Oral caffeine pretreatment produced modest increases in smoked cocaine self-administration in rhesus monkeys

Application of dose-addition analyses to characterize the abuse-related effects of drug mixtures

Polysubstance use makes up a majority of drug use, yet relatively few studies investigate the abuse-related effects of drug mixtures. Dose-addition analyses provide a rigorous and quantitative method to determine the nature of the interaction (i.e., supraadditive, additive, or subadditive) between two or more drugs. As briefly reviewed here, studies in rhesus monkeys have applied dose-addition analyses to group level data to characterize the nature of the interaction between the reinforcing effects of stimulants and opioids (e.g., mixtures of cocaine + heroin). Building upon these foundational studies, more recent work has applied dose-addition analyses to better understand the nature of the interaction between caffeine and illicit stimulants such as MDPV and methamphetamine in rats. In addition to utilizing a variety of operant procedures, including drug discrimination, drug self-administration, and drug-primed reinstatement, these studies have incorporated potency and effectiveness ratios as a method for both statistical analysis and visualization of departures from additivity at both the group and individual subject level. As such, dose-addition analyses represent a powerful and underutilized approach to quantify the nature of drug-drug interactions that can be applied to a variety of abuse-related endpoints in order to better understand the behavioral pharmacology of polysubstance use.

Interactions between reinforcement history and drug-primed reinstatement: Studies with MDPV and mixtures of MDPV and caffeine

Many drugs of abuse are mixed with other psychoactive substances (e.g., caffeine) prior to their sale or use. Synthetic cathinones (e.g., 3,4-methylenedioxypyrovalerone [MDPV]) are commonly mixed with caffeine or other cathinones (e.g., 3,4-methylenedioxy-N-methylcathinone [methylone]), and these "bath salts" mixtures (e.g., MDPV + caffeine) can exhibit supra-additive interactions with regard to their reinforcing and discriminative stimulus properties. However, little is known about relapse-related effects of drug mixtures. In these studies, male Sprague-Dawley rats self-administered 0.032 mg/kg/inf MDPV or a mixture of MDPV + caffeine (0.029 + 0.66 mg/kg/inf, respectively) and then underwent multiple rounds of extinction and reinstatement testing to evaluate the influence of reinforcement history and drug-associated stimuli on the effectiveness of saline (drug-paired stimuli alone), MDPV (0.032-1.0 mg/kg), caffeine (1.0-32 mg/kg), and mixtures of MDPV:caffeine (in 3:1, 1:1, and 1:3 ratios, relative to each drug's ED₅₀ ) to reinstate responding. Dose-addition analyses were used to determine the nature of the drug-drug interaction for each mixture. MDPV and caffeine dose-dependently reinstated responding and were equally effective, regardless of reinforcement history. Most fixed ratio mixtures of MDPV + caffeine exhibited supra-additive interactions, reinstating responding to levels greater than was observed with caffeine and/or MDPV alone. Drug-associated stimuli also played a key role in reinstating responding, especially for caffeine. Together, these results demonstrate that the composition of drug mixtures can impact relapse-related effects of drug mixtures, and "bath salts" mixtures (MDPV + caffeine) may be more effective at promoting relapse-related behaviors than the constituents alone. Further research is needed to determine how other polysubstance reinforcement histories can impact relapse-related behaviors.

Self-administration of benzodiazepine and cocaine combinations by male and female rhesus monkeys in a choice procedure: role of α1 subunit-containing GABAA receptors

RATIONALE

Compounds lacking efficacy at the α1 subunit-containing GABA_A (α1GABA_A) receptor appear to have reduced abuse potential compared with those having measurable efficacy at this receptor, though their self-administration in nonhuman primates is dependent upon past drug experience.

OBJECTIVES

We used a drug vs. drug choice procedure to evaluate the hypothesis that L-838,417, a compound lacking efficacy at αGABA_A receptors, would not enhance cocaine choice in monkeys trained to self-administer cocaine. We also hypothesized that zolpidem, a compound with preferential modulation of ⍺1GABA_A receptors and midazolam, a nonselective benzodiazepine, would enhance cocaine choice in this procedure.

