Elevations in extracellular potassium (K+e) accompany seizure-like events (SLEs), but elevated K+ may also participate in seizure cessation. The objective of this thesis was to investigate the possibility that K+ may undergo a pro- to anti-convulsant switch in the epileptic juvenile (postnatal day 17-21) rat hippocampus. Field recordings were performed in the CA1 pyramidal layer. SLEs and primary afterdischarges (PADs) were induced with 0.25 mM Mg/5 mM K+ perfusion or tetanic stimulation of the Schaffer collaterals respectively. In these seizure models, elevating [K+]e beyond 7.5 mM showed anticonvulsant properties. The addition of ZD7288, a blocker of the hyperpolarization activated nonspecific cationic current (Ih) and allowed SLEs to continue even in elevated [K+]e. This suggests that [K+]e switches from being pro- to anti-convulsant, in part due to an elevated [K+]e-induced potentiation of Ih. Ih likely contributes to this anticonvulsant behavior by decreasing membrane resistance and subsequently attenuating summation of incoming EPSPs.

Since caloric restriction (CR) can modify multiple pathways central to the ischemic cascade and enhance neuroplasticity mechanisms, we hypothesized that CR should exert protective effects following brain ischemia. Previous studies have suggested benefit when CR was administered prior to ischemia. This study investigated whether prolonged CR beginning after global ischemia would result in lasting protection as assessed by performance in the open field, as a measure of functional outcome, and hippocampal CA1 neuronal counts. Adult male Mongolian gerbils were subjected to five minute bilateral carotid artery occlusion (I) or sham surgery (S) with tympanic temperature maintained at 36.5 ± 0.2ºC during the intra-ischemic period. After screening out gerbils with incomplete ischemia, each of the two surgical groups were randomly assigned to control diet (CON) or 30% CR for the duration of the study (60d). Gerbils were tested in the open field on d3, 7, 10, 30 and 60. Ischemic animals on control diet showed a significantly higher level of activity in the open field (impaired habituation) compared to SCON gerbils on all test days (p<0.001). Open field activity was decreased 9% in the ICR group versus ICON gerbils on d7 (p=0.024), suggesting a transient neuroprotective effect. Open field activity of the SCR gerbils began increasing relative to that of SCON gerbils during the last 30 days of the study (p=0.055 on d60), raising the question of suitability of the open field test for long-term studies of CR and ischemia. Brain sections obtained at d60 were stained with hematoxylin & eosin. Hippocampal CA1 neuron counts were reduced 88% by ischemia (p<0.001), and there was no sparing effect of CR. These findings suggest that prolonged CR administered beginning after global ischemia cannot diminish brain injury or enhance long-term recovery.

Preliminary novel research findings indicate that a subset of individuals with moderate to severe traumatic brain injury show bilateral hippocampal atrophy progressing beyond the acute stage post-injury. The present study proposes a novel, integrated model of neuroprotection against subacute hippocampal atrophy (i.e., atrophy occurring beyond the initial 3 months post-injury) via environmental enrichment, drawing on theoretical models and research findings from the fields of environmental enrichment, brain and cognitive reserve, and neuroplastic models of functional recovery from brain injury. Objectives: (a) to examine the relationship between environmental enrichment factors and subacute hippocampal atrophy and (b) to examine the relationship between subacute hippocampal atrophy and return to productivity. Design: Retrospective observational within-subjects. Participants: Patients (N=21) with moderate to severe TBI. Measures: Primary predictors: Self-report ratings of environmental enrichment factors (i.e., hours of cognitive, physical, and social activities, meditation/prayer, and therapy). Primary outcome: hippocampal volume change between 5 months and 24+ months post-injury based on initial and follow-up MRI scans; Brain Injury Community Rehabilitation Outcome Scales-39 (BICRO-39). Results: Generalized environmental enrichment (i.e., an aggregate of cognitive, physical, and social activity) was significantly negatively correlated with subacute bilateral hippocampal atrophy (p<.05). Cognitive activity was the environmental enrichment element that accounted for the greatest degree of variance (32%) in subacute bilateral hippocampal atrophy (p<.01). Frequency of meditation/prayer was significantly negatively correlated with right hippocampal subacute atrophy (<.05) after controlling for socioeconomic status and generalized environmental enrichment. Level of education and pre-injury occupational attainment did not correlate with subacute hippocampal atrophy. Conclusion: Findings suggest that a fixed degree of neural reserve at the time of brain injury may not confer neuroprotection against structural pathology in the manner suggested by the present study’s proposed model of neuroprotection via environmental enrichment. Instead, findings suggest that in order for environmental enrichment to positively modulate susceptibility to subacute hippocampal atrophy post-TBI, environmental enrichment exposure must occur during the subacute phase post-injury rather than prior to injury.

