Chronic ethanol intoxication enhances [3H]CCPA binding and does not reduce A1 adenosine receptor function in rat cerebellum

Stimulation of dorsal hippocampal histaminergic transmission mitigates the expression of ethanol withdrawal-induced despair in mice

Garnered literature points toward the role of the dorsal hippocampus (CA1) in ethanol withdrawal-induced responses, wherein a strong presence of the histaminergic system is also reported. Therefore, the present study investigated the effect of an enhanced CA1 histaminergic transmission on the expression of chronic ethanol withdrawal-induced despair in mice on the tail suspension test (TST). The results revealed that mice who were on an ethanol-fed diet (5.96%, v/v) for 8 days exhibited maximum immobility time on the TST, and decreased locomotion at 24 h post-ethanol withdrawal (10th day), indicating ethanol withdrawal-induced despair. Enhancement of CA1 histaminergic activity achieved by the treatment of intra-CA1 microinjection of histaminergic agents such as histamine (0.1, 10 μg/mouse, bilateral), the histamine precursor l-histidine (1, 10 μg/mouse, bilateral), the histamine neuronal releaser/H₃ receptor antagonist thioperamide (2, 10 μg/mouse, bilateral), the histamine H₁ receptor agonist FMPH (2, 6.5 μg/mouse, bilateral), or the H₂ receptor agonist amthamine (0.1, 0.5 μg/mouse, bilateral) to ethanol-withdrawn mice, 10 min before the 24-h post-ethanol withdrawal time point, significantly alleviated the expression of ethanol withdrawal-induced despair in mice on the TST. On the other hand, only the pre-treatment of the histamine H₁ receptor agonist FMPH (2, 6.5 μg/mouse, intra-CA1 bilateral) reversed the reduction in locomotor activity induced in ethanol-withdrawn mice, whereas other employed histaminergic agents were devoid of any effect on this behavior. Therefore, our findings indicate that an enhanced CA1 histaminergic transmission, probably via stimulation of CA1 postsynaptic histamine H₁ or H₂ receptor, could preclude the behavioral despair, while H₁ stimulation affects motor deficit expressed after ethanol withdrawal.

Sequential Changes in Brain Glutamate and Adenosine A1 Receptors May Explain Severity of Adolescent Alcohol Withdrawal after Consumption of High Levels of Alcohol

There is an excellent correlation between the age when alcohol consumption begins and the likelihood of lifelong problems with alcohol abuse. Alcohol use often begins in adolescence, a time marked by brain development and maturation of numerous brain systems. Rats are an important model, wherein the emergence of alcohol withdrawal symptoms serves as a gauge of dependency following chronic alcohol consumption. Previous work has shown that adolescent Long-Evans rats consume high levels of alcohol and develop a severe alcohol withdrawal syndrome when fed alcohol as part of a liquid diet. Acutely, alcohol inhibits two important excitatory receptors for glutamate (NMDA and AMPA) and may further decrease glutamate activity through modulatory adenosine receptors. The present study focuses on potential adaptive changes in expression of these receptors that may create a receptor imbalance during chronic alcohol consumption and lead to severe overexcitation of the adolescent brain during alcohol withdrawal. Levels of brain expression of NMDA, AMPA, and adenosine A1 and A2a receptors were determined by Western blotting after adolescent rats consumed an alcohol-containing liquid diet for 4, 11, or 18 days. Severity of alcohol withdrawal was also assessed at these time points. Levels increased for both AMPA and NMDA receptors, significant and approaching maximal by day 11. In contrast, A1 receptor density showed a slow decline reaching significance at 18 days. There were no changes in expression of adenosine A2a receptor. The most severe withdrawal symptoms appear to coincide with the later downregulation of adenosine A1 receptors coming on top of maximal upregulation of excitatory AMPA and NMDA glutamate receptors. Thus, loss of adenosine "brakes" on glutamate excitation may punctuate receptor imbalance in alcohol-consuming adolescents by allowing the upregulation of the excitatory receptors to have full impact during early alcohol withdrawal.

