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

Toluene alters the intrinsic excitability and excitatory synaptic transmission of basolateral amygdala neurons

Introduction

Inhalant abuse is an important health issue especially among children and adolescents who often encounter these agents in the home. Research into the neurobiological targets of inhalants has lagged behind that of other drugs such as alcohol and psychostimulants. However, studies from our lab and others have begun to reveal how inhalants such as the organic solvent toluene affect neurons in key addiction related areas of the brain including the ventral tegmental area, nucleus accumbens and medial prefrontal cortex. In the present study, we extend these findings and examine the effect of toluene on electrophysiological responses of pyramidal neurons in the basolateral amygdala BLA, a region important for generating emotional and reward based information needed to guide future behavior.

Methods

Whole-cell patch-clamp electrophysiology recordings of BLA pyramidal neurons in rat brain slices were used to assess toluene effects on intrinsic excitability and excitatory glutamatergic synaptic transmission.

Results

Acute application of 3 mM but not 0.3 mM toluene produced a small but significant (~20%) increase in current-evoked action potential (AP) firing that reversed following washout of the toluene containing solution. The change in firing during exposure to 3 mM toluene was accompanied by selective changes in AP parameters including reduced latency to first spike, increased AP rise time and decay and a reduction in the fast after-hyperpolization. To examine whether toluene also affects excitatory synaptic signaling, we expressed channelrhodopsin-2 in medial prefrontal cortex neurons and elicited synaptic currents in BLA neurons via light pulses. Toluene (3 mM) reduced light-evoked AMPA-mediated synaptic currents while a lower concentration (0.3 mM) had no effect. The toluene-induced reduction in AMPA-mediated BLA synaptic currents was prevented by the cannabinoid receptor-1 antagonist AM281.

Discussion

These findings are the first to demonstrate effects of acute toluene on BLA pyramidal neurons and add to existing findings showing that abused inhalants such as toluene have significant effects on neurons in brain regions involved in natural and drug induced reward.

Abuse-like toluene exposure during early adolescence alters subsequent ethanol and cocaine behavioral effects and brain monoamines in male mice

Currently, there is a gap in understanding the neurobiological impact early adolescent toluene exposure has on subsequent actions of other drugs. Adolescent (PND 28-32) male Swiss-Webster mice (N = 210) were exposed to 0, 2000, or 4000 ppm of toluene vapor for 30 min/day for 5 days. Immediately following the last toluene exposure (PND 32; n = 15) or after a short delay (PND 35; n = 15), a subset of subjects' brains was collected for monoamine analysis. Remaining mice were assigned to one of two abstinence periods: a short 4-day (PND 36) or long 12-day (PND 44) delay after toluene exposure. Mice were then subjected to a cumulative dose response assessment of either cocaine (0, 2.5, 5, 10, 20 mg/kg; n = 60), ethanol (0, 0.5, 1, 2, 4 g/kg; n = 60), or saline (5 control injections; n = 60). Toluene concentration-dependently increased locomotor activity during exposure. When later challenged, mice exposed previously to toluene were significantly less active after cocaine (10 and 20 mg/kg) compared to air-exposed controls. Animals were also less active at the highest dose of alcohol (4 g/kg) following prior exposure to 4000 ppm when compared to air-exposed controls. Analysis of monoamines and their metabolites using High Pressure Liquid Chromatography (HPLC) within the medial prefrontal cortex (mPFC), nucleus accumbens (NAc), dorsal striatum (dSTR), and ventral tegmental area (VTA) revealed subtle effects on monoamine or metabolite levels following cumulative dosing that varied by drug (cocaine and ethanol) and abstinence duration. Our results suggest that early adolescent toluene exposure produces behavioral desensitization to subsequent cocaine-induced locomotor activity with subtle enhancement of ethanol's depressive effects and less clear impacts on levels of monoamines.

