The Impact of Caffeine on the Behavioral Effects of Ethanol Related to Abuse and Addiction: A Review of Animal Studies

Receptor and metabolic insights on the ability of caffeine to prevent alcohol-induced stimulation of mesolimbic dopamine transmission

The consumption of alcohol and caffeine affects the lives of billions of individuals worldwide. Although recent evidence indicates that caffeine impairs the reinforcing properties of alcohol, a characterization of its effects on alcohol-stimulated mesolimbic dopamine (DA) function was lacking. Acting as the pro-drug of salsolinol, alcohol excites DA neurons in the posterior ventral tegmental area (pVTA) and increases DA release in the nucleus accumbens shell (AcbSh). Here we show that caffeine, via antagonistic activity on A2A adenosine receptors (A2AR), prevents alcohol-dependent activation of mesolimbic DA function as assessed, in-vivo, by brain microdialysis of AcbSh DA and, in-vitro, by electrophysiological recordings of pVTA DA neuronal firing. Accordingly, while the A1R antagonist DPCPX fails to prevent the effects of alcohol on DA function, both caffeine and the A2AR antagonist SCH 58261 prevent alcohol-dependent pVTA generation of salsolinol and increase in AcbSh DA in-vivo, as well as alcohol-dependent excitation of pVTA DA neurons in-vitro. However, caffeine also prevents direct salsolinol- and morphine-stimulated DA function, suggesting that it can exert these inhibitory effects also independently from affecting alcohol-induced salsolinol formation or bioavailability. Finally, untargeted metabolomics of the pVTA showcases that caffeine antagonizes alcohol-mediated effects on molecules (e.g. phosphatidylcholines, fatty amides, carnitines) involved in lipid signaling and energy metabolism, which could represent an additional salsolinol-independent mechanism of caffeine in impairing alcohol-mediated stimulation of mesolimbic DA transmission. In conclusion, the outcomes of this study strengthen the potential of caffeine, as well as of A2AR antagonists, for future development of preventive/therapeutic strategies for alcohol use disorder.

The effects of alcohol on anxiety-like, depression-like, and social behavior immediately and a day after binge drinking

The aim of the present study was to determine the effects of binge drinking on anxiety-like, depression-like, and social behavior. The participation of the corticotropin-releasing factor (CRF) receptors (CRF1 and CRF2) in these effects was also investigated. Therefore, male C57BL/6 mice were exposed to drinking in the dark, a classical animal model for binge drinking, and treated intracerebroventricularly (icv) with selective CRF1 antagonist antalarmin or selective CRF2 antagonist astressin2B, immediately or 24 h after binge drinking. After 30 min, the animals were investigated in an elevated plus-maze test and a forced swim test for anxiety-like and depression-like signs, respectively. In addition, mice were tested in a three-chamber social interaction arena for sociability and preference for social novelty. Immediately after binge drinking, mice exposed to alcohol expressed anxiolytic and antidepressant effects, which were reduced by astressin2B, but not antalarmin. Moreover, mice exposed to alcohol showed increased sociability and preference for social novelty immediately after binge drinking. In contrast, 24 h after binge drinking mice exposed to alcohol presented anxiety-like and depression-like signs, which were reversed by antalarmin, but not astressin2B. However, mice exposed to alcohol did not show any significant change in social interaction after 24 h. The present study demonstrates that alcohol exerts different effects on anxiety-like, depression-like, and social behavior immediately and a day after binge drinking, and that the anxiolytic and antidepressant effects produced by binge drinking are mediated by CRF2, whereas the anxiety-like and depression-like signs observed the next day are promoted by CRF1.

Ethanol and caffeine age-dependently alter brain and retinal neurochemical levels without affecting morphology of juvenile and adult zebrafish (Danio rerio)

Adolescent alcohol exposure in humans is predictive of adult development of alcoholism. In rodents, caffeine pre-exposure enhances adult responsiveness to ethanol via a pathway targeted by both compounds. Embryonic exposure to either compound adversely affects development, and both compounds can alter zebrafish behaviors. Here, we evaluate whether co-exposure to caffeine and/or alcohol in adolescence exerts neurochemical changes in retina and brain. Zebrafish (Danio rerio) were given daily 20 min treatments to ethanol (1.5% v/v), caffeine (25-100 mg/L), or caffeine + ethanol for 1 week during mid-late adolescence (53-92 days post fertilization (dpf)) or early adulthood (93-142 dpf). Immediately after exposure, anatomical measurements were taken, including weight, heart rate, pigment density, length, girth, gill width, inner and outer eye distance. Brain and retinal tissue were subsequently collected either (1) immediately, (2) after a short interval (2-4d) following exposure, or (3) after a longer interval that included an acute 1.5% ethanol challenge. Chronic ethanol and/or caffeine exposure did not alter anatomical parameters. However, retinal and brain levels of tyrosine hydroxylase were elevated in fish sacrificed after the long interval following exposure. Protein levels of glutamic acid decarboxylase were also increased, with the highest levels observed in 70-79 dpf fish exposed to caffeine. The influence of ethanol and caffeine exposure on neurochemistry demonstrates specificity of their effects during postembryonic development. Using the zebrafish model to assess neurochemistry relevant to reward and anxiety may inform understanding of the mechanisms that reinforce co-addiction to alcohol and stimulants.

