Effects of differential rearing on amphetamine-induced c-fos expression in rats

Highlights

• Differential rearing alters c-fos expression following amphetamine treatment in a regionally specific pattern.

• Rats reared in an impoverished condition and treated with amphetamine demonstrate greater c-fos expression in the NAcc than enriched rats.

• Rats reared in an impoverished condition and treated with amphetamine displayed greater c-fos expression in the NAcc than rats treated with saline; while rats reared in an enriched condition and treated with saline displayed greater c-fos expression in the prelimbic cortex compared to enriched rats treated with amphetamine.

Abstract

Background

Rearing rats in environmental enrichment alters psychostimulant-induced locomotor activity as well as neurotransmitter expression. Exposure to novelty and psychostimulants induces c-fos expression in neurons in the mesolimbic dopamine (DA) pathway. Here we investigated changes in the expression of the immediate early gene c-fos in the mesolimbic DA pathway of enriched, isolated, or socially reared rats due to the neurobiological changes that result from rearing conditions and influence drug taking behavior.

Methods

Rats were reared in either enriched (EC), isolated (IC), or social (SC) conditions for 30 days, after which they received an acute amphetamine or saline injection (1.0 mg/kg) and locomotor activity was measured. Following immunohistochemical staining c-fos positive neurons were quantified in the NAcc, mPFC, and amygdala.

Results

Greater locomotor activity was observed in differentially reared rats treated with amphetamine compared to saline, as well as in SC compared to EC and IC rats. Rats reared in an IC context demonstrated greater c-fos expression than EC rats in the NAcc when treated with amphetamine, and EC saline rats demonstrated greater c-fos expression in the cingulate and prelimbic cortices compared to SC saline rats. Additionally, IC amphetamine rats displayed greater c-fos expression in the NAcc compared to IC saline rats, while EC saline rats displayed greater c-fos expression in the prelimbic cortex compared to EC amphetamine rats.

Conclusions

These results suggest regional specificity of psychostimulant-induced c-fos expression in the prelimbic/NAcc pathway that is altered in differential rearing, and influences initial c-fos activation following psychostimulant exposure.

Introduction

Drug abuse is a multi-faceted disorder that is influenced by both environmental and genetic factors. Environmental enrichment causes behavioral, neurobiological, and neurochemical changes in the brain; thus, the current study investigated the impact of rearing conditions on neurological changes associated with psychostimulant abuse (Bardo and Dwoskin, 2004, Bardo et al., 2001, Simpson and Kelly, 2011, Stairs and Bardo, 2009).

The enrichment paradigm is composed of the enriched (EC), isolated (IC), and social condition (SC), which are differentiated by rats’ exposure to novel objects and social interaction (Greenough et al., 1972, Renner and Rosenzweig, 1987). In general, the effects of differential rearing on the response to psychostimulants are most robust when low unit doses of psychostimulants are administered (Bardo and Dwoskin, 2004, Simpson and Kelly, 2011, Stairs and Bardo, 2009). For example, environmental enrichment decreases acute amphetamine-induced hyperactivity when low (Cain et al., 2012), but not moderate/high doses of amphetamine are administered (Bardo et al., 1995, Cain et al., 2012, Gill et al., 2012).

Differential rearing leads to several neurological changes in rats. Lower levels of dopamine (DA) clearance, DA transporter (DAT) surface expression, DA uptake, and D1 receptor density are found in the medial prefrontal cortex (mPFC) of EC rats compared to IC rats (Bardo and Dwoskin, 2004, Wooters et al., 2011, Zhu et al., 2005, Zhu et al., 2004). In the nucleus accumbens (NAcc), however, the effects of enrichment on psychostimulant-induced DA activity are inconsistent (Bardo et al., 1999, Bardo et al., 1995, Del Arco et al., 2007, Segovia et al., 2010, Zakharova et al., 2009). As the mesolimbic DA pathway is the primary pathway involved in the rewarding effects of psychostimulants (Badiani et al., 1998, Bardo et al., 1995, Koob, 1999), and is activated following exposure to a novel environment (Bardo and Dwoskin, 2004, Koob, 1999), we hypothesize that changes in DA following amphetamine exposure, differential rearing, and exposure to a novel environment will alter DA associated c-fos expression (Umino et al., 1995).

The mesolimbic DA system in particular is hypothesized to be the commonality for c-fos expression as the NAcc (Graybiel et al., 1990, Umino et al., 1995), mPFC (Tani et al., 2001, Uslaner et al., 2001), central nucleus of the amygdala (ACe; Engber et al., 1998, Howes et al., 2000, Umino et al., 1995), and basolateral amygdala (BLA; Day et al., 2001, Neisewander et al., 2000, Umino et al., 1995), all express c-fos following psychostimulant exposure. c-fos Expression is also altered in the mesolimbic DA system following exposure to novelty (Badiani et al., 1997, Badiani et al., 1998, Day et al., 2001, Ostrander et al., 2003).

To further clarify the interaction between rearing-induced neuroanatomical changes and acute psychostimulant action, the present study examined the effect of a moderate dose of amphetamine on amphetamine-induced hyperactivity, and quantified c-fos stained neurons in the NAcc, mPFC, and amygdala, in EC, IC, and SC rats tested in a novel environment. Since the prelimbic cortex projects to the NAcc, ACe, and BLA, while the infralimbic cortex only has amygdalar projections (Vertes, 2004), we hypothesized that differential rearing would primarily alter c-fos expression in the prelimbic pathway.