GABA_A receptor activation can have an excitatory action in developing neurones, associated with a reversal of the Cl^– electrochemical gradient (Owens et al. 1996). During this early developmental period, depolarising GABAergic potentials increase [Ca²⁺]_i, a key regulator of gene expression (Ghosh & Greenberg, 1995). It has been previously shown that GABA_A receptor gene expression is altered upon long-term receptor activation, although the cellular mechanisms remain unknown (Holt et al. 1997). Here we provide data on GABA_A receptor-mediated [Ca²⁺]_i changes in 3 day in vitro cultured rat cerebellar granule neurones (CGN). Rats were killed by cervical dislocation according to Schedule 1 procedures.

Neuronal [Ca²⁺]_i of CGN, isolated from 6- to 8-day-old pups, was measured using fluorescence from the calcium indicators fura-PE3 or fluo-4. Normal and drug-supplemented bath solutions were delivered at room temperature with a rapid exchange bath perfusion system (4 ml min^-1). Data are given as means ± S.E.M. (using n values), where n represents the number of neurone recordings and y represents the number of individual experiments.

Administration of the selective GABA_A receptor agonist muscimol (0.1 mM, n = 54, y = 3) or the non-selective GABA receptor agonist GABA (0.1 mM, n = 54, y = 3) both caused a rapid, transient increase in [Ca²⁺]_i. The EC₅₀ for GABA was 0.06 mM (n = 201, y = 20). The GABA_B receptor agonist baclofen (0.1 mM) produced little or no effect (n = 58, y = 3), demonstrating that GABA_B receptors are not involved. The selective GABA_A receptor antagonists (0.01 mM) picrotoxin and gabazine blocked the 0.1 mM muscimol-induced [Ca²⁺]_i response by 30.2 ± 12 % (n = 53, y = 3) and 76.4 ± 10 % (n = 59, y = 3), respectively.

The L-type Ca²⁺ channel blocker nifedipine (0.01 mM) attenuated the muscimol-induced [Ca²⁺]_i response by 91 ± 4 % (n = 45, y = 3), with the peak [Ca²⁺]_i rise occurring 70 ± 10 s later than the peak muscimol-induced [Ca²⁺]_i response.

From these data we conclude that the [Ca²⁺]_i of CGN increases upon activation of GABA_A, rather than GABA_B receptors. This occurs via a nifedipine sensitive pathway, suggesting that GABA_A receptor activation might cause depolarisation which in turn activates L-type voltage-sensitive Ca²⁺ channels. These data along with the established role of Ca²⁺ in regulating gene expression suggest a possible pathway for GABA_A receptor activation to modify its own gene expression in immature neurones.

R.B. Liversage has a MRC PhD Sudentship.

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