Distribution of GABAA receptors in the limbic system of alcohol-preferring and non-preferring rats: in situ hybridisation histochemistry and receptor autoradiography

Animal Models of Depression: What Can They Teach Us about the Human Disease?

Depression is apparently the most common psychiatric disease among the mood disorders affecting about 10% of the adult population. The etiology and pathogenesis of depression are still poorly understood. Hence, as for most human diseases, animal models can help us understand the pathogenesis of depression and, more importantly, may facilitate the search for therapy. In this review we first describe the more common tests used for the evaluation of depressive-like symptoms in rodents. Then we describe different models of depression and discuss their strengths and weaknesses. These models can be divided into several categories: genetic models, models induced by mental acute and chronic stressful situations caused by environmental manipulations (i.e., learned helplessness in rats/mice), models induced by changes in brain neuro-transmitters or by specific brain injuries and models induced by pharmacological tools. In spite of the fact that none of the models completely resembles human depression, most animal models are relevant since they mimic many of the features observed in the human situation and may serve as a powerful tool for the study of the etiology, pathogenesis and treatment of depression, especially since only few patients respond to acute treatment. Relevance increases by the fact that human depression also has different facets and many possible etiologies and therapies.

Dynamic changes in extracellular release of GABA and glutamate in the lateral septum during social play behavior in juvenile rats: Implications for sex-specific regulation of social play behavior

Social play is a motivated and rewarding behavior that is displayed by nearly all mammals and peaks in the juvenile period. Moreover, social play is essential for the development of social skills and is impaired in social disorders like autism. We recently showed that the lateral septum (LS) is involved in the regulation of social play behavior in juvenile male and female rats. The LS is largely modulated by GABA and glutamate neurotransmission, but their role in social play behavior is unknown. Here, we determined whether social play behavior is associated with changes in the extracellular release of GABA and glutamate in the LS and to what extent such changes modulate social play behavior in male and female juvenile rats. Using intracerebral microdialysis in freely behaving rats, we found no sex difference in extracellular GABA concentrations, but extracellular glutamate concentrations are higher in males than in females under baseline conditions and during social play. This resulted in a higher glutamate/GABA concentration ratio in males vs. females and thus, an excitatory predominance in the LS of males. Furthermore, social play behavior in both sexes is associated with significant increases in extracellular release of GABA and glutamate in the LS. Pharmacological blockade of GABA-A receptors in the LS with bicuculline (100 ng/0.5 μl, 250 ng/0.5 μl) dose-dependently decreased the duration of social play behavior in both sexes. In contrast, pharmacological blockade of ionotropic glutamate receptors (NMDA and AMPA/kainate receptors) in the LS with AP-5+CNQX (2mM+0.4mM/0.5 μl, 30 mM+3mM/0.5 μl) dose-dependently decreased the duration of social play behavior in females, but did not alter social play behavior in males. Together, these data suggest a role for GABA neurotransmission in the LS in the regulation of juvenile social play behavior in both sexes, while glutamate neurotransmission in the LS is involved in the sex-specific regulation of juvenile social play behavior.

γ-Aminobutyric acid neural signaling in the lateroanterior hypothalamus modulates aggressive behavior in adolescent anabolic/androgenic steroid-treated hamsters

Male Syrian hamsters (Mesocricetus auratus) treated with anabolic/androgenic steroids (AAS) during adolescence (P27-P56) display highly escalated and mature forms of offensive aggression correlated with increased γ-aminobutyric acid (GABA) afferent development as well as decreased GABAA receptors in the lateroanterior hypothalamus (LAH) - an area of convergence for developmental and neuroplastic changes that underlie offensive aggressive behaviors in hamsters. This study investigated whether microinfusion of a GABAA receptor agonist (muscimol; 0.01-1.0 pmol/l) or antagonist (bicuculline; 0.04-4.0 pmol/l) directly into the LAH modulate adolescent AAS-induced offensive aggression. Activation of LAH GABAA receptors enhanced adolescent AAS-induced offensive aggression, beginning at the 0.1 pmol/l dose, when compared with AAS-treated animals injected with saline into the LAH. Importantly, GABAA receptor agonism within the LAH significantly increased the frequency of belly/rear attacks, while simultaneously decreasing the frequency of frontal attacks. These data identify a neuroanatomical locus where GABAA receptor activation functions to enhance aggression in adolescent AAS-treated animals, while also promoting the display of mature forms of aggression and suppressing juvenile play behaviors.

