Post-sensitisation conditioned hyperlocomotion induced by cocaine is augmented as a function of dose in C57BL/6J mice

Drug-free and context-dependent locomotor hyperactivity in DBA/2 J mice previously treated with repeated cocaine: Relationship with behavioral sensitization and role of noradrenergic receptors

Drug-associated contexts and discrete cues can trigger motivational states responsible for drug-seeking behavior and relapse. In preclinical research, drug-free conditioned hyperactivity has been used to investigate the expression of memories associated with psychostimulant drug effects. Addictive drugs can produce long-lasting sensitization to their psychomotor actions, a phenomenon known as behavioral sensitization. The neuroplasticity underlying behavioral sensitization appears to be involved in pathological drug pursuit and abuse. In the present study we evaluated drug-free, context-dependent hyperactivity in DBA/2 J mice previously treated with cocaine and we explored whether this conditioned effect was related to behavioral sensitization. Given the role of noradrenergic (NA) neurotransmission in memory retrieval, consolidation and reconsolidation processes, we also investigated whether conditioned hyperactivity in a drug-free state was mediated by NA receptors. Animals underwent a sensitization protocol with six cocaine injections (0, 5, 10 or 20 mg/kg) paired to a particular floor cue. Three days after this sensitization phase, all animals were exposed to the same familiar floor environment without drug treatment. A second test with an unfamiliar floor was conducted 24 h later. Conditioned hyperactivity was also explored after one or three cocaine pairings and was evaluated for its duration (with repeated familiar vs. unfamiliar floor tests). In a series of pharmacological experiments, we evaluated the effects propranolol (a non-selective antagonist of β1- and β2-receptors) and prazosin (α1-receptor antagonist) on conditioned hyperactivity. Cocaine treatment produced both robust sensitization and drug-free conditioned hyperactivity, an effect that lasted up to 17 days (with cocaine 20 mg/kg). A significant correlation between the magnitude of cocaine sensitization and the level of conditioned hyperactivity was found. Propranolol, but not prazosin, blocked context-dependent hyperlocomotion in a drug-free state. Our data, together with a vast body of literature, indicate that the NA system plays a key role in the retrieval and behavioral expression of drug-associated memories.

Effects of ketamine on the unconditioned and conditioned locomotor activity of preadolescent and adolescent rats: impact of age, sex, and drug dose

RATIONALE

Ketamine is used by preadolescent and adolescent humans for licit and illicit purposes.

OBJECTIVE

The goal of the present study was to determine the effects of acute and repeated ketamine treatment on the unconditioned behaviors and conditioned locomotor activity of preadolescent and adolescent rats.

METHODS

To assess unconditioned behaviors, female and male rats were injected with ketamine (5-40 mg/kg), and distance traveled was measured on postnatal day (PD) 21-25 or PD 41-45. To assess conditioned activity, male and female rats were injected with saline or ketamine in either a novel test chamber or the home cage on PD 21-24 or PD 41-44. One day later, rats were injected with saline and conditioned activity was assessed.

RESULTS

Ketamine produced a dose-dependent increase in the locomotor activity of preadolescent and adolescent rats. Preadolescent rats did not exhibit sex differences, but ketamine-induced locomotor activity was substantially stronger in adolescent females than males. Repeated ketamine treatment neither caused a day-dependent increase in locomotor activity nor produced conditioned activity in preadolescent or adolescent rats.

CONCLUSIONS

The activity-enhancing effects of ketamine are consistent with the actions of an indirect dopamine agonist, while the inability of ketamine to induce conditioned activity is unlike what is observed after repeated cocaine or amphetamine treatment. This dichotomy could be due to ketamine's ability to both enhance DA neurotransmission and antagonize N-methyl-D-aspartate (NMDA) receptors. Additional research will be necessary to parse out the relative contributions of DA and NMDA system functioning when assessing the behavioral effects of ketamine during early ontogeny.

