Pharmacological identification of the K(+) currents mediating the hypoglycemic hyperpolarization of rat midbrain dopaminergic neurones.

Hypoglycemia (zero glucose) initially depolarized the membrane and increased the spontaneous firing of rat midbrain dopaminergic neurones (more than 50%) intracellularly recorded in an in vitro slice preparation. Under single-electrode voltage-clamp mode (V(h) -55 mV), this transient phase correlated with an inward current of -18 pA. In all the cells tested (n=30), an inhibition fully developed over 16.9 min of hypoglycemia and was associated with a hyperpolarization of the membrane (7.7 mV) or outward current (95.6 pA). Upon re-application of a control solution (glucose 10 mM) a rebound hyperpolarization/outward current developed. The depression of firing was only seen when the artificial cerebrospinal fluid (ACSF) contained less than 1 mM glucose. In addition, the period of time required to block the spontaneous activity decreased, by diminishing the extracellular concentration of glucose from 1 to 0 mM. The hypoglycemia-induced outward current was associated with an increase in membrane conductance and reversed polarity at -100.4 mV, close to the reversal potential of K(+). The post-hypoglycemic outward current was not associated with an increase in membrane conductance and did not reverse. The K(+)-ATP channel blockers, tolbutamide (300 microM-1 mM) and glibenclamide (3-30 microM) reduced the hypoglycemia-induced inhibition. In addition, the blocker of the Ca(++)-activated K(+)-channels, charybdotoxin (100-400 nM) partially counteracted the hypoglycemic hyperpolarization. Furthermore, barium (100-300 microM) fully antagonized the hypoglycemia-induced inhibition. The post-hypoglycemic hyperpolarization/outward current was not observed in cells treated with the Na(+)/K(+) ATPase pump inhibitor strophanthidin (1-3 microM). Our data suggest that midbrain dopaminergic cells respond to glucose deprivation with a hyperpolarization generated by the opening of several K(+) channels (sulphonylurea-sensitive, charybdotoxin-sensitive and sulphonylurea and charybdotoxin-insensitive) and by the activation of the Na(+)/K(+) ATPase pump after the hypoglycemic period.