Modeling depression: social dominance-submission gene expression patterns in rat neocortex.

Gene expression profiles in the cortex of adult Long-Evans rats as a function of a stressful social loss and victory in inter-male fighting encounters were examined. This social dominance and subordination model has been postulated to simulate early changes in the onset of depression in the losers. Microarrays were fabricated containing 45mer oligonucleotides spotted in quadruplicate and representing 1178 brain-associated genes. Dynamic range, discrimination power, accuracy and reproducibility were determined with standard mRNA "spiking" studies. Gene expression profiles in dominant and subordinate animals were compared using a "universal" reference design [Churchill GA (2002) Fundamentals of experimental design for cDNA microarrays. Nat Genet 32 (Suppl):490-495]. Data were analyzed by significance analysis of microarrays using rank scores [Tusher VG, Tibshirani R, Chu G (2001) Significance analysis of microarrays applied to the ionizing radiation response. Proc Natl Acad Sci USA 98:5116-5121; van de Wiel MA (2004) Significance analysis of microarrays using rank scores. Kwantitatieve Methoden 71:25-37]. Ontological analyses were then performed using the GOMiner algorithm [Zeeberg BR, Feng W, Wang G, Wang MD, Fojo AT, Sunshine M, Narasimhan S, Kane DW, Reinhold WC, Lababidi S, Bussey KJ, Riss J, Barrett JC, Weinstein JN (2003) GoMiner: a resource for biological interpretation of genomic and proteomic data. Genome Biol 4(4):R28]. And finally, genes of special interest were further studied using quantitative reverse transcriptase polymerase chain reaction. Twenty-two transcripts were statistically significantly differentially expressed in the neocortex between dominant and subordinate animals. Ontological analyses revealed that significant gene changes were clustered primarily into functional neurochemical pathways associated with protein biosynthesis and cytoskeletal dynamics. The most robust of these were the increased expression of interleukin-18, heat shock protein 27, beta3-tubulin, ribosome-associated membrane protein 4 in subordinate animals. Interleukin-18 has been found to be over-expressed in human depression and panic disorder as well as other physiological stress paradigms [Takeuchi M, Okura T, Mori T, Akita K, Ohta T, Ikeda M, Ikegami H, Kurimoto M (1999) Intracellular production of interleukin-18 in human epithelial-like cell lines is enhanced by hyperosmotic stress in vitro. Cell Tissue Res 297(3):467-473] and heat shock proteins have been shown to be involved in the pathogenesis of many neurodegenerative and psychiatric disorders [Iwamoto K, Kakiuchi C, Bundo M, Ikeda K, Kato T (2004) Molecular characterization of bipolar disorder by comparing gene expression profiles of postmortem brains of major mental disorders. Mol Psychiatry 9(4):406-416; Pongrac JL, Middleton FA, Peng L, Lewis DA, Levitt P, Mirnics K (2004) Heat shock protein 12A shows reduced expression in the prefrontal cortex of subjects with schizophrenia. Biol Psychiatry 56(12):943-950]. Thus, the gene expression changes that we have observed here are consistent with and extend the observations found in the clinical literature and link them to the animal model used here thereby reinforcing its use to better understand the genesis of depression and identify novel therapeutic targets for its treatment.