Role Of Endogenous Opioid System Research Articles

Endogenous opioid systems alterations in pain and opioid use disorder.

Decades of research advances have established a central role for endogenous opioid systems in regulating reward processing, mood, motivation, learning and memory, gastrointestinal function, and pain relief. Endogenous opioid systems are present ubiquitously throughout the central and peripheral nervous system. They are composed of four families, namely the μ (MOPR), κ (KOPR), δ (DOPR), and nociceptin/orphanin FQ (NOPR) opioid receptors systems. These receptors signal through the action of their endogenous opioid peptides β-endorphins, dynorphins, enkephalins, and nociceptins, respectfully, to maintain homeostasis under normal physiological states. Due to their prominent role in pain regulation, exogenous opioids-primarily targeting the MOPR, have been historically used in medicine as analgesics, but their ability to produce euphoric effects also present high risks for abuse. The ability of pain and opioid use to perturb endogenous opioid system function, particularly within the central nervous system, may increase the likelihood of developing opioid use disorder (OUD). Today, the opioid crisis represents a major social, economic, and public health concern. In this review, we summarize the current state of the literature on the function, expression, pharmacology, and regulation of endogenous opioid systems in pain. Additionally, we discuss the adaptations in the endogenous opioid systems upon use of exogenous opioids which contribute to the development of OUD. Finally, we describe the intricate relationship between pain, endogenous opioid systems, and the proclivity for opioid misuse, as well as potential advances in generating safer and more efficient pain therapies.

The partial \xb5-opioid agonist buprenorphine in autism spectrum disorder: a case report

BackgroundThere are currently no approved medications for impaired social cognition and function, core symptoms of autism spectrum disorder. We describe marked improvement of these symptoms with long-term low-dose administration of the partial µ-opioid agonist buprenorphine. We discuss these observations in the context of a role for endogenous opioid systems in social attachment, and theories integrating those findings mechanistically with autism spectrum disorder.Case presentationM, a 43-year-old Caucasian male, is medically healthy. Despite social difficulties since childhood, he completed high school with better-than-average grades, but failed university education. A psychiatric evaluation in his twenties diagnosed attention deficit hyperactivity disorder but also noted symptoms of coexisting autism spectrum disorder. M accidentally came across buprenorphine in his late twenties and experienced progressively improved social functioning on a low daily dosage (0.5–1.0 mg/day), an effect maintained for 15 years. He lived independently and maintained a part-time occupation. After abrupt discontinuation of treatment, his autistic symptoms returned, and function deteriorated. Following evaluation by our team, buprenorphine was resumed, with gradual return to prior level of functioning. An attempt to formally evaluate M both on and off medication was agreed with him and approved by the Swedish Ethics Authority, but medication had to be resumed when the patient worsened following discontinuation.ConclusionsAccording to the µ-opioid receptor balance model, both excessive and deficient μ-receptor activity may negatively influence social behavior, and accordingly both opioid agonist and opioid antagonist treatment may be able to improve social functioning, depending on an individual’s opioid tone before treatment. Our case report is consistent with these hypotheses, and given the extensive unmet medical needs in individuals with autism spectrum disorders, randomized controlled trial appears warranted.

Investigating the role of endogenous opioid system in chloroquine-induced phospholipidosis in rat liver by morphological, biochemical and molecular modelling studies.

Drug-induced phospholipidosis (DIPL) is characterized by phospholipid storage in the lysosomes of affected tissues. Many severe effects and toxicities have been linked to DIPL. The aim of this study was to determine whether the endogenous opioid system is involved in chloroquine-induced phospholipidosis. The effect of naltrexone as an antagonist of opioid receptors in chloroquine-induced phospholipidosis in rat liver was investigated by morphological, biochemical, and molecular modelling studies. Transmission electron microscopy (TEM) showed that morphological characteristic changes of rat liver, including the number of lamellar bodies, grade of vacuolization and cell steatosis, were markedly attenuated in rats treated with naltrexone alone or in combination with chloroquine, in comparison with chloroquine-treated rats. The results of liquid chromatography mass spectrometry (LC/MS) showed that the concentrations of phenylacetylglycine (PAG) and hippuric acid (HA) were significantly decreased and increased, respectively, in target groups. Besides, the concentration ratio of PAG/HA was significantly decreased. Spectrophotometry resulted in a notable decrease in alanine aminotransferase (ALT) and alkaline phosphatase (ALP) activities in target groups. The results from the molecular docking and molecular dynamic simulation studies demonstrated clear chloroquine interaction with the active site cavity of the µopioid receptor. These data suggest that administration of naltrexone alone, or in combination with chloroquine, notably attenuates the side effects of chloroquine-induced phospholipidosis, as well as demonstrating an increased probability of the endogenous opioid system involvement in chloroquine-induced phospholipidosis in rat liver.

