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Environmental enrichment reverses memory impairment induced by toluene in mice

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Highlights

  • Repeated toluene exposure impaired learning and memory in mice.

  • Impairment was independent of toluene concentration and chronicity of exposure.

  • Environmental enrichment reversed toluene-induced cognitive alterations.

  • Glutamate and GABA tissue content were not involved in the effect of toluene on learning and memory.

Abstract

Toluene is the main component of a variety of inhalants that are used for intoxication purposes. Alterations in memory have been reported in inhalant users; however, it is unclear whether these impairments could be reversed, and the mechanisms involved in the putative recovery. Therefore, the main purpose of this study was to model the deleterious effects of toluene on memory in mice and to evaluate the effect of environmental enrichment on that response. In the second part of the study, the concentrations of glutamate and GABA, following chronic toluene exposure and after environmental enrichment treatment, were evaluated. Adolescent mice were exposed to either a single or repeated schedule of toluene administration and their responses to object recognition were analyzed. An independent group of mice was repeatedly exposed to toluene and then housed either under environmental enrichment or standard conditions for four weeks. At the end of the housing period, the rodents' performance in object recognition test, as well as the concentrations of neurotransmitters, were analyzed. The results showed that toluene caused memory impairment in mice that received a single or repeated solvent exposure. Remarkably, environmental enrichment could reverse memory deficits induced by repeated administration of toluene. Cessation of toluene exposure in mice in standard housing did not produce that response. The glutamate and GABA tissue contents were not involved in the effects of toluene or environmental enrichment of memory.

Introduction

Inhalant abuse is a health problem in both developed and developing countries (Dell et al., 2011). Inhalants are among the main drugs of choice for young people, with an age of onset between 10 and 17 years (Dell et al., 2011). In Mexico, recent surveys have detected inhalant abuse in socially marginalized populations as well as an increasing prevalence among young students living at home (Cruz, 2014, Villatoro et al., 2011). Toluene-based products (paint thinner, glues, and Activo [a toluene-enriched formulation distributed by drug dealers]) are the most commonly used for intoxication in Mexico (Villatoro et al., 2011). Toluene exposure has been associated with many health effects, mainly neurotoxicity and neuropsychiatric disorders (Ridenour et al., 2007). Despite their important deleterious health consequences, inhalants continue to be one of the least studied psychoactive substances. Therefore, treatment and prevention research has only recently emerged (Howard et al., 2011).

Significant memory and attention impairments have been described as the most disabling effects of inhalant abuse (Filley et al., 2004, Howard et al., 2011). Similarly, preclinical studies have found memory disruptions in various toluene administration patterns. For instance, exposure to a low toluene concentration (80 ppm, 6 h/day) for four weeks led to memory deficiencies in rats subjected to the Morris water maze (von Euler et al., 2000). Repeated exposure to a high toluene concentration (6000 ppm) also resulted in memory deficits in rats that were evaluated with the object recognition test and inhibitory avoidance task (Huerta-Rivas et al., 2012). Additionally, intraperitoneal administration of toluene (600 and 750 mg/kg for 9 days) resulted in memory impairment in mice that were subjected to the object recognition test (Lin et al., 2010).

Pharmacological and psychosocial interventions have been tested among inhalant users (Dell and Hopkins, 2011, Howard et al., 2011, Shaw et al., 2011); nevertheless, their impact on memory performance is still unclear. At the preclinical level, environmental enrichment (EE) is a housing condition that adds objects to a subject's environment to increase the levels of sensory, cognitive, and motor stimuli. Under these housing conditions, delayed onset and reversal of cognitive deficits have been observed in various models of memory (For review, see Nithianantharajah and Hannan, 2006, Hannan, 2014). For instance, mice with EE showed improved performance compared to mice under standard housing conditions in the Morris water maze, T-maze and radial arm water maze (Nithianantharajah and Hannan, 2006, Pang and Hannan, 2013, van Praag et al., 2000). Additionally, EE exposure improves memory in animal models of pathological conditions, such as brain injury or trauma, and in mice with poor learning task performance (Hannan, 2014, van Praag et al., 2000). Moreover, previous findings showed that improved memory is associated with enhanced neurogenesis (Kempermann et al., 1998, van Praag et al., 1999). Repeated toluene administration (2000 ppm for four weeks) reduced neurogenesis in the dorsal hippocampus, whereas EE reversed toluene-induced alterations in doublecortin-labeled cells (Paez-Martinez et al., 2013). Furthermore, EE has shown the ability to reverse the negative effects induced by some drugs of abuse, such as cocaine, heroin and amphetamine (Bardo et al., 2001, El Rawas et al., 2009, Solinas et al., 2008, Solinas et al., 2010). Hence, the overall purpose of this study was to screen toluene-induced memory impairment at different extents of exposure. Then, we evaluated the putative positive effect of EE to reverse memory impairment induced by chronic toluene exposure.

