A slow EPSP in mammalian inferior mesenteric ganglion persists after in vivo capsaicin

Navigating for reward

An organism's survival can depend on its ability to recall and navigate to spatial locations associated with rewards, such as food or a home. Accumulating research has revealed that computations of reward and its prediction occur on multiple levels across a complex set of interacting brain regions, including those that support memory and navigation. However, how the brain coordinates the encoding, recall and use of reward information to guide navigation remains incompletely understood. In this Review, we propose that the brain's classical navigation centres - the hippocampus and the entorhinal cortex - are ideally suited to coordinate this larger network by representing both physical and mental space as a series of states. These states may be linked to reward via neuromodulatory inputs to the hippocampus-entorhinal cortex system. Hippocampal outputs can then broadcast sequences of states to the rest of the brain to store reward associations or to facilitate decision-making, potentially engaging additional value signals downstream. This proposal is supported by recent advances in both experimental and theoretical neuroscience. By discussing the neural systems traditionally tied to navigation and reward at their intersection, we aim to offer an integrated framework for understanding navigation to reward as a fundamental feature of many cognitive processes.

Patterns of innervation of sympathetic vascular neurons by peptide-containing primary sensory fibers

The purpose of this study was to determine whether there is a specific organization of the primary sensory innervation on to identified vascular neurons in the inferior mesenteric ganglion (IMG) in guinea-pig. Retrograde tracers were placed intraluminally in inferior mesenteric artery (IMA) or inferior mesenteric vein (IMV) in vitro to identify ganglionic neurons as arterial, venous or unlabeled neurons. The distribution of primary sensory nerve fibers containing calcitonin gene-related peptide (CGRP), neuronal nitric oxide synthase (NOS) and substance P immunoreactivity (SP-IR) was compared before and after treatment with capsaicin. In control animals the density of immunoreactivity varied both with the transmitter and the type of neuron innervated. The density of immunoreactivity for all the three substances was reduced by capsaicin treatment. The degree of reduction of immunoreactivity in the fibers varied with the transmitter and the type of neuron. The density of CGRP and SP immunoreactive fibers was greatest around unlabeled neurons; 78% of the CGRP fibers were of primary sensory origin and all of the SP fibers were primary sensory. Around arterial neurons 44% of the CGRP fibers were of primary sensory origin and around venous 68% were primary sensory. NOS positive innervation around venous neurons was denser than around arterial neurons and all of it was completely (97%) eliminated by capsaicin, indicating that it was solely of primary sensory origin. We conclude that the primary sensory fibers innervating the IMG are differentially distributed to arterial and venous neurons and that the pattern of distribution is characteristic for each sensory neurotransmitter.

Immediate-early gene expression in the inferior mesenteric ganglion and colonic myenteric plexus of the guinea pig

Activation of neurons in the inferior mesenteric ganglion (IMG) was assessed using c-fos, JunB, and c-Jun expression in the guinea pig IMG and colonic myenteric plexus during mechanosensory stimulation and acute colitis in normal and capsaicin-treated animals. Intracolonic saline or 2% acetic acid was administered, and mechanosensory stimulation was performed by passage of a small (0.5 cm) balloon either 4 or 24 hr later. Lower doses of capsaicin or vehicle were used to activate primary afferent fibers during balloon passage. c-Jun did not respond to any of the stimuli in the study. c-fos and JunB were absent from the IMG and myenteric plexus of untreated and saline-treated animals. Acetic acid induced acute colitis by 4 hr, which persisted for 24 hr, but c-fos was found only in enteric glia in the myenteric plexus and was absent from the IMG. Balloon passage induced c-fos and JunB in only a small subset of IMG neurons and no myenteric neurons. However, balloon passage induced c-fos and JunB in IMG neurons (notably those containing somatostatin) and the myenteric plexus of acetic acid-treated animals. After capsaicin treatment, c-fos and JunB induction by balloon passage was inhibited in the IMG, but there was enhanced c-fos expression in the myenteric plexus. c-fos and JunB induction by balloon stimulation was also mimicked by acute activation of capsaicin-sensitive nerves. These data suggest that colitis enhances reflex activity of the IMG by a mechanism that involves activation of both primary afferent fibers and the myenteric plexus.

