Regional Localization and Subcellular Compartmentalization of Thyrotropin‐Releasing Hormone in Adult Human Brain

Abstract: In the current study, we sought to define the subcellular compartmentalization of thyrotropin‐releasing hormone (TRH) in adult human brain tissues. Upon evaluating tissues (3–24 h post mortem) from 62 humans, ranging in age from 5 to 75 years, we found that TRH was widely distributed throughout the brain. The highest TRH concentration (ng/mg protein) was in the stalk‐median eminence region of the hypothalamus (19.3 ± 3.3, mean ± SE); the TRH concentration in the hypothalamus, exclusive of the stalk‐median eminence, was much lower (1.7 ± 0.2). Substantial quantities of TRH also were detected in the medulla oblongata (0.26 ± 0.08), mammillary bodies (0.33 ± 0.25), and optic chiasm (0.14 ± 0.07). Lower levels of TRH were found in the amygdala (0.060 ± 0.015) and the corpus striatum (0.033 ± 0.010). TRH was near or below the limits of detection in tissues of the cerebral and cerebellar cortices, the olfactory bulbs, the pons, and the hippocampus. When homogenates of medial basal hypothalamic tissue (preparedin 0.32 M sucrose‐10 μM CaCl₂) were fractionated by means of differential centrifugation, most of the TRH was recovered in subcellular particles which were pelleted at 10,000 ×g and which contained the highest amounts of occluded LDH activity. When the nuclei‐free supernatant fluid (900 ×g S) was fractionated on discontinuous sucrose density gradients or continuous sucrose density gradients, most of the TRH was recovered in subcellular fractions containing synaptosomes. The subcellular distribution of TRH appeared to be stable for up to 24 h post mortem in rat and human brain tissue. In contrast to findings with homogenates of rat hypothalami, exposure of subcellular particles of human hypothalamic origin to hypoosmotic shock caused only minor release of particle bound TRH, which was recovered in the cytosolic fraction. Also in contrast to findings with rat hypothalamic tissue, we were unable to demonstrate convincingly the presence of TRH containing subsynaptosomal particles in homogenates of human hypothalami which were resistant to hypoosmotic shock.