Presynaptic GABA(B) receptor modulation of glutamate exocytosis from rat cerebrocortical nerve terminals: receptor decoupling by protein kinase C

• Published in: Journal of neurochemistry Vol. 70; no. 4; p. 1513
• Main Authors: Perkinton, M S, Sihra, T S
• Format: Journal Article
• Language: English
• Published: England 01.04.1998
• Subjects: 4-Aminopyridine - pharmacology; Animals; Baclofen - pharmacology; Calcium - physiology; Cerebral Cortex - metabolism; Electrophysiology; Excitatory Amino Acid Antagonists - pharmacology; Exocytosis - physiology; GABA Agonists - pharmacology; Glutamic Acid - metabolism; Male; Membrane Potentials - drug effects; Presynaptic Terminals - metabolism; Protein Kinase C - physiology; Rats; Rats, Sprague-Dawley; Receptors, GABA-B - physiology; Stereoisomerism; Synaptosomes - drug effects; Synaptosomes - physiology
• ISSN: 0022-3042
• DOI: 10.1046/j.1471-4159.1998.70041513.x

GABA and the GABA(B) receptor agonist (-)-baclofen inhibited 4-aminopyridine (4AP)- and KCl-evoked, Ca2+-dependent glutamate release from rat cerebrocortical synaptosomes. The GABA(B) receptor antagonist CGP 35348, prevented this inhibition of glutamate release, but phaclofen had no effect. (-)-Baclofen-mediated inhibition of glutamate release was insensitive to 2 microg/ml pertussis toxin. As determined by examining the mechanism of GABA(B) receptor modulation of glutamate release, (-)-baclofen caused a significant reduction in 4AP-evoked Ca2+ influx into synaptosomes. The agonist did not alter the resting synaptosomal membrane potential or 4AP-mediated depolarization; thus, the inhibition of Ca2+ influx could not be attributed to GABA(B) receptor activation causing a decrease in synaptosomal excitability. Ionomycin-mediated glutamate release was not affected by (-)-baclofen, indicating that GABA(B) receptors in this preparation are not coupled directly to the exocytotic machinery. Instead, the data invoke a direct coupling of GABA(B) receptors to voltage-dependent Ca2+ channels linked to glutamate release. This coupling was subject to regulation by protein kinase C (PKC), because (-)-baclofen-mediated inhibition of 4AP-evoked glutamate release was reversed when PKC was stimulated with phorbol ester. This may therefore represent a mechanism by which inhibitory and facilitatory presynaptic receptor inputs interplay to fine-tune transmitter release.