1. Highly localized microinjections of serotonin into the nucleus tractus solitarius in rats produced dose-dependent hypotension and bradycardia and electroencephalogram (EEG) changes consistent with an alerting response. 2. Microinjection of the serotinin antagonist metergoline blocked the cardiovascular and EEG effects of serotonin but did not affect the same responses elicited by the microinjection of l-glutamate. 3. Microinjections of l-glutamate produced similar effects; acetylcholine, which lowered arterial pressure and heart rate, did not affect the EEG. 4. Bilateral injections of metergoline into the nucleus tractus solitarius occasionally produced a rise in arterial pressure but did not affect the baroreceptor reflex. 5. Serotonin may integrate cardiovascular and behavioural responses mediated by the nucleus tractus solitarius. 6. Neuronal populations within the nucleus tractus solitarius may be neurochemically and functionally differentiated.
1. Changes in arterial pressure associated with a repertoire of natural behaviour patterns in the rat were examined. 2. A hierarchy of such changes was found. Eating and drinking were associated with higher pressures than grooming and exploration, which in turn were associated with higher pressures than resting. 3. Desynchronized sleep was associated with higher pressures than slow-wave sleep. 4. Lesions of the catecholamine neurons of the A2 region of the medulla did not disrupt the normal hierarchy but resulted in an exaggerated pressor response.
1. In rats, electrolytic lesions of the A2 group of catecholamine neurons result in lability of arterial pressure without hypertension. 2. To establish whether labile arterial pressure, when chronic, will lead to fixed hypertension, we placed lesions in the A2 area of adult male Sprague-Dawley rats and measured mean arterial pressure, heart rate and their variability (expressed as the standard deviation) 11 months later. Controls were age-matched, unoperated or sham-operated rats. 3. In rats with A 2 lesions: (a) the mean arterial pressure was lower (103 ± 7.5 mmHg; n = 6; P <0.05) than in sham-operated (123 ± 4.7 mmHg; n = 4) or unoperated (120 ± 3.1 mmHg; n = 9) controls; (b) the standard deviation of mean arterial pressure was higher (16 ± 1.8 mmHg; P <0.001) than in sham-operated (5 ± 0.7 mmHg) or unoperated controls (7 ± 0.6 mmHg); (c) the mean and standard deviation of heart rate did not differ between groups. No histopathological changes were detected in the A2 group. 4. Chronic lability of arterial pressure does not evolve into sustained hypertension nor does it induce systemic lesions.