Selective neuronal degeneration in the retrosplenial cortex impairs the recall of contextual fear memory

Modulation of contextual fear acquisition and extinction by acute and chronic relaxin-3 receptor (RXFP3) activation in the rat retrosplenial cortex

The retrosplenial cortex (RSC) plays a central role in processing contextual fear conditioning. In addition to corticocortical and thalamocortical projections, the RSC receives subcortical inputs, including a substantial projection from the nucleus incertus in the pontine tegmentum. This GABAergic projection contains the neuropeptide, relaxin-3 (RLN3), which inhibits target neurons via its Gi/o-protein-coupled receptor, RXFP3. To assess this peptidergic system role in contextual fear conditioning, we bilaterally injected the RSC of adult rats with an adeno-associated-virus (AAV), expressing the chimeric RXFP3 agonist R3/I5 or a control AAV, and subjected them to contextual fear conditioning. The R3/I5 injected rats did not display any major differences to control-injected and naïve rats but displayed a significantly delayed extinction. Subsequently, we employed acute bilateral injections of the specific RXFP3 agonist peptide, RXFP3-Analogue 2 (A2), into RSC. While the administration of A2 before each extinction trial had no impact on the extinction process, treatment with A2 before each acquisition trial resulted in delayed extinction. In related anatomical studies, we detected an enrichment of RLN3-immunoreactive nerve fibers in deep layers of the RSC, and a higher level of co-localization of RXFP3 mRNA with vesicular GABA transporter (vGAT) mRNA than with vesicular glutamate transporter-1 (vGLUT1) mRNA across the RSC, consistent with an effect of RLN3/RXFP3 signalling on the intrinsic, inhibitory circuits within the RSC. These findings suggest that contextual conditioning processes in the RSC involve, in part, RLN3 afferent modulation of local inhibitory neurons that provides a stronger memory acquisition which, in turn, retards the extinction process.

Developing the theory of the extended amygdala with the use of the cupric-silver technique

The amygdaloid complex is a crucial component of the basal forebrain that participates in the modulation of many homeostatic functions, emotional behaviors, and learning. These features require a widespread pattern of connections with several brain structures. In the past, the amygdaloid complex was divided into corticomedial and basolateral groups. The existence of a neuronal continuum linking the central amygdaloid nucleus to the lateral bed nucleus of stria terminalis through the subpallidal area was first revealed by José de Olmos (1932-2008) with the aid of his cupric-silver technique. This observation gave birth to the concept of the extended amygdala, a conceptual framework that is useful for understanding the anatomofunctional organization of the amygdaloid complex, with relevance for basic neuroscience and clinical interventions. Traditional tract-tracing staining methods were complicated and tedious to reproduce. Axonal terminal endings were lost among a myriad of normal fibers. The need to visualize these terminals drove de Olmos to develop cupric-silver methods that revealed disintegrating synaptic terminals, without staining normal fibers. In this article, we describe the historical events leading to the development of the cupric-silver technique that evolved into the amino-cupric-silver technique, which developed hand-in-hand over the years.

Examining a role for the retrosplenial cortex in age-related memory impairment

Aging is often characterized by changes in the ability to form and accurately recall episodic memories, and this is especially evident in neuropsychiatric conditions including Alzheimer's disease and dementia. Memory impairments and cognitive decline associated with aging mirror the impairments observed following damage to the retrosplenial cortex, suggesting that this region might be important for continued cognitive function throughout the lifespan. Here, we review lines of evidence demonstrating that degeneration of the retrosplenial cortex is critically involved in age-related memory impairment and suggest that preservation of function in this region as part of a larger circuit that supports memory maintenance will decrease the deleterious effects of aging on memory processing.

Hyperexcitable Neurons Enable Precise and Persistent Information Encoding in the Superficial Retrosplenial Cortex

The retrosplenial cortex (RSC) is essential for memory and navigation, but the neural codes underlying these functions remain largely unknown. Here, we show that the most prominent cell type in layers 2/3 (L2/3) of the mouse granular RSC is a hyperexcitable, small pyramidal cell. These cells have a low rheobase (LR), high input resistance, lack of spike frequency adaptation, and spike widths intermediate to those of neighboring fast-spiking (FS) inhibitory neurons and regular-spiking (RS) excitatory neurons. LR cells are excitatory but rarely synapse onto neighboring neurons. Instead, L2/3 is a feedforward, not feedback, inhibition-dominated network with dense connectivity between FS cells and from FS to LR neurons. Biophysical models of LR but not RS cells precisely and continuously encode sustained input from afferent postsubicular head-direction cells. Thus, the distinct intrinsic properties of LR neurons can support both the precision and persistence necessary to encode information over multiple timescales in the RSC.

