Acta Pharmacologica Sinica 2008 November; 29 (11): 1289-1295; doi: 10.1111/j.1745-7254.2008.00882.x

Aim: Excessive glutamate release has been proposed to be involved in the pathogenesis of several neurological diseases. In this study, we investigated the effect of HDT-1 (3, 4, 4a, 5, 8, 8a-hexahydro-6,7-dimethyl-4a-(phenylsulfonyl)-2-tosylisoquinolin-1(2H)-one), a novel synthetic compound, on glutamate release in rat cerebrocortical nerve terminals and explored the possible mechanism.

Methods: The release of glutamate was evoked by the K⁺ channel blocker 4-aminopyridine (4-AP) or the high external [K⁺] and measured by one-line enzyme-coupled fluorometric assay. We also determined the loci at which HDT-1 impinges on cerebrocortical nerve terminals by using membrane potential-sensitive dye to assay nerve terminal excitability and depolarization, and Ca²⁺ indicator Fura-2 to monitor Ca²⁺ influx.

Results: HDT-1 inhibited the release of glutamate evoked by 4-AP and KCl in a concentration-dependent manner. HDT-1 did not alter the resting synaptosomal membrane potential or 4-AP-evoked depolarization. Examination of the effect of HDT-1 on cytosolic [Ca²⁺] revealed that the diminution of glutamate release could be attributed to reduction in voltage-dependent Ca²⁺ influx. Consistent with this, the HDT-1-mediated inhibition of glutamate release was significantly prevented in synaptosomes pretreated with the N- and P/Q-type Ca²⁺ channel blocker ω-conotoxin MVIIC.

Conclusion: In rat cerebrocortical nerve terminals, HDT-1 inhibits glutamate release through a reduction of voltage-dependent Ca²⁺ channel activity and subsequent decrease of Ca²⁺ influx into nerve terminals, rather than any upstream effect on nerve terminal excitability.

Keywords: isoquinoline alkaloid; neuroprotection; glutamate release; voltage-dependent Ca²⁺ channel; synaptosome; cerebral cortex