Allelic variation in the α5 nicotinic acetylcholine receptor (nAChR) subunit gene, CHRNA5, increases vulnerability to tobacco addiction. Here, we investigated the role of α5* nAChRs in the effects of nicotine on brain reward systems.
Materials and methods
Effects of acute (0.03125-0.5 mg/kg SC) or chronic (24 mg/kg per day; osmotic minipump) nicotine, and mecamylamine-precipitated withdrawal, on intracranial self-stimulation (ICSS) thresholds were assessed in wildtype and α5 nAChR subunit knockout mice. Noxious effects of nicotine were further investigated using a conditioned taste aversion (CTA) procedure.
Lower nicotine doses (0.03125-0.125 mg/kg) decreased ICSS thresholds in wildtype and α5 knockout mice. At higher doses (0.25-0.5 mg/kg), threshold-lowering effects of nicotine were diminished in wildtype mice, whereas nicotine lowered thresholds across all doses tested in α5 knockout mice. Nicotine (1.5 mg/kg) conditioned a taste aversion to saccharine equally in both genotypes. Mecamylamine (5 mg/kg) elevated ICSS thresholds by a similar magnitude in wildtype and α5 knockout mice prepared with minipumps delivering nicotine. Unexpectedly, mecamylamine also elevated thresholds in saline-treated α5 knockout mice.
α5* nAChRs are not involved in reward-enhancing effects of lower nicotine doses, the reward-inhibiting effects of nicotine withdrawal, or the general noxious effects of higher nicotine doses. Instead, α5* nAChRs regulate the reward-inhibiting effects nicotine doses that oppose the reward-facilitating effects of the drug. These data suggest that disruption of α5* nAChR signaling greatly expands the range of nicotine doses that facilitate brain reward activity, which may help explain the increased tobacco addiction vulnerability associated with CHRNA5 risk alleles.
CHRNA5; α5 nicotinic receptors; nicotine; reward; aversion; habenula; interpeduncular nucleus; conditioned taste aversion
Research has begun to examine how acute cognitive impairment from alcohol could contribute to alcohol abuse. Specifically, alcohol-induced impairment of inhibitory control could compromise the drinker’s ability to stop the self-administration of alcohol, increasing the risk of binge drinking.
The present study was designed to test this hypothesis by examining the relation between acute alcohol impairment of inhibitory control and alcohol consumption during a single drinking episode.
Materials and methods
Twenty-six healthy adults performed a cued go/no-go task that measured inhibitory control. The study tested the degree to which their inhibitory control was impaired by a moderate dose of alcohol (0.65 g/kg) versus a placebo and the extent to which individual differences in this impairment predicted levels of alcohol consumption as assessed by ad lib drinking in the laboratory.
In accord with the hypothesis, greater impairment of inhibitory control from alcohol was associated with increased ad lib consumption.
Acute impairment of inhibitory control might be an important cognitive effect that contributes to abuse in addition to the positive rewarding effects of the drug.
Alcohol; Inhibition; Ad lib consumption; Cued go/no-go task; Neurocognitive mechanisms; Abuse potential
Alcohol effects on behavioral and cognitive mechanisms influence impaired driving performance and decisions to drive after drinking (Barry 1973; Moskowitz and Robinson 1987). To date, research has focused on the ascending limb of the blood alcohol curve, and there is little understanding of how acute tolerance to impairment of these mechanisms might influence driving behavior on the descending limb.
To provide an integrated examination of the degree to which alcohol impairment of motor coordination and inhibitory control contributes to driving impairment and decisions to drive on the ascending and descending limbs of the blood alcohol curve.
Social-drinking adults (N=20) performed a testing battery that measured simulated driving performance and willingness to drive, as well as mechanisms related to driving: motor coordination (grooved pegboard), inhibitory control (cued go/no-go task), and subjective intoxication. Performance was tested in response to placebo and a moderate dose of alcohol (0.65 g/kg) twice at comparable blood alcohol concentrations: once on the ascending limb and again on the descending limb.
Impaired motor coordination and subjective intoxication showed acute tolerance, whereas driving performance and inhibitory control showed no recovery from impairment. Greater motor impairment was associated with poorer driving performance under alcohol, and poorer inhibitory control was associated with more willingness to drive.
