Cognitive recovery during and after treatment for volatile solvent abuse

The Effects of the Inhalant Toluene on Cognitive Function and Behavioral Flexibility: A Review of Recent Findings

Substance use disorder (SUD) is characterized, in part, by lack of control over drug seeking and taking. The prefrontal cortex (PFC) is highly involved in control of behavior and deficits in PFC structure and function have been demonstrated in clinical and preclinical studies of SUD. Of the various classes of drugs associated with the development of SUD, inhalants are among the least studied despite their widespread use among adolescents and children. In this work, we review what is currently known regarding the sites and mechanisms of action of inhalants with a focus on the volatile solvent toluene that is contained in a wide variety of legal and easily obtained products. We then describe how inhalants including toluene affect various behaviors with an emphasis on those associated with PFC function and how chronic use of inhalants alters brain structure and neuronal signaling. Findings from these studies highlight advances made in recent years that have expanded our understanding of the effects of inhalants on brain structure and reinforce the need for continued work in this field.

What is the optimum thiamine dose to treat or prevent Wernicke's encephalopathy or Wernicke-Korsakoff syndrome? Results of a randomized controlled trial

Background

The primary cause of Wernicke–Korsakoff syndrome (WKS) is thiamine deficiency, and more than 90% of cases are reported in alcohol‐dependent patients. While observational studies show parenteral thiamine administration drastically reduced WKS‐related mortality, relevant treatment trials have never been conducted to determine the optimum thiamine dose.

Methods

Two double‐blind, parallel groups, randomized controlled trials (RCTs) were conducted to determine the optimal thiamine dose required for (1) the prevention of Wernicke's encephalopathy (WE), the acute phase of WKS, in asymptomatic but “at‐risk” alcohol misuse patients (Study 1) and (2) the treatment of WE in symptomatic alcohol misuse patients (Study 2). Each study had a dosage regimen comprising three parenteral thiamine doses that were allocated at a ratio of 1:1:1. Study 1: Asymptomatic At‐Risk patients (N = 393) received either 100 mg daily, 100 mg thrice daily, or 300 mg thrice daily, for 3 days. Study 2: Symptomatic patients (N = 127) received either 100 mg thrice daily, 300 mg thrice daily, or 500 mg thrice daily, for 5 days. Cognitive function was the primary outcome, assessed using the Rowland Universal Dementia Assessment Scale, two Cogstate subtests, and an adapted Story Memory Recall test. Secondary analyses examined differences in neurological function (ataxia, oculomotor abnormalities, and confusion) at follow‐up.

Results

No significant differences were observed between any of the dosage conditions for either Study 1 or Study 2 on cognition or neurological functioning. This real‐world study found that having a clinically unwell target population with high comorbidity and multiple presentations, coupled with challenges in cross‐cultural assessment is likely to complicate RCT findings.

Conclusions

The results of this study showed no clear benefit of high dose thiamine over intermediate or lower doses of thiamine, over the time intervals examined, for the treatment and prevention of cognitive and neurological abnormalities related to WKS. Several study limitations temper the interpretation of these findings. Nevertheless, the absence of conclusive evidence for the superiority of high‐dose thiamine supports a recommendation for patient‐specific treatment, while ensuring that the potential impact of other biochemical factors (e.g., magnesium and other B vitamin deficiencies) are considered and corrected if necessary.

Neurotoxicity induced by toluene: In silico and in vivo evidences of mitochondrial dysfunction and dopaminergic neurodegeneration

