Serotonin activates a Ca(2+)-dependent K(+) current in identified peptidergic neurons from the crayfish

The effects of 5-hydroxytryptamine (5-HT) were investigated in red pigment concentrating hormone (RPCH)-containing neurons isolated from the X-organ of the crayfish (Procambarus clarkii). Under current-clamp conditions and using the gramicidin-perforated-patch configuration, 5-HT elicited a prolonged hyperpolarization that suppressed neuronal firing concomitant with an increase in membrane conductance. Under voltage-clamp conditions, 5-HT evoked an outward current at a holding potential of -50 mV. This current reversed at an E(K) of -90 mV, which shifted by 30 mV when the extracellular K(+) concentration was increased from 5.4 to 19 mmol l(-1). The effect of 5-HT was dose-dependent within the range 1-100 micromol l(-1) and followed simple Michaelis-Menten kinetics, with a half-maximal response being elicited at 10 micromol l(-1). Preincubation with charybdotoxin (100 nmol l(-1)), tetraethylammonium (500 micromol l(-1)) or methysergide (100 micromol l(-1)) was effective in blocking the response to 5-HT. These results suggest that 5-HT is an inhibitory mediator of the release of red pigment concentrating hormone by acting on a Ca(2+)-dependent K(+) current.