Antagonistic Oxytocin/α2‐Adrenoreceptor Interactions in the Nucleus Tractus Solitarii: Relevance for Central Cardiovascular Control

The modulation of the central cardiovascular effects of α2‐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 α2−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 Kd and Bmax values of [3H]p‐aminoclonidine binding sites in the nucleus tractus solitarii compatible with a possible antagonistic interaction with the α2‐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/α2‐adrenoceptor interaction in nucleus tractus solitarii that may be of relevance for the demonstrated modulation of α2−adrenoceptor induced cardiovascular responses by oxytocin.

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