The neurokinin-3 receptor agonist senktide facilitates the integration of memories for object, place and temporal order into episodic memory

How does neurokinin 3 receptor agonism affect pathological and cognitive impairments in an Alzheimer's disease-like rat model?

Alzheimer's disease (AD) is accepted as a form of progressive dementia. Cholinergic systems are commonly affected in AD. Neurokinin 3 receptor (NK3R) is involved in learning memory-related processes. It is known that the activation of NK3R affects the release of many neurotransmitters. The aim of this project was to investigate the effects of NK3R agonist senktide administration on neurobehavioral mechanisms in the experimental AD-like rat model. 50 male Wistar albino rats were divided into Control (C), AD, Control + NK3R agonist (CS), AD + NK3R agonist (ADS), AD + NK3Ragonist + antagonist groups (ADSO). We designed AD-like model by intrahippocampal administration of Aβ1-42. After NK3R agonist + antagonist injections, open field (OF), Morris water maze (MWM) tests were applied. Cholinergic mechanism analysis from hippocampus-cortex tissues was performed by ELISA and catecholamine analysis from brain stem tissue were performed by HPLC method. The transitions from edge to center, rearing, grooming parameters were found to be reduced in final values of OF. While the group-time interaction was significant in the OF test findings, there was no significant difference between the groups. In MWM test, ADS group showed a learning level close to control group and animals in AD and ADSO groups could not learn target quadrant in MWM test. The brain stem NA and DA concentrations were not statistically significant. Hippocampal AChE-ChAT levels were supported by positive effects of senktide on learning via the cholinergic mechanisms. As a result, NK3R agonists were found to be effective in improving cognitive functions in rats with AD pathology. In the experimental AD model, positive effects of NK3R on learning memory may be mediated by cholinergic mechanisms.

Probing the nature of episodic memory in rodents

Episodic memory (EM) specifies the experience of retrieving information of an event at the place and time of occurrence. Whether non-human animals are capable of EM remains debated, whereas evidence suggests that they have a memory system akin to EM. We here trace the development of various behavioral paradigms designed to study EM in non-human animals, in particular the rat. We provide an in-depth description of the available behavioral tests which combine three spontaneous object exploration paradigms, namely novel object preference (for measuring memory for "what"), novel location preference (for measuring memory for "where") and temporal order memory (memory for "when"), into a single trial to gauge a memory akin to EM. Most important, we describe a variation of such a test in which each memory component interacts with the others, demonstrating an integration of diverse mnemonic information. We discuss why a behavioral model of EM must be able to assess the ability to integrate "what", "where" and "when" information into a single experience. We attempt an interpretation of the various tests and review the studies that have applied them in areas such as pharmacology, neuroanatomy, circuit analysis, and sleep. Finally, we anticipate future directions in the search for neural mechanisms of EM in the rat and outline model experiments and methodologies in this pursuit.

Post-learning caffeine administration improves 'what-when' and 'what-where' components of episodic-like memory in rats

Episodic-like memory (ELM) consists in the capacity of nonhuman animals to remember 'where' and 'when' a specific episode occurred ('what'). Previous studies have showed that Wistar rats can form an ELM, but not after a 24 h retention delay. On the other hand, it has been demonstrated that caffeine can improve episodic memory consolidation in humans. Therefore, we verified whether acute post-sample caffeine administration could improve ELM consolidation in Wistar rats, as well if it could be related to neurochemical changes in the prefrontal cortex and hippocampus - regions related to episodic-like memory processing. 46 Male Wistar Rats, approximately 3 months-old, were divided into four groups as follows: untreated (n = 11), saline (n = 11), caffeine 10 mg ∕kg i.p (n = 12); caffeine 15 mg∕kgi.p (n = 12) and tested in WWWhen/ELM task. The animals treated with caffeine in different dosages (10 mg/kg and 15 mg/kg) discriminated temporally and spatially the objects, respectively. These groups also showed a dopamine renewal rate in the hippocampus, suggesting that there was an increase in the turnover compared with the groups with no caffeine administration. We can conclude that caffeine leads to an improvement in the consolidation of the temporal ('what-when') and spatial ('what-where') aspects of episodic-like memory.

Nk3R blockade has sex-divergent effects on memory in mice

Background

Memory consolidation is a process required for the formation of long-term memories. The G-protein-coupled receptor (GPCR) neurokinin-3-receptor (Nk3R) and its interactions with sex hormones seem important for the modulation of fear memory consolidation: Nk3R antagonism in male mice impairs fear memory, but enhances it in females. However, the involvement of the Nk3R as a modulator of other memories in both sexes remains unexplored.

