Vinpocetine inhibits glutamate release induced by the convulsive agent 4-aminopyridine more potently than several antiepileptic drugs.

4-Aminopyridine (4-AP) is a convulsing agent that in vivo preferentially releases Glu, the most important excitatory amino acid neurotransmitter in the brain. Here the ionic dependence of 4-AP-induced Glu release and the effects of several of the most common antiepileptic drugs (AEDs) and of the new potential AED, vinpocetine on 4-AP-induced Glu release were characterized in hippocampus isolated nerve endings pre-loaded with labelled Glu ([3H]Glu). 4-AP-induced [3H]Glu release was composed by a tetrodotoxin (TTX) sensitive and external Ca2+ dependent fraction and a TTX insensitive fraction that was sensitive to the excitatory amino acid transporter inhibitor, TBOA. The AEDs: carbamazepine, phenytoin, lamotrigine and oxcarbazepine at the highest dose tested only reduced [3H]Glu release to 4-AP between 50-60%, and topiramate was ineffective. Vinpocetine at a much lower concentration than the above AEDs, abolished [3H]Glu release to 4-AP. We conclude that the decrease in [3H]Glu release linked to the direct blockade of presynaptic Na+ channels, may importantly contribute to the anticonvulsant actions of all the drugs tested here (except topiramate); and that the significantly greater vinpocetine effect in magnitude and potency on [3H]Glu release when excitability is exacerbated like during seizures, may involve the increase additionally exerted by vinpocetine in some K+ channels permeability.

A role for calcium/calmodulin kinase(s) in the regulation of GABA exocytosis.

A possible role for protein kinases in the regulation of GABA exocytosis in nerve endings was investigated. The effect on the release of the radioactive neurotransmitter ([3H]GABA) from mouse brain synaptosomes of several protein kinase inhibitors was estimated after treatment with 37 mM K+ in the absence of external Na+, a condition under which [3H]GABA release is completely Ca2+ dependent. Among the inhibitors one group inhibit the kinases by the catalytic site (i.e. staurosporine and H7) and others (TFP, sphingosine and W7) act on the regulatory site of protein kinases. The compounds of the second group, which are reported to inhibit calmodulin dependent events and the increase in cytosolic Ca2+ (Cai) induced by high K+ depolarization, were the most efficient inhibitors of [3H]GABA release. The selective inhibitor of CaMPK II, KN-62, also markedly diminished [3]GABA release as well as the increase in Cai induced by high K+. The kinase inhibitors from the first group that are unable to diminish the increase in Cai induced by high K+ were also less efficient inhibitors of [3H]GABA release even at high concentrations. The present results indicate that at the doses tested all the drugs inhibit to some extent the release of the Ca2+ dependent fraction of [3H]GABA perhaps by inhibiting a CaMPK II mediated phosphorylation step triggered by depolarization and facilitated by the elevation of Cai. In addition, the second group of antagonists and KN-62 inhibit the elevation of Cai to high K+ thus exhibiting a higher efficiency on [3H]GABA release than the first group of antagonists.

Dopamine transporter mediated release of dopamine: role of chloride.

Using a rapid (0.5 ml/min) flow rate superfusion system, the dopamine (DA) transporter mediated release of DA is further explored, and compared to the depolarization evoked release of DA in rat striatal synaptosomes preloaded with radioactive DA (3H-DA). In this system external DA in the low microM range efficaciously releases the preloaded transmitter, the maximal response being reached at 3 microM DA. The external DA stimulated release is Ca(2+)-independent, Cl(-)-dependent, and blocked by both bupropion and nomifensine. The atypical antidepressant bupropion inhibits 3H-DA accumulation to rat striatal synaptosomes with a calculated IC50 of 1.3 x 10(-6) M. Among DA uptake blockers some are known to act as DA releasing agents. Here we found that the DA uptake blocker nomifensine (30 microM) is unable to modify the baseline release of 3H-DA, whereas bupropion (10 microM) clearly elevates the baseline release of 3H-DA in a Ca(2+)-independent and Cl(-)-dependent manner. The non releasing agent nomifensine blocks the release of 3H-DA induced by bupropion. The Ca(2+)-dependent, high K+ depolarization evoked release of 3H-DA is not modified by nomifensine and does not depend on the external Cl- concentration. When the depolarizing medium contains DA the carrier mediated release of 3H-DA induced by the external DA is additive to the high K+ induced response. A drastic drop in the external Cl- concentration induces 3H-DA release. This release of 3H-DA induced by low external Cl- levels is completely blocked by nomifensine, which only slightly diminished the release of 3H-DA induced by the absence of external Na+. On the basis of these results, it is concluded that: 1) Rapid perfusion flow rates eliminate DA reuptake. 2) DA uptake inhibitors either with or without DA releasing capabilities block the release of DA induced by microM levels of external DA. 3) By preventing translocation of the DA transporter mobile moiety, nomifensine may inhibit the release of DA induced by external DA or bupropion and by drastic drops in the external Cl- concentration. 4) In the absence of nomifensine, the DA transporter works under both resting and depolarized conditions, but in contrast to the GABA transporter (Sitges et al.: Neurochem Res 18:1081-1087, 1993), the DA transporter does not contribute to the amount of the DA released by depolarization. 5) Reversal of the DA uptake carrier is favored by conditions increasing the internal DA levels. 6) Cl- rather than Na+ is a major determinant in 3H-DA movements through the DA transporter.