Adenosine and pituitary-adrenocortical axis activity in the rat

As shown by an increase in plasma corticosterone concentrations, adenosine administration stimulated pituitary-adrenocortical activity. This effect was prevented by dexamethasone (2 mg/kg i.p.). Added in vitro, adenosine reduced both adrenal basal and adrenocorticotropic hormone (ACTH)-stimulated corticosterone release, while it stimulated pituitary ACTH release. This ACTH response was blocked by dexamethasone but not by Tyr-somatostatin. Restraint stress increased adenosine content in the anterior pituitary, suggesting its possible involvement in hormonal stress response. Because the effect of adenosine on plasma corticosterone was still present in rats with a pharmacological block of the endogenous corticotropin-releasing factor release, we propose that adenosine is involved in the regulation of adrenocortical secretion at the level of the anterior pituitary and that this role is exerted through an interaction with a stimulatory adenosine receptor.     

Effects of a novel 21-amino steroid, U74006F, on the rat pituitary-adrenocortical axis

The 21-amino steroid U74006F is a potent inhibitor of lipid peroxidation and has been shown to affect beneficially the acutely injured central nervous system. Therapeutically, it is desirable for this compound to be devoid of steroid side-effects. We have demonstrated a significant (P less than 0.001) inhibition of basal ACTH secretion from cultured rat pituitary cells during a 24-h incubation at concentrations (10-100 mumols/l) previously demonstrated to inhibit lipid peroxidation in vitro. U74006F also inhibited corticotrophin-releasing factor (CRF)-stimulated ACTH secretion significantly and the combination of dexamethasone and U74006F completely blocked CRF-41-stimulated ACTH secretion. Administration of U74006F in vivo (30 mg/kg, orally, every 6 h for 30 h) had no effect on ACTH levels in normal rats (84 +/- 38 vs 45 +/- 6 ng/l in control animals) but increased ACTH levels in adrenalectomized rats (1330 +/- 295 vs 464 +/- 79 ng/l in control animals, P less than 0.02). This increase in ACTH was not observed when adrenalectomized animals were maintained on the same regime of U74006F for 5 days. Our data suggest that U74006F is capable of exerting inhibitory effects on ACTH secretion in vitro. In vivo, effects on ACTH secretion were stimulatory rather than inhibitory and only occurred short-term in adrenalectomized animals or chronically in adrenalectomized rats maintained on dexamethasone. No effects on the pituitary-adrenocortical axis were seen following short-term or chronic administration of U74006F in normal rats.     

The pituitary-adrenocortical axis and the immune system

Recent findings indicate that interactions between the pituitary-adrenocortical axis and the immune system involve more than simply the effects of glucocorticoid hormones. This altered view has resulted from the observations that: 1) cells of the immune system have receptors for and are directly acted upon by ACTH and endorphins, and 2) the immune system is an important non-pituitary source of these peptide hormones. In this chapter, while we review in a cursory way the older findings with glucocorticoid hormones, we concentrate on the newer developments which suggest that leukocyte- and pituitary-derived ACTH and endorphins perform regulatory functions within and between the immune system and the pituitary-adrenocortical axis.     

The response of the hypothalamic-pituitary-gonadal axis to fasting is modulated by leptin

Reproductive function is intimately related to caloric consumption. During fasting states, the hormones regulating reproduction, those of the hypothalamic-pituitary-gonadal axis, in particular, are severely altered. With the exciting observations that the obese (ob) gene product leptin, may also modulate neuroendocrine functions, we examined leptin's ability to prevent the consequences of fasting on reproductive hormones. Two groups of male rats, aged 65 days old, were either fasted and saline-injected or fasted and leptin-treated for approximately three days. Another group was given free access to rat chow. Leptin was able to prevent the fasting-induced fall of serum testosterone. Similar to testosterones dependence on leptin, leptin concentrations were somewhat dependent on testosterone. Castration accelerated the normal, age-related increase in serum leptin. Leptin also prevented the fasting-induced fall in luteinizing hormone (LH). The increase of beta-LH mRNA seen in the fasting state was prevented by leptin. There were no differences noted in luteinizing hormone releasing hormone (LHRH) mRNA among any of the groups. While neither fasting nor fasting plus leptin caused changes in serum prolactin, the increase in prolactin mRNA seen in fasted animals was prevented by leptin treatment. These data support the hypothesis that leptin plays a specific role in mediating the response of reproductive hormones to the nutritional status of the organism.     

