Effects of chronic caffeine pre-exposure on conditioned and unconditioned psychomotor activity induced by nicotine and amphetamine in rats

The Intergenerational Transmission of Developmental Nicotine Exposure-Induced Neurodevelopmental Disorder-Like Phenotypes is Modulated by the Chrna5 D397N Polymorphism in Adolescent Mice

Maternal tobacco smoking during pregnancy constitutes developmental nicotine exposure (DNE) and is associated with nicotine dependence and neurodevelopmental disorders in both children and grandchildren as well as animal models thereof. Genetic variants such as the CHRNA5 single nucleotide polymorphism (SNP) rs16969968, which leads to an aspartic acid to asparagine substitution at amino acid position 398 (D398N) in the alpha-5 nicotinic acetylcholine receptor subunit, can also confer risk for nicotine dependence and neurodevelopmental disorders in the absence of DNE. However, the degrees to which, the consequences of maternal smoking on offspring outcomes are influenced by genetic variants and interactions therewith are not well understood. Addressing this void in the literature, the present study utilizes a DNE mouse model engineered to possess the equivalent of the human D398N SNP in CHRNA5 (D397N SNP in mice) to assess how the N397 risk allele impacts the induction and intergenerational transmission of a range of neurodevelopmental disorder-related behavioral phenotypes in first- and second-generation DNE offspring. Results reveal that offspring possessing the N397 variant in the absence of DNE as well as DNE offspring and grand offspring possessing theD397 variant exhibit analogous neurodevelopmental disorder-like phenotypes including hyperactivity, risk-taking behaviors, aberrant rhythmicity of activity, and enhanced nicotine consumption. DNE amplified these behavioral anomalies in first-generation N397 progeny, but the severity of DNE-evoked behavioral perturbations did not significantly differ between first-generation D397 and N397 DNE mice for any measure. Remarkably, the behavioral profiles of second-generation N397 DNE progeny closely resembled DNE-naive D397 mice, suggesting that the N397 variant may protect against the intergenerational transmission of DNE-induced neurodevelopmental disorder-like behaviors.

Existence of wild brown rats (Rattus norvegicus) that are indifferent to novel objects

Exposure to novel objects typically evokes avoidance behavior in wild animals, which is called neophobia. We previously found that wild brown rats (Rattus norvegicus) that were trapped in a park in downtown Tokyo, Japan, exhibited neophobia. We also found that this behavior was accompanied by the activation of the basolateral complex of the amygdala (BLA). Previous studies have suggested that genetic factors are the primary determinants of neophobia. Since rats in cities form populations with distinct genetic characteristics, it is reasonable to assume that wild rats caught at different locations in urban centers will exhibit different levels of neophobia. Here we assessed the intensity of neophobia in wild rats trapped at a wholesale market in Tokyo. Although we performed exactly the same experiment in which neophobia was observed in wild rats trapped at the park, the presence of novel objects did not affect the behaviors of wild rats trapped at the market. Conversely, laboratory rats showed approach and exploratory behaviors as seen in the previous study, suggesting that the experiment was performed appropriately. Compared to the laboratory rats, the lack of behavioral changes in the wild rats was accompanied by fewer Fos immunoreactive cells in the BLA. In addition, the numbers of Fos immunoreactive cells in the bed nucleus of the stria terminalis and ventromedial hypothalamus were similar between the two types of rats. The results demonstrated the existence of wild rats that were indifferent to novel objects.

Caffeine, a common active adulterant of cocaine, enhances the reinforcing effect of cocaine and its motivational value

RATIONALE

Caffeine is one of the psychoactive substances most widely used as an adulterant in illicit drugs, such as cocaine. Animal studies have demonstrated that caffeine is able to potentiate several cocaine actions, although the enhancement of the cocaine reinforcing property by caffeine is less reported, and the results depend on the paradigms and experimental protocols used.

OBJECTIVES

We examined the ability of caffeine to enhance the motivational and rewarding properties of cocaine using an intravenous self-administration paradigm in rats. Additionally, the role of caffeine as a primer cue during extinction was evaluated.

METHODS

In naïve rats, we assessed (1) the ability of the cocaine (0.250-0.125 mg/kg/infusion) and caffeine (0.125-0.0625 mg/kg/infusion) combination to maintain self-administration in fixed ratio (FR) and progressive ratio (PR) schedules of reinforcement compared with cocaine or caffeine alone and (2) the effect of caffeine (0.0625 mg/kg/infusion) in the maintenance of responding in the animals exposed to the combination of the drugs during cocaine extinction.