Toluene is an air pollutant widely used as an organic solvent in industrial production and emitted by fossil fuel combustion, in addition to being used as a drug of abuse. Its toxic effects in the central nervous system have not been well established, and how and which neurons are affected remains unknown. Hence, this study aimed to fill this gap by investigating three central questions: 1) How does toluene induce neurotoxicity? 2) Which neurons are affected? And 3) What are the long-term effects induced by airborne exposure to toluene? To this end, a Caenorhabditis elegans model was employed, in which worms at the fourth larval stage were exposed to toluene in the air for 24 h in a vapor chamber to simulate four exposure scenarios. After the concentration-response curve analysis, we chose scenarios 3 (E3: 792 ppm) and 4 (E4: 1094 ppm) for the following experiments. The assays were performed 1, 48, or 96 h after removal from the exposure environments, and an irreversible reduction in neuron fluorescence and morphologic alterations were observed in different neurons of exposed worms, particularly in the dopaminergic neurons. Moreover, a significant impairment in a dopaminergic-dependent behavior was also associated with negative effects in healthspan endpoints, and we also noted that mitochondria may be involved in toluene-induced neurotoxicity since lower adenosine 5'-triphosphate (ATP) levels and mitochondrial viability were observed. In addition, a reduction of electron transport chain activity was evidenced using ex vivo protocols, which were reinforced by in silico and in vitro analysis, demonstrating toluene action in the mitochondrial complexes. Based on these findings model, it is plausible that toluene neurotoxicity can be initiated by complex I inhibition, triggering a mitochondrial dysfunction that may lead to irreversible dopaminergic neuronal death, thus impairing neurobehavioral signaling.

Gasoline fume inhalation induces apoptosis, inflammation, and favors Th2 polarization in C57BL/6 mice

Gasoline exposure has been widely reported in the literature as being toxic to human health. However, the exact underlying molecular mechanisms triggered by its inhalation have not been thoroughly investigated. We herein present a model of sub-chronic, static gasoline vapor inhalation in adult female C57BL/6 mice. Animals were exposed daily to either gasoline vapors (0.86 g/animal/90 minutes) or ambient air for five days/week over seven consecutive weeks. At the end of the study period, toxic and molecular mechanisms, underlying the inflammatory, oxidative, and apoptotic effects triggered by gasoline vapors, were examined in the lungs and liver of gasoline exposed mice. Static gasoline exposure induced a significant increase (+21 %) in lungs/body weight ratio in gasoline-exposed (GE) versus control (CON) mice along with a pulmonary inflammation attested by histological staining. The latter was consistent with increases in the transcript levels of proinflammatory cytokines [Interleukins (ILs) 4 and 6], respectively by ~ 6-, 4-fold in the lungs of GE mice compared to CON. Interestingly, IL-10 expression was also increased by ~ 10-fold in the lungs of GE mice suggesting an attempt to counterbalance the established inflammation. Moreover, the pulmonary expression of IL-12 and TNF-α was downregulated by 2- and 4-fold, respectively, suggesting the skewing toward Th2 phenotype. Additionally, GE mice showed a significant upregulation in Bax/Bcl-2 ratio, caspases 3, 8 and 9 with no change in JNK expression in the lungs, suggesting the activation of both intrinsic and extrinsic apoptotic pathways. Static gasoline exposure over seven consecutive weeks had a minor hepatic portal inflammation attested by H&E staining along with an increase in the hepatic expression of the mitochondrial complexes in GE mice. Therefore, tissue damage biomarkers highlight the health risks associated with vapor exposure and may present potential therapeutic targets for recovery from gasoline intoxication.

The Abused Inhalant Toluene Impairs Medial Prefrontal Cortex Activity and Risk/Reward Decision-Making during a Probabilistic Discounting Task

Inhalant (e.g., toluene) misuse is linked to behavioral and cognitive deficits in humans, yet preclinical studies of the effect of inhalants on higher-order cognition are limited. We addressed this gap in the literature by examining the effect of toluene vapor exposure on risk/reward decision-making in male and female Sprague-Dawley rats using a probabilistic discounting task. In this task, rodents chose a risky/large reward or a safe/small reward, with the odds of risky reinforcement descending or ascending throughout the test session. We observed a dose-dependent, sex-independent deficit in behavioral flexibility during probabilistic discounting caused by acute toluene exposure. Rats exposed to toluene vapor during adolescence and tested as adults performed comparably to air-treated controls and were susceptible to the effects of an acute toluene challenge. These behavioral flexibility deficits observed suggests dysfunctional medial prefrontal cortex (mPFC) activity. To address this hypothesis, we virally expressed the genetically encoded calcium sensor GCaMP6f in glutamatergic mPFC neurons and monitored calcium transients in real-time using in vivo fiber photometry. mPFC activity peaked before either lever press during free-choice trials in toluene- and air-treated animals. During forced-choice trials, GCaMP6f transients shifted from pre-risky to pre-safe choice, an effect mitigated by acute toluene exposure. mPFC activity decreased during rewarded trials, with larger decreases following risky/large wins compared with safe/small wins. Toluene-treated animals also had decreased mPFC activity during rewarded trials, but there was no distinction between risky/large wins and safe/small wins. These results provide physiological evidence for mPFC-dependent behavioral deficits caused by toluene.SIGNIFICANCE STATEMENT Inhalants (e.g., toluene) are an understudied class of drugs of abuse that cause devastating behavioral and cognitive deficits in humans. Understanding the neurobiological interactions of toluene vapor using animal models is important for developing effective treatment strategies for inhalant addicts. Here we find that toluene vapor reduces behavioral flexibility in rodents making risk/reward-based decisions. The medial prefrontal cortex (mPFC) drives behavioral flexibility during this type of decision-making and we show that toluene reduces the ability of mPFC neurons to track optimal choices as reward probabilities change. Toluene also reduces these neurons' ability to distinguish between small and large rewards. A combination of these factors likely leads to the impaired performance in probabilistic discounting following acute toluene exposure.

