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Defensive behavior and hippocampal cell proliferation : differential modulation by naltrexone during stress Holmes, Melissa Marie


The present study was designed to investigate the role of endogenous opioids in both the expression of defensive behaviors and the suppression of cell proliferation that are induced by exposure to predator odor, trimethyl thiazoline (TMT). Adult male rats were injected with either naltrexone (5mg/kg) or saline 30 minutes prior to being transferred to a testing chamber where they were exposed to either TMT (250 ul) or control odor (distilled water 250 u.1). Rats were videotaped and the expression of defensive and non-defensive behaviors was scored over the first fifteen minutes of exposure. Bromodeoxyuridine (BrdU, 200mg/kg), a thymidine analogue, was injected i.p. 15 minutes after exposure to the odors and the rats were perfused 1 hour after BrdU administration. As previously reported, the expression of defensive behaviors was increased after exposure to TMT. Pre-treatment with naltrexone attenuated the expression of defensive behaviors (defensive burying and directed stretch approach), independent of any drug effects on ability to perform the required motor patterns. TMT exposure rapidly suppressed the number of proliferating (BrdU-ir positive) cells in the dentate gyrus. In addition, naltrexone administration alone suppressed cell proliferation in the dentate gyrus. Thus, consistent with other reports, endogenous opioids mediate the expression of defensive behaviors in response to predator odor exposure. Furthermore, endogenous opioids may play a regulatory role in the control of cell proliferation in the dentate gyrus of adult male rats.

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