Expression of the growth hormone gene and the pituitary-specific transcription factor GHF-1 in diabetic rats.

Diabetes in the rat is associated with poor growth and decreased GH in the pituitary. In this study we have examined whether this reduction reflects an impairment of GH gene expression. Diabetes was induced by the administration of streptozotocin (7 mg/100 g BW), and 18 days later, GH content, GH mRNA, and GH transcription rate were determined. GH mRNA levels were reduced by more than 80% in the pituitaries of diabetic rats, which had a similarly reduced GH content. The differences observed in transcription fully account for the changes in mRNA concentration, since the transcription rate of the gene was also reduced by a factor of 10 in the diabetic pituitaries. Insulin therapy (3 U/15 days) partially restored these parameters. The expression of the specific transcription factor GHF-1/Pit-1 in diabetic rats was also analyzed. Both GHF-1 mRNA levels and the binding of nuclear proteins to an oligodeoxynucleotide conforming to the GHF-1 proximal binding site in the promoter of the GH gene were normal in the diabetic pituitaries, thus excluding the possibility that decreased availability of this factor could be responsible for the decreased GH transcription. Since diabetes produced an approximately 3-fold reduction of circulating T3, the potential role of thyroid hormones on GH gene expression was also evaluated in thyroidectomized and thyroidectomized diabetic rats. Thyroidectomy decreased GH and GH mRNA to less than 5% of the values found in intact animals, and a single saturating injection of T3 (250 micrograms/100 g BW) resulted in a 8- to 10-fold induction of GH mRNA after 6 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin regulation of malic enzyme gene expression in rat liver: evidence for nuclear proteins that bind to two putative insulin response elements.

Diabetes in rats is characterized by insulin deficiency accompanied by a decrease in lipogenic enzymes. The malic enzyme (ME) gene, which encodes an important lipogenic enzyme, was used to investigate insulin regulation of gene expression. ME mRNA levels were reduced by more than 90% in the liver of diabetic rats. The administration of insulin (3 U/15 days) to either control or diabetic rats increased ME mRNA by 2- to 10-fold, respectively. Since diabetes reduces circulating T3 and the levels of nuclear T3-receptors, the potential role of thyroid hormone on insulin regulation of ME gene expression was also evaluated in thyroidectomized-diabetic rats. In these animals the levels of ME mRNA were undetectable but were increased by insulin even in the absence of thyroid hormones. These in vivo effects of insulin and T3 were not additive. The transcription rate of the gene was also reduced in the diabetic liver and recovered after insulin therapy. By computer analyses we have identified two different putative insulin response elements (IREs) in the ME gene promoter, hereafter referred to as IRE-I (-683 to -692), which is similar to the phosphoenol pyruvate carboxy kinase promoter IRE and IRE-II (-161 to -170), which is similar to the glyceraldehyde phosphate dehydrogenase gene promoter IRE-A. Results from gel retardation assays suggest that a single nuclear protein binds to IRE-I whereas two different nuclear proteins bind to IRE-II. The protein/IRE-I complex increased in liver nuclear extracts from diabetic rats and decreased after insulin administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Response of anterior pituitary hormones to chronic stress. The specificity of adaptation.

The effect of chronic noise stress on the response of anterior pituitary hormones to the same or to another stressor (forced swimming) was studied in adult male Wistar rats. Both acute stressors increased corticosterone, prolactin, LH and TSH secretion and inhibited GH secretion. Previous chronic exposure to noise reduced corticosterone response to the same stimulus without modifying corticosterone response to a novel acute stress. Neither prolactin nor TSH responses to acute noise were reduced by previous chronic exposure to noise. Since chronic noise increased basal levels of LH and decreased those of GH, the response of these hormones to acute stress was expressed as percent changes of their respective basal values. It was found that chronically stressed rats showed diminished LH response to noise but not to forced swimming. GH showed the same pattern without reaching statistical significance. These data indicate that the response of some anterior pituitary hormones can adapt after repeated exposure to the same stressor. When adaptation occurred, this was specific for the stressor which the animals were repeatedly exposed to. The pituitary-adrenal axis appears to be the most reliable index of adaptation to chronic stress among all the anterior pituitary endocrine axes.

Diabetes decreases liver and kidney nuclear 3,5,3'-triiodothyronine receptors in rats.

