Chronic cadmium exposure attenuates conditioned place preference produced by cocaine and other drugs

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 concurrent administration of +/-3,4-methylenedioxymethamphetamine and cocaine on conditioned place preference in the adult male rat

Conditioned place preference (CPP), a commonly used model for studying the role of contextual cues in drug reward and drug seeking, was employed to explore possible behavioral interactions between (+/-)3,4-methylenedioxymethamphetamine (MDMA; "ecstasy") and cocaine. On each of four occasions, adult male rats received one of three doses of MDMA (0 mg/kg, 5 mg/kg, 10 mg/kg; administered subcutaneously [s.c.]) combined with one of three doses of cocaine (0 mg/kg, 2.5 mg/kg, 5 mg/kg; administered intraperitoneally [i.p.]), and were then tested in a CPP paradigm. The results showed MDMA-induced CPP at a unit dose of 5 mg/kg, but at the 10 mg/kg dose there was a return to baseline (control) performance levels. For cocaine alone, CPP increased in a linear fashion as the drug dose was increased. Concurrent administration resulted in antagonism of each drug, but there was evidence that this pattern was reversible at higher doses of the respective drugs. These data are instructive insofar as they suggest that the behavioral and neurochemical effects of MDMA and cocaine presented in isolation are dramatically altered when the two drugs are presented in combination.

Biological research on drug abuse and addiction in Hispanics: current status and future directions

Impressive progress has been made in the understanding of biological contributions to drug abuse and addiction. An area that has only recently begun to receive attention is potential ethnic and racial differences in biological systems that contribute to, or protect from, problem drug use. This article reviews recent research on drug abuse and addiction in Hispanics in which biological questions have been addressed, including work on genes, gene products (proteins), physiology and pharmacotherapy. Taken together, work to date suggests that there are both similarities and differences between Hispanics and other ethnic groups in biological factors related to drug abuse and addiction. Although the results are intriguing, relatively few studies have been done, and those that have been done have often been inconclusive due to low numbers of Hispanic subjects. Moreover, studies have often failed to recognize the complexity and heterogeneity of Hispanic populations in the United States and around the world. After reviewing the current status of the field, recommendations are given for future research in both humans and relevant animal models that will lead to a better understanding of drug abuse and addiction in Hispanics.

Dose response effects of lithium chloride on conditioned place aversions and locomotor activity in rats

The present study examined the multi-variable locomotor activity effects of lithium chloride (LiCl) treatment in male rats. Of interest was a determination of which variables might show a dose-response relationship in LiCl-induced conditioned place aversions. Automated open-fields were partitioned into two chambers distinct in tactile and visual cues. A control group [n=8] received saline (NaCl; 0.15 M) paired with both chambers while three LiCl groups (0.15 M; 32 mg/kg [n=7], 95 mg/kg [n=7], 127 mg/kg [n=7]) received LiCl paired with the normally preferred chamber and saline paired with the non-preferred chamber. During extinction trials, rats were allowed to choose between the two chambers to provide an index of conditioned place aversions. Locomotor activity and its distribution within the chambers were also assessed during both conditioning and extinction trials. Dose-dependent decreases occurred in all measures of locomotor activity following LiCl administration during conditioning. During extinction trials, place aversions developed in animals conditioned with LiCl. LiCl-treated rats spent significantly less time in the LiCl-paired chamber relative to controls but not in a dose-dependent manner. Animals that had been conditioned with 95 or 127 but not 32 mg/kg LiCl, displayed significantly more vertical activity in the LiCl-paired chamber than controls during extinction trials. These findings indicate that, in addition to producing dose-dependent unconditioned effects on locomotor activity, LiCl also produces dose-dependent conditioned effects on vertical activity. These conditioned rearing response effects provide a valid measure of the conditioned avoidance response that provides evidence for dose-dependent LiCl-induced conditioned place aversions.

Exposure to Cadmium During Gestation and Lactation Decreases Cocaine Self-Administration in Rats

This investigation examined the effects of perinatal cadmium exposure on subsequent self-administration of cocaine during the adult cycle. Female Sprague-Dawley rats were gavaged daily with 0.0 (14% sucrose solution, w/v) or 5.0 mg cadmium chloride (dissolved in 14% sucrose solution, w/v) for 30 days prior to breeding with non-exposed males. Dams continued to experience cadmium exposure through gestation and until pups were weaned at postnatal day (PND) 21. On PND 70, offspring were anesthetized and chronic indwelling jugular catheters were implanted. Following recovery, test subjects were trained in operant chambers to self-administer 0.500 mg/kg infusion (inf) intravenous cocaine on a fixed-ratio (FR) 2 schedule of reinforcement. Following acquisition, self-administration rates were tested for saline, 0.030, 0.060, 0.125, 0.250, and 0.500 mg/kg inf cocaine. Rats exposed developmentally to cadmium self-administered significantly less than controls at saline, 0.030, and 0.060 mg/kg inf cocaine. These data indicate that early-life cadmium exposure, a common exposure vector of which is the use of tobacco products, may affect cocaine sensitivity.

