ACTH: a single pretreatment enhances the analgesic efficacy of and prevents the development of tolerance to morphine

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.

The effect of dexamethasone on spinal glutamine synthetase and glutamate dehydrogenase expression in morphine-tolerant rats

BACKGROUND

Excitatory amino acids play an important role in morphine tolerance. Recently, we demonstrated that a single morphine challenge induces an increase in spinal cerebrospinal fluid excitatory amino acid concentrations in morphine-tolerant rats, and that dexamethasone inhibits the development of morphine tolerance. We further examined the effect of intrathecal dexamethasone infusion on the development of morphine tolerance and on expression of the intracellular glutamate metabolizing enzymes, glutamate dehydrogenase and glutamine synthetase, in the spinal cord.

METHODS

Male Wistar rats, implanted with an intrathecal catheter, were divided into four groups that were infused for 5 days with intrathecal morphine (15 microg/h), saline (1 microL/h), dexamethasone (2 microg/h), or dexamethasone (2 microg/h) plus morphine (15 microg/h). On Day 5, the spinal cords were removed and prepared for Western blot analysis of glutamate dehydrogenase and glutamate synthetase.

RESULTS

Glutamate dehydrogenase and glutamate synthetase concentrations were downregulated in the morphine-tolerant rat spinal cords. Concurrent infusion of dexamethasone attenuated morphine tolerance and the associated glutamate dehydrogenase and glutamate synthetase downregulation.

CONCLUSION

Intrathecal dexamethasone attenuates long-term morphine infusion-induced glutamate dehydrogenase and glutamate synthetase downregulation and antinociceptive tolerance.

Dexamethasone mimics the inhibitory effect of chronic pain on the development of tolerance to morphine analgesia and compensates for morphine induced changes in G proteins gene expression

It is previously reported that the HPA axis plays role in the inhibitory effect of pain on tolerance development to analgesic effect of opioids. The present study was designed to investigate whether the chronic co-administration of dexamethasone as a glucocorticoid is also able to prevent or reverse analgesic tolerance to morphine and to compare the expression of G(alphai/o) and G(beta) subunits of G proteins in the context of chronic dexamethasone, development of morphine tolerance and their combination. Analgesic tolerance to morphine was induced by chronic intraperitoneally (i.p.) administration of morphine 20 mg/kg to male Wistar rats weighing 200-240 g within 4 consecutive days and analgesia was assessed using tail-flick test. Chronic dexamethasone was applied using 4 daily i.p. injections. Lumbar spinal tissues were assayed for the expression of G(alphai/o) and G(beta) proteins using "semiquantitative PCR" normalized to beta-actin gene expression. Results showed that chronic administration of dexamethasone could reduce and reverse the development of tolerance in rats that received chronic i.p. injections of morphine. Chronic administration of dexamethasone significantly increased the expression of G(alphai/o), while chronic administration of morphine did not change its expression. The expression of G(beta), however, was increased after the chronic administration of morphine, but did not change after the administration of chronic dexamethasone. None of these increases were observed when morphine and dexamethasone were co-administered. We conclude that the development of tolerance to analgesic effect of morphine could be prevented and reversed by dexamethasone co-administration. The increase in G(alphai/o) genes expression produced by chronic dexamethasone may facilitate the opioid signaling pathway and compensate for morphine-induced tolerance.

Changes in G proteins genes expression in rat lumbar spinal cord support the inhibitory effect of chronic pain on the development of tolerance to morphine analgesia

There are some reports regarding the inhibitory effect of pain on tolerance development to analgesic effect of opioids. The present study was designed to investigate whether the chronic formalin induced pain is able to reverse analgesic tolerance to morphine and to evaluate the expression of G(alpha i/o) and G(beta) subunits of G proteins in the context of chronic pain, development of morphine tolerance and their combination. Morphine tolerance was induced by chronic systemic (intraperitoneally, i.p.) or spinal (intrathecally, i.t.) administration of morphine to male Wistar rats weighing 200-240 g and analgesia was assessed using tail flick test. Chronic pain was induced by 4 daily intraplantar injections of 50 microl of 5% formalin. Lumbar spinal tissues were assayed for the expression of G(alpha i/o) and G(beta) proteins using "semiquantitative PCR" normalized to beta-actin gene expression. Results showed that chronic formalin induced pain could reduce and reverse the development of tolerance in rats that had received chronic (i.p. or i.t.) administration of morphine. Chronic administration of morphine did not change G(alpha i/o) gene expression, while chronic pain significantly increased its expression. The expression of G(beta), however, was increased after the chronic administration of morphine, but did not change after the induction of chronic pain. None of these increases were observed when morphine and formalin were administered at the same time. Due to synchronous development of morphine tolerance and changes in expression of G(beta), it may be concluded that the development of tolerance to analgesic effect of morphine is partially mediated by increase in G(beta) gene expression. The increase in G(alpha i/o) genes expression produced by chronic pain may facilitate the opioid signaling pathway and compensate for morphine-induced tolerance.

