No effect of glucose administration in a novel contextual fear generalization protocol in rats

Isolation of the differential effects of chronic and acute stress in a manner that is not confounded by stress severity

Firm conclusions regarding the differential effects of the maladaptive consequences of acute versus chronic stress on the etiology and symptomatology of stress disorders await a model that isolates chronicity as a variable for studying the differential effects of acute versus chronic stress. This is because most previous studies have confounded chronicity with the total amount of stress. Here, we have modified the stress-enhanced fear learning (SEFL) protocol, which models some aspects of posttraumatic stress disorder (PTSD) following an acute stressor, to create a chronic variant that does not have this confound. Comparing results from this new protocol to the acute protocol, we found that chronic stress further potentiates enhanced fear-learning beyond the nonassociative enhancement induced by acute stress. This additional component is not observed when the unconditional stimulus (US) used during subsequent fear learning is distinct from the US used as the stressor, and is enhanced when glucose is administered following stressor exposure, suggesting that it is associative in nature. Furthermore, extinction of stressor-context fear blocks this additional associative component of SEFL as well as reinstatement of generalized fear, suggesting reinstatement of generalized fear may underlie this additional SEFL component.

Post-stress glucose consumption facilitates hormesis and resilience to severe stress

Oral ingestion of a glucose solution following severe stress is a simple and effective way of preventing several of the negative sequelae of stress in rats. Similar resilience is obtained through hormetic training - pre-exposure to mild-to-moderate stress prior to severe stress. Here, we examined whether hormetic training is facilitated when a glucose solution is available following each hormetic training session. In Experiment 1, all rats were pre-exposed to a 30 min hormetic session of 25 inescapable tailshocks on each of 3 days. The schedule or hormesis differed between groups. The hormetic sessions occurred on either 3 consecutive days or with an interpolated day of rest between each hormetic session. Furthermore, in each of these conditions, one group had access to water and one group had access to a 40% glucose solution immediately after each hormetic session to complete a 2x2 factorial design. All groups were exposed to 100 inescapable tailshocks on the day following the end of hormetic training. Shuttle-escape testing occurred 24 h later. In Experiment 2, rats received two consecutive days of 100 inescapable tailshocks. Water or glucose was available following each session. Testing occurred 24 h after the second shock exposure. Experiment 1 replicated previous findings that rats exposed to hormetic training with interpolated rest did not show exaggerated fear responding or shuttle-escape deficits that normally result from 100 inescapable tailshocks, but training was ineffective if no rest was given between stress sessions. However, all post-stress glucose groups showed an elimination of helpless behavior. In Experiment 2, it was revealed that even 100 tailshocks can be made hormetic by post-stress glucose consumption.

Lack of drug-induced post-retrieval amnesia for auditory fear memories in rats

BACKGROUND

Long-term memory formation is generally assumed to involve the permanent storage of recently acquired memories, making them relatively insensitive to disruption, a process referred to as memory consolidation. However, when retrieved under specific circumstances, consolidated fear memories are thought to return to a labile state, thereby opening a window for modification (e.g., attenuation) of the memory. Several interventions during a critical time frame after this destabilization seem to be able to alter the retrieved memory, for example by pharmacologically interfering with the restabilization process, either by direct protein synthesis inhibition or indirectly, using drugs that can be safely administered in patients (e.g., propranolol). Here, we find that, contrary to expectations, systemic pharmacological manipulations in auditory fear-conditioned rats do not lead to drug-induced post-retrieval amnesia.

RESULTS

In a series of well-powered auditory fear conditioning experiments (four with propranolol, 10 mg/kg, two with rapamycin, 20-40 mg/kg, one with anisomycin, 150 mg/kg and cycloheximide, 1.5 mg/kg), we found no evidence for reduced cued fear memory expression during a drug-free test in adult male Sprague-Dawley rats that had previously received a systemic drug injection upon retrieval of the tone fear memory. All experiments used standard fear conditioning and reactivation procedures with freezing as the behavioral read-out (conceptual or exact replications of published reports) and common pharmacological agents. Additional tests confirmed that the applied drug doses and administration routes were effective in inducing their conventional effects on expression of fear (propranolol, acutely), body weight (rapamycin, anisomycin, cycloheximide), and consolidation of extinction memories (cycloheximide).

