Infraorbital nerve transection alters serotonin transporter expression in sensory pathways in early postnatal rat development

The serotonin transporter MRNA has been found throughout the trigeminal sen sory system late in gestation and during early postnatal development, a per iod known to be critical for maturation of the sensory circuitry. The purpo se of the present study was to determine whether sensory denervation in new born rat pups would alter either the density or pattern of expression of th e 5-HT transporter (5-HTT) within the trigeminal system. We combined autora diographic localization of 5-HT transporters and in situ hybridization tech niques to visualize both the transporter protein and mRNA in thalamic senso ry neurons and in the somatosensory cortex following unilateral infraorbita l nerve transection at postnatal day 1. For comparative purposes, similar m easurements were conducted in thalamic visual neurons as well as in the vis ual cortex. Lesion of the infraorbital nerve decreased the [H-3]citalopram labelling of 5-HT transporters in the ventral basal and ventral medial area s of the thalamus contralateral to the lesion, while labelling of 5-HT tran sporters was decreased in both contralateral and ipsilateral sides of the l ateral genicuate (visual thalamus). Citalopram labelling of 5-HT transporte rs was not significantly altered in somatosensory or in cingulate cortex, h owever a significant decrease was observed in the visual cortex. In contras t, there were no obvious changes in the intensity of the 5-HT mRNA hybridiz ation signal in sensory or visual thalamic areas. Given that the serotonin transporter regulates extracellular concentrations of 5-HT, the present dat a suggest that altered peripheral innervation and thereby altered sensory i nputs to the thalamus during fetal development can potentially influence 5- HT transporter densities and thus, may influence extracellular levels of 5- HT in thalamus and cortex during a critical period of synapse formation. In turn, modulation of 5-HT transporter levels may influence extracellular co ncentrations of 5-HT in thalamus and cortex during a critical period of syn apse formation. (C) 1998 Elsevier Science B.V. All rights reserved.