Effects of ibotenic acid lesions of the ventral striatum and the medial prefrontal cortex on ethanol consumption in the rat

Alcohol and the dopamine system

The mesolimbic dopamine pathway plays a major role in drug reinforcement and is likely involved also in the development of drug addiction. Ethanol, like most addictive drugs, acutely activates the mesolimbic dopamine system and releases dopamine, and ethanol-associated stimuli also appear to trigger dopamine release. In addition, chronic exposure to ethanol reduces the baseline function of the mesolimbic dopamine system. The molecular mechanisms underlying ethanol´s interaction with this system remain, however, to be unveiled. Here research on the actions of ethanol in the mesolimbic dopamine system, focusing on the involvement of cystein-loop ligand-gated ion channels, opiate receptors, gastric peptides and acetaldehyde is briefly reviewed. In summary, a great complexity as regards ethanol´s mechanism(s) of action along the mesolimbic dopamine system has been revealed. Consequently, several new targets and possibilities for pharmacotherapies for alcohol use disorder have emerged.

Argyrophilic grain disease in individuals younger than 75 years: clinical variability in an under-recognized limbic tauopathy

BACKGROUND AND PURPOSE

Argyrophilic grain disease (AGD) is a limbic-predominant 4R-tauopathy. AGD is thought to be an age-related disorder and is frequently detected as a concomitant pathology with other neurodegenerative conditions. There is a paucity of data on the clinical phenotype of pure AGD. In elderly patients, however, AGD pathology frequently associates with cognitive decline, personality changes, urine incontinence and cachexia. In this study, clinicopathological findings were analysed in individuals younger than 75.

METHODS

Patients were identified retrospectively based on neuropathological examinations during 2006-2017 and selected when AGD was the primary and dominant pathological finding. Clinical data were obtained retrospectively through medical records.

RESULTS

In all, 55 patients (2% of all examinations performed during that period) with AGD were identified. In seven cases (13%) AGD was the primary neuropathological diagnosis without significant concomitant pathologies. Two patients were female, median age at the time of death was 64 years (range 51-74) and the median duration of disease was 3 months (range 0.5-36). The most frequent symptoms were progressive cognitive decline, urinary incontinence, seizures and psychiatric symptoms. Brain magnetic resonance imaging revealed mild temporal atrophy.

CONCLUSIONS

Argyrophilic grain disease is a rarely recognized limbic tauopathy in younger individuals. Widening the clinicopathological spectrum of tauopathies may allow identification of further patients who could benefit from tau-based therapeutic strategies.

The nucleus accumbens and alcoholism: a target for deep brain stimulation

Alcohol use disorder (AUD) is a difficult to treat condition with a significant global public health and cost burden. The nucleus accumbens (NAc) has been implicated in AUD and identified as an ideal target for deep brain stimulation (DBS). There are promising preclinical animal studies of DBS for alcohol consumption as well as some initial human clinical studies that have shown some promise at reducing alcohol-related cravings and, in some instances, achieving long-term abstinence. In this review, the authors discuss the evidence and concepts supporting the role of the NAc in AUD, summarize the findings from published NAc DBS studies in animal models and humans, and consider the challenges and propose future directions for neuromodulation of the NAc for the treatment of AUD.

Inactivation of the lateral orbitofrontal cortex increases drinking in ethanol-dependent but not non-dependent mice

Long-term consumption of ethanol affects cortical areas that are important for learning and memory, cognition, and decision-making. Deficits in cortical function may contribute to alcohol-abuse disorders by impeding an individual's ability to control drinking. Previous studies from this laboratory show that acute ethanol reduces activity of lateral orbitofrontal cortex (LOFC) neurons while chronic exposure impairs LOFC-dependent reversal learning and induces changes in LOFC excitability. Despite these findings, the role of LOFC neurons in ethanol consumption is unknown. To address this issue, we examined ethanol drinking in adult C57Bl/6J mice that received an excitotoxic lesion or viral injection of the inhibitory DREADD (designer receptor exclusively activated by designer drug) into the LOFC. No differences in ethanol consumption were observed between sham and lesioned mice during access to increasing concentrations of ethanol (3-40%) every other day for 7 weeks. Adulterating the ethanol solution with saccharin (0.2%) or quinine (0.06 mM) enhanced or inhibited, respectively, consumption of the 40% ethanol solution similarly in both groups. Using a chronic intermittent ethanol (CIE) vapor exposure model that produces dependence, we found no difference in baseline drinking between sham and lesioned mice prior to vapor treatments. CIE enhanced drinking in both groups as compared to air-treated animals and CIE treated lesioned mice showed an additional increase in ethanol drinking as compared to CIE sham controls. This effect persisted during the first week when quinine was added to the ethanol solution but consumption decreased to control levels in CIE lesioned mice in the following 2 weeks. In viral injected mice, baseline drinking was not altered by expression of the inhibitory DREADD receptor and repeated cycles of CIE exposure enhanced drinking in DREADD and virus control groups. Consistent with the lesion study, treatment with clozapine-N-oxide (CNO) further enhanced consumption only in CIE exposed DREADD mice with no change in air-treated mice. These results suggest that the LOFC is not critical for the initiation and maintenance of ethanol drinking in non-dependent mice, but may regulate the escalated drinking observed during dependence.

