beta-Endorphin stimulates corticosterone synthesis in isolated rat adrenal cells

Evidence-based literature pertaining to thyroid dysfunction and Cushing's syndrome in the horse

The evidence-based literature pertaining to thyroid dysfunction and Cushing's syndrome is discussed in this article. Summaries of and recommendations for the treatment of these conditions are made. There is a need for reliable diagnostic tests for these conditions in horses.

Separation induced changes in squirrel monkey hypothalamic-pituitary-adrenal physiology resemble aspects of hypercortisolism in humans

When separated from groups, squirrel monkeys respond with significant increases in plasma cortisol and adrenocorticotropic hormone (ACTH). While cortisol remains elevated above pre-separation levels, significant reductions occur in ACTH. Monkeys that respond with greater increases in cortisol subsequently exhibit greater reductions in ACTH, which suggests that reductions in ACTH are mediated by corticosteroid feedback. Monkeys that respond with greater increases in cortisol also tend to exhibit greater cerebrospinal fluid levels of the dopamine metabolite HVA, but not the norepinephrine metabolite MHPG, or corticotropin-releasing factor (CRF). Attenuation of corticosteroid feedback with metyrapone results in significant increases in circulating ACTH, and in older monkeys increases plasma HVA. Similar findings in humans have been reported in clinical studies of hypercortisolism and major depression.

Psychobiological consequences of social relationships

Social separations can induce long-lasting increases in cortisol, whereas companionship can result in social buffering. Preliminary evidence from studies of squirrel monkeys suggests that social separation-induced hypersecretion of cortisol is initially driven by hypersecretion of ACTH. From 1-21 days postseparation, however, cortisol remains elevated above pre-separation controls, while ACTH levels are consistently reduced. Hypercortisolism is maintained despite reductions in ACTH, because adrenal responsiveness to ACTH is enhanced. Low circulating ACTH, in turn, is maintained by robust feedback mechanisms that apparently inhibit biosynthesis or release of pituitary ACTH. These findings are consistent with neuroendocrine interactions known or hypothesized to occur during major depressive disorders in humans and raise unique possibilities for comparative research in human and nonhuman primates.

Ontogenetic studies of tolerance development: effects of chronic morphine on the hypothalamic-pituitary-adrenal axis

Endogenous opiates are important regulators of the hypothalamic-pituitary-adrenal (HPA) axis in rats. Tolerance clearly develops to morphine-induced stimulation of the HPA axis in adult rats (Ignar and Kuhn 1990). The goal of the present study was to determine whether tolerance to morphine-induced stimulation of the HPA axis developed in neonatal and weanling rats treated chronically with morphine. Rats were injected with morphine or saline between days 4-8 postnatal (pups) or days 21-25 (weanlings) and tolerance assessed by determining dose-response curves for ACTH and corticosterone secretion following an acute morphine challenge. Weanlings displayed marked tolerance to the stimulation of ACTH and corticosterone secretion by morphine. Tolerance was also observed in pups to morphine-stimulated ACTH and corticosterone release. These findings suggest that the relative adaptability of the HPA axis to chronic morphine in neonatal and weanling rats is similar.

β-endorphin binding in cultured adrenal cortical cells

The polypeptide β-endorphin binds to cultured bovine adrenal cortical cells in a naloxone insensitive manner, β-endorphin and N-Acetyl-β-endorphin are equipotent in inhibiting binding. The amino terminal 27 amino acid fragment referred to as β-endorphin[1-27] shows no ability to inhibit binding, whereas the carboxy-terminal tetrapeptide Lys-Lys-Gly-Glu partially inhibits binding. ACTH, angiotensin II and met-enkephalin show little or no ability to inhibit β-endorphin binding. Competition bin-ding reveals an apparently single affinity class with Kd of 33 nM. Molecular cross linking experiments reveal putative receptor subunits of 85 kD, 64 kD, 54 kD and 44 kD. The lower molecular weight bands are preferentially cross-linked by a hydrophobic cross linking reagent, in contrast to the two higher molecular weight bands, which are cross linked equally by hydrophobic and water soluble cross linking reagents. The β-endorphin binding characteristics of adrenal cortical cells revealed here are quite similar to those of a class of non-opioid β-endorphin receptors previously shown to exist in cells of the immune system.

Neuropeptides in the adrenal gland: distribution, localization of receptors, and effects on steroid hormone synthesis

In this review we defined and classified the neuropeptides (NPs) related to the adrenal gland, according to Palkovits (Frontiers Neuroendocrinol 10:1 1988). The concentration (RIA) and distribution (immunohistochemistry) of NPs, as well as the localization of the receptors (radioligand studies) were summarized. Direct effects of NPs on aldosterone and corticosterone synthesis obtained by in vivo, in situ perfusion, and in vitro experimental approaches were reviewed. Data (from different rat strains and genders) for 35 NPs are presented.