METHODS

One female and three male rhesus monkeys chose between cocaine alone (0.1 mg/kg/injection) vs. the same dose of cocaine combined with midazolam (0.003-0.1 mg/kg/injection), zolpidem (0.003-0.3 mg/kg/injection), or L-838-417 (0.01-0.1 mg/kg/injection). In addition, we evaluated choice between saline and L-838,417 at select doses to determine whether L-838,417 would function as a reinforcer on its own.

RESULTS

Consistent with our hypotheses, midazolam- and zolpidem-cocaine mixtures were chosen over cocaine alone at sufficiently high doses. However, L-838,417-cocaine mixtures also were chosen over cocaine alone in three of four subjects with at least one dose. When available alone vs. saline, L-838,417 did not function as a reinforcer in any subject.

CONCLUSION

Compounds that lack efficacy at α1GABA_A receptors may have low abuse potential compared to classic benzodiazepines, but self-administration of these compounds is context-dependent.

Reinforcing Effects of Binary Mixtures of Common Bath Salt Constituents: Studies with 3,4-Methylenedioxypyrovalerone (MDPV), 3,4-Methylenedioxymethcathinone (methylone), and Caffeine in Rats

Bath salts use is associated with high rates of abuse, toxicity, and death. Bath salt preparations often contain mixtures of drugs including multiple synthetic cathinones (eg, 3,4-methylenedioxypyrovalerone (MDPV) or 3,4-methylenedioxymethcathinone (methylone)) or synthetic cathinones and caffeine; however, little is known about whether interactions among bath salt constituents contribute to the abuse-related effects of bath salts preparations. This study used male Sprague-Dawley rats responding under a progressive ratio schedule to quantify the reinforcing effectiveness of MDPV, methylone, and caffeine, administered alone and as binary mixtures (n=12 per mixture). Each mixture was evaluated at four ratios (10 : 1, 3 : 1, 1 : 1, and 1 : 3) relative to the mean ED₅₀ for each drug alone. Dose-addition analyses were used to determine the predicted, additive effect for each dose pair within each drug mixture. MDPV, methylone, and caffeine maintained responding in a dose-dependent manner, with MDPV being the most potent and effective, and caffeine being the least potent and effective of the three bath salts constituents. High levels of responding were also maintained by each of the bath salts mixtures. Although the nature of the interactions tended toward additivity for most bath salts mixtures, supra-additive (3 : 1 MDPV : caffeine, and 3 : 1 and 1 : 1 methylone : caffeine) and sub-additive (3 : 1, 1 : 1, and 1 : 3 MDPV : methylone) interactions were also observed. Together, these findings demonstrate that the composition of bath salts preparations can have an impact on both their reinforcing potency and effectiveness, and suggest that such interactions among constituent drugs could contribute to the patterns of use and effects reported by human bath salts users.

The Role of Adenosine Receptors in Psychostimulant Addiction

Adenosine receptors (AR) are a family of G-protein coupled receptors, comprised of four members, named A1, A2A, A2B, and A3 receptors, found widely distributed in almost all human body tissues and organs. To date, they are known to participate in a large variety of physiopathological responses, which include vasodilation, pain, and inflammation. In particular, in the central nervous system (CNS), adenosine acts as a neuromodulator, exerting different functions depending on the type of AR and consequent cellular signaling involved. In terms of molecular pathways and second messengers involved, A1 and A3 receptors inhibit adenylyl cyclase (AC), through Gi/o proteins, while A2A and A2B receptors stimulate it through Gs proteins. In the CNS, A1 receptors are widely distributed in the cortex, hippocampus, and cerebellum, A2A receptors are localized mainly in the striatum and olfactory bulb, while A2B and A3 receptors are found at low levels of expression. In addition, AR are able to form heteromers, both among themselves (e.g., A1/A2A), as well as with other subtypes (e.g., A2A/D2), opening a whole range of possibilities in the field of the pharmacology of AR. Nowadays, we know that adenosine, by acting on adenosine A1 and A2A receptors, is known to antagonistically modulate dopaminergic neurotransmission and therefore reward systems, being A1 receptors colocalized in heteromeric complexes with D1 receptors, and A2A receptors with D2 receptors. This review documents the present state of knowledge of the contribution of AR, particularly A1 and A2A, to psychostimulants-mediated effects, including locomotor activity, discrimination, seeking and reward, and discuss their therapeutic relevance to psychostimulant addiction. Studies presented in this review reinforce the potential of A1 agonists as an effective strategy to counteract psychostimulant-induced effects. Furthermore, different experimental data support the hypothesis that A2A/D2 heterodimers are partly responsible for the psychomotor and reinforcing effects of psychostimulant drugs, such as cocaine and amphetamine, and the stimulation of A2A receptor is proposed as a potential therapeutic target for the treatment of drug addiction. The overall analysis of presented data provide evidence that excitatory modulation of A1 and A2A receptors constitute promising tools to counteract psychostimulants addiction.

