Guanine nucleotides modulate TRH-receptor binding in sheep anterior pituitary

Diversity of thyrotropin-releasing hormone receptors in the pituitary and discrete brain regions of rats

In order to analyze the receptor properties of central nervous system (CNS)-stimulant thyrotropin-releasing hormone (L-pyroglutamyl-L-histidyl-L-prolinamide, TRH), we evaluated the binding of TRH and its analog taltirelin hydrate ((-)-N-[(S)-hexahydro-1-methyl-2,6-dioxo-4-pyrimidinylcarbonyl]-L- histidyl-L-prolinamide tetrahydrate; taltirelin, TA-0910) in rat anterior pituitary and several brain regions. There was a specific binding of [3H]methyl TRH (MeTRH) in the anterior pituitary, hypothalamus, brain stem, cerebral cortex and cerebellum with Kd values of 1.0-1.6 nM. The inhibition of [3H]MeTRH binding by TRH and taltirelin was monophasic in the anterior pituitary, hypothalamus and brain stem with Ki values of 6.3-8.0 nM and 145.5-170.4 nM for TRH and taltirelin, respectively. In contrast, the biphasic inhibition was revealed in the cerebral cortex and cerebellum. The Ki values for TRH and taltirelin were 4.1-4.3 nM and 67.8-73.4 nM for the high affinity binding site and 3.6-4.2 microM and 82.3-197.5 microM for the low affinity binding site, respectively. Addition of 100 microM GTP or its analog 5'-guanylylimidodiphosphate (Gpp[NH]p) affected neither the biphasic inhibition by TRH nor that by taltirelin. Thus the results suggest the presence of distinct high and low affinity TRH receptors in the CNS in contrast to the pituitary.

Is signal transduction modulated by an interaction between heterotrimeric G-proteins and tubulin?

Although it is generally accepted that tubulin plays an important role in G-protein-mediated signal transduction in a variety of systems, the mechanism of this phenomenon is not completely understood. G-protein-tubulin interaction at the cell membrane and the cytosol, and the influence of such an interaction on cellular signaling are discussed in this review article. Because the diameter of a microtubule is 25 nm and the plasma membrane is 9-11 nm thick, it is not possible for membrane-associated tubulin to assemble into a complete microtubule in the membrane environment. However, tubulin heterodimers may be able to function in the membrane environment as individual heterodimers or as polymers arranged into short protofilaments. At the cell membrane, membrane-associated tubulin may influence hormone-receptor interaction, receptor-G-protein coupling, and G-protein-effector coupling. Structural proteins, such as tubulin, can participate in cellular signaling by communicating through physical forces. By virtue of its interaction with the submembranous network of cytoskeletal proteins, tubulin, when perturbed in one locus, can transmit large changes in conformations to other points. Thus, GTP binding to membrane-associated tubulin might lead to a conformational change in either receptors or G proteins. This may, in turn, influence the binding of an agonist to its receptor. On the other hand, in the cell cytosol, subsequent to agonist-induced translocation of G-proteins from the membrane compartment to the cytosol, G-proteins may affect microtubule formation. In GH3 and AtT-20 cells (stably expressing TRH receptor), transiently transfected with Gq alpha cDNA, soluble tubulin levels decreased in Gq alpha-transfected GH3 and AtT-20 cells, by 33% and 52%, respectively. These results suggest that G-proteins may have a direct effect on the microtubule function in vivo. Because tubulin and G-protein families are ubiquitous and highly conserved, an interaction between these two protein families may occur in vivo, and this, in turn, can have an impact on signal transduction. However, the physiological significance of this interaction remains to be demonstrated.

