Baclofen reestablishes ?-opioid receptor levels modified by morphine withdrawal syndrome in either sex

Baclofen prevents morphine rewarding effects and associated biochemical alterations in male and female mice

Previous studies from our laboratory have shown sex differences in the behavioral, molecular, and neurochemical manifestations of morphine withdrawal and they were related to an increased sensitivity to morphine effects in males. In addition, we observed an interaction between the GABAergic and opioid systems that could also be sex-dependent. Baclofen, a GABAB receptor agonist, prevented the somatic expression and the molecular and neurochemical changes induced by morphine withdrawal syndrome in mice. On the contrary, little is known about baclofen effects in the rewarding properties of morphine in male and female mice. The present study aimed to explore the effect of baclofen (1, 2 and 3 mg/kg, i.p.) pretreatment in the rewarding effects induced by morphine (7 mg/kg, s.c.) and its effect on c-Fos and brain-derived neurotrophic factor (BDNF) expression induced by the rewarding properties of morphine in prepubertal male and female mice. Baclofen (2 mg/kg) pretreatment prevented the rewarding effects of morphine only in male mice, while baclofen (3 mg/kg) reduced these effects in both sexes. Moreover, the rewarding effects of morphine were associated with a decrease of BDNF and c-Fos expression cingulate cortex, nucleus accumbens shell, cornu ammonis 1 (CA1), and cornu ammonis 3 (CA3) areas of the hippocampus only in male mice. In addition, baclofen pretreatment prevented these changes in BDNF, but not in c-Fos expression. In conclusion, our results show that GABAB receptors have a regulatory role in the rewarding effects of morphine that could be of interest for a potential future therapeutic application in opioid use disorders.

Review of addiction risk potential associated with adolescent opioid use

Adolescence is a critical period of development with robust behavioral, morphological, hormonal, and neurochemical changes including changes in brain regions implicated in the reinforcing effects of drugs such as opioids. Here we examine the preclinical and, where appropriate complementary clinical literature, for the behavioral and neurological changes induced by adolescent opioid exposure/use and their long-term consequences during adulthood. Adolescent opioid exposure results in a widened biphasic shift in reinforcement with increased impact of positive rewarding aspects during initial use and profound negative reinforcement during adulthood. Females may have enhanced vulnerability due to fast onset of antinociceptive tolerance and reduced severity of somatic withdrawal symptoms during adolescence. Overall, adolescent opioid exposure, be it legally prescribed protracted intake or illicit consumption, results in significant and prolonged consequences of increased opioid reward concomitant with reduced analgesic efficacy and exacerbated somatic withdrawal severity during opioid use/exposure in adulthood. These findings are highly relevant to physicians, parents, law makers, and the general public as adolescent opioid exposure/misuse results in heightened risk for substance use disorders.

Mecamylamine-precipitated nicotine withdrawal syndrome and its prevention with baclofen: an autoradiographic study of α4β2 nicotinic acetylcholine receptors in mice

A previous study from our laboratory showed that baclofen (BAC, GABAB receptor agonist) was able to prevent the behavioral expression of nicotine (NIC) withdrawal syndrome. To further investigate the mechanisms underlying this effect, we conducted this study, with the aims of analyzing α4β2 nicotinic receptor density during NIC withdrawal and, in case we found any changes, of determining whether they could be prevented by pretreatment with BAC. Swiss Webster albino mice received NIC (2.5 mg/kg, s.c.) 4 times daily, for 7 days. On the 8th day, NIC-treated mice received the nicotinic antagonist mecamylamine (MEC; 2 mg/kg, i.p.) 1 h after the last dose of NIC. A second group of NIC-treated mice received BAC (2 mg/kg, i.p.) prior to MEC administration. Thirty minutes after MEC, mice were sacrificed and brain autoradiography with [(3)H]epibatidine was carried out at five different anatomical levels. Autoradiographic mapping showed a significant increase of α4β2 nicotinic receptor labeling during NIC withdrawal in the nucleus accumbens shell (AcbSh), medial habenular nucleus (HbM), thalamic nuclei, dorsal lateral geniculate (DLG) nucleus, fasciculus retroflexus (fr), ventral tegmental area, interpeduncular nucleus and superior colliculus. BAC pretreatment prevented the increased α4β2 nicotinic receptor binding sites in the AcbSh, MHb, thalamic nuclei, DLG nucleus and fr. The present results suggest a relationship between BAC's preventive effect of the expression of NIC withdrawal signs, and its ability to restore the changes in α4β2 nicotinic receptor labeling, evidenced in specific brain areas in NIC withdrawn animals.

Ability of baclofen to prevent somatic manifestations and neurochemical changes during nicotine withdrawal

BACKGROUND

Nicotine (NIC), the major active component of tobacco, is critical in the maintenance of the smoking habit. The aims of the present study were to analyze the behavioural and neurochemical variations during NIC withdrawal syndrome in mice, and whether they are prevented with baclofen (BAC, GABA(B) receptor agonist).

METHODS

Swiss-Webster albino mice received NIC (2.5 mg/kg, s.c.) 4 times daily, for 7 consecutive days. On day 8 (the day of the experiment), NIC-treated mice received the nicotine antagonist mecamylamine (MEC, 2 mg/kg, i.p.) 1h after the last dose of NIC. A second group of dependent mice received BAC (2mg/kg, i.p.) before MEC-precipitated abstinence. The somatic signs were measured for 30 min. Dopamine (DA), serotonin (5-hydroxytryptamine; 5-HT) and its metabolites concentrations were determined by HPLC in the striatum, cortex and hippocampus.