CONCLUSIONS

In contrast with previously published studies, we did not find evidence for drug-induced post-retrieval amnesia, underlining that this effect, as well as its clinical applicability, may be considerably more constrained and less readily reproduced than what the current literature would suggest.

Novelty exposure hinders aversive memory generalization and depends on hippocampal protein synthesis

Fear generalization is defined as the transferring of fear experienced during a traumatic event to safe conditions resembling or not the traumatic event. It has been related to several psychological disorders. Here we set out to determine whether novelty exposure can be effective to avoid fear generalization. We evaluated the effect of a novelty exposure on fear memory generalization using an aversive memory task, the inhibitory avoidance (IA). Male Wistar rats were trained in IA (day 1) and 24 h after (day 2) they were exposed to a new context similar to the original (modified IA - MIA), with some rats being exposed to a novelty just before the exposure to the MIA, while others were not (controls). On day 3, retention tests for IA and MIA contexts were performed. The control rats generalized the memory, expressing aversive behavioral in both contexts whereas rats exposed to novelty only expressed aversion on IA. Furthermore, both anisomycin, an inhibitor of ribosomal protein synthesis, and rapamycin, an inhibitor of mTOR-mediated protein synthesis, injected in the CA1 region of dorsal hippocampus blocked the novelty effect, promoting memory generalization. We conclude that novelty exposure hinders aversive memory generalization depending on hippocampal protein synthesis.

Prior stress promotes the generalization of contextual fear memories: Involvement of the gabaergic signaling within the basolateral amygdala complex

Fear generalization occurs when a response, previously acquired with a threatening stimulus, is transferred to a similar one. However, it could be maladaptive when stimuli that do not represent a real threat are appraised as dangerous, which is a hallmark of several anxiety disorders. Stress exposure is a major risk factor for the occurrence of anxiety disorders and it is well established that it influences different phases of fear memory; nevertheless, its impact on the generalization of contextual fear memories has been less studied. In the present work, we have characterized the impact of acute restraint stress prior to contextual fear conditioning on the generalization of this fear memory, and the role of the GABAergic signaling within the basolateral amygdala complex (BLA) on the stress modulatory effects. We have found that a single stress exposure promoted the generalization of this memory trace to a different context that was well discriminated in unstressed conditioned animals. Moreover, this effect was dependent on the formation of a contextual associative memory and on the testing order (i.e., conditioning context first vs generalization context first). Furthermore, we observed that increasing GABA-A signaling by intra-BLA midazolam administration prior to the stressful session exposure prevented the generalization of fear memory, whereas intra-BLA administration of the GABA-A antagonist (Bicuculline), prior to fear conditioning, induced the generalization of fear memory in unstressed rats. We concluded that stress exposure, prior to contextual fear conditioning, promotes the generalization of fear memory and that the GABAergic transmission within the BLA has a critical role in this phenomenon.

Forebrain-specific, conditional silencing of Staufen2 alters synaptic plasticity, learning, and memory in rats

Background

Dendritic messenger RNA (mRNA) localization and subsequent local translation in dendrites critically contributes to synaptic plasticity and learning and memory. Little is known, however, about the contribution of RNA-binding proteins (RBPs) to these processes in vivo.

Results

To delineate the role of the double-stranded RBP Staufen2 (Stau2), we generate a transgenic rat model, in which Stau2 expression is conditionally silenced by Cre-inducible expression of a microRNA (miRNA) targeting Stau2 mRNA in adult forebrain neurons. Known physiological mRNA targets for Stau2, such as RhoA, Complexin 1, and Rgs4 mRNAs, are found to be dysregulated in brains of Stau2-deficient rats. In vivo electrophysiological recordings reveal synaptic strengthening upon stimulation, showing a shift in the frequency-response function of hippocampal synaptic plasticity to favor long-term potentiation and impair long-term depression in Stau2-deficient rats. These observations are accompanied by deficits in hippocampal spatial working memory, spatial novelty detection, and in tasks investigating associative learning and memory.