Effects of acute ethanol administration and cold exposure on the hypothalamic-pituitary-thyroid axis

A single dose of ethanol increases cellular levels of the mRNA encoding thyrotropin-releasing hormone (TRH) in neurons of the paraventricular nucleus (PVN), but blocks the cold-induced increase in the levels of this mRNA. Because the thyrotropic response to cold is dependent upon TRH secretion, we hypothesized that ethanol uncouples the stimulus-induced regulation of TRH secretion from the stimulus-induced regulation of TRH expression. We employed two complementary strategies to test this hypothesis. The first was to determine whether ethanol alters pituitary sensitivity to TRH. Animals given a single intraperitoneal injection of ethanol (3 g/kg) that produced a blood alcohol concentration of nearly 300 mg/100 mL exhibited the same increase in circulating levels of TSH following an intravenous infusion of TRH. Thus, ethanol does not appear to alter pituitary sensitivity to TRH. Second, we tested whether ethanol blocks the cold induction of c-fos expression in TRH neurons of the PVN. Both cold exposure and ethanol induced the expression of c-fos in the PVN and in TRH neurons; the effects of cold and ethanol on c-fos expression were additive. Thus, ethanol clearly does not block the cold activation of TRH neurons.     

Food intake-induced leptin secretion modulates hypothalamo-pituitary-adrenal axis response and hypothalamic Ob-Rb expression to insulin administration

The mutation of the ob gene is known to induce a phenotype of obesity accompanied by symptoms including enhanced production of glucocorticoid. Chronic administration to ob/ob mice of leptin, the ob gene product, reverses hypercorticosteronemia. This establishes a clear relation between adipocyte and hypothalamo-pituitary-adrenal (HPA) axis functions. In the present study we investigated the acute modulatory effects of food intake-stimulated leptin secretion on HPA axis activity and hypothalamic leptin receptor (Ob-Rb) expression in 24-hour fasting, adult female, BALB/c mice after insulin-induced hypoglycemia. Our results indicate that: (1) food supply for 45 min to 24-hour fasting mice increased plasma glucose levels and reversed both hypercorticosteronemia and hypoleptinemia; (2) the insulin-induced hypoglycemia produced a marked HPA axis activation in animals with no access to food but this response was fully prevented by food intake and the consecutive increase in plasma leptin levels; (3) the inhibitory effect of leptin on the HPA axis response to insulin-induced hypoglycemia was corroborated by i.p. administration of murine leptin, and (4) fasting-induced hypothalamic Ob-Rb overexpression is not modulated by insulin itself but by leptin, since increase in leptin levels by food intake or by administration of exogenous leptin completely reversed this Ob-Rb overexpression. These results confirm the inhibitory effect of leptin on the HPA axis response to various stress stimuli. They clearly demonstrate that acute food intake in 24-hour fasting mice: (a) rapidly reduced fasting-induced hypercorticosteronemia by enhancing both spontaneous and insulin-elicited endogenous leptin secretion; (b) fully prevented HPA axis response to insulin administration, by rapidly increasing endogenous leptin secretion and probably also by diminishing the extent and the duration of insulin-induced hypoglycemia, and (c) abolished hypothalamic Ob-Rb overexpression induced by fasting itself combined with insulin treatment. The present data strongly suggests an inhibitory effect of endogenous leptin on insulin-induced HPA axis response, an interaction relevant to the physiological adaptation to starvation and caloric excess, and further supports the pivotal role played by the hypothalamus in restoring homeostasis in different allostatic states.     

