“…Large numbers of enkephalin-immunoreactive neurons are observed in the amygdala (Roberts, 1992;Veinante et al, 1997;Fallon and Leslie, 1986;Gray et al, 1984;Harlan et al, 1987), and the central nucleus of the amygdala (CEA) receives enkephalin afferents from the bed nucleus of the stria terminalis (BNST) as well as other amygdalar nuclei (Poulin et al, 2006). The number of enkephalin neurons and enkephalin mRNA expression show differences between high-anxiety and low-anxiety mice, enkephalin mRNA expression in the amygdala is altered by exposure to predator odor, and pre-proenkephalin-knockout mice show increased levels of anxiety-like behaviors (Konig et al, 1996;Hebb et al, 2004;Filliol et al, 2000). Enkephalins interact with both mu-opioid receptors (MORs) and delta-opioid receptors (DORs), both of which are seen in the amygdala (Mansour et al, 1993(Mansour et al, , 1994a(Mansour et al, , b, 1987(Mansour et al, , 1995Poulin et al, 2006;Wilson et al, 2002;Goodman et al, 1980) and could mediate the effects of amygdalar enkephalin in anxiety or fear responses.…”