Paraventricular stimulation with glutamate elicits bradycardia and pituitary responses

The excitatory neurotransmitter, L-glutamate (0.5 M, pH 7.4), or the organic acid, acetate (0.5 M, pH 7.4), was microinjected (50 nl over 2 min) directly into the paraventricular nuclei (PVN) of pentobarbital sodium-anesthetized rats while arterial blood pressure and heart rate and plasma adrenocorticotropic hormone (ACTH), vasopressin, and oxytocin were measured. Activation of PVN neurons with L-glutamate led to increases in plasma ACTH, vasopressin, and oxytocin and a profound bradycardia (approximately 80 beats/min) with little change in arterial blood pressure. Microinjection of acetate had no effect on the above variables. The decrease in heart rate was shown to be dependent on the concentration of glutamate injected and the volume of injectate. The bradycardia was mediated through the autonomic nervous system because ganglionic blockade (pentolinium tartrate) eliminated the response; atropine and propranolol severely attenuated the bradycardia. The bradycardia was greatest when L-glutamate was microinjected into the caudal PVN. Injections into the rostral PVN or into nuclei surrounding the PVN led to small or nonsignificant decreases in heart rate. Focal electric stimulation (2-50 microA) of the PVN also led to decreases in heart rate and arterial blood pressure. These data suggest that activation of PVN neurons leads to the release of ACTH, vasopressin, and oxytocin from the pituitary and a bradycardia that is mediated by the autonomic nervous system.

Steroid inhibition of canine ACTH: in vivo evidence for feedback at the corticotrope

We infused submaximal feedback doses of either dexamethasone (DEX; 0.1 microgram.kg-1.min-1) or corticosterone and cortisol (B+F; 1.5 micrograms.kg-1.min-1) intravenously for 40 min into conscious dogs and measured the adrenocorticotropic hormone (ACTH) responses to hypoglycemia induced by insulin (0.1 U/kg) or to ovine corticotropin-releasing factor (oCRF; 1 microgram/kg); both agents were injected at 120 min. The dose of DEX was chosen to produce suppression of the ACTH response to oCRF equivalent to that produced by B+F. The purpose of the study was to determine 1) whether CRF- and hypoglycemia-induced ACTH secretion are equally inhibited by glucocorticoid treatment and 2) whether DEX and B+F have differential effects in the inhibition of stress-induced ACTH secretion. We found that peak ACTH responses to hypoglycemia and CRF were equally inhibited by DEX (36 +/- 6 and 52 +/- 9%, respectively). The peak ACTH responses to hypoglycemia and CRF were also equally inhibited after B+F infusion (45 +/- 13 and 65 +/- 5%, respectively). There was no significant interaction between the steroid administered and the stimulus given in controlling the ACTH response (by 2-way analysis of variance). The results suggest that pituitary feedback is of primary importance in suppression of canine ACTH secretion by delayed feedback and that the natural and synthetic steroids both act at this site.

Radio-frequency vs. electrolytic VMH lesions: differential effects on plasma hormones

Electrolytic lesions of the ventromedial hypothalamus (VMH) result in marked hyperphagia and obesity, but several studies have found the excess food intake and weight gain to be greatly reduced when lesions are produced by electrocauterization with radio-frequency current. Unlike electrolytic lesions, radio-frequency lesions leave few or no deposits of metallic ions that can potentially stimulate adjacent tissue. In the present experiment, weight gain and several endocrine responses previously associated with the VMH syndrome were compared in female rats given either electrolytic, radio-frequency, or sham VMH lesions. Both groups with VMH lesions became obese, but rats with radio-frequency lesions displayed only 63.2% of the weight gain of animals with irritative lesions (120.0 vs. 189.8 g in 20 days). Only rats with electrolytic lesions displayed elevated plasma insulin levels during an initial period of food restriction, but both groups with lesions were hyperinsulinemic when allowed to overeat. Plasma growth hormone levels were decreased by electrolytic lesions but unaffected by radio-frequency lesions. Morning corticosterone levels were elevated in both VMH groups, but only the rats with electrolytic lesions were found to have elevations in plasma adrenocorticotropin. It is concluded that some of the endocrine dysfunctions resulting from electrolytic VMH lesions are due to irritative stimulation rather than to tissue ablation.

