A state-dependent learning effect produced by amylobarbitone sodium

Apparent reconsolidation interference without generalized amnesia

Memories remain dynamic after consolidation, and when reactivated, they can be rendered vulnerable to various pharmacological agents that disrupt the later expression of memory (i.e., amnesia). Such drug-induced post-reactivation amnesia has traditionally been studied in AAA experimental designs, where a memory is initially created for a stimulus A (be it a singular cue or a context) and later reactivated and tested through exposure to the exact same stimulus. Using a contextual fear conditioning procedure in rats and midazolam as amnestic agent, we recently demonstrated that drug-induced amnesia can also be obtained when memories are reactivated through exposure to a generalization stimulus (GS, context B) and later tested for that same generalization stimulus (ABB design). However, this amnestic intervention leaves fear expression intact when at test animals are instead presented with the original training stimulus (ABA design) or a novel generalization stimulus (ABC design). The underlying mechanisms of post-reactivation memory malleability and of MDZ-induced amnesia for a generalization context remain largely unknown. Here, we evaluated whether, like typical CS-mediated (or AAA) post-reactivation amnesia, GS-mediated (ABB) post-reactivation amnesia displays key features of a destabilization-based phenomenon. We first show that ABB post-reactivation amnesia is critically dependent on prediction error at the time of memory reactivation and provide evidence for its temporally graded nature. In line with the known role of GluN2B-NMDA receptor activation in memory destabilization, we further demonstrate that pre-reactivation administration of ifenprodil, a selective antagonist of GluN2B-NMDA receptors, prevents MDZ-induced ABB amnesia. In sum, our data reveal that ABB MDZ-induced post-reactivation amnesia exhibits the hallmark features of a destabilization-dependent phenomenon. Implication of our findings for a reconsolidation-based account of post-reactivation amnesia are discussed.

Using New Approaches in Neurobiology to Rethink Stress-Induced Amnesia

Purpose of Review

Psychological stress can impact memory systems in several different ways. In individuals with healthy defense and coping systems, stress results in the formation of negatively valenced memories whose ability to induce emotional and somatic distress subsides with time. Vulnerable individuals, however, go on to develop stress-related disorders such as post-traumatic stress disorder (PTSD) and suffer from significant memory abnormalities. Whether expressed as intrusive trauma memories, partial amnesia, or dissociative amnesia, such abnormalities are thought to be the core source of patients' symptoms, which are often debilitating and implicate an entire socio-cognitive-affective spectrum.

Recent Findings

With this in mind, and focusing on stress-responsive hippocampal microcircuits, this article highlights recent advances in the neurobiology of memory that allow us to (1) isolate and visualize memory circuits, (2) change their activity using genetic tools and state-dependent manipulations, and (3) directly examine their impact on socio-affective circuits and global network connectivity. By integrating these approaches, we are now in a position to address important questions that have troubled psychiatry for a long time-questions such as are traumatic memories special, and why are stress effects on memory diverse.

Summary

Furthering our fundamental understanding of memory in the framework of adaptive and maladaptive stress responses has the potential to boost the development of new treatments that can benefit patients suffering from psychological trauma.

Neurobiological correlates of state-dependent context fear

Retrieval of fear memories can be state-dependent, meaning that they are best retrieved if the brain states at encoding and retrieval are similar. Such states can be induced by activating extrasynaptic γ-aminobutyric acid type A receptors (GABA_AR) with the broad α-subunit activator gaboxadol. However, the circuit mechanisms and specific subunits underlying gaboxadol's effects are not well understood. Here we show that gaboxadol induces profound changes of local and network oscillatory activity, indicative of discoordinated hippocampal–cortical activity, that were accompanied by robust and long-lasting state-dependent conditioned fear. Episodic memories typically are hippocampus-dependent for a limited period after learning, but become cortex-dependent with the passage of time. In contrast, state-dependent memories continued to rely on hippocampal GABAergic mechanisms for memory retrieval. Pharmacological approaches with α-subunit-specific agonists targeting the hippocampus implicated the prototypic extrasynaptic subunits (α₄) as the mediator of state-dependent conditioned fear. Together, our findings suggest that continued dependence on hippocampal rather than cortical mechanisms could be an important feature of state-dependent memories that contributes to their conditional retrieval.

Dissociative states in multiple personality disorder: a quantitative study

Multiple personality disorder (MPD) patients may experience themselves as several discrete alter personalities who do not share consciousness or memories with one another. In this study, we asked whether MPD patients are different from controls in their ability to learn and remember, and their ability to compartmentalize information. MPD patients were not found to differ from controls in overall memory level. Learning of information by MPD patients in disparate personality states did not result in greater compartmentalization than that of which control subjects were capable. However, there were qualitative differences between the cognitive performance of patients and that of controls attempting to role-play alter personalities. Our results suggest that simple confabulation is not an adequate model for the MPD syndrome, and we consider a possible role for state-dependent learning in the phenomenology of MPD.

Asymmetrical transfer: an inherent weakness of repeated-measure drug experiments

When subjects act as their own controls in drug experiments there is a risk of asymmetrical transfer between treatment conditions, with the result that treatment effects are determined largely by order of administration. In such cases the effect of a drug may be reduced or exaggerated, and prior treatment with a drug may affect placebo conditions. Asymmetrical transfer is probably much more common than is generally realized. Suggested causes include inadequate wash-out between treatments, state-dependent learning, adoption of learning strategies, and practice effects. The problem is illustrated by the reanalysis of a paper on the effects of benzhexol on memory, published recently in this journal (Potamianos and Kellet, 1982). The advantages of separate-group experimental design are reconsidered.

Clinical trial design: the neglected problem of asymmetrical transfer in cross-over trials

The paper illustrates drawbacks of the popular cross-over design in clinical trials in psychiatry where treatment effects become distorted by, and confounded with, the effects of their order of administration. It is argued that research workers should analyse data from cross-over trials for asymmetries due to order of treatment and seriously doubt the reliability of data if such effects are found. The problem is avoided by the use of separate-groups designs.

State-dependent learning in children receiving methylphenidate

Methylphenidate state-dependent learning was investigated using a paired-associate learning task of 16 pairs of pictures. Subjects were 15 children for whom methylphenidate was currently being prescribed. Each child was seen for two learning sessions and two testing sessions. Learning took place under placebo and under the child's regular dosage of methylphenidate. During retention testing all combinations of learning drug-state and testing drug-state were observed for each child. Data were analyzed using contrast variables which took into account the repeated-measures nature of the design. The results indicated that there was a trend in the direction of state-dependence in the whole sample. Significant symmetrical state-dependent effects were obtained in those subjects whose original learning was facilitated by methylphenidate.