Serotonin activates a Ca2+-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 o f the crayfish (Procambarus clarkii). Under current-clamp conditions and us ing the gramicidin-perforated-patch configuration, 5-HT elicited a prolonge d hyperpolarization that suppressed neuronal firing concomitant with an inc rease in membrane conductance. Under voltage-clamp conditions, 5-HT evoked an outward current at a holding potential of -50mV. This current reversed a t an E-K of -90 mV, which shifted by 30 mV when the extracellular K+ concen tration was increased from 5.4 to 19 mmol l(-1). The effect of 5-HT was dos e-dependent within the range 1-100 mu mol l(-1) and followed simple Michael is-Menten kinetics, with a half-maximal response being elicited at 10 mu mo l l(-1). Preincubation with charybdotoxin (100nmol l(-1)), tetraethylammoni um (500 mu mol l(-1)) or methysergide (100 mu mol l(-1)) was effective in b locking the response to 5-HT. These results suggest that 5-HT is an inhibit ory mediator of the release of red pigment concentrating hormone by acting on a Ca2+-dependent K+ current.