David N. Stephens

Adolescent impulsivity phenotypes characterized by distinct brain networks

The impulsive behavior that is often characteristic of adolescence may reflect underlying neurodevelopmental processes. Moreover, impulsivity is a multi-dimensional construct, and it is plausible that distinct brain networks contribute to its different cognitive, clinical and behavioral aspects. As these networks have not yet been described, we identified distinct cortical and subcortical networks underlying successful inhibitions and inhibition failures in a large sample (n = 1,896) of 14-year-old adolescents. Different networks were associated with drug use (n = 1,593) and attention-deficit hyperactivity disorder symptoms (n = 342). Hypofunctioning of a specific orbitofrontal cortical network was associated with likelihood of initiating drug use in early adolescence. Right inferior frontal activity was related to the speed of the inhibition process (n = 826) and use of illegal substances and associated with genetic variation in a norepinephrine transporter gene (n = 819). Our results indicate that both neural endophenotypes and genetic variation give rise to the various manifestations of impulsive behavior.

Cognitive and emotional consequences of binge drinking: role of amygdala and prefrontal cortex

Binge drinking is an increasingly recognized problem within the UK. We have studied the relationship of binge drinking to cognitive and emotional functioning in young adults, and have found evidence for increased impulsivity, impairments in spatial working memory and impaired emotional learning. Since in human studies it is difficult to understand whether such behavioural changes pre-date or are a consequence of binge drinking, we have also studied parallel behaviours in a rodent model, in which rats are exposed to intermittent episodes of alcohol consumption and withdrawal. In this model, and in parallel with our findings in human binge drinkers, and alcoholic patients who have undergone multiple episodes of detoxification, we have found evidence for impairments in aversive conditioning as well as increased impulsivity. These behavioural changes are accompanied by facilitated excitatory neurotransmission and reduced plasticity (long-term potentiation (LTP)) in amygdala and hippocampus. The impaired LTP is accompanied by both impaired associative learning and inappropriate generalization of previously learned associations to irrelevant stimuli. We propose that repeated episodes of withdrawal from alcohol induce aberrant neuronal plasticity that results in altered cognitive and emotional competences.

Impairment in Cognitive Functions After Multiple Detoxifications in Alcoholic Inpatients

BACKGROUND Repeated experience of withdrawal from alcohol results in a kindling-like process leading to increased likelihood and severity of convulsions during detoxification. The aim of this study was to determine whether repeated withdrawals affect cognitive function. METHODS We investigated alcoholic patients undergoing detoxification in an inpatient setting, using tasks sensitive to dysfunction of prefrontal areas. The tasks applied were two maze tasks from the Wechsler Intelligence Scale for Children, the color Stroop task, and the vigilance task for adults and the delay task from the Gordon Diagnostic System. Forty-two abstinent alcoholic patients who were no longer receiving pharmacotherapy for detoxification participated. RESULTS Compared to a group of forty-three social drinkers matched for age, sex, and verbal IQ, the alcoholic patients took more time to complete maze 1 and made more errors in both mazes. Alcoholics made more commission errors and gave fewer correct answers in the vigilance task. No differences were found in the color Stroop task between alcoholic patients and social drinkers. Patients with 2 or more detoxifications were more impaired in the maze 1, in the vigilance task and in the delay task than patients with a single, or no previous detoxification. When patients were reclassified on the basis of the total number of attempts at withdrawing from alcohol (including the medically supervised) only the deficit in the vigilance task was associated with the number of withdrawal attempts. The effects of medically supervised detoxifications on maze 1 and vigilance task were confounded with other factors related to the history of alcoholism, alcohol use, age of starting heavy drinking and years of problem drinking. CONCLUSIONS Repeated experience of withdrawal from alcohol is thus associated with impaired cognitive function although it appears that for some of these effects, other factors associated with the history of alcoholism might also be involved.

Discriminative stimulus properties of β-carbolines characterized as agonists and inverse agonists at central benzodiazepine receptors

The discriminative stimulus properties of three β-carboline derivatives were studied in three groups of rats trained, respectively, to discriminate diazepam (2.5 mg/kg IP), chlordiazepoxide (CDP, 5 mg/kg IP) or pentylenetetrazol (PTZ, 15 mg/kg IP) from saline in standard procedures employing two-lever operant chambers. Two β-carbolines, ZK 91296 and ZK 93423, substituted for the benzodiazepines in both CDP- and diazepam-trained rats. The neutral benzodiazepine antagonist Ro 15-1788 blocked the diazepam discriminative stimulus and the ability of ZK 91296 to substitute for diazepam. A third β-carboline, FG 7142, was not identified as benzodiazepine-like in generalization tests in either diazepam- or CDP-trained rats, but when administered together with CDP antagonized the benzodiazepine discriminative stimulus. In rats trained to discriminate PTZ from saline (a discrimination which is thought to depend on the anxiogenic properties of PTZ) the PTZ cue was antagonized by diazepam and ZK 93423, and partially antagonized by ZK 91296. The PTZ cue generalized to FG 7142 and this generalization was partially antagonized by Ro 15-1788. These results suggest that the three β-carbolines provide more than one kind of discriminative stimulus, consistent with the classification of ZK 93423 as an agonist at central benzodiazepine receptors, with ZK 91 296 as a partial agonist, and with FG 7142 as an inverse agonist. Pharmacologically, ZK 93 423 and ZK 91 296 may exhibit anxiolytic qualities, whereas FG 7142 produces anxiogenic effects.

