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

Viscerofugal neurons recorded from guinea-pig colonic nerves after organ culture

BACKGROUND

Enteric viscerofugal neurons provide cholinergic synaptic inputs to prevertebral sympathetic neurons, forming reflex circuits that control motility and secretion. Extracellular recordings of identified viscerofugal neurons have not been reported.

METHODS

Preparations of guinea pig distal colon were maintained in organotypic culture for 4-6 days (n = 12), before biotinamide tracing, immunohistochemistry, or extracellular electrophysiological recordings from colonic nerves.

KEY RESULTS

After 4-6 days in organ culture, calcitonin gene-related peptide and tyrosine hydroxylase immunoreactivity in enteric ganglia was depleted, and capsaicin-induced firing (0.4 μmol L(-1) ) was not detected, indicating that extrinsic sympathetic and sensory axons degenerate in organ culture. Neuroanatomical tracing of colonic nerves revealed that viscerofugal neurons persist and increase as a proportion of surviving axons. Extracellular recordings of colonic nerves revealed ongoing action potentials. Interestingly, synchronous bursts of action potentials were seen in 10 of 12 preparations; bursts were abolished by hexamethonium, which also reduced firing rate (400 μmol L(-1) , P < 0.01, n = 7). DMPP (1,1-dimethyl-4-phenylpiperazinium; 10(-4) mol L(-1) ) evoked prolonged action potential discharge. Increased firing preceded both spontaneous and stretch-evoked contractions (χ(2) = 11.8, df = 1, P < 0.001). Firing was also modestly increased during distensions that did not evoke reflex contractions. All single units (11/11) responded to von Frey hairs (100-300 mg) in hexamethonium or Ca(2+) -free solution.

CONCLUSIONS & INFERENCES

Action potentials recorded from colonic nerves in organ cultured preparations originated from viscerofugal neurons. They receive nicotinic input, which coordinates ongoing burst firing. Large bursts preceded spontaneous and reflex-evoked contractions, suggesting their synaptic inputs may arise from enteric circuitry that also drives motility. Viscerofugal neurons were directly mechanosensitive to focal compression by von Frey hairs.

Effects of extrinsic autonomic inputs on expression of c-Fos immunoreactivity in myenteric neurons of the guinea pig distal colon

c-Fos protein is a nuclear protein coded by c-fos proto-oncogene subsequent to synaptic activation of the neurons. We used immunohistochemical methods to visualize the expression of c-Fos protein in myenteric neurons of the guinea pig distal colon and examined the effects of the extrinsic autonomic inputs on the enteric circuits. No c-Fos immunoreactivity was observed in the colonic segments fixed immediately after removal from the animal body. A number of c-Fos-immunoreactive nuclei of myenteric neurons, however, appeared in all preparations that were incubated in Krebs solution in vitro (n=10). Application of tetrodotoxin (0.2 microM) abolished the expression of c-Fos-immunoreactivity (n=6), but hexamethonium (100 microM) failed to decrease the number of c-Fos-positive neurons despite a complete suppression of spontaneous peristaltic movements (n=5). Neither the electrical stimulation (n=8) nor the severing of the pelvic nerves (n=5) changed the number of c-Fos-positive neurons. Application of clonidine, an alpha(2)-agonist, (0.1 microM) abolished the expression of c-Fos protein in all preparations (n=5), while denervation of the sympathetic fibers in the lumbar colonic and hypogastric nerves in vivo increased the number of c-Fos-positive neurons (n=5). The results indicate that the enteric circuit in the distal part of the gastrointestinal tract is under tonic inhibition by the sympathetic nervous system from the lumbar spinal cord. c-Fos immunoreactivity expressed in the colonic preparations in vivo might be the results of enhanced activation of non-nicotinic receptors after removal of the sympathetic inhibition.

Chronic decentralization of the heart differentially remodels canine intrinsic cardiac neuron muscarinic receptors

The objective of the study was to determine if chronic interruption of all extrinsic nerve inputs to the heart alters cholinergic-mediated responses within the intrinsic cardiac nervous system (ICN). Extracardiac nerve inputs to the ICN were surgically interrupted (ICN decentralized). Three weeks later, the intrinsic cardiac right atrial ganglionated plexus (RAGP) was removed and intrinsic cardiac neuronal responses were evaluated electrophysiologically. Cholinergic receptor abundance was evaluated using autoradiography. In sham controls and chronic decentralized ICN ganglia, neuronal postsynaptic responses were mediated by acetylcholine, acting at nicotinic and muscarinic receptors. Muscarine- but not nicotine-mediated synaptic responses that were enhanced after chronic ICN decentralization. After chronic decentralization, muscarine facilitation of orthodromic neuronal activation increased. Receptor autoradiography demonstrated that nicotinic and muscarinic receptor density associated with the RAGP was unaffected by decentralization and that muscarinic receptors were tenfold more abundant than nicotinic receptors in the right atrial ganglia in each group. After chronic decentralization of the ICN, intrinsic cardiac neurons remain viable and responsive to cholinergic synaptic inputs. Enhanced muscarinic responsiveness of intrinsic cardiac neurons occurs without changes in receptor abundance.

