Acute morphine treatment and morphine tolerance/dependence alter immunoreactive oxytocin levels in the mouse hippocampus

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.

Severity of Withdrawal Symptoms, Plasma Oxytocin Levels, and Treatment Outcome in Heroin Users Undergoing Acute Withdrawal

Pre-clinical studies show that, following chronic opioid exposure, oxytocin neurons exhibit over-excitation upon withdrawal, causing an increase in oxytocin brain and plasma levels. Relevant clinical data on humans are scarce. This study investigates the opioid withdrawal stress effect on oxytocin plasma levels in humans. We evaluated 57 male chronic heroin users in a residential detoxification program. We determined plasma oxytocin levels by ELISA and measured the stress effects of withdrawal using the COWS scale for opioid withdrawal, the VAS scale for craving, and the Hamilton scales for anxiety and depression on the second day of admission. Out of the 57 patients enrolled in the study, 27 completed the 21-day program, while the remaining 30 dropped out prior to completion. Plasma oxytocin levels were significantly higher in those individuals who dropped out than in those who completed the program. Participants who dropped out at some stage scored higher in the COWS, VAS-Craving, and Hamilton-anxiety scales, indicating a higher stress and explaining the higher oxytocin levels. In addition, plasma oxytocin levels correlated positively with the scores achieved in the COWS and Hamilton-anxiety scales. Higher withdrawal stress levels are associated with higher plasma oxytocin levels and early treatment discharge.

The effects of intranasal oxytocin in opioid-dependent individuals and healthy control subjects: a pilot study

RATIONALE

There has been an explosion of research on the potential benefits of the social neuropeptide oxytocin for a number of mental disorders including substance use disorders. Recent evidence suggests that intranasal oxytocin has both direct anti-addiction effects and pro-social effects that may facilitate engagement in psychosocial treatment for substance use disorders.

OBJECTIVES

We aimed to assess the tolerability of intranasal oxytocin and its effects on heroin craving, implicit association with heroin and social perceptual ability in opioid-dependent patients receiving opioid replacement therapy (ORT) and healthy control participants.

METHODS

We performed a randomized, double-blind, placebo-controlled, within- and between-subjects, crossover, proof-of-concept trial to examine the effects of oxytocin (40 international units) on a cue-induced craving task (ORT patients only), an Implicit Association Task (IAT), and two social perception tasks: the Reading the Mind in the Eyes Task (RMET) and The Awareness of Social Inference Test (TASIT).

RESULTS

Oxytocin was well tolerated by patients receiving ORT but had no significant effects on craving or IAT scores. There was a significant reduction in RMET performance after oxytocin administration versus placebo in the patient group only, and a significant reduction in TASIT performance after oxytocin in both the patient and healthy control groups.

CONCLUSIONS

A single dose of intranasal oxytocin is well tolerated by patients receiving ORT, paving the way for future investigations. Despite no significant improvement in craving or IAT scores after a single dose of oxytocin and some evidence that social perception was worsened, further investigation is required to determine the role oxytocin may play in the treatment of opioid use disorder.

CLINICAL TRIAL REGISTRATION

Methadone Oxytocin Option. ClinicalTrials.gov identifier: NCT01728909.

Central opioids and consumption of sweet tastants: when reward outweighs homeostasis

Numerous reports have described opioids as peptides involved in the regulation of food intake. The role of these endogenous substances appears to be linked with reward-dependent feeding, since injection of opioid receptor ligands alters consumption of palatable foods and solutions more readily than of non-palatable ones, and intake of such tastants affects the activity of the opioid system within the brain. Among a variety of available foods, those rich in sucrose and other sweet tastants, are extremely appealing to humans and laboratory animals. In the current review, we focus on the rewarding aspects of consummator behavior driven by opioids. We attempt to delineate opioid-dependent central mechanisms responsible for overconsumption of "rewarding" palatable diets, especially foods high in sugar that can potentially jeopardize homeostasis.

Influence of oxytocin on nociception and morphine antinociception

In the hot plate test the intracerebroventricular (i.c.v.) injection of oxytocin produced a significant decrease in nociception, starting from the dose of 1 microgram/rat. A comparable effect was obtained with 10-200 times higher intraperitoneal (i.p.) doses. The i.c.v. injection of the oxytocin antagonist d(CH2)5-Tyr(Me)-[Orn8]-vasotocin, while having no influence per se, completely prevented the antinociceptive effect of an equal i.c.v. dose of oxytocin. The antinociceptive effect of oxytocin was also prevented by naltrexone, and oxytocin caused a small but significant increase of the antinociceptive effect of morphine and of its duration. These data indicate that pharmacological amounts of oxytocin produce antinociception, that occurs through the activation of oxytocin receptors; endogenous opioid systems seem to be involved altogether.

Chromatographically identified oxytocin in the human peripheral nervous system

Immunoreactive oxytocin (OXT) detected in extracts of human coeliac ganglia and nn. vagi was characterized by high-performance liquid chromatography (HPLC). HPLC/RIA examinations demonstrated that a major part of the immunoreactive material in both investigated areas co-eluted with a reference synthetic OXT, but in the extracts of coeliac ganglia a second immunoreactive peak was also observed.