Age-dependent differences in the strength and persistence of psychostimulant-induced conditioned activity in rats: effects of a single environment-cocaine pairing

The aim of the present study was to determine the strength and persistence of cocaine-induced conditioned activity in young and adult rats. A one-trial protocol has proven useful for studying the ontogeny of psychostimulant-induced behavioral sensitization; therefore, a similar procedure was used to examine conditioned activity. On postnatal day (PD) 19 or PD 80, rats were injected with saline or cocaine in either a novel test chamber or the home cage. After various drug abstinence intervals (1-21 days), rats were injected with saline and returned to the test chamber, where conditioned activity was assessed. In a separate experiment, we examined whether cocaine-induced conditioned activity was a consequence of Pavlovian conditioning or a failure to habituate to the test environment. The results indicated that adult rats showed strong one-trial conditioned activity that persisted for at least 21 days, whereas young rats did not show a conditioned locomotor response. The conditioned activity shown by adult rats did not result from a failure to habituate to the cocaine-paired environment. These results indicate that cocaine-paired contextual stimuli differentially affect behavior depending on the age of the animal. The data obtained from adult rats have potential translational relevance for humans because a single environment-drug pairing caused long-term alterations in behavior.

Development and persistence of methamphetamine-conditioned hyperactivity in Swiss-Webster mice

These experiments examined the development and persistence of methamphetamine-conditioned hyperactivity in Swiss-Webster mice. Experiments 1 and 2 examined the development of conditioned hyperactivity, varying the methamphetamine dose (0.25-2.0 mg/kg), the temporal injection parameters (continuous; experiment 1 or intermittent; experiment 2), and the comparison control group (saline; experiment 1 or unpaired; experiment 2). Experiment 3 examined the persistence of methamphetamine-conditioned hyperactivity by comparing mice 1 (immediate) or 28 (delay) days after drug withdrawal. In each experiment, several behavioral measures (vertical counts, distance traveled, and velocity) were recorded and temporal analyses conducted to assess methamphetamine-conditioned hyperactivity. In experiments 1 and 2, it was found that methamphetamine-conditioned hyperactivity was (i) dose-dependent, (ii) detected early in the session, and (iii) detected by a behavioral measure indicative of general activity (i.e. distance traveled), and (iv) varied as a function of the number of conditioning sessions. In experiment 3, it was found that conditioned hyperactivity persisted for 28 days, though was weakened by nonassociative factors, following methamphetamine withdrawal. Collectively, these results suggest that conditioned hyperactivity to methamphetamine is robust and persists after prolonged periods of drug withdrawal in mice. Furthermore, these results are consistent with an excitatory classical conditioning interpretation of conditioned hyperactivity.

Chronic cocaine exposure in the SCID mouse model of HIV encephalitis

Clinical and preclinical evidence suggests that cocaine exposure hastens progression of the HIV disease process. An established active, euphoric dose of cocaine (20 mg/kg) was administered to SCID mice according to a regimen consistent with exposure to the drug by cocaine-abusing HIV-infected patients to determine the effects of cocaine on four previously established pathological characteristics of HIV encephalitis: cognitive deficits, fatigue, astrogliosis, and microgliosis. Mice were intracranially inoculated with either HIV-infected, or uninfected macrophages and then injected with either cocaine or saline in a 2 (Infection)x2 (Cocaine) factorial design. Cognition was assessed by acquisition and retention of a spatially cued learning task. Fatigue was assessed by monitoring motor activity following a 2 min forced swim. Mice were then sacrificed to determine the extent of astrogliosis and microgliosis in the four groups. Results indicated that in comparison to uninfected controls, HIV positive mice had increased astrogliosis and microgliosis, cognitive deficits, and recovered more slowly from fatigue. However, despite evidence that the cocaine exposure regimen activated the central nervous system and had long-term CNS effects, the drug did not alter the behavioral or the neuropathological deficits noted in HIV-infected SCID mice.

