Cholecystokinin-8S-induced intracellular calcium signaling in acutely isolated periaqueductal gray neurons of the rat

Abstract

Many behavior studies indicate that cholecystokinin (CCK) is related to nociception and anxiety/panic actions in the midbrain periaqueductal gray (PAG). We previously reported that a sulfated form of CCK octapeptide (CCK-8S) produced excitatory effects at both pre- and postsynaptic loci in PAG neurons using slice preparations and whole-cell patch-clamp recordings. Here, we further examined the detailed mechanism of CCK-8S in acutely isolated PAG neurons of the rat using fura-2-based imaging of intracellular Ca 2+ concentration ([Ca 2+] i) and whole-cell patch-clamp recordings. Application of 1 μM CCK-8S produced an increase of [Ca 2+] i, and its effect did not desensitize. This CCK-8S-induced [Ca 2+] i increase was inhibited by the CCK 2 receptor antagonist L-365260 but not by the CCK 1 receptor antagonist L-364718. In addition, the effect of CCK-8S was eliminated by removing extracellular Ca 2+, but not by an addition of the intracellular Ca 2+ reuptake inhibitor thapsigargin. When simultaneous recordings of [Ca 2+] i imaging and whole-cell patch-clamp were performed, CCK-8S-induced [Ca 2+] i increase was significantly reduced at a membrane holding potential of -60 mV while CCK-8S-induced inward current was still observed. Current-voltage plots revealed that CCK-8S-induced inward current reversed near the equilibrium potential for K + ions with a decreased membrane conductance. However, CCK-8S produced a significant inhibition on high-voltage-activated Ca 2+ channel currents. These results suggest that CCK-8S can excite PAG neurons by inhibiting K + channels, and CCK-8S-induced [Ca 2+] i increase occurs secondary to depolarization. The evidence presented here expands our understanding of cellular mechanisms for CCK-mediated anti-analgesic and anxiogenic actions in the PAG. © 2007 Pharmaceutical Society of Japan.