LOCALIZATION OF THE SEROTONIN TRANSPORTER IN RAT SPINAL-CORD

The spinal cord is richly innervated by serotoninergic fibres originat ing from the raphe nuclei. The localization of the terminating compone nt of serotoninergic neurotransmission, the serotonin transporter SERT 1, was found in both the dorsal and ventral horns, especially at the l evel of the cervical and lumbar segments. Within the thoracic region, we observed a heavily labelled bundle in the intermediolateral nucleus of lamina VII. A low density of stained fibres was encountered in the sacral spinal cord. in contrast to homogeneous staining of motor nucl ei, a differential labelling of laminae was seen in the dorsal horn, w ith laminae I, III and IV exhibiting a higher density of immunopositiv e terminals than the medial part of lamina If. High magnification reve aled a preferential accumulation of serotonin transporter staining wit hin nerve endings and varicosities of thin fibres. Double immunofluore scence staining demonstrated a co-localization of serotonin and its up take system within these varicosities. These results show that the ser otonin transporter is highly expressed in the rat spinal cord and that its distribution parallels the serotoninergic innervation. They also reinforce the view that varicosities are important neuronal structures , which modulate the function of dorsal and ventral horn neurons by re leasing serotonin.