Alcohol and energy drinks: individual contribution of common ingredients on ethanol-induced behaviour

Since energy drinks (EDs) were sold to the general public as soft drinks and recreational beverages, mixing EDs with ethanol has grown in popularity, particularly among younger people. Given the research that links these drinks with higher risk behaviors and increased ethanol intake, ethanol combined with EDs (AmEDs) is a particularly worrying combination. EDs generally commonly include a variety of ingredients. Sugar, caffeine, taurine, and B-group vitamins are almost always present. Studies on the combined effect of ethanol and sugar and caffeine on ethanol-induced behaviors are extensive. Not so much in regards to taurine and vitamins. This review briefly summarises available information from research on the isolated compounds on EtOH-induced behaviors first, and secondly, the combination of AmEDs on EtOH effects. The conclusion is that additional research is needed to fully comprehend the characteristics and consequences of AmEDs on EtOH-induced behaviors.

Combined Exposure to Chronic Sleep Deprivation and Caffeine Potentiates Behavioural Deficits by Altering Neurochemical Profile and Synaptophysin Expression in Long-Evans Rats

Using the Unpredictable Chronic Sleep Deprivation (UCSD) paradigm we developed, the combined effects of chronic sleep deprivation and high caffeine intake on prefrontal cortical synaptophysin expression, neurochemical profiles, and behavioural outcomes in Long-Evans rats were evaluated. The combination of chronic sleep deprivation and high-dose caffeine treatment produced varying degrees of behavioural impairments, depletion of antioxidants, serotonin, and an upregulation of acetylcholinesterase (AChE) activity in the prefrontal cortex. An immunohistochemical assessment revealed a reduction in synaptophysin protein expression in the prefrontal cortex following exposure to high-dose caffeine and chronic sleep deprivation. Overall, our findings support the advocacy for adequate sleep for optimal mental performance as a high intake of caffeine to attenuate the effects of sleep deprivation that may alter the neurochemical profile and synaptic plasticity in the prefrontal cortex, significantly increasing the risk of neuropsychiatric/degenerative disorders.

Neurochemical and Behavioral Consequences of Ethanol and/or Caffeine Exposure: Effects in Zebrafish and Rodents

Zebrafish are increasingly being utilized to model the behavioral and neurochemical effects of pharmaceuticals and, more recently, pharmaceutical interactions. Zebrafish models of stress establish that both caffeine and ethanol influence anxiety, though few studies have implemented co-administration to assess the interaction of anxiety and reward-seeking. Caffeine exposure in zebrafish is teratogenic, causing developmental abnormalities in the cardiovascular, neuromuscular, and nervous systems of embryos and larvae. Ethanol is also a teratogen and, as an anxiolytic substance, may be able to offset the anxiogenic effects of caffeine. Co-exposure to caffeine and alcohol impacts neuroanatomy and behavior in adolescent animal models, suggesting stimulant substances may moderate the impact of alcohol on neural circuit development. Here, we review the literature describing neuropharmacological and behavioral consequences of caffeine and/or alcohol exposure in the zebrafish model, focusing on neurochemistry, locomotor effects, and behavioral assessments of stress/anxiety as reported in adolescent/juvenile and adult animals. The purpose of this review is twofold: (1) describe the work in zebrafish documenting the effects of ethanol and/or caffeine exposure and (2) compare these zebrafish studies with comparable experiments in rodents. We focus on specific neurochemical pathways (dopamine, serotonin, adenosine, GABA, adenosine), anxiety-type behaviors (assessed with novel tank, thigmotaxis, shoaling), and locomotor changes resulting from both individual and co-exposure. We compare findings in zebrafish with those in rodent models, revealing similarities across species and identifying conservation of mechanisms that potentially reinforce co-addiction.