Circadian variations in plasma corticosterone permit normal termination of adrenocorticotropin responses to stress

We previously reported that adrenalectomized rats given constant corticosterone via a sc pellet (B-PELLET) hypersecrete ACTH in response to stress. Although lacking a feedback signal, B-PELLET rats do not secrete ACTH indefinitely after stress; plasma ACTH levels in these animals returned to those in sham-operated (SHAM) rats within 1-4 h after 2-min restraint. To distinguish between the requirement for circadian or stress-induced increases in corticosterone, we compared changes in ACTH and corticosterone levels after stress in SHAM and B-PELLET rats with those in cyanoketone-treated rats (CK) and adrenalectomized rats given corticosterone in their drinking fluid (B-WATER). B-WATER rats exhibited sustained increases in plasma corticosterone after lights-off, correlating with the nocturnal feeding period. Morning plasma corticosterone levels in B-WATER rats were constant and even lower than those in B-PELLET rats; however, B-WATER rats did not differ from SHAM rats in their ACTH response to ip injection. CK rats, which have an approximately normal circadian corticosterone rhythm but do not have significant corticosterone responses to acute stimuli, also exhibited plasma ACTH levels similar to those of SHAM rats at all times after 5-min restraint. Compared with SHAM and B-WATER rats in the same experiment, B-PELLET rats tended to hypersecrete ACTH 60 min after 5 min of restraint, but only had significantly elevated plasma ACTH relative to both groups 45 min after 10 min of restraint. We conclude that circadian, rather than stress-induced, increases in corticosterone may be sufficient for normal termination of ACTH responses to stress.

Constant corticosterone replacement normalizes basal adrenocorticotropin (ACTH) but permits sustained ACTH hypersecretion after stress in adrenalectomized rats

To characterize further the effects of providing a constant corticosterone signal after bilateral adrenalectomy, we have compared the effects of bilateral adrenalectomy with no replacement (ADX) and with replacement with a corticosterone pellet implanted sc at surgery (B-PELLET) to those of sham-adrenalectomy (SHAM) on pituitary and plasma ACTH concentrations during the first 3 postoperative days. In ADX rats, plasma ACTH concentrations were elevated at all times compared to those in the SHAM group; pituitary ACTH content decreased during the first 12 h, then increased and was not different from that in the SHAM group thereafter. Replacement of corticosterone at the time of adrenal surgery in B-PELLET rats resulted in no differences in pituitary and plasma ACTH concentrations from SHAM values, suggesting that immediate steroid replacement prevents the major adrenalectomy-induced changes in central regulatory components governing basal activity of the adrenocortical system. Although B-PELLET rats had normal basal morning ACTH concentrations 5 days after surgery, they exhibited augmented and sustained ACTH responses to five different ACTH-releasing stimuli (injection, restraint, chlorpromazine, and, under pentobarbital anesthesia, morphine or sham adrenalectomy). The circulating corticosterone concentrations were maintained at relatively constant, low levels (3-6 micrograms/dl). Because these concentrations appear to restore basal morning ACTH concentrations to normal, but do not restore the ACTH response to stress to normal, we conclude that a different corticosterone signal is required to normalize stress-induced ACTH responses.

Corticosterone acts on the brain to inhibit adrenalectomy-induced adrenocorticotropin secretion

