K-ATP CHANNEL MEDIATION OF ANOXIA-INDUCED OUTWARD CURRENT IN RAT DORSAL VAGAL NEURONS IN-VITRO

1. Thin brainstem slices (150 mu m thickness) were taken from mature rats, and membrane potentials (E(m)) and currents (I-m) in the dorsal vagal neurons (DVN) were analysed with whole-cell patch clamp techniques during anoxia. 2. At a holding potential (V-h) of -50 mV, a sustained anoxia-induced outward current (AOC) of 92 +/- 44 pA (reversal potential (E(rev)), -78 +/- 12 mV) and a concomitant increase of membrane conductance (g(m)) from 2.2 +/- 0.45 to 5.9 +/- 2.4 nS were revealed in 40% of 142 DVN analysed. The AOC led to a hyperpolarization of the cells by 14.4 +/- 6.1 mV from a mean resting E(m) of -51 +/- 6 mV, and to blockade of spontaneous action potential discharges. In the remaining DVN, anoxia had almost no effect on E(m), I-m or g(m) and did not block spontaneous action potential discharges. 3. The AOC was not affected by 0.5 mu M tetrodotoxin (TTX), 2 mM Mn2+, 50 mu m cyanonitroquinoxaline dione (CNQX) or 100 mu M bicuculline. 4. Elevation of the extracellular [K+] from 3 to 10 mM resulted in a positive shift of E(rev) of the AOC by 23 mV, whereas an increase in the [Cl-] of the patch pipette solution from 5 to 144 mM had no effect on E(rev). 5. In DVN responding with an AOC, addition of 200 mu M diazoxide, an activator of ATP-sensitive K+ (K-ATP) channels, to oxygenated solutions elicited a similar outward current (E(rev) = -79 +/- 5.5 mV, n = 12) and increase in g(m). Diazoxide did not affect E(m), I-m or g(m) in cells which did not show an BOG. 6. In a subpopulation of DVN (n = 26), spontaneous activation of a K-ATP current with an E(rev) of -80 +/- 6 mV was observed. As analysed in four of these cells, an AOC was revealed during the initial phase of development of the spontaneous outward current but not under steady-state conditions. 7. The AOC, the diazoxide-induced current, and the spontaneous outward current were completely blocked upon bath application of the K-ATP channel blocker tolbutamide (100-200 mu M). 8. The results indicate that the sustained anoxia-induced outward current of dorsal vagal neurons is due to activation of K-ATP channels. A possible physiological role of functional inactivation of these cells during metabolic disturbances is discussed.