Release of monoamines and nitric oxide is involved in the modulation of hyperpolarization-activated inward current during acute thalamic hypoxia

Using slices of the dorsal lateral geniculate nucleus, it has been shown th at, in the presence of excitatory and inhibitory amino acid antagonists, br ief periods of hypoxia (3-4 min of 95% N-2/5% CO2) induce in thalamocortica l neurons an increase in instantaneous input conductance (GN) accompanied b y an inward shift in baseline holding current (I-BH) These effects have bee n suggested to be mediated, at least in part, by a positive shift in the vo ltage-dependence of the hyperpolarization-activated, mixed Na+/K+ current ( I-h) and a change in its activation kinetics which transforms it into an al most instantaneously activated current. In this study, using the whole-cell patch-clamp technique, the contribution of an increased Ca2+-dependent tra nsmitter release to the hypoxic response of thalamocortical neurons was fur ther investigated using (i) blockers of calcineurin, a Ca2+/calmodulin-acti vated phosphatase that selectively regulates Ca2+-dependent release, and (i i) antagonists of neurotransmitters that are known to modulate I-h Thalamoc ortical neurons (n = 23) recorded with electrodes filled with calcineurin a utoinhibitory fragment (30-250 mu M), a membrane impermeable blocker of cal cinuerin, showed no difference either in resting, or in the hypoxia-induced changes in, G(N), I-BH and I-h, when compared to thalamocortical cells pat ched with electrodes that did not contain calcineurin autoinhibitory fragme nt. In contrast, in 18 of these neurons recorded with calcineurin autoinhib itory fragment-filled electrodes, bath application either of cyclosporin-A (20 mu M) or tacrolimus (50-100 mu M), two membrane permeable blockers of c alcineurin, abolished the effects of hypoxia on G(N), I-BH, and I-h Separat e application of noradrenaline, serotonin, histamine and nitric oxide antag onists produced only a small depression of the hypoxic response, while conc omitant bath application of these antagonists decreased the hypoxia-induced changes in G(N) and I-BH by 55 and 42%, respectively (n = 12). Concomitant bath application of 8-bromo-adenosine-3'5'-cyclicmonophosphate and 8-bromo -guanosine-3'5'-cyclicmonophosphate (both 1 mM), which are known to mediate the action of these transmitters on I-h, increased G(N) (40%), decreased I -h time-constant of activation (30%) and significantly occluded (50%) the h ypoxia-induced effect on G(N) and I-BH Thalamocortical neurons (n = 6) patc hed with electrodes filled with 8-bromo-adenosine-3'5'-cyclicmonophosphate and 8-bromo-guanosine-3 '5' -cyclicmonophosphate (both 1 mM) showed a large r G(N) than the one recorded with the standard internal solution, and a sig nificant depression of the hypoxia-induced changes in G(N) and I-BH These results indicate that during acute thalamic hypoxia an increased rele ase of noradrenaline, serotonin, histamine and nitric oxide is responsible for transforming I-h into an instantaneously activating current via cyclic AMP- and cyclic GMP-mediated mechanisms. (C) 2000 IBRO. Published by Elsevi er Science Ltd.