Immunohistochemical and electrophysiological evidence that locust ocellar photoreceptors contain and release histamine.

Immunohistochemistry with an antiserum directed against histamine revealed histamine-like immunoreactivity in ocellar photoreceptors of Locusta migratoria. Prefixative incubation of the tissue in Ringer solution containing 10 microM histamine increased number and intensity of labeled photoreceptors. Additionally, glial cells became immunoreactive. High extracellular potassium concentration abolished labeling in retinula cells but not in glial cells. These results suggest that locust ocellar photoreceptors contain histamine and release it upon depolarization. Histamine itself or/and a metabolite is taken up by photoreceptors and glial cells. In high-potassium/calcium-free Ringer solution labeling in photoreceptors persisted, indicating that release is calcium-dependent. Electrophysiological recordings confirm the hyperpolarizing and localized action of histamine onto dendrites of second-order neurons in the ocellar cup. These results support the hypothesis that histamine is a neurotransmitter in insect photoreceptor cells.