Exploring the reliability and acceptability of cognitive tests for Indigenous Australians: a pilot study

BACKGROUND

Reliable cognitive assessment for Indigenous Australians is difficult given that mainstream tests typically rely on Western concepts, content and values. A test's psychometric properties should therefore be assessed prior to use in other cultures. The aim of this pilot study was to examine the reliability and acceptability of four cognitive tests for Australian Aboriginal people.

METHODS

Participants were 40 male and 44 female (N = 84) Aboriginal patients from Alice Springs Hospital. Four tests were assessed for reliability and acceptability - Rowland Universal Dementia Assessment Screen (RUDAS) (n = 19), PEBL Corsi Blocks (Corsi) (n = 19), Story Memory Recall Test (SMRT) (n = 17) and a CogState battery (n = 18). Participants performed one to three of the tests with repeated assessment to determine test-retest reliability. Qualitative interviews were conducted and analysed based on an adapted phenomenological approach to explore test acceptability. An Indigenous Reference Group gave advice and guidance.

RESULTS

Intra-class correlations (ICC) for test retest reliability ranged from r = 0.58 (CogState One Back accuracy) to 0.86 (RUDAS). Themes emerged relating to general impressions, impacts on understanding and performance, appropriateness, task preferences and suggested improvements.

CONCLUSIONS

RUDAS, CogState Identification task, and SMRT showed the highest reliabilities. Overall the tests were viewed as a positive challenge and an opportunity to learn about the brain despite provoking some anxiety in the patients. Caveats for test acceptability included issues related to language, impacts of convalescence and cultural relevance.

Serial neuropsychological assessment of an adolescent girl after suffering a sudden out-of-hospital-cardiac-arrest following recreational inhalant use

Sudden out-of-hospital-cardiac-arrest (OHCA) following recreational inhalant use is well documented in the literature. The present case study reports the long-term neuropsychological sequelae of a 14-year-old girl who suffered an OHCA secondary to recreational butane gas inhalation. The patient was assessed on day-13 as an inpatient, and again at 3- and 12-month outpatient follow-ups. Acutely the patient demonstrated significant impairment on tasks associated with verbal and non-verbal memory and executive functioning. Re-testing at 3-months post-acute state suggested improved executive and non-verbal functions while showing continued deficits in verbal memory. At 12-months she was cognitively performing at levels expected for her age. This case is reported for its rarity in successfully being able to track the patient's clinical course from hospital discharge to the successful reintegration back into school and community settings.

Adolescent inhalant use prevention, assessment, and treatment: A literature synthesis

Inhalant use refers to the use of substances such as gases, glues, and aerosols in order to achieve intoxication, while inhalant use disorder (IUD) encompasses both DSM-IV-TR criteria for inhalant abuse and dependence. Inhalant use among adolescents is an international public health concern considering the severe medical and cognitive consequences and biopsychosocial correlates. In this paper, we summarize the current state of the literature on inhalant use among adolescents focusing on social context, prevention, assessment, and treatment strategies. Psychoeducation, skills training, and environmental supply reduction are helpful strategies for preventing adolescent inhalant use, while parent and adolescent self-report as well as physician report of medical signs and symptoms can aid in assessment and diagnosis. Although research has only begun to explore the treatment of inhalant use, preliminary findings suggest that a multimodal approach involving individual counselling (i.e., CBT brief intervention), family therapy, and activity and engagement programs is the first-line treatment, with residential treatment programs indicated for more severe presentations. The limited nature of treatments developed specifically for inhalant use combined with high prevalence rates and potential for significant impairment within the adolescent population indicate the need for further research. Research should focus on understanding the social context of use, establishing the efficacy of current adolescent substance use treatments adapted for inhalant use, and exploring long-term outcomes.

