The role of serotonin in non-cholinergic excitatory transmission in the guinea pig inferior mesenteric ganglion

Post-stimulus potentiation of transmission in pelvic ganglia enhances sympathetic dilatation of guinea-pig uterine artery in vitro

Vasodilatation produced by stimulation of preganglionic neurones in lumbar and sacral pathways to pelvic ganglia was studied using an in vitro preparation of guinea-pig uterine artery and associated nerves in a partitioned bath allowing selective drug application to the ganglia or artery. Arterial diameter was monitored using real time video imaging. Vasodilatations produced by hypogastric nerve stimulation (HN; 300 pulses, 10 Hz) were significantly larger and longer in duration than with pelvic nerve stimulation (N = 18). Stimulation of ipsilateral lumbar splanchnic nerves or ipsilateral third lumbar ventral roots also produced prolonged vasodilatations. Blockade of ganglionic nicotinic receptors (0.1-1 mM hexamethonium) delayed the onset and sometimes reduced the peak amplitude of dilatations, but slow dilatations persisted in 16 of 18 preparations. These dilatations were not reduced further by 3 microM capsaicin applied to the artery and ganglia, or ganglionic application of 1 microM hyoscine, 30-100 microM suramin or 10 microM CNQX. Dilatations were reduced slightly by ganglionic application of NK1 and NK3 receptor antagonists (SR140333, SR142801; 1 microM), but were reduced significantly by bathing the ganglia in 0.5 mM Ca2+ and 10 mM Mg2+. Intracellular recordings of paracervical ganglion neurones revealed fast excitatory postsynaptic potentials (EPSPs) in all neurones on HN stimulation (300 pulses, 10 Hz), and slow EPSPs (3-12 mV amplitude) in 25 of 37 neurones. Post-stimulus action potential discharge associated with slow EPSPs occurred in 16 of 37 neurones (firing rate 9.4 +/- 1.5 Hz). Hexamethonium (0.1-1 mM) abolished fast EPSPs. Hexamethonium and hyoscine (1 microM) did not reduce slow EPSPs and associated post-stimulus firing in identified vasodilator neurones (with VIP immunoreactivity) or non-vasodilator paracervical neurones. These results demonstrate a predominantly sympathetic origin of autonomic pathways producing pelvic vasodilatation in females. Non-cholinergic mediators of slow transmission in pelvic ganglia produce prolonged firing of postganglionic neurones and long-lasting dilatations of the uterine artery. This mechanism would facilitate maintenance of pelvic vasodilatation on stimulation of preganglionic neurones during sexual activity.

Serotonin receptor subtypes that depolarize guinea pig inferior mesenteric ganglion neurons

Our previous studies indicated that serotonin (5-HT) depolarized a majority of guinea pig inferior mesenteric ganglion (IMG) neurons and may be another transmitter for the noncholinergic late slow excitatory postsynaptic potential (ls-EPSP) in the IMG. However, the subtypes of 5-HT receptor mediating these responses have not yet been identified. Using intracellular recording, we examined the effect of 5-HT receptor antagonists with specificity to various 5-HT receptor subtypes on the 5-HT-mediated depolarization and ls-EPSP in IMG neurons in vitro. Cyproheptadine, a 5-HT(1/2) receptor antagonist, reversibly inhibited the slow, but not the fast, depolarization and ls-EPSP in the 5-HT-sensitive neurons. Both mianserin and spiperone, 5-HT(2) and 5-HT(1A) receptor antagonists, did not significantly alter either the fast or slow depolarizing responses or the ls-EPSP. The 5-HT(3) receptor antagonist MDL 72222 (Bemesetron) completely inhibited the fast depolarization with little diminution of the slow depolarization and ls-EPSP. Superfusion of putative 5-HT(1P) receptor antagonist, BRL 24924 (Renzapride), reversibly attenuated both the depolarization and ls-EPSP. However, 5-HT-insensitive neurons with ls-EPSP were found to be insensitive to both cyproheptadine and BRL 24924. In most 5-HT-sensitive neurons, the 5-HT(3) receptor agonist, 2-methyl-5-HT, and the selective 5-HT(1P) agonist, MCPP or 5-OHIP, evoked a fast and a slow depolarization in 55.6 and 71.4% of the neurons, respectively, without a significant effect on the membrane potential in 85.7 and 100% of the 5-HT-insensitive neurons. In 5-HT-sensitive neurons, MDL 72222 reversibly abolished the fast depolarization induced by 2-methyl-5-HT; BRL 24924 significantly inhibited the slow depolarization induced by MCPP or 5-OHIP, but not by SP. Prolonged superfusion of 5-HT-sensitive neurons with MCPP abolished the evoked ls-EPSP without inhibition of action potential. These results suggest that the fast and slow depolarizations in these neurons are mediated by 5-HT(3) and 5-HT(1P) receptor subtypes, respectively. The latter may also mediate the ls-EPSP in 5-HT-sensitive neurons.

