Morphine preexposure facilitates morphine place preference and attenuates morphine taste aversion

Sensitization processes in drug addiction

In 1993, Robinson and Berridge published their first review that laid out the incentive sensitization theory of addiction (Robinson and Berridge 1993 Brain Res Rev 18:247). Its basic point is that repeated exposure to drugs of abuse causes hypersensitivity to drugs and drug-associated stimuli of the neural circuits mediating incentive salience, an important way in which motivational stimuli influence behavior. In laymen's terms, it states that this drug-induced hypersensitivity of motivational circuitry would mediate an increase in drug "wanting," thus being responsible for the dramatically exaggerated motivation for drugs displayed by addicts. This theory has been exceptionally influential, as evidenced by the fact that the original review paper about this theory (Robinson and Berridge 1993 Brain Res Rev 18:247) has been cited 2,277 times so far, and subsequent updates of this view (Robinson and Berridge 2000 Addiction 95(Suppl 2):S91; Robinson and Berridge 2001 Addiction 96:103; Robinson and Berridge 2003 Ann Rev Psychol 54:25) have been cited 274, 297, and 365 times, respectively, adding up to more than 3,200 citations within 15 years. The present chapter aims to delineate the merits and limitations of the incentive sensitization view of addiction, and whether incentive sensitization occurs in humans. We conclude that since incentive sensitization most prominently occurs after the first few drug exposures, it may represent an important initial step in the addiction process. During the expression of full-blown addiction, characterized by loss of control over drug intake and use of large quantities of drugs, the expression of incentive sensitization may be transiently suppressed. However, detoxification and the gradual disappearance of tolerance and withdrawal symptoms may unmask sensitization, which could then play an important role in the high risk of relapse.

Delta-opioid receptor antagonists prevent sensitization to the conditioned rewarding effects of morphine

BACKGROUND

Functional interactions between mu- and delta-opioid receptors (MOPr and DOPr, respectively) are implicated in morphine tolerance and dependence. The contribution of DOPr to the conditioned rewarding effects of morphine and the enhanced conditioned response that occurs after repeated morphine administration is unknown. This issue was addressed with the conditioned place preference procedure (CPP).

METHODS

Rats received home cage injections of saline or morphine (5.0 mg/kg/day x 5 days) before conditioning. For sensitization studies, DOPr antagonists (DOPr1/2: naltrindole, DOPr2: naltriben, DOPr1: 7-benzylidenenaltrexone) were administered before morphine injections. Conditioning sessions (2 morphine; 2 saline) commenced 3 days later. To assess the influence of acute DOPr blockade on the conditioning of morphine reward in naïve animals, 3 morphine and 3 saline conditioning sessions were employed. Antagonists were administered before morphine conditioning sessions.

RESULTS

Morphine was ineffective as a conditioning stimulus after two conditioning sessions in naïve rats. However, doses > or = 3.0 mg/kg produced significant CPP in morphine pre-exposed rats, confirming that sensitization develops to the conditioned rewarding effects of morphine. In animals that received morphine pre-exposure with naltrindole or naltriben but not 7-benzylidenenaltrexone, sensitization was prevented. No attenuation of morphine CPP was observed in animals that received DOPr antagonists acutely, before conditioning sessions.

CONCLUSIONS

These data indicate a critical role of DOPr systems in mediating sensitization to the conditioned rewarding effects of morphine. The efficacy of naltrindole and naltriben in preventing the enhanced response to morphine suggest the specific involvement of DOPr2 in the sensitization process.

