Development of morphine tolerance under tonic control of brain oxytocin

Establishment of a biomarker of peripheral stress in opioid addicts based on the hypothalamic-pituitary-adrenal axis-The improvement effect of exercise

PURPOSE

This study aims to investigate the relationship between peripheral blood oxytocin (OT), vasopressin (AVP), and subjectively perceived stress and cortisol in male opioid addicts based on the hypothalamic-pituitary-adrenal (HPA) axis. We also investigate the impact of exercise on reducing subjectively perceived stress, craving level, negative reinforcement, anxiety, sleep quality, plasma OT, AVP, and cortisol levels.

METHODS

Participants were divided into 28 subjects in the low-stress control group (LSC group), 29 subjects in the medium-high stress control group (MTHSC group), and 28 subjects in the moderate-high-stress exercise group (MTHSE group), based on their subjectively perceived stress levels. Subjects in the MTHSE group performed 12 weeks of combined aerobic resistance training (60 min per day, 5 days per week). Plasma OT, AVP, and cortisol concentrations were analyzed via Elisa. PSQI was used to assess the subjective perceived stress, craving, negative reinforcement, anxiety, and sleep quality level, respectively. Mixed-effects ANOVA and Pearson correlation analysis were employed to explore the impact and correlation between different parameters.

RESULT

Plasma OT levels significantly increased (95% CI: -7.48, -2.26), while plasma AVP (95% CI: 2.90, 4.10), and cortisol (95% CI: 19.76, 28.17) levels significantly decreased in the MTHSE group after exercise. The PSS (95% CI: 1.756, 4.815), "Desire and Intention" (95% CI: 1.60, 2.71), and "Negative reinforcement" (95% CI: 0.85, 1.90) (DDQ), SAS (95% CI: 17.51, 26.06), and PSQI (95% CI: 1.18, 3.25) scores of the MTHSE group were significantly decreased after exercise. Plasma OT, plasma cortisol, craving, negative reinforcement and anxiety were negatively correlated. Plasma AVP was positively correlated with craving.

CONCLUSION

As an auxiliary treatment, exercise improves the plasma OT, AVP, and cortisol levels of opioid addicts, and reduces their subjective perceived stress level, desire, negative reinforcement level, anxiety level, and sleep quality. In addition, peripheral plasma OT, AVP, and cortisol may play a role as potential peripheral biomarkers to predict stress in male opioid addicts.

Oxytocin signaling in the treatment of drug addiction: Therapeutic opportunities and challenges

Drug addiction is one of the leading causes of mortality worldwide. Despite great advances were achieved in understanding the neurobiology of drug addiction, the therapeutic options are severely limited, with poor effectiveness and serious side effects. The neuropeptide oxytocin (OXT) is well known for its effects on uterine contraction, sexual/maternal behaviors, social affiliation, stress and learning/memory by interacting with the OXT receptor and other neuromodulators. Emerging evidence suggests that the acute or chronic exposure to drugs can affect the OXT system. Additionally, OXT administration can ameliorate a wide range of abused drug-induced neurobehavioral changes. Overall, OXT not only suppresses drug reward in the binge stage of drug addiction, but also reduces stress responses and social impairments during the withdrawal stage and, finally, prevents drug/cue/stress-induced reinstatement. More importantly, clinical studies have also shown that OXT can exert beneficial effects on reducing substance use disorders of a series of drugs, such as heroin, cocaine, alcohol, cannabis and nicotine. Thus, the present review focuses on the role of OXT in treating drug addiction, including the preclinical and clinical therapeutic potential of OXT and its analogs on the neurobiological perspectives of drugs, to provide a better insight of the efficacy of OXT as a clinical addiction therapeutic agent.

The impact of early life stress on the central oxytocin system and susceptibility for drug addiction: Applicability of oxytocin as a pharmacotherapy

Early life trauma is strongly associated with an increased vulnerability to abuse illicit drugs and the impairment of neural development. This includes alterations to the development of the oxytocin system, which plays a pivotal role in the regulation of social behaviours and emotion. Dysregulation of this important system also contributes to increased susceptibility to develop drug addiction. In this review, we provide an overview of the animal models of early life stress that are widely used, and discuss the impact that early life stress has on drug-taking behaviour in adolescence and adulthood in both sexes. We link this to the changes that early life stress has on the endogenous oxytocin system, and how exogenously administered oxytocin may help to re-establish functioning of the system, and in turn, reduce drug-taking behaviour.