The Role of Endogenous Opioid System in the Regulation of Heart Tolerance to Stress-Induced Damage

In Wistar rats, stress was modeled by 24-h immobilization in a supine posture and stress-induced damage to the heart was assessed by accumulation of 99mTc-pyrophosphate in the myocardium. The intensity of stress reaction was measured by serum levels of cortisol and insulin. Both stressinduced damage to the heart and intensity of stress reaction were examined under control conditions and in rats treated with opioid receptor antagonists naltrexone, methylnaltrexone bromide, MR2266, and ICI174.864. Activation of central μ-opioid receptors with endogenous opioids aggravated stress-induced cardiomyopathy, while stimulation of peripheral μ-opioid receptors produced a cardioprotective effect. The stress-induced damage to the heart was not directly related to up-regulation of cortisol secretion in response to 24-h immobilization. Blockade of the central opioid receptors promoted a decrease in cortisol level in stressed rats.

Role of Endogenous Opioid System in Ischemic-Induced Late Preconditioning.

BackgroundOpioid receptors (OR) are involved in myocardial late preconditioning (LPC) induced by morphine and δ1-opioid receptor (δ1-OR) agonists. The role of OR in ischemic-induced LPC is unknown. We investigated whether 1) OR are involved in the trigger and/or mediation phase of LPC and 2) a time course effect on the expression of different opioid receptors and their endogenous ligands exists.MethodsMale Wistar rats were randomly allocated to four groups (each group n = 8). Awake animals were ischemic preconditioned by a 5 minutes coronary occlusion. 24 hours later, anesthetized animals underwent 25 minutes coronary occlusion followed by 2 hours of reperfusion. The role of OR was investigated by treatment with intraperitoneal naloxone (Nal) 10 minutes prior to LPC (Nal-LPC; trigger phase) or 10 min prior to sustained ischemia (LPC-Nal; mediation phase).ResultsLPC reduced infarct size from 61±10% in controls to 25±9% (P<0.001). Naloxone during trigger or mediation phase completely abolished LPC-induced cardioprotection (59±9% and 62±9%; P<0.001 vs. LPC). 8, 12 and 24 hours after the ischemic stimulus, expression of δ-OR in the heart was increased, whereas μ-opioid receptor (μ-OR) and κ-opioid receptor (κ-OR) were not. Plasma concentrations of β-endorphin and leu-enkephalin but not dynorphin were increased by LPC.ConclusionIschemic LPC is triggererd and mediated by OR. Expression of δ-OR and plasma levels of endogenous opioid peptides are increased after ischemic LPC.

Neuropharmacological basis of rTMS-induced analgesia: The role of endogenous opioids

