responses of rat spinal neurones to natural and electrical stimulation of colonic afferents: effect of inflammation

Single unit electrical activity has been recorded from 107 neurones excited by electrical stimulation of the pelvic nerve in or around lamina X of the L6-S1 spinal cord in anaesthetised rats. Responses to colorectal distension (CRD; 30 s, 5-80 mmHg) and to somatic electrical and mechanical stimulation were characterised. Of 107 neurones excited by pelvic nerve stimulation, 58 (54%) were affected by CRD: 46 neurones were excited (39 with a sustained response and 7 with an on-off response) and 12 neurones were inhibited. The vast majority of the neurones affected by CRD (54/58) had nociceptive somatic receptive fields. Neurones excited by CRD showed graded stimulus response functions in the noxious range (20-80 mmHg), except for two neurones which only encoded stimulus intensity below 20 mmHg. Neurones inhibited by CRD had significantly larger somatic receptive fields, and more superficial recording sites than those excited by CRD. A group of 12 neurones with sustained excitatory responses to CRD were characterised before and 45 min after intracolonic instillation of 1 ml 0.6% acetic acid. Colon inflammation provoked a significant increase in responses to CRD and to pelvic nerve stimulation (n=12), but no significant change in responses to pinch of their somatic receptive field (n=10). We conclude that of these neurones, the population with excitatory sustained responses to CRD are those likely responsible for processing information leading to acute pain sensations from the colon, and also show central sensitisation after colon inflammation, suggesting they play an important role in development of colonic hyperalgesia.     

Protease-activated receptor-4 (PAR4): a role as inhibitor of visceral pain and hypersensitivity

Abstract Protease‐activated receptor‐4 (PAR₄) belongs to the family of receptors activated by the proteolytic cleavage of their extracellular N‐terminal domain and the subsequent binding of the newly released N‐terminus. While largely expressed in the colon, the role of PAR₄ in gut functions has not been defined. We have investigated the effects of PAR₄ agonist on colonic sensations and sensory neuron signalling, and its role in visceral pain. We observed that a single administration of the PAR₄ agonist peptide (AYPGKF‐NH₂), but not the control peptide (YAPGKF‐NH₂) into the colon lumen of mice significantly reduced the visceromotor response to colorectal distension at different pressures of distension. Further, intracolonic administration of the PAR₄ agonist, but not the control peptide, was able to significantly inhibit PAR₂ agonist‐ and transcient receptor potential vanilloid‐4 (TRPV4) agonist‐induced allodynia and hyperalgesia in response to colorectal distension. Protease‐activated receptor‐4 was detected in sensory neurons projecting from the colon, and isolated from the dorsal root ganglia, where it co‐expressed with PAR₂ and TRPV4. In total sensory neurons, PAR₄ agonist exposure inhibited free intracellular calcium mobilization induced by the pro‐nociceptive agonists of PAR₂ and TRPV4. Finally, PAR₄‐deficient mice experienced increased pain behaviour in response to intracolonic administration of mustard oil, compared with wild‐type littermates. These results show that PAR₄ agonists modulate colonic nociceptive response, inhibit colonic hypersensitivity and primary afferent responses to pro‐nociceptive mediators. Endogenous activation of PAR₄ also plays a major role in controlling visceral pain. These results identify PAR₄ as a previously unknown modulator of visceral nociception.     

Effect on urinary bladder function and arterial blood pressure of the activation of putative purine receptors in brainstem areas