The mammalian sympathetic prevertebral ganglia: integrative properties and role in the nervous control of digestive tract motility

The prevertebral ganglia which are a constitutive part of the sympathetic system have long been considered as a simple relay on this efferent pathway. In fact, these ganglia must be considered as true peripheral nervous centres. They possess various integrative properties, such as projections of central and peripheral inputs onto the ganglionic neurones, gating of these projections and pacemaker activity of the ganglionic neurones. These properties explain the ability of these ganglia to participate in the regulation of various visceral functions, including digestive tract motility.

Laser irradiation abates neuronal responses to nociceptive stimulation of rat-paw skin

The effects of diode laser irradiation on peripheral nerves was examined by monitoring neuronal discharges elicited by application of various stimuli to the hind-paw skin of rats. Neuronal discharges elicited by brush, pinch, cold, and/or heat stimulation, as well as chemical stimulation by injection of turpentine (0.1 ml, SC) were recorded from L5 dorsal roots in urethane-anesthetized rats. Diode laser irradiation (830 nm, 40 mW, 3 min, continuous wave) of the saphenous nerve exposed from the muscle of the lower leg significantly inhibited neuronal discharges elicited by pinch (68.4 +/- 6.5%), cold (45.4 +/- 9.2%), and heat stimulation (49.2 +/- 11.3%). Neuronal discharges induced by brush stimulation (104.3 +/- 4.7%) were not affected by laser irradiation. Injection of turpentine, a chemical irritant, into the hind-paw skin (0.1 ml, SC) elicited neuronal discharges in the ipsilateral dorsal root, and these discharges were significantly inhibited or abolished by laser irradiation. In 6- to 7-week-old rats treated neonatally with capsaicin (10 mg/kg, SC), injection of turpentine into the hind-paw skin did not elicit neuronal discharges and laser irradiation did not affect the background discharges. These data suggest that laser irradiation may selectively inhibit nociceptive neuronal activities.

A capsaicin-sensitive inhibitory reflex from the colon to mesenteric arteries in the guinea-pig

1. The present in vitro study examined the effect of distension of the distal colon on membrane potential in the inferior mesenteric artery of the guinea-pig. 2. Distension of the distal colon up to an intraluminal pressure of 25 cmH2O induced a hyperpolarization in the inferior mesenteric artery. The average amplitude of hyperpolarizations induced by 2 min distensions of the colon was 3 mV and their average duration was 268 s. 3. Distension-induced hyperpolarizations (DIHs) were abolished in the presence of tetrodotoxin or a low-Ca2+ (0.5 mM) superfusion solution. 4. Superfusion of capsaicin (10 microM) induced slow hyperpolarizing responses in mesenteric arteries. Following application of capsaicin (10 microM), DIHs were abolished. 5. These findings provide strong evidence that mesenteric arteries receive an inhibitory, capsaicin-sensitive sensory innervation from the distal colon which is activated during periods of colon distension to induce hyperpolarization of the arterial smooth muscle. This extramural inhibitory reflex pathway may play a physiological role in co-ordinating mesenteric blood flow with changes in gut motility.

Comparison of central versus peripheral nerve pathways to the guinea pig inferior mesenteric ganglion determined electrophysiologically after chronic nerve section

