Antagonistic Oxytocin/α2-adrenoreceptor interactions in the nucleus tractus solitarii:: Relevance for central cardiovascular control

The modulation of the central cardiovascular effects of alpha (2)-adrenoceptor activation by oxytocin in the nucleus tractus solitarii has been evaluated by cardiovascular analysis and by quantitative receptor autoradiography: Microinjections in the nucleus tractus solitarii of a threshold dose of oxytocin effectively and significantly counteracted the vasodepressor and bradycardic actions of an ED50 dose of the alpha (2)-adrenoceptor agonist clonidine. The coinjection of a threshold dose of oxytocin with a threshold dose of clonidine did not produce any changes in the mean arterial pressure but a tachycardic response was observed. Receptor autoradiographical experiments 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 solitarii 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 relevance for the demonstrated modulation of alpha (2)-adrenoceptor induced cardiovascular responses by oxytocin.