Examination of the role of FMRFamide-related peptides in the circadian clock of the cockroach Leucophaea maderae

Distribution and daily oscillation of GABA in the circadian system of the cockroach Rhyparobia maderae

Gamma-aminobutyric acid (GABA) is the prevalent inhibitory neurotransmitter in nervous systems promoting sleep in both mammals and insects. In the Madeira cockroach, sleep-wake cycles are controlled by a circadian clock network in the brain's optic lobes, centered in the accessory medulla (AME) with its innervating pigment-dispersing factor (PDF) expressing clock neurons at the anterior-ventral rim of the medulla. GABA is present in cell clusters that innervate different circuits of the cockroach's AME clock, without colocalizing in PDF clock neurons. Physiological, immunohistochemical, and behavioral assays provided evidence for a role of GABA in light entrainment, possibly via the distal tract that connects the AME's glomeruli to the medulla. Furthermore, GABA was implemented in clock outputs to multiple effector systems in optic lobe and midbrain. Here, GABAergic brain circuits were analyzed further, focusing on the circadian system in search for sleep/wake controlling brain circuits. All GABA-immunoreactive neurons of the cockroach brain were also stained with an antiserum against the GABA-synthesizing enzyme glutamic acid decarboxylase. We found strong overlap of the distribution of GABA-immunoreactive networks with PDF clock networks in optic lobes and midbrain. Neurons in five of the six soma groups that innervate the clock exhibited GABA immunoreactivity. The intensity of GABA immunoreactivity in the distal tract showed daily fluctuations with maximum staining intensity in the middle of the day and weakest staining at the end of the day. Quantification via enzyme-linked immunosorbent assay and quantitative liquid chromatography coupled to electrospray ionization tandem mass spectrometry, likewise, showed higher GABA levels in the optic lobe during the inactivity phase of the cockroach during the day and lower levels during its activity phase at dusk. Our data further support the hypothesis that light- and PDF-dependently the circadian clock network of the cockroach controls GABA levels and thereby promotes sleep during the day.

Candidates for photic entrainment pathways to the circadian clock via optic lobe neuropils in the Madeira cockroach

The compound eye of cockroaches is obligatory for entrainment of the Madeira cockroach's circadian clock, but the cellular nature of its entrainment pathways is enigmatic. Employing multiple-label immunocytochemistry, histochemistry, and backfills, we searched for photic entrainment pathways to the accessory medulla (AME), the circadian clock of the Madeira cockroach. We wanted to know whether photoreceptor terminals could directly contact pigment-dispersing factor-immunoreactive (PDF-ir) circadian pacemaker neurons with somata in the lamina (PDFLAs) or somata next to the AME (PDFMEs). Short green-sensitive photoreceptor neurons of the compound eye terminated in lamina layers LA1 and LA2, adjacent to PDFLAs and PDFMEs that branched in LA3. Long UV-sensitive compound eye photoreceptor neurons terminated in medulla layer ME2 without direct contact to ipsilateral PDFMEs that arborized in ME4. Multiple neuropeptide-ir interneurons branched in ME4, connecting the AME to ME2. Before, extraocular photoreceptors of the lamina organ were suggested to send terminals to accessory laminae. There, they overlapped with PDFLAs that mostly colocalized PDF, FMRFamide, and 5-HT immunoreactivities, and with terminals of ipsi- and contralateral PDFMEs. We hypothesize that during the day cholinergic activation of the largest PDFME via lamina organ photoreceptors maintains PDF release orchestrating phases of sleep-wake cycles. As ipsilateral PDFMEs express excitatory and contralateral PDFMEs inhibitory PDF autoreceptors, diurnal PDF release keeps both PDF-dependent clock circuits in antiphase. Future experiments will test whether ipsilateral PDFMEs are sleep-promoting morning cells, while contralateral PDFMEs are activity-promoting evening cells, maintaining stable antiphase via the largest PDFME entrained by extraocular photoreceptors of the lamina organ.