Conclusions

Together, these experiments reveal a critical contribution of Stau2 to various forms of synaptic plasticity including spatial working memory and cognitive management of new environmental information. These findings might contribute to the development of treatments for conditions associated with learning and memory deficits.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-017-1350-8) contains supplementary material, which is available to authorized users.

A preregistered, direct replication attempt of the retrieval-extinction effect in cued fear conditioning in rats

In 2009, Monfils and colleagues proposed a behavioral procedure that was said to result in a permanent attenuation of a previously established fear memory, thereby precluding a possible return of fear after extinction (Monfils, Cowansage, Klann, & LeDoux, 2009). By presenting a single retrieval trial one hour before standard extinction training, they found an enduring reduction of fear. The retrieval-extinction procedure holds great clinical potential, particularly for anxiety patients, but the findings are not undisputed, and several conceptual replications have failed to reproduce the effect. These failures have largely been attributed to small procedural differences. This preregistered study is the first endeavor to exactly replicate three key experiments of the original report by Monfils et al. (2009), thereby gauging the robustness of their seminal findings. Despite adhering to the original procedures as closely as possible, we did not find any evidence for reduced return of fear with the retrieval-extinction procedure relative to regular extinction training, as assessed through spontaneous recovery, reinstatement and renewal. Behavior of animals in the control condition (extinction only) was comparable to that in the original studies and provided an adequate baseline to reveal differences with the retrieval-extinction condition. Our null findings indicate that the effect sizes in the original paper may have been inflated and question the legitimacy of previously proposed moderators of the retrieval-extinction effect. We argue that direct experimental evaluation of purported moderators of the retrieval-extinction effect will be key to shed more light on its nature and prerequisites.

Low-dose systemic scopolamine disrupts context conditioning in rats

Cholinergic neurotransmission plays a key role in learning and memory. Prior research with rats indicated that a low dose of pre-training scopolamine (0.1 mg/kg), a cholinergic receptor antagonist, did not affect cued fear conditioning, but did block renewal when injected before extinguishing a conditioned tone, opening up opportunities to pharmacologically improve exposure therapy for anxiety patients. Before translating these findings to the clinic, it is important to carefully examine how scopolamine affects contextual fear memories. Here, we investigated the effects of scopolamine on encoding of contextual anxiety and its generalization in male Wistar rats. We found a profound disruption of context conditioning, suggesting that, even at a low dose, systemic scopolamine may influence contextual encoding in the hippocampus, particularly when the context is the best predictor for the presence of shocks.

Temporal parameters of post-stress prophylactic glucose treatment in rats

Acute trauma can lead to life-long changes in susceptibility to psychiatric disease, such as post-traumatic stress disorder (PTSD). Rats given free access to a concentrated glucose solution for 24 h beginning immediately after trauma failed to show stress-related pathology in the learned helplessness model of PTSD and comorbid major depression. We assessed effective dosing and temporal constraints of the glucose intervention in three experiments. We exposed 120 male Sprague-Dawley rats to 100, 1 mA, 3-15 s, inescapable and unpredictable electric tail shocks (over a 110-min period) or simple restraint in the learned helplessness procedure. Rats in each stress condition had access to a 40% glucose solution or water. We measured fluid consumption under 18-h free access conditions, or limited access (1, 3, 6, 18 h) beginning immediately after trauma, or 3-h access with delayed availability of the glucose solution (0, 1, 3, 6 h). We hypothesized that longer and earlier access following acute stress would improve shuttle-escape performance. Rats exposed to traumatic shock and given 18-h access to glucose failed to show exaggerated fearfulness and showed normal reactivity to foot shock during testing as compared to their water-treated counterparts. At least 3 h of immediate post-stress access to glucose were necessary to see these improvements in test performance. Moreover, delaying access to glucose for more than 3 h post-trauma yielded no beneficial effects. These data clearly identify limits on the post-stress glucose intervention. In conclusion, glucose should be administered almost immediately and at the highest dose after trauma.