Cold-restraint stress reduces [3H]etorphine binding to rat brain membranes: influence of acute and chronic morphine and naloxone

Sex differences in opioid receptor mediated effects: Role of androgens

An abundance of data indicates there are sex differences in endogenous opioid peptides and opioid receptors, leading to functional differences in sensitivity to opioid receptor mediated behaviors between males and females. Many of these sex differences are mediated by the effects of gonadal hormones on the endogenous opioid system. Whereas much research has examined the role of ovarian hormones on opioid receptor mediated endpoints, comparatively less research has examined the role of androgens. This review describes what is currently known regarding the influence of androgens on opioid receptor mediated endpoints and how androgens may contribute to sex differences in these effects. The review also addresses the clinical implications of androgenic modulation of opioid receptor mediated behaviors and suggests future lines of research for preclinical and clinical investigators. We conclude that further investigation into androgenic modulation of opioid receptor mediated effects may lead to new options for addressing conditions such as chronic pain and substance use disorders.

Repeated Restraint Stress Reduces Opioid Receptor Binding in Different Rat CNS Structures

Different effects of exposure to acute or to repeated stress have been observed upon the nociceptive response in rats. In the present study, we repeatedly submitted Wistar rats to restraint for 40 days, a treatment known to induce an increase in the nociceptive response in the tail-flick test. Afterwards, the effect of repeated restraint stress on the density of opioid receptors in rat spinal cord, frontal cortex, and hippocampus was investigated. Results showed that repeatedly stressed rats displayed a significant decrease in opioid receptors density in all structures studied; cortex (141.3 +/- 5.7 for control and 103.3 +/- 15.9 for stressed rats), hippocampus (92.4 +/- 7.2 for control and 64.8 +/- 7.7 for stressed rats), and spinal cord (122.2 +/- 12.8 for control and 79.7 +/- 9.7 for stressed rats). These findings suggest opioid mediation of the altered responses observed in these repeatedly-stressed animals, although the participation of non-opioid mechanisms in this phenomenon cannot be ruled out.

Ablation of pituitary pro-opiomelanocortin (POMC) cells produces alterations in hypothalamic POMC mRNA levels and midbrain mu opioid receptor binding in a conditional transgenic mouse model

The hypothalamic-pituitary-adrenal (HPA) axis is regulated by stress-related excitatory inputs, and various inhibitory and negative-feedback controls by glucocorticoids and opioids, including pro-opiomelanocortin (POMC)-derived peptides. The role of POMC-derived peptides of pituitary origin in the modulation of brain POMC mRNA expression and opioid receptor binding was investigated using a line of transgenic mice that express a fusion gene composed of the pituitary expression-specific promoter region of the POMC gene driving the herpes simplex viral-1 thymidine kinase (TK). Male adult mice were treated with the antiherpes agent ganciclovir that selectively ablates cells expressing TK. Following treatment, POMC mRNA levels, measured by quantitative solution hybridization/RNase protection assays, were decreased by 48% in the pituitary of the TK+/+ mice, reflecting an expected loss of the pituitary corticotrope POMC cells. This treatment also significantly lowered pituitary beta-endorphin immunoreactivity content and plasma concentrations of corticosterone. In contrast, POMC mRNA levels were increased by 79% in the hypothalamus of the TK+/+ mice with pituitary POMC cell ablation. Binding of [(3)H]DAMGO to mu opioid receptors, as measured by quantitative autoradiography, was significantly reduced in several brain regions including the central grey, median raphe and superficial grey layer of the superior colliculus. These regions are innervated by hypothalamic POMC neurones. No significant differences in binding to either kappa or delta opioid receptors were found in the brain regions studied. These results suggest that POMC-derived peptides of pituitary origin may exert a tonic negative-feedback effect on hypothalamic POMC neurones. In turn, the downregulation of central mu opioid receptors in this model may be mediated through a mechanism related to hypothalamic POMC overexpression.

Stress-induced behavioral responses and multiple opioid systems in the brain

Various stressor produce a wide range of behavioral responses such as analgesia, catalepsy and motor suppression, which are sensitive to opioid receptor antagonists. These behavioral responses in stress are accompanied by changes in the contents of opioid peptides, the mRNAs encoding their precursors and opioid receptor binding in the brain. In the present article, experimental data concerning stress-induced analgesia and motor suppression is reviewed and discussed in relation to a possible involvement of different opioid systems in the various observed behavioral responses in stress. Pharmacological studies with subtype-selective antagonists have demonstrated that not only mu- but also delta- and/or kappa-opioid receptors are involved in opioid-mediated stress-induced analgesia. There are two types of stress-induced analgesia referred to as opioid-mediated and non-opioid mediated forms. It has been proposed that the intensity and temporal pattern of stressor may be a critical factor determining the nature of stress-induced analgesia. Accumulated evidence demonstrate that these two forms of pain inhibitory systems interact each other according to a collateral inhibition model. Recent studies show that parallel activation of multiple opioid receptors mediates non-opioid froms of stress-induced analgesia. Dynorphins, by acting at kappa-opioid receptors, may play a pivotal role in the expression of stress-induced motor suppression, whereas enkephalins may act to attenuate this response.

