Epinephrine release and presynaptic alpha 2-adrenoceptic regulation in the guinea pig hypothalamus

The release of epinephrine (E) and its regulation system were investigated in the slices of guinea pig hypothalamus. Electrical stimulation produced an efflux of [3H]E from tissues preloaded with [3H]E, in a current- and frequency-dependent manner. Stimulated [3H]E release was inhibited by tetrodotoxin and by a calcium-free medium containing ethylene glycol bis(beta-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA). Stimulated [3H]E release was enhanced by yohimbine and this effect was suppressed by clonidine. These results provide strong evidence for the neurotransmitter role of E in the hypothalamus, and suggest the possible existence of regulatory mechanism of E release via presynaptic alpha 2-receptors.