Revealing a Cortical Circuit Responsive to Predatory Threats and Mediating Contextual Fear Memory

The ventral part of the anteromedial thalamic nucleus (AMv) receives substantial inputs from hypothalamic sites that are highly responsive to a live predator or its odor trace and represents an important thalamic hub for conveying predatory threat information to the cerebral cortex. In the present study, we begin by examining the cortico-amygdalar-hippocampal projections of the main AMv cortical targets, namely, the caudal prelimbic, rostral anterior cingulate, and medial visual areas, as well as the rostral part of the ventral retrosplenial area, one of the main targets of the anterior cingulate area. We observed that these areas form a clear cortical network. Next, we revealed that in animals exposed to a live cat, all of the elements of this circuit presented a differential increase in Fos, supporting the idea of a predator threat-responsive cortical network. Finally, we showed that bilateral cytotoxic lesions in each element of this cortical network did not change innate fear responses but drastically reduced contextual conditioning to the predator-associated environment. Overall, the present findings suggest that predator threat has an extensive representation in the cerebral cortex and revealed a cortical network that is responsive to predatory threats and exerts a critical role in processing fear memory.

Retrograde and anterograde contextual fear amnesia induced by selective elimination of layer IV-Va neurons in the granular retrosplenial cortex (A29)

Differences in cytoarchitectural organization and connectivity distinguishes granular (or area 29, A29) and dysgranular (or area 30, A30) subdivisions of the retrosplenial cortex (RSC). Although increasing evidence supports the participation of RSC in contextual fear learning and memory, the contribution of each RSC subdivision remains unknown. Here we used orchiectomized rats and intraperitoneal (i.p.) injections of saline (control) or 5 mg/kg MK801, to trigger selective degeneration of pyramidal neurons in layers IV-Va of A29 (A29^MK801 neurons). These treatments were applied 3 days before or two days after contextual fear conditioning, and contextual fear memory was evaluated by scoring freezing in the conditioned context five days after training. Afterwards, brains were fixed and c-Fos and Egr-1 expression were assessed as surrogates of neuronal activity elicited by the recall in A29, A30 and in limbic areas. We found that eliminating A29^MK801 neurons after training reduces conditioned freezing to 43.1 ± 9.9% respect to control rats. This was associated with a significant reduction of c-Fos and Egr-1 expression in A30, but not in other limbic areas. On the other hand, eliminating A29^MK801 neurons before training caused a mild but significant reduction of conditioned freezing to 79.7 ± 6.8%, which was associated to enhanced expression of c-Fos in A29, A30 and CA1 field of hippocampus, while Egr-1 expression in caudomedial (CEnt) entorhinal cortex was not depressed as in control animals. These observations show that severeness of amnesia differs according to whether A29^MK801 neurons were eliminated before or after conditioning, likely because loss of A29^MK801 neurons after conditioning disrupt memory engram while their elimination before training allow recruitment of other neurons in A29 for partial compensation of contextual fear learning and memory. These observations add further support for the critical role of A29^MK801 neurons in contextual fear learning and memory by connecting limbic structures with A30.

Fear-context association during memory retrieval requires input from granular to dysgranular retrosplenial cortex

The contribution of the granular (area 29, A29) and dysgranular (area 30, A30) subdivisions of the retrosplenial cortex (RSC) to contextual fear memory (CFM) retrieval remains elusive. Here, intact and orchiectomized (ORC) male rats received an intraperitoneal (I.P.) injection of saline (control) or 5 mg/Kg MK801 after training and memory formation. In ORC, but not in intact males, this MK801 treatment selectively induces overt loss of neurons in layers IV-Va of A29 (A29^MK801 neurons) (Sigwald et al., 2016). Compared to ORC-saline, ORC-MK801 rats showed impaired CFM retrieval in an A-B-A design for contextual fear conditioning (CFC), however context recognition was not affected. In ORC-MK801 rats, neither novel object recognition nor object-in-context discrimination were impaired, further indicating that A29^MK801 neurons are not required for contextual recognition. Elevated plus maze test showed that anxiety-like behavior was not affected in ORC-MK801 animals, suggesting that loss of A29^MK801 neurons does not affect the emotional state that could impair freezing during test. Importantly, in a sensory preconditioning test, higher order CFM retrieval was abolished in ORC-MK801, but not in male-MK801. Collectively, these observations indicate that A29^MK801 neurons are critically required for retrieving fear-context association. For dissecting the anatomofunctional contribution of A29^MK801 neurons to CFM retrieval, expression of c-Fos and Egr-1 was used to map brain-wide neuronal activity. In control male rats CFC and CFM retrieval was associated with significant enhancement of both proteins in limbic structures and A30, but not in A29, suggesting that neurons in A30 and limbic structures encode and store the associative experience. Notably, in ORC but not in intact males, MK801 impairs CFM retrieval and expression of c-Fos and Egr-1 proteins in A30, without affecting their expression in limbic structures. Thus, the loss of A29^MK801 neurons after CFM formation precludes activation of associative neurons in A30, impairing CFM recall. FluoroGold retrograde track-tracing confirmed that A29^MK801 neurons project to A30. Silver staining provide evidence that MK801 in ORC rats induces axonal deafferentation of A29^MK801 neuron in A30. Collectively, our experiments provide the first evidence that A30 neurons participate in encoding and storing CFM while A29 is required for their activation during recall.