Methods

We use the novel object recognition paradigm to test the effect of a systemic blockade of Nk3R during memory consolidation. Further, we assess the expression of estrogen receptor α, estrogen receptor β, and androgen receptor and heterodimerization with Nk3R in the medial prefrontal cortex (mPFC) and dorsal hippocampus (DH) of mice.

Results

Nk3R systemic antagonism elicited decreased memory consolidation in males while it enhanced it in females during proestrus. Nk3R analysis in the different subregions of the mPFC and the DH showed a higher expression in males than females. Moreover, females presented upregulation of the androgen receptor in the CA1 and the estrogen receptor beta in the cingulate cortex, CA1, and dentate gyrus. Overall, males presented an upregulation of the estrogen receptor alpha. We also explored the heterodimerization of GCPR membrane sex hormone receptors with the Nk3R. We found a higher percentage of Nk3R-membrane G-protein estrogen receptors heterodimers in the prelimbic cortex of the mPFC in females, suggesting an interaction of estradiol with Nk3R in memory consolidation. However, males presented a higher percentage of Nk3R-membrane G-protein androgen receptors heterodimers compared to females, pointing to an interaction of testosterone with Nk3R in memory consolidation.

Conclusion

These data propose novel ideas on functional interactions between Nk3R, sex hormones, estrogen receptors, and androgen receptors in memory consolidation.

Nk3R antagonism reduces recognition memory consolidation in male mice and increases it in proestrus females.

Androgen receptor expression is higher in the CA1 compared to DG, CA3, and the mPFC.

Estrogen repcetor α expression is higher in males than in females in the DH and mPFC.

Estrogen receptor β expression is greater in females than in males in the DG, CA1, and CG.

Over 60% of Nk3R in the DH and mPFC is heterodimerized with membrane estrogen receptor and androgen receptor.

Nk3R–GPAR is more abundant in males than in proestrus females, whereas Nk3R–GPER is greater in proestrus females compared to males.

A Second Wave for the Neurokinin Tac2 Pathway in Brain Research

Despite promising advances in basic research of the neurokinin B/Tac2 pathway in both animals and humans, clinical applications are yet to be implemented. This is likely because of our limited understanding of the action of the pathway in the brain. While this system controls neuronal activity in multiple regions, the precise impact of Tac2-induced cellular responses on behavior remains unclear. Recently, elegant studies revealed a key contribution to stress-related behaviors and memory. Here, we discuss the crucial importance of bridging the gap between the Tac2 pathway's involvement in cell physiology and cognition to comprehend its role in health and disease. We propose that a better understanding of the Tac2 pathway in the brain could provide an essential perspective for basic investigations, which in turn will feed clinical research.

Conspecific Presence Improves Episodic-Like Memory in Rats

A number of studies have provided evidence that animals, including rats, remember past episodes. However, few experiments have addressed episodic-like memory from a social perspective. In the present study, we evaluated Wistar rats in the WWWhen/ELM task as single setups and in dyads, applying a long retention interval. We also investigated behaviors that could subserve the emergence of this type of memory. We found that only rats tested in the social setting were able to recollect an integrated episodic-like memory that lasted 24 h. Additionally, rats in dyads presented higher levels of exploration during the task. When exposed to the testing environment, the dyads exhibited affiliative behavior toward each other and presented fewer anxiety-like responses. Our findings indicate that the presence of a conspecific could act as a facilitating factor in memory evaluations based on spontaneous exploration of objects and provide empirical support for applying more naturalistic settings in investigations of episodic-like memory in rats.

The medial prefrontal cortex - hippocampus circuit that integrates information of object, place and time to construct episodic memory in rodents: Behavioral, anatomical and neurochemical properties

Rats and mice have been demonstrated to show episodic-like memory, a prototype of episodic memory, as defined by an integrated memory of the experience of an object or event, in a particular place and time. Such memory can be assessed via the use of spontaneous object exploration paradigms, variably designed to measure memory for object, place, temporal order and object-location inter-relationships. We review the methodological properties of these tests, the neurobiology about time and discuss the evidence for the involvement of the medial prefrontal cortex (mPFC), entorhinal cortex (EC) and hippocampus, with respect to their anatomy, neurotransmitter systems and functional circuits. The systematic analysis suggests that a specific circuit between the mPFC, lateral EC and hippocampus encodes the information for event, place and time of occurrence into the complex episodic-like memory, as a top-down regulation from the mPFC onto the hippocampus. This circuit can be distinguished from the neuronal component memory systems for processing the individual information of object, time and place.

