Thyrotropin-releasing Hormone Receptor Binding Research Articles

Regulation of excitation-secretion coupling by thyrotropin-releasing hormone (TRH): evidence for TRH receptor-ion channel coupling in cultured pituitary cells.

The electrophysiological and secretory properties of a well-studied clonal line of rat anterior pituitary cells (GH3) have been compared with a new line of morphologically distinct cells derived from it (XG-10). The properties of the latter cells differ from the parent cells in that they do not have receptors for thyrotropin-releasing hormone and their basal rate of secretion is substantially higher (ca. three- to fivefold). While both cell types generate Ca++ spikes, the duration of the spike in XG-10 cells (ca. 500 msec) is about 2 orders of magnitude longer than that in GH3 cells (5-10 msec). The current-voltage characteristics of the two cell types are markedly different; the conductance of GH3 cells is at least 20-fold higher than XG-10 cells when cells are depolarized to more positive potentials than the threshold for Ca++ spikes (approximately -35 mV). While treatment of GH3 cells with the secretagogues tetraethylammonium chloride or thyrotropin-releasing hormone decreases the conductance in this voltage region to approximately the same as that for XG-10 cells, the electrophysiological and secretory properties of XG-10 cells are unaffected by treatment with either of these agents. Results of this comparative study suggest that XG-10 cells lack tetraethylammonium-sensitive K+ channels. The parallel loss of thyrotropin-releasing hormone receptor binding activity and of a K+ channel in XG-10 cells implies that the thyrotropin-releasing hormone receptor may be coupled with, or be an integral part of, this channel. Apparently thyrotropin-releasing hormone, like tetraethylammonium chloride, acts by inhibiting K+ channels resulting in a prolongation of the action potential, promoting Ca++ influx and subsequently enhancing hormone secretion.

Thyrotropin releasing hormone and analogues rapidly lose their TRH receptor binding capacities in solution--a discrepancy from degradation.

Thyrotropin releasing hormone and analogues rapidly lose their TRH receptor binding capacities in solution--a discrepancy from degradation.

TRH receptor binding in retina and pituitary: Major species variation

Retinas and anterior pituitary glands of nine readily available mammalian and one avian species have been examined for their TRH-sensitive binding of [3H]-[3-Me-His2]TRH. Among mammals, major species variations in TRH receptor binding have been detected in both tissues, amounting to about 100-fold in the retina and 20-fold in the pituitary. In the retina, TRH receptor binding was very high in the rat, quite high in sheep and guinea pig, intermediate in rabbit, pig, and mouse, and low but detectable in chicken, cat, calf and dog. In the pituitary gland, binding was very high in sheep, quite high in rabbit, rat, pig, calf and guinea pig, intermediate in chicken, and fairly low in mouse, cat, and dog. A number of possible interfering variables, including affinity differences, albino vs. non-albino strains, barbiturate anesthesia, time after death, sex and estrous cycle, age, and history of light exposure, were considered and/or tested directly, with generally negative results.

Characteristics of thyrotropin releasing hormone (TRH) receptors in rat brain

Characteristics of TRH-receptors were studied in the rat central nervous system (CNS). Ion species, pH and temperature importantly influenced TRH-receptor binding. Subcellular distribution of TRH-receptor binding revealed that synaptic membranes had the greatest percentage of total sites. Scatchard analysis suggested that the rat CNS had two distinct TRH binding sites with apparent dissociation constants (K d) of 5×10 −9 M and 13×10 −8 M. Receptor activity is sensitive to trypsin and phospholipase A digestion, suggesting that protein and phospholipid moieties are essential for the binding of [ 3H]TRH. Thiol reagents reduced the binding activity of the receptor, suggesting that an intrachain disulfide bond may form an important constituent of the binding site for TRH. The TRH-receptor in the rat brain was successfully solubilized with non-ionic detergent Triton X-100. On gel chromatography with Sepharose 6B column, the solubilized TRH-receptor molecule eluted at the fraction corresponding to an apparent molecular weight of 300,000 daltons, with Stokes' radius of 5.8 nm. The regional distribution of TRH-receptor binding was examined to clarify the site of TRH action. The highest level of binding was in the hypothalamus, cerebral cortex and hippocampus, indicating that TRH affects the CNS function mainly through the limbic system, cerebral cortex and hypothalamus. Moreover, tricyclic anti-depressants and Li + decreased the binding of [ 3H]TRH. These findings suggest that endogenous TRH and TRH receptor may play the role of a neurotransmission modulator in the brain to control emotional and mental functions.

TRH receptor binding in avian pituitary and brain

The pituitary gland of the domestic fowl binds [ 3H]-[3-methyl-His 2]thyrotropin-releasing hormone ([ 3H]MeTRH) with properties very similar to those exhibited by mammalian TRH receptors, including affinity ( K D = 4.9 n M), density of binding sites ( B max = 5.1 pmol/g), and pharmacology for eight TRH analogs. Chicken brain appears to contain similar binding sites, but the level of binding was too low for detailed characterization.

Discrete regional distributions of thyrotropin releasing hormone (TRH) receptor binding in monkey central nervous system

Thyrotropin releasing hormone (TRH) directly influences central nervous system (CNS) function, independent of its pituitary action. Although these CNS effects have been behaviorally characterized, information is not yet available on the precise regional distribution of its receptor. TRH receptor binding was examined in the monkey CNS by the radioreceptor assay for clarifying the site of TRH action. TRH was bound to brain tissue membranes via high-affinity ( K d = 5.9 × 10 −9M) and low-affinity ( K d = 11.2 × 10 −8M) components. TRH receptor binding varied dramatically throughout the monkey brain, with more than a 40-fold variation. The limbic system contained the greatest amount of binding. The next highest areas were the cerebral cortex, hypothalamus, interpeduncular nucleus and periaqueductal gray matter of the midbrain. Receptor binding was very low or not detectable in the medial thalamus, cerebellum, brain stem, spinal cord and white matter. These data suggest that TRH has an effect on the CNS via limbic system, cerebral cortex and midbrain.