To determine the site (brain and/or pituitary) at which corticosterone (Cort) acts to inhibit adrenalectomy-induced ACTH secretion, the following experiments were performed in male rats. Rats in which brain and pituitary feedback sites were to be left intact were subjected to sham hypothalamic lesions. Rats in which only pituitary sites were to be left were subjected to either medial basal hypothalamic (MBH) or para-ventricular nuclei (PVN) lesions. Two days later all rats were adrenalectomized and replaced with varying amounts of Cort (by sc pellet). Lesioned rats also received sc pumps that delivered 0-5 micrograms/day rat CRF. All rats were killed 5 days after adrenalectomy. Sham-lesioned groups exhibited the expected dose-related inhibition of plasma and pituitary ACTH concentrations by Cort, with normal values obtained at plasma Cort levels between 4.4 and 7.7 micrograms/dl. By contrast, rats with MBH and PVN lesions exhibited no ACTH responses to adrenalectomy when CRF infusions were between 0 and 1 micrograms/day. In rats with MBH and PVN lesions receiving 5 micrograms/day CRF, plasma ACTH concentrations were elevated and were not inhibited by plasma Cort values up to 6 micrograms/dl. Plasma ACTH was inhibited in rats with MBH lesions infused with 5 micrograms/day CRF when plasma Cort levels were 30.6 micrograms/dl. These rats also exhibited marked thymic atrophy. We conclude from these results that Cort normally acts only on a brain site to inhibit adrenalectomy-induced increases in ACTH secretion. Only when plasma Cort concentrations are markedly elevated can evidence for pituitary feedback be demonstrated in vivo.

Paraventricular lesions: hormonal and cardiovascular responses to hemorrhage

The responses of adrenocorticotropin (ACTH), renin, epinephrine and norepinephrine and arterial pressure and heart rate (HR) to hypotensive hemorrhage were examined before and 1 h after lesion of the paraventricular nuclei (PVN) in pentobarbital-anesthetized rats and 1 day before and 4 days after lesion of the PVN in conscious rats. The ACTH response to hemorrhage was abolished 1 h (n = 8) and 4 days (n = 14) after PVN lesion whereas the ACTH response in the sham groups (in both anesthetized and conscious studies, n = 8 and 16 respectively) remained intact. PVN lesion had no effect on basal ACTH levels 4 days after lesion. The responses of renin, epinephrine, norepinephrine and mean arterial blood pressure (MABP) and HR to hemorrhage were not affected 1 h or 4 days after PVN lesion. Resting levels of the above variables did not change 4 days after lesion. The PVN lesion had a small (but significant) effect on the baroreceptor reflex in the conscious study (reflex changes in HR caused by phenylephrine- or nitroglycerin-induced change in MABP) and had no effect on the baroreceptor reflex in the anesthetized study. The group with PVN lesions gained more weight 6 days after lesion than the group with sham lesions. We conclude that the PVN are part of a neural pathway involved in ACTH regulation during perturbations of the cardiovascular system and on weight gain and that PVN lesions have little or no effect on resting or stimulated (hemorrhage) levels of renin, epinephrine, norepinephrine, HR and MABP or on the baroreceptor reflex.

Characterization of corticosterone feedback regulation of ACTH secretion

Adrenalectomy-induced increases in ACTH secretion in rats are returned to normal by an action of corticosterone on the brain, not on the pituitary. Five days after adrenalectomy with constant steroid replacement, the concentration of free corticosterone in plasma which reduces plasma ACTH by 50% is approximately 0.8 nM. By contrast, the concentration of free plasma corticosterone required for 50% reduction of thymus wet weight or plasma transcortin concentration (both targets for glucocorticoid action) is about 4.5 nM. These results suggested that the inhibition of ACTH by corticosterone might be mediated by association of the steroid with high affinity, type I corticosteroid receptors, whereas the inhibition of thymus weight and transcortin might be mediated by association of the steroid with lower affinity, type II receptors. The results of studies comparing the ability of corticosterone, dexamethasone and aldosterone to inhibit adrenalectomy-induced ACTH secretion support the hypothesis that basal ACTH secretion in rats is mediated by association of corticosterone with type I receptors.

The suprachiasmatic nuclei stimulate evening ACTH secretion in the rat

The effect of bilateral lesions of the suprachiasmatic nuclei (SCN) on the circadian rhythm in ACTH was studied in rats that were adrenalectomized and implanted with a subcutaneous corticosterone (B) pellet. Rats wee chronically cannulated to allow for repeated blood sampling. In rats with B pellets, bilateral lesions of the SCN eliminated the circadian rise in plasma ACTH seen in sham-lesioned animals. This is consistent with the idea that the SCN stimulate ACTH secretion in the evening.