Release of oxytocin into blood and into cerebrospinal fluid induced by naloxone in anaesthetized morphine-dependent rats: the role of the paraventricular nucleus

Abstract Opioid actions on oxytocin secretion into blood and cerebrospinal fluid (CSF) were investigated in urethane-anaesthetized female rats after intracerebroventricular (icv) infusion of morphine sulphate or vehicle for 5 days. Serial femoral arterial blood samples and cisterna magna CSF samples were collected for radioimmunoassay. Naloxone was given to assess endogenous opioid tone in icv vehicle-infused rats and to precipitate withdrawal in morphine-dependent animals. Initial plasma oxytocin concentration was not affected by icv morphine infusion. In control rats receiving icv vehicle, naloxone increased plasma oxytocin 11-fold within 5 min, and in icv morphine-infused rats, naloxone increased plasma oxytocin 80-fold within 5 min. In both groups, 90 min after naloxone plasma oxytocin was still 5 and 10 times, respectively, the initial concentration. Without naloxone, neither plasma nor CSF oxytocin concentration changed significantly with time (up to 90 min) in either icv treatment group. In the icv vehicle group, there was a 2-fold increase in CSF oxytocin 90 min after naloxone. In the icv morphine-infused group, CSF oxytocin was increased 5-fold 40 min after naloxone. In another group of icv morphine-infused rats, intravenous infusion of oxytocin to achieve plasma levels similar to those seen after naloxone, did not significantly increase CSF oxytocin. In a further group of icv morphine-infused rats, [(3)H]oxytocin was infused intravenously immediately after naloxone was given; in these rats oxytocin transfer from blood to CSF could account at most for only 20% of the increase in CSF oxytocin after naloxone. A further group of rats underwent bilateral microknife ablation of the paraventricular nuclei (PVN) 9 days before icv vehicle or morphine infusions were started; blood and CSF samples were collected under urethane anaesthesia. Initial concentrations of oxytocin in CSF and in plasma were similar in both groups with PVN ablation. In all PVN-lesioned rats initial plasma concentrations of oxytocin were undetectable (<5 pg/ml) and thus less than in intact rats. In contrast, initial levels of oxytocin in CSF were 8-fold greater in PVN-lesioned rats than in intact animals. Naloxone increased plasma oxytocin concentration in the icv vehicle group at least 10-fold within 30 min and in the icv morphine group at least 100-fold within 5 min. CSF oxytocin in the icv vehicle group was not altered by naloxone, but in the icv morphine group CSF oxytocin was increased 5-fold 40 min after naloxone. There were no consistent differences between the icv vehicle- and icv morphine-treated groups in the initial plasma levels of vasopressin, growth hormone and adrenocorticotrophin; PVN ablation did not affect adrenocorticotrophin levels. After naloxone growth hormone levels did not change, vasopressin concentration rose moderately only after 90 min and only in the icv vehicle-treated group, and adrenocorticotrophin concentrations decreased with time whether or not naloxone was given. The results imply an endogenous opioid tone on neurons releasing oxytocin into CSF, and morphine-dependence of these neurons. Furthermore, in PVN-lesioned rats, magnocellular supraoptic neurons could be a source of oxytocin release into CSF.

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.

Characterization of oxytocin immunoreactivity in human sympathetic paravertebral ganglia

Immunoreactive oxytocin (IR-OXT) detected in extracts of human lumbar sympathetic paravertebral ganglia was characterized by high-performance liquid chromatography (HPLC). The immunoreactive substance was found to elute at the same position as the reference preparation of oxytocin (OXT). The results revealed the presence of chromatographically identified OXT in human sympathetic ganglia.

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.

Presence of chromatographically identified oxytocin in human sensory ganglia

Oxytocin-like immunoreactivity (IR-OXT) was detected in extracts of human spinal L5 and Gasserian ganglia by a radioimmunoassay (RIA) specific to oxytocin (OXT) and was identified by high-performance liquid chromatography (HPLC). One of the two immunoreactive peaks obtained on HPLC was found to elute at the same position as the OXT standard. The results reveal the presence of chromatographically identified OXT immunoreactivity in human sensory ganglia.

Effects of beta-endorphin2-9 on arginine-8-vasopressin and oxytocin levels in hypothalamic and limbic brain regions

Immunoreactive arginine-8-vasopressin (AVP) and oxytocin (OXT) were measured in rat hypothalamic and limbic brain regions after the intracerebroventricular administration of beta-endorphin fragment 2-9 (beta E2-9). The peptide decreased the AVP content of the hippocampus and the OXT levels in the septum and amygdala. The present data favor the view that beta E2-9 interacts with limbic AVP- and OXT-systems.

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.

Development of morphine tolerance under tonic control of brain oxytocin

Acute morphine tolerance was induced in mice by subcutaneous (s.c.) injection of a high dose (30 or 100 mg/kg) of morphine. The degree of tolerance was estimated 5 h later. Intracerebroventricular (i.c.v.) injection of graded doses of oxytocin (OXT) dose-dependently attenuated the development of tolerance. i.c.v. injection of a specific anti-OXT serum, on the other hand, facilitated the development of tolerance. Neither OXT nor anti-OXT serum had any effect on the pain sensitivity in morphine-naive mice; nor did these treatments modify the antinociceptive action of a single morphine treatment. It is concluded that the endogenous OXT of the mouse brain is normally involved in the adaptive response of the organism, leading to the development of morphine tolerance.