Understanding the Causes of Reduced Startle Reactivity in Stress-Related Mental Disorders

Abstract

Many questions have plagued the study of the etiology and subsequent treatment of mental illness. In part, it is simply because, as far as we know, some mental illnesses are somewhat unique to the human condition. Moreover, clinical studies have produced many different results concerning potential biomarkers for conditions such as post-traumatic stress disorder PTSD and major depressive disorder MDD . In this chapter, we review how we approached modeling a specific behavioral condition, suppression of the startle reflex, by examining whether one of two commonly associated peripheral biomarkers of anxiety and depression could potentially cause this rather specific symptom. The two peripheral systems under investigation were the hypothalamic-pituitary-adrenal HP" axis and the peripheral pro-inflammatory immune response, both of which have been implicated as a vulnerability factor, causal factor, or resultant perpetuating effect of PTSD and MDD.Our general theory is that peripheral endocrine and immune signals, measured to be abnormal in patients with either PTSD or MDD as well as other mental disorders , are actually perpetuating the behavioral features of these disorders. "t the same time, if an individual has an immunosensitivity or has an overactive adrenal gland, s/he would be more likely to experience some of the symptoms associated with one of the particular mental illnesses. This may then lead the brain to compensate for those peripheral abnormalities, but, at the same time, cause other imbalances, which lead to the experience of a decline into mental illness. Thus, treating these peripheral markers as part of the mental disorder may be quite beneficial in normalizing certain aspects of the diagnosed abnormal behavior.© 2013 Beck and Ca""zzi; licensee InTech. This is an open access ar"icle dis"rib""ed "nder "he "erms of "he Crea"ive Commons A""rib""ion License (h""p://crea"ivecommons.org/licenses/by/3.0), which permi"s "nres"ric"ed "se, dis"rib""ion, and reprod"c"ion in any medi"m, provided "he original work is properly ci"ed. . The startle reflex and mental health . . The startle reflex as an assessment toolOne of the major impediments to the mechanistic study of mental illness is establishing analogs of the abnormal behaviors expressed in humans in animal models, especially in sub-primate species. This has led some to adopt reflex-based measures such that the face and construct validity of the behavior change can be readily translated between the model and patient populations. Consequently, a popular measures for the study of anxiety disorders has been the potentiation of the startle reflex however, there is growing evidence that a dampening of the startle response may be indicative of changes in physiology that underlie different mental disorders.The startle reflex comprises a -synaptic sensory-motor neuronal pathway that serves as a defensive behavioral response to abrupt, usually intense, stimuli. The acoustic startle response "SR is the most commonly used form for studying this reflex. "s shown in Fig...