Thyrotropin releasing hormone: apparent receptor binding in retina

Pharmacologically distinct binding sites in rat brain for [3H]thyrotropin-releasing hormone (TRH) and [3H][3-methyl-histidine(2)]TRH

We have used a directed peptide library, in which the histidyl residue of thyrotropin-releasing hormone (TRH) was systematically replaced by a series of 24 natural and unnatural amino acids, to characterise TRH binding sites in rat brain cortex. This was achieved by measuring the ability of library peptides to compete with [3H][3-Me-His(2)]TRH or [3H]TRH binding to rat cortical homogenates. [3H][3-Me-His(2)]TRH was observed to bind to a single population of high-affinity, low-capacity sites (K(d): 4.54+/-0.62 nM, N=5; B(max): 4.38+/-0.21 fmol/mg wet weight tissue, N=5), consistent with them being central TRH receptors. Displacement studies showed TRH to bind to these sites with an apparent K(i) of 22 nM. K(i) values for the library peptides at [3H][3-Me-His(2)]TRH-labelled sites varied from 10(-3) to 10(-9)M; the potency order was: [3-Me-His(2)]>His>Thi>Leu,Phe,Asn>Gln, Arg, Thr, Ala, HomoPhe. All other replacements had K(i) values >10(-4)M. [3H]TRH was observed to label a single population of low-affinity, high-capacity sites (K(d): 7.55+/-1.23 microM, N=6; B(max): 3.40+/-0.63 pmol/mg wet weight tissue, N=6). The affinities of the synthetic peptides for [3H]TRH-labelled sites did not correlate with their affinities for [3H][3-Me-His(2)]TRH-labelled sites (r=0.33, N=18, P>0.1). They did, however, correlate significantly with previously reported binding affinities for TRH-degrading ectoenzyme (r=0.72, N=12, P<0.01). These results strongly indicate that the identity of the low-affinity, [3H]TRH-labelled site is the membrane-bound enzyme, TRH-degrading ectoenzyme, not a subpopulation of TRH receptors. They also provide the first comprehensive description of the influence of the histidyl residue in TRH on binding of TRH to brain receptors.

Identification of thyrotropin-releasing hormone receptor messenger RNA in the rat central nervous system and eye

TRH exerts a wide variety of neuropharmacological actions by interacting with specific receptors in the central nervous system (CNS). Specific binding sites for TRH have been identified also in the mammalian retina. However, whether TRH receptors (TRH-R) in the brain and retina are identical in structure with those in the anterior pituitary gland is presently unknown. In this study, TRH-R gene expression was examined by Northern blot analysis in the CNS and eye using a cloned rat pituitary TRH-R cDNA. Northern analysis demonstrated a specific hybridization band of approximately 3.8 kb in hypothalamus, cerebrum, cerebellum, brain stem, spinal cord, and eye, indistinguishable from that characterized in pituitary gland. These data strongly support the hypothesis that a TRH receptor similar or identical to that cloned from the pituitary occurs in the retina and throughout the CNS.

Norvaline2-TRH: binding to TRH receptors in rat brain homogenates

Norvaline2-thyrotropin-releasing hormone ([Nva2]TRH) has been described as a thyrotropin-releasing hormone (TRH) analog with no thyrotropin (TSH)-releasing capacity but enhanced analeptic activity compared with TRH, as shown by the reversal of haloperidol-induced catalepsy. We have evaluated the receptor-binding properties of [Nva2]TRH in homogenates of rat anterior pituitary, hypothalamus, brainstem and cortex tissue, using [3H]TRH and [3H][3-Me-His2]TRH as radioligands. Apparent Ki values at high affinity TRH-binding sites, labelled predominantly by [3H][3-Me-His2]TRH, ranged from 17.0 to 36.9 microM in all tested regions. Additionally, [Nva2]TRH was shown to compete with [3H]TRH at low affinity TRH-binding sites with similar affinities. It is concluded that the loss of TSH-releasing activity of [Nva2]TRH appears to be due to a drastic reduction in binding affinity to the high affinity TRH receptor subtype. Its analeptic activity, however, may be mediated by low affinity TRH binding sites which are predominantly labelled by [3H]TRH or by yet unidentified mechanisms.

