Pigment-dispersing hormone (PDH)-immunoreactive neurons form a direct coupling pathway between the bilaterally symmetric circadian pacemakers of the cockroach Leucophaea maderae

Abstract

Circadian locomotor activity rhythms of the cockroach Leucophaea maderae are driven by two bilaterally paired and mutually coupled pacemakers that reside in the optic lobes of the brain. Transplantation studies have shown that this circadian pacemaker is located in the accessory medulla (AMe), a small neuropil of the medulla of the optic lobe. The AMe is densely innervated by about 12 anterior pigment-dispersing-hormone-immunoreactive (PDH-ir) medulla (PDHMe) neurons. PDH-ir neurons are circadian pacemaker candidates in the fruitfly and cockroach. A subpopulation of these neurons also appears to connect both optic lobes and may constitute at least one of the circadian coupling pathways. To determine whether PDHMe neurons directly connect both accessory medullae, we injected rhodamine-labeled dextran as neuronal tracer into one AMe and performed PDH immunocytochemistry. Double-labeled fibers in the anterior, shell, and internodular neuropil of the AMe contralaterally to the injection site showed that PDH-ir fibers directly connect both accessory medullae. This connection is formed by three anterior PDHMe neurons of each optic lobe, which, thus, fulfill morphological criteria for a direct circadian coupling pathway. Our double-label studies also showed that all except one of the midbrain projection areas of anterior PDHMe neurons were innervated ipsilaterally and contralaterally. Thus, anterior PDHMe neurons seem to play multiple roles in generating circadian rhythms. They also deliver timing information output and perform mutual pacemaker coupling in L. maderae.

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

“…The PDF-IR neurons, which branch in the internodular neuropil, control locomotor rhythms via projections to locomotor control centers in the superior lateral protocerebrum (Stengl and Homberg, 1994;Reischig and Stengl, 2003a). In addition, three of the 12 PDF-IR neurons connect both accessory medullas (AMae) and apparently serve to synchronize both pacemakers (Reischig and Stengl, 2004). Consistent with this hypothesis, PDF acts as a nonphotic input signal into the clock, delaying circadian locomotor activity rhythms (Petri and Stengl, 1997).…”

“…AMe neurons are synchronized by PDF In L. maderae, the 12 PDF-IR neurons of the AMe apparently serve in different circuits of the circadian clock (Reischig and Stengl, 2002, 2003a,b, 2004. The small, weakly staining PDF-IR neurons appear to be local neurons of the AMe; two large and one medium PDF-IR neurons directly connect both AMae, whereas the other large and medium cells form outputs to various midbrain and optic lobe targets (Reischig and Stengl, 2004). Thus, in L. maderae, as in Drosophila, PDF-IR neurons are circadian pacemaker candidates and, at the same time, serve as nonphotic clock inputs and outputs.…”

Circadian clocks in crustaceans: identified neuronal and cellular systems

“…In particular, phase locking between eyestalk ERG rhythms has been attributed to PDH being released from the sinus gland of one light-stimulated eye (122,224). However, as an alternative pathway, large contralateral projections via the protocerebrum exist between eyestalk ganglia of both sides; these large projections are very similar to those of insect PDF-neurons (225)(226)(227)(228) and arise from the large PDH-tracts in the medullae terminales of eyestalks formed by PDH-ir C-type cells in decapods (138,141,229) (arrows in Figure 4A,B) and PGR1-and PGR3-type medulla-associated PDH-ir neurons in the isopod Oniscus asellus (152).…”

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

“…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).…”

“…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). Neurons immunoreactive for ␤-pigment-dispersing factor (PDF) may serve as pacemakers of the clock; some of these neurons with projections to the lamina and selected parts of the midbrain might also serve as outputs of the clock (Reischig et al, 2004), while others with fibers in the anterior and posterior optic commissure transmit coupling information into the contralateral clock (Petri and Stengl, 1997;Reischig et al, 2004). GABA-immunoreactive neurons of the distal tract and allatotropin-related peptides in local interneurons of the AMe are part of the ipsilateral light entrainment pathway, as suggested by injection experiments (Petri et al, 2002).…”