AJP - Regulatory, Integrative and Comparative Physiology, Vol 245, Issue 1 95-R99, Copyright © 1983 by American Physiological Society
Neonatal hyperthyroidism: effects on sympathetic responses to stress in adult rats
R. McCarty, R. F. Kirby and P. C. Brunjes
Treatment of developing rats with thyroid hormone results in accelerated
maturation of sympathetic and adrenal medullary responses to reflex
activation of central sympathetic outflow. In this study, we examined the
effects of neonatal hyperthyroidism on the responsiveness of the
sympathetic nervous system of adult rats to acute stress. Hyperthyroidism
was produced in Long-Evans hooded rats by injections of thyroxine (neo-T4,
1 mg/kg body wt) on postnatal days 1-4. Littermate controls received
injections of vehicle only. In adulthood, male rats of the two groups were
prepared with chronic tail artery catheters to allow repeated sampling of
blood and direct measurements of mean arterial pressure (MAP, mmHg) and
heart rate (HR, beats/min). Two days after surgery, rats were stressed by
exposure to 1 min of inescapable foot shock (2.0 mA, 0.6-s duration, every
6 s). The activity of the sympathetic nervous system was assessed by
measuring plasma levels of norepinephrine (NE) and epinephrine (E). Basal
plasma levels of NE and E and resting MAP did not differ between neo-T4 and
control rats. However, basal HR was elevated in neo-T4 rats.
Footshock-induced increments in plasma levels of both catecholamines were
greater in neo-T4 compared with control rats even though behavioral
responses to footshock were similar across groups. However, neo-T4 rats
were more active when tested in an open field on each of 3 consecutive
days. These findings indicate that neonatal treatment with T4 results in
hyperresponsiveness of the sympathoadrenal medullary system to acute stress
that persists into adulthood.