Effects of pro-opiomelanocortin peptides on adrenocortical steroidogenesis

Whilst studying the effects of pro-opiomelanocortin (POMC) peptides on adrenocortical steroidogenesis we showed that alpha-melanocyte-stimulating hormone (alpha-MSH) has a specific glomerulotropic effect and potentiates both the mineralocorticotropic and glucocorticotropic effects of ACTH. The fragments ACTH-(4-10) and ACTH-(11-13) are responsible for the glomerulotropic effect of alpha-MSH. beta-Endorphin enhances, inhibits or has no effect on corticosteroidogenesis, depending on the dose and on the functional state of the adrenocortical cells (saturation of the receptors). beta-Endorphin antagonizes the effect of alpha-MSH on aldosterone production. It is suggested that the modulatory effects of POMC peptides on adrenocortical steroidogenesis may be of physiological significance.

In vivo and in vitro effects of β-endorphin and naloxone on corticosterone and cortisol release in male and female water frog, Rana esculenta

1. beta-Endorphin and naloxone effects on corticosterone and cortisol production in male and female Rana esculenta, were studied in vivo and in vitro. 2. The in vivo and in vitro results were in agreement. 3. beta-Endorphin caused a decrease in corticosterone and cortisol release. 4. Naloxone induced an increase in the two corticosteroids at the same times as the decrease caused by beta-endorphin. 5. beta-Endorphin plus naloxone treatment did not change corticosterone and cortisol levels. 6. These results suggest that in Rana esculenta opioids are involved in the regulation of the hypothalamo-pituitary-interrenal axis; in particular, opioids directly modulated interrenal steroidogenesis.

Interaction of ACTH, beta-endorphin and alpha-melanocyte stimulating hormone in relation to the corticosteroid production of isolated rat adrenocortical zona fasciculata and zona glomerulosa cells

The combined effects of ACTH, beta-endorphin (beta-EP) and alpha-MSH were studied on the corticosteroidogenesis of isolated rat adrenocortical zona fasciculata and zona glomerulosa cells. beta-EP potentiated the effects of ACTH and alpha-MSH on the zona fasciculata corticosterone production but inhibited those on the zona glomerulosa aldosterone production. beta-EP did not affect the combined action of 4 x 10(-11) M ACTH and 5 x 10(-9) M alpha-MSH on the zona fasciculata or the zona glomerulosa cells, but it inhibited the stimulatory action of the combination of 1.6 x 10(-10) M ACTH and 10(-9) M alpha-MSH on the zona glomerulosa aldosterone production. An interaction of ACTH, beta-EP and alpha-MSH in relation to the zona fasciculata and zona glomerulosa corticosteroid production was found.

Ovarian opioids and the reproductive cycle of the frog Rana esculenta

In mammals, proopiomelanocortin-related peptides are involved in reproductive processes both at the hypothalamo-pituitary and ovarian levels. Using immunocytochemical, biochemical and physiological "in vitro" studies, we provide here evidence for a diffuse POMC-related opioid system in the frog Rana esculenta. Ovarian beta-endorphin (beta-EP) is expressed in thecal cells and changes during the reproductive cycle in an inverse relationship with follicular development. Seasonal changes in the ovary are different to those in the brain or in the pituitary. The ratio of acetylated vs native beta-EP in the ovary also changes over the reproductive period, affecting the biological activity of the peptide. During both the reproductive spring period and the summer post-reproductive phase pMol amounts of beta-EP stimulate follicular androgen secretion in vitro, in a naloxone-reversible way. In either period, an inhibition of estradiol, possibly mediated via other factors, is the result of opioid action. In conclusion, these data demonstrate for the first time the widespread presence of beta-EP-related peptides in the frog Rana esculenta. Both immunocytochemical and biochemical evidence, as well as in vitro responses, support a physiological role for beta-EP in ovarian seasonality during the reproductive cycle of this amphibian.

The substance P(1-7) fragment is a potent modulator of substance P actions in the brain

The neuropeptide, substance P (SP), produces a spectrum of behavioural effects. When given locally into the substantia nigra, SP induces dopamine release in the ipsilateral striatum and produces contralateral rotation in a dose-dependent, but bell-shaped, manner. Similar dose-response relationships have been observed for SP and other peptides in different bioassays. To test whether SP fragmentation is responsible for this phenomenon, SP(1-7), which is the main SP fragment in rat CNS, was injected intranigrally. SP(1-7) was found to act as a very potent antagonist against the SP-induced responses and was formed locally in the nigra after SP injection. It is proposed that SP(1-7) is an endogenous modulator of SP actions. Generation of peptide fragments, which retain receptor affinity but not efficacy, may be a general mechanism for autoregulation in peptidergic systems.

