Fear memories allow organisms to avoid danger, thereby increasing their chances of survival. Fear memories can be retrieved long after learning1,2, but little is known about how retrieval circuits change with time3,4. Here we show that the dorsal midline thalamus of rats is required for retrieval of auditory conditioned fear at late timepoints (24 h, 7 d, 28 d), but not early timepoints (0.5 h, 6 h) after learning. Consistent with this, the paraventricular subregion of the dorsal midline thalamus (PVT) showed increased cFos expression only at late timepoints, indicating that PVT is gradually recruited for fear retrieval. Accordingly, the conditioned tone responses of PVT neurons increased with time following training. The prelimbic (PL) prefrontal cortex, which is necessary for fear retrieval5–7, sends dense projections to PVT8. Retrieval at late timepoints activated PL neurons projecting to PVT, and optogenetic silencing of these projections impaired retrieval at late, but not early times. In contrast, silencing of PL inputs to the basolateral amygdala (BLA) impaired retrieval at early, but not late times, indicating a time-dependent shift in retrieval circuits. Retrieval at late timepoints also activated PVT neurons projecting to the central nucleus of the amygdala (CeA), and silencing these projections at late, but not early, times induced a persistent attenuation of fear. Thus, PVT may serve as a critical thalamic node recruited into cortico-amygalar networks for retrieval and maintenance of long-term fear memories.

Previous rodent studies have implicated the infralimbic (IL) subregion of the medial prefrontal cortex in extinction of auditory fear conditioning. However, these studies used pharmacological inactivation or electrical stimulation techniques, which lack temporal precision and neuronal specificity. Here, we used an optogenetic approach to either activate (with channelrhodopsin) or silence (with halorhodopsin) glutamatergic IL neurons during conditioned tones delivered in one of two phases: extinction training or extinction retrieval. Activating IL neurons during extinction training reduced fear expression and strengthened extinction memory the following day. Silencing IL neurons during extinction training had no effect on within-session extinction, but impaired the retrieval of extinction the following day, indicating that IL activity during extinction tones is necessary for the formation of extinction memory. Surprisingly, however, silencing IL neurons optogenetically or pharmacologically during the retrieval of extinction 1 day or 1 week following extinction training had no effect. Our findings suggest that IL activity during extinction training likely facilitates storage of extinction in target structures, but contrary to current models, IL activity does not appear to be necessary for retrieval of extinction memory.

SUMMARY The paraventricular nucleus of the thalamus (PVT) is thought to regulate behavioral responses under emotionally arousing conditions. Reward-associated cues activate PVT neurons, however, the specific PVT efferents regulating reward-seeking remain elusive. Using a cued sucrose-seeking task, we manipulated PVT activity under two emotionally distinct conditions: 1) when reward was available during the cue as expected, or 2) when reward was unexpectedly omitted during the cue. Pharmacological inactivation of the anterior PVT (aPVT), but not the posterior PVT, increased sucrose-seeking only when reward was omitted. Consistent with this, photoactivation of aPVT neurons abolished sucrose-seeking, and the firing of aPVT neurons differentiated reward availability. Photoinhibition of aPVT projections to the nucleus accumbens or to the amygdala increased or decreased, respectively, sucrose-seeking only when reward was omitted. Our findings suggest that PVT bidirectionally modulates sucrose-seeking under the negative (frustrative) conditions of reward omission.

RationaleThere is a high degree of comorbidity between alcohol use disorder and post-traumatic stress disorder (PTSD), but little is known about the interactions of ethanol with traumatic memories.ObjectivesUsing auditory fear conditioning in rats, we asked if repeated exposure to ethanol could modify the retrieval of fear memories acquired prior to ethanol exposure.MethodsFollowing auditory fear conditioning, Sprague-Dawley rats were given daily injections of ethanol (1.5 g/kg) or saline over 5 days. Two days later, they were given 20 trials of extinction training and then tested for extinction memory the following day. In a separate experiment, conditioned rats were given repeated ethanol injections and processed for c-Fos immunohistochemistry following a fear retrieval session.ResultsTwo days following the cessation of ethanol, the magnitude of conditioned fear responses (freezing and suppression of bar pressing) was significantly increased. This increase persisted the following day. Waiting 10 days following cessation of ethanol eliminated the effect on fear retrieval. In rats conditioned with low shock levels, repeated exposure to ethanol converted a sub-threshold fear memory into a supra-threshold fear memory. It also increased c-Fos expression in the prelimbic prefrontal cortex, paraventricular thalamus, and the central and basolateral nuclei of the amygdala, areas implicated in the retrieval of fear memories.ConclusionsThese results suggest that repeated exposure to ethanol may exacerbate pre-existing traumatic memories.

The long-lasting nature of fear memories is essential for survival, but the neural circuitry for retrieval of these associations changes with the passage of time. We previously reported a time-dependent shift from prefrontal-amygdalar circuits to prefrontal-thalamic circuits for the retrieval of auditory fear conditioning. However, little is known about the time-dependent changes in the originating site, the prefrontal cortex. Here we monitored the responses of prelimbic (PL) prefrontal neurons to conditioned tones at early (2 h) vs. late (4 days) timepoints following training. Using c-Fos, we find that PL neurons projecting to the amygdala are activated early after learning, but not later, whereas PL neurons projecting to the paraventricular thalamus (PVT) show the opposite pattern. Using unit recording, we find that PL neurons in layer V (the origin of projections to amygdala) showed cue-induced excitation at earlier but not later timepoints, whereas PL neurons in Layer VI (the origin of projections to PVT) showed cue-induced inhibition at later, but not earlier, timepoints, along with an increase in spontaneous firing rate. Thus, soon after conditioning, there are conditioned excitatory responses in PL layer V which influence the amygdala. With the passage of time, however, retrieval of fear memories shifts to inhibitory responses in PL layer VI which influence the midline thalamus.