Abstract

Elevation of potassium concentrations ([K+]) in the presence of Ca2+ is the most common method of evoking neurotransmitter release from synaptosomes. However, we have been investigating a method of releasing dopamine from synaptosomes that does not involve using elevated [K+]. In this paradigm of neurotransmitter release, dopamine is released from synaptosomes, previously exposed to micromolar or lower [Ca2+], by 1.25 mM Ca2+ in the presence of non-depolarizing [K+] (4.5 mM). The present experiments characterize the Ca2+ channel(s) involved in the Ca2(+)-evoked release of dopamine from synaptosomes, and determine whether the release is mediated by acetylcholine, glutamate or aspartate. omega-Conotoxin (10 nM), which blocks N-, L- and possibly T-type voltage-sensitive Ca2+ channels (VSCC), inhibited the Ca2(+)-evoked [3H]dopamine release from either striatal or olfactory tubercle synaptosomes to less than 50% of control. Neither 1 microM nifedipine nor 1 microM verapamil, which block L-type VSCC, affected Ca2(+)-evoked release. The N- and T-type VSCC blocker neomycin and the nonspecific Ca2+ antagonist, cobalt2+, inhibited release to a greater extent than omega-conotoxin. At 1 mM, both compounds inhibited release to approximately 30% of control. Neither the excitatory neurotransmitter glutamate nor aspartate (2mM) affected 1 microM LY-171555 (a dopamine D2 agonist) inhibition of Ca2(+)-evoked [3H]dopamine release. Also, the glutamate antagonist, glutamic acid diethyl ester, did not affect either Ca2(+)-evoked release or 1 microM LY-171555 inhibition thereof. The nicotinic antagonist hexamethonium (10 microM) and the muscarinic antagonist atropine (1 microM) were also ineffective in inhibiting Ca2(+)-evoked release or LY-171555 inhibition of release.(ABSTRACT TRUNCATED AT 250 WORDS)