Naloxone inhibits and morphine potentiates the adrenal steroidogenic response to ACTH

A Runner's High for New Neurons? Potential Role for Endorphins in Exercise Effects on Adult Neurogenesis

Physical exercise has wide-ranging benefits to cognitive functioning and mental state, effects very closely resembling enhancements to hippocampal functioning. Hippocampal neurogenesis has been implicated in many of these mental benefits of exercise. However, precise mechanisms behind these effects are not well known. Released peripherally during exercise, beta-endorphins are an intriguing candidate for moderating increases in neurogenesis and the related behavioral benefits of exercise. Although historically ignored due to their peripheral release and status as a peptide hormone, this review highlights reasons for further exploring beta-endorphin as a key mediator of hippocampal neurogenesis. This includes possible routes for beta-endorphin signaling into the hippocampus during exercise, direct effects of beta-endorphin on cell proliferation and neurogenesis, and behavioral effects of manipulating endogenous opioid signaling. Together, beta-endorphin appears to be a promising mechanism for understanding the specific ways that exercise promotes adult neurogenesis specifically and brain health broadly.

Adrenal insufficiency in patients on long-term opioid analgesia

OBJECTIVE

Opioid analgesia has been implicated as a cause of secondary adrenal insufficiency, but little is known of the prevalence of this potentially serious adverse effect in patients with chronic pain.

DESIGN

Cross-sectional study of chronic pain patients on long-term opioid analgesia.

PATIENTS

Patients attending tertiary chronic pain clinics at the Western General Hospital, Edinburgh, treated with long-term opioid analgesia (n = 48) with no recent exposure to exogenous glucocorticoids.

RESULTS

Four patients (8·3%) had basal morning plasma cortisol concentrations below 100 nmol/l, of whom three failed to achieve a satisfactory cortisol response to exogenous ACTH_1-24 stimulation (peak cortisol >430 nmol/l). Basal cortisol was positively associated with age (R = 0·398, P = 0·005) and negatively associated with BMI (R = -0·435, P = 0·002).

CONCLUSIONS

Suppression of the hypothalamic-pituitary-adrenal axis is present in a clinically significant proportion of chronic pain patients treated with opioid analgesia. Studies of larger populations should be conducted to better define the prevalence and potential clinical consequences of adrenal insufficiency in this context.

Opioid-induced biochemical alterations of the neuroendocrine axis

Exogenous opioids have been used for decades to palliate cancer-related pain and other cancer-related manifestations and, more recently, to treat patients with pain not related to oncologic disease. While the goal of opioid treatment is symptomatic relief and improved quality of life, these patients often suffer from adverse side effects, including endocrine system abnormalities, of which hypogonadism is the best known. Opioids may interact with other hypothalamic-pituitary pathways and endocrine end organs, and in most cases these interactions are subtle and the effects unclear. The long-term effects of these agents on the endocrine system are still largely unknown. This article discusses the various effects of opioid agents on the endocrine system and provides information that allows early recognition of side effects that may alter the quality of life of patients affected by pain, awareness of the potential complications in opioid addicts, and detection and treatment of side effects in participants of an opioid detoxification program.

The effects of opioids and opioid analogs on animal and human endocrine systems

Opioid abuse has increased in the last decade, primarily as a result of increased access to prescription opioids. Physicians are also increasingly administering opioid analgesics for noncancer chronic pain. Thus, knowledge of the long-term consequences of opioid use/abuse has important implications for fully evaluating the clinical usefulness of opioid medications. Many studies have examined the effect of opioids on the endocrine system; however, a systematic review of the endocrine actions of opioids in both humans and animals has, to our knowledge, not been published since 1984. Thus, we reviewed the literature on the effect of opioids on the endocrine system. We included both acute and chronic effects of opioids, with the majority of the studies done on the acute effects although chronic effects are more physiologically relevant. In humans and laboratory animals, opioids generally increase GH and prolactin and decrease LH, testosterone, estradiol, and oxytocin. In humans, opioids increase TSH, whereas in rodents, TSH is decreased. In both rodents and humans, the reports of effects of opioids on arginine vasopressin and ACTH are conflicting. Opioids act preferentially at different receptor sites leading to stimulatory or inhibitory effects on hormone release. Increasing opioid abuse primarily leads to hypogonadism but may also affect the secretion of other pituitary hormones. The potential consequences of hypogonadism include decreased libido and erectile dysfunction in men, oligomenorrhea or amenorrhea in women, and bone loss or infertility in both sexes. Opioids may increase or decrease food intake, depending on the type of opioid and the duration of action. Additionally, opioids may act through the sympathetic nervous system to cause hyperglycemia and impaired insulin secretion. In this review, recent information regarding endocrine disorders among opioid abusers is presented.