Restraint stress in biomedical research: an update

Since the publication of our initial review of restraint stress in 1986, much work has continued with this technique, either as a tool for the investigation of other pharmacological, physiological, or pathologic phenomena or with restraint stress itself serving as the object of the study. As we noted in 1986, the major use of restraint has been for the induction of stress responses in animals and, more specifically, for the investigation of drug effects, particularly as they affect typical stress-related pathology--gastrointestinal, neuroendocrine, and immunological agents have been extensively studied. In compiling this update on restraint stress and its effects, we noted an increasing emphasis on central nervous system mechanisms in peripheral disease, especially gastrointestinal disease. In particular, many CNS-active agents have been tested for their effects on gastric and duodenal lesion formation and gastric secretion, including antidepressants, antipsychotics, anxiolytics, noradrenergic, serotonergic, dopaminergic, and peptidergic compounds. Some of these agents are especially active in the gastrointestinal tract even when administered centrally, further solidifying the concept of a brain-gut axis. The present update includes studies of: methods and procedures, pre-restraint manipulations, post-restraint/healing effects, and drug effects. In addition, a current bibliography of reports that have employed restraint is included.

Effects of stress on opioid receptor binding in the rat central nervous system

The endogenous opioid peptides are known to play a significant role in the modulation and/or mediation of numerous environmental or experimental stressors. However, the specific opioid peptide(s) and receptor type(s) involved, under what physiologic conditions they are engaged and within which regions of the CNS is not well understood. We therefore examined the effects of both a chronic and an acute stressor-90-h water deprivation and a single 20-min foot shock on opioid receptor binding in 17 specific rat brain nuclei. [3H]DSTLE (Tyr-D-Ser-Gly-Phe-Leu-Thr) and [3H]DAGO(Tyr-D-Gly-Phe-NMe-Phe-Gly-ol) were used to label delta and mu receptors, respectively. Foot shock induced profound antinociception as measured by tail-flick latency which outlasted the stressor by several minutes. However, only the septum responded with a decrease in [3H]DAGO binding to this type of stress-induced analgesia. No other alterations in either [3H]DAGO or [3H]DSTLE binding were seen in response to foot shock. In contrast, water deprivation induced increases in [3H-DAGO] binding in the septum as well as increases in [3H]DSTLE binding in the caudate and accumbens nuclei. Moreover, the presumptive mild stress of handling in the foot shock control group was sufficient to decrease mu or delta receptor binding in seven out of 17 brain regions investigated (including the frontal cortex and olfactory tubercle where both mu and delta binding were increased) when compared to unhandled deprivation control animals. These changes in opioid receptor binding may have been the result of alterations in treatment-induced peptide release, receptor regulation, or interactions with other released neurotransmitter ligand/receptor complexes.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of stress in the effects of morphine on pituitary-adrenocortical activity in the guinea-pig

1. Modifications by stress of the effects of morphine on pituitary-adrenocortical activity were examined in the guinea-pig at different lengths of time. 2. An increase in plasma cortisol but not in beta-endorphin levels was obtained in guinea-pigs stressed for 60 min. 3. Morphine (50 mg kg i.p.) enhanced beta-endorphin and cortisol levels 5 min after injection and decreased beta-endorphin concentration 30 and 90 min after its administration. 4. In stressed guinea-pigs plasma beta-endorphin levels were reduced 5 min after morphine injection. 5. These results indicate that stress can alter the effects of morphine on pituitary-adrenal activity in the guinea-pig and that the type of modification may be dependent on the time of the sampling.

Altered diurnal pattern of arousal following naloxone administration in opioid-naive rats

Heightened arousal occurring in response to physical or psychological stressors in associated with increased levels of endogenous opioid peptides in peripheral circulation and at binding sites in the central nervous system. When administered as a series of bolus injections during active periods, the opioid antagonist naloxone increased delta wave activity and total spectral power in the EEG of opioid-naive rats. A single 1.0 mg/kg injection of naloxone had a similar effect when given at the onset of night-time active periods, but not if administered during the day when rats are normally inactive. These results are consistent with a diminished level of arousal following blockade of endogenous opioid activity, and suggest an excitatory effect of opioid peptides in certain behavioral settings.

Neurobiological background of pain and analgesia: the attempt at revaluation according to position of the organism's adaptive activity

The most adequate and successful way to understand the essence of any complex psychophysiological phenomenon, including pain, is obviously the study of its origin, its genesis, i.e., its biological background. Based on critical analysis of recent literature and our own electrophysiological, biochemical and pharmacological data we tried to overcome the difficulties and contradictions derived from the traditional reflex approach and analytical orientation in understanding the experimental investigation of pain-related problems and to determine the neurobiological background of pain and analgesia through the notion of the organism's adaptive activity. Interrelations between the notion of pain and other biological and psychological ideas, the place and functional significance of pain and endogenous analgesic mechanisms in the organization, maintenance and regulation of the organism's adaptive activity, characterization of the involvement of endogenous opioid peptides and monoamines in central processes associated with pain and analgesia, the essence and mechanisms of pain-depressing activity of the opiates are the main stages in our neurobiological consideration of the phenomenon of pain and its natural and pharmacological regulation.