Plasma beta-endorphin and cortisol levels in morphine-tolerant rats and in naloxone-induced withdrawal

The relationship between morphine tolerance and pituitary-adrenocortical activity was examined. In rats made tolerant to morphine by implantation of morphine-containing pellets, there was a significant reduction in plasma levels of beta-endorphin-like immunoreactivity (beta-END-LI), whereas no significant changes in cortisol levels were seen. Naloxone treatment induced an increase in plasma beta-END-LI and cortisol levels in morphine-tolerant animals. Additionally, acute morphine administration induced an increase in plasma levels of beta-END-LI and cortisol, an effect which was prevented by naloxone. These results are consistent with an increased release of pro-opiomelanocortin-derived peptides after acute morphine and with a decreased release of these peptides in tolerant rats, and suggest that opioid peptides play an important role in the regulation of pituitary-adrenocortical function.

Interactions of anxiolytic and antidepressant drugs with hormones of the hypothalamic-pituitary-adrenal axis

Changes in hormones of the hypothalamic-pituitary-adrenal (HPA) axis in patients suffering from anxiety and depressive disorders are reviewed, and the changes that occur when animals are exposed to test situations used preclinically to model anxiety or depression. The effects of exogenous administration of HPA hormones both clinically and in animal tests is discussed and the effects of clinically used anxiolytics and antidepressants on hormones of the HPA axis. The final section discusses stress-induced changes in the CNS.

Neuroendocrine responses to emotional stress: possible interactions between circulating factors and anterior pituitary hormone release

We have shown that a psychological stressor can elicit increases in plasma AVP levels in normal human subjects. Since AVP can enhance the release of ACTH, and the pituitary gland is outside the blood-brain barrier, AVP present in the general circulation might extend the time course of stress-induced, CRF-mediated release of ACTH from the anterior lobe. Since PRA is involved in the synthesis of angiotensin I, the precursor of AII, and AII is known to enhance CRF-mediated release of ACTH from pituitary cells and to stimulate release of AVP, it is possible that the increase in PRA also contributed to the release of AVP and ACTH in this study. Reports differ as to whether circulating catecholamines can release ACTH in vivo by direct action on the pituitary. Finally, it has been reported that beta-EP enhances the release of PRL, and inhibits release of AVP. Since the increase in beta-EP in the present study was quite robust, it might have extended the PRL release, and truncated the AVP response.

Effects of bromocriptine on pituitary and adrenal cortex in pre-adrenarchal rabbits

The effect of bromocriptine on the morphological picture and steroid content of the adrenal gland, and on certain pro-opiomelanocortin (C) peptides in the pituitary gland was evaluated in preadrenarchal rabbits. Eighteen immature male rabbits (5 weeks of age), were treated for 10 days with saline (n = 10,2 ml sc) or bromocriptine mesylate (n = 8, 3 mg/kg sc) two times/day. After the last administration all animals received dexamethasone (0.25 mg im) and the next morning, 60 min after ACTH injection (0.25 mg im), plasma was drawn and they were sacrificed. Adrenals and pituitaries were immediately removed. For each animal, one adrenal gland was fixed, dehydrated and embedded in paraffin for histology; the other one was stored in saline for determination of androstenedione (A), dehydroepiandrosterone (DHA), 17-OH progesterone (17 P), and cortisol. Steroids were analyzed by RIA after previous extraction and celite-ethyleneglycol chromatography, or directly (cortisol). The immunoreactivities (ir) related to beta-Endorphin (B-EP), ACTH and alpha-MSH were evaluated in pituitary homogenates using specific RIAs. The bromocriptine-treated rabbits showed a significant increase in the percentage of the adrenal zona reticularis (21.5 +/- 3.9% of total cortex vs. 12.7 +/- 1.3% in controls, p less than 0.05, mean +/- SE), and a decrease of the zona fasciculata (57.6 +/- 3.13% vs. 67.7 +/- 2.05% in controls, p less than 0.05). No significant changes were observed in the relative percentage of the zona glomerulosa.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of stress and ethanol: effect on beta-endorphin and catecholamines