Opioid and cocaine combined effect on cocaine-induced changes in HPA and HPG axes hormones in men

Nalbuphine, a mixed micro-/kappa-opioid analgesic, may have potential as a new medication for the treatment of cocaine abuse. Kappa-opioid agonists functionally antagonize some abuse-related and locomotor effects of cocaine, and both kappa-selective and mixed micro-/kappa-opioids reduce cocaine self-administration by rhesus monkeys. Because cocaine's interactions with the hypothalamic-pituitary-adrenal and (HPA) hypothalamic-pituitary-gonadal (HPG) axes may contribute to its reinforcing properties, we examined the effects of cocaine alone and in combination with nalbuphine. Neuroendocrine effects of a single dose of cocaine alone (0.2 mg/kg, IV), with nalbuphine (5 mg/70 kg, IV)+cocaine (0.2 mg/kg, IV) in combination were compared in seven adult men (ages 18-35) who met DSM-IV criteria for current cocaine abuse. Cocaine alone, and in combination with nalbuphine was administered on separate test days under placebo-controlled, double blind conditions. Cocaine stimulated ACTH, cortisol, and LH, whereas cocaine+nalbuphine in combination produced a smaller increase in ACTH, and decreased cortisol and LH. Thus it appears that nalbuphine attenuated cocaine's effects on ACTH, cortisol, and LH. These data are consistent with our earlier report that nalbuphine modestly attenuated cocaine's positive subjective effects, and that the subjective and cardiovascular effects of cocaine+nalbuphine in combination were not additive.

Nonresponsiveness of the rat hypothalamo-pituitary-adrenocortical axis to parturition-related events: inhibitory action of endogenous opioids

During the last 2 days of pregnancy in rats, basal corticosterone secretion is enhanced, although the response of the hypothalamo-pituitary-adrenocortical (HPA) axis to emotional and physical stressors is blunted, independent of the action of endogenous opioids. In this study, alterations in the reactivity of the HPA axis, which may accompany parturition-related stimuli, and the involvement of endogenous opioids were examined in chronically catheterized rats. In vehicle-treated controls (n = 9), ACTH and corticosterone secretion decreased in preparation for birth (P < 0.01) and further declined immediately after delivery of the second pup (P < 0.01), remaining low for 150 min. In contrast, in animals injected with the opiate antagonist naloxone (5 mg ml(-1) kg(-1), i.v., n = 6) after delivery of the second pup, ACTH and corticosterone release were enhanced within 20 min (ACTH, 5.0-fold; corticosterone, 2.3-fold; P < 0.01 vs. controls) and returned to control levels after 90 min. In confirmation of previous reports, oxytocin secretion into blood was elevated in control rats after the onset of parturition (P < 0.01) and was further enhanced in the naloxone group (1.4-fold, P < 0.01 vs. control). Plasma lactate concentration was increased, 30 min after the onset of delivery (1.9-fold, P < 0.01), independent of the treatment. The data indicate that parturition-related events do not trigger HPA axis hormone release because of an effective inhibition by endogenous opioids. This nonresponsiveness of the HPA axis is likely to protect the pups' well-being during birth.

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.

