Thyrotropin‐releasing hormone (TRH)‐like immunoreactivity in the grey monkey (Macaca fascicularis) spinal cord and medulla oblongata with special emphasis on the bulbospinal tract

Abstract

The distribution of thyrotropin‐releasing hormone (TRH)‐like immunoreactivity (LI) has been studied in the grey monkey (Macaca fascicularis) spinal cord and medulla oblongata by the use of indirect immunofluorescence and the peroxidase–antiperoxidase (PAP) technique. Furthermore, double‐labeling experiments were performed in order to study colocalization of 5‐hydroxytryptamine (5‐HT)‐ and substance P‐LI. A dense innervation of TRH‐immunoreactive (IR) varicose fibers was found in the ventral horn motor nuclei, in the region surrounding the central canal, in the intermediolateral cell column, and in the dorsal horn laminae II and III. In addition, cell bodies harboring TRH‐LI were found in the dorsal horn laminae II–IV. In the ventral horn, many of the large cell bodies and their proximal dendrites were totally encapsulated by TRH‐IR fibers. From double‐labeled sections a high degree of coexistence could be established between TRH‐/5‐HT‐LI, TRH‐/substance P‐LI, and 5‐HT‐/substance P‐LI in fibers in the motor nuclei; as a consequence, a large proportion of these fibers should harbor TRH‐/5‐HT‐/substance P‐LI. A coexistence between TRH‐/5‐HT‐LI could also be demonstrated in the intermediolateral cell column. However, no unequivocal coexistence could be found between TRH‐/substance P‐LI and 5‐HT‐/substance P‐LI in this region. In the dorsal horn, no clear coexistence could be encountered for any of the above indicated combinations. Electron microscopic analysis of material from the lumbar lateral motor nucleus demonstrated TRH‐IR terminals making synapses with large cell bodies and dendrites. In addition, contacts lacking synaptic specializations could also be verified.In the medulla oblongata, with the use of the PAP technique, a large number of cell bodies containing TRH‐LI were encountered in the midline raphe nuclei and in nucleus reticularis lateralis. A similar distribution pattern could be found for 5‐HT‐LI, but no cell bodies containing substance P‐LI could be seen in these regions.Chemical analysis of specimens from cervical, thoracic, and lumbar spinal cord revealed higher concentrations of TRH‐ and 5‐HT‐LI in the ventral quadrants, whereas substance P‐LI dominated in the dorsal quadrants. Thus, the concentrations of TRH‐, 5‐HT‐, and substance P‐LI was in accordance with the observed regional variation in density of IR‐fibers and varicosities found in the spinal cord.We have shown that TRH‐LI has a distribution in the monkey spinal cord and medulla oblongata similar to that previously demonstrated in other species. Furthermore, the TRH‐IR terminals found in the motor nuclei and nucleus intermediolateralis seem to be identical to the 5–HT innervation. In addition, TRH/5‐HT‐containing fibers in the motor nucleus also harbor substance P‐LI. The findings are discussed in relation to previous results in other species.