No abstract

We conducted a series of experiments to investigate the neural basis of the immediate extinction deficit, the lack of extinction when the interval between fear memory acquisition and extinction is short. In experiment 1, rats were given extinction training composed of 15 conditioned stimuli (CSs) either 15 min (immediate extinction: I-EXT) or 24 h (delayed extinction: D-EXT) after five tone-shock pairings. In the retention test performed 48 h after conditioning, I-EXT group exhibited significantly higher freezing than D-EXT group. In experiment 2, functional activation in the medial prefrontal cortex (mPFC) was detected using c-fos immunoreactivity. The number of Fos-positive neurons in the mPFC was significantly lower in I-EXT group than in D-EXT group. In experiment 3, rats received immediate extinction with microstimulation of the infralimbic region (IL) of the mPFC, either contingently paired or unpaired with the CS. In a subsequent retention test, the paired stimulation group exhibited decreased freezing relative to the unpaired stimulation group. Together, our results suggest that the immediate extinction deficit may be linked to the lack of neuronal activity in the IL.

Summary: Generalized fear is a maladaptive behavior in which non-threatening stimuli elicit a fearful response. Here we demonstrate that discrimination between predictive and non-predictive threat stimuli is highly sensitive to probabilistic discounting and increasing threat intensity in mice. We find that dopamine neurons of the ventral tegmental area (VTA) encode both the negative valence of threat-predictive cues and the certainty of threat prediction. As fear generalization emerges the dopamine neurons that are activated by a threat predictive cue (CS+) decrease the amplitude of activation and an equivalent signal emerges to a non-predictive cue (CS−). Temporally precise enhancement of dopamine neurons during threat conditioning to high-threat levels or uncertain threats can prevent generalization. Moreover, phasic enhancement of genetically captured dopamine neurons activated by threat cues can reverse fear generalization. These findings demonstrate the dopamine neurons reflect the certainty of threat prediction that can be used to inform and update the fear engram.

The hippocampus and the perirhinal cortex (PR) are reciprocally connected both directly and indirectly via the entorhinal cortex. Although it has been hypothesized that the two regions should have intimate functional interactions with each other on the basis of the anatomical connectivity, many lesion studies have demonstrated functional dissociations instead between the hippocampus and PR. To show a tight functional relationship between the two regions, we used reversible inactivation techniques targeting both the hippocampus and PR within subjects, combined with a biconditional memory task in which the rat must consider information about objects and their locations. Specifically, rats were implanted with two sets of bilateral cannulas into the hippocampus and PR, and were tested in an object-place paired-associate task in a radial maze. While alternating between two arms, the rats were required to choose one of the objects exclusively associated with a given arm for food. Bilateral muscimol (MUS) injections into either the hippocampus or PR equally produced chance level performance. When a functional disconnection procedure was used to disrupt the interaction between the hippocampus and PR, contralateral MUS injections into the hippocampus and PR resulted in severe impairment in performance. However, inactivating the hippocampus and PR ipsilaterally did not affect the performance. In a simple object discrimination task, the same functional disconnection protocol with MUS did not affect the performance. The results powerfully demonstrate that the hippocampus, the PR, and their functional interactions are all indispensable when objects and their spatial locations must be processed at the same time.