Role of nitric oxide in the periaqueductal gray in defensive behavior in mice: influence of prior local N-methyl-D-aspartate receptor activation and aversive condition

Glutamate N-methyl-D-aspartate (NMDA) receptor activation within the dorsal column of the periaqueductal gray (dPAG) leads to antinociceptive, autonomic, and behavioral responses characterized as the fear reaction. Activation of NMDA receptors in the brain increases nitric oxide (NO) synthesis, and NO has been proposed to be a mediator of the aversive action of glutamate. This paper reviews a series of studies investigating the effects of neuronal NO synthase (nNOS) inhibition in the dPAG of mice in different aversive conditions. nNOS inhibition by infusion of Nω-propyl-L-arginine (NPLA) prevents fear-like reactions (e.g., jumping, running, freezing) induced by NMDA receptor stimulation within the dPAG and produces anti-aversive effects when injected into the same midbrain site in mice confronted with a predator. Interestingly, nNOS inhibition within the dPAG does not change anxiety-like behavior in mice exposed to the elevated plus maze (EPM), but it reverses the effect of an anxiogenic dose of NMDA injected into the same site in animals subjected to the EPM. Altogether, the results support a role for glutamate NMDA receptors and NO in the dPAG in the regulation of defensive behaviors in mice. However, dPAG nitrergic modulation of anxiety-like behavior appears to depend on the magnitude of the aversive stimulus.

Fos-like immunoreactivity in central nervous system of mice simultaneously exposed to the elevated plus-maze and nociception

It has been demonstrated that mice exhibit antinociception when they are exposed to the elevated plus-maze (EPM), an animal model of anxiety. To investigate which brain structures are activated during EPM exposure, the present study assessed the immunohistochemical staining for Fos-like immunoreactivity (Fos-LI) in mice intraperitoneally injected with saline or 0,6 por cente acetic acid (which produces nociception) and cofined in the open arm (threatening situation) or enclosed arm (Control) of the EPM. The following strutuctures were investigated: magnus, dorsal and median raphe nuclei (MR, DR and MnR), periaqueductal gray matter (PAG), dorsal and ventral hippocampus (DH and VH), amygdala (AMY), hypothalamus (HYP) and superior and inferior colliculi (SC and IC)...