The effect on bladder function and arterial blood pressure of adenosine-5'-triphosphate (ATP) and its synthetic analogue, alpha,beta-methylene ATP (alpha,beta-meATP) applied by microinjection to brainstem areas was assessed in the anaesthetised, paralysed and artificially ventilated female rat. Recordings of bladder pressure, changes in the pelvic nerve activity, arterial blood pressure and heart rate were evaluated. The purinergic drugs were microinjected into two brainstem areas the periaqueductal grey matter (PAG) and the area of the Barrington nucleus/locus coeruleus (LC) - only after electrical stimulation (50 Hz, 1 ms, 30-50 microA; n(PAG) = 17; n(LC) = 18) and the microinjection of glutamate (2 mM, pH 7.4+/-0.1; n(PAG) = 16; n(LC) = 16) had shown increases of bladder pressure and/or rate of bladder contractions and/or pelvic nerve activity at specific sites. Electrical and glutamate activation of PAG evoked an increase of arterial blood pressure. Microinjections of ATP (20 mM, pH 7.4+/-0.1; n(PAG) = 11; n(LC) = 11) and alpha,beta-meATP (2 mM, pH 7.4+/-0.1; n(PAG) = 10; n(LC) = 9) both evoked consistent increases of bladder pressure and/or pelvic nerve activity. Stimulation with ATP elicited a biphasic change of arterial blood pressure characterised by an increase followed by a decrease which was accompanied by a rise of heart rate. Microinjection of alpha,beta-meATP into PAG did not elicit a consistent response: a decrease of arterial blood pressure was evoked in five rats, while in two other rats an increase occurred. Electrical stimulation and glutamate activation of Barrington's nucleus/LC evoked an increase of arterial blood pressure, but a decrease was observed after microinjection of both ATP and alpha,beta-meATP. At some sites (n = 8) the effect of alpha,beta-meATP after a pre-injection at the same site of the P2 purino receptor antagonist, suramin (20 mM, pH 7.4+/-0.1) was smaller than the control. At three sites within PAG and two within LC located more medially to sites where an excitatory response had been observed, electrical stimulation evoked a small decrease or no change in bladder pressure. Following the stimulus, a rise in bladder pressure was preceded by an increase of pelvic nerve activity. A similar effect of glutamate was observed in one case. These data suggest that activation of P2 purine receptors in both PAG and Barrington's nucleus/LC is implicated in the neuronal mechanisms that generate patterns of activity in the parasympathetic innervation of the bladder and that purines also act at this level to modify sympathetic outflow to the cardiovascular system.     

Responses of neurons in ventroposterolateral nucleus of primate thalamus to urinary bladder distension.

The purpose of this study was to examine effects of a noxious visceral stimulus, urinary bladder distension (UBD), on cells in the ventroposterolateral (VPL) nucleus of anesthetized monkeys. We hypothesized that processing of visceral information in the VPL nucleus of the thalamus is similar to spinothalamic tract (STT) organization of visceral afferent input. Urinary bladder distension excites sacral and upper-lumbar STT cells that have somatic input from proximal somatic fields; whereas, thoracic STT cells are inhibited by UBD. Extracellular action potentials of 67 neurons were recorded in VPL nucleus. Urinary bladder distension excited 22 cells, inhibited 9 cells, and did not affect activity of 36 cells. Seventeen of 22 cells excited by UBD also received convergent somatic input from noxious squeeze of the hip, groin, or perineal regions. No cells activated only by innocuous somatic stimuli were excited by UBD. Five of 9 cells inhibited by UBD had upper-body somatic fields. There was a significant tendency for VPL neurons excited by UBD to have proximal lower-body somatic fields that were excited by noxious stimulation of skin and underlying muscle (P less than 0.001). Antidromic activation of 4 thalamic neurons affected by UBD showed that visceral input stimulated by UBD reached the primary somatosensory (SI) cortex.     

An electrophysiological study of the forebrain projection of nucleus commissuralis: Preliminary identification of presumed A2 catecholaminergic neurons

Using a double-labeling technique (HRP combined with catecholamine fluorescence), up to 80% of all CA-containing neurons visualized in the nucleus commissuralis were found to project to or through the median forebrain bundle area (MFB). In addition at least 90% of all nucleus commissuralis neurons projecting through the MFB were found to be catecholaminergic. In a series of chloral hydrate-anesthetized rats, nucleus commissuralis neurons projecting through the MFB were identified with single-unit recordings by antidromic (AD) activation. These cells had a conduction velocity of about 0.5 m/s and a firing rate of 0-14 spikes/s. The pattern of discharge of these neurons was not correlated with the heart rate; they were unaffected by single-unit stimulation applied to the sciatic nerve but were powerfully excited by vagus nerve stimulation. For comparative purposes, NE-containing neurons were also recorded in the locus coeruleus (A6) in the course of the same experiments; in contrast with MFB-activated commissuralis neurons, A6 neurons were excited by both visceral (vagus nerve) and somatic (sciatic nerve) stimulation. The spontaneous firing rate of MFB-activated commissuralis neurons was inhibited by the intravenous administration of a low dose of the centrally acting antihypertensive agent clonidine (ED50: 28 micrograms/kg).     