Activation of GABAA and GABAB receptors in the lateral septum increases sucrose intake by differential stimulation of sucrose licking activity

The present study was aimed to determine how direct injections into the lateral septum (LS) of muscimol and baclofen, GABAA and GABAB receptor agonists, respectively, affect intake of 10% sucrose and sucrose licking activity in rats. The effects of muscimol and baclofen on the 1-h intake of sucrose and sucrose licking activity were tested at low (350pmol), medium (876pmol), and high (1752pmol) doses. The medium and high doses of muscimol and the high dose of baclofen significantly increased 1-h sucrose intake. The total sucrose lick number was significantly increased by the medium dose of muscimol and the high dose of baclofen. An increase in sucrose licking activity induced by muscimol but not baclofen occurred in the first 15min after injections. The medium and high doses of muscimol but not baclofen significantly decreased latency to initiate the first lick of sucrose. The total licking time calculated as the sum of the duration of all sucrose lick clusters showed a significant increase by the high dose of baclofen but not by any dose of muscimol. Therefore, the GABAA and GABAB LS mechanisms appear to be involved in stimulating sucrose intake, but this stimulation occurs by differential regulation of the sucrose licking activity. Muscimol intra-LS administration led to a short-latency rapid increase in sucrose licking. In contrast, baclofen did not decrease latency to initiate licking, but significantly increased total licking duration.

Relaxin-3 mRNA levels in nucleus incertus correlate with alcohol and sucrose intake in rats

BACKGROUND

Chronic alcohol intake produces multiple neuroadaptive changes, including up- and down-regulation of neuropeptides and receptors. There are widespread projections of relaxin-3 containing neurons to, and abundant relaxin family peptide 3 receptor (RXFP3) expression within, brain regions involved in modulating alcohol intake. Recently we demonstrated the involvement of relaxin-3/RXFP3 signalling in alcohol-seeking in rats; therefore in this study we examined whether relaxin-3 and/or RXFP3 expression were altered by chronic alcohol intake in alcohol-preferring iP rats.

METHODS

Expression of relaxin-3 mRNA in the hindbrain nucleus incertus and RXFP3 radioligand binding levels in discrete forebrain regions were investigated following voluntary intake of alcohol or sucrose for 12 weeks, with a 2 day washout, using quantitative in situ hybridisation histochemistry and in vitro receptor autoradiography, respectively, in cohorts of adult, male iP rats.

RESULTS

Levels of relaxin-3 mRNA in the hindbrain nucleus incertus were positively correlated with the level of intake of both alcohol (r(12)=0.59, p=0.03) and sucrose (r(7)=0.70, p=0.04) in iP rats. Dense binding of the RXFP3-selective radioligand, [(125)]-R3/I5, was detected in hypothalamic and extrahypothalamic sites, but no significant changes in the density of RXFP3 were observed in the brain regions quantified following chronic sucrose or ethanol intake.

CONCLUSIONS

Our findings suggest high endogenous relaxin-3 expression may be associated with higher intake of rewarding substances, rather than its expression being regulated in response to their intake, consistent with an active role for the relaxin-3/RXFP3 system in modulating ingestive and alcohol-related behaviours.

Neurobiology of consummatory behavior: mechanisms underlying overeating and drug use

Consummatory behavior is driven by both caloric and emotional need, and a wide variety of animal models have been useful in research on the systems that drive consumption of food and drugs. Models have included selective breeding for a specific trait, manipulation of gene expression, forced or voluntary exposure to a substance, and identification of biomarkers that predict which animals are prone to overconsuming specific substances. This research has elucidated numerous brain areas and neurochemicals that drive consummatory behavior. Although energy homeostasis is primarily mediated by the hypothalamus, reinforcement is more strongly mediated by nuclei outside the hypothalamus, in mesocorticolimbic regions. Orexigenic neurochemicals that control food intake can provide a general signal for promoting caloric intake or a more specific signal for stimulating consumption of a particular macronutrient, fat, carbohydrate, or protein. The neurochemicals involved in controlling fat ingestion--galanin, enkephalin, orexin, melanin-concentrating hormone, and the endocannabinoids--show positive feedback with this macronutrient, as these peptides both increase fat intake and are further stimulated by its intake. This positive association offers some explanation for why foods high in fat are so often overconsumed. Consumption of ethanol, a drug of abuse that also contains calories, is similarly driven by the neurochemical systems involved in fat intake, according to evidence that closely relates fat and ethanol consumption. Further understanding of the systems involved in consummatory behavior will enable the development of effective therapies for the treatment of both overeating and drug abuse.

Chronic binge-like moderate ethanol drinking in rats results in widespread decreases in brain serotonin, dopamine, and norepinephrine turnover rates reversed by ethanol intake

This research was initiated to assess the turnover rates (TORs) of dopamine (DA), norepinephrine (NA), serotonin (5-HT), aspartate, glutamate, and GABA in brain regions during rodent ethanol/sucrose (EtOH) and sucrose (SUC) drinking and in animals with a history of EtOH or SUC drinking to further characterize the neuronal systems that underlie compulsive consumption. Groups of five male rats were used, with two trained to drink EtOH solutions, two to drink SUC and one to serve as a non-drinking control. When stable drinking patterns were obtained, rats were pulse labeled intravenously and killed 60 or 90 min later and the TORs of DA, norepinephrine, 5-HT, aspartate, glutamate, and GABA determined in brain regions. Changes in the TOR of 5-HT, DA, and NA were detected specific to EtOH drinking, SUC drinking or a history of EtOH or SUC drinking. An acute EtOH deprivation effect was detected that was mostly reversed with EtOH drinking. These results suggest that binge-like drinking of moderate amounts of EtOH produces a deficit in neuronal function that could set the stage for the alleviation of anhedonic stimuli with further EtOH intake that strengthen EtOH seeking behaviors which may contribute to increased EtOH use in at risk individuals.

