Chronic prolactin, gonadal and thyroid hormone treatments in vivo alter levels of TRH and muscarinic receptors in male and female rat tissues

Chronic subcutaneous administration of prolactin into female rats during proestrus led to a 20% (P < 0.05) decrease in retinal and a 32% (n = 20; P < 0.01) decrease in pituitary TRH receptors as compared to controls. In parallel experiments prolactin treatment during diestrus failed to influence TRH receptor levels in both tissues compared to vehicle-treated rats. Intraperitoneal injections of triiodothyronine for 8 weeks resulted in a selective 41% increase (P < 0.02) in retinal TRH receptor levels without any changes in the pituitary and 4 other brain regions. Administration of 17-β-estradiol for 2 weeks into male rats 1 month after castration resulted in a 68% increase (P < 0.02) in pituitary TRH receptor content without any changes in the retina, amygdala or hypothalamus. Testosterone administration for 2 weeks into castrated male rats 30 days post-castration did not alter TRH receptor content in the latter 4 tissues but caused a 27% (n = 10; P < 0.05) and a 40% increase (n = 5; P < 0.05) in muscarinic cholinergic receptor levels in the superior cervical ganglia and anterior pituitary gland, respectively. In conclusion, these data have demonstrated that chronic administration of exogenous hormones selectively up- or down-regulates TRH and muscarinic receptors in a region-specific manner depending on the physiological state of the animal and the tissue under study, and provide further new evidence for feedback hormonal control of these neurotransmitter receptors.     

Corticosterone regulation of Type I and Type II adrenal steroid receptors in brain, pituitary, and immune tissue

Type I and Type II adrenal steroid receptor levels were compared in the brain, pituitary and immune system of adrenalectomized rats in the presence or absence of several replacement doses of corticosterone. Six days of adrenalectomy produced an up-regulation of Type II adrenal steroid receptors in the brain and spleen. The lowest replacement dose of corticosterone (equivalent to resting levels of this hormone) blocked this Type II receptor up-regulation, while higher replacement doses of corticosterone were associated with widespread Type I and Type II adrenal steroid receptor down-regulation. However, the dose of corticosterone required for receptor down-regulation varied between tissues. Specifically, hippocampal receptors were most sensitive to corticosterone, whereas pituitary receptors were the least sensitive. All tissues examined, except the pituitary, exhibited a down-regulation of Type II receptors with a high corticosterone replacement dose which approximated acute stress levels of this hormone. In summary, physiologically relevant concentrations of corticosterone were capable of down-regulating Type I and Type II adrenal steroid receptors in multiple brain areas and peripheral immune tissues, including peripheral blood mononuclear cells. In contrast, adrenal steroid receptor levels in the pituitary were relatively insensitive to regulation by corticosterone.     

Age-related changes in the concentrations of cytosol receptors for sex steroid hormones in the hypothalamus and pituitary gland of the rat

Estrogen and androgen receptors were measured in cytosols from hypothalamus, pituitary and uterus or prostate of rats at 3 stages in life, from 90 to 650 days old in females and from 90 to 550 days old in males. Saturation analysis of cytosol 17β-estradiol receptors in females demosntrated a significant age-associated reduction in maximum binding capacity in hypothalamus and uterus already at 300–330 days of life, but there was no significant change in pituitary gland. However, there was no difference in binding affinity, steroid specificity, sedimentation coefficient, chemical nature and heat lability of cytosol 17β-estradiol binding of these tissues among the 3 age groups. In males, each receptor for 17β-estradiol, testosrone and 5α-dihydrotestosterone was isolated by sucrose density gradient centrifugation from hypothalamic, pituitary and prostate cytosols. These receptors showed the same sedimentation coefficient of 8–9S in all age groups. Androgen binding by cytosols already decreased at 300–330 days of life, but estrogen binding was lower at 500–550 days of life than in younger adults. The increase in the serum luteinizing hormone level after gonadectomy was significantly depressed with aging in both females and males.These findings suggested that the age-associated reduction in cytosol sex steroid hormone receptors was ascribable to changes of numbers of binding sites. These age-related changes may be concerned with feedback system dysfunction of hypothalamic-pituitary-gonadal axis in aged rats.     