Discriminative Stimulus Effects of Binary Drug Mixtures: Studies with Cocaine, MDPV, and Caffeine

Illicit drug preparations often include more than one pharmacologically active compound. For example, cocaine and synthetic cathinones [e.g., 3,4-methylenedioxypyrovalerone (MDPV)] are often mixed with caffeine before sale. Caffeine is likely added to these preparations because it is inexpensive and legal; however, caffeine might also mimic or enhance some of the effects of cocaine or MDPV. In these studies, male Sprague-Dawley rats were trained to discriminate 10 mg/kg cocaine from saline, and the discriminative stimulus effects of cocaine, caffeine, and MDPV were evaluated alone and as binary mixtures (cocaine and caffeine, MDPV and caffeine, and cocaine and MDPV) at fixed-dose ratios of 3:1, 1:1, and 1:3 relative to the dose of each drug that produced 50% cocaine-appropriate responding. Dose-addition analyses were used to determine the nature of the drug-drug interactions for each mixture (e.g., additive, supra-additive, or subadditive). Although additive interactions were observed for most mixtures, supra-additive interactions were observed at the 50% effect level for the 1:1 mixture of cocaine and caffeine and at the 80% effect level for all three mixtures of cocaine and caffeine, as well as for the 3:1 and 1:3 mixtures of cocaine and MDPV. These results demonstrate that with respect to cocaine-like discriminative stimulus effects, caffeine can function as a substitute in drug preparations containing either cocaine or MDPV, with enhancements of cocaine-like effects possible under certain conditions. Further research is needed to determine whether similar interactions exist for other abuse-related or toxic effects of drug preparations, including cocaine, synthetic cathinones, and caffeine.

Pre-exposure to ethanol, but not to caffeine and nicotine, induced place preference and self-administration of the NMDA receptor antagonist-benzodiazepine combination, Zoletil®

Zoletil® is an equal amount combination of the NMDA receptor antagonist, tiletamine, and the benzodiazepine, zolazepam, usually used as a veterinary anesthetic. Previous studies have shown that pre-exposure to Zoletil® and other psychoactive drugs (e.g. ketamine, diazepam) plays a significant role in the abuse liability of the compound. However, these studies were only focused on illicit psychoactive drugs and not on the more widely used licit psychoactive substances. Thus, the goal of the present work is to investigate whether pre-exposure to the three most commonly used licit psychoactive substances (caffeine, nicotine, and ethanol) affects the rewarding and reinforcing effects of Zoletil®. Rats were pretreated with caffeine (1.25 or 2.5 mg/kg), nicotine (125 or 250 μg/kg), ethanol (0.5, 2, or 4 g/kg), or saline (1 ml/kg) for 14 days, and evaluated for subsequent Zoletil® place preference (2.5 mg/kg) and self-administration (250 μg/kg). Zoletil® produced neither place preference nor self-administration in saline-pretreated rats. Pre-exposure to caffeine or nicotine does not have significant effects on Zoletil®'s abuse potential. However, pretreatment of ethanol significantly produced Zoletil® place preference and self-administration. These results suggest that individuals who are exposed to ethanol may have a high propensity to use/abuse Zoletil®. More importantly, the present result advocates the careful monitoring on the use and dispensation of Zoletil® or related substances.

