Expression of the mu-opioid receptor in the anterior pituitary gland is influenced by age and sex

The opioid peptide leucine enkephalin modulates hypothalamic-hypophysial axis in the cichlid fish Oreochromis mossambicus

In vertebrates, opioid peptides are thought to be involved in the regulation of reproduction; however, the significance of enkephalins in testicular function remains unclear. We examined the influence of δ-opioid receptor agonist leucine enkephalin (L-ENK) on the hypophysial-testicular axis of the cichlid fish Oreochromis mossambicus. Treatment with a low dose of L-ENK (60 µg) caused a significant increase in the numbers of primary and secondary spermatocytes and early and late spermatids, concomitant with intense immunolabelling of testicular androgen receptors, but did not significantly alter serum luteinizing hormone (LH) and 11-ketotestosterone (11-KT) levels compared to those of controls. Nevertheless, treatment with a high dose of L-ENK (200 µg) caused a significant reduction in the numbers of secondary spermatocytes as well as late spermatids associated with marginal immunolabelling of androgen receptors and significantly lower concentrations of serum 11-KT and LH compared to controls. In addition, the serum cortisol level was not affected in low-dose L-ENK-treated fish, but its level was significantly increased in the high-dose L-ENK-treated group. Together, these findings indicate that a low dose of L-ENK stimulates the germ cells at the meiosis stage and promotes further stages of spermatogenesis, whereas a high concentration of L-ENK inhibits spermatogenesis at the advanced stages. This effect appears to be mediated through the suppression of testicular steroidogenesis and the reduction of LH release in the pituitary gland of tilapia. The findings also suggest that elevated L-ENK levels in teleosts may exert their inhibitory influence on the hypophysial-testicular axis via glucocorticoids.

Evaluating the expression pattern of the opioid receptor in pituitary neuroendocrine tumors (PitNET) and the role of morphine and naloxone in the regulation of pituitary cell line growth and apoptosis

PURPOSE

The expression pattern of the opioid receptor (MOR) in pituitary neuroendocrine tumors (PitNET) and the possible effect of morphine and naloxone on GH3 cell growth and apoptosis were evaluated.

METHODS

The 114 pituitary tissues including non-functioning, GH-producing and ACTH-producing PitNET and healthy cadaver pituitary tissues were included. The expression level of the MOR gene and protein was assessed using real-time PCR and Western blot. The association with patient demographic characteristics was assessed. Morphine and naloxone were applied to assess their possible pharmacological role in GH3 pituitary adenoma cell death. The cytotoxic effect, the apoptosis rate, the cell cycle distribution, the content of reactive oxygen species and the caspase 3 activity were measured.

RESULTS

MOR gene levels increased significantly in pituitary neuroendocrine tumors (PitNET) compared to the healthy pituitary samples. The increased level of MOR gene expression was prominent in invasive functional and non-functional pituitary tumors. A consistent expression pattern was demonstrated for MOR protein levels in PitNET samples. A dose- and time-dependent reduction in the rate of GH3 pituitary cells was observed after morphine treatment with an IC50 of 483 µM after 24 h of incubation. Morphine induced early apoptosis, accumulation of cells in sub-G1 phase, increase in cellular ROS levels and caspase-3 activity. The observed effects of morphine were reversed after MOR blockade using 10 and 25 µM naloxone.

CONCLUSION

The possible contributing role of the MOR in pituitary tumor cell growth and the putative pharmaceutical effect of morphine in pituitary neuroendocrine tumor cell death (PitNET) is illustrated.

