5‐Hydroxytryptamine, substance P, and thyrotropin‐releasing hormone in the adult cat spinal cord segment L7: Immunohistochemical and chemical studies

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Abstract
The terminal projections of the descending 5‐hydroxytryptamine (5‐HT) bulbospinal pathway and the coexistence among 5‐HT‐, substance P (SP)‐, and thyrotropin‐releasing hormone (TRH)‐like immunoreactivities (LI) in fibers innervating the L7 segement in the cat spinal cord were studied quantitively by use of the indirect double‐staining immunofluorescence technique. The content of 5‐HT, SP, and TRH in different parts of the spinal cord was determined by use of radioimmunoassay (RLA) (SP and TRH) and high‐performance liquid chromatography with electrochemical detection (HPLC‐ECD) (5‐HT).For all three substances studied, immunoreactive (IR) axon terminals were found in all parts of the gray matter, but with clear regional variation in the density of innervation. Thus, all three substances showed a dense innervation in the motor nucleus, particularly in the ventral part of the nucles, while the superficial dorsal horn was very densely innervated by SP‐IR fibers (laminae I and II) and TRH‐IR fibers (laminae II and III). In the motor nucleus, the studied substances coexisted to a very high degree, but some 5‐HT‐IR fibers (about 10%) lacked peptide‐LI and some SP‐IR fibers (about10%) lacked peptide‐LI and some SP‐IR fibers (about10%) lacked 5‐HT‐LI while virtually all TRH‐IR fibers also contained 5‐HT‐LI. In the superficial dorsal horn (laminae I‐III), no coexistence was detected, while other parts of the gray matter displayed various degrees of coexistence in between those found in the motor nucleus and laminae I‐III.The quantitative analyis of IR varicosities in the motor nucleus suggested that the unilateral L7 motor nucleus is innerated by about 55–110 × 106 5‐HT‐IR nerve trminals, which may indicate as many as 4,000 boutons per descending 5‐HT cell body in the brain stem only with this restricted projection. When combining these results with the biochemical data, it could be calculated that the concentration of 5‐HT in IR varicosities is about 3–6 × 10−3M, while the corresponding figures for SP and TRH was 0.3–0.5 × 10−3 M and 0.1–0.2 ×10−3 M, respectively.In cats subject to spinal cord transection at the lower thoracic level, all 5‐HT‐IR fibers in the L7 segement had disapperared 44 days after the lesion, indicating a strict suprasegmental origion of 5‐HT‐IR fibers in this segment. A differential effect of the lesion was noticed, however, in that the dorsally projecting 5‐HT fibers without peptide‐LI had vanished already 74 days postoperatively, while a large part of the 5‐HT‐IR fibers to the motor nucleus with peptide‐LI still remained at this time interval. Furthermore, after the lesion increased numbers of SP‐IR cell bodies could be observed in the gray matter.Combined spinal cord transection and unilateral dorsal rhizotomy caused a transient decrease of the dense SP‐IR plexus in laminae I and II on the rhizotomized side, whereas in this part of the cord CGRP‐LI completely disappeared.It is concluded that the descending 5‐HT system constitutes a major innervation of the spinal cord and that it seems especially prominent in the motor nucleus, where it, in all probability, exerts direct effects on the motoneurons. The findings are discussed in relation to recent findings regarding possible functional implications of the 5‐HT pathway on spinal motoneurons in the cat.

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