The contributions of central versus peripheral nerve pathways to neurons of the inferior mesenteric ganglion of guinea pigs 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. Guinea pigs were divided into two experimental groups: (1) those that had the lumbar sympathetic chain ganglia (LSG) L2 through L4 removed and (2) those that had the intermesenteric, lumbar colonic and hypogastric nerves sectioned leaving central connections intact. After 6-8 days fast excitatory postsynaptic potentials (EPSPs) and slow EPSPs were recorded intracellularly in randomly selected principal ganglionic neurons. The threshold stimulus voltage to elicit a fast EPSP, the amplitude of the slow EPSP and the number of neurons in which each type of synaptic potential occurred in response to stimulation of each of the nerve trunks was compared between surgically-sectioned animals and sham-operated controls. Neither section of preganglionic nerve trunks nor of postganglionic nerve trunks eliminated all synaptic input to neurons in the ganglion, indicating that neurons with cell bodies located central to the ganglion as well as in visceral target organs made synaptic connections in the ganglion. Both fast and slow synaptic potentials could be evoked by stimulation of postganglionic nerve trunks even after they were sectioned provided that preganglionic nerves were intact, indicating that axons of central origin which synapse in the ganglion may continue out into postganglionic nerve trunks. In like manner, evidence was obtained indicating that fibers from peripheral nerve trunks which initiate either fast or slow synaptic potentials in ganglionic neurons may continue out into the lumbar splanchnic nerves. These studies demonstrate that the synaptic potentials recorded in the inferior mesenteric ganglion arise not only from neurons with cell bodies central to the ganglion but also from neurons with cell bodies located in the visceral organs which this ganglion subserves.

Neurotensin facilitates release of substance P in the guinea-pig inferior mesenteric ganglion

1. Intracellular, electrophysiological techniques were combined with radio-immunological, chromatographic and pharmacological techniques to determine if nerve terminals containing substance P mediated transient depolarizing responses of principal ganglion cells induced by neurotensin. Experiments were performed in vitro on guinea-pig inferior mesenteric ganglia. 2. In 61% of principal ganglion cells tested in normal ganglia, neurotensin caused a transient membrane depolarization. In ganglia which were removed from animals which had been pre-treated with capsaicin, transient responses to neurotensin were virtually abolished. 3. In normal ganglia, neurotensin increased the amplitude and duration of noncholinergic slow EPSPs evoked by electrical stimulation of the lumbar colonic nerve. Such increases were absent in ganglia obtained from animals pre-treated with capsaicin. 4. In guinea-pigs pre-treated with capsaicin, the content of substance P-like material was significantly reduced in inferior mesenteric and coeliac ganglia, dorsal root ganglia and lumbar spinal cord, compared to control animals. The content of substance P-like material in segments of distal colon was slightly reduced. The content of vasoactive intestinal polypeptide-, cholecystokinin- and bombesin-like material in the same tissues from animals pre-treated with capsaicin was not significantly different from control animals. 5. Chromatographic analysis using HPLC (high-performance liquid chromatography) techniques revealed that the material depleted from inferior mesenteric and coeliac ganglia, dorsal root ganglia and lumbar spinal cord by capsaicin pre-treatment co-eluted with synthetic substance P. 6. Electrical stimulation of the lumbar colonic nerve released substance P-like material from isolated inferior mesenteric ganglia as determined by radioimmunoassay of samples of superfusate. Exogenous administration of neurotensin caused a significant increase in the amount of substance P-like material released during nerve stimulation. 7. Transient depolarizing responses evoked by neurotensin were markedly attenuated when ganglion cells were postsynaptically desensitized to exogenously administered substance P. 8. Taken together, these findings suggest that transient depolarizations mediated by an indirect action of neurotensin and facilitation of electrically evoked non-cholinergic slow EPSPs by neurotensin involved presynaptic release of substance P from collateral nerve terminals of primary afferent nerve fibres in the inferior mesenteric ganglion. 9. It was suggested that under normal in vivo conditions, neurotensin or a C-terminal-related peptide contained in central preganglionic nerve endings might function as an excitatory neuromodulator to enhance the release of substance P from primary afferent nerve terminals thereby facilitating non-cholinergic peripheral afferent synaptic input to prevertebral ganglion cells.

The electrophysiological effects of neurotensin on neurones of guinea-pig prevertebral sympathetic ganglia

