Activation of alpha 6 GABAA receptors on depolarized cerebellar parallel fibers elicits glutamate release through anion channels.
Rat cerebellar synaptosomes labeled with [3H]D-aspartate ([3H]D-ASP) were exposed in superfusion to muscimol. The GABA(A) receptor agonist did not affect [3H]D-ASP basal release or the overflow provoked by 15mM K(+); muscimol potentiated the 35mM K(+)-evoked overflow of [3H]D-ASP or endogenous glutamate. Membrane potential measured by Rhodamine 6G fluorescence was -65mV under resting conditions and -32mV in the presence of 35mM K(+). The membrane potential was not significantly affected by muscimol. The muscimol effect on the K(+)(35mM)-evoked [3H]D-ASP overflow was not inhibited by omitting external Ca(2+) or by entrapping BAPTA to chelate cytosolic Ca(2+). Muscimol lost its ability to release glutamate following superfusion with D-aspartate to deplete cytosolic glutamate by heteroexchange suggesting that GABA(A) receptor activation elicits release of cytosolic glutamate. The non-transportable glutamate carrier blockers dihydrokainate or DL-TBOA did not reduce the muscimol potentiation. This was abolished by the anion channel blockers niflumic acid and NPPB. To conclude, when cerebellar parallel fiber terminals are sufficiently depolarized, activation of alpha6 GABA(A) receptors on these terminals mediates glutamate release in addition to that evoked by depolarization. This extra-release does not occur by exocytosis or transporter reversal but involves the opening of anion channels present on parallel fiber terminals.
Raiteri
L
L
Dipartimento di Medicina Sperimentale, Sezione di Farmacologia e Tossicologia, Università di Genova, Viale Cembrano 4, 16148, Genova, Italy.
Schmid
G
G
Prestipino
S
S
Raiteri
M
M
Bonanno
G
G
D000818:Animals; D015220:Calcium Channels / Q000502:physiology*; D002531:Cerebellum / Q000187:drug effects / Q000378:metabolism*; D018755:GABA Agonists / Q000494:pharmacology; D058785:GABA-A Receptor Agonists; D018698:Glutamic Acid / Q000378:metabolism*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009118:Muscimol / Q000494:pharmacology; D009412:Nerve Fibers / Q000187:drug effects / Q000378:metabolism*; D015221:Potassium Channels / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000378:metabolism*; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D015220:Calcium Channels; D018755:GABA Agonists; D058785:GABA-A Receptor Agonists; D015221:Potassium Channels; D011963:Receptors, GABA-A; D009118:Muscimol; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid