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Journal of Neuroscience, Vol 15, 2839-2848, Copyright © 1995 by Society for Neuroscience
Pharmacological types of calcium channels and their modulation by baclofen in cerebellar granules
C Amico, C Marchetti, M Nobile and C Usai
Instituto di Cibernetica e Biofisica, Consiglio Nazionale delle Ricerche, Genova, Italy.
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
microM) 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 microM) 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 GABAB
agonist (+/-)baclofen, or (- )baclofen (100 microM), reduced the calcium
current by 30 +/- 5%, with a IC50 4 microM. 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 GABAB receptors does not clearly discriminate
between pharmacologically distinct calcium channels and (2) L-type calcium
channels represent an heterogeneous population in these neurons.
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