Contransmitters: differential effects of serotonin (5-HT)-depleting drugs on levels of 5-HT and TRH and their receptors in rat brain and spinal cord

Following codepletion of endogenous serotonin (5-HT, >90%) and thyrotropin-releasing hormone (TRH, 66%) by neonatal treatment with the serotonergic neurotoxin, 5,7-dihydroxytryptamine (DHT), a 33% (n = 12, P < 0.01) increase in specific TRH receptor binding was observed in adult rat spinal cord (SC) homogenates. A 20–21% increase in TRH receptors was also observed in the medulla/pons (MP) (n = 12, P < 0.05) and midbrain (MB) (n = 12, P < 0.02), but no changes were detected in 6 rostral brain regions. The depletion of 5-HT after DHT-treatment was also accompanied by a 34–42% increase in 5-HT₁ binding in the SC, MP and MB. Eadle-Hofstee analysis revealed that the changes in TRH receptor levels observed after DHT-lesions were due to an increase in receptor number rather than any significant changes in receptor affinity. Chronic treatment of adult rats with the 5-HT-depleting drugs, p-chlorophenylalanine (PCPA) and reserpine, produced a 90–97% decrease in 5-HT in the SC, MP and MB and elevated 5-HT₁ binding in any of these tissues. In conclusion, these results have provided further support for the coexistence of 5-HT and TRH in the MP and SC and revealed possible new areas of such colocalization in the MB. Furthermore, these data have demonstrated that only DHT-treatment, as apposed to PCPA or reserpine, can produce long-lasting codepletion of 5-HT and TRH with simultaneous compensatory up-regulation of their receptor systems in the SC and other caudal tissues.     

Adrenal steroid receptor activation in rat brain and pituitary following dexamethasone: implications for the dexamethasone suppression test.

The dexamethasone suppression test (DST) has been used extensively to evaluate feedback inhibition of the hypothalamic-pituitary-adrenal (HPA) axis by adrenal steroids. Nevertheless, it remains unclear at what level of the HPA axis and through which adrenal steroid receptor subtype dexamethasone exerts its inhibitory effect. Because adrenal steroid receptor activation is an important prerequisite for dexamethasone to affect cellular function, HPA axis tissues that exhibit evidence of receptor activation following dexamethasone administration are likely site(s) of action for this synthetic hormone to inhibit HPA axis activity. Therefore, type-I and type-II adrenal steroid receptor activation was assessed in the pituitary, hypothalamus, and hippocampus of intact and adrenalectomized rats after overnight exposure to various oral doses of dexamethasone. Results with dexamethasone were compared to similar studies using corticosterone, the endogenous glucocorticoid of the rat. All dexamethasone doses led to significant type-II receptor activation in the pituitary, whereas only an exceedingly high dexamethasone dose activated type-II receptors in the hippocampus and hypothalamus. Dexamethasone had little effect on type I receptors in any tissue at any dose. In contrast, corticosterone significantly activated type-I receptors in all tissues, whereas it activated type-II receptors in the brain and not the pituitary at physiological concentrations. Because dexamethasone activated pituitary type-II receptors at blood concentrations that did not activate type-II receptors in the brain, these results suggest that the DST in humans may primarily be a measure of type-II adrenal steroid receptor feedback inhibition at the level of the pituitary.     

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

Specific binding of³H-labeled[3-Me-His²]TRH([³H]MeTRH) to membranes of rabbit spinal cord (thoracic) involved a homogeneous population of high-affinity sites (K_d = 2.7 ± 0.17 (n= 5)nM, B_max= 204 – 12(5)fmol/mg protein). TRH analogs competed for the binding in the following rank order of potency: MeTRH>TRH>TRH-Gly-NH₂ >Ser-His-Pro-NH₂ >Thr-His-Pro-NH₂ > pGlu-His-Pro-NH-C₂H₅>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.     

Seasonal and state-dependent changes in brain TRH receptors in hibernating ground squirrels.

Quantitative autoradiography was used to localize and quantify thyrotropin-releasing hormone (TRH) receptors in the brain of hibernating (H), winter euthermic (WE), and summer euthermic (SE) animals to further explore the state-dependent physiological and behavioral effects of TRH in ground squirrels. The pattern of [3H]MeTRH binding (Kd 6.7 +/- 0.1 nM) was heterogeneous and highly concentrated in structures primarily associated with the limbic forebrain. Statistically significant seasonal changes (SE vs. WE) were reflected by decreases in TRH receptor binding in the arcuate nucleus, dorsomedial nucleus, and ventral pallidum of WE animals. Increased binding in WE animals was evident in the suprachiasmatic nucleus and choroid plexus of the lateral ventricles. Significant state-dependent changes (WE vs. H) were characterized by decreases in TRH receptor binding in the hypothalamic paraventricular nucleus, medial preoptic area, ventral tegmental area, and choroid plexus of the lateral ventricles of H animals. Increased binding occurred in the anterior cortical nucleus of the amygdala in H animals. The results suggest that naturally occurring changes in central TRH systems may be important in the mediation of physiological and behavioral processes that undergo seasonal and state-dependent adjustments in hibernating mammals.     