To examine the interaction of ethanol (ET) and stress on beta-endorphin and catecholamine (CA) levels, male rats pretreated with ET (3.0 g/kg, i.p.) or saline were immobilized for 30 min and killed 90 min after the initial injection. Stress resulted in (a) an increase in plasma levels of norepinephrine (NE, 243%), epinephrine (E, 175%), beta-endorphin (220%) and corticosterone (CS, 151%) and a decrease in dopamine (DA, 54%); (b) a decrease in hypothalamic NE (15%) and beta-endorphin (33%) levels and an increase E (23%) and DA (58%) levels; (c) a decrease in pituitary beta-endorphin levels in both the neurointermediate (23%) and anterior (131%) lobes. Treatment with ET resulted in: (a) an increase in plasma NE (81%), E (53%), CS (71%), and beta-endorphin (33%) levels and decrease in DA (54%); (b) a decrease in the hypothalamic NE (12%) levels and an increase DA (27%) and beta-endorphin (46%) levels, and (c) a decrease in beta-endorphin (15.5%) in the intermediate lobe of the pituitary. Treatment with ET of stressed animals had only a small effect: (a) in plasma NE, E, CS, and beta-endorphin levels decreased by 30, 31, 14, and 36%, respectively; (b) in the hypothalamus DA levels decreased by 40% and beta-endorphin increased by 71%; (c) in the pituitary beta-endorphin increased in both the intermediate lobe (25%) and anterior (50%) lobes. Thus when the data of the stressed ET-treated group is compared to that of the nonstressed saline injected group, none of the measures differ significantly. These results confirm our earlier work indicating a significant interaction of ET and stress.

The melanotropin potentiating factor and beta-endorphin do not modulate the alpha-melanotropin-or adrenocorticotropin-induced corticosteroidogenesis in purified isolated rat adrenal cells

The ability of two pro-opiomelanocortin-derived peptides, the melanotropin potentiating factor (MPF) and beta-endorphin (beta EP), to affect corticosteroid production was studied in purified isolated rat adrenal cells. Addition of MPF or beta EP, in doses from 5 pg to 5 micrograms, alone did not result in a corticosterone production. Furthermore, no effect of MPF or beta EP in doses from 5 pg to as high as 5 micrograms for both peptides upon the ACTH or alpha-MSH-induced corticosteroidogenesis was observed (p greater than 0.1). It is concluded that both MPF and beta EP do not influence the steroidogenic activity in the adrenal gland. Use of these peptides for discrimination of the ACTH/alpha-MSH receptor interactions is suggested.

Effects of beta-lipotropin, beta-endorphin, gamma 2-melanotropin and corticotropin on steroid production by isolated human adrenocortical cells

Normal human adrenocortical cells (from multi-organ donors) were incubated with corticotropin (tetracosactide), highly purified human beta-lipotropin, synthetic human beta-lipotropin, gamma 2-melanocyte stimulating hormone and beta-endorphin. Corticotropin stimulated cortisol, aldosterone and 18-hydroxycorticosterone production starting at 10(-12)-10(-11) mol/l in normal adrenocortical cells. Purified human beta-lipotropin also stimulated steroidogenesis but 100-1000-fold higher concentrations of the peptide were needed. In contrast, synthetic human beta-lipotropin was without any effect on steroid production up to concentrations of 10(-7) mol/l. Synthetic beta-lipotropin (5 x 10(-10) mol/l) did not significantly change the dose-response curve for corticotropin (10(-13) mol/l-10(-9) mol/l) versus the three steroids measured. gamma 2-Melanotropin and beta-endorphin (10(-6) mol/l) stimulated the secretion of cortisol, but not of aldosterone. Since synthetic human beta-lipotropin has no effect on human adrenocortical cells, the purified beta-lipotropin must be contaminated with traces of corticotropin. Since pathologically elevated levels of proopiomelanocortin-derived peptides will rarely exceed plasma concentrations of 10(-10) mol/l, our findings in vitro do not support a physiological or pathophysiological role of the peptides examined in the regulation of adrenal steroid secretion.

Hypothalamo-pituitary-interrenal responses to opioid substances in the trout. II. Effects of morphine and D-Ala2, Met5-enkephalinamide on plasma cortisol titres in vivo

The influence of morphine, D-Ala2, Met5-enkephalinamide (DALA), and naloxone on plasma cortisol titres has been studied in vivo in fingerling and adult trout. The responses were complex and variable. A single ip injection of morphine or DALA into fingerlings usually resulted in a rise in plasma cortisol after 0.5 hr followed by a fall below control values within 2 hr. In similar experiments with adult trout, only an inhibitory effect was observed. Naloxone reduced the rise in plasma cortisol following saline injection, but only when the hypothalamo-pituitary-interrenal response was intense. The antagonist also blocked the morphine-induced rise in cortisol secretion. Prolonged morphine treatment diminished both the postinjection and stress-induced secretion of cortisol in adult fish. Morphine had no effect on the spontaneous or ACTH-induced secretion of cortisol by interrenal tissue incubated in vitro. The results support the concept of inhibitory and stimulatory sites of action by opiates and opioid substances on the hypothalamo-pituitary-interrenal axis. These findings are discussed with reference to the action of morphine on hypothalamic and pituitary tissue of the trout in vitro and with the opioid control of hypothalamo-pituitary-adrenal function in mammals.