Effects of leptin on rat ventromedial hypothalamic neurons

Neurons in the ventromedial and arcuate hypothalamic nuclei (VMN and ARC, respectively) mediate many of leptin's effects on energy homeostasis. Some are also glucosensing, whereby they use glucose as a signaling molecule to regulate their firing rate. We used fura-2 calcium (Ca2+) imaging to determine the interactions between these two important mediators of peripheral metabolism on individual VMN neurons and the mechanisms by which leptin regulates neuronal activity in vitro. Leptin excited 24%, inhibited 20%, and had a biphasic response in 10% of VMN neurons. Excitation occurred with a EC50 of 5.2 fmol/liter and inhibition with a IC50 of 4.2 fmol/liter. These effects were independent of the ambient glucose levels, and both glucosensing and non-glucosensing neurons were affected by leptin. In contrast, the ARC showed a very different distribution of leptin-responsive neurons, with 40% leptin excited, 10% leptin inhibited, and 2% having a biphasic response (chi2=60.2; P<0.0001). Using pharmacological manipulations we found that leptin inhibits VMN neurons via activation of phosphoinositol-3 kinase and activation of the ATP-sensitive K+ channel. In addition, leptin inhibition was antagonized by 5'-AMP-activated protein kinase activation in 39% of neurons but was unaffected by 5'-AMP-activated protein kinase inhibition. No mechanism was delineated for leptin-induced excitation. Thus, within the physiological range of brain glucose levels, leptin has a differential effect on VMN vs. ARC neurons, and acts on both glucosensing and non-glucosensing VMN neurons in a glucose-independent fashion with inhibition primarily dependent upon activation of the ATP-sensitive K+ channel.     

Effects of phenobarbital on hypothalamic-pituitary-thyroid axis in the rat

It has been reported that phenobarbital (PB) increases the peripheral clearance of T4 and T3 and decreases serum T4 and T3 concentrations in the rat, but serum TSH remains unchanged. To explore a possible direct effect of PB on TSH secretion at the hypothalamic-pituitary level, adult male rats were given PB 100 mg/kg or vehicle IP for 10 days. No difference in their thyroid weights was observed. In the PB-treated group serum T4 was decreased (PB, 3 +/- 0.2 micrograms/dl vs. control, 3.8 +/- 0.1 micrograms/dl, mean +/- SE, p less than .002), as was serum T3 (PB, 51 +/- 6 ng/dl vs. control, 70 +/- 5 ng/dl, p less than .05), but serum TSH remained unchanged. Pituitary TSH and hypothalamic TRH contents also were unchanged. Further studies were carried out similarly in the thyroidectomized hypothyroid rat to eliminate the effect of PB on serum T4 and T3 levels. PB or vehicle were started two days after thyroidectomy. By postoperative day 12, TSH levels in the PB-treated rats were lower than in the controls (PB, 697 +/- 62 microU/ml vs. control, 891 +/- 53 microU/ml, p less than .05). Pituitary TSH and hypothalamic TRH contents again were similar in both groups. When TRH (500 ng/kg body weight, IV) was given, the increment in serum TSH at 10 minutes was significantly lower in the PB group (PB, 53 +/- 26 microU/ml vs. control, 131 +/- 18 microU/ml, p less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Acute leptin deficiency, leptin resistance, and the physiologic response to leptin withdrawal

Food restriction and weight loss result in reduced plasma leptin, which is associated with a pleiotropic biologic response. However, because weight loss itself is also associated with changes in numerous other humoral and metabolic signals, it can be difficult to determine the precise features of the biologic response to acute leptin deficiency. To study this response in the absence of changes in nutritional state, we have developed a protocol that allows such analysis in normal, non-food-restricted animals. Wild-type mice are treated with high-dose leptin until fat mass is depleted and, as a consequence, endogenous leptin production is reduced. At this point, exogenous leptin is abruptly withdrawn, thus inducing a state of leptin deficiency in otherwise normal mice. Leptin deficiency is sustained by feeding the animals only as much as they consumed voluntarily before leptin withdrawal. The biologic response to leptin deficiency induced in this manner includes altered neuropeptide levels, decreased energy expenditure, and impaired reproductive and immune function. Replacement of leptin at physiological concentrations after withdrawal of high-dosage leptin blunts, but does not completely block, the hyperphagia and weight regain caused by acute leptin deficiency, nor does it correct the resulting reproductive and immune dysfunction. This suggests that high-dosage leptin treatment induces a state of partial leptin resistance. In aggregate, these studies establish the role of acute hypoleptinemia in regulating energy balance, the immune system, and reproductive function, and further suggest that high-dosage leptin treatment can induce a state of acquired leptin resistance.     

Activation of the pituitary-adrenocortical axis in day-old rats by insulin-induced hypoglycemia.

