Whole cell patch-clamp recordings of rat midbrain dopaminergic neurons isolate a sulphonylurea- and ATP-sensitive component of potassium currents activated by hypoxia

The effects of brief (2-4 min) hypoxia on presumed dopaminergic 'principal' neurons of the rat ventral mesencephalon were investigated by using either intracellular or whole cell patch-clamp recordings in in vitro conditions. Under single electrode voltage clamp, with sharp microelectrode (V(h) -60 mV), a brief hypoxia caused an outward current (hypo(out)) of 110.2 ± 15.2 (SE) pA (n = 18), which was followed by a posthypoxic outward current (posthypo(out)) of 149.6 ± 10.6 pA (n = 18). Although the hypo(out) reversed at -83.7 ± 3.8 mV (n = 18), the posthypo(out) did not reverse. The K⁺(ATP)- blocking sulphonylureas tolbutamide (100 μM) and glibenclamide (30 μM), significantly reduced the peak of the hypo(out) by 47.6 ± 7.7% (n = 16) and 54.18 ± 7.5% (n = 3), respectively. In contrast, they did not affect the posthypo(out). Extracellular barium (300 μM to 1 mM) almost abolished the hypo(out), leaving the posthypo(out) unchanged. The large K⁺ channel blocker charybdotoxin (1050 nM), depressed the hypo(out) after tolbutamide treatment. To investigate whether or not cytosolic factors might control the development of the hypo(out), we dialyzed the principal neurons by patchclamp recordings (V(h)60 mV). Under whole cell recordings hypoxia evoked an hypo(out) of 70.2 ± 14.5 pA that reversed polarity at -87.9 ± 5.1 mV (n = 8). A small posthypoxic response was detected upon reoxygenation in a few neurons (4 out of 14). Three different sulphonylureas, tolbutamide (100 μM), glibenclamide (10-30 μM), and glipizide (100 nM) completely blocked the hypo(out) in patch-clamped neurons. The hypo(out) was also abolished by extracellular BaCl₂ (300 μM). When the content of ATP in the dialyzate was raised from 2 to 10 mM no outward current/hyperpolarization was: evoked by hypoxia. These data suggest that the hypo(out), in principal neurons, is a complex response sustained by at least two barium-sensitive components: 1) an ATP-dependent, sulphonylurea-sensitive K⁺ conductance which could be isolated by the patch- clamp techniques and 2) a K⁺ conductance remaining after tolbutamide in intracellularly recorded neurons, which is sensitive to charybdotoxin and dependent on dialyzable cytosolic factors.