Effects of POMC-derived peptides on aldosterone secretion in vivo

1. To investigate a role for peptides derived from the precursor molecule pro-opiomelanocortin (POMC) on the control of aldosterone secretion (ASR), alpha-, beta-, gamma 1-, and gamma 2-melanocyte stimulating hormone (MSH), corticotropin-like intermediate lobe peptide (CLIP) or beta-endorphin were infused into the adrenal arterial supply of sheep with an adrenal cervical autotransplant. 2. None of the peptides had any significant effect on aldosterone secretion rate in Na replete, unstressed, conscious animals. In contrast, ACTH-stimulated ASR approximately twofold. 3. POMC-derived peptides other than ACTH appear to have little or no effect on the short-term control of aldosterone secretion in vivo, although a role in control and modulation of adrenal function over the longer term cannot be discounted.

Cortisol secretion in vitro by the interrenal of coho salmon (Oncorhynchus kisutch) during smoltification: relationship with plasma thyroxine and plasma cortisol

An in vitro system for the incubation of interrenal tissue (head kidney fragments) from coho salmon, Oncorhynchus kisutch, was developed in order to examine changes in interrenal sensitivity to ACTH1-24 during smoltification, using cortisol secretion as the endpoint. Time-course studies indicated that maximal cortisol accumulation in incubation media was achieved after 3 hr exposure to ACTH. There was no correlation between head kidney weight, body weight, or sex and the response of the interrenal to ACTH1-24 in vitro. Approximately monthly or bi-weekly experiments were performed during the smoltification period (February-July): tissue was preincubated in hormone-free media for 3 hr, washed twice, and then challenged with 5 X 10(-10)-5 X 10(-7) M (1.5-1500 ng/ml) ACTH1-24 for 3 hr. The pattern of cortisol secretion was similar in February, early March, and late March in the dose range of 5 X 10(-10)-5 X 10(-8) M ACTH1-24. A marked, significant increase in sensitivity to ACTH and in the steroidogenic capacity of the tissue occurred in April, but by May the response was similar to that in the pre-April period. Enhanced sensitivity and steroidogenic capacity were found in interrenal tissue taken from coho salmon in June and July. Maximal in vitro responsiveness of interrenal tissue to ACTH in April was correlated with peak plasma thyroxine titers and enhanced hypoosmoregulatory ability, but not with peak plasma cortisol titers, which occurred in May.

Corticosteroidogenesis modulation by beta-endorphin and dynorphin1-17 in isolated rat adrenocortical cells

Two opioid peptides, beta-endorphin and dynorphin1-17 were bioassayed with isolated rat adrenocortical cells. beta-Endorphin increases basal production of corticosterone as well as the adrenal responsiveness to low doses of ACTH, these effects being partially reversed by naloxone. Dynorphin1-17, without affecting basal corticosterone synthesis, increases adrenocortical responsiveness to ACTH; naloxone does not influence this effect. It is suggested that peripheral opioid peptides may participate in the maintenance of the homeostatic balance by modulating adrenal corticosteroidogenesis.

Responses of rat adrenal glomerulosa and inner zone cells to synthetic ACTH analogs and proopiomelanocortin-derived peptides

A comparison of the responsiveness of isolated rat adrenal decapsular and glomerulosa cells to corticotrophin 1-39 (ACTH 1-39), synthetic ACTH analogs (characterized by a shorter amino acid chain length, the substitution of certain amino acids in the natural sequence by other amino acid residues, the replacement of the C-terminal carboxyl group by an amide), and proopiomelanocortin-derived peptides was performed by measuring corticosterone and aldosterone production, respectively. The potencies of the synthetic ACTH analogs correlated closely with the length of the peptides, similarly in both zones. No activity was observed with the proopiomelanocortin-derived peptides in either zone, with the exception of beta-LPH and alpha-MSH.

