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Stimulation of Melatonin Receptors Decreases Calcium Levels in Xenopus Tectal Cells by Activating GABA_C Receptors

¹Neuroscience Program, ²Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, New York; ³Department of Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma; and ⁴Department of Anatomy and Neurobiology, Boston University, Boston Massachusetts

Submitted 15 December 2004; accepted in final form 2 April 2005

To investigate the physiological effects of melatonin receptors in the Xenopus tectum, we have used the fluorescent indicator Fluo-4 AM to monitor calcium dynamics of cells in tectal slices. Bath application of KCl elicited fluorescence increases that were reduced by melatonin. This effect was stronger at the end of the light period than at the end of the dark period. Melatonin increased -aminobutyric acid-C (GABA_C)–receptor activity, as demonstrated by the ability of the GABA_C-receptor antagonists, picrotoxin and TPMPA, to abolish the effects of melatonin. In contrast, neither the GABA_A-receptor antagonist bicuculline nor the GABA_B-receptor antagonist CGP 35348 diminished the effects of melatonin. RT-PCR analyses revealed expression of the 3 known melatonin receptors, MT1 (Mel1_a), MT2 (Mel1_b), and Mel1_c. Because the effect of melatonin on tectal calcium increases was antagonized by an MT2-selective antagonist, 4-P-PDOT, we performed Western blot analyses with an antibody to the MT2 receptor; the data indicate that the MT2 receptor is expressed primarily as a dimeric complex and is glycosylated. The receptor is present in higher amounts at the end of the light period than at the end of the dark period, in a pattern complementary to the changes in melatonin levels, which are higher during the night than during the day. These results imply that melatonin, acting by MT2 receptors, modulates GABA_C receptor activity in the optic tectum and that this effect is influenced by the light–dark cycle.

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