Sex specific recruitment of a medial prefrontal cortex-hippocampal-thalamic system during context-dependent renewal of responding to food cues in rats

Abstract

Renewal, or reinstatement, of responding to food cues after extinction may explain the inability to resist palatable foods and change maladaptive eating habits. Previously, we found sex differences in context-dependent renewal of extinguished Pavlovian conditioned responding to food cues. Context-induced renewal involves cue-food conditioning and extinction in different contexts and the renewal of conditioned behavior is induced by return to the conditioning context (ABA renewal). Male rats showed renewal of responding while females did not. In the current study we sought to identify recruitment of key neural systems underlying context-mediated renewal and sex differences. We examined Fos induction within the ventromedial prefrontal cortex (vmPFC), hippocampal formation, thalamus and amygdala in male and female rats during the test for renewal. We found sex differences in vmPFC recruitment during renewal. Male rats in the experimental condition showed renewal of responding and had more Fos induction within the infralimbic and prelimbic vmPFC areas compared to controls that remained in the same context throughout training and testing. Females in the experimental condition did not show renewal or an increase in Fos induction. Additionally, Fos expression differed between experimental and control groups and between the sexes in the hippocampal formation, thalamus and amygdala. Within the ventral subiculum, the experimental groups of both sexes had more Fos compared to control groups. Within the dorsal CA1 and the anterior region of the paraventricular nucleus of the thalamus, in males, the experimental group had higher Fos induction, while both females groups had similar number of Fos-positive neurons. Within the capsular part of the central amygdalar nucleus, females in the experimental group had higher Fos induction, while males groups had similar amounts. The differential recruitment corresponded to the behavioral differences between males and females and suggests the medial prefrontal cortex-hippocampal-thalamic system is a critical site of sex differences during renewal of appetitive Pavlovian responding to food cues. These findings provide evidence for novel neural mechanisms underlying sex differences in food motivation and contextual processing in associative learning and memory. The results should also inform future molecular and translational work investigating sex differences and maladaptive eating habits.

Keywords: Appetitive; Conditioning; Context; Medial prefrontal cortex; Renewal.

Figure 1

Experimental design. A denotes training in Context A, B denotes training in Context B (contexts were counterbalanced). Each training session consisted of eight presentations of either CS-US (denoted as +), or CS alone (denoted as −). All animals were sacrificed 90 minutes after the end of Renewal test and brains were collected for Fos induction detection by immunohistochemistry.

Figure 2

Sampling areas in the hippocampal formation. Dark gray shading denotes sampling areas in dorsal (left) and ventral (right) hippocampal formation. Illustrations were made on modified templates from the Swanson atlas (2004), and numbers in the upper left corner of each denote atlas levels. Abbreviations: CA1- field CA1, Ammon’s horn; DG- dentate gyrus; SUB-subiculum.

Figure 3

Conditioned responses during acquisition and extinction. Percentage of time rats expressed food cup behavior (mean ± SEM) during the preCS and CS periods during training sessions. PreCS values are the average across all sessions for acquisition and extinction, respectively. Acquisition is shown as the average responding during each session. Extinction is shown as the average responding in 4-trial blocks (2 blocks per session; blocks 1&2 were trials during Session 1 and blocks 3&4 during Session 2).

Figure 4

Conditioned responses during the test for renewal. Percentage of time rats expressed food cup behavior (mean ± SEM) during preCS and CS periods; * indicates p < .001, # indicates within-group preCS vs CS difference p < 0.05.

Figure 5

Fos induction in the medial prefrontal cortex. Representative photomicrographs of Fos induction in the infralimbic area (ILA) are shown on right (Scale bar = 200 µm), and thionin-stained adjacent sections are shown on left (level 9; Scale bar = 500 µm). Each box depicts the area shown in the corresponding Fos image.

Figure 6

Fos induction. Total number of Fos-positive neurons in the infralimbic area, ILA (A), prelimbic area, PL (B), ventral subiculum, SUBv (C), dorsal CA1 (D), anterior part of the paraventricular nucleus, PVTa (E), capsular part of the central amygdalar nucleus, CEAc (F). * indicates p < .05

Figure 7

Medial prefrontal cortex-hippocampal-thalamic system recruitment during context-mediated renewal of conditioned responding to food cues. Summary of major Fos induction differences in males and females are shown on a connectional diagram. Males are represented by filled arrows and females are represented by open arrows. Arrow up indicates higher Fos induction compared to same sex control, arrow down indicates lower Fos induction, line indicates no change. See text for details and for additional areas, including amygdala.