The liver and kidney nuclear T3 content and the maximal nuclear T3-binding capacity (MBC) were measured 1 month after streptozotocin administration and compared with values in controls either fed ad libitum (C) or offered a restricted diet (FR). A group of insulin-treated diabetic (D+I) rats was also included. Plasma T4 and T3 concentrations decreased to low levels in diabetic (D) rats. Plasma T3 levels were decreased in FR rats, whereas circulating T4 was in the normal range for C animals. The MBC (nanograms of T3 per mg DNA) for liver and kidney nuclear T3 was determined by an in vivo saturation technique. The respective results for all groups were as follows (asterisks denote values differing from C with P values less than 0.05): C, 0.601 and 0.414; FR, 0.583 and 0.369; D, 0.310 and 0.220; D+I, 0.630 and 0.394. Nuclear T4 and T3 concentrations were determined by an isotopic equilibrium technique. Nuclear T3 (nanograms per mg DNA) for liver and kidney were, respectively, 0.298 and 0.176 for C, 0.208 and 0.135 for FR, 0.109 and 0.070 for D, and 0.270 and 0.168 for D+I rats. The decreased liver and kidney nuclear T3 content in D rats appears to be due to a marked reduction of their available intracellular T4 pool, from which T3 could be generated, but most likely represents a decreased T3 uptake into liver and kidney nuclei, as the nuclear to plasma ratios of labeled T3 were decreased in D rats. The low levels of T3 in nuclei of FR rats could be attributed to an inhibition of T4 to T3 conversion, since the intracellular pool of T4 appears to be normal. The possibility that diabetes and food restriction might affect the thyroid activity was examined by measurement of the activities of alpha-glycerophosphate dehydrogenase and cytosol malic enzyme, two liver and kidney enzymes regulated by thyroid hormone. Furthermore, although the measurements made in FR rats excluded the possibility that the alterations in MBC found in D animals were nutrition dependent, the reduced nuclear T3 content concomitant with food restriction may account for some of the quantitative changes in the alpha-glycerophosphate dehydrogenase and cytosol malic enzyme activity found in D rat tissues. In conclusion, the present findings suggest that the observed changes in indices of thyroid hormone action in liver and kidney of D rats could be related to alterations in nuclear T3 receptor concentrations and the concentration of T3 bound to the receptor.

Response of hepatic mitochondrial alpha-glycerophosphate dehydrogenase and malic enzyme to 3,5,3'-triiodothyronine in streptozotocin-diabetic rats.

Hepatic mitochondrial alpha-glycerophosphate dehydrogenase (alpha-GPD) and cytosolic malic enzyme (ME) response to a single injection of a receptor-saturating dose of T3 were measured 10, 20, and 30 days after diabetes induction, and compared with values in controls either fed ad libitum (C) or under a restricted diet (FR). An insulin-treated diabetic (D + I) group was also included. Basal enzyme levels as well as enzyme response to T3 injection were correlated with nuclear T3 content, maximal nuclear T3-binding capacity (MBC) and equilibrium association constant (Ka). Diabetes for 10, 20, and 30 days was associated with a progressive decrease in the MBC; the mean decrease was 17%, 50%, and 59%, respectively, from the corresponding C values. The MBC in FR animals did not change appreciably during the experimental period. Moreover, neither the decreased MBC in D groups nor MBC in C, FR, or D + I animals were influenced by T3 injection. The Ka values were comparable in all experimental groups. Specifically bound nuclear T3 was decreased within the experimental period between 33% and 73% in D rats and 6% and 39% in FR rats respect to C values. T3 injection raised the mean nuclear T3 content in all groups. However, at each time interval the mean values of the nuclear T3 in D groups was significantly lower than that in C, FR, or D + I groups after T3 injection. The basal alpha-GPD activity tended to be relatively stable during the experimental period in both D and FR rats, whereas ME activity in D and FR groups was decreased, respectively, 52-64% and 18-39% from C values. The response of both alpha-GPD and ME to T3 injection in FR rats was comparable to that of C groups. The alpha-GPD response to T3 in D rats was not different from that of C rats on days 10 and 20 of the experiment, but on day 30 it decreased by 26%. In contrast, the induction of ME by T3 was severely decreased (by 66-88% of C values) within the experimental diabetes period. Thus, the measurements made in FR rats excluded the possibility that the quantitative changes in the enzyme response to T3 in D rats were nutrition-dependent. The differences between the response of alpha-GPD and ME to T3 in D rats suggest that cellular factors play a role in inhibiting or increasing the response to a given concentration of the T3-receptor complex.

