Impact of continuous testosterone exposure on reproductive physiology, activity, and pain-related behavior in young adult female rats

Testosterone may reduce pain in cisgender women and transgender men. Rodents can provide a useful model for investigating physiological effects of hormone therapy. To this end, continuous-release testosterone or blank (placebo) capsules were implanted s.c. into young adult female rats, and three weeks later rats were either ovariectomized or sham-ovariectomized. Testosterone treatment that mimicked previously reported endogenous levels in males eliminated estrous cycling and decreased uterine weight. Testosterone also significantly increased body weight and suppressed the increases in daily wheel running observed in placebo controls over time. Subsequent ovariectomy or sham-ovariectomy decreased wheel running in all groups, but testosterone-treated rats recovered significantly more quickly than did placebo-treated rats. Neither testosterone nor ovariectomy significantly altered hindpaw mechanical threshold. Two weeks after sham/ovariectomy surgery, injection of Complete Freund Adjuvant (CFA) into one hindpaw reduced wheel running and mechanical threshold in all groups; running significantly decreased from the first to second day after CFA in testosterone- but not in placebo-treated rats. Morphine 1.0 but not 3.2 mg/kg increased CFA-suppressed wheel running similarly in all groups, whereas both doses of morphine increased CFA-suppressed mechanical threshold. These data suggest that weeks-long testosterone treatment with or without ovariectomy may provide a useful physiological model of testosterone therapy as used in human gender transition. Although testosterone administered at levels similar to those in gonadally intact males tended to hasten female rats' recovery from surgery, it did not decrease maximal pain-related behaviors after surgery or hindpaw inflammatory insult, nor did it alter opioid antinociception.

Continuous fentanyl administration and spontaneous withdrawal decreases home cage wheel running in rats with and without hindpaw inflammation

Fentanyl is a potent analgesic with a rapid onset and short half-life that make it a useful treatment for pain and a lethal drug of abuse. The present study used voluntary home cage wheel running to assess the effect of hindpaw inflammation, fentanyl administration, and spontaneous fentanyl withdrawal. Fentanyl (0.32 or 1.0 mg/kg/day) or placebo osmotic pumps were implanted subcutaneously and rats received an intraplantar injection of Complete Freund's Adjuvant (CFA) or saline. Rats with hindpaw inflammation caused by CFA administration were less active than rats injected with saline into the hindpaw. The antinociceptive effect of 0.32 mg/kg/day of fentanyl was evident as a recovery of wheel running in these rats. Administration of 1 mg/kg/day of fentanyl almost completely inhibited wheel running during the first day in rats with and without hindpaw inflammation. Wheel running increased each subsequent day until the pumps were surgically removed after day 3. Withdrawal from 0.32 or 1 mg/kg/day of fentanyl caused a decrease in wheel running that lasted 2 days in rats without hindpaw inflammation. In contrast, withdrawal was only evident following termination of 1 mg/kg/day of fentanyl in rats with hindpaw inflammation. This decrease in running seemed to persist beyond the 3 days of assessment. These data demonstrate that fentanyl can either depress or restore activity depending on the dose and pain condition. Moreover, termination of 3 days of continuous fentanyl administration resulted in a dose and time dependent decrease in wheel running consistent with opioid withdrawal.

Low-Dose Δ9-THC Produces Antinociception in Female, But Not Male Rats

Introduction: The analgesic effects of delta-9-tetrahydrocannabinol (THC), the primary psychoactive compound in cannabis, have been widely promoted. Unfortunately, animal research is limited by the use of high doses and pain-evoked tests. Motor and psychoactive effects of THC may suppress evoked responses in the absence of antinociceptive effects. Materials and Methods: This study overcomes these problems by assessing the antinociceptive effect of low doses of subcutaneous THC on depression of home cage wheel running caused by hindpaw inflammation. Female and male Long-Evans rats were individually housed in a cage with a running wheel. Results: Female rats ran significantly more than male rats. Administration of Complete Freund's Adjuvant into the right hindpaw produced inflammatory pain that significantly depressed wheel running in female and male rats. Administration of a low dose of THC (0.32, but not 0.56 or 1.0 mg/kg) restored wheel running in the hour after administration in female rats. Administration of these doses had no effect on pain-depressed wheel running in male rats. Conclusions: These data are consistent with previous studies showing greater antinociceptive effects of THC in female compared with male rats. These data extend previous findings by showing that low doses of THC can restore pain-depressed behaviors.

Sex differences in the impact of pain, morphine administration and morphine withdrawal on quality of life in rats

The disruptive effects of pain on quality of life are greater in men than in women, but the disruptive effects of opioid administration and withdrawal tend to be greater in women. These sex differences in pain, acute opioid effects, and opioid withdrawal tend to be opposite to sex differences reported in laboratory rats. We hypothesized that sex differences in humans and rats would more closely align if animal research measured quality of life as opposed to traditional evoked behaviors of pain (e.g., nociceptive reflexes) and opioid withdrawal (e.g., wet dog shakes). The present study assessed quality of life in adult female and male rats by measuring voluntary wheel running in the rat's home cage. Hindpaw inflammation induced by administration of Complete Freund's Adjuvant (CFA) into the right hindpaw caused a greater depression of wheel running in male compared to female rats. Twice daily injections of high morphine doses (5-20 mg/kg) and the subsequent morphine withdrawal caused a greater depression of wheel running in female compared to male rats. These sex differences are consistent with human data that shows the impact of pain on quality of life is greater in men than women, but the negative effects of opioid administration and withdrawal are greater in women. The present data indicate that the clinical significance of animal research would be enhanced by shifting the endpoint from pain and opioid evoked behaviors to measures of quality of life such as voluntary wheel running.

Death Cafes: An Exploration of the Setting, the Players, and the Conversation

The overarching mission of Death Cafes is to raise people's consciousness about life's temporality so that they make the most of their finite lives. Death Cafés create a space for people to gather and have an open dialogue about death and dying. The current qualitative research study explores the experience of nine participants who engage in Death Cafes as facilitators or attendees. Findings including themes of "The Setting", "The Players", and "The Conversation" explore the experiences of Death Cafe participation. Discussion of findings and implications include death rituals, commercialization of death experiences, and the supportive community of the Death Cafe.

Comparative benefits of social housing and buprenorphine on wheel running depressed by morphine withdrawal in rats

RATIONALE

Social support and opioid replacement therapy are commonly used to treat opioid withdrawal.

OBJECTIVE

The present study tested the hypothesis that social housing and buprenorphine administration can restore wheel running depressed by morphine withdrawal in rats.

RESULTS

Experiment 1 assessed disruptive side effects of buprenorphine and found that administration of low doses (3.2, 10, & 32 µg/kg, s.c.) had no impact on voluntary wheel running. Experiment 2 assessed the impact of social housing and acute buprenorphine administration (10 µg/kg) on morphine withdrawal. Two 75 mg morphine pellets were implanted for 3 days to induce dependence. Removal of the morphine pellets caused a decrease in body weight, increase in wet dog shakes, and depression of wheel running during the normally active dark phase of the circadian cycle. Social housing restored wheel running and reduced the number of wet dog shakes but did not affect body weight. Administration of buprenorphine restored wheel running depressed by morphine withdrawal for 2 days in individually housed rats and produced time-dependent changes in socially housed rats: Depression of wheel running in the 3 h following administration and restoration of running subsequently compared to saline-treated controls.

CONCLUSIONS

The impact of buprenorphine and social housing to reduce the effect of morphine withdrawal in rats is consistent with the use of opioid substitution therapy and psychotherapy/social support to treat opioid withdrawal in humans. These data provide further validation for the clinical relevance for the use of wheel running to assess spontaneous opioid withdrawal.

