Histamine-immunoreactive neurons in the brain of the cockroach Leucophaea maderae

Histamine is the neurotransmitter of insect photoreceptor cells but has als o been found in a small number of interneurons in the insect brain. In orde r to investigate whether the accessory medulla (AMe), the putative circadia n pacemaker of the cockroach Leucophaea maderae receives direct visual inpu t from histaminergic photoreceptors, we analyzed the distribution of histam ine-like immunoreactivity in the optic lobe and midbrain of the cockroach. Intense immunostaining was detected in photoreceptor cells of the compound eye, which terminated in the first optic neuropil, the lamina, and in a dis tal layer of the medulla the second optic neuropil. Histamine immunostainin g in parts of the AMe, however, originated from a centifungal neuron of the midbrain. Within the midbrain 21-23 bilaterally symmetric pairs of cell bo dies were stained. Most areas of the brain were innervated by one or more o f these neurons, but the protocerebral bridge and the mushroom bodies were devoid of histamine immunoreactivity. The branching patterns of most histam ine-immunoreactive neurons could be reconstructed individually. While the m ajority of identified neurons arborized in both brain hemispheres, five cel ls were local neurons of the antennal lobe. A comparison with other insect species shows striking similarities in the position of certain histamine-im munoreactive neurons, but considerable variations in the presence and branc hing patterns of others. The data suggest a role for histamine in a non-pho tic input to the circadian system of the cockroach. (C) 1999 Elsevier Scien ce B.V. All rights reserved.