Triiodothyronine-receptor complex in rat brain: effects of thyroidectomy, fasting, food restriction, and diabetes.

In vitro saturation analysis combined with quantification of T3, by an isotopic equilibrium technique or RIA, were used to examine the effects of thyroidectomy, fasting, diabetes, and food restriction on T3 concentration and specific binding in cerebral cortex and cerebellum. Fasting and food restriction did not affect the T3 binding parameters in the brain areas studied. Both thyroidectomy and diabetes were accompanied by a reduction in T3 content in nuclei from both cerebral cortex and cerebellum, but a decrease in T3 binding sites was only observed in both brain areas of diabetic animals. No significant differences in the binding affinity values among the experimental groups were seen. The diabetes-induced decrease in T3 content and receptor number were completely reversed by insulin treatment. Studies with fractionated nuclei from cerebral cortex and cerebellum showed that diabetes resulted in a reduction in T3 content and the number of receptors in glial nuclei from both brain areas. Although T3 content was also decreased in neuronal nuclei, the receptor concentration in these nuclear preparations did not change in concentration or affinity under the same conditions. These observations indicate that glial cells, not only have T3-binding characteristics similar to those of neuronal cells, but the T3 receptor number is decreased in the diabetes state.

Effect of streptozotocin diabetes on the hypothalamic-pituitary-thyroid axis in the rat.

Studies of the hypothalamic-pituitary-thyroid axis have been performed in streptozotocin (STZ)-diabetic Wistar rats and their controls. Plasma PBI concentration, plasma and pituitary TSH, contents, and hypothalamic TRH content were measured by RIA in basal and stimulated conditions. Compared to controls, rats made diabetic by 6.0 or 7.5 mg STZ/100 g BW showed decreased plasma PBI and TSH and diminished pituitary TSH content, with greater alterations in rats receiving the highest STZ dose. Both diabetic groups showed an almost 50% reduction of hypothalamic TRH content in comparison with the mean control value. After thyroidectomy, pituitary TSH secretion increased in diabetic, ad libitum fed, and semistarved animals, but it was lower in the diabetic group in which the reduction in plasma PBI was similar or greater. To evaluate pituitary sensitivity to the inhibitory action of L-T4 on TSH secretion in diabetes, thyroidectomized control (Thx-C), thyroidectomized diabetic (Thx-D), and thyroidectomized semistarved (Thx-S) rats were injected twice daily for 7 days with either saline or a fractional L-T4 dose of 0.25, 0.50, or 1.00 microgram/100 microgram/100 g BW. In Thx-D rats, a daily dose of 1.00 microgram L-T4 was sufficient to normalize pituitary TSH secretion, while a dose of 2.00 microgram was required to induce a similar effect in the Thx-C and Thx-S animals. Pituitary TSH content was increased in the Thx-C group with increasing T4 doses. No modification in this parameter was seen in the Thx-D and Thx-S animals. The fact that diabetes caused a reduction in the hypothalamic TRH content indicates that the primary cause of pituitary-thyroid alterations in STZ-diabetic rats lies in the hypothalamus, although the metabolic imbalance induced by diabetes and, in less degree, by undernutrition could also be partly responsible for some of the described modifications.

Effect of restricted feeding, fasting, and diabetes on the relationship between thyroid hormone receptor occupancy, growth hormone induction, and inhibition of thyrotropin release in thyroidectomized rats.

