Effects of ethanol on monoamine and amino acid release from cerebral cortical slices of the alcohol-preferring P line of rats

Ethanol-Associated Changes in Glutamate Reward Neurocircuitry: A Minireview of Clinical and Preclinical Genetic Findings

Herein, we have reviewed the role of glutamate, the major excitatory neurotransmitter in the brain, in a number of neurochemical, -physiological, and -behavioral processes mediating the development of alcohol dependence. The findings discussed include results from both preclinical as well as neuroimaging and postmortem clinical studies. Expression levels for a number of glutamate-associated genes and/or proteins are modulated by alcohol abuse and dependence. These changes in expression include metabotropic receptors and ionotropic receptor subunits as well as different glutamate transporters. Moreover, these changes in gene expression parallel the pharmacologic manipulation of these same receptors and transporters. Some of these gene expression changes may have predated alcohol abuse and dependence because a number of glutamate-associated polymorphisms are related to a genetic predisposition to develop alcohol dependence. Other glutamate-associated polymorphisms are linked to age at the onset of alcohol-dependence and initial level of response/sensitivity to alcohol. Finally, findings of innate and/or ethanol-induced glutamate-associated gene expression differences/changes observed in a genetic animal model of alcoholism, the P rat, are summarized. Overall, the existing literature indicates that changes in glutamate receptors, transporters, enzymes, and scaffolding proteins are crucial for the development of alcohol dependence and there is a substantial genetic component to these effects. This indicates that continued research into the genetic underpinnings of these glutamate-associated effects will provide important novel molecular targets for treating alcohol abuse and dependence.

Influence of the novel histamine H₃ receptor antagonist ST1283 on voluntary alcohol consumption and ethanol-induced place preference in mice

RATIONALE

Growing evidence supports a role for the central histaminergic system to have a modulatory influence on drug addiction in general and alcohol-use disorders in particular through histamine H3 receptors (H3R).

OBJECTIVE

In the present study, the effects of systemic injection of the newly synthesized H3R antagonist ST1283 on ethanol (EtOH) voluntary intake and EtOH-conditioned reward in mice have been investigated.

METHODS

Oral EtOH, saccharin, and quinine intake was assessed in a two-bottle choice paradigm using escalating concentrations of alcohol or tastant solutions. EtOH-induced place preference (CPP), EtOH-induced locomotor activity, and blood ethanol concentration (BEC) were also measured.

RESULTS

Following administration of the H3R antagonist (2.5, 5, and 10 mg/kg, i.p.), there was a significant dose-dependent decrease in alcohol consumption and preference. Importantly, vehicle- and ST1283 (5 mg/kg)-treated mice showed similar consumption and preference to increasing concentration of both sweet and bitter tastes. More interestingly, systemic administration of ST1283 inhibited EtOH-CPP and EtOH-enhanced locomotion. This inhibition was blocked when mice were pretreated with the selective H3R agonist R-(alpha)-methyl-histamine (10 mg/kg). Finally, vehicle- and ST1283-treated mice had similar BECs.

CONCLUSION

Our results show that ST1283 may decrease voluntary EtOH consumption and EtOH-CPP by altering its reinforcing effects, suggesting a novel role for histamine signaling in regulation of alcoholism. Lastly, the results add to the growing literature on H3R modulation in the pharmacotherapy of EtOH addiction.

Accumbens Homer2-mediated signaling: a factor contributing to mouse strain differences in alcohol drinking?

Alcohol-induced increases in nucleus accumbens glutamate actively regulate alcohol consumption, and the alcohol responsiveness of corticoaccumbens glutamate systems relates to genetic variance in alcohol reward. Here, we extend earlier data for inbred mouse strain differences in accumbens glutamate by examining for differences in basal and alcohol-induced changes in the striatal expression of glutamate-related signaling molecules between inbred C57BL/6J and DBA2/J mice. Repeated alcohol treatment (8 × 2 g/kg) increased the expression of Group1 metabotropic glutamate receptors, the NR2a/b subunits of the N-methyl-D-aspartate receptor, Homer2a/b, as well as the activated forms of protein kinase C (PKC) epsilon and phosphoinositol-3-kinase within ventral, but not dorsal, striatum. Regardless of prior alcohol experience, C57BL/6J mice exhibited higher accumbens levels of mGluR1/5, Homer2a/b, NR2a and activated kinases vs. DBA2/J mice, whereas an alcohol-induced rise in dorsal striatum mGluR1/5 expression was observed only in C57BL/6J mice. We next employed virus-mediated gene transfer approaches to ascertain the functional relevance of the observed strain difference in accumbens Homer2 expression for B6/D2 differences in alcohol-induced glutamate sensitization, as well as alcohol preference/intake. Manipulating nucleus accumbens shell Homer2b expression actively regulated these measures in C57BL/6J mice, whereas DBA2/J mice were relatively insensitive to the neurochemical and behavioral effects of virus-mediated changes in Homer2 expression. These data support the over-arching hypothesis that augmented accumbens Homer2-mediated glutamate signaling may be an endophenotype related to genetic variance in alcohol consumption. If relevant to humans, such data pose polymorphisms affecting glutamate receptor/Homer2 signaling in the etiology of alcoholism.

