Browsing by Subject "NMDA receptors"

Now showing 1 - 5 of 5
  • Thumbnail Image
    Item
    Astrocyte beta-adrenergic receptor activity regulates neuronal NMDA receptor signaling
    (2023-06) Del Franco, Armani
    Glutamate spillover from the synapse is tightly regulated by astrocytes, limiting the activation of extrasynaptically located NMDA receptors (NMDAR). The processes of astrocytes are dynamic and can affect synaptic physiology. Though norepinephrine (NE) and β-adrenergic receptor (β-AR) activity can modify astrocyte volume, this has yet to be confirmed outside of sensory cortical areas, nor has the effect of noradrenergic signaling on glutamate spillover and neuronal NMDAR activity been explored. We monitored changes to astrocyte process volume in response to noradrenergic agonists in the medial prefrontal cortex of male and female mice. Both NE and the β-AR agonist isoproterenol (ISO) increased process volume by 18%, significantly higher than changes seen in aCSF or when astrocytes had G-protein signaling blocked by GDPβS. We then measured the effect of β-AR signaling on evoked NMDAR currents. While ISO did not affect single stimulus excitatory currents of Layer 5 pyramidal neurons, ISO reduced NMDAR currents evoked by 10 stimuli at 50 Hz, which elicits glutamate spillover, by 18%. After isolating extrasynaptic NMDARs by blocking synaptic NMDARs with the activity-dependent NMDAR blocker MK-801, ISO similarly reduced extrasynaptic NMDAR currents in response to 10 stimuli by 16%. Finally, blocking β-AR signaling in the astrocyte network, by loading them with GDPβS, reversed the ISO effect on 10 stimuli-evoked NMDAR currents. These results demonstrate that astrocyte β-AR activity reduces glutamate spillover and extrasynaptic NMDAR recruitment.
  • Thumbnail Image
    Item
    D-serine: a study of its function and regulation in the retina.
    (2009-10) Gustafson, Eric Charles
    Activation of the NMDA-type glutamate receptor requires the simultaneous binding of both glutamate and a coagonist, either glycine or D-serine. In the inner retina, glutamate released from bipolar cells excites NMDA receptors on retinal ganglion cells and some amacrine cells. The identity of the coagonist, however, has remained unknown. Early on, the relatively high levels of glycine and its use in the retina as an inhibitory transmitter by a subset of amacrine cells led many to believe that glycine was the endogenous coagonist. The discovery that D-serine and its synthesizing enzyme, serine racemase, are both present in the retina suggested that D-serine may play a role as well. This manuscript reports results that have examined the role of D-serine in the retinas of larval tiger salamanders and in mice. These studies suggest that D-serine is the major endogenous coagonist during light-induced responses in the inner retina of both species. In addition, the regulatory mechanisms of glycine transport and of the endogenous D-serine degrading enzyme, D-amino acid oxidase, have been shown to be essential in maintaining coagonist levels below that needed to saturate the NMDA receptor. Together, the results position D-serine as a major contributor and potential modulator of excitatory neurotransmission in the retina.
  • Thumbnail Image
    Item
    HIV gp120-induced synaptic changes -- modulation by the endocannabinoid system and NMDARs
    (2019-10) Zhang, Xinwen
    37.9 million people worldwide are living with HIV. Nearly half of these individuals develop HIV-associated neurocognitive disorder (HAND). Symptoms range from subclinical cognitive deficits to severe dementia that impairs daily function and may cause death. Combination antiretroviral therapy (cART) has significantly decreased the incidence of severe HIV dementia and encephalitis; however, HAND prevalence remains high and may be increasing due to the prolonged life span of HIV patients. Currently, there is no effective treatment for improving cognitive function in HAND patients. Thus, understanding the mechanism and possible avenues for modulating HIV-induced cognitive decline is important. HIV neurotoxicity is mainly mediated by factors released by infected cells. The HIV envelope protein, gp120, is shed by infected cells and is a potent neurotoxin causing excitotoxicity and loss of excitatory synapses. Synaptic damage is reversible and correlates with cognitive decline in HAND patients. In this dissertation, I explored mechanisms of HIV gp120-induced changes in the number of excitatory and inhibitory synapses. Balance between excitatory and inhibitory synapses is important for controlling network excitability and maintaining normal neural function. The endocannabinoid (eCB) system and NMDA receptors (NMDARs) were investigated for their role in gp120-induced synaptotoxicity. HIV gp120 induces loss of excitatory synapses via a neuroinflammatory pathway; the endocannabinoid (eCB) system is a potential target to modulate neuroinflammation. In the first study, I demonstrated that inhibition of monoacylglycerol lipase (MGL), the enzyme that degrades the eCB 2-arachidonoylglycerol (2-AG), using the specific inhibitor JZL184, blocked gp120-induced excitatory synapse loss. Inhibition of MGL suppressed gp120-induced release of the inflammatory cytokine interleukin-1β (IL-1β) through enhanced activation of cannabinoid type 2 receptors (CB2Rs), and decreased production of prostaglandin E2 (PGE2) further decreases neuroinflammation. In the second study, I showed that gp120 increases the number of inhibitory synapses through the same IL-1β-mediated neuroinflammatory pathway. Activation of the tyrosine kinase Src potentiated GluN2A NMDARs to overcome a tonic suppression of inhibitory synapses by p38 mitogen-activated protein kinase. In the third study, a mechanism for excitatory synaptogenesis was examined. I showed that presynaptic GluN2B NMDARs control spontaneous glutamate release, and inhibition of these receptors induces synaptogenesis when evoked neurotransmission is impaired. Taken together, these studies elucidate mechanisms of gp120-induced synaptotoxicity. Decreased excitatory and increased inhibitory synaptic input may be adaptive mechanisms through which neurons counteract excessive excitation-induced by HIV neurotoxins. Because these synaptic changes correlate with cognitive decline, they may indicate that a neuroprotective mechanism has gone awry. By determining the pathways activated by HIV gp120, this dissertation provides new insight into synaptotoxicity-associated with HAND and identifies novel targets for therapeutic agents that provide neuroprotection.
