An Unusual Synaptic Response Mediated by a Serotonin Neurone

Abstract

SUMMARYImpulse activity in an identified serotonin-containing neurone produces a delayed slow excitatory response in another identified neurone. An axon of the serotonin neurone passes very close to the follower neurone perikaryon. The synaptic response is frequently accompanied by oscillations of the membrane potential. Constant-current pulsing experiments suggest that it could result from a reduction in membrane conductance. The response is markedly voltagesensitive, being greatly reduced at potentials in excess of about -55 mV. Serotonin, locally applied, produces a very similar effect. The response to serotonin does not involve a change in conductance to either Na+ or Cl-, but Ca2+ appears to be involved, either by virtue of its influence on K+ conductance, or directly in its transfer of charge across the membrane.

Physiological Role of a Slow, Voltage‐sensitive, Synaptic Response Mediated by an Identified Serotonin‐containing Neurone

“…The depolarizing response observed in one large neurone, the A neurone, is, however, atypical. It is delayed in onset, long lasting and markedly voltage-sensitive, being diminished but not inverted on hyperpolarization, while showing marked potentiation with depolarization (Cottrell, 1971(Cottrell, , 1981. In these respects, it parallels the serotonin-induced calcium current response described by Pellmar & Carpenter (1980) and Pellmar (1981).…”

“…The experiments in Helix described above provide direct experimental evidence in support of an action ofendogenously released serotonin on action potential duration. It is interesting to note that the slow inward current response can also be produced by applied dibutyryl cyclic adenosine monophosphate, is not dependent on sodium ions but is blocked by calcium ion antagonists (Cottrell, 1981;G. A. Cottrell, unpublished data).…”

“…Electrophysiological recordings were made using methods described in an earlier paper (Cottrell, 1981). Cerebral g.s.n.s were impaled on a double-barrelled electrode.…”

Enhancement of inward current by serotonin in neurons of Aplysia

“…These data suggest that serotonin increases calcium conductance. Cottrell (1981) and Paupardin-Tritsch, DeTerre, and Gerschenfeld (1981) report a similar voltage-dependent response in neurons of Helix. This response 14 PELLMAR in Helix was, in many respects, like that described in Aplysia by Pellmar (Pellmar andCarpenter, 1979,1980;Pellmar and Wilson, 1977).…”

Multiple actions of a molluscan cardioexcitatory neuropeptide and related peptides on identified Helix neurones.

“…Other voltage-dependent responses, inward in sign, have been observed with 5-hydroxytryptamine (Pellmar & Wilson, 1977;Pellmar & Carpenter, 1980; Paupardin-Tritsch, Deterre & Gerschenfeld, 1981;Siegelbaum, Camardo & Kandel, 1982) and following activation of an identified serotonergic input (Cottrell, 1981). In bull-frog sympathetic ganglion cells, luteinizing hormone releasing factor, and also muscarinic agonists, suppress a voltage-sensitive K current which is not sensitive to Ca2+ ions .…”