The neurocircuitry involved in oxytocin modulation of methamphetamine addiction

The role of oxytocin in attenuating the abuse of licit and illicit drugs, including the psychostimulant methamphetamine, has been examined with increased ferocity in recent years. This is largely driven by the potential application of oxytocin as a pharmacotherapy. However, the neural mechanisms by which oxytocin modulates methamphetamine abuse are not well understood. Recent research identified an important role for the accumbens core and subthalamic nucleus in this process, which likely involves an interaction with dopamine, glutamate, GABA, and vasopressin. In addition to providing an overview of methamphetamine, the endogenous oxytocin system, and the effects of exogenous oxytocin on drug abuse, we propose a neural circuit through which exogenous oxytocin modulates methamphetamine abuse, focusing on its interaction with neurochemicals within the accumbens core and subthalamic nucleus. A growing understanding of exogenous oxytocin effects at a neurochemical and neurobiological level will assist in its evaluation as a pharmacotherapy for drug addiction.

Vasoactive intestinal polypeptide induces analgesia and impairs the antinociceptive effect of morphine in mice

Vasoactive intestinal polypeptide (VIP) has numerous regulatory roles in peripheral, endocrine organs and in the central nervous system. The present study related to the effects of centrally (intracerebroventricularly) administered VIP on pain sensitivity and on opiate tolerance and dependence in intact male CFLP mice. VIP was analgesic when administered alone centrally. Naloxone treatment abolished this analgesic effect. VIP decreased the analgesic effect of a single subcutaneous morphine injection and the development of chronic tolerance to morphine. Morphine withdrawal signs were not significantly affected after VIP pretreatment. These findings indicate that VIP may play a role in pain sensitivity and that an opiate component may participate in this effect.

Oxytocin and addiction: a review

Neuropeptides affect adaptive central nervous system processes related to opiate ethanol and cocaine addiction. Oxytocin (OXT), a neurohypophyseal neuropeptide synthesized in the brain and released at the posterior pituitary, also is released in the central nervous system (CNS). OXT acts within the CNS and has been shown to inhibit the development of tolerance to morphine, and to attenuate various symptoms of morphine withdrawal in mice. In rats, intravenous self-administration of heroin was potently decreased by OXT treatment. In relation to cocaine abuse, OXT dose-dependently decreased cocaine-induced hyperlocomotion and stereotyped grooming behavior. Following chronic cocaine treatment, the behavioral tolerance to the sniffing-inducing effect of cocaine was markedly inhibited by OXT. Behavioral sensitization to cocaine, on the other hand, was facilitated by OXT. OXT receptors in the CNS--mainly those located in limbic and basal forebrain structures--are responsible for mediating various effects of OXT in the opiate- and cocaine-addicted organism. Dopaminergic neurotransmission--primarily in basal forebrain structures--is another important biochemical mediator of the central nervous system effects of OXT. Tolerance to ethanol (e.g. hypothermia-inducing effect of ethanol) also was inhibited by OXT.

Is systemically administered oxytocin an analgesic in rats?

The effect of systemically administered oxytocin and a specific oxytocin antagonist, 1-deamino-2-D-Tyr(OEt)-4-Thr-8-Orn-oxytocin, on heat pain sensitivity was examined in rats. Intraperitoneal (i.p.) oxytocin at 1 mg/kg, but not at 0.1 and 0.3 mg/kg, significantly increased response latencies on the hot-plate test. However, the rats displayed clear signs of sedation, motor impairment and vasoconstriction after 1 mg/kg oxytocin. Skin temperature on the plantar surface of the hind paws was also significantly decreased by this dose of oxytocin. The oxytocin antagonist (1 mg/kg i.p.) did not influence response latency. Since increased response latency was not the only behavioral effect of oxytocin, we conducted electrophysiological experiments to examine the effect of systemic oxytocin on the nociceptive flexor reflex in decerebrate, spinalized, unanesthetized rats. Oxytocin at 0.1 mg/kg i.p. did not influence flexor reflex magnitude, mean blood pressure or heart rate. Oxytocin at 0.3 and 1 mg/kg caused a gradual increase in blood pressure with stronger effect observed with 1 mg/kg. Neither 0.3 nor 1 mg/kg oxytocin significantly influenced the flexor reflex magnitude and heart rate. We thus conclude that systemic oxytocin did not produce analgesia in rats and the observed increase in response latency in the hot-plate test may result from the sedative and vasoconstrictive effects of this peptide. Furthermore, since the oxytocin antagonist did not significantly alter response latency on the hot-plate test, it is unlikely that endogenous oxytocin exerts a tonic effect on the pain threshold in rats.