The opioid/anti-opioid balance in shock: a new target for therapy in resuscitation

Teleologically, pain is of paramount importance for survival and induces the organism to cope in an active way with aggressions from a basically hostile environment. While the activation of endogenous analgesic (opioid) systems typically occurs in conditions of surrender (pre-terminal conditions, sustained tortures, etc.), the activation of endogenous anti-analgesic systems triggers mechanisms of active or passive defence (such as camouflage) aimed at survival. The distinctive features of the main anti-analgesic systems (melanocortinergic, cholecystokininergic, thyroliberinergic) and the dramatic results obtained in experimental pre-terminal conditions (hemorrhagic shock, prolonged respiratory arrest) with the administration of their neuropeptide transmitters (ACTH and several ACTH-fragments, including alpha-MSH, CCK peptides and thyrotropin-releasing hormone) are here reviewed. The study of the mechanisms underlying the resuscitating effects of these neuropeptides has led to the discovery of the (often extremely potent) resuscitating effect of other drugs (protoveratrines, nicotine, centrally-acting cholinergic agents, ganglion-stimulating drugs). It is particularly remarkable that in pre-terminal conditions these neuropeptides and drugs have highly impressive effects on cardiocirculatory parameters at doses that are almost or actually inactive under normal conditions, and that their resuscitating effect is obtained without the need for any other supportive treatment and at dose-levels well below toxic ranges. Finally, in hemorrhage-shocked animals, the treatment with anti-analgesic neuropeptides shortly after bleeding considerably extends the time-limit for an effective and definitively curing blood reinfusion. This would be of self-evident importance in clinical practice, because an extremely simple, non-toxic first-aid treatment in the field, shortly after a massive hemorrhage, could resuscitate the patient for a period sufficient to effectively set up the most appropriate in-hospital treatment.

Effect of chronic morphine treatment on the adrenaline biosynthesis in adrenals and brain regions of the rat

Phenylethanolamine-N-methyltransferase (PNMT) activity and tissue catecholamine content were examined after 13 days morphine treatment. Prolonged morphine treatment did not alter the PNMT activity in brain regions (A1/C1 and A2/C2 cell groups, medial basal hypothalamus, median eminence). However, the enzyme activity and the adrenaline content were increased in the adrenal medulla of male rats. In parallel, morphine treatment induced adrenal hypertrophy. In female or hypophysectomized male animals the chronic morphine treatment had no effect on adrenal weight but evoked the increase of PNMT activity. It is concluded that the morphine-induced increased adrenaline biosynthesis in the adrenal gland is not fully dependent on the intact pituitary-adrenal axis and may be mediated partly by a neural mechanism (increased splanchnic nerve activity) or by a direct effect of morphine.

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.

Adrenocorticotropin reversal of experimental hemorrhagic shock is antagonized by morphine

ACTH-(1-24) dose-dependently improved cardiovascular function in rats and dogs subjected to experimental hemorrhagic shock, and intravenous dose of 160 and 100/microgram/kg, respectively, completely restoring arterial blood pressure and pulse amplitude. All saline-treated animals died within 30 min of bleeding, while all ACTH-treated animals were still alive at the end of the observation period (2 hr). The injection of ACTH-(1-24) also dramatically improved the respiratory function. Morphine, i.v. injected into rats at the dose of 2.5 mg/kg, antagonised the effect of ACTH-(1-24) to a greater or lesser degree, depending on the dose of peptide employed: at 160/microgram/kg, antagonism was complete, at 320/microgram/kg antagonism was only partial, while at 480/microgram/kg antagonism was almost completely overcome. These data further support the idea that melanocortins are physiological antagonists of opioids, and suggest that melanocortin peptides may prove to be rational and effective drugs in the treatment of hypovolemic shock.

ACTH-(1-24) and alpha-MSH antagonize feeding behavior stimulated by kappa opiate agonists

ACTH-(1-24) and alpha-MSH, intracerebroventricularly (ICV) injected at the doses of 4 and 10 micrograms/animal, respectively, markedly inhibited spontaneous feeding in adult Sprague-Dawley rats, the effect remaining significant for 6-9 hours. At these same doses, ACTH-(1-24) and alpha-MSH abolished the feeding-stimulatory effect of the kappa opiate receptor agonist pentazocine, intraperitoneally (IP) injected at the dose of 10 mg/kg. The same antagonism was obtained by ICV injection of ACTH-(1-24) into rats IP treated with other kappa opiate agonists, bremazocine and tifluadom, at the doses of 1 and 5 mg/kg, respectively. These data suggest that melanocortin peptides play an inhibitory role in the complex regulation of food intake, and further support and extend the hypothesis of a melanocortin-opioid homeostatic system, its two neuropeptide components usually having opposite, mutually-balancing effects.