Reverse pharmacogenetic modulation of the nucleus accumbens reduces ethanol consumption in a limited access paradigm

Bilateral stereotactic lesioning of the nucleus accumbens (NAc) core reduces relapse rates in alcohol-dependent patients but may cause irreversible cognitive deficits. Deep brain stimulation has similar effects but requires costly implanted hardware and regular surgical maintenance. Therefore, there is considerable interest in refining these approaches to develop reversible, minimally invasive treatments for alcohol dependence. Toward this end, we evaluated the feasibility of a reverse pharmacogenetic approach in a preclinical mouse model. We first assessed the predictive validity of a limited access ethanol consumption paradigm by confirming that electrolytic lesions of the NAc core decreased ethanol consumption, recapitulating the effects of similar lesions in humans. We then used this paradigm to test the effect of modulating activity in the NAc using the Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) hM3Dq and hM4Di. We found that increasing activity with hM3Dq had no effect, but suppressing activity with hM4Di reduced alcohol consumption to a similar extent as lesioning without affecting consumption of water or sucrose. These results may represent early steps toward a novel neurosurgical treatment modality for alcohol dependence that is reversible and externally titratable, yet highly targetable and less invasive than current approaches such as lesioning or deep brain stimulation.

Deletion of the gabra2 gene results in hypersensitivity to the acute effects of ethanol but does not alter ethanol self administration

Human genetic studies have suggested that polymorphisms of the GABRA2 gene encoding the GABA_A α2-subunit are associated with ethanol dependence. Variations in this gene also convey sensitivity to the subjective effects of ethanol, indicating a role in mediating ethanol-related behaviours. We therefore investigated the consequences of deleting the α2-subunit on the ataxic and rewarding properties of ethanol in mice. Ataxic and sedative effects of ethanol were explored in GABA_A α2-subunit wildtype (WT) and knockout (KO) mice using a Rotarod apparatus, wire hang and the duration of loss of righting reflex. Following training, KO mice showed shorter latencies to fall than WT littermates under ethanol (2 g/kg i.p.) in both Rotarod and wire hang tests. After administration of ethanol (3.5 g/kg i.p.), KO mice took longer to regain the righting reflex than WT mice. To ensure the acute effects are not due to the gabra2 deletion affecting pharmacokinetics, blood ethanol concentrations were measured at 20 minute intervals after acute administration (2 g/kg i.p.), and did not differ between genotypes. To investigate ethanol’s rewarding properties, WT and KO mice were trained to lever press to receive increasing concentrations of ethanol on an FR4 schedule of reinforcement. Both WT and KO mice self-administered ethanol at similar rates, with no differences in the numbers of reinforcers earned. These data indicate a protective role for α2-subunits, against the acute sedative and ataxic effects of ethanol. However, no change was observed in ethanol self administration, suggesting the rewarding effects of ethanol remain unchanged.

Infusion of gliotoxins or a gap junction blocker in the prelimbic cortex increases alcohol preference in Wistar rats

Postmortem research has revealed that there is a lower density of glial cells in regions of the prefrontal cortex (PFC) of uncomplicated alcoholics when compared with control subjects. Impairment of astrocyte function in the PFC may contribute to malfunction in circuits involved in emotion- and reward-related subcortical centers, heavily connected with the PFC and directly involved in the pathophysiology of addictive behaviours. The hypothesis was tested that infusion of gliotoxins known to injure astrocytes or of a gap junction blocker into the prelimbic area of the rat PFC results in increased preference for ethanol in rats exposed to free choice between water and 10% ethanol. Fluorocitric acid, L-alpha-aminoadipic acid (AAD) or the gap junction blocker 18-alpha-glycyrrhetinic acid (AGA) were bilaterally infused once into the rat prelimbic cortex and alcohol preference (ratio of 10% ethanol consumed to total liquid ingested) was measured before and after infusion. Infusion of AAD or AGA dissolved in their vehicles, but not of their vehicles alone, resulted in significant transient increase of preference for 10% ethanol. The present data suggest that impaired integrity of glial cells or the gap junctional communication between them in the rat PFC may contribute to changes in ethanol preference.