Gonadotropin-releasing hormone and thyrotropin-releasing hormone regulation of g protein function in the rat anterior pituitary lobe

Abstract In order to evaluate the role of guanine nucleotide-dependent transducer proteins (G proteins) in hormone-mediated signal transduction in the anterior pituitary lobe, we examined the effect of gonadotropin-releasing hormone (GnRH) and thyrotropin-releasing hormone (TRH) on two parameters of G protein function, namely [(35) S]GTP(gamma)S binding and low K(m)GTPase activity. Plasma membranes were prepared from anterior pituitary lobes of adult male rats using conventional procedures. GTP binding was determined by incubating 2 to 5 mug membrane protein with approximately 100,000 cpm [(35) S]GTP(gamma)S in a buffer containing 20 mM Tris- HCl, 1 mM EDTA, 1 mM dithiothreitol, and 100 mM NaCI at a pH of 7.4 for 10 or 15 min at 37 degrees C GnRH agonist and TRH stimulated high affinity [(35) S]GTP(gamma)S binding in a concentration-dependent manner. GTP binding was maximally stimulated by GnRH agonist (1 muM) and TRH (0.1 muM) by up to 27% and 34%, respectively. A time-course study revealed that 1 muM GnRH agonist stimulated GTP binding by 30% at 15 min; 0.1 muM TRH stimulated GTP binding by 23% at 1 min, 18% at 5 min and 25% at 10 min. A stable GTP analog, 5'-guanylylimidodiphosphate, inhibited GnRH- as well as TRH-stimulated GTP binding. GnRH antagonist did not affect GTP binding. However, in the presence of the antagonist, stimulation of GTP binding by the GnRH agonist was completely blocked. The low K(m)GTPase activity (EC 3.6.1.-), another parameter of G protein function, was assayed in 2 to 5 mug membrane protein using [gamma-(32) P]GTP at 37 degrees C in an ATP-regenerating buffer containing 1 muM unlabeled GTP. GnRH agonist (0.1 muM) and TRH (1 muM) maximally stimulated this GTPase activity by up to 50% and 40%, respectively. GnRH agonist (1 muM) stimulated the GTPase activity by 30% at 10 min and 48% at 30 min. TRH (1 muM) stimulated the GTPase activity at all time points monitored; stimulation was 46% at 5 min, 49% at 20 min, and 41% at 30 min. Interestingly, the GnRH antagonist stimulated GTPase activity by about 20%, but inhibited GnRH agonist-stimulated GTPase activity in a concentration-dependent manner. These results indicate that the binding of GnRH and TRH to their receptors results in interaction of the receptor with a G protein and activation of the G protein cycle.

Guanine nucleotide regulation of receptor binding of thyrotropin-releasing hormone (TRH) in rat brain regions, retina and pituitary

Guanine nucleotides inhibited the specific binding of [3H](3-Me-His2)thyrotropin-releasing hormone ([3H]MeTRH) to receptors for TRH in washed homogenates of rat anterior pituitary gland in a dose-related manner. The order of potency (at 100 and 500 microM final) was Gpp(NH)p (a stable analog of GTP) greater than GTP much greater than GDP much greater than cGMP (with the adenine nucleotides being inactive) in the pituitary and various brain regions. Gpp(NH)p at 1 mM caused 17-35% inhibition of [3H]MeTRH binding to different tissues including the pituitary, hypothalamus, retina and nucleus accumbens. A statistically significant nucleotide effect was not observed, however, in the olfactory bulb and medulla/pons membranes. Gpp(NH)p (1 mM) increased the dissociation constants for [3H]MeTRH binding by 1.9- to 2.4-fold in the pituitary, n. accumbens and retinal preparations without altering the apparent binding capacity. These data suggest that TRH receptor binding can be allosterically regulated by guanine nucleotides and provide further evidence for the existence of guanine nucleotide binding protein(s) coupled to the TRH receptor.

Kinetic analysis of thyrotropin-releasing hormone binding in the central nervous system: evidence for receptor desensitization

To investigate the mechanism(s) of experimentally and clinically observed refractoriness of spinal lower motor neurons (LMNs) to the excitatory effects of high-dose TRH, we examined the kinetics of dissociation of [3H]TRH from its CNS-receptor. At 23 degrees C, the receptor was rapidly (40 min) and completely converted from a form with fast dissociation kinetics (complex I; t1/2 20-30 min) to one from which the peptide dissociated much more slowly (complex II; t1/2 greater than 120 min). This conversion required the presence of added agonist ([3H]TRH) and was not prevented by the GTP-analog Gpp(NH)p. We suggest that complexes I and II may respectively represent active and inactive (desensitized) forms of the TRH-receptor and that TRH-induced I to II conversion of the receptor is responsible for refractoriness of LMNs to the drug.

