Serotonin is a powerful neuromodulator of locomotor activity in the mammalian spinal cord. We have recently shown that locomotor-activated neurons are innervated by serotoninergic fibers and that a significant percentage of them possess 5HT7 receptors on their soma and proximal dendrites (Johnson et al. SFN Abstr. 2002). Since serotonergic denervation of these cells after spinal cord injury (SCI) will likely further compromise locomotor function, serotonin could possibly be used in transmitter replacement therapies to reverse these effects. We, therefore, examined the effect of chronic SCI by spinal transection on the pattern and distribution of these receptors to determine whether there are any long-term changes. The spinal cords of 9 cats were transected at the T8/9 level and the animals allowed to survive for 1, 3 and 6 months. Their lumbar spinal cords were processed for quantitative receptor autoradiography using an antibody directed against the receptor. Spinal cords were compared to control animals....