Caffeine provokes adverse interactions with 3,4-methylenedioxymethamphetamine (MDMA, 'ecstasy') and related psychostimulants: mechanisms and mediators

Concomitant consumption of caffeine with recreational psychostimulant drugs of abuse can provoke severe acute adverse reactions in addition to longer term consequences. The mechanisms by which caffeine increases the toxicity of psychostimulants include changes in body temperature regulation, cardiotoxicity and lowering of the seizure threshold. Caffeine also influences the stimulatory, discriminative and reinforcing effects of psychostimulant drugs. In this review, we consider our current understanding of such caffeine-related drug interactions, placing a particular emphasis on an adverse interaction between caffeine and the substituted amphetamine, 3,4-methylenedioxymethamphetamine (MDMA, 'ecstasy'), which has been most recently described and characterized. Co-administration of caffeine profoundly enhances the acute toxicity of MDMA in rats, as manifested by high core body temperature, tachycardia and increased mortality. In addition, co-administration of caffeine enhances the long-term serotonergic neurotoxicity induced by MDMA. Observations to date support an interactive model of drug-induced toxicity comprising MDMA-related enhancement of dopamine release coupled to a caffeine-mediated antagonism of adenosine receptors in addition to inhibition of PDE. These experiments are reviewed together with reports of caffeine-related drug interactions with cocaine, d-amphetamine and ephedrine where similar mechanisms are implicated. Understanding the underlying mechanisms will guide appropriate intervention strategies for the management of severe reactions and potential for increased drug-related toxicity, resulting from concomitant caffeine consumption.

Methylxanthines and drug dependence: a focus on interactions with substances of abuse

This chapter examines the psychostimulant actions of methylxanthines, with a focus on the consequences of their excessive use. Consumption of methylxanthines is pervasive and their use is often associated with that of substances known to produce dependence and to have abuse potential. Therefore, the consequences of this combined use are taken into consideration in order to evaluate whether, and to what extent, methylxanthines could influence dependence on or abuse of other centrally active substances, leading to either amplification or attenuation of their effects. Since the methylxanthine that mostly influences mental processes and readily induces psychostimulation is caffeine, this review mainly focuses on caffeine as a prototype of methylxanthine-produced dependence, examining, at the same time, the risks related to caffeine use.

Adenosine A(2A) receptors in psychopharmacology: modulators of behavior, mood and cognition

The adenosine A(2A) receptor (A(2A)R) is in the center of a neuromodulatory network affecting a wide range of neuropsychiatric functions by interacting with and integrating several neurotransmitter systems, especially dopaminergic and glutamatergic neurotransmission. These interactions and integrations occur at multiple levels, including (1) direct receptor- receptor cross-talk at the cell membrane, (2) intracellular second messenger systems, (3) trans-synaptic actions via striatal collaterals or interneurons in the striatum, (4) and interactions at the network level of the basal ganglia. Consequently, A(2A)Rs constitute a novel target to modulate various psychiatric conditions. In the present review we will first summarize the molecular interaction of adenosine receptors with other neurotransmitter systems and then discuss the potential applications of A(2A)R agonists and antagonists in physiological and pathophysiological conditions, such as psychostimulant action, drug addiction, anxiety, depression, schizophrenia and learning and memory.

"Sex, drugs and the brain": the interaction between drugs of abuse and sexual behavior in the female rat

Preclinical and clinical research investigating female sexual motivation has lagged behind research on male sexual function. The present review summarizes recent advances in our understanding of the specific roles of various brain areas, as well as our understanding of the role of dopaminergic neurotransmission in sexual motivation of the female rat. A number of behavioral paradigms that can be used to thoroughly evaluate sexual behavior in the female rat are first discussed. Although traditional assessment of the reflexive, lordosis posture has been useful in understanding the neuroanatomical and neurochemical systems that contribute to copulatory behavior, the additional behavioral paradigms described in this review have helped us expand our understanding of appetitive and consumatory behavioral patterns that better assess sexual motivation - the equivalent of "desire" in humans. A summary of numerous lesion studies indicates that different areas of the brain, including forebrain and midbrain structures, work together to produce the complex repertoire of female sexual behavior. In addition, by investigating the effects of commonly addictive drugs, we are beginning to elucidate the role of dopaminergic neurotransmission in female sexual motivation. Consequently, research in this area may contribute to meaningful advances in the treatment of human female sexual dysfunction.

