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In this thesis, I describe the use of two different techniques for the targeting and functional inactivation of individual populations of GABAergic interneurons located within the cerebellum and dentate gyrus of the hippocampus. Through functional inactivation of these interneuron types, I demonstrate their behavioural relevance for the processes of learning and memory. In chapter 2, I describe a genetic approach for the removal of GABAA-mediated signalling from molecular layer interneurons (MLIs) onto Purkinje cells within the cerebellum. Using the Cre lox P system to delete post-synaptic GABAA receptors on Purkinje cells, I have shown a previously unappreciated role for MLIs in fear memory. Deficits were specific to the acquisition and long-term retention of fear memories suggesting that feed-forward inhibition from MLIs onto Purkinje cells is critical for these processes. In chapter 3, I describe a further development to this project through the creation of a novel dual recombinase mouse line. This intersectional approach of combining Cre and Flpo recombinase systems together would allow direct targeting of MLIs for the first time and circumvent drawbacks associated with using a static genetic knockout approach. In chapter 4, I describe an adeno-associated viral (AAV) approach to target a specific population of GABAergic interneurons located within the hilar region of the dentate gyrus. These hilar perforant path-associated (HIPP) cells are characterised by their expression of the neuropeptide somatostatin (SST) and regulate granule cell activity through feedback inhibition. However, up until now their behavioural relevance has been unknown. Through Cre-mediated viral expression of tetanus toxin light chain (TeLC), neurotransmission was prevented in SST interneurons revealing an involvement in spatial working memory and spatial reference memory precision. In addition, preliminary immediate early gene data suggests that SST interneurons increase memory precision through maintaining the sparse activity of the granule cell population through feedback inhibition.

Rationale: Lead-exposure during developmental periods may alter reinforcing patterns of drugs of abuse in adulthood. Anxiety related mechanisms may also influence drug intake. Interactions between the two altering factors may exist. Objectives: The present study examined the effects of perinatal lead-exposure on cocaine self-administration after a GABAA antagonist pre-treatment. Methods: Female rats were exposed to a regimen of 16 mg lead daily for 30 days prior to breeding with un-exposed males. This continued throughout gestation and lactation until postnatal day (PND) 21. On PND 63, animals were implanted with indwelling jugular catheters. After a 7 day recovery period, animals were trained to self-administer 0.50 mg/kg cocaine intravenously [IV]. After stable responding had been established, testing procedures began using combinations of 0.03 and 0.06 mg/kg cocaine [IV] and 0.00, 0.50, 1.00 and 2.00 mg/kg bicuculline (a GABAA antagonist) intraperitoneal [IP]. Results: Bicuculline pre-treatment caused directionally opposite effects in both treatment groups (Group 0-Lead and Group 16-Lead) at the 0.06 mg/kg cocaine dose. Group 0-Lead animals showed an increase in self-administration, while Group 16-Lead animals showed a decrease in responding on the active (cocaine) lever. Results at the 0.03 mg/kg cocaine dose showed no discernable pattern. Group 0-Lead animals decreased in active lever responding at the 2.00 mg/kg bicuculline dose. Group 16-Lead animals showed no differences in responding at any dose of bicuculline. Conclusions: These data further suggest the influential role of GABA in mediating cocaine reward and the ability of developmental lead-exposure to alter mechanisms mediating drug responsiveness even after exposure has terminated.

In the mature dentate gyrus (DG) new neurons are continuously produced in a process known as adult neurogenesis. These neurons are thought to contribute to learning and memory processes, but their precise function is still not fully understood. The rate of neuronal production and the development of young neurons are affected by stimuli such as physical exercise and learning events. The neurotransmitter γ-aminobutyric acid (GABA) has recently emerged as a possible key mediator of this activity-dependent regulation. My study examined the role of a subtype of GABAA receptors that contains the δ subunit (δGABA(A)R) in the regulation of adult hippocampal neurogenesis. Mice lacking the δ subunit (Gabrd−/− or δ-null mice) displayed decreased maturation, migration and dendritic complexity of adult-born neurons. Conversely, following treatment with a selective δGABA(A)R agonist, neuronal maturation was promoted in wild-type, but not δ-null mice. These results indicate a key role for δGABA(A)R in activity-dependent regulation of adult neurogenesis. When administered to rats, δGABA(A)R agonists promoted neuronal survival as well as maturation. The effect on maturation was blocked by the N-Methyl-D-aspartate (NMDA) blocker AP-5, suggesting that some aspects of δGABA(A)R-mediated regulation require the activation of NMDA receptors. To further understand the contribution of adult born neurons to memory function, neurogenesis in rats was alternatively suppressed using ionizing radiation, or enhanced by allowing the rats to engage in running. The rats were then trained and tested in a behavioral paradigm designed to assess their susceptibility to memory interference, a phenomenon which occurs when similar memories are not sufficiently distinguished from one another. Irradiated rats exhibited increased susceptibility to memory interference, indicating an important role for adult-born neurons in the encoding and/or retrieval of distinct memories. Remarkably, irradiated rats that engaged in running exhibited increased neuronal growth and a complete reversal of the memory impairment,emphasizing the beneficial effects of physical exercise on cognitive functions. Taken together, the findings reported in this dissertation offer novel information about the process of adult neurogenesis and its physiological significance.