Congenital hypothyroidism (CH) is a pediatric endocrine disorder caused by an insufficiency of thyroid hormone. Despite treatment following newborn screening, CH is associated with persisting memory weaknesses. Given animal research has shown thyroid hormone plays a crucial role in the development of the hippocampus, a brain structure required for normal declarative memory, it is possible that altered hippocampally-dependent processes underlie the memory weakness associated with CH. Previous studies of individuals with CH have found reduced memory abilities and left hippocampal volumes but no study has thoroughly assessed memory abilities or how the hippocampus functions to support memory. Thus, the present study compared individuals with CH and typically developing adolescents using clinical memory tests and two associative memory tasks shown in adults to activate the hippocampus during functional magnetic resonance imaging (fMRI). Results indicated groups did not differ in memory accuracy on clinical measures or either fMRI task. However, fMRI revealed hippocampal activation differed between the groups when performing the associative memory tasks. The first task utilized a visuospatial paired associates novelty detection paradigm to show the CH group increased activation relative to controls in left hippocampus and recruited right hippocampus when controls did not. Since previous research suggested the left hippocampus and verbal memory were more vulnerable to the effects of CH, the second task utilized a verbal paired associates paradigm to demonstrate that when making old and new judgments about associations versus items, the CH group increased activation relative to controls in left hippocampus. Further investigation revealed that when recognizing old associations versus items, the CH group had increased bilateral posterior hippocampal activation whereas controls showed increased activation in right anterior hippocampus, a distinction noted in previous research with this paradigm which suggests individuals with CH may retrieve associations in a less flexible manner than controls. In addition, worse memory performance and increased hippocampal activation, particularly on the left, was predicted by severity of hypothyroidism experienced early in life. In conclusion, these studies demonstrate that early thyroid hormone insufficiency associated with CH alters the functioning of the hippocampus and engenders use of compensatory mechanisms to support associative memory functions.

Elevations in extracellular potassium (K+e) accompany seizure-like events (SLEs), but elevated K+ may also participate in seizure cessation. The objective of this thesis was to investigate the possibility that K+ may undergo a pro- to anti-convulsant switch in the epileptic juvenile (postnatal day 17-21) rat hippocampus. Field recordings were performed in the CA1 pyramidal layer. SLEs and primary afterdischarges (PADs) were induced with 0.25 mM Mg/5 mM K+ perfusion or tetanic stimulation of the Schaffer collaterals respectively. In these seizure models, elevating [K+]e beyond 7.5 mM showed anticonvulsant properties. The addition of ZD7288, a blocker of the hyperpolarization activated nonspecific cationic current (Ih) and allowed SLEs to continue even in elevated [K+]e. This suggests that [K+]e switches from being pro- to anti-convulsant, in part due to an elevated [K+]e-induced potentiation of Ih. Ih likely contributes to this anticonvulsant behavior by decreasing membrane resistance and subsequently attenuating summation of incoming EPSPs.

