TRANSIENT POSTNATAL THYROXINE TREATMENT LEADS TO AN INCREASED NUMBER OF CHOLINERGIC NEURONS IN THE MEDIAL SEPTUM AND TO A HIGHER DENSITY OFCHOLINERGIC FIBERS IN HIPPOCAMPAL CA3 IN RATS

Newborn male pups of Sprague-Dawley rats received daily injections of buffered L-thyroxine or saline during the first 12 days postnatally. A t the age of 12 weeks, the number of immunocytochemically labeled chol inergic neuronal profiles in the medial septum/diagonal band (MS/vDB) was counted, and the density of acetylcholinesterase (AChE)-positive f ibers in four hippocampal subregions was measured. Thyroxine-treated r ats had more ChAT-positive neuronal profiles in the MS/vDB and a highe r density of AChEpositive fibers in hippocampal CA3 as compared to con trols. From this study it is concluded that transient postnatal hypert hyroidism leads to an increased cholinergic innervation of hippocampal CA3.