Autoradiographic localization of thyrotropin releasing hormone receptors in human brain

We used quantitative autoradiography to localize thyrotropin releasing hormone (TRH) receptors in human brain. Highest concentrations of TRH receptors were localized within the cortical, basal, and lateral nuclei of the amygdala and the molecular layer of the hippocampus. Low levels were found in the cortex, diencephalon, and basal ganglia. The radioligand bound with similar affinity and pharmacology to pituitary gland as to brain. These data suggest that authentic TRH receptors in the hippocampus and amygdala may mediate the putative effects of TRH on the human brain.     

Autoradiographic localization of thyrotropin-releasing hormone and substance P receptors in the rat dorsal vagal complex

We utilized quantitative autoradiography to localize receptors for thyrotropin-releasing hormone (TRH) and substance P in individual subnuclei of the rat nucleus tractus solitarii (NTS) and the dorsal vagal complex. Within the NTS, TRH receptor concentrations were highest within the gelatinosus and centralis subnuclei and the medial subnucleus rostral to the area postrema, moderate within the intermediate subnucleus and the medial subnucleus adjacent to the area postrema, and low within the ventrolateral and commissural subnuclei and the medial subnucleus caudal to the area postrema. In contrast, substance P receptor concentrations were high throughout the medial subnucleus, moderate in all other subnuclei medial to the tractus solitarius, and relatively low in subnuclei lateral to the tractus solitarius. The dorsal motor nucleus of the vagus contained high concentrations of both TRH and substance P receptors, whereas we observed low TRH and moderate substance P receptors in the area postrema. High TRH and moderate substance P receptors were observed in the adjacent hypoglossal nucleus. In addition, we compared the concentrations of TRH receptors between chloroform-defatted and nondefatted tissue sections, and noted little effect of white matter tritium quench upon the observed TRH receptor concentrations. These results suggest that neurotransmitter receptors within the rat dorsal vagal complex are organized in a manner consistent with previous cytoarchitectural and hodological partitioning of the NTS and that the distribution of an individual neurotransmitter receptor in the NTS may correspond to the role of that transmitter in modulating autonomic function.     

Distribution of somatostatin in the brain and of somatostatin and thyrotropin-releasing hormone in peripheral tissues of the chicken

Our research group recently presented the distribution of thyrotropin-releasing hormone (TRH) in the chicken brain. In this study we measured somatostatin (SRIH) concentrations in different brain parts and nuclei. The distribution of SRIH and TRH in peripheral tissues was also studied. Although the highest SRIH content was found in endocrine areas like diencephalon and median eminence (ME), high levels were also recorded in brain stem and several hypothalamic nuclei which do not project to the ME. SRIH immunoreactivity was also found within the pituitary. In peripheral tissues, SRIH was mainly present in gonads, thyroid and intestine. Low amounts were found in duodenum, kidney, heart and lung. SRIH concentrations were barely detectable (liver, blood cells) or undetectable (muscle, skin, spleen) in other peripheral tissues investigated. Although TRH was found in all tissues collected, it was also most abundant in brain, pituitary, thyroid and gonads. Our results suggest that also in the chicken SRIH and TRH are implicated in the control of several physiological processes like growth, reproduction and digestion.     

The effects of desipramine (DMI) and electroconvulsive shock (ECS) on the function of the hypothalamo-pituitary-thyroid axis in the rat

Monoaminergic systems influence the hypothalamo-pituitary-thyroid (HPT) axis. Since two different antidepressant treatments, desipramine (DMI) and electroconvulsive shock (ECS), are known to alter monoaminergic function in the rat central nervous system (CNS), the effects of DMI and ECS on the function of the HPT axis in the rat were examined. Animals were treated with either DMI (5 mg/kg) twice daily for 14 days (DMI x 14) or once only (DMI x 1) or ECS five times in 10 days (ECS x 5) or once only (ECS x 1). Three and 24 hours after the final treatment, blood samples were taken for measurement of plasma total thyroxine (TT4), total tri-iodothyronine (TT3), free thyroxine (FT4), free tri-iodothyronine (FT3) and thyroid stimulating hormone (TSH). Plasma TSH concentrations were decreased by the DMI x 14 and increased by the ECS x 5 regimen. Small decreases in thyroid hormones (T3 and T4) occurred after DMI x 14. No other consistent changes were observed in the animals treated with ECS. The effect of DMI or ECS treatment on the responsiveness of pituitary thyrotrophs was assessed in vitro. Isolated superfused pituitary glands from rats treated in vivo with either DMI x 14 or ECS x 5 were exposed to a pulse of thyrotropin releasing hormone (TRH; 1 ng/ml). No significant change in TSH secretion was observed in response to TRH in either case. Therefore, the changes observed in circulating plasma TSH levels are unlikely to have resulted from either direct or indirect effects on pituitary thyrotroph TRH receptor sensitivity.     