Choline acetyltransferase-immunoreactive neurones in a prevertebral sympathetic ganglion, the inferior mesenteric ganglion

Using immunohistochemical techniques a small population of choline acetyltransferase (ChAT) immunoreactive (IR) neurones has been identified in the inferior mesenteric ganglion (IMG) of guinea pig (4.6% of all neurones), ferret (6.4%) and rat (0.4%). A detailed study in the guinea-pig IMG revealed that the vast majority of cholinergic neurones did not express tyrosine hydroxylase (TH)-IR, indicating that they were non-catecholaminergic. The cholinergic neurones were significantly larger than the TH-positive neurones. The majority of the ChAT-IR cells (64%) was observed in small clusters which were consistently located in the caudal lobe of the IMG close to the entry of the hypogastric nerves. 83% of the ChAT-IR cells also contained neuropeptide Y (NPY). Since the vast majority of TH-negative cells were ChAT-positive (94%), the TH negativity was taken as an indirect indication for ChAT-IR. NPY-IR, somatostatin (SOM)-IR and vasoactive intestinal peptide (VIP)-IR were found in both the TH-IR cells (22, 84 and 1%, respectively) and the putative cholinergic population (95, 84 and 70, respectively). Thus the majority of cholinergic neurones in the IMG were likely to contain NPY, SOM and VIP. TH-IR cells exhibited an extensive innervation of fibers immunoreactive for ChAT, VIP, ENK and NOS. In contrast, only a sparse plexus of ChAT-, ENK-, NOS-, NPY- and SOM-positive fibres was found around the TH-negative cells. VIP-IR fibres did not appear to innervate ChAT neurones.

Electrophysiological analysis of the convergence of peripheral inputs onto neurons of the coeliac ganglion in the guinea pig

The convergence of intestinofugal axons from different intestinal regions onto individual neurons in the coeliac ganglion of the guinea pig was investigated using intracellular recording methods in vitro. Peripheral nerve trunks from the distal ileum, the most proximal colon and the colon near the colonic flexure were electrically stimulated along with preganglionic fibres running in the splanchnic nerve. Fast cholinergic excitatory synaptic potentials (EPSPs) were seen in ganglion cells in response to stimulation of each nerve trunk. Roughly half of 78 neurons impaled received inputs from stimulation of peripheral nerves, and almost all of these received input from the proximal colon. Most cells responded to stimulation of more than one peripheral nerve indicating that coeliac neurons receive converging inputs from intestinofugal neurons located in more than one intestinal region. In a second series of experiments, segments of intestine were left attached to the ganglion and distended with saline to stimulate peripheral mechanosensory input to the coeliac ganglion. In each experiment, two segments were stimulated. A subgroup of ganglion cells exhibited spontaneous fast EPSPs and the frequency of these potentials was increased by distension of one or other of the attached intestinal segments. However, few neurons responded to distension of both of the attached intestinal segments suggesting that some of the intestinofugal inputs to the coeliac ganglion identified by electrical stimulation may be sensitive to sensory modalities other than distension. Hexamethonium (0.5 mM) applied to the intestine, and not to the coeliac ganglion, reduced the frequency of the spontaneous synaptic potentials seen in coeliac ganglion cells, but did not abolish the response to distension of the colon (n = 8). When the Ca2+ concentration of the solution bathing the proximal colon was reduced to block all synaptic transmission in the enteric plexuses the background synaptic input was further depressed, but again the response to distension was little changed (n = 4). This suggests that at least some of the neurons projecting from the colon to the coeliac ganglion are first order mechanosensory neurons.

Activity following colonic distension in enteric sensory fibres projecting to the inferior mesenteric ganglion in the guinea pig

In this investigation the characteristics of the response to colonic distension of afferent fibres projecting to the inferior mesenteric ganglion (IMG) of the guinea pig were studied in vitro. Intracellular membrane potential recordings were made from neurons in the IMG. Post-synaptic potentials arising from activity in afferent fibres were recorded in these cells following distensions of a small segment of colon placed in a separate and independently perfused organ bath. These afferent fibres showed both transient and sustained responses to distension. Application of a solution containing 0.25 mM Ca2+ and 10 mM2+ Mg2+ to the colon (but not to the IMG) reduced the overall response but did not abolish the activity in these fibres. It is concluded that some afferent fibres which respond to colonic distension project directly to the IMG.

The origin and distribution of neurons with projections passing through the inferior mesenteric ganglion of the guinea-pig

Retrograde tracing with the fluorescent dye, Fast Blue, was used to examine the origin and distribution of neurons whose axons project through the inferior mesenteric ganglion (IMG) of the guinea-pig. These studies were performed by applying the tracer to (a) the rostral cut-end of the hypogastric nerves and (b) the caudal cut-end of the inter-mesenteric nerve (IMN). After application of tracer to the hypogastric nerves retrogradely labelled cell profiles were observed in the IMG and the superior mesenteric ganglion (SMG). The number of labelled cell profiles in the SMG was consistently about 15% of the number in the IMG. In only one of seven animals tested were labelled cells seen in the wall of the colon. Application of tracer to the IMN labelled cells in the IMG and in the wall of the colon. The distribution of the labelled enteric neurons was skewed towards the anal end of the colon. These results confirm that postganglionic sympathetic neurons in the SMG project axons through the guinea-pig IMG and describe the colonic distribution of enteric neurons that project through the IMG and into the IMN.

Inhibition of peripheral fast synaptic inputs to celiac ganglion neurons by splanchnic preganglionic fibers in the cat

Fast nicotinic transmission was studied in vitro in isolated cat celiac ganglion neurons using intracellular recording techniques. Fast synaptic activation was evoked by stimulation (0.1 Hz) of the anterior peripheral rami. A long-lasting inhibition of this response was triggered by repetitive splanchnic stimulation (30-50 Hz) for 10 s. Evidence is given that this inhibition occurs at presynaptic level. Our results indicate that central inputs modulate transmission of fast synaptic peripheral inputs to prevertebral ganglionic neurons. This would be another integrative mechanism in the prevertebral ganglia.

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.