The cyclo-oxygenase inhibitor nimesulide induces conditioned place preference in rats

Epoxy fatty acids mediate analgesia in murine diabetic neuropathy

BACKGROUND

Neuropathic pain is a debilitating condition with no adequate therapy. The health benefits of omega-3 fatty acids are established, however, the role of docosahexaenoic acid (DHA) in limiting pain has only recently been described and the mechanisms of this action remain unknown. DHA is metabolized into epoxydocosapentanoic acids (EDPs) via cytochrome P450 (CYP450) enzymes which are substrates for the soluble epoxide hydrolase (sEH) enzyme. Here, we tested several hypotheses; first, that the antinociceptive action of DHA is mediated by the EDPs. Second, based on evidence that DHA and CYP450 metabolites elicit analgesia through opioid signalling, we investigated this as a possible mechanism of action. Third, we tested whether the analgesia mediated by epoxy fatty acids had similar rewarding effects as opioid analgesics.

METHODS

We tested diabetic neuropathic wild-type and sEH null mice in a conditioned place preference assay for their response to EDPs, sEHI and antagonism of these treatments with naloxone, a mu-opioid receptor antagonist.

RESULTS

The EDPs and sEH inhibitors were efficacious against chronic pain, and naloxone antagonized the action of both EDPs and sEH inhibitors. Despite this antagonism, the sEH inhibitors lacked reward side effects differing from opioids.

CONCLUSIONS

The EpFA are analgesic against chronic pain differing from opioids which have limited efficacy in chronic conditions.

SIGNIFICANCE

EDPs and sEHI mediate analgesia in modelled chronic pain and this analgesia is blocked by naloxone. However, unlike opioids, sEHI are highly effective in neuropathic pain models and importantly lack rewarding side effects.

Peroxisome proliferator-activated receptors-alpha modulate dopamine cell activity through nicotinic receptors

BACKGROUND

Modulation of midbrain dopamine neurons by nicotinic acetylcholine receptors (nAChRs) plays an important role in behavior, cognition, motivation, and reward. Specifically, nAChRs containing beta2 subunits (beta2-nAChRs) switch dopamine cells from a resting to an excited state. However, how beta2-nAChRs can be modulated and thereby how dopamine firing activity is affected remains elusive. Because changes in dopamine cell activity are reflected in the dynamics of microcircuits generating altered responses to stimuli and inputs, factors regulating their state are fundamental. Among these, endogenous ligands to the nuclear receptor-transcription factor peroxisome proliferator-activated receptors type-alpha (PPARalpha) have been recently found to suppress nicotine-induced responses of dopamine neurons.

METHODS

We used both in vitro and in vivo electrophysiological techniques together with behavioral analysis to investigate on the effects of modulation of PPARalpha in Sprague-Dawley rat and C57BLJ/6 mouse dopamine neurons and their interactions with beta2-nAChRs. To this aim, we took advantage of a selective reexpression of beta2-nAChR exclusively in dopamine cells by stereotaxically injecting a lentiviral vector in the mouse ventral tegmental area.

RESULTS

We found that activation of PPARalpha decreases in vitro both dopamine cell activity and ventral tegmental area net output through negative modulation of beta2-nAChRs. Additionally, PPARalpha activation in vivo reduces both the number of spontaneously active dopamine neurons and nicotine-induced increased locomotion.

CONCLUSIONS

Our combined findings suggest PPARalpha ligands as important negative modulators of beta2-nAChRs on dopamine neurons. Thus, PPARalpha ligands might prove beneficial in treating disorders in which dopamine dysfunction plays a prominent role, such as schizophrenia and nicotine addiction.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.