PHARMACOLOGICAL TYPES OF CALCIUM CHANNELS AND THEIR MODULATION BY BACLOFEN IN CEREBELLAR GRANULES

Voltage-dependent calcium currents were measured by whole-cell recording technique in cultured cerebellar granule neurons from 8 d old rats, in 10 mM BaCl2 and with a holding potential of -80 mV. A saturating dose (10 mu M) of the dihydropyridine nimodipine reversibly inhibited the maximum current by 25% and the dose dependence showed IC50 close to 50 nM. omega-Conotoxin GVIA (cgtx, 5 mu M) and omega-agatoxin IVA (agatx, 200 nM) irreversibly inhibited the current by 17% and by 47%, respectively. The effect of nimodipine was additive with that of the toxins. The GABA, agonist (+/-)baclofen, or (-)baclofen (100 mu M), reduced the calcium current by 30 +/- 5%, with a IC50 4 mu M. The effect was mediated by a pertussis toxin-sensitive G-protein, In cells treated with cgtx during the experiment or preincubated with the toxin for 30 min, the effect of baclofen was significantly reduced. However, the action of baclofen was not confined to cgtx-sensitive channels: application of nimodipine or agatx resulted in a 50% reduction of the baclofen effect as well. In contrast, baclofen inhibited approximately the same amount of current both before and after the increase caused by the dihydropyridine agonist BayK 8644 and did not modify the slow BayK-induced tail current. These results indicate (1) the modulation through GABA(B) receptors does not clearly discriminate between pharmacologically distinct calcium channels and (2) L-type calcium channels represent an heterogeneous population in these neurons.