Alcohol disrupts sleep homeostasis

Alcohol is a potent somnogen and one of the most commonly used "over the counter" sleep aids. In healthy non-alcoholics, acute alcohol decreases sleep latency, consolidates and increases the quality (delta power) and quantity of NREM sleep during the first half of the night. However, sleep is disrupted during the second half. Alcoholics, both during drinking periods and during abstinences, suffer from a multitude of sleep disruptions manifested by profound insomnia, excessive daytime sleepiness, and altered sleep architecture. Furthermore, subjective and objective indicators of sleep disturbances are predictors of relapse. Finally, within the USA, it is estimated that societal costs of alcohol-related sleep disorders exceeds $18 billion. Thus, although alcohol-associated sleep problems have significant economic and clinical consequences, very little is known about how and where alcohol acts to affect sleep. In this review, we have described our attempts to unravel the mechanism of alcohol-induced sleep disruptions. We have conducted a series of experiments using two different species, rats and mice, as animal models. We performed microdialysis, immunohistochemical, pharmacological, sleep deprivation and lesion studies which suggest that the sleep-promoting effects of alcohol may be mediated via alcohol's action on the mediators of sleep homeostasis: adenosine (AD) and the wake-promoting cholinergic neurons of the basal forebrain (BF). Alcohol, via its action on AD uptake, increases extracellular AD resulting in the inhibition of BF wake-promoting neurons. Since binge alcohol consumption is a highly prevalent pattern of alcohol consumption and disrupts sleep, we examined the effects of binge drinking on sleep-wakefulness. Our results suggest that disrupted sleep homeostasis may be the primary cause of sleep disruption observed following binge drinking. Finally, we have also shown that sleep disruptions observed during acute withdrawal, are caused due to impaired sleep homeostasis. In conclusion, we suggest that alcohol may disrupt sleep homeostasis to cause sleep disruptions.

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

The impact of caffeine on the behavioral effects of ethanol, including ethanol consumption and abuse, has become a topic of great interest due to the rise in popularity of the so-called energy drinks. Energy drinks high in caffeine are frequently taken in combination with ethanol under the popular belief that caffeine can offset some of the intoxicating effects of ethanol. However, scientific research has not universally supported the idea that caffeine can reduce the effects of ethanol in humans or in rodents, and the mechanisms mediating the caffeine-ethanol interactions are not well understood. Caffeine and ethanol have a common biological substrate; both act on neurochemical processes related to the neuromodulator adenosine. Caffeine acts as a nonselective adenosine A1 and A2A receptor antagonist, while ethanol has been demonstrated to increase the basal adenosinergic tone via multiple mechanisms. Since adenosine transmission modulates multiple behavioral processes, the interaction of both drugs can regulate a wide range of effects related to alcohol consumption and the development of ethanol addiction. In the present review, we discuss the relatively small number of animal studies that have assessed the interactions between caffeine and ethanol, as well as the interactions between ethanol and subtype-selective adenosine receptor antagonists, to understand the basic findings and determine the possible mechanisms of action underlying the caffeine-ethanol interactions.

Alcohol and Caffeine: The Perfect Storm

Although it is widely believed that caffeine antagonizes the intoxicating effects of alcohol, the molecular mechanisms underlying their interaction are incompletely understood. It is known that both caffeine and alcohol alter adenosine neurotransmission, but the relationship is complex, and may be dose dependent. In this article, we review the available literature on combining caffeine and alcohol. Ethical constraints prohibit laboratory studies that would mimic the high levels of alcohol intoxication achieved by many young people in real-world settings, with or without the addition of caffeine. We propose a possible neurochemical mechanism for the increase in alcohol consumption and alcohol-related consequences that have been observed in persons who simultaneously consume caffeine. Caffeine is a nonselective adenosine receptor antagonist. During acute alcohol intake, caffeine antagonizes the "unwanted" effects of alcohol by blocking the adenosine A₁ receptors that mediate alcohol's somnogenic and ataxic effects. The A₁ receptor-mediated "unwanted" anxiogenic effects of caffeine may be ameliorated by alcohol-induced increase in the extracellular concentration of adenosine. Moreover, by means of interactions between adenosine A_2A and dopamine D₂ receptors, caffeine-mediated blockade of adenosine A_2A receptors can potentiate the effects of alcohol-induced dopamine release. Chronic alcohol intake decreases adenosine tone. Caffeine may provide a "treatment" for the withdrawal effects of alcohol by blocking the effects of upregulated A₁ receptors. Finally, blockade of A_2A receptors by caffeine may contribute to the reinforcing effects of alcohol.

