Unexpected inhibitory regulation of glutamate release from rat cerebrocortical nerve terminals by presynaptic 5-hydroxytryptamine-2A receptors

Presynaptic 5-HT2A receptor modulation of glutamate release from rat cerebrocortical nerve terminals (synaptosomes) was investigated by using the 5-HT2A/2C receptor agonist (+/-)-1-[2,5-dimethoxy-4-iodophenyl]-2aminopropane (DOI). DOI potently inhibited 4-aminopyridine (4AP)-evoked glutamate release. Involvement of presynaptic 5-HT2A receptors in this modulation of 4AP-evoked release was confirmed by blockade of the DOI-mediated inhibition by the 5-HT2A receptor antagonist ketanserin but not by the 5-HT2C receptor antagonist RS102221. Inhibition of glutamate release by DOI was associated with a reduction of 4AP-evoked depolarization and downstream elevation of cytoplasmic free calcium concentration ([Ca2+](C)) mediated via P/Q- and N-type voltage-dependent Ca2+ channels (VDCCs). In contrast to the DOI effect on 4AP-evoked release, the agonist had no effect on high external [K+] (30 mM)induced (KCI) stimulation of VDCCs or glutamate release. Likewise, release mediated by direct Ca2+ entry with Ca2+ ionophore (ionomycin) or by hypertonic sucrose was unaffected by DOI. Mechanistically, DOI modulation of 4AP-evoked glutamate release appeared to involve a phospholipase C/protein kinase C signaling cascade, insofar as pretreatment of synaptosomes with the phospholipase C inhibitor U73122 or protein kinase C inhibitors Ro320432 or GF109203X all effectively occluded the inhibitory effect of the agonist. Together, these results suggest that presynaptic 5-HT2A receptors present on glutamatergic terminals effect an unexpected depression of glutamate release by negatively modulating nerve terminal excitability and downstream VDCC activation through a signaling cascade involving phospholipase C/protein kinase C. These observations invoke presynaptic inhibitory 5-HT2A receptor function as a potential target for drugs to mitigate the effects of excessive glutamatergic transmission. (c) 2006 Wiley-Liss, Inc.