Neural pathways that mediate the effects of afferent stimuli on paraventricular nucleus multiunit activity in freely moving rats

The direct involvement of the hypothalamic paraventricular nucleus (PVN) in the control of adrenocortical secretion is now generally accepted. In order to contribute to our understanding of the electrical activity of cells in this region during adrenocortical activation, we have recorded multiunit electrical activity (MUA) in response to acute neural stimuli in freely moving male rats and have examined the pathways involved. Photic, acoustic, olfactory, and sciatic nerve stimulation all increased PVN MUA by between 130% and 250%. These responses were selectively blocked, according to the stimulus modality tested, by radiofrequency lesions of central neural structures. Thus PVN responses to photic stimulation were blocked by lesions of the suprachiasmatic nuclei and reduced by mammillary peduncle lesions but were unaffected by lesions of the bed nuclei of the stria terminalis. Responses to acoustic stimulation were blocked by lesions of the mammillary peduncles but not by those placed in the suprachiasmatic nuclei, the septum, or the bed nuclei of the stria terminalis. Lesions of the septum blocked the response to sciatic nerve stimulation but did not affect the response to olfactory stimulation with amyl acetate fumes, which was blocked by lesions of the bed nuclei of the stria terminalis. The data confirm those obtained in endocrine studies concerning the neural pathways involved in the transmission of neural stimuli that produce adrenocortical activation.     

Peripheral nerve pathways to neurons in the guinea pig inferior mesenteric ganglion determined electrophysiologically after chronic nerve section

Peripheral synaptic pathways to neurons in the guinea pig inferior mesenteric ganglion (IMG) were studied. Nerve trunks innervating neurons in the ganglion were surgically sectioned and intracellular electrical responses to nerve stimulation were measured 6-8 days after surgery. In all animals ganglia were decentralized by removal of the lumbar sympathetic chain ganglia L2 through L4 and in addition two peripheral nerves were sectioned leaving the ganglion innervated by only one peripheral nerve. Fast and slow excitatory postsynaptic potential (EPSP) were evoked with electrical stimulation of each of the nerve trunks and with distension of the colon. The thresholds to evoke fast EPSPs and the amplitude of slow EPSPs were compared for each nerve trunk among the different surgical groups including sham-operated controls and completely denervated ganglia. Both fast and slow EPSPs could be evoked electrically from each intact peripheral nerve trunk after the other three nerve trunks had been sectioned, which demonstrates that nerve fibers with cell bodies in the regions innervated by the peripheral nerves make functional synaptic connections with neurons in the inferior mesenteric ganglion. In general, nerve sections increased the threshold for evoking fast EPSPs and decreased the amplitude of electrically-evoked slow EPSPs compared to control ganglia. Synaptic potentials could also be evoked with stimulation of cut nerve trunks, demonstrating that branches of nerve fibers from peripheral nerves enter other nerve trunks. The hypogastric nerve was unique in that branches of axons eliciting fast but not slow synaptic potentials in the ganglion entered this nerve trunk. Distension-induced fast and slow EPSPs were present only if the lumbar colonic nerve was intact and they were not altered by section of the other nerve trunks. In contrast, the slow EPSPs evoked with electrical stimulation of the lumbar colonic nerve were significantly smaller when at least one other nerve trunk was sectioned suggesting that the axon branches from other nerve trunks which enter the lumbar colonic nerve are not activated by distension. These studies demonstrate that neurons eliciting either fast or slow synaptic potentials with cell bodies in regions innervated by the peripheral nerve trunks make functional synaptic connections with neurons of the inferior mesenteric ganglion. The results also suggest that the majority of mechanosensory neurons mediating excitatory synaptic responses to colon distension are neurons with a peripheral cell body.     

PAR4: A new role in the modulation of visceral nociception

Abstract  Protease-activated receptors (PARs) are a family of G-protein-coupled receptors with a widespread distribution that are involved in various physiological functions including inflammation and nociception. In a recent study in Neurogastroenterology and Motility, Augé et al. describe for the first time the presence of PAR4 on visceral primary afferent neurons and its role in modulating colonic nociceptive responses, colonic hypersensitivity and primary afferent responses to PAR2 and Transient Receptor Potential Vanilloid-4 (TRPV4). Using the model of visceromotor response (VMR) to colorectal distension (CRD), they show that a PAR4 agonist delivered into the colon lumen decreases basal visceral response to CRD and reduces the exacerbated VMR to CRD induced by treatment with PAR2 or TRPV4 agonists. In isolated sensory neurons, they show that a PAR4 agonist inhibits calcium mobilization induced by PAR2 or TRPV4 agonists. Finally, they describe increased pain behaviour evoked by luminal application of mustard oil in PAR4 deficient mice compared to wild type controls. The newly discovered role of PAR4 in modulating visceral pain adds to our growing understanding of the contribution of colonic proteases and PARs to the mechanisms involved in colonic hypersensitivity and their potential role as therapeutic targets for irritable bowel syndrome.