We investigated the role of endogenous opioid systems in the analgesic effects induced by repetitive transcranial magnetic stimulation (rTMS). We compared the analgesic effects of motor cortex (M1) or dorsolateral prefrontal cortex (DLPFC) stimulation before and after naloxone or placebo treatment, in a randomized, double-blind crossover design, in healthy volunteers. Three groups of 12 volunteers were selected at random and given active stimulation (frequency 10Hz, at 80% motor threshold intensity, 1500 pulses per session) of the right M1, active stimulation of the right DLPFC, or sham stimulation, during two experimental sessions 2weeks apart. Cold pain thresholds and the intensity of pain induced by a series of fixed-temperature cold stimuli (5, 10, and 15°C) were used to evaluate the analgesic effects of rTMS. Measurements were made at the left thenar eminence, before and 1hour after the intravenous injection of naloxone (bolus of 0.1mg/kg followed by a continuous infusion of 0.1mg/kg/h until the end of rTMS) or placebo (saline). Naloxone injection significantly decreased the analgesic effects of M1 stimulation, but did not change the effects of rTMS of the DLPFC or sham rTMS. This study demonstrates, for the first time, the involvement of endogenous opioid systems in rTMS-induced analgesia. The differential effects of naloxone on M1 and DLPFC stimulation suggest that the analgesic effects induced by the stimulation of these 2 cortical sites are mediated by different mechanisms.Endogenous opioids are shown to be involved in the analgesic effects of repetitive transcranial magnetic stimulation of the motor cortex.

Dynorphin A (1–13) and responsiveness of the newborn rat to a surrogate nipple: Immediate behavioral consequences and reinforcement effects in conditioning

The role of endogenous opioid system in learning (Pavlovian conditioning) and reinforcement was studied in newborn rats by pairing central injection of dynorphin A (1–13), an endogenous ligand with high affinity for kappa opioid receptors, with a surrogate nipple. For conditioning, the nipple served as the conditioned stimulus and dynorphin as the unconditioned stimulus. Dynorphin was found to elicit an immediate unconditioned, dose-dependent increase in the neonatal responsiveness to a surrogate nipple providing water, an effect apparently mediated through the kappa opioid receptors. This immediate effect of dynorphin was evident in the context of suckling behavior but not in the context of adult-like drinking when water was delivered through an intra-oral cannula. One hour after injection, the unconditioned stimulatory effect of dynorphin had dissipated and no longer affected responsiveness to the nipple. However, pairing of centrally injected dynorphin and suckling on a nipple delivering water substantially enhanced responsiveness to a nipple 1 h later. This suggested conditioning, and hence reinforcement, by the centrally injected dynorphin. The conditioned sustained nipple attachment was mediated through both kappa and mu opioid receptors. These experiments confirm a central role for the brain opioid systems in reinforcement of neonatal behavior on the nipple.

Opioid peptides in cancer.

Opioid and somatostatin systems are two main inhibitory systems in mammals implicated in a variety of processes from hormone secretion to the modulation of cell proliferation. Opioids and opioid receptors were found in a great diversity of primary human tumors and in various cancer cell lines. A new area of interest that has developed since the early 1980s has been concerned with the role of endogenous opioid systems in the growth of normal and abnormal tissues and cells. This review presents recent developments on the identification of opioid ligands and receptors in different types of human neoplasia. It also deals with the mechanisms of opioid peptide action in carcinoma and the involvement of opioids in the regulation of tumor growth.

Drug dependence and the endogenous opioid system

The discovery of endogenous opioids has markedly influenced the research on the biology of drug dependence. Evidence has been presented that these brain substances are self-administered by laboratory animals. This finding, among others, has led to the hypothesis that endogenous opioids are involved in reinforcing habits, including dependence on drugs of abuse. The course of drug dependence is presented as a continuum from no drug use via controlled use to an actual dependence on the drug. Specific brain opioid systems belonging to four conceptualized brain circuits are described to be involved during the different phases of the drug dependence continuum. More recent research to delineate the role of endogenous opioid systems in drug dependence has focussed on genetic research in humans and animals. Among others, the findings obtained from studies of opioid receptor and opioid peptide precursor knockout mice provided further support for a role of endogenous opioid systems in drug dependence, in agreement with previous pharmacological studies.