Findings suggest that acute tolerance to impairment of motor coordination is insufficient to promote recovery of driving performance and that the persistence of alcohol-induced disinhibition might contribute to risky decisions to drive on the descending limb.
Acute tolerance; Driving; Inhibition; Motor control; Subjective intoxication
Poor behavioral control and heightened attentional bias toward alcohol-related stimuli have independently received considerable attention in regard to their roles in alcohol abuse. Theoretical accounts have begun to speculate as to potential reciprocal interactions between these two mechanisms that might promote excessive alcohol consumption, yet experimental evidence is lacking.
The objective of the study was to integrate these two lines of research through the development of a novel laboratory task that examines the degree to which alcohol cues serve to disrupt mechanisms of behavioral control.
Fifty adult drinkers were recruited to perform the attentional bias–behavioral activation (ABBA) task. The ABBA task, an adaptation of traditional cued go/no-go tasks, is a reaction time model that measures the degree to which alcohol-related stimuli can increase behavioral activation of a drinker and reduce the ability to inhibit inappropriate responses. Participants also completed a novel measure of attentional bias, the scene inspection paradigm (SIP), that measures fixation time on alcohol content imbedded in complex scenes.
As hypothesized, the proportion of inhibitory failures on the ABBA task was significantly higher following alcohol images compared to neutral images. Correlational analyses showed that heightened attentional bias on the SIP was associated with greater response activation following alcohol images on the ABBA task.
These findings suggest that alcohol stimuli serve to disrupt mechanisms of behavioral control, and that heightened attentional bias is associated with greater disruption of control mechanisms following alcohol images.
Alcohol; Behavioral control; Attentional bias; Response activation; Inhibition
Orolingual motor deficits, such as dysarthria and dysphagia, contribute to increased morbidity and mortality in the elderly. In preclinical studies we and others have reported age-related decreases in tongue motility in both F344 and F344/BN rats. The fact that these deficits are associated with nigrostriatal dopamine (DA) tissue measures suggests that increasing dopamine function might normalize tongue motility.
The purpose of the current study was to determine whether two indirect dopamine agonists with locomotor-enhancing effects, d-amphetamine (amphetamine; 1 & 2 mg/kg) and GBR-12909 (5, 10, and 20 mg/kg), can improve tongue motility in aged F344/BN rats.
Young (6 months) and aged (30 months) F344/BN rats licked water from an isometric force disc so that tongue motility (licks/second) and tongue force could be measured as a function of age and drug dose.
Consistent with our previous studies, tongue force was greater and tongue motility was lower in the aged group. Tongue motility was increased by amphetamine but not by GBR-12909. Amphetamine decreased peak tongue force, primarily in the young group. GBR-12909 did not affect tongue force. GBR-12909 increased the number of licks/session in the young group but not in the aged group, while amphetamine increased this measure in both groups.
These results demonstrate differential effects of these drugs on orolingual motor function and suggest that blocking DA uptake is insufficient to increase tongue motility in aging.
aging; aging; oromotor; bradykinesia; movement; licking; operant; tongue
Cue exposure therapy, which attempts to limit relapse by reducing reactivity to cocaine-paired cues through repeated exposures, has had limited success.
The current experiments examined cocaine cue-induced anxiogenesis, and investigated whether a model of cue-exposure therapy would reduce reinstatement of cocaine seeking in rats with a history of cocaine self-administration.
Male rats experienced daily intravenous cocaine self-administration. Rats then experienced exposure to either the self-administration context or the context plus noncontingent presentations of cocaine-paired cues. Immediately following exposure, anxiety-like behavior was measured using elevated plus maze and defensive burying tests. In a second group of rats, self-administration was followed by seven days of exposure to the context, context + noncontingent cue exposure, lever extinction, or cue + lever extinction. All animals then underwent two contingent cue-induced reinstatement tests separated by 7 days of lever extinction.
Exposure to noncontingent cocaine-paired cues in the self-administration context increased anxiety-like behavior on the defensive burying test. Animals that experienced lever + cue extinction displayed the least cocaine seeking on the first reinstatement test, and lever extinction reduced cocaine seeking below context exposure or context + noncontingent cue exposure. All animals had similar levels of cocaine seeking on the second reinstatement test.
