• Activity-dependent short-term depression of inhibitory synaptic currents in the hippocampus

      Whittington, Kevin C.; Alger, Bradley Eugene (2010)
      An understanding of the regulation of GABAergic synaptic inhibition is important because of its influence on neuronal excitability, synaptic plasticity, and disease processes in the central nervous system. Aberrant inhibition is believed to be the cause of pathophysiological processes, such as epilepsy and mood disorders (e.g. anxiety). I seek to determine what regulates inhibitory transmission at short time scales (milliseconds - minutes) in the hippocampus, the structure that provides a cognitive map of the physical environment and, in addition, is the locus of explicit memory formation. I use electrophysiological and pharmacological tools to study inhibitory transmission at certain interneuron-pyramidal neuron synapses in the CA1 field of the rat hippocampus. Certain interneurons, those that contain cholecystokinin, also express the presynaptic cannabinoid receptor (CB1) as well as the GABAB autoreceptor, both key mediators of presynaptic inhibition. Hippocampal interneurons can fire in rhythmic, brief bursts. However, the roles of endogenous cannabinoids (endocannabinoids; eCBs) and GABAB autoreceptors in regulating inhibitory postsynaptic currents (IPSCs) elicited by such bursts has not been described. The fundamental hypothesis is that certain features of the short-term depression induced by these bursts are mediated by eCBs and others by GABA acting on GABAB autoreceptors. I find that eCB-mediated depression of pyramidal cell IPSCs develops slowly (tau ~ 30 s). IPSC depression mediated by the GABAB autoreceptor occurs in parallel with the eCB-mediated depression, but it is restricted to the time domain of hundreds of milliseconds (tau ~ 100 ms). Interestingly, although CB1 and GABAB receptors are expressed on the same nerve terminals, have the same effector (the Gi/o G protein), and the same molecular targets (Ca2+ and K+ channels), they reduce inhibitory transmission via non-interacting and distinct mechanisms. I also discovered that eCBs are released from cells via a novel mechanism. I conclude that a new form of short-term depression mediated by eCBs is present at certain inhibitory synapses of the hippocampus, and may help understand the functions of hippocampal neuronal circuits.
    • Hormonal modulation of neuronal-glial interactions in the arcuate nucleus of the adult female rat

      Blutstein, Tamara; Mong, Jessica Aurora (2009)
      Morphological plasticity in response to estradiol is a hallmark of astrocytes in the arcuate nucleus, with estradiol causing an increase in astrocytic surface area. The increase in surface area correlates with changes in synaptic patterns in both the developing and adult brain. The functional consequences of these morphological changes have remained relatively unexplored. The data presented here demonstrate a novel role for estradiol in modulating neuronal-glial interactions in the arcuate nucleus and its implications in the regulation of feeding behavior. Estradiol is a potent inhibitor of food intake, but the mechanisms of this phenomenon are largely unknown; however, it is clear that the arcuate nucleus is a key feeding nuclei and amino acid neurotransmission may be important. The glutamate-glutamine cycle is a classic example of a neuronal-glial cooperation, which is essential to the maintenance of synaptic transmission. We have shown that in the arcuate nucleus estradiol increased the protein levels of the two enzymes in the glutamate-glutamine cycle, glutamine synthetase (GS) and glutaminase. These changes in enzyme protein were thought to underlie functional changes in neurotransmitter availability as (1) total glutamate concentration in the arcuate nucleus is significantly increased and (2) microdialysis revealed a significant increase in extracellular glutamate levels following a synaptic challenge in the presence of estradiol. However, in vivo enzyme activity assays demonstrated that the estradiol mediated increase in glutamate-glutamine cycle enzymes in the arcuate nucleus led to an increase in GABA production and is likely not related to the increase in extracellular glutamate. Therefore, we have observed two independent effects of estradiol on amino acid neurotransmission in the arcuate nucleus, which we hypothesized to be involved in the regulation of food intake. To test this hypothesis we measured the food intake of ovariectomized adult female rats treated with estradiol or oil vehicle and either saline or MSO, a specific inhibitor of GS activity. Inhibition of GS activity accelerated and prolonged the well-established estradiol-mediated decrease in food intake, demonstrating a role for neuronal-glial cooperation in the regulation of hypothalamic function.
    • The role of fast-spiking parvalbumin interneurons in prefrontal mediated cognition