RESULTS

Cocaine combined with caffeine and cocaine alone was self-administered on FR and PR schedules of reinforcement. Interestingly, the breaking point determined for the cocaine + caffeine group was significantly higher than the cocaine group. Moreover, caffeine, that by itself did not maintain self-administration behavior in naïve rats, maintained drug-seeking behavior of rats previously exposed to combinations of cocaine + caffeine.

CONCLUSIONS

Caffeine enhances the reinforcing effects of cocaine and its motivational value. Our results highlight the role of active adulterants commonly used in cocaine-based illicit street drugs.

Extending the Role of Associative Learning Processes in Nicotine Addiction

Compulsive smoking is a worldwide public health problem. Although research has confirmed the importance of associative learning processes in nicotine addiction, therapies targeting nicotine-associated cues still have a high relapse rate. Most theories conceptualize nicotine as an ‘outcome’ that reinforces behaviors and/or changes the affective value of stimuli. Albeit important, this view does not capture the complexity of associative processes involved in nicotine addiction. For example, nicotine serves as a conditional stimulus acquiring new appetitive/affective properties when paired with a non-drug reward. Also, nicotine functions as an occasion setter that participates in higher-order associative processes that likely permit a more pervasive influence of conditioned cues that are resistant to typically cue-exposure therapy techniques. Finally, nicotine appears to amplify the salience of other stimuli that have some incentive value resulting in enhanced nicotine selfadministration and conditioned reinforcement processes. Future smoking intervention strategies should take into consideration these additional associative learning processes.

Caffeine increases the motivation to obtain non-drug reinforcers in rats

BACKGROUND

Caffeine is widely considered to be a reinforcer in humans, but this effect is difficult to measure in non-human animals. We hypothesized that caffeine may have dual reinforcing effects comparable to nicotine--limited primary reinforcing effects, but potent reinforcement enhancing effects. The present studies tested this hypothesis by investigating the effect of caffeine on responding for non-drug rewards.

METHODS

In two experiments, rats were shaped to respond on a progressive ratio (PR) schedule for sucrose solution (20%, w/v; experiment 1) or a fixed ratio 2 (FR2) schedule for a moderately reinforcing visual stimulus (VS; experiment 2). Pretreatment with various doses of caffeine (0-50 mg/kg, intraperitoneal injection) were administered prior to tests over successive week days (M-F). In experiment 1, acute administration of low-moderate caffeine doses (6.25-25 mg/kg) increased responding for sucrose under the PR schedule. This effect of caffeine declined over the initial 15 test days. In experiment 2, only acute pretreatment with 12.5mg/kg caffeine increased responding for the visual stimulus and complete tolerance to this effect of caffeine was observed over the 15 days of testing. In follow up tests we found that abstinence periods of 4 and 8 days resulted in incomplete recovery of the enhancing effects of caffeine.

CONCLUSION

The findings suggest that caffeine enhances the reinforcing effects of non-drug stimuli, but that the pharmacological profile of these effects may differ from other psychomotor stimulants.

Effects of caffeine on persistence and reinstatement of nicotine-seeking behavior in rats: interaction with nicotine-associated cues

RATIONALE

Caffeine and nicotine are the most commonly co-used psychostimulants. However, it is still unclear whether caffeine exposure enhances nicotine-seeking behavior.

OBJECTIVE

The present study examined the effects of caffeine on nicotine-seeking in rats trained to self-administer nicotine with and without presession administration of caffeine.

METHODS

Male Sprague-Dawley rats were trained to intravenously self-administer nicotine (0.03 mg/kg/infusion, freebase) on a fixed ratio 5 schedule of reinforcement and associate a stimulus cue with each nicotine administration. Five minutes before the sessions, the rats received an intraperitoneal administration of caffeine (5 mg/kg). Extinction tests were conducted under four conditions: presession caffeine administration, response-contingent presentation of nicotine cues, neither condition, or both conditions. Reinstatement tests were conducted after responding was extinguished by withholding presession caffeine, nicotine, and its cues. A separate group of rats trained without presession caffeine exposure was also subjected to the reinstatement tests.

