Concentration-dependent conditioned place preference to inhaled toluene vapors in rats

Effects of housing condition on the development and persistence of addictive-like behavior induced by toluene

Environmental factors can modify addictive responses induced by drugs of abuse; however, little is known about the impact of environmental conditions on behavioral responses induced by inhalants. In this study, we analyzed the effects of housing conditions, considering environmental enrichment (EE; n = 10), social isolation (SI; n = 10), and standard housing (STD; n = 10), as positive, negative, and control environments, respectively, on the development and persistence of behavioral sensitization induced by toluene. Mice exposed to air were used as a comparative control groups for each housing condition (EE: n = 11, SI: n = 10 and STD: n = 11). Results showed that a history of toluene exposure induced the development of locomotor sensitization in mice, independent of their housing conditions. However, SI increased the expression of behavioral sensitization to toluene after a drug-free period.

The last two decades on preclinical and clinical research on inhalant effects

This paper reviews the scientific evidence generated in the last two decades on the effects and mechanisms of action of most commonly misused inhalants. In the first section, we define what inhalants are, how they are used, and their prevalence worldwide. The second section presents specific characteristics that define the main groups of inhalants: (a) organic solvents; (b) aerosols, gases, and volatile anesthetics; and (c) alkyl nitrites. We include a table with the molecular formula, structure, synonyms, uses, physicochemical properties and exposure limits of representative compounds within each group. The third and fourth sections review the direct acute and chronic effects of common inhalants on health and behavior with a summary of mechanisms of action, respectively. In the fifth section, we address inhalant intoxication signs and available treatment. The sixth section examines the health effects, intoxication, and treatment of nitrites. The seventh section reviews current intervention strategies. Finally, we propose a research agenda to promote the study of (a) solvents other than toluene; (b) inhalant mixtures; (c) effects in combination with other drugs of abuse; (d) age and (e) sex differences in inhalant effects; (f) the long-lasting behavioral effects of animals exposed in utero to inhalants; (g) abstinence signs and neurochemical changes after interrupting inhalant exposure; (h) brain networks involved in inhalant effects; and finally (i) strategies to promote recovery of inhalant users.

Self-Administration of Toluene Vapor in Rats

Inhalants, including volatile organic solvents such as toluene, continue to be one of the most prevalent, and often first substances abused by adolescents. Like other drugs of abuse, toluene affects the function of neurons within key brain reward circuits including the prefrontal cortex, ventral tegmental area, and nucleus accumbens. However, preclinical models used to study these toluene-induced adaptations generally employ passive exposure paradigms that do not mirror voluntary patterns of solvent exposure observed in humans. To address this shortcoming, we developed an inhalation chamber containing active and inactive nose pokes, cue lights, flow-through vaporizers, and software-controlled valves to test the hypothesis that rats will voluntarily self-administer toluene vapor. Following habituation and self-administration (SA) training rats achieve vapor concentrations associated with rewarding effects of toluene, and maintain responding for toluene vapor, but not for air. During extinction trials, rats showed an initial burst of drug-seeking behavior similar to that of other addictive drugs and then reduced responding to Air SA levels. Responding on the active nose poke recovered during cue-induced reinstatement but not following a single passive exposure to toluene vapor. The results from these studies establish a viable toluene SA protocol that will be useful in assessing toluene-induced changes in addiction neurocircuitry.

Positive reinforcement and c-Fos expression following abuse-like thinner inhalation in mice: Behavioural and immunohistochemical assessment

Thinners are organic solvents widely used in industrial applications, but they have also been subject to abuse by inhalation for their psychoactive and rewarding properties. In spite of the prevalence of inhalant abuse, the addictive potential and pathways mediating their reinforcing effects are not yet fully understood and thus still subject of further investigations. Here, we assessed in mice the locomotor activity and the ability of paint thinner to reinforce the conditioning in the place preference paradigm following acute (1 day), subchronic (6 weeks) and chronic (12 weeks) exposures to 300 and 600 ppm of thinner vapor. While locomotor activity was unaffected by the different thinner treatments, a positive conditioned place preference to inhaled thinner was found upon subchronic and chronic exposures. To investigate the activated brain structures underlying such behavioural changes, we analyzed the distribution of c-Fos immunoreactivity, a marker for neuronal activation, following acute and repeated exposures to 600 ppm of thinner. Notably, thinner exposure increased the number of c-Fos immunoreactive neurons with increasing duration of exposure in the majority of structures examined; including those typically involved in the processing of rewarding or emotionally stimuli (e.g., ventral tegmental area, core and shell of nucleus accumbens, amygdala, bed nucleus of the stria terminalis, and cingulate cortex), and olfactory stimuli (e.g., piriform cortex and olfactory tubercle). Moreover, prolonged, but not acute thinner inhalation significantly increased c-Fos immunoreactivity in all hippocampal subregions. Taken together, the expanded distribution of thinner-induced c-Fos expression may underlie the observed positive reinforcement upon long-term thinner inhalation.

