Ionic mechanisms and receptor properties underlying the responses of molluscan neurones to 5-hydroxytryptamine.
1. Molluscan neurones have been found to show six different types of response (three excitatory and three inhibitory) to the iontophoretic application of 5-hydroxytryptamine (5-HT). The pharmacological properties of the receptors and the ionic mechanisms associated with these responses have been analysed.2. Four of the responses to 5-HT (named A, A', B and C) are consequent upon an increase in membrane conductance whereas the other two (named alpha and beta) are caused by a decrease in membrane conductance.3. The A-response to 5-HT consists of a ;fast' depolarization due to an increase mainly in Na(+)-conductance; the A'-response is a ;slow' depolarization also associated with a Na(+)-conductance increase. Receptors mediating the A- and A'-depolarizations have different pharmacological properties and may exist side by side on the same neurone.4. Both the B- and C-responses are inhibitory. The B-response is a ;slow' hyperpolarization due to an increase in K(+)-conductance, the C-response is a fast hyperpolarization associated with an increase in Cl(-)-conductance.5. The alpha-response to 5-HT is a depolarization which becomes reduced in amplitude with cell hyperpolarization and reverses at -75 mV; it is caused by a decrease in K(+)-conductance. The beta-response is an hyperpolarization which increases in amplitude with cell hyperpolarization and reverses at -20/-30 mV. It results from a decrease in conductance to both Na(+) and K(+) ions.6. The receptors involved in the 5-HT responses associated with a conductance increase may be recognized by the action of specific antagonists: 7-methyltryptamine blocks only the A-receptors, 5-methoxygramine only the B-receptors and neostigmine only the C-receptors. Curare blocks the A- and C-receptors and bufotenine, the A-, A'- and B-receptors. No specific antagonists have yet been found for the 5-HT responses caused by a conductance decrease.7. The significance of the multiplicity of receptors is discussed. Their functional significance at synapses is analysed in the following paper.
D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000818:Animals; D002027:Bufotenin / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism / Q000502:physiology; D002712:Chlorides / Q000378:metabolism; D003470:Culture Media; D003472:Curare / Q000494:pharmacology; D004123:Dimethylamines / Q000494:pharmacology; D004553:Electric Conductivity; D005724:Ganglia / Q000502:physiology; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D007478:Iontophoresis; D008564:Membrane Potentials; D008738:Methyl Ethers / Q000494:pharmacology; D008974:Mollusca; D009388:Neostigmine / Q000494:pharmacology; D009474:Neurons / Q000187:drug effects*; D011188:Potassium / Q000378:metabolism; D011955:Receptors, Drug; D012701:Serotonin / Q000494:pharmacology*; D012908:Snails; D012964:Sodium / Q000378:metabolism; D014363:Tryptamines / Q000494:pharmacology
D000319:Adrenergic beta-Antagonists; D002712:Chlorides; D003470:Culture Media; D004123:Dimethylamines; D007211:Indoles; D008738:Methyl Ethers; D011955:Receptors, Drug; D014363:Tryptamines; D002027:Bufotenin; D012701:Serotonin; D009388:Neostigmine; D003472:Curare; D012964:Sodium; D011188:Potassium