SEROTONIN MODULATES A PHOTIC RESPONSE IN CIRCADIAN LOCOMOTOR RHYTHMICITY OF ADULTS OF THE BLOW FLY, CALLIPHORA-VICINA

The present experiments were undertaken to explore a role for serotoni n (5-hydroxytryptamine, 5-HT) in modulating photic signal transduction in photoreceptors of the blow fly, Calliphora vicina. Injection of p- chlorophenylalanine (pCPA) into the haemolymph appeared to reduce sens itivity to the photic effects of constant 'bright' light (LL hyperacti vity and circadian arrhythmicity). After drug injection in bright LL, flies continued with a free-running rhythm as in constant darkness (DD ) or with a lengthened period tau as in 'dim' LL. When 5-HT was inject ed into flies kept in dim LL, they became hyperactive and arrhythmic a s in bright LL. This finding suggests a potential role for serotonin a s mediator of circadian changes in the insect visual system including extraretinal photoreceptors.