      Bissonette, Gregory; Powell, Elizabeth M. (Elizabeth Mary) (2011)
      In a constantly changing environment, the ability to shift from one learned behavioral strategy to another more adaptive strategy is imperative. Research suggests there may be common underlying causes for the similar cognitive etiologies observed in many psychiatric disorders. One of these causes appear to be alterations in cortical GABAergic tone in the prefrontal cortices, particularly in the Orbital Frontal Cortex (OFC) which is known for its role in reversal learning and the Medial Prefrontal Cortex (mPFC) which mediates a form of behavioral flexibility. We tested a mouse model of defective frontal lobe inhibitory GABAergic anatomy on cognitive tasks, including a mouse reversal/set-shift test and fear-conditioning paradigm. We used several lines of mice: a mouse lacking the urokinase plasminogen activator receptor (uPAR) gene with a decreased GABA interneuron phenotype, a hepatocyte growth factor/scatter factor (HGF/SF) overexpressing mouse (Gfap-HGF), and a cross between the uPAR-/- and the Gfap-HGF mice, in which the interneuron deficit appears to be corrected. We have also developed a mouse serial reversal task in which we can record in vivo single unit activity in awake behaving animals, to evaluate murine OFC function during reversal learning. Further, we have studied the role developmental alterations to cortical GABAergic tone play in reversal learning. Using a transgenic animal model to produce a specific frontal cortical GABAergic deficit in adult mice, we have assessed reversal learning through behavioral and in vivo psychological techniques, using single cell and local field potential recordings. By studying genetically altered mice, our research illuminated a common neural substrate between mouse circuitry and behavior and human cortical function in psychiatric disease states. We have shown that mice have functional and dissociable prefrontal cortical structures that match rat, primate and human data. We have shown that GABAergic deficits specific to PV+ interneurons impact prefrontal mediated cognition and that OFC and mPFC cortices are differentially sensitive to growth factor alterations. We further showed that high frequency oscillations are reduced in Plaur mice performing a serial reversal task, and that murine OFC plays a critical role in mediating behavioral flexibility in a first, but not subsequent reversals.
    • The role of urokinase plasminogen activator receptor (uPAR) on forebrain GABAergic interneuron development

      Bae, Mihyun; Powell, Elizabeth M. (Elizabeth Mary) (2009)
      Although GABAergic interneurons are only a small portion of neurons in the cerebral cortex, they have powerful influence on excitatory neurons and the overall function of neural networks. The disruption of GABAergic interneuron development causes an imbalance between excitation and inhibition and, consequently, a perturbation in overall circuitry. It has been reported that altered interneuron development causes a number of neurological disorders including epilepsy. Studies about the molecular mechanisms involved in GABAergic interneurons development are important to understand the mechanisms underlying neurological disorders. When the urokinase type plasminogen activator receptor (uPAR, gene symbol: Plaur) is genetically eliminated, migration of GABAergic interneurons is altered embryonically. This defect has been attributed to the reduction of hepatocyte growth factor/scatter factor (HGF/SF) levels. In the adult, lack of uPAR causes the reduction of GABAergic interneurons in cerebral cortex and consequently, increased seizure susceptibility and enhanced anxiety. Here I investigate the role of uPAR on postnatal GABAergic interneurons and if the GABAergic interneuron defects in the absence of Plaur can be rescued by postnatal supplementation of HGF/SF. Loss of uPAR alters HGF/SF levels and affects cell proliferation, maturation, and survival during postnatal GABAergic interneuron development in the somatosensory cortex. The defects of GABAergic interneurons are also found in rostral part of forebrain, including the striatum and frontal cortical areas. Postnatal supplementation of HGF/SF can recover some of the GABAergic interneuron development defects, and GABAergic interneurons appear to be properly functional. My thesis work suggests that uPAR regulates postnatal GABAergic interneuron development in the forebrain as a regulator of HGF/SF levels. This molecular mechanism is important to develop potential therapeutic methods for epilepsy.