Cardiovascular responses to microinjection of ANF into dorsal medulla of rats

Atrial natriuretic factor (ANF) has been suggested as a putative neurotransmitter in central pathways involved in the control of the cardiovascular system. To investigate this possibility, 50 nl of 10(-7) M ANF were microinjected into discrete sites in the nucleus of the tractus solitarius (NTS) where baro- and chemoreceptor afferents terminate. Injections into 36 of a total of 66 sites in the NTS of paralyzed artificially ventilated Wistar rats under urethan anesthesia were found to produce a significant decline in heart rate [HR; -9.2 +/- 2.9 (SE) beats/min, P less than 0.05] and mean arterial pressure [MAP; -11.1 +/- 1.2 (SE) mmHg, P less than 0.01]. Similar responses were also present in anesthetized animals breathing spontaneously. Microinjection of an inactive peptide analogue or of saline did not produce cardiovascular changes. It was also found that ANF injection into the cuneate nucleus (20 of 38 sites) and the spinal trigeminal complex (28 of 42 sites) produced a decrease in MAP and HR that were of the same magnitude as those seen in the NTS. Injections of ANF into the medial longitudinal fasciculus (n = 22), hypoglossal nucleus (n = 9), area postrema (n = 16), and dorsal motor nucleus of the vagus (n = 11) did not change HR or MAP. These results suggest that ANF may serve as a neurotransmitter involved in cardiovascular reflexes mediated by specific nuclei in the dorsal medulla.