Exposure to the Abused Inhalant Toluene Alters Medial Prefrontal Cortex Physiology

Inhalants, including toluene, target the addiction neurocircuitry and are often one of the first drugs of abuse tried by adolescents. The medial prefrontal cortex (mPFC) is involved in regulating goal-directed/reward-motivated behaviors and different mPFC sub-regions have been proposed to promote (prelimbic, PRL) or inhibit (infralimbic, IL) these behaviors. While this dichotomy has been studied in the context of other drugs of abuse, it is not known whether toluene exposure differentially affects neurons within PRL and IL regions. To address this question, we used whole-cell electrophysiology and determined the intrinsic excitability of PRL and IL pyramidal neurons in adolescent rats 24 h following a brief exposure to air or toluene vapor (10 500 p.p.m.). Prior to exposure, fluorescent retrobeads were injected into the NAc core (NAcc) or shell (NAcs) sub-regions to identify projection-specific mPFC neurons. In toluene treated adolescent rats, layer 5/6 NAcc projecting PRL (PRL5/6) neurons fired fewer action potentials and this was associated with increased rheobase, increased spike duration, and reductions in membrane resistance and amplitude of the I_h current. No changes in excitability were observed in layer 2/3 NAcc projecting PRL (PRL2/3) neurons. In contrast to PRL neurons, layer 5 IL (IL5) and layer 2/3 (IL2/3) NAcc projecting neurons showed enhanced firing in toluene-exposed animals and in IL5 neurons, this was associated with a reduction in rheobase and AHP. For NAcs projecting neurons, toluene exposure significantly decreased firing of IL5 neurons and this was accompanied by an increased rheobase, increased spike duration, and reduced I_h amplitude. The intrinsic excitability of PRL5, PRL2/3, and IL2/3 neurons projecting to the NAcs was not affected by exposure to toluene. The changes in excitability observed 24 h after toluene exposure were not observed when recordings were performed 7 days after the exposure. Finally, there were no changes in intrinsic excitability of any region in rats exposed to toluene as adults. These findings demonstrate that specific projections of the reward circuitry are uniquely susceptible to the effects of toluene during adolescence supporting the idea that adolescence is a critical period of the development that is vulnerable to drugs of abuse.

Chemogenetic Excitation of Accumbens-Projecting Infralimbic Cortical Neurons Blocks Toluene-Induced Conditioned Place Preference

Abuse rates for inhalants among adolescents continue to be high, yet preclinical models for studying mechanisms underlying inhalant abuse remain limited. Our laboratory has previously shown that, in male rats, an acute binge-like exposure to toluene vapor that mimics human solvent abuse modifies the intrinsic excitability of mPFC pyramidal neurons projecting to the NAc. These changes showed region (infralimbic; IL vs prelimbic; PRL), layer (shallow; 2/3 vs deep; 5/6), target (core vs shell), and age (adolescent vs adult) dependent differences (Wayman and Woodward, 2017). To expand these findings using reward-based models that may better mimic human drug abuse, we used whole-cell electrophysiology and drug receptors exclusively activated by designer drugs to examine changes in neuronal function and behavior in rats showing a conditioned place preference (CPP) to toluene. Repeated pairings of adolescent rats to binge concentrations of toluene vapor previously shown to enhance dopamine release in reward-sensitive areas of the brain produced CPP that persisted for 7 but not 30 d. Toluene-induced CPP was associated with increased excitability of IL5/6 mPFC neurons projecting to the core of the NAc and reduced excitability of those projecting to the NAc shell. No changes in PRL-NAc-projecting neurons were found in toluene-CPP rats. Chemogenetic reversal of the toluene-induced decrease in IL5/6-NAc shell neurons blocked the expression of toluene-induced CPP while manipulating IL5/6-NAc core neuron activity had no effect. These data reveal that alterations in selective mPFC-NAc pathways are required for expression of toluene-induced CPP.SIGNIFICANCE STATEMENT Disturbed physiology of pyramidal neurons projecting from the mPFC to the NAc has been shown to have different roles in drug-seeking behaviors for a number of drugs (e.g., methamphetamine, cocaine, ecstasy, alcohol, heroin). Here, we report that rats repeatedly exposed to the volatile organic solvent toluene, a member of the class of abused inhalants often used for intoxicating purposes by adolescents, induces a preference for the drug-paired environment that is accompanied by altered physiology of a specific population of NAc-projecting mPFC neurons. Chemogenetic correction of this deficit before testing prevented expression of drug preference. Overall, these findings highlight the importance of corticolimbic circuitry in mediating the rewarding properties of abused inhalants.

