Differential effects of neurokinin-1 receptor activation in subregions of the periaqueductal gray matter on conditional and unconditional fear behaviors in rats.

Central neurokinin-1 (NK-1) receptors are thought to modulate aversion, whereas the periaqueductal gray matter (PAG) is a common pathway for the integration of fear behaviors. The authors determined whether injection of an NK-1 agonist (GR73632) into subregions of the PAG would alter fear-related behaviors. Behavioral inactivity was increased by GR73632 injected into the caudodorsal PAG or the dorsal raphe. Flight behavior induced by stimulation of the dorsal PAG or by a footshock was decreased after injection of GR73632 into the dorsal PAG. Rats that had 6 pairings of a tone with a footshock after injection of GR73632 into the dorsal PAG displayed more freezing behavior than controls at the beginning of the session. However, there was no change in the shock- or the tone-induced freezing because some GR73632-treated rats, but no controls, froze during the baseline period. It is concluded that NK-1 receptors in the dorsal PAG modulate the unconditional but not the mnemonic aspects of fear behaviors.