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Previously, we have described prolonged cAMP-induced inhibition of a K+ current in cultured colliculi neurons. The aim of the present study was to characterize the channel responsible for this cAMP-dependent effect. We detected the presence of a non-inactivating voltage-dependent 16-pS K+ channel that displayed long-lasting inhibition upon a brief application of cAMP and greater sensitivity to tetraethylammonium than to 4-aminopyridine. In addition to this channel, colliculi neurons express two other voltage-sensitive, non-inactivating K+ channels (8 and 49 pS) whose activity is facilitated by a brief application of cAMP, the effect of which is also long-lasting. These results suggest the presence of common sustained cAMP-dependent processes responsible for both up- and down-regulation of these channels in the neurons studied. They indicate that the 16-pS, but not the 8-pS or the 49-pS channels, participates in the cAMP-inhibited macroscopic K+ current.

In cultured cerebellar granule cells, we examined the effects of dihydropyridines (DHPs) on K+ currents, using the whole-cell recording configuration of the patch-clamp technique and on Ca(2+)-activated K+ channels ("maxi K+ channels") using outside-out patches. We found that micromolar concentrations of nicardipine, nifedipine, (+) and (-) BAY K 8644, nitrendipine, nisoldipine and (-) nimodipine block 10-60% of macroscopic K+ currents. The most potent of these DHPs was nicardipine and the least potent, (-) BAY K 8644. (+) Nimodipine had no effect on this current. The inhibitory effects of nifedipine and nicardipine were not additive with those of 1 mM tetraethylammonium (TEA). Outside-out recordings of "maxi K+ channels" showed a main conductance of 200 pS (in 77% of the patches) and two subconductance states (in 23% of the patches). Neither nifedipine nor nicardipine affected the main conductance, but decreased the values of the subconductance levels. In 10% of these patches, nicardipine induced a flickering activity of the channel. These findings show that both Ca2+ and K+ channels have DHP-sensitive sites, suggesting similarity in electrostatic binding properties of these channels. Furthermore, cerebellar granule cells may express different subtypes of "maxi K+ channels" having different sensitivities to DHPs. These drugs may provide new tools for the molecular study of K+ channels.

Cell-attached patch recordings were used to study calcium channels on the dendritic membrane of rat cerebellar Purkinje cells maintained in culture. Experiments were performed with isotonic BaCl2 (110 mM) in the pipette and isotonic potassium gluconate in the bath to zero the cell membrane potential. Two distinct types of voltage-activated calcium channels were identified. The first one had a small conductance (9 pS), was activated at a low threshold (congruent to -50 mV) and could be inactivated by holding the membrane potential at -30 mV. This channel had the same characteristics as the T channel described in other neuronal preparations. The second type of Ca channel activated at a high threshold (-30 or +10 mV depending on whether BAY K 8644 was added or not to the pipette solution) and was still activatable even when the membrane was held at -40 mV. In the presence of BAY K 8644 this channel had a conductance of 21 pS with long openings. All these characteristics are similar to those of the S (L) Ca channel described in many preparations. The present study is in agreement with our previous experiments on Purkinje dendrites, where we identified low and high threshold Ca currents using the whole-cell configuration. Up to now, no channel corresponding to the N current has been observed but we cannot exclude its presence.