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

Histamine is the neurotransmitter of insect photoreceptor cells but has also been found in a small number of interneurons in the insect brain. In order to investigate whether the accessory medulla (AMe), the putative circadian pacemaker of the cockroach Leucophaea maderae receives direct visual input from histaminergic photoreceptors, we analyzed the distribution of histamine-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 distal layer of the medulla, the second optic neuropil. Histamine immunostaining in parts of the AMe, however, originated from a centrifugal neuron of the midbrain. Within the midbrain 21–23 bilaterally symmetric pairs of cell bodies were stained. Most areas of the brain were innervated by one or more of these neurons, but the protocerebral bridge and the mushroom bodies were devoid of histamine immunoreactivity. The branching patterns of most histamine-immunoreactive neurons could be reconstructed individually. While the majority of identified neurons arborized in both brain hemispheres, five cells were local neurons of the antennal lobe. A comparison with other insect species shows striking similarities in the position of certain histamine-immunoreactive neurons, but considerable variations in the presence and branching patterns of others. The data suggest a role for histamine in a non-photic input to the circadian system of the cockroach.