The role of mesoprefrontal dopamine neurons in the acquisition and expression of conditioned fear in the rat.

The mesoprefrontal dopamine neurons are sensitive to physical, pharmacological and psychological stressors. In this report, the role of these neurons in the response to classical fear conditioning was investigated. 6-Hydroxydopamine lesions to the medial prefrontal cortex reduced dopamine levels to about 13% of controls but did not alter behavior during the acquisition of fear conditioning. As expected, conditioned fear increased dopamine metabolism (3,4-dihydroxyphenylacetic acid/dopamine ratio) in the nucleus accumbens in sham-lesion rats. The medial prefrontal 6-hydroxydopamine lesions did not alter this effect. During the expression, however, lesioned rats demonstrated a delayed extinction of the conditioned response without an overall increase in the initial conditioned response. This effect was consistent in rats receiving 6-hydroxydopamine lesions before or after the acquisition period. The calculated rates of extinction showed that the 6-hydroxydopamine lesioned rats had a reduced rate of extinction, but not acquisition, of fear conditioning. The results presented in this manuscript indicate that the mesoprefrontal dopamine neurons are involved in co-ordinating the normal extinction of a fear response but do not alter the acquisition of fearful behaviors. These data are consistent with the conclusion that the mesoprefrontal dopamine neurons are involved in maintaining the animal's response adaptability with regards to stress-related changes in the external environment.