Experimental study of pre- and postnatal caffeine exposure and its observable effects on selected neurotransmitters and behavioural attributes at puberty : Caffeine exposure and its observable effects on selected neurotranmitters and behaviour

Caffeine is globally consumed as a stimulant in beverages. It is also ingested in purified forms as power and tablets. Concerns have been raised about the potential consequences of intrauterine and early life caffeine exposure on brain health. This study modeled caffeine exposure during pregnancy and early postanal life until puberty, and the potential consequences. Caffeine powder was dissolved in distilled water. Thirty-two (n = 32) pregnant mice (Mus musculus) (dams) were divided into four groups- A, B, C and D. Group A animals served as a control, receiving placebo. Caffeine doses in mg/kg body weight were administered as follows: Group B, 10 mg/kg; Group C, 50 mg/kg; Group D, 120 mg/kg. Prenatal caffeine exposure [phase I] lasted throughout pregnancy. Half the number of offspring (pups) were sacrificed at birth; the rest were recruited into phase II and the experiment continued till day 35, marking puberty. Brain samples were processed following sacrifice. γ-aminobutyric acid (GABA), acetylcholine (ACh), and serotonin (5Ht) neurotransmitters were assayed in homogenates to evaluate functional neurochemistry. Anxiety and memory as neurobehavioural attributes were observed using the elevated plus and Barnes' mazes respectively. Continuous caffeine exposure produced positive effects on short and long-term memory parameters; the pattern interestingly was irregular and appeared more effective with the lowest experimental dose. Anxiety test results showed no attributable significant aberrations. Caffeine exposure persistently altered the neurochemistry of selected neurotransmitters including ACh and 5Ht, including when exposure lasted only during pregnancy. ACh significantly increased in group B^C+ to 0.3475μgg-¹ relative to control's 0.2508μgg-¹; pre-and continuous postnatal exposure in Group B increased 5Ht to 0.2203 μgg-¹ and 0.2213 μgg-¹ respectively relative to control's 0.1863 μgg-¹. From the current investigation, caffeine exposure in pregnancy had persistent effects on brain functional attributes including neurotransmitters activities, memory and anxiety. Caffeine in moderate doses affected memory positively but produced negative effects at the higher dosage including increased anxiety tendencies.

Effect of caffeine on alcohol drinking in mice

Mixing alcohol (ethanol) with caffeinated beverages continues to be a common and risky practice. Energy drinks are one type of caffeinated beverage that may be especially problematic when used as mixers, due to their relatively high caffeine content in combination with their highly sweetened flavor profile. The present study used a mouse model of limited-access drinking and lickometer circuitry to examine the effects of an energy drink anid its caffeine content on ethanol consumption. Predictably, the highly sweetened energy drink significantly increased ethanol intake compared to a plain ethanol solution (6.34 ± 0.2 vs. 5.01 ± 0.3 g/kg; Cohen's d = 1.79). Interestingly, adulterating a plain ethanol solution with the same concentration of caffeine (without sweetener) found in the energy drink also increased ethanol intake (5.47 ± 0.3 vs. 4.11 ± 0.3 g/kg; Cohen's d = 1.4). A lower concentration of caffeine was without effect on ethanol drinking. Interestingly, plain caffeine solutions at both tested concentrations provoked high numbers of bottle contacts, indicating that the mice found the solution palatable. These findings suggest that altering the bitterness profile of an ethanol solution with the addition of caffeine can increase intake in a similar manner as sweetening the solution. Further, the findings underscore the importance of taste in motivating ethanol consumption and the potential role that caffeine can have in this process.

Altered Heterosynaptic Plasticity Impairs Visual Discrimination Learning in Adenosine A1 Receptor Knock-Out Mice

Theoretical and modeling studies demonstrate that heterosynaptic plasticity-changes at synapses inactive during induction-facilitates fine-grained discriminative learning in Hebbian-type systems, and helps to achieve a robust ability for repetitive learning. A dearth of tools for selective manipulation has hindered experimental analysis of the proposed role of heterosynaptic plasticity in behavior. Here we circumvent this obstacle by testing specific predictions about the behavioral consequences of the impairment of heterosynaptic plasticity by experimental manipulations to adenosine A1 receptors (A1Rs). Our prior work demonstrated that the blockade of adenosine A1 receptors impairs heterosynaptic plasticity in brain slices and, when implemented in computer models, selectively impairs repetitive learning on sequential tasks. Based on this work, we predict that A1R knock-out (KO) mice will express (1) impairment of heterosynaptic plasticity and (2) behavioral deficits in learning on sequential tasks. Using electrophysiological experiments in slices and behavioral testing of animals of both sexes, we show that, compared with wild-type controls, A1R KO mice have impaired synaptic plasticity in visual cortex neurons, coupled with significant deficits in visual discrimination learning. Deficits in A1R knockouts were seen specifically during relearning, becoming progressively more apparent with learning on sequential visual discrimination tasks of increasing complexity. These behavioral results confirm our model predictions and provide the first experimental evidence for a proposed role of heterosynaptic plasticity in organism-level learning. Moreover, these results identify heterosynaptic plasticity as a new potential target for interventions that may help to enhance new learning on a background of existing memories.SIGNIFICANCE STATEMENT Understanding how interacting forms of synaptic plasticity mediate learning is fundamental for neuroscience. Theory and modeling revealed that, in addition to Hebbian-type associative plasticity, heterosynaptic changes at synapses that were not active during induction are necessary for stable system operation and fine-grained discrimination learning. However, lacking tools for selective manipulation prevented behavioral analysis of heterosynaptic plasticity. Here we circumvent this barrier: from our prior experimental and computational work we predict differential behavioral consequences of the impairment of Hebbian-type versus heterosynaptic plasticity. We show that, in adenosine A1 receptor knock-out mice, impaired synaptic plasticity in visual cortex neurons is coupled with specific deficits in learning sequential, increasingly complex visual discrimination tasks. This provides the first evidence linking heterosynaptic plasticity to organism-level learning.

