CA2-ACTIVATED K+ CURRENTS IN RAT LOCUS-COERULEUS NEURONS INDUCED BY EXPERIMENTAL-ISCHEMIA, ANOXIA, AND HYPOGLYCEMIA()

The effects of metabolic inhibition on membrane currents and N-methyl- D-aspartic acid (NMDA)-induced currents were investigated in dissociat ed rat locus coeruleus (LC) neurons by using the nystatin perforated p atch recording mode under voltage-clamp conditions. Changes in the int racellular Ca2+ concentration ([Ca2+](i)) during the metabolic inhibit ion were also investigated by using the microfluometry with a fluoresc ent probe, Indo-1. Removal of both the oxygen and glucose (experimenta l ischemia), deprivation of glucose (hypoglycemia), and a blockade of electron transport by sodium cyanide (NaCN) or a reduction of the mito chondrial membrane potential with carbonyl cyanide-p-trifluoromethoxyp henyl-hydrazone (FCCP) as experimental anoxia all induced a slowly dev eloping outward current (I-OUT) at a holding potential of -40 mV. The application of 10(-4) M NMDA induced a rapid transient peak and a succ essive steady state inward current and a transient outward current imm ediately after washout. All treatments related to metabolic inhibition increased the NMDA-induced outward current (INMDA-OUT) and prolonged the one-half recovery time of INMDA-OUT. The reversal potentials of bo th I-OUT and INMDA-OUT were close to the K+ equilibrium potential (E-K ) of -82 mV. Either charybdotoxin or tolbutamide inhibited the I-OUT a nd INMDA-OUT, suggesting the contribution of Ca2+ activated and ATP-se nsitive K+ channels, even though the inhibitory effect of tolbutamide gradually diminished with time. Under the metabolic inhibition, the ba sal level of [Ca2+](i) was increased and the one-half recovery time of the NMDA-induced increase in [Ca2+](i) was prolonged. The I-OUT induc ed by NaCN was inhibited by a continuous treatment of thapsigargin but not by ryanodine, indicating the involvement of inositol 1, 4, 5-tris phosphate (IP3)-induced Ca2+ release(IICR) store. These findings sugge st that energy deficiency causes Ca2+ release from the IICR store and activates continuous Ca2+-activated K+ channels and transient ATP-sens itive K+ channels in acutely dissociated rat LC neurons.