Spatial sequence memory and spatial error monitoring in the Groton Maze Learning Task (GMLT): A validation study of GMLT sub-measures in healthy children

The stepping-stone variant of the hidden pathway maze learning (HPML) task paradigm has been extensively used to investigate cognitive functions in neuropsychology and neuropharmacology. Previous studies have used total error across trials, as well as rule-break errors and learning errors, to define spatial memory and/or executive function in healthy and impaired adults and children. However, the construct validity of performance measures on HPML tasks has not been established in healthy children. To assess the construct validity of measures of exploratory and rule-break errors on the Groton Maze Learning Task (GMLT) measures of spatial sequence memory (Corsi Blocks Task) and spatial error monitoring (Continuous Paired Associate Learning; CPAL) were used. The results indicate that Corsi span predicted GMLT spatial sequence memory and CPAL accuracy predicted GMLT spatial error monitoring. The construct validity of the GMLT as a measure of spatial memory and executive function are discussed with regard to prior research using HPML tasks in neuropsychological contexts.

Deficits in behavioural inhibition in substance abuse and addiction: a meta-analysis

AIMS

Deficits in behavioural inhibitory control are attracting increasing attention as a factor behind the development and maintenance of substance dependence. However, evidence for such a deficit is varied in the literature. Here, we synthesised published results to determine whether inhibitory ability is reliably impaired in substance users compared to controls.

METHODS

The meta-analysis used fixed-effects models to integrate results from 97 studies that compared groups with heavy substance use or addiction-like behaviours with healthy control participants on two experimental paradigms commonly used to assess response inhibition: the Go/NoGo task, and the Stop-Signal Task (SST). The primary measures of interest were commission errors to NoGo stimuli and stop-signal reaction time in the SST. Additionally, we examined omission errors to Go stimuli, and reaction time in both tasks. Because inhibition is more difficult when inhibition is required infrequently, we considered papers with rare and equiprobable NoGo stimuli separately.

RESULTS

Inhibitory deficits were apparent for heavy use/dependence on cocaine, MDMA, methamphetamine, tobacco, and alcohol (and, to a lesser extent, non-dependent heavy drinkers), and in pathological gamblers. On the other hand, no evidence for an inhibitory deficit was observed for opioids or cannabis, and contradictory evidence was observed for internet addiction.

CONCLUSIONS

The results are generally consistent with the view that substance use disorders and addiction-like behavioural disorders are associated with impairments in inhibitory control. Implications for treatment of substance use are discussed, along with suggestions for future research arising from the limitations of the extant literature.

Effects of acute and chronic inhalation of paint thinner in mice: behavioral and immunohistochemical study

Abuse of volatile inhalants has become a worldwide issue mainly among adolescents of low income social class. Acute and chronic exposure to these substances results in serious neurological and behavioral impairments. Although real exposure consists largely of simultaneous inhalation of multiple solvents, the vast majority of basic research studies have evaluated the actions of a single volatile component leaving the behavioral and neuronal effects of chemical mixture not fully understood. In this study, we investigated the acute behavioral effects of 300, 450 and 600 ppm of paint thinner inhalation on anxiety, locomotor activity and spatial memory. Additionally, the cognitive impairments related to chronic exposure of the same concentrations of thinner for 45 days were assessed. To understand the neuronal correlates of acute exposure to thinner, we used c-Fos immunohistochemistry as an endogenous marker of neuronal activation following 600 ppm of thinner. The results reveal that (i) chronically thinner exposed mice showed cognitive deficits in Morris water maze and object recognition tasks; (ii) acute inhalation of thinner induces a wide range of behavioral changes. These changes include an anxiolytic effect toward the aversive environmental bright light and a dose dependent effect on explorative locomotion. The wide range of behavioral alterations induced by acute thinner inhalation is consistent with the widespread distribution of thinner-induced c-Fos expression in multiple brain structures.

