Simultaneous anhedonia and exaggerated locomotor activation in an animal model of depression

Rationale  Anhedonia, or hyposensitivity to normally pleasurable stimuli, is a cardinal symptom of depression. As such, reward circuitry may comprise a substrate with relevance to this symptom of depression. Objectives  Our aim was to characterize in the rat changes in the rewarding properties of a pharmacological and a natural stimulus following olfactory bulbectomy (OBX), a pre-clinical animal model of depression. Methods  We measured amphetamine enhancement of brain stimulation reward, changes in sucrose intake, as well as striatal cAMP response element binding protein (CREB) activity, a molecular index previously associated with depressant-like behavior. Moreover, since alteration of psychomotor activity is also a common symptom of depression, and psychostimulant reward and locomotion are thought to share common neurobiology, we used the same treatment schedule of amphetamine to probe for changes in locomotion. Results  Our findings show that OBX produces a behavioral phenotype characterized by both anhedonia and exaggerated locomotor activation. Thus, we observed a blunted response to the rewarding properties of amphetamine (1 mg/kg, 21 days post-lesion), a long-lasting reduction in sucrose intake and increased striatal CREB activity. In addition, the same dose of amphetamine, at a coincident time post-lesion, triggered an exaggerated response to its locomotor-stimulant actions. Conclusions  These paradoxical findings are not consistent with the notion that reward and locomotion are mediated by a common substrate; this dissociation may be useful in modeling psychiatric disorders such as mixed depressive states. In addition, our findings suggest that central reward circuitry may constitute a possible target for rationally designed therapeutics for depression.