Impact of Caffeine on Ethanol-Induced Stimulation and Sensitization: Changes in ERK and DARPP-32 Phosphorylation in Nucleus Accumbens

BACKGROUND

Caffeine is frequently consumed with ethanol to reduce the impairing effects induced by ethanol, including psychomotor slowing or incoordination. Both drugs modulate dopamine (DA)-related markers in accumbens (Acb), and Acb DA is involved in voluntary locomotion and locomotor sensitization. The present study determined whether caffeine can affect locomotion induced by acute and repeated ethanol administration in adult male CD-1 mice.

METHODS

Acute administration of caffeine (7.5 to 30.0 mg/kg) was evaluated for its effects on acute ethanol-induced (1.5 to 3.5 g/kg) changes in open-field horizontal locomotion, supported rearing, and rearing not supported by the wall. DA receptor-dependent phosphorylation markers were assessed: extracellular signal-regulated kinase (pERK), and dopamine-and cAMP-regulated phosphoprotein Mr32kDa phosphorylated at threonine 75 site (pDARPP-32-Thr75) in Acb core and shell. Acutely administered caffeine was also evaluated in ethanol-sensitized (1.5 g/kg) mice.

RESULTS

Acute ethanol decreased both types of rearing. Caffeine increased supported rearing but did not block ethanol -induced decreases in rearing. Both substances increased horizontal locomotion in a biphasic manner, and caffeine potentiated ethanol-induced locomotion. Although ethanol administered repeatedly induced sensitization of locomotion and unsupported rearing, acute administration of caffeine to ethanol-sensitized mice in an ethanol-free state resulted in blunted stimulant effects compared with those seen in ethanol-naïve mice. Ethanol increased pERK immunoreactivity in both subregions of the Acb, but coadministration with caffeine blunted this increase. There were no effects on pDARPP-32(Thr75) immunoreactivity.

CONCLUSIONS

The present results demonstrated that, after the first administration, caffeine potentiated the stimulating actions of ethanol, but did not counteract its suppressant or ataxic effects. Moreover, our results show that caffeine has less activating effects in ethanol-sensitized animals.

Alcohol mixed with energy drinks: what about taurine?

RATIONALE

Since energy drinks (EDs) were marketed to the general public as recreational and soft drinks, mixing these with alcohol has become a popular practice, especially in the younger population. Alcohol mixed with EDs (AmEDs) is a particularly alarming combination, given the evidence that consistently associate these drinks with increased risk behaviours and greater alcohol consumption. Caffeine and taurine are commonly found in EDs. In contrast to caffeine, the studies on taurine psychoactive properties and how this amino acid influences ethanol intake alone or in combination with caffeine are not so numerous.

OBJECTIVES

We summarised relevant and available data on the studies focusing on taurine as a psychoactive agent and its influence on ethanol (EtOH)-induced behaviours. Given the increased risk that represents mixing alcohol with energy drinks, we put emphasis on the research exploring the impact of these combinations on motivated behaviour towards EtOH consumption.

RESULTS

The research on taurine properties on motivated behaviour towards EtOH consumption is limited, and mostly all done in combination with caffeine or other molecules. This makes it difficult to elucidate the effect of this amino acid when combined with alcohol.

CONCLUSIONS

Incomplete understanding of the properties and effects of AmEDs is unavoidable until more studies are performed on the influence of taurine on motivation to consume alcohol. Taurine should be further explored, particularly in regard to its potential beneficial applications, motivational properties and synergies with other psychoactive ingredients (i.e. caffeine).

Effects of caffeine on ethanol-elicited place preference, place aversion and ERK phosphorylation in CD-1 mice

BACKGROUND

Epidemiological studies indicate a rise in the combined consumption of caffeinated and alcoholic beverages, which can lead to increased risk of alcoholic-beverage overconsumption. However, the effects of the combination of caffeine and ethanol in animal models related to aspects of drug addiction are still underexplored.

AIMS

To characterize the pharmacological interaction between caffeine and ethanol and establish if caffeine can affect the ability of ethanol (2 g/kg) to elicit conditioned place preference and conditioned place aversion, we administered caffeine (3 or 15 mg/kg) to male CD-1 mice before saline or ethanol. Moreover, we determined if these doses of caffeine could affect ethanol (2 g/kg) elicited extracellular signal-regulated kinase phosphorylation in brain areas, nucleus accumbens, bed nucleus of stria terminalis, central nucleus of the amygdala, and basolateral amygdala, previously associated with this type of associative learning.