Corticotropin releasing factor increases the adrenocortical responsiveness to adrenocorticotropin

In the course of studying the plasma adrenocorticotropic hormone (ACTH) and corticosterone responses to synthetic corticotropin releasing factor (CRF), we noted some disparity in the responses. A higher dose (20 micrograms compared with 5 micrograms per rat i.a.) produced an equal plasma ACTH but greater plasma corticosterone response in adult male rats. Thus, we examined the possibility that CRF increases adrenocortical responsiveness to ACTH. CRF significantly (p less than 0.0005) increased the plasma corticosterone response to ACTH in rats pretreated with dexamethasone. Thus, synthetic CRF increases corticosterone secretion in rats not only by stimulating ACTH secretion, but also by increasing the adrenocortical responsiveness to ACTH.

The Met-enkephalin analog D-Ala2-Met-enkephalinamide decreases the adrenocortical response to ACTH in dispersed rat adrenal cells

The effects of the Met-enkephalin analog D-Ala2-Met-enkephalinamide (DALA) on basal and ACTH-stimulated corticosterone secretion from dispersed adrenal cells were investigated. Low doses (10(-10) and 10(-12) M) of DALA resulted in no apparent alteration in the response to ACTH (8 X 10(-9), 3.2 X 10(-8) or 1.6 X 10(-7) M). High doses of DALA (10(-8) and 10(-6) M) produced a decline in the steroidogenic response to ACTH. The opiate receptor antagonist naloxone (10(-4)-10(-10) M) did not influence the basal production of corticosterone or the stimulating action exerted by ACTH. However, the presence of naloxone reversed the blocking action on corticosterone production that was exerted by DALA. These findings indicate that enkephalins may decrease adrenocortical responsiveness to ACTH.

The hypothalamic-pituitary-adrenal axis

Anterior pituitary corticotrophin cells secrete ACTH as part of a larger precursor molecule, pro-opiomelanocortin. Post-translational cleavage of this precursor yields three major peptides: ACTH, beta-LPH and N-POMC. Experiments both in vivo and in vitro suggest that N-POMC may act as a prohormone amplifier for ACTH-induced adrenal steroidogenesis and as regulator of adrenocortical cell growth. The secretion of POMC is under the control of CRF. These findings are discussed in relation to the pathophysiology of corticotrophinoma. The primary defect in this condition appears to reside at the level of the anterior pituitary cell and is readily amenable to treatment by trans-sphenoidal microsurgery. The estimation of plasma ACTH concentrations is proving useful in the monitoring of various clinical conditions including Addison's disease and congenital adrenal hyperplasia.

Naloxone potentiates ACTH and angiotension II but not potassium stimulated aldosterone secretion, in vitro

The effects of naloxone on basal and ACTH, Angiotensin II (AII) and [K+] o stimulated aldosterone secretion from superfused rat adrenocortical tissue were investigated. A high dose (10(-6) M) of naloxone inhibited while a smaller dose (10(-10) M) potentiated and doses of 10(-8) or 10(-12) M naloxone were without an effect on ACTH stimulated aldosterone secretion. A potentiation of AII stimulated aldosterone secretion was observed beginning 2 hrs after 10(-6) or 10(-10) M naloxone was administered while no effect was observed with 10(-4) M naloxone. No effects of 10(-6), 10(-8), 10(-12) M naloxone were detected on aldosterone secretion stimulated by transiently elevating extracellular potassium. Naloxone from 10(-4) to 10(-12) M did not appear to significantly influence basal steroidogenic activity under these conditions. These findings demonstrate that the "opioid antagonist" naloxone has prominent actions on adrenocortical tissue. Both the specificity and lack of specificity of the action of this agent to influence the activity of the 3 secretagogues suggest that naloxone and possibly a naturally occurring endogenous ligand interacts with one or more membrane receptor distinct from the ACTH receptor. A naturally occurring ligand for this receptor could play a prominent role in the physiological regulation of adrenal steroid secretion.

Naloxone has a local effect on the testis of immature rats

Local effect of naloxone on testicular weight, compensatory hypertrophy, and differentiation of the testis and serum testosterone level has been studied in immature rats. A single injection of naloxone in a dose of 2 micrograms/testis under the testicular capsule significantly increased the testicular weight, promoted testicular differentiation and enhanced the rate of compensatory hypertrophy of the remaining testis in unilaterally orchidectomized animals. Data suggest that naloxone has a direct effect on the testis of immature rats.

