A study on seasonal variation in the development of morphine dependence in rats

Involvement of supraspinal and peripheral naloxonazine-insensitive opioid receptor sites in the expression of μ-opioid receptor agonist-induced physical dependence

Withdrawal syndrome after the cessation of μ-opioid receptor agonists remains an obstacle in the clinical treatment of pain. There is limited information available on the mechanisms that underlie the expression of the withdrawal signs of opioids, and especially regarding the involvement of μ-opioid receptor subtypes and the location of the responsible opioid receptors. Therefore, the present study was designed to determine the mechanism of the expression of withdrawal signs in μ-opioid receptor agonist-dependent mice. Morphine-, oxycodone- and fentanyl-dependent mice showed a marked loss of body-weight and other signs of withdrawal after a naloxone challenge. Interestingly, the phenotype of the withdrawal signs for morphine and oxycodone was different from that of fentanyl. Furthermore, pretreatment with naloxonazine (so-called μ1-opioid receptor antagonist), did not significantly alter the withdrawal signs precipitated by naloxone in these μ-opioid receptor agonist-dependent mice, whereas the peripherally limited opioid receptor antagonist naloxone methiodide significantly increased the loss of body-weight accompanied by diarrhea, indicating that a peripheral naloxonazine-insensitive site for opioid receptors, as an adaptation mechanism, plays an important role in the expression of at least the loss of body-weight. On the other hand, i.c.v. treatment with naloxone methiodide potently induced jumping behavior and trembling in morphine-dependent mice. These results indicate that the prolonged activation of supraspinal μ-opioid receptors plays a role in most of the physical dependence induced by μ-opioid receptor agonists in mice. Thus, the withdrawal symptoms observed after the cessation of μ-opioid receptor agonists are distinctly regulated though supraspinal and peripheral naloxonazine-insensitive sites of μ-opioid receptors.

Lack of sex-related differences in the prevention by baclofen of the morphine withdrawal syndrome in mice

In previous studies we have demonstrated a possible interaction between the GABAergic and opioid systems involved in the antinociceptive effect of the GABA(B) agonist, baclofen (BAC). On the other hand, sex differences have been observed for the antinociceptive effect of morphine (MOR). In the present study, we analyzed sex-related differences in the MOR abstinence syndrome and its prevention with BAC. Prepubertal male and female Swiss-Webster albino mice (27-33 g) were rendered dependent by intraperitoneal (i.p.) injection of MOR (2, 4 and 8 mg/kg), twice daily for 9 days. On the tenth day the dependent animals were divided into two groups: one received naloxone (NAL) (6 mg/kg, i.p.) 60 min after the last dose of MOR, to precipitate the abstinence syndrome; the other group received BAC (2 mg/kg, i.p.) followed by NAL (6 mg/kg, i.p.), injected 30 and 60 min after the last dose of MOR, respectively. Behavioral signs were recorded in the open field for 30 min. Although there were sex differences in the MOR withdrawal syndrome, we found a lack of sex differences in the prevention of the MOR abstinence syndrome by BAC.