Phenotype-dependent inhibition of glutamatergic transmission on nucleus accumbens medium spiny neurons by the abused inhalant toluene

Abused inhalants are voluntarily inhaled at high concentrations to produce intoxicating effects. Results from animal studies show that the abused inhalant toluene triggers behaviors, such as self-administration and conditioned place preference, which are commonly associated with addictive drugs. However, little is known about how toluene affects neurons within the nucleus accumbens (NAc), a brain region within the basal ganglia that mediates goal-directed behaviors and is implicated in the development and maintenance of addictive behaviors. Here we report that toluene inhibits a component of the after-hyperpolarization potential, and dose-dependently inhibits N-methyl-D-aspartate (NMDA)-mediated currents in rat NAc medium spiny neurons (MSN). Moreover, using the multivariate statistical technique, partial least squares discriminative analysis to analyze electrophysiological measures from rat NAc MSNs, we show that toluene induces a persistent depression of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-mediated currents in one subtype of NAc MSNs, and that the electrophysiological features of MSN neurons predicts their sensitivity to toluene. The CB1 receptor antagonist AM281 blocked the toluene-induced long-term depression of AMPA currents, indicating that this process is dependent on endocannabinoid signaling. The neuronal identity of recorded cells was examined using dual histochemistry and shows that toluene-sensitive NAc neurons are dopamine D2 MSNs that express preproenkephalin mRNA. Overall, the results from these studies indicate that physiological characteristics obtained from NAc MSNs during whole-cell patch-clamp recordings reliably predict neuronal phenotype, and that the abused inhalant toluene differentially depresses excitatory neurotransmission in NAc neuronal subtypes.

GABAA receptor drugs and neuronal plasticity in reward and aversion: focus on the ventral tegmental area

GABA_A receptors are the main fast inhibitory neurotransmitter receptors in the mammalian brain, and targets for many clinically important drugs widely used in the treatment of anxiety disorders, insomnia and in anesthesia. Nonetheless, there are significant risks associated with the long-term use of these drugs particularly related to development of tolerance and addiction. Addictive mechanisms of GABA_A receptor drugs are poorly known, but recent findings suggest that those drugs may induce aberrant neuroadaptations in the brain reward circuitry. Recently, benzodiazepines, acting on synaptic GABA_A receptors, and modulators of extrasynaptic GABA_A receptors (THIP and neurosteroids) have been found to induce plasticity in the ventral tegmental area (VTA) dopamine neurons and their main target projections. Furthermore, depending whether synaptic or extrasynaptic GABA_A receptor populations are activated, the behavioral outcome of repeated administration seems to correlate with rewarding or aversive behavioral responses, respectively. The VTA dopamine neurons project to forebrain centers such as the nucleus accumbens and medial prefrontal cortex, and receive afferent projections from these brain regions and especially from the extended amygdala and lateral habenula, forming the major part of the reward and aversion circuitry. Both synaptic and extrasynaptic GABA_A drugs inhibit the VTA GABAergic interneurons, thus activating the VTA DA neurons by disinhibition and this way inducing glutamatergic synaptic plasticity. However, the GABA_A drugs failed to alter synaptic spine numbers as studied from Golgi-Cox-stained VTA dendrites. Since the GABAergic drugs are known to depress the brain metabolism and gene expression, their likely way of inducing neuroplasticity in mature neurons is by disinhibiting the principal neurons, which remains to be rigorously tested for a number of clinically important anxiolytics, sedatives and anesthetics in different parts of the circuitry.

