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 Ca2+-free medium and with voltage-sensitive Ca2+ channel (VSCC) blockers, the pH 8.7-induced release of SRIF-LI and CCK-LI was only in part (30-50%) dependent on external Ca2+ and Ca2+ channel activation. In contrast, the alkalinization-evoked release of [3H]noradrenaline was highly sensitive to external Ca2+ removal and to blockade of Ca2+ channels with omega-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 Ca2+-independent mechanism. Differently, the release of noradrenaline provoked by alkalinization occurs entirely by conventional exocytosis.
Gemignani
Anita
A
Sezione di Farmacologia e Tossicologia, Dipartimento di Medicina Sperimentale, Università di Genova, Viale Cembrano 4, Genova 16148, Italy.
Paudice
Paolo
P
Longordo
Fabio
F
Raiteri
Maurizio
M
D000818:Animals; D001905:Botulinum Toxins / Q000494:pharmacology; D002118:Calcium / Q000502:physiology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D002540:Cerebral Cortex / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D002766:Cholecystokinin / Q000378:metabolism*; D004791:Enzyme Inhibitors; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology*; D005998:Glycine / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008297:Male; D016202:N-Methylaspartate / Q000494:pharmacology*; D009411:Nerve Endings / Q000187:drug effects / Q000378:metabolism*; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors; D052248:Nitric Oxide Synthase Type I; D009638:Norepinephrine / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D011863:Radioimmunoassay; D051381:Rats; D017207:Rats, Sprague-Dawley; D012694:Serine / Q000494:pharmacology; D013004:Somatostatin / Q000378:metabolism*; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D004791:Enzyme Inhibitors; D018690:Excitatory Amino Acid Agonists; D012694:Serine; D013004:Somatostatin; D016202:N-Methylaspartate; D002766:Cholecystokinin; D019001:Nitric Oxide Synthase; D052248:Nitric Oxide Synthase Type I; C496322:Nos1 protein, rat; D001905:Botulinum Toxins; D011188:Potassium; D002118:Calcium; D005998:Glycine; D009638:Norepinephrine