Specific impairments in instrumental learning following chronic intermittent toluene inhalation in adolescent rats

RATIONALE

Inhalant abuse is prevalent in adolescent populations, with chronic use resulting in neurobiological and cognitive abnormalities in adulthood. However, the nature and persistence of cognitive dysfunction, particularly following adolescent inhalant abuse, remain equivocal.

OBJECTIVE

The present study assessed specific cognitive processes beginning in late adolescence and adulthood following adolescent inhalation of toluene, a main component of many compounds readily abused.

METHODS

Adolescent male Wistar rats (postnatal day (PN) 27) were exposed to chronic intermittent inhaled toluene (10,000 ppm) for 1 h/day, 3 days/week for 4 weeks (PN 27-52) to mimic the patterns observed in human adolescent inhalant abusers. Following toluene exposure, motor and cognitive function was assessed.

RESULTS

Adolescent toluene exposure did not alter motor learning in the Rotarod task (PN 58) or acquisition, reversal, or retention of spatial learning in the Morris water maze (PN 55-64). In contrast, it delayed acquisition of instrumental responding for sucrose (5 % w/v) and impaired operant reversal learning and cue-induced reinstatement of sucrose seeking in adulthood (PN 57-100).

CONCLUSION

This study demonstrates that exposure to toluene at an abuse concentration during adolescence results in specific impairments in aspects of instrumental learning, without altering motor function and spatial learning in late adolescence/early adulthood. Our data imply that persistent alterations in reward processing may occur following adolescent inhalant misuse.

Adolescent inhalant use and executive cognitive functioning

BACKGROUND

This study investigates the association between inhalant use and executive cognitive functioning (ECF) and processing speed (PS) in 754 adjudicated poly-substance users on a series of neuropsychological tests. Poly-substance users who used inhalants (PSI = 262) and poly-substance users who did not use inhalants (PSO = 492) neuropsychological tests scores were compared. Hispanic Americans comprised 72% of the participants; European Americans, African Americans and Asian Americans comprised 28% of the participants.

METHODS

Standardized neuropsychological tests were used to assess ECF and PS. Psychosocial and substance abuse standardized surveys were used to assess drug use severity and psychosocial problems associated with substance use.

RESULTS

Multiple Analysis of Covariance shows that PSI users who used inhalants performed worse on ECF and PS measures in comparison to non-inhalant poly-substance users (PSO). PSI users were younger, used more drugs and had more psychiatric admissions than PSO users.

CONCLUSION

PSI users initiate substances at a younger age and experienced more ECF and PS deficits, and behavioural problems in comparison to PSO users. The results of this study suggest that PS has direct effect on ECF and psychosocial outcomes in PSI users.

Volatile solvents as drugs of abuse: focus on the cortico-mesolimbic circuitry

Volatile solvents such as those found in fuels, paints, and thinners are found throughout the world and are used in a variety of industrial applications. However, these compounds are also often intentionally inhaled at high concentrations to produce intoxication. While solvent use has been recognized as a potential drug problem for many years, research on the sites and mechanisms of action of these compounds lags behind that of other drugs of abuse. In this review, we first discuss the epidemiology of voluntary solvent use throughout the world and then consider what is known about their basic pharmacology and how this may explain their use as drugs of abuse. We next present data from preclinical and clinical studies indicating that these substances induce common addiction sequelae such as dependence, withdrawal, and cognitive impairments. We describe how toluene, the most commonly studied psychoactive volatile solvent, alters synaptic transmission in key brain circuits such as the mesolimbic dopamine system and medial prefrontal cortex (mPFC) that are thought to underlie addiction pathology. Finally, we make the case that activity in mPFC circuits is a critical regulator of the mesolimbic dopamine system's ability to respond to volatile solvents like toluene. Overall, this review provides evidence that volatile solvents have high abuse liability because of their selective effects on critical nodes of the addiction neurocircuitry, and underscores the need for more research into how these compounds induce adaptations in neural circuits that underlie addiction pathology.