Benzodiazepine-like discriminative stimulus effects of toluene vapor

In vitro studies show that the abused inhalant toluene affects a number of ligand-gated ion channels.The two most consistently implicated of these are γ-aminobutyric acid type A(GABAA) receptors which are positively modulated by toluene and N-methyl-D-aspartate(NMDA) receptors which are negatively modulated by toluene. Behavioral studies also suggest an interaction of toluene with GABAA and/or NMDA receptors but it is unclear if these receptors underlie the abuse-related intoxicating effects of toluene. Seventeen B6SJLF1/J mice were trained using a two-choice operant drug discrimination procedure to discriminate 10 min of exposure to 2000 ppm toluene vapor from 10 min of exposure to air. The discrimination was acquired in a mean of 65 training sessions. The stimulus effects of 2000 ppm toluene vapor were exposure concentration-dependent but rapidly diminished following the cessation of vapor exposure. The stimulus effects of toluene generalized to the chlorinated hydrocarbon vapor perchloroethylene but not 1,1,2-trichloroethane nor the volatile anesthetic isoflurane. The competitive NMDA antagonist CGS-19755, the uncompetitive antagonist dizocilpine and the glycine-site antagonist L701,324 all failed to substitute for toluene. The classical nonselective benzodiazepines midazolam and chlordiazepoxide produced toluene-like stimulus effects but the alpha 1 subunit preferring positive GABAA modulator zaleplon failed to substitute for toluene. The barbiturates pentobarbital and methohexital and the GABAA positive modulator neurosteroid allopregnanolone did not substitute for toluene. These data suggest that the stimulus effects of toluene may be at least partially mediated by benzodiazepine-like positive allosteric modulation of GABAA receptors containing alpha 2, 3 or 5 subunits.

Assessment of reinforcement enhancing effects of toluene vapor and nitrous oxide in intracranial self-stimulation

RATIONALE

Despite widespread abuse, there are few validated methods to study the rewarding effects of inhalants. One model that may have utility for this purpose is intracranial self-stimulation (ICSS).

OBJECTIVES

This study aims to compare and contrast the ICSS reward-facilitating effects of abused inhalants to other classes of abused drugs. Compounds were examined using two different ICSS procedures in mice to determine the generality of each drug's effects on ICSS and the sensitivity of the procedures.

METHODS

Male C57BL/6J mice with electrodes implanted in the medial forebrain bundle were trained under a three-component rate-frequency as well as a progressive ratio (PR) ICSS procedure. The effects of nitrous oxide, toluene vapor, cocaine, and diazepam on ICSS were then examined.

RESULTS

Concentrations of 1,360-2,900 parts per million (ppm) inhaled toluene vapor significantly facilitated ICSS in the rate-frequency procedure and 1,360 ppm increased PR breakpoint. A concentration of 40 % nitrous oxide facilitated ICSS in the rate-frequency procedure but reduced PR breakpoint. Doses of 3-18 mg/kg cocaine facilitated ICSS in the rate-frequency procedure, and 10 and 18 mg/kg increased PR breakpoint. Doses of 1 and 3 mg/kg diazepam facilitated ICSS in the rate-frequency procedure, and 3 mg/kg increased PR breakpoint.

CONCLUSIONS

The reinforcement-facilitating effect of toluene in ICSS is at least as great as diazepam. By contrast, nitrous oxide weakly enhances ICSS in only the rate-frequency procedure. The data suggest that the rate-frequency procedure may be more sensitive than the PR schedule to the reward-facilitating effects of abused inhalants.

Effects of the abused inhalant toluene on the mesolimbic dopamine system

Toluene is a representative member of a class of inhaled solvents that are voluntarily used by adolescents and adults for their euphorigenic effects. Research into the mechanisms of action of inhaled solvents has lagged behind that of other drugs of abuse despite mounting evidence that these compounds exert profound neurobehavioral and neurotoxicological effects. Results from studies carried out by the authors and others suggest that the neural effects of inhalants arise from their interaction with a discrete set of ion channels that regulate brain activity. Of particular interest is how these interactions allow toluene and other solvents to engage portions of an addiction neurocircuitry that includes midbrain and cortical structures. In this review, we focus on the current state of knowledge regarding toluene's action on midbrain dopamine neurons, a key brain region involved in the initial assessment of natural and drug-induced rewards. Findings from recent studies in the authors' laboratory show that brief exposures of adolescent rats to toluene vapor induce profound changes in markers of glutamatergic plasticity in VTA DA neurons. These changes are restricted to VTA DA neurons that project to limbic structures and are prevented by transient activation of the medial prefrontal cortex prior to toluene exposure. Together, these data provide the first evidence linking the voluntary inhalation of solvents to changes in reward -sensitive dopamine neurons.