Aripiprazole blocks reinstatement but not expression of morphine conditioned place preference in rats

Aripiprazole is an atypical antipsychotic drug primarily characterized by partial agonist activity at dopamine(DA) D2 receptors and serotonin-1A (5-hydroxytryptamine, 5-HT1A) receptors and minimal side effects.Based on its pharmacological profile, including stabilization of mesocorticolimbic DA activity (a pathway implicated in drug addiction), we investigated the effects of aripiprazole on relapse to morphine seeking in rats. In experiment 1, rats underwent morphine-induced conditioned place preference (CPP) training with alternate injections of morphine (5 mg/kg, s.c.) and saline (1 ml/kg, s.c.) for 8 consecutive days. To examine the effect of aripiprazole on the expression of morphine-induced CPP, rats received aripiprazole (0, 0.03, 0.1,and 0.3 mg/kg, i.p.) 30 min before testing for the expression of CPP. In experiment 2, rats underwent the same CPP training as in experiment 1 and subsequent extinction training. To examine the effect of aripiprazole on reinstatement of morphine-induced CPP, rats received aripiprazole 30 min before testing for reinstatement of CPP. In experiment 3, to assess the effects of aripiprazole on locomotor activity, aripiprazole was administered 30 min before testing for locomotor activity. Aripiprazole significantly decreased the reinstatement of CPP induced by a priming injection of morphine but had no effect on the expression of morphine-induced CPP or locomotor activity. The D2 and 5-HT1A partial agonist and 5-HT2A antagonist properties of aripiprazole likely account for the blockade of relapse to drug seeking. These findings suggest that aripiprazole may have therapeutic value for reducing craving and preventing relapse to drug seeking.

Adenosine A2A receptors in ventral striatum, hypothalamus and nociceptive circuitry implications for drug addiction, sleep and pain

Adenosine A2A receptors localized in the dorsal striatum are considered as a new target for the development of antiparkinsonian drugs. Co-administration of A2A receptor antagonists has shown a significant improvement of the effects of l-DOPA. The present review emphasizes the possible application of A2A receptor antagonists in pathological conditions other than parkinsonism, including drug addiction, sleep disorders and pain. In addition to the dorsal striatum, the ventral striatum (nucleus accumbens) contains a high density of A2A receptors, which presynaptically and postsynaptically regulate glutamatergic transmission in the cortical glutamatergic projections to the nucleus accumbens. It is currently believed that molecular adaptations of the cortico-accumbens glutamatergic synapses are involved in compulsive drug seeking and relapse. Here we review recent experimental evidence suggesting that A2A antagonists could become new therapeutic agents for drug addiction. Morphological and functional studies have identified lower levels of A2A receptors in brain areas other than the striatum, such as the ventrolateral preoptic area of the hypothalamus, where adenosine plays an important role in sleep regulation. Although initially believed to be mostly dependent on A1 receptors, here we review recent studies that demonstrate that the somnogenic effects of adenosine are largely mediated by hypothalamic A2A receptors. A2A)receptor antagonists could therefore be considered as a possible treatment for narcolepsy and other sleep-related disorders. Finally, nociception is another adenosine-regulated neural function previously thought to mostly involve A1 receptors. Although there is some conflicting literature on the effects of agonists and antagonists, which may partly be due to the lack of selectivity of available drugs, the studies in A2A receptor knockout mice suggest that A2A receptor antagonists might have some therapeutic potential in pain states, in particular where high intensity stimuli are prevalent.

"Coffee, tea and me": moderate doses of caffeine affect sexual behavior in female rats

The present study evaluated the effects of acute caffeine administration on paced mating behavior and partner preference in ovariectomized rats primed with estrogen and progesterone. In Experiment 1, female rats were tested for paced mating behavior following acute administration of caffeine (15 mg/kg). Caffeine shortened the latency to return to a male following an ejaculation. Although this dose of caffeine did not alter the likelihood of leaving a male after receiving sexual stimulation, locomotor activity did increase significantly. Experiment 2 evaluated the dose response characteristics of caffeine (7.5, 15, 30 mg/kg) administration on paced mating behavior. Replicating Experiment 1, caffeine at the lower doses shortened the latency to return to a male following an ejaculation. Finally, to determine whether the effects of caffeine (15 mg/kg) on contact-return latency reflect a change in sexual motivation or merely an inability to inhibit locomotion, rats were tested for partner preference (intact male vs. estrous female) following caffeine administration (Experiment 3). Although caffeine did not disrupt preference for a sexual partner, caffeine selectively increased visits to the male when physical contact was possible. Collectively, these results suggest that the effects of caffeine on female mating behavior may reflect an increase in both sexual motivation and locomotor activity.

