The excitatory effect of cholecystokinin on rat neostriatal neurons: Ionic and molecular mechanisms

Whole-cell patch-clamp recordings were performed to study ionic and molecular mechanisms by which cholecystokinin (CCK) peptides modulate the membrane excitability of acutely dissociated rat neostriatal neurons. Immunohistochemical staining studies indicated that about 95% of acutely isolated neostriatal neurons were GABA(γ-aminobutyric acid)ergic medium-sized cells. During current-clamp recordings, sulfated cholecystokinin octapeptide (CCK-8) depolarized neostriatal neurons and evoked action potentials. During voltage-clamp recordings, CCK-8 induced inward currents at negative membrane potentials by increasing the voltage-insensitive and non-selective cationic conductance. Cholecystokinin tetrapeptide (CCK-4), a selective CCK(B) receptor agonist, also evoked cationic currents. The CCK-8-induced cation currents were antagonized by PD135,158 (4-{[2-[[3-(1H-indol-3yl)-2-methyl-1-oxo-2-[[[1.7.7.-tri methyl-bicyclo[2.2.1]hept-2-yl)oxy]carbonyl]amino]propyl] amino]-1-phenyl]ethyl]amino-4-oxo-[1S-1α,2β[S*(S*)]4α)}- butanoate N-methy]-D-glucamine), a highly specific and potent CCK(B) receptor antagonist. The CCK-8-evoked inward currents were blocked by the internal perfusion of 1 mM GDP-β-S. In neostriatal neurons dialyzed with 0.5 mM GTP-γ-S, the cationic currents produced by CCK-8 became irreversible. Pretreating neostriatal neurons with 500 ng/ml pertussis toxin did not prevent CCK-8 from evoking cationic currents. Internal administration of heparin (2 mg/ml), an inositol 1,4,5-trisphosphate (IP₃) receptor antagonist, and buffering of intracellular calcium with the Ca²⁺-chelator, BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, 10 mM), suppressed CCK-8-evoked cationic currents. These findings suggest that, by activating CCK, receptors, CCK-8 excites rat neostriatal neurons through enhancing a non-selective cationic conductance and that pertussis toxin-insensitive G-proteins mediate CCK-8 enhancement of the cationic conductance. The coupling mechanism via G-proteins is likely to involve the production of IP₃, and the subsequent IP₃-evoked Ca²⁺ release leads to the opening of non-selective cation channels.