RESULTS

In the rats trained with presession caffeine exposure, continued caffeine administration sustained nicotine-seeking responses and interacted with nicotine cues to significantly delay the extinction of nicotine-seeking behavior. Readministration of caffeine after extinction effectively reinstated nicotine-seeking behavior. In caffeine-naive rats, caffeine administration did not reinstate extinguished nicotine-seeking behavior but significantly potentiated the cue-induced reinstatement of nicotine-seeking.

CONCLUSION

These data demonstrate that caffeine administration sustained and reinstated nicotine-seeking behavior, possibly via its acquired discriminative-stimulus properties predictive of nicotine availability. These findings suggest that smokers who attempt to quit may benefit from stopping caffeine consumption.

Guidelines on nicotine dose selection for in vivo research

RATIONALE

This review provides insight for the judicious selection of nicotine dose ranges and routes of administration for in vivo studies. The literature is replete with reports in which a dosaging regimen chosen for a specific nicotine-mediated response was suboptimal for the species used. In many cases, such discrepancies could be attributed to the complex variables comprising species-specific in vivo responses to acute or chronic nicotine exposure.

OBJECTIVES

This review capitalizes on the authors' collective decades of in vivo nicotine experimentation to clarify the issues and to identify the variables to be considered in choosing a dosaging regimen. Nicotine dose ranges tolerated by humans and their animal models provide guidelines for experiments intended to extrapolate to human tobacco exposure through cigarette smoking or nicotine replacement therapies. Just as important are the nicotine dosaging regimens used to provide a mechanistic framework for acquisition of drug-taking behavior, dependence, tolerance, or withdrawal in animal models.

RESULTS

Seven species are addressed: humans, nonhuman primates, rats, mice, Drosophila, Caenorhabditis elegans, and zebrafish. After an overview on nicotine metabolism, each section focuses on an individual species, addressing issues related to genetic background, age, acute vs chronic exposure, route of administration, and behavioral responses.

CONCLUSIONS

The selected examples of successful dosaging ranges are provided, while emphasizing the necessity of empirically determined dose-response relationships based on the precise parameters and conditions inherent to a specific hypothesis. This review provides a new, experimentally based compilation of species-specific dose selection for studies on the in vivo effects of nicotine.

The selective dopamine D3 receptor antagonist SB-277011A reduces nicotine-enhanced brain reward and nicotine-paired environmental cue functions

Increasing evidence suggests that enhanced dopamine (DA) neurotransmission in the nucleus accumbens (NAc) may play a role in mediating the reward and reinforcement produced by addictive drugs and in the attentional processing of drug-associated environmental cues. The meso-accumbens DA system is selectively enriched with DA D3 receptors, a DA receptor subtype increasingly implicated in reward-related brain and behavioural processes. From a variety of evidence, it has been suggested that selective DA D3 receptor antagonism may be a useful pharmacotherapeutic approach for treating addiction. The present experiments tested the efficacy of SB-277011A, a selective DA D3 receptor antagonist, in rat models of nicotine-enhanced electrical brain-stimulation reward (BSR), nicotine-induced conditioned locomotor activity (LMA), and nicotine-induced conditioned place preference (CPP). Nicotine was given subcutaneously within the dose range of 0.25-0.6 mg/kg (nicotine-free base). SB-277011A, given intraperitoneally within the dose range of 1-12 mg/kg, dose-dependently reduced nicotine-enhanced BSR, nicotine-induced conditioned LMA, and nicotine-induced CPP. The results suggest that selective D3 receptor antagonism constitutes a new and promising pharmacotherapeutic approach to the treatment of nicotine dependence.

Sensitization to caffeine and cross-sensitization to amphetamine: Influence of individual response to caffeine

The present study evaluated the ability of a subchronic intermittent administration of caffeine to induce a sensitized motor response and correlated the individual susceptibility of rats to acute caffeine to the development of sensitization. Moreover, individual susceptibility to caffeine and development of motor behaviour sensitization were correlated to the behavioural response obtained after a challenge with amphetamine. To this end, rats were subdivided in "low" and "high" responders according to their individual susceptibility to acute caffeine established on the basis of the motor activity observed after the first caffeine administration. "Low" and "high" responder rats were then repeatedly and intermittently treated with caffeine (15 mg/kg, i.p.), or vehicle, every other day for fourteen days. Three days after treatment discontinuation, behavioural activation induced by acute amphetamine (0.5 mg/kg, s.c.) was measured in vehicle- and caffeine-pretreated rats. Subchronic caffeine resulted in motor sensitization of a variable degree among rats and no difference were observed between "low" and "high" responders. Moreover, caffeine pretreatment potentiated the behavioural effects of amphetamine according to the degree of caffeine sensitization but not to individual susceptibility to acute caffeine. These results demonstrate that individual susceptibility to acute caffeine does not influence the modifications in caffeine motor effects produced by its subchronic administration and does not affect the enhancement of acute behavioural effects of amphetamine in caffeine-pretreated rats, rather sensitization to subchronic caffeine administration critically influences the behavioural effects of amphetamine.

