Pressor effect mediated by bradykinin in the paratrigeminal nucleus of the rat.

1. The participation of the paratrigeminal nucleus (Pa5) in the pressor response produced by bradykinin in the dorsolateral medulla of rats was investigated. The microinjection of 6 pmol of bradykinin directly over the paratrigeminal nucleus of unanaesthetized rats produced a significant increase in arterial pressure and a moderate increase in heart rate. 2. Bradykinin microinjections in different sites surrounding the Pa5 compromising the external cuneate nucleus, the trigeminal nucleus, the lateral and ventral spinal trigeminal tract and the dorsal trigeminal tract rostral and caudal to the Pa5 did not elicit significant pressor responses. In contrast, microinjections in the paratrigeminal nucleus produced pressor effects. Injections in the dorsolateral medulla directly over the paratrigeminal nucleus produced larger responses than when injections were made in the nucleus. Saline injections in the different nuclei did not produce pressor effects. 3. Neurochemical lesioning of the Pa5, with microinjections of ibotenic acid in the Pa5, abolished the pressor response to bradykinin injected over the lesioned nucleus. The effect was present, however, when bradykinin was injected on the contralateral side to the lesion, over the intact nucleus of the same animal. Pretreatment with capsaicin (injected in the lateral cerebral ventricle), which causes selective degeneration of afferent sensory fibres, did not alter the pressor effect of bradykinin injected over the paratrigeminal nucleus. 4. Dose-related responses were produced by different concentrations of bradykinin (0.6-1.8 pmol) microinjected over the nucleus. The bradykinin receptor antagonist HOE 140, injected over the paratrigeminal nucleus 30 min earlier, abolished the pressor response caused by bradykinin. 5. Low doses of bradykinin injected in or directly over the paratrigeminal nucleus increased arterial pressure and caused a small increase in heart rate by stimulating kinin receptors of the paratrigeminal nucleus in the dorsolateral medulla of awake and unrestrained rats. The pattern of the response was consistent with that of sympathetic stimulation. The paratrigeminal nucleus, which receives primary afferents and projects to the nucleus tractus solitarii and the rostral ventral lateral medulla, may be positioned as relay nucleus possibly connecting sensory input to structures that regulate blood pressure.