The abuse of methylenedioxymethamphetamine (MDMA) during pregnancy is of concern. MDMA treatment of rats during a period of brain growth analogous to late human gestation leads to neurochemical and behavioral changes. MDMA from postnatal day (P)11–20 in rats produces reductions in serotonin and deficits in spatial and route-based navigation. In this experiment we examined the impact of MDMA from P11 to P20 (20 mg/kg twice daily, 8 h apart) on neuronal architecture. Golgi impregnated sections showed significant changes. In the nucleus accumbens, the dendrites were shorter with fewer spines, whereas in the dentate gyrus the dendritic length was decreased but with more spines, and for the entorhinal cortex, reductions in basilar and apical dendritic lengths in MDMA animals compared with saline animals were seen. The data show that neuronal cytoarchitectural changes are long-lasting following developmental MDMA exposure and are in regions consistent with the learning and memory deficits observed in such animals.

Previous research investigating the neural circuitry underlying memory consolidation has primarily focused on single “nodes” in the circuit rather than the neural connections between brain regions, despite the likely importance of these connections in mediating different aspects or forms of memory. This focus has, in part, been due to technical limitations; however the advent of optogenetics has altered our capabilities in this regard, enabling optical control over neural pathways with temporal and spatial precision. The current set of experiments took advantage of optogenetics to control activity in specific pathways connecting brain regions in rats immediately after different kinds of learning. Chapter 2 first established the use of optogenetics to manipulate activity in the basolateral amygdala (BLA), which has been shown to modulate memory consolidation for a variety of types of learning likely through its connections to various downstream regions. Using a one-trial inhibitory avoidance task, a simple and robust fear learning paradigm, we found that both post-training stimulation and inhibition of BLA activity could enhance or impair later retention of the task, respectively. Enhancement was specific to stimulation using trains of 40, but not 20, Hz light pulses. Chapters 3 and 4 examined the projections from the BLA to the ventral hippocampus (VH) and medial entorhinal cortex (mEC) as the BLA’s ability to influence the consolidation for many types of memory is believed to be mediated through discrete projections to distinct brain regions. Indeed, the BLA innervates both structures, and prior studies suggest that the mEC and VH have distinct roles in memory processing related to contextual and nociceptive (footshock) learning, such as those involved in contextual fear conditioning (CFC). Optogenetic stimulation or inhibition of the BLA-VH or BLA-mEC pathway after training on a modified CFC task, in which the nociceptive or emotional stimulus (the footshock) and the context are separated, enabled experimental manipulations to selectively affect the consolidation for learning about one component and not the other. Optogenetic stimulation/inhibition was given to each candidate pathway immediately after the relevant training to determine its role in influencing consolidation for that component of the CFC learning. Chapter 3 results showed that stimulation of the BLA-VH pathway following footshock, but not context, training enhanced retention, an effect that was specific to trains of 40 Hz stimulation. Post-footshock photoinhibition of the same pathway impaired retention for the task. Similar investigations of the BLA-mEC pathway in Chapter 4 produced complementary findings. Post-context, but not footshock, stimulation of the pathway enhanced retention. In this particular case, only trains of 8 Hz stimulation were effective at enhancing retention. These results are the first, to our knowledge, to find that BLA inputs to different structures selectively modulate consolidation for different aspects of learning, thus enhancing our understanding of the neural connections underlying the consolidation of contextual fear conditioning and providing a critical foundation for future research.

Consolidation of associative memories may depend on communication between the lateral entorhinal cortex (LEC) and hippocampus (HPC) for recently learned memories and the LEC and medial prefrontal cortex (mPFC) for remote memories. To determine whether direct connections between these regions are necessary for the retrieval of a recently or remotely learned memory, rats acquired an associative memory through trace eyeblink conditioning and were tested for memory retention after inactivating the regions of interest with the GABAA agonist, muscimol. Inactivating the LEC-HPC connection did not impair memory retrieval. However, inactivating the LEC-mPFC connection impaired remote, but not recent, memory retrieval. Thus, the LEC and mPFC connection is necessary for the retrieval of a remotely, but not recently learned associative memory. Increased reliance on the entorhinal-prefrontal connection indicates the strengthening of functional connectivity between the two regions, which may be a biological correlate for the proposed reorganization during systems consolidation.

