Equal sensitivity to cocaine reward in addiction-prone and addiction-resistant rat genotypes

Fischer 344 and Lewis Rat Strains as a Model of Genetic Vulnerability to Drug Addiction

Today it is well acknowledged that both nature and nurture play important roles in the genesis of psychopathologies, including drug addiction. Increasing evidence suggests that genetic factors contribute for at least 40–60% of the variation in liability to drug dependence. Human genetic studies suggest that multiple genes of small effect, rather than single genes, contribute to the genesis of behavioral psychopathologies. Therefore, the use of inbred rat strains might provide a valuable tool to identify differences, linked to genotype, important in liability to addiction and related disorders. In this regard, Lewis and Fischer 344 inbred rats have been proposed as a model of genetic vulnerability to drug addiction, given their innate differences in sensitivity to the reinforcing and rewarding effects of drugs of abuse, as well their different responsiveness to stressful stimuli. This review will provide evidence in support of this model for the study of the genetic influence on addiction vulnerability, with particular emphasis on differences in mesolimbic dopamine (DA) transmission, rewarding and emotional function. It will be highlighted that Lewis and Fischer 344 rats differ not only in several indices of DA transmission and adaptive changes following repeated drug exposure, but also in hypothalamic-pituitary-adrenal (HPA) axis responsiveness, influencing not only the ability of the individual to cope with stressful events, but also interfering with rewarding and motivational processes, given the influence of corticosteroids on dopamine neuron functionality. Further differences between the two strains, as impulsivity or anxiousness, might contribute to their different proneness to addiction, and likely these features might be linked to their different DA neurotransmission plasticity. Although differences in other neurotransmitter systems might deserve further investigation, results from the reviewed studies might open new vistas in understanding aberrant deviations in reward and motivational functions.

The development and maintenance of drug addiction

What is the defining property of addiction? We dust off a several-decades-long debate about the relative importance of two forms of reinforcement—positive reinforcement, subjectively linked to drug-induced euphoria, and negative reinforcement, subjectively linked to the alleviation of pain—both of which figure importantly in addiction theory; each of these forms has dominated addiction theory in its time. We agree that addiction begins with the formation of habits through positive reinforcement and that drug-opposite physiological responses often establish the conditions for negative reinforcement to come into play at a time when tolerance, in the form of increasing reward thresholds, appears to develop into positive reinforcement. Wise’s work has tended to focus on positive-reinforcement mechanisms that are important for establishing drug-seeking habits and reinstating them quickly after periods of abstinence, whereas Koob’s work has tended to focus on the negative-reinforcement mechanisms that become most obvious in the late stages of sustained addiction. While we tend to agree with each other about the early and late stages of addiction, we hold different views as to (i) the point between early and late at which the diagnosis of ‘addiction’ should be invoked, (ii) the relative importance of positive and negative reinforcement leading up to this transition, and (iii) the degree to which the specifics of negative reinforcement can be generalized across the range of addictive agents.

Fischer rats are more sensitive than Lewis rats to the suppressive effects of morphine and the aversive kappa-opioid agonist spiradoline

Data have suggested that rats avoid intake of an otherwise palatable saccharin cue when paired with a drug of abuse, at least, in part, because the value of the taste cue pales in anticipation of the availability of the highly rewarding drug. Earlier support for this hypothesis was provided by the finding that, relative to the less sensitive Fischer rats, Lewis rats exhibit greater avoidance of a saccharin cue when paired with a rewarding sucrose or cocaine unconditioned stimulus (US), but not when paired with the aversive agent, lithium chloride. More recent data, however, have shown that Fischer rats actually exhibit greater, not less, avoidance of the same saccharin cue when morphine serves as the US. Therefore, Experiment 1 evaluated morphine-induced suppression of intake of the taste cue in Lewis and Fischer rats when the morphine US was administered subcutaneously, rather than intraperitoneally. Experiment 2 examined the effect of strain on the suppression of intake of the saccharin cue when paired with spiradoline, a selective kappa-opioid receptor agonist. The results confirmed that Fischer rats are more responsive to the suppressive effects of morphine than Lewis rats, and that Fischer rats also exhibit greater avoidance of the saccharin cue when paired with spiradoline, despite the fact that spiradoline is devoid of reinforcing properties. Taken together, the data suggest that the facilitated morphine-induced suppression observed in Fischer rats, compared with Lewis rats, may reflect an increased sensitivity to the aversive, kappa-mediated properties of opiates.