Preexposure to nicotine alters the subsequent locomotor stimulant effects of bupropion in rats

Little is known about the interaction between nicotine and bupropion (Zyban), but many studies suggest they have neurological and behavioral similarities. One feature of drugs with common profiles is the ability to cross-sensitize possibly through neurological changes in the reward pathway. Activation of this pathway might explain the effectiveness of bupropion as a smoking cessation aid. The present research investigated whether repeated nicotine administration altered the subsequent locomotor effects of bupropion. In experiment 1, rats were preexposed to nicotine (0.4 mg/kg subcutaneously) or saline on eight separate occasions in the home cage and then tested with bupropion (0, 20, or 30 mg/kg) in locomotor chambers. The acute stimulant effect of 30 mg/kg of bupropion was potentiated by nicotine preexposure. In experiment 2, rats received nicotine repeatedly paired with the locomotor chambers or home cages. An additive effect was observed between acute bupropion and nicotine-conditioned hyperactivity in the chamber-paired group. This enhancement of the acute locomotor effects of bupropion might reflect alterations in common dopaminergic processes.

Subchronic-intermittent caffeine amplifies the motor effects of amphetamine in rats

Caffeine, the most widely consumed psychostimulant drug, acutely stimulates motor behaviour and enhances dopamine agonists actions whilst chronically it induces tolerance to either caffeine- or dopamine agonist-induced motor activating effects. The present study examined whether subchronic caffeine administration (15 mg/kg, on alternate days for 14 days) induces enduring modifications in caffeine- and amphetamine-mediated motor activity. To this end, motor activation and rotational behaviour stimulated by either caffeine or D-amphetamine (0.5, 2 mg/kg), given 3 days after the last caffeine administration, were evaluated in neurologically intact and unilaterally 6-hydroxydopamine-lesioned rats respectively. Subchronic caffeine resulted in an increase in caffeine-induced motor and turning behaviour. Furthermore, caffeine pretreatment potentiated the motor effects of amphetamine in both intact and 6-hydroxydopamine-lesioned rats. These results suggest that subchronic caffeine treatment results in an enhancement of its motor stimulant effects, rather than in tolerance, and induces neuroadaptive facilitatory changes in dopamine transmission.

Potentiation of amphetamine-mediated responses in caffeine-sensitized rats involves modifications in A2A receptors and zif-268 mRNAs in striatal neurons

Subchronic intermittent administration of caffeine induces sensitization of motor behaviour and promotes cross-sensitization to amphetamine motor activity. In order to evaluate the possible mechanisms at the basis of these effects, modifications in A(2A) receptor and zif-268 mRNAs were evaluated in rats subchronically treated with caffeine (15 mg/kg i.p.) and challenged with caffeine (15 mg/kg i.p.) or amphetamine (0.5, 1 mg/kg s.c.) 3 days after discontinuation of treatment. Results showed that the sensitized motor response to caffeine was associated with a decrease of adenosine A(2A) receptor and zif-268 mRNA levels in the striatum and nucleus accumbens, whereas cross-sensitization to amphetamine was linked to a more pronounced increase of zif-268 mRNA levels in the striatum, but not in the nucleus accumbens. Single-cell analysis showed that zif-268 mRNA modifications occurred in Enk(+) striatopallidal neurons after acute or subchronic treatment with caffeine and in Enk(-) striatonigral neurons after acute amphetamine administration. Potentiation of amphetamine effects was not associated with modifications of amphetamine-induced dopamine release in nucleus accumbens in caffeine-pretreated rats compared with vehicle-pretreated rats. Results demonstrate that sensitization to caffeine and cross-sensitization to amphetamine are associated with post-synaptic neuroadaptive changes in selective neuronal populations of the striatum.

