Novel animal models of affective disorders

Is there an appropriate animal model for human affective disorders? The traditional difficulties in accepting animal models for psychopathology stem from the argument that there is no evidence for concluding that what occurs in the brain of the animal is equivalent to what occurs in the brain of a human. However, if one models any or some core aspects of affective disorder, this model can become an invaluable tool in the analysis of the multitude of causes, genetic, environmental, or pharmacological, that can bring about symptoms homologous to those of patients with affective disorders. Animal models can also allow the study of the mechanisms of specific behaviors, their pathophysiology, and can aid in developing and predicting therapeutic responses to pharmacologic agents. Although animals exhibit complex and varied social and emotional behaviors for which well-validated and standardized measures exist, an understanding that a precise replica of human affective disorders cannot be expected in a single animal model is crucial. Instead, a good animal model of a human disorder should fulfill as many of the four main criteria as possible: (1) strong behavioral similarities, (2) common cause, (3) similar pathophysiology, and (4) common treatment. An animal model fulfilling any or most of these criteria can be used to elucidate the mechanisms of the specific aspect of the model that is homologous to the human disorder. A wide range of animal models of affective disorders, primarily depression, has been developed to date. They include models in which "depressive behavior" is the result of genetic selection or manipulation, environmental stressors during development or in adulthood, or pharmacologic treatments. The assessment of these animal models is based either on behavioral tests measuring traits that are homologous to symptoms of the human disorder they model, or behavioral tests responsive to appropriate pharmacologic treatments. The goal of this review is to focus on relatively recent developments of selected models, to aid in understanding their strengths and weaknesses, and to help those choosing the difficult task of developing novel animal models of affective disorders. The ideal animal model of affective disorders of the future would be an endogenous, genetic model that reiterates the essential, core aspects of the human disease and responds to the standard regimens of therapy. Because complex diseases have been approached from the genetic startpoint by using rodent models, a genetic model of affective disorder would open up possibilities for genetic analysis of polygenic traits that seem to underlie these disorders.