Lack of self-administration and behavioural sensitisation to morphine, but not cocaine, in mice lacking NK1 receptors

Mice lacking the NK1 receptor, the preferred receptor for substance P, demonstrate normal analgesic responses to morphine on the hot plate assay, but have been predicted, on the basis of conditioned place preference studies, to be insensitive to the rewarding properties of opiates. In this study, self-administration and the development and maintenance of locomotor sensitisation of both morphine and cocaine were investigated in mice that lacked the NK1 gene (NK1 knockout mice, NK1(-/-)). Both wildtype and NK1(-/-) mice learned an operant lever-press response to obtain food. When intravenous infusions of morphine (0.2 mg/kg/infusion) were substituted for the food reward, the wildtype mice initially reduced rates of lever pressing, but then increased them on the rewarded lever to obtain approx. 10 infusions per 90 min session; in contrast, NK1(-/-) mice continued to operate both the rewarded, and non-rewarded levers at low rates. Additionally, NK1(-/-) mice failed, following repeated administration, to sensitise to the locomotor stimulant effects of morphine (15 mg/kg, i.p.). These deficits were specific to opiates, since NK1(-/-) mice responding for food or cocaine self-administration (0.65 mg/kg/infusion) did not differ from wildtypes, and they showed normal behavioural sensitisation to repeated cocaine administration (10 mg/kg, i.p.). These results demonstrate that NK1 receptors are critical for the reinforcing properties of morphine, and for adaptive responses elicited by repeated opiate administration. It is postulated that substance P and the NK1 receptor may be necessary for the development of opiate, but not cocaine addiction.

Both alpha2 and alpha3 GABAA receptor subtypes mediate the anxiolytic properties of benzodiazepine site ligands in the conditioned emotional response paradigm

Mice with point‐mutated α2 GABAA receptor subunits (rendering them diazepam insensitive) are resistant to the anxiolytic‐like effects of benzodiazepines (BZs) in unconditioned models of anxiety. We investigated the role of the α2 GABAA subtype in a model of conditioned anxiety. α2(H101R) and wildtype mice were trained in a conditioned emotional response (CER) task, in which lever‐pressing for food on a variable interval (VI) schedule was suppressed during the presentation of a conditioned stimulus (CS+) that predicted footshock. The ability of diazepam, ethanol and pentobarbital to reduce suppression during the CS+ was interpreted as an anxiolytic response. Diazepam (0, 0.5, 1, 2, 4 and 8 mg/kg) induced a dose‐dependent anxiolytic‐like effect in wildtype mice. At high doses, diazepam (2, 4 and 8 mg/kg) was sedative in α2(H101R) mice. Analysis of the anxiolytic properties of nonsedative diazepam doses (0.5 and 1 mg/kg), showed that α2(H101R) mice were resistant to the anxiolytic effects of diazepam. Equivalent anxiolytic properties of pentobarbital (20 mg/kg) and ethanol (1 and 2 g/kg) were seen in both genotypes. These findings confirm the critical importance of the α2 GABAA subtype in mediating BZ anxiolysis. However, as a compound, L‐838417, with agonist properties at α2, α3 and α5‐containing receptors, gave rise to anxiolytic‐like activity in α2(H101R) mice in the CER test, α3‐containing GABA receptors are also likely to contribute to anxiolysis. Observations that α2(H101R) mice were more active, and displayed a greater suppression of lever pressing in response to fear‐conditioned stimuli than wildtype mice, suggests that the α2(H101R) mutation may not be behaviourally silent.