1. The membrane effects of neurotensin on neurons of guinea-pig prevertebral ganglia were investigated by means of intracellular recording techniques in vitro. 2. Neurotensin (2-5 microM) applied by superfusion caused depolarizing responses in fifty-seven of seventy-four neurones tested in the inferior mesenteric ganglion and thirty-seven of forty-seven neurones tested in the coeliac plexus. The remaining neurones tested showed no membrane response. 3. Responses to neurotensin could be discriminated into two different types of membrane depolarizations on the basis of their different time courses and pharmacological characteristics: a steady-state type of depolarization and a transient type of depolarization. Seven of fifty-seven responsive neurones tested in the inferior mesenteric ganglion and ten of thirty-seven responsive neurones tested in the coeliac plexus responded to neurotensin with a depolarization which was maintained constant as long as neurotensin was superfused over the preparation (steady-state type). Forty-eight of fifty-seven responsive neurones tested in the inferior mesenteric ganglion and twenty of thirty-seven responsive neurones tested in the coeliac plexus responded with a transient depolarization which was followed by a repolarization in the maintained presence of neurotensin (transient type). A combination of both types of responses was observed in two neurones tested in the inferior mesenteric ganglion and in seven neurones tested in the coeliac plexus. 4. Steady-state type responses were characterized by a slowly developing membrane depolarization which reached a plateau and lasted throughout the presence of neurotensin. Amplitude and time course of this response were not altered in a solution containing hexamethonium (10 microM) and atropine (10 microM) or by a solution low in calcium (1 mM) and high in magnesium (15 mM). 5. Transient type depolarizations evoked by neurotensin were faster in reaching their maximum and were followed by a repolarization during the maintained presence of neurotensin. Responses similar in time course and amplitude were obtained in solutions containing hexamethonium (10-100 microM) and atropine (10 microM). However, transient responses were abolished in a solution low in calcium (1 mM) and high in magnesium (15 mM) and were markedly attenuated in ganglia treated with capsaicin (3 microM). 6. Both types of depolarizations were associated with increases in membrane input resistance. Both responses converted subthreshold depolarizing electrotonic potentials and subthreshold fast EPSPs to action potentials. 7. Both types of depolarizations were observed when the C-terminal hexapeptide fragment neurotensin 8-13 was used.(ABSTRACT TRUNCATED AT 400 WORDS)

The electrophysiological effects of vasoactive intestinal polypeptide in the guinea-pig inferior mesenteric ganglion

1. The effects of vasoactive intestinal polypeptide (VIP) on the inferior mesenteric ganglion of the guinea-pig were studied in vitro. 2. In 67% of the neurones tested, application of VIP (1-7.5 X 10(-5) M) by pressure ejection caused a depolarization of the membrane potential which averaged 8.6 +/- 0.4 mV. 3. In 52% of the cells that were responsive to VIP, the membrane depolarization was accompanied by a decrease in membrane input resistance. In another 48% of the cells tested, there was an increase in membrane input resistance. 4. Membrane depolarization caused by VIP enhanced the excitability of post-ganglionic neurones and converted subthreshold electrotonic and subthreshold synaptic potentials to action potentials. 5. The effects of VIP persisted during nicotinic and muscarinic synaptic blockade. The effects of VIP also persisted in a low-Ca2+, high-Mg2+ solution. Thus, the site of action of VIP was on the postsynaptic membrane. 6. Electrical stimulation of the lumbar colonic nerves evoked a slow noncholinergic depolarization of the membrane potential. 7. VIP appeared to be one of the transmitters involved in the electrically evoked e.p.s.p. because both prior desensitization with exogenous VIP and VIP antiserum reduced the amplitude of the slow, non-cholinergic e.p.s.p. 8. Radial distension of a segment of colon attached to the inferior mesenteric ganglion (i.m.g.) evoked a non-cholinergic depolarization of the membrane potential in neurones in the i.m.g. 9. The distension-induced non-cholinergic depolarization was reduced by VIP antiserum. 10. The data support the hypothesis that a population of the mechanosensory afferent nerves running between the colon and the i.m.g. utilize VIP or a VIP-like peptide as a transmitter to modulate reflex activity between the colon and the i.m.g.

