Synaptic transmission from the submucosal plexus to the myenteric plexus in Guinea-pig ileum.

Stimulation of the myenteric plexus results in activation of submucosal neurons and dilation of arterioles, one way that motility and secretion can be coupled together. The present study aimed to examine the converse, whether myenteric neurons receive synaptic input from the submucosal plexus (SMP). Intracellular recordings were made from guinea-pig ileal myenteric neurons while the SMP was electrically stimulated. Of the 29 neurons studied (13 S and 16 AH neurons), stimulation of the SMP evoked a synaptic potential in only seven cells, or 24% of neurons. When the SMP was situated oral to the myenteric plexus, 4 of 13 (31%) myenteric neurons had synaptic input. When it was situated circumferential, 2 of 8 (25%) had input, and when the SMP was situated anal 1 of 8 (13%) had input. Overall, 5 of the 13 (38%) S neurons responded with fast excitatory post-synaptic potentials (EPSPs), one of which also showed a slow EPSP, while 2 of the 16 (13%) AH neurons responded with a slow EPSP. This study indicates that the synaptic input from the SMP to myenteric neurons is relatively sparse. Whether this input is less important than the myenteric to submucosal input or simply represents a more selective form of control is unknown.

Slow excitatory post-synaptic potentials in myenteric AH neurons of the guinea-pig ileum are reduced by the 5-hydroxytryptamine7 receptor antagonist SB 269970.

Serotonin (5-HT) is a key modulator of neuronal excitability in the central and peripheral nervous system. In the enteric nervous system, 5-HT causes a slow depolarization in the intrinsic sensory neurons, but the receptor responsible for this has not been correlated with known gene products. The aim of this study was to determine whether the newly characterized 5-HT7 receptor may participate in the 5-HT-mediated depolarization of, and synaptic transmission to, the intrinsic sensory neurons of the guinea-pig ileum. Intracellular electrophysiological recordings were made from intrinsic sensory neurons identified as myenteric AH neurons from guinea-pig ileum. 5-HT (5 microM) applied to the cell body evoked both a fast depolarization (5-HT3 mediated) and/or a slow depolarization (5-HT1P-like). The 5-HT1/5/7 receptor agonist 5-carboxamidotryptamine (5-CT) (5 microM) evoked only a slow depolarization. When the fast depolarization evoked by 5-HT was blocked with granisetron (1 microM, 5-HT3 receptor antagonist), only a slow depolarization remained; this was abolished by the 5-HT7 receptor antagonist SB 269970 (1 microM, control: 14+/-2 mV, granisetron+SB 269970: -1+/-2 mV). The slow depolarization evoked by 5-CT was also significantly reduced by SB 269970 (control: 14+/-1 mV, SB 269970: 5+/-2 mV) suggesting a 5-HT7 receptor was activated by exogenous application of 5-CT and 5-HT. Slow excitatory postsynaptic potentials evoked by stimulating descending neural pathways (containing serotonergic fibers) were reduced by SB 269970 (control: 8+/-3 mV, SB 269970: 3+/-1 mV). However, SB 269970 had no effect on slow excitatory postsynaptic potentials evoked by stimulation of circumferential (tachykinergic) pathways (control: 7+/-1 mV, SB 269970: 6+/-1 mV). These data are consistent with the presence on enteric AH neurons of functional 5-HT7 receptors that participate in slow synaptic transmission.

ATP participates in three excitatory postsynaptic potentials in the submucous plexus of the guinea pig ileum.

Synaptic transmission between neurones intrinsic to the wall of the intestine involves multiple neurotransmitters. This study aimed to identify neurotransmitters responsible for non-cholinergic excitatory synaptic transmission in the submucous plexus of the guinea pig ileum. Intracellular recordings were made from secretomotor and vasodilator neurones. A single electrical stimulus to a fibre tract evoked excitatory postsynaptic potentials (EPSPs) with three different time courses - fast, slow and an EPSP with an intermediate time course (latency 96 ms, duration 1.2 s). In all neurones, blocking nicotinic receptors reduced fast EPSPs, but they were abolished in only 57 of 78 neurones. Fast EPSPs were also reduced by P2 purinoceptor blockade (5 of 27 neurones) or 5-HT(3) receptor blockade (3 of 20 neurones). The intermediate EPSP was abolished by P2 receptor blockade (13 of 13 neurones) or by the specific P2Y(1) receptor antagonist MRS 2179 (5 of 5 neurones) and was always preceded by a nicotinic or mixed nicotinic/purinergic fast EPSP. Intermediate EPSPs were observed in over half of all neurones including most non-cholinergic secretomotor neurones identified by immunoreactivity for vasoactive intestinal peptide. The slow EPSP evoked by a single pulse stimulus was also abolished by P2 receptor blockade (5 of 5 neurones) or by MRS 2179 (3 of 3 neurones). We conclude that fast EPSPs in submucous neurones are mediated by acetylcholine acting at nicotinic receptors, ATP acting at P2X receptors and 5-HT acting at 5-HT(3) receptors. Both the intermediate EPSP and the single stimulus slow EPSP are mediated by ATP acting at P2Y(1) receptors.

ATP and 5-HT are the principal neurotransmitters in the descending excitatory reflex pathway of the guinea-pig ileum.

Neurotransmission underlying descending excitatory reflexes evoked by distension was studied in opened segments of guinea-pig ileum and compared with peristalsis in intact segments. The opened segments were distended by inflating a balloon against the serosa at the oral end and changes in muscle length recorded from the anal end. Distension elicited contractions in both circular (CM) and longitudinal (LM) muscle layers. Granisetron, a 5-HT(3) receptor antagonist (10 nmol L-1 to 1 micromol L-1) reduced CM contractions (24% control), without affecting the LM. The P2 receptor antagonist, pyridoxal phosphate-6-azopheyl-2',4'-disulphonic acid (PPADS; 10 micromol L-1), reduced CM contractions to 31% and LM contractions to 39%. Hexamethonium (500 micromol L-1) enhanced LM contractions, but had no effect on CM contractions. Granisetron (1 micromol L-1) had no significant effect on the threshold for peristaltic contractions in a modified Trendelenburg preparation, but decreased the decay time of these contractions by 37%. PPADS (10 micromol L-1) had no significant effect in this preparation. Thus, the descending excitatory pathways to CM and LM can be distinguished pharmacologically; the former depend on 5-HT(3) and P2 ATP receptors, the latter are independent of 5-HT(3) receptors. Nicotinic receptors may have little part in either pathway. These properties differ from conventional peristaltic reflexes, which are effectively abolished by nicotinic blockade.