Development of anxiety-like behaviors during adolescence: Persistent effects of adolescent morphine exposure in male rats

Epidemiological studies show the prevalence of opioid use, misuse and abuse in adolescents, which imposes social and economic accountability worldwide. Chronic opioid exposure, especially in adolescents, may have lasting effects on emotional behaviors that persist into adulthood. The current experiments were therefore designed to study the effects of sustained opioid exposure during adolescence on anxiety-like behaviors. Adolescent male Wistar rats underwent increasing doses of morphine for 10 days (PNDs 31-40). After that the open field test (OFT) and elevated plus maze (EPM) test were performed over a 4-week postmorphine treatment from adolescence to adulthood. Moreover, the weight of the animals was measured at these time points. We found that chronic adolescent morphine exposure reduces the weight gain during the period of morphine treatment and 4 weeks after that. It had no significant effect on the locomotor activity in the animals. Moreover, anxiolytic-like behavior was observed in the rats exposed to morphine during adolescence evaluated by OFT and EPM test. Thus, long-term exposure to morphine during adolescence has the profound potential of altering the anxiety-like behavior profile in the period from adolescence to adulthood. The maturation of the nervous system can be affected by drug abuse during the developmental window of adolescence and these effects may lead to behaviorally stable alterations.

Opioid-Induced Sexual Dysfunction in Cancer Patients

Simple Summary

Sexual disorders affect up to 80% of cancer patients, depending on the type of cancer, yet they are commonly overlooked and untreated. Opioid-induced sexual dysfunction (OISD) is reported in half of opioid users. The pathophysiology of OISD—still a subject for research—may include disorders of both the endocrine and nervous systems, expressed in, among other things, erectile dysfunction and declined sexual desire, sexual arousal, orgasm, and general satisfaction with one’s sex life. The etiology of sexual dysfunction in cancer patients is usually multifactorial, so the management should be multifaceted and individualized by targeting pathophysiological factors. The treatment options for OISD are few and include testosterone replacement therapy, bupropion, opioid antagonists, phosphodiesterase type 5 inhibitors, plant-derived substances, and non-pharmacological treatments, although the evidence is insufficient. One of the treatment options may also be a choice of an opioid that is less likely to cause sexual dysfunction, yet further research is necessary.

Abstract

Sexual dysfunction is common in patients with advanced cancer, although it is frequently belittled, and thus consistently underdiagnosed and untreated. Opioid analgesics remain fundamental and are widely used in cancer pain treatment. However, they affect sexual functions primarily due to their action on the hypothalamus–pituitary–gonadal axis. Other mechanisms such as the impact on the central and peripheral nervous systems are also possible. The opioid-induced sexual dysfunction includes erectile dysfunction, lack of desire and arousal, orgasmic disorder, and lowered overall sexual satisfaction. Around half of the individuals taking opioids chronically may be affected by sexual dysfunction. The relative risk of sexual dysfunction in patients on chronic opioid therapy and opioid addicts increased two-fold in a large meta-analysis. Opioids differ in their potential to induce sexual dysfunctions. Partial agonists and short-acting opioids may likely cause sexual dysfunction to a lesser extent. Few pharmaceutical therapies proved effective: testosterone replacement therapy, PDE5 inhibitors, bupropion, trazodone, opioid antagonists, and plant-derived medicines such as Rosa damascena and ginseng. Non-pharmacological options, such as psychosexual or physical therapies, should also be considered. However, the evidence is scarce and projected primarily from non-cancer populations, including opioid addicts. Further research is necessary to explore the problem of sexuality in cancer patients and the role of opioids in inducing sexual dysfunction.

The effect of methadone on the hypothalamic pituitary gonadal axis and sexual function: A systematic review

BACKGROUND

Opioid abuse is a public health crisis. As opioid misuse worsens, efforts are being made to increase access to medication-assisted treatments. Methadone is a medication-assisted treatment used to treat opioid dependence and chronic pain. While methadone is beneficial in the treatment of opiate abuse and chronic pain, side effects of the medication include hormonal and sexual function changes. The purpose of this report is to review the effects of methadone on the hypothalamic pituitary gonadal axis hormones and sexual functioning in males and females.

METHODS

A search of PubMed was conducted using pre-defined criteria, resulting in the evaluation of 295 articles. A total of 72 articles, including 52 human studies and 20 animal studies, met the selection criteria and were reviewed. The included studies examined the effects of methadone on the hypothalamic pituitary gonadal axis and/or sexual function.