A brief exposure to toluene vapor alters the intrinsic excitability of D2 medium spiny neurons in the rat ventral striatum

Although volatile organic solvents such as toluene are used for commercial and industrial uses, they are often voluntarily inhaled for their intoxicating and euphoric effects. Research into the effects of inhalants such as toluene on brain function have revealed actions on a variety of ligand-gated and voltage-activated ion channels involved in regulating neuronal excitability. Previous work from this laboratory has also shown that brief exposures to toluene vapor induce changes in the intrinsic excitability and synaptic transmission of neurons within the medial prefrontal cortex and ventral tegmental area that vary depending on projection target. In the present study, we recorded current-evoked spiking of medium spiny neurons (MSNs) in the nucleus accumbens (NAc) core and shell in adolescent rats exposed to an intoxicating concentration of toluene vapor. Compared to air controls, firing of NAc core MSNs in Sprague-Dawley rats was not altered 24 h after exposure to 10,500 ppm toluene vapor while spiking of NAc shell MSNs was enhanced at low current steps but reduced at higher current steps. When the rheobase current was used to putatively identify MSN subtypes, both "D1-like" and "D2-like" MSNs within the NAc shell but not core showed toluene-induced changes in firing. As toluene may itself have altered the rheobase resulting in misclassification of neuron subtype, we conducted additional studies using adolescent D2-Cre rats infused with a Cre-dependent mCherry reporter virus. Following toluene vapor exposure, spiking of NAc shell D2+ MSNs was enhanced at low current steps but inhibited at higher currents as compared to air controls while there were no differences in the firing of NAc shell D2- MSNs. The toluene-induced change in NAc D2+ shell MSN firing was accompanied by alterations in membrane resistance, rheobase, action potential rise time and height with no changes noted in D2- MSNs. Overall, these data add to a growing literature showing that brief exposures to intoxicating concentrations of toluene vapor causes selective alterations in the excitability of neurons within the addiction neurocircuitry that vary depending on sub-region, cell-type and projection target.

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.

α2δ-1 protein drives opioid-induced conditioned reward and synaptic NMDA receptor hyperactivity in the nucleus accumbens

Glutamate NMDA receptors (NMDARs) in the nucleus accumbens (NAc) are critically involved in drug dependence and reward. α2δ-1 is a newly discovered NMDAR-interacting protein that promotes synaptic trafficking of NMDARs independently of its conventional role as a calcium channel subunit. However, it remains unclear how repeated opioid exposure affects synaptic NMDAR activity and α2δ-1-NMDAR interaction in the NAc. In this study, whole-cell patch-clamp recordings showed that repeated treatment with morphine in mice markedly increased the NMDAR-mediated frequency of miniature excitatory postsynaptic currents (mEPSCs) and amplitude of puff NMDAR currents in medium spiny neurons in the NAc core region. Morphine treatment significantly increased the physical interaction of α2δ-1 with GluN1 and their synaptic trafficking in the NAc. In Cacna2d1 knockout mice, repeated treatment with morphine failed to increase the frequency of mEPSCs and amplitude of puff NMDAR currents in the NAc core. Furthermore, inhibition of α2δ-1 with gabapentin or disruption of the α2δ-1-NMDAR interaction with the α2δ-1 C terminus-interfering peptide blocked the morphine-elevated frequency of mEPSCs and amplitude of puff NMDAR currents in the NAc core. Correspondingly, systemically administered gabapentin, Cacna2d1 ablation, or microinjection of the α2δ-1 C terminus-interfering peptide into the NAc core attenuated morphine-induced conditioned place preference and locomotor sensitization. Our study reveals that repeated opioid exposure strengthens presynaptic and postsynaptic NMDAR activity in the NAc via α2δ-1. The α2δ-1-bound NMDARs in the NAc have a key function in the rewarding effect of opioids and could be targeted for treating opioid use disorder and addiction.

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.

Modulating the Neuromodulators: Dopamine, Serotonin, and the Endocannabinoid System

Dopamine (DA), serotonin (5-hydroxytryptamine, 5-HT), and endocannabinoids (ECs) are key neuromodulators involved in many aspects of motivated behavior, including reward processing, reinforcement learning, and behavioral flexibility. Among the longstanding views about possible relationships between these neuromodulators is the idea of DA and 5-HT acting as opponents. This view has been challenged by emerging evidence that 5-HT supports reward seeking via activation of DA neurons in the ventral tegmental area. Adding an extra layer of complexity to these interactions, the endocannabinoid system is uniquely placed to influence dopaminergic and serotonergic neurotransmission. In this review we discuss how these three neuromodulatory systems interact at the cellular and circuit levels. Technological advances that facilitate precise identification and control of genetically targeted neuronal populations will help to achieve a better understanding of the complex relationship between these essential systems, and the potential relevance for motivated behavior.

