Infants dying of Sudden Infant Death Syndrome (SIDS) have reduced brainstem serotonin (5-hydroxytryptamine, 5-HT) where some cases die following episodes of severe bradycardia and hypoxemia. The specific role of central 5-HT in resting arterial blood pressure (BP) and on baroreflex sensitivity during neonatal life has not been studied. In adult animals, systemic depletion of 5-HT increases BP with no effect on heart rate (HR) and reduces the sensitivity of the baroreflex. Other studies have also shown that a loss of central 5-HT beginning in embryogenesis reduces resting BP and HR in adulthood. Based on these reports, we hypothesized that loss of brainstem 5-HT neurons in the neonatal period would reduce baseline BP and HR as well as reduce baroreflex gain. To test this, we utilized 3-week old Sprague Dawley rats treated centrally with 5,7-dihydroxytryptamine (5,7-DHT, n=4; ~120 ug in saline, i.c.v.), a chemical that is toxic to serotonergic neurons. Littermate controls were injected with saline (CTRL, n=5, ~3ul, i.c.v.). We measured BP with a femoral artery catheter. HR was derived from BP. Following a recovery period, we measured resting variables for 15 minutes and then injected phenylephrine (PE; 3mg/kg s.c.) followed by sodium nitroprusside (SNP; 2.5mg/kg s.c.), separated by 15 minutes, to induce pressor or depressor responses, respectively. For both responses, baroreflex gain was calculated as the [[Unable to Display Character: ∆]]HR at the maximum [[Unable to Display Character: ∆]]BP following drug injection. We found that a loss of 5-HT neurons did not alter baseline BP (p>0.05) but did reduce baseline HR when compared to control littermates (p<0.02). 5-HT neuron deficiency tended to reduce baroreflex gain in response to PE (CTRL: -2.756 ± 0.483 beats/mmHg; 5,7-DHT: -1.499 ± 0.348 beats/mmHg; p=0.058), but not SNP (CTRL: -2.408 ± 0.351 beats/mmHg; 5,7-DHT: -3.316 ± 1.214 beats/mmHg; p>0.05). Our data indicate that brainstem 5-HT maintains resting HR, and is involved in baroreflex control of HR in response to hypertensive stimuli. Reduced brainstem 5-HT may predispose an infant to SIDS via altered autonomic control of BP and HR. The role of 5-HT in BP regulation during hypoxic conditions remains to be elucidated.
Role of the carotid body chemoreceptors in baroreflex control of blood pressure during hypoglycaemia in humans