Neuroadaptations in adenosine receptor signaling following long-term ethanol exposure and withdrawal

Ethanol affects the function of neurotransmitter systems, resulting in neuroadaptations that alter neural excitability. Adenosine is one such receptor system that is changed by ethanol exposure. The current review is focused on the A(1) and the A(2A) receptor subtypes in the context of ethanol-related neuroadaptations and ethanol withdrawal because these subtypes (i) are activated by basal levels of adenosine, (ii) have been most well-studied for their role in neuroprotection and ethanol-related phenomena, and (iii) are the primary site of action for caffeine in the brain, a substance commonly ingested with ethanol. It is clear that alterations in adenosinergic signaling mediate many of the effects of acute ethanol administration, particularly with regard to motor function and sedation. Further, prolonged ethanol exposure has been shown to produce adaptations in the cell surface expression or function of both A(1) and the A(2A) receptor subtypes, effects that likely promote neuronal excitability during ethanol withdrawal. As a whole, these findings demonstrate a significant role for ethanol-induced adaptations in adenosine receptor signaling that likely influence neuronal function, viability, and relapse to ethanol intake following abstinence.

Implication of the purinergic system in alcohol use disorders

In the central nervous system, adenosine and adenosine 5'-triphosphate (ATP) play an important role in regulating neuronal activity as well as controlling other neurotransmitter systems, such as, GABA, glutamate, and dopamine. Ethanol increases extracellular adenosine levels that regulate the ataxic and hypnotic/sedative effects of ethanol. Interestingly, ethanol is known to increase adenosine levels by inhibiting an ethanol-sensitive adenosine transporter, equilibrative nucleoside transporter type 1 (ENT1). Ethanol is also known to inhibit ATP-specific P2X receptors, which might result in such similar effects as those caused by an increase in adenosine. Adenosine and ATP exert their functions through P1 (metabotropic) and P2 (P2X-ionotropic and P2Y-metabotropic) receptors, respectively. Purinergic signaling in cortex-striatum-ventral tegmental area (VTA) has been implicated in regulating cortical glutamate signaling as well as VTA dopaminergic signaling, which regulates the motivational effect of ethanol. Moreover, several nucleoside transporters and receptors have been identified in astrocytes, which regulate not only adenosine-ATP neurotransmission, but also homeostasis of major inhibitory-excitatory neurotransmission (i.e., GABA or glutamate) through neuron-glial interactions. This review will present novel findings on the implications of adenosine and ATP neurotransmission in alcohol use disorders.

Role of adenosine and wake-promoting basal forebrain in insomnia and associated sleep disruptions caused by ethanol dependence

Insomnia is a severe symptom of alcohol withdrawal; however, the underlying neuronal mechanism is yet unknown. We hypothesized that chronic ethanol exposure will impair basal forebrain (BF) adenosinergic mechanism resulting in insomnia-like symptoms. We performed a series of experiments in Sprague-Dawley rats to test our hypothesis. We used Majchrowicz's chronic binge ethanol protocol to induce ethanol dependency. Our first experiment verified the effects of ethanol withdrawal on sleep-wakefulness. Significant increase in wakefulness was observed during ethanol withdrawal. Next, we examined c-Fos expression (marker of neuronal activation) in BF wake-promoting neurons during ethanol withdrawal. There was a significant increase in the number of BF wake-promoting neurons with c-Fos immunoreactivity. Our third experiment examined the effects of ethanol withdrawal on sleep deprivation induced increase in BF adenosine levels. Sleep deprivation did not increase BF adenosine levels in ethanol dependent rats. Our last experiment examined the effects of ethanol withdrawal on equilibrative nucleoside transporter 1 and A1 receptor expression in the BF. There was a significant reduction in A1 receptor and equilibrative nucleoside transporter 1 expression in the BF of ethanol dependent rats. Based on these results, we suggest that insomnia observed during ethanol withdrawal is caused because of impaired adenosinergic mechanism in the BF.