Tetrahydrocannabinol (THC) Exacerbates Inflammatory Bowel Disease in Adolescent and Adult Female Rats

Inflammatory Bowel Disease (IBD) is a life-long disorder that often begins between the ages of 15 and 30. Anecdotal reports suggest cannabinoids may be an effective treatment. This study sought to determine whether home cage wheel running is an effective method to assess IBD, and whether Tetrahydrocannabinol (THC), the primary psychoactive compound in cannabis, can restore wheel running depressed by IBD. Adolescent and adult female Sprague-Dawley rats were individually housed in a cage with a running wheel. Rats were injected with trinitrobenzene sulphonic acid (TNBS) into the rectum to induce IBD-like symptoms. One day later, both vehicle and TNBS treated rats were injected with a low dose of THC (0.32 mg/kg, s.c.) or vehicle. Administration of TNBS depressed wheel running in adolescent and adult rats. No antinociceptive effect of THC was evident when administered 1 day after TNBS. In fact, administration of THC prolonged TNBS-induced depression of wheel running for over 5 days in adolescent and adult rats. These results show that home cage wheel running is depressed by TNBS-induced IBD, making it a useful tool to evaluate the behavioral consequences of IBD, and that administration of THC, instead of producing antinociception, exacerbates TNBS-induced IBD. PERSPECTIVE: This article advances research on inflammatory bowel disease in two important ways: 1) Home cage wheel running is a new and sensitive tool to assess the behavioral consequences of IBD in adolescent and adult rats; and 2) Administration of the cannabinoid THC exacerbates the negative behavioral effects of IBD.

Morphine restores and naloxone-precipitated withdrawal depresses wheel running in rats with hindpaw inflammation

Opioids such as morphine are the most effective treatment for pain, but termination of opioid use can produce severe withdrawal symptoms. The present study models this process by using home cage wheel running to assess well-being as a result of pain, morphine analgesia, and opioid withdrawal. Injection of CFA into the right hindpaw caused a dramatic decrease in wheel running and body weight. Implantation of two morphine pellets (75 mg each) resulted in an increase in body weight on Day 1 of administration and a more gradual restoration of wheel running that was only evident during the dark phase of the circadian cycle on Days 3 and 4 of morphine administration. Continuous morphine administration decreased wheel running during the relatively inactive light phase. These findings are consistent with the clinical goal of pain therapeutics to restore normal activity during the day and facilitate sleep at night. Administration of naloxone (1 mg/kg) on Day 5 of morphine administration depressed wheel running for approximately 4 h and caused an increase in wet dog shakes. Naloxone-precipitated changes were no longer evident 6 h after administration. These findings demonstrate that the use of morphine to treat pain does not protect against opioid withdrawal. Moreover, this study provides additional support for the use of home cage wheel running as a method to assess changes in well-being as a result of pain, analgesia, and opioid withdrawal.

'Reinventing the wheel' to advance the development of pain therapeutics

Chronic pain affects approximately one-third of the population worldwide. The primary goal of animal research is to understand the neural mechanisms underlying pain so better treatments can be developed. Despite an enormous investment in time and money, almost no novel treatments for pain have been developed. There are many factors that contribute to this lack of translation in drug development. The mismatch between the goals of drug development in animals (inhibition of pain-evoked responses) and treatment in humans (restoration of function) is a major problem. To solve this problem, a number of pain-depressed behavioral tests have been developed to assess changes in normal behavior in laboratory animals. The use of home cage wheel running as a pain assessment tool is especially useful in that it is easy to use, provides an objective measurement of the magnitude and duration of pain, and is a clinically relevant method to screen novel drugs. Pain depresses activity in humans and animals, and effective analgesic treatments restore activity. Unlike traditional pain-evoked tests (e.g., hot plate, tail flick, von Frey test), restoration of home cage wheel running evaluates treatments for both antinociceptive efficacy and the absence of disruptive side effects (e.g., sedation, paralysis, nausea). This article reviews the literature using wheel running to assess pain and makes the case for home cage wheel running as an effective and clinically relevant method to screen novel analgesics for therapeutic potential.

Use of home cage wheel running to assess the behavioural effects of administering a mu/delta opioid receptor heterodimer antagonist for spontaneous morphine withdrawal in the rat

Opioid abuse is a major health problem. The objective of the present study was to evaluate the potentially disruptive side effects and therapeutic potential of a novel antagonist (D24M) of the mu-/delta-opioid receptor (MOR/DOR) heterodimer in male rats. Administration of high doses of D24M (1 & 10 nmol) into the lateral ventricle did not disrupt home cage wheel running. Repeated twice daily administration of increasing doses of morphine (5-20 mg/kg) over 5 days depressed wheel running and induced antinociceptive tolerance measured with the hot plate test. Administration of D24M had no effect on morphine tolerance, but tended to prolong morphine antinociception in non-tolerant rats. Spontaneous morphine withdrawal was evident as a decrease in body weight, a reduction in wheel running and an increase in sleep during the normally active dark phase of the circadian cycle, and an increase in wheel running and wakefulness in the normally inactive light phase. Administration of D24M during the dark phase on the third day of withdrawal had no effect on wheel running. These data provide additional evidence for the clinical relevance of home cage wheel running as a method to assess spontaneous opioid withdrawal in rats. These data also demonstrate that blocking the MOR/DOR heterodimer does not produce disruptive side effects or block the antinociceptive effects of morphine. Although administration of D24M had no effect on morphine withdrawal, additional studies are needed to evaluate withdrawal to continuous morphine administration and other opioids in rats with persistent pain.

Social housing promotes recovery of wheel running depressed by inflammatory pain and morphine withdrawal in male rats

The increased use of opioids to treat pain has led to a dramatic increase in opioid abuse. Our previous data indicate that pain may facilitate the development of opioid abuse by increasing the magnitude and duration of opioid withdrawal. The present study tested the hypothesis that social housing would facilitate recovery of activity depressed by pain and opioid withdrawal. Male Sprague Dawley rats were housed either in pairs or alone and then moved to a cage with a running wheel for 6 h daily to assess pain- and opioid withdrawal-induced depression of wheel running. Rats were implanted with two morphine (75 mg each) or placebo pellets to induce opioid dependence and were simultaneously injected with Complete Freund's Adjuvant or saline into the right hind paw to induce persistent inflammatory pain. Hind paw inflammation depressed wheel running whether rats were implanted with a morphine or placebo pellet. Pair-housed rats showed greater recovery of wheel running than individually housed rats. Spontaneous morphine withdrawal precipitated by removal of the morphine pellets caused a reduction in wheel running that was greater in rats with hind paw inflammation compared to pain free rats. Social housing facilitated recovery from withdrawal in rats with hind paw inflammation, but slowed recovery in pain free rats. These data suggest that social housing facilitates recovery by reducing pain both before and during opioid withdrawal. Our findings are consistent with previous studies showing that social buffering reduces pain-evoked responses.

Differences in antinociceptive signalling mechanisms following morphine and fentanyl microinjections into the rat periaqueductal gray

BACKGROUND

Morphine and fentanyl are two of the most commonly used opioids to treat pain. Although both opioids produce antinociception by binding to mu-opioid receptors (MOR), they appear to act via distinct signalling pathways.

OBJECTIVE

This study will reveal whether differences in morphine and fentanyl antinociception are the result of selective activation of G-protein signalling and/or selective activation of pre- or postsynaptic MORs.

METHODS

The contribution of each mechanism to morphine and fentanyl antinociception was assessed by microinjecting drugs to alter G-protein signalling or block potassium channels linked to pre- and postsynaptic MORs in the ventrolateral periaqueductal gray (PAG) of male Sprague-Dawley rats.

RESULTS

Both morphine and fentanyl produced a dose-dependent antinociception when microinjected into the PAG. Enhancement of intracellular G-protein signalling by microinjection of the Regulator of G-protein Signalling 4 antagonist CCG-63802 into the PAG enhanced the antinociceptive potency of morphine, but not fentanyl. Microinjection of α-dendrotoxin into the PAG to block MOR activation of presynaptic K_v ⁺ channels caused a significant rightward shift in the dose-response curve of both morphine and fentanyl. Microinjection of tertiapin-Q to block MOR activation of postsynaptic GIRK channels caused a larger shift in the dose-response curve for fentanyl than morphine antinociception.

