NEURAL SYSTEMS FOR THE EXPRESSION OF HYPOALGESIA DURING NONASSOCIATIVE FEAR

A single brief exposure to moderately intense white noise is sufficien t to produce opioid-mediated antinociception in rats. This form of str ess-induced hypoalgesia represents a response to unconditional fear or anxiety. Three experiments compared the neural circuits responsible f or learned versus unlearned fear responses. Male rats received lesions of the medial geniculate nucleus, lateral or central nuclei of the am ygdala, or the ventral, dorsal lateral, or dorsal medial periaqueducta l gray (PAG). Controls showed a pronounced elevation in tail-flick lat ency following presentation of 90-dB white noise. All lesions, with th e exception of dorsolateral and dorsomedial PAG, significantly blocked this response. These results support the idea that hypoalgesia produc ed by aversive auditory stimuli uses a common neural circuit regardles s of whether the response is a product of associative learning or unco nditional fear/anxiety.