Stimulation of adrenal cortisol biosynthesis by epidermal growth factor

The effect of epidermal growth factor (EGF) on adrenal cortisol biosynthesis in ewes was determined both in vivo and in vitro. There was a progressive rise in plasma cortisol levels after infusion of 2.9 micrograms/kg/h EGF into 5 ewes over 24 h. Mean plasma cortisol rose to a sustained plateau of 700% of the saline control value and fell towards saline control values before the end of the infusion. In vitro, adrenal slices taken from ewes synthesized and released significantly greater amounts of cortisol in response to the addition of EGF or ACTH1-24. EGF and ACTH1-24 gave a maximal stimulation of cortisol biosynthesis between 10 and 100 ng/ml and above 100 ng/ml, respectively. Biologically inactive reduced S-carboxyamidomethyl EGF as well as a number of polypeptide hormones and growth factors were without effect at these dose levels. Stimulation by EGF of cortisol synthesis and secretion was dependent on protein synthesis as it was for ACTH. However, EGF did not synergise with ACTH in promoting cortisol production. These results suggest that EGF may be a regulatory factor affecting adrenal cortisol synthesis and secretion.

Co-localization of vasoactive intestinal peptide (VIP) and enkephalins in chromaffin cells of the adrenal gland of amphibia. Stimulation of corticosteroid production by VIP

Recent studies have shown that biologically active peptides and monoaminergic neurotransmitters coexist in certain neuronal cell populations. Using the immunofluorescence technique, we have examined the localization of enkephalins, vasoactive intestinal peptide (VIP) and tyrosine hydroxylase in the adrenal gland of the frog Rana ridibunda. Most chromaffin cells which stained for tyrosine hydroxylase contained VIP-like immunoreactivity, whereas methionine- (Met-) and leucine- (Leu-) enkephalin-like immunoreactivity was detected in about 40% of the cells revealed by the anti-tyrosine hydroxylase serum. No VIP- or enkephalin-like immunoreactive nerve fibres were observed. Since in the frog, the chromaffin cells are in close contact with the adrenocortical (interrenal) tissue, a possible action of VIP and opiates on corticosteroidogenesis has been investigated. At doses 10(-6) and 10(-5) M, 20-min infusions of synthetic porcine or chicken VIP elicited a significant increase in corticosterone and aldosterone production by perifused frog adrenals, in a dose-dependent manner. As compared to ACTH, VIP was several orders of magnitude less effective in stimulating corticosteroid production. Morphine, Met- and Leu-enkephalins (10(-5) M) had no effect on spontaneous secretion of corticosteroids. In addition, Met- and Leu-enkephalins (10(-5) M) did not alter the production of corticosterone induced by ACTH. THese results suggest that VIP contained in the chromaffin cells of the frog adrenal gland may exert a local action in stimulating corticosteroid production by the interrenal tissue.

The role of endorphins in exercise: a review of current knowledge

Exercise training is used increasingly to prevent and treat disease, and millions of healthy persons participate in various aerobic-type sports; yet, the mechanisms by which exercise produces various clinical effects is imperfectly understood. Emerging evidence suggests that the endogeneous opioid endorphins may be involved in two widely varying aspects of exercise: endocrine control and behavior and mood adaptation. The present paper summarizes these findings. The relationship of endorphins and adrenocorticotropin to stress and the aspects of endorphins' involvement in the concept of "runner's high" are discussed. J Orthop Sports Phys Ther 1983;4(3):169-173.

ACTH and corticosterone response to naloxone and morphine in normal, hypophysectomized and dexamethasone-treated rats

The effect of opiate receptors blocker naloxone on ACTH and corticosterone secretion in normal, dexamethasone-treated and hypophysectomized rats was studied. A dose-related increase in plasma corticosterone level was found at 45 min after s.c. injection of naloxone in a dose range of 0.25-2.0 mg kg-1. The rise in plasma corticosterone was preceded by a slight increase in plasma ACTH. Acute morphine administration in a relatively low dose (6 mg kg-1 s.c.) induced a significant rise in both plasma ACTH and corticosterone levels. Dexamethasone treatment was followed by low basal corticosterone level, by total inhibition of the stress response and response to morphine injection, while the response to ACTH administration was normal. Under these circumstances as well as in rats 6 days after hypophysectomy, naloxone failed to increase plasma corticosterone levels. It is concluded that a direct stimulation of corticosteroid biosynthesis in adrenal cortex is not involved in the mechanism of naloxone-induced activation of pituitary-adrenocortical function.