Pancreatic acinar cell amylase gene expression: selective effects of adrenalectomy and corticosterone replacement

To examine the role of glucocorticoids in the regulation of the acinar pancreas, adult male rats were adrenalectomized (Adx) and replaced with no corticosterone (B), normal B, or high B. Plasma B concentration, body weight gain, and thymus weight were used as independent measures of treatment efficacy. Compared to controls, Adx animals had a 75 +/- 0.5% (n = 30) reduction in pancreatic amylase content; a 50% decrease occurred within 1 day and the maximal 75% decrease was observed after 5 days. In Adx animals, amylase content was normalized by normal B replacement and was increased to 235 +/- 39% (n = 30) of control by high B replacement. Furthermore, in all Adx rats, pancreatic content of amylase and plasma B concentration was significantly correlated (r = 0.81, n = 30). The effect of adrenalectomy was selective for amylase; contents of ribonuclease, chymotrypsin, and elastase were not altered. However, the effects of high B replacement were not selective, and increased the content of all digestive enzymes. To determine whether the changes in enzyme content were associated with changes in messenger RNA (mRNA), pancreatic RNA was probed with 32P-labeled complementary DNAs for amylase, ribonuclease, and chymotrypsin. After adrenalectomy and B replacement there was a significant correlation only between amylase mRNA (r = 0.87, n = 13) and plasma B concentration. These data indicate that physiological levels of B have a selective effect on pancreatic amylase gene expression. In contrast, high levels of B have the separate, nonselective effect of increasing the content of all digestive enzymes without increasing corresponding mRNA levels.

Plasma adrenocorticotropin is more sensitive than transcortin production or thymus weight to inhibition by corticosterone in rats

After adrenalectomy, ACTH, corticosterone-binding globulin (CBG), and thymus wet weight increase in rats as a consequence of the removal of corticosterone (B) and decrease again in response to replacement with glucocorticoids. We have studied the effect of replacing adrenalectomized male rats with a variety of different concentrations of B. Plasma concentrations of ACTH and CBG and thymus wet weights were related to the measured concentration of free ultrafilterable B in plasma. In other experiments, the clearance of [125I]CBG was determined in adrenalectomized rats with and without B replacement, and the time required for the changes in plasma CBG concentrations after removal and/or replacement of B in adrenalectomized rats was determined. Five to 7 days after adrenalectomy and institution of a relatively constant B replacement signal, plasma CBG concentrations were highly correlated with circulating B concentrations (r2 = 0.745; P less than 0.001). The effect of B was on the CBG production rate, since clearance did not change. Because CBG concentrations decrease as total B concentrations increase, there is an amplification of free B concentrations with increasing total B. The relationships of plasma ACTH and CBG and thymus wet weight to circulating free B levels showed that 50% inhibition of ACTH was achieved at a free B concentration of 0.8 +/- 0.05 nM, whereas 50% inhibition of CBG and thymus wet weight were achieved at free B concentrations of 4.6 +/- 0.9 and 4.4 +/- 0.6 nM, respectively. These inhibition values correlate well with the known Kd values for the high affinity type I B receptor (0.5 nM) and for the lower affinity type II glucocorticoid receptor (2.5-5 nM), respectively, suggesting that ACTH secretion, CBG production, and thymus wet weight are regulated by the association of B with these receptor types.

Chronic administration of corticotropin-releasing factor increases pituitary corticotroph number

The effect of chronic administration of CRF on rat pituitary morphology was studied. Experimental animals received CRF (10 micrograms/day) over a period of 52 days by means of sc osmotic pumps changed at 10- to 14-day intervals. The average 0800 h plasma corticosterone levels in the treated animals were significantly greater than control values [7.52 +/- 0.99 (+/- SE) vs. 1.14 +/- 0.5 micrograms/dl; P less than 0.001]. The CRF-treated animals also had a significantly greater adrenal weight (16.44 +/- 1.38 vs. 12.24 +/- 0.85 mg; P less than 0.05) and lower thymus weight (164 +/- 12 vs. 248 +/- 27 mg; P less than 0.005). There was a marked increase in the number of ACTH-producing cells in the anterior pituitaries of the rats that received CRF (13.3 +/- 0.8% vs. 4.5 +/- 0.3% ACTH-producing cells; P less than 0.001), as determined by immunocytochemical methods. Corticotrophs of rats treated with CRF manifested a significant increase in nuclear area (24.0 +/- 0.7 vs. 21.4 +/- 0.4 micron 2; P less than 0.001) and an increased diameter of forming and storage granules (191.1 +/- 1.1 vs. 158.6 +/- 3.5 nm and 196.1 +/- 1.2 vs. 170.1 +/- 3.7 nm, respectively; P less than 0.001). There was no demonstrable increase in ACTH cell area. These data indicate that long term administration of CRF is capable of increasing the number of pituitary corticotrophs. It also supports the view that the corticotroph hyperplasia occurring after adrenalectomy, in unusual cases of ectopic CRF production, and in rare instances of Cushing's disease is a proliferative response to CRF.