Calmodulin inhibitor trifluoperazine attenuates the development and expression of morphine-induced conditioned place preference in rats

The effect of trifluoperazine, a calmodulin inhibitor, on morphine-induced conditioned place preference was examined in rats. Morphine (5, 10 mg/kg, i.p.) produced significant place preference for the drug-associated place. Trifluoperazine significantly suppressed the development as well as the expression of morphine-induced place preference in a dose-dependent manner, but it neither produced place preference or aversion, nor affected locomotor activity. Injection of 0.5 and 1.0 mg/kg apomorphine, a dopamine receptor agonist, did not alter the inhibition by trifluoperazine of morphine-induced place preference. Verapamil, at the dose that failed to change the place preference induced by morphine, enhanced the inhibition by trifluoperazine of morphine-induced place preference. These findings provide the first demonstration that trifluoperazine attenuates morphine-induced conditioned place preference in rats. The action of trifluoperazine might be produced through its inhibition of calmodulin, but is probably not related to dopamine receptor blockade.

Developmental exposure to cadmium alters responsiveness to cocaine in the rat

The purpose of the present report was to investigate the potential interactive relation between perinatal (gestation/lactation) cadmium exposure and changes in responsiveness to cocaine. In Experiment 1, adult female rats were exposed to a diet containing 50 ppm cadmium (as cadmium chloride) or a diet containing no added cadmium for 30 days prior to breeding with nonexposed males. The metal-exposure regimen continued throughout gestation, and for 15 days of lactation, at which time all animals were placed on standard rat chow diets containing no added cadmium for the remainder of the investigation. Atomic absorption assays confirmed that cadmium concentrations were significantly elevated in metal-exposed dams, littermates, and test animals. Offspring were weaned on postnatal day (PND) 21 and commenced cocaine sensitization testing on PND 70. Testing operations for controls and animals perinatally exposed to cadmium consisted of 21 daily i.p. injections of vehicle (saline) or 10 mg/kg cocaine HCl, and subsequent recording of locomotor activity. Subsequently, across successive days, all animals received 0, 10, and 20 mg/kg cocaine challenges. The results showed that cocaine sensitization was attenuated in animals perinatally exposed to cadmium. A similar pattern of antagonism was observed in Experiment 2 where a higher dose of cocaine was required to produce conditioned place preference (CPP) in cadmium-exposed animals. The implications of these findings with respect to the interactive role of cadmium in the dynamics of cocaine use/abuse remain unclear.

Perinatal exposure to lead attenuates the conditioned reinforcing properties of cocaine in male rats

The purpose of this study was to examine the effects of developmental lead exposure on drug responsiveness later in the life cycle. Adult female rats were gavaged daily with 0, 8, or 16 mg lead for 30 days before breeding with non-exposed males. The respective exposure regimens were maintained throughout gestation and lactation (perinatal exposure). In Experiment 1, at postnatal day (PND) 30 or 90, pups were trained with 0, 1.25, 2.5, or 5 mg/kg cocaine HCl (IP) in a biased conditioned place preference (CPP) procedure. At both PND 30 and 90, an attenuation in CPP was present in animals exposed to 8 or 16 mg lead relative to control rats. Using an identical lead-exposure regimen, a conditioned place aversion (CPA) procedure with 0, 10, 20, or 40 mg/kg lithium chloride (IP) was employed for Experiment 2. No significant differences were present among pups from each lead-exposure group conditioned and tested at PND 30 or 90, thus suggesting that an impairment of associative mechanisms was not solely responsible for the pattern of attenuation present in Experiment 1. Subsequent analyses of blood-lead in all experiments demonstrated concentrations below 5 microg/dl for all animals at PND 30 and below detectable limits (<1 microg/dl) at PND 90. The findings suggested attenuation in cocaine reinforcement with perinatal lead exposure even though the metal apparently had gained clearance from soft tissue.