Induction and maintenance of ganglionic long-term potentiation require activation of 5-hydroxytryptamine (5-HT3) receptors

1. An extracellular recording technique was used to study the effects of 5-hydroxytryptamine (5-HT, serotonin) on the tetanus-induced long-term potentiation (LTP) of the nicotinic pathway of transmission in the superior cervical ganglion (SCG) of the rat. The postganglionic compound action potential (CAP), made submaximal by treatment with hexamethonium (O.4 mM), was used as an index of transmission in the ganglion. 2. Serotonin (10 microM) markedly enhanced the magnitude of LTP without affecting the post-tetanic potentiation (PTP). The serotonin (2-30 microM) concentration-response curve for LTP was bell shaped as no enhancement was seen with 30 microM serotonin. This may largely be due to activation of a 5-HT1 receptor subtype and not to desensitization. 3. When superfused before tetanus, the 5-HT1A receptor agonist 8-hydroxydipropylamino-tetralin (8-OH-DPAT, 5 microM) prevented the expression of LTP without affecting PTP. 4. Pretreatment of ganglia with the 5-HT2 receptor agonist R-(+)-dimethoxy-4-iodoamphetamine (R-(+)-DOI, 1 microM) enhanced the tetanus-induced LTP. Similar treatment with the 5-HT2 receptor antagonist ketanserin (3 microM) had no significant effect on LTP. 5. Pretreatment of ganglia with the 5-HT3 receptor agonist 1-m-(chlorophenyl) biguanide (m-CPGB, 1 microM), markedly increased (300%) the tetanus-induced LTP. Similar pretreatment with the 5-HT3 receptor antagonist 3-tropanyl-3,5-dichlorobenzoate (MDL 72222, 0.5 microM) completely prevented the expression of LTP. Fully expressed LTP was reversibly blocked by MDL 72222 when applied during the maintenance phase of LTP. 6. Tetanic stimulation of monoamine-depleted ganglia (from reserpine-pretreated rats, 3 mg kg-1 for 24 h) failed to induced LTP. 7. In monoamine-depleted ganglia, tetanus preceded by superfusion with m-CPBG readily induced LTP. MDL 72222 completely blocked this LTP. However in these ganglia tetanus failed to induced LTP when m-CPBG was given 2 min (during PTP) or 1 h after tetanus. 8. Tetanic stimulation of monoamine-depleted ganglia in the presence of R-(+)-DOI failed to induced LTP. 9. We conclude that tetanus-induced LTP of the SCG of the rat requires activation of 5-HT3 receptors both for induction and maintenance.

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.

Involvement of NK1 receptors in synaptic transmission in the guinea pig coeliac ganglion

Using intracellular recording techniques, we examined the effects of tachykinin receptor agonists and antagonists on electrophysiologically identified tonic neurons of the isolated guinea pig coeliac ganglion. In most of the tonic neurons, substance P (SP), neurokinin A (NKA) and/or senktide induced a depolarization. The effects of SP and NKA were blocked by the NK1-selective antagonist, GR71251 (5 microM), but not by the NK2-selective antagonist, L659,877 (10 microM), whereas the effect of senktide was not affected by these antagonists. The NK1-selective agonists, [Sar9,Met(O)2(11)]SP and SP methyl ester, and the NK3-selective agonist, [MePhe7]neurokinin B, also evoked depolarizations in tonic neurons. By contrast, the NK2-selective agonists, [Nle10]NKA4-10, [beta-Ala8]NKA4-10 and GR64349, at 1 microM each, did not evoke any significant depolarizing response. Repetitive electrical stimulation of the mesenteric nerves induced slow excitatory postsynaptic potentials (EPSPs) in the majority of tonic neurons, which were depressed by GR71251 (5 microM). These results suggest that NK1 and NK3 receptors but not NK2 receptors are involved in the tachykinin-induced depolarization of tonic neurons, and that the NKA-induced response is due to the activation of NK1 receptors. This study also suggests the involvement of NK1 receptors in the slow EPSPs in tonic neurons.

Response to 5-hydroxytryptamine on neurons of guinea pig celiac and inferior mesenteric ganglia in primary culture

The electrophysiological effects of serotonin, a putative neurotransmitter in prevertebral sympathetic ganglia, were evaluated in cultured celiac and inferior mesenteric ganglia (IMG) neurons. Intracellular microelectrode recordings were performed in neurons that were maintained in culture an average of 26 days. Seventy-eight of 85 neurons responded when serotonin (10 microM) was applied by pressure ejection from a micropipette to the surface of the isolated cells. The majority of the neurons (n = 48) generated fast depolarizations, although slow depolarizations (n = 17), bipolar responses (n = 5), hyperpolarizations (n = 7), and a biphasic response (n = 1), were also seen. Hyperpolarizing responses were evoked in celiac neurons only. All responses were inhibited by the 5-HT3 antagonist MDL 72,222 (5 microM). Fast responses were not inhibited by tetrodotoxin (n = 3). These results demonstrate that serotonin evokes a variety of membrane potential changes in cultured prevertebral sympathetic neurons by activating 5-HT3 receptors.