Evidence That Histamine is a Neurotransmitter of Photoreceptors in the Locust Ocellus

Abstract

The results presented here are consistent with the hypothesis that histamine is the major neurotransmitter released by photoreceptors of locust ocelli. 1. When histamine is injected by ionophoresis into the locust ocellar neuropile, large second-order neurones (L-neurones) hyperpolarize in a dose-dependent manner, and responses to light in these neurones are diminished in amplitude. Both histamine and the illumination of ocellar photoreceptors caused an outward current across the membrane. 2. Hyperpolarizing potentials in L-neurones evoked by histamine had the same reversal potential as hyperpolarizing potentials evoked by photoreceptor illumination. 3. When applied ionophoretically in the ocellus, other biogenic amines, including octopamine, dopamine and noradrenaline, had no effect on the L-neurones. Both gamma-aminobutyricacid and acetylcholine, however, depolarized L-neurones and diminished responses to light. 4. Curare blocked the L-neurone's responses to histamine and light. The histamine response recovered fully. The initial peak hyperpolarizing response to increased light recovered, but the more sustained plateau hyperpolarizing potential did not. 5. Hexamethonium bromide prolonged the response of an L-neurone to histamine, and increased the tonic component of the response to light.