GABA modulates day-night variation in melatonin levels in the cerebral ganglia of the damselfly Ischnura graellsii and the grasshopper Oedipoda caerulescens

The relationship between daily rhythms in GABA content and melatonin (MEL) content, as well as the effect of GABA treatment during either the day time and night time phases on MEL levels and N-acetyltransferase (NAT) activity, were studied in the brains of two insect species, the grasshopper Oedipoda caerulescens and the damselfly Ischnura graellsii. In O. caerulescens, levels of GABA in the optic lobes showed significant daily variation, with a marked increase during the light-to-dark transition period. In contrast, in the brain of I. graellssi, two daily peaks in GABA levels were observed, during the light-to-dark and the dark-to-light transition periods. In both insects the maximal levels of GABA occurred 4-6 h in advance of the nocturnal MEL peak, which was associated with a reduction in GABA levels. In both insects, treatment with GABA (1 μg/μl, intracranial injection) during the night was followed by a significant reduction in melatonin levels and NAT activity. In contrast, GABA administered during the day time increased brain MEL levels and synthesis. These data suggest that GABA acts as a modulator of light/dark-dependent melatonin synthesis in the insect brain.