Dexamethasone modulates the development of morphine tolerance and expression of glutamate transporters in rats

We recently demonstrated an increase in spinal cerebrospinal fluid (CSF) excitatory amino acids (EAAs) in morphine-tolerant rats after morphine challenge. The present study examined whether co-infusion of the glucocorticoid dexamethasone (DEX) co-infusion inhibited morphine tolerance and the morphine challenge-induced EAAs increase after long-term morphine infusion. Intrathecal (i.t.) catheters and one microdialysis probe were implanted to male Wistar rats. Rats were divided into four groups: i.t. morphine (15 microg/h), saline (1 microl/h), DEX (2 microg/h), or DEX (2 microg/h) plus morphine (15 microg/h) infusion for 5 days. Tail-flick responses were examined before drug infusion and daily after the start of infusion for 5 days. Moreover, on day 5 after morphine challenge (50 microg, i.t.), CSF EAAs was also measured. Rat spinal cords were removed on day 5, and prepared for Western blot analysis of different glutamate transporters (GTs). The AD50 (analgesic dose) on day 5 was 1.33 microg in saline-infused rats, 83.84 microg in morphine-tolerant rats, and 10.15 microg in DEX plus morphine co-infused rats. Single DEX (2 microg, i.t.) injection did not enhance morphine's antinociceptive effect in either naïve or morphine-tolerant rats. No difference in CSF EAA level was observed in all groups between baseline (before drug infusion) and on day 5 after tolerance developed. Surprisingly, on day 5, after morphine challenge, an increase in glutamate and aspartate (284+/-47% and 201+/-18% of basal) concentration was observed, and morphine lost its antinociceptive effect (maximum percent effect, MPE = 41+/-12%), whereas DEX/morphine co-infusion inhibited morphine-evoked EAA increase with a MPE = 97+/-2%. DEX co-infusion prevented the downregulation of glial glutamate transporters (GLAST (Glu-Asp transporter) and GLT-1 (Glu transporter-1)), but not the neuronal GT EAAC1 (excitatory amino acid carrier). Upregulation of GLT-1 was also observed (204+/-20% of basal). DEX co-infusion inhibits the morphine-challenge induced EAA increase and prevents the loss of morphine's antinociceptive effect after long-term morphine infusion.

Maternal hypothalamic-pituitary-adrenal disregulation during the third trimester influences human fetal responses

Maternal peptides from the hypothalamic-pituitary-adrenal (HPA) axis rise during human pregnancy. The effects of circulating maternal adrenocorticotropin (ACTH) and beta-endorphin (BE) on human fetal behavior was determined in 135 women during their 32nd week of gestation. Fetal behavior was measured by assessing heart rate habituation to a series of repeated vibroacoustic stimuli. Individual differences in habituation were determined by computing the number of consecutive responses above the standard deviation during a control period. There was no significant relation between levels of ACTH, BE and the rate of fetal heart rate habituation. However, an index of HPA disregulation (uncoupling of ACTH and BE) was related significantly to fetal behavior. Fetal exposure to high levels of maternal BE relative to ACTH was associated with significantly lower rates of habituation. Results indicate that maternal stress and stress-related peptides influence fetal response patterns. It is possible that this influence persists over the life span.

Opioid poorly-responsive cancer pain. Part 2: basic mechanisms that could shift dose response for analgesia

Basic research in experimental pain models may illuminate the phenomenon of cancer pain that is poorly responsive to opioid drugs. Research findings can be valuable in formulating new strategies in clinical practice. This review evaluated experimental observations in terms of the events that occur in cancer patients receiving opioid therapy for pain.

