Serotonin modulates photoresponses in Hermissenda type-B photoreceptors

Light responses of Hermissenda type-B photoreceptors evoked by flashes of light were enhanced and prolonged by bath application of serotonin (5-HT). 5-HT appears to act directly on the B photoreceptors and not indirectly through 5-HT-sensitive interneurons since similar effects were observed in the presence of tetrodotoxin and in axotomized preparations which eliminated synaptic input to the B photoreceptors. We also examined the effect of 5-HT on light-evoked membrane currents with voltage-clamp techniques. The increase in the duration of the light response in 5-HT can be explained by the prolonged decay of the light-activated current evoked by constant-intensity test flashes. 5-HT appears to prolong the light response by slowing a late process in the decay of light-activated membrane current. In contrast to the effects of 5-HT an alpha 2-agonist, clonidine, enhanced the peak photoresponse but did not prolong the light response. Serotoninergic modulation of light-dependent processes in the B photoreceptors may contribute to the development of long-term changes produced by conditioning.