The present study was undertaken to test the effect of food restriction, fasting, and diabetes on the relationship between thyroid hormone receptor occupancy and two biological end points, GH production and the inhibition of TSH secretion, in thyroidectomized rats. The estimated maximal binding capacity (MBC) in diabetic (D) and fasting (F) rats and in animals limited to 25% (FR25) of the food consumption of normal (C) rats was decreased to 57%, 73%, and 76%, respectively, of C values (P < 0.01-0.001), whereas normal values were found in thyroidectomized (Tx) rats and in animals limited to 50% (FR50) of the food intake of C animals. The nuclear T3 content and T3 receptor occupancy were reduced, respectively, to 25% and 29% in Tx, 77% and 81% in FR50, 52% and 69% in FR25, 49% and 66% in F, and 36% and 64% in D rats of the corresponding C values (P < 0.05-0.001). Pituitaries from Tx, FR50, FR25, F, and D rats contained less GH than C pituitaries (0.14%, 81%, 69%, 88%, and 51%, respectively, of C pituitaries; P < 0.05-0.001). Plasma TSH was lower in FR50, FR25, F, and D rats than in C animals (78%, 57%, 52%, and 48%, respectively (P < 0.01-0.001)), and markedly increased in Tx animals. Administration of a single dose of 2, 5, or 10 micrograms T3/100 g BW to Tx C, Tx FR50, TX FR25, Tx F, and Tx D rats resulted in a similar and progressive increase in nuclear T3 in all groups, except for lower values in Tx D animals. However, receptor occupancy did not differ among the different groups at each T3 dose. This treatment resulted in a progressive increase in pituitary GH in all groups; however, in Tx FR50, Tx FR25, Tx F, and Tx D pituitaries, the GH responses to 10 micrograms T3 were only 77%, 59%, 44%, and 27%, respectively, of that in Tx C rats. (P < 0.05-0.001). Moreover, significant differences in the GH response to 10 micrograms T3 were observed among Tx FR50, Tx FR25, Tx F, and Tx D animals (P < 0.01-0.001). In addition plasma TSH levels in untreated Tx FR50, Tx FR25, Tx F, and Tx D rats were only 88%, 82%, 79%, and 72%, respectively, of that in Tx C animals (P < 0.05-0.01).(ABSTRACT TRUNCATED AT 400 WORDS)

Diurnal variations of plasma growth hormone, thyrotropin, thyroxine, and triiodothyronine in streptozotocin-diabetic and food-restricted rats.

The pattern of spontaneous GH, TSH, T4, and T3 secretion has been studied in male rats in response to a 15-day period of streptozotocin diabetes or food restriction. Beginning at 0900 h, groups of control (C), food-restricted (FR), diabetic (D), and insulin-treated D rats were killed every 60-90 min for a 8-h period. Food restriction resulted in a significant depression of the GH, TSH, T4, and T3 peaks, whereas diabetes caused complete suppression of episodic secretion of each hormone. Insulin (6 U/100 g BW X day for 12 days) administration to D rats restored the normal pattern of secretion. In D and FR rats, pituitary GH concentrations were lower than in C rats, whereas pituitary TSH concentrations were similar to those in controls. Thus, as compared to C rats, FR and D rats showed an inhibition in GH, TSH, T4, and T3 secretion, most marked in D animals. Since diabetes is associated with a deficiency of circulating thyroid hormones, the potential roles of T4 and T3 on pituitary GH concentration and secretion in D rats were evaluated. Treatment of D rats with insulin (3 U/100 g BW X day), T4 (1.8 micrograms/100 g BW X day), or T3 (0.30 microgram/100 g BW X day) for 12 days resulted in a significant but limited increase in pituitary GH content. When administered together with insulin, the net effects of T4 or T3 with insulin appeared additive. T4 administration to D rats produced a significant though limited increase in plasma GH concentrations and weight gain, whereas both values were unaffected by T3. Simultaneous administration of T4 and insulin resulted in significant increased plasma GH concentration to levels greater than those in C rats. However, plasma GH levels in rats treated with T3 plus insulin were greater than those in D rats, but lower than in C animals. The results indicate that the decreased pituitary GH content of D rats can be corrected, at least in part, by T4 and T3.

Effect of perinatal hypothyroidism on the developmental regulation of rat pituitary growth hormone and thyrotropin genes.

We studied the effects of thyroid hormone (T3) on GH, TSH, and T3 receptor (TR) gene expression as well as deiodinase activities during rat pituitary development. By reverse transcriptase-polymerase chain reaction, GH and TSH beta transcripts were detectable on fetal day 15. Although with certain differences, the expression of both GH and TSH beta genes was under T3 control during fetal and neonatal life. Differences in plasma, but not pituitary, TSH concentrations were observed between control and hypothyroid animals throughout the period studied. Both TR alpha and TR beta genes were expressed in the fetal pituitary. TR alpha 1, TR beta 2, and c-erbA alpha 2 transcripts displayed a developmental profile different from that of TR beta 1. Thyroid hormone repressed TR alpha 1, TR beta 2, and c-erbA alpha 2 and stimulated TR beta 1. Type I and type II deiodinase activities (5'DI and 5'DII, respectively) had different ontogenic patterns; 5'D-II was the predominant activity in fetuses, with levels similar to those in adults, whereas the level of 5'D-I was low and increased with age. T3 stimulated 5'D-I and decreases 5'D-II. These results demonstrate that in somatotroph and thyrotroph cells, the mechanisms responsible for T3 action are mature and active very early in development and suggest an involvement of this hormone in the establishment and/or maintenance of the somatotroph and thyrotroph phenotype.

