Antidepressant-like Effects of Uridine and Omega-3 Fatty Acids Are Potentiated by Combined Treatment in Rats William A. Carlezon, Jr., Stephen D. Mague, Aimee M. Parow, Andrew L. Stoll, Bruce M. Cohen, and Perry F. Renshaw Background: Brain phospholipid metabolism and membrane fluidity may be involved in the pathophysiology of mood disorders. We showed previously that cytidine, which increases phospholipid synthesis, has antidepressant-like effects in the forced swim test (FST) in rats, a model used in depression research. Because cytidine and uridine both stimulate synthesis of cytidine 5= -diphosphocholine (CDP-choline, a critical substrate for phospholipid synthesis), we examined whether uridine would also produce antidepressant-like effects in rats. We also examined the effects of omega-3 fatty acids (OMG), which increase membrane fluidity and reportedly have antidepressant effects in humans, alone and in combination with uridine. Methods: We first examined the effects of uridine injections alone and dietary supplementation with OMG alone in the FST. We then combined sub-effective treatment regimens of uridine and OMG to determine whether these agents would be more effective if administered together. Results: Uridine dose-dependently reduced immobility in the FST, an antidepressant-like effect. Dietary supplementation with OMG reduced immobility when given for 30 days, but not for 3 or 10 days. A sub-effective dose of uridine reduced immobility in rats given sub-effective dietary supplementation with OMG. Conclusions: Uridine and OMG each have antidepressant-like effects in rats. Less of each agent is required for effectiveness when the treatments are administered together. Key Words: Depression, phospholipids, membrane, energy, model, rat T he neurobiology of depression and its response to treat- ment are not well understood. Because most antidepres- sants with clinical efficacy act upon monoamines (primar- ily norepinephrine [NE] and serotonin [5HT]), much research on depression has focused upon interactions between these neuro- transmitters and their reuptake and receptor proteins. However, most pharmacotherapies for depression require weeks or months of treatment despite immediate effects on brain monoamine transmission (for review, see Nestler 1998), suggesting that other mechanisms may be involved in re-establishing normal mood. As one example, recent research has become focused less on monoamines and their receptors and more on intracellular mechanisms that change during antidepressant treatments (Du- man et al 1997; Nestler et al 2002; Coyle and Duman 2003), with the goal of discovering new targets and developing novel therapeutics that act faster, are more efficacious, and have fewer side effects. Accumulating evidence suggests alterations of brain phos- pholipid composition and membrane fluidity can affect extracel- lular processes such as neurotransmitter-receptor binding, intra- cellular processes such as signal transduction and mitochondrial function, as well as eicosanoid-mediated processes, all of which may be involved in the pathophysiology of mood disorders (Lands 1992; Pacheco and Jope 1996; Shetty et al 1996; Exton 1997; Nomura et al 2001). Indeed, depression has been linked to abnormalities in both membrane synthesis and fluidity (Moore et al 1997; Sonawalla et al 1999; Ende et al 2000; Moore et al 2000; Steingard et al 2000; Hirashima et al 2004). These observations raise the possibility that administration of agents that affect the metabolism of phospholipids or their incorporation into neural membranes may represent an alternative approach to the treat- ment of depression. There is already some evidence that treatments which affect phospholipid metabolism and membrane fluidity have efficacy in the treatment of depressive symptoms, although the effects are often modest and causal relationships are difficult to prove. For example, populations with diets rich in fish tend to show a lower prevalence of major depression (Hibbeln 1998; Tanskanen et al 2001a; 2001b). The fish in these diets are high in omega-3 fatty acids, which are long-chain polyunsaturated fatty acids that are incorporated into neuronal membranes (for review, see Freeman 2000). The double bonds in the structure of omega-3 fatty acids result in conformations that prevent dense packing of phospho- lipids, thereby increasing membrane fluidity (Popp-Snijders et al 1984; Cartwright et al 1985). Treatment with omega-3 fatty acids in humans decreases brain water proton transverse relaxation times (T2s), consistent with increased membrane fluidity (Hirashima et al 2004). In addition, numerous studies have shown correlations between depressive symptoms and low levels of omega-3 fatty acids in plasma and erythrocyte phospho- lipids (Adams et al 1996) and red blood cell membranes (Ed- wards et al 1998; Peet et al 1998). In clinical studies, omega-3 fatty acids reportedly have beneficial effects in major depression in some (Locke and Stoll 2001; Nemets et al 2002; Peet and Horrobin 2002; Su et al 2003) but not all (Maes et al 1999; Llorente et al 2003; Marangell et al 2003; Marangell et al 2004) studies. Omega-3 fatty acids have also been shown to improve depressive symptoms in patients with bipolar disorder (Stoll et al 1999). Similarly, some symptoms of cocaine withdrawal, which often involves depressive symptoms, can be treated with citico- line (Renshaw et al 1999). Citicoline (a synthetic form of CDP- choline, a critical substrate for phospholipid synthesis) is metab- From the Department of Psychiatry, Harvard Medical School, McLean Hos- pital, Belmont, Massachusetts. Address reprint requests to Bill Carlezon, Ph.D., Department of Psychiatry, McLean Hospital, MRC 217, 115 Mill Street, Belmont, MA 02478; E-mail: carlezon@mclean.harvard.edu. Received August 20, 2004; revised November 15, 2004; accepted November 22, 2004. BIOL PSYCHIATRY 2005;57:343–350 0006-3223/05/$30.00 doi:10.1016/j.biopsych.2004.11.038 © 2005 Society of Biological Psychiatry