Inverse relationship between CSF TRH concentrations and the TSH response to TRH in abstinent alcohol-dependent patients.

The authors performed the thyrotropin-releasing hormone (TRH) stimulation test and measured CSF concentrations of TRH in 13 abstinent alcohol-dependent subjects. They found an inverse correlation between the thyrotropin (TSH) response to TRH and endogenous CSF TRH concentrations. This finding supports the hypothesis that as the concentration of CSF TRH increases, anterior pituitary TRH receptor density decreases, resulting in a blunted TSH response to TRH stimulation.     

Change in Brain Thyrotropin-Releasing Hormone (TRH) Mechanism of Amygdaloid Kindled Rats

Abstract: We reported previously that DN-1417, a potent analog of thyrotropin-releasing hormone (TRH), suppressed both the progression of amygdaloid (AM) kindling and AM kindled seizure.
To study a functional role of the cerebral TRH mechanism in AM kindling, immuno-reactive TRH (IR-TRH) and specific TRH receptor binding were examined in the rat brains kindled from the left AM. The IR-TRH concentration elevated significantly in the amygdala plus piriform cortex and the hippocampus 24 and 48 hours after the AM kindled convulsion. Such an elevation of IR-TRH was not found 7 days after the last convulsion, indicating that the elevation of IR-TRH was a transient change seen after the AM kindled convulsion. By contrast, the specific TRH receptor binding in the striatum increased 48 hours, 7 and 21 days after the AM kindled convulsion. This indicates that the increase of the specific TRH binding in the striatum was a long-lasting change.
The present study suggests that the change in the striatal TRH receptors may be associated with a long-lasting seizure susceptibility of AM kindled rats.     

Change in brain thyrotropin-releasing hormone (TRH) mechanism of amygdaloid kindled rats

We reported previously that DN-1417, a potent analog of thyrotropin-releasing hormone (TRH), suppressed both the progression of amygdaloid (AM) kindling and AM kindled seizure. To study a functional role of the cerebral TRH mechanism in AM kindling, immunoreactive TRH (IR-TRH) and specific TRH receptor binding were examined in the rat brains kindled from the left AM. The IR-TRH concentration elevated significantly in the amygdala plus piriform cortex and the hippocampus 24 and 48 hours after the AM kindled convulsion. Such an elevation of IR-TRH was not found 7 days after the last convulsion, indicating that the elevation of IR-TRH was a transient change seen after the AM kindled convulsion. By contrast, the specific TRH receptor binding in the striatum increased 48 hours, 7 and 21 days after the AM kindled convulsion. This indicates that the increase of the specific TRH binding in the striatum was a long-lasting change. The present study suggests that the change in the striatal TRH receptors may be associated with a long-lasting seizure susceptibility of AM kindled rats.     

Thyrotropin-releasing hormone (TRH) inchronic schizophrenia. A controlled study.

A controlled study on the effect of synthetic Thyrotropin-Releasing Hormone (TRH) versus placebo in 10 drug-free chronic schizophrenics is presented. TRH 600 mug or placebo was administered intravenously on 4 consecutive days in a double-blind cross-over design and the schizophrenic symptoms were scored daily by use of two rating scales (CPRS and NOSIE). Blood samples were taken before and 30 and 45 minutes after the first and the fourth injection and analyzed for TSH. No changes in symptom intensity were found during the TRH or placebo treatment period and no difference was obtained between the two treatments. The dose of TRH used caused a significant increase in the plasma level of TSH which indicated a normal pituitary responsiveness to TRH. The data do not support the notion that TRH has a beneficial effect in chronic schizophrenic patients.     