Expression of thyroglobulin gene in maternal and fetal thyroid in rats.

The relationship between the changes in thyroglobulin (Tg) mRNA and Tg proteins during thyroid development in the fetus and in maternal thyroid glands during gestation and lactation is studied. While the appearance of Tg mRNA (fetal day 15) showed good temporal correlation with that of 12S Tg, no 19S Tg could be detected until 3 days later. The 12S Tg was the predominant protein on days 18 and 19 of gestation in the fetus, while 19S Tg was the predominant protein on fetal days 21-22 and during the postnatal period in the offspring; by the 20th postnatal day, the 19S Tg content per gland was 4 times the amount of 12S (155 vs. 37 micrograms/gland; P less than 0.001). The 19S iodine content in the fetus was the same as that in 12S up to the 21st day of gestation, except for lower values on day 18. From fetal day 22 and through the postnatal period, the iodine content in 19S was 1.6-5.9 times greater than that in 12S. Therefore, the ratio of atoms of iodine per mol Tg during the experimental period changed from 0.75 to 19.5 for 19S and from 0.72 to 7.2 for 12S. The levels of all of the iodoamino acids were low on fetal days 17-19, after which they increased at different rates for each protein. The greatest increase in monoiodotyrosine and T3 corresponded to 12S, while diiodotyrosine and especially T4 showed a greater increase in 19S than in 12S Tg; 20 days after birth, the T4 content in 19S was about 3 times greater than that in 12S Tg. The soluble thyroid proteins from pregnant, lactating, and nonpregnant female controls contained a main protein, 19S, and a smaller amount of 27S. Both 19S Tg and 19S iodine contents were already lower than those in nonpregnant rats at 14 days of pregnancy, and the levels continued to decrease during the experimental period. In contrast, the 27S Tg and 27S iodine levels remained constant and similar to nonpregnant values. Surprisingly, a decrease in the level of Tg mRNA was observed during pregnancy and lactation. We have no explanation for the dramatic decrease in Tg mRNA during the last days of pregnancy. Further studies should help to elucidate the mechanism responsible for the changes in Tg gene expression in the thyroids of pregnant and lactating rats.

Comparative effects of food restriction, fasting, diabetes and thyroidectomy on growth hormone and thyrotropin gene expression in the rat pituitary.

To examine the molecular basis for the decreased pituitary growth hormone (GH) and thyrotropin (TSH) content during restricted feeding, fasting and diabetes, we measured steady-state levels of mRNA for TSH-alpha, TSH-beta and GH in the pituitary from normal rats either fed ad libitum (C), limited to 75%, 50% and 25% (FR75, FR50, FR25, respectively) of ad libitum intake, or deprived of food for 2 and 4 days (F2 and F4, respectively), and also in streptozotocin-diabetic (D) and D insulin-treated animals. The results from these experimental groups were compared with those in thyroidectomized (Tx) rats. Pituitary mRNA was quantified by Northern blot hybridization with cDNA probes specific for rat TSH-alpha, TSH-beta and GH. Although changes in the pituitary GH mRNA during restricted feeding, fasting and diabetes were similar qualitatively to those induced by hypothyroidism, GH mRNA levels in Tx rats (> 10% of C values) were less than in the other experimental groups (p < 0.001). Pituitaries from FR50, FR25 and D rats also contained less GH mRNA than F2 and F4 animals (p < 0.05). Thyroidectomy resulted in a marked increase in both TSH-beta and TSH-alpha mRNAs, the changes in TSH-beta mRNA being greater than those in TSH-alpha mRNA. In contrast, FR50, FR25, F2, F4 and D rats exhibited a decrease in pituitary TSH-beta mRNA (60%, 50%, 35%, 36% and 33%, respectively, of C values; p < 0.01-0.05) and in TSH-alpha mRNA levels (81%, 64%, 46%, 43% and 36%, respectively, of normal values; p < 0.02-0.05), TSH-beta mRNA showing the greater changes.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute stress attenuates but does not abolish circadian rhythmicity of serum thyrotrophin and growth hormone in the rat.

