Circadian pacemaker neurons of the Madeira cockroach are inhibited and activated by GABAA and GABAB receptors.

GABA is the most abundant neurotransmitter in the circadian pacemaker circuits of mammals and insects. In the Madeira cockroach the accessory medulla (AME) in the brain's optic lobes is the circadian clock that orchestrates rest-activity rhythms in synchrony with light dark cycles. Three prominent GABAergic tracts connect the AME to termination sites of compound eye photoreceptors in the lamina and medulla. Parallel GABAergic light entrainment pathways were suggested to either advance or delay the clock for adjustment to changing photoperiods. In agreement with this hypothesis GABA activated or inhibited AME clock neurons, allowing for the distinction of three different GABA response types. Here, we examined which GABA receptors are responsible for these response types. We found that both ionotropic GABAA receptors and metabotropic GABAB receptors were expressed in AME clock cells. Via different signalling pathways, either one of them could account for all three GABA response types. The muscimol-dependently activated GABAA receptor formed a chloride channel, while the SKF 97541-dependently activated GABAB receptor signalled via G-proteins, apparently targeting potassium channels. Expression of chloride exporters or importers determined whether GABAA receptor activation hyper- or depolarized AME neurons. For GABAB receptor responses second messenger gated channels present in the clock cells appeared to decide about the polarity of the GABA response. In summary, circadian clock neurons co-expressed inhibitory and/or excitatory GABAA and GABAB receptors in various combinations, while cotransporter expression and the set of second messenger gated ion channels present allowed for distinct signalling in different clock neurons.

Time-Dependent Trapping of Pollinators Driven by the Alignment of Floral Phenology with Insect Circadian Rhythms.

Several evolutionary lineages in the early divergent angiosperm family Annonaceae possess flowers with a distinctive pollinator trapping mechanism, in which floral phenological events are very precisely timed in relation with pollinator activity patterns. This contrasts with previously described angiosperm pollinator traps, which predominantly function as pitfall traps. We assess the circadian rhythms of pollinators independently of their interactions with flowers, and correlate these data with detailed assessments of floral phenology. We reveal a close temporal alignment between patterns of pollinator activity and the floral phenology driving the trapping mechanism (termed 'circadian trapping' here). Non-trapping species with anthesis of standard duration (c. 48 h) cannot be pollinated effectively by pollinators with a morning-unimodal activity pattern; non-trapping species with abbreviated anthesis (23-27 h) face limitations in utilizing pollinators with a bimodal circadian activity; whereas species that trap pollinators (all with short anthesis) can utilize a broader range of potential pollinators, including those with both unimodal and bimodal circadian rhythms. In addition to broadening the range of potential pollinators based on their activity patterns, circadian trapping endows other selective advantages, including the possibility of an extended staminate phase to promote pollen deposition, and enhanced interfloral movement of pollinators. The relevance of the alignment of floral phenological changes with peaks in pollinator activity is furthermore evaluated for pitfall trap pollination systems.