Consolidation of associative memories may depend on communication between the lateral entorhinal cortex (LEC) and hippocampus (HPC) for recently learned memories and the LEC and medial prefrontal cortex (mPFC) for remote memories. To determine whether direct connections between these regions are necessary for the retrieval of a recently or remotely learned memory, rats acquired an associative memory through trace eyeblink conditioning and were tested for memory retention after inactivating the regions of interest with the GABAA agonist, muscimol. Inactivating the LEC-HPC connection did not impair memory retrieval. However, inactivating the LEC-mPFC connection impaired remote, but not recent, memory retrieval. Thus, the LEC and mPFC connection is necessary for the retrieval of a remotely, but not recently learned associative memory. Increased reliance on the entorhinal-prefrontal connection indicates the strengthening of functional connectivity between the two regions, which may be a biological correlate for the proposed reorganization during systems consolidation.

The aim of this thesis is to examine the underlying cognitive and neural processes at play during retrieval of temporal and contextual source information. This was assessed across three experimental chapters. In the first experimental chapter, Chapter 2, the neural loci of context associations were assessed. Rats trained on an odour-context association task were given lesions to either the Lateral Entorhinal Cortex (LEC) or sham lesions. After surgery, performance on the odour-context task was assessed. It was hypothesised that memory for previously learned odour-context associations would be impaired following LEC lesions but not sham lesions. The results supported this hypothesis, demonstrating impaired memory for the previously learned odour-context associations in the LEC lesion group compared to the Sham lesion. In Chapter 3, the underlying retrieval processes used to retrieve time and context in human memory was assessed across three experiments. It was hypothesised that time would be remembered accurately using both recollection and familiarity, while correct context memory should rely on recollection alone. Two out of the three experiments supported this hypothesis, demonstrating that temporal information can be retrieved using familiarity in certain instances. The final experimental Chapter 4 used fMRI to extend Chapter 3 and examine whether neural activity would be greater in regions associated with recollection during memory for context, while activity in familiarity-related regions would be higher during memory for time. Results revealed no support for these predictions with no regions linked to recollection showing greater context-related activity, and no regions previously linked to familiarity exhibiting increased activation as temporal information was retrieved. The results are discussed in relation to established recollection and familiarity frameworks and previous work examining the neural substrates supporting memory for time and context.

OBJECTIVE: To determine whether volumetric measures of the hippocampus or entorhinal cortex in combination with other cortical measures can differentiate between cognitively normal individuals and participants with amnestic mild cognitive impairment (MCI). METHODS: T1-weighted magnetic resonance imaging (MRI) data acquired from 46 cognitively normal participants and 50 participants with amnestic MCI as part of the Boston University Alzheimer's Disease Center research registry and the Alzheimer's Disease Neuroimaging Initiative were used in this cross-sectional study. Cortical and subcortical volumes, including hippocampal subfield volumes, were automatically generated from each participant’s structural MRI data using FreeSurfer v6.0. Nominal logistic regression models containing these variables were used to evaluate their ability to identify participants with MCI. RESULTS: A model containing 11 regions of interest (insula, superior parietal cortex, rostral middle frontal cortex, middle temporal cortex, pars opercularis, paracentral lobule, whole hippocampus, subiculum, superior temporal cortex, precentral cortex and caudal anterior cingulate cortex) fit the data best (R2 = 0.7710, whole model test chi square = 102.4794, p < 0.0001). CONCLUSIONS: Volumetric measures acquired from MRI were able to correctly identify most healthy control subjects and those with amnestic MCI using measures of selected medial temporal lobe structures in combination with those from other cortical areas yielding an overall classification of 95.83% for this dataset. These findings support the notion that while clinical features of amnestic MCI may reflect medial temporal atrophy, differences that can be used to distinguish between these two populations are present elsewhere in the brain. This finding further affirming that atrophy can be identified before clinical features are expressed. Additional studies are needed to assess how well other imaging modalities, such as resting state functional connectivity, diffusion imaging, and amyloid and tau position emission tomography (PET), perform in classifying participants who are cognitively normal versus those who are amnestic MCI.

One of the few areas in the brain that still exhibits experience-dependent neuroplasticity in adulthood is found in the medial temporal lobe (MTL) system. Within the MTL, this plasticity has been observed in the hippocampus in both humans and animal models. Rodent model studies focusing on the effect of aerobic exercise have shown a positive increase of neuroplasticity in the dentate gyrus subregion of the hippocampus. Another area in the MTL, the entorhinal cortex (EC), serves as a primary input to the hippocampus, and studies on environmental enrichment have reported greater EC volume in rodents supplied with toys and running wheels. Previous work in our lab working with healthy young adults showed a positive correlation between right EC volume, and aerobic fitness (VO2 max). In this thesis, I examined two aims, first whether aerobic fitness predicts changes in thickness or volume of the MTL as well as performance in an MTL dependent task in healthy young adults. Additionally, whether the brain morphology measures of the MTL can predict performance on the memory task. The second aim looks at the longitudinal effect a 12-week exercise intervention has on thickness or volume in the MTL and performance on an MTL dependent task in the same population. Results indicate that there is a positive baseline correlation between aerobic fitness and thickness of the EC on the left hemisphere but there are no longitudinal changes in morphology after the exercise intervention. These data extend previous work on the effects aerobic exercise has on MTL structure and offer interesting venues to combat neurodegenerative diseases that affect the MTL memory system like Alzheimer’s disease.