Some metabolic and behavioral effects of adrenalectomy on obese Zucker rats

Ten-week-old female obese and lean Zucker rats were given access to three separate macronutrient sources (casein, starch, and lard) for 7 days. They were then either adrenalectomized (ADX) or given a sham operation. Rats were assigned to one of three groups and given a daily injection of either 0, 2, or 10 mg of corticosterone. They continued to select a diet for another 17 days, after which they were killed, and their blood was assayed for corticosterone, adrenocorticotropin hormone (ACTH), insulin, glucose, and triglyceride. Retroperitoneal and parametrial fat depots were excised and sampled for lipoprotein lipase activity, fat cell size, and number. Body composition was also determined. Selection patterns of lean and obese rats were markedly affected by both ADX and corticosterone replacement. All three groups of sham-operated obese rats ate significantly more fat than did sham-operated lean rats. Adrenalectomy significantly reduced fat intakes in both obese and lean rats. Corticosterone therapy restored fat appetites of lean and obese rats in a dose-dependent fashion. In comparison to ADX lean rats, ADX obese rats reduced their normally elevated levels of blood glucose, plasma triglycerides, and insulin to within normal limits. Similarly, adipose cellularity of the ADX obese rats was reduced to that of sham-operated lean rats. Carcass fat was significantly reduced after adrenalectomy. Corticosterone therapy prevented the reduction in a dose-dependent way.

Reset of feedback in the adrenocortical system: an apparent shift in sensitivity of adrenocorticotropin to inhibition by corticosterone between morning and evening

There is evidence in man and rats that higher circulating levels of glucocorticoids are required to normalize basal unstimulated ACTH levels at the peak of the circadian rhythm than at the trough. To explore this phenomenon, we tested the inhibitory effect of constant levels of corticosterone on plasma ACTH in the morning (AM) and evening (PM) in young male rats implanted with fused pellets of corticosterone-cholesterol at the time of adrenalectomy (ADX+B) and studied 5 days later. There was a marked shift of the plasma corticosterone-ACTH inhibition curve to the right between AM and PM, demonstrating that the efficacy of corticosterone feedback inhibition of ACTH is less in the PM. Comparison of plasma ACTH and corticosterone levels during 24 h in sham-adrenalectomized rats (SHAM-ADX), adrenalectomized rats (ADX), and ADX+B revealed constantly low ACTH in SHAM-ADX, constantly high ACTH in ADX, and biphasic ACTH levels in ADX+B. Corticosterone levels were biphasic in SHAM-ADX and were constant in the other two groups. These results again showed a shift in corticosterone feedback efficacy as a function of the time of day and also suggested that basal ACTH secretion is maintained in the low normal range in intact rats because of the marked diurnal rhythm in corticosterone. The sensitivity of the pituitary ACTH response to exogenous CRF did not change between AM and PM in either intact or ADX+B showing that the shift in feedback sensitivity to corticosterone does not reside in the pituitary. The response of the entire adrenocortical system to histamine stress was shown to be equivalent in both the AM and PM, suggesting that feedback sensitivity of the entire system to corticosterone does not change as a function of the time of day. We conclude from these results that there is an apparent diurnal change in ACTH sensitivity to corticosterone feedback that can be defined operationally as reset. We believe that the site of feedback being tested shifts solely from the pituitary in the AM (at the nadir of the rhythm) to the brain and the pituitary in the PM (at the peak of the rhythm). The lack of the normally high transients of corticosterone that occur in SHAM-ADX rats results in increased brain drive of the pituitary in ADX+B.