Ethanol-induced locomotor sensitization in DBA/2J mice is associated with alterations in GABA(A) subunit gene expression and behavioral sensitivity to GABA(A) acting drugs

Repeated exposure to ethanol may produce increased sensitivity to its acute locomotor stimulant actions, a process referred to as locomotor sensitization. Neuroadaptation within certain brain circuits, including those possessing GABA(A) receptors, may underlie locomotor sensitization to ethanol. Indeed, GABA(A) receptors are documented mediators of ethanol's cellular and behavioral actions. Moreover, because subunit composition of this receptor is predictive of its pharmacology, it is possible that alterations in subunit composition contribute to the expression of locomotor sensitization to ethanol. The goal of the present study was to determine if alterations in GABA(A) subunit composition are associated with the expression of locomotor sensitization in DBA/2J mice, a strain known to be particularly susceptible to the development of this behavioral phenomenon. Following a modified 14day sensitization procedure (Phillips et al., 1994) relative changes in GABA(A) subunit gene expression were assessed in discrete mesolimbic brain regions. To determine if the observed changes in gene expression produced functional changes in the locomotor responses to drugs known to either preferentially or generally activate GABA(A) receptors normally possessing the significantly altered subunits, separate cohorts of animals were challenged with one of several low doses of zolpidem (alpha1-selective), etomidate (beta2/3-selective), or flurazepam (gamma2-directed) and assessed for locomotor alterations. Sensitized animals displayed increased expression of the alpha1, beta2, and gamma2 (v1) subunits in the Nucleus Accumbens (NAc) but not Ventral Tegmental Area (VTA). Additionally, sensitized animals displayed altered sensitivity to the locomotor actions of etomidate and flurazepam. These results support the hypothesis that neuroadaptive changes in GABA(A) subunit composition participate in the expression of locomotor sensitization.

A conductor hidden in the orchestra? Role of the habenular complex in monoamine transmission and cognition

Influences of the habenular complex on electrophysiological and neurochemical aspects of brain functioning are well known. However, its role in cognition has been sparsely investigated until recently. The habenular complex, composed of medial and lateral subdivisions, is a node linking the forebrain with midbrain and hindbrain structures. The lateral habenula is the principal actor in this direct dialogue, while the medial habenula mostly conveys information to the interpeduncular nucleus before this modulates further regions. Here we describe neuroanatomical and physiological aspects of the habenular complex, and its role in cognitive processes, including new behavioral, electrophysiological and imaging findings. Habenular complex lesions result in deficits in learning, memory and attention, some of which decline during repeated testing, while others become worse, consistent with multiple roles in cognition. The habenular complex is particularly responsive to feedback about errors. Electrophysiological studies indicate a role in metaplasticity, the modulation of neuroplasticity. These studies thus reveal important roles of the habenular complex in learning, memory and attention.

Depressive-like behavior and high alcohol drinking co-occur in the FH/WJD rat but appear to be under independent genetic control

This review will consider the evidence supporting the view that a specific substrain of Fawn-Hooded rat (FH/Wjd) exhibits co-occurring depressive-like behavior and high alcohol intake independently. First, the FH/Wjd rat is compared with other Fawn-Hooded substrains (FH/Har, FHH/Eur, FHL/Eur) and it is concluded that only the FH/Wjd rat is both highly immobile in the forced swim test and drinks substantial amounts of 5-10% alcohol voluntarily. Next it is demonstrated that the FH/Wjd rat fulfils many of the criteria proposed for an animal model of alcoholism (becomes tolerant, becomes dependent and expresses withdrawal symptoms, bar-presses for alcohol). Other literature in addition to the high swim test immobility suggests that the FH/Wjd rat may also be an animal model of depression (high basal corticosterone levels, blunted hormonal responses to serotonergic agonists). To study the phenotypes more closely an inbred strain (ACI/N) of rat that drank little alcohol voluntarily and exhibited considerable swimming in the forced swim test (i.e., low immobility) was obtained. A systematic intercrossing of the parental strains and the resulting F1 progeny was carried out to generate more than 800 F2s. Swim test immobility, alcohol intake and preference and saccharin intake are four of the 7 variables assessed in each of these rats. Using classical quantitative genetics methods, it was determined that these four phenotypes exhibited modest heritability and were influenced by multiple genes. Correlation coefficients between immobility and the other measures were near zero, whereas alcohol intake and preference were highly correlated (r=0.9) and alcohol and saccharin intakes were modestly correlated (r=0.3). A final study showed that chronic fluoxetine treatment counteracted the high immobility but did not affect alcohol intake, similar to human studies. These findings suggest that although depressive-like behavior and high alcohol intake co-occur in the FH/Wjd rat, they are independently regulated.