Noncontingent cue exposure causes anxiety, and noncontingent cue and context exposure are less effective at reducing contingent cue-induced reinstatement than lever or lever + cue extinction. These data indicate that active extinction of the drug-taking response may be critical for reduction of relapse proclivity in former cocaine users.
anxiety; cocaine; cues; extinction; reinstatement; relapse
Pharmacological magnetic resonance imaging (phMRI) provides an approach to study effects of drug challenges on brain processes. Elucidating mechanisms of drug action helps us to better understand the workings of neurotransmitter systems, map brain function or facilitate drug development. phMRI is increasingly used in preclinical research employing rodent models; however, data interpretation and integration are complicated by the use of different experimental approaches between laboratories. In particular, the effects of different anaesthetic regimes upon neuronal and haemodynamic processes and baseline physiology could be problematic.
This paper investigates how differences in phMRI research methodologies are manifested and considers associated implications, placing particular emphasis on choice of anaesthetic regimes.
A systematic review of rodent phMRI studies was conducted. Factors such as those describing anaesthetic regimes (e.g. agent, dosage) and parameters relating to physiological maintenance (e.g. ventilatory gases) and MRI method were recorded.
We identified 126 eligible studies and found that the volatile agents isoflurane (43.7 %) and halothane (33.3 %) were most commonly used for anaesthesia, but dosage and mixture of ventilatory gases varied substantially between laboratories. Relevant physiological parameters were usually recorded, although 32 % of studies did not provide cardiovascular measures.
Anaesthesia and animal preparation can influence phMRI data profoundly. The variation of anaesthetic type, dosage regime and ventilatory gases makes consolidation of research findings (e.g. within a specific neurotransmitter system) difficult. Standardisation of a small(er) number of preclinical phMRI research methodologies and/or increased consideration of approaches that do not require anaesthesia is necessary to address these challenges.
Electronic supplementary material
The online version of this article (doi:10.1007/s00213-014-3855-0) contains supplementary material, which is available to authorized users.
phMRI; Cerebral blood flow; Anesthesia; Neuroimaging; fMRI; Pharmacological magnetic resonance imaging; Animal model; Systematic review
Neuroactive steroids and benzodiazepines can positively modulate GABA by acting at distinct binding sites on synaptic GABAA receptors. Although these receptors are thought to mediate the behavioral effects of both benzodiazepines and neuroactive steroids, other receptors (e.g., extrasynaptic GABAA, NMDA, σ1, or 5-HT3 receptors) might contribute to the effects of neuroactive steroids, resulting in differences among positive modulators.
The current study established the neuroactive steroid pregnanolone as a discriminative stimulus to determine whether actions in addition to positive modulation of synaptic GABAA receptors might contribute to its discriminative stimulus effects.
Four rhesus monkeys discriminated 5.6 mg/kg pregnanolone while responding under a fixed-ratio 10 schedule of stimulus-shock termination.
Positive modulators acting at benzodiazepine, barbiturate, or neuroactive steroid sites produced ≥80% pregnanolone-lever responding, whereas drugs acting primarily at receptors other than synaptic GABAA receptors, such as extrasynaptic GABAA, NMDA, σ1, and 5-HT3 receptors, produced vehicle-lever responding. Flumazenil antagonized the benzodiazepines midazolam and flunitrazepam, with Schild analyses yielding slopes that did not deviate from unity and pA2 values of 7.39 and 7.32, respectively. Flumazenil did not alter the discriminative stimulus effects of pregnanolone.
While these results do not exclude the possibility that pregnanolone acts at receptors other than synaptic GABAA receptors, they indicate a primary if not exclusive role of synaptic GABAA receptors in its discriminative stimulus effects. Reported differences in the chronic effects of benzodiazepines and neuroactive steroids are not due to differences in their actions at synaptic GABAA receptors.
pregnanolone; benzodiazepines; drug discrimination; rhesus monkeys
Initial screening of new medications for potential efficacy (i.e. FDA early Phase 2), such as in aiding smoking cessation, should be efficient in identifying which drugs do, or do not, warrant more extensive (and expensive) clinical testing.
This focused review outlines our research on development, evaluation, and validation of an efficient crossover procedure for sensitivity in detecting medication efficacy for smoking cessation. First-line FDA-approved medications of nicotine patch, varenicline, and bupropion were tested, as model drugs, in 3 separate placebo-controlled studies. We also tested specificity of our procedure in identifying a drug that lacks efficacy, using modafinil.