Naloxone inhibits and morphine potentiates the adrenal steroidogenic response to ACTH

The administration of morphine to hypophysectomized rats potentiated the steroidogenic response of the adrenal cortex to exogenous adrenocorticotrophic hormone (ACTH) in a dose-dependent fashion. Conversely, the opiate antagonist naloxone inhibited the adrenal response to ACTH. Naloxone pretreatment also antagonized the potentiating effect of morphine on ACTH-induced steroidogenesis in a dose-dependent manner. Neither morphine nor naloxone, administered to hypophysectomized rats, had any direct effect on adrenal steroidogenesis. These adrenal actions were stereospecific since neither the (+)-stereoisomer of morphine, nor that or naloxone, had any effect on the adrenal response to ACTH. The administration of human beta-endorphin to hypophysectomized rats had no effect on the adrenal corticosterone concentration nor did it alter the response of the adrenal gland to ACTH. These results indicate that morphine can potentiate the action of ACTH on the adrenal by a direct, stereospecific, dose-dependent mechanism that is prevented by naloxone pretreatment and which may involve competition for ACTH receptors on the corticosterone-secreting cells of the adrenal cortex.

The endocrinology of the opiates and opioid peptides

Since the isolation of the enkephalins five yr ago, there has been an explosive increase in knowledge concerning the effects of the opiates and opioid peptides. This review deals with the interactions of opiates with the endocrine system in rat and man. The opioid peptides have been demonstrated to exert a variety of effects on pituitary hormone secretion in rat and man. In the rat, opiates stimulate growth hormone, prolactin and ACTH release and inhibit the release of the glycoprotein hormones. In man, the physiologic role of the endogenous opiates appears to be involved predominantly in ACTH and gonadotrophin regulation. Opiate effects are mainly exerted at the level of the hypothalamus but further modulating effects may occur at the pituitary and at end-organs. Opiate-induced hormonal effects appear to be mediated through dopaminergic and/or serotonergic mechanisms. Recent studies have also suggested a possible local neuromodulatory role for the opioid peptides in the control of carbohydrate metabolism and reproductive processes.

Adrenocortical response to corticotropin is potentiated by part of the amino-terminal region of pro-corticotropin/endorphin

Five peptides derived from pro-corticotropin/endorphin (pro-ACTH/endorphin), the pituitary corticotroph cell prohormone, were bioassayed with isolated rat adrenocortical cells: alpha- and beta-melanotropin, beta-lipotropin, beta-endorphin, and the amino-terminal region of pro-ACTH/endorphin known as "16k fragment." The effect of each on steroidogenesis was measured at potentially physiological concentrations (0.01-1 nM) in both the absence and presence of varying concentrations of ACTH-(1-24). Of the peptides tested, only 16k fragment, the amino-terminal region of pro-ACTH/endorphin, has a slight but significant potentiating effect on ACTH-(1-24) action. Prior treatment of 16k fragment with trypsin for 30 sec dramatically increases this dose-dependent synergism. Experiments performed in vivo with hypophysectomized female rats indicate that the trypsin digest of 16k fragment stimulates cholesterol ester hydrolase (cholesterol esterase; sterol-ester acylhydrolase, EC 3.1.1.13) activity in the adrenal cortex but fails to activate cholesterol side-chain cleavage. The effect of the trypsinized material can therefore be qualitatively distinguished from that of ACTH-(1-24). When both ACTH-(1-24) and the digest are administered together, a synergistic increase in serum corticosterone concentration results. We propose that a portion of 16k fragment molecule may play a hormonal role in the control of adrenocortical steroidogenesis.

The role of endorphins in animal learning and behavior

The present review examined the influence of endorphins in animal learning and behavior. It was suggested that in learning paradigms involving stress, the stressor elicits the release of endorphins. Given the evidence on endorphin-mediated, stress-induced analgesia, it was further suggested that the stress-induced release of endorphins modulates the aversiveness of the stressor, and as such, affects the learning based on this stressor. A number of learning paradigms, e.g., the conditioned emotional response, preference for signaled shock, conditioned taste aversions, and learned helplessness, were presented in support of this mediation of learning by the endorphins. A possible interaction between the endorphins and adrenocorticotropic hormone was offered as a physiological basis for this mediation.

The role of endorphins in stress: evidence and speculations

Several lines of evidence suggest that the endogenous opioid peptides endorphins may play a role in the defensive response of the organism to stress. The present paper summarizes these findings as well as evidence linking endorphins to the anterior pituitary polypeptide hormone adrenocorticotropin (ACTH). Evidence is presented that endorphins may function as trophic hormones in peripheral target organs such as the adrenal medulla and the pancreas. As such they may be part of the physiological mechanisms that mediate adrenaline and glucagon release in response to stress. Endorphins (enkephalins) are also suggested to play a role in the control of the pituitary gland during stress. In such capacity they may act as hormone-releasing or inhibiting factors. Finally, endorphins appear to play a role in the behavioral concomitants of stress. In such capacity endorphins are suggested to function as modulators of neural systems that mediate the elaboration and expression of the reactive/affective components of stress. Speculations on the mode of interaction between endorphins and ACTH in the global response to stress are discussed.