Trimethyltin (TMT) is known to produce substantial damage to the hippocampal formation. It also destroys neurons within the entorhinal cortex, thereby causing degeneration of perforant path afferents that terminate in the outer molecular layer (OML) of the dentate gyrus. Surgical destruction of the entorhinal cortex also causes the perforant path to degenerate. This leads to reactive synpatogenesis (axonal sprouting) of septal afferents to the dentate gyrus. The purpose of the present study was to determine whether administration of 6 mg/kg of TMT by gavage to rats would cause axonal sprouting within the septodentate projection. A histochemical stain for acetycholinesterase (AChE) was used. Compared to control subjects rats given TMT exhibited significantly denser AChE staining in the dentate OML. This is putative indication of reactive synaptogenesis within the cholinergic projection to this layer of the dentate and is somewhat surprising because other neurotoxins, such as lead and ethanol, that affect neurons within the hippocampal formation reduce the capacity for reactive synaptogenesis in response to lesions of the entorhinal cortex.

The visual system is able to form relationships across a variety of timescales. These relationships could allow the temporal continuity of the retinal image and the underlying temporal structure of the world to serve as key cues in invariant object recognition (the ability of the visual system to recognize objects across a variety of angles, distances, and other conditions) as well as other visual processes at longer timescales. To utilize this temporal continuity and temporal structure the visual system needs a continuous temporal signal that spans multiple timescales and a computational mechanism for forming relationships across this temporal signal. Two studies (Chapters 2 and 3) showed evidence for a temporal signal that could be used in vision in the monkey brain. Time cells, neurons that fire at particular time intervals relative to a stimulus, could be a component of this temporal signal. Evidence of time cells was found through analysis of neural recording from monkey HPC and PFC during a memory task that requires the monkey to associate visual stimuli separated by about a second in time. After the first stimulus was presented, large numbers of units in both HPC and PFC fired in sequence. Many units fired only when a particular stimulus was presented at a particular time in the past. The temporal information of time cells might originate in another form of temporal coding: temporal context cells. Temporal context cells are neurons that quickly change in firing rate in response to a stimuli then slowly relax back to a baseline firing rate. Evidence of temporal context cells was found by analyzing the temporal responses of neural recordings from the entorhinal cortex of macaque monkeys as they viewed complex images. Many neurons in the entorhinal cortex were responsive to image onset, showing large deviations from baseline firing shortly after image onset but relaxing back to baseline at different rates. This range of relaxation rates allowed for the time since image onset to be decoded on the scale of seconds. Further, these neurons carried information about image content, suggesting that neurons in the entorhinal cortex carry information not only about when an event took place but also the identity of that event. Taken together, these findings suggest that the primate entorhinal cortex uses a spectrum of time constants to construct a temporal record of the past in support of episodic memory. A computational model was implemented that can construct and use this putative temporal record to form relationships across timescales. This model is supported by empirical results in visual experiments at timescales of saccades, seconds, and tens of seconds. At the saccadic timescale, this association across time could be relevant to forming invariant object representations.

Regulation of metabolism is vital to health and lies at the core of many different diseases. The breakdown of metabolisms' regulation within the brain can lead to neurological disease like Alzheimer's Disease (AD). AD is known to affect brain regions responsible for memory and memory processing like the hippocampus and entorhinal cortex. The regulation of these regions' protein quality, synthesis, and degradation deviate from 'normal' or 'healthy' levels when AD is happening. It is known there is a breakdown of regulation in those regions; however, little is known about the specifics of regulation in healthy brains regions or how it changes with disease. Using the sample collection method of microsampling in combination with kinetic proteomics we investigated proteostasis control in regions known to be affected by AD relative to a control region. This provides a baseline for proteins and ontologies found in the proteomes under healthy circumstances. The regions are all the same tissue type; however, since different regions of the brain perform different functions, the metabolism and therefore the regulation of proteostasis are different. By understanding how regional brain proteomes are regulated in young healthy mice, we are prepared for comparisons against diseased tissue in future work.