GABA- and serotonin-expressing neurons take part in inhibitory as well as excitatory input pathways to the circadian clock of the Madeira cockroach Rhyparobia maderae

In the Madeira cockroach, pigment-dispersing factor-immunoreactive (PDF-ir) neurons innervating the circadian clock, the accessory medulla (AME) in the brain's optic lobes, control circadian behaviour. Circadian activity rhythms are entrained to daily light-dark cycles only by compound eye photoreceptors terminating in the lamina and medulla. Still, it is unknown which neurons connect the photoreceptors to the clock to allow for light entrainment. Here, we characterized by multiple-label immunocytochemistry the serotonin (5-HT)-ir anterior fibre fan and GABA-ir pathways connecting the AME- and optic lobe neuropils. Colocalization of 5-HT with PDF was confirmed in PDF-ir lamina neurons (PDFLAs). Double-labelled fibres were traced to the AME originating from colabelled PDFLAs branching in accessory laminae and proximal lamina. The newly discovered GABA-ir medial layer fibre tract connected the AME to the medulla's medial layer fibre system, and the distal tract fibres connected the AME to the medulla. With Ca²⁺ imaging on primary cell cultures of the AME and with loose-patch-clamp recordings in vivo, we showed that both neurotransmitters either excite or inhibit AME clock neurons. Because we found no colocalization of GABA and 5-HT in any optic lobe neuron, GABA- and 5-HT neurons form separate clock input circuits. Among others, both pathways converged also on AME neurons that coexpressed mostly inhibitory GABA- and excitatory 5-HT receptors. Our physiological and immunocytochemical studies demonstrate that GABA- and 5-HT-immunoreactive neurons constitute parallel excitatory or inhibitory pathways connecting the circadian clock either to the lamina or medulla where photic information from the compound eye is processed.

Ecdysis behaviors and circadian rhythm of ecdysis in the stick insect, Carausius morosus

Successful ecdysis in insects depends on proper timing and sequential activation of an elaborate series of motor programs driven by a relatively conserved network of neuropeptides. The behaviors must be activated at the appropriate times to ensure successful loosening and shedding of the old cuticle, and can be influenced by environmental cues in the form of immediate sensory feedback and by circadian rhythms. We assessed the behaviors, components of the neural network and the circadian basis of ecdysis in the stick insect, Carausius morosus. C. morosus showed many of the characteristic pre-ecdysis and ecdysis behaviors previously described in crickets and locusts. Ecdysis was described in three phases, namely the (i) preparatory or pre-ecdysis phase, (ii) the ecdysial phase, and (iii) the post-ecdysis or exuvial phase. The frequencies of push-ups and sways during the preparatory phase were quantified as well as durations of all the phases. The regulation of ecdysis appeared to act via elevation of cGMP, as described in many other insects, although eclosion hormone-like immunoreactivity was not noted using a lepidopteran antiserum. Finally, C. morosus showed a circadian rhythm to the onset of ecdysis, with ecdysis occurring just prior to or at lights on. Ecdysis could be induced precociously with mechanical stimulation.

Calcium responses of circadian pacemaker neurons of the cockroach Rhyparobia maderae to acetylcholine and histamine

The accessory medulla (aMe) is the pacemaker that controls circadian activity rhythms in the cockroach Rhyparobia maderae. Not much is known about the classical neurotransmitters of input pathways to the cockroach circadian system. The circadian pacemaker center receives photic input from the compound eye, via unknown excitatory and GABAergic inhibitory entrainment pathways. In addition, neuropeptidergic inputs couple both pacemaker centers. A histamine-immunoreactive centrifugal neuron connects the ventral aMe with projection areas in the lateral protocerebrum and may provide non-photic inputs. To identify neurotransmitters of input pathways to the circadian clock with Fura-2-dependent Ca(2+) imaging, primary cell cultures of the adult aMe were stimulated with acetylcholine (ACh), as the most prominent excitatory, and histamine, as common inhibitory neurotransmitter. In most of aMe neurons, ACh application caused dose-dependent increases in intracellular Ca(2+) levels via ionotropic nicotinic ACh receptors. These ACh-dependent rises in Ca(2+) were mediated by mibefradil-sensitive voltage-activated Ca(2+) channels. In contrast, histamine application decreased intracellular Ca(2+) levels in only a subpopulation of aMe cells via H2-type histamine receptor chloride channels. Thus, our data suggest that ACh is part of the light entrainment pathway while histamine is involved in a non-photic input pathway to the ventral circadian clock of the Madeira cockroach.

