RELATION OF [CA2+](I) TO DOPAMINE RELEASE IN STRIATAL SYNAPTOSOMES - ROLE OF CA2+ CHANNELS

We compared the effects of KCl and 4-aminopyridine (4-AP) stimulation on the coupling of Ca2+ channel activation to [H-3]dopamine ([H-3]DA) release in rat striatal synaptosomes and used specific Ca2+ channel blockers to discriminate between the different VSCC's activated by the two stimulatory agents. We found that whereas [H-3]DA release is strictly Ca2+-dependent in the case of KCl depolarization, 4-AP, at concentrations above 100 mu M, progressively causes a large Ca2+-independent release of [H-3]DA. Thus, at 1 to 3 mM 4-AP, as much as 80-95% of the [H-3]DA release is Ca2+-independent and can be partially blocked by nomifensine, indicating that some [H-3]DA release is occurring through reversal of the DA carrier. Therefore, in the studies relating [Ca2+](i) to [H-3]DA release we selected 4-AP concentrations lower than 100 mu M and corrected for the Ca2+-independent release. Under these conditions, we determined that: (1) Ca2+ entry through N-type VSCC's is involved in [H-3]DA release both in the case of KCl depolarization (35% inhibition by omega-CgTx) and in 4-AP stimulation (23% inhibition by omega-CgTx); (2) Ca2+ entering through P-type and/or Q-type VSCC's is also involved in [H-3]DA release due to 4-AP stimulation (26% inhibition by 200 nM omega-Aga IVA); (3) Neomycin (0.35 mM) inhibited the [H-3]DA release due to 4-AP stimulation by about 20% and decreased the KCl induced [H-3]DA release by 55%; the effects of neomycin (0.35 mM) and omega-CgTx were additive in both cases, indicating that, at this concentration, the antibiotic does not affect significantly N-type Ca2+ channels; (4) When applied together, omega-CgTx and omega-Aga IVA inhibited the 4-AP stimulated [H-3]DA release by about 40-50%, suggesting that the remaining large fraction of the VSCC's activated by 4-AP stimulation are non-N, non-P VSCC's and are coupled to Ca2+-dependent [H-3]DA release; (5) The contribution of L-type VSCC's is uncertain, since there seemed to be a small contribution in the case of KCl depolarization, but not in the case of 4-AP stimulation. On the whole, the results suggest that the release of [H-3]DA in the rat striatal nerve terminals depends on Ca2+ entry through N-, P-, possibly Q-, and other non-N-, non-P-type VSCC's when either KCl or 4-AP stimulation is utilized.