Modulation by 5-hydroxytryptamine of nicotinic acetylcholine responses recorded from an identified cockroach (Periplaneta americana) motoneuron.
Recordings from the soma of the cockroach (Periplaneta americana) fast coxal depressor motoneuron (Df) were made while acetylcholine (ACh) was regularly pressure-applied locally from a micropipette. The modulatory effects upon these nicotinic ACh responses of bath-applied 5-hydroxytryptamine (5-HT, serotonin), dopamine and octopamine were investigated under either current-clamp or voltage-clamp conditions. The biogenic amines reversibly suppressed, but never totally abolished, ACh responses, 5-HT being the most potent, with a threshold near 10(-6) m (EC50 = 5 x 10(-5) m). Occlusion experiments indicate that the amines share a common mechanism at the level of either receptors or second messenger pathways. The amines also modulated responses to nicotine or carbachol (each of which resists hydrolysis by acetylcholinesterases), indicating that the amines did not act by accelerating ACh degradation. Pharmacological antagonists were used in an attempt to characterize the receptor responsible for amine-mediated modulation. Although a number of antagonists mimicked the action of amines rather than producing blockade, the antagonistic actions of LSD and RS23597 pointed strongly to a receptor-mediated mechanism, but did not allow receptor identification. The magnitude of the modulatory effect of 5-HT was significantly reduced by intracellular guanosine-5'-O-(2-thiodiphosphate) (GDP-beta-S), indicating involvement of a G-protein. Intracellular injection of the calcium chelator BAPTA did not block the modulatory effect of 5-HT, showing that the amines do not operate through the calcium-dependent pathway by which muscarinic receptors act on nicotinic currents. The adenylate cyclase inhibitor dideoxyadenosine (DDA), on the other hand, did attenuate the action of 5-HT, suggesting involvement of cyclic AMP.
Butt
Simon J B
SJ
School of Biomedical Sciences, Gatty Marine Laboratory, University of St Andrews, Fife KY16 8LB, UK.
Pitman
Robert M
RM
D000109:Acetylcholine / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology; D002490:Central Nervous System / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D004298:Dopamine / Q000378:metabolism / Q000494:pharmacology; D018492:Dopamine Antagonists / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009046:Motor Neurons / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D018727:Muscarinic Antagonists / Q000494:pharmacology; D009655:Octopamine / Q000378:metabolism / Q000494:pharmacology; D010526:Periplaneta / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000378:metabolism; D011978:Receptors, Nicotinic / Q000187:drug effects* / Q000378:metabolism; D012701:Serotonin / Q000378:metabolism / Q000494:pharmacology*; D012702:Serotonin Antagonists / Q000494:pharmacology; D013570:Synaptic Membranes / Q000187:drug effects* / Q000378:metabolism; D009435:Synaptic Transmission / Q000187:drug effects* / Q000502:physiology
D018492:Dopamine Antagonists; D018727:Muscarinic Antagonists; D011976:Receptors, Muscarinic; D011978:Receptors, Nicotinic; D012702:Serotonin Antagonists; D009655:Octopamine; D012701:Serotonin; D019204:GTP-Binding Proteins; D000109:Acetylcholine; D004298:Dopamine