Co-release of endogenous ATP and [ 3H]noradrenaline from rat hypothalamic slices: origin and modulation by α2-adrenoceptors

Abstract

The release of endogenous ATP, measured by the luciferin-luciferase assay, and of [ 3H]noradrenaline from the in vitro superfused rat hypothalamic slices were studied. ATP and [ 3H]noradrenaline were released simultaneously during resting conditions and in response to low and high frequency field electrical stimulation; the release of both substances were frequency dependent between 2 Hz and 16 Hz. The stimulation-induced release of ATP and [ 3H]noradrenaline was diminished by more than 80% under Ca 2+-free conditions. Tetrodotoxin inhibited the majority of the evoked release of both ATP and [ 3H]noradrenaline, however, it was less effective in reducing the release of [ 3H]noradrenaline, than that of ATP. Bilateral stereotaxic injection of 6-hydroxydopamine (4 μg/side) to the ventral part of the ventral noradrenergic bundle, originating from the A1 cell group in the brainstem, resulted in a 55% reduction of endogenous noradrenaline content of the hypothalamic slices, and the tritium uptake and the stimulation-evoked release of [ 3H]noradrenaline was also markedly reduced. While the basal release of ATP was not affected, the evoked release was diminished by 72% by this treatment. Perfusion of the slices with noradrenaline (100 μM) initiated rapid and continuous tritium release; on the other hand, it did not release any ATP. In contrast, 6 min perfusion of (−)nicotine and 1,1-dimethyl-4-phenyl-piperazinium iodide evoked parallel release of ATP and [ 3H]noradrenaline which was inhibited by the nicotinic receptor antagonist mecamylamine; 6-hydroxydopamine lesion of the ventral part of the ventral noradrenergic bundle did not affect the nicotine-evoked ATP and [ 3H]noradrenaline release. While CH 38083, a non subtype-selective α 2-antagonist and BRL44408, the subtype-selective α 2AD antagonist augmented the evoked release of [ 3H]noradrenaline, ARC239, a selective α 2BC antagonist was without effect. In contrast, neither of the α 2-antagonists significantly affected the evoked-release of ATP. In summary, we report here that endogenous ATP and [ 3H]noradrenaline are co-released stimulation-dependently from superfused rat hypothalamic slices. A significant part of the release of both compounds is derived from the nerve terminals, originating from the A1 catecholaminergic cell group of brainstem nuclei. Unlike that from the peripheral sympathetic transmission, noradrenaline and α 1-adrenoceptor agonists were unable to promote the release of ATP. Conversely, parallel ATP and noradrenaline release could be induced by nicotine receptor activation, but this release does not originate from the same nerve endings. The evoked-release of [ 3H]noradrenaline is inhibited by endogenous noradrenaline via α 2AD subtype of adrenoreceptors, while the release of ATP is not subject to this autoinhibitory modulation. In conclusion, our results support the view that ATP is involved in the neurotransmission in the hypothalamus, but the sources of the released ATP and noradrenaline seem to be not identical under different stimulatory and modulatory conditions.