LESIONS OF THE PERIAQUEDUCTAL GRAY AND ROSTRAL VENTROMEDIAL MEDULLA DISRUPT ANTINOCICEPTIVE BUT NOT CARDIOVASCULAR AVERSIVE CONDITIONAL RESPONSES

The presentation of an auditory stimulus that signals a noxious event such as foot shock results in the simultaneous expression of multiple aversive conditional responses (CRs), which include a transient elevat ion of arterial blood pressure (ABP) and an opioid-mediated form of hy poalgesia. Recent evidence suggests that the neural circuits responsib le for the expression of these two aversive responses may overlap. In the present study, rats were trained using a Pavlovian fear conditioni ng paradigm in which white noise was repeatedly paired with shock. Aft er training, groups of animals received electrolytic lesions centered in the dorsal or ventral periaqueductal gray (PAG) or in the medial or lateral rostral medulla. In sham-lesioned animals that were given pai red presentations of noise and shock, subsequent presentation of the a uditory stimulus caused a significant transient elevation of ABP and t ime-dependent inhibition of the tail flick reflex evoked by radiant he at. Lesions of either the dorsal or the ventral PAG blocked the antino ciceptive CR but did not significantly affect ABP responses. Lesions o f the ventromedial, but not the lateral, rostral medulla blocked hypoa lgesia. Rostral medullary lesions did not reliably affect stimulus-evo ked cardiovascular responses or baseline ABP. These results indicate t hat antinociceptive and cardiovascular conditional responses are anato mically dissociable and support our proposal that conditional hypoalge sia is mediated by a serial neural circuit that includes the amygdala, PAG, and rostral ventromedial medulla.