Muscarine receptor-mediated modulation of [3H]dopamine and [14C]acetylcholine release from rat neostriatal slices: selective antagonism by gallamine but not pirenzepine.

The [3H]dopamine release induced from superfused rat neostriatal slices by 15 mM K+ was enhanced and that of [14C]acetylcholine was inhibited by oxotremorine in a concentration-dependent manner with a similar EC50 value (0.1 microM). Pirenzepine antagonized the modulatory effects of 1 microM oxotremorine on the release of both neurotransmitters with the same EC50 value (0.3 microM), whereas gallamine up to a concentration of 30 microM antagonized the inhibitory effect of oxotremorine on [14C]acetylcholine release only (EC50: 3 microM). These data strongly suggest that, although these functionally different populations of muscarine receptors cannot be regarded as being pharmacologically different (i.e. as M-1 and M-2 receptors, respectively), autoreceptors may still be selectively modified by drugs such as gallamine possibly acting on binding sites adjacent to the conventional muscarine receptor recognition site.