TI - Forced Swimming-induced changes in cFos+ neurons in the nucleus accumbens . AB - The nucleus accumbens is a mesolimbic brain region involved in behavioral responses to stress . It has also been suggested to act as a neuroanatomical SUBstrate for immobility/floating behavior in the forced swim test [35] . Therefore , we investigated whether forced swimming would lead to a differential c-Fos induction in the nucleus accumbens of exercised rats as compared to control animals . These animals were the same as those used for the dentate gyrus analyses . Figure 6A and B show representative images of the nucleus accumbens of rats killed under baseline conditions or at 2 h after forced swimming . We counted c-Fos+ neurons in an area of the nucleus accumbens showing highest numbers of immuno-positive neurons , comprising parts of both the core and shell region . Forced swimming induced a marked increase in the number of c-Fos+ neurons in the nucleus accumbens of both control and exercised animals . The increase , however , was similar in both groups (Fig 6C) . The similar degree of c-Fos induction in exercised and control animals suggests that any differential behavioral responses to stress in the exercisers are unlikely to be mediated by the nucleus accumbens . We also analyzed the hypothalamic paraventricular nucleus ( PVN ) , a stress-sensitive nucleus that plays a principal role in hypothalamic-pituitary-adrenocortical ( HPA ) axis regulation [17] . We found that the increases in c-Fos levels were similar in control and exercised rats after forced swimming ( data not shown ) .