CONCLUSIONS

These findings reveal different PAG signalling mechanisms for morphine and fentanyl antinociception. In contrast with fentanyl, the antinociceptive effects of morphine are mediated by G-protein signalling primarily activated by presynaptic MORs.

SIGNIFICANCE

Microinjection of the opioids morphine and fentanyl into the periaqueductal gray (PAG) produce antinociception via mu-opioid receptor signalling. This study reveals differences in the signalling mechanisms underlying morphine and fentanyl antinociception in the PAG. In contrast with fentanyl, morphine antinociception is primarily mediated by presynaptic opioid receptors and is enhanced by blocking RGS proteins.

Pros and Cons of Clinically Relevant Methods to Assess Pain in Rodents

The primary objective of preclinical pain research is to improve the treatment of pain. Decades of research using pain-evoked tests has revealed much about mechanisms but failed to deliver new treatments. Evoked pain-tests are often limited because they ignore spontaneous pain and motor or disruptive side effects confound interpretation of results. New tests have been developed to focus more closely on clinical goals such as reducing pathological pain and restoring function. The objective of this review is to describe and discuss several of these tests. We focus on: Grimace Scale, Operant Behavior, Wheel Running, Burrowing, Nesting, Home Cage Monitoring, Gait Analysis and Conditioned Place Preference/ Aversion. A brief description of each method is presented along with an analysis of the advantages and limitations. The pros and cons of each test will help researchers identify the assessment tool most appropriate to meet their particular objective to assess pain in rodents. These tests provide another tool to unravel the mechanisms underlying chronic pain and help overcome the translational gap in drug development.

Lack of Antinociceptive Cross-Tolerance With Co-Administration of Morphine and Fentanyl Into the Periaqueductal Gray of Male Sprague-Dawley Rats

Tolerance to the antinociceptive effect of mu-opioid receptor agonists, such as morphine and fentanyl, greatly limits their effectiveness for long-term use to treat pain. Clinical studies have shown that combination therapy and opioid rotation can be used to enhance opioid-induced antinociception once tolerance has developed. The mechanism and brain regions involved in these processes are unknown. The purpose of this study was to evaluate the contribution of the ventrolateral periaqueductal gray (vlPAG) to antinociceptive tolerance and cross-tolerance between administration and co-administration of morphine and fentanyl. Tolerance was induced by pretreating rats with morphine or fentanyl or low-dose combination of morphine and fentanyl into the vlPAG followed by an assessment of the cross-tolerance to the other opioid. In addition, tolerance to the combined treatment was assessed. Cross-tolerance did not develop between repeated vlPAG microinjections of morphine and fentanyl. Likewise, there was no evidence of cross-tolerance from morphine or fentanyl to the co-administration of morphine and fentanyl. Co-administration did not cause cross-tolerance to fentanyl. Cross-tolerance was only evident to morphine or morphine and fentanyl combined in rats pretreated with co-administration of low doses of morphine and fentanyl. This finding is consistent with the functionally selective signaling that has been reported for antinociception and tolerance after morphine and fentanyl binding to the mu-opioid receptor. This research supports the notion that combination therapy and opioid rotation may be useful clinical practices to decrease opioid tolerance and other side effects. PERSPECTIVE: This preclinical study shows that there is a decrease in cross-tolerance between morphine and fentanyl within the periaqueductal gray, which is a key brain region in opioid antinociception and tolerance.

Medication overuse headache following repeated morphine, but not [INCREMENT]9-tetrahydrocannabinol administration in the female rat

The potential of [INCREMENT]-tetrahydrocannabinol (THC) as a treatment for migraine depends on antinociceptive efficacy with repeated administration. Although morphine has good antinociceptive efficacy, repeated administration causes medication overuse headache (MOH) - a condition in which the intensity/frequency of migraine increases. The present study compared the effect of repeated morphine or THC administration on the magnitude and duration of migraine-like pain induced by a microinjection of allyl isothiocyanate (AITC) onto the dura mater of female rats. Acute administration of THC or morphine prevented AITC-induced depression of wheel running. This antinociception was maintained in rats treated repeatedly with THC, but not following repeated administration of morphine. Moreover, repeated morphine, but not THC administration, extended the duration of AITC-induced depression of wheel running. These data indicate that tolerance and MOH develop rapidly to morphine administration. The lack of tolerance and MOH to THC indicates that THC may be an especially effective long-term treatment against migraine.

Anti-migraine effect of ∆9-tetrahydrocannabinol in the female rat

Current anti-migraine treatments have limited efficacy and many side effects. Although anecdotal evidence suggests that marijuana is useful for migraine, this hypothesis has not been tested in a controlled experiment. Thus, the present study tested whether administration of ∆⁹-tetrahydrocannabinol (THC) produces anti-migraine effects in the female rat. Microinjection of the TRPA1 agonist allyl isothiocyanate (AITC) onto the dura mater produced migraine-like pain for 3h as measured by depression of home cage wheel running. Concurrent systemic administration of 0.32 but not 0.1mg/kg of THC prevented AITC-induced depression of wheel running. However, 0.32mg/kg was ineffective when administered 90min after AITC. Administration of a higher dose of THC (1.0mg/kg) depressed wheel running whether rats were injected with AITC or not. Administration of a CB₁, but not a CB₂, receptor antagonist attenuated the anti-migraine effect of THC. These data suggest that: 1) THC reduces migraine-like pain when administered at the right dose (0.32mg/kg) and time (immediately after AITC); 2) THC's anti-migraine effect is mediated by CB₁ receptors; and 3) Wheel running is an effective method to assess migraine treatments because only treatments producing antinociception without disruptive side effects will restore normal activity. These findings support anecdotal evidence for the use of cannabinoids as a treatment for migraine in humans and implicate the CB₁ receptor as a therapeutic target for migraine.

Depression of home cage wheel running is an objective measure of spontaneous morphine withdrawal in rats with and without persistent pain

Opioid withdrawal in humans is often subtle and almost always spontaneous. In contrast, most preclinical studies precipitate withdrawal by administration of an opioid receptor antagonist such as naloxone. These animal studies rely on measurement of physiological symptoms (e.g., wet dog shakes) in the period immediately following naloxone administration. To more closely model the human condition, we tested the hypothesis that depression of home cage wheel running will provide an objective method to measure the magnitude and duration of spontaneous morphine withdrawal. Rats were allowed access to a running wheel in their home cage for 8days prior to implantation of two 75mg morphine or placebo pellets. The pellets were removed 3 or 5days later to induce spontaneous withdrawal. In normal pain-free rats, removal of the morphine pellets depressed wheel running for 48h compared to rats that had placebo pellets removed. Morphine withdrawal-induced depression of wheel running was greatly enhanced in rats with persistent inflammatory pain induced by injection of Complete Freund's Adjuvant (CFA) into the hindpaw. Removal of the morphine pellets following 3days of treatment depressed wheel running in these rats for over 6days. These data demonstrate that home cage wheel running provides an objective and more clinically relevant method to assess spontaneous morphine withdrawal compared to precipitated withdrawal in laboratory rats. Moreover, the enhanced withdrawal in rats with persistent inflammatory pain suggests that pain patients may be especially susceptible to opioid withdrawal.

Depression of home cage wheel running: a reliable and clinically relevant method to assess migraine pain in rats

BACKGROUND

The development of new anti-migraine treatments is limited by the difficulty inassessing migraine pain in laboratory animals. Depression of activity is one of the few diagnostic criteria formigraine that can be mimicked in rats. The goal of the present study was to test the hypothesis thatdepression of home cage wheel running is a reliable and clinically relevant method to assess migraine painin rats.

METHODS

Adult female rats were implanted with a cannula to inject allyl isothiocyanate (AITC) onto the dura to induce migraine pain, as has been shown before. Rats recovered from implantation surgery for 8 days in cages containing a running wheel. Home cage wheel running was recorded 23 h a day. AITC and the migraine medication sumatriptan were administered in the hour prior to onset of the dark phase.