Preliminary novel research findings indicate that a subset of individuals with moderate to severe traumatic brain injury show bilateral hippocampal atrophy progressing beyond the acute stage post-injury. The present study proposes a novel, integrated model of neuroprotection against subacute hippocampal atrophy (i.e., atrophy occurring beyond the initial 3 months post-injury) via environmental enrichment, drawing on theoretical models and research findings from the fields of environmental enrichment, brain and cognitive reserve, and neuroplastic models of functional recovery from brain injury. Objectives: (a) to examine the relationship between environmental enrichment factors and subacute hippocampal atrophy and (b) to examine the relationship between subacute hippocampal atrophy and return to productivity. Design: Retrospective observational within-subjects. Participants: Patients (N=21) with moderate to severe TBI. Measures: Primary predictors: Self-report ratings of environmental enrichment factors (i.e., hours of cognitive, physical, and social activities, meditation/prayer, and therapy). Primary outcome: hippocampal volume change between 5 months and 24+ months post-injury based on initial and follow-up MRI scans; Brain Injury Community Rehabilitation Outcome Scales-39 (BICRO-39). Results: Generalized environmental enrichment (i.e., an aggregate of cognitive, physical, and social activity) was significantly negatively correlated with subacute bilateral hippocampal atrophy (p<.05). Cognitive activity was the environmental enrichment element that accounted for the greatest degree of variance (32%) in subacute bilateral hippocampal atrophy (p<.01). Frequency of meditation/prayer was significantly negatively correlated with right hippocampal subacute atrophy (<.05) after controlling for socioeconomic status and generalized environmental enrichment. Level of education and pre-injury occupational attainment did not correlate with subacute hippocampal atrophy. Conclusion: Findings suggest that a fixed degree of neural reserve at the time of brain injury may not confer neuroprotection against structural pathology in the manner suggested by the present study’s proposed model of neuroprotection via environmental enrichment. Instead, findings suggest that in order for environmental enrichment to positively modulate susceptibility to subacute hippocampal atrophy post-TBI, environmental enrichment exposure must occur during the subacute phase post-injury rather than prior to injury.

Introduction: Alzheimer’s disease (AD) and dementias constitute a significant public health burden and it is estimated that one in 85 people may be living with AD by 2050. Dementias are a spectrum of diseases with common traits including amyloid protein growth, neurodegradation, neurofibrillary plaque and tangle formation, and which may be influenced by pro- and anti- inflammatory immune mechanisms. Even a modest delay in onset could result in significant reductions in the social and economic burdens of dementias. An important lifestyle factor identified in risk reduction is physical activity (PA). Although the association between dementia risk and PA has been established, the exact physiological mechanisms through which protection occurs are not known. This research consists of two experiments that were designed to explore the effects of physical activity on pro- and anti-inflammatory cytokines and apoptosis in the mouse hippocampus, a brain region implicated in learning, memory, and cognition. Methods: Study #1: Female C57BL/6 mice, 4-5 months of age, were divided into three groups: sedentary controls (NOTREAD) (n = 22), treadmill exercise with immediate sacrifice (TREAD-Imm) (n = 21), or treadmill exercise with sacrifice after 2 hours (TREAD-2h) (n = 20). TNF-α, IL-6, and IL-1β expression in the hippocampus and intestinal lymphocytes were measured by Western blot analysis. Percentages of hippocampal cells undergoing apoptosis (Annexin+) or necrosis (Propidium Iodide+) were determined through flow cytometry. Plasma levels of 8-isoprostane and corticosterone were measured using commercially available EIA kits. Study # 2: Female C57BL/6 mice, 3-4 weeks of age, were assigned to wheel running (WR; n = 20) or a control condition (No WR; n = 22) and sacrificed after the 16 weeks. Data collected included measures of training status (running volume, body weight, run-to-exhaustion time, and skeletal muscle cytochrome c oxidase activity), flow cytometric analysis of hippocampal cell phenotypes and apoptosis (CD45+, CD11b+, Annexin+, Annexin+/PI+, PI+), and cytokine concentrations (TNF-α, IL-1β, IL-12, IL-6, IL-1ra, and IL-10) in cell lysates. Results: Study #1: Acute treadmill exercise lead to significant decreases in TNF-α (p<0.05) and increases in IL-6 (p<0.05) expression in the hippocampus of healthy mice. No effects of acute exercise on the apoptotic status of hippocampal cells were observed. In intestinal lymphocytes, the exercise bout lead to significant increases in TNF-α (p<0.05), IL-6 (p<0.05), and IL-1β (p<0.05). Acute exercise was associated with a significant increase in both plasma 8-isoprostane (p<0.05) and corticosterone (p<0.05) levels. Study #2: WR mice had measurable training effects and significantly lower TNF-α (p<0.05) and higher IL-6 (p<0.05), IL-1ra (p<0.05) and IL-12 (p<0.05) expression in the hippocampus compared to controls. IL-1β, IL-10, and the percent of apoptotic, dead cells, and cell phenotypes did not change due to training. Conclusion: Exercise chronicity (acute vs. chronic), stress characteristics of the exercise (forced vs. voluntary) and tissue location (systemic vs. central) emerged as important variables with effects on both cytokine concentrations and plasma levels of stress hormones. Physical activity may protect the hippocampus against inflammatory damage caused by TNF-α, and the suppression of this cytokine may be due to increased glucocorticoid secretion during acute exercise. It is also proposed that elevated IL-6 expression (central and systemic) may mediate this protection by creating an anti-apoptotic environment in the hippocampus. Less apoptosis may also contribute to maintenance of cognitive function during acute and long-term physical activity.

