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 cerebellar granule cells stimulated with electrical pulses (5-20 Hz) or depolarized with KCl (15-40 mM). ACh, 0.3-1000 nM, greatly facilitated the 10-Hz-evoked tritium efflux while its effect on 20 mM KCl-evoked efflux was significantly smaller. ACh, 10-1000 nM, enhanced [Ca2+](i) levels 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 activation 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 action of M-2-receptors being blocked by AF-DX 116. In conclusion, at very low concentrations, ACh greatly facilitated the electrically evoked [H-3]D-aspartate efflux through M-3-receptors, while at a higher concentrations, it inhibited, through M-2-receptors, the rise in [Ca2+](i) caused by prolonged cell depolarization. (C) 1997 Elsevier Science B.V.