Role of oxytocin in the neuroadaptation to drugs of abuse

Oxytocin (OXT), a neurohypophyseal hormone, has a wide range of behavioral effects outside its classic peripheral endocrine functions. OXT involvement in adaptive central nervous system processes has been demonstrated as an inhibitory, amnestic action on learning and memory in different paradigms. Because adaptation and learning are likely to be involved in the neural events leading to drug tolerance and dependence, the question logically arose whether OXT is able to influence the development of tolerance of and dependence on abused drugs. In this review, we summarize our results on the effects of OXT on opiate (including morphine, heroin, and the endogenous opiates beta-endorphin and enkephalin) tolerance and dependence, heroin self-administration, psychostimulant-induced behavioral changes, and behavioral tolerance and sensitization. The sites and mechanisms of action and the possible physiological role of OXT are also discussed. In the first part of this review the effects of exogenously administered OXT on both the acute and chronic behavioral effects of opiates and psychostimulants have been summarized. OXT inhibited the development of tolerance to morphine, heroin, beta-endorphin, and enkephalin, OXT also inhibited the development of cross-tolerance between the predominantly mu-agonist heroin and the predominantly delta-agonist enkephalin in mice. Naloxone-precipitated morphine withdrawal syndrome was also attenuated by OXT. Heroin self-administration was decreased by OXT administration in heroin-tolerant rats. OXT inhibited cocaine-induced exploratory activity, locomotor hyperactivity, and stereotyped behavior in rats and in mice. Behavioral tolerance to cocaine was also attenuated by OXT. On the contrary, OXT stimulated the development of behavioral sensitization to cocaine. OXT did not alter the stereotyped behavior induced by amphetamine. In the second series of experiments, the sites of action of OXT on drug-related behavior were investigated. Intracerebro-ventricular (ICV) and intracerebral (IC) administration of an OXT-receptor antagonist inhibited the effects of peripherally administered OXT on morphine tolerance, heroin self-administration, and cocaine-induced sniffing behavior. This suggests the central, intracerebral location of OXT target sites. Local IC microinjection of OXT in physiological doses into the posterior olfactory nucleus, tuberculum olfactorium, nucleus accumbens, central amygdaloid nucleus, and the hippocampus inhibited the development of tolerance to and dependence on morphine as well as cocaine-induced sniffing behavior and tolerance to cocaine. The physiological role of endogenous OXT in acute morphine tolerance has also been demonstrated, since OXT antiserum (ICV) and OXT-receptor antagonist (injected into the basal forebrain structures) potentiated the development of morphine tolerance. Finally, we investigated the possible mechanisms of action of OXT on drug related behavior. Both morphine tolerance and dependence, and cocaine administration, increased dopamine utilization in the mesencephalon and in the nucleus accumbens, respectively. OXT treatment decreased the alpha-methylparatyrosine-induced dopamine utilization in the mesencephalon and in the nucleus accumbens-septal complex. Chronic OXT treatment decreased the number of apparent binding sites of dopamine in the basal forebrain area. It also inhibited a cocaine-induced increase in dopamine utilization in the nucleus accumbens, but not in the striatum. In light of this information, it appears that OXT inhibits the development of opiate tolerance, dependence, and self-administration as well as the acute behavioral actions of and chronic tolerance to cocaine. This suggests the possible role of this neuropeptide in the regulation of drug abuse. Therefore, OXT may act as a neuromodulator on dopaminergic neurotransmission in limbic-basal forebrain structures to regulate adaptive CNS processes leading to drug addiction.

