Transient postnatal thyroxine treatment leads to an increased number of cholinergic neurons in the medial septum and to a higher density of cholinergic 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. At the age of 12 weeks, the number of immunocytochemically labeled cholinergic neuronal profiles in the medial septum/diagonal band (MS/vDB) was counted, and the density of acetylcholinesterase (AChE)-positive fibers in four hippocampal subregions was measured. Thyroxine-treated rats had more ChAT-positive neuronal profiles in the MS/vDB and a higher density of AChE-positive fibers in hippocampal CA3 as compared to controls. From this study it is concluded that transient postnatal hyperthyroidism leads to an increased cholinergic innervation of hippocampal CA3.