Ca2+ changes induced by different presynaptic nicotinic receptors in separate populations of individual striatal nerve terminals

Presynaptic nicotinic acetylcholine receptors likely play a modulatory role in the nerve terminal. Using laser-scanning confocal microscopy, we have c haracterized physiological responses obtained on activation of presynaptic nicotinic receptors by measuring calcium changes in individual nerve termin als (synaptosomes) isolated from the rat corpus striatum. Nicotine (500 nM) induced Ca2+ changes in a subset (10-25%) of synaptosomes. The Ca2+ respon ses were dependent on extracellular Ca2+ and desensitized very slowly (seve ral minutes) on prolonged exposure to agonist. The nicotine-induced Ca2+ re sponses were dose-dependent and were completely blocked by dihydro-beta -er ythroidine (5 muM), differentially affected by mecamylamine (10 muM) and al pha -conotoxin MII (100 nM), and not affected by alpha -bungarotoxin (500 n M). Immunocytochemical studies using well-characterized monoclonal antibodi es revealed the presence of the alpha4 and alpha3/alpha5 nicotinic subunits , The nicotine-induced responses were unaffected by prior depolarization or by a mixture of Ca2+ channel toxins including omega -conotoxin MVIIC (500 nM), omega -conotoxin GVIA (500 nM) and agatoxin TK (200 nM). Our results i ndicate that nicotinic receptors present on striatal nerve terminals induce Ca2+ entry largely without involving voltage-gated Ca2+ channels, most lik ely by direct permeation via the receptor channel itself, In addition, at l east two subpopulations of presynaptic nicotinic receptors reside on separa te terminals in the striatum, suggesting distinct modulatory roles.