Responses of ACTH, epinephrine, norepinephrine, and cardiovascular system to hemorrhage

Hemorrhages of various magnitudes were performed on conscious rats, and arterial pressure, heart rate, and plasma levels of adrenocorticotropin hormone (ACTH), epinephrine, and norepinephrine were measured. Eight rats were prepared with chronic femoral arterial cannulas and received a 10, 15, or 20 ml/kg X 3 min hemorrhage in random order on day 4, 7, or 10 after surgery. Mean arterial blood pressure, heart rate, and plasma ACTH, epinephrine, and norepinephrine concentrations were determined before and 20 min after hemorrhage. Arterial blood pressure decreased significantly immediately after each hemorrhage and slowly recovered over the next 20 min. Heart rate did not change during the 10 ml/kg X 3 min hemorrhage but decreased significantly after 15 and 20 ml/kg X 3 min hemorrhages. Plasma ACTH and epinephrine levels increased significantly 20 min after the 15 and 20 ml/kg X 3 min hemorrhages but not after 10 ml/kg X 3 min hemorrhage. Norepinephrine increased significantly 20 min after the 20 ml/kg X 3 min hemorrhage but not after the 10 or 15 ml/kg X 3 min hemorrhage. There was no significant effect of time and repeated hemorrhages on resting levels of plasma ACTH, epinephrine, norepinephrine, osmolality, or proteins. Since hemorrhage leads to a fall in arterial pressure and a subsequent rise in plasma ACTH, the relationship between plasma ACTH and mean arterial blood pressure during hemorrhage was examined in both conscious and acutely prepared pentobarbital sodium-anesthetized rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Medullary lesions eliminate ACTH responses to hypotensive hemorrhage

The adrenocorticotropin (ACTH) response to hemorrhage (15 ml . kg-1 . 3 min-1) before and 30 min or 4 days after placement of bilateral electrolytic lesions of the nucleus tractus solitarius (NTS) were examined in anesthetized and in conscious rats. Two groups of rats were anesthetized with pentobarbital sodium (45 mg/kg). Femoral arterial and venous cannulas were placed acutely in the anesthetized group and chronically in the conscious group. Each rat received a hemorrhage 30 min before and 30 min after NTS lesions (in the anesthetized group) and 1 day before and 4 days after NTS lesions (in the conscious group). Plasma ACTH was determined before and 20 min after hemorrhage, and mean arterial blood pressure and heart rate were measured throughout. The baroreceptor reflex (bradycardia caused by a phenylephrine-induced rise in MABP) was determined 5 min before hemorrhage (in the anesthetized group) and 1 day before hemorrhage (in the conscious group) to assess the effectiveness of lesion. Hexamethonium was given to rats that developed hypertension postlesion and to sham-lesioned controls. Plasma ACTH did not increase after hemorrhage 30 min or 4 days after NTS lesions when compared with the other groups (sham, sham with hexamethonium, and missed lesion) and to prelesion controls. Also, lesions of the NTS had no effect on resting ACTH levels 4 days later. Mean arterial pressure and heart rate decreased during hemorrhage to similar extents before and after lesions in all groups. This study demonstrates that lesions of the NTS eliminate the ACTH response to hemorrhage immediately and 4 days after the lesions but have no effect on resting ACTH levels. The result suggests that the NTS is an essential part of the neural pathway for ACTH release after hemorrhage.

Corticotrope response to removal of releasing factors and corticosteroids in vivo

