S. Akhondzadeh et Tw. Stone, MUSCIMOL-INDUCED LONG-TERM DEPRESSION IN THE HIPPOCAMPUS - LACK OF DEPENDENCE ON EXTRACELLULAR CALCIUM, Neuroscience, 71(2), 1996, pp. 581-588
We have recently reported a new protocol for inducing long-term depres
sion through activation of GABA, receptors in the hippocampal slice. T
his long-term depression is reversed by bicuculline and potentiated by
neurosteroids such as alphaxalone and 5 alpha-pregnan-3 alpha-ol-20-o
ne. It was also shown that glutamate receptor activity is not involved
in the induction of this type of long-term depression. The present st
udy investigates the role of calcium in the induction of this novel fo
rm of long-term depression and attempts to determine the mechanism of
reversal of muscimol-induced long-term depression. Extracellular recor
dings were made in the CAI pyramidal cell layer of rat hippocampal sli
ces following orthodromic stimulation of Schaffer collateral fibres in
stratum radiatum (0.01 Hz). It was observed that the muscimol-induced
long-term depression can be obtained in the absence of calcium in the
bathing medium. In addition to this, the long-term depression was rev
ersed by N-methyl-D-aspartate, kainic acid, high potassium medium, ver
atrine and the calcium ionophore A23187 but not high calcium (10 mM) m
edium. High potassium medium in the absence of calcium reversed the lo
ng-term depression induced by muscimol 10 mu M. The results suggest th
at this type of glutamate-independent long-term depression can be indu
ced in the absence of extracellular calcium. Extracellular calcium is
not necessary for reversal of the long-term depression, although when
intracellular calcium levels are raised, as by A23187, this is capable
of inducing reversal.