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 sensory system late in gestation and during early postnatal development, a period known to be critical for maturation of the sensory circuitry. The purpose of the present study was to determine whether sensory denervation in newborn rat pups would alter either the density or pattern of expression of the 5-HT transporter (5-HTT) within the trigeminal system. We combined autoradiographic localization of 5-HT transporters and in situ hybridization techniques to visualize both the transporter protein and mRNA in thalamic sensory neurons and in the somatosensory cortex following unilateral infraorbital nerve transection at postnatal day 1. For comparative purposes, similar measurements were conducted in thalamic visual neurons as well as in the visual cortex. Lesion of the infraorbital nerve decreased the [3H]citalopram labelling of 5-HT transporters in the ventral basal and ventral medial areas of the thalamus contralateral to the lesion, while labelling of 5-HT transporters was decreased in both contralateral and ipsilateral sides of the lateral genicuate (visual thalamus). Citalopram labelling of 5-HT transporters was not significantly altered in somatosensory or in cingulate cortex, however a significant decrease was observed in the visual cortex. In contrast, there were no obvious changes in the intensity of the 5-HT mRNA hybridization signal in sensory or visual thalamic areas. Given that the serotonin transporter regulates extracellular concentrations of 5-HT, the present data suggest that altered peripheral innervation and thereby altered sensory inputs 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 concentrations of 5-HT in thalamus and cortex during a critical period of synapse formation.