The long-term goal of the project is to understand the neural systems responsible for anxiety, fear and aversive motivational/emotional processes. A model system is proposed in which the hypoalgesic reactions of rats briefly exposed to intense white noise is measured. This will allow us to explore the functional relationship between forebrain structures known to be important for certain forms of fear and emotional processing and lower brainstem systems involved in the expression of hypoalgesia and defensive behavior. Importantly, this will be done using a paradigm which is widely accepted as a laboratory model of acute anxiety for both human psychophysiological studies and more basic research with laboratory animals. A series of five experiments has been designed to determine: 1) the effects of manipulating the intensity and duration of noise stress on the magnitude and duration of hypoalgesia, 2) the relationship between hypoalgesia and noise stress-induced increases in arterial blood pressure, 3) the conditions under which endogenous opioid peptides are involved, 4) if the specific nuclei within the amygdaloid complex which are known to be critical for other forms of fear and anxiety contribute to hypoalgesia in this paradigm, 5) if noise stress hypoalgesia represents the activation of documented endogenous antinociceptive systems which include the periaqueductal gray and nucleus raphe magnus. This paradigm may prove to be an interesting alternative to many of the similar currently accepted models which employ electric shock since noise stress appears to have fewer non-specific long-term effects. The fact that identical procedures that do not require the application of nociceptive stimuli may be used to produce anxiety and hypoalgesia in humans and rats may also prove useful eventually.