5-Hydroxytryptamine decreases the sensitivity of nicotinic acetylcholine receptor in bull-frog sympathetic ganglion cells.

The post-synaptic effects of 5-hydroxytryptamine (5-HT) were examined in neurones of bull-frog sympathetic ganglia with intracellular micro-electrode and voltage-clamp recording techniques. Atropine (1 microM) was used to block the muscarinic cholinoceptors. 5-HT reduced the amplitude of the fast excitatory post-synaptic potential (fast e.p.s.p.). 5-HT also reduced the mean amplitude of the miniature excitatory post-synaptic potentials (m.e.p.s.p.s) without affecting their frequency. Voltage-clamp studies showed that 5-HT decreased in a dose-dependent manner the amplitude of the acetylcholine (ACh) current produced by ionophoretic application of ACh to sympathetic neurones. The relationship between the log of the ACh dose, applied ionophoretically, and the peak ACh current (the dose-response curve) was examined in voltage-clamped neurones. 5-HT caused a parallel shift to the right of the dose-response curve for ACh. Analysis using a double reciprocal plot (Lineweaver-Burk plot) revealed that 5-HT increased the apparent dissociation constant (Km) of ACh for the receptor without changing the maximum ACh current (Vmax), suggesting a competitive antagonism. The relationship between the 5-HT dose and the magnitude of inhibition of the ACh current was obtained using two different amplitudes for the ACh response. The dose-response curve of 5-HT-induced inhibition using a relatively high amplitude ACh current, S1, was parallel with that for a relatively low amplitude ACh current, S2. The Dixon plot of these two curves yielded an apparent inhibition constant (Ki) of 42 microM. Both fast excitatory post-synaptic currents (fast e.p.s.c.s) and miniature excitatory post-synaptic currents (m.e.p.s.c.s) had single-exponential decay time courses. The time constants of fast e.p.s.c. decay (tau e) and m.e.p.s.c. decay (tau m) were not altered by 5-HT, suggesting that 5-HT does not change the kinetics of opening and closing of the ionic channel associated with the nicotinic receptor. 5-HT did not alter the reversal potential of the fast e.p.s.c. These results suggest that 5-HT decreases the sensitivity of the nicotinic receptor of sympathetic neurones, by interfering with ACh binding at the active site on the receptor-ionic-channel complex. 5-HT may physiologically inhibit cholinergic transmission as it is an endogenous substance which antagonizes the nicotinic receptor in post-ganglionic neurones of bull-frog sympathetic ganglia.