CHOLINERGIC MODULATION OF N-METHYL-D-ASPARTATE-EVOKED [H-3] NOREPINEPHRINE RELEASE FROM RAT CORTICAL SLICES

Previous work has shown that N-methyl-D-aspartate (NMDA) receptor activation inhibits muscarinic receptor-mediated phosphoinositide hydrolysis in brain slices. To further explore the potential interactions between NMDA receptors and cholinergic receptors, the effects of cholinergic agonists and NMDA on [H-3] norepinephrine (NE) release from rat cortical slices were determined. Slices were labeled with [H-3]NE, washed and treated with various agonists by transferring the slices through a series of vials at 1-min intervals. Radioactivity remaining in the medium was then quantitated to determine the fractional release of [H-3] NE from the slices. Carbachol (30-3000 muM) slightly stimulated [H-3]NE release from a basal level of 0.10 to approximately 0.35 fractional release by itself and significantly enhanced the effect of 250 muM NMDA (3.6 fractional release for NMDA and 5.3 for carbachol + NMDA) in a concentration-dependent manner. Carbachol (1 mM) increased the maximal response but had no effect on the EC50 of NMDA. Atropine (1 muM) significantly attenuated the effect of carbachol alone and the potentiation of NMDA-evoked [H-3]NE release by carbachol, whereas d-tubocurarine (10 muM) inhibited the effect of carbachol alone but had no effect on the enhancement of the NMDA response by carbachol. Mecamylamine (100 muM) inhibited the effect of carbachol alone, but also inhibited the NMDA-evoked response with an IC50 of 16 muM. The nicotinic agonist, dimethylphenylpiperazinium (DMPP) stimulated [H-3]NE release (approximately 0.4 fractional release at 30 muM) and also potentiated NMDA-stimulated [H-3]NE release (2.0 above NMDA alone). d-Tubocurarine, but not atropine, partially inhibited DMPP-stimulated [H-3]NE release, but neither antagonist altered the enhancement of NMDA-stimulated [H-3]NE release by DMPP. These results suggest that both muscarinic and nicotinic receptor mechanisms enhance the release of [H-3]NE in cortex and that a synergistic interaction occurs between these mechanisms and NMDA receptor-mediated [H-3]NE release in this brain region.