RESULTS

There was evidence of methadone-induced hormonal changes, disruptions in the hypothalamic pituitary gonadal axis, and sexual dysfunction, although there was some variability in the results of the reviewed studies. Differences in methadone dose and length of exposure to treatment appears to influence the variability in the results. Much of the literature examines the effects of methadone in males, with very limited research examining the effects in females.

CONCLUSIONS

Despite its effectiveness for opiate abuse and chronic pain treatment, methadone has disruptive effects on the hypothalamic pituitary gonadal axis and sexual function. Further research is warranted to better define potential methadone-induced endocrine consequences and to further examine the effects of methadone in females.

Opioid induced adrenal insufficiency: what is new?

PURPOSE OF REVIEW

Despite the declaration of an opioid epidemic, opioid use remains prevalent. Side-effects of chronic opioid use continue to be problematic. Opioid-induced endocrinopathies have been well documented, yet opioid-induced adrenal insufficiency (OIAI) remains underappreciated. This review summarizes what is currently known regarding the prevalence, predictive factors for the development and effect of treatment of OIAI.

RECENT FINDINGS

Although several case reports have highlighted the development of adrenal crisis among those receiving chronic opioids, only a few studies have systematically assessed patients for OIAI. The heterogeneity of these small studies presents challenges when trying to assess prevalence of or potential risk factors for OIAI. The estimated prevalence of OIAI among those treated with chronic opioids ranges from 8.3 to 29% and is more likely in those receiving higher doses of opioids. Reduced health-related quality of life variables and altered pain perception has been associated with lower cortisol levels; however, the effect of glucocorticoid replacement on the parameters remains unknown.

SUMMARY

Further research is critical to better identify those at greatest risk and guide optimal management of OIAI. Frontline providers should remain vigilant for possibility of OIAI among chronic opioid users.

Gi/o-Protein Coupled Receptors in the Aging Brain

Cells translate extracellular signals to regulate processes such as differentiation, metabolism and proliferation, via transmembranar receptors. G protein-coupled receptors (GPCRs) belong to the largest family of transmembrane receptors, with over 800 members in the human species. Given the variety of key physiological functions regulated by GPCRs, these are main targets of existing drugs. During normal aging, alterations in the expression and activity of GPCRs have been observed. The central nervous system (CNS) is particularly affected by these alterations, which results in decreased brain functions, impaired neuroregeneration, and increased vulnerability to neuropathologies, such as Alzheimer's and Parkinson diseases. GPCRs signal via heterotrimeric G proteins, such as G_o, the most abundant heterotrimeric G protein in CNS. We here review age-induced effects of GPCR signaling via the G_i/o subfamily at the CNS. During the aging process, a reduction in protein density is observed for almost half of the G_i/o-coupled GPCRs, particularly in age-vulnerable regions such as the frontal cortex, hippocampus, substantia nigra and striatum. G_i/o levels also tend to decrease with aging, particularly in regions such as the frontal cortex. Alterations in the expression and activity of GPCRs and coupled G proteins result from altered proteostasis, peroxidation of membranar lipids and age-associated neuronal degeneration and death, and have impact on aging hallmarks and age-related neuropathologies. Further, due to oligomerization of GPCRs at the membrane and their cooperative signaling, down-regulation of a specific G_i/o-coupled GPCR may affect signaling and drug targeting of other types/subtypes of GPCRs with which it dimerizes. G_i/o-coupled GPCRs receptorsomes are thus the focus of more effective therapeutic drugs aiming to prevent or revert the decline in brain functions and increased risk of neuropathologies at advanced ages.