Contribution of early environmental stress to alcoholism vulnerability

The most problematic aspects of alcohol abuse disorder are excessive alcohol consumption and the inability to refrain from alcohol consumption during attempted abstinence. The root causes that predispose certain individuals to these problems are poorly understood but are believed to be produced by a combination of genetic and environmental factors. Early environmental trauma alters neurodevelopmental trajectories that can predispose an individual to a number of neuropsychiatric disorders, including substance abuse. Prenatal stress (PNS) is a well-established protocol that produces perturbations in nervous system development, resulting in behavioral alterations that include hyperresponsiveness to stress, novelty, and psychomotor stimulant drugs (e.g., cocaine, amphetamine). Moreover, PNS animals exhibit enduring alterations in basal and cocaine-induced changes in dopamine and glutamate transmission within limbic structures, which exhibit pathology in drug addiction and alcoholism, suggesting that these alterations may contribute to an increased propensity to self-administer large amounts of drugs of abuse or to relapse after periods of drug withdrawal. Given that cocaine and alcohol have actions on common limbic neural substrates (albeit by different mechanisms), we hypothesized that PNS would elevate the motivation for, and consumption of, alcohol. Accordingly, we have found that male C57BL/6J mice subject to PNS exhibit higher operant responding and consume more alcohol during alcohol reinforcement as adults. Alterations in glutamate and dopamine neurotransmission within the forebrain structures appear to contribute to the PNS-induced predisposition to high alcohol intake and are induced by excessive alcohol intake. Accordingly, we are exploring the interactions between neurochemical changes produced by PNS and changes induced by consumption of alcohol in adulthood to model the biological bases of high vulnerability to alcohol abuse.

Binge drinking upregulates accumbens mGluR5-Homer2-PI3K signaling: functional implications for alcoholism

The glutamate receptor-associated protein Homer2 regulates alcohol-induced neuroplasticity within the nucleus accumbens (NAC), but the precise intracellular signaling cascades involved are not known. This study examined the role for NAC metabotropic glutamate receptor (mGluR)-Homer2-phosphatidylinositol 3-kinase (PI3K) signaling in regulating excessive alcohol consumption within the context of the scheduled high alcohol consumption (SHAC) model of binge alcohol drinking. Repeated bouts of binge drinking ( approximately 1.5 g/kg per 30 min) elevated NAC Homer2a/b expression and increased PI3K activity in this region. Virus-mediated knockdown of NAC Homer2b expression attenuated alcohol intake, as did an intra-NAC infusion of the mGluR5 antagonist MPEP [2-methyl-6-(phenylethynyl)pyridine hydrochloride] (0.1-1 microg/side) and the PI3K antagonist wortmannin (50 ng/side), supporting necessary roles for mGluR5/Homer2/PI3K in binge alcohol drinking. Moreover, when compared with wild-type littermates, transgenic mice with an F1128R point mutation in mGluR5 that markedly reduces Homer binding exhibited a 50% reduction in binge alcohol drinking, which was related to reduced NAC basal PI3K activity. Consistent with the hypothesis that mGluR5-Homer-PI3K signaling may be a mechanism governing excessive alcohol intake, the "anti-binge" effects of MPEP and wortmannin were not additive, nor were they observed in the mGluR5(F1128R) transgenic mice. Finally, mice genetically selected for a high versus low SHAC phenotype differed in NAC mGluR, Homer2, and PI3K activity, consistent with the hypothesis that augmented NAC mGluR5-Homer2-PI3K signaling predisposes a high binge alcohol-drinking phenotype. Together, these data point to an important role for NAC mGluR5-Homer2-PI3K signaling in regulating binge-like alcohol consumption that has relevance for our understanding of the neurobiology of alcoholism and its pharmacotherapy.