Intracranial self-stimulation in FAST and SLOW mice: effects of alcohol and cocaine

RATIONALE

Sensitivity to the stimulant and rewarding effects of alcohol may be genetically correlated traits that predispose individuals to develop an alcohol use disorder.

OBJECTIVE

This study aimed to examine the effects of alcohol and cocaine on intracranial self-stimulation (ICSS) in FAST and SLOW mice, which were selectively bred for extremes in alcohol stimulation.

METHODS

Male FAST and SLOW mice were conditioned to respond for reinforcement by direct electrical stimulation of the medial forebrain bundle (i.e., brain stimulation reward). ICSS responses were determined immediately before and after oral gavage with water or alcohol (0.3-2.4 g/kg) or intraperitoneal injection with saline or cocaine (1.0-30.0 mg/kg). In separate FAST and SLOW mice, the locomotor effects of these treatments were measured in activity chambers.

RESULTS

Alcohol dose-dependently lowered the threshold for self-stimulation (θ (0)) and the frequency that maintained 50% of maximal responding (EF50) in FAST mice but did not significantly affect these parameters in SLOW mice. The largest effects of alcohol were after the 1.7- and 2.4-g/kg doses and were about 40% compared to water injection. Alcohol did not affect MAX response rates, but dose-dependently stimulated locomotor activity in FAST mice. Cocaine lowered thresholds equally in FAST and SLOW mice, although cocaine-stimulated locomotor activity was higher in the FAST than in the SLOW mice.

CONCLUSIONS

Selective breeding for alcohol locomotor stimulation also renders the mice more sensitive to the effects of alcohol, but not cocaine, on ICSS.

Qualitative differences between C57BL/6J and DBA/2J mice in morphine potentiation of brain stimulation reward and intravenous self-administration

RATIONALE

The C57BL/6J (C57) and DBA/2J (DBA) mice are the most common genotypes used to identify chromosomal regions and neurochemical mechanisms of interest in opioid addiction. Unfortunately, outside of the oral two-bottle choice procedure, limited and sometimes controversial evidence is available for determining their relative sensitivity to the rewarding effects of morphine.

OBJECTIVES

The purpose of this study was to utilize classically accepted models of drug abuse liability to determine relative susceptibility to the rewarding effects of morphine.

METHODS

The ability of morphine or amphetamine to potentiate lateral hypothalamic brain stimulation and intravenous morphine self-administration (across three doses in a fixed ratio schedule and at the highest dose in progressive ratio schedules) was investigated in both genotypes.

RESULTS

In both measures, C57 and DBA mice differed dramatically in their response to morphine. Morphine potentiated rewarding stimulation in the C57 mice but antagonized it in the DBA mice. Consistent with these findings, intravenous morphine did not serve as a positive reinforcer in DBA mice under conditions that were effective in the C57 mice using a fixed ratio schedule and failed to sustain levels of responding sufficient to maintain a constant rate of drug intake under a progressive ratio schedule. In contrast, amphetamine potentiated the rewarding effects of brain stimulation similarly in the two genotypes.

CONCLUSIONS

These findings provide strong evidence that morphine is rewarding in the C57 genotype and not in the DBA genotype. Understanding their relative susceptibility is important given the prominence of these genotypes in candidate gene identification and gene mapping.

Individual differences in the sensitivity to serotonergic drugs: a pharmacobehavioural approach using rats selected on the basis of their response to novelty

RATIONALE

The mechanisms underlying individual differences in the response to serotonergic drugs are poorly understood. Rat studies may contribute to our knowledge of the neuronal substrates that underlie these individual differences.

OBJECTIVES

A pharmacobehavioural study was performed to assess individual differences in the sensitivity to serotonergic drugs in rats that were selected based on their response to a novel environment.

METHODS

Low responders (LR) and high responders (HR) to novelty rats were tested on the elevated T-maze following systemic injections of increasing doses of various serotonergic agents. The duration of avoidance of the open arms was scored for five trials.

