Ontogeny of functional sympathetic innervation to the heart and adrenal medulla in the preweanling rat.

The development of functional sympathetic innervation to the heart and adrenal medulla was examined in Long-Evans and Sprague-Dawley rats during the preweanling period. Insulin-induced hypoglycemia was used to produce a centrally mediated activation of the sympathetic nervous system and the induction of ornithine decarboxylase activity in the heart and the depletion of adrenal epinephrine were used as indices of sympathetically mediated responses in these target organs. First, dose by time response curves were performed in 11-13 day old Sprague-Dawley pups to establish an insulin dose which would simultaneously produce stimulation of cardiac ornithine decarboxylase activity and depletion of adrenal medullary epinephrine at an appropriate time post-injection. This dose of insulin was then administered to pups of different postnatal ages to determine the onset of functional sympathetic innervation to these target organs. Significant increases in heart ornithine decarboxylase activity were seen by two postnatal days of age in both rat strains examined. In contrast, adrenal epinephrine depletions were not found until 8-12 postnatal days of age. These results indicate that functional sympathetic neurotransmission to the heart is present in the first few days after birth, and this precedes the onset of sympathetic control of adrenal epinephrine release, which matures near the end of the first postnatal week in laboratory rats.