The magnitude and the extinction duration of the cocaine-induced conditioned locomotion-activated response are related to the number of cocaine injections paired with the testing context in C57BL/6J mice

Behavioural activation repeatedly induced by a stimulant in rodents can persist in the absence of the drug if the animals are tested in the context where the drug was previously given, a phenomenon often explained in terms of Pavlovian conditioning. The aim of this study was to verify whether the amplitude of the putative CR (the drug-like activity) increases with the number of the US-CS associations (the number of drug-context pairings), one of the most representative rules of Pavlovian conditioning. The effect of the number of trials on the speed of extinction was also considered. C57BL/6J mice received 3, 6 or 12 once-daily injections of either saline or 12 mg/kg (-)-cocaine hydrochloride (s.c.) in the same test context, a photocell activity-box in which they were tested for 60 min after every injection. Other groups received the same treatments outside of the test context (being placed in a novel cage tub after each injection). Twenty-four hours after the last treatment session, all mice were challenged with saline in the test context (test for conditioned activity), extinction sessions taking place on the three subsequent days. Sensitisation to the locomotor-activating effect of cocaine developed only amongst the animals injected 6 or 12 times, the magnitude of the last sensitised response being comparable for these two injections regimen. On saline challenge, only the animals that had received 6 or 12 cocaine injections showed significant conditioned activity (CR), with the greatest response occurring following 12 injections. The 6-trial group reached the level of non-significance after fewer extinction sessions than the 12-trial group; however, the rates of extinction did not differ (comparable regression coefficients and quasi-parallel curves). These results suggest that the amplitude of the CR (cocaine-like stimulation after saline), and perhaps less convincingly the duration of extinction, are functions of the number of the US-CS (cocaine-context) pairings, supporting the Pavlovian nature of post-sensitisation placebo drug-like effects.

Sensitised locomotion does not predict conditioned locomotion in cocaine-treated mice: further evidence against the excitatory conditioning model of context-dependent sensitisation

The excitatory conditioning model of contextual sensitisation proposes that the progressive emergence of the locomotion-activating effect of cocaine (or any other stimulant drug) characterising that phenomenon is due to a growing conditioned response (the test context cues) that mimics the unchanging unconditioned response (the drug effect). The present study aimed at verifying whether the relationship between the amplitude of sensitisation and the size of the conditioned response was positive, a direct implication of that view. Sensitisation to the locomotion-activating effect of cocaine (10 mg/kg, s.c.) was firstly generated over 10 daily sessions in 25 mice (strain C57Bl/6J), another lot of 25 mice receiving the same dose of cocaine outside of the testing context. Conditioned locomotion was assessed 24 h later. No significant linear correlations were found between the magnitude of the conditioned response and the magnitude of the sensitised response (delta scores), the rate of sensitisation (individual regression coefficients) or the magnitude of the initial unconditioned response to cocaine (scores in the first session of sensitisation treatment). Accordingly, there was no significant correlation between the magnitude of the initial unconditioned response and the magnitude of the sensitised response or that of the initial unconditioned response. Therefore, the conditioned response is neither necessary nor sufficient for the development of context-dependent sensitisation of the locomotion-activating effect of cocaine, a conclusion that refutes the excitatory conditioning model of that chronic effect.

Quasi-asymptotic development of conditioned hyperactivity induced by intermittent injections of cocaine in C57BL/6J mice

The emergence of a conditioned cocaine-induced hyperlocomotion was examined in C57BL/6J mice using a procedure that has not been used previously. Two days after a session of preexposure to the test chambers under saline, a first group of mice (cocaine-cued) received five once-daily injections of 10-mg/kg s.c. cocaine every other day (on the odd days of the chronic treatment period) and a saline injection on the 5 days following each cocaine injection day (on the even days of the treatment period), in all cases before being placed in the test chamber. Another group of mice (saline-cued) received 10 injections of saline on both the even and the odd days in the same context, and a third group of mice (cocaine-uncued) received five injections of saline on the even days in the test context and five injections of cocaine on the odd days in an alternative context. On the odd days sessions, the cocaine-cued group showed significant repeated increases in locomotion without behavioural sensitisation being induced, whereas the saline-cued levels of locomotion remained on baseline levels. On the first even session, the three groups did not differ from each other and showed lower levels of locomotion than on the preexposure session. During the two following even sessions, the cocaine-cued group showed an increase in locomotion that levelled off on the two remaining sessions, whereas the saline-cued and the cocaine-uncued groups (which presented comparable values) exhibited significantly lower levels of locomotion. That pattern of successive placebo responses resembles the typical S-shaped development of a Pavlovian conditioned response, albeit the increase described here was quite rapid. The protocol used here may provide an additional method for the experimental analysis of stimulant-induced conditioned placebo activity.