Review of toluene action: clinical evidence, animal studies and molecular targets

It has long been known that individuals will engage in voluntary inhalation of volatile solvents for their rewarding effects. However, research into the neurobiology of these agents has lagged behind that of more commonly used drugs of abuse such as psychostimulants, alcohol and nicotine. This imbalance has begun to shift in recent years as the serious effects of abused inhalants, especially among children and adolescents, on brain function and behavior have become appreciated and scientifically documented. In this review, we discuss the physicochemical and pharmacological properties of toluene, a representative member of a large class of organic solvents commonly used as inhalants. This is followed by a brief summary of the clinical and pre-clinical evidence showing that toluene and related solvents produce significant effects on brain structures and processes involved in the rewarding aspects of drugs. This is highlighted by tables highlighting toluene's effect on behaviors (reward, motor effects, learning, etc.) and cellular proteins (e.g. voltage and ligand-gated ion channels) closely associated the actions of abused substances. These sections demonstrate not only the significant progress that has been made in understanding the neurobiological basis for solvent abuse but also reveal the challenges that remain in developing a coherent understanding of this often overlooked class of drugs of abuse.

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.

Repeated toluene exposure increases c-Fos in catecholaminergic cells of the nucleus accumbens shell

Toluene is a frequently abused solvent. Previous studies have suggested that toluene acts like other drugs of abuse, specifically on the dopaminergic system in the nucleus accumbens (NAc) and ventral tegmental area (VTA) of the mesolimbic pathway. Although changes in dopamine (DA) levels and c-Fos have been observed in both acute and repeated exposure paradigms, the extent to which c-Fos is localized to catecholaminergic cells is unknown. The present study tested the effects of repeated toluene exposure (1000-4000ppm) on locomotor activity and cells containing c-Fos, tyrosine hydroxylase (TH), or both in the core and shell of the NAc, as well as the anterior and posterior VTA. We focused our study on adolescents, since adolescence is a time of great neural change and a time when individuals tend to be more susceptible to drug abuse. In early tests, toluene dose-dependently increased locomotor activity. Repeated exposure to the highest concentration of toluene resulted in sensitization to toluene's effects on locomotor activity. Although the number of cells immunopositive for c-Fos or TH did not significantly differ across groups, cells immunopositive for TH+c-Fos were higher in the NAc shell of animals exposed to 4000ppm than in animals exposed to air (control) or 1000ppm. Taken together, these findings demonstrate that repeated high dose toluene exposure increases locomotor activity as well as activation of catecholaminergic cells in the shell of the NAc.

Medial prefrontal cortex inversely regulates toluene-induced changes in markers of synaptic plasticity of mesolimbic dopamine neurons

Toluene is a volatile solvent that is intentionally inhaled by children, adolescents, and adults for its intoxicating effects. Although voluntary use of toluene suggests that it possesses rewarding properties and abuse potential, it is unknown whether toluene alters excitatory synaptic transmission in reward-sensitive dopamine neurons like other drugs of abuse. Here, using a combination of retrograde labeling and slice electrophysiology, we show that a brief in vivo exposure of rats to a behaviorally relevant concentration of toluene vapor enhances glutamatergic synaptic strength of dopamine (DA) neurons projecting to nucleus accumbens core and medial shell neurons. This effect persisted for up to 3 d in mesoaccumbens core DA neurons and for at least 21 d in those projecting to the medial shell. In contrast, toluene vapor exposure had no effect on synaptic strength of DA neurons that project to the medial prefrontal cortex (mPFC). Furthermore, infusion of GABAergic modulators into the mPFC before vapor exposure to pharmacologically manipulate output, inhibited, or potentiated toluene's action on mesoaccumbens DA neurons. Together, the results of these studies indicate that toluene induces a target-selective increase in mesolimbic DA neuron synaptic transmission and strongly implicates the mPFC as an important regulator of drug-induced plasticity of mesolimbic dopamine neurons.