Sex-specific enhancement of palatability-driven feeding in adolescent rats

It has been hypothesized that brain development during adolescence perturbs reward processing in a way that may ultimately contribute to the risky decision making associated with this stage of life, particularly in young males. To investigate potential reward dysfunction during adolescence, Experiment 1 examined palatable fluid intake in rats as a function of age and sex. During a series of twice-weekly test sessions, non-food-deprived rats were given the opportunity to voluntarily consume a highly palatable sweetened condensed milk (SCM) solution. We found that adolescent male, but not female, rats exhibited a pronounced, transient increase in SCM intake (normalized by body weight) that was centered around puberty. Additionally, adult females consumed more SCM than adult males and adolescent females. Using a well-established analytical framework to parse the influences of reward palatability and satiety on the temporal structure of feeding behavior, we found that palatability-driven intake at the outset of the meal was significantly elevated in adolescent males, relative to the other groups. Furthermore, although we found that there were some group differences in the onset of satiety, they were unlikely to contribute to differences in intake. Experiment 2 confirmed that adolescent male rats exhibit elevated palatable fluid consumption, relative to adult males, even when a non-caloric saccharin solution was used as the taste stimulus, demonstrating that these results were unlikely to be related to age-related differences in metabolic need. These findings suggest that elevated palatable food intake during adolescence is sex specific and driven by a fundamental change in reward processing. As adolescent risk taking has been hypothesized as a potential result of hypersensitivity to and overvaluation of appetitive stimuli, individual differences in reward palatability may factor into individual differences in adolescent risky decision making.

Sex Differences in μ-Opioid Receptor Regulation of the Rat Locus Coeruleus and Their Cognitive Consequences

Stress-related neuropsychiatric pathologies are more prevalent in females compared with males. An important component of the stress response is activation of the locus coeruleus (LC)-norepinephrine system. Because LC activation is tempered by endogenous opioid release during stress, the magnitude of opioid regulation of the LC could determine stress vulnerability. Here we report convergent evidence for decreased μ-opioid receptor (MOR) function in the female rat LC. The selective MOR agonist, DAMGO (10 pg), completely inhibited LC discharge of male but not female rats and DAMGO (30 pg) produced no further inhibition of female LC neurons. Consistent with a decreased maximum DAMGO response, MOR protein and mRNA expression were decreased in female compared with male LC. These molecular and cellular sex differences were associated with sexually distinct effects of LC-MOR activation on cognitive processing in an operant strategy-shifting task. Although DAMGO (10 pg intra-LC) increased the number of trials to reach criterion for both sexes, it increased the duration to complete the task and the total number of errors selectively in males. Specifically, DAMGO increased premature responses, regressive errors, and random errors in males and perseverative errors in females. The sexually distinct cognitive consequences of activating LC-MOR may contribute to sex differences in opioid abuse patterns and may guide sex-specific therapies. Finally, given evidence that endogenous opioids restrain stress-induced LC activation and promote recovery of activity to pre-stress levels, decreased MOR function in the female LC could contribute to LC-NE overactivity that underlies the hyperarousal symptoms of stress-related psychiatric diseases.

Sex differences in peripheral mu-opioid receptor mediated analgesia in rat orofacial persistent pain model

Unilateral ligation of the tendon of anterior superficial part of rat masseter muscle (TASM) leads to long-lasting allodynia. Sex differences in peripheral mu-opioid receptor (MOR)-mediated analgesia under persistent myogenic pain are not well understood. In this study, we examined (1) whether locally applied MOR agonists attenuate persistent pain following TASM ligation in a sex dependent manner, (2) whether there are sex differences of MOR expression changes in rat trigeminal ganglia (TG). The effects of MOR agonist, D-Ala2, N-Me-Phe4, Gly5-ol]-Enkephalin acetate salt (DAMGO), were assessed 14 days after TASM ligation in male, female and orchidectomized (GDX) male rats. MOR mRNA and protein levels in TG 14 days following tendon ligation were also determined. The mechanical thresholds of the injured side were significantly decreased in both male and female rats, from 3 days to 28 days after TASM ligation. A10 μg DAMGO significantly attenuated allodynia in male rats. A 10-fold higher dose of DAMGO was required in female and GDX male rats to produce the level of anti- allodynia achieved in male rats. The level of MOR mRNA in TG from male rats was significantly greater 14 days after TASM ligation compared with the sham-operated male rats, but not from female and GDX male rats. After TASM ligation, males had significantly more MOR immunoreactivity in TG compared to sham-operated males. The MOR levels increased to 181.8% of the sham level in male rats receiving tendon injury. But there was no significant change in female rats receiving tendon injury compared to the sham female rats. Taken together, our data suggest that there were sex differences in the effects of peripheral MOR agonists between male and female rats under TASM ligation developing long-lasting pain condition, which is partly mediated by sex differences in the changes of MOR expressions and testosterone is an important factor in the regulation of MOR.

