Anatomy and physiology of neurons with processes in the accessory medulla of the cockroach Leucophaea maderae

The accessory medulla (AMe), a small neuropil in the insect optic lobe, has been proposed to serve a circadian pacemaker function analogous to the rol e of the suprachiasmatic nucleus in mammals. Building upon considerable kno wledge of the circadian system of the cockroach Leucophaea maderae, we inve stigated the properties of AMe neurons in this insect with intracellular re cordings combined with dye injections. Responses of neurons with processes in the AMe to visual stimuli, including stationary white light, moving obje cts, and polarized light were compared with the responses of adjacent medul la tangential neurons. Neurons with processes in the AMe and additional ram ifications in the medulla strongly responded to stationary light stimuli an d might, therefore, be part of photic entrainment pathways to the clock. Ac cessory medulla neurons lacking significant processes in the medulla but wi th projections to the midbrain or to the contralateral optic lobe, in contr ast, responded weakly or not at all to light and, thus, seem to be part of the clock's output pathway. Two types of commissural neurons with tangentia l arborizations in both medullae were sensitive to polarized light, suggest ing a role of these neurons in celestial navigation. Sidebranches in the AM ae of one of the two cell types are discussed with respect to a possible in volvement of the AMe in polarization vision. Finally, neurons responding to movement stimuli did not arborize in the AMe. The results show that the AM e receives photic input and support a role of this neuropil in circadian ti mekeeping functions. (C) 2001 Wiley-Liss, Inc.