Piperine-mediated suppression of voltage-dependent Ca2+ influx and glutamate release in rat hippocampal nerve terminals involves 5HT1A receptors and G protein βγ activation
Piperine is the crucial alkaloid component of black pepper (Piper nigrum Linn.) and has neuroprotective effects. Because inhibition of glutamatergic excitatory neurotransmission is a possible mechanism involved in neuroprotection, we investigated the effect of piperine on the 4-aminopyridine (4-AP)-evoked release of glutamate from rat hippocampal synaptosomes. Piperine inhibited 4-AP-evoked glutamate release, and the inhibition was prevented by the chelation of extracellular Ca2+ ions and a vesicular transporter inhibitor. Piperine reduced the 4-AP-evoked elevation of intrasynaptosomal Ca2+ levels but did not affect the synaptosomal membrane potential. In the presence of ω-conotoxin MVIIC, an N- and P/Q-type channel blocker, the piperine-mediated inhibition of 4-AP-evoked glutamate release was markedly reduced; however, dantrolene and CGP37157, which are intracellular Ca2+-release inhibitors, did not alter the piperine effect. In addition, immunocytochemical analysis confirmed the presence of presynaptic 5-hydroxytryptamine 1A (5-HT1A) receptor proteins. The glutamate release-inhibiting effect of piperine was discovered to be prevented by the 5-HT1A receptor antagonist WAY100635 and the G protein βγ subunit inhibitor gallein; however, it was unaffected by the adenylate cyclase inhibitor SQ22536 or the protein kinase A inhibitor PKI622. These results suggest that piperine inhibits glutamate release from rat hippocampal nerve terminals by reducing Ca2+ influx through N- and P/Q-type Ca2+ channels and that the activation of presynaptic 5-HT1A receptors and the G protein βγ subunit is involved in this effect.