Effect of ACTH, beta-endorphin, morphine and naloxone on the release of cortisol by isolated adrenal glands

The release of cortisol (determined by RIA) from isolated slices of adrenal glands of guinea pigs is stimulated by ACTH, by beta-endorphin, and by morphine in a concentration-dependent way; naloxone gives a small stimulation which is not related to its concentration. Naloxone inhibits the effect of ACTH (1.11 X 10(-11) M) in a competitive manner with an IC50 of about 3.10(-9) M. Also morphine and beta-endorphin inhibit the effect of ACTH, but not in competitive manner. Naloxone (10(-9)-10(-7) M) gives a concentration-related inhibition of the increase of cortisol release produced by morphine (10(-8) M) and by beta-endorphin (1.44 X 10(-10) M). These data suggest a similarity in the conformation of ACTH, beta-endorphin, morphine and naloxone towards the binding sites of ACTH of the guinea pig adrenal glands.

Effects of opiate antagonists on hormones and behavior of male and female rhesus monkeys

Opiate antagonists, naloxone (100 micrograms/kg) and naltrexone (1 mg/kg) were given to singly housed adult male or female rhesus prior to a 20-minute behavioral test with an oppositely sexed stimulus monkey. Four of the intact adult males were socially and sexually experienced. The remaining two intact males and two castrated males had been reared in socially restricted conditions and were psychosexually deficient. Adult females were ovariectomized, and the effects of opiate antagonists were examined with or without concurrent estradiol treatment. Both antagonists inhibited sexual behavior of the socially reared, sexually active, intact males. No stimulatory effects on sexual behavior were observed for sexually deficient males, whether intact or castrated. Females showed little change in sexual behavior following opiate antagonist treatment, regardless of endocrine status. The proportion of approaches of the female to the male was increased when naloxone, but not naltrexone, was given. Specific endocrine effects of the opiate antagonists were only found in intact males. Naltrexone significantly increased LH concentrations in the two males tested, while the increase in LH in the four males receiving naloxone was not significant. In all intact males, increases in LH were accompanied by statistically significant increases in circulating concentrations of testosterone following naloxone and naltrexone. The gonadotropic stimulating effect of the opiate antagonists was specific to LH, and no changes were observed in circulating concentrations of FSH in either sex.

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.

Beta endorphin selectively stimulates aldosterone secretion in hypophysectomized, nephrectomized dogs

We examined the effects of synthetic human beta-endorphin (beta END) and a stable methionine (Met)-enkephalin analogue on aldosterone and cortisol secretion rates in anesthetized, hypophysectomized, and nephrectomized dogs and compared them to those of (1-39) ACTH. The circulation of the adrenal glands was completely isolated on the arterial and venous sides (Hilton Pouch). The peptides were infused to deliver 3 pmol/min into the aortic "pouch." Blood was collected from the vena caval pouch, which received blood only from the adrenal gland. Secretion rates of aldosterone and cortisol were calculated as the product of adrenal blood flow and venous steroid concentration. Duplicate steroid measurements were obtained during a control period, at 10, 30, and 50 min of peptide infusion and during a postcontrol period. BetaEND increased aldosterone secretion rate from 2.4 +/- 0.5 ng/min (mean +/- SEM) to 3.2 +/- 0.9 ng/ min at 10 min (N.S.), 8.2 +/- 2.5 ng/min (P less than 0.05) at 30 min and 11.0 +/- 3.7 ng/ min (P less than 0.05) at 50 min of infusion. Cortisol secretion rate was not affected by infusion of betaEND. Infusion of the stable Met-enkephalin analogue D-alanine2; Metphenylalanine4, Met(O)-enkephalin-ol or saline alone had no effect on aldosterone or cortisol secretion rates. ACTH infusion increased mean aldosterone secretion rate by approximately 215% and significantly stimulated cortisol secretion rate. These results indicate that beta END selectively stimulates aldosterone secretion with a potency similar to that of an equimolar dose of ACTH.