Central functions of oxytocin

Oxytocin, the peptide well-known for its hormonal role in parturition and lactation, is present in several extrahypothalamic brain areas besides the neurohypophyseal system. The peptide is found in neurons which send their projections to brain areas containing specific oxytocin-binding sites. Oxytocin is also released from its synapses in a calcium-dependent fashion and may be the precursor of potent behaviorally active neuropeptides. These findings suggest that this ancient neuropeptide acts as a neurotransmitter in the central nervous system. We have attempted to review the most recent behavioral, morphological, electrophysiological and neurochemical studies providing evidence that oxytocin plays an important role in the expression of central functions, such as maternal behavior, sexual behavior (penile erection, lordosis and copulatory behavior), yawning, memory and learning, tolerance and dependence mechanisms, feeding, grooming, cardiovascular regulation and thermoregulation.

Selective attenuation of cocaine-induced stereotyped behaviour by oxytocin: putative role of basal forebrain target sites

The effects of oxytocin (OXT), arginine- and lysine-vasopressin (AVP and LVP) and an OXT-receptor antagonist on cocaine-induced sniffing behaviour were investigated in rats. OXT, but not AVP or LVP injected subcutaneously (s.c.) attenuated cocaine-induced sniffing. The effect of OXT (s.c.) was inhibited by an OXT-receptor antagonist administered intracerebroventricularly (i.c.v.). I.c.v. administration of different doses of OXT in nanogram quantities caused a dose-dependent attenuation of cocaine-induced sniffing. Local cerebral microinjection of OXT into the accumbens nucleus and olfactory tubercle but not into the olfactory nucleus, central amygdaloid nucleus or caudate nucleus, inhibited the cocaine-induced sniffing behaviour. These results demonstrate that OXT selectively attenuates the cocaine-induced stereotyped behaviour through basal forebrain target sites.

Endogenous oxytocin inhibits morphine tolerance through limbic forebrain oxytocin receptors

It has previously been shown that endogenous oxytocin (OXT) inhibits the development of acute morphine tolerance. The role of OXT receptors in the central nervous system (CNS) was therefore studied by using a specific OXT receptor antagonist, N-acetyl-(2-O-methyltyrosin)-OXT (ACME-OXT). ACME-OXT (1 pg) was injected into the posterior olfactory nucleus, central amygdaloid nucleus, ventral hippocampus, caudate nucleus or lateral cerebral ventricle. The antagonist facilitated the development of tolerance to morphine when injected into the posterior olfactory nucleus, central amygdaloid nucleus or ventral hippocampal areas, which are known to contain OXT binding sites. When administered into the caudate nucleus (with no OXT binding sites) or the lateral cerebral ventricle, it had no effect on morphine tolerance. Our results suggest that the limbic forebrain OXT receptors play an important inhibitory role in adaptive responses to morphine.

Limbic oxytocin and arginine 8-vasopressin in morphine tolerance and dependence

Immunoreactive oxytocin (OXT) and arginine8-vasopressin (AVP) levels were measured in limbic areas of the mouse brain (hippocampus, amygdala and basal forebrain). Peptides were measured by radioimmunoassay (RIA). Acute morphine treatment caused a naloxone-reversible increase in OXT content in all three brain regions. The AVP contents of the same brain areas, on the other hand, were not affected by acute morphine treatment. In mice rendered tolerant to/dependent on morphine with subcutaneous morphine pellets, the OXT levels in the limbic brain structures were in the control range (basal forebrain and amygdala) or even decreased (hippocampus). In the latter brain structure of the tolerant animals, the AVP content was also decreased. Naloxone-precipitated withdrawal syndrome in the tolerant/dependent animals resulted in abrupt increases in the OXT and AVP levels of the hippocampus and in the OXT content of the basal forebrain structures.

Effect of oxytocin on acute enkephalin tolerance in mice

Intracerebroventricular (i.c.v.) administration of a test dose of 0.5 microgram D-Met2-Pro5-enkephalinamide (ENK) resulted in analgesia. Acute tolerance developed if a tolerance-inducing high dose of 5 micrograms ENK was administered i.c.v. prior to the test dose. Tolerance appeared as a reduction of the analgesic effect of the test dose. Subcutaneous (s.c.) injection of oxytocin (OXT) inhibited the development of tolerance. It is suggested that OXT is able to modify the development of tolerance to enkephalins, which are ligands of delta-opiate rather than mu-opiate receptors.