Strain differences in alcohol-induced neurochemical plasticity: a role for accumbens glutamate in alcohol intake

BACKGROUND

Repeated alcohol administration alters nucleus accumbens (NAC) basal glutamate content and sensitizes the capacity of alcohol to increase NAC extracellular glutamate levels. However, the relevance of alcohol-induced changes in NAC glutamate for alcohol drinking behavior is under-investigated.

METHODS

To examine the relationship between genetic variance in alcohol consumption and alcohol-induced neuroadaptations within the NAC, in vivo microdialysis was conducted in the alcohol-preferring C57BL/6J (B6) and alcohol-avoiding DBA2/J (D2) mouse strains on injections 1 and 8 of repeated alcohol treatment (8 x 2 g/kg, IP). To confirm an active role for NAC glutamate in regulating alcohol drinking behavior, the glutamate reuptake inhibitor dl-threo-beta-benzyloxyaspartic acid (TBOA) (300 microM) and the Group 2 metabotropic glutamate autoreceptor agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) (50 microM) were infused into the NAC of B6 and D2 mice prior to alcohol consumption in a 4 bottle-choice test.

RESULTS

While strain differences were not apparent for NAC basal levels of dopamine, serotonin or gamma-amino butyric acid (GABA), repeated alcohol treatment elevated NAC basal glutamate content only in B6 mice. Strain differences in both the acute and the sensitized neurochemical responses to 2 g/kg alcohol were observed for all neurotransmitters examined. While the alcohol-induced rise in NAC dopamine and glutamate levels sensitized in B6 mice, a sensitization was not observed in D2 animals. Moreover, B6 mice exhibited a sensitized serotonin and GABA response to alcohol followed repeated treatment, whereas neither tolerance nor sensitization was observed in D2 animals. An intra-NAC APDC infusion reduced alcohol intake in both B6 and D2 mice by approximately 50%. In contrast, TBOA infusion elevated alcohol intake selectively in B6 mice.

CONCLUSIONS

These data indicate an active role for NAC glutamate in regulating alcohol consumption in mice and support the hypothesis that predisposition to high alcohol intake involves genetic factors that facilitate alcohol-induced adaptations in glutamate release within the NAC.

Accumbens neurochemical adaptations produced by binge-like alcohol consumption

RATIONALE

The Scheduled High Alcohol Consumption (SHAC) binge drinking model is a simple, partial murine model with which to investigate some of the neurobiological underpinnings of alcoholism.

OBJECTIVES

The SHAC model was used to characterize monoamine and amino acid adaptations produced in the nucleus accumbens (NAC) by repeated bouts of high alcohol consumption.

METHODS

In vivo microdialysis was conducted in the NAC of C57BL/6J (B6) mice during consumption of water, a 5% alcohol (v/v) solution for the first time (SHAC1) or a 5% alcohol solution for the sixth time (SHAC6). A second set of microdialysis experiments assessed the neurotransmitter response to an alcohol challenge injection (1.5 or 2 g/kg, IP).

RESULTS

In both drinking experiments, SHAC1 and SHAC6 mice consumed comparable amounts of alcohol during the 40-min period of alcohol availability (approximately 1.5 g/kg) and total fluid intake was similar between water and SHAC1/6 mice. Despite the similarity in alcohol consumption, alcohol-mediated increases in the extracellular concentration of GABA and serotonin were reduced, but glutamate was increased in the NAC of SHAC6 mice, relative to SHAC1 animals. No differences were observed in extracellular dopamine between SHAC1 and SHAC6 mice during alcohol consumption. After alcohol injection, SHAC6 mice also exhibited sensitized glutamate release, but did not differ from water or SHAC1 animals for any of the other neurotransmitters examined. Brain alcohol concentrations did not differ between groups after injection.

CONCLUSIONS

Repeated bouts of high alcohol consumption induce an imbalance between inhibitory and excitatory neurotransmission within the NAC that may drive excessive drinking behavior.