RESULTS

In the place-conditioning paradigm, caffeine did not have an effect on its own, whereas ethanol elicited significant conditioned-place preference and conditioned-place aversion. Caffeine (15 mg/kg) significantly prevented the acquisition of ethanol-elicited conditioned-place preference and, at both doses, also prevented the acquisition of ethanol-elicited conditioned-place aversion. Moreover, both doses of caffeine also prevented ethanol-elicited extracellular signal-regulated kinase phosphorylation expression in all brain areas examined.

CONCLUSIONS

The present data indicate a functional antagonistic action of caffeine and ethanol on associative learning and extracellular signal-regulated kinase phosphorylation after an acute interaction. These results could provide exciting grounds for further studies, also in a translational perspective, of their pharmacological interaction modulating other processes involved in drug consumption and addiction.

Ethnopharmacological Applications Targeting Alcohol Abuse: Overview and Outlook

Excessive alcohol consumption is the cause of several diseases and thus is of a major concern for society. Worldwide alcohol consumption has increased by many folds over the past decades. This urgently calls for intervention and relapse counteract measures. Modern pharmacological solutions induce complete alcohol self-restraint and prevent relapse, but they have many side effects. Natural products are most promising as they cause fewer adverse effects. Here we discuss in detail the medicinal plants used in various traditional/folklore medicine systems for targeting alcohol abuse. We also comprehensively describe preclinical and clinical studies done on some of these plants along with the possible mechanisms of action.

Energy drink constituents (caffeine and taurine) selectively potentiate ethanol-induced locomotion in mice

Mixing alcohol with energy drinks has emerged as a popular trend over the last decade. However, epidemiological studies have found this consumption to be associated with increased hazards, such as binge drinking, increased alcohol-related harm and risk of developing alcohol use disorder. The mechanisms underlying these effects are not clear, but much attention has been attributed to caffeine. However, taurine, another common ingredient in energy drinks, has also been associated with the dopamine elevating properties of ethanol, and may in this respect contribute to the increased liability associated with the mixture of alcohol and energy drinks. In the present study we measured locomotor activity, a phenomenon previously linked to the dopamine activating and reinforcing properties of the drug, following acute systemic administration with caffeine (1, 5, 15, 30 mg/kg), taurine (30, 60, 300, 600 mg/kg) and ethanol (1.75, 2.5, 3.25 g/kg), alone or in combination. We found that ethanol and caffeine, but not taurine, increased locomotion compared to vehicle. In addition, when combined with ethanol, caffeine, but not taurine, increased the locomotor stimulatory effect of ethanol. Furthermore, the combination of caffeine and taurine were able to further enhance the ethanol-induced locomotor response. Eleven days of intermittent caffeine exposure produced a sensitized response to the caffeine-induced locomotion, but did not alter the additive effect produced by the combination of caffeine and taurine on ethanol-induced locomotion. Based on the present study we suggest that the combination of caffeine and taurine, at a specific dose range, enhances the locomotor stimulatory properties of ethanol, a phenomenon previously linked to the reinforcing properties of the drug.

Early exposure to ethanol is able to affect the memory of adult zebrafish: Possible role of adenosine

Ethanol is one of the most widely consumed drugs in the world, and the effects of ethanol during early development include morphological and cognitive problems. The regulation of adenosine levels is essential for the proper function of major neurotransmitter systems in the brain, particularly glutamate and dopamine; thus, the investigation of the relation of adenosine and memory after early ethanol exposure becomes relevant. Embryos of zebrafish were exposed to 1% ethanol during two distinct developmental stages: gastrula/segmentation or pharyngula. The evaluation of memory, morphology, and locomotor parameters was performed when fish were 3 months old. The effect of ecto-5'-nucleotidase and adenosine deaminase inhibition on the consequences of ethanol exposure with regard to memory formation was observed. Morphological evaluation showed decreases in body length and the relative telencephalic and cerebellar areas in ethanol exposed animals. The locomotor parameters evaluated were not affected by ethanol. In the inhibitory avoidance paradigm, ethanol exposure during the gastrula/segmentation and pharyngula stages decreased zebrafish memory retention. When ethanol was given in the pharyngula stage, the inhibition of ecto-5'-nucleotidase in the acquisition phase of memory tests was able to revert the effects of ethanol on the memory of adults. These findings suggest that the increased adenosine levels caused by ethanol could alter the neuromodulation of important components of memory formation, such as neurotransmitters. The adjustment of adenosine levels through ecto-5'-nucleotidase inhibition appears to be effective at restoring normal adenosine levels and the acquisition of memory in animals exposed to ethanol during the pharyngula stage.