The effects of acute immobilization (IMO) on daily rhythms of corticosterone, thyroid-stimulating hormone (TSH) and growth hormone (GH) were studied in adult male rats. Two hours of IMO increased serum corticosterone, this increase still being observed 3 h after finishing stress exposure. In the dark period corticosterone levels did not differ in control and IMO rats, but higher levels were observed again in the morning of the day after. Immobilization lowered serum GH and TSH levels throughout the 24-h period that followed exposure to the stressor. Such an effect was more marked in GH than in TSH. In addition, GH, but not TSH, levels were found to be reduced significantly by IMO at 08.30 h of the next day. None the less, daily rhythms of GH and TSH were still persistent and roughly similar to those of control rats. The daily rhythm of food intake was measured in a separate experiment and it was observed, as expected, that IMO reduced food intake only in the dark period of the lighting cycle. It appears therefore unlikely that IMO-induced anorexia was the major factor responsible for the inhibition of GH and TSH caused by IMO at 11.00 and 19.00 h, considering that the amount of food intake was very low and similar in control and IMO rats during this period. However, anorexia might have contributed to inhibition of GH and TSH secretion afterwards. Thus, in a third experiment we studied the contribution of IMO-induced anorexia to the changes in hormone levels observed 24 h after stress by introducing a group of pair-fed rats. It was found that IMO, but not pair-feeding, reduced TSH levels, whereas a similar reduction of GH was found in the two conditions. It might be concluded that acute stress transiently altered corticosterone secretion, the only long-lasting effect being a slight increase in its morning levels on the following stress. Immobilization also causes an inhibition of GH and TSH secretion in the rat that persists for several hours after finalization of exposure to the stressor, but daily rhythms were still apparent. It appears that the contribution of stress-induced anorexia is different in GH than in TSH. In conclusion, an acute severe stressor such as IMO, although modifying circulating levels of some hormones, particularly in the hours following exposure to the stressor, did not appear to interfere greatly with the expression of circadian rhythms of anterior pituitary hormones.

Effect of regularity of exposure to chronic immobilization stress on the circadian pattern of pituitary adrenal hormones, growth hormone, and thyroid stimulating hormone in the adult male rat.

Circadian variation of serum levels of adrenocorticotropin hormone (ACTH), corticosterone, growth hormone (GH), and thyroid-stimulating hormone (TSH) were studied in three groups of adult male rats exposed to chronic intermittent immobilization stress (IMO) for 2 hr daily under different schedules. IMO resulted in reduced food intake, body weight loss, and increased adrenal weight. ACTH levels were not affected but corticosterone levels were increased in all IMO rats as compared to control ones during the diurnal phase of the circadian cycle. IMO decreased serum GH and TSH levels but the circadian pattern of secretion was influenced in a complex way depending on the specific pattern of daily exposure to IMO. Differences observed between the IMO groups were not caused by differences in food intake because its circadian rhythm was very similar in all IMO groups. These results suggest that regularity of exposure to immobilization alters in a complex fashion circadian GH and TSH rhythms.

The effects of chronic intermittent stress on basal and acute stress levels of TSH and GH, and their response to hypothalamic regulatory factors in the rat.

The effect of chronic stress on basal and stress-induced alterations of TSH and GH was studied in adult male rats. Chronically stressed rats were subjected 6 days per week for 4 weeks to several acute stressors including saline injections, noise, ether and forced swimming. Each day, one stressor was chosen randomly. Twenty hr after the last stress session, basal levels of TSH were normal or increased, with no altered pituitary response to TRH. In contrast, the TSH rise induced by acute stress was blunted in chronically stressed rats. Chronic stress resulted in lower basal and acute stress levels of GH. These modifications were probably due to changes in the release of hypothalamic regulatory hormones, because no evidence for altered TSH response to TRH, and GH response either to GHRH or to somatostatin, was found. Some abnormal responses of GH to TRH and of TSH to GHRH were observed in chronically stressed rats. These data indicate that this type of chronic stress induced significant changes in basal and acute stress levels of GH and TSH in the rat.