Role of wake-promoting basal forebrain and adenosinergic mechanisms in sleep-promoting effects of ethanol

BACKGROUND

Ethanol intake has significant impact on sleep. However, the cellular substrates responsible for sleep promotion following ethanol intake are unknown. The purine nucleoside, adenosine, is responsible for mediating many neuronal and behavioral responses to ethanol. Studies performed in cell cultures suggest that ethanol inhibits equilibrative nucleoside transporter 1 to block the reuptake of adenosine resulting in increased extracellular adenosine. Adenosine also has a pivotal role in sleep regulation. Adenosine acts via A1 receptor to inhibit the wake-promoting neurons of the basal forebrain (BF) resulting in the promotion of sleep. Is ethanol-induced sleep associated with the inhibition of the BF wake-promoting neurons? Do adenosinergic mechanisms in the BF have a role in sleep-promoting effects of ethanol?

METHODS

To address these questions, we performed 3 experiments in Sprague-Dawley rats. First, we verified the effect of ethanol on sleep promotion. Second, we evaluated the effect of ethanol on c-Fos expression (a marker of neuronal activation) in the BF wake-promoting neurons and third we monitored the effects of A1 receptor blockade in the BF on ethanol-induced sleep.

RESULTS

Significant increase in non-rapid eye movement (NREM) sleep with a concomitant decrease in wakefulness was observed during the first 12 hours postethanol. REM sleep remained unaffected. Ethanol administration caused a significant decrease in the number of BF wake-promoting neurons with c-Fos immunoreactivity. Bilateral microinjections of a selective A1R receptor antagonist 8-cyclopentyl-1, 3-dipropylxanthine into the BF significantly attenuated sleep-promoting effects of ethanol.

CONCLUSION

These results suggest that the inhibition of BF wake-promoting neurons by adenosinergic mechanism may be responsible for the sleep promoting effects of ethanol. We believe our study is the first to investigate the cellular mechanisms responsible for the somnogenic effects of ethanol.

Sex differences in caffeine neurotoxicity following chronic ethanol exposure and withdrawal

AIMS

Caffeine is a central nervous system stimulant that produces its primary effects via antagonism of the A(1) and A(2A) adenosine receptor subtypes. Previous work demonstrated a sex difference in neurotoxicity produced by specific adenosine A(1) receptor antagonism during ethanol withdrawal (EWD) in vitro that was attributable to effects downstream of A(1) receptors at NMDA receptors. The current studies were designed to examine the effect of non-specific adenosine receptor antagonism with caffeine during ethanol withdrawal on hippocampal toxicity in cultures derived from male and female rats.

METHODS

At 5 days in vitro (DIV), half of the male and female organotypic hippocampal slice cultures were exposed to 50 mM ethanol (EtOH) in culture media for 10 days before exposure to caffeine (5, 20 and 100 microM) for the duration of a 24 h EWD period. In keeping with this timeline, the remaining ethanol-naïve cultures were given media changes at 10 and 15 DIV and exposed to caffeine (5, 20 and 100 microM) for 24 h at 15 DIV. Cytotoxicity was assessed by fluorescent microscopy and quantification of propidium iodide (PI) uptake in the pyramidal cell layers of the CA1 and CA3 regions and the granule cell layer of the dentate gyrus (DG). A two-way (sex x treatment) ANOVA was conducted within each hippocampal region.

