Involvement of N- and P/Q- but not L- or T-type voltage-gated calcium channels in ischaemia-induced striatal dopamine release in vitro

Abstract

Calcium influx and transmitter efflux are central events in the neuropathological cascade that occurs during and following cerebral ischaemia. This study explored the role of voltage-gated calcium channels (VGCCs) in ischaemia-induced striatal dopamine (DA) release in vitro. Slices (350 μm thickness) of rat neostriatum were superfused (400 ml/h) with an artificial cerebrospinal fluid (aCSF) at 34°C and subjected to episodes of `ischaemia' by reduction of the glucose concentration from 4 to 2 mM and gassing with 95% N 2/5% CO 2. DA release was monitored with fast cyclic voltammetry at implanted carbon fibre microelectrodes. The time to onset, time to peak, rate and magnitude of DA release were measured. Non-selective blockade of VGCCs with a high concentration of Ni 2+ (2.5 mM), markedly delayed ( P<0.01) and slowed ( P<0.05) DA release but preferential blockade of T-type VGCCs with a lower concentration (200 μM) had no effect. DA release was also unaffected by selective antagonism of L-type VGCCs with nimodipine and nicardipine (10 μM each). Selective blockade of N-type VGCCs with ω-conotoxin GVIA (100 nM) delayed DA release ( P<0.05) but did not affect its rate or magnitude. Blockade of P- and possibly Q-type VGCCs with ω-agatoxin IVA (up to 200 nM) both delayed ( P<0.05) and slowed ( P<0.05) DA release. Preferential blockade of P- type VGCCs with neomycin (500 μM) also delayed ( P<0.05) and slowed ( P<0.05) DA release. These findings suggest that N-, P- and possibly Q- but not L- or T-type VGCCs mediate ischaemia-induced DA release. Although it is not possible to say, on the basis of these results, that the effects are directly upon the dopamine terminals, these calcium channels nevertheless constitute promising targets for therapeutic intervention.