Browsing School, Graduate by Subject "glutamate"
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Hormonal modulation of neuronal-glial interactions in the arcuate nucleus of the adult female ratMorphological 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.
Prefrontal kappa-opioid regulation of local neurotransmissionKappa-opioid receptors (KORs) are mediators of motivational processes, mood/emotion, and stress reactivity. KORs are enriched in brain regions that mediate such behaviors, including the medial prefrontal cortex (mPFC). The mPFC guides behavior through its connections with limbic brain regions such as the amygdala, hippocampus, and ventral tegmental area. A critical knowledge gap exists in our understanding of the role of KOR systems in modulating mPFC afferents. Thus, we designed a series of studies to understand the role of KORs in regulating dopamine (DA) and limbic glutamatergic afferents to the mPFC. First, we determined the role of KOR systems in regulating neurotransmission in the mesocortical DA pathway by utilizing a combination of neurochemical techniques in rats and in transgenic mice. We found that mPFC KOR activation and antagonism decreased and increased DA output, respectively. Genetic ablation of KOR in DA neurons abolished the inhibitory effect of mPFC KOR signaling on local DA output. These findings suggest that mPFC KORs negatively modulate the mesocortical DA pathway by directly acting on DA varicosities in the mPFC. Second, we examined the role of mPFC KOR systems in regulating mPFC extracellular glutamate and glutamatergic limbic inputs utilizing neurochemical and electrophysiological techniques in rats. We found that extracellular glutamate and glutamate-driven GABA levels were inhibited by mPFC KORs. mPFC KOR activation inhibited synaptic transmission in the BLA to mPFC pathway in a manner not overcome by BLA burst stimulation. KORs do not inhibit the hippocampus to mPFC pathway. These findings show that mPFC KORs inhibit glutamatergic afferents in a pathway-specific manner. Lastly, we characterized heterosynaptic interactions between the BLA and hippocampus in the mPFC, and determined the role of KORs in these interactions. BLA train stimulation inhibits hippocampal inputs to the mPFC in an activity-dependent manner, while hippocampus stimulation is without effect on BLA-evoked responses. KOR antagonism does not modify BLA-evoked heterosynaptic suppression, suggesting that BLA-evoked heterosynaptic suppression is KOR-independent. This series of studies demonstrate that mPFC KORs regulate mesocortical DA and limbic pathways, but may not be involved in limbic interactions. These findings provide a biological framework whereby mPFC KOR signaling alters mPFC-dependent behaviors.
Serotonergic Modulation of Glutamate Transmission: Bridging Two Theories of DepressionMajor Depression is a debilitating disease affecting close to 10% of the world population. Although there are many antidepressant drugs available, 30% of patients are unresponsive and the rest must wait 3-4 weeks for therapeutic efficacy. The serendipitous discovery of these drugs in the 1950s initiated the monoamine theory of depression. This theory postulates that depression is caused by a depletion of synaptic serotonin and that effective therapies work by restoring this imbalance. However, the many gaps in this theory together with inefficient therapies have led to the search for a new explanation of the pathology of depression. Recent data has implicated a dysfunction of the glutamate system to be at fault for the symptoms of depression. Both of these theories have been developed independently, however I propose that they are not in fact distinct but may actually complement each other. Therefore I hypothesized that glutamatergic dysfunction underlies the etiology of depression, but that serotonin is capable of modulating glutamatergic transmission in a manner that rescues this defect. I investigated the effect of serotonin elevation on glutamate transmission and how this phenomenon may be altered in an animal model of depression. I found that elevation of endogenous serotonin activates 5-HT1BRs which in turn signal to phosphorylate glutamatergic AMPA receptors. This potentiation of the glutamatergic response is enhanced in animals subjected to chronic unpredictable stress and absent in na?ve animals chronically treated with antidepressants. This finding pointed to a decrease in basal glutamatergic transmission in depressed animals, which I confirmed by measuring AMPAR/NMDAR ratios. Finally, I found that activation of 5-HT1BRs and subsequent phosphorylation of the GluA1 subunit of the AMPAR is necessary for the therapeutic effects of antidepressants, and that phosphorylation of S831 is necessary for normal basal affective state in a number of behavior measures. Together my data present a novel pathway through which 5-HT1BR activation can specifically enhance AMPAR function in the hippocampus and provide a connection between two previously disparate theories of depression. These findings provide insight into the locus of dysfunction in depression and also point to new potential targets in the treatment of this disease.