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

Threonine-for-leucine mutation within domain M2 of the neuronal alpha(7) nicotinic receptor converts 5-hydroxytryptamine from antagonist to agonist

A study was made of the effects of 5-hydroxytryptamine (5HT) on homomeric neuronal nicotinic receptors (nAcChoR) expressed in Xenopus oocytes after injection of cDNA encoding the wild-type chicken alpha(7) subunit. Acetylcholine (AcCho) elicited large currents (IAcCho) that were reduced by 5HT in a reversible and dose-dependent manner, with a half-inhibitory concentration (IC50) of 56 microM and a Hill coefficient (nH) of 1.2. The inhibition of IAcCho by 5HT was noncompetitive and voltage independent, a behavior incompatible with a channel blockade mechanism. 5HT alone did not elicit membrane currents in oocytes injected with the wild-type alpha(7) subunit cDNA. In contrast, 5HT elicited membrane currents (I5HT) in oocytes injected with cDNA encoding an alpha(7) mutant subunit with a threonine-for-leucine-247 substitution (L247T alpha(7)). I5HT was inhibited by the potent nicotinic receptor blockers alpha-bungarotoxin (100 nM) and methyllycaconitine (1 microM). Furthermore, the characteristics of I5HT, including its voltage dependence, were similar to those of IAcCho. The 5HT dose-I5HT response gave an apparent dissociation constant EC50 of 23.5 microM and a Hill coefficient nH of 1.7, which were not modified by the presence of AcCho. Similarly, the apparent affinity of L247T alpha(7) for AcCho as well as its cooperativity were not influenced by 5HT, indicating a lack of mutual interactions between 5HT and AcCho. These results show that 5HT is a potent noncompetitive antagonist of neuronal alpha(7) nAcChoR, but it becomes a noncompetitive agonist following mutation of the highly conserved leucine residue 247 located in the channel domain M2.

Effect of (+/-)-DOI on neuromuscular transmission: a microelectrode study

DOI (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) significantly depressed end plate current (EPC) amplitude. It decreased quantum content, increased the extent of neurally evoked EPC rundown during the train, produced a nonlinear current-voltage relationship, shortened time constant of decay, and depressed iontophoretically evoked EPC. The depressant response of DOI on EPC amplitude was antagonized by 5-HT1-like receptor antagonists, but was resistant to 5-HT2 and 5-HT3 receptor antagonists. This suggests that inhibitory 5-HT receptors roughly correspond to 5-HT1-like receptors.

In vivo labeling of serotonin-containing neurons by 5,7-dihydroxytryptamine in Aplysia

Intrahemocoelial administration of 5,7-dihydroxytryptamine (5,7-DHT) to Aplysia californica induces a transient (less than 4 h) behavioral alteration. About 5 weeks after 5,7-DHT treatment, 5-hydroxytryptamine (5-HT)-containing neurons develop dark brown pigmentation. These labeled 5-HT neurons have normal physiological and pharmacological properties when investigated electrophysiologically. This contrasts with the long-term neurotoxic effect of 5,7-DHT on vertebrate neurons. This technique will greatly facilitate visual identification of 5-HT-containing neurons and study of their physiology and actions.

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.