Behavioural NeuroscienceResearch PaperEstrous cycle phase and gonadal hormones influence conditioned fear extinction
Section snippets
Subjects
A total of 22 male and 150 female Sprague–Dawley rats (approximately 300 g and 8 weeks of age) were housed individually at the Massachusetts General Hospital Center for Comparative Medicine and handled for 5 min/d for 2 days following their arrival. Rats were maintained on a 12 h light/dark cycle and restricted to ∼15 g/d of laboratory rat chow, with free access to water. They were transported to a holding room in our laboratory in the morning and returned to the facility at the end of each
Pro/Met during fear conditioning
Rats that underwent fear conditioning during the proestrus phase (Pro_Cond group) were compared to those that underwent conditioning during the metestrus phase (Met_Cond group). Student's t-test conducted on the acquisition phase and extinction recall phase revealed no significant difference in either phase (t(22)=1.20, P=0.24, t(22)=0.95, P=0.35, for conditioning and extinction recall, respectively). Repeated measures ANOVA conducted on the extinction learning phase revealed a significant main
Discussion
The data gathered in the present study show that naturally cycling gonadal hormones influence fear extinction in that elevated gonadal hormone levels during the proestrus phase appear to facilitate extinction recall. This was further supported by (1) the fact that administration of exogenous estrogen and progesterone prior to extinction learning on day 2 facilitated extinction recall tested on day 3 and (2) the fact that blockade of estrogen and progesterone receptors prior to extinction
Acknowledgments
The work was supported by a grant from the National Institute of Mental Health (K01 MH080346) and a Rapapport Fellowship to M.R.M. The authors would like to thank Dr. David Rubinow and Dr. Gregory J. Quirk for helpful comments on the manuscript.
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