Neonatal sympathectomy by 6-hydroxydopamine: cardiovascular responses in the paralyzed rat

Rats who had been neonatally sympathectomized by peripheral injections of 6-hydroxydopamine or vehicle were paralyzed by succinylcholine and received either atropine or saline injection. Heart rate and blood pressure responses to foot shocks and to a conditioned stimulus (CS) for shock were continuously recorded. Sympathectomized rats showed lower basal systolic blood pressure, less second to second blood pressure variability and attenuated pressor responses to the shock and the CS. Basal heart rate and the heart rate response to shock were were unaffected by sympathectomy, but cardiac rate deceleration to the CS was ablished. Atropine increased basal heart rate, decreased heart rate variability and abolished the cardiac deceleration to the CS. These data were interpreted as suggesting that conditioned cardiac deceleration in the rat is a vagal mediated response compensatory to prior blood pressure increases. Sympathectomized rats also showed higher core temperatures under paralysis.