RESULTS

The duration of avoidance behaviour was larger in saline-treated LR rats compared to saline-treated HR rats. The 5-HT1A agonist 8-OH-DPAT and the 5-HT2 agonists mCPP and DOI decreased the duration of avoidance behaviour in LR rats, but increased it in HR rats. The 5-HT3 agonist SR57227A and the 5-HT releaser/reuptake inhibitor d-fenfluramine increased the duration of avoidance behaviour in both types of rat. However, higher doses of SR57227A were required to alter avoidance behaviour in HR than in LR rats. The onset of the effects of SR57227A, d-fenfluramine and WAY100635 was faster in LR than in HR rats. The described effects were receptor specific. A model explaining the data is presented.

CONCLUSIONS

These data demonstrate that LR and HR rats differ in their sensitivity to serotonergic drugs that act at 5-HT3, 5-HT2 and 5-HT1A receptors. The implications of these individual differences for individual-specific treatment of substance abuse are briefly discussed.

Mesolimbic alpha-, but not beta-adrenoceptors control the accumbal release of dopamine that is derived from reserpine-sensitive storage vesicles

Mesolimbic beta-, but not alpha-adrenoceptors control the accumbal release of dopamine that is derived from alpha-methyl-para-tyrosine-sensitive pools of newly synthesized neurotransmitter. The aim of this study was to investigate which of these adrenoceptors control the accumbal release of dopamine that is derived from reserpine-sensitive pools of previously stored neurotransmitter. Rats, that were divided in low-responders and high-responders to novelty, were pretreated with 1 mg/kg of reserpine before the alpha-adrenergic-agent phentolamine or the beta-adrenergic-agent isoproterenol was locally applied into the nucleus accumbens. The original finding that phentolamine and isoproterenol increased accumbal dopamine levels in low-responders and high-responders was replicated. Reserpine reduced the phentolamine-induced increase of accumbal dopamine in both types of rat. However, phentolamine could still increase accumbal dopamine levels in reserpine-treated high-responders, but not anymore in reserpine-treated low-responders. Reserpine did not reduce the isoproterenol-induced increase of accumbal dopamine in any type of rat. This study demonstrates that mesolimbic alpha-, but not beta-adrenoceptors control the accumbal release of dopamine that is derived from reserpine-sensitive storage vesicles. In addition, these data confirm our previous finding that dopamine can still be released from storage vesicles of reserpinized high-responders, but not of reserpinized low-responders. The collected data underline our notion that alpha- and beta-adrenergic drugs may have therapeutic effects in patients suffering from diseases in which accumbal dopamine is involved.

A unified framework for addiction: vulnerabilities in the decision process

The understanding of decision-making systems has come together in recent years to form a unified theory of decision-making in the mammalian brain as arising from multiple, interacting systems (a planning system, a habit system, and a situation-recognition system). This unified decision-making system has multiple potential access points through which it can be driven to make maladaptive choices, particularly choices that entail seeking of certain drugs or behaviors. We identify 10 key vulnerabilities in the system: (1) moving away from homeostasis, (2) changing allostatic set points, (3) euphorigenic "reward-like" signals, (4) overvaluation in the planning system, (5) incorrect search of situation-action-outcome relationships, (6) misclassification of situations, (7) overvaluation in the habit system, (8) a mismatch in the balance of the two decision systems, (9) over-fast discounting processes, and (10) changed learning rates. These vulnerabilities provide a taxonomy of potential problems with decision-making systems. Although each vulnerability can drive an agent to return to the addictive choice, each vulnerability also implies a characteristic symptomology. Different drugs, different behaviors, and different individuals are likely to access different vulnerabilities. This has implications for an individual's susceptibility to addiction and the transition to addiction, for the potential for relapse, and for the potential for treatment.