Thyrotropin-releasing hormone stimulates GTP hydrolysis by membranes from GH4C1 rat pituitary tumor cells

Thyrotropin-releasing hormone (TRH) stimulates prolactin production by GH4C1 rat pituitary tumor cells, which possess high-affinity membrane receptors for the peptide. TRH caused up to a 50% increase in the activity of a low-Km GTPase in membranes from GH4C1 cells. The TRH stimulatory effect was maximal at GTP concentrations of 1 microM or lower. TRH caused an increase in GTPase activity of between 0.2 and 20 pmol of GTP hydrolyzed per mg of protein per min, depending on GTP concentration, while TRH binding was 0.3 pmol/mg of protein. TRH did not stimulate GTPase activity in membranes from GH12C1, or GH-Y cells, two pituitary lines lacking TRH receptors. Stimulation of GTPase depended on occupancy of the TRH receptor; half-maximal increases in GTPase activity required 46 nM TRH and 25 nM [N3-methyl-His]TRH, but the TRH free acid was inactive. The apparent Kds of these peptides for receptors were similar when measured under the same conditions. The fact that TRH binding to receptors is regulated by guanyl nucleotides, together with the demonstration of TRH stimulation of low-Km GTPase activity, suggests that the TRH receptor is associated with a guanyl nucleotide regulatory protein in the lactotroph membrane.

Receptors for thyrotropin-releasing hormone (TRH) in rabbit spinal cord

Specific binding of 3H-labeled [3-Me-His2]TRH ([3H]MeTRH) to membranes of rabbit spinal cord (thoracic) involved a homogeneous population of high-affinity sites (Kd = 2.7 +/- 0.17 (n = 5) nM, Bmax = 204 +/- 12(5) fmol/mg protein). TRH analogs competed for the binding in the following rank order of potency: MeTRH greater than TRH greater than TRH-Gly-NH2 greater than Ser-His-Pro-NH2 greater than Thr-His-Pro-NH2 greater than pGlu-His-Pro-NH-C2H5 greater than TRH-free acid. Competition experiments with rat amygdala, run in parallel with rabbit spinal cord, revealed a closely similar pattern of activity. These properties help identify binding sites in the rabbit spinal cord as physiological receptors for TRH. The binding sites resemble receptors previously demonstrated in pituitary and CNS tissues of other species. The densities of [3H]MeTRH binding sites in different segments were generally similar, although density in the thoracic segment appeared to be somewhat higher. In all segments, binding seemed to be enriched in the dorsal gray matter. Dorsal roots and their associated ganglia, however, displayed little or no binding.

Properties of [3H](3-Me-His2)TRH binding to apparent TRH receptors in the sheep central nervous system

[3H](3-methyl-His2)thyrotropin releasing hormone ([3H]MeTRH) binds to sites in the sheep central nervous system (CNS) whose properties closely resemble both those of CNS binding sites for [3H]TRH and those of pituitary binding sites for [3H]MeTRH. Detailed studies for binding of [3H]MeTRH in the sheep nucleus accumbens and retina have yielded equilibrium dissociation constants of about 4 nM and densities of binding sites of about 3 and 2 pmol/g wet weight, respectively. The binding affinity of [3H]MeTRH was 8- to 10-fold higher than that of [3H]TRH, resulting in much lower non-specific binding with the new ligand. The association reaction had a rate constant of about 2-3 x 10(7) M-1 min-1, while the biphasic dissociation reaction had rate constants of 8-9 x 10(-2) min-1 for the fast phase and 1-2 x 10(-2) min-1 for the slow phase. The regional distribution of binding in the sheep CNS was similar to that observed previously with [3H]TRH. Highest binding outside the pituitary was in the nucleus accumbens area and retina, with another peak in the amygdala-temporal cortex area. Binding was widely distributed, so that no CNS region appeared totally devoid of binding. Nineteen TRH analogs, ranging in potency over 6 orders of magnitude, showed nearly identical abilities to complete for binding of [3H]MeTRH in the CNS areas and in the sheep anterior pituitary gland in side-by-side experiments. These findings argue strongly for identification of [3H]MeTRH binding sites in the CNS as TRH receptors.