Fig. 3. Schematic diagram of neurotransmission in the dPAG in acute and persistent aversive stimulation of the dPAG.

Acute fear stimuli activate the dPAG, which produces defense reactions, characterized by behavioral and autonomic responses. The neural substrates of fear in these defense reactions are subjected to excitatory (+) and inhibitory (−) modulatory influences. The passage of aversive information to higher encephalic structures is under the inhibitory control of dopaminergic mechanisms at the midbrain level. NK also mediates the processing of aversive information at the amygdala level. GABA, γ-aminobutyric acid; BZD, benzodiazepines; 5-HT, 5-hydroxytryptamine (serotonin); EAA, excitatory amino acids; NO, nitric oxide