Most newborn mammals, including humans, are susceptible to neonatal hypoglycemia shortly after birth, especially if fasted. Glycogenolysis provides short term benefit, but gluconeogenesis is required for glucose homeostasis at this stage of development. A functional hypothalamic-pituitary-adrenocortical (HPA) axis is essential for glucose homeostasis in adults, but little is known of the importance of this endocrine axis in this regard during the neonatal period. The neonatal rat is an excellent model for study of both glucose metabolism and the HPA, and shares many similarities with other mammals. For the first 2 weeks of postnatal life, the rat HPA appears to be relatively unresponsive to stress, however, which would seem maladaptive during the critical first 24 h. The present study examined the ability of day-old rats to respond to hypoglycemia with activation of the HPA system. Each of three doses of insulin resulted in hypoglycemia (glucose, less than 40 mg/dl), with the greatest decrease in glucose 120 min after an insulin dose of 2.5 U/kg BW. At this dose, plasma corticosterone was increased to levels comparable to those observed in adults. The steroid response was associated with a small but significant increase in ACTH, and both endocrine responses were delayed compared to those in adults. There was no difference in adrenal response time to ACTH in incubations of adrenal cells isolated from day-old or adult rats, however. The stimulatory effect of insulin was progressively diminished in older animals (2-6 days postnatal). Insulin had no direct effect on adrenal steroidogenesis either alone or in combination with ACTH when tested on dispersed cells in vitro. We conclude that 1) the adrenal gland is very sensitive to the stress of hypoglycemia for 1 day after birth, but rapidly declines in activity over the first 6 days of life; 2) the response time of the HPA axis to hypoglycemia stress is delayed in day-old rats, and this delay occurs above the level of the adrenal; and 3) there may be factors in addition to ACTH that promote steroidogenesis on the first postnatal day, or there may be bioactive or immunoreactive differences in the ACTH molecule during this time.     

Influence of tri-iodothyronine or lipid administration on the response of the pituitary-thyroid axis to exposure to cold in the newborn lamb

The influence of the administration of tri-iodothyronine (T3) or a solution of soya oil and egg lecithin on the response of the pituitary-thyroid axis to moderate exposure to cold (4 degrees C for 4 h) was studied in 24-h old lambs. In control lambs, plasma concentrations of TSH. T3 and total and free thyroxine (T4) rose significantly whereas plasma concentrations of reverse T3 remained unchanged during the test. In lambs injected i.v. with a small amount of T3 (1.23 nmol/kg) 30 min before the onset of exposure to cold, plasma concentrations of TSH, reverse T3 and total and free T4 did not change during the test. Administration of lipid 30 min before exposure to cold induced, as expected, a sharp rise in plasma free fatty acid (FFA) concentrations and a transient increase in free T4 concentrations. In these animals, plasma concentrations of TSH increased during the test as observed in control lambs, but plasma concentrations of T3, reverse T3 and total T4 did not show any significant change, whereas free T4 levels decreased during the first 2 h. These results strongly suggest, in contrast to previous results, that T3 exerts a negative feedback upon the hypothalamic-pituitary-thyroid axis in the newborn lamb. Moreover, it appears that a rise in plasma concentrations of FFA could affect neonatal thyroid function.     

Evidence for a specific role of vasopressin in sustaining pituitary-adrenocortical stress response in the rat

In the adult male Wistar rat a 2-fold 2-min restraint stress exposure, repeated 15 min apart, activated the adrenocortical secretion more than a single one would have. However, in rats with a pharmacological block of the endogenous CRF release, exogenous CRH (0.3 micrograms/kg iv), administered 15 min after a first similar dose, was unable to stimulate pituitary-adrenocortical activity above the level attained with the first peptide injection. On the contrary, in the same conditions exogenous arginine vasopressin (AVP) (0.3 micrograms/kg iv) administered 15 min after CRH, was able to further stimulate pituitary-adrenocortical activity. Using the same experimental procedure, oxytocin (0.3 micrograms/kg iv) was found to be totally inactive. The physiological import of these findings was investigated in the Brattleboro rat, genetically lacking in endogenous AVP, in which, unlike the control Long-Evans strain, the 2-fold stress exposure did not cause an increase in plasma corticosterone concentration greater than that of a single exposure. These results suggest that endogenous AVP is essential in sustaining adrenocortical activation in circumstances in which pituitary refractoriness towards CRH stimulation intervenes.     

