Electrophysiological evidence for presynaptic inhibition of acetylcholine release by 5-hydroxytryptamine in the enteric nervous system

Evidence that 5-hydroxytryptamine does not mediate GABA-induced contractile responses in the guinea-pig proximal ileum

Segments of the isolated proximal ileum of the guinea-pig were set up in the organ bath to record longitudinal muscle contractions. Both gamma-aminobutyric acid (GABA) and 5-hydroxytryptamine induced neurally mediated cholinergic contractile responses; desensitization to 5-HT did not consistently depress contractile responses to GABA, whilst quipazine antagonized the neuronal responses induced by 5-HT without affecting responses to GABA. It is thus concluded that 5-HT does not mediate GABA-induced responses in the ileum.

Excitatory synaptic potentials due to activation of neurons with short projections in the myenteric plexus

Intracellular microelectrodes have been used to examine the effects, on excitatory inputs to myenteric nerve cells, of lesions of intrinsic pathways in the myenteric plexus of the guinea-pig small intestine. The lesions consisted of circumferential cuts (myotomies) which severed the external musculature to the depth of the submucosa and thus interrupted pathways in the myenteric plexus. Sufficient time was allowed between creating the lesions and recording from the neurons for the endings of severed neurites to degenerate and this was confirmed histochemically by examining the distribution of varicose fibres with 5-hydroxytryptamine immunoreactivity in myenteric ganglia from which recordings were made. Two types of excitatory input, eliciting fast and slow excitatory post-synaptic potentials, respectively, were demonstrable in response to focal stimulation of nerves in the ganglia from which recordings were made. There were no differences in the proportions of neurons in which fast or slow excitatory synaptic potentials were evoked in unoperated preparations (controls), in islands 1.5-4 mm wide between myotomies, or within 1 mm on the oral or anal sides of myotomies. Possible differences in the amplitudes, durations at half amplitude, and threshold numbers of stimuli for initiation of slow excitatory synaptic potentials were analyzed. The only significant differences were found when data from control and oral areas were pooled and compared with combined data from island and anal areas (this assessed differences that could arise from severing nerve fibres running from oral to anal).(ABSTRACT TRUNCATED AT 250 WORDS)

5-HT antagonists and blockade of neuronal (5-HT) receptors on ganglion cells

Potential changes in superior cervical ganglion cells evoked by 5-HT or the nicotinic agonist, dimethyl-phenyl piperazinium (DMPP), were recorded using the sucrose-gap method and a number of putative 5-HT antagonists tested for potency and selectivity. Selective blockade of 5-HT responses was produced by 5-HT itself and, in increasing order of potency, by cocaine, metoclopramide and quipazine. A non-selective blockade was observed with bufotenine and d-tubocurarine. Substances which had no effect on 5-HT responses included methysergide and other compounds related to LSD, cinanserin, cyproheptadine, phenylbiguanide and morphine. The results provide further information about the 5-HT receptor on sympathetic ganglion cells and support the view that this receptor is distinct from neuronal receptors in the myenteric plexus and on cholinergic nerve terminals.

5-HT mediated GABA excitatory responses in the guinea-pig proximal ileum

GABA (3--100 microM) and 5-HT (0.03--30 microM) caused concentration-dependent transient contractions of the longitudinal muscle of the guinea-pig ileum. The contractile response to GABA was antagonized by hyoscine (2.2 microM). TTX (0.7 microM), bicuculline (3 microM), furosemide (25 microM) and desensitization to GABA itself, while hexamethonium (20 microM) and methysergide (20 microM) were without effect. The contractile response to 5-HT was antagonized by hyoscine (2.2 microM), TTX (0.7 microM) and desensitization to 5-HT itself and was unaffected by bicuculline (10 microM), hexamethonium (20 microM), furosemide (25 microM) and methysergide (20 microM). A desensitization procedure that caused a 84.7-fold increase in the 5-HT EC50 also resulted in a 74.1-fold increase of the GABA EC50. Desensitization to GABA caused a reduction of 5-HT induced response but only in preparations desensitized by high (50 microM) concentrations of GABA. The results indicate that GABA-induced contractions in the guinea-pig ileum are mediated by activation of cholinergic motor neurones. This effect appears to be mediated by interneuronal release of 5-HT rather than by a direct stimulatory action of GABA on the effector neurones.