Adolescent exposure to nicotine alters the aversive effects of cocaine in adult rats

Nicotine is one of the most commonly used drugs in adolescence and has been shown to alter the rewarding effects of cocaine when administered in adulthood. Although the abuse potential of a drug has been suggested to be a balance between its rewarding and aversive effects, the long-term effects of nicotine on the aversive properties of other drugs had not been studied. To that end, in the present study rats exposed to nicotine (0.4 mg/kg) during adolescence (postnatal days 35-44) were tested for the acquisition and extinction of a cocaine-induced conditioned taste aversion (10, 18 or 32 mg/kg) in adulthood. Conditioning consisted of four saccharin-drug pairings followed by six extinction trials. Although cocaine-induced aversions at all doses, no effect of nicotine preexposure was seen during acquisition. During extinction, the nicotine-preexposed groups conditioned with 10 and 18 mg/kg cocaine displayed a decreased rate of extinction compared to their respective controls. These results suggest that while adolescent nicotine exposure does not appear to directly alter the aversive properties of cocaine it may affect other processes related to the response to drugs given in adulthood.

Simultaneous expression of cocaine-induced behavioral sensitization and conditioned place preference in individual rats

Conditioned place preference and locomotor sensitization are rodent behavioral models commonly used to investigate the actions of drugs of abuse. However, few studies have examined both paradigms in the same group of animals. We were interested in developing a combined protocol which successfully induced both conditioned place preference and sensitization simultaneously in cocaine-treated Sprague-Dawley rats in order to test the hypothesis that the magnitude of these two phenomena would be positively correlated. We used an open-field with a removable place preference insert to assess these measures independently. Cocaine-conditioned animals demonstrated a significant shift in preference for the drug-paired compartment and a sensitized locomotor response which was not observed in saline-conditioned animals challenged with cocaine. There was no significant relationship between locomotor sensitization and conditioned place preference in individual animals. We further examined these results with respect to each rat's initial response to cocaine, response to a novel environment and central zone entries in an open-field. Locomotor sensitization demonstrated an inverse correlation with the initial cocaine response. In contrast, conditioned place preference demonstrated an inverse correlation with the centre response. These results demonstrate that the combination of the acute cocaine response and the centre response in a novel open-field environment can be used to indicate the propensity of a given rat to exhibit either behavioral sensitization or conditioned place preference; however, it seems that sensitization and place preference are not necessarily co-expressed to a similar extent in the same individual animal.

The effects of nicotine on ethanol-induced conditioned taste aversions in Long-Evans rats

RATIONALE

Overall drug acceptability is thought to be a function of the balance between its rewarding and aversive effects, the latter of which is reportedly affected by polydrug use.

OBJECTIVES

Given that nicotine and alcohol are commonly co-used, the present experiments sought to assess nicotine's impact on ethanol's aversive effects within a conditioned taste aversion design.

MATERIALS AND METHODS

Experiment 1 examined various doses of nicotine (0, 0.4, 0.8, 1.2 mg/kg) to determine a behaviorally active dose, and experiment 2 examined various doses of ethanol (0, 0.5, 1.0, 1.5 g/kg) to determine a dose that produced intermediate aversions. Experiment 3 then examined the aversive effects of nicotine (0.8 mg/kg) and ethanol (1.0 g/kg) alone and in combination. Additionally, nicotine's effects on blood alcohol concentrations (BAC) and ethanol-induced hypothermia were examined.

RESULTS

Nicotine and ethanol combined produced aversions significantly greater than those produced by either drug alone or the summed aversive effects of the individual compounds. These effects were unrelated to changes in BAC, but nicotine and ethanol combined produced a prolonged hypothermic effect which may contribute to the increased aversions induced by the combination.

CONCLUSIONS

These data demonstrate that nicotine may interact with ethanol, increasing ethanol's aversive effects. Although the rewarding effects of concurrently administered nicotine and ethanol were not assessed, these data do indicate that the reported high incidence of nicotine and ethanol co-use is unlikely due to reductions in the aversiveness of ethanol with concurrently administered nicotine. It is more likely attributable to nicotine-related changes in ethanol's rewarding effects.

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.

