Antagonistic Oxytocin/alpha(2)-adrenoreceptor interactions in the nucleus tractus solitarii: Relevance for central cardiovascular control

The modulation of the central cardiovascular effects of alpha (2)-adrenocep tor activation by oxytocin in the nucleus tractus solitarii has been evalua ted by cardiovascular analysis and by quantitative receptor autoradiography : Microinjections in the nucleus tractus solitarii of a threshold dose of o xytocin effectively and significantly counteracted the vasodepressor and br adycardic actions of an ED50 dose of the alpha (2)-adrenoceptor agonist clo nidine. The coinjection of a threshold dose of oxytocin with a threshold do se of clonidine did not produce any changes in the mean arterial pressure b ut a tachycardic response was observed. Receptor autoradiographical experim ents showed that oxytocin (3 nM) significantly increased the K-d and B-max values of [H-3]p-aminoclonidine binding sites in the nucleus tractus solita rii compatible with a possible antagonistic interaction with the alpha (2)- adrenoceptors, and this effect was blocked by the presence of the specific oxytocin receptor antagonist 1-deamino-2-D-Tyr-(OEt)-4-Thr-8-Orn-oxytocin. These findings suggest the existence of an antagonistic oxytocin/alpha (2)- adrenoceptor interaction in nucleus tractus solitarii that may be of releva nce for the demonstrated modulation of alpha (2)-adrenoceptor induced cardi ovascular responses by oxytocin.