The abused inhalant toluene differentially modulates excitatory and inhibitory synaptic transmission in deep-layer neurons of the medial prefrontal cortex

Volatile organic solvents such as toluene are voluntarily inhaled for their intoxicating effects. Solvent use is especially prevalent among adolescents, and is associated with deficits in a wide range of cognitive tasks including attention, behavioral control, and risk assessment. Despite these findings, little is known about the effects of toluene on brain areas mediating these behaviors. In this study, whole-cell patch-clamp recordings were used to determine the effect toluene on neurons within the medial PFC, a region critically involved in cognitive function. Toluene had no effect on measures of intrinsic excitability, but enhanced stimulus-evoked γ-amino butyric acid A-mediated inhibitory postsynaptic currents (IPSCs). In the presence of tetrodotoxin (TTX) to block action potentials, toluene increased the frequency and amplitude of miniature IPSCs. In contrast, toluene induced a delayed but persistent decrease in evoked or spontaneous AMPA-mediated excitatory postsynaptic currents (EPSCs). This effect was prevented by an intracellular calcium chelator or by the ryanodine receptor and SERCA inhibitors, dantrolene or thapsigargin, respectively, suggesting that toluene may mobilize intracellular calcium pools. The toluene-induced reduction in AMPA EPSCs was also prevented by a cannabinoid receptor (CB1R) antagonist, and was occluded by the CB1 agonist WIN 55,212-2 that itself induced a profound decrease in AMPA-mediated EPSCs. Toluene had no effect on the frequency or amplitude of miniature EPSCs recorded in the presence of TTX. Finally, toluene dose-dependently inhibited N-methyl-D-aspartate (NMDA)-mediated EPSCs and the magnitude and reversibility of this effect was CB1R sensitive indicating both direct and indirect actions of toluene on NMDA-mediated responses. Together, these results suggest that the effect of toluene on cognitive behaviors may result from its action on inhibitory and excitatory synaptic transmission of PFC neurons.

Reproductive toxicology and teratology of abused toluene

Toluene is an organic solvent that is widely used by industry and is ubiquitous in our environment. As a result, exposure to solvents like toluene in work-related settings (i.e., relatively constant, low-level exposures) or through inhalant abuse (i.e., relatively intermittent, high-level exposures) is increasing for many women of reproductive age. Evidence suggests that the risk for pregnancy problems, as well as developmental delays and neurobehavioral difficulties, is higher for the children of women who have been exposed to high concentrations of organic solvents during pregnancy than for those who have not. These risks appear to be higher in cases of abuse exposure to solvents such as toluene, particularly in comparison to the risk for teratogenic outcomes with occupational solvent exposure. Despite this, the reproductive toxicology and teratology following abuse of toluene and other inhalants remains under-investigated. This brief review describes the current state of our understanding of the reproductive and teratogenic risk of gestational toluene abuse. The data to date suggest that the high levels of toluene exposure typical with inhalant abuse are more detrimental to fetal development than typical occupational exposure, and preclinical paradigms can be beneficial for investigating the processes and risks of prenatal solvent exposure. While substantial research has been done on the reproductive effects of occupational exposures to organic solvents, more research is needed on the outcomes and mechanisms of exposures typical of inhalant abuse.

Neurochemical changes after acute binge toluene inhalation in adolescent and adult rats: a high-resolution magnetic resonance spectroscopy study

Inhalant abuse in young people is a growing public health concern. We reported previously that acute toluene intoxication in young rats, using a pattern of exposures that approximate abuse patterns of inhalant use in humans, significantly altered neurochemical measures in select brain regions. In this study, adolescent and young adult rats were exposed similarly to an acute (2 x 15 min), high dose (8000-12,000 ppm) of toluene and high-resolution magic angle spinning proton magnetic resonance spectroscopy (HR-MAS 1H-MRS) was used to assess neurochemical profiles of tissue samples from a number of brain regions collected immediately following solvent exposure. The current investigation focused on N-acetyl-aspartate (NAA), choline-containing compounds, creatine, glutamate, GABA, and glutamine. Contrary to our predictions, no significant alterations were found in the levels of NAA, choline, creatine, glutamate, or glutamine in adolescent animals. In contrast to these minimal effects in adolescents, binge toluene exposure altered several neurochemical parameters in young adult rats, including decreased levels of choline and GABA in the frontal cortex and striatum and lowered glutamine and NAA levels in the frontal cortex. One of the more robust findings was a wide-ranging increase in lactate after toluene exposure in adult animals, an effect not observed in adolescents. These age-dependent effects of toluene are distinct from those reported previously in juvenile rats and suggest a developmental difference in vulnerability to the effects of inhalants. Specifically, the results suggest that the neurochemical response to toluene in adolescents is attenuated compared to adults, and imply an association between these neurochemical differences and age-influenced differences in solvent abuse in humans.