Role of endogenous opioid system in the regulation of the stress response

1. Numerous studies and reviews support an important contribution of endogenous opioid peptide systems in the mediation, modulation, and regulation of stress responses including endocrine (hypothalamopituitary-adrenal, HPA axis), autonomic nervous system (ANS axis), and behavioral responses. Although several discrepancies exist, the most consistent finding among such studies using different species and stressors is that opioids not only diminish stress-induced neuroendocrine and autonomic responses, but also stimulate these effector systems in the non-stressed state. 2. A distinctive feature of the analgesic action of opioids is the blunting of the distressing, affective component of pain without dulling the sensation itself. Therefore, opioid peptides may diminish the impact of stress by attenuating an array of physiologic responses including emotional and affective states. 3. The widespread distribution of enkephalin (ENK) throughout the limbic system (including the extended amygdala, cingulate cortex, entorhinal cortex, septum, hippocampus, and the hypothalamus) is consistent with a direct role in the modulation the stress responses. 4. The predictability of stressful events reduces the impact of a wide range of stressors and ENK appears to play an important role in this process. Therefore, ENK and its receptors could represent a major modulatory system in the adaptation of an organism to stress, balancing the response that the stressor places on the central stress system with the potentially detrimental effects that a sustained stress may produce. Chronic neurogenic stressors will induce changes in specific components of the stress-induced ENKergic system, including ENK, δ- and μ-opioid receptors. 5. This review presents evidences for adaptive cellular mechanisms underlying the response of the central stress system when assaulted by repeated psychogenic stress, and the involvement of ENK in these processes.

Disparate spinal and supraspinal opioid antinociceptive responses in β-endorphin-deficient mutant mice

The role of endogenous opioid systems in the analgesic response to exogenous opiates remains controversial. We previously reported that mice lacking the peptide neurotransmitter β-endorphin, although unable to produce opioid-mediated stress-induced antinociception, nevertheless displayed intact antinociception after systemic administration of the exogenous opiate morphine. Morphine administered by a peripheral route can activate opioid receptors in both the spinal cord and brain. However, β-endorphin neuronal projections are confined predominantly to supraspinal nociceptive nuclei. Therefore, we questioned whether the absence of β-endorphin would differentially affect antinociceptive responses depending on the route of opiate administration. Time- and dose–response curves were obtained in β-endorphin-deficient and matched wild-type C57BL/6 congenic control mice using the tail-immersion/withdrawal assay. Null mutant mice were found to be more sensitive to supraspinal (i.c.v.) injection of the μ-opioid receptor-selective agonists, morphine and d-Ala 2-MePhe 4-Gly-ol 5 enkephalin. In contrast, the mutant mice were less sensitive to spinal (i.t.) injection of these same drugs. Quantitative receptor autoradiography revealed no differences between genotypes in the density of μ, δ, or κ opioid receptor binding sites in either the spinal cord or pain-relevant supraspinal areas. Thus we report that the absence of a putative endogenous ligand for the μ-opioid receptor results in opposite changes in morphine sensitivity between discrete areas of the nervous system, which are not simply caused by changes in opioid receptor expression.

The Use of Naloxone for Investigating Disorders of the Hypothalamic-Pituitary-Adrenal Axis

The hypothalamic-pituitary-adrenal (HPA) axis forms an integral and vital part of the stress response system. Pituitary ACTH secretion is regulated by two hypothalamic peptides, corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP), with CRH being the primary mediator in humans. Numerous neuronal circuits within the brain influence the activity of the CRH neurons in the paraventricular nuclei of the hypothalamus, including endogenous opioid systems. Opioid receptors are widely distributed in the brain but have a particular association with three major neural networks: the sensory, limbic and neuroendocrine systems. Opioidergic agents affect HPA activity in humans and animals, al though important species differences exist. In vitro and in vivo studies provide considerable evidence for a centrally mediated effect of opioidergic agents on the HPA axis, most likely by direct action on the hypothalamus. Naloxone is a competitive antagonist at multiple opioid receptor subtypes. Clinical investigations utilizing naloxone administration in humans have had three primary aims:1) to elucidate the role of endogenous opioid systems in the normal physiology of the HPA axis; 2) to study the pathophysiology of disorders associated with or caused by dysfunction of the HPA axis; 3) to develop new diagnostic tests. High doses of naloxone stimulate the human HPA ards causing rises in plasma ACTH and cortisol concentrations. Pharmacological stud ies strongly suggest that naloxone blocks a tonic inhibitory effect of endogenous opioids on central alpha-adrenergic pathways, which in turn stimulate ACTH secretion via CRH release from the hypothalamus. Therefore, the naloxone test provides a means of evaluating hypothalamic CRH reserve and assesses the integrity of the entire HPA axis. Several disorders are associated with dysregulation of the HPA axis including major depression, post-traumatic stress disorder, alcoholism, chronic fatigue syndrome, Cushing's syndrome, and obesity. Major depression is a condition often associated with clinical and biochemical evidence of hypercortisolism (pseudo-Cushing's syndrome) and may be very difficult to distinguish from patients with true Cushing's syndrome. Patients with pseudo-Cushing's syndrome are postulated to have hypersecretion of hypothalamic CRH producing an upregulation of an otherwise normal HPA axis. Patients with major depression have an ACTH hyperresponse to naloxone compared with healthy subjects and patients with Cushing's disease. This increased ACTH response is further exaggerated by combining naloxone administration with the direct corticotrope stimulation provided by exogenous AVP. The naloxone test also has potential diagnostic utility-when central adrenal insufficiency is suspected. Naloxone applies a specific pharmacological stimulus at the hypothalamic level, therefore a normal ACTH and cortisol response implies functional integrity of all three components of the HPA axis. Preliminary studies comparing naloxone administration with the insulin hypoglycemia and metyrapone tests are promising. The naloxone test may provide an alternative in patients with suspected central adrenal insufficiency who are unable to undergo an insulin hypoglycemia test or a metyrapone test because of safety issues.

