Mechanisms Underlying Positive Modulation of a Current through P-Type Calcium Channels in Purkinje Neurons by an Agonist of Opioid Receptors

摘要

In experiments on isolated rat Purkinje neurons using the whole-cell patch-clamp technique, the addition of 10 nM of an agonist of mu-opioid receptors (mu-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 GTP beta S (an irreversible blocker of G-proteins) or the same amount of GTP gamma S (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 mu M 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 mu-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.