An inverse agonist selective for α5 subunit-containing GABAA receptors improves encoding and recall but not consolidation in the Morris water maze

RationaleCompounds selective for the GABAA receptors containing an α5 subunit have been reported to enhance performance in the hippocampally mediated delayed-matching-to-position version of the Morris water maze, in which reduction in the time required to find a hidden platform relative to an initial trial is used as an index of learning and memory.ObjectiveIn the present study, we have used one such compound, α5IA-II, to examine whether these effects occur during the encoding, consolidation or recall phases of this paradigm.Methodsα5IA-II was administered in the absence or presence of the benzodiazepine site antagonist flumazenil, so as to limit its action to periods associated with encoding, consolidation and recall. Drug doses and timings of administrations were defined using occupancy data derived from an in vivo [3H]flumazenil binding assay. Similar experiments were carried out to study the memory-disruptive properties of chlordiazepoxide (CDP).ResultsThe trial 1 to trial 2 difference was increased when α5IA-II was given before either trial 1 or trial 2, indicating an effect on the encoding and recall phases, respectively, of learning and memory. Conversely, α5IA-II had no effect on performance when given immediately after trial 1, suggesting that it had no effect on the consolidation phase. In contrast to the facilitation of performance produced by the α5-selective inverse agonist α5IA-II given during the encoding and recall but not the consolidation phase, the non-selective agonist CDP impaired performance when given during the encoding and recall phases, whilst having no effect on the consolidation phase.ConclusionsThese data further highlight the cognition-enhancing properties of GABAA α5-selective inverse agonists and define the functional specificity of these effects in terms of encoding and recall processes in the Morris water maze.

Repeated ethanol exposure and withdrawal impairs human fear conditioning and depresses long-term potentiation in rat amygdala and hippocampus.

BACKGROUND In rats, repeated episodes of alcohol consumption and withdrawal (RWD) impair fear conditioning to discrete cues. METHODS Fear conditioning was measured in human binge drinkers as the increased startle response in the presence of a CS+ conditioned to aversive white noise. Secondly, the ability of tone CSs, paired with footshock, to induce c-fos expression, a marker of neuronal activity, in limbic structures subserving emotion was studied in rats. Additionally, consequences of RWD on subsequent induction of long term potentiation (LTP) in external capsule/lateral amygdala and Schaffer collateral/hippocampus CA1 pathways were studied in rat brain slices. RESULTS Fear conditioning was impaired in young human binge drinkers. The ability of fear-conditioned CSs to increase c-fos expression in limbic brain areas was reduced following RWD, as was LTP induction. Rats conditioned prior to RWD, following RWD showed generalization of conditioned fear from the tone CS+ to a neutral control stimulus, and a novel tone. CONCLUSIONS Binge-like drinking impairs fear conditioning, reduces LTP, and results in inappropriate generalization of learned fear responses. We propose a mechanism whereby RWD-induced synaptic plasticity reduces capacity for future learning, while allowing unconditioned stimuli access to neuronal pathways underlying conditioned fear.

Ethanol consumption: how should we measure it? Achieving consilience between human and animal phenotypes.

There is only modest overlap in the most common alcohol consumption phenotypes measured in animal studies and those typically studied in humans. To address this issue, we identified a number of alcohol consumption phenotypes of importance to the field that have potential for consilience between human and animal models. These phenotypes can be broken down into three categories: (1) abstinence/the decision to drink or abstain; (2) the actual amount of alcohol consumed; and (3) heavy drinking. A number of suggestions for human and animal researchers are made in order to address these phenotypes and enhance consilience. Laboratory studies of the decision to drink or to abstain are needed in both human and animal research. In human laboratory studies, heavy or binge drinking that meets cut‐offs used in epidemiological and clinical studies should be reported. Greater attention to patterns of drinking over time is needed in both animal and human studies. Individual differences pertaining to all consumption phenotypes should be addressed in animal research. Lastly, improved biomarkers need to be developed in future research for use with both humans and animals. Greater precision in estimating blood alcohol levels in the field, together with consistent measurement of breath/blood alcohol levels in human laboratory and animal studies, provides one means of achieving greater consilience of alcohol consumption phenotypes.

Reward sensitivity: issues of measurement, and achieving consilience between human and animal phenotypes

Reward is a concept fundamental to discussions of drug abuse and addiction. The idea that altered sensitivity to either drug–reward, or to rewards in general, contributes to, or results from, drug‐taking is a common theme in several theories of addiction. However, the concept of reward is problematic in that it is used to refer to apparently different behavioural phenomena, and even to diverse neurobiological processes (reward pathways). Whether these different phenomena are different behavioural expressions of a common underlying process is not established, and much research suggests that there may be only loose relationships among different aspects of reward. Measures of rewarding effects of drugs in humans often depend upon subjective reports. In animal studies, such insights are not available, and behavioural measures must be relied upon to infer rewarding effects of drugs or other events. In such animal studies, but also in many human methods established to objectify measures of reward, many other factors contribute to the behaviour being studied. For that reason, studying the biological (including genetic) bases of performance of tasks that ostensibly measure reward cannot provide unequivocal answers. The current overview outlines the strengths and weaknesses of current approaches that hinder the conciliation of cross‐species studies of the genetics of reward sensitivity and the dysregulation of reward processes by drugs of abuse. Some suggestions are made as to how human and animal studies may be made to address more closely homologous behaviours, even if those processes are only partly able to isolate ‘reward’ from other factors contributing to behavioural output.