Neuropeptide Y increases the inhibitory effects of clonidine on potassium evoked 3H-noradrenaline but not 3H-5-hydroxytryptamine release from synaptosomes of the hypothalamus and the frontoparietal cortex of the male Sprague-Dawley rat

The release of 3H-noradrenaline (3H-NA) and of 3H-5-hydroxytryptamine (3H-5-HT) evoked by high-K+ (15 mM) was studied in synaptosomes isolated from the hypothalamus and the frontoparietal cortex of the male Sprague-Dawley rat using a superfusion apparatus. Based on concentration-response curves obtained by analyzing the full-time course of the inhibitory effects of clonidine on 3H-NA and on 3H-5-HT release neuropeptide Y (NPY) (1 nM) was shown to significantly increase the ability of clonidine to inhibit 3H-NA release in synaptosomes isolated from the hypothalamus and from the frontoparietal cortex. NPY (1 nM) alone had no effect on K+-evoked 3H-NA release from these regions. In contrast, NPY (1 nM) did not modulate the inhibitory effects of clonidine on 3H-5-HT release in the above mentioned regions. These results indicate that NPY can increase the sensitivity of the alpha 2-autoreceptors belonging to hypothalamic NA and/or to adrenaline nerve terminals and to cortical NA nerve terminals, while the alpha 2-heteroreceptors inhibiting 3H-HT release in the same brain regions appear not to be regulated by high affinity NPY receptors. Thus, alpha 2-autoreceptors and alpha 2-heteroreceptors appear to be differentially controlled by high affinity NPY receptors at least with regard to regulation of 3H-NA and 3H-5-HT release, respectively.