Seizure vulnerability and anxiety responses following chronic co-administration and acute withdrawal of caffeine and ethanol in a rat model

BACKGROUND

Caffeine antagonizes the intoxicating effects of alcohol. Consequently, there has been a dramatic global increase in the consumption of caffeinated drinks together with alcohol, especially among young adults. We assessed the seizure vulnerability and anxiety responses following the chronic co-administration of, and withdrawal from, caffeine and ethanol in male rats.

METHODS

The rats were randomly assigned to six groups consisting of 10 animals each: 10 mg/kg of caffeine, 20 mg/kg of caffeine, 4 g/kg of 20% ethanol, combined caffeine (20 mg/kg) and ethanol (4 g/kg of 20%), 4 mL/kg distilled water, and an untreated control group. The test substances were administered intragastrically twice daily for 29 days. On day 29, the rats were tested on the elevated plus maze to assess anxiety-related responses. On day 30, pentylenetetrazol (PTZ), a chemoconvulsant, was administered intraperitoneally at a dose of 40 mg/kg to the animals. Seizure responses and mortality up to 72 h were recorded.

RESULTS

Compared with the control group, the rats that received chronic treatment with low-dose caffeine, ethanol alone, and combined caffeine and ethanol exhibited significant anxiogenic-like effects, unlike with high-dose caffeine. Both low- and high-dose caffeine significantly increased PTZ seizure latency. Ethanol alone and combined caffeine and ethanol both lowered PTZ seizure latency. No significant difference occurred between the controls and the untreated group for either anxiety or seizure expression. Combined caffeine and ethanol increased the seizure-induced mortality from withdrawal effects at 72 h.

CONCLUSIONS

These findings suggest that the chronic co-administration of caffeine and ethanol and the acute withdrawal from these drugs lead to anxiogenic effects and increased seizure vulnerability.

Adenosine A2A receptor mediates hypnotic effects of ethanol in mice

Ethanol has extensive effects on sleep and daytime alertness, causing premature disability and death. Adenosine, as a potent sleep-promoting substance, is involved in many cellular and behavioral responses to ethanol. However, the mechanisms of hypnotic effects of ethanol remain unclear. In this study, we investigated the role of adenosine in ethanol-induced sleep using C57BL/6Slac mice, adenosine A_2A receptor (A_2AR) knockout mice, and their wild-type littermates. The results showed that intraperitoneal injection of ethanol (3.0 g/kg) at 21:00 decreased the latency to non-rapid eye movement (NREM) sleep and increased the duration of NREM sleep for 5 h. Ethanol dose-dependently increased NREM sleep, which was consistent with decreases in wakefulness in C57BL/6Slac mice compared with their own control. Caffeine (5, 10, or 15 mg/kg), a nonspecific adenosine receptor antagonist, dose-dependently and at high doses completely blocked ethanol-induced NREM sleep when administered 30 min prior to (but not after) ethanol injection. Moreover, ethanol-induced NREM sleep was completely abolished in A_2AR knockout mice compared with wild-type mice. These findings strongly indicate that A_2AR is a key receptor for the hypnotic effects of ethanol, and pretreatment of caffeine might be a strategy to counter the hypnotic effects of ethanol.

Time-dependent interacting effects of caffeine, diazepam, and ethanol on zebrafish behaviour

Zebrafish have become a popular animal model for behavioural pharmacology due to their small size, rapid development, and amenability to high throughput behavioural drug screens. Furthermore, water-soluble compounds can be administered via immersion of the fish in the drug solution, which provides a non-invasive drug delivery method. Numerous studies have demonstrated stimulant effects of alcohol. Diazepam and caffeine, on the other hand have been found to have inhibitory effect on locomotor activity in zebrafish. However, the time-dependent changes induced by these psychoactive drugs are rarely reported, and potential drug interactions have not been examined in zebrafish, despite the translational relevance of this question. In the current study, we examine time- and dose-dependent changes in zebrafish following exposure to caffeine, diazepam, and ethanol quantifying four different behavioural parameters over a 30min recording session. We subsequently analyze potential drug-drug interactions by co-administering the three drugs in different combinations. Our time-course and dose-response analyses for each of the three drugs represent so far the most detailed studies available serving as a foundation for future psychopharmacology experiments with zebrafish. Furthermore, we report significant interactions between the three drugs corroborating findings obtained with rodent models as well as in humans, providing translational relevance for the zebrafish model.

