Effects of chronic environmental and social stimuli during adolescence on mesolimbic dopaminergic circuitry markers

Previously, we have shown that chronic exposure to environmental and social stimuli (ESS) during adolescence prevents the development of behavioral sensitization to amphetamine in adult rats. At the onset of the peripubertal-juvenile period (28-d) male rats were subjected to a 28-d long intermittent ESS protocol or handled as controls (NO-ESS). Twenty-four hours after the last session of ESS or NO-ESS, all rats started a regimen of behavioral sensitization to amphetamine (1 mg/kg, i.p.), in which rats were injected every third day with amphetamine or saline on four occasions. Then following one week abstinence all rats were challenged with a lower dose of amphetamine (0.5 mg/kg, i.p.) and their locomotor activity monitored for 2 h. Our results showed that while NO-ESS rats developed behavioral sensitization to amphetamine, ESS rats did not develop this behavior. All rats were then sacrificed 3 days following the challenge to allow for amphetamine clearance. Since mesolimbic dopamine has been implicated in behavioral sensitization to amphetamine we compared messenger RNA (mRNA) expression of key dopamine-related molecules in the mesolimbic circuitry in ESS and NO-ESS rats. A decrease in dopaminergic D1 receptor (D1R) gene expression in the caudate-putamen (CPu) was associated with amphetamine sensitization in the controls, possibly as a result of a chronic increase in DA release. In contrast, amphetamine treatment did not modulate D1R mRNA levels in ESS rats. No change has been detected in any other dopaminergic markers [D2R, D3R, tyrosine hydroxylase (TH) or dopamine transporter (DAT) mRNAs]. Consequently, we conclude that ESS may inhibit the development of behavioral sensitization to amphetamine through preventing the decrease in CPu D1R mRNA levels.