Exploring the involvement of Tac2 in the mouse hippocampal stress response through gene networking

Tachykinin 2 (Tac2) is expressed in a number of areas throughout the brain, including the hippocampus. However, knowledge about its function has been only well explored in the hypothalamus in the context of reproductive health. In this study, we identified and validated increased hippocampal Tac2 mRNA expression in response to chronic mild stress in mice. Expression quantitative trait locus (eQTL) analysis showed Tac2 is cis-regulated in the hippocampus. Using a systems genetics approach, we constructed a Tac2 co-expression network to better understand the relationship between Tac2 and the hippocampal stress response. Our network identified 69 total genes associated with Tac2, several of which encode major neuropeptides involved in hippocampal stress signaling as well as critical genes for producing neural plasticity, indicating that Tac2 is involved in these processes. Pathway analysis for the member of Tac2 gene network revealed a strong connection between Tac2 and neuroactive ligand-receptor interaction, calcium signaling pathway, as well as cardiac muscle contraction. In addition, we also identified 46 stress-related phenotypes, specifically fear conditioning response, that were significantly correlated with Tac2 expression. Our results provide evidence for Tac2 as a strong candidate gene who likely plays a role in hippocampal stress processing and neural plasticity.

Episodic-like memory impairment induced by sub-anaesthetic doses of ketamine

Episodic-like memory refers to integration of where and when a certain event (what) happened. The glutamatergic neurotransmission, particularly AMPA and NMDA receptors, in the dorsal hippocampus mediates episodic recall. Ketamine is a non-competitive NMDA antagonist with effect on cognitive performance and plasticity. The goal of this study was to evaluate the acute action of ketamine on behavioural and neurochemical aspects of episodic-like memory (WWWhen/ELM task) through immediate-early gene expression (IEG), c-Fos, in the dorsal hippocampus. Animals received saline 0.9% or ketamine at 8 mg/kg or 15 mg/kg (i.p.) immediately after the second sample. Our data indicate that untreated and saline rats integrate the three elements of episodic-like memory. Conversely, animals treated with ketamine showed impairment of ELM formation. In addition, the highest dose of ketamine increased c-Fos expression in dorsal CA1 subregion when compared to saline rats. Our results indicate that the antagonism of NMDA concurrently impair ELM formation of all three aspects of ELM and increase neuronal activation in dorsal CA1.

Substance P signalling in primary motor cortex facilitates motor learning in rats

Among the genes that are up-regulated in response to a reaching training in rats, Tachykinin 1 (Tac1)-a gene that encodes the neuropeptide Substance P (Sub P)-shows an especially strong expression. Using Real-Time RT-PCR, a detailed time-course of Tac1 expression could be defined: a significant peak occurs 7 hours after training ended at the first and second training session, whereas no up-regulation could be detected at a later time-point (sixth training session). To assess the physiological role of Sub P during movement acquisition, microinjections into the primary motor cortex (M1) contralateral to the trained paw were performed. When Sub P was injected before the first three sessions of a reaching training, effectiveness of motor learning became significantly increased. Injections at a time-point when rats already knew the task (i.e. training session ten and eleven) had no effect on reaching performance. Sub P injections did not influence the improvement of performance within a single training session, but retention of performance between sessions became strengthened at a very early stage (i.e. between baseline-training and first training session). Thus, Sub P facilitates motor learning in the very early phase of skill acquisition by supporting memory consolidation. In line with these findings, learning related expression of the precursor Tac1 occurs at early but not at later time-points during reaching training.

CA1 inactivation impairs episodic-like memory in rats

Episodic memory was initially believed to be unique to humans. However, studies demonstrate that nonhuman species discriminate items based on the triad what, where and when. Here we addressed the role of the dorsal hippocampal subfield CA1 in an integrative what-where-when task in Wistar rats. We performed bilateral inactivation of dorsal CA1 with the GABA_A agonist muscimol previously to the task. As expected, sham-operated animals recollected an integrative memory for objects (what), their places (where) and temporal order (when). However, the inactivation of CA1 impaired the performance of the three components of episodic-like memory. In addition, total time of objects exploration and distance traveled were not different between groups, indicating that rats had similar levels of motivation, thus, alterations in exploration does not account for impaired locomotor performance. Altogether, our data provides evidence that CA1 plays an important role in episodic-like memory.

