External pH changes affect NMDA-evoked and spontaneous release of cholecystokinin, somatostatin and noradrenaline from rat cerebrocortical nerve endings

It was previously reported that the K +-evoked release of somatostatin-like immunoreactivity (SRIF-LI) and of cholecystokinin-like immunoreactivity (CCK-LI) from superfused rat cerebrocortical synaptosomes can be enhanced by NMDA or d-serine alone. We here studied the effects of extraterminal pH changes on SRIF-LI and CCK-LI release. Lowering pH from 7.4 to 6.9 or 6.4 abolished the effects of NMDA or d-serine on the K +-evoked peptide release. Identical results were obtained when external pH was raised to 8 or 8.7. Sudden alkalinization of the superfusion medium, in absence of K +-depolarization, induced SRIF-LI or CCK-LI release which was insensitive to NMDA. Based on experiments in Ca 2+-free medium and with voltage-sensitive Ca 2+ channel (VSCC) blockers, the pH 8.7-induced release of SRIF-LI and CCK-LI was only in part (30–50%) dependent on external Ca 2+ and Ca 2+ channel activation. In contrast, the alkalinization-evoked release of [ 3 H ]noradrenaline was highly sensitive to external Ca 2+ removal and to blockade of Ca 2+ channels with ω-conotoxins. The pH 8.7-evoked SRIF-LI and CCK-LI was about halved in synaptosomes intoxicated with botulinum toxin C1. The results suggest that the pH-sensitive NMDA receptors mediating somatostatin and cholecystokinin release contain NR1 subunits lacking the exon-5 cassette. Alkalinization represents a novel releasing stimulus which elicits neuropeptide release in part by conventional exocytosis and largely by an external Ca 2+-independent mechanism. Differently, the release of noradrenaline provoked by alkalinization occurs entirely by conventional exocytosis.