Descending serotonergic spinal projections and modulation of locomotor rhythmicity in Rana temporaria embryos

The neuroanatomy of descending spinal projections from serotonergic raphe interneurons in embryos of the amphibian, Rana temporaria, has been examined around the time of hatching by using immunocytochemical techniques. The results illustrate that at this early stage in development the ventrolateral spinal cord is richly innervated by 5HT immunoreactive (5HTi) raphe spinal axons and associated growth cones. Other regions are devoid of processes. In conjunction, the effects of bath applied 5-hydroxytryptamine (5HT, serotonin) and its metabolic precursor, 5-hydroxytryptophan (5HTP) on locomotor activity, was also investigated by monitoring ventral root activity during fictive swimming in immobilized animals. Fictive swimming activity is similarly modulated by both exogenously applied 5HT and enhanced endogenous release of 5HT (using 5HTP). These agents increase the duration and intensity of ventral root burst, decrease cycle frequency, lengthen rostrocaudal phase delays and reduce swimming episode duration. We conclude that by the time of hatching in Rana temporaria a functional endogenous serotonergic system is established in the spinal cord which modulates the output of the central pattern generator for swimming. We compare and contrast these results with homologous descending pathways in other vertebrates, especially in a related amphibian Xenopus laevis at equivalent stages in development.