Pre- and post-natal ontogeny of thyrotropin-releasing-hormone in the rat spinal cord: an immunocytochemical study.

This work aimed at providing by means of immunocytochemical techniques a detailed study of the ontogeny of thyrotropin-releasing hormone (TRH) in the spinal cord of the rat. We report the first appearance of TRH-immunoreactive fibers in the ventral funiculus of thoracic and lumbar levels at embryonic day 17. At embryonic day 18, fibers penetrated the ventral gray matter towards the central canal. At embryonic day 19, the first immunoreactive fibers were seen in the intermediolateral cell column at upper thoracic levels. This region was invaded at lower thoracic levels on the day of birth. At this time, TRH-immunoreactive axodendritic synapses were observed in the ventral horn and in the intermediolateral cell column. Immunoreactivity increased in these regions until post-natal day 21 when the adult pattern of TRH immunoreactivity was established in the sympathetic nuclei and in the ventral horn. However, a transient TRH-like immunoreactivity was detected in lamina IIi of the dorsal horn between post-natal days 14 and 30: at ultrastructural level, immunoreactive varicosities were seen to establish axodendritic synapses. In conclusion, TRH is one of the earliest peptidergic systems established in the spinal cord and it presents extensive temporal and topographical similarities with the serotonergic system with which it could be colocalized.