  • Thumbnail Image
    Item
    Modulation of the Junctional Conductance of Retinal AII Amacrine Cell Electrical Synapses
    (2023-10) Cable, Chloe
    Retinal AII amacrine cells are extensively coupled together by electrical synapses. Changes to the strength of these synapses affect how signals are routed through rod and cone retinal pathways during scotopic and photopic vision. Plasticity at these electrical synapses have not, to date, been characterized using electrophysiological approaches. We investigated the effects of adenosine (AR) and N-methyl-D-aspartate receptor (NMDAR) activation on the electrical coupling between AII cells using dual whole-cell patch-clamp electrophysiology in mouse retinal slices. While neither AR activation nor inhibition affected junctional conductance, NMDAR activation substantially decreased junctional conductance between AII cells. Relieving the Mg2+ block of NMDARs through bath application of Mg2+-free solution or by depolarizing AII cells to 0 mV reduced junctional conductance. Exogenous application of NMDA decreased conductance between cells, a decrease which was blocked by the non-selective NMDAR antagonist APV but not by Ro 25-6981, a selective GluN2B-NMDAR antagonist. Addition of either D-serine or glycine, both NMDAR coagonists, without NMDA, reduced the junctional conductance and addition of either coagonist to NMDA-treated retinas further decreased conductance. Experiments were conducted in inositol 1,4,5-trisphosphate receptor type 2 KO and serine racemase KO mice and in WT mice with D-amino acid oxidase to reduce retinal D-serine levels. Under these conditions, the NMDA-mediated conductance decrease was maintained, indicating that D-serine is not necessary for NMDAR-mediated plasticity. These results demonstrate that NMDAR activation results in a decrease in electrical coupling between AII amacrine cells and suggests that both D-serine and glycine can serve as NMDAR coagonists for this plasticity.
  • Thumbnail Image
    Item
    Neural adaptations following exposure to HIV proteins
    (2018-07) Green, Matthew
    HIV associated neurocognitive disorders (HAND) affect nearly half of the population infected with HIV. Symptoms range from subclinical cognitive deficits to severe dementia which can affect daily living. The neurotoxicity produced is likely indirect as the virus does not infect neurons but infects other cells in the brain such as microglia and astrocytes which can release viral proteins and inflammatory cytokines that then affect neurons and lead to synaptodendritic damage. Thus, understanding how these toxic viral factors affect neurons and the mechanisms underlying these changes will aid in the discovery of therapies for HAND. In this dissertation, I report viral proteins altering two types of receptors involved in regulating neuronal excitability and synaptic transmission, the excitatory NMDA receptor (NMDAR) and the inhibitory GABAAR. NMDARs initially become overactivated but then downregulate in hippocampal cultures exposed to the HIV protein transactivator of transcription (Tat). This downregulation was mediated by GluN2A-containing NMDARs signaling to the kinase Akt and the E3 ubiquitin ligase, Mdm2. This novel mechanism delineates a signaling pathway activated by viral proteins that regulates NMDAR-dependent loss of excitatory synapses. Second, I determined how the viral protein gp120 affects GABAergic inhibition. In hippocampal cultures, gp120 caused an increase in tonic GABAAR currents mediated by extrasynaptic GABAARs and an increase in the number of inhibitory synapses. Both increases in inhibition were dependent on microglial activation and release of interleukin-1β which activated interleukin-1 receptors. The increase in inhibitory synapses was dependent on IL-1R-mediated Src activation and subsequent potentiation of GluN2A-containing NMDARs and protein synthesis. On the other hand, the increase in tonic inhibition was dependent on IL-1R-mediated p38 MAPK activation and selective upregulation of α5-containing GABARs. The increases in inhibition likely dampen neuronal excitability and network function and may contribute to the cognitive deficits in HAND. These studies elucidate changes in two types of receptors affected by viral proteins and identify novel signaling pathways that may lead to therapeutic targets for HAND.