Effects of chronic immobilization stress on GH and TSH secretion in the rat: response to hypothalamic regulatory factors.

The effect of chronic immobilization (2 h/day) for 13 days on basal and stress levels of GH and TSH, and their response to various hypothalamic regulatory factors was studied in male Sprague-Dawley rats. Chronic immobilization (IMO) resulted in reduced serum TSH levels in stress situations but not in resting conditions. GH secretion was inhibited both in resting and stress situations. Chronic IMO impaired both GH and TSH responses to GRH and TRH, respectively, but also to another peptide (VIP) stimulatory for the two hormones. Whereas somatostatin administration inhibited GH secretion in control but not in chronic IMO rats, its inhibitory effect on TSH was slight and similar in the two experimental groups. The present results suggest that chronic exposure to a severe stressor such as IMO alters GH and TSH secretion, at least in part by changes in the response of the pituitary to the hypothalamic regulatory factors. The actual influence of chronic IMO on the release of these peptides into the median eminence remains to be studied.

The effects of chronic stress on corticosterone, GH and TSH response to morphine administration.

The effects of a chronic stress model in which several acute stressors were applied on a random basis on corticosterone, growth hormone (GH) and thyroid stimulating hormone (TSH) responses to morphine administration were studied in adult male rats. Chronic stress resulted in lower corticosterone response to the drug. In contrast, GH response to morphine was enhanced in the former animals and TSH response remained unchanged. The physiological role of changes in hormone response to opiates remains to be established, but the present results suggest that central opioid pathways involved in the neuroendocrine control of the anterior pituitary did not respond homogeneously to chronic stress.

Blockade of opioid receptors with naltrexone inhibits thyrotropin increase after noise stress but does not prevent the decrease caused by immobilization.

The influence of naltrexone-induced opioid receptor blockade on the response of thyrotropin to two different acute stressors was studied in adult male rats. Naltrexone slightly but significantly reduced basal thyrotropin levels and abolished the increase in serum thyrotropin caused by acute noise stress. In contrast, the opioid antagonist did not prevent the decrease in serum thyrotropin caused by another much more severe stressor such as immobilization. The present data offer the first evidence that endogenous opioids could play a stimulatory role in the control of thyrotropin secretion in a presumably physiological condition such as the response to a mild stressor. In addition, factors other than opioids could be involved in the inhibition of thyrotropin secretion under severe stress.

Influence of intensity and duration of exposure to various stressors on serum TSH and GH levels in adult male rats.

The effect of stressor intensity and duration of exposure to the stimuli on adrenocorticotropin (ACTH), somatotropin (GH) and thyrotropin (TSH) concentration in serum was studied in adult male Sprague-Dawley rats. The stressors used were noise, restraint in plastic tubes and immobilization on wood boards. The greatest ACTH release was found in immobilized rats and the smallest in noise-exposed animals. The inhibition of GH secretion was related to the intensity of ACTH release in that maximal GH inhibition was observed in immobilized rats and minimal in noise-exposed rats. The TSH response was more complex. Noise increased TSH release at all periods observed (10, 30 and 60 min); the stimulation of TSH release caused by restraint was significant at 30 and 60 min and was always of lesser magnitude than that in response to noise. Finally, immobilization significantly increased TSH levels at 10 min and decreased them at 30 and 60 min. These results suggest that, under appropriate conditions, all hormones studied discriminate between different stressor intensities. However, the complexity of the TSH response to stressors indicates that this hormone is not an adequate index of the stress experienced by the animals.

Sensitivity of anterior pituitary hormones to graded levels of psychological stress.

The effect of graded levels of stressor intensity on anterior pituitary hormones was studied in adult male rats. Corticosterone, considered as a reflection of ACTH release, and prolactin responses showed a good correlation with the intensity of the stressors. On the contrary, neither LH, GH nor TSH release showed a parallelism with the intensity of the stressors in spite of the fact that they clearly responded to all the stimuli. It appears that the hormones of the anterior pituitary might be divided into two groups: those whose response is sensitive to the levels of emotional arousal elicited by stress, and those displaying a clear but stereotyped response during stress. However, other alternative explanations might exist to justify the present results. The neural mechanisms underlying the two types of response are at present unknown. These data indicate that only the pituitary-adrenal axis and prolactin have some potential utilities as quantitative indices of emotional arousal elicited by currently applied stressors in the rat.