RESULTS

Administration of AITC caused a concentration-dependent decrease in wheel running that lasted 3 h. The duration and magnitude of AITC-induced depression of wheel running was consistent following three repeated injections spaced 48 h apart. Administration of sumatriptan attenuated AITC-induced depressionof wheel running when a large dose (1 mg/kg) was administered immediately following AITC administration. Wheel running patterns did not change when sumatriptan was given to naïve rats.

CONCLUSIONS

These data indicate that home cage wheel running is a sensitive, reliable, and clinically relevant method to assess migraine pain in the rat.

Analysis of inflammation-induced depression of home cage wheel running in rats reveals the difference between opioid antinociception and restoration of function

Opioids are effective at inhibiting responses to noxious stimuli in rodents, but have limited efficacy and many side effects in chronic pain patients. One reason for this disconnect is that nociception is typically assessed using withdrawal from noxious stimuli in animals, whereas chronic pain patients suffer from abnormal pain that disrupts normal activity. We hypothesized that assessment of home cage wheel running in rats would provide a much more clinically relevant method to assess opioid efficacy to restore normal behavior. Intraplantar injection of Complete Freund's Adjuvant (CFA) into the right hindpaw depressed wheel running and caused mechanical allodynia measured with the von Frey test in both male and female rats. Administration of an ED₅₀ dose of morphine (3.2mg/kg) reversed mechanical allodynia, but did not reverse CFA-induced depression of wheel running. In contrast, administration of a low dose of morphine (1.0mg/kg) restored running for one hour in both sexes, but had no effect on mechanical allodynia. Administration of the atypical opioid buprenorphine had no effect on inflammation-induced depression of wheel running in male or female rats, but attenuated mechanical allodynia in male rats. Administration of buprenorphine and higher doses of morphine depressed wheel running in non-inflamed rats, suggesting that the side effects of opioids interfere with restoration of function. These data indicate that restoration of pain-depressed function requires antinociception in the absence of disruptive side effects. The disruptive side effects of opioids are consistent with the major limitation of opioid use in human pain patients.

Relative contribution of the dorsal raphe nucleus and ventrolateral periaqueductal gray to morphine antinociception and tolerance in the rat

The dorsal raphe nucleus (DRN) is embedded in the ventral part of the caudal periaqueductal gray (PAG). Electrical or chemical activation of neurons throughout this region produces antinociception. The objective of this manuscript is to determine whether the ventrolateral PAG and DRN are distinct antinociceptive systems. This hypothesis was tested by determining the antinociceptive potency of microinjecting morphine into each structure (Experiment 1), creating a map of effective microinjection sites that produce antinociception (Experiment 2) and comparing the development of antinociceptive tolerance to repeated microinjections of morphine into the ventrolateral PAG and DRN (Experiment 3). Morphine was more potent following cumulative injections (1.0, 2.2, 4.6 & 10 μg/0.2 μL) into the ventrolateral PAG (D₅₀  = 3.3 μg) compared to the lateral (4.3 μg) or medial DRN (5.8 μg). Antinociception occurred following 94% of the morphine injections into the ventrolateral PAG, whereas only 68.3% and 78.3% of the injections into the lateral and medial aspects of the DRN produced antinociception. Repeated microinjections of morphine into the ventrolateral PAG produced tolerance as indicated by a 528% difference in potency between morphine and saline pretreated rats. In contrast, relatively small changes in potency occurred following repeated microinjections of morphine into the lateral and medial aspects of the DRN (107% and 49%, respectively). These data indicate that the ventrolateral PAG and DRN are distinct antinociceptive structures. Antinociception is greater with injections into the ventrolateral PAG compared to the DRN, but this antinociception disappears rapidly because of the development of tolerance.

Analysis of morphine-induced changes in the activity of periaqueductal gray neurons in the intact rat

Microinjection of morphine into the periaqueductal gray (PAG) produces antinociception. In vitro slice recordings indicate that all PAG neurons are sensitive to morphine either by direct inhibition or indirect disinhibition. We tested the hypothesis that all PAG neurons respond to opioids in vivo by examining the extracellular activity of PAG neurons recorded in lightly anesthetized and awake rats. Spontaneous activity was less than 1Hz in most neurons. Noxious stimuli (heat, pinch) caused an increase in activity in 57% and 75% of the neurons recorded in anesthetized and awake rats, respectively. The same noxious stimuli caused a decrease in activity in only 17% and 6% of neurons recorded in anesthetized and awake rats. Systemic administration of morphine caused approximately equal numbers of neurons to increase, decrease, or show no change in activity in lightly anesthetized rats. In contrast, administration of morphine caused an increase in the activity of 22 of the 27 neurons recorded in awake rats. No change in activity was evident in the remaining five neurons. Changes in activity caused by morphine were surprisingly modest (a median increase from 0.7 to 1.3Hz). The small inconsistent effects of morphine are in stark contrast to the large changes produced by morphine on the activity of rostral ventromedial medulla (RVM) neurons or the widespread inhibition and excitation of PAG neurons treated with opioids in in vitro slice experiments. The relatively modest effects of morphine in the present study suggest that morphine produces antinociception by causing small changes in the activity of many PAG neurons.

Home cage wheel running is an objective and clinically relevant method to assess inflammatory pain in male and female rats

BACKGROUND

The assessment of nociception in preclinical studies is undergoing a transformation from pain-evoked to pain-depressed tests to more closely mimic the effects of clinical pain. Many inflammatory pain-depressed behaviors (reward seeking, locomotion) have been examined, but these tests are limited because of confounds such as stress and difficulties in quantifying behavior.

NEW METHOD

The present study evaluates home cage wheel running as an objective method to assess the magnitude and duration of inflammatory pain in male and female rats.

RESULTS

Injection of Complete Freund's Adjuvant (CFA) into the right hindpaw to induce inflammatory pain almost completely inhibited wheel running for 2 days in male and female rats. Wheel running gradually returned to baseline levels within 12 days despite persistent mechanical hypersensitivity (von Frey test).

COMPARISON WITH EXISTING METHODS

Continuously monitoring home cage wheel running improves on previous studies examining inflammatory pain-depressed wheel running because it is more sensitive to noxious stimuli, avoids the stress of removing the rat from its cage for testing, and provides a complete analysis of the time course for changes in nociception.

CONCLUSIONS

The present data indicate that home cage wheel running is a clinically relevant method to assess inflammatory pain in the rat. The decrease in activity caused by inflammatory pain and subsequent gradual recovery mimics the changes in activity caused by pain in humans. The tendency for pain-depressed wheel running to be greater in female than male rats is consistent with the tendency for women to be at greater risk of chronic pain than men.

Functionally selective signaling for morphine and fentanyl antinociception and tolerance mediated by the rat periaqueductal gray

Functionally selective signaling appears to contribute to the variability in mechanisms that underlie tolerance to the antinociceptive effects of opioids. The present study tested this hypothesis by examining the contribution of G protein-coupled receptor kinase (GRK)/Protein kinase C (PKC) and C-Jun N-terminal kinase (JNK) activation on both the expression and development of tolerance to morphine and fentanyl microinjected into the ventrolateral periaqueductal gray of the rat. Microinjection of morphine or fentanyl into the periaqueductal gray produced a dose-dependent increase in hot plate latency. Microinjection of the non-specific GRK/PKC inhibitor Ro 32-0432 into the periaqueductal gray to block mu-opioid receptor phosphorylation enhanced the antinociceptive effect of morphine but had no effect on fentanyl antinociception. Microinjection of the JNK inhibitor SP600125 had no effect on morphine or fentanyl antinociception, but blocked the expression of tolerance to repeated morphine microinjections. In contrast, a microinjection of Ro 32-0432 blocked the expression of fentanyl, but not morphine tolerance. Repeated microinjections of Ro 32-0432 blocked the development of morphine tolerance and inhibited fentanyl antinociception whether rats were tolerant or not. Repeated microinjections of SP600125 into the periaqueductal gray blocked the development of tolerance to both morphine and fentanyl microinjections. These data demonstrate that the signaling molecules that contribute to tolerance vary depending on the opioid and methodology used to assess tolerance (expression vs. development of tolerance). This signaling difference is especially clear for the expression of tolerance in which JNK contributes to morphine tolerance and GRK/PKC contributes to fentanyl tolerance.

