Thesis (MScMedSc (Biomedical Sciences. Medical Physiology)--Stellenbosch University, 2008. / For many centuries, scientists have engaged in a theoretical debate concerning the etiology of mood disorders, with very few ancient scholars speculating about the importance of genetic factors and affective temperaments as factors in the etiology of depression. Mood, emotion and cognition have been shown to be modulated by the serotonergic midbrain raphe system; implicated in the pathogenesis of psychiatric disorders like those of the affective spectrum. Evidence from neuroscience, genetics, and clinical investigation demonstrate that depression is a disorder of the brain. Brain imaging research is revealing that in depression, neural circuits responsible for moods, thinking, sleep, appetite, and behavior fail to function properly, and that the regulation of critical neurotransmitters is impaired. Genetics research, including studies of twins, indicates that genes play a role in depression. Vulnerability to depression appears to result from the influence of multiple genes acting together with environmental factors. Other research has shown that stressful life events, particularly in the form of loss such as the death of a close family member, may trigger major depression in susceptible individuals. Depression and anxiety have often been successfully treated by means of selective serotonin reuptake inhibitors. However, selective serotonin reuptake inhibitors do not solve all the problems inherent to the treatment of depression, for approximately 30 % of depressed patients do not respond to treatment and 20 % experience relapses whilst on treatment. Of consideration is the fact that the majority of drugs today are based on proteins, with 50 % of therapeutics on the market targeting cell membrane proteins. Up to this day the precise pathophysiology of mood disorders remains obscure, as does the neurobiology of normal mood regulation. Accordingly, there is a need for methods to identify the structural and/or signaling components which lead to changes in the brain, particularly the hippocampus, of subjects having mood disorders such as bipolar depressive disorder, chronic major depressive disorder and the like. Similarly, there is a need for the early detection, screening and diagnosis of individuals at risk for a mood disorder. As the serotonin tranpsorter is the primary target for therapeutic intervention in the treatment of numerous psychiatric disorders and considering the fact that at the structural level this protein’s function as transporter in membranes remains incompletely understood, investigating its function in psychiatric disorders are of importance . The objective of this study was to determine the role of the serotonin transporter in wild type and serotonin knockout rats, with regards to the hippocampus. Rat hippocampi were fractionated into cytosolic and membrane components, which were run and further separated in two dimensions. Firstly separation occurred by isoelectrical focusing (pI), follwed by gel iii electrophoresis (molecular weight). Gels were compared to see whether protein spots have changed between animals that have been differentially bred. Differentially expressed protein spots, as determined by PD Quest software, were excised, digested and analyzed by means of mass spectrometry. Our results indicated that metabolic, structural and cell signaling proteins were differentially expressed in both the ventral and dorsal hippocampus of the serotonin knockout rat. Futhermore, cellular stress proteins were found to be only differentially expressed in the ventral hippocampus. The majority of proteins identified in both hippocampal areas as well as both fractions, were assigned to energy metabolism. The cytosolic protein profile mirrored the pattern of the membrane protein profile. In conclusion, this proteomic study identified various protein groups that interacted with one another, thus establishing compensation for disrupted serotonin homeostasis.