HYPOXIC CHANGES IN RAT LOCUS-COERULEUS NEURONS IN-VITRO

1. Intracellular recordings were made in a pontine slice preparation of the rat brain containing the nucleus locus coeruleus (LC). Locus coeruleus neurons responded to brief hypoxic stimuli (replacement of 95% O-2-5% CO2 with 95% N-2-5% CO2) with hyperpolarization and a cessation of spontaneous action potentials. When the cells were continuously hyper polarized by about 15 mV in order to abolish spontaneous firing, hypoxia induced an early depolarization (HD), followed by a hypoxic hyperpolarization (HH) and after reoxygenation, a posthypoxic hyperpolarization (PHH). These responses were accompanied by a decrease in input resistance, which was larger during HH than during HD but, thereafter, became smaller during PHH. 2. The hypoxia-induced currents associated with the changes in membrane potential, at a holding potential of -70 mV, were an early inward current (HIC), a subsequent outward current (HOC) and after reoxygenation, another outward current (PHOC). The HIC did not change with an increasing holding potential. In contrast, the HOC reversed its amplitude at about -95 mV. Finally, the PHOC decreased, but did not reverse its polarity at more negative holding potentials. When the external K+ was elevated from 2.5 to 10.5 mM, the current-voltage (I-V) relation of the HOC and its reversal potential were shifted to the right. 3. In the presence of tetrodotoxin, the HH decreased. A low Ca2+-high Mg2+ medium depressed both the HH and PHH. Rauwolscine did not alter either response to hypoxia, while 8-cyclopentyl-1,3-dipropylxanthine decreased the PHH only. S-(p-Nitrobenzyl)-6-thioguanosine potentiated both HH and PHH. 4. Whereas tolbutamide markedly lowered the HH and PHH, glibenclamide was ineffective. Tetraethylammonium also failed to alter the hypoxic responses. Furthermore, ouabain or the removal of K+ from the superfusion medium, depressed PHH. 5. Pressure application of adenosine inhibited the spontaneous firing of LC neurons. DPCPX did not alter the firing, but antagonized the effect of adenosine. Tolbutamide also counteracted the inhibitory effect of adenosine and, additionally, facilitated the firing rate in some neurons. Moreover, tolbutamide abolished the adenosine-induced outward current. 6. Early hypoxic depolarization and PHH are mostly due to the blockade and subsequent reactivation of the K+-Na+ pump, respectively. The HH is caused by the opening of ATP-sensitive K+ (K-ATP) channels in response to the hypoxia-induced decline of intracellular ATP Adenosine released by hypoxic stimuli may lead to an adenosine A(1)-receptor-mediated opening of K-ATP channels during the HH and more markedly during the PHH.