Neural control of the sphincter of Oddi. A physiological role of 5-hydroxytryptamine in the regulation of basal sphincter of Oddi motor activity in the cat

The effect of 5-hydroxytryptamine (5-HT) on the sphincter of Oddi (SO) was studied in the cat. The SO had two motor responses to 5-HT: the most common was an initial contraction followed by a more prolonged relaxation, and the other was an exclusive relaxation. Tetrodotoxin did not impair the magnitude of the net contraction induced by 5-HT, but it completely blocked the relaxation. Methysergide partially inhibited the SO contraction in response to submaximal doses of 5-HT (5-20 micrograms/kg). Atropine decreased the SO excitatory response to all doses of 5-HT. The combination of atropine and methysergide completely antagonized the 5-HT excitatory effect, which changed the SO biphasic response to an exclusive relaxation. After tetrodotoxin, the effect of 5-HT was almost completely antagonized by methysergide alone. The SO contraction and relaxation caused by 5-HT were almost completely blocked by 5-HT tachyphylaxis. In contrast, a 5-HT depletion with reserpine enhanced the sensitivity of the SO to 5-HT, responding to doses a thousand times smaller than in control animals. Hexamethonium, phentolamine, propranolol, and 5-methoxy-N,N-dimethyltryptamine did not antagonize the 5-HT-induced contraction or relaxation. These findings indicate that 5-HT caused SO contraction by stimulating postganglionic cholinergic neurons and the smooth muscle directly and caused relaxation by stimulating postganglionic, noncholinergic, nonadrenergic inhibitory neurons. 5-HT blockade or depletion resulted in a significant reduction in basal tonic pressures and in the amplitude of phasic contractions, which suggested that serotonergic neurons may play a physiologic role in the regulation of basal SO motor activity.

Effects of serotonin (5-hydroxytryptamine) on amphibian neuromuscular junction

A study of the effects of serotonin transmission was carried out on the frog neuromuscular junction by means of microelectrode methods. Serotonin was employed in concentrations of 5-100 microM. Serotonin did not affect membrane characteristics or the resting potential whether at non-neuronal (muscular fiber) or endplate segments of the junction. While serotonin did not affect the frequency of the miniature endplate potentials (MEPPs), it significantly decreased evoked release of acetylcholine. Serotonin significantly decreased, in a dose-dependent fashion, the amplitude of acetylcholine potentials, endplate currents (EPCs), endplate potentials (EPPs) and MEPPs. Also, serotonin shortened significantly the EPC time course and half-decay time, and caused loss of membrane voltage sensitivity of the half-decay time. While it did not affect the null potential, serotonin changed the voltage-EPC relationship from linear to non-linear, and markedly attenuated the dependence of EPC amplitude on membrane potential. These results demonstrate that serotonin induces depressant effects at both pre- and post-synaptic sites of amphibian neuromuscular junction and that its post-synaptic action is directed at the receptor-channel macromolecule rather than at either the channel or the receptor alone.

Neurons with 5-hydroxytryptamine-like immunoreactivity in the enteric nervous system: their projections in the guinea-pig small intestine

Changes in the distribution of 5-hydroxytryptamine-like immunoreactivity have been examined in enteric neurons at various times after microsurgical lesions of the enteric plexuses. In the myenteric plexus, varicose immunoreactive nerve fibres disappeared or were reduced in number in ganglia anal to an interruption of the myenteric plexus. Up to about 2 mm on the anal side, all varicose immunoreactive fibers disappeared from the ganglia. At about 14-16 mm below an interruption, there were about 50% of the normal number of fibres in the myenteric ganglia and at about 24 mm the innervation was normal. In the submucosa, fibres immunoreactive for 5-hydroxytryptamine were absent from an area on the anal side following interruption of the myenteric plexus. From consideration of the pattern of disappearance, it is deduced that some myenteric nerve cell bodies send immunoreactive axons in an anal direction to supply submucous ganglia. The axons run for about 8 mm in the myenteric plexus, enter the submucosa and then run for a further 4 mm approximately. Thus, varicose fibres immunoreactive for 5-hydroxytryptamine, which occur around the enteric ganglion cells of both plexuses arise from nerve cell bodies in myenteric ganglia than send their axons in an anal direction.

Clonidine activates membrane potassium conductance in myenteric neurones

1 Intracellular recordings were made from neurones in the myenteric plexus of the ileum removed from guinea-pigs. The effects of clonidine and adrenaline on membrane potential and resistance were observed.2 Clonidine (100 pM-30 nM) caused a concentration-dependent membrane hyperpolarization associated with a fall in neurone input resistance.3 The amplitude of the clonidine hyperpolarization, but not the conductance increase, was greater in cells with lower resting potentials and smaller in more polarized neurones. In a given cell, membrane hyperpolarization decreased and membrane depolarization increased the clonidine effect.4 Low potassium solutions enhanced and high potassium solutions reduced the hyperpolarizing action of clonidine but did not significantly change the conductance increase caused by clonidine.5 The concentration-effect curve for clonidine was displaced to the left when the extracellular calcium concentration was reduced. Conversely, clonidine was almost ineffective in elevated calcium concentrations. This was true for both the hyperpolarization and the conductance increase.6 It is suggested that clonidine activates a potassium conductance by causing an elevation in the free intracellular calcium concentration.7 Clonidine reversibly depressed the amplitude of the nicotinic fast excitatory postsynaptic potential and the noncholinergic slow excitatory postsynaptic potential.8 All the effects of clonidine were shared by adrenaline and the actions of both were reversed or prevented by phentolamine (100 nM-1 muM).

