Peripheral opioid receptors may mediate a portion of the aversive and depressant effect of EtOH: CPP and locomotor activity

Male and female C57BL/6 mice display drug-induced aversion and reward in the combined conditioned taste avoidance/conditioned place preference procedure

BACKGROUND

Drugs of abuse have rewarding and aversive effects that, in balance, impact abuse potential. Although such effects are generally examined in independent assays (e.g., CPP and CTA, respectively), a number of studies have examined these effects concurrently in rats in a combined CTA/CPP design. The present study assessed if similar effects can be produced in mice which would allow for determining how each is affected by subject and experiential factors relevant to drug use and abuse and the relationship between these affective properties.

METHODS

Male and female C57BL/6 mice were exposed to a novel saccharin solution, injected (IP) with saline or 5.6, 10 or 18 mg/kg of the synthetic cathinone, methylone, and placed on one side of the place conditioning apparatus. The following day, they were injected with saline, given access to water and placed on the other side of the apparatus. After four conditioning cycles, saccharin avoidance and place preferences were assessed in a final two-bottle CTA test and a CPP Post-Test, respectively.

RESULTS

In the combined CTA/CPP design, mice acquired a significant dose-dependent CTA (p = 0.003) and a significant CPP (p = 0.002). These effects were independent of sex (all p's > 0.05). Further, there was no significant relationship between the degree of taste avoidance and place preference (p > 0.05).

CONCLUSIONS

Similar to rats, mice displayed significant CTA and CPP in the combined design. It will be important to extend this design in mice to other drugs and to examine the impact of different subject and experiential factors on these effects to facilitate predictions of abuse liability.

Evidence for conditioned place preference to a moderate dose of ethanol in adult male Sprague-Dawley rats

The present series of experiments examined affective properties of a moderate dose of ethanol using the conditioned place preference (CPP) paradigm in ethanol-naïve, adult male Sprague-Dawley rats. The apparatus and the procedure used were both unbiased. In Experiment 1, rats were given four 30 min conditioning sessions with 1.5 g/kg ethanol (i.p.) or an equivalent volume of saline on the paired side. Animals were found to demonstrate CPP to the ethanol-paired side, an unexpected finding at this relatively high dose in rats. To replicate this finding, and to examine the possibility of non-associative conditioning, an unpaired control group was included in Experiment 2. Once again, rats showed a CPP to the side paired with ethanol relative to either control group. Given that testing in an unfamiliar environment typically results in elevated levels of anxiety and that animals in Experiments 1 and 2 were not exposed to the apparatus prior to conditioning, Experiment 3 was conducted to examine the potential role of context unfamiliarity for induction of ethanol CPP in this test situation by varying whether animals were exposed to the apparatus prior to conditioning. In this study, pre-exposure to the CPP apparatus was found to eliminate the CPP to ethanol observed in rats who were not familiarized with the apparatus. Collectively, these studies demonstrate that ethanol-naïve rats can find ethanol reinforcing as indexed by the CPP test, and provide some evidence for the conditions under which this uncommon finding is observed.

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.

Conditioned place aversion induced by intragastric administration of ethanol in rats

Most experiments investigating ethanol-induced place conditioning in rats have produced conditioned place aversion (CPA). In one of the few reports of ethanol-induced conditioned place preference (CPP) in rats, selectively bred alcohol-preferring (msP) rats showed CPP in a biased procedure when ethanol was administered via intragastric (IG) catheter but not when ethanol was administered via intraperitoneal injection or by gavage. This finding suggests the importance of both route of administration and genetic variables to the outcome of place conditioning studies. We conducted three experiments examining place conditioning induced by IG ethanol in genetically heterogeneous rats to test the generality of the earlier finding. We employed an unbiased procedure that is more sensitive to detecting preference changes in either direction (preference or aversion). Ethanol-naive (Experiment 1) and ethanol-experienced Sprague-Dawley rats (Experiment 2) showed robust CPA. In Experiment 3, infusion rate was varied to see if the CPA observed in Experiments 1 and 2 was a result of the rapidity of the transition from the sober to the intoxicated states. Both groups showed strong CPA. Overall, the present findings are consistent with previous findings of CPA in heterogeneous rats, suggesting that the aversive postabsorptive effects of ethanol produce CPA.

Functional effects of systemically administered agonists and antagonists of mu, delta, and kappa opioid receptor subtypes on body temperature in mice

We have investigated the roles of peripheral and central mu, delta, and kappa opioid receptors and their subtypes in opioid-induced hypothermia in mice. Measuring rectal temperature after i.p. injection, opioid agonists [morphine, fentanyl, SNC80 ((+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)3-methoybenzyl]-N,N-diethylbenzamide), U50,488H ((trans-(dl)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide), and loperamide)] were tested alone or with opioid antagonists [naloxone, beta-funaltrexamine, naloxonazine, naltrindole, 7-benzylidenenaltrexone (BNTX), naltriben, nor-binaltorphimine, 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-(3-isothiocyanatophenyl)-2-(1-pyrrolidinyl)ethyl]acetamide (DIPPA), and methyl-naltrexone] given 15 min after the agonist. All agonists produced dose-related hypothermia, although at low doses, morphine and U50,488H produced hyperthermia. The effects of morphine and fentanyl were antagonized by naloxone and by the mu(1) antagonist naloxonazine. The delta(2) antagonist naltriben potentiated the hypothermic effect of mu agonists. SNC80-induced hypothermia was blocked by the delta antagonist naltrindole but not by the delta(1) antagonist BNTX. Depending on the dose, the delta(2) antagonist naltriben produced either a potentiation or an attenuation of the effect of SNC80. U50,488H-induced hypothermia was antagonized by the kappa antagonist nor-binaltorphimine but not by acute treatment with the irreversible kappa antagonist DIPPA. The peripherally acting opioid loperamide produced hypothermia that could be blocked by several mu-, delta-, or kappa-selective antagonists as well as the peripherally acting antagonist methyl-naltrexone. Methyl-naltrexone produced a weak potentiation of morphine-, fentanyl-, and U50,488H-induced hypothermia, whereas a significant attenuation of SNC80-induced hypothermia was observed. In conclusion, at high doses, morphine- and fentanyl-induced hypothermia may involve composite action on mu, kappa, and possibly delta opioid receptors after initial activation. In the mediation of delta opioid-induced hypothermia, no clear selectivity between the delta(1) and delta(2) subtypes was defined. The studies provide new evidence that maintenance of the initial effects of agonist/receptor activation vary with the agonist and the receptor. The existence of both central and peripheral components of opioid-induced hypothermia is also emphasized.