• 5-HT receptors;
  • α2-adrenergic receptors;
  • enteric nerves;
  • ganglionic neurotransmission;
  • presynaptic inhibition

Abstract  These studies investigated receptors modulating release of mediators of fast excitatory postsynaptic potentials (fEPSPs) in guinea pig ileum myenteric plexus using electrophysiological methods. Fast EPSPs inhibited by >95% by hexamethonium (100 μmol L−1) were cholinergic; mixed fEPSPs were inhibited <95% by hexamethonium. Non-cholinergic fEPSPs were studied in the presence of hexamethonium. The α2-adrenergic receptor agonist UK 14304 inhibited cholinergic (maximum inhibition = 76%, EC50 = 18 nmol L−1), mixed (81%, 21 nmol L−1) and non-cholinergic (76%, 44 nmol L−1) fEPSPs equally. The 5-HT1 receptor agonist 5-carboxamidotryptamine inhibited cholinergic, mixed and non-cholinergic fEPSPs equally. Renzapride, increased non-cholinergic (33%) less than mixed (97%, 13 μmol L−1) fEPSPs. Renzapride inhibited the purely cholinergic fEPSPs (−29%) but potentiated the cholinergic component of mixed fEPSPs (39%). Prucalopride potentiated all fEPSPs equally (30–33%). 5-HT (0.1 μmol L−1) induced potentiation of cholinergic (75%), mixed (97%) and non-cholinergic (84%) fEPSPs was not statistically different. The potentiating effects of renzapride and 5-HT on fEPSPs were inhibited by the 5-HT4 receptor antagonist, SB 204070 (10 nmol L−1). Renzapride (0.3 μmol L−1) blocked 5-HT-induced increases in cholinergic fEPSPs. α2-Adrenergic and 5-HT1 receptors mediate inhibition of transmitter release from cholinergic and mixed terminals. 5-HT and prucalopride, acting at 5-HT4 receptors, facilitate all fEPSPs; renzapride facilitates the cholinergic and non-cholinergic components of mixed fEPSPs but not purely cholinergic fEPSPs. Cholinergic synapses may express few 5-HT4 receptors or a renzapride-insensitive 5-HT4 receptor isoform.