Targeted disruption of the orphanin FQ/nociceptin gene increases stress susceptibility and impairs stress adaptation in mice

The neuropeptide orphanin FQ (also known as nociceptin; OFQ/N) has been implicated in modulating stress-related behavior. OFQ/N was demonstrated to reverse stress-induced analgesia and possess anxiolytic-like activity after central administration. To further study physiological functions of OFQ/N, we have generated OFQ/N-deficient mice by targeted disruption of the OFQ/N gene. Homozygous mice display increased anxiety-like behavior when exposed to a novel and threatening environment. OFQ/N-null mice show elevated basal pain threshold but develop normal stress-induced analgesia. Interestingly, these mice show impaired adaptation to repeated stress when compared with wild-type mice, whereas their performance in spatial learning remained unaffected. Basal and poststress plasma corticosterone levels were found to be elevated in OFQ/N-deficient animals. Thus, OFQ/N appears to be crucially involved in the neurobiological regulation of stress-coping behavior and fear.

The role of corticosterone in the blockade of tolerance to morphine analgesia by formalin-induced pain in the rat

We previously reported that morphine fails to produce analgesic tolerance when administered in the presence of formalin-induced pain, which may be related to activity of the hypothalamic-pituitary-adrenal axis. In the present study, we examined whether suppression of corticosterone secretion during pain prevents the blockade of tolerance to morphine analgesia. Male Long-Evans rats were injected with morphine (20 mg/kg) or saline for 4 consecutive days in the presence or absence of formalin-induced pain. To suppress corticosterone activity, some animals were injected daily with the corticosterone synthesis inhibitor, metyrapone (100 mg/kg), 24 h and 30 min before formalin injections. The analgesic effect of a test dose of morphine (10 mg/kg) was then measured in the tail-flick test 24 h after tolerance induction (i.e. day 5). The presence of pain during tolerance induction prevented the development of analgesic tolerance. Furthermore, inhibition of corticosterone synthesis by metyrapone prevented the blockade of tolerance by pain. These results suggest that the blockade of tolerance to morphine analgesia by formalin-induced pain depends on stress-induced corticosterone increases.

Blockade of tolerance to stress-induced analgesia by MK-801 in mice

The N-methyl-D-aspartate (NMDA) receptor has been implicated in mechanisms of tolerance to morphine-induced analgesia. The present study examined the role of the NMDA receptor in the development of tolerance to stress-induced analgesia (SIA). In the first experiment, mice were exposed to a stressor (a 3-min forced swim in water maintained at 32 degrees C) once daily for 15 consecutive days. Analgesia was measured 2 min after stress on the first and last day using the hot-plate test. To examine the role of the NMDA receptor in the development of tolerance to SIA mice were treated daily with the non-competitive NMDA receptor antagonist, MK-801, 15 min before swimming. Pretreatment with MK-801 was found to block both analgesia and tolerance. In a second experiment, to examine whether SIA and tolerance to SIA are mediated by similar or different mechanisms, mice were injected daily with MK-801 after analgesia had dissipated (1 h following swim). Tolerance to SIA was blocked by delayed injections of MK-801. These results suggest that the NMDA receptor is involved in mechanisms of tolerance to SIA, independent of its role in analgesia.

Stress-induced analgesia. The role of hormones produced by the hypophyseal-adrenocortical system

Experimental studies on the effects of stress on blood corticosteroid levels and the appearance of analgesia were carried out on rats anesthetized with Nembutai (4 mg/100 g). Stress, consisting of stimulation of the hind footpad with a current at 0.7 mA, produced parallel changes in plasma corticosteroid concentrations and the threshold of a pain response. Functional blockade of the hypophyseal-adrenocortical system, produced by systemic administration of hydrocortisone (15 mg/100 g) or by implantation of dexamethasone (200 micrograms) above the paraventricular nucleus of the hypothalamus, resulted in reductions in stress-induced analgesia. Dosage with naloxone (1 and 10 mg/kg) had no effect on the level of analgesia or corticosteroid concentrations. It is concluded that stress-induced analgesia not mediated by opioids is corticosteroid-dependent.

