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Proc Natl Acad Sci U S A. 1980 Jun;77(6):3729-32.

Enkephalin and fear-motivated behavior.


Leu[Enkephalin (400.0 micrograms/kg) and the enkephalin analog [DAla,DLeu]enkephalin (0.4, 4.0, and 40.0 micrograms/kg) were intraperitoneally administered to rats 5 min before they were trained on aversively motivated tasks. The peptides impaired acquisition of a one-way active avoidance response, facilitated acquisition of an inhibitory avoidance response, and had no effect on acquisition of a swim-escape response. The data indicate that the enkephalin effects are not mediated through actions on locomotor activity, pain perception or reactivity, or light sensitivity. Rather, we suggest that [Leu]enkephalin and its analog strengthen the tendency of rats to suppress behavior in the presence of cues previously associated with aversive stimulation. This effect may be due to an enkephalin-induced increase in fear or arousal. The enkephalin effect on behavior was obtained at low doses and was observed within a few minutes after administration. These findings are consistent with the interpretation that enkephalin effects may be initiated at a peripheral site.

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