Nortriptyline influences protein pathways involved in carbohydrate metabolism and actin-related processes in a rat gene–environment model of depression

Abstract

Although most available antidepressants increase monoaminergic neurotransmission, their therapeutic efficacy is likely mediated by longer-term molecular adaptations. To investigate the molecular changes induced by chronic antidepressant treatment we analysed proteomic changes in rat pre-frontal/frontal cortex and hippocampus after nortriptyline (NT) administration. A wide-scale analysis of protein expression was performed on the Flinders Sensitive Line (FSL), a genetically-selected rat model of depression, and the control Flinders Resistant Line (FRL). The effect of NT treatment was examined in a gene–environment interaction model, applying maternal separation (MS) to both strains.

In the forced swim test, FSL rats were significantly more immobile than FRL animals, whereas NT treatment reduced immobility time. MS alone did not modify immobility time, but it impaired the response to NT in the FSL strain.

In the proteomic analysis, in FSL rats NT treatment chiefly modulated cytoskeleton proteins and carbohydrate metabolism. In the FRL strain, changes influenced protein polymerization and intracellular transport. After MS, NT treatment mainly affected proteins in nucleotide metabolism in FSL rats and synaptic transmission and neurite morphogenesis pathways in FRL rats. When the effects of NT treatment and MS were compared between strains, carbohydrate metabolic pathways were predominantly modulated.

Introduction

Although most available antidepressants acutely increase monoaminergic neurotransmitter concentration in synapses, their therapeutic efficacy is supposed to be mediated by longer-term molecular adaptations (Berton and Nestler, 2006, Schloss and Henn, 2004). The delay usually observed between start of treatment and clinical improvement supports the hypothesis that the regulation of neurotransmitter availability in brain synapses is followed by a cascade of molecular events which stand at the basis of symptom amelioration. Since the discovery of effective antidepressants, several lines of research were committed to the investigation of long-term effects of antidepressant treatments, with the aim of gaining further knowledge about the molecular basis of their therapeutic efficacy. Antidepressant treatments are reported to induce receptor desensitisation, regulation of G protein and second messengers, modulation of cytoskeletal microtubule dynamics, alterations in gene expression, changes in synaptic plasticity, increased neurotrophic regulation, support of neurogenetic events, modulation of stress responses, and anti-inflammatory actions; these mechanisms are believed to play a role in their therapeutic efficacy (Bianchi et al., 2005, Castren, 2004, Donati and Rasenick, 2003, Dranovsky and Hen, 2006, Malberg and Blendy, 2005, O'Brien et al., 2004, Pittenger and Duman, 2008, Schloss and Henn, 2004, Tanis and Duman, 2007). Recently, the availability of large-scale analysis methods allowed the investigation of antidepressant mechanism of action with unbiased approaches (Carboni et al., 2006b, Conti et al., 2007, Khawaja et al., 2004, Landgrebe et al., 2002, Sillaber et al., 2008). A major challenge facing these lines of research is the choice of a suitable experimental design to discriminate molecular changes related to antidepressant therapeutic efficacy from neutral or even toxicology-related modifications. Selecting an appropriate model is critical since the therapeutic potential of antidepressant treatment is not elicited in healthy people; thus studies in healthy animals may pose difficult challenges to distinguish relevant from non relevant results. Since all complex features of major depressive disorder (MDD) cannot be reproduced in rodents, the choice falls on disease models able to suitably mimic some relevant symptoms and/or reproduce significant etiological factors of the disease (Cryan and Slattery, 2007, Holmes, 2003). MDD is recognised to be associated to an interaction between genetic predisposition and environmental challenges (aan het Rot et al., 2009, Bartolomucci and Leopardi, 2009). Therefore, we addressed our investigations on molecular changes brought about by antidepressant treatment in an animal model including features related to genetic predisposition and environmental challenge. These studies were performed within GENDEP, an integrated project combining large-scale pharmacogenomic studies on depressed patients with preclinical investigations on animal models of disease, focusing on treatment with pro-serotonergic and pro-noradrenergic antidepressants (Uher et al., 2010). In this framework, to gain a better understanding of the molecular changes induced by antidepressant treatment, we have analysed proteomic changes in rat brain regions after chronic administration with a pro-noradrenergic tricyclic antidepressant, nortriptyline (NT) (Gillman, 2007). Wide-scale analyses of protein expression were performed by 2-D electrophoresis on the Flinders Sensitive Line (FSL), a genetically selected rat model of depression that displays good face, predictive and construct validity (Overstreet et al., 2005, Yadid et al., 2000). The Flinders Resistant Line (FRL), which does not show the depressive-like behaviour, was used as a reference, since this strain was derived in parallel with FSL from the same Sprague–Dawley strain (Overstreet et al., 2005). In MDD, the environmental challenges are often related to stressful experiences, especially in early ages (Hammen, 2005, Horesh et al., 2008); we thus included in the experimental design the exposure to maternal separation (MS).