This crossover procedure showed sensitivity (increased days of abstinence) during week-long “practice” quit attempts with each of the active cessation medications (positive controls) vs. placebo, but not with modafinil (negative control) vs. placebo, as hypothesized. Sensitivity to medication efficacy signal was observed only in smokers high in intrinsic quit motivation (i.e. already preparing to quit soon) and not smokers low in intrinsic quit motivation, even if monetarily reinforced for abstinence (i.e., given extrinsic motivation).
A crossover procedure requiring less time and fewer subjects than formal trials may provide an efficient strategy for a go/no-go decision whether to advance to subsequent Phase 2 randomized clinical trials with a novel drug. Future research is needed to replicate our results and evaluate this procedure with novel compounds, identify factors that may limit its utility, and evaluate its applicability to testing efficacy of compounds for treating other forms of addiction.
Medication screening; Pharmacotherapy; Nicotine dependence; Addiction
Contextual control over drug relapse depends on the successful reconsolidation and retention of context-response-cocaine associations in long-term memory stores. The basolateral amygdala (BLA) plays a critical role in cocaine memory reconsolidation and subsequent drug context-induced cocaine-seeking behavior; however, less is known about the cellular mechanisms of this phenomenon.
The present study evaluated the hypothesis that protein kinase A (PKA) and calcium/calmodulin-dependent protein kinase II (CaMKII) activation in the BLA is necessary for the reconsolidation of context-response-cocaine memories that promote subsequent drug context-induced cocaine-seeking behavior.
Rats were trained to lever-press for cocaine infusions in a distinct context, followed by extinction training in a different context. Rats were then briefly re-exposed to the previously cocaine-paired context or an unpaired context in order to reactivate cocaine-related contextual memories and initiate their reconsolidation or to provide a similar behavioral experience without explicit cocaine-related memory reactivation, respectively. Immediately after this session, rats received bilateral microinfusions of vehicle, the PKA inhibitor, Rp-Adenosine 3′,5′-cyclic monophosphorothioate triethylammonium salt (Rp-cAMPS), or the CaMKII inhibitor, KN-93, into the BLA or the posterior caudate putamen (anatomical control region). Rats were then tested for cocaine-seeking behavior (responses on the previously cocaine-paired lever) in the cocaine-paired context and the extinction context.
Intra-BLA infusion of Rp-cAMPS, but not KN-93, following cocaine memory reconsolidation impaired subsequent cocaine-seeking behavior in a dose-dependent, site-specific, and memory reactivation-dependent fashion.
PKA, but not CaMKII, activation in the BLA is critical for cocaine memory re-stabilization processes that facilitate subsequent drug context-induced instrumental cocaine-seeking behavior.
memory reconsolidation; context; cocaine-seeking; protein kinase A; basolateral amygdala; self-administration; calcium/calmodulin-dependent protein kinase 2
Chronic pain is becoming a more common medical diagnosis and is especially prevalent in older individuals. As such, prescribed use of opioids is on the rise, even though the efficacy for pain management in older individuals is unclear.
Thus the present preclinical study assessed the effectiveness of chronic fentanyl administration to produce antinociception in aging rats (16, 20, 24 months).
Animals were tested in a thermal sensitivity procedure known to involve neural circuits implicated in chronic pain in humans. Sensitivity to heat and cold thermal stimulation was assessed during 28 days of fentanyl administration (1.0 mg/kg/day), and 28 days of withdrawal.
Fentanyl resulted in decreased thermal sensitivity to heat but not cold stimulation indicated by more time spent in the hot compartment relative to time spent in the cold or neutral compartments. Unlike previous findings using a hot-water tail withdrawal procedure, tolerance did not develop to the antinociceptive effects of fentanyl over a 28-day period of drug administration. The oldest animals were least sensitive, and the youngest animals most sensitive to the locomotor-stimulating effects of fentanyl. The effect on the antinociceptive response to fentanyl in the oldest group of rats was difficult to interpret due to profound changes in the behavior of saline-treated animals.
Overall, aging modifies the behavioral effects of opioids, a finding that may inform future studies for devising appropriate treatment strategies.
