Alterations in reward, fear and safety cue discrimination after inactivation of the rat prelimbic and infralimbic cortices

Abstract

Accurate discrimination of environmental cues predicting reward, fear, or safety is important for survival. The prelimbic and infralimbic cortices are implicated in regulating reward-seeking and fear behaviors; however, no studies have examined their roles in discriminating among reward, fear, and safety cues. Using a discriminative conditioning task that includes presentations of a reward cue (paired with a reward pellet), fear cue (paired with footshock), and a compound fear+safety cue (no footshock) within the same sessions allowed us to assess the flexibility and precision of fear and reward-seeking behaviors to these cues. We found that fear behavior was appropriately limited to the fear cue in untreated rats, but during infralimbic cortical inactivation, similar levels of fear were seen to the fear and compound fear+safety cues. Reward-seeking behavior was also appropriately limited to the reward cue in untreated rats. Inactivating the prelimbic cortex altered discriminative reward seeking as rats with prelimbic inactivation did not increase their reward seeking behavior during the reward cue to the same degree as saline controls. Our results imply dissociable roles of the two cortical regions: the prelimbic cortex in precise discriminative reward seeking and the infralimbic cortex in discriminating between fear and safety cues. These data suggest that alterations in the balance of activity between areas homologous to the prelimbic and infralimbic cortices may be involved in the processes that go awry in anxiety and addiction disorders.

Figure 1

(a) Schematic of the experimental design. Rats were pretrained on the reward cue–reward pellet association followed by three discriminative conditioning sessions in which the reward cue (+reward pellet), fear cue (+footshock), compound fear+safety cues (no footshock), and safety cue (no footshock) were presented. Rats then received either saline or muscimol/baclofen (M/B) infusions during another discriminative conditioning session followed by the opposite drug treatment the next day. During extinction acquisition, all rats received unreinforced presentations of the fear and reward cues. Rats then received either saline or M/B infusions during an extinction recall test followed by the opposite drug treatment the next day. (b) Infusion needle placements in the prelimbic cortex (PL). (c) Infusion needle placements in the infralimbic cortex (IL). In panels b and c, numbers indicate the distance of the histology plate anterior to bregma.

Figure 2

Prelimbic (PL) inactivation impairs fear expression. (a) Reduced freezing in the presence of the safety cue under drug-free conditions. Averaged percentage of time freezing was significantly higher during the fear cue compared with all other cues (*p<0.05). (b) Saline-treated rats froze significantly more to the fear cue than to any other cue (*p<0.05). PL inactivated rats froze significantly less to the fear cue when compared with saline-treated rats (*p<0.05). (c) Significant within-session extinction of freezing behavior to the fear cue under drug-free conditions. (d) During extinction recall, both saline-treated and PL-inactivated rats showed a significant reduction in freezing to the fear cue compared with the beginning of extinction acquisition (*p<0.05), demonstrating good fear extinction recall.

Figure 3

Prelimbic (PL) inactivation impairs discriminative reward seeking. (a) Discriminative reward seeking under drug-free conditions. Percentage of time spent in port was significantly higher during the reward cue compared with all other cues (*p<0.05) under drug-free conditions. (b) (left) Saline-treated rats spent significantly more time in the port during the reward cue compared with all other cues (*p<0.05). PL inactivated rats also spent significantly more time in port during the reward cue compared with all other cues (*p<0.05), but it was significantly less time than the saline-treated rats spent (*p<0.05). (b) (right) The ratio of time spent in port during the reward cue vs non-reward cues was calculated for saline-treated and PL-inactivated rats in order to assess the degree of discriminative reward seeking. The ratio for saline-treated rats was significantly higher than PL-inactivated rats (*p<0.05), indicating that PL inactivation reduced the level of discrimination. (c) Significant within-session extinction of reward-seeking behavior to the reward cue under drug-free conditions. (d) During extinction recall, both saline-treated and PL-inactivated rats showed a significant reduction in reward seeking to the reward cue compared with the beginning of extinction acquisition (*p<0.05), demonstrating good reward extinction recall.

Figure 4

Infralimbic (IL) inactivation impairs fear and safety cue discrimination and recall of fear extinction. (a) Reduced freezing in the presence of the safety cue under drug-free conditions. Freezing was significantly higher during the fear cue compared with all other cues (*p<0.05). (b) During session 4, saline-treated rats froze significantly more to the fear cue than to any other cue (*p<0.05). Freezing was not significantly different during presentations of either the fear cue or compound fear+safety cue during IL inactivation. (c) Significant within-session extinction of freezing behavior to the fear cue under drug-free conditions. (d) During extinction recall, saline-treated rats showed a significant reduction in freezing to the fear cue compared with the beginning of extinction acquisition (*p<0.05). IL inactivation resulted in freezing levels to the fear cue similar to the beginning of extinction acquisition, demonstrating poor fear extinction recall.

Figure 5

Infralimbic (IL) inactivation does not impair discriminative reward seeking or recall of reward extinction. (a) Discriminative reward seeking under drug-free conditions in session 3. Percentage of time spent in port was significantly higher during the reward cue compared with all other cues (*p<0.05) under drug-free conditions. (b) (left) Saline-treated rats spent significantly more time in the port during the reward cue compared with all other cues (*p<0.05). IL-inactivated rats also spent significantly more time in port during the reward cue compared with all other cues (*p<0.05). (b) (right) The ratio of time spent in port during the reward cue vs non-reward cues was calculated for saline-treated and IL-inactivated rats in order to assess the degree of discriminative reward seeking. The ratio for saline-treated rats was not significantly different than IL-inactivated rats, indicating similar levels of discrimination. (c) Significant within-session extinction of reward-seeking behavior to the reward cue under drug-free conditions. (d) During extinction recall, both saline-treated and IL-inactivated rats showed a significant reduction in reward seeking to the reward cue compared with the beginning of extinction acquisition (*p<0.05), demonstrating good reward extinction recall.