The goal of this research project was to synthesize a small library of 3- and 5-arylisoxazoles. These compounds are of interest because of potential biological activity similar to Fipronil. Fipronil is used commercially in the agrochemical industry and exhibits pesticidal activity as a noncompetitive inhibitor of the GABA receptor. By deleting the amino group normally at the 5-position and the cyano group normally at the 3-position and changing the atoms in the heterocyclic ring from containing two nitrogen atoms to one nitrogen and one oxygen atom, we hope to create changes in the binding so the geometry of the GABA receptor may be better understood.The synthesis of our target compounds consisted of many steps. First, brominated intermediates were made from commercially available compounds. The brominated intermediates were converted to aldehydes, which then produced oximes. The oximes were then combined with alkynes through a 1,3-dipolar cycloaddition to form the arylisoxazoles. / Department of Chemistry

In the mature dentate gyrus (DG) new neurons are continuously produced in a process known as adult neurogenesis. These neurons are thought to contribute to learning and memory processes, but their precise function is still not fully understood. The rate of neuronal production and the development of young neurons are affected by stimuli such as physical exercise and learning events. The neurotransmitter γ-aminobutyric acid (GABA) has recently emerged as a possible key mediator of this activity-dependent regulation. My study examined the role of a subtype of GABAA receptors that contains the δ subunit (δGABA(A)R) in the regulation of adult hippocampal neurogenesis. Mice lacking the δ subunit (Gabrd−/− or δ-null mice) displayed decreased maturation, migration and dendritic complexity of adult-born neurons. Conversely, following treatment with a selective δGABA(A)R agonist, neuronal maturation was promoted in wild-type, but not δ-null mice. These results indicate a key role for δGABA(A)R in activity-dependent regulation of adult neurogenesis. When administered to rats, δGABA(A)R agonists promoted neuronal survival as well as maturation. The effect on maturation was blocked by the N-Methyl-D-aspartate (NMDA) blocker AP-5, suggesting that some aspects of δGABA(A)R-mediated regulation require the activation of NMDA receptors. To further understand the contribution of adult born neurons to memory function, neurogenesis in rats was alternatively suppressed using ionizing radiation, or enhanced by allowing the rats to engage in running. The rats were then trained and tested in a behavioral paradigm designed to assess their susceptibility to memory interference, a phenomenon which occurs when similar memories are not sufficiently distinguished from one another. Irradiated rats exhibited increased susceptibility to memory interference, indicating an important role for adult-born neurons in the encoding and/or retrieval of distinct memories. Remarkably, irradiated rats that engaged in running exhibited increased neuronal growth and a complete reversal of the memory impairment,emphasizing the beneficial effects of physical exercise on cognitive functions. Taken together, the findings reported in this dissertation offer novel information about the process of adult neurogenesis and its physiological significance.

The mechanisms underlying the transition to seizure are still unresolved. Proposed mechanisms include excitatory GABAergic drive, loss of interneuron-mediated inhibition, and glutamatergic input potentiation. The objective of this thesis was to investigate the relative contributions of synchronized glutamatergic and GABAergic inputs and their functional roles during ictogenesis in the epileptic neonatal (postnatal days 6-12) mouse hippocampus, induced with 0.25mM Mg2+/5mM K+ perfusion. Simultaneous field and whole-cell patch-clamp recordings were obtained from CA3 stratum-oriens interneurons and pyramidal cells. The antagonists for GABAA and glutamate receptors abolished the preictal and ictal discharges, respectively, suggesting that the preictal state is mediated by the coherent discharges of GABAergic inhibitory interneurons, whereas the recurrent excitatory inputs are required for ictogenesis. Synaptic charge transfers underlying the synchronized discharges showed a dynamic change in the balance between the inputs: GABAergic currents markedly diminished by ictal onset whereas glutamatergic currents dominated at ictal onset and throughout the ictus.

The mechanisms underlying the transition to seizure are still unresolved. Proposed mechanisms include excitatory GABAergic drive, loss of interneuron-mediated inhibition, and glutamatergic input potentiation. The objective of this thesis was to investigate the relative contributions of synchronized glutamatergic and GABAergic inputs and their functional roles during ictogenesis in the epileptic neonatal (postnatal days 6-12) mouse hippocampus, induced with 0.25mM Mg2+/5mM K+ perfusion. Simultaneous field and whole-cell patch-clamp recordings were obtained from CA3 stratum-oriens interneurons and pyramidal cells. The antagonists for GABAA and glutamate receptors abolished the preictal and ictal discharges, respectively, suggesting that the preictal state is mediated by the coherent discharges of GABAergic inhibitory interneurons, whereas the recurrent excitatory inputs are required for ictogenesis. Synaptic charge transfers underlying the synchronized discharges showed a dynamic change in the balance between the inputs: GABAergic currents markedly diminished by ictal onset whereas glutamatergic currents dominated at ictal onset and throughout the ictus.

Thesis (Ph.D.) -- University of Adelaide, Dept. of Medicine, 1999. / Bibliography: leaves 181-233.

Thesis (Ph. D.)--Auburn University, 2008. / Abstract. Vita. Includes bibliographical references.