RESPONSES OF RAT MESENCEPHALIC DOPAMINERGIC-NEURONS TO A PROLONGED PERIOD OF OXYGEN DEPRIVATION

We employed intracellular electrophysiological techniques to examine t he effects of a prolonged anoxia (more than 7 min superfusion with art ificial cerebrospinal fluid saturated with 95% N-2-5% O-2) on dopamine rgic neurons of the rat ventral mesencephalon maintained in vitro. A p rolonged anoxia caused an inhibition of the spontaneous firing and a s ustained (mean 16 min) and slowly declining hyperpolarization of the m embrane in 30 dopaminergic cells. This was associated with a decrease of the apparent input resistance at 5, 10, 15 and 20 min of O-2 depriv ation by 38% (n = 18), 42% (n = 18), 48% (n = 18) and 54% (n = 8) of c ontrol, respectively. The continuation of anoxia, 1-4 min after the hy perpolarizing period, induced an irreversible depolarization (n = 8). More than 50% of the cells (17 of 30) fully recovered their electrophy siological properties after 15 min of O-2 deprivation. Since the intra cellular diffusion of cesium (a potassium channel blocker) was able to block the hyperpolarization and to reveal a depolarization caused by anoxia, we tested whether the blockade of the hyperpolarization modifi ed the resistance of the cells to O-2 deprivation. We observed that th e cells loaded with cesium were depolarized and damaged in a period of O-2 deprivation less than 10 min. The apparent input resistance of th ese neurons was irreversibly reduced by 36% of the control at 5 min of anoxia (n = 6). Furthermore, in order to ascertain whether an impairm ent of the sodium/potassium pump due to energy failure is involved in the anoxia-induced depolarization, we blocked the Na+/K+ ATPase pump w ith the inhibitor ouabain. In the presence of ouabain (1-10 mu M), the depolarization/inward current induced by a short-term anoxia (2-5 min ) in cesium-loaded cells was augmented (n = 8) or became irreversible( n = 5). Our results suggest that (i) the dopaminergic neurons stay hyp erpolarized during a prolonged period of O-2 deprivation and later dep olarize after a certain time during anoxia, (ii) the inhibition of the Na+/K+ ATP-dependent pump enhances the anoxia-induced depolarization and (iii) the hyperpolarization of the membrane that develops during a noxia may play an important role in prolonging neuronal survival.