Behavioral characteristics and neurobiological substrates shared by Pavlovian sign-tracking and drug abuse

Drug abuse researchers have noted striking similarities between behaviors elicited by Pavlovian sign-tracking procedures and prominent symptoms of drug abuse. In Pavlovian sign-tracking procedures, repeated paired presentations of a small object (conditioned stimulus, CS) with a reward (unconditioned stimulus, US) elicits a conditioned response (CR) that typically consists of approaching the CS, contacting the CS, and expressing consummatory responses at the CS. Sign-tracking CR performance is poorly controlled and exhibits spontaneous recovery and long-term retention, effects that resemble relapse. Sign-tracking resembles psychomotor activation, a syndrome of behavioral responses evoked by addictive drugs, and the effects of sign-tracking on corticosterone levels and activation of dopamine pathways resemble the neurobiological effects of abused drugs. Finally, the neurobiological profile of individuals susceptible to sign-tracking resembles the pathophysiological profile of vulnerability to drug abuse, and vulnerability to sign-tracking predicts vulnerability to impulsive responding and alcohol self-administration. Implications of sign-tracking for models of drug addiction are considered.

Microarray analysis of the effects of a gamma-protein kinase C null mutation on gene expression in striatum: a role for transthyretin in mutant phenotypes

A constitutive null mutation of the neural-specific isotype of protein kinase C (gamma-PKC) in mice produces alterations in behavioral traits and responses to ethanol suggesting that gamma-PKC-mediated phosphorylation is essential for regulation of some behaviors. However, it is possible that some of the effects of gamma-PKC gene deletion also may be due to altered gene expression. To examine alterations in gene expression, microarray analyses were performed on striatal tissue from wild types and mutants. A total of 143 genes and ESTs were identified as potential candidates related to differences between null mutants and wild types. Confirmation studies using qRT-PCR indicated that the expression of transthyretin was increased about 8-fold in striatum of naïve mutants compared to wild types. The effect of chronic ethanol treatment on transthyretin expression was analyzed because gamma-PKC mutants do not develop tolerance to chronic ethanol treatment. Ethanol treatment of mutants reversed the dramatic increase in transthyretin expression seen in naïve and control-diet treated mutants, but did not affect transthyretin expression in wild types.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Increased alcohol intake and behavioral disinhibition in rats with ventral striatal neuron loss

A previous study of ours reported excessive alcohol intake, enhanced defensive aggressiveness (hyperreactivity towards the experimenter), impulsive behavior, and reduced cortical serotonin levels in rats following extensive basal forebrain axon-sparing lesions involving the septal area and the ventral striatum. This constellation of signs resembles that seen clinically in "Dionysian" alcoholics. The present investigation aimed at examining the effect of ibotenic acid lesions restricted to the septal area or the ventral striatum on this behavioral profile. Experiment 1 indicated that medium-sized lesions (induced by infusing 0.35 microl ibotenic acid in each hemisphere) encompassing the septal area or the ventral striatum elicited a qualitatively similar behavioral profile. Both lesion types markedly enhanced the intake of 6% ethanol, and both groups were significantly more hyperreactive towards the experimenter. A brief doorbell signal elicited significantly more fleeing in rats with basal forebrain lesions, and licking from an electrified waterspout in the punished drinking test caused lesser suppression of locomotor activity than normal. Both groups also showed significant deficits in food hoarding. Histological examination revealed that the posterior portion of the ventral striatal lesion typically overlapped with the anterior portion of the septal lesion. Experiment 2 avoided this neuropathological overlap, and examined groups bearing small discrete lesions (induced by infusing 0.15 microl ibotenic acid in each hemisphere) restricted to either the accumbens part of the ventral striatum or the dorsal septal area. Lesions to the nucleus accumbens were associated with an increase in home-cage alcohol drinking, no hyperreactivity towards the experimenter, potentiation of fleeing at the expense of freezing in response to a sudden auditory signal, and disinhibited behavior in the punished drinking test with increased punished responding and reduced behavioral suppression. Rats with small septal lesions showed a weak enhancement of defensive aggression, but no other behavioral alterations. Our results suggest that ventral striatal neuron loss gives rise to excessive alcohol drinking and enhanced impulsivity.