Proenkephalin transgene regulation in the paraventricular nucleus of the hypothalamus by lipopolysaccharide and interleukin-1?

Immunologic challenge with lipopolysaccharide (LPS) or interleukin-1beta (IL-1beta) produces a functional response within the paraventricular nucleus of the hypothalamus (PVN) and leads to changes in gene expression within PVN neurons. Regulated expression of neuropeptide genes within neurons of the PVN is a potential mechanism by which an organism can adapt to stressful challenges. Here, the authors used a transgenic mouse model in which expression of a readily measurable beta-galactosidase reporter was driven in PVN neurons by human proenkephalin regulatory sequences. This proenkephalin-beta-galactosidase transgene has been demonstrated previously to respond appropriately to a variety of stressors. It is demonstrated that expression of the proenkephalin transgene product was up-regulated significantly in a subset of PVN neurons 6 hours following intraperitoneal LPS (16-400 microg/kg) administration, remained elevated at 12 hours, and fell below basal levels by 24 hours. A more rapid and transient pattern of transgene up-regulation in the PVN followed administration of intraperitoneal IL-1beta (10 microg/kg) with significant induction by 2 hours, peak levels reached by 4 hours, and a return toward basal levels by 6 hours. IL-1beta (10-50 ng/mouse) administered intracerebroventricularly also led to up-regulation of the transgene 6 hours following infusion. Transgene expression was not up-regulated in hypothalamic slice cultures treated directly with IL-1beta (5-10 ng/ml media). Up-regulation of transgene expression does not appear to result from local action of IL-1beta at the level of the PVN but, rather, through as yet unidentified intermediates. The authors demonstrate phosphorylation of the cyclic amino-3-hydroxy-5-methyl-4-isoxazolepropionate response element binding protein, a transcription factor known to interact with proenkephalin regulatory sequences within the transgene, in the PVN following LPS administration. LPS induced up-regulation of the transgene was blocked by pretreatment with naltrexone, indicating an additional role for endogenous opioid systems in regulation of the PVN response to immune challenge.

Proenkephalin transgene regulation in the paraventricular nucleus of the hypothalamus by lipopolysaccharide and interleukin-1beta.