Aging; Antinociception; Chronic administration; Geriatric pharmacotherapy; Pain; Thermal sensitivity; Tolerance; Withdrawal
The interaction between ethanol (EtOH) and anxiety plays an integral role in the development and maintenance of alcoholism. Many medications in pre-clinical or clinical trials for the treatment of alcoholism share anxiolytic properties. However, these drugs typically have untoward side effects, such as sedation or impairment of motor function that may limit their clinical use. We have recently demonstrated that BRL 37344 (BRL), a selective β3-adrenoceptor agonist, enhances a discrete population of GABAergic synapses in the basolateral amygdala (BLA) that mediates feed-forward inhibition from lateral paracapsular (LPC) GABAergic interneurons onto BLA pyramidal cells. Behavioral studies revealed that intra-BLA infusion of BRL significantly reduced measures of unconditioned anxiety-like behavior without locomotor depressant effects.
The present studies tested the effect of BRL (0.1, 0.5, or 1.0μg/side) on EtOH self-administration using an intermittent access (IA) home cage two-bottle choice procedure, and limited access operant responding for EtOH or sucrose.
Intra-BLA infusion of BRL did not reduce homecage, intermittent EtOH self-administration. However, using an operant procedure that permits the discrete assessment of appetitive (seeking) and consummatory measures of EtOH self-administration, BRL reduced measures of EtOH and sucrose seeking, but selectively reduced operant responding for EtOH during extinction probe trials. BRL had no effect on consummatory behaviors for EtOH or sucrose.
Together, these data suggest that intra-BLA infusion of BRL significantly reduces motivation to seek EtOH and provide initial evidence that β3-ARs and LPC GABAergic synapses may represent promising targets for the development of novel pharmacotherapies for the treatment of alcoholism.
alcoholism; anxiety; GABA; negative reinforcement; norepinephrine
Drug effects on delay discounting are thought to reflect changes in sensitivity to reinforcer delay, although other behavioral mechanisms might be involved. One strategy for revealing the influence of different behavioral mechanisms is to alter features of the procedures in which they are studied.
This experiment examined whether the order of delay presentation under within-session delay discounting procedures impacts drug effects on discounting.
Rats responded under a discrete-trial choice procedure in which responses on one lever delivered 1 food pellet immediately and responses on the other lever delivered 3 food pellets either immediately or after a delay. The delay to the larger reinforcer (0, 4, 8, 16, and 32 s) was varied within session and the order of delay presentation (ascending or descending) varied between groups.
Amphetamine (0.1–1.78 mg/kg) and methylphenidate (1.0–17.8 mg/kg) shifted delay functions upward in the ascending group (increasing choice of the larger reinforcer) and downward in the descending group (decreasing choice of the larger reinforcer). Morphine (1.0–10.0 mg/kg) and delta-9-tetrahydrocannabinol (0.32–5.6 mg/kg) tended to shift the delay functions downward, regardless of order of delay presentation, thereby reducing choice of the larger reinforcer, even when both reinforcers were delivered immediately.
The effects of amphetamine and methylphenidate under delay discounting procedures differed depending on the order of delay presentation indicating that drug-induced changes in discounting were due, in part, to mechanisms other than altered sensitivity to reinforcer delay. Instead, amphetamine and methylphenidate altered responding in a manner consistent with increased behavioral perseveration.
amphetamine; methylphenidate; morphine; delta-9-tetrahydrocannabinol; delay discounting; order of delay presentation; perseveration; lever press; rats
Abuse of synthetic cathinones, popularized as “bath salts,” has increased dramatically in the United States since their debut in 2010. Preclinical behavioral studies may clarify determinants of the abuse-related effects produced by these compounds.
This study examined behavioral effects of (±)-methcathinone, (±)-3,4-methylenedioxypyrovalerone (MDPV), (±)-3,4-methylenedioxymethcathinone (methylone) and (±)-4-methylmethcathinone (mephedrone) in rats using intracranial self-stimulation (ICSS).
Male Sprague-Dawley rats (n=18) with electrodes targeting the medial forebrain bundle responded for multiple frequencies of brain stimulation and were tested in two phases. First, dose-effect curves for methcathinone (0.1–1.0 mg/kg), MDPV (0.32–3.2 mg/kg), methylone (1.0–10 mg/kg) and mephedrone (1.0–10 mg/kg) were determined. Second, time courses were determined for effects produced by the highest dose of each compound.