Change in functional selectivity of morphine with the development of antinociceptive tolerance

BACKGROUND AND PURPOSE

Opioids, such as morphine, are the most effective treatment for pain but their efficacy is diminished with the development of tolerance following repeated administration. Recently, we found that morphine activated ERK in opioid-tolerant but not in naïve rats, suggesting that morphine activation of μ-opioid receptors is altered following repeated morphine administration. Here, we have tested the hypothesis that μ-opioid receptor activation of ERK in the ventrolateral periaqueductal gray (vlPAG) is dependent on dynamin, a protein implicated in receptor endocytosis.

EXPERIMENTAL APPROACH

Rats were made tolerant to repeated microinjections of morphine into the vlPAG. The effects of dynamin on ERK activation and antinociception were assessed by microinjecting myristoylated dominant-negative dynamin peptide (Dyn-DN) or a scrambled control peptide into the vlPAG. Microinjection of a fluorescent dermorphin analogue (DERM-A594) into the vlPAG was used to monitor μ-opioid receptor internalization.

KEY RESULTS

Morphine did not activate ERK and Dyn-DN administration had no effect on morphine-induced antinociception in saline-pretreated rats. In contrast, morphine-induced ERK activation in morphine-pretreated rats that was blocked by Dyn-DN administration. Dyn-DN also inhibited morphine antinociception. Finally, morphine reduced DERM-A594 internalization only in morphine-tolerant rats indicating that μ-opioid receptors were internalized and unavailable to bind DERM-A594.

CONCLUSIONS AND IMPLICATIONS

Repeated morphine administration increased μ-opioid receptor activation of ERK signalling via a dynamin-dependent mechanism. These results demonstrate that the balance of agonist signalling to G-protein and dynamin-dependent pathways is altered, effectively changing the functional selectivity of the agonist-receptor complex.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Chronic inflammatory pain prevents tolerance to the antinociceptive effect of morphine microinjected into the ventrolateral periaqueductal gray of the rat

The ventrolateral periaqueductal gray (vlPAG) contributes to morphine antinociception and tolerance. Chronic inflammatory pain causes changes within the PAG that are expected to enhance morphine tolerance. This hypothesis was tested by assessing antinociception and tolerance following repeated microinjections of morphine into the vlPAG of rats with chronic inflammatory pain. Microinjection of morphine into the vlPAG reversed the allodynia caused by intraplantar administration of complete Freund's adjuvant and produced antinociception on the hot plate test. Although there was a gradual decrease in morphine antinociception with repeated testing, there was no evidence of tolerance when morphine- and saline-treated rats with hind paw inflammation were tested with cumulative doses of morphine. In contrast, repeated morphine injections into the vlPAG caused a rightward shift in the morphine dose-response curve in rats without hind paw inflammation, as would be expected with the development of tolerance. The lack of tolerance in complete Freund's adjuvant-treated rats was evident whether rats were exposed to repeated behavioral testing or not (experiment 2) and whether they were treated with 4 or 8 prior microinjections of morphine into the vlPAG (experiment 3). These data demonstrate that chronic inflammatory pain does not disrupt the antinociceptive effect of microinjecting morphine into the vlPAG, but it does disrupt the development of tolerance.

PERSPECTIVE

The present data show that induction of chronic inflammatory pain does not disrupt the antinociceptive effect of microinjecting morphine into the vlPAG, but it does attenuate the development of tolerance. This finding indicates that tolerance to opioids in rats with inflammatory pain is mediated by structures other than the vlPAG.

Columnar distribution of catecholaminergic neurons in the ventrolateral periaqueductal gray and their relationship to efferent pathways

The periaqueductal gray (PAG) is a critical brain region involved in opioid analgesia and provides efferents to descending pathways that modulate nociception. In addition, the PAG contains ascending pathways to regions involved in the regulation of reward, including the substantia nigra (SN) and the ventral tegmental area (VTA). SN and VTA contain dopaminergic neurons that are critical for the maintenance of positive reinforcement. Interestingly, the PAG is also reported to contain a population of dopaminergic neurons. In this study, the distribution of catecholaminergic neurons within the ventrolateral (vl) PAG was examined using immunocytochemical methods. In addition, the catecholaminergic PAG neurons were examined to determine whether these neurons are integrated into ascending (VTA, SN) and descending rostral ventral medulla (RVM) efferent pathways from this region. The immunocytochemical analysis determined that catecholaminergic neurons in the PAG are both dopaminergic and noradrenergic and these neurons have a distinct rostrocaudal distribution within the ventrolateral column of PAG. Dopaminergic neurons were concentrated rostrally and were significantly smaller than noradrenergic neurons. Combined immunocytochemistry and tract tracing methods revealed that catecholaminergic neurons are distinct from, but closely associated with, both ascending and descending efferent projection neurons. Finally, by electron microscopy, catecholaminergic neurons showed close dendritic appositions with other neurons in PAG, suggesting a possible nonsynaptic mechanism for regulation of PAG output by these neurons. In conclusion, our data indicate that there are two populations of catecholaminergic neurons in the vlPAG that form dendritic associations with both ascending and descending efferents suggesting a possible nonsynaptic modulation of vlPAG neurons.

The periaqueductal gray contributes to bidirectional enhancement of antinociception between morphine and cannabinoids

Co-administration of opioids and cannabinoids can enhance pain relief even when administered on different days. Repeated systemic administration of morphine has been shown to enhance the antinociceptive effect of tetrahydrocannabinol (THC) administered 12h later, and repeated microinjection of the cannabinoid receptor agonist HU-210 into the ventrolateral periaqueductal gray (PAG) has been shown to enhance the antinociceptive effect of morphine administered 1 day later. The primary objective of the present study was to test the hypothesis that this cannabinoid/opioid interaction is bidirectional. Experiment 1 showed that microinjection of morphine into the ventrolateral PAG of male Sprague-Dawley rats twice daily for 2 days enhanced the antinociceptive effect of HU-210 measured 1 day later. In Experiment 2, twice daily systemic injections of THC enhanced the antinociceptive effect of morphine administered 1 day later. These results complement the previously mentioned studies by showing that morphine and cannabinoid interactions are bidirectional and that the ventrolateral PAG plays an important role in this effect. In contrast to the PAG, repeated administration of HU-210 or the cannabinoid receptor agonist, WIN 55,212-2, into the RVM had a neurotoxic effect. Rats became ill following repeated cannabinoid administration whether given alone or with morphine. Presumably, this neurotoxic effect was caused by the high cannabinoid concentration following RVM microinjection because rats did not become ill following repeated systemic THC administration. These findings indicate that alternating opioid and cannabinoid treatment could produce a longer lasting and more potent analgesia than either compound given alone.

Differential development of antinociceptive tolerance to morphine and fentanyl is not linked to efficacy in the ventrolateral periaqueductal gray of the rat

Systemic administration of morphine typically produces greater tolerance than higher efficacy mu-opioid receptor (MOPr) agonists such as fentanyl. The objective of the present study was to test this relationship by measuring antinociceptive efficacy and tolerance to morphine and fentanyl microinjected into the ventrolateral periaqueductal gray (vlPAG). MOPr agonist efficacy was evaluated by microinjecting the irreversible opioid receptor antagonist β-funaltrexamine hydrochloride (β-FNA) into the vlPAG prior to a dose-response analysis of morphine and fentanyl antinociception. In contrast to systemic administration of morphine and fentanyl, microinjection of these drugs into the vlPAG had similar efficacy as measured by similar reductions in maximal antinociception following β-FNA administration. Analysis of tolerance revealed a rightward shift in the dose-response curve to a single pretreatment with morphine, but not fentanyl. The magnitude of tolerance to morphine was comparable following 1, 4, or 8 pretreatments. Tolerance to fentanyl also was evident following 4 or 8 microinjections. These data are surprising in that antinociceptive efficacy appears to vary depending on the site of administration. Moreover, the similar efficacy following microinjection of morphine and fentanyl into the vlPAG was associated with comparable tolerance, with the 1 exception of no tolerance to acute administration of fentanyl.