A prospective study of neurocognitive changes 15 years after chronic inhalant abuse

AIMS

In a previous study, neurological and cognitive deficits reflecting central nervous system (CNS) disruption from chronic inhalant abuse showed substantial recovery after 2 years' abstinence. Functional recovery was progressive, with recovery rates dependent on the degree of impairment prior to abstinence, and severity and duration of initial abuse. Persistent deficits occurred in those with previous 'lead encephalopathy' from leaded petrol abuse. The current study examined recovery in the same cohort 15 years after baseline.

DESIGN

Prospective cohort design.

SETTING

Two remote Aboriginal communities in Arnhem Land, Australia.

PARTICIPANTS

Using baseline group classifications, 27 healthy controls, 60 ex-chronic inhalant abusers and an additional 17 with previous lead encephalopathy were assessed.

MEASUREMENTS

Standard neurological, ocular-motor and cognitive functions and blood lead levels.

FINDINGS

Chronic (non-encephalopathic) inhalant abusers showed elevated blood lead levels and abnormal scores on most tasks at baseline. At 2 years' abstinence, blood lead was reduced but remained elevated and most scores had normalized. By 15 years, blood lead and all performance scores were equivalent to healthy controls for this group (P > 0.05). The encephalopathic group was more severely impaired on all scores at baseline and showed little improvement, if any, across all tests after both 2 and 15 years' abstinence. Blood lead for this group declined, and was not significantly different to controls after 15 years.

CONCLUSIONS

Some inhalant abusers experience severe and persistent neurological deficits, suggesting irrecoverable damage attributable to lead encephalopathy. In the absence of this encephalopathy long-term abstinence from inhalants may allow recovery of normal brain function.

Adolescent toluene inhalation in rats affects white matter maturation with the potential for recovery following abstinence

Inhalant misuse is common during adolescence, with ongoing chronic misuse associated with neurobiological and cognitive abnormalities. While human imaging studies consistently report white matter abnormalities among long-term inhalant users, longitudinal studies have been lacking with limited data available regarding the progressive nature of such abnormalities, including the potential for recovery following periods of sustained abstinence. We exposed adolescent male Wistar rats (postnatal day 27) to chronic intermittent inhaled toluene (3,000 ppm) for 1 hour/day, 3 times/week for 8 weeks to model abuse patterns observed in adolescent and young adult human users. This dosing regimen resulted in a significant retardation in weight gain during the exposure period (p<0.05). In parallel, we performed longitudinal magnetic resonance imaging (T₂-weighted) and diffusion tensor imaging prior to exposure, and after 4 and 8 weeks, to examine the integrity of white matter tracts, including the anterior commissure and corpus callosum. We also conducted imaging after 8 weeks of abstinence to assess for potential recovery. Chronic intermittent toluene exposure during adolescence and early adulthood resulted in white matter abnormalities, including a decrease in axial (p<0.05) and radial (p<0.05) diffusivity. These abnormalities appeared region-specific, occurring in the anterior commissure but not the corpus callosum and were not present until after at least 4 weeks of exposure. Toluene-induced effects on both body weight and white matter parameters recovered following abstinence. Behaviourally, we observed a progressive decrease in rearing activity following toluene exposure but no difference in motor function, suggesting cognitive function may be more sensitive to the effects of toluene. Furthermore, deficits in rearing were present by 4 weeks suggesting that toluene may affect behaviour prior to detectable white matter abnormalities. Consequently, exposure to inhalants that contain toluene during adolescence and early adulthood appear to differentially affect white matter maturation and behavioural outcomes, although recovery can occur following abstinence.

Recovery from central nervous system changes following volatile substance misuse

This review examines cognitive, neurological, and neuroanatomical recovery associated with abstinence from volatile substance misuse (VSM). Articles describing functional or structural brain changes longitudinally or cross-sectional reports comparing current and abstinent users were identified and reviewed. A significant lack of empirical studies investigating central nervous system recovery following VSM was noted. The few case reports and group studies identified indicated that cognitive and neurological impairments appear to follow a progression of decline and progression of recovery model, with the severity of impairment related to the duration and severity of misuse, blood lead levels among leaded petrol misusers, and the duration of abstinence for recovery. By contrast, severe neurological impairment known as lead encephalopathy from sniffing leaded petrol occurred as more catastrophic or abrupt damage to cerebellar processes that may never fully recover. Neuroanatomical damage may not recover even with prolonged abstinence.