Modulation of high alcohol drinking in the inbred Fawn-Hooded (FH/Wjd) rat strain: implications for treatment

The Fawn-Hooded rat (FH/Wjd) is an inbred alcohol-preferring rat strain, unlike most of the other strains that were selectively bred for high alcohol intake and preference. It was chosen for study some 16 years ago because of a reported mutation that disrupted platelet serotonin function. Although the FH/Wjd rat has high alcohol intake (>5 g/kg/day) and preference (>65%), interbreeding with an alcohol-non-preferring inbred strain suggested that these measures are unrelated to the serotonin abnormality. Similarly, the exaggerated immobility of the FH/Wjd rats in the forced swim test did not correlate with the high alcohol intake. Many compounds have been tested in the FH/Wjd rats after both acute and chronic treatment and a substantial number of them have proved effective. However, as the case with opiate antagonists, tolerance to the effects of the drug can develop. An up-regulation of opioid receptors accompanied the chronic treatment and this mechanism may account for the development of tolerance. Tolerance to opiate antagonists has also been demonstrated in two of the selectively bred alcohol-preferring rat lines, but it is unknown whether this process may contribute to the relapses seen in individuals being treated with naltrexone. Other drugs that reliably decrease alcohol intake in the FH/Wjd rats include the 5-hydroxytryptamine-2A receptor antagonist, amperozide, the mGlu5 receptor antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) and herbal derivatives such as ibogaine, St. John's wort and kudzu extract. Thus, studies in the FH/Wjd rat have led to the discovery of a wide variety of targets for the development of novel agents to treat alcoholism. The fact that several of these drugs were shown to reduce alcohol intake in some of the selectively bred alcohol-preferring rat lines and/or alcohol-preferring vervet monkeys increases our confidence that they are good candidates for further development.

Comparison of ethanol preference and neurochemical measures of mesolimbic dopamine and adenosine systems across different strains of mice

BACKGROUND

To extend the known phenotype of strains commonly used in the development of mutant mice, ethanol, saccharin, and caffeine preferences were examined in C57Bl/6J, CD-1, and hybrid C57Bl/6J x CD-1 mice. As dopaminergic mechanisms are inherently involved in the neuronal processing of many drugs of abuse (including ethanol), and an important role for adenosine-dopamine interactions has also been reported, the dopaminergic and purinergic neurochemical profiles of mice were compared against the consummatory phenotype observed.

METHODS

Ethanol (5% v/v), saccharin (0.1% w/v), and caffeine (0.1% w/v) consumption and preference were examined using a 2-bottle free-choice paradigm. Dopamine and adenosine receptor and transporter mRNA and protein density were quantified using in situ hybridization histochemistry and in vitro autoradiography, respectively.

RESULTS

C57Bl/6J and hybrid C57Bl/6J x CD-1 mice demonstrated a clear ethanol preference, voluntarily consuming large quantities of ethanol when given the choice between drinking vessels containing either ethanol or water. Conversely, CD-1 mice were characterized as ethanol-avoiding under the present paradigm. Differences in D(1) receptor mRNA between the strains were consistent with the observed behavioral differences in ethanol preference. The high ethanol-preferring phenotype of C57Bl/6J mice could not be directly linked to alterations in dopamine transporter neurochemistry and/or enkephalin levels as proposed by earlier researchers. Ethanol-seeking behavior appeared to correlate with D2 receptor expression, however, with evidence that ethanol-preferring mice also exhibit an increased density of D2 receptors within limbic dopaminergic projection nuclei. Interestingly, strain differences in the expression of the ethanol-sensitive nucleoside transporter paralleled differences in ethanol consumption, a novel finding consonant with purinergic involvement in dopamine-related behaviors.

CONCLUSIONS

This study has highlighted the relevance of alterations in dopamine receptor expression and purinergic modulation within the mesolimbic pathway and predisposition toward the development of ethanol-seeking behavior.

The GABAB receptor allosteric modulator CGP7930, like baclofen, reduces operant self-administration of ethanol in alcohol-preferring rats

GABA systems have been implicated as targets for ethanol at the cellular, molecular and behavioural level. The present study was designed to further examine the potential of the GABA(B) receptor as a target for regulating operant alcohol responding. Given that the prototypic agonist, baclofen, reduces the self-administration of alcohol, we hypothesized that the GABA(B) receptor allosteric modulator, CGP7930, might have similar actions but a reduced side-effect profile. In this context, inbred alcohol-preferring (iP) rats were trained to respond for 10% v/v ethanol in a fixed ratio paradigm; all drug testing was performed under an FR3 schedule. Both baclofen and CGP7930 independently reduced voluntary responding for 10% ethanol in a dose-related manner. Neither drug impacted upon responding for water. A combination of subthreshold doses of baclofen and CGP7930 was also able to reduce operant responding for ethanol, suggesting that CGP7930 is indeed acting to facilitate GABA(B) receptor-mediated signalling in this paradigm. These data demonstrate the potential of positive allosteric modulators of metabotropic GABA(B) receptors to regulate alcohol responding.