Chronic corticotropin-releasing hormone and vasopressin regulate corticosteroid receptors in rat hippocampus and anterior pituitary

Corticotropin-releasing hormone (CRH) and vasopressin (AVP) participate in the endocrine, autonomic, immunological and behavioral response to stress. CRH and AVP receptors are found in hippocampus and anterior pituitary, where mineralocorticoid (MR) and glucocorticoid (GR) receptors are abundant. We investigated the possible influence of CRH and AVP on the regulation of MR and GR in both tissues. CRH, AVP, or their antagonists were administered to adrenalectomized rats substituted with corticosterone, to avoid interference with adrenal secretion. Repeated i.c.v. oCRH injections (10 microgram) for 5 days significantly decreased MR and GR mRNA in hippocampus and MR mRNA in anterior pituitary. AVP significantly increased both corticosteroid receptor mRNAs, as repeated i.c.v. injections (5 microgram) for 5 days in hippocampus, and as continuous i.c.v. infusion (10 ng/h/5 days) in anterior pituitary. The i.c.v. infusion of 5 or 10 microgram/day of the alpha-helical CRH antagonist during intermittent restraint stress (5 days), induced a significant decrease in hippocampal MR binding. In anterior pituitary, 5 microgram/day significantly decreased MR binding, while 10 microgram/day significantly increased GR binding. Under the same conditions of stress, the infusion of 15 microgram/day of the vasopressin V1a/1b receptor antagonist [dP Tyr (Me)(2)AVP] significantly increased MR and GR binding in hippocampus and anterior pituitary; 5 microgram/day significantly decreased pituitary MR binding. Our results show that CRH and AVP regulate MR and GR in hippocampus and anterior pituitary. This reveals another important function of CRH and AVP, which could be relevant to understand stress adaptation and the pathophysiology of stress-related disorders like major depression.     

Triiodothyronine attenuates estradiol-induced increases in dopamine D-2 receptor number in rat anterior pituitary

Estrogens promote adenohypophyseal enlargement and tumor transformation, and thyroid hormones antagonize these effects. Hormone-induced pituitary enlargement may be mediated by alterations in pituitary dopaminergic function. The present study examined the effects of chronic (20 days) administration of estradiol benzoate (EB), triiodothyronine (T3), or EB and T3 (T3 + EB) on dopamine (D-2) receptors in rat anterior pituitary. D-2 receptor number increased after EB without altered receptor affinity. T3 alone did not affect D-2 receptor number in the anterior pituitary but significantly attenuated the effect of EB. T3 administration also inhibited EB-induced anterior pituitary hyperplasia. D-2 receptor upregulation by EB more likely could reflect a compensatory response to decreased receptor occupation. The present results suggest that D-2 receptors could play an important role in estrogen-induced adenohypophyseal tumor formation and hyperprolactinemia and that thyroid hormones may inhibit estrogen-induced pituitary tumor development via adenohypophyseal D-2 receptors.     

Thyrotropin releasing hormone (TRH): apparent receptor binding in rat brain membranes.

Thyrotropin releasing hormone (TRH) binds to membranes of rat brain tissue via high- and low-affinity binding components. The high-affinity binding of TRH to brain membranes resembles binding to pituitary membranes in terms of its affinity and specificity for most peptides. In equilibrium studies, the affinity and specificity for most peptides. In equilibrium studies, the dissociation constant for high-affinity binding to brain membranes is about 50 dissociation constant for high-affinity binding to brain membranes is about 50nM, which is about the same as for aat pituitary membranes, while low-affinity binding to brain membranes has a dissociation constant of about 5 muM. Liver membranes display low-affinity binding for TRH with a dissociation constant similar to the low-affinity binding component of brain membranes. No high-affinity binding can be detected with liver membranes. Negligible saturable binding to TRH can be detected with membranes of any tissues examined other than liver, pituitary and brain...     

Involvement of both opiate and catecholaminergic receptors of ventromedial hypothalamus in the locomotor stimulant action of thyrotropin-releasing hormone

Summary To explore the mode of the locomotor stimulant action of thyrotropin-releasing hormone (TRH), rats with or without administration of opiate or catecholaminergic receptor antagonists were infused with TRH through previously implanted hypothalamic cannula. Administration of TRH, but not the normal saline or TSH, into the ventromedial hypothalamus caused an enhancement in both the gross movements (including stimulation of forward locomotion, head and body rearing) and fine movements (including increased grooming and head swaying). The locomotor stimulant action provoked by TRH was antagonized by pretreatment of ventromedial hypothalamus with either an alpha-adrenergic receptor antagonist (yohimbine), a dopaminergic receptor antagonist (haloperidol) or an opiate receptor antagonist (naloxone), but not with a beta-adrenergic receptor antagonist (propranolol). The results indicate that all the adrenergic, dopaminergic and opiate receptors in the ventromedial hypothalamus are involved in the TRH-induced hyperactivity in the rat.