Immunologic challenge with lipopolysaccharide (LPS) or interleukin-1beta (IL-1beta) produces a functional response within the paraventricular nucleus of the hypothalamus (PVN) and leads to changes in gene expression within PVN neurons. Regulated expression of neuropeptide genes within neurons of the PVN is a potential mechanism by which an organism can adapt to stressful challenges. Here, the authors used a transgenic mouse model in which expression of a readily measurable beta-galactosidase reporter was driven in PVN neurons by human proenkephalin regulatory sequences. This proenkephalin-beta-galactosidase transgene has been demonstrated previously to respond appropriately to a variety of stressors. It is demonstrated that expression of the proenkephalin transgene product was up-regulated significantly in a subset of PVN neurons 6 hours following intraperitoneal LPS (16-400 microg/kg) administration, remained elevated at 12 hours, and fell below basal levels by 24 hours. A more rapid and transient pattern of transgene up-regulation in the PVN followed administration of intraperitoneal IL-1beta (10 microg/kg) with significant induction by 2 hours, peak levels reached by 4 hours, and a return toward basal levels by 6 hours. IL-1beta (10-50 ng/mouse) administered intracerebroventricularly also led to up-regulation of the transgene 6 hours following infusion. Transgene expression was not up-regulated in hypothalamic slice cultures treated directly with IL-1beta (5-10 ng/ml media). Up-regulation of transgene expression does not appear to result from local action of IL-1beta at the level of the PVN but, rather, through as yet unidentified intermediates. The authors demonstrate phosphorylation of the cyclic amino-3-hydroxy-5-methyl-4-isoxazolepropionate response element binding protein, a transcription factor known to interact with proenkephalin regulatory sequences within the transgene, in the PVN following LPS administration. LPS induced up-regulation of the transgene was blocked by pretreatment with naltrexone, indicating an additional role for endogenous opioid systems in regulation of the PVN response to immune challenge.

The role of endogenous opioids in the luteinizing hormone surge in rats: studies with clocinnamox, a long-lasting opioid receptor antagonist

Endogenous opioid peptides have been demonstrated to regulate luteinizing hormone (LH) secretion in a variety of species. Studies in rodents suggest a role of opioid peptide systems in controlling the timing of the LH surge, which is entrained to the circadian rhythm. The current studies utilize clocinnamox, a novel long-lasting opioid receptor antagonist that is capable of occupying μ-opioid receptors for periods of one week or more, to examine the role of endogenous opioid systems on the LH surge. Administration of clocinnamox [14b-( p-chlorocinamoylamino)-7,8-dihydro- N-cyclopropylmethyl normophineone mesylate]) on the morning of proestrus advanced the LH surge by several hours. Despite the blockade of opioid receptors and analgesia for more than one week, administration of clocinnamox on the evening of diestrus II had no effect on the timing of the LH surge but significantly increased plasma LH levels throughout the day of proestrus. These data suggest that removal of opioid tone is unlikely to be the critical signal controlling the initiation of the LH surge in rodents, although it does appear to be permissive for the surge. Furthermore, the μ-opioid receptor appears to be the receptor involved in the regulation of the LH surge.

O-67 - Studies on drug dependence (Rept. 284) : Role of endogenous opioid system in cthanol-induced place preference under foot shock stress in rats

O-67 - Studies on drug dependence (Rept. 284) : Role of endogenous opioid system in cthanol-induced place preference under foot shock stress in rats

Multiple effects of morphine on social play

To elucidate the role of endogenous opioid systems in social play, we investigated the effects of morphine on playful and non-playful social behaviors in juvenile rats. These effects were studied under various circumstances, being 1, 3.5 or 24 hours of social isolation before testing, known to yield a minimal, half-maximal and maximal induction of social play (1), and different levels of illumination during testing. It appeared that morphine increased playful rather than nonplayful social behaviors, in a dose-dependent manner. This increasing effect was apparent under dim light circumstances, independent of the duration of previous isolation and with a maximal effect at a dose of 1.0 mg/kg of morphine. This effect can be interpreted as an effect on the rewarding aspects of play

Effects of chronic stimulation or antagonism of opiate receptors on GH secretion in male and female rats

The present study was undertaken to assess the role of endogenous opioid systems in the sexually dimorphic pattern of growth hormone (GH) secretion. To this end, male rats were treated chronically (6 to 12 h) with morphine and estrogen-exposed, ovariectomized female rats with morphine or naloxone. Chronic morphine exposure of male rats caused a 12-fold increase in basal GH levels and a modest rise in GH pulse frequency. These two changes resulted in a 3-fold increase in both mean GH concentration and total GH secretion over 6 h. In female rats, chronic morphine reduced GH pulse amplitudes but did not significantly affect other parameters of GH secretion. By contrast, chronic naloxone treatment of female rats reduced basal GH levels by 64% without affecting GH pulse amplitudes or pulse frequency. These data suggest that endogenous opioid systems are involved in the regulation of the basal GH secretion in both male and female rats.