In the intact rat, adrenalectomy (ADX) is known to result in increased ACTH synthesis, content, and secretion from the anterior pituitary compared with those in the sham-adrenalectomized control. Treatment of adrenalectomized, rats with corticosterone prevents or reverses these changes in ACTH. Because corticosterone is known to act both at the corticotrope and at the level of CRF secretion, it is not clear to what extent the ACTH response to ADX is a result of removal of glucocorticoids from the pituitary per se. To test the role of brain input as well as the role of glucocorticoids on the corticotrope response to ADX, we performed the following experiment. Rats were prepared with anterolateral hypothalamic deafferentations (lesion) which severed CRF and arginine vasopressin cell bodies in the hypothalamus from their axonal endings in the median eminence and posterior pituitary. Control rats were subjected to sham lesions. Two days later, half of the rats in each group were subjected to either ADX or sham ADX; a subgroup of the lesioned rats was provided at the time of adrenal surgery with a constant infusion of rat CRF. Five days later, all rats were killed, and anterior pituitary levels of proopiomelanocortin (POMC) mRNA, ACTH, and protein; plasma ACTH and corticosterone, and adrenal and thymus weights were measured. In sham-lesioned rats, ADX resulted in increases in POMC mRNA, and plasma ACTH of 2.5- and 12-fold, respectively, compared to sham-adrenalectomized controls. In the absence of hypothalamic drive (lesion only), there were no responses of any of these variables to ADX. In lesioned rats driven with CRF, ADX resulted in increases in POMC mRNA and plasma ACTH of 2.2- and 2.6-fold, respectively, compared to sham ADX. After consideration of the three variables indicating ACTH synthesis, storage, and secretion and comparison of the results of ADX vs. sham ADX within and across the sets of animals, we conclude that 1) there is no autonomous response of the corticotrope to ADX; 2) the removal of corticosterone from the anterior pituitary may account for the majority of the effects of ADX on ACTH synthesis; and 3) the normal response to ADX requires secretion of CRF and increased secretion of another ACTH-releasing factor (possibly arginine vasopressin) that causes increased secretion but little synthesis of ACTH.

Acute volume expansion decreases adrenocortical sensitivity to ACTH and angiotensin II

This study examined the plasma aldosterone and corticosteroid responses to a 60-min infusion of adrenocorticotropin (ACTH) or angiotensin (ANG) II started immediately after an acute isotonic saline volume expansion (0.5 ml . kg-1 . min-1 for 30 min). Five conscious dogs of either sex with exteriorized carotid loops were used in this repeated-design study. Volume expansion per se caused a 10% decrease in hematocrit, a 12.5% decrease in plasma protein, and a 2.7-mmHg increase in central venous pressure with no change in mean arterial pressure, heart rate, or plasma sodium. Volume expansion per se also resulted in significant reductions in vasopressin, plasma renin activity, ACTH, aldosterone, and corticosteroid levels. The aldosterone responses to ACTH and ANG II were significantly inhibited (46-71%) by acute volume expansion. The corticosteroid response to ACTH was 19-29% inhibited by volume expansion. We conclude that acute volume expansion significantly inhibits the adrenocortical sensitivity to its tropic hormones probably via alterations of synergistic factors.

Corticosterone: narrow range required for normal body and thymus weight and ACTH

ACTH secretion appears to be under fairly tight negative feedback control by corticosteroids secreted from the adrenal cortex. In these studies we determined the circulating levels of a constant corticosterone signal that best restored body weight gain, thymus weight and ACTH levels to normal in bilaterally adrenalectomized rats given saline to drink. Young male rats were treated at the time of adrenalectomy with subcutaneously implanted pellets of wax or various ratios of corticosterone-cholesterol. Sham-adrenalectomized rats and adrenalectomized rats given corticosterone in the drinking fluid served as comparison groups. Rats were killed 3, 7, or 14 days after adrenalectomy. There was no difference in levels of plasma corticosterone in the morning and in the evening in pellet-implanted rats in contrast to the diurnal variation in the reference groups. Circulating corticosterone levels that best restored body weight, thymus weight, and resting and stress-induced ACTH levels to normal ranged between 4.5 and 7.4 micrograms/dl. Plasma corticosterone levels of 8-11 micrograms/dl were excessive and levels of 2-4 micrograms/dl were not adequate. We conclude that there is a very narrow range of plasma corticosterone compatible with normal growth rate, thymus mass and ACTH secretion. These results reveal the necessity for strict negative feedback regulation of ACTH secretion by corticosteroids.