Higher locomotor response to cocaine in female (vs. male) rats selectively bred for high (HiS) and low (LoS) saccharin intake

Rats selectively bred for high saccharin consumption (HiS) self-administer more oral ethanol and i.v. cocaine than those selectively bred for low saccharin consumption (LoS). Male and female drug-seeking-prone (HiS) and -resistant (LoS) rats were used in the present experiment to test the prediction that cocaine-induced locomotor activity and sensitization varied with sex and their selective breeding status (HiS and LoS). All rats were intermittently exposed over 2 weeks to pairs of sequential saline and cocaine injections, separated by 45 min. The first 5 pairs of injections, each separated by 2-3 days (10-12 days total), were given to examine the development of cocaine-induced locomotor activity and the development of locomotor sensitization, which was determined by comparing the effects of cocaine injection 1 with injection 6 (given 2 weeks after the 5 pairs of intermittent injections). Results indicated that after the first injection pair (saline, cocaine) the HiS and LoS groups did not differ (saline vs. cocaine) in locomotor activity; however, after cocaine injection pairs 1, 5, and 6, HiS females were more active than HiS males and LoS females. There were also significant phenotype differences (HiS>LoS) in locomotor activity after cocaine injections 5 and 6. There was a weak sensitization effect in cocaine-induced locomotor activity in HiS females after cocaine injection 5 (compared to 1); however it was not present after injection 6 or in other groups. The lack of a strong sensitization effect under these temporal and dose conditions was inconsistent with previous reports. However, the results showing HiS>LoS and females>males on cocaine-induced activity measures are consistent with several measures of cocaine-seeking behavior (acquisition, maintenance, escalation, extinction, and reinstatement), and they suggest that cocaine-induced locomotor activity and sensitization are behavioral markers of drug-seeking phenotypes.

Inbred Lewis and Fischer 344 rat strains differ not only in novelty- and amphetamine-induced behaviors, but also in dopamine transporter activity in vivo

Inbred Lewis (LEW) and Fischer 344 (F344) rats are differentially sensitive to drugs of abuse, making them useful for studying addiction-related neural mechanisms. Here, we investigated whether strain differences in dopamine transporters (DATs) in dorsal striatum (dSTR) and/or nucleus accumbens (NAc) may help to explain their behavioral differences. The behavior of male LEW and F344 rats was assessed in an open-field arena during habituation to novelty and after an i.v. infusion of saline and/or 0.5 mg/kg d-amphetamine (AMPH). In vitro measures of DAT binding, protein and cell-surface expression, as well as in vitro and in vivo measures of function, were used to compare DATs in dSTR and NAc of these two strains. We found that LEW rats exhibited higher novelty- and AMPH-induced locomotion, but F344 rats exhibited greater AMPH-induced rearing and stereotypy. An initial habituation session with i.v. saline minimized the strain differences in AMPH-induced behaviors except that the more frequent AMPH-induced rearing in F344 rats persisted. Strain differences in DAT total protein and basal activity were also observed, with LEW rats having less protein and slower in vivo clearance of locally applied DA in dSTR and NAc. AMPH inhibited in vivo DA clearance in dSTR and NAc of both strains, but to a greater extent in F344 rats. Taken together, the lower basal DAT function in LEW rats is consistent with their greater novelty-induced locomotor activation, whereas the greater inhibition of DA clearance by AMPH in F344 rats is consistent with their marked AMPH-induced rearing behavior.

Differential contribution of storage pools to the extracellular amount of accumbal dopamine in high and low responders to novelty: effects of reserpine

The present study examined the effects of reserpine on the extracellular concentration of accumbal dopamine in high responders (HR) and low responders (LR) to novelty rats. Reserpine reduced the baseline concentration of extracellular accumbal dopamine more in HR than in LR, indicating that the dopamine release is more dependent on reserpine-sensitive storage vesicles in non-challenged HR than in non-challenged LR. In addition, reserpine reduced the novelty-induced increase of the extracellular concentration of accumbal dopamine in LR, but not in HR, indicating that the dopamine release in response to novelty depends on reserpine-sensitive storage vesicles only in LR, not in HR. Our data clearly demonstrate that HR and LR differ in the characteristics of those monoaminergic storage vesicles that mediate accumbal dopamine release.

The role of CYP2A6 in the emergence of nicotine dependence in adolescents

OBJECTIVES

The objectives of our study were to evaluate whether genetic variation in nicotine metabolic inactivation accounted for the emergence of nicotine dependence from mid- to late adolescence and whether initial smoking experiences mediated this effect.

