GABA modulates Drosophila circadian clock neurons via GABA_B receptors and decreases in calcium

Circadian clocks play vital roles in the control of daily rhythms in physiology and behavior of animals. In Drosophila, analysis of the molecular and behavioral rhythm has shown that the master clock neurons are entrained by sensory inputs and are synchronized with other clock neurons. However, little is known about the neuronal circuits of the Drosophila circadian system and the neurotransmitters that act on the clock neurons. Here, we provide evidence for a new neuronal input pathway to the master clock neurons, s-LN_vs, in Drosophila that utilizes GABA as a slow inhibitory neurotransmitter. We monitored intracellular calcium levels in dissociated larval s-LN_vs with the calcium-sensitive dye Fura-2. GABA decreased intracellular calcium in the s-LN_vs and blocked spontaneous oscillations in calcium levels. The duration of this response was dose-dependent between 1 nM and 100 μM. The response to GABA was blocked by a metabotropic GABA_B receptor (GABA_B-R) antagonist, CGP54626, but not by an ionotropic receptor antagonist, picrotoxin. The GABA_B-R agonist, 3-APMPA, produced a response similar to GABA. An antiserum against one of the Drosophila GABA_B-Rs (GABA_B-R2) labeled the dendritic regions of the s-LN_vs in both adults and larvae, as well as the dissociated s-LN_vs. We found that some GABAergic processes terminate at the dendrites of the LN_vs, as revealed by GABA immunostaining and a GABA-specific GAL4 line (GAD1-gal4). Our results suggest that the s-LN_vs receive slow inhibitory GABAergic inputs that decrease intracellular calcium of these clock neurons and block their calcium cycling. This response is mediated by postsynaptic GABA_B receptors. © 2005 Wiley Periodicals, Inc. J Neurobiol, 2005