RESULTS

Twenty-four-hour withdrawal from 10-day exposure to 50 mM ethanol did not produce increased PI uptake in any hippocampal region. Caffeine exposure (5, 20 and 100 microM) in ethanol-naïve cultures did not produce toxicity in the DG or CA1 region, but 20 microM caffeine produced modest toxicity in the CA3 region. Exposure to 20 microM caffeine during EWD produced cytotoxicity in all hippocampal regions, though toxicity was sex-dependent in the DG and CA1 region. In the DG, both 5 and 20 microM caffeine produced significantly greater PI uptake in ethanol-exposed female cultures compared to ethanol-naïve female cultures and all male cultures. Similarly, 20 microM caffeine caused markedly greater toxicity in female cultures as compared to male cultures in the CA1 region.

CONCLUSIONS

Twenty-four-hour exposure to caffeine during EWD produced significant toxicity in the pyramidal cell layer of the CA3 region in male and female cultures, though toxicity in the granule cell layer of the DG and pyramidal cell layer of the CA1 region was observed only in female cultures. Greater sensitivity of the female slice cultures to toxicity upon caffeine exposure after prolonged ethanol exposure is consistent with previous studies of effects of a specific A(1) receptor antagonism during EWD on toxicity and indicate that this effect is independent of the hormonal milieu. Together, these data suggest that the A(1) receptor subtype is predominant in mediating caffeine's neurotoxic effects during EWD. These findings demonstrate the importance of considering gender/sex when examining neuroadaptive changes in response to ethanol exposure and withdrawal.

Sex differences in the neurotoxic effects of adenosine A1 receptor antagonism during ethanol withdrawal: reversal with an A1 receptor agonist or an NMDA receptor antagonist

BACKGROUND

Neuronal adaptations that occur during chronic ethanol (EtOH) exposure have been observed to sensitize the brain to excitotoxic insult during withdrawal. The adenosine receptor system warrants further examination in this regard, as recent evidence has implicated adenosine receptor involvement in the behavioral effects of both EtOH exposure and withdrawal.

METHODS

The current studies examined effects of adenosine A(1) receptor manipulation on neuronal injury in EtOH-naive and EtOH-withdrawn male and female rat hippocampal slice cultures. EtOH-naive and EtOH pretreated (43.1 to 26.9 mM from days 5 to 15 DIV) cultures were exposed to the A(1) receptor agonist 2-Chloro-N(6)-cyclopentyladenosine (CCPA; 10 nM), the A(1) receptor antagonist 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX;10 nM), or the N-methyl-D-aspartate (NMDA) receptor antagonist D,L,-2-amino-5-phosphovalerate (APV; 20 microM) at 15 days in vitro (DIV). Cytotoxicity was measured in the primary neuronal layers of the dentate gyrus, CA3 and CA1 hippocampal regions by quantification of propidium iodide (PI) fluorescence after 24 hours. Immunohistochemical analysis of A(1) receptor abundance was conducted in EtOH-naive and EtOH pretreated slice cultures at 15 DIV.

RESULTS

Twenty-four hour exposure to DPCPX in EtOH-naive slice cultures did not produced neurotoxicity in any region of slice cultures. Though withdrawal from 10 day EtOH exposure produced no toxicity in either male or female slice cultures, exposure to DPCPX during 24 hours of EtOH withdrawal produced a marked increase in PI uptake in all hippocampal culture subregions in female cultures (to approximately 160% of control values). A significant effect for sex was observed in the CA1 region such that toxicity in females cultures exposed to the A(1) antagonist during withdrawal was greater than that observed in male cultures. These effects of DPCPX in EtOH withdrawn female and male slices were prevented by co-exposure to either the A(1) agonist CCPA or the NMDA receptor antagonist APV for 24 hours. No differences in the abundance of A(1) receptors were observed in male and female EtOH-naive or EtOH pretreated cultures.

CONCLUSIONS

The current findings suggest that the female hippocampus possesses an innate sensitivity to effects of EtOH exposure and withdrawal on neuronal excitability that is independent of hormonal influences. Further, this sex difference is not related to effects of EtOH exposure on A(1) receptor abundance, but likely reflects increased NMDA receptor-mediated signaling downstream of A(1) inhibition in females.