The adenosine receptor antagonist CGS15943 reinstates cocaine-seeking behavior and maintains self-administration in baboons

RATIONALE

Caffeine and the adenosine A(1) and A(2A) receptor antagonist CGS15943 produce many behavioral effects that are similar to those produced by classic stimulant drugs (e.g. cocaine and amphetamines).

OBJECTIVES

The current study evaluated whether CGS15943 would maintain self-administration and reinstate extinguished lever responding previously maintained by cocaine (i.e. cocaine-seeking) or by food (i.e. food-seeking). Reinstatement with CGS15943 was compared to cocaine, caffeine, and alprazolam.

METHODS

Up to 30 injections of 0.032 mg/kg cocaine or 30 deliveries of 1-g food pellets were available under a fixed ratio (FR10) schedule of reinforcement during daily 2-h sessions. For reinstatement tests, lever responses were extinguished by substituting saline for cocaine or by removing pellets from the mechanical feeder. After extinction of lever responding, acute "priming" doses (mg/kg, IV) of cocaine (0.1-3.2), the adenosine receptor antagonists caffeine (0.1-1.8) and CGS15943 (0.032-0.32) or the benzodiazepine receptor agonist alprazolam (0.1-1.8 mg/kg) were administered. The intravenous reinforcing effects of CGS15943 were also evaluated; each dose of CGS15943 (0.001-0.032 mg/kg) was substituted for cocaine for at least 10 days and until self-injection was relatively stable.

RESULTS

Cocaine, caffeine and CGS15943, dose-dependently increased cocaine-seeking, where as alprazolam did not. Cocaine, caffeine and CGS15943 did not increase food-seeking. CGS15943 reinstated cocaine-seeking at rates that were comparable to those produced by cocaine. Pretreatment with the adenosine A(2) agonist CGS21680 decreased CGS15943-induced reinstatement of cocaine-seeking. In self-injection testing, CGS15943 was self-administered at levels greater than vehicle. An inverted U-shaped dose-effect function was obtained with peak mean rates maintained by 0.01 mg/kg CGS15943.

CONCLUSIONS

The adenosine antagonist CGS15943 reinstated cocaine-seeking and functioned as an intravenous reinforcer. The finding that CGS21680 produced a rightward shift in the CGS15943 reinstatement dose-effect curve supports a role of adenosine mechanisms in the reinstatement of cocaine-seeking behavior.

Differences in the effect of chronic and acute caffeine on self-administration of cocaine in mice

We have compared the ability of an acute injection of caffeine (3 mg/kg, i.p.) and long-term peroral caffeine consumption for 10 days ( approximately 150 mg/kg/day in tap water) to affect cocaine self-administration in mice. The peak plasma and brain levels of caffeine and its metabolites were similar in the two experimental set-ups. Moreover, the levels reached are close to those obtained in humans upon coffee ingestion. Neither type of caffeine administration affected the reinforcing effect of cocaine, defined as a selective increase in nose-poke responses in mice self-administering cocaine compared to their yoked controls. However, caffeine injection increased the amount of cocaine self-administered whereas caffeine drinking reduced it. A low dose of cocaine, by itself essentially ineffective, produced an increase in c-fos and NGFI-A mRNA in the cerebral cortex in mice that had been drinking caffeine. An acute caffeine injection also enhanced the immediate early gene response to cocaine, but to a lesser degree. Cocaine and caffeine also synergistically increased NGFI-A expression in caudate-putamen. Thus, regular caffeine drinking decreased the cocaine intake without significantly affecting its reinforcing properties, perhaps because it enhanced the activation of the predominantly inhibitory frontal cortical areas produced by low doses of cocaine.