Relationship between hypothalamic-pituitary-adrenal activity and blockade of tolerance to morphine analgesia by pain: a strain comparison

We previously reported that morphine fails to produce analgesic tolerance when administered in the presence of formalin-induced pain. The hypothalamic-pituitary-adrenal (HPA) axis is known to respond to stressful stimuli, including pain. To examine whether the blockade of tolerance by pain is related to HPA activity, we assessed the development of tolerance to morphine analgesia in an inbred strain of rats that lack typical stress-induces HPA responses (Lewis strain). Lewis rats lack typical stress-induced activation of corticotropin-releasing hormone, adrenocorticotropin hormone and glucocorticoids. Female Lewis rats were injected with morphine (20 mg/kg, i.p.) or saline for 4 consecutive days in the presence or absence of pain induced by injection of formalin into the hind-paw. The analgesic effect of morphine (5, 10 or 20 mg/kg, i.p.) was then measured in the tail-flick test 24 h after tolerance induction. Inbred female Fischer rats, which show significant stress-induced HPA activity, were used for comparison. Analgesic tolerance was produced in both strains when morphine was delivered in the absence of pain. However, the presence of pain during morphine administration prevented the development of analgesic tolerance in Fischer, but not in Lewis, rats. The differential of pain on the development of tolerance to morphine analgesia are suggested to be related to genetically determined differences in stress-induced HPA activity.

Morphine fails to produce tolerance when administered in the presence of formalin pain in rats

The present study examined the development of tolerance to morphine analgesia under conditions in which morphine was administered in the presence or absence of pain induced by subcutaneous injection of 50 microliters of 2.5% formalin into the hind paw of rats. Animals were injected with morphine (25 mg/kg, i.p.) or saline for 3 consecutive days either in the presence of pain (10 min after formalin injection) or in the absence of pain (6 h prior to formalin injection). On the 4th day, tolerance to the analgesic effect of test doses of morphine (6 or 10 mg/kg) was assessed in the formalin and tail-flick tests, respectively. Significant tolerance in both tests was observed in animals receiving morphine in the absence of pain during the tolerance induction period, but not in animals receiving morphine in the presence of pain.

Formalin-induced pain antagonizes the development of opiate dependence in the rat

Clinical studies have suggested that patients who take morphine for pain relief do not show a high degree of dependence. The present study examined the development of naloxone-precipitated withdrawal in rats receiving morphine in the presence or absence of formalin-induced pain. Morphine (10 mg/kg, i.p.) or saline was administered for 4 consecutive days 10 min after a subcutaneous injection of 50 microliters of 2.5% formalin or saline into the hind-paw. On the 5th day, rats were injected with naloxone (1 mg/kg, i.p.) and observed for signs of precipitated withdrawal (ptosis, teeth chattering and excretion/diarrhea). Naloxone-precipitated withdrawal symptoms were significantly greater in rats that received morphine in the absence of pain than in rats that received morphine in the presence of pain.

Stress-induced hypoalgesia and opioid inhibition of pigs' responses to restraint

Pigs' responses to physical restraint were examined in order to detect a stress-induced increase in endogenous opioid activity. Tail-flick latencies in response to a source of thermal energy were used to assess the sensitivity of pigs to pain. Restraining pigs for 15 min with a nose snare resulted in a temporary increase in tail-flick latencies that was apparent after 5 min, absent after 30 min and was blocked by naloxone. Tail-flick latencies were unaffected by IV ACTH injections and were not related to plasma cortisol concentrations. Naloxone increased the pigs' vocalization during the restraint and prolonged the elevation of plasma cortisol levels that followed the restraint. The cortisol response to naloxone was larger than when the animals were not restrained. The results indicate a transient, opioid-based hypoalgesia following restraint stress that is not a result of ACTH or cortisol secretion. Endogenous opioids inhibit the pigs' behavioral and pituitary-adrenocortical responses to the restraint stress.

ACTH produces long-lasting recovery following partial extinction of an active avoidance response

Recent data suggest that ACTH administration produces recovery of an extinguished passive avoidance response at an unusually long injection-to-test interval. The present experiment sought to explore further the durability of recovery by examining the effect of ACTH following extinction of one-way active avoidance. Adult rats were injected with 16 IU ACTH, an equivalent volume of the ACTH vehicle gel, or saline 48 h after a previously learned active avoidance response was partially extinguished. Different groups from each treatment condition were tested 15 min, 24 h, or 7 days after injection. ACTH improved avoidance performance at all injection-to-test intervals relative to saline and vehicle gel injected controls. These data indicate that unlike reversal of other types of performance decrements, in which the effect of ACTH appears to be transient, administration of the hormone following an extinction treatment can produce enduring improvement of avoidance performance.