RESULTS

The global score was greater in the abstinent group compared to the control group. Moreover, the global score time course showed a higher increase at 10 min compared to the global score at 5 min or 30 min after MEC-precipitated NIC withdrawal. In addition, the global score was attenuated by BAC. The DA and dihydroxyphenyl acetic acid (DOPAC) cortical levels decreased in the abstinent group, while BAC reestablished these levels 10 min after NIC withdrawal. Furthermore, DA and 5-HT striatal levels decreased during NIC withdrawal, and BAC reverted this decrease.

CONCLUSION

In conclusion, the prevention of NIC withdrawal signs by BAC could be related to changes in dopaminergic and serotonergic activity.

Sex differences in opioid analgesia, hyperalgesia, tolerance and withdrawal: central mechanisms of action and roles of gonadal hormones

This article reviews sex differences in opiate analgesic and related processes as part of a Special Issue in Hormones and Behavior. The research findings on sex differences are organized in the following manner: (a) systemic opioid analgesia across mu, delta and kappa opioid receptor subtypes and drug efficacy at their respective receptors, (b) effects of the activational and organizational roles of gonadal steroid hormones and estrus phase on systemic analgesic responses, (c) sex differences in spinal opioid analgesia, (d) sex differences in supraspinal opioid analgesia and gonadal hormone effects, (e) the contribution of genetic variance to analgesic sex differences, (f) sex differences in opioid-induced hyperalgesia, (g) sex differences in tolerance and withdrawal-dependence effects, and (h) implications for clinical therapies.

Pharmacokinetic aspects of naloxone-precipitated morphine withdrawal in male and female prepubertal mice

It has been shown that the expression of the morphine (MOR) withdrawal syndrome precipitated by naloxone (NAL) is more intense in male mice than in females, but the reasons for this phenomenon remain uncertain. The purpose of the present study was to evaluate whether this sexual dimorphism might be due to differences in MOR and/or NAL plasma levels after a chronic treatment with MOR. Prepubertal Swiss male and female mice were rendered dependent by intraperitoneal (i.p.) injection of MOR (2 mg/kg), twice daily for 9 days. On day 10 dependent mice received NAL (6 mg/kg, i.p.) 60 min after MOR injection. Blood samples were taken at different times in order to determine MOR and NAL plasma levels by gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC), respectively. Pharmacokinetic analysis showed no differences between male and female mice either for MOR or for NAL. In conclusion, although males and females respond differentially to NAL-precipitated withdrawal, this dimorphic behavior would not be influenced by a pharmacokinetic factor.

Morphine withdrawal syndrome and its prevention with baclofen: Autoradiographic study of mu-opioid receptors in prepubertal male and female mice

Although the expression of the morphine (MOR) withdrawal syndrome is more marked in male mice than in females, we have demonstrated that the GABAB agonist baclofen (BAC) is able to attenuate MOR withdrawal signs in either sex. In order to extend these previous observations, the aim of the present study was to evaluate the mu-opioid receptor labeling in various brain areas in mice of either sex, during MOR withdrawal and its prevention with BAC. Prepubertal Swiss-Webster mice were rendered dependent by intraperitonial (i.p.) injection of MOR (2 mg/kg) twice daily for 9 days. On the 10th day, dependent animals received naloxone (NAL; 6 mg/kg, i.p.) 60 min after MOR, and another pool of dependent mice received BAC (2 mg/kg, i.p.) previous to NAL. Thirty minutes after NAL, mice were sacrificed and autoradiography with [3H]-[D-Ala2, N-Me-Phe4, -glycol5] enkephalin (DAMGO) was carried out on mice brains at five different anatomical levels. Autoradiographic mapping showed a significant increase of mu-opioid receptor labeling during MOR withdrawal in nucleus accumbens core (NAcC), caudate putamen (CPu), mediodorsal thalamic nucleus (MDTh), basolateral and basomedial amygdala, and ventral tegmental area vs. respective control groups in male mice. In contrast, opiate receptor labeling was not significantly modified in any of the brain areas studied in withdrawn females. BAC reestablished mu-opioid receptor binding sites during MOR withdrawal only in NAcC of males, and a similar tendency was observed in CPu and MDTh, even when it was not statistically significant. The sexual dimorphism observed in the present study confirms previous reports indicating a greater sensitivity of males in response to MOR pharmacological properties. The present results suggest that the effect of BAC in preventing the expression of MOR withdrawal signs could be related with the ability of BAC to reestablish the mu-opioid receptor labeling in certain brain areas.

Morphine withdrawal syndrome: Involvement of the dopaminergic system in prepubertal male and female mice

Morphine (MOR) withdrawal signs are more marked in males than in females. Considering that the influence of the dopaminergic system on these differences is unclear, we analyzed dopamine (DA) and dihydroxyphenylacetic-acid (DOPAC) brain levels during naloxone (NAL)-precipitated withdrawal as well as the involvement of D(1) and D(2) receptors in the expression of MOR withdrawal in either sex. Prepubertal Swiss-Webster mice received MOR (2 mg/kg, i.p.) twice daily for 9 days. On the tenth day, dependent animals received NAL (6 mg/kg, i.p.) after MOR and were sacrificed 30 min later. DA and DOPAC concentrations were determined in different brain areas using HPLC with electrochemical detection. Other pool of mice received either a D(1) (SCH 23390; 0.2 mg/kg, i.p.) or D(2) (raclopride; 0.3 mg/kg, i.p.) receptor antagonist before NAL and withdrawal signs were evaluated. DA and DOPAC levels only decreased in striatum and cortex of withdrawn males. Conversely, both DA receptor antagonists decreased the expression of MOR withdrawal signs in either sex. The neurochemical sex differences described here could partially explain the behavioral sex differences observed during MOR withdrawal. Additionally, SCH-23390 and raclopride effects suggest an important role of both DA receptors in the expression of MOR withdrawal in males and females.

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.