CA2- ROLE OF INTERNAL MG2+ IN ALTERING CHARACTERISTICS AND ANTAGONISTEFFECTS( CURRENTS IN CEREBELLAR GRANULE NEURONS )

Using the whole-cell patch-clamp technique, Ca2+ channel currents were measured in cultured rat cerebellar granule neurones in the presence of 10 mM Ba2+. Two different solutions were used to fill patch pipette s, one containing mainly tetraethylammonium acetate (TEA-Ac solution), and the other mainly caesium and HEPES (Cs-HEPES solution). Under the se two different intracellular conditions markedly different Ca2+ chan nel currents were recorded. When TEA-Ac solution was used intracellula rly, small, Cd2+-sensitive inward currents (approx. -55 pA) that were inhibited by the dihydropyridine antagonist (-)-202-791 and the GABA(B ) agonist (-)-baclofen were observed. These currents were insensitive to the Ca2+ channel clocking toxins omega-conotoxin GVIA (omega-CgTX) and omega-agatoxin IVA and were enhanced by the dihydropyridine agonis t (+)-202-791. In contrast, when the Cs-HEPES solution was used, curre nts were 2-3 times larger (approx. -130 pA), inhibited by (-)-202-791, omega-CgTX and omega-agatoxin IVA but were unaffected by (-)-baclofen . Furthermore, both (+)-202-791 and Bay K8644 in the presence of Cs-HE PES solution produced only a transient enhancement that was followed b y an inhibition. Analysis of steady-state inactivation revealed two co mponents of current in both cases, with similar voltage dependencies. The factor(s) giving rise to these differences were investigated in te rms of current amplitude and responses to (-)-baclofen and omega-CgTX and were found to be mainly due to the concentrations of Mg2+ and ATP added to the patch pipette solutions. Furthermore, free internal Mg2concentrations of greater than 0.2 mM selectively inhibited omega-CgTX -sensitive Ca2+ channels. Preliminary evidence indicates that the same may be true of omega-Aga IVA-sensitive P-type current. These data sug gest that the N-type Ca2+ channels in these cells are preferentially i nhibited by intracellular Mg2+ and this may provide an explanation for discrepancies between the results of different groups investigating C a2+ channel currents in similar cell types.