Evidence of the modulatory role of serotonin in acetylcholine release from striatal interneurons

The release of acetylcholine was studied in isolated striatal slices of the rat. The spontaneous and ouabain-stimulated release of acetylcholine was higher in those slices where serotonergic input was somehow impaired: raphe nuclei lesion or p-chlorophenylalanine pretreatment or 5, 7-dihydroxytryptamine pretreatment resulted in a higher release. L-(m-chlorophenyl)-piperazine, a pure serotonin receptor stimulant and D-fenfluramine, a serotonin releaser significantly reduced the release of acetylcholine evoked by ouabain. Serotonin antagonists (cyproheptadine, mianserine and methysergide) prevented the effect of serotonin agonists. When the serotonergic neurons were destroyed either by p-chlorophenylalanine or by 5, 7-dihydroxytryptamine pretreatment D-fenfluramine had no inhibitory action; however, the effect of L-(m-chlorophenyl)-piperazine was not affected. It is suggested that there is a link between serotonergic and cholinergic neurons in the striatum: serotonin released from raphe-striatal neurons is able to inhibit the release of acetylcholine from striatal interneurons.

A synaptosomal preparation from the guinea pig ileum myenteric plexus

Our interest in investigating the presynaptic modulation of acetylcholine release led to the development of a synaptosomal preparation from the guinea pig ileum myenteric plexus-longitudinal muscle. A crude synaptosomal fraction (P2) was obtained by homogenization and differential centrifugation. The preparation exhibited a specific uptake system for choline and for noradrenaline (NA), but not for 5-hydroxytryptamine (5-HT). Synaptosomes were isolated from this P2 fraction by an isoosmotic density gradient prepared from sucrose and metrizamide. The resultant synaptosomal fraction was enriched about sevenfold in both choline uptake and in choline acetyltransferase (ChAT). Choline was transported by a high-affinity system was a Km of 6.5 X 10(-7) M and a Vmax of 41 pmol/mg protein/min. Electron microscopy confirmed the synaptosomal nature of the gradient fraction. Some synaptosomal profiles contained only small, translucent vesicles whereas others also contained large (approx. 100 nm diameter) electron-opaque vesicles. The crude synaptosomal fraction synthesized acetylcholine (ACh) from exogenous choline and it released the synthesized ACh in a calcium-dependent manner.

Mediators of slow synaptic potentials in the myenteric plexus of the guinea-pig ileum

1. Intracellular recordings were made from neurones in the myenteric plexus of the ileum isolated from adult guinea-pigs.2. Three synaptic potentials were evoked in different myenteric neurones by focal stimulation of the ganglion surface at a distance of up to 100 mum from the cell body from which the recording was made. These were the fast cholinergic excitatory post-synaptic potential (e.p.s.p.), the slow e.p.s.p. and the slow inhibitory post-synaptic potential (i.p.s.p.).3. 5-hydroxytryptamine and substance P were applied to the neurones by superfusion (10 nm-1 mum) or by electrophoresis within 5 mum of the neurone cell body. 5-HT depolarized, hyperpolarized or had no effect on approximately equal numbers of neurones, whereas substance P depolarized 90% of neurones.4. Many neurones with a depolarizing slow e.p.s.p. were hyperpolarized by superfusion or electrophoretic application of 5-HT.5. Superfusion with 5-HT reversibly depressed the fast e.p.s.p., slow e.p.s.p. and slow i.p.s.p. Superfusion with substance P depressed the slow e.p.s.p.6. Methysergide (10-30 mum) reduced the amplitude of the fast e.p.s.p., the slow e.p.s.p. and the slow i.p.s.p.7. Chymotrypsin (200 mug/ml.) reversibly reduced the amplitude of the slow e.p.s.p., but had no effect on membrane potential, the action potential or the fast e.p.s.p. Chymotrypsin reduced or abolished the depolarization caused by electrophoretic application of substance P, but had no effect on the depolarization or hyperpolarization caused by 5HT.8. The results provide evidence that 5-HT is not the transmitter which mediates the slow e.p.s.p. in myenteric neurones. The slow e.p.s.p. may be caused by substance P or another similar peptide.