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

Abstract

The accessory medulla (AMe), a small neuropil in the insect optic lobe, has been proposed to serve a circadian pacemaker function analogous to the role of the suprachiasmatic nucleus in mammals. Building upon considerable knowledge of the circadian system of the cockroach Leucophaea maderae, we investigated the properties of AMe neurons in this insect with intracellular recordings combined with dye injections. Responses of neurons with processes in the AMe to visual stimuli, including stationary white light, moving objects, and polarized light were compared with the responses of adjacent medulla tangential neurons. Neurons with processes in the AMe and additional ramifications in the medulla strongly responded to stationary light stimuli and might, therefore, be part of photic entrainment pathways to the clock. Accessory medulla neurons lacking significant processes in the medulla but with projections to the midbrain or to the contralateral optic lobe, in contrast, 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 tangential arborizations in both medullae were sensitive to polarized light, suggesting a role of these neurons in celestial navigation. Sidebranches in the AMae of one of the two cell types are discussed with respect to a possible involvement of the AMe in polarization vision. Finally, neurons responding to movement stimuli did not arborize in the AMe. The results show that the AMe receives photic input and support a role of this neuropil in circadian timekeeping functions.

Evidence for a role of orcokinin-related peptides in the circadian clock controlling locomotor activity of the cockroachLeucophaea maderae

“…The neurons project via the anterior (two neurons) and posterior (one neuron) optic commissures, innervate the internodular and shell neuropils of the AMe, and send a fan of fibers along the distal surface of the medulla toward the first optic chiasm and lamina. Neurons of this morphological type were found to be unresponsive to light stimuli in intracellular recordings (Loesel and Homberg, 2001). Together with the non-photic phase-response curve obtained by PDF injection, Petri and Stengl (Petri and Stengl, 2002) and Reischig et al (Reischig et al, 2004), therefore concluded that the three PDF-ir VNe transmit phase information to the contralateral pacemaker.…”

“…Neural connections between both brain hemispheres of L. maderae serve for bilateral coupling of the clocks and, in addition, for light entrainment of the clock through the contralateral compound eye (Page, 1978;Page, 1981;Page, 1983a). Commissural neurons between both AMae, which might mediate these functions, have been identified (Loesel and Homberg, 2001;Reischig and Stengl, 2002;Reischig et al, 2004). Immunocytochemical studies and injections of neuroactive substances suggest that several neuropeptides and ␥-aminobutyric acid (GABA) play roles as neuromediators in the circadian system of the cockroach (Petri et al, 1995;Petri et al, 2002;Petri and Stengl, 1997;Schneider and Stengl, 2005).…”

Visual and olfactory input segregation in the mushroom body calyces in a basal neopteran, the American cockroach

“…Recent findings that both anterior and posterior optic tracts deliver polarization information in the locust S. gregaria (Mappes and Homberg, 2007;Jundi and Homberg, 2010) suggests the possibility that these visual neurons respond to polarized light as well as ordinary visual stimuli in the cockroach, as is demonstrated in a posterior optic tract neuron (Loesel and Homberg, 2001). Future work using controlled visual stimuli (such as color, motion and polarized light stimuli) will be necessary to elucidate the physiological properties of visual input neurons.…”

Pigment-Dispersing Factor and GABA Synchronize Cells of the Isolated Circadian Clock of the CockroachLeucophaea maderae

“…Whether all regularly discharging cells of the cockroach circadian clock are also endogenous circadian pacemaker neurons and vice versa remains to be examined. PDF-IR neurons apparently generate very regular interspike intervals, because intracellular recordings from neurons with the same branching pattern as PDF-IR neurons showed spontaneous discharge rates with regularly spaced spikes (Loesel and Homberg, 2001). In addition, regularly bursting neurons with arborizations in the circadian clock were recorded Homberg, 1998, 2001).…”