Ž . Developmental Brain Research 111 1998 305–314 Interactive report Infraorbital nerve transection alters serotonin transporter expression in sensory pathways in early postnatal rat development 1 Stefan R. Hansson a, ),2 , Theresa M. Cabrera-Vera a,b , Beth J. Hoffman a,c a Unit on Molecular Pharmacology, Laboratory of Cellular and Molecular Regulation, National Institute of Mental Health, Bethesda, MD 20892, USA b Institute For Neuroscience, Northwestern UniÕersity, Chicago, IL 60611, USA c Lilly Research Laboratories, Lilly Corporate Center, Indianapolis, IN 46285, USA Accepted 4 October 1998 Abstract 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 Ž . 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 w 3 x neurons as well as in the visual cortex. Lesion of the infraorbital nerve decreased the H 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 Ž . 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. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Somatosensory cortex; Whisker; Lateral geniculate; Vibrissae; Cingulate cortex; Autoradiography; In situ hybridization 1. Introduction. In rats, the whisker region relays its sensory informa- tion through the trigeminal system. Two major relay sta- Ž tions i.e. the trigeminal nuclei in the brainstem and the . ventrobasal complex of the thalamus process the informa- tion from one individual vibrissa to the somatosensory Ž . cortex S1 via the infraorbital nerve. The cortical somato- topic representation of the whisker region develops during ) Corresponding author. Department of Physiology and Neuroscience, Wallenberg Neuroscience Center, University of Lund, Solvegatan 17, ¨ S-223 62 Lund, Sweden; Tel.: q46-46-2220571; Fax: q46-46-2220567; E-mail: Stefan.Hansson@mphy.lu.se 1 Published on the World Wide Web on 23 October 1998. 2 On leave of absence from the Division of Molecular Neurobiology, Wallenberg Neuroscience Center, University of Lund, Sweden. wx the first postnatal weeks 2 . Every individual whisker is represented in S1 by discrete cytoarchitectonic units of w x stellate neurons in layer IV known as cortical barrels 52 . The development of this precise somatotopic representa- w x tion is dependent on peripheral cues 34,41 . For example, w x if the vibrissa follicles 25,45,53 or the infraorbital nerve Ž ION; the trigeminal V branch that innervates the vibrissa . follicles is injured during a critical period of development w x 26,27,45 , severe structural alterations occur with disrup- tion of the normal thalamocortical afferents clustering in somatosensory cortex, ventrobasal thalamus and the wx trigeminal nucleus 8 . w x In addition to overt peripheral cues 30,44 , it has been Ž suggested that the neurotransmitter, serotonin 5-hydroxy- . tryptamine; 5-HT , might play an important, transient role during development of thalamocortical projections. During 0165-3806r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. Ž . PII: S0165-3806 98 00148-5