Histamine: the case for a photoreceptor's neurotransmitter.

The evidence is strong that histamine is the transmitter from photoreceptors of the barnacle, several insect species, and Limulus. Histamine is synthesized and stored in the photoreceptors and mimics the effect of the natural transmitter on the postsynaptic cell. An activity-dependent uptake mechanism exists to recapture the histamine for reuse at this continuously active synapse. Only light-dependent release of this compound remains to be demonstrated. A primary job of the photoreceptor's synapse in the barnacle and also in insects is to convert photoreceptor hyperpolarizations at the offset of light (which presumably lead to decreases in transmitter release) to a depolarizing off-response in the postsynaptic cell. Hardie (10) has established that histamine has cooperative binding and a relatively low affinity for its receptor, leading to a short open time for the postsynaptic channel. These features could allow the postsynaptic cell to respond relatively quickly to decreases in transmitter concentration. Perhaps the uptake mechanism demonstrated in barnacle photoreceptors will also prove to be important in the generation of the postsynaptic response by quickly clearing the cleft of released histamine. It will be of interest to determine what factors control the rate of uptake of histamine and whether blocking the uptake mechanism affects signals set up in the postsynaptic cell.