Inhalant abuse among adolescents: neurobiological considerations

Experimentation with volatile substances (inhalants) is common during early adolescence, yet limited work has been conducted examining the neurobiological impact of regular binge use during this key stage of development. Human studies consistently demonstrate that chronic use is associated with significant toxic effects, including neurological and neuropsychological impairment, as well as diffuse and subtle changes in white matter. However, most preclinical research has tended to focus on acute exposure, with limited work examining the neuropharmacological or toxicological mechanisms underpinning these changes or their potential reversibility with abstinence. Nevertheless, there is growing evidence that commonly abused inhalants share common cellular mechanisms, and have similar actions to other drugs of abuse. Indeed, the majority of acute behavioural effects appear to be underpinned by changes in receptor and/or ion channel activity (for example, GABA(A), glycine and 5HT(3) receptor activation, NMDA receptor inhibition), although nonspecific interactions can also arise at high concentrations. Recent studies examining the effects of toluene exposure during the early postnatal period are suggestive of long-term alterations in the function of NMDA and GABA(A) receptors, although limited work has been conducted investigating exposure during adolescence. Given the critical role of neurotransmitter systems in cognitive, emotional and brain development, future studies will need to take account of the substantial neuromaturational changes that are known to occur in the brain during childhood and adolescence, and to specifically investigate the neuropharmacological and toxicological profile of inhalant exposure during this period of development.

The effects of inhaled acetone on place conditioning in adolescent rats

INTRODUCTION

Acetone is an ubiquitous ingredient in many household products (e.g., glue solvents, air fresheners, adhesives, nail polish, and paint) that is putatively abused; however, there is little empirical evidence to suggest that acetone alone has any abuse liability. Therefore, we systematically investigated the conditioned response to inhaled acetone in a place conditioning apparatus.

METHOD

Three groups of male, Sprague-Dawley rats were exposed to acetone concentrations of 5000, 10,000 or 20,000 ppm for 1 h in a conditioned place preference apparatus alternating with air for 6 pairing sessions. A place preference test ensued in an acetone-free environment. To test the preference of acetone as a function of pairings sessions, the 10,000 ppm group received an additional 6 pairings and an additional group received 3 pairings. The control group received air in both compartments. Locomotor activity was recorded by infrared photocells during each pairing session.

RESULTS

We noted a dose response relationship to acetone at levels 5000-20,000 ppm. However, there was no correlation of place preference as a function of pairing sessions at the 10,000 ppm level. Locomotor activity was markedly decreased in animals on acetone-paired days as compared to air-paired days.

CONCLUSION

The acetone concentrations we tested for these experiments produced a markedly decreased locomotor activity profile that resemble CNS depressants. Furthermore, a dose response relationship was observed at these pharmacologically active concentrations, however, animals did not exhibit a positive place preference.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

The abused inhalant toluene increases dopamine release in the nucleus accumbens by directly stimulating ventral tegmental area neurons

Recreational abuse of toluene-containing volatile inhalants by adolescents is a significant public health problem. The mechanisms underlying the abuse potential of such substances remain unclear, but could involve increased activity in mesoaccumbal dopamine (DA) afferents innervating the nucleus accumbens (ACB). Here, using in vitro electrophysiology, we show that application of behaviorally relevant concentrations of toluene directly stimulates DA neurons in the ventral tegmental area (VTA), but not surrounding midbrain regions. Toluene stimulation of VTA neurons persists when synaptic transmission is reduced. Moreover, unlike non-DA neurons, the magnitude of VTA DA neuron firing does not decline during longer exposures designed to emulate 'huffing'. Using dual-probe in vivo microdialysis, we show that perfusion of toluene directly into the VTA increases DA concentrations in the VTA (somatodendritic release) and its terminal projection site, the ACB. These results provide the first demonstration that even brief exposure to toluene increases action potential drive onto mesoaccumbal VTA DA neurons, thereby enhancing DA release in the ACB. The finding that toluene stimulates mesoaccumbal neurotransmission by activating VTA DA neurons directly (independently of transynaptic inputs) provide insights into the neural substrates that may contribute to the initiation and pathophysiology of toluene abuse.