EVIDENCE THAT HISTAMINE IS A NEUROTRANSMITTER IN AN INSECT EXTRAOCULAR PHOTORECEPTOR PATHWAY

The pair of vasopressin-like immunoreactive (VPLI) neurones of the loc ust Locusta migratoria have cell bodies in the suboesophageal ganglion and extensive arborization throughout the central nervous sytem. The activity of the VPLI neurone is regulated by a spontaneously active ex citatory descending interneurone (DI) that is, in turn, inhibited by a n uncharacterised extraocular photoreceptor (EOP) system located in th e brain. Light directed at the brain results in inhibition of DI activ ity, which thereby deprives the VPLI neurone of its major synaptic inp ut. We present evidence that histamine plays an important role in the EOP-DI-VPLI pathway. Histamine mimics the EOP-mediated inhibition of t he DI, and the H2-specific histamine antagonists cimetidine and raniti dine block its inhibitory action. Histamine application to various are as of the brain localises the area where histaminergic inhibition occu rs; this region is confined to the medial protocerebrum. At least six bilaterally paired histaminelike immunoreactive neurones send axonal p rojections into this area. Depolarisation of the brain region containi ng the soma of these neurones with high-K+ saline deactivates the VPLI neurone through the removal of the DI excitatory synaptic input.