Receptor crosstalk: characterization of mice deficient in dopamine D1 and adenosine A2A receptors

Here we report the development of D1A2A receptor knockout mice to investigate whether interactions between dopamine D1 and adenosine A2A receptors participate in reward-related behavior. The combined deletion of D1 and A2A receptors resulted in mice with decreased weight and appetitive processes, reduced rearing and exploratory behaviors, increased anxiety, and a significantly poorer performance on the rotarod, compared to wild-type littermates. D1A2A receptor knockout mice shared phenotypic similarities with mice deficient in D1 receptors, while also paralleling behavioral deficits seen in A2A receptor knockout mice, indicating individual components of the behavioral phenotype of the D1A2A receptor knockout attributable to the loss of both receptors. In contrast, ethanol and saccharin preference in D1A2A receptor knockout mice were distinctly different from that observed in derivative D1 or A2A receptor-deficient mice. Compared to wild types, preference and consumption of ethanol were decreased in D1A2A receptor knockout mice, the reduction in ethanol consumption greater even than that seen in D1 receptor-deficient mice. Preference and consumption of saccharin were also reduced in D1A2A receptor knockout mice, whereas saccharin preference was similar in wild-type, D1, and A2A receptor knockout mice. These data suggest an interaction of D1 and A2A receptors in the reinforcement processes underlying the intake of rewarding substances, whereby the A2A receptor seems involved in goal-directed behavior and the motor functions underlying the expression of such behaviors, and the D1 receptor is confirmed as essential in mediating motivational processes related to the repeated intake of novel substances and drugs.

Genetic interdependence of adenosine and dopamine receptors: Evidence from receptor knockout mice

Dopamine and adenosine receptors are known to share a considerable overlap in their regional distribution, being especially rich in the basal ganglia. Dopamine and adenosine receptors have been demonstrated to exhibit a parallel distribution on certain neuronal populations, and even when not directly co-localized, relationships (both antagonistic and synergistic) have been described. This study was designed to investigate dopaminergic and purinergic systems in mice with ablations of individual dopamine or adenosine receptors. In situ hybridization histochemistry and autoradiography was used to examine the level of mRNA and protein expression of specific receptors and transporters in dopaminergic pathways. Expression of the mRNA encoding the dopamine D2 receptor was elevated in the caudate putamen of D1, D3 and A2A receptor knockout mice; this was mirrored by an increase in D2 receptor protein in D1 and D3 receptor knockout mice, but not in A2A knockout mice. Dopamine D1 receptor binding was decreased in the caudate putamen, nucleus accumbens, olfactory tubercle and ventral pallidum of D2 receptor knockout mice. In substantia nigra pars compacta, dopamine transporter mRNA expression was dramatically decreased in D3 receptor knockout mice, but elevated in A2A receptor knockout mice. All dopamine receptor knockout mice examined exhibited increased A2A receptor binding in the caudate putamen, nucleus accumbens and olfactory tubercle. These data are consistent with the existence of functional interactions between dopaminergic and purinergic systems in these reward and motor-related brain regions.

Repeated cocaine exposure during adolescence alters glutamic acid decarboxylase-65 (GAD65) immunoreactivity in hamster brain: correlation with offensive aggression

Male Syrian hamsters (Mesocricetus auratus) treated with low-dose (0.5 mg/kg/day) cocaine throughout adolescence (P27-P56) display highly escalated offensive aggression. The current study examined whether adolescent cocaine exposure influenced the immunohistochemical localization of glutamic acid decarboxylase-65 (GAD65), the rate-limiting enzyme in the synthesis of gamma-aminobutyric acid (GABA), a fast-acting neurotransmitter implicated in the modulation of aggression in various species and models of aggression. Hamsters were administered low doses of cocaine throughout adolescence, scored for offensive aggression using the resident-intruder paradigm, and then examined for changes in GAD65 immunoreactivity in areas of the brain implicated in aggression control. When compared with saline-treated control animals, aggressive cocaine-treated hamsters showed significant differences in the area covered by GAD65 puncta in several notable aggression regions, including the anterior hypothalamus, the medial and central amygdaloid nuclei, and the lateral septum. However, no differences in GAD65 puncta were found in other aggression areas, such as the bed nucleus of the stria terminalis, the ventrolateral hypothalamus, and the corticomedial amygdala. Together, these results suggest that altered GABA synthesis and function in specific aggression areas may be involved in adolescent cocaine-facilitated offensive aggression.

High trait anxiety and hyporeactivity to stress of the dorsomedial prefrontal cortex: a combined phMRI and Fos study in rats

The neural basis of trait anxiety is poorly understood. In genetically selected hyperanxious (high anxiety-related behavior; HAB) rats, diazepam induces a stronger anxiolytic response than in hypoanxious (low anxiety-related behavior; LAB) rats. A screen for neuronal response differences to diazepam between HAB and LAB rats using pharmacologic fMRI (phMRI) at 7 T revealed a blunted diazepam-induced neuronal deactivation in the dorsomedial prefrontal cortex (dmPFC) of HABs. This was not due to reduced benzodiazepine (BDZ) receptor densities in this region. Instead, dmPFC tissue oxygenation at baseline was found to be significantly lower in HABs. This suggests a tonic relative hypoactivity under the highly stressful phMRI conditions, offering an explanation for the reduced responsivity to the neural depressant effect of diazepam in the sense of a floor effect. Subsequently, Fos immunoreactivity (Fos-IR) showed that ethologically relevant stressors also cause less dmPFC activation in HABs. In the context of an anxiety-inhibiting role of the dmPFC, we propose that failure to sufficiently activate this region in stressful situations may contribute to high trait anxiety.

