In experiments on isolated rat Purkinje neurons using the whole-cell patch-clamp technique, the addition of 10 nM of an agonist of μ-opioid receptors (μ-ORs), DAMGO, to the bath solution led to moderate but highly significant intensification of the current through high-threshold calcium channels of the P type (increment 9-10%). We found that this effect was independent of the kind of cations (Ca2+ or Ba2+) coming via the plasma membrane. This effect (positive modulation of the current) was practically preserved in the case where a prepulse shifting the membrane potential to +50 mV preceded the test pulse, i.e., the effect was voltage-independent. The above-mentioned effect was almost unchanged under conditions where the intracellular solution contained 0.5 mM GTPβS (an irreversible blocker of G-proteins) or the same amount of GTPγS (a nonspecific activator of these proteins) instead of GTP. The addition of 0.5 mM cAMP to the intracellular solution also did not practically influence positive modulation of the P-current under the action of DAMGO. Preliminary 10-min-long incubation of the examined cells in a solution containing 0.5 μM calmidazolium (an antagonist of calmodulin-regulated enzymes) induced a twofold decrease in the DAMGO-evoked increment of the P-current. Based on the obtained data, we hypothesize that there is a high-affinity allosteric site of binding with agonists of μ-ORs in the molecule of the calcium P-channel, and that voltage-, calcium-, and G-protein-independent positive modulation of the current through these channels is realized by just such a mechanism.
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