Morphine administration during low ovarian hormone stage results in transient over expression of fear memories in females

Abstract

Acute exposure to morphine after a traumatic event reduces trauma related symptoms in humans and conditioned fear expression in male rats. We aimed to determine whether acute administration of morphine alters consolidation of fear learning and extinction. Male and female rats in proestrus and metaestrus (high and low ovarian hormones respectively) underwent fear conditioning and received saline or morphine (2.5 mg/kg s.c.). The next day they underwent extinction. Results showed increased freezing during extinction only in the morphine metaestrus group while morphine did not affect males or proestrus females. Recall of extinction was similar on all groups. On a second experiment, a subset of rats conditioned during metaestrus was administered morphine prior to extinction producing no effects. We then measured mu opioid receptor (MOR) expression in the amygdala and periaqueductal gray (PAG) at the end of extinction (day 2). In males and proestrus females, morphine caused an increase in MOR in the amygdala but no in the PAG. In metaestrus females, morphine did not change MOR expression in either structure. These data suggests that ovarian hormones may interact with MORs in the amygdala to transiently alter memory consolidation. Morphine given after trauma to females with low ovarian hormones might increase the recall of fear responses, making recovery harder.

Keywords: estrous cycle; fear conditioning; fear extinction; morphine; sex-differences; trauma.

Figure 1

Percent freezing to the tone in males, proestrus and metaestrus female rats shown in blocks of two trials. Acute morphine immediately after conditioning resulted in an increased level of conditioned freezing in the metaestrus group compare to controls and acute morphine 24 h after conditioning had no significant effect in extinction learning in metaestrus female rats. (A) Experimental timeline. (B) Post-training injections of morphine given after the metaestrus stage had no effect on extinction, but over expression of conditioned fear (morphine n = 14, saline = 14). (C) Post-training injections of morphine given during the proestrus stage had no effect either in the consolidation of fear conditioning or within session extinction (morphine n = 13, saline n = 14). (D) Post-training injections of morphine had no effect either in the consolidation of fear conditioning or within session extinction in male rats (morphine n = 15, saline n = 14). (E) Experimental timeline. (F) Pre-extintion injections of morphine given after the metaestrus stage had no effect either in the consolidation of fear conditioning or extinction (morphine n = 8, saline n = 6).

Figure 2

Ratio of mu opioid receptor (MOR) changes from control group in males, proestrus and metaestrus female rats. (A) In males the amygdala showed an increase in MOR expression produced by morphine (p < 0.05 compared to baseline control). (B) In proestrus female the amygdala showed an increase in MOR expression produced by morphine (p < 0.05). (C) In metaestrus female rats the amygdala and periaqueductal gray (PAG) showed no significant difference in MORs expression compared to controls.