Behavioral and neurochemical interactions between Group 1 mGluR antagonists and ethanol: potential insight into their anti-addictive properties

Blockade of the mGluR5 subtype of Group 1 metabotropic glutamate receptor (mGluRs) reduces the rewarding effects of ethanol (EtOH), while the effects of mGluR1a blockade remain under-investigated. The present study compared the effects of pretreatment with the mGluR5 antagonist MPEP and the mGluR1a antagonist CPCCPOEt upon behavioral and neurochemical variables associated with EtOH reward in alcohol-preferring C57BL/6J mice. Pretreatment with either antagonist (0-10 mg/kg, IP) dose-dependently reduced measures of EtOH reward in an operant self-administration paradigm and the maximally effective antagonist dose (10 mg/kg) also blocked the expression of EtOH-induced place conditioning, as well as EtOH consumption under 24-h free-access conditions. MPEP pretreatment did not significantly alter the EtOH dose-locomotor response function; however, it prevented EtOH-induced changes in extracellular dopamine, glutamate and GABA in the nucleus accumbens (NAC). In contrast, CPCCOEt shifted the EtOH dose-response function downwards, enhanced the capacity of higher EtOH doses to elevate NAC levels of GABA and lowered extracellular dopamine and glutamate below baseline following EtOH injection. It is suggested that the "anti-alcohol" effects of MPEP may involve an attenuation of the neurochemical signals mediating EtOH reward, whereas those of CPCCOEt may involve an increased sensitivity to the inhibitory effects of EtOH upon brain and behavior.

Homer2 is necessary for EtOH-induced neuroplasticity

Homer proteins are integral to the assembly of proteins regulating glutamate signaling and synaptic plasticity. Constitutive Homer2 gene deletion [knock-out (KO)] and rescue with adeno-associated viral (AAV) transfection of Homer2b was used to demonstrate the importance of Homer proteins in neuroplasticity produced by repeated ethanol (EtOH) administration. Homer2 KO mice avoided drinking high concentrations of EtOH and did not develop place preference or locomotor sensitization after repeated EtOH administration. The deficient behavioral plasticity to EtOH after Homer2 deletion was paralleled by a lack of augmentation in the rise in extracellular dopamine and glutamate elicited by repeated EtOH injections. The genotypic differences in EtOH-induced change in behavior and neurochemistry were essentially reversed by AAV-mediated transfection of Homer2b into accumbens cells including, differences in EtOH preference, locomotor sensitization, and EtOH-induced elevations in extracellular glutamate and dopamine. These data demonstrate a necessary and active role for accumbens Homer2 expression in regulating EtOH-induced behavioral and cellular neuroplasticity.

Lower packing density of glial fibrillary acidic protein-immunoreactive astrocytes in the prelimbic cortex of alcohol-naive and alcohol-drinking alcohol-preferring rats as compared with alcohol-nonpreferring and Wistar rats

BACKGROUND

Low packing density of glial cells, possibly astrocytes, has been described in the prefrontal cortex and hippocampus of "uncomplicated" alcoholics. Astrocytes perform crucial support functions in the processing of neurotransmitters and transfer of energy substrates from blood to cortical neurons. It is still unknown whether attrition in the numbers of astrocytes is only a consequence of prolonged alcohol abuse or also predates the exposure to alcohol in subjects at risk for alcohol dependence.

METHODS

We used alcohol-preferring (P) rats exposed ad libitum for 2 or 6 months to either water only or 10% ethanol and alcohol-nonpreferring (NP) rats and nonselected Wistar rats exposed only to water for 2 months. Sections through the rat frontal cortex were immunostained for glial fibrillary acidic protein (GFAP), a specific marker of astrocytes. The packing density of GFAP-immunoreactive (IR) astrocytes and the area fraction of GFAP immunoreactivity were measured in the prelimbic cortex (PLC) using the dissector probe and analysis of binary images of GFAP immunostaining, respectively.

RESULTS

The packing density of GFAP-IR astrocytes was significantly lower in both alcohol-naive and alcohol-exposed P rats than in NP rats or Wistar rats. The area fraction of GFAP immunoreactivity was significantly lower in the alcohol-exposed P rats than in NP rats, Wistar rats, and alcohol-naive P rats.

CONCLUSION

These results suggest that low density of GFAP-IR astrocytes in the PLC of P rats predates the exposure to alcohol and might be a factor contributing to the increased risk for alcohol dependence. In addition, prolonged free-choice alcohol drinking may reduce the extent of GFAP-IR processes in the PLC of P rats.