Methcathinone produced dose- and time-dependent facilitation of ICSS. MDPV, methylone and mephedrone produced dose- and time-dependent increases in low rates of ICSS maintained by low brain stimulation frequencies, but also produced abuse-limiting depression of high ICSS rates maintained by high brain stimulation frequencies. Efficacies to facilitate ICSS were methcathinone ≥ MDPV ≥ methylone > mephedrone. Methcathinone was the most potent compound, and MDPV was the longest acting compound.
All compounds facilitated ICSS at some doses and pretreatment times, which is consistent with abuse liability for each of these compounds. However, efficacies of compounds to facilitate ICSS varied, with methcathinone displaying the highest efficacy and mephedrone the lowest efficacy to facilitate ICSS.
methcathinone; methylenedioxypyrovalerone; methylone; mephedrone; intracranial self-stimulation; rat; drug abuse
18-25-year-olds show the highest rates of alcohol use disorders (AUD) and heavy drinking, which may have critical neurocognitive implications. Regions subserving memory may be particularly susceptible to alcohol-related impairments.
We used blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) to examine the neural correlates of visual encoding and recognition among heavy drinking college students. We predicted that heavy drinkers would show worse memory performance and increased frontal/parietal activation and decreased hippocampal response during encoding.
Participants were 23 heavy drinkers and 33 demographically matched light drinkers, ages 18-20, characterized using quantity/frequency of drinking and AUD diagnosis. Participants performed a figural encoding and recognition task during fMRI. BOLD response during encoding was modeled based on whether each stimulus was subsequently recognized or forgotten (i.e., correct vs. incorrect encoding).
There were no group differences in behavioral performance. Compared to light drinkers, heavy drinkers showed: 1) greater BOLD response during correct encoding in right hippocampus/medial temporal, right dorsolateral prefrontal, left inferior frontal, and bilateral posterior parietal cortices; 2) less left inferior frontal activation and greater bilateral precuneus deactivation during incorrect encoding; and 3) less bilateral insula response during correct recognition (clusters >10,233ul, p<.05 whole-brain).
This is the first investigation of the neural substrates of figural memory among heavy drinking older adolescents. Heavy drinkers demonstrated compensatory hyperactivation of memory-related areas during correct encoding, greater deactivation of default mode regions during incorrect encoding, and reduced recognition-related response. Results could suggest use of different encoding and recognition strategies among heavy drinkers.
Alcohol; Adolescent; Young Adult; fMRI; Learning; Memory; Cognition
Ventricular enlargement is a robust phenotype of the chronically dependent alcoholic human brain, yet the mechanism of ventriculomegaly is unestablished. Heterogeneous stock Wistar rats administered binge EtOH (3 g/kg intragastrically every 8 h for 4 days to average blood alcohol levels (BALs) of 250 mg/dL) demonstrate profound but reversible ventricular enlargement and changes in brain metabolites (e.g., N-acetylaspartate (NAA) and choline-containing compounds (Cho)).
Here, alcohol-preferring (P) and alcohol-nonpreferring (NP) rats systematically bred from heterogeneous stock Wistar rats for differential alcohol drinking behavior were compared with Wistar rats to determine whether genetic divergence and consequent morphological and neurochemical variation affect the brain’s response to binge EtOH treatment.
The three rat lines were dosed equivalently and approached similar BALs. Magnetic resonance imaging and spectroscopy evaluated the effects of binge EtOH on brain.
As observed in Wistar rats, P and NP rats showed decreases in NAA. Neither P nor NP rats, however, responded to EtOH intoxication with ventricular expansion or increases in Cho levels as previously noted in Wistar rats. Increases in ventricular volume correlated with increases in Cho in Wistar rats.
The latter finding suggests that ventricular volume expansion is related to adaptive changes in brain cell membranes in response to binge EtOH. That P and NP rats responded differently to EtOH argues for intrinsic differences in their brain cell membrane composition. Further, differential metabolite responses to EtOH administration by rat strain implicate selective genetic variation as underlying heterogeneous effects of chronic alcoholism in the human condition.
Alcoholism; Genetics; Selective breeding; Ventriculomegaly; Choline; Magnetic resonance (MR) imaging; MR spectroscopy