Bicuculline increases Ca2+ transients in rat cerebellar granule cells through non-GABAA receptor associated mechanisms

doi:10.1016/S0304-3940(99)00213-X

Neuroscience Letters

Volume 265, Issue 2, 16 April 1999, Pages 95-98

https://doi.org/10.1016/S0304-3940(99)00213-X Get rights and content

Abstract

The effects of bicuculline methiodide (bicuculline) and gabazine, two GABA_A antagonists, on cytosolic calcium increases (Ca²⁺ transients) in rat cerebellar granule cells were examined using Fluo-3-spectrofluorometry. Bicuculline (25 μM) markedly potentiated Ca²⁺ transients caused by KCl (25 mM) and by A23187 (4 μM) whereas gabazine (25 μM) had no effect. Calcium increases caused by glutamate (2 μM), N-methyl-d-aspartic acid (200 μM), trans-1-amino-cyclopentane-1,3 dicarboxylate (200 μM), thapsigargin (1 μM) or caffeine (5 mM) were not altered by bicuculline. Thapsigargin, which depletes intracellular Ca²⁺ stores, had no effect on either KCl- or A23187-induced Ca²⁺ transients, but completely blocked bicuculline-induced potentiation of Ca²⁺ increases. Our data suggest that bicuculline triggers calcium release when calcium entry is evoked by KCl or A23187 and that this effect is not mediated via GABA_A receptor blockade.

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This indicates that convergent intracellular Ca2+ mobilization mechanisms are activated by the two receptors as has been previously shown for histamine1R and histamine2R in splenocytes [48], or P2XR and P2YR in retinal cells [49]. Another explanation may be a GABAAR-independent potentiation effect of bicuculline on the Ca2+ transients described previously in rat cerebellar granule cells [50]. Finally, this effect may be caused by the increased GABABR activity due an elevated level of ambient GABA [51] following blockade of GABAAR.
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However, the unilateral injection of bicuculline into the POA was efficacious in suppressing the effects of PGE2 when administered contralateral, as well as ipsilateral, to the PGE2 injection site. Since bicuculline may affect the level of intracellular Ca2+, which effect is not mediated through GABAA receptor mechanisms (Mestdagh and Wülfert, 1999), another GABAA receptor antagonist, gabazine, was also employed. The unilateral microinjection of 0.57–1.1 pmol PGE2 into the peri-OVLT elicited simultaneous increases in the rate of whole-body O2 consumption (VO2), heart rate, and colonic temperature (Tc) and a decrease in tail skin temperature (Ts) (Fig. 1).
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In the previous study of the rat frontal cortex slices oxygen consumption (QO₂), polarographically determined using the biological oxygen monitor, a moderate respiratory depressant action of midazolam ex vivo (1.0 mg/kg) has been observed. Antagonist of the benzodiazepine binding site, flumazenil, blocked the effect of the agonist. However, midazolam–γ-aminobutyric acid (GABA) interactions pointed to the possibility that a part of midazolam action is independent of the classical GABA potentiation. To test this presumption, GABA_A receptor antagonists bicuculline and picrotoxin were administered. Both blockers antagonized the QO₂ reducing effect of the combination of per se effective doses of midazolam (1.0 mg/kg) and GABA (5 × 10⁻⁴ mol/l), as well as of GABA (5 × 10⁻⁴ mol/l) itself. However, neither effects of midazolam (1.0 mg/kg) on its own, nor those of midazolam in presence of the physiological, per se ineffective, concentration of GABA (10⁻⁶ mol/l), were susceptible to antagonism. These results show that ex vivo influence of midazolam on cerebral metabolic activity should be partly ascribed to some of its cellular mechanisms probably associated to the GABA modulation, but distinct from the standard GABA-potentiating effects of benzodiazepines.

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Bicuculline increases Ca2+ transients in rat cerebellar granule cells through non-GABAA receptor associated mechanisms

Abstract

Neuroscience

J. Biol. Chem.

J. Biol. Chem.

Semin. Neurosci.

Semin. Neurosci.

Neurosci. Lett.

Neurosci. Lett.

Trends Pharmacol. Sci.

Neurosci. Lett.

Bicuculline increases Ca²⁺ transients in rat cerebellar granule cells through non-GABA_A receptor associated mechanisms