Changes in distribution of serotonin induced by spinal injury in larval lampreys: Evidence from immunohistochemistry and HPLC

Larval lampreys are known to successfully recover normal behavior following spinal cord injury. More recently, we showed temperature can influence functional recovery, with colder temperatures more likely producing behavioral abnormality despite the cold being the animals' normal temperature. Here we analyze the differences associated with temperature effects. We examine serotonergic expression along the spinal cord following midbody lesions considering time from injury, temperature during recovery and fiber location. We also examine the relationship between regeneration and locomotion, insofar as the pattern of muscle potentials during unrestrained swimming is normal or abnormal. At 26 weeks after spinal cord injury in all groups of animals, immunohistochemistry and HPLC for serotonin and serotonin expression above and below the lesion can be significantly changed in all regional sources of serotonin independent of the temperature animals recover from their injuries. Animals from warmer tanks recover serotonin expression in the segment immediately caudal to the lesion site with little further away from the lesion; animals from the cold room aquaria have significantly less recovery of expression caudal to the lesion and none further away. There was no apparent relationship between the distribution of serotonin and recovery. The changes suggest that some intraspinal reorganization has occurred. We propose a relationship between the observed results and functional recovery, but it remains conjectural. The fact that some animals recover normal function suggests plasticity must occur in animals successful in recovering normal function. Thus, the lamprey can be used as a model system to study the adaptive changes that permit or prevent functional recovery.