Ontogenesis of behavioral sensitization and conditioned place preference induced by psychostimulants in laboratory rodents

The present review deals mainly with the ontogenesis of two important phenomena involved in vulnerability to several neuropsychiatric disorders, namely with drug-induced sensitization (both contextual and non-contextual) and with conditioned place preference. The term 'infancy' covers the first three postnatal weeks during development in rats and mice. Conversely, the term 'adolescence' may cover the whole postnatal period ranging from weaning (PND 21) to adulthood (at least PND 60) or specifically the period around the onset of puberty (animals aged 33-44 days). Recent studies in rats demonstrated that the establishment of a context-dependent sensitization appears during the first (for repeated drug administration) or during the second (for a single drug administration) postnatal week. However, the memory of drug-context association is transient in developing pups (lasting one or two days following the drug pretreatment). The long-term retention of drug-context associations matures progressively, and is complete by the third week of postnatal life. Finally, those mechanisms responsible for an adult-like profile of context-independent pharmacological sensitization appear later during ontogenesis, being mature by the fourth week of postnatal life. Another set of experiments extended this ontogenetic characterization by comparing adolescent and adult mice. When compared to the latter, the former subjects exhibit a greater amphetamine-induced locomotor sensitization, almost no sensitization of aversive stereotyped behaviors, and a less marked place conditioning. The strength of the drug-induced place conditioning was also directly compared with the unconditioned novelty-seeking drive. In conclusion, neonatal rats are able to show a relatively short-lasting retention of sensitized drug effects (short-term sensitization), whereas the ability to exhibit relatively long-lasting sensitized effects matures progressively during infancy (long-term sensitization). On the other hand, adolescent mice show a reduced sensitization of drug-induced psychotic symptoms, together with a more marked sensitization of arousing and euphorigenic properties of the drug and a reduced incentive memory of its hedonic effects. These age-related changes do imply very different degrees of vulnerability to drug addiction and several other neuropsychiatric disorders.

Effects of the social conditions of housing through testing on cocaine-induced contextual sensitisation and conditioned locomotion in C57BL/6J mice

The potential differential effects of isolated and collective housing through the testing phase on sensitisation to cocaine-induced locomotion, the subsequent conditioned locomotion and the context-dependent expression of sensitisation were examined in C57Bl/6J male mice. Sensitisation was first generated in mice receiving seven once-daily subcutaneous injections of either saline or 10 mg/kg cocaine, before being placed in a testing chamber (singly) or in their home cage. On Day 8, mice were tested for conditioned locomotion (under saline). On Day 12, after three daily sessions of reinstatement of sensitisation, they were tested for contextual sensitisation (under cocaine). Whereas little or no effect of housing on the development of sensitisation was found, postsensitisation conditioned activity was significantly greater in isolation-kept mice (in comparison with the group-kept animals) and the likelihood of inducing a context-dependent expression of sensitisation was greater in grouped-housed mice. The results indicate that some of the aspects of contextual sensitisation can be influenced by the social conditions of lodging, not only when these conditions start several weeks before or during previous developmental periods of animals life (as previously published), but also when social isolation or social grouping are initiated after sexual maturity and applied through the period of treatments and testing. Moreover, the differential effect of social conditions of lodging on conditioned activity and contextual expression of sensitisation disagrees with the excitatory conditioning account of contextual sensitisation.