Caffeine and the dopaminergic system

Caffeine is the most widely consumed psychostimulant substance, being self-administered throughout a wide range of conditions and present in numerous dietary products. Due to its widespread use and low abuse potential, caffeine is considered an atypical drug of abuse. The main mechanism of action of caffeine occurs via the blockade of adenosine A1 and A2A receptors. Adenosine is a modulator of CNS neurotransmission and its modulation of dopamine transmission through A2A receptors has been implicated in the effects of caffeine. This review provides an updated summary of the results reported in the literature concerning the behavioural pharmacology of caffeine and the neurochemical mechanisms underlying the psychostimulant effects elicited by caffeine. The review focuses on the effects of caffeine mediated by adenosine A2A receptors and on the influence that pre-exposure to caffeine may exert on the effects of classical drugs of abuse.

Stimulus properties of nicotine, amphetamine, and chlordiazepoxide as positive features in a pavlovian appetitive discrimination task in rats

Recent experiments from our laboratory have demonstrated that drug states can signal when environmental cues will be followed by rewarding outcomes (ie Pavlovian conditioning). However, little is known about the generality of this approach and whether it can be used for studying the pharmacological properties of drug states. Accordingly, the present experiments tested the pharmacological specificity of nicotine (0.4 mg/kg), amphetamine (1 mg/kg), and chlordiazepoxide (CDP, 5 mg/kg) in this Pavlovian drug discrimination procedure. Following drug administration, presentation of a conditional stimulus (CS) was followed by brief access to sucrose. When saline was administered, the same CS was presented but sucrose was withheld. In substitution tests, rats in each condition received varying doses of all training drugs and caffeine. Anticipatory food seeking developed during the CS on drug sessions but not on saline sessions for all drug features (ie drug state-specific conditional response (CR)). In generalization tests, this CR decreased as a function of decreases in the training dose. Median effective doses (ED50s) were calculated for nicotine (0.054 mg/kg), amphetamine (0.26 mg/kg), and CDP (2.48 mg/kg). No compound tested substituted for the CDP training drug. Partial substitution was evident between nicotine and amphetamine; CDP did not substitute for either of these drug features. Caffeine fully substituted for nicotine (ED50 = 15.45 mg/kg) and amphetamine (ED50 = 3.70 mg/kg), but not for CDP. These results are consistent with the hypothesis that drug states can occasion appetitive Pavlovian CRs in a pharmacologically specific manner.

Novelty reward as a measure of anhedonia

A decrease in sensitivity to pleasurable stimuli, anhedonia, is a major symptom of depression in humans. Several animal models have been developed to simulate this symptom (e.g. drug withdrawal, learned helplessness) using reward-sensitive procedures such as intracranial self-stimulation and progressive ratio responding as a measure of reward function. Recently, we introduced the use of another procedure, novel-object place conditioning in rats, to measure reward function in an associative learning situation. Withdrawal from chronic nicotine blocked a place preference conditioned by access to novel objects. This blockade was not due to impairment of object interaction, general activity, novelty detection, environmental familiarization, or expression of learning. Consequently, nicotine withdrawal directly reduced the rewarding properties of novelty. It is proposed that the novel-object place conditioning procedure could be usefully extended to other experimental situations and to genetically altered mice, so as to better understand the processes underlying changes in reward function.

Timing of conditioned responding in a nicotine locomotor conditioning preparation: manipulations of the temporal arrangement between context cues and drug administration

Using a locomotor conditioning preparation, we examined whether manipulating time between exposure to distinct environmental cues and nicotine administration affected conditioned responding. Rats that received nicotine (0.42 mg/kg base) immediately before placement in an environment for 30 min on eight separate occasions displayed hyperactivity relative to controls in a subsequent injection/drug-free test. This conditioned hyperactivity was weaker if nicotine was administered 15 min before environment exposure. Conditioning was not evidenced when nicotine was administered 15 min after placement or upon removal from the environment. In a follow-up experiment, rats received 45 min in the environment; nicotine was administered 15 min after placement. This group showed conditioning that was localized to the last two-thirds of a 45 min test indicating that a 15 min delay did not prevent conditioning given 30 min of environment/nicotine overlap. This apparent timing of conditioned responding was not due to increasing environment exposure to 45 min. Further, a state-dependent environmental familiarization account of locomotor hyperactivity during testing was eliminated by the finding that rats displayed temporally specific increases in activity on the test day despite the fact that the context was previously experienced without drug for 15 min on eight consecutive days.