Acceptability of a Plasticity-Focused Serious Game Intervention for Posttraumatic Stress Disorder: User Requirements Analysis

Background

Trauma-focused cognitive behavioral therapy (TF-CBT) is a first-line treatment for posttraumatic stress disorder (PTSD). Despite a solid evidence base, TF-CBT response and attrition rates vary considerably. Plasticity-focused interventions, including the use of serious games, have the potential to improve TF-CBT response and treatment retention.

Objective

The aim of this study was to assess the acceptability of a mobile phone–delivered plasticity-focused serious game to improve response to TF-CBT for PTSD, and carry out a user requirements analysis should the development of a prototype be warranted.

Methods

We conducted 2 one-to-one interviews (n=2), one focus group involving service users who had received a diagnosis of PTSD (n=3) and one focus group involving psychological trauma service clinicians (n=4).

Results

We found that the concept of a plasticity-focused mobile phone intervention for PTSD is acceptable to patients and clinicians. Service users and clinicians both believed that the usage should be guided by a therapist, and both contributed useful inputs regarding the audiovisual aspects of the proposed serious game. It was accepted that the game would not be suitable for all patients and that clinicians would need to appropriately prescribe the usage of the game.

Conclusions

The findings highlight the acceptability of the proposed serious game and clarify the user requirements for such an intervention. It is the intention of the authors to carry out a user experience evaluation using a prototype serious game in a clinical population.

Distinct retrosplenial cortex cell populations and their spike dynamics during ketamine-induced unconscious state

Ketamine is known to induce psychotic-like symptoms, including delirium and visual hallucinations. It also causes neuronal damage and cell death in the retrosplenial cortex (RSC), an area that is thought to be a part of high visual cortical pathways and at least partially responsible for ketamine's psychotomimetic activities. However, the basic physiological properties of RSC cells as well as their response to ketamine in vivo remained largely unexplored. Here, we combine a computational method, the Inter-Spike Interval Classification Analysis (ISICA), and in vivo recordings to uncover and profile excitatory cell subtypes within layers 2&3 and 5&6 of the RSC in mice within both conscious, sleep, and ketamine-induced unconscious states. We demonstrate two distinct excitatory principal cell sub-populations, namely, high-bursting excitatory principal cells and low-bursting excitatory principal cells, within layers 2&3, and show that this classification is robust over the conscious states, namely quiet awake, and natural unconscious sleep periods. Similarly, we provide evidence of high-bursting and low-bursting excitatory principal cell sub-populations within layers 5&6 that remained distinct during quiet awake and sleep states. We further examined how these subtypes are dynamically altered by ketamine. During ketamine-induced unconscious state, these distinct excitatory principal cell subtypes in both layer 2&3 and layer 5&6 exhibited distinct dynamics. We also uncovered different dynamics of local field potential under various brain states in layer 2&3 and layer 5&6. Interestingly, ketamine administration induced high gamma oscillations in layer 2&3 of the RSC, but not layer 5&6. Our results show that excitatory principal cells within RSC layers 2&3 and 5&6 contain multiple physiologically distinct sub-populations, and they are differentially affected by ketamine.

Retrosplenial Cortical Neurons Encode Navigational Cues, Trajectories and Reward Locations During Goal Directed Navigation

The retrosplenial cortex (RSC) plays an important role in memory and spatial navigation. It shares functional similarities with the hippocampus, including the presence of place fields and lesion-induced impairments in spatial navigation, and the RSC is an important source of visual-spatial input to the hippocampus. Recently, the RSC has been the target of intense scrutiny among investigators of human memory and navigation. fMRI and lesion data suggest an RSC role in the ability to use landmarks to navigate to goal locations. However, no direct neurophysiological evidence of encoding navigational cues has been reported so the specific RSC contribution to spatial cognition has been uncertain. To examine this, we trained rats on a T-maze task in which the reward location was explicitly cued by a flashing light and we recorded RSC neurons as the rats learned. We found that RSC neurons rapidly encoded the light cue. Additionally, RSC neurons encoded the reward and its location, and they showed distinct firing patterns along the left and right trajectories to the goal. These responses may provide key information for goal-directed navigation, and the loss of these signals may underlie navigational impairments in subjects with RSC damage.