Cortisol axis abnormalities early after stroke--relationships to cytokines and leptin

OBJECTIVE: To assess the relationships between circulating levels of proinflammatory cytokines and adrenocortical hormones and leptin early after stroke. DESIGN: Blood samples were collected four times daily the first two days after stroke, twice daily the next 4 days and four times at day 7. Cognitive function and functional outcome was measured at admittance and at day 7. SETTING: Consecutive inclusion of patients admitted to the stroke unit at Umea University Hospital. SUBJECTS: Eight men and 4 women with acute stroke and 10 healthy volunteers. MAIN OUTCOME MEASURES: Levels and diurnal variations of plasma proinflammatory cytokines interleukin-6 (IL-6) and tumour necrosis factor-alpha (TNF-alpha), serum adrenocortical hormones (cortisol and DHEA) and leptin, and MMSE, SSS, and ADL scores. RESULTS: A significant correlation was present between IL-6 and cortisol levels the first two days after stroke (P < 0.05). In patients with a disturbed diurnal rhythm of cortisol, cortisol and leptin levels were increased (68% and 81% increase, respectively), whilst DHEA levels were unaltered. Half of the patients displayed an abnormal diurnal rhythmicity of leptin at the end of the week. Median TNF-alpha levels for the first two days after stroke also correlated to median leptin levels at the end of the week (P < 0.05). Median IL-6 levels correlated to severity of paresis on days 1 and 7 and to MMSE scores on day 7 (P < 0.05). CONCLUSIONS: Neuroendocrine disturbances are common and often profound early after stroke. Cytokines seem to be important modulators of these disturbances, including diurnal rhythmicity of cortisol and leptin.     

Effects of castration and testosterone on the pituitary and adrenal responses of the newborn rat to ether inhalation

In 8-day-old rat newborns, the pituitary response to 2 min of ether inhalation was noted to vary according to sex. Plasma ACTH levels were similarly increased in males and females at the end of ether exposure; however, during the following 30 min, ACTH levels were always higher in females than in males. In order to verify that the putative masculinization of some neuroendocrine pathways involved in the pituitary response to ether stress was the result of the transitory surge of testosterone at birth, fetuses at term were delivered by cesarean section and thereafter immediately castrated or sham-castrated under cold anesthesia (males), injected with testosterone heptylate (1 mg s.c.) or olive oil used as solvent (females) before being put in the care of a nurse. The rise in plasma testosterone levels during the 1st h after birth was prevented or stopped in males put at 2 degrees C. At the 8th postnatal day, the newborns were subjected to 2 min of ether inhalation; they were sacrificed either just, before or after the end (0 and 30 min) of the stress procedure. Plasma immunoreactive ACTH level and adrenal corticosterone content were measured. The pituitary response, shown by the ACTH increase, in castrated or sham-castrated males and testosterone-injected females was similar to that of intact males but very different from that observed in olive-oil-injected or intact females. The rise in adrenal corticosterone content 30 min after ether inhalation was greater in intact and olive-oil-injected females than in testosterone-injected ones or in males; adrenal response was well correlated with the maintenance of ACTH release in the former and the decrease following transitory surge in the latter.(ABSTRACT TRUNCATED AT 250 WORDS)     

Maturation of the inhibitory response of growth hormone secretion to ether stress in postnatal rat

To study the maturation of inhibitory influences on growth hormone (GH) secretion the effect of ether stress on plasma GH levels was studied during postnatal ontogenesis in female rats. Ether stress did not affect plasma GH levels in 1-day-old pups. A distinct decrease of plasma GH was found in 3- and 9-day-old pups, and the response was prevented by treatment of 3-day-old animals with somatostatin antiserum. No effect of ether stress on plasma GH was noted in 12-, 15-, 18- and 21-day-old rats. Treatment of intact 12-day-old pups with the somatostatin antiserum increased plasma GH level under basal conditions. The inhibitory effect of ether stress on plasma GH was noted again at the age 30 days and in adult animals. It is concluded that the hypothalamus of 3-day-old rats is able to release enough somatostatin to inhibit GH secretion after stress. At the period 12-18 days a phase of pituitary refractoriness was noted: ether stress as well as TRH injection (our previous observation) fail to affect plasma GH in female pups, probably due to high somatostatin secretion under basal conditions and (or) low capacity of pituitary to release GH. It is suggested that regulation of GH secretion is not mature until after the 21st day of life.