Neurotensin increases the cationic conductance of rat substantia nigra dopaminergic neurons through the inositol 1,4,5-trisphosphate-calcium pathway

Abstract

Whole-cell patch-clamp recordings were used to investigate electrophysiological effects of neurotensin on acutely isolated dopaminergic (DA) neurons of the rat substantia nigra pars compacta (SNC). During current-clamp recordings, neurotensin depolarized DA neurons and triggered action potentials. Under voltage-clamp recordings, neurotensin evoked an inward current at a holding potential of −50 mV. Neurotensin-induced inward currents reversed the direction at −5 mV and became smaller as the membrane potential was hyperpolarized from −75 mV. With potassium-free recording solutions, neurotensin evoked voltage-insensitive cationic currents. With sodium-free external solution, neurotensin also caused inward currents by reducing the inwardly rectifying potassium conductance. Neurotensin-induced inward currents mainly resulted from an increase in a non-selective cationic conductance. Neurotensin-evoked cationic currents were inhibited by the intracellular perfusion of 1 mM guanosine-5′-O-(2-thiodiphosphate). In DA neurons internally perfused with 0.5 mM guanosine-5′-O-(3-thiotriphosphate), the cationic current produced by neurotensin became irreversible. Pretreating DA neurons with 500 ng/ml pertussis toxin (PTX) did not significantly affect the ability of neurotensin to evoke cationic currents. Internal perfusion of heparin (2 mg/ml), an inositol 1,4,5-trisphosphate (IP 3) receptor antagonist, and buffering intracellular calcium with the Ca 2+-chelator BAPTA (10 mM) suppressed neurotensin-induced cationic currents. Dialyzing DA neurons with protein kinase C (PKC) inhibitors, staurosporine and PKC(19–31), failed to prevent neurotensin from evoking cationic currents. It is concluded that PTX-insensitive G-proteins mediate neurotensin-induced enhancement of the cationic conductance of SNC DA neurons. The coupling mechanism via G-proteins is likely to involve the generation of IP 3, and subsequent IP 3-evoked Ca 2+ release from the intracellular store results in activating the non-selective cationic conductance.