Quantitation and Characterization of Thyrotropin‐Releasing Hormone in Vagal Nuclei and Other Regions of the Medulla Oblongata of the Rat

Abstract: Since evidence is now available to support a nonendocrine autonomic function for thyrotropin‐releasing hormone (TRH), quantitative measurements of TRH were made in nuclei of the vagal complex and other areas of the caudal medulla oblongata of the rat. Regions containing the dorsal motor nucleus of the vagus (DMN), nucleus tractus solitarius (NTS), hypoglossal nucleus, dorsal column nuclei, descending nucleus V (DNV), nucleus ambiguus (NA), raphe nuclei (MR) dorsomedial and ventromedial reticular formation, and inferior olivary nuclei were isolated from 300‐μm‐thick frozen sections of medulla by the micropunch technique. Each region was pooled bilaterally, homogenized in 0.1 M HCl, and vacuum‐dried. Extracts were assayed for TRH by specific radioimmunoassay (RIA). TRH levels varied 100‐fold among medulla nuclei. Highest content (ng/mg protein ± SEM) was found in DMN (14 ± 1.38) and NTS (4.7 ± 0.68), whereas lowest levels occurred in the DNV and MR (0.13, 0.06). Nearly 65% of the total medullary TRH was localized in nuclei associated with vagal complex (DMN, NTS, NA). Characterization of tissue immunoreactivity (TRHi) in these regions suggests the presence of TRH, since (1) medullary tissue extracts competed with 125 I‐TRH for antibody binding sites with the same affinity as authentic TRH; (2) TRHi in tissue extracts co‐migrated with synthetic TRH when subjected to reverse‐phase high performance liquid chromatography and Sephadex G‐10 chromatography; and (3) rat serum TRH peptidases degraded TRHi and authentic TRH at similar rates. Another group of rats was subjected to unilateral (right side) vagotomy. At 33 weeks post‐vagotomy, the vagal preganglionic cell population in the ipsilateral DMN was depleted 50–75%, while the contralateral side was unaffected. Interestingly, the content of TRH in the ipsilateral (right) DMN remained unchanged, whereas TRH in the contralateral DMN increased by 50%. In contrast, TRH was significantly elevated in the NA on the ipsilateral side of the lesion. TRH in both ipsi‐ and contralateral NTS was unchanged when compared with sham‐operated controls. These results indicate that (1) TRH is present in several specific loci of the medulla; (2) very high levels are found in the vagal complex; and (3) vagotomy may alter TRH in the contralateral DMN and ipsilateral NA.