Chronic cold stress regulates ascending noradrenergic pathways

Chronic exposure to cold in rats alters the activity of locus coeruleus (LC) neurons. In this study we aimed to examine the cellular effect of cold stress on catecholamine neurons, and determine whether this is specific to the LC compared with other catecholamine cell groups. Male Sprague-Dawley rats were subjected to 21 days of isolation under ambient conditions, chronic cold exposure at 5 degrees C, or after chronic cold followed by return to ambient temperature for 7 or 14 days. In the LC, chronic cold exposure significantly reduced tyrosine hydroxylase (TH) mRNA expression by approximately 45% compared with control rats, and remained significantly reduced (approximately 36%) after return to ambient conditions for 7 days; however, expression returned to normal after 14 days' recovery. There were no significant changes in TH mRNA in the substantia nigra or ventral tegmental area. Chronic cold increased expression of alpha2A adrenoceptor mRNA in the LC (approximately 27%). There were decreases in alpha2A expression in the nucleus tractus solitarius; however, this was seen only in rats returned to ambient conditions for 7 days. Additionally, alpha2A mRNA in the caudal ventrolateral medulla (A1 region) increased following cold exposure (approximately 84%) compared with controls. Binding of [125I]iodoclonidine to alpha2-like protein increased in the olfactory bulbs but decreased in the medial amygdala following cold exposure. Collectively, these data indicate robust effects of cold on central catecholamine neurons, not necessarily specific to the LC.

Glutamic acid decarboxylase (GAD65) immunoreactivity in brains of aggressive, adolescent anabolic steroid-treated hamsters

Chronic anabolic-androgenic steroid (AAS) treatment during adolescence facilitates offensive aggression in male Syrian hamsters (Mesocricetus auratus). The current study assessed whether adolescent AAS exposure influenced the immunohistochemical localization of glutamic acid decarboxylase (GAD65), the rate-limiting enzyme in the synthesis of gamma-aminobutyric acid (GABA), in areas of hamster brain implicated in aggressive behavior. Hamsters were administered high dose AAS throughout adolescence, scored for offensive aggression, and then examined for differences in GAD65 puncta to regions of the hamster brain important for aggression. When compared with control animals, aggressive AAS-treated hamsters showed significant increases in the area covered by GAD65 immunoreactive puncta in several of these aggression regions, including the anterior hypothalamus, ventrolateral hypothalamus, and medial amygdala. Conversely, aggressive AAS-treated hamsters showed a significant decrease in GAD65-ir puncta in the lateral septum when compared with oil-treated controls. However, no differences in GAD65 puncta were found in other aggression areas, such as the bed nucleus of the stria terminalis and central amygdala. Together, these results support a role for altered GAD65 synthesis and function in adolescent AAS-facilitated offensive aggression.

The effect of isolation rearing on volitional ethanol consumption and central CCK/dopamine systems in Fawn-Hooded rats

Numerous studies have demonstrated that socially isolating rats (from weaning) produces a sustained anxious phenotype and an enhanced response to psychostimulant drugs such as amphetamine and cocaine. In addition, isolation rearing has been shown to induce significant changes in the mesolimbic dopamine system. These data indicate that isolation rearing not only induces an anxiogenic phenotype but also induces neurochemical changes in reward nuclei of the brain, which is correlated with an enhanced response to psychostimulants. For these reasons, the effect of isolation rearing on volitional ethanol consumption was examined in Fawn-Hooded (FH) rats and correlated with neurochemical changes in central dopamine and cholecystokinin systems. Social isolation from weaning produced an anxiogenic phenotype as measured by a decreased time spent on the open arms of an elevated plus-maze. Interestingly, isolation-rearing induced a greater proportion of FH rats to acquire preference for ethanol while having no effect on the amount of ethanol consumed by alcohol-preferring rats. In addition, isolation rearing induced a number of changes in central CCK/dopamine systems.