Receptor binding of fluorinated histidine analogs of thyrotropin-releasing hormone in various regions of the rat brain

Binding properties of [4(5)-fluoro-imidazole-His2]-TRH (4(5)-F-TRH), [2-trifluoromethyl-imidazole-His2]-TRH (2-CF3-TRH) and [4(5)-trifluoromethyl-imidazole-His2]-TRH (4(5)-CF3-TRH), three novel TRH analogs, have been evaluated in rat pituitary, hypothalamus, brainstem and cortex tissue. 4(5)-F-TRH, previously shown to elicit arterial pressor responses and prolactin release similar to those of TRH, binds to TRH receptors with low, micromolar affinity (Ki = 7.5-13.5 microM). 2-CF3-TRH, an analog of less cardiovascular but increased prolactin-releasing activity, shows Ki values of 3.3-4.9 microM. 4(5)-CF3-TRH, which shows comparable biological activity to 2-CF3-TRH, demonstrates a binding affinity which is virtually nonspecific (Ki = 0.39-1.01 mM). It is therefore concluded that the biological effects of these analogs are mediated either through low affinity TRH binding sites not recognized by [3H][3Me-His2]-TRH or through mechanisms not involving TRH receptors as such.

Identification and characterization of atrial natriuretic factor receptors in the rat retina

The characteristics of atrial natriuretic factor (ANF) receptors where studied in rat retinal particulate preparations. Specific 125I-ANF binding to retinal particulate preparations was greater than 90% of total binding and saturable at a density (Bmax) of 40 +/- 8 fmol/mg protein with an apparent dissociation constant (Kd) of 6.0 +/- 2.0 pM (n = 3). Apparent equilibrium conditions were established within 30 min. The Kd value of 125I-ANF binding calculated by kinetic analysis was 4.0 pM. The Bmax of 60 +/- 10 fmol/mg protein and the Kd of 5 +/- 2 pM, calculated by competition analysis, were in close agreement with the values obtained from Scatchard plots or kinetic analysis. The 125I-ANF binding to retinal particulate preparations was not inhibited by 1 microM concentration of somatostatin, vasopressin, vasoactive intestinal peptide, adrenocorticotropin, thyrotropin releasing hormone, or leu-enkephalin. The rank order of potency of the unlabelled atrial natriuretic peptides for competing with specific 125I-ANF (101-126) binding sites was rANF (92-126) greater than rANF (101-126) greater than rANF (99-126) greater than rANF (103-126) greater than Tyro-Atriopeptin I greater than hANF (105-126) greater than rANF (1-126). Similar results have been obtained in peripheral tissues and mammalian brain, indicating that central and peripheral ANF-binding sites have somewhat similar structural requirements. Affinity cross-linking of 125I-ANF to retinal particulate preparations resulted in the labelling of two sites of molecular weight 140 and 66 kDa, respectively. This demonstration of specific high-affinity ANF receptors suggests that the peptide may act as a neurotransmitter or neuromodulator in the retina.

TRH receptors in fish

The brains of goldfish and other teleosts contain high-affinity binding sites for [3H][3-methyl-His2]thyrotropin-releasing hormone [( 3H]MeTRH) which closely resemble TRH receptors in mammalian brain and pituitary gland in apparent dissociation constant (KD = 3-4 nM), in pharmacology for eight TRH analogs, and in exhibiting marked regional differences in the density of binding sites, with highest binding in the cerebrum and lowest in the cerebellum. Fish brain differs from mammalian brain in containing a prominent additional class of much lower affinity [3H]MeTRH binding sites (KD about 15 microM), of unknown nature, which also exhibit regional differences. Fish pituitary glands contain high-affinity [3H]MeTRH binding sites, but insufficient tissue has prevented full characterization. Little or no saturable binding of [3H]MeTRH was detected in several goldfish peripheral tissues.