Activation of adenosine A1 receptors reduces anxiety-like behavior during acute ethanol withdrawal (hangover) in mice

Elevated signs of anxiety are observed in both humans and rodents during withdrawal from chronic as well as acute ethanol exposure, and it represents an important motivational factor for ethanol relapse. Several reports have suggested the involvement of brain adenosine receptors in different actions produced by ethanol such as motor incoordination and hypnotic effects. In addition, we have recently demonstrated that adenosine A1 receptors modulate the anxiolytic-like effect induced by ethanol in mice. In the present study, we evaluated the potential of adenosine A1 and A2A receptor agonists in reducing the anxiety-like behavior during acute ethanol withdrawal (hangover) in mice. Animals received a single intraperitoneal administration of saline or ethanol (4 g/kg) and were tested in the elevated plus maze after an interval of 0.5-24 h. The results indicated that hangover-induced anxiety was most pronounced between 12 and 18 h after ethanol administration, as indicated by a significant reduction in the exploration of the open arms of the maze. At this time interval, ethanol was completely cleared. The acute administration of 'nonanxiolytic' doses of adenosine and the selective adenosine A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA), but not the adenosine A2A receptor agonist N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl]adenosine (DPMA), at the onset of peak withdrawal (18 h), reduced this anxiogenic-like response. In addition, the effect of CCPA on the anxiety-like behavior of ethanol hangover was reversed by pretreatment with the selective adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). These results reinforce the notion of the involvement of adenosine receptors in the anxiety-like responses and indicate the potential of adenosine A1 receptor agonists to reduce the anxiogenic effects during ethanol withdrawal.

Role of the GABAA system in behavioral, motoric, and cerebellar protection by estrogen during ethanol withdrawal

Results of studies from our laboratory have shown that administration of 17beta-estradiol (E(2)) reduces cerebellar neuronal damage during ethanol withdrawal (EW). In the current study, we investigated whether the GABAergic system is involved in the protective effects of E(2) against the EW syndrome. To test this hypothesis, we examined the effects of GABAergic drugs, with and without E(2), on EW sign scores, motoric capacity, and caspase activation. Ovariectomized rats implanted with an E(2) or an oil pellet received liquid ethanol [7.5% weight/volume (wt./vol.)] for 5 weeks or dextrin diet, followed by 2 weeks of EW. A gamma-aminobutyric acid type A (GABA(A)) agonist, muscimol (0.125 or 0.25 mg/kg), and antagonist, bicuculline (1.25 mg/kg), were administered (intraperitoneally; three times a day for 4 days) starting 1 day before the onset of EW. On termination of chronic administration of ethanol diet, rats were tested for overt withdrawal signs and latency to fall from a rotarod. The initial latency was measured separately to assess motoric capacity before learning occurred. Cerebelli were subsequently collected for immunohistochemistry to detect caspase activation. Results showed that treatment with E(2) lowered EW sign scores and improved initial as well as subsequent rotarod latencies compared with findings without treatment with E(2) (control group). These effects of E(2) were enhanced by combined treatment with muscimol and diminished by bicuculline. Results also showed that ethanol-withdrawn rats had more caspase-3-positive cells than observed for the dextrin diet-fed group in a manner reversed by E(2) and exacerbated by bicuculline. Bicuculline also caused partial antagonism of the protective effect of E(2). These findings support the suggestion that GABA(A) agonists ameliorate, and GABA(A) antagonists exacerbate, EW signs, cerebellar neuronal damage, and motoric impairment in ethanol-withdrawn rats. Also, results of the current study provide indirect evidence that the GABAergic system is involved in protective effects of E(2) against the EW syndrome.