Sex differences in μ-opioid receptor expression in trigeminal ganglia under a myositis condition in rats

BACKGROUND

Peripheral opioid receptor expression is up-regulated under inflammatory conditions, which leads to the increased efficacy of peripherally administered opioids. Sex differences in the effects of inflammation, cytokines and gonadal hormones on μ-opioid receptor (MOR) expression in trigeminal ganglia (TG) are not well understood.

METHODS

MOR mRNA and protein levels in TG from male and female Sprague Dawley rats following complete Freund's adjuvant (CFA)-induced muscle inflammation were assessed. Cytokine-induced changes in MOR mRNA expression from TG cultures prepared from intact and gonadectomized male and female, and gonadectomized male rats with testosterone replacement were examined. Behavioural experiments were then performed to examine the efficacy of a peripherally administered MOR agonist in male, female and gonadectomized male rats under a myositis condition.

RESULTS

CFA and cytokine treatments induced significant up-regulation of MOR expression in TG from male, but not from female, rats. The cytokine-induced up-regulation of MOR mRNA expression was prevented in TG from orchidectomized (GDX) male rats, which was restored with testosterone replacement. Peripherally administered DAMGO, a specific MOR agonist, significantly attenuated CFA-induced masseter mechanical hypersensitivity only in intact male rats.

CONCLUSIONS

Collectively, these data indicate that testosterone plays a key role in the regulation of MOR in TG under inflammatory conditions, and that sex differences in the anti-hyperalgesic effects of peripherally administered opioids are, in part, mediated by peripheral opioid receptor expression levels.

Morphine treatment selectively regulates expression of rat pituitary POMC and the prohormone convertases PC1/3 and PC2

The prohormone convertases, PC1/3 and PC2 are thought to be responsible for the activation of many prohormones through processing including the endogenous opioid peptides. We propose that maintenance of hormonal homeostasis can be achieved, in part, via alterations in levels of these enzymes that control the ratio of active hormone to prohormone. In order to test the hypothesis that exogenous opioids regulate the endogenous opioid system and the enzymes responsible for their biosynthesis, we studied the effect of short-term morphine or naltrexone treatment on pituitary PC1/3 and PC2 as well as on the level of pro-opiomelanocortin (POMC), the precursor gene for the biosynthesis of the endogenous opioid peptide, β-endorphin. Using ribonuclease protection assays, we observed that morphine down-regulated and naltrexone up-regulated rat pituitary PC1/3 and PC2 mRNA. Immunofluorescence and Western blot analysis confirmed that the protein levels changed in parallel with the changes in mRNA levels and were accompanied by changes in the levels of phosphorylated cyclic-AMP response element binding protein. We propose that the alterations of the prohormone processing system may be a compensatory mechanism in response to an exogenous opioid ligand whereby the organism tries to restore its homeostatic hormonal milieu following exposure to the opioid, possibly by regulating the levels of multiple endogenous opioid peptides and other neuropeptides in concert.