Memory is a cognitive process that involves several neurotransmitters, including GABA and glutamate (Kandel et al., 2013, Mohler, 2007). Alterations in GABA and glutamate levels have been observed after toluene exposure. For instance, acute 2000 ppm toluene exposure decreased extracellular GABA levels within the globus pallidus (Stengard and O'Connor, 1994), while in adolescent rats, a high toluene concentration (12,000 ppm) decreased the GABA levels in the hippocampus and striatum (O'Leary-Moore et al., 2007). Similarly, in adult rats, toluene (12,000 ppm) reduced the GABA levels in the striatum and frontal cortex (O'Leary-Moore et al., 2009). On the other hand, acute exposure to 12,000 ppm toluene decreased the glutamate levels in the hippocampus of adolescent rats, but not in the hippocampus of adult rats (O'Leary-Moore et al., 2007). Repeated toluene exposure (8000 and 12,000, for 7 days) in adolescent rats reduced the glutamate levels in the prefrontal cortex 1 day after the last exposure, while this exposure enhanced the glutamate levels in the hippocampus 7 days after stopping toluene administration (Perrine et al., 2011). Accordingly, in the second part of this study, we evaluated the putative changes in these neurotransmitters after both repeated toluene exposure and the EE paradigm.

Section snippets

Animals

A total of 178 mice were used in the study; there were 42 animals for the acute toluene exposure protocol, 56 animals for the repeated toluene exposure protocol, and 80 animals for EE. Inbred male Swiss Webster mice were provided from our breeding facilities (originally obtained from Taconic, Hudson, NY). The study started on postnatal day 35 to 40 of the animals (PN 35 to 40). This stage was selected because mice at this age exhibit behavioral, neurochemical and endocrine patterns of human

Acute toluene exposure impaired memory in mice

Mice exposed to a single dose of toluene had a decreased recognition index compared to control animals in the ORT (Fig. 2). This reduction was statistically significant from 2000 to 6000 ppm (F4,37 = 5.791; p < 0.001). These results suggest that a single exposure to toluene impaired memory in mice.

Mice exposed to toluene from 1000 to 4000 ppm did not have statistically significant changes in the cumulative exploration time during the first trial. Although exposure to 6000 ppm showed a tendency to

Discussion

Based on the findings of this study, acute toluene exposure induced memory impairment in adolescent mice. In addition, repeated toluene exposure also resulted in memory impairment; however, this effect did not depend on the toluene concentration used or chronicity of exposure. Remarkably, EE in animals previously exposed to toluene resulted in rescue of memory performance. Our analysis of glutamate and GABA showed that the neurotransmitters had no change after repeated exposure to toluene.

Conclusions

Detrimental effects on memory were observed in mice that had received either a single or repeated toluene administration. These effects endured even after several weeks of cessation of toluene exposure. On the other hand, EE reversed the cognitive deficits induced by toluene using an administration schedule that simulated the exposure patterns of inhalant users. Our results suggest that cognitive stimulation promotes the development of brain plasticity that can rescue cognitive alterations

Disclosure

The authors declare that there are no conflicts of interest.

Transparency document

Transparency document

Acknowledgements

This paper includes data from the Master's dissertation by Rocío del Carmen Solís Guillén and Bachelor's thesis by David García Jácome. Authors wish to thank to Beatriz Cruz-López, M. Yepci Guadalupe Yee-Ríos, Ariel Alaín Oros-González, Ricardo Mosco-Aquino and Susan Muñoz-Aquiahuatl for their excellent technical assistance. This study was performed with the support of the Instituto Nacional de Psiquiatría Ramón de la Fuente Muñiz NC103380.2, projects SIP-Instituto Politécnico Nacional and

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