Pigment-dispersing hormone-immunoreactive neurons in the cockroach Leucophaea maderae share properties with circadian pacemaker neurons.

Neurons immunoreactive with antisera against the crustacean peptide beta-pigment dispersing hormone fulfill several anatomical criteria proposed for circadian pacemakers in the brain of the cockroach Leucophaea maderae. These include position of somata, projections to the lamina and midbrain and possible coupling pathways between the two pacemakers through commissural fibers. In behavioral experiments combined with lesion studies and immunocytochemical investigations we examined whether the presence of pigment-dispersing hormone-immunoreactive arborizations in the midbrain of the cockroach correlates with the presence of circadian locomotor activity. No rhythm was detected after severing both optic stalks in any animal for at least 12 days. Within the same time pigment-dispersing hormone-immunoreactive fibers in the midbrain disappeared. Two to seven weeks after the operation some of the cockroaches regained circadian locomotor activity, while others remained arrhythmic. In all cockroaches which regained rhythmic behavior pigment-dispersing hormone-immunoreactive fibers had regenerated and had largely found their original targets within the brain. In all arrhythmic cockroaches either none or very little regeneration had occurred. The period of the regained circadian activity inversely correlated with the number of regenerated immunoreactive commissural fibers. These data provide further evidence for the involvement of pigment-dispersing hormone-immunoreactive neurons in circadian clocks of orthopteroid insects.