VENTRAL AND DORSOLATERAL REGIONS OF THE MIDBRAIN PERIAQUEDUCTAL GRAY (PAG) CONTROL DIFFERENT STAGES OF DEFENSIVE BEHAVIOR - DORSOLATERAL PAG LESIONS ENHANCE THE DEFENSIVE FREEZING PRODUCED BY MASSED AND IMMEDIATE SHOCK

Rats that receive nociceptive electric shock in an environment normall y show the conditional fear-induced defensive response of freezing whe n returned to that environment. If several electric shocks are given i n a massed manner they will condition less freezing than the same shoc ks given in a distributed manner. If a single shock is given immediate ly after placement in the chamber it does not support any conditioning , although the same shock given after a brief delay does. Electrolytic lesions of the dorsolateral periaqueductal gray (FAG), which damaged dorsomedial, dorsolateral, and lateral FAG, enhanced freezing under th ese conditions. Lesions of the ventral FAG, which caused extensive dam age to the central gray below the aqueduct, reduced conditioning under the more optimal parameters (distributed or delayed shock). This was taken to indicate that both of these regions support different modes o f defensive behavior and that when activated, the dorsolateral FAG inh ibits conditional fear-induced defensive behavior. (C) 1995 Wiley-Liss , Inc.