METHODS

Participants were 222 adolescents of European ancestry who participated in a longitudinal cohort study of the biobehavioral determinants of adolescent smoking. Survey data were collected annually from grade 9 to the end of grade 12. Self-report measures included nicotine dependence, smoking, age first smoked, initial smoking experiences, peer and household member smoking, and alcohol and marijuana use. DNA collected via buccal swabs was assessed for CYP2A6 alleles that are common in white people and are demonstrated to decrease enzymatic function (CYP2A6*2, *4, *9, *12).

RESULTS

Latent growth-curve modeling indicated that normal metabolizers (individuals with no detected CYP2A6 variants) progressed in nicotine dependence at a faster rate and that these increases in nicotine dependence leveled off more slowly compared with slower metabolizers (individuals with CYP2A6 variants). Initial smoking experiences did not account for how CYP2A6 genetic variation impacts nicotine dependence.

CONCLUSIONS

These findings may help to promote a better understanding of the biology of smoking behavior and the emergence of nicotine dependence in adolescents and inform future work aimed at understanding the complex interplay between genetic, social, and psychological factors in adolescent smoking behavior.

Escalation of intravenous cocaine self-administration, progressive-ratio performance, and reinstatement in rats selectively bred for high (HiS) and low (LoS) saccharin intake

RATIONALE

Rats selectively bred for high saccharin (HiS) intake consume more alcohol and acquire intravenous (IV) cocaine self-administration more rapidly than their low saccharin (LoS) consuming counterparts.

OBJECTIVES

The purpose of the present study was to determine whether HiS and LoS rats also differ in the escalation, maintenance, and reinstatement of IV cocaine self-administration.

METHODS

LoS and HiS female rats were allowed to self-administer cocaine [0.4 mg/kg; fixed ratio (FR) 1] under short (ShA, 2 h per day) or long (LgA, 12 h per day) access conditions for 21 days. Session lengths were subsequently equated (2 h) and (1) FR1-maintained cocaine (0.4 mg/kg) self-administration, (2) progressive ratio (PR)-maintained cocaine (0.2-1.6 mg/kg) self-administration, and (3) saline-induced and cocaine (10 mg/kg, IP)-induced reinstatement of drug-seeking behavior were examined.

RESULTS

HiS LgA rats escalated their cocaine intake more rapidly and self-administered more cocaine (mg/kg) than LoS LgA rats; however, there was no LoS versus HiS phenotype difference in the number of infusions self-administered by Day 21. Post-escalation cocaine self-administration under an FR1 schedule did not differ as a function of phenotype (LoS versus HiS) or access condition (ShA versus LgA); however, LoS rats responded more for cocaine under the PR schedule than HiS rats, and they showed a greater reinstatement of cocaine-seeking behavior than HiS rats. In contrast, ShA versus LgA did not affect PR or reinstatement performance in the LoS and HiS groups.

CONCLUSIONS

These results suggest that LoS and HiS rats have distinct drug-seeking and drug-taking profiles that differ as a function of the experimental phase and access condition. The LoS and HiS rats differ along a wide range of behavioral dimensions and represent an important model to study the interactions of feeding, emotionality, and other factors related to vulnerability to drug abuse.

Individual responses to novelty predict qualitative differences in d-amphetamine-induced open field but not reward-related behaviors in rats

Differences in the locomotor response of rats to a novel environment (high responders [HR] versus low responders [LR]) have been associated with differences in vulnerability to psychostimulants. In the present study we profiled extensively the behavioral repertoire of HR and LR rats (differentiated on the basis of vertical activity) during exposure to a novel environment and in response to d-amphetamine (d-amp; 1.5 mg/kg, i.p.). Moreover, we ascertained whether HR and LR rats differ in the rewarding effects of medial forebrain bundle electrical self-stimulation and in the ability of d-amp to increase the reinforcing efficacy of self-stimulation. Apart from rearing, HR animals displayed increased moving, sniffing, but decreased standing and yawning compared with LR. Factor analysis revealed a more complex behavioral structure consisting of locomotion, exploration, vertical activity and self-directed behavior for HR compared with LR rats. Qualitative, but not quantitative differences, between the two groups of rats in their behavioral responses to d-amp were found. In particular, a more complex profile mainly characterized by self-directed behavior, locomotion and vertical activity was manifested for HR as compared with LR rats. Baseline brain stimulation reward thresholds did not differ between the two groups of rats. Additionally, brain stimulation reward thresholds for the two groups were not differentially affected by d-amp. The above results suggest that HR and LR can be further differentiated upon exposure to a novel environment and in response to d-amp. This differentiation is primarily based on qualitative cohorts of their behavioral structure, but not on deviations in the reward processes as assessed by intracranial self-stimulation.