Since caloric restriction (CR) can modify multiple pathways central to the ischemic cascade and enhance neuroplasticity mechanisms, we hypothesized that CR should exert protective effects following brain ischemia. Previous studies have suggested benefit when CR was administered prior to ischemia. This study investigated whether prolonged CR beginning after global ischemia would result in lasting protection as assessed by performance in the open field, as a measure of functional outcome, and hippocampal CA1 neuronal counts. Adult male Mongolian gerbils were subjected to five minute bilateral carotid artery occlusion (I) or sham surgery (S) with tympanic temperature maintained at 36.5 ± 0.2ºC during the intra-ischemic period. After screening out gerbils with incomplete ischemia, each of the two surgical groups were randomly assigned to control diet (CON) or 30% CR for the duration of the study (60d). Gerbils were tested in the open field on d3, 7, 10, 30 and 60. Ischemic animals on control diet showed a significantly higher level of activity in the open field (impaired habituation) compared to SCON gerbils on all test days (p<0.001). Open field activity was decreased 9% in the ICR group versus ICON gerbils on d7 (p=0.024), suggesting a transient neuroprotective effect. Open field activity of the SCR gerbils began increasing relative to that of SCON gerbils during the last 30 days of the study (p=0.055 on d60), raising the question of suitability of the open field test for long-term studies of CR and ischemia. Brain sections obtained at d60 were stained with hematoxylin & eosin. Hippocampal CA1 neuron counts were reduced 88% by ischemia (p<0.001), and there was no sparing effect of CR. These findings suggest that prolonged CR administered beginning after global ischemia cannot diminish brain injury or enhance long-term recovery.

Understanding the neural mechanisms underlying the extinction of maladaptive behaviors has become increasingly relevant. Extinction, or the reduction of a response due to lack of reinforcement, is believed to be “new learning.” Most extinction paradigms involve the performance of the previously reinforced response in the absence of reinforcement in order for extinction to occur. Conversely, latent extinction is a cognitive form of learning in which the previously rewarded response is not made during extinction training. However, until now the neurobiological basis of latent extinction has remained unknown. This dissertation has three aims to examine the neurobiological basis of latent extinction. Previous research has shown latent extinction to be impaired following hippocampal inactivation and the goal of Aim 1 was to examine other neural systems potentially involved in latent extinction through examination of brain structures such as the dorsal striatum, medial prefrontal cortex, and basolateral amygdala. Additionally, the neurochemical basis of latent extinction is unidentified; therefore Aim 2 addressed this question, specifically investigating the glutamatergic system through both NMDA receptor agonism and antagonism. Finally, understanding latent extinction may be useful for the extinction of drug addiction. Aim 3 was to examine some clinical implications for the extinction of drug addiction utilizing latent extinction following maze running for an oral cocaine reward. Reversible neural inactivation studies using the sodium channel blocker bupivacaine demonstrated a selective impairment of response extinction following dorsal striatum inactivation, but no effect on either latent or response extinction following medial prefrontal cortex or basolateral amygdala inactivation. These results, coupled with previous data from our lab demonstrate a double dissociation for extinction behavior. Further, peripheral NMDA receptor agonism with D-cyloserine enhances latent extinction and intra-hippocampal NMDA receptor antagonism with AP5 impairs latent extinction, identifying a role for the glutamatergic system in latent extinction. Finally, oral cocaine administration during acquisition selectively impairs latent extinction indicating that drug use affects the relive use of multiple memory systems during extinction. Overall, the multiple memory systems theory and latent extinction provide a framework with which to further understand the neural mechanisms of extinction behavior.

Thesis (M.Sc.)--Memorial University of Newfoundland, 2001. / Pages 250-291 are duplicates of pages 328-369. Pages 292-327 are non existant. Table of contents (page v) does not refer to pages 250-291 but does refer to pages 328-369. Bibliography: leaves 117-128.