GABAergic control of Arg-vasopressin release from suprachiasmatic nucleus slice culture.

gamma-Aminobutyric acid (GABA) is contained in many neurons in the suprachiasmatic nucleus (SCN), and is considered to be a circadian entraining factor. Arg-vasopressin (AVP)-containing neurons represent one of the output paths from the SCN to other brain areas. We examined the effects of GABA, muscimol (GABA-A agonist), bicuculline (GABA-A antagonist), baclofen (GABA-B agonist) and phaclofen (GABA-B antagonist) on AVP release using SCN slice preparations in culture. SCN slices were prepared from coronally sliced brain tissue and cultured in organic tissue culture dishes with DMEM/N2 medium in a CO2 (5%) incubator. The culture medium was changed at 3-h intervals until 9 h after 3 h application of each drug. Concentrations of AVP in 1 ml aspirates of the medium were analyzed by EIA. Muscimol (1, 10 microM) increased and bicuculline (1, 10, 100 microM) decreased the AVP release 3-6 h after application. However, baclofen and phaclofen had no apparent effects on AVP release. Riluzole (0.1 mM) and nipecotic acid (1 mM), GABA uptake inhibitors, increased AVP release 3-6 h after application. These results indicate that GABA promotes AVP release mediated by GABA-A receptors in the SCN.