Endogenous opioid system modulation in anginal pain: Demonstration of its central activity

Plasma β-endorphin levels provide controversial results on the role of endogenous opioid system in modulation of anginal pain. As an alternative, the action of plasmatic luteinizing hormone after administration of naloxone was investigated: naloxone blocks the tonic endogenous opioid system inhibition of gonadotropin release; thus, the level of luteinizing hormone after naloxone administration is an index of central endogenous opioid system activity. Twenty patients with coronary artery disease and positive results of stress tests were selected: 10 had angina (group I) and 10 did not (group II). Ten healthy subjects were also studied as a control group (group III). In all patients basal plasma β-endorphin levels, basal luteinizing hormone plasma levels (every 15 minutes for 1 hour) and luteinizing hormone plasma levels after administration of 0.1 mg/kg naloxone over 4 minutes (every 15 minutes for 2 hours) were determined. In 15 patients the test was performed after luteinizing hormone releasing hormone was given. The integral concentration time of luteinizing hormone plasma level during baseline (LHiB) and after administration of naloxone (LHiN) or luteinizing hormone releasing hormone (LHiRH), the ratio (LHiN:LHiB and LHiRH:LHiB) and the differences (LHiN-LHiB and LHiRH − LHiB) between the postinfusion period and baseline were calculated. No difference was found in β-endorphin plasma levels and luteinizing hormone response after luteinizing hormone releasing hormone infusion. On the contrary, after naloxone administration, group II showed significantly higher responses (LHiN − LHiB, 2413 ± 380 mlU/ml; LHiN:LHiB, 5 ± 0.4) with respect to group I (LHiN − LHiB, 1150 ± 304 mlU/ml; p < 0.001; LHiN:LHiB, 2.7 ± 0.6; p < 0.001) and to group III (LHiN − LHiB, 1048 ± 279 mlU/ml; p < 0.001; LHiN:LHiB, 2.6 ± 0.4, p < 0.001). In comparison with β-endorphin levels, luteinizing hormone response to naloxone provides a better discrimination between patients with symptoms and those without symptoms, which demonstrates the role of the endogenous opioid system in modulating anginal pain.

Endogenous opioid systems and the regulation of dendritic growth and spine formation.

The role of endogenous opioid systems (endogenous opioids and opioid receptors) in neuronal development was examined in 10- and 21-day-old rats by utilizing an opioid antagonist (naltrexone) paradigm. Throughout the first 3 weeks of life, Sprague-Dawley rats were given daily subcutaneous injections of either 50 mg/kg naltrexone, a dosage that invoked a complete (24 hours/day) receptor blockade, or 1 mg/kg naltrexone, a dosage which intermittently blocked (4-6 hours/day) opioid receptors and exacerbated opioid action; animals injected with sterile water served as controls. Pyramidal cells from the frontoparietal cortex (layer III) and hippocampal field CA1, and cerebellar Purkinje cells, were impregnated by using the Golgi-Kopsch method; total and mean dendrite segment length, branch frequency, and spine concentration were analyzed morphometrically. Perturbations of endogenous opioid systems caused region-dependent alterations in dendrite complexity and/or spine concentration in all brain areas. Continuous opioid receptor blockade resulted in dramatic increases in dendrite and/or spine elaboration compared to controls at 10 days in all brain regions; however, these increases were only evident in the hippocampus at 21 days. With intermittent blockade, dendrite and/or spine growth were often subnormal, being predominant at day 21. Our results indicate that endogenous opioid systems are critical regulators of neuronal differentiation, and they control growth through an inhibitory mechanism. Considering previous findings demonstrating that neurobehavioral ontogeny is dependent on endogenous opioid-opioid receptor interactions, the present results suggest an opioid-dependent, structure-function relationship between neuronal and behavioral maturation.