Circadian rhythms and endocrine functions in adult insects

Many behavioral and physiological processes in adult insects are influenced by both the endocrine and circadian systems, suggesting that these two key physiological systems interact. We reviewed the literature and found that experiments explicitly testing these interactions in adult insects have only been conducted for a few species. There is a shortage of measurements of hormone titers throughout the day under constant conditions even for the juvenile hormones (JHs) and ecdysteroids, the best studied insect hormones. Nevertheless, the available measurements of hormone titers coupled with indirect evidence for circadian modulation of hormone biosynthesis rate, and the expression of genes encoding proteins involved in hormone biosynthesis, binding or degradation are consistent with the hypothesis that the circulating levels of many insect hormones are influenced by the circadian system. Whole genome microarray studies suggest that the modulation of farnesol oxidase levels is important for the circadian regulation of JH biosynthesis in honey bees, mosquitoes, and fruit flies. Several studies have begun to address the functional significance of circadian oscillations in endocrine signaling. The best understood system is the circadian regulation of Pheromone Biosynthesis Activating Neuropeptide (PBAN) titers which is important for the temporal organization of sexual behavior in female moths. The evidence that the circadian and endocrine systems interact has important implications for studies of insect physiology and behavior. Additional studies on diverse species and physiological processes are needed for identifying basic principles underlying the interactions between the circadian and endocrine systems in insects.

Ca²⁺-dependent ion channels underlying spontaneous activity in insect circadian pacemaker neurons

Electrical activity in the gamma frequency range is instrumental for temporal encoding on the millisecond scale in attentive vertebrate brains. Surprisingly, also circadian pacemaker neurons in the cockroach Rhyparobia maderae (Leucophaea maderae) employ fast spontaneous rhythmic activity in the gamma band frequency range (20-70  Hz) together with slow rhythmic activity. The ionic conductances controlling this fast spontaneous activity are still unknown. Here, Ca(2+) imaging combined with pharmacology was employed to analyse ion channels underlying spontaneous activity in dispersed circadian pacemakers of the adult accessory medulla, which controls circadian locomotor activity rhythms. Fast spontaneous Ca(2+) transients in circadian pacemakers accompany tetrodotoxin (TTX)-blockable spontaneous action potentials. In contrast to vertebrate pacemakers, the spontaneous depolarisations from rest appear to be rarely initiated via TTX-sensitive sustained Na(+) channels. Instead, they are predominantly driven by mibefradil-sensitive, low-voltage-activated Ca(2+) channels and DK-AH269-sensitive hyperpolarisation-activated, cyclic nucleotide-gated cation channels. Rhythmic depolarisations activate voltage-gated Na(+) channels and nifedipine-sensitive high-voltage-activated Ca(2+) channels. Together with Ca(2+) rises, the depolarisations open repolarising small-conductance but not large-conductance Ca(2+) -dependent K(+) channels. In contrast, we hypothesise that P/Q-type Ca(2+) channels coupled to large-conductance Ca(2+) -dependent K(+) channels are involved in input-dependent activity.

Light Affects the Branching Pattern of Peptidergic Circadian Pacemaker Neurons in the Brain of the Cockroach Leucophaea maderae

Pigment-dispersing factor–immunoreactive neurons anterior to the accessory medulla (aPDFMes) in the optic lobes of insects are circadian pacemaker neurons in cockroaches and fruit flies. The authors examined whether any of the aPDFMes of the cockroach Leucophaea maderae are sensitive to changes in period and photoperiod of light/dark (LD) cycles as a prerequisite to adapt to changes in external rhythms. Cockroaches were raised in LD cycles of 11:11, 13:13, 12:12, 6:18, or 18:6 h, and the brains of the adults were examined with immunocytochemistry employing antisera against PDF and orcokinin. Indeed, in 11:11 LD cycles, only the number of medium-sized aPDFMes specifically decreased, while it increased in 13:13. In addition, 18:6 LD cycles increased the number of large- and medium-sized aPDFMes, as well as the posterior pPDFMes, while 6:18 LD cycles only decreased the number of medium-sized aPDFMes. Furthermore, PDF-immunoreactive fibers in the anterior optic commissure and orcokinin-immunoreactive fibers in both the anterior and posterior optic commissures were affected by different lengths of light cycles. Thus, apparently different groups of the PDFMes, most of all the medium-sized aPDFMes, which colocalize orcokinin, respond to changes in period and photoperiod and could possibly allow for the adjustment to different photoperiods.