Individual differences in pavlovian autoshaping of lever pressing in rats predict stress-induced corticosterone release and mesolimbic levels of monoamines

Pavlovian autoshaping CRs are directed and reflexive consummatory responses targeted at objects repeatedly paired with rewarding substances. To evaluate the hypothesis that autoshaping may provide an animal learning model of vulnerability to drug abuse, this study relates individual differences in lever-press autoshaping CR performance in rats to stress-induced corticosterone release and tissue monoamine levels in the mesolimbic dopamine tract. Long-Evans rats (n = 14) were given 20 sessions of Pavlovian autoshaping training wherein the insertion of a retractable lever CS was followed by the response-independent presentation of food US. Large between-subjects differences in lever-press autoshaping CR performance were observed, with group high CR frequency (n = 5) performing many more lever press CRs than group low CR frequency (n = 9). Tail-blood samples were obtained before and after the 20th autoshaping session, then 24 h later the rats were sacrificed and dissection yielded tissue samples of nucleus accumbens (NAC), prefrontal cortex (PFC), caudate putamen (CP), and ventral tegmental area (VTA). Serum levels of postsession corticosterone were elevated in group high CR frequency. HPLC revealed that group high CR frequency had higher tissue levels of dopamine and DOPAC in NAC, lower levels of DOPAC/DA turnover in CP, and lower levels of 5-HIAA and lower 5-HIAA/5-HT turnover in VTA. The neurochemical profile of rats that perform more autoshaping CRs share some features of vulnerability to drug abuse.

Behavioral disinhibition following basal forebrain excitotoxin lesions: alcohol consumption, defensive aggression, impulsivity and serotonin levels

Research on alcoholism have identified a subgroup in which the drinking problem is associated with high rates of violence, an impulsive disposition and signs of reduced serotonin functioning in the brain. The present study reports that male Wistar rats with ibotenic acid-induced (5 micrograms/0.5 microliter) neuron loss in the basal forebrain (ventral striatum, septal area and adjacent structures) showed behavioral and neurochemical signs not unlike this subtype of alcoholics. Thus, rats with this lesion exhibited excessive 6% alcohol drinking in a two-bottle choice test and showed augmentation of certain defensive behaviors, including defensive aggression and increased activity-during signal. In the punished drinking test, a passive avoidance task which taps psychological mechanisms underlying impulsivity [56], experimental rats were not different from sham-operated controls with regard to the number of punished licks, but punishment evoked less disruption of ongoing behavior in subjects with basal forebrain damage. The virtual absence of food hoarding in the face of normal feeding may constitute yet another sign of increased impulsivity, indicating as it does a diminished influence of future rewards on behavior. As expected, in view of ibotenic acid's selectivity for neuronal perikarya, the concentrations of dopamine, serotonin and norepinephrine were normal in the lesioned area. However, the levels of serotonin and norepinephrine in the cortex were reduced. A separate experiment examined the extent to which serotonin depletion alone reproduced the behavioral profile induced by basal forebrain neuron loss. However, measures of alcohol consumption, defensive behavior and impulsivity were not different from controls in rats given intracerebroventricular 5,7-DHT (150 micrograms/20 microliters), except for a modest increase in defensive aggression.

Left and right 6-hydroxydopamine lesions of the medial prefrontal cortex differentially affect voluntary ethanol consumption

Dopaminergic projections to the medial prefrontal cortex (mPFC) were unilaterally lesioned with 6-hydroxydopamine (6-OHDA) to examine how dopamine (DA) asymmetry in the mPFC influences voluntary ethanol consumption. Differences in nucleus accumbens (NAS) DA neurotransmission have been related to individual differences in locomotor activity and in the rewarding efficacy of ethanol. Therefore, differences in locomotor activity were used to further characterize the effects of unilateral mPFC 6-OHDA lesions on ethanol consumption. Male Long Evans rats were assessed for high versus low levels of spontaneous locomotor activity. DA terminals in the left or right mPFC were unilaterally lesioned with 6-OHDA, resulting in an average DA depletion of 54% and 50%, respectively. After a minimum seven-day recovery period, preference for a 10% ethanol solution vs. water was determined in a 24-h 2-bottle home-cage free-choice paradigm. Left mPFC 6-OHDA lesions increased and right lesions decreased ethanol consumption. These differential effects of left and right lesions were primarily attributable to rats exhibiting low locomotor activity prior to surgery. The present data suggest that right greater than left cortical DA asymmetry in combination with low endogenous NAS DA (predicted by low locomotor activity levels) may increase the vulnerability to abuse ethanol.