Amygdala-Dependent Molecular Mechanisms of the Tac2 Pathway in Fear Learning

Recently we determined that activation of the tachykinin 2 (Tac2) pathway in the central amygdala (CeA) is necessary and sufficient for the modulation of fear memories. The Tac2 pathway includes the Tac2 gene, which encodes the neuropeptide neurokinin B and its corresponding receptor neurokinin 3 receptor (NK3R). In this study, using Tac2-cre and Tac2-GFP mice, we applied a combination of in vivo (optogenetics) and multiple in vitro techniques to further explore the mechanisms of action within the Tac2 pathway. In transgenic mice that express ChR2 solely in Tac2 neurons, in vivo optogenetic stimulation of CeA Tac2-expressing neurons during fear acquisition enhanced fear memory consolidation and drove action potential firing in vitro. In addition, Tac2-CeA neurons were shown to co-express striatal-enriched protein tyrosine phosphatase, which may have an important role in regulating Nk3R signaling during fear conditioning. These data extend our current understanding for the underlying mechanism(s) for the role of the Tac2 pathway in the regulation of fear memory, which may serve as a new therapeutic target in the treatment of fear-related disorders.

The mental time travel continuum: on the architecture, capacity, versatility and extension of the mental bridge into the past and future

Mental time travel (MTT) is the ability to remember past events and to anticipate or imagine events in the future. MTT globally serves to optimize decision-making processes, improve problem-solving capabilities and prepare for future needs. MTT is also essential in providing our concept of self, which includes knowledge of our personality, our strengths and weaknesses, as well as our preferences and aversions. We will give an overview in which ways the capacity of animals to perform MTT is different from humans. Based on the existing literature, we conclude that MTT might represent a quantitative rather than qualitative entity with a continuum of MTT capacities in both humans and nonhuman animals. Given its high complexity, MTT requires a large processing capacity in order to integrate multimodal stimuli during the reconstruction of past and/or future events. We suggest that these operations depend on a highly specialized working memory subsystem, ‘the MTT platform’, which might represent a necessary additional component in the multi-component working memory model by Alan Baddeley.

Concurrent assessment of memory for object and place: Evidence for different preferential importance of perirhinal cortex and hippocampus and for promnestic effect of a neurokinin-3 R agonist

We here explore the utility of a paradigm that allows the simultaneous assessment of memory for object (what) and object location (where) and their comparative predominance. Two identical objects are presented during a familiarity trial; during the test trial one of these is displaced, and a new object is presented in a familiar location. When tested 5 or 80min later, rats explored both the novel and the displaced objects more than two familiar stationary objects, indicating intact memory for both, object and place. When tested 24h later rats explored the novel object more than the displaced familiar one, suggesting that forgetting differently influenced object and place memory, with memory for object being more robust than memory for place. Animals that received post-trial administration of the neurokinin-3 receptor agonist senktide and were tested 24h later, now explored the novel and displaced objects equally, suggesting that the treatment prevented the selective decay of memory for location. Next, animals received NMDA lesions in either the perirhinal cortex or the hippocampus, which are hypothesized to be preferentially involved in memory for objects and memory for place, respectively. When tested 5 or 80min later, the perirhinal cortex lesion group explored the displaced object more, indicating relatively deficient object memory, while the hippocampal lesion led to the opposite pattern, demonstrating comparatively deficient place memory. These results suggest different preferential engagement of the perirhinal cortex and hippocampus in their processing of memory for object and place. This preference test lends itself to application in the comparison of selective lesions of neural sites and projection systems as well as to the assessment of possible preferential action of pharmacological agents on neurochemical processes that subserve object vs place learning.

Time decay of object, place and temporal order memory in a paradigm assessing simultaneously episodic-like memory components in mice

A common trait of numerous memory disorders is the impairment of episodic memory. Episodic memory is a delay-dependant memory, especially associating three components, the "what", "where" and "when" of a unique event. To investigate underlying mechanisms of such memory, several tests, mainly based on object exploration behaviour, have been set up in rodents. Recently, a three-trial object recognition task has been proposed to evaluate simultaneously the different components of episodic-like memory in rodents. However, to date, the time course of each memory component in this paradigm is not known. We characterised here the time course of memory decay in adult mice during the three-trial object recognition task, with inter-trial interval (ITI) ranging from 1h to 4h. We found that, with 1h and 2h, but not 4h ITI, mice spent more time to explore the displaced "old object" relative to the displaced "recent object", reflecting memory for "what and when". Concomitantly, animals exhibited more exploration time for the displaced "old object" relative to the stationary "old object", reflecting memory for "what and where". These results provide strong evidence that mice establish an integrated memory for unique experience consisting of the "what", "where" and "when" that can persist until 2h ITI.