Repeated toluene exposure increases c-Fos in catecholaminergic cells of the nucleus accumbens shell ☆ Michelle L. Tomaszycki a,c, ⁎, Kelsey E. Aulerich a , Scott E. Bowen a,b, ⁎⁎ a Department of Psychology, Wayne State University, Detroit, MI, United States b Behavioral Pharmacology and Toxicology Laboratory, Wayne State University, Detroit, MI, United States c The Behavioral Neuroscience of Social Relationships Laboratory, Wayne State University, Detroit, MI, United States abstract article info Article history: Received 31 May 2013 Received in revised form 3 September 2013 Accepted 5 September 2013 Available online 12 September 2013 Keywords: Toluene Nucleus accumbens Ventral tegmental area c-Fos Tyrosine hydroxylase 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 (1000–4000 ppm) on loco- motor 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 4000 ppm than in animals exposed to air (control) or 1000 ppm. Taken together, these findings demonstrate that repeated high dose toluene exposure increases locomotor activity as well as ac- tivation of catecholaminergic cells in the shell of the NAc. © 2013 Elsevier Inc. All rights reserved. 1. Introduction Drug use, especially abuse of solvents, tends to be higher in adoles- cents than in adults (Doremus-Fitzwater et al., 2010; SAMHSA, 2008). For solvents, the route of administration is typically via inhalation of vapors, a practice that can result in severe intoxication of the user (Garland and Howard, 2010). Toluene (i.e., methylbenzene) is the most commonly abused solvent due to its availability in industrial and common household products such as paint thinners/removers, gasoline, spray paint, and glue (Lubman et al., 2008). Because of this, it is one of the most studied substances in inhalant abuse research. The behavioral effects of acute inhalant exposure are well under- stood [for review see Bowen et al. (2006) and Evans and Balster (1991)]. Acute exposure causes reversible, dose-dependent, behavioral effects that occur below concentrations that produce toxic effects. A bi-phasic locomotor dynamic results from acute inhalation of toluene; higher levels of activity occur at concentrations below 6000 ppm, while sedative-like effects occur at concentrations greater than 6000 ppm (Bowen and Balster, 1998; Bowen et al., 2010; Conti et al., 2012; Hinman, 1987). Toluene exposure can produce a conditioned place pref- erence, and rats will self-administer toluene, which emphasizes the re- warding properties of this inhalant (Blokhina et al., 2004; Lee et al., 2006). Drug-discrimination trials have shown that toluene exhibits simi- larities to ethanol, amphetamine, and PCP (Bowen, 2006, 2009; Bowen et al., 1999; Rees et al., 1987), which strongly suggests that toluene shares mechanisms with several drugs of abuse. Although drugs of abuse differ in both molecular structure and initial activity, they share features important to understanding addiction, pri- marily activation of the dopaminergic system in the mesolimbic path- way (Everitt and Robbins, 2005). Administration of a drug of abuse generally results in the increased firing of dopaminergic neurons in the ventral tegmental area (VTA), which in turn, increases dopamine (DA) levels in the nucleus accumbens (NAc) (Nestler, 2001). Furthermore, the posterior VTA tends to express more tyrosine hydroxylase (TH), the rate limiting enzyme for DA, than does the anterior VTA (Ikemoto, 2007), making this region of particular interest. The NAc, too, demon- strates neurochemical differences, with the NAc shell releasing more ex- tracellular DA in response to intravenous cocaine, amphetamine, or Neurotoxicology and Teratology 40 (2013) 28–34 ☆ This work was supported in part by a WSU Undergraduate Creative Projects Grant (KA), WSU Start Up Funds (MLT), and WSU Bridge Funding (SEB). ⁎ Correspondence to: M.L. Tomaszycki, Department of Psychology, Wayne State University, 5057 Woodward Avenue, Detroit, MI 48202, United States. Tel.: +1 313 577 0341; fax: +1 313 577 7636. ⁎⁎ Correspondence to: S.E. Bowen, Department of Psychology, Wayne State University, 5057 Woodward Avenue, Detroit, MI 48202, United States. Tel.: +1 313 577 9546; fax: +1 313 577 7636. E-mail addresses: michelletomas@wayne.edu (M.L. Tomaszycki), Scott.Bowen@wayne.edu (S.E. Bowen). 0892-0362/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.ntt.2013.09.001 Contents lists available at ScienceDirect Neurotoxicology and Teratology journal homepage: www.elsevier.com/locate/neutera