Is histamine a neurotransmitter in insect photoreceptors?

Intracellular recordings were made from the large monopolar cells (LMC's) in the first visual neuropil (lamina) of the fly Musca, whilst applying pharmacological agents from a three-barrelled ionophoretic pipette. Most of the known neurotransmitter candidates (except the neuropeptides) were tested. The LMC's were most sensitive to histamine, saturating with ionophoretic pulses of less than 2 nC. The responses to histamine were fast hyperpolarizations with maximum amplitudes similar to that of the light-induced response. Like the light response, the histamine response was associated with a conductance increase. The histamine responses were not blocked by a synaptic blockade induced by ionophoretic application of cobalt ions. Several histamine antagonists, and also atropine, were effective at blocking or reducing both the response to histamine and the response to light. Other transmitter candidates having marked effects on the LMC's were: a) the acidic amino-acids, L-aspartate and L-glutamate, which evoked slower hyperpolarizations that could be blocked by cobalt; b) GABA, which induced a depolarization associated with an inhibition of the light response; and c) acetylcholine which also caused a depolarization. Substances with no obvious effect on the LMC's included serotonin (5-HT), beta-alanine, dopamine, octopamine, glycine, taurine and noradrenalin. Together with the evidence of Elias and Evans (1983), which shows the presence, synthesis and inactivation of histamine in the retina and optic lobes of the locust, the data suggest that histamine is a neurotransmitter in insect photoreceptors.