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 [3H]dopamine ([3H]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 [3H]DA release is strictly Ca(2+)-dependent in the case of KCl depolarization, 4-AP, at concentrations above 100 microM, progressively causes a large Ca(2+)-independent release of [3H]DA. Thus, at 1 to 3 mM 4-AP, as much as 80-95% of the [3H]DA release is Ca(2+)-independent and can be partially blocked by nomifensine, indicating that some [3H]DA release is occurring through reversal of the DA carrier. Therefore, in the studies relating [Ca2+]i to [3H]DA release we selected 4-AP concentrations lower than 100 microM and corrected for the Ca(2+)-independent release. Under these conditions, we determined that: (1) Ca2+ entry through N-type VSCC's is involved in [3H]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 [3H]DA release due to 4-AP stimulation (26% inhibition by 200 nM omega-Aga IVA); (3) Neomycin (0.35 mM) inhibited the [3H]DA release due to 4-AP stimulation by about 20% and decreased the KCl induced [3H]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 [3H]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 Ca(2+)-dependent [3H]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 [3H]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.
Carvalho
C M
CM
Departamento de Zoologia, Universidade de Coimbra, Portugal.
Ferreira
I L
IL
Duarte
C B
CB
Malva
J O
JO
Tretter
L
L
Adam-Vizi
V
V
Carvalho
A P
AP
D015102:3,4-Dihydroxyphenylacetic Acid / Q000378:metabolism; D015761:4-Aminopyridine / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D003342:Corpus Striatum / Q000187:drug effects* / Q000378:metabolism; D004298:Dopamine / Q000378:metabolism*; D008297:Male; D008564:Membrane Potentials; D009568:Nitrendipine / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D013574:Synaptosomes / Q000502:physiology*
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D015102:3,4-Dihydroxyphenylacetic Acid; D011189:Potassium Chloride; D009568:Nitrendipine; D015761:4-Aminopyridine; D002118:Calcium; D004298:Dopamine