Comparative analysis of hepatic ethanol metabolism in Fawn-Hooded and Wistar-Kyoto rats

Results of a number of studies have supported the suggestion that a correlation exists between voluntary ethanol consumption and enhanced ethanol metabolism in some (but not all) rodent strains. However, as yet, the capacity for alcohol-preferring Fawn-Hooded (FH) rats to metabolize ethanol has not been investigated. Hence, the aim of the current study was to compare the activities of the major hepatic enzymes involved in ethanol metabolism--cytosolic alcohol dehydrogenase (ADH) and mitochondrial aldehyde dehydrogenase (ALDH)--in the FH rat and its alcohol-nonpreferring counterpart, the Wistar-Kyoto (WKY) rat. In addition, the effect of chronic (5 weeks in vivo) ethanol pretreatment on the activity of these enzymes was investigated. Alcohol-naive FH rats were found to have significantly higher ADH activity (+61%) and no significant change in ALDH activity when compared with findings for WKY rats. In addition, chronic ethanol self-administration produced a small increase in ADH activity (+14%) in WKY rats only. Taken as a whole, these findings are the first to demonstrate an increased in vitro hepatic ethanol metabolism in alcohol-preferring FH rats and further demonstrate an association between hepatic ethanol metabolism and voluntary ethanol self-administration in rodents.

The effect of chronic ethanol consumption and withdrawal on mu-opioid and dopamine D(1) and D(2) receptor density in Fawn-Hooded rat brain

Previous studies have implicated the dopamine and opioid systems in the induction and maintenance of ethanol consumption. This study investigated, in alcohol-preferring Fawn-Hooded (FH) rats, whether chronic free-choice ethanol consumption and subsequent withdrawal cause alterations in central mu-opioid, dopamine D(1), and D(2) receptor density using autoradiography. FH rats were given a free choice between a 5% ethanol solution and tap water (n = 25) and displayed a mean ethanol consumption of 5.6 g/kg/day. A parallel group of FH rats (n = 5) only had access to tap water. Rats were then withdrawn from ethanol for 0, 1, 2, 5, or 10 days and killed by cervical dislocation and decapitation. Increases in mu-opioid receptor density were observed in the nucleus accumbens and ventral tegmental area upon withdrawal compared with the ethanol naive group. In the lateral amygdala, binding in all withdrawal groups was significantly different from the ethanol naive FH rats, and also from the chronic ethanol rats. An increase in dopamine D(1) receptor density was observed in the substantia nigra, pars reticulata in the 5- and 10-day withdrawal groups compared with ethanol naive. Accumbal dopamine D(2) receptor density (+25-30%) increased in the 10-day withdrawal group compared with both naive and chronic ethanol groups. These findings demonstrate that the opioid and dopamine systems are susceptible to modulation by chronic ethanol consumption and withdrawal in the FH rat. Furthermore, although acute ethanol withdrawal results in modulation of mu-opioid receptors, effects on dopamine receptors are delayed and only become evident 5 to 10 days after withdrawal.

The emergence test: effects of psychotropic drugs on neophobic disposition in Wistar Kyoto (WKY) and Sprague Dawley rats

1. The Emergence Test (ET), a variation of the open field test in which the rat is not handled, and is purported to measure neophobia, was applied to Wistar Kyoto (WKY) and Sprague Dawley (S-D) rats. 2. While no-stress control WKY rats were less active in the ET, pre-treatment with shock stress exacerbated strain differences. WKY rats, previously exposed to shock, did not emerge from the home cage start box during repeated testing, whereas previously stressed S-D rats vacated the home cage quickly and revealed increasing behavioral agitation. 3. Diazepam reduced emergence latency only in S-D rats, whereas nomifensine significantly increased head poke responses in WKY rats. 4. WKY rats responded to the ET with characteristically depressive behavior, whereas S-D rats responded to the same ET with behavioral agitation and anxiety. The implications of these behavior patterns for discriminating between anxiety and depressive behavior are presented.

Regional variations in the effects of chronic ethanol administration on GABA(A) receptor expression: potential mechanisms

Gamma-aminobutyric acid type A (GABA(A)) receptors in brain adapt to chronic ethanol exposure via changes in receptor function and subunit expression. The present review summarizes currently available data regarding changes in GABA(A) receptor subunit mRNA and peptide expression. Data are presented from various different brain regions and the variations between specific brain regions used to draw conclusions about mechanisms that may underlie GABA(A) receptor adaptations during chronic ethanol exposure. In the whole cerebral cortex, chronic ethanol exposure leads to a reduction of GABA(A) receptor alpha1 subunit mRNA and peptide levels and a near equivalent increase in alpha4 subunit mRNA and peptide levels. This observation is the primary support for the hypothesis that altered receptor composition is a mechanism for GABA(A) receptor adaptation produced by chronic ethanol exposure. However, other brain regions do not display similar patterns of subunit changes. Moreover, subregions within cortex (prefrontal, cingulate, parietal, motor, and piriform) exhibit patterns of changes in subunit expression that differ from whole cortex. Therefore, regional differences in GABA(A) receptor subunit expression are evident following chronic ethanol administration, thus suggesting that multiple mechanisms contribute to the regulation of GABA(A) receptor expression. These mechanisms may include the involvement of other neurotransmitter systems, endogenous steroids and second or third messenger cross-talk.