Caffeine promotes ethanol drinking in rats. Examination using a limited-access free choice paradigm

There is growing evidence that caffeine may alter the pattern of intake of a variety of drugs. The present study was designed to assess the effect of caffeine pretreatment on voluntary ethanol consumption. The first experiment examined the effect of caffeine on the acquisition of ethanol intake in a limited-access-choice procedure in which water and ethanol were presented concurrently. Male Wistar rats, exposed to food and water ad lib, were presented with a daily 1-h choice session between water and progressively increasing concentrations of ethanol (2-10%). Each ethanol concentration was made available for 4-6 days for a total of 20 days of access to ethanol. Intraperitoneal injections of caffeine (5 or 10 mg/kg) or saline were administered to the rats 30 min prior to each choice session. Caffeine produced a dose-related facilitation in ethanol drinking whereby the lower caffeine dose produced enhancement in ethanol drinking. The second experiment examined the effect of caffeine on the maintenance of established ethanol consumption. Male Wistar rats, initially acclimatized to increasing concentrations of ethanol (2%-10), were presented with an additional 18 ethanol (10%) presentations, comprised of a 6-day baseline period followed by 6 days of treatment where animals were given one of three doses of caffeine (2.5, 5 or 10 mg/kg) or saline prior to ethanol presentation. A final 6-day post-treatment period followed treatment. These results revealed an inverted-U effect of caffeine dose on ethanol ingestion where the low and high caffeine doses produced no effect but the moderate dose of 5 mg/kg enhanced ethanol drinking that persisted throughout the post-treatment period. A third experiment revealed that caffeine did not alter levels of blood ethanol within the time period used for the ethanol drinking session.

Alteration of the behavioral effects of nicotine by chronic caffeine exposure

The prevalence of tobacco smoking and coffee drinking place nicotine and caffeine among the most used licit drugs in many societies and their consumption is often characterised by concurrent use. The pharmacological basis for any putative interaction between these drugs remains unclear. Some epidemiological reports support anecdotal evidence, which suggests that smokers consume caffeine to enhance the effects of nicotine. This paper reviews various aspects of the pharmacology of caffeine and nicotine, in humans and experimental animals, important for the understanding of the interactions between these drugs. In particular, recent experiments are reviewed in which chronic exposure to caffeine in the drinking water of rats facilitated acquisition of self-adminstration behavior, enhanced nicotine-induced increases in dopamine levels in the shell of the nucleus accumbens and altered the dopaminergic component of a nicotine discrimination. These studies provide evidence that the rewarding and subjective properties of nicotine can be changed by chronic caffeine exposure and indicate that caffeine exposure may be an important environmental factor in shaping and maintaining tobacco smoking.

Caffeine withdrawal in normal school-age children

OBJECTIVE

Caffeine is widely consumed by children around the world. The purpose of this study was to determine whether children manifest withdrawal effects after cessation of caffeine intake.

METHOD

Thirty normal children completed the single-blind, within-subjects, repeated-measures study with weekly sessions. Subjects were tested four times: (1) baseline (on regular caffeine diet); (2) on caffeine (approximately 120 to 145 mg/day); (3) during withdrawal (24 hours after discontinuation of caffeine taken for 13 consecutive days); and (4) at return to baseline. Subjects were evaluated with self-report measures of symptoms and objective measures of attention, motor performance, processing speed, and memory.

RESULTS

During caffeine withdrawal, there was a significant deterioration on response time of a visual continuous performance test of attention. This finding is consistent with caffeine withdrawal. The deterioration in response time appeared to persist for 1 week.

CONCLUSIONS

Twenty-four hours after children discontinued caffeine, there was a decrease in performance on reaction time of a task requiring sustained attention. Further work is indicated to determine whether children manifest caffeine withdrawal effects after cessation of caffeine intake.

Subjective effects of oral caffeine in formerly cocaine-dependent humans

Eleven formerly cocaine-dependent (FCD) adults (mean 4 years in recovery) and 11 with no substance dependence history (ND) drank one cup of coffee (caffeine content 0, 50, or 100 mg) per hour for 5 h (for a total of 0, 250, or 500 mg caffeine) in a double-blind, randomized crossover procedure. Participants completed self-report scales before the first cup and 50 min after each cup. Caffeine did not increase cocaine-like effect or desire-for-cocaine ratings among the FCD subjects. Ratings of 'jittery' (P < 0.05) and 'anxious/tense/nervous' (P < 0.10) increased more with caffeine in the FCD group than among ND subjects. Self-report measures of caffeine reinforcement did not differ between FCD and ND groups. These results suggest that, among FCD adults, (a) caffeine does not produce cocaine-like effects, (b) caffeine reinforcement is neither greater nor lesser than that among ND adults, and (c) chronic cocaine use may induce sensitization to some effects of stimulants.