Phorbol Ester Differentiates the Levels of [3H]MK-801 Binding in Rats Lines Selected for Differential Sensitivity to the Hypnotic Effects of Ethanol

These studies addressed the possible involvement between sensitivity to the hypnotic action of ethanol and function of the NMDA receptor. The studies were carried out using high-alcohol sensitive (HAS) and low-alcohol sensitive (LAS) rats, two rats having differential sensitivity to the acute hypnotic action of ethanol. The animal models were developed by a selective breeding experiment. Using a quantitative autoradiograph technique, it was demonstrated that [3H]MK-801 binding to the NMDA receptor was highest in hippocampus in both HAS and LAS rats, but significant [3H]MK-801 binding was also detected in cortex, caudate-putamen, and thalamus of HAS and LAS rats. The density of [3H]MK-801 binding was lower only in cerebellar granule layers of untreated HAS rats as compared to the same brain area in untreated LAS rats. Activation of protein kinase C (PKC) by 100 nM PDBu, increased [3H]MK-801 binding in cortex, caudate-putamen, thalamus, central gray, and cerebellum of HAS rats but activation of PKC did not influence [3H]MK-801 binding in LAS rats. These activation of PKC differentiates between [3H]MK-801 binding of HAS and LAS rats in frontal cortex (layer II-IV and cingulate), caudate-putamen, and ventral lateral thalamic nuclei. The basal level of PKC-gamma mRNA was higher in HAS rats than that of LAS rats. These results suggest that the activation of PKC potentiates NMDA receptor function of the rat line which is more sensitive to alcohol (HAS) but does not affect [3H]MK-801 binding of alcohol resistant (LAS) rats.

Involvement of the corticostriatal glutamatergic pathway in ethanol-induced ascorbic acid release in rat striatum

The mechanism of ethanol-induced ascorbic acid (AA) release in striatum is not well understood. In the present work, the possible involvement of NMDA receptors in the corticostriatal pathway was studied by microdialysis coupled to high performance liquid chromatography with electrochemical detection. Ethanol (3.0 g/kg i.p.) stimulated significant striatal AA release to more than 200% above the baseline. This effect of ethanol could be partially antagonized by amantadine, a non-selective NMDA receptor antagonist and dopamine releaser, at a dose of 200 mg/kg i.p. and significantly antagonized by MK-801, a non-competitive NMDA receptor antagonist, at the doses of 0.5 and 1.0 mg/kg i.p. Furthermore, deafferentation of the glutamatergic projection from cortex to striatum by undercutting the prefrontal cortex completely eliminated ethanol-induced AA release in rat striatum. The basal level of AA in striatum could only be reduced by high doses of MK-801, but not by low doses of MK-801, amantadine or decortication. The results further confirm that NMDA receptors are involved in ethanol-induced AA release and provide the first evidence for the necessity of the activation of corticostriatal glutamatergic pathway in ethanol-induced AA release in rat striatum.

Possible role of glutamatergic neurotransmission in regulating ethanol-evoked brain ascorbate release

It was found that systemic application of ethanol induced brain ascorbate (AA) release. In order to study the mechanism of ethanol-evoked AA release, the role of brain glutamatergic neurotransmission was investigated using in vivo voltammetry in the striatum of freely moving rats. Pretreatment with L-trans-pyrrolidine-2,4-dicarboxylate (PDC, 10 nmol, i.c.v.), a glutamate (Glu) uptake blocker, potentiated ethanol (1 g/kg, intraperitoneal injection, i.p.)-evoked release of brain AA. N-methyl-D-aspartate (NMDA, 1 nmol, i.c.v.) produced a fast transient increase in extracellular AA, whereas alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA, 1 nmol, i.c.v.) produced a decrease in extracellular AA (75.8 +/- 3% of control). Kainate (KA, 1 nmol, i.c.v.) produced an initial decrease (48.7 +/- 11.7% of control) then an increase (250 +/- 68.5% of control) in extracellular AA. These results suggest that systemic administration of ethanol may affect the release or uptake of brain glutamatergic neurotransmitters which appear to regulate brain AA release. The NMDA, but not the non-NMDA, type of Glu receptor may be responsible for this regulation.

NMDA enhances the central depressant properties of ethanol in mice

Male Swiss-Webster mice were used to examine the effect of NMDA on the ethanol-induced loss of the righting reflex (LORR). The LORR was used as a measure of CNS depression. Immediately after animals regained the righting reflex following ethanol injection (4.0 g/kg, IP) mice received an ICV injection of saline or NMDA (10, 50, 100, or 500 nmol/kg) in a volume of 5 microliters. Upon ICV injection of NMDA, mice again lost the righting reflex and this effect of NMDA in the presence of ethanol occurred rapidly and in a dose-dependent manner. In another experiment DL-2-amino-5-phosphonovaleric acid (APV), a competitive antagonist of NMDA, was given ICV with NMDA (50 nmol/kg) in the presence of ethanol. APV (10 and 100 nmol/kg, ICV) significantly attenuated the response of NMDA to enhance the depressant action of ethanol. When bicuculline methiodide, an antagonist of GABA, was given ICV with NMDA (50 nmol/kg), bicuculline methiodide reduced the effect of NMDA to produce a second loss of the righting reflex (return to the LORR) in the presence of ethanol. When NMDA (100 nmol/kg, ICV) was injected in the absence of ethanol into mice, NMDA by itself did not produce a loss of the righting reflex. In this investigation, the results suggest that NMDA can augment ethanol-induced depression possibly through an interaction between glutamatergic and GABAergeric systems in the CNS.