Non-cholinergic transmission in a sympathetic ganglion of the guinea-pig elicited by colon distension

Sensory transmission from the colon was studied using a preparation of inferior mesenteric ganglion (i.m.g.) attached to a segment of distal colon in guinea-pigs, in vitro. Electrical responses to colon distension were recorded intracellularly from neurones of the i.m.g. Distension of the distal colon up to an intraluminal pressure of 20 cmH2O caused an increase in resting asynchronous synaptic activity and a concomitant slow depolarization. The asynchronous synaptic activity, but not the slow depolarization, was abolished by cholinergic antagonists. Distension-induced non-cholinergic depolarizations were elicited in 44% of i.m.g. neurons sampled. For distensions of 1 min at 10-20 cmH2O, depolarizations reached a mean amplitude of 3.4 +/- 0.3 mV and lasted 108 +/- 7 s. Continuous distension resulted in a tachyphylaxis of the depolarization. Tetrodotoxin (3 X 10(-7) M) superfused over the i.m.g. reversibly abolished the distension-induced non-cholinergic depolarization. Distension-induced non-cholinergic depolarizations were accompanied by an increase in input resistance of 21%. Neuronal excitability also increased, as sub-threshold potentials produced by intracellular current injection reached threshold for firing action potentials during colon distension. The amplitude of non-cholinergic depolarizations increased with colonic intraluminal pressure between 2 and 20 cmH2O, although the slope of the mean amplitude-pressure curve decreased progressively at higher pressures. The amplitude of distension-induced non-cholinergic depolarizations increased as membrane potential was manually hyperpolarized to approximately -80 mV, whereupon further hyperpolarization resulted in a decrease in response amplitude. Non-cholinergic slow excitatory post-synaptic potentials (e.p.s.p.s) evoked by repetitive presynaptic nerve stimulation were reversibly attenuated by 19 +/- 8% during depolarizations produced by distension. Systemic administration of capsaicin (50-350 mg/kg) reduced the number of i.m.g. neurones exhibiting the non-cholinergic mechanosensory response; direct superfusion of capsaicin over the i.m.g. attenuated the response in some neurones but had no effect in others. These results demonstrate the existence of a non-cholinergic mechanosensory pathway from the colon to the i.m.g., and suggest that non-cholinergic transmission in the ganglion participates in mediating gastrointestinal reflexes. One transmitter utilized by the non-cholinergic mechanosensory pathway may be substance P.

Fast and slow synaptic potentials produced in a mammalian sympathetic ganglion by colon distension

Radial distension of the large intestine produced a slow depolarization in a population of neurons in the inferior mesenteric ganglion of the guinea pig. The slow potentials often occurred simultaneously with cholinergic fast potentials [( excitatory postsynaptic potentials (EPSPs]) yet persisted in the presence of nicotinic and muscarinic cholinergic antagonists when all fast EPSPs were absent. The amplitude of the distension-induced noncholinergic slow depolarization increased with increasing distension pressure. For distensions of 1-min duration at pressures of 10-20 cm of water, the mean depolarization amplitude was 3.4 mV. The slow depolarization was associated with an increase in membrane resistance, and prolonged periods of colon distension resulted in a tachyphylaxis of the depolarization. Desensitization of ganglion cells to the peptide substance P attenuated the distension-induced slow potential by an average of 49% +/- 17%. Thus, two colonic mechanosensory afferent pathways converge on principal ganglion cells in the inferior mesenteric ganglion: one was previously described to be mediated by acetylcholine, and the other is described here, whose transmitter remains to be determined but which preliminary evidence suggests is mediated in part by substance P. The noncholinergic afferent pathway may enhance the intestinal inhibitory reflex mediated by cholinergic mechanosensory afferent input to the abdominal prevertebral sympathetic ganglia.

Vasopressin-mediated slow EPSPs in a mammalian sympathetic ganglion

Vasopressin is one of numerous neuropeptides contained in sympathetic ganglia, but whose function remains unresolved. In this report, we present electrophysiological evidence that arginine-vasopressin (AVP) is a neurotransmitter in guinea pig inferior mesenteric ganglion (IMG). AVP superfused over the IMG, in vitro, produced in a population of neurons a membrane depolarization accompanied by a resistance increase, both of which were blocked by a specific V1 receptor antagonist. Moreover, slow excitatory postsynaptic potentials (EPSPs) elicited by repetitive nerve stimulation were attenuated in 75% of cells tested in the presence of excess AVP, and occasionally in the presence of the antagonist. Thus, AVP joins substance P as a putative transmitter of slow potentials in the guinea pig IMG.