Reduced inhibitory effects of clonidine and neuropeptide Y on 3H-noradrenaline release from synaptosomes of the medulla oblongata of the spontaneously hypertensive rat.

The release of 3H-noradrenaline (3H-NA) evoked by high-K+ (15 mM) was studied in synaptosomes isolated from the medulla oblongata of the normotensive Wistar-Kyoto and spontaneously hypertensive male rat (14 weeks old) using a superfusion apparatus. Based on concentration-response curves clonidine was shown to have a reduced ability to inhibit 3H-NA release in synaptosomes isolated from the spontaneously hypertensive rat (SHR) versus the normotensive rat. Furthermore, only a high concentration of NPY (100 nM) had the ability to enhance the inhibitory effects of clonidine on 3H-NA release in synaptosomes isolated from the medulla oblongata of the SH male rat, while 1 nM of NPY was effective in synaptosomes isolated from the medulla oblongata of the normotensive Wistar-Kyoto rat. These results may indicate a reduced presynaptic alpha 2-adrenoceptor and NPY receptor function to inhibit 3H-NA release from NA and/or adrenaline (A) nerve terminals in the medulla oblongata of the adult 14 weeks old SHR.