The effects of cocaine preexposure on cocaine-induced taste aversion learning in Fischer and Lewis rat strains

The Fischer (F344) and Lewis (LEW) inbred rat strains differ on a number of behaviors, including those induced by a variety of drugs of abuse. Although a number of physiological and biochemical differences between the strains have been reported following both single and repeated drug administration, studies assessing changes in the affective properties of drugs after repeated exposure are limited. To that end, using the F344 and LEW strains, the present study examined the effects of repeated exposure to cocaine on the subsequent acquisition of cocaine-induced conditioned taste aversions, a preparation often used in assessing the development of tolerance to the drug's aversive effects. Specifically, separate groups of male F344 and LEW rats received five injections of 32 mg/kg cocaine (or vehicle) prior to taste aversion conditioning with 32 mg/kg cocaine (or vehicle). Vehicle-preexposed subjects of both strains acquired aversions to the cocaine-associated taste with no differences in the strength of the aversions. Further, cocaine-preexposed subjects displayed significantly attenuated aversions, an effect consistent with prior work with outbred animals. There was no difference between the two strains in this attenuation, suggesting that there were no genotype-specific differences in tolerance to cocaine's aversive effects. The data were discussed in relation to genetic/environmental interactions in the vulnerability to drugs of abuse.

Separation stress, litter size, and the rewarding effects of low-dose morphine in the dams of maternally separated rats

Potential differences in sensitivity to the rewarding effects of morphine as a function of litter separation stress were assessed in post-weaning rat dams. During the first two weeks postnatal, Sprague-Dawley rat litters were subjected to daily 15- or 180-min sessions of dam-pup separation while control litters only experienced twice-weekly animal facility care. One week after weaning, the dams (n=7 per group) underwent a fully unbiased conditioned place preference (CPP) procedure to 1 mg/kg subcutaneous morphine. CPP responses after each conditioning cycle were recorded. Rates of acquisition and asymptotic levels of CPP were comparable in all groups; however, an inverse relationship between litter size and magnitude of morphine CPP was revealed. Although these initial data indicate no differential sensitivity to the rewarding effects of low-dose morphine produced by the stress of litter separation, this assessment of litter size and drug-induced place conditioning in post-weaning litter-separated dams is the first of its kind. Potential effects of other doses, drugs of abuse and post-partum manipulations remain to be evaluated within this emerging etiological model.

Morphine-induced place conditioning in Fischer and Lewis rats: acquisition and dose-response in a fully biased procedure

The Fischer (F344) and Lewis (LEW) rat strains differ on a variety of behavioral assays examining the effects of morphine, with many of the differences observed during acquisition of behavioral responses. The results of these studies and others examining endogenous physiology and the biochemical effects of morphine suggest that F344 rats are more sensitive to morphine than LEW rats. However, LEW animals have shown greater conditioned place preferences (CPP) to 4 mg/kg than F344 rats. CPP is a popular assay of drug reward in which acquisition of the preference can be measured across multiple conditioning cycles, yet this aspect of CPP has not been assessed in F344 and LEW rats. As part of an ongoing effort to fully characterize the conditioned rewarding effects of abused drugs in these strains, the present study assessed the effects of 0, 1, 4 and 10 mg/kg subcutaneous (SC) morphine in adult male F344 and LEW rats (n=12/strain/dose). A fully biased place conditioning procedure was employed where morphine's effects were paired with the initially non-preferred chamber on Day 1, saline was paired with the preferred chamber on Day 2 and drug-free access to the entire apparatus was allowed on Day 3. This conditioning and testing regimen was repeated for four consecutive cycles. The F344 animals acquired CPP at 1 mg/kg only; this effect emerged after only two conditioning cycles. LEW rats never acquired a CPP at any dose tested. Peak blood morphine levels following SC injections of 1, 4 or 10 mg/kg revealed no significant strain or dose effects. These behavioral data are consistent with the hypothesis that F344 rats are more sensitive to the rewarding effects of morphine than LEW rats. Additional implications for the Fischer-Lewis model of drug abuse and the utility of CPP acquisition procedures are discussed.

Assessment of the aversive and rewarding effects of alcohol in Fischer and Lewis rats

RATIONALE

Application of the Fischer-Lewis genetic model of drug abuse to the study of alcohol's motivational properties has been limited.

OBJECTIVES

To assess the aversive and rewarding effects of ethanol in Fischer and Lewis rats.