Modulation of audiogenic seizures by histamine and adenosine receptors in the inferior colliculus

Susceptibility to behaviorally similar audiogenic seizures (AGS) occurs genetically and is inducible during ethanol withdrawal (ETX). Comparisons between AGS mechanisms of genetically epilepsy-prone rats (GEPR-9s) and ethanol-withdrawn rats (ETX-Rs) are yielding information about general pathophysiological mechanisms of epileptogenesis. The inferior colliculus (IC) is the AGS initiation site. Excitatory amino acid (EAA) abnormalities in the IC are implicated in AGS, and histamine and adenosine receptor activation each reduce EAA release and inhibit several seizure types. Previous studies indicate that focal infusion of an adenosine receptor agonist into the IC blocked AGS in GEPR-9s, but the effects of adenosine receptor activation in the IC on AGS in ETX-Rs are unknown. The effects of histamine receptor activation on either form of AGS are also unexamined. The present study evaluated effects of histamine or a nonselective adenosine A(1) agonist, 2-chloroadenosine, on AGS by focal microinjection into the IC. Ethanol dependence and AGS susceptibility were induced in normal rats by intragastric ethanol. Histamine (40 or 60 nmol/side) significantly reduced AGS in GEPR-9s, but histamine in doses up to 120 nmol/side did not affect AGS in ETX-Rs. 2-Chloroadenosine (5 or 10 nmol/side) did not affect AGS in ETX-Rs, despite the effectiveness of lower doses of this agent in GEPR-9s reported previously. Thus, histamine and adenosine receptors in the IC modulate AGS of GEPR-9s, but do not modulate ETX-induced AGS. The reasons for this difference may involve the chronicity of AGS susceptibility in GEPR-9s, which may lead to more extensive neuromodulation as compensatory mechanisms to limit the seizures compared to the acute AGS of ETX-Rs.

Role of adenosine A1 and A2A receptors in the alcohol withdrawal syndrome

The role of adenosine receptor-mediated signaling was examined in the alcohol withdrawal syndrome. CD-1 mice received a liquid diet containing ethanol (6.7%, v/v) or a control liquid diet that were abruptly discontinued after 14 days of treatment. Mice consuming ethanol showed a progressive increase in signs of intoxication throughout the drinking period. Following abrupt discontinuation of ethanol diet, mice demonstrated reversible signs of handling-induced hyperexcitability that were maximal between 5-8 h. Withdrawing mice received treatment with adenosine receptor agonists at the onset of peak withdrawal (5.5 h) and withdrawal signs were blindly rated (during withdrawal hours 6 and 7). Adenosine A1-receptor agonist R-N6(phenylisopropyl)adenosine (0.15 and 0.3 mg/ kg) reduced withdrawal signs 0.5 and 1.5 h after drug administration in a dose-dependent fashion. Adenosine A2A-selective agonist 2-p-(2-carboxyethyl)phenylethyl-amino-5'-N-ethylcarboxamidoadenosine (0.3 mg/kg) reduced withdrawal signs at both time points. In ethanol-withdrawing mice, there were significant decreases in adenosine transporter sites in striatum without changes in cortex or cerebellum. In ethanol-withdrawing mice, there were no changes in adenosine A1 and A2A receptor concentrations in cortex, striatum, or cerebellum. There appears to be a role for adenosine A1 and A2A receptors in the treatment of the ethanol withdrawal syndrome. Published by Elsevier Science Inc.

The effects of adenosine ligands R-PIA and CPT on ethanol withdrawal

The potential anxiogenic or anxiolytic effects of R(-)-N6-(2-phenylisopropyl)adenosine (R-PIA), an adenosine agonist, and 8-cyclopentyl-1,3,dimethylxanthine (CPT), an adenosine antagonist, were tested during chronic exposure to ethanol and to ethanol-induced withdrawal in rats. Effects on anxiety were measured by the elevated plus maze and dark-light box. Ethanol consumption and preference was tested in an additional experiment. In testing of elevated plus maze performance during withdrawal from ethanol, R-PIA produced no change in the anxiety-related behaviors of total arm entries and percent open arm entries, but produced a significant decrease in percent open arm time. CPT produced at least partial recovery from the anxiogenic effects of ethanol withdrawal on all three measures of elevated plus maze performance, although peak effects were seen at the intermediate dose of CPT (0.08 mg/kg) for total arm entries and percent open arm time. CPT also showed anxiolytic effects at low to intermediate doses (0.04, 0.08 mg/kg) in the dark-light box. CPT did not reduce the preference for ethanol over water or the total consumption of ethanol over a range of ethanol doses. In summary, the adenosine agonist, R-PIA, exacerbated the effects of ethanol withdrawal, whereas the adenosine antagonist, CPT, at least partially blocked the anxiogenic effects produced by ethanol withdrawal. These results suggest that adenosine antagonists, at least at some doses, may be useful for ameliorating the anxiogenic effects produced by ethanol withdrawal, although it does not appear useful for reducing consumption.