Ethanol and Caffeine Effects on Social Interaction and Recognition in Mice: Involvement of Adenosine A2A and A1 Receptors

Ethanol and caffeine are frequently consumed in combination and have opposite effects on the adenosine system: ethanol metabolism leads to an increase in adenosine levels, while caffeine is a non-selective adenosine A1/A2A receptor antagonist. These receptors are highly expressed in striatum and olfactory tubercle, brain areas involved in exploration and social interaction in rodents. Ethanol modulates social interaction processes, but the role of adenosine in social behavior is still poorly understood. The present work was undertaken to study the impact of ethanol, caffeine and their combination on social behavior, and to explore the involvement of A1 and A2A receptors on those actions. Male CD1 mice were evaluated in a social interaction three-chamber paradigm, for preference of conspecific vs. object, and also for long-term recognition memory of familiar vs. novel conspecific. Ethanol showed a biphasic effect, with low doses (0.25 g/kg) increasing social contact and higher doses (1.0-1.5 g/kg) reducing social interaction. However, no dose changed social preference; mice always spent more time sniffing the conspecific than the object, independently of the ethanol dose. Ethanol, even at doses that did not change social exploration, produced amnestic effects on social recognition the following day. Caffeine reduced social contact (15.0-60.0 mg/kg), and even blocked social preference at higher doses (30.0-60.0 mg/kg). The A1 antagonist Cyclopentyltheophylline (CPT; 3-9 mg/kg) did not modify social contact or preference on its own, and the A2A antagonist MSX-3 (1.5-6 mg/kg) increased social interaction at all doses. Ethanol at intermediate doses (0.5-1.0 g/kg) was able to reverse the reduction in social exploration induced by caffeine (15.0-30.0 mg/kg). Although there was no interaction between ethanol and CPT or MSX-3 on social exploration in the first day, MSX-3 blocked the amnestic effects of ethanol observed on the following day. Thus, ethanol impairs the formation of social memories, and A2A adenosine antagonists can prevent the amnestic effects of ethanol, so that animals can recognize familiar conspecifics. On the other hand, ethanol can counteract the social withdrawal induced by caffeine, a non-selective adenosine A1/A2A receptor antagonist. These results show the complex set of interactions between ethanol and caffeine, some of which could be the result of the opposing effects they have in modulating the adenosine system.

Concomitant Caffeine Increases Binge Consumption of Ethanol in Adolescent and Adult Mice, But Produces Additive Motor Stimulation Only in Adolescent Animals

BACKGROUND

Binge co-consumption of highly caffeinated energy drinks with alcohol (ethanol [EtOH]) has become a common practice among adolescents/young adults and has been associated with an increased incidence of hazardous behaviors. Animal models are critical in advancing our understanding the neurobehavioral consequences of this form of binge drinking. Surprisingly, virtually no work has explored caffeine and EtOH co-consumption or its long-term consequences in adolescent animals. The primary objective of the current study was to extend a previously established mouse model of voluntary binge caffeine and EtOH co-consumption to explore adolescent consumption and responses compared to adults.

METHODS

Adolescent and adult male C57BL/6J mice had daily limited access to caffeine (0.03% w/v), EtOH (20% v/v), a combined EtOH/caffeine solution, or water for 14 days via the binge-like drinking paradigm, drinking-in-the-dark (DID). Home cage locomotor activity was measured during DID in a subset of mice. Following DID, all mice rested for 18 days so that adolescents reached adulthood, whereupon all mice underwent 7 days of continuous access 2-bottle choice drinking for 10% (v/v) EtOH or water.

RESULTS

Co-consumption with caffeine significantly increased EtOH intake and resultant blood ethanol concentrations in both adolescent and adult mice. In addition, adolescent mice exhibited a uniquely robust locomotor stimulant response to caffeine and EtOH co-consumption. Later EtOH intake and preference was not influenced, however, by prior fluid consumption history via DID.

CONCLUSIONS

Together with findings from the human literature, our results suggest that caffeine co-consumption may positively influence binge alcohol consumption in adolescents/young adults. Importantly, this age group may be particularly sensitive to the additive stimulant effects of caffeinated alcohol consumption, an effect which may be related to the high incidence of associated negative outcomes in this population. These observations are particularly concerning considering the heightened plasticity of the adolescent brain.

Rodent models and mechanisms of voluntary binge-like ethanol consumption: Examples, opportunities, and strategies for preclinical research

Binge ethanol consumption has widespread negative consequences for global public health. Rodent models offer exceptional power to explore the neurobiology underlying and affected by binge-like drinking as well as target potential prevention, intervention, and treatment strategies. An important characteristic of these models is their ability to consistently produce pharmacologically-relevant blood ethanol concentration. This review examines the current available rodent models of voluntary, pre-dependent binge-like ethanol consumption and their utility in various research strategies. Studies have demonstrated that a diverse array of neurotransmitters regulate binge-like drinking, resembling some findings from other drinking models. Furthermore, repeated binge-like drinking recruits neuroadaptive mechanisms in mesolimbocortical reward circuitry. New opportunities that these models offer in the current context of mechanistic research are also discussed.

