Weakening of negative relative to positive associations with cocaine-paired cues contributes to cue-induced responding after drug removal

On the positive and negative affective responses to cocaine and their relation to drug self-administration in rats

RATIONALE

Acute cocaine administration produces an initial rewarding state followed by a dysphoric/anxiogenic "crash."

OBJECTIVE

The objective of this study was to determine whether individual differences in the relative value of cocaine's positive and negative effects would account for variations in subsequent drug self-administration.

METHODS

The dual actions of cocaine were assessed using a conditioned place test (where animals formed preferences for environments paired with the immediate rewarding effects of 1.0mg/kg i.v. cocaine or aversions of environments associated with the anxiogenic effects present 15-min postinjection) and a runway test (where animals developed approach-avoidance "retreat" behaviors about entering a goal box associated with cocaine delivery). Ranked scores from these two tests were then correlated with each other and with the escalation in the operant responding of the same subjects observed over 10 days of 1- or 6-h/day access to i.v. (0.4 mg/inj) cocaine self-administration.

RESULTS

Larger place preferences were associated with faster runway start latencies (r s = -0.64), but not with retreat frequency or run times; larger place aversions predicted slower runway start times (r s = 0.62), increased run times (r s = 0.65), and increased retreats (r s = 0.62); response escalation was observed in both the 1- and 6-h self-administration groups and was associated with increased CPPs (r s = 0.58) but not CPAs, as well as with faster run times (r s = -0.60).

CONCLUSIONS

Together, these data suggest that animals exhibiting a greater positive than negative response to acute (single daily injections of) cocaine are at the greatest risk for subsequent escalated cocaine self-administration, a presumed indicator of cocaine addiction.

Adaptations underlying the development of excessive alcohol intake in selectively bred mice

BACKGROUND

This commentary discusses the important contributions of the article in this issue by Matson and colleagues entitled "Selectively bred crossed high-alcohol-preferring mice drink to intoxication and develop functional tolerance, but not locomotor sensitization during free-choice ethanol access" as well as providing comparison to studies on other drugs of abuse.

METHODS

The findings of the target article are evaluated and compared to the larger literature of intake escalation and vulnerability to addiction observed with other drugs of abuse.

RESULTS

In their study, Matson and colleagues report that mice derived by crossing different lines selectively bred for high alcohol intake exhibit initial alcohol intakes associated with motor impairment followed by marked escalation of consumption and tolerance to the effects of alcohol on motor coordination. In contrast, no evidence of pharmacokinetic tolerance or sensitization of alcohol-induced locomotion was observed. These results demonstrate that the cHAP mice constitute an appropriate model for the study of excessive drinking, which is produced by escalated alcohol intake and functional changes, leading to excessive intoxication.

CONCLUSIONS

Future work should assess adaptations in motivational processes and subjective effects of alcohol as well as the potential genetic and epigenetic bases of escalated alcohol intake.

On the persistence of cocaine-induced place preferences and aversions in rats

RATIONALE

Rats develop preferences for places associated with the immediate rewarding effects of cocaine and aversions for places paired with the drug's delayed negative effects. The motivation to seek cocaine should therefore depend upon the relative magnitude of these two opposing effects of the drug.

OBJECTIVE

The current study tested this notion by assessing the relative persistence of the positive and negative associations formed between environmental cues and the immediate or delayed effects of cocaine.

METHODS

Rats were administered 1.0 mg/kg intravenous cocaine and placed into a distinctive environment either immediately or 15-min after injection, alternating daily with pairings of a second environment with saline. After four drug-place and four saline-place pairings, rats were returned to their home cages for 1, 7, or 21 days after which a 15-min place preference test was conducted. In a second experiment, the effectiveness of a single reconditioning session (one drug-place and one saline-place pairing) to reactivate learned cocaine-place associations was assessed after 1 or 3 weeks of drug abstinence.

RESULTS

Places associated with the immediate effects of cocaine were preferred (CPP), while places associated with the delayed effects of cocaine were avoided (CPA). The persistence of these effects differed with CPP remaining viable at 3 weeks of withdrawal, while CPA was no longer present after 1 week. Reconditioning with an additional cocaine-place pairing failed to reinstate the CPA.

CONCLUSIONS

Cue-induced "relapse" of cocaine-seeking behavior may be fueled in part by an increased persistence of positive relative to negative associations with drug-paired stimuli.

Neural processing of a cocaine-associated odor cue revealed by functional MRI in awake rats

Using an olfactory conditioning procedure, brain stimulation reward threshold measurements, and functional magnetic resonance imaging (fMRI), we investigated brain stimulation reward threshold change and fMRI neural activation in response to a cocaine-associated odor cue. In the first brain stimulation experiment, over 10 days of rate-frequency curve-shift testing, rats were administered intravenous cocaine (1.0mg/kg) paired with a contextual cue of peppermint odor previously placed in the operant chamber or they were given vehicle treatment (no cocaine) in the presence of no olfactory cue. Following a 14-day drug-free rest period, rats were again given the rate-frequency curve-shift threshold test with or without the odor cue. In a second experiment, rats were similarly conditioned with a peppermint odor but with intraperitoneally delivered cocaine (10mg/kg). After a 14 day rest period, rats were imaged on a 7-T MRI for their blood oxygen level dependent (BOLD) in response to the cocaine-paired peppermint odor versus an unpaired neutral lemon odor. In the brain stimulation experiment, expected significant reward threshold shifts were produced by cocaine and, importantly, about half that level of shift was produced by the paired contextual olfactory cue. In the fMRI experiment, the insular cortex showed a significantly greater BOLD activation in cocaine-treated versus saline-treated animals to the olfactory cue, but not with the unpaired lemon scent. These data are in agreement with previous studies suggesting a role of the insular cortex in attributing reward value (positive or negative) to conditioned odor stimuli.