Neurobiology of fear and stress

Abstract

A large body of literature has accumulated over the last half century on the anatomy, physiology and pharmacology of areas or projection systems in the CNS, which, when stimulated electrically or chemically result in physiological and behavioral changes indicative of states of agony or displeasure as expressed by vegetative nervous system indicators, attempts to escape, and displays of fear and aggressive behavior. lt is generally assumed that brain sites, which mediate such behavioral changes, integrate information of aversive nature into appropriate adaptive emotional and behavioral reactions, including fear-related behaviors. The medial hypothalamus, amygdale and dorsal periaqueductal gray (dPAG) have been traditionally grouped together as a "brain aversion system". More recently, a continuous strip of midbrain structures composed of superior and inferior colliculus have also been proposed as parts of this "system". In this project we will focus on the neural substrates of defensive behavior in the midbrain tectum (dPAG, superior and inferior colliculi), and their relevance for understanding fear and anxiety. The proposed link between the defense behavior, fear and anxiety is consistent with many behavioral, electrophysiological and immunohistochemical studies showing expressive activation of these regions by threatening stimuli or conditions. The periaqueductal gray matter participates in the expressions of major brain functions, such as pain processing and modulation, vocalization, autonomic regulation, fear and anxiety. lt has been proposed that neurons in the inferior colliculus are filters for sounds that require immediate action, such as certain sounds made by prey, predators and co specifics. The stimulation of the midbrain tectum may represent an animal model for all these situations, since a gradual increase in the intensity of the electrical stimulation of the midbrain tectum structures produce initially alertness, then freezing, and finally escape reactions. Furthermore, electrical stimulation of the dPAG and inferior colliculus has been consistently shown to induce aversive effects since it elicits defensive behaviors, sustains Iearned operant escape responses and also supports Iearning of conditioned emotional responses. Recent studies using c-ias immunoreactivity bring further support to the notion that amygdale, medial hypothalamus, dP AG, dlSC and inferior colliculus appear to be constituents of a neuronal system or systems that control defensive behavior in the brain by showing that these areas are labeled following exposure or the animals to aversive environmental stimulation… (AU)