Inhibition of neuronal Ca2+ influx by gabapentin and subsequent reduction of neurotransmitter release from rat neocortical slices

Cytosolic calcium ion concentrations ([Ca2+](i)) were measured in rat neocortical synaptosomes using fura-2, and depolarization of synaptosomal membranes was induced by K+ (30 mM). The release of the endogenous excitatory. amino acids glutamate and aspartate was evoked by K+ (50 mM) and determined by HPLC. The release of [H-3]-noradernaline from rat neocortical synaptosomes or slices was evoked by K+ (15 and 25 mM) and measured by liquid scintillation counting. 2 Gabapentin produced a concentration-dependent inhibition of the K+-induced[Ca2+](i) increase in synaptosomes (IC50 = 14 mu M; maximal inhibition by 36%). The inhibitory effect of gabapentin was abolished in the presence of the P/Q-type Ca2+ channel blocker omega-agatoxin IVA, but not by the N-type Ca2+ channel antagonist omega-conotoxin GVIA. 3 Gabapentin (100 mu M) decreased the K+-evoked release of endogenous aspartate and glutamate in neocortical slices by 16 and 18% respectively. 4 Gabapentin reduced the K+-evoked [H-3]-noradrenaline release in neocortical slices (IC50 = 48 mu M; maximal inhibition of 46%) but not from synaptosomes. 5 In the presence of the AMPA receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 2.3-dioxo-6-nitro-1,2,3,4-tetrahydro[f]quinoxaline-7-sulphonamide (NBQX), gabapentin did not reduce [H-3]-noradrenaline release. Gabapentin did, however, cause inhibition in the presence of the NMDA receptor antagonist DL-(E)-2-amino-4-methyl-5-phosphono-3-pentanoic acid (CGP 37849). 6 Gabapentin is concluded to reduce the depolarizatian-induced [Ca2+](i) increase in excitatory amino acid nerve terminals by inhibiting P/Q-type Ca2+ channels; this decreased Ca2+ influx subsequently attenuates K+-evoked excitatory amino acid release. The latter effect leads to a reduced activation of AMPA receptors which contribute to K+-evoked noradrenaline release from noradrenergic varicosities, resulting in an indirect inhibition of noradrenaline release.