Effect of ethanol on ascorbate release in the nucleus accumbens and striatum of freely moving rats

An in vivo voltammetry technique was used to monitor the extracellular ascorbate (AA) concentration in the nucleus accumbens and striatum of unanesthetized, freely moving rats. A single injection of ethanol, 1.0 g/kg intraperitoneally (IP), induced a significant increase in extracellular AA concentration in both the nucleus accumbens and striatum. This effect was dose dependent within a dose range from 0.5-2.0 g/kg. 4-Methylpyrazole (50 mg/kg, IP), which inhibits alcoholdehydrogenase, could not prevent the increase in AA concentration, evoked by ethanol. Furthermore, systemic administration of acetaldehyde (20 mg/kg, IP), the main metabolite of ethanol, did not have any effect on the level of AA in the nucleus accumbens or striatum. These results show that ethanol can alter the brain extracellular AA levels and that this effect seems to be attributed to ethanol itself and not to acetaldehyde. Consequently, these results indicate that a role for AA in the action of ethanol in the brain should be considered.

NMDA agonists and antagonists alter the hypnotic response to ethanol in LS and SS mice

Involvement of glutamate neurotransmission in the differential response of long-sleep (LS) and short-sleep (SS) mice to acute ethanol was examined by measuring the effect of centrally administered glutamate receptor agonists and antagonists on blood ethanol concentration (BEC) at loss of righting response following intragastric administration of ethanol. NMDA coinjected with glycine, and quinolinic acid (QA), decreased sensitivity to ethanol in both lines of mice. SS mice were more sensitive to QA than were LS. The NMDA antagonists 2-amino-5-phosphonovaleric acid (APV), MK-801 and an inhibitor of glutamate synthesis, methionine sulfoximine, increased sensitivity to ethanol in both lines of mice. MK-801 effects were line dependent with SS being more sensitive. In addition, coinjection of APV, Mg++ or Zn++ with QA blocked the decrease in sensitivity seen with QA alone. These results demonstrate that NMDA agonists and antagonists alter the acute hypnotic response to ethanol in both LS and SS mice, and support the hypothesis that ethanol exerts its effects in part by altering glutamatergic neurotransmission.

Temperature dependence of the microsomal oxidation of ethanol by cytochrome P450 and hydroxyl radical-dependent reactions

The temperature dependence and activation energies for the oxidation of ethanol by microsomes from controls and from rats treated with pyrazole was evaluated to determine whether the overall mechanism for ethanol oxidation by microsomes was altered by the pyrazole treatment. Arrhenius plots of the temperature dependence of ethanol oxidation by pyrazole microsomes were linear and exhibited no transition breaks, whereas a slight break was observed at about 20 +/- 2.5 degrees C with control microsomes. Energies of activation (about 15-17 kcal/mol) were identical for the two microsomal preparations. Although transition breaks were noted for the oxidation of substrates such as dimethylnitrosamine and benzphetamine, activation energies for these two substrates were similar for control microsomes and microsomes from the pyrazole-treated rats. The addition of ferric-EDTA to the microsomes increased the rate of ethanol oxidation by a hydroxyl radical (.OH)-dependent pathway. Arrhenius plots of the .OH-dependent oxidation of ethanol by both microsomal preparations were linear with energies of activation (about 7 kcal/mol) that were considerably lower than values found for the P450-dependent pathway. These results suggest that, at least in terms of activation energy, the increase in microsomal ethanol oxidation by pyrazole treatment is not associated with any apparent change in the overall mechanism or rate-limiting step for ethanol oxidation but likely reflects induction of a P450 isozyme with increased activity toward ethanol. The lower activation energy for the .OH-dependent oxidation of ethanol suggests that different steps are rate limiting for oxidation of ethanol by .OH and by P450, which may reflect the different enzyme components of the microsomal electron transfer system involved in these reactions.