THE MUSCARINIC MODULATION OF [H-3] D-ASPARTATE EFFLUX AND [CA2+](I) LEVELS IN RAT CEREBELLAR GRANULE CELLS

The effects of ACh on [H-3]D-aspartate efflux and on calcium levels ([ Ca2+](i)) were studied at the same time in sister cultures of rat cere bellar granule cells stimulated with electrical pulses (5-20 Hz) or de polarized with KCl (15-40 mM). ACh, 0.3-1000 nM, greatly facilitated t he 10-Hz-evoked tritium efflux while its effect on 20 mM KCl-evoked ef flux was significantly smaller. ACh, 10-1000 nM, enhanced [Ca2+](i) le vels to a limited extent under both experimental conditions. Therefore , ACh facilitation was evident above all on the electrically evoked [H -3]D-aspartate efflux. The ACh-mediated responses depended on the acti vation of M-3-muscarinic receptors since these responses were blocked by 4-DAMP. ACh, 50 mu M, reduced the [Ca2+](i) plateau, determined by prolonged electrical or KCl stimulation. This effect was due to its ac tion of M-2-receptors being blocked by AF-DX 116. In conclusion, at ve ry low concentrations, ACh greatly facilitated the electrically evoked [H-3]D-aspartate efflux through M-3-receptors, while at a higher conc entrations, it inhibited, through M-2-receptors, the rise in [Ca2+](i) caused by prolonged cell depolarization. (C) 1997 Elsevier Science B. V.