Therapeutically relevant concentrations of neomycin selectively inhibit P-type Ca2+ channels in rat striatum

The effects of neomycin on voltage-activated Ca(2+) channels (VACCs) were studied by Ca(2+)-dependent K(+)- and veratridine-evoked [3H]dopamine release from rat striatal slices. Neomycin (0.01-1 mM) concentration dependently reduced K(+)-evoked [3H]dopamine release (IC(50) approximately 25 microM), producing approximately 98% inhibition at 1 mM. Contribution of N-, P- and Q-type Ca(2+) channels to this neomycin-sensitive [3H]dopamine release was tested by the combined application of 100 microM neomycin and selective Ca(2+) channel blockers. The effects of neomycin combined with 1 microM of omega-conotoxin GVIA (N-type Ca(2+) channels) or with 100 nM of omega-conotoxin MVIIC (Q-type Ca(2+) channels) were additive, excluding involvement of N- and Q-type Ca(2+) channels. However, the combined effects of neomycin with 30 nM of omega-agatoxin-IVA (P-type Ca(2+) channels) were not additive, suggesting involvement of P-type Ca(2+) channels in neomycin-induced inhibition of [3H]dopamine release. On the other hand, veratridine-evoked [3H]dopamine release was shown to be mediated by Q-type Ca(2+) channels only. In addition, neither the inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase thapsigargin (500 nM) nor the blocker of sarcoplasmic reticulum ryanodine Ca(2+) channels ryanodine (30 microM) modulate veratridine-evoked [3H]dopamine release, suggesting no contribution of intracellular Ca(2+) stores. Neomycin (up to 100 microM) did not affect veratridine-evoked [3H]dopamine release, suggesting that intracellular Ca(2+) stores are not a prerequisite for the action of neomycin. Lack of inhibitory effect of neomycin is taken as additional indirect evidence for the involvement of P-type Ca(2+) channels. In conclusion, therapeutically relevant concentrations of neomycin preferentially block P-type Ca(2+) channels which regulate dopamine release in rat striatum. This block could be responsible for aminoglycoside-induced toxicity.