Pharmacological characterization of presynaptic alpha-adrenoceptors in the nucleus tractus solitarii and the cerebral cortex of the rat

The presynaptic alpha-adrenoceptors mediating inhibition of K+-induced release of 3H]noradrenaline (NA) from superfused slices and synaptosomes obtained from the nucleus tractus solitarii (NTS) region were characterized pharmacologically and compared to those in the cerebral cortex. Clonidine concentration-dependently (pD2 7.80) reduced the K+-induced 3H]NA release from slices and synaptosomes from the nucleus tractus solitarii of both normotensive and spontaneously hypertensive rats; 10(-6) M clonidine inhibited the 3H]NA release induced by 10 mM K+ to 40% of control. This inhibitory effect of clonidine was antagonized by the alpha2-adrenoceptor antagonist yohimbine (10(-7) M), but not by the alpha1-adrenoceptor antagonist prazosin (10(-7) M). The rank order of apparent affinities of agonists for the presynaptic alpha-adrenoceptors mediating inhibition of 3H]NA release from slices of nucleus tractus solitarii induced by 20 mM K+, was: oxymetazoline greater than clonidine greater than adrenaline greater than alpha-methylnoradrenaline greater than noradrenaline much greater than methoxamine. A similar rank order was found for cerebral cortex slices. The rank order of intrinsic activities in both brain regions was found to be: noradrenaline approximately equal to alpha-methylnoradrenaline approximately equal to adrenaline greater than clonidine greater than oxymetazoline much greater than methoxamine. The data indicate that presynaptic alpha-adrenoceptors mediating inhibition of NA release both in the nucleus tractus solitarii and the cerebral cortex belong to the alpha2-type. The presynaptic alpha2-adrenoceptors in the two brain regions appear to be similar both functionally and pharmacologically. It is suggested that these receptors in the nucleus tractus solitarii may play a role in the hypotensive effect of clonidine.