Enhanced Fear Memories and Altered Brain Glucose Metabolism (18F-FDG-PET) following Subanesthetic Intravenous Ketamine Infusion in Female Sprague-Dawley Rats

Abstract

Although women and men are equally likely to receive ketamine following traumatic injury, little is known regarding sex-related differences in the impact of ketamine on traumatic memory. We previously reported that subanesthetic doses of an intravenous (IV) ketamine infusion following fear conditioning impaired fear extinction and altered regional brain glucose metabolism (BGluM) in male rats. Here, we investigated the effects of IV ketamine infusion on fear memory, stress hormone levels, and BGluM in female rats. Adult female Sprague-Dawley rats received a single IV ketamine infusion (0, 2, 10, or 20 mg/kg, over a 2-h period) following auditory fear conditioning (three pairings of tone and footshock). Levels of plasma stress hormones, corticosterone (CORT) and progesterone, were measured after the ketamine infusion. Two days after ketamine infusion, fear memory retrieval, extinction, and renewal were tested over a three-day period. The effects of IV ketamine infusion on BGluM were determined using ¹⁸F-fluoro-deoxyglucose positron emission tomography (¹⁸F-FDG-PET) and computed tomography (CT). The 2 and 10 mg/kg ketamine infusions reduced locomotor activity, while 20 mg/kg infusion produced reduction (first hour) followed by stimulation (second hour) of activity. The 10 and 20 mg/kg ketamine infusions significantly elevated plasma CORT and progesterone levels. All three doses enhanced fear memory retrieval, impaired fear extinction, and enhanced cued fear renewal in female rats. Ketamine infusion produced dose-dependent effects on BGluM in fear- and stress-sensitive brain regions of female rats. The current findings indicate that subanesthetic doses of IV ketamine produce robust effects on the hypothalamic-pituitary-adrenal (HPA) axis and brain energy utilization that may contribute to enhanced fear memory observed in female rats.

Keywords: brain imaging; fear memory; intravenous ketamine; post-traumatic stress disorder (PTSD); sex differences; stress hormone.

Figure 1

Effects of subanesthetic IV ketamine infusion on locomotor activity in female rats. (A) Experimental design of fear conditioning/ketamine infusion and fear memory testing. (B) Freezing behavior during fear conditioning (fear acquisition). Freezing gradually increased to repeated tone and footshock pairing but there were no group differences at each time point. (C) Time course of locomotor activity during the ketamine infusion (2 h). (D) Total activity during the first hour of ketamine infusion. (E) Total activity during the second hour of ketamine infusion. * p < 0.05 compared to the saline controls.

Figure 2

Effects of subanesthetic IV ketamine infusion on fear memory retrieval, extinction, and renewal in female rats. (A) Ketamine infusion increased cued fear memory retrieval with all three doses (2, 10, and 20 mg/kg, 2 h). (B) Ketamine infusion impaired fear extinction retrieval in female rats. Each Block consists of average freezing of two consecutive auditory tone (CS) presentations. (C) Higher doses of ketamine (10 and 20 mg/kg) increased contextual fear memory when animals were tested in the conditioning chamber (context A). (D) Higher doses of ketamine (10 and 20 mg/kg) increased cued fear memory renewal (context A) after two sessions of cued fear extinction (context B). * p < 0.05 compared to saline controls.

Figure 3

No effects of estrous cycle on ketamine-induced fear memory enhancement. (A) Representative images of vaginal cell types (from top to bottom): Cornified epithelial cells (C), Leukocytes (L), and Nucleated epithelial cells (N). Scale: 30 µm. (B) Freezing behavior during Cue Test 1. (C) Freezing during Cue Test 2. (D) Freezing behavior during contextual fear test 3. (E): Freezing behavior during cued fear renewal test.

Figure 4

Effects of subanesthetic doses of IV ketamine infusion on plasma stress hormone (CORT and progesterone) levels in female rats. (A) Higher doses of ketamine (10 and 20 mg/kg) significantly elevated plasma CORT levels. (B) Higher doses of ketamine (10 and 20 mg/kg) also elevated plasma progesterone levels. * p < 0.05 compared to saline controls.

Figure 5

Effects of subanesthetic doses of IV ketamine infusion on BGluM of female rats. (A) Representative images of ¹⁸F-FDG-PET/CT of a rat brain. Three images including sagittal, planar, and coronal sections from left to right are shown in the first image. The second image shows a coronal section with a rat brain atlas registered to it. (B) Both 10 and 20 mg/kg ketamine increased BGluM in the cortex. (C) The 10 mg/kg dose reduced BGluM in the thalamus. (D) The 20 mg/kg dose reduced BGluM in the hypothalamus. (E) Both 10 and 20 mg/kg ketamine doses reduced BGluM in the midbrain. Baseline BGluM levels in these brain regions are not significantly different between the groups as shown in scan 1. * p < 0.05 compared to saline controls.