A history of chronic morphine exposure during adolescence increases despair-like behaviour and strain-dependently promotes sociability in abstinent adult mice

A crucial issue in treating opiate addiction, a chronic relapsing disorder, is to maintain a drug-free abstinent state. Prolonged abstinence associates with mood disorders, strongly contributing to relapse. In particular, substance use disorders occurring during adolescence predispose to depression later in adulthood. Using our established mouse model of opiate abstinence, we characterized emotional consequences into adulthood of morphine exposure during adolescence. Our results indicate that morphine treatment in adolescent mice has no effect on anxiety-like behaviours in adult mice, after abstinence. In contrast, morphine treatment during adolescence increases behavioural despair in adult mice. We also show that morphine exposure strain-dependently enhances sociability in adult mice. Additional research will be required to understand where and how morphine acts during brain maturation to affect emotional and social behaviours into adulthood.

Activation of peripheral delta-opioid receptors leads to anti-hyperalgesic responses in the masseter muscle of male and female rats

In this project, we examined peripheral δ-opioid receptor (DOR)-mediated anti-hyperalgesic responses in the context of an acute orofacial muscle pain condition in both male and female rats. We also investigated whether the ATP-sensitive K+ channel (KATP), a downstream target of OR signaling, contributes to DOR-mediated anti-hyperalgesic responses. Local pretreatment of the masseter with a DOR agonist, [D-Pen², D-Pen⁶]-enkephalin (DPDPE), dose-dependently attenuated capsaicin-induced mechanical hypersensitivity in both male and female rats. However, there were sex differences in the potency of local DPDPE in that a 10-fold higher dose of DPDPE was required in female rats to produce the level of anti-hyperalgesia achieved in male rats. The sex differences in the DPDPE effect may not be fully explained by DOR expression level since there was no significant sex difference in DOR mRNA levels in trigeminal ganglia (TG). Finally, pretreatment of the masseter with the KATP antagonist, glibenclamide, significantly blocked the effects of DPDPE in male rats suggesting that the peripheral DOR effect is mediated by the KATP. These studies revealed novel information about sex differences with regards to peripherally localized DOR-mediated anti-hyperalgesia under an orofacial muscle pain condition.

UMB-3, a novel rabbit monoclonal antibody, for assessing μ-opioid receptor expression in mouse, rat and human formalin-fixed and paraffin-embedded tissues

BACKGROUND

The immunohistochemical localization of the μ-opioid receptor (MOR, MOP) has been studied in detail in mouse and rat brain using a variety of polyclonal antibodies. However, biochemical analysis of the MOR signaling complex in vivo has been hampered by the lack of suitable monoclonal antibodies for efficient immunoprecipitation of the receptor protein from native sources. Moreover, previous immunohistochemical investigations were restricted to frozen sections from perfusion-fixed rodent brain, largely due to the limited availability of MOR antibodies that effectively stain paraffin-embedded tissues.

METHODS

Here, we extensively characterized the novel rabbit monoclonal anti-MOR antibody UMB-3 using transfected cells and MOR-deficient mice. UMB-3 was also subjected to a comparative immunohistochemical study of formalin-fixed, paraffin-embedded mouse and rat organ samples as well as human normal and neoplastic tissues.

RESULTS

Specificity of UMB-3 was demonstrated by detection of a broad band migrating at M(r) 70,000-80,000 in immunoprecipitates from crude brain homogenates of MOR+/+ mice but not of MOR⁻/⁻ mice; cell surface staining of MOR-transfected cells; translocation of MOR receptor immunostaining after agonist exposure; distinct immunostaining of neuronal cell bodies and fibers in MOR-expressing brain regions; absence of staining in MOR-deficient mice; and abolition of tissue immunostaining by preadsorption of UMB-3 with its immunizing peptide.

CONCLUSIONS

The rabbit monoclonal antibody UMB-3 is an excellent tool for immunoprecipitation of MOR from native sources as well as for immunohistochemical staining of MOR in paraffin-embedded tissue samples of rodent and human origin.