Genetics of alcoholism: rapid development of a new high-ethanol-preferring (HEP) strain of female and male rats

A genetically based animal model of alcoholism has been developed in a relatively short period of 3 years. The new strain is characterized by an intense preference for ethanol over water as well as unique behavioral, neurochemical and other attributes. This new strain, termed high-ethanol-preferring (HEP) rats, was derived initially from selective cross-breeding of a variant strain of female Harlan Sprague-Dawley (SD) rats with the outbred Wistar line of male ethanol-preferring (P) rats. In this study, drinking patterns of both genders were obtained over 10 days by presenting water and ethanol in concentrations ranging from 3% to 30%. To expedite the development of the new strain, only three to five female and male rats served as breeders, which were chosen from all litters on the basis of their maximum g/kg intake integrated with proportion of ethanol to total fluid values. Profiles of intake of preferred concentrations of ethanol were obtained over 24 h of unlimited access as well as during 2-h intervals of limited access to ethanol. Levels of blood ethanol were measured in both female and male HEP animals during bouts of ethanol drinking in the limited access paradigm. By the sixth generation of HEP rats, ethanol consumption of the females often exceeded that of any other rat genetically bred to drink ethanol (e.g., at a concentration of 15.7%, 10.3 g/kg per day). Seven additional characteristics are notable: 1) the HEP rats prefer ethanol in the presence of a nutritious chocolate drink or nonnutrient sweetened solution (aspartame); 2) high levels of blood ethanol are associated with their drinking; 3) females drink significantly greater g/kg amounts of ethanol than HEP males and prefer a higher percent concentration of ethanol; 4) the drinking of ethanol by the female HEP animals does not fluctuate during the estrous cycle; 5) neurochemical assays show differential profiles of 5-HT, dopamine, and their metabolites in different regions of the brain; 6) measures of activity using the elevated plus maze, open field, and cork gnawing reveal differences between genders of HEP rats and SD rats; and 7) the HEP animals are without phenotypically expressed abnormalities. Finally, one cardinal principle derived from this study revealed that the breeding strategy to develop high-ethanol-drinking rats centers on the use of multiple solutions of ethanol whereby the intakes of ethanol in concentration of 9% through 20% dictate the ultimate selection of breeding pairs over successive F generations. Further, it is concluded that because of an intense rise in ethanol drinking of the F1 generation of female HEP rats well above that of the parental SD female breeders, the complex genotypic characteristic of the male P rat is predominantly responsible for evoking ethanol drinking in female offspring.

An animal model for type 2 alcoholism? Alcohol consumption and aggressive behavior following lesions in the raphe nuclei, medial hypothalamus, or ventral striatum-septal area

Given the conspicuous association between aggressive antisocial traits and alcoholism in men, we investigated whether or not a link between defensive aggressive behavior and homecage alcohol consumption could be demonstrated in the laboratory rat. This was accomplished by observing ethanol intake and hyperreactivity towards the experimenter in rats made hyperdefensive by brain lesions. Rats with medial hypothalamic electrocoagulations showed a remarkable degree of hyperdefensiveness, lasting throughout the entire 6-week postoperative period. Alcohol intake, on the other hand, was not different from sham-operated controls when the beverage was offered as a plain 6% solution or in a 0.2% saccharin vehicle. When subjected to the stress of food restriction, which enhances ethanol intake in normal rats, medial hypothalamic subjects actually decreased their alcohol consumption. Electrolytic lesions in the dorsal and median raphe brought about a transient increase in defensive aggression, but no alteration in ethanol drinking. Animals with ibotenic acid-induced extensive lesions to the ventral striatum and septal area were not only viciously aggressive, but also drank considerably more alcohol than controls. Ibotenic acid-lesioned rats did not respond to the saccharin or food-restriction conditions by increasing their alcohol intake further, perhaps because they drank at a maximal rate already during the plain ethanol-phase of the experiment. These observations show that basal forebrain dysfunction in the rat can give rise to excessive alcohol intake and heightened aggression, a constellation of behavioral symptoms observed in male type 2 alcoholics.