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.

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.

Persistent cognitive and morphological alterations induced by repeated exposure of adolescent rats to the abused inhalant toluene

While thepsychoactive inhalant toluene causes behavioral effects similarto those produced by other drugs of abuse, the persistent behavioral and anatomical abnormalities induced by toluene exposure are not well known. To mimic human "binge-like" inhalant intoxication, adolescent, male Sprague-Dawley rats were exposed to toluene vapor (5700ppm) twice daily for five consecutive days. These rats remained in their home cages until adulthood (P60), when they were trained in operant boxes to respond to a palatable food reward and then challenged with several different cognitive tasks. Rats that experienced chronic exposure to toluene plus abstinence ("CTA") showed enhanced performance in a strategy set-shifting task using a between-session, but not a within-session test design. CTA also blunted operant and classical conditioning without affecting responding during a progressive ratio task. While CTA rats displayed normal latent inhibition, previous exposure to a non-reinforced cue enhanced extinction of classically conditioned approach behavior of these animals compared to air controls. To determine whether CTA alters the structural plasticity of brain areas involved in set-shifting and appetitive behaviors, we quantified basal dendritic spine morphology in DiI-labeled pyramidal neurons in layer 5 of the medial prefrontal cortex (mPFC) and medium spiny neurons in the nucleus accumbens (NAc). There were no changes in dendritic spine density or subtype in the mPFC of CTA rats while NAc spine density was significantly increased due to an enhanced prevalence of long-thin spines. Together, these findings suggest that the persistent effects of CTA on cognition are related to learning and memory consolidation/recall, but not mPFC-dependent behavioral flexibility.

Viral vector strategies for investigating midbrain dopamine circuits underlying motivated behaviors

Midbrain dopamine (DA) neurons have received significant attention in brain research because of their central role in reward processing and their dysfunction in neuropsychiatric disorders such as Parkinson's disease, drug addiction, depression and schizophrenia. Until recently, it has been thought that DA neurons form a homogeneous population whose primary function is the computation of reward prediction errors. However, through the implementation of viral vector strategies, an unexpected complexity and diversity has been revealed at the anatomical, molecular and functional level. In this review, we discuss recent viral vector approaches that have been leveraged to dissect how different circuits involving distinct DA neuron subpopulations may contribute to the role of DA in reward- and aversion-related behaviors. We focus on studies that have used cell type- and projection-specific optogenetic manipulations, discuss the strengths and limitations of each approach, and critically examine emergent organizational principles that have led to a reclassification of midbrain DA neurons.

The First Alcohol Drink Triggers mTORC1-Dependent Synaptic Plasticity in Nucleus Accumbens Dopamine D1 Receptor Neurons

Early binge-like alcohol drinking may promote the development of hazardous intake. However, the enduring cellular alterations following the first experience with alcohol consumption are not fully understood. We found that the first binge-drinking alcohol session produced enduring enhancement of excitatory synaptic transmission onto dopamine D1 receptor-expressing neurons (D1+ neurons) in the nucleus accumbens (NAc) shell but not the core in mice, which required D1 receptors (D1Rs) and mechanistic target of rapamycin complex 1 (mTORC1). Furthermore, inhibition of mTORC1 activity during the first alcohol drinking session reduced alcohol consumption and preference of a subsequent drinking session. mTORC1 is critically involved in RNA-to-protein translation, and we found that the first alcohol session rapidly activated mTORC1 in NAc shell D1+ neurons and increased synaptic expression of the AMPAR subunit GluA1 and the scaffolding protein Homer. Finally, D1R stimulation alone was sufficient to activate mTORC1 in the NAc to promote mTORC1-dependent translation of the synaptic proteins GluA1 and Homer. Together, our results indicate that the first alcohol drinking session induces synaptic plasticity in NAc D1+ neurons via enhanced mTORC1-dependent translation of proteins involved in excitatory synaptic transmission that in turn drives the reinforcement learning associated with the first alcohol experience. Thus, the alcohol-dependent D1R/mTORC1-mediated increase in synaptic function in the NAc may reflect a neural imprint of alcohol's reinforcing properties, which could promote subsequent alcohol intake. Significance statement: Consuming alcohol for the first time is a learning event that drives further drinking. Here, we identified a mechanism that may underlie the reinforcing learning associated with the initial alcohol experience. We show that the first alcohol experience induces a persistent enhancement of excitatory synaptic transmission on NAc shell D1+ neurons, which is dependent on D1R and mTORC1. We also find that mTORC1 is necessary for the sustained alcohol consumption and preference across the initial drinking sessions. The first alcohol binge activates mTORC1 in NAc D1+ neurons and increases levels of synaptic proteins involved in glutamatergic signaling. Thus, the D1R/mTORC1-dependent plasticity following the first alcohol exposure may be a critical cellular component of reinforcement learning.