Enkephalin-dopamine interactions in the central amygdalar nucleus during gastric stress ulcer formation in rats

Intra-amygdalar (i/am) microinjections of the enkephalin analog, (D-Ala2)-Met-enkephalinamide (DAME, 3, 10 and 30 micrograms) into the central amygdalar nucleus (CEA) produced a dose-related, naltrexone-reversible attenuation of cold restraint (3 h at 4 degrees C)-induced gastric mucosal lesions in rats. Similarly, gastric stress ulcer formation was also inhibited by i/am dopamine (DA, 10 micrograms) - an effect which was reversed by the DA-antagonist, clozapine (5 mg/kg) pretreatment. Further, pretreatment of rats with clozapine or the DA-neurotoxin, 6-hydroxydopamine (6-OHDA, 10 micrograms, i/am) clearly reversed and/or antagonized the gastric cytoprotective effect of DAMEA (30 micrograms). The results indicate interactions between enkephalinergic and DAergic systems at the level of the CEA in the maintenance of gastric mucosal integrity during immobilization stress.

Morphine: some puzzles of well-known substance

The study of morphines influence on vocalization and movement, nociceptive reactivity in rats in free behavior, during trauma of an extremity, intraperitoneal acetic acid administration and two-hour restraint allow us to reveal specific analgesic actions of this substance in doses comparable to that used for pain relief in man. This action consisted in the almost disappearance of trauma-induced hyperalgesia or a significant reduction of visceral stimulation-induced hypalgesia without any significant changes in movement reactivity depression typical to these states. Moreover, analysis or morphines influence on movement reactivity depression in free behaving rats of different groups or parts of one group previously housed in different aversive conditions allow to understand the reasons of variability in the action of opiates and the significance of previous living conditions in determination of their effects. Furthermore, the enhancement of morphines action on movement reactivity 3-4 hours after a single naloxone administration was observed, which indicates the possibility of artificial regulation of opiate effects by direct action on opiate receptors by naloxone. Subchronic naloxone administration (0.5 mg/kg, 3 times per day for 3 days) led to substantial and longterm enhancement of morphines depressive effect on movement reactivity (20 and 105 hours after the last naloxone use) and to decrease in movement reactivity depression typical to restraint stress.

Opiate mechanisms in the central amygdala and gastric stress pathology in rats

Bilateral microinjections of the opiate antagonist naloxone (0.1, 1.0 and 10.0 micrograms) into the central nucleus of the amygdala (CEA) produced a significant potentiation of cold restraint-induced gastric pathology in rats. The opiate agonist, beta-endorphin (0.1, 1.0 and 10.0 micrograms), on the other hand, inhibited stress ulcer formation in a dose-related manner. Stress ulcer-attenuating effects were also seen with intra-CEA injections of the enkephalin analogs [D-Ala2,D-Leu5]enkephalin (10.0 micrograms) and [D-Ala2]Met-enkephalinamide (10.0 micrograms). Pretreatment of rats with naloxone (1.0 microgram) completely antagonized and even reversed the gastric cytoprotective effects of beta-endorphin (1.0 and 10.0 micrograms). The results indicate that the CEA is important in the gastric cytomodulatory effects of endogenous opiates during stressful experiences.

Effects of stress and beta-funal trexamine pretreatment on morphine analgesia and opioid binding in rats

This study was essentially an in vivo protection experiment designed to test further the hypothesis that stress induces release of endogenous opioids which then act at opioid receptors. Rats that were either subjected to restraint stress for 1 hr or unstressed were injected ICV with either saline or 2.5 micrograms of beta-funaltrexamine (beta-FNA), an irreversible opioid antagonist that alkylates the mu-opioid receptor. Twenty-four hours later, subjects were tested unstressed for morphine analgesia (tail-flick assay) or were sacrificed and opioid binding in brain was determined. [3H]D-Ala2NMePhe4-Gly5(ol)enkephalin (DAGO) served as a specific ligand for mu- opioid receptors, and [3H]-bremazocine as a general ligand for all opioid receptors. Rats injected with saline while stressed were significantly less sensitive to the analgesic action of morphine 24 hr later than were their unstressed counterparts. Beta-FNA pretreatment attenuated morphine analgesia in an insurmountable manner. Animals pretreated with beta-FNA while stressed were significantly more sensitive to the analgesic effect of morphine than were animals that received beta-FNA while unstressed, consistent with the hypothesis that stress induces release of endogenous opioids that would protect opioid receptors from alkylation by beta-FNA. beta-FNA caused small and similar decreases in [3H]-DAGO binding in brain of both stressed and unstressed animals. Stressed rats injected with saline tended to have increased levels of [3H]DAGO and [3H]-bremazocine binding compared to the other groups. This outcome may be relevant to the tolerance to morphine analgesia caused by stress.

Endogenous opiates: 1986

This is the ninth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1986. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic processes; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; activity; sex, pregnancy, and development; and some other behaviors.