Withania somnifera Dunal (Indian ginseng) impairs acquisition and expression of ethanol-elicited conditioned place preference and conditioned place aversion

Withania somnifera Dunal (Indian Ginseng) has recently been shown to impair ethanol self-administration. In order to gain further insights on the ability of the Withania somnifera standardised root extract (WSE) to affect the motivational properties of ethanol, this study investigated whether WSE may also affect ethanol (2 g/kg)-elicited conditioned place preference (CPP) and aversion (CPA). To this end male CD-1 mice were conditioned under two distinct schedules: in backward conditioning experiments ethanol was administered before mice were placed in the conditioning apparatus (CPP) while, in forward conditioning experiments, ethanol was administered immediately after removing mice from the apparatus (CPA). Following these schedules, mice developed significant CPP and CPA, respectively. Administration of WSE significantly impaired both the acquisition (50 and 100 mg/kg) and the expression (50 mg/kg) of CPP and CPA without affecting spatial memory (50 mg/kg), as determined by a two-trial memory recognition task. Overall, the study highlights the ability of WSE to interfere with both positive and negative motivational properties of ethanol and suggests that the effects of WSE may target both ethanol's motivational properties and underpinning associative learning mechanisms. In conclusion, these results cast new light on Withania somnifera as an agent potentially useful to counteract distinct aspects of ethanol effects.

Sensitization and Tolerance Following Repeated Exposure to Caffeine and Alcohol in Mice

BACKGROUND

Energy drinks are popular mixers with alcohol. While energy drinks contain many ingredients, caffeine is an important pharmacologically active component and is generally present in larger amounts than in other caffeinated beverages. In these studies, we investigated the hypothesis that caffeine would influence the effects of alcohol (ethanol [EtOH]) on conditioned taste aversion (CTA), ataxia, and locomotor activity (LA) after repeated exposure.

METHODS

Four groups of mice were exposed by oral gavage twice daily to vehicle, EtOH (4 g/kg), caffeine (15 mg/kg), or the EtOH/caffeine combination. CTA to saccharin and ataxia in the parallel rod task was evaluated after 8 or 16 gavages, respectively, using EtOH (1 to 3 g/kg) or EtOH/caffeine (3 mg/kg + 2 g/kg) challenges. In addition, LA was evaluated initially and after repeated exposure to oral gavage of these drugs and doses.

RESULTS

Repeated oral gavage of EtOH produced significant locomotor sensitization, with those mice increasing total distance traveled by 2-fold. The locomotor response to caffeine, while significantly greater than vehicle gavage, did not change with repeated exposure. On the other hand, repeated gavage of caffeine/EtOH combination produced a substantial increase in total distance traveled after repeated exposure (~4-fold increase). After repeated EtOH exposure, there was significant tolerance to EtOH in the CTA and parallel rod tests. However, neither a history of caffeine exposure nor including caffeine influenced EtOH-induced CTA. Interestingly, a history of caffeine exposure increased the ataxic response to the caffeine/EtOH combination and appeared to reduce the ataxic response to high doses of EtOH.

CONCLUSIONS

The data support the general hypothesis that repeated exposure to caffeine influences the response to EtOH. Together with previously published work, these data indicate that caffeine influences some EtOH-related behaviors, notably locomotion and ataxia, but appears not to influence the expression of conditioned behaviors.

"Wired," yet intoxicated: modeling binge caffeine and alcohol co-consumption in the mouse

BACKGROUND

The combination of highly caffeinated "energy drinks" with alcohol (ethanol [EtOH]) has become popular among young adults and intoxication via such beverages has been associated with an elevated risk for harmful behaviors. However, there are discrepancies in the human literature regarding the effect of caffeine on alcohol intoxication, perhaps due to confounding factors such as personality type, expectancy, and history of exposure. Animal models of co-exposure are resistant to such issues; however, the consequences of voluntary co-consumption have been largely ignored in the animal literature. The primary goal of this work was to characterize a mouse model of binge caffeine and EtOH co-consumption employing the limited access "Drinking-in-the-Dark" (DID) paradigm.

METHODS

Caffeine was added to a 20% alcohol solution via DID. Alcohol/caffeine intake, locomotor behavior, ataxia, anxiety-like behavior, and cognitive function were evaluated as a consequence of co-consumption in adult male C57BL/6J mice.

RESULTS

Caffeine did not substantially alter binge alcohol intake or resultant blood EtOH concentrations (BECs), nor did it alter alcohol's anxiolytic effects on the elevated plus maze or cognitive-interfering effects in a novel object-recognition task. However, no evidence of alcohol-induced sedation was observed in co-consumption groups that instead demonstrated a highly stimulated state similar to that of caffeine alone. The addition of caffeine was also found to mitigate alcohol-induced ataxia.

CONCLUSIONS

Taken together, our mouse model indicates that binge co-consumption of caffeine and alcohol produces a stimulated, less ataxic and anxious, as well as cognitively altered state; a state that could be of great public health concern. These results appear to resemble the colloquially identified "wide awake drunk" state that individuals seek via consumption of such beverages. This self-administration model therefore offers the capacity for translationally valid explorations of the neurobiological consequences of binge co-consumption to assess the public health risk of this drug combination.