Involvement of the retrosplenial cortex in the processing of the temporal aspect of episodic-like memory in rats

Disrupting direct inputs from the dorsal subiculum to the granular retrosplenial cortex impairs flexible spatial memory in the rat

In a changing environment, animals must process spatial signals in a flexible manner. The rat hippocampal formation projects directly upon the retrosplenial cortex, with most inputs arising from the dorsal subiculum and terminating in the granular retrosplenial cortex (area 29). The present study examined whether these same projections are required for spatial working memory and what happens when available spatial cues are altered. Consequently, injections of iDREADDs were made into the dorsal subiculum of rats. In a separate control group, GFP-expressing adeno-associated virus was injected into the dorsal subiculum. Both groups received intracerebral infusions within the retrosplenial cortex of clozapine, which in the iDREADDs rats should selectively disrupt the subiculum to retrosplenial projections. When tested on reinforced T-maze alternation, disruption of the subiculum to retrosplenial projections had no evident effect on the performance of those alternation trials when all spatial-cue types remained present and unchanged. However, the same iDREADDs manipulation impaired performance on all three alternation conditions when there was a conflict or selective removal of spatial cues. These findings reveal how the direct projections from the dorsal subiculum to the retrosplenial cortex support the flexible integration of different spatial cue types, helping the animal to adopt the spatial strategy that best meets current environmental demands.

Assessing episodic memory in rodents using spontaneous object recognition tasks

Models of episodic memory are successfully established using spontaneous object recognition tasks in rodents. In this review, we present behavioral techniques devised to investigate this type of memory, emphasizing methods based on associations of places and temporal order of items explored by rats and mice. We also provide a review on the areas and circuitry of the medial temporal lobe underlying episodic-like memory, considering that a large number of neurobiology data derived from these protocols. Although spontaneous recognition tasks are commonplace in this field, there is need for careful evaluation of factors affecting animal performance. Such as the ongoing development of tools for investigating the neural basis of memory, efforts should be put in the refinement of experimental designs, in order to provide reliable behavioral evidence of this complex mnemonic system.

Neuronal circuitry for recognition memory of object and place in rodent models

Rats and mice are used for studying neuronal circuits underlying recognition memory due to their ability to spontaneously remember the occurrence of an object, its place and an association of the object and place in a particular environment. A joint employment of lesions, pharmacological interventions, optogenetics and chemogenetics is constantly expanding our knowledge of the neural basis for recognition memory of object, place, and their association. In this review, we summarize current studies on recognition memory in rodents with a focus on the novel object preference, novel location preference and object-in-place paradigms. The evidence suggests that the medial prefrontal cortex- and hippocampus-connected circuits contribute to recognition memory for object and place. Under certain conditions, the striatum, medial septum, amygdala, locus coeruleus and cerebellum are also involved. We propose that the neuronal circuitry for recognition memory of object and place is hierarchically connected and constructed by different cortical (perirhinal, entorhinal and retrosplenial cortices), thalamic (nucleus reuniens, mediodorsal and anterior thalamic nuclei) and primeval (hypothalamus and interpeduncular nucleus) modules interacting with the medial prefrontal cortex and hippocampus.

Towards an animal model of consciousness based on the platform theory

The evolution of intellectual capacities has brought forth a continuum of consciousness levels subserved by neuronal networks of varying complexity. Brain pathologies, neurodegenerative, and mental diseases affect conscious cognition and behavior. Although impairments in consciousness are among the most devastating consequences of neurological and mental diseases, valid and reliable animal models of consciousness, that could be used for preclinical research are missing. The platform theory holds that the brain enters a conscious operation mode, whenever mental representations of stimuli, associations, concepts, memories, and experiences are effortfully maintained (in working memory) and actively manipulated. We used the platform theory as a framework and evaluation standard to categorize behavioral paradigms with respect to the level of consciousness involved in task performance. According to the platform theory, a behavioral paradigm involves conscious cognitive operations, when the problem posed is unexpected, novel or requires the maintenance and manipulation of a large amount of information to perform cognitive operations on them. Conscious cognitive operations are associated with a relocation of processing resources and the redirection of attentional focus. A consciousness behavioral test battery is proposed that is composed of tests which are assumed to require higher levels of consciousness as compared to other tasks and paradigms. The consciousness test battery for rodents includes the following tests: Working memory in the radial arm maze, episodic-like memory, prospective memory, detour test, and operant conditioning with concurrent variable-interval variable-ratio schedules. Performance in this test battery can be contrasted with the performance in paradigms and tests that require lower levels of consciousness. Additionally, a second more comprehensive behavioral test battery is proposed to control for behavioral phenotypes not related to consciousness. Our theory could serve as a guidance for the decryption of the neurobiological basis of consciousness.

Peering into the Brain through the Retrosplenial Cortex to Assess Cognitive Function of the Injured Brain

The retrosplenial cortex (RSC) is a posterior cortical area that has been drawing increasing interest in recent years, with a growing number of studies studying its contribution to cognitive and sensory functions. From an anatomical perspective, it has been established that the RSC is extensively and often reciprocally connected with the hippocampus, neocortex, and many midbrain regions. Functionally, the RSC is an important hub of the default-mode network. This endowment, with vast anatomical and functional connections, positions the RSC to play an important role in episodic memory, spatial and contextual learning, sensory-cognitive activities, and multi-modal sensory information processing and integration. Additionally, RSC dysfunction has been reported in cases of cognitive decline, particularly in Alzheimer's disease and stroke. We review the literature to examine whether the RSC can act as a cortical marker of persistent cognitive dysfunction after traumatic brain injury (TBI). Because the RSC is easily accessible at the brain's surface using in vivo techniques, we argue that studying RSC network activity post-TBI can shed light into the mechanisms of less-accessible brain regions, such as the hippocampus. There is a fundamental gap in the TBI field about the microscale alterations occurring post-trauma, and by studying the RSC's neuronal activity at the cellular level we will be able to design better therapeutic tools. Understanding how neuronal activity and interactions produce normal and abnormal activity in the injured brain is crucial to understanding cognitive dysfunction. By using this approach, we expect to gain valuable insights to better understand brain disorders like TBI.

Episodic-like memory of rats as retrospective retrieval of incidentally encoded locations and involvement of the retrosplenial cortex

To examine episodic memory in rats, we trained rats to perform two tasks and tested them for memory of past self-behavior without making them expect to be asked about the memory later when encoding. One of the trained tasks was a delayed matching-to-position task in which the rats were required to remember the location of a presented lever. The other was a tone discrimination task in which the rats were required to discriminate between two pure tones. After learning both tasks, the rats were unexpectedly asked the location of the pressed lever after responding to the cue tone in probe trials during test sessions. The rats demonstrated a response bias that suggests that they have the ability to retrospectively recollect their self-behavior, i.e., episodic memory. We next made excitotoxic lesions in the retrosplenial cortex (RSC) and investigated the effects of the lesions on the unexpected recollection. In the rats with lesions of the RSC, the response bias disappeared. This suggests that the RSC has a role in retrospectively answering unexpected questions about self-behavior.