CCK/dopamine interactions in Fawn-Hooded and Wistar-Kyoto rat brain

The aim of this study was to compare the actions of CCK neuropeptides within the nucleus accumbens (N.Acc) of alcohol preferring (Fawn-Hooded, FH) and alcohol nonpreferring (Wistar-Kyoto, WKY) rats. CCK-8S (30-300 nM) facilitated the K(+) stimulated release of [(3)H]dopamine (DA) from N.Acc prisms in both rat strains, whereas CCK-4 (30 nM-1 microM) caused a significant decrease of evoked [(3)H]DA in the FH rat only. A scattered distribution of CCK-A and -B receptor immunopositive varicose fibers were visualized throughout the N.Acc of both rat strains along with a topographic distribution of CCK receptor positive cells throughout the ventral mesencephalon.

Up-regulation of cortical AMPA receptor binding in the fawn-hooded rat following ethanol withdrawal

The present study has employed quantitative receptor autoradiography to compare the binding of (S)-[3H]5-fluorowillardiine to (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors in the brains of alcohol-preferring Fawn-Hooded (FH) rats, alcohol non-preferring Wistar-Kyoto (WKY) rats, and FH rats following a 28-day period of 5% ethanol consumption with or without ethanol withdrawal. Significantly higher binding of [3H]5-fluorowillardiine was found in the cingulate cortex (+12%) and claustrum (+13%) in alcohol naïve FH rats compared to WKY rats. Chronic ethanol consumption decreased binding of (S)-[3H]5-fluorowillardiine in four cortical regions (frontal, parietal, occipital and temporal cortex), hippocampus and septohippocampal nucleus. In contrast, ethanol withdrawal induced a significant "rebound" increase in binding by +22% in frontal and parietal cortex, by +17% in cingulate cortex and +13% in claustrum, and by +14% in the septohippocampal nucleus compared to chronic ethanol-exposed FH rats. The findings suggest that AMPA receptors in frontal cortical regions are sensitive to ethanol and therefore may be implicated in the predisposition of alcohol preference in FH rats.

The role of opioid-dopamine interactions in the induction and maintenance of ethanol consumption

1. Alcohol is one of the most widely used recreational drugs, but also one of the most widely abused, causing vast economic, social and personal damage. 2. Several animal models are available to study the reinforcing mechanisms that are the basis of the abuse liability of ethanol. Innate differences in opioid or dopamine neurotransmission may enhance the abuse liability of ethanol, as indicated by animal and human studies. 3. Opioid antagonists have been shown to be effective, both experimentally and clinically, in decreasing ethanol consumption, presumably since ethanol induces the release of endogenous opioid peptides in vivo. However, ethanol may also stimulate the formation of opiate-like compounds, which could interact with opioid (or dopamine) receptors. Ethanol may cause changes in neurotransmission mediated via opioid receptors that determines whether alcohol abuse is more or less likely. 4. Ethanol appears to facilitate dopamine release by increasing opioidergic activity, disinhibiting dopaminergic neurons (by inhibition of GABAergic neurotransmission) via mu-opioid receptors in the ventral tegmental area (VTA) and delta-opioid receptors in the nucleus accumbens (NAcc). The effects of ethanol would be antagonised by presynaptic kappa-opioid receptors present on dopaminergic terminals in the NAcc. 5. Mesolimbic dopamine release induced by ethanol consumption seems to indicate ethanol-related stimuli are important, focussing attention on and enabling learning of the stimuli. However, studies indicate that there are redundant pathways, and neural pathways 'downstream' of the mesolimbic dopamine system, which also enable the reinforcing properties of ethanol to be mediated.

Distribution of opioid peptide gene expression in the limbic system of Fawn-Hooded (alcohol-preferring) and Wistar-Kyoto (alcohol-non-preferring) rats

Preprodynorphin and preproenkephalin mRNA expression was examined in the CNS of two rat strains, the alcohol-preferring Fawn-Hooded (FH) and the alcohol-non-preferring Wistar-Kyoto (WKY), using in situ hybridisation histochemistry. Relative to the WKY, the FH showed significantly lower levels of preproenkephalin mRNA in the striatum and nucleus accumbens (-24% and -17% respectively), but a higher level of preprodynorphin mRNA in the hippocampus (+33%). The depressed level of preproenkephalin mRNA in the nucleus accumbens may be implicated in alcohol-seeking behaviour.

[3H]zolpidem binding in alcohol-preferring and non-preferring rat brain

The present study has employed in vitro autoradiography to study the distribution and density of [3H]zolpidem binding sites, which are regarded as an index of ethanol-sensitive gamma-aminobutyric acid (GABA)A receptors, in the brains of alcohol-preferring Fawn-Hooded (FH) rats compared to non-alcohol preferring Wistar-Kyoto (WKY) rats. Binding of [3H]zolpidem showed a similar distribution profile in both rat strains examined and included cerebellum, globus pallidus, nucleus of the solitary tract and a number of midbrain/hindbrain nuclei. Densitometric quantitation of binding revealed that FH rats possessed a significantly higher density of [3H]zolpidem binding compared to WKY rats in cortical regions, substantia nigra pars reticulata and the ventral pallidum. These data indicate that FH rats may have an increased number of ethanol-sensitive GABA(A) receptors in regions intimately involved in reward processes, and may partially explain the alcohol-seeking nature of the FH rat.