Sex differences in affective response to opioid withdrawal during adolescence

Drug withdrawal is suggested to play a role in precipitating mood disorders in individuals with familial predisposition. Age-related differences in affective responses to withdrawal might explain the increased risk of mental illnesses when drug use begins during adolescence. Recently we observed that, in contrast to adult male mice, adolescent males exhibited a decrease in immobility in the forced swim test on the third day of withdrawal, as compared with controls. Thus, the present study examined forced swim test behaviors of adolescent female mice during opioid withdrawal. Similar to the male study, adolescent female mice were injected with two morphine regimens which differed in dosage. Three and nine days following discontinuation of morphine administration, forced swim test immobility time and locomotion were evaluated. In contrast to males, which exhibited a decrease in immobility, no significant differences in immobility were observed in female adolescents undergoing withdrawal as compared with saline-injected controls. This sex difference in forced swim test behaviors was not due to changes in overall motor activity, since differences in locomotion were not observed in either male or female adolescent mice. Thus, this study demonstrates sex differences in forced swim test behavior during opioid withdrawal. Forced swim test behaviors are classically used to evaluate mood in rodents, thus this study suggests that opioid withdrawal might affect mood differentially across sexes.

Different affective response to opioid withdrawal in adolescent and adult mice

AIMS

Drug withdrawal is suggested to play a role in precipitating mood disorders in individuals with familial predisposition. Age-related differences in affective responses to withdrawal might explain the increased risk of mental illnesses when drug use begins during adolescence. Since there is a lack of animal research examining the effects of opioid withdrawal during adolescence, the present study examined whether there are age-related differences in affective responses to opioid withdrawal.

MAIN METHODS

Adolescent and adult mice were injected with two different morphine regimens, namely low and high, which differed in the dosage. Three and nine days following discontinuation of morphine administration, immobility time in the forced swim test (FST) and locomotion (total distance traveled) were evaluated.

KEY FINDINGS

On withdrawal day 3 (WD3), adolescent mice exhibited a decrease in immobility as compared to controls. No significant differences in immobility were observed on withdrawal day 9 (WD9). This effect on FST behaviors was not due to changes in overall motor activity, since no differences in locomotion were observed on either WD3 or WD9 in adolescent mice. In adults, no differences in either FST or locomotor behaviors were observed on WD3. As expected, on WD9, adult mice exhibited an increase in immobility and a decrease in locomotion.

SIGNIFICANCE

This study demonstrates age-dependent differences in both FST scores and locomotor behaviors during opioid withdrawal. FST behaviors are classically used to evaluate mood in rodents, thus this study suggests that opioid withdrawal might affect mood differentially across age.

Lipopolysaccharide induces preproenkephalin transcription in hypophysiotropic neurons of the rat paraventricular hypothalamic nucleus suggesting a neuroendocrine role for enkephalins during immune stress

Opioids have impact on stress responses and possess immune modulatory functions. We have previously shown that immune stress elevates the levels of preproenkephalin transcript in a variety of autonomic structures in the rat brain, including the paraventricular hypothalamic nucleus. By using in situ hybridization with an intronic probe recognizing the preproenkephalin heteronuclear RNA combined with retrograde tract tracing, we examined the efferent target of the enkephalinergic neurons in the paraventricular hypothalamic nucleus that display induced transcriptional activity during immune challenge. Rats were first given i.p. injections of the tracer substance Fluoro-Gold, which following this route of administration is taken up only by nerve terminals residing outside the blood-brain barrier, and were then given an i.v. injection of lipopolysaccharide. Neuronal cell bodies retrogradely labeled with Fluoro-Gold were detected by immunohistochemistry, and-using a dual-labeling approach-the same cells were then examined for their expression of preproenkephalin heteronuclear RNA. We found that over 90% of the neurons that expressed preproenkephalin heteronuclear RNA also contained Fluoro-Gold. In addition, approximately 40% of the neurons expressing preproenkephalin heteronuclear RNA co-expressed mRNA for corticotropin-releasing hormone, the main adrenocorticotropic hormone secretagogue. These data show that the paraventricular hypothalamic neurons that display induced preproenkephalin transcription following immune challenge are almost exclusively hypophysiotropic neurons, indicating a role for enkephalin in the hypothalamic control of hormone release during infectious and inflammatory conditions.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.