Circadian pacemaker coupling by multi-peptidergic neurons in the cockroach Leucophaea maderae

Lesion and transplantation studies in the cockroach, Leucophaea maderae, have located its bilaterally symmetric circadian pacemakers necessary for driving circadian locomotor activity rhythms to the accessory medulla of the optic lobes. The accessory medulla comprises a network of peptidergic neurons, including pigment-dispersing factor (PDF)-expressing presumptive circadian pacemaker cells. At least three of the PDF-expressing neurons directly connect the two accessory medullae, apparently as a circadian coupling pathway. Here, the PDF-expressing circadian coupling pathways were examined for peptide colocalization by tracer experiments and double-label immunohistochemistry with antisera against PDF, FMRFamide, and Asn¹³-orcokinin. A fourth group of contralaterally projecting medulla neurons was identified, additional to the three known groups. Group one of the contralaterally projecting medulla neurons contained up to four PDF-expressing cells. Of these, three medium-sized PDF-immunoreactive neurons coexpressed FMRFamide and Asn¹³-orcokinin immunoreactivity. However, the contralaterally projecting largest PDF neuron showed no further peptide colocalization, as was also the case for the other large PDF-expressing medulla cells, allowing the easy identification of this cell group. Although two-thirds of all PDF-expressing medulla neurons coexpressed FMRFamide and orcokinin immunoreactivity in their somata, colocalization of PDF and FMRFamide immunoreactivity was observed in only a few termination sites. Colocalization of PDF and orcokinin immunoreactivity was never observed in any of the terminals or optic commissures. We suggest that circadian pacemaker cells employ axonal peptide sorting to phase-control physiological processes at specific times of the day.

A review of FMRFamide- and RFamide-like peptides in metazoa

Neuropeptides are a diverse class of signalling molecules that are widely employed as neurotransmitters and neuromodulators in animals, both invertebrate and vertebrate. However, despite their fundamental importance to animal physiology and behaviour, they are much less well understood than the small molecule neurotransmitters. The neuropeptides are classified into families according to similarities in their peptide sequence; and on this basis, the FMRFamide and RFamide-like peptides, first discovered in molluscs, are an example of a family that is conserved throughout the animal phyla. In this review, the literature on these neuropeptides has been consolidated with a particular emphasis on allowing a comparison between data sets in phyla as diverse as coelenterates and mammals. The intention is that this focus on the structure and functional aspects of FMRFamide and RFamide-like neuropeptides will inform understanding of conserved principles and distinct properties of signalling across the animal phyla.

RFamide-related peptide and its cognate receptor in the sheep: cDNA cloning, mRNA distribution in the hypothalamus and the effect of photoperiod

Photoperiodic responses enable animals to adapt their physiology to predictable patterns of seasonal environmental change. In mammals, this depends on pineal melatonin secretion and effects in the hypothalamus, but the cellular and molecular substrates of its action are poorly understood. The recent identification of a mammalian orthologue of the avian gonadotrophin-inhibitory hormone gene has led to interest in its possible involvement in seasonal breeding. In long-day breeding Syrian hamsters, hypothalamic RFamide-related peptide (RFRP) expression is increased by exposure to long photoperiod. Because, opposite to hamsters, sheep are short-day breeders, we predicted that a conserved role in mammalian reproductive activation would decrease RFRP expression in sheep under a long photoperiod. We cloned the ovine RFRP cDNA and examined its expression pattern in Soay sheep acclimated to a 16 : 8 h or 8 : 16 h light /dark cycle (LP and SP, respectively). RFRP was expressed widely in the sheep hypothalamus and increased modestly overall with exposure to LP. Interestingly, RFRP expression in the ependymal cells surrounding the base of the third ventricle was highly photoperiodic, with levels being undetectable in animals held on SP but consistently high under LP. These data are inconsistent with a conserved reproductive role for RFRP across mammals. Additionally, we cloned the ovine homologue of the cognate RFRP receptor, rfr-2 (NPFF1) and found localised expression in the suprachiasmatic nuclei and in the pars tuberalis. Taken together, these data strengthen the emerging view that interplay between ependymal cells and the pars tuberalis might be important for the seasonal timing system.