Characterization of [3H]GABA release from striatal slices: evidence for a calcium-independent process via the GABA uptake system.
GABA can be released by depolarization even in the absence of external Ca2+. To investigate the underlying mechanism of this phenomenon, GABA release was studied using slices prepared from rat striatum. Slices were preincubated with [3H]GABA in the presence of beta-alanine and superfused with Krebs buffer. Total tritium efflux was measured as an index of GABA release. Electrical stimulation at 2 Hz for 3 min elevated resting tritium efflux approximately two-fold. Decreasing external Ca2+ to 0.1 mM increased basal tritium efflux and reduced electrically evoked overflow, while omitting Ca2+ entirely (and adding 1 mM EGTA) increased both basal efflux and evoked overflow. Tetrodotoxin (5 microM) abolished the evoked release of tritium but did not affect the resting outflow in either normal or Ca2+-deficient conditions. In the presence of normal Ca2+, nipecotic acid (0.1-1 mM), an inhibitor of GABA transport into neurons as well as glia, enhanced both spontaneous efflux and evoked overflow of tritium. Nipecotic acid also increased spontaneous release when external Ca2+ was reduced or removed; however, under these conditions electrically evoked overflow was reduced. These results suggest that the electrically evoked release of [3H]GABA from striatal slices is of neuronal origin, but can occur in part in the absence of external Ca2+. They further suggest that this Ca2+-independent release, which may co-exist with the Ca2+-dependent release, takes place via the same carrier system utilized for high-affinity GABA uptake.
Bernath
S
S
Department of Behavioral Neuroscience, University of Pittsburgh, PA 15260.
Zigmond
M J
MJ
D000818:Animals; D002118:Calcium / Q000502:physiology*; D003342:Corpus Striatum / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D004558:Electric Stimulation; D066298:In Vitro Techniques; D008297:Male; D009557:Nipecotic Acids / Q000494:pharmacology; D011392:Proline / Q000031:analogs & derivatives*; D051381:Rats; D011919:Rats, Inbred Strains; D013779:Tetrodotoxin / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D009557:Nipecotic Acids; C030278:nipecotic acid; D013779:Tetrodotoxin; D005680:gamma-Aminobutyric Acid; D011392:Proline; C010983:homoproline; D002118:Calcium