PERSPECTIVE

These data reveal that antinociceptive tolerance following vlPAG administration of opioids develops rapidly and is evident with both morphine and fentanyl, and the magnitude is relatively consistent regardless of the number of pretreatments.

Distribution of CB1 cannabinoid receptors and their relationship with mu-opioid receptors in the rat periaqueductal gray

The periaqueductal gray (PAG) is part of a descending pain modulatory system that, when activated, produces widespread and profound antinociception. Microinjection of either opioids or cannabinoids into the PAG elicits antinociception. Moreover, microinjection of the cannabinoid 1 (CB1) receptor agonist HU-210 into the PAG enhances the antinociceptive effect of subsequent morphine injections, indicating a direct relationship between these two systems. The objective of this study was to characterize the distribution of CB1 receptors in the dorsolateral and ventrolateral PAG in relationship to mu-opioid peptide (MOP) receptors. Immunocytochemical analysis revealed extensive and diffuse CB1 receptor labeling in the PAG, 60% of which was found in somatodendritic profiles. CB1 and MOP receptor immunolabeling were co-localized in 32% of fluorescent Nissl-stained cells that were analyzed. Eight percent (8%) of PAG neurons that were MOP receptor-immunoreactive (-ir) received CB1 receptor-ir appositions. Ultrastructural analysis confirmed the presence of CB1 receptor-ir somata, dendrites and axon terminals in the PAG. These results indicate that behavioral interactions between cannabinoids and opioids may be the result of cellular adaptations within PAG neurons co-expressing CB1 and MOP receptors.

Analysis of opioid efficacy, tolerance, addiction and dependence from cell culture to human

Opioid agonists are the most effective treatment for pain, but their use is limited by side effects, tolerance and fears of addiction and dependence. A major goal of opioid research is to develop agonists that have high analgesic efficacy and a low profile for side effects, tolerance, addiction and dependence. Unfortunately, there is a serious lack of experimental data comparing the degree to which different opioids produce these effects in humans. In contrast, a wide range of experimental techniques from heterologous expression systems to behaviour assessment in whole animals have been developed to study these problems. The objective of this review is to describe and evaluate these techniques as they are used to study opioid efficacy, tolerance, addiction and dependence.

Chronic psychostimulant exposure to adult, but not periadolescent rats reduces subsequent morphine antinociception

Preweanling methylphenidate (MPH) exposure produces a long lasting enhanced sensitivity to opioids. Two important questions are whether this enhancement is specific to the age of psychostimulant exposure and the type of psychostimulant. To answer these questions periadolescent (PD 35) and adult (PD 55) rats received daily injections of saline, MPH, or methamphetamine (METH) for 10 consecutive days. Two weeks later, acute morphine antinociception was assessed on the hot plate using a cumulative dose response procedure. Following acute antinociceptive testing, morphine tolerance was induced in half the animals by administering morphine twice a day over 2 days. Rats pretreated with MPH and METH during the periadolescent period of ontogeny showed no change in acute morphine antinociception, but rats exposed to a relatively high METH dose (3 mg/kg) displayed enhanced morphine tolerance compared to saline pretreated controls. MPH and METH pretreatment during adulthood led to a reduction in morphine antinociceptive potency and an apparent reduction in morphine tolerance. When combined with our previously published findings, these data indicate that the developmental stage during which MPH and METH exposure occurs differentially alters adult morphine responsiveness. That is, psychostimulant exposure to preweanling rats enhances morphine antinociception and facilitates the development of tolerance, whereas psychostimulant exposure to adult rats reduces subsequent morphine antinociception and tolerance. These alterations indicate that it could be important for physicians to know about prior psychostimulant use when prescribing opioids for pain relief.

Contribution of the rostral ventromedial medulla to post-anxiety induced hyperalgesia

Rats exposed to an elevated plus maze (EPM) with four open arms display antinociception while on the maze and hyperalgesia immediately upon removal. Little is known about the neural mechanisms underlying EPM-induced antinociception and the subsequent hyperalgesia except that the antinociception is not mediated by endogenous opioids. The objective of the present study was to test the hypothesis that endogenous cannabinoids and/or the rostral ventromedial medulla (RVM) contributes to EPM-induced antinociception. Administration of the CB1 receptor antagonist AM251 (1mg/kg, i.p.) had no effect on baseline nociception to formalin administration into the hindpaw or on the antinociception produced by placing a rat on the open EPM. Likewise, inactivation of the RVM by microinjecting the GABA(A) receptor agonist muscimol (10 ng/0.5 μL) had no effect on the antinociceptive effect of placing a rat in the EPM. However, RVM inactivation blocked the hyperalgesia produced upon removal from the EPM. Although distinct classes of RVM neurons inhibit and facilitate nociception, the present data demonstrate that the antinociception induced by the EPM and the subsequent hyperalgesia is mediated by distinct neural pathways.

Environmentally induced antinociception and hyperalgesia in rats and mice

Stress can enhance and inhibit nociception depending on the situation. Thus, simply shifting the context from the elevated plus maze (EPM) which has been shown to produce stress-induced antinociception to a different environment could produce drastic and rapid changes in nociception. The present experiment tested this hypothesis by assessing nociception in rats and mice during and immediately after removal from the maze. Experiment 1 found hyperalgesia in female and male rats tested on the hot plate immediately after exposure to the elevated plus maze. This hyperalgesia occurred with or without the added stress of a hind paw formalin injection and regardless of whether rats were exposed to an EPM with open (oEPM) or enclosed (eEPM) arms despite a clear antinociceptive effect while on the oEPM. Experiment 2 showed a similar shift from antinociception to nociception on the formalin test in mice immediately after removing them from the EPM. These data demonstrate that a mild stressor such as the EPM can produce both antinociception and hyperalgesia depending on the context. This shift from antinociception to hyperalgesia occurs rapidly and is evident in mice, male and female rats, and with the hot plate and formalin tests.

Tolerance to the antinociceptive effect of morphine in the absence of short-term presynaptic desensitization in rat periaqueductal gray neurons

Opioids activate the descending antinociceptive pathway from the ventrolateral periaqueductal gray (vlPAG) by both pre- and postsynaptic inhibition of tonically active GABAergic neurons (i.e., disinhibition). Previous research has shown that short-term desensitization of postsynaptic μ-opioid receptors (MOPrs) in the vlPAG is increased with the development of opioid tolerance. Given that pre- and postsynaptic MOPrs are coupled to different signaling mechanisms, the present study tested the hypothesis that short-term desensitization of presynaptic MOPrs also contributes to opioid tolerance. Twice-daily injections of morphine (5 mg/kg s.c.) for 2 days caused a rightward shift in the morphine dose-response curve on the hot plate test (D(50) = 9.9 mg/kg) compared with saline-pretreated (5.3 mg/kg) male Sprague-Dawley rats. In vitro whole-cell patch-clamp recordings from vlPAG slices revealed that inhibition of evoked inhibitory postsynaptic currents (eIPSCs) by the MOPr-selective agonist [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin was decreased in morphine-tolerant (EC(50) = 708 nM) compared with saline-pretreated rats (EC(50) = 163 nM). However, short-term desensitization of MOPr inhibition of eIPSCs was not observed in either saline- or morphine-pretreated rats. Reducing the number of available MOPrs with the irreversible opioid receptor antagonist, β-chlornaltrexamine decreased maximal MOPr inhibition with no evidence of desensitization, indicating that the lack of observed desensitization is not caused by receptor reserve. These results demonstrate that tolerance to the antinociceptive effect of morphine is associated with a decrease in presynaptic MOPr sensitivity or coupling to effectors, but this change is independent of short-term MOPr desensitization.

