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The cardiovascular effects of selective alpha 1 and alpha 2 agonists and antagonists injected into the nucleus tractus solitarii (NTS) were studied in urethane-anesthetized rats. Methoxamine (0.3-3 micrograms) injected bilaterally into the NTS caused a dose-dependent increase in blood pressure and heart rate. Phenylephrine (6 micrograms) and an imidazolidine derivative St 587 (3 micrograms) similarly injected also produced an increase in blood pressure, whereas alpha-methylnoradrenaline and an azepine derivative B-HT 920 (1 and 3 micrograms) caused a decrease in blood pressure and heart rate. The pressor response to methoxamine (1 microgram) was markedly inhibited by prazosin (0.3 microgram) injected into the same sites or hexamethionum (25 mg/kg, i.v.). Prazosin (0.3 microgram) alone injected bilaterally into the NTS did not affect the blood pressure, while yohimbine (0.1 microgram) similarly injected increased the pressure. These results suggest that in the rat NTS there exist alpha 1 adrenoceptors responsible for an increase in arterial pressure. The NTS alpha 2 adrenoceptors seem to be involved in the tonic regulation of arterial pressure.
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