Sulfhydryl groups in receptor binding of thyrotropin-releasing hormone to rat amygdala

Binding of [3H]-[3-Me-His2]thyrotropin-releasing hormone [( 3H]MeTRH) to TRH receptors in rat amygdala was decreased by sulfhydryl reagents in a time-, temperature-, and concentration-dependent manner. A pronounced reduction in receptor density, with little or no change in binding affinity, was apparent following disulfide bond reduction by dithiothreitol (DTT), alkylation of thiol groups by N-ethylmaleimide (NEM), and their oxidation by 5,5'-dithiobis (2-nitrobenzoic acid). Heavy metals (Cd2+, Hg2+), which complex with reactive -SH residues, also potently inhibited binding. The pharmacological specificity of residual [3H]MeTRH binding in chemically modified amygdala membranes was the same as that in control preparations. Sequential exposure to thiol reagents, in the presence or absence of cations, revealed possible additive effects. Pretreatment of membranes with TRH (10(-8) - 10(-6) M), and its continued presence during modification, afforded protection against DTT and NEM. These results indicate the possible importance of thiol groups in the maintenance of TRH receptor conformation.

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.

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.

Species differences in the brain regional distribution of receptor binding for thyrotropin-releasing hormone

A survey of the regional distribution of binding of 1 nM [3H](3-Me-His2)thyrotropin-releasing hormone ([3H]MeTRH) to TRH receptors in the brains of eight mammalian species revealed major species differences in both the absolute and relative values of TRH receptor binding in different brain regions. Several brain regions exhibited binding equal to or exceeding that in the anterior pituitary gland of the same species, including the amygdala in the guinea pig and rat, the hypothalamus in the guinea pig, the nucleus accumbens in the rabbit, and all these and other regions in the cat and dog, for which pituitary binding was exceptionally low. Species could be divided into two groups according to which brain region appeared highest in binding: rabbits, sheep, and cattle had highest binding in the nucleus accumbens/septal area, whereas guinea pigs, rats, dogs, cats, and pigs had highest binding in the amygdala/temporal cortex area. The nucleus accumbens consistently exceeded the caudate-putamen in receptor binding. For most brain regions, rabbits, rodents, and sheep tended to be higher than carnivores, cattle, or pigs. Further regions that exhibited appreciable binding in most species included the olfactory bulb and tubercle, hippocampus, and various cortical and brain stem areas. In fact, essentially all brain regions appeared to have detectable levels of TRH receptors in at least some species, but no rat peripheral tissues have yet shown detectable receptor binding. The species differences appeared to reflect largely if not entirely differences in receptor density, although this was not tested in every species.

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.

Guanine nucleotides modulate TRH-receptor binding in sheep anterior pituitary

Binding of thyrotropin-releasing hormone (TRH, thyroliberin) to receptors in chilled membrane resuspensions from sheep anterior pituitary glands is reduced in affinity almost 2-fold by micromolar concentrations of guanosine 5'-triphosphate (GTP) and the hydrolysis-resistant analog guanyl-5'-yl imidodiphosphate (GppNHp) added to tissue during a 10-min preincubation at 37 degree C. Guanosine 5'-diphosphate (GDP) produced a similar effect at 1 mM, while GMP, ATP, ADP and AMP appeared relatively inactive. The number of TRH-binding sites was not affected by the nucleotide treatments. These results are consistent with reports of guanine nucleotide effects on other receptor types and with evidence suggesting an adenylate cyclase mechanism for some of TRH's effects in the anterior pituitary.

Pituitary cell cultures contain muscarinic receptors

The presence of muscarinic receptors in 5-day dissociated cell cultures of rat anterior pituitary glands was detected by atropine-sensitive binding of 3H-quinuclidinyl benzilate (3H-QNB). Muscarinic receptor levels in cultures were compared to levels of receptors for dopamine and thyrotropin releasing hormone.