Primary structure of the (1-->3,1-->4)-beta-D-glucan 4-glucohydrolase from barley aleurone

During germination of barley grains, the cell walls of the starchy endosperm are degraded by (1-->3,1-->4)-beta-glucanases (EC 3.2.1.73) secreted from the aleurone and scutellar tissues. The complete sequence of the aleurone (1-->3,1-->4)-beta-glucanase isoenzyme II comprises 306 amino acids and was determined by sequencing nine tryptic peptides (110 residues) and aligning them with the amino acid sequence deduced from a cDNA clone encoding the 291 NH(2)-terminal residues. Although no amino acid sequence homology with a bacterial (1-->3)(1-->4)-beta-glucanase is apparent, close to 50% homology is found with two large regions of a (1-->3)-beta-glucanase from tobacco pith tissue. The gene for barley (1-->3,1-->4)-beta-glucanase isoenzyme II shares with that for the alpha-amylase isoenzyme 1 a strongly preferred use of codons with G and C in the wobble position (94% and 90%, respectively). Both enzymes are secreted from the aleurone cells during germination. Such one-sided codon usage is not characteristic for the gene encoding the (1-->3)-beta-glucanase of tobacco pith tissue or the hor2-4 gene encoding the B(1) hordein storage protein in the endosperm.

Drug dependent sex-differences in periaqueducatal gray mediated antinociception in the rat

Mu-opioid receptor (MOPr) agonists, such as morphine, produce greater antinociception in male compared to female rats. The ventolateral periaqueductal gray (vlPAG) appears to contribute to this sex-difference despite fewer vlPAG output neurons projecting to the rostral ventromedial medulla in male compared to female rats. This greater projection in female rats suggests that non-opioid activation of vlPAG output neurons should produce greater antinociception in female compared to male rats. This hypothesis was tested by comparing the time course and antinociceptive potency of microinjecting MOPr agonists (morphine, DAMGO, fentanyl) and non-opioid compounds (bicuculline, kainic acid) into the vlPAG of female and male rats. Microinjection of morphine or DAMGO produced antinociception that had a slow onset (peak from 15 to 30min) and long duration (60min) compared to the antinociception produced following microinjection of fentanyl, bicuculline, or kainic acid (peak effect at 3min; duration less than 30min). No sex-differences in the time courses were evident. All five compounds caused a dose-dependent antinociception when microinjected into the vlPAG. Antinociceptive potency was significantly greater in male compared to female rats following microinjection of morphine, DAMGO, and bicuculline, but not following microinjection of fentanyl or kainic acid. In no case did activation of the vlPAG produce greater antinocicepiton in female compared to male rats. These findings demonstrate that the vlPAG can produce comparable antinociception in female and male rats, but antinociception produced by inhibition of GABAergic neurons (whether by morphine or the GABA(A) receptor antagonist bicuculline) produces greater antinociception in males.

Extracellular signal-regulated kinase 1/2 activation counteracts morphine tolerance in the periaqueductal gray of the rat

Repeated administration of opioids produces long-lasting changes in micro-opioid receptor (MOR) signaling that underlie behavioral changes such as tolerance. Mitogen-activated protein kinase (MAPK) pathways, including MAPK extracellular signal-regulated kinases (ERK1/2), are modulated by opioids and are known to produce long-lasting changes in cell signaling. Thus, we tested the hypothesis that ERK1/2 activation contributes to the development and/or expression of morphine tolerance mediated by the periaqueductal gray (PAG). Changes in phosphorylated ERK1/2 expression were assessed with confocal microscopy and compared to behavioral measures of tolerance to the antinociceptive effects of chronic morphine administration. Repeated microinjection of morphine into the PAG produced tolerance and caused a significant increase in ERK1/2 phosphorylation, an effect not evident with acute morphine microinjection. Microinjection of the MAPK/ERK kinase inhibitor, 1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene ethanolate (U0126), into the PAG had no effect on antinociception produced by acute morphine administration. However, repeated coadministration of U0126 and morphine into the PAG blocked ERK1/2 phosphorylation and enhanced the development of morphine tolerance. Coadministration of U0126 with morphine only on the test day also enhanced the expression of morphine tolerance. Administration of the irreversible opioid receptor antagonist beta-chlornaltrexamine blocked the activation of ERK1/2 caused by repeated morphine microinjections, demonstrating that ERK1/2 activation was a MOR-mediated event. In summary, these studies show that chronic morphine administration alters ERK1/2 signaling and that disruption of ERK1/2 signaling enhances both the development and expression of morphine tolerance. Contrary to expectations, these data indicate that ERK1/2 activation opposes the development of morphine tolerance.

Glutamate modulation of antinociception, but not tolerance, produced by morphine microinjection into the periaqueductal gray of the rat

The periaqueductal gray (PAG) plays an important role in morphine antinociception and tolerance. Co-localization of mu-opioid and NMDA receptors on dendrites in the PAG suggests that glutamate may modulate morphine antinociception. Moreover, the involvement of glutamate in spinally mediated tolerance to morphine suggests that glutamate receptors may contribute to PAG mediated tolerance. These hypotheses were tested by microinjecting glutamate receptor antagonists and morphine into the ventrolateral PAG (vPAG) of the rat. Microinjection of the non-specific glutamate receptor antagonist kynurenic acid or the NMDA receptor antagonist MK-801 into the vPAG did not affect nociception. However, co-administration of these antagonists with morphine into the vPAG enhanced the acute antinociceptive effects of morphine as measured by a leftward shift in the morphine dose-response curves. Repeated microinjections of morphine into the vPAG caused a rightward shift in the dose-response curve for antinociception whether the glutamate receptor antagonists kynurenic acid or MK-801 were co-administered or not. The lack of effect of microinjecting glutamate receptor antagonists into the vPAG indicates that tonic glutamate release in the PAG does not contribute to nociceptive tone. That these antagonists enhance morphine antinociception indicates that endogenous glutamate counteracts the antinociceptive effect of morphine in the vPAG. However, this compensatory glutamate release does not contribute to tolerance to the antinociceptive effects of microinjecting morphine into the vPAG. Previous research showing that glutamate contributes to spinal mechanisms of tolerance indicate that different tolerance mechanisms are engaged in the vPAG and spinal cord.

Early methylphenidate exposure enhances morphine antinociception and tolerance in adult rats

Methylphenidate (MPH) is often used to reduce the symptoms of Attention-Deficit/Hyperactivity Disorder. Early MPH treatment in rats has been shown to enhance adult morphine-induced antinociception. Although this enhanced antinociception could improve pain treatment, it could also lead to enhanced tolerance to morphine. This hypothesis was tested by examining the effects of MPH administration during the pre-weanling period on morphine-induced antinociception and tolerance in adulthood. Male and female Sprague-Dawley rats received daily IP injections of saline or MPH (2 or 5 mg/kg) for 10 consecutive days beginning on post-natal day (PD) 11. At 60 days of age, morphine (0, 1.8, 3.2, 5.6, 10.0, and 18 mg/kg) antinociception was assessed. Beginning one day later, rats received two daily injections of either saline or morphine (5 mg/kg) for two consecutive days to induce tolerance. On PD 63 cumulative doses of morphine were administered as before to assess the development of tolerance. Rats pretreated with MPH showed enhanced acute morphine antinociception compared to saline pretreated controls. In addition, tolerance to morphine was greater in rats pretreated with MPH early in life. The magnitude of this decrease in morphine potency was dependent on the dose of MPH, such that animals that received 5 mg/kg of MPH from PD 11 to 20 showed the greatest tolerance. These findings demonstrate that MPH exposure during the pre-weanling period has long-lasting effects that include enhanced morphine antinociception and tolerance.

