Persisting alterations in monoaminergic innervation patterns have been observed following various environmental manipulations and neuro-psychopharmacological treatments during fetal or early postnatal life. The present study investigates the question how differences in initial growth conditions at birth might interfere with subsequent development of both serotonergic and noradrenergic innervation in the rat frontal cortex (FC) and brain stem. For this purpose, newborn rat littermates were divided into two groups, a low and a high birth weight group, and the densities of both serotonin (5-HT) and norepinephrine (NE) transporters in the FC and brain stem were analyzed at adulthood. 5-HT transporter density in the FC was significantly higher in the high birth weight group as compared with the low birth weight group. No significant differences were observed between both groups in the density of 5-HT transporters in the brain stem and in the densities of NE transporters in FC and brain stem. It is discussed that differences in birth weight may affect the postnatal development of 5-HT projections to the frontal cortex.

The reasons are ambiguous. A possible answer could be an adaptation in higher fT4 levels in cause of the reported chronic LT4 overdosage.