Differential sensitivity to neurotensin-induced hypothermia, but not analgesia, in Fischer and Lewis rats

Neurotensin (NT) produces behavioral and physiological effects, including analgesia and hypotheria, when administered into the CNS. Fischer and Lewis rats exhibit differential behavioral responses to central NT receptor activation. To further characterize these differences, we assessed central NT-induced analgesia and hypothermia in independent groups of rats from each strain. Fischer and Lewis rats showed a similar dose-orderly analgesic response in a hot-plate test. Such an isosensitivity was not observed for NT-induced hypothermia. Although NT produced a dose-orderly decrease in mean rectal temperature in both strains, the magnitude of the hypothermic response was significantly smaller in Fischer than in Lewis rats. These findings provide further evidence of genetic differences in central neurotensinergeric neurotransmission in these two strains.

Variability of drug self-administration in rats

RATIONALE

Although temporal patterns of drug self-administration in animals are known to be highly regular, this regularity has rarely been quantified or systematically compared across reinforcers.

OBJECTIVES

Over a range of unit doses, this study assessed: (1) the within-subject variability of inter-infusion intervals (latencies); (2) the estimated whole-body drug level at the time of self-infusion; (3) the within-subject variability of these drug levels; and (4) the statistical dependence between successive latencies, to determine whether regularity is maintained by compensatory, moment-to-moment adjustment of latencies.

METHODS

Rats were trained with cocaine (10-1000 microg/kg per infusion, IV), remifentanil (an ultra-short acting opioid; 0.25-32 microg/kg per infusion, IV), or food (20-180 mg/delivery).

RESULTS

Within subjects, latencies were most consistent from infusion to infusion at unit doses on the descending limb of the dose-response curve. However, the drug level at the time an infusion was initiated was actually least consistent at these doses. Sequential latencies showed only a weak autocorrelation for both drugs.

CONCLUSION

These results suggest that highly consistent response patterns are not simply a product of precise titration of drug levels. The weak autocorrelation between sequential latencies suggests that temporal regularity of responding is not maintained through compensatory adjustments of post-infusion pauses.

Enhanced motivation to self-administer cocaine is predicted by self-grooming behaviour and relates to dopamine release in the rat medial prefrontal cortex and amygdala

Rats, like humans, show strong individual differences in their response to anxiogenic and stressful stimuli. In the present study we evaluated whether differences in stress-induced self-grooming behaviour may predict an individual's vulnerability to engage in drug self-administration behaviour. From a population of Wistar rats, the lower and upper quartile with respect to time spent self-grooming on an elevated plus maze (EPM) were selected and trained to intravenously self-administer cocaine under fixed and progressive ratio schedules of reinforcement. High grooming (HG) rats reached considerably higher breakpoints than low grooming (LG) rats but showed no differences in acquisition rate and dose-response relationships. Further, EPM exposure elicited higher anxiety levels and enhanced plasma corticosterone secretion in HG rats. In addition, HG rats did not display enhanced novelty-seeking and still spent more time self-grooming during an EPM re-test following the cocaine self-administration procedure, indicating that stress-induced self-grooming is a stable behavioural trait marker. Neurochemically, electrically evoked [(3)H]dopamine release in vitro was profoundly lower in brain slices from the substantia nigra, medial prefrontal cortex and amygdala of naive HG rats as compared to LG rats, whereas no differences were found in the nucleus accumbens shell and core, the ventral tegmental area and caudate putamen. In conclusion, stress-induced self-grooming specifically predicts enhanced motivation to self-administer cocaine rather than sensitivity to its reinforcing effects. Responsiveness of dopaminergic nerve terminals in the medial prefrontal cortex and amygdala may represent pre-existing underlying factors.