The Anterior Retrosplenial Cortex and Recognition of Objects, Locations, and Object-in-Place Associations: Exploring Short-term Memory Retrieval in Male and Female Long Evans Rats
Date
2025-04-22
Authors
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Journal ISSN
Volume Title
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ORCID
0000-0002-9797-1676
Type
Thesis
Degree Level
Doctoral
Abstract
Associative recognition learning is a complex process involving encoding, storage, and retrieval of integrated multimodal sensory information. The ability to retrieve information regarding the past is foundational for learning and memory, including an array of cognitive functions important for survival and navigation in uncertain environments. In recent decades, the retrosplenial cortex (RSC) has gained increased attention for its role in multimodal sensory integration and associative recognition learning, specifically in rodent models. In this thesis, a series of experiments were conducted using common behavioral recognition paradigms to further investigate the role of the anterior RSC (aRSC) in short-term recognition retrieval, in male and female long Evans rats. In the first round of experiments (reported in Chapter 2), we tested the role of the aRSC in short-term visuospatial recognition retrieval, using the 1-hour delay object-in-place (1-h OiP) test. Findings showed that naïve male and female Long Evans rats performed equally in up to three repeated OiP test sessions. Next, bilateral cannulae were implanted into the aRSC of male and female rats prior to OiP testing, facilitating intracranial aRSC drug infusions prior to OiP test phases. Following temporary inactivation of the aRSC by infusing a mixture of GABAA and GABAB receptor agonists, significant OiP retrieval deficits were observed in both sexes. Similarly, OiP retrieval deficits were observed in both sexes following infusion of AP-5 (NMDA receptor antagonist) or CNQX (AMPA receptor antagonist) into the aRSC. Findings highlighted a key role for ionotropic glutamate receptors in the aRSC for the retrieval of short-term associative (e.g., visual-spatial) recognition retrieval. The next set of experiments (reported in Chapter 3) explored the role of CaMKIIa-expressing neurons in the aRSC during 1-hour recognition retrieval in the object recognition (OR), object location (OL), and OiP tests. In these experiments, stereotaxic surgery and chemogenetic approaches were used to infuse control (AAV5-CaMKIIα-mCherry) or active (AAV5-CaMKIIα-hM4D(Gi)-mCherry) viral vectors bilaterally into the aRSC of male and female Long Evans rats, enabling selective inhibition of neurons with the inhibitory designer receptor exclusively activated by designer drugs (DREADD) hM4D(Gi), using the DREADD agonist Compound 21 (C-21). Findings revealed that rats infused with the active inhibitory DREADD and treated with C-21 prior to test phases were significantly impaired in short-term OR retrieval, but not OL retrieval, regardless of sex. Interestingly, treatment with C-21 in rats treated with the inhibitory DREADD only impaired OiP recognition retrieval in male rats, highlighting a nuanced role for CaMKIIα-expressing neurons in the aRSC that varies by sex and type of memory being tested. In the final experiment (reported in Chapter 4), sex differences in exploration and search strategy were further characterized in OiP test phases (e.g., habituation, sample, test). Using animal behavioral tracking software, a variety of behavioral metrics (i.e., velocity, inner versus outer zone behavior, mobility traits) were compared between male and female rats. Findings suggested that, while unique sex differences in exploration strategy were present in OiP phases (e.g., females moved faster and farther than males), those differences were not predictive of recognition performance, suggesting behavioral sex differences may indicate motivation and search strategy, rather than recognition capacity. In summary, while male and female rodents exhibited unique explorative behavior during recognition test phases, both sexes performed well in repeated recognition tests and critically relied on the aRSC for OiP retrieval. Furthermore, glutamatergic signalling in the aRSC was shown to be important for short-term OR retrieval, but not OL retrieval, in both sexes; however, males increasingly relied on CaMKIIα-expressing neurons for 1-h OiP retrieval. Thus, glutamatergic signalling in the aRSC is important for short-term recognition retrieval, but depends on a variety of factors including biology, context, and memory test.
Description
Keywords
object-place, recognition memory, short-term retrieval, NMDA receptor, AMPA receptor, sex differences, chemogenetic inhibition, DREADD, hM4D(Gi), Compound 21, rodent, locomotor activity, exploration, search strategy, object interaction
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Anatomy, Physiology, and Pharmacology
Program
Anatomy, Physiology, and Pharmacology