CORELEASE OF ENDOGENOUS ATP AND [H-3] NORADRENALINE FROM RAT HYPOTHALAMIC SLICES - ORIGIN AND MODULATION BY ALPHA(2)-ADRENOCEPTORS

The release of endogenous ATP, measured by the luciferin-luciferase as say, and of [H-3]noradrenaline from the in vitro superfused rat hypoth alamic slices were studied. ATP and [H-3]noradrenaline were released s imultaneously during resting conditions and in response to low and hig h frequency held electrical stimulation; the release of both substance s were frequency dependent between 2 Hz and 16 Hz. The stimulation-ind uced release of ATP and [H-3]noradrenaline was diminished by more than 80% under Ca2+-free conditions. Tetrodotoxin inhibited the majority o f the evoked release of both ATP and [H-3]noradrenaline, however, it w as less effective in reducing the release of [H-3]noradrenaline, than that of ATP. Bilateral stereotaxic injection of 6-hydroxydopamine (4 m u g/side) to the ventral part of the ventral noradrenergic bundle, ori ginating from the A1 cell group in the brainstem, resulted in a 55% re duction of endogenous noradrenaline content of the hypothalamic slices , and the tritium uptake and the stimulation-evoked release of [H-3]no radrenaline was also markedly reduced. While the basal release of ATP was not affected, the evoked release was diminished by 72% by this tre atment. Perfusion of the slices with noradrenaline (100 mu M) initiate d 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-d imethyl-4-phenyl-piperazinium iodide evoked parallel release of ATP an d [H-3]noradrenaline which was inhibited by the nicotinic receptor ant agonist mecamylamine; 6-hydroxydopamine lesion of the ventral part of the ventral noradrenergic bundle did not affect the nicotine-evoked AT P and [H-3]noradrenaline release. While CH 38083, a non subtype-select ive alpha(2)-antagonist and BRL44408, the subtype-selective alpha(2AD) antagonist augmented the evoked release of [H-3]noradrenaline, ARC239 , a selective alpha(2BC) antagonist was without effect. In contrast, n either of the alpha(2)-antagonists significantly affected the evoked-r elease of ATP. In summary, we report here that endogenous ATP and [H-3 ]noradrenaline are co-released stimulation-dependently from superfused rat hypothalamic slices. A significant part of the release of both co mpounds is derived from the nerve terminals, originating from the A1 c atecholaminergic cell group of brainstem nuclei. Unlike that from the peripheral sympathetic transmission, noradrenaline and alpha(1)-adreno ceptor agonists were unable to promote the release of ATP. Conversely, parallel ATP and noradrenaline release could be induced by nicotine r eceptor activation, but this release does not originate from the same nerve endings. The evoked-release of [H-3]noradrenaline is inhibited b y endogenous noradrenaline via alpha(2AD) subtype of adrenoreceptors, while the release of ATP is not subject to this autoinhibitory modulat ion. In conclusion, our results support the view that ATP is involved in the neurotransmission in the hypothalamus, but the sources of the r eleased ATP and noradrenaline seem to be not identical under different stimulatory and modulatory conditions. (C) 1997 IBRO. Published by El sevier Science Ltd.