Responding for a conditioned reinforcer or unconditioned sensory reinforcer in mice: interactions with environmental enrichment, social isolation, and monoamine reuptake inhibitors.

RATIONALE

Environmental factors influence the etiology of many psychiatric disorders. Likewise, environmental factors can alter processes central to motivation. Therefore, motivational deficits present in many disorders may be influenced by early life environmental conditions.

OBJECTIVE

We examined whether housing animals in different environmental conditions influenced the ability of sensory stimuli to acquire incentive value and whether elevated monoamine activity altered responsing for these stimuli.

METHODS

Isolation-housed (IH), pair-housed (PH), and environmentally enriched (EE) male C57BL/6N mice were examined in tests of responding for a conditioned reinforcer (CRf) or an unconditioned sensory reinforcer (USRf). The CRf was previously paired with saccharin delivery through Pavlovian conditioning, while the USRf was not conditioned with a reward. Following baseline tests of responding for the CRf or USRf, the effects of elevated monoamine activity were examined.

RESULTS

At baseline, PH and EE mice responded similarly for the CRf or USRf. IH mice responded more for the CRf but exhibited slower acquisition of responding for the USRf. Administration of citalopram, a serotonin transporter blocker, or atomoxetine, a norepinephrine transporter blocker, decreased responding for the CRf and USRf in all groups. The dopamine transporter blocker GBR 12909 generally increased responding for the CRf and USRf, but further analysis revealed enhanced responding for both reinforcers only in EE mice.

CONCLUSIONS

Baseline incentive motivation is strongly influenced by the social component of housing conditions. Furthermore, environmental enrichment increased the sensitivity to elevated dopamine activity, while acute elevations in serotonin and norepinephrine inhibit incentive motivation irrespective of housing condition.