Increased abundance of GABAA receptor subunit mRNAs in the brain of Long-Evans Cinnamon rats, an animal model of Wilson's disease

The abundance of mRNAs encoding various subunits of the gamma-aminobutyric acid type A (GABAA) receptor was examined in different regions of the brain of Long-Evans Cinnamon (LEC) rats, an animal model of Wilson's disease (WD). The measurements were performed at two different stages of disease: at 9 weeks of age, when no symptoms are evident, and at 15 weeks of age, when 90% of the animals develop jaundice. The amounts of the gamma2L and gamma2S subunit mRNAs in the striatum, cerebellum, and cerebral cortex of LEC rats at 9 weeks of age were increased (+25 to +35%) compared with those in LE rats of the same age; these differences were no longer apparent in 15-week old animals. The amount of alpha1 subunit mRNA was also significantly increased (+30%) in the cerebellum of LEC rats at 9 weeks of age; although a smaller increase (+20%) was still evident at 15 weeks of age, this was not statistically significant. The amount of beta2 subunit mRNA was increased in the cerebellum (+32%) and hippocampus (+21%) of LEC rats at 9 weeks of age, but no differences with LE rats were apparent at 15 weeks. The onset of isoniazid-induced seizures in LEC rats at 9 weeks of age was significantly delayed compared with that in LE rats. These results demonstrate abnormal expression of GABAA receptor subunit genes in the brain of LEC rats, and they suggest that this altered expression is associated with an increase in GABAergic tone.

Ethanol and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.

Single and repeated episodes of ethanol withdrawal increase adenosine A1, but not A2A, receptor density in mouse brain

A history of multiple ethanol withdrawal experiences has been shown to exacerbate the severity of future withdrawal episodes, and this sensitization of the withdrawal response has been hypothesized to represent a 'kindling' phenomenon. Since adenosine functions as an inhibitory modulator of seizure activity and may interact with ethanol to influence neuronal excitability, the present study was conducted to examine the effects of single and repeated episodes of ethanol withdrawal on adenosine A1 and A2A receptors in adult C3H/He mice. Mice were chronically exposed to ethanol vapor in inhalation chambers and tested for withdrawal seizures following multiple withdrawal (MW) experience (four cycles of 16 h ethanol intoxication interrupted by 8 h periods of abstinence), single withdrawal experience following 16 h (SW) or 64 h (CE) continuous ethanol intoxication, or no ethanol exposure (controls). Separate groups of mice from each withdrawal condition were used to generate pooled cortical and striatal tissue for ligand saturation experiments using [3H]cyclohexyladenosine to label A1 receptors and [3H]CGS 21680 to label A2A receptors. Results indicated that withdrawal seizures were significantly more severe in mice with multiple withdrawal experience in comparison to animals that experienced only a single withdrawal episode, even when total amount of ethanol exposure was equated among groups. The density of A1 receptors in cerebral cortex was significantly increased over controls 8 h following final ethanol withdrawal by approximately 35% in SW and CE groups, with the largest increase observed in the MW group (56%). Withdrawal treatment groups did not differ in cortical A1 binding sites immediately upon withdrawal from ethanol, and no significant differences in binding of [3H]CGS 21680 to striatal A2A receptors were observed following ethanol withdrawal. Ethanol exposure and withdrawal did not significantly alter ligand affinity for either adenosine receptor. These results indicate that adenosine A1 receptors are selectively upregulated during ethanol withdrawal and that the degree of upregulation may be enhanced following multiple withdrawal episodes. Further, these observations suggest that the upregulation of brain A1 receptors during ethanol withdrawal may represent a compensatory inhibitory response to increased seizure severity associated with repeated episodes of ethanol withdrawal.