MATERIALS AND METHODS

Fischer and Lewis rats underwent a four-trial combined conditioned taste aversion/conditioned place preference procedure (CTA/CPP; 0, 1, 1.25, or 1.5 g/kg IP ethanol). Others received 0, 1, or 1.5 g/kg followed by tail blood sampling at 15-, 60- and 180-min post-injection. In additional groups, hypothermia to 0, 1.5, and 3 g/kg was assessed before and 30- and 60-min post-injection.

RESULTS

All alcohol-treated groups except low-dose Lewis acquired CTA after one trial. Fischer rats developed stronger CTAs than Lewis at 1.25 and 1.5 g/kg. Ethanol-induced reward in taste or place conditioning was not evident in either strain. Lewis animals showed overall higher peak blood alcohol concentrations, but hypothermia did not vary by strain.

CONCLUSION

Compared to Fischer, Lewis rats are less sensitive to alcohol's aversive effects as assessed in the CTA paradigm. The behavioral differences observed are not due to hypothermia, but pharmacokinetic differences may contribute. These data underscore the importance of genetic factors and the aversive effects of initial drug exposures in modeling vulnerability to abuse. In addition to its application with other drugs, the Fischer-Lewis model may be useful for investigating the biobehavioral bases of alcohol abuse.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Naloxone, not proglumide or MK-801, alters effects of morphine preexposure on morphine-induced taste aversions

Both cholecystokinin (CCK) antagonists and N-methyl-D-aspartate (NMDA) antagonists block or reduce the development of morphine tolerance in several analgesic assays. The present experiments were performed to assess the ability of the CCK antagonist proglumide and the NMDA antagonist MK-801 to affect tolerance to the aversive properties of morphine as indexed by conditioned taste aversion (CTA) learning. Specifically, male Sprague-Dawley rats were exposed to either vehicle or morphine (5 mg/kg) in combination with either proglumide (5 mg/kg; Experiment 1), MK-801 (0.1 mg/kg; Experiment 2) or naloxone (1, 3.2 mg/kg; Experiment 3). Saccharin was then presented and was followed by an injection of either vehicle or morphine (10 mg/kg). Animals preexposed to and conditioned with morphine acquired an attenuated morphine-induced aversion to saccharin. While neither proglumide nor MK-801 had an effect on this attenuation, naloxone blocked the effects of morphine preexposure, suggesting that neither CCK nor NMDA may be involved in the aversive effects of morphine (or their modulation by drug exposure). That the attenuating effects of morphine preexposure on a morphine-induced CTA can be blocked suggests that the weakening of the aversive effects of morphine with chronic use can be prevented, an effect that may have implications for overall drug acceptability.

Apparatus bias and the use of light and texture in place conditioning

In a typical conditioned place preference (CPP) preparation, animals alternately experience drug and vehicle effects in distinct chambers of an apparatus, spending more time in the drug-paired chamber post-conditioning. However, if all animals prefer the same chamber before conditioning, data interpretation may be compromised. Unbiased apparatus has been systematically validated with ethanol in mice ([Cunningham, C.L., Feree, N.K., Howard, M.A. Apparatus bias and place conditioning with ethanol in mice. Psychopharmacology (Berl) 2003;170:409-422]); the present study sought to identify and eliminate bias in a standard black-and-white apparatus and validate that apparatus for CPP with morphine and cocaine in rats. Apparatus bias was assessed in 24 adult female Sprague-Dawley rats. Subjects preferred the black chamber under bright lighting conditions, with no preference in the dark. Subjects then underwent a counterbalanced CPP regimen to 5 mg/kg SC morphine (n=12) or 20 mg/kg IP cocaine (n=12) using only tactile conditioned stimuli. Significant absolute preferences for the drug-paired chamber were produced by both drugs, with no effect of drug-paired chamber assignment on CPP expression; vehicle-treated controls (n=12) showed no preferences. Bias-free CPP to morphine and cocaine using standard apparatus in rats is possible. Implications for place conditioning are discussed, including the potential value of systematically exploiting apparatus bias in addition to eliminating it.