Inhibition of neuronal Ca 2+ influx by gabapentin and subsequent reduction of neurotransmitter release from rat neocortical slices

Cytosolic calcium ion concentrations ([Ca 2+ ] i ) were measured in rat neocortical synaptosomes using fura‐2, and depolarization of synaptosomal membranes was induced by K + (30 m M ). The release of the endogenous excitatory amino acids glutamate and aspartate was evoked by K + (50 m M ) and determined by HPLC. The release of [ 3 H]‐noradrenaline from rat neocortical synaptosomes or slices was evoked by K + (15 and 25 m M ) and measured by liquid scintillation counting. Gabapentin produced a concentration‐dependent inhibition of the K + ‐induced [Ca 2+ ] i increase in synaptosomes (IC 50 =14 μ M ; maximal inhibition by 36%). The inhibitory effect of gabapentin was abolished in the presence of the P/Q‐type Ca 2+ channel blocker ω‐agatoxin IVA, but not by the N‐type Ca 2+ channel antagonist ω‐conotoxin GVIA. Gabapentin (100 μ M ) decreased the K + ‐evoked release of endogenous aspartate and glutamate in neocortical slices by 16 and 18%, respectively. Gabapentin reduced the K + ‐evoked [ 3 H]‐noradrenaline release in neocortical slices (IC 50 =48 μ M ; maximal inhibition of 46%) but not from synaptosomes. 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 [ 3 H]‐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). Gabapentin is concluded to reduce the depolarization‐induced [Ca 2+ ] i increase in excitatory amino acid nerve terminals by inhibiting P/Q‐type Ca 2+ channels; this decreased Ca 2+ 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.British Journal of Pharmacology (2000) 130 , 900–906; doi: 10.1038/sj.bjp.0703380