Prenatal and postnatal hypothyroidism abolishes lesion-induced noradrenergic sprouting in the adult rat

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Abstract
The effects of altered thyroid states on regeneration in the nervous system are equivocal. This work was undertaken to examine the influence of propylthiouracil‐induced (PTU‐induced) pre‐ and postnatal hypothyroidism on collateral sprouting of noradrenergic (NA) axons in the habenula (Hb), following lesions in the stria medullaris (SM) of the adult rat. Ten pregnant dams were divided into control and PTU‐treated groups. Control rats had free access to food and water and the hypothyroid group received, in addition, 0.05% (w/v) of PTU in their drinking water, beginning on day 12 of gestation and continuing post partum through lactation until the offspring were weaned at age 22 days. The pregnant dams received, in addition, daily injections of thyroxine (25 μg/kg, ip) and only male pups were used in the study. At weaning he pups were removed from the dams and placed in cages with free access to food and water. The hypothyroid offspring received 0.05% (w/v) PTU in the drinking water until sacrificed at 10 weeks of age. At 6 weeks, some rats in each group received bilateral lesions in the SM and the remainder were sham‐operated. All rats were sacrificed 4 weeks after operation. Thus, four groups wre formed: (1) control/sham, (2) control/lesion, (3) PTU/sham, (4) PTU/lesion. Sprouting of NA fibers in discrete regions of the Hb was identified by norepinephrine (NE) levels and by fluorescence histochemistry. Blood levels of thyroid stimulating hormone (TSH) were determined. The results show that pre‐ and postnatal induced chronic hypothyroidism abolished NA sprouing in the partially deafferented Hb. Furthermore, lesions of the SM resulted in a marked decrease in serum TSH levels.