Temporal profiles dissociate regional extracellular ethanol versus dopamine concentrations

In vivo monitoring of dopamine via microdialysis has demonstrated that acute, systemic ethanol increases extracellular dopamine in regions innervated by dopaminergic neurons originating in the ventral tegmental area and substantia nigra. Simultaneous measurement of dialysate dopamine and ethanol allows comparison of the time courses of their extracellular concentrations. Early studies demonstrated dissociations between the time courses of brain ethanol concentrations and dopaminergic responses in the nucleus accumbens (NAc) elicited by acute ethanol administration. Both brain ethanol and extracellular dopamine levels peak during the first 5 min following systemic ethanol administration, but the dopamine response returns to baseline while brain ethanol concentrations remain elevated. Post hoc analyses examined ratios of the dopamine response (represented as a percent above baseline) to tissue concentrations of ethanol at different time points within the first 25-30 min in the prefrontal cortex, NAc core and shell, and dorsomedial striatum following a single intravenous infusion of ethanol (1 g/kg). The temporal patterns of these "response ratios" differed across brain regions, possibly due to regional differences in the mechanisms underlying the decline of the dopamine signal associated with acute intravenous ethanol administration and/or to the differential effects of acute ethanol on the properties of subpopulations of midbrain dopamine neurons. This Review draws on neurochemical, physiological, and molecular studies to summarize the effects of acute ethanol administration on dopamine activity in the prefrontal cortex and striatal regions, to explore the potential reasons for the regional differences observed in the decline of ethanol-induced dopamine signals, and to suggest directions for future research.

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.

Chronic intermittent toluene inhalation initiated during adolescence in rats does not alter voluntary consumption of ethanol in adulthood

Voluntary inhalation of organic solvents, such as toluene, is particularly prevalent in adolescent populations and is considered to be a contributing factor to substance use and dependence later in life. While inhalants are often the initial "drug" experienced during this period, alcohol is another substance readily abused by adolescent populations. Although both substances are thought to have similar actions within the brain, our understanding of the implications of adolescent inhalant abuse upon subsequent exposure to alcohol remains to be investigated. Thus, this study aimed to assess locomotor responses to acute ethanol and voluntary ethanol consumption following a period of toluene inhalation throughout adolescence/early adulthood. Adolescent male Wistar rats (postnatal day [PN] 27) inhaled air or toluene (3000 ppm) for 1 h/day, 3 days/week for 4 (PN 27-52) or 8 weeks (PN 27-80) to mimic the patterns observed in human inhalant abusers. Following the exposure period, cross-sensitization to acute ethanol challenge (0.5 g/kg, intra-peritoneally [i.p.]), and voluntary consumption of 20% ethanol in a chronic intermittent 2-bottle choice paradigm, were assessed. Hepatic ethanol and acetaldehyde metabolism and liver histopathology were also investigated. Chronic intermittent toluene (CIT) exposure throughout adolescence for up to 8 weeks did not alter the behavioral response to acute ethanol or voluntary consumption of ethanol in adulthood, although an age-dependent effect on ethanol consumption was observed (p<0.05). Both liver function and pathology did not differ between treatment groups. Thus, in the paradigm employed, CIT exposure throughout adolescence and early adulthood did not predispose rats to subsequent locomotor sensitivity or voluntary consumption of ethanol in adulthood.

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.

