Maria M. Backer

The antinociceptive effect of venlafaxine in mice is mediated through opioid and adrenergic mechanisms.

The antinociceptive effects of the novel phentylethylamine antidepressant drug venlafaxine and its interaction with various opioid, noradrenaline and serotonin receptor subtypes were evaluated. When mice were tested with a hotplate analgesia meter, venlafaxine induced a dose-dependent antinociceptive effect following i.p. administration with an ED50 of 46.7 mg/kg (20.5; 146.5; 95% CL). Opioid, adrenergic and serotoninergic receptor antagonists were tested for their ability to block venlafaxine antinociception. Venlafaxine-induced antinociception was significantly inhibited by naloxone, nor-BNI and naltrindole but not by beta-FNA or naloxonazine, implying involvement of kappa1- and delta-opioid mechanisms. When adrenergic and serotoninergic antagonists were used, yohimbine (P < 0.005) but not phentolamine or metergoline, decreased antinociception elicited by venlafaxine, implying a clear alpha2- and a minor alpha1-adrenergic mechanism of antinociception. When venlafaxine was administered together with various agonists of the opioid and alpha2- receptor subtypes, it significantly potentiated antinociception mediated by kappa1- kappa3- and delta-opioid receptor subtypes. The alpha2-adrenergic agonist clonidine significantly potentiated venlafaxine-mediated antinociception. Summing up these results, we conclude that the antinociceptive effect of venlafaxine is mainly influenced by the kappa- and delta-opioid receptor subtypes combined with the alpha2-adrenergic receptor. These results suggest a potential use of venlafaxine in the management of some pain syndromes. However, further research is needed in order to establish both the exact clinical indications and the effective doses of venlafaxine when prescribed for pain.

The antinociceptive effect of fluvoxamine

The authors conducted a study in order to evaluate the antinociceptive effects of the serotonin-selective reuptake inhibitor (SSRI) antidepressant fluvoxamine and its interaction with various opioid receptor subtypes. Male ICR mice were tested with a hotplate analgesia meter. Fluvoxamine elicited antinociceptive effect in a dose-dependent manner following i.p., i.t. and i.c.v. injection. Naloxone 10 mg/kg s.c. did not abolish the fluvoxamine antinociceptive effect. At the next stage fluvoxamine was administered together with various agonists of opioid receptors. When administered together with opiates, fluvoxamine significantly potentiated analgesia at the kappa(3)-opioid receptor subtype (P < .005) and to a lesser extent, at the mu-, delta-, and kappa(1)-opioid receptors. We conclude that fluvoxamine alone induces an antinociceptive effect. This effect is mediated by non-opioid mechanism of action. These results suggest a potential role for fluvoxamine in the management of pain when co-administered with opioids at low doses.

Augmentation of opioid induced antinociception by the atypical antipsychotic drug risperidone in mice

Risperidone is a novel atypical neuroleptic with a favorable profile of side effects due to its unique pharmacological activity: it exhibits both potent dopamine D2 and 5-HT2 receptor blocking activity, as well as high affinity for alpha1 and alpha2 adrenergic receptors and histamine H1 receptor. We found that risperidone has a potent antinociceptive effect in the tailflick assay with an ED50 of 26.4 mg/kg. This effect of risperidone was antagonized by naloxone (P < 0.05). This sensitivity to naloxone indicates that at least some of the analgesic effects of risperidone are mediated by an opioid mechanism of action. beta-FNA (mu1 mu2-antagonist), naloxonazine (mu1-antagonist) and norbinaltorphamine (nor-BNI; kappa1-analgesia) reversed risperidone antinociceptive effect (P < 0.05). Naltrindole (delta-antagonist) only partially reversed risperidone antinociceptive effect. We found that the sensitivity of risperidone antinociceptive effect to selective antagonists implies involvement of mu1-, mu2- and kappa1-opioid and to a lesser extent delta-opioid mechanisms. These results suggest a possible role for risperidone both in the management of pain and in the management of opiate withdrawal and detoxification.

Interaction between the tetracyclic antidepressant mianserin HCl and opioid receptors

The antinociceptive effects of the tetracyclic antidepressant mianserin and its interaction with various opioid receptor subtypes was evaluated. Mice were tested with a hotplate analgesia meter. Mianserin elicited an antinociceptive effect in a dose-dependent manner following doses from 1-25 mg/kg. As the mianserin dose increased beyond 30 mg/kg, latencies returned to baseline, yielding a biphasic effect. This effect of mianserin was antagonized by naloxone (P<0.005), implying a possible opioid mechanism of action involved in the mianserin induced antinociceptive effect. When administered with various opioid antagonists, the sensitivity of mianserin to selective opioid antagonists was found significant for mu and kappa1 opioid receptor subtypes (P<0.005), but not for delta-receptor. At the next stage mianserin was administered together with various agonists of opioid receptors. When administered together with opiates, mianserin significantly potentiates analgesia at the mu, kappa1 and kappa3 opioid receptor subtype (P<0.005) and to a lesser extent, at the delta opioid receptors. These results suggest a potential use of mianserin in the management of some pain syndromes. However, further research is needed in order to establish both the exact clinical indications and the effective doses of mianserin when prescribed for pain.

Mouse strains differ in their sensitivity to alprazolam effect in the staircase test.

The behavioral responses of five mouse strains (inbred: C57 and BALB/c; outbred: Swiss, ICR and HS/Ibg) to alprazolam was examined in the staircase test, an animal model sensitive to benzodiazepines (BZs). Alprazolam administration resulted in a dose-dependent suppression of rearing behavior, but to a different extent among the strains. By contrast, the number of stairs ascended was not suppressed by alprazolam at doses of 0.25 and 0.5 mg/kg, except in the C57 mice. The addition of flumazenil antagonized the alprazolam effect on rearing and climbing in all strains. There was a consistency within strains in sensitivity to alprazolam, with some strains being highly sensitive (C57 and HS) or less sensitive (Swiss, ICR and BALB/c) with regard to both rearing and climbing behaviors. Serum alprazolam levels did not differ significantly among the strains. This strain-dependent pattern of response to alprazolam seems to indicate a genetic component, rather than pharmacokinetic, in the behavior sensitivity to the BZ, with a spectrum of degree of responsivity among strains.