Periaqueductal gray neurons project to spinally projecting GABAergic neurons in the rostral ventromedial medulla

The analgesic effects of morphine are mediated, in part, by periaqueductal gray (PAG) neurons that project to the rostral ventromedial medulla (RVM). Although much of the neural circuitry within the RVM has been described, the relationship between RVM neurons and PAG input and spinal output is not known. The objective of this study was to determine whether GABAergic output neurons from the PAG target RVM reticulospinal neurons. Immunocytochemistry and confocal microscopy revealed that PAG neurons project extensively to RVM neurons projecting to the spinal cord, and two-thirds of these reticulospinal neurons appear to be GABAergic (contain GAD67 immunoreactivity). The majority (71%) of PAG fibers that contact RVM reticulospinal GAD67-immunoreactive neurons also contained GAD67 immunoreactivity. Thus, there is an inhibitory projection from PAG to inhibitory RVM reticulospinal neurons. However, there were also PAG projections to the RVM that did not contain GAD67 immunoreactivity. Additional experiments were conducted to determine whether the heterogeneity in this projection can be explained by the electrophysiological character of the RVM target neurons. PAG projections to electrophysiologically defined and juxtacellularly filled ON, OFF, and Neutral cells in the RVM were examined. Similar to the pattern reported above, both GAD67- and non-GAD67-immunoreactive PAG neurons project to RVM ON, OFF, and Neutral cells in the RVM. These inputs include a GAD67-immunoreactive projection to a GAD67-immunoreactive ON cell and non-GAD67 projections to GAD67-immunoreactive OFF cells. This pattern is consistent with PAG neurons producing antinociception by direct excitation of RVM OFF cells and inhibition of ON cells.

Sexually dimorphic activation of the periaqueductal gray-rostral ventromedial medullary circuit during the development of tolerance to morphine in the rat

The midbrain periaqueductal gray (PAG) and its descending projections to the rostral ventromedial medulla (RVM) provides an essential neural circuit for the antinociceptive effects of opiates, and has been implicated in the development of tolerance to morphine. Systemic morphine activates a greater proportion of PAG-RVM neurons in male vs female rats, and induces tolerance to a greater degree in males. The present studies tested the hypothesis that if the PAG-RVM pathway is essential for the development of tolerance, then: (i) morphine activation of the PAG-RVM pathway should decline as tolerance develops; and (ii) sex differences in the development of tolerance to morphine should be reflected as a greater decline in the activation of this pathway in males. These hypotheses were tested in male and female rats using behavioral testing (hot-plate) and immunohistochemistry to map the activation of the PAG-RVM pathway following repeated morphine administration (5 mg/kg; s.c.). In males, morphine potency decreased from 3.0 to 6.3 mg/kg, indicating tolerance, and this was paralleled by a steady decline in the percentage of PAG-RVM output neurons activated by morphine. In contrast, in females the shift in morphine potency was significantly attenuated (D(50) 6-8.3 mg/kg), and no significant difference in the activity of PAG-RVM output neurons was noted. These results demonstrate that the greater development of tolerance to morphine administration in male rats corresponds with a significant reduction in the activation of the PAG-RVM circuit and suggest a central role for the PAG in the development of tolerance to morphine.

Microinjection of the vehicle dimethyl sulfoxide (DMSO) into the periaqueductal gray modulates morphine antinociception

Dimethyl sulfoxide (DMSO) is commonly used as a solvent for water-insoluble drugs. Given that DMSO has varying cellular and behavioral effects ranging from increased membrane permeability to toxicity, microinjection of DMSO as a vehicle could confound the effects of other drugs. For example, DMSO is often used as a vehicle for studies examining the neurochemical mechanisms underlying morphine antinociception. Given that the ventrolateral periaqueductal gray (vlPAG) plays a major role in morphine antinociception and tolerance, the effects of DMSO on morphine antinociception mediated by the vlPAG needs to be evaluated. The present experiment tested whether co-administration of DMSO (0, 0.2, 2, or 20%) would alter the antinociceptive effect of microinjecting morphine into the vlPAG. DMSO had no effect on nociception when microinjected into the vlPAG alone, but 2% DMSO enhanced morphine potency when co-administered with morphine. In contrast, twice daily microinjections of DMSO (5 or 20%) for two days reduced the potency of subsequent microinjections of morphine into the vlPAG--an effect that persisted for at least one week. A similar rightward shift in the morphine dose-response curve was caused by morphine tolerance. Co-administration of morphine and DMSO during the pretreatment did not cause a greater shift in the morphine dose-response curve compared to morphine pretreated alone. In conclusion, DMSO can alter morphine antinociception following both acute (enhancement) and chronic (inhibition) administration depending on the concentration. These data reinforce the need to be cautious when using DMSO as a vehicle for drug administration.

Analgesic tolerance to microinjection of the micro-opioid agonist DAMGO into the ventrolateral periaqueductal gray

Repeated administration of the relatively low-efficacy micro-opioid receptor agonist morphine induces tolerance to its antinociceptive effects. High-efficacy agonists such as D-Ala2NMePhe4,Gly-ol5 (DAMGO) have been shown to be less effective at producing tolerance, suggesting that different neural mechanisms underlie tolerance to these agonists. However, the correlation between agonist efficacy and tolerance development has not been examined within the ventrolateral periaqueductal gray (vPAG), a brain area known to be crucial for the development of morphine tolerance. The current studies examined whether tolerance to DAMGO occurs within the vPAG, and whether repeated treatment with DAMGO into the vPAG alters the development of morphine tolerance. The results showed that repeated vPAG microinjections of DAMGO induced robust tolerance and cross-tolerance to morphine. Further, co-administration of a low dose of DAMGO with morphine potentiated morphine tolerance. These findings indicate that similar mechanisms underlie tolerance to morphine and DAMGO within the vPAG.

Morphine preferentially activates the periaqueductal gray-rostral ventromedial medullary pathway in the male rat: a potential mechanism for sex differences in antinociception

The midbrain periaqueductal gray (PAG), and its descending projections to the rostral ventromedial medulla (RVM), provide an essential neural circuit for opioid-produced antinociception. Recent anatomical studies have reported that the projections from the PAG to the RVM are sexually dimorphic and that systemic administration of morphine significantly suppresses pain-induced activation of the PAG in male but not female rats. Given that morphine antinociception is produced in part by disinhibition of PAG output neurons, it is hypothesized that a differential activation of PAG output neurons mediates the sexually dimorphic actions of morphine. The present study examined systemic morphine-induced activation of PAG-RVM neurons in the absence of pain. The retrograde tracer Fluorogold (FG) was injected into the RVM to label PAG-RVM output neurons. Activation of PAG neurons was determined by quantifying the number of Fos-positive neurons 1 h following systemic morphine administration (4.5 mg/kg). Morphine produced comparable activation of the PAG in both male and female rats, with no significant differences in either the quantitative or qualitative distribution of Fos. While microinjection of FG into the RVM labeled significantly more PAG output neurons in female rats than male rats, very few of these neurons (20%) were activated by systemic morphine administration in comparison to males (50%). The absolute number of PAG-RVM neurons activated by morphine was also greater in males. These data demonstrate widespread disinhibition of PAG neurons following morphine administration. The greater morphine-induced activation of PAG output neurons in male compared with female rats is consistent with the greater morphine-induced antinociception observed in males.

Behavioral and electrophysiological evidence for opioid tolerance in adolescent rats

Morphine and other opiates are successful treatments for pain, but their usefulness is limited by the development of tolerance. Given that recent studies have observed differential sensitivity to drugs of abuse in adolescents, the aim of this study was to assess antinociceptive tolerance to morphine in adolescent rats using both behavioral and cellular measures. Early (28-35 days postnatal) and late (50-59 days) adolescent and adult (73-75 days) male rats were injected with morphine (5 mg/kg, s.c.) or saline twice a day for two consecutive days. On Day 3, tolerance to morphine was evident in morphine-pretreated rats when tested on the hot plate test. Although baseline latencies for the early compared to late adolescent rats were faster, the magnitude of the shift in ED(50) for morphine was similar for the two adolescent groups. However, the shift in ED(50) tended to be greater in adolescent compared to adult rats. Subsequent to behavioral testing, whole cell patch-clamp recordings were made from ventrolateral PAG neurons. The opioid agonist, met-enkephalin (ME), activated similar outward currents in PAG neurons of early and late adolescent rats. However, reversal potentials of ME-induced currents were shifted to more hyperpolarized potentials in cells from morphine-pretreated rats. In addition, ME induced larger currents in morphine-pretreated rats with faster hot plate latencies compared to the mean (more tolerant) than in rats with slower latencies. These results indicate that repeated intermittent administration of morphine produces tolerance in adolescent rats that is associated with novel changes in opioid-sensitive ventrolateral PAG neurons.