Flavor, forced choice and deprivation affect corticosterone selection by the adrenalectomized rat

Adrenalectomized rats were given access to corticosterone-supplemented (20 micrograms/ml) and -unsupplemented saline (0.9% NaCl) in two separate bottles and preferences were measured for each. Half of the rats received corticosterone in maple-flavored saline (MAPLE group), while the other half received corticosterone in anise-flavored saline (ANISE group). Each rat also received unsupplemented saline in the alternate flavor. After an initial preference test, preferences were measured again following a period of forced choice (where rats were given the supplemented saline only) and again following a period of deprivation (where rats were given the unsupplemented saline only). All three independent variables (flavor, forced-choice and deprivation) played roles in determining self-administration of corticosterone. Results from the initial preference test demonstrated that both groups preferred the maple-flavored saline whether it contained corticosterone or not. However, consumption of corticosterone-supplemented saline regardless of flavor increased following both forced-choice and deprivation suggesting that the adrenalectomized rats were controlling their corticosterone levels. Strong negative correlations were found between intake of corticosterone-supplemented saline and body weight suggesting that the adrenalectomized rat may be controlling corticosterone levels based on some correlate of body weight.

ACTH and corticosterone secretion in rats following removal of the neurointermediate lobe of the pituitary gland

Surgical removal of the neurointermediate lobe of the pituitary gland (NIL-X) in the rat resulted in two abnormalities in ACTH secretion: (1) plasma ACTH and corticosterone levels were elevated in the morning and over a 24-hour period compared with levels in control (NIL-C) rats, and (2) although NIL-X and NIL-C rats had acute increases in plasma ACTH and corticosterone of equal magnitude after interoceptive stimuli (hemorrhage, surgery), NIL-X rats demonstrated markedly smaller elevations in plasma levels of these hormones after neurotropic stimuli (noise, novel environment). This subnormal adrenocortical response of NIL-X rats was not due to an impairment in perception of a neurotropic stimulus; these rats had normal latencies to paw licking and to jumping off a heated surface, yet smaller increases in plasma corticosterone after the stimulus. The impairment in ACTH response was not related to stimulus intensity, as NIL-X and NIL-C rats had equal ACTH and corticosterone secretion during both low and high levels of insulin-induced hypoglycemia. NIL-X rats demonstrated a significant elevation in daily water intake, although hematocrit, plasma Na+, K+, osmolality and protein were normal. Significant diurnal rhythms in plasma corticosterone levels and in water intake were maintained as well. The elevated morning plasma ACTH levels, the blunted hormone increases after noise, and the increase in water intake persisted in NIL-X rats 2 months after surgery. These data indicate that removal of the NIL results in (1) chronic elevations in basal ACTH and corticosterone secretion, and (2) chronic impairment in adrenocortical responses to neurotropic stimuli, but not to interoceptive stimuli. The deficit is not due to impaired perceptual capacity nor to the intensity of the stimulus.

Closed-loop feedback control of the nyctohemeral rise in adrenocortical system function

We tested the hypothesis that the increase in adrenocortical system activity is controlled by an error signal generated by the difference between a central reference, or set point, value and some aspect of circulating corticosterone levels in rats maintained on a 12-h light, 12-h dark cycle. The test rats were treated with cyanoketone, an inhibitor that blocks conversion of pregnenolone to progesterone; control rats were treated with vehicle. Test and control rats were fitted with arterial cannulas and, after recovery, continuously collected blood was sampled for plasma corticosterone at 5-min intervals over 2-h periods at five times during the day. Samples were collected at the end of each period for plasma adrenocorticotropin (ACTH). Corticosterone and ACTH levels were similar in the two groups of rats at the nadir (0-2 h after lights-on) and at the peak (0-2 h after lights-off) of the diurnal rhythm. During the time of the maximal diurnal rise in corticosterone (8-10 and 10-12 h after lights-on), test rats had lower mean corticosterone and higher ACTH levels than controls. Infusion of 8 micrograms corticosterone during the 2 h before lights-out did not significantly elevate mean corticosterone levels but decreased end ACTH levels in test rats, whereas similar infusions in control rats resulted in decreased mean corticosterone levels and no change in end ACTH. We conclude that the nyctohemeral rise in ACTH is driven by an error signal resulting from the difference between a central set point value and some aspect of plasma corticosterone levels.