Lipopolysaccharide-induced anhedonia is abolished in male serotonin transporter knockout rats: An intracranial self-stimulation study

A growing body of evidence suggests that pro-inflammatory cytokines contribute to the pathogenesis of depression. Previously, it has been shown that cytokines (e.g. interferon-α therapy) induce major depression in humans. In addition, administration of the cytokine-inducer lipopolysaccharide (LPS) provokes anhedonia (i.e. the inability to experience pleasure) in rodents. Furthermore, serum pro-inflammatory cytokine levels are increased in depressed patients. Nevertheless, the etiology of cytokine-induced depression is largely unknown. Previously, it has been shown that selective serotonin re-uptake inhibitors decrease serum pro-inflammatory cytokine levels and that pro-inflammatory cytokines increase activity of the serotonin transporter (SERT). The purpose of this study was to explore the effect of partial and complete lack of the SERT in LPS-induced anhedonia assessed in the intracranial self-stimulation (ICSS) paradigm. A single intraperitoneal injection of LPS was used to induce a pro-inflammatory immune response in male serotonin transporter wild type (SERT^+/+), heterozygous (SERT^+/-) and knockout (SERT^-/-) rats. Body weight and ICSS thresholds were measured daily. Although LPS reduced body weight in all genotypes, loss of body weight was less pronounced in SERT^-/- compared to SERT^+/+ rats. Remarkably, LPS-induced anhedonia was totally abolished in SERT^-/- rats and as expected was still present in SERT^+/+ and to a lesser extent in SERT^+/- rats. Therefore, it is concluded that an intact SERT function is needed for pro-inflammatory cytokine-induced anhedonia and weight loss in rats.