Designer receptors show role for ventral pallidum input to ventral tegmental area in cocaine seeking

Ventral pallidum (VP) is centrally positioned within mesocorticolimbic reward circuits, and its dense projection to ventral tegmental area (VTA) regulates neuronal activity there. However, VP is a heterogeneous structure, and how this complexity affects its role within wider reward circuits is unclear. Here we demonstrate that projections to VTA from rostral (RVP), but not caudal VP (CVP) are robustly Fos-activated during cue-induced reinstatement of cocaine seeking—a rat model of relapse in addiction. Moreover, designer receptor-mediated transient inactivation of RVP neurons, their terminals in VTA, or functional connectivity between RVP and VTA dopamine neurons blocks the ability of drug-associated cues (but not a cocaine prime) to reinstate cocaine seeking. In contrast, CVP neuronal inhibition instead blocked cocaine-primed, but not cue-induced reinstatement. This novel double dissociation in VP sub-regional roles in drug seeking is likely important for understanding mesocorticolimbic circuits underlying reward seeking and addiction.

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.

Pharmacological classification of the abuse-related discriminative stimulus effects of trichloroethylene vapor

Inhalants are distinguished as a class primarily based upon a shared route of administration. Grouping inhalants according to their abuse-related in vivo pharmacological effects using the drug discrimination procedure has the potential to provide a more relevant classification scheme to the research and treatment community. Mice were trained to differentiate the introceptive effects of the trichloroethylene vapor from air using an operant procedure. Trichloroethylene is a chlorinated hydrocarbon solvent once used as an anesthetic as well as in glues and other consumer products. It is now primarily employed as a metal degreaser. We found that the stimulus effects of trichloroethylene were similar to those of other chlorinated hydrocarbon vapors, the aromatic hydrocarbon toluene and the vapor anesthetics methoxyflurane and isoflurane. The stimulus effects of trichloroethylene overlapped with those of the barbiturate methohexital, to a lesser extent the benzodiazepine midazolam and to ethanol. NMDA antagonists, the kappa opioid agonist U50,488 and the mixed 5-HT agonist mCPP largely failed to substitute for trichloroethylene. These data suggest that stimulus effects of chlorinated hydrocarbon vapors are mediated at least partially by GABA_A receptor positive modulatory effects.

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.

Alterations in ethanol-induced behaviors and consumption in knock-in mice expressing ethanol-resistant NMDA receptors

Ethanol's action on the brain likely reflects altered function of key ion channels such as glutamatergic N-methyl-D-aspartate receptors (NMDARs). In this study, we determined how expression of a mutant GluN1 subunit (F639A) that reduces ethanol inhibition of NMDARs affects ethanol-induced behaviors in mice. Mice homozygous for the F639A allele died prematurely while heterozygous knock-in mice grew and bred normally. Ethanol (44 mM; ∼0.2 g/dl) significantly inhibited NMDA-mediated EPSCs in wild-type mice but had little effect on responses in knock-in mice. Knock-in mice had normal expression of GluN1 and GluN2B protein across different brain regions and a small reduction in levels of GluN2A in medial prefrontal cortex. Ethanol (0.75-2.0 g/kg; i.p.) increased locomotor activity in wild-type mice but had no effect on knock-in mice while MK-801 enhanced activity to the same extent in both groups. Ethanol (2.0 g/kg) reduced rotarod performance equally in both groups but knock-in mice recovered faster following a higher dose (2.5 g/kg). In the elevated zero maze, knock-in mice had a blunted anxiolytic response to ethanol (1.25 g/kg) as compared to wild-type animals. No differences were noted between wild-type and knock-in mice for ethanol-induced loss of righting reflex, sleep time, hypothermia or ethanol metabolism. Knock-in mice consumed less ethanol than wild-type mice during daily limited-access sessions but drank more in an intermittent 24 h access paradigm with no change in taste reactivity or conditioned taste aversion. Overall, these data support the hypothesis that NMDA receptors are important in regulating a specific constellation of effects following exposure to ethanol.

Reward and aversion in a heterogeneous midbrain dopamine system

The ventral tegmental area (VTA) is a heterogeneous brain structure that serves a central role in motivation and reward processing. Abnormalities in the function of VTA dopamine (DA) neurons and the targets they influence are implicated in several prominent neuropsychiatric disorders including addiction and depression. Recent studies suggest that the midbrain DA system is composed of anatomically and functionally heterogeneous DA subpopulations with different axonal projections. These findings may explain a number of previously confusing observations that suggested a role for DA in processing both rewarding as well as aversive events. Here we will focus on recent advances in understanding the neural circuits mediating reward and aversion in the VTA and how stress as well as drugs of abuse, in particular cocaine, alter circuit function within a heterogeneous midbrain DA system. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.