Simulation of Postsynaptic Glutamate Receptors Reveals Critical Features of Glutamatergic Transmission

Activation of several subtypes of glutamate receptors contributes to changes in postsynaptic calcium concentration at hippocampal synapses, resulting in various types of changes in synaptic strength. Thus, while activation of NMDA receptors has been shown to be critical for long-term potentiation (LTP) and long term depression (LTD) of synaptic transmission, activation of metabotropic glutamate receptors (mGluRs) has been linked to either LTP or LTD. While it is generally admitted that dynamic changes in postsynaptic calcium concentration represent the critical elements to determine the direction and amplitude of the changes in synaptic strength, it has been difficult to quantitatively estimate the relative contribution of the different types of glutamate receptors to these changes under different experimental conditions. Here we present a detailed model of a postsynaptic glutamatergic synapse that incorporates ionotropic and mGluR type I receptors, and we use this model to determine the role of the different receptors to the dynamics of postsynaptic calcium with different patterns of presynaptic activation. Our modeling framework includes glutamate vesicular release and diffusion in the cleft and a glutamate transporter that modulates extracellular glutamate concentration. Our results indicate that the contribution of mGluRs to changes in postsynaptic calcium concentration is minimal under basal stimulation conditions and becomes apparent only at high frequency of stimulation. Furthermore, the location of mGluRs in the postsynaptic membrane is also a critical factor, as activation of distant receptors contributes significantly less to calcium dynamics than more centrally located ones. These results confirm the important role of glutamate transporters and of the localization of mGluRs in postsynaptic sites in their signaling properties, and further strengthen the notion that mGluR activation significantly contributes to postsynaptic calcium dynamics only following high-frequency stimulation. They also provide a new tool to analyze the interactions between metabotropic and ionotropic glutamate receptors

Prefrontal Synaptic Glutamate Transmission Dynamics across Psychostimulants and Behavioral Paradigms of Drug Addiction

The medial prefrontal cortex (mPFC) is an important node in the brain’s reward-seeking circuit and neuronal activity within this region is modulated by exposure to discrete cues and contexts previously associated with a drug experience. The mPFC is anatomically divided into the dorsal mPFC (dmPFC: containing the cingulate and dorsal prelimbic areas) and ventral mPFC (vmPFC: ventral prelimbic and infralimbic areas). Several studies have explored the functional distinctions between the dorsal and ventral mPFC with pharmacologic or genetic manipulations of its afferent glutamatergic projections to the ventral striatum. This line of research has uncovered opposing roles between these two mPFC subregions. Specifically, increases in activity within the prelimbic area (PLmPFC) have been shown to drive drug-seeking behavior while excitatory drive of the infralimbic area (IL-mPFC) inhibits this behavior following extinction training. However, the basal features of glutamatergic synaptic transmission that underlie this functional distinction and the synaptic plasticity generated by drug experience or exposure to drug-associated stimuli in PL- and IL-mPFC pyramidal projection neurons are not known. This dissertation addresses the hypothesis that glutamate synaptic transmission in deep layer 5/6 pyramidal neurons of the mPFC exhibits basal differences between mPFC subregions that are altered in response to drug-related cues and context, and that drug-seeking behavior, specifically psychostimulants such as cocaine and methamphetamine, is in part regulated by these plastic changes in ionotropic excitatory synaptic transmission. To test this, we made brain slice recordings in two widely used models of drug addiction in rats: the conditioned place preference paradigm (CPP) and the reinstatement model of drug self-administration. Following self-administration or experimenter administration (in the CPP paradigm) of psychostimulants (i.e. cocaine or methamphetamine), we tested whether exposure to discrete cues (reinstatement model) or the context (CPP) previously associated with the drug, produced alterations in synaptic excitatory ionotropic glutamate receptor transmission that could account for the following behavioral responses: retention of CPP after different abstinence intervals or cue-induced reinstatement of drug seeking. Our results suggest that cocaine produces different effects on PL and IL neurons that are dependent on the behavioral paradigm that is utilized. Specifically, cocaine self-administration followed by extinction alone, or cue-induced reinstatement did not produce any measurable differences in glutamate transmission compared to saline yoked rats. Cocaine-CPP on the other hand, produced several changes in glutamate transmission in both PL and IL neurons. These neuroadaptations were dependent on the length of abstinence and were reversed by context re-exposure. Lastly, contrary to the effects of cocaine self-administration, methamphetamine self-administration followed by 8 days of abstinence produced pre- and postsynaptic changes in glutamate transmission in mPFC neurons. In summary, these results provide evidence that general changes in mPFC synaptic glutamate transmission account for aspects of drug-seeking behavior that are not responsive to exposure to drug-associated cues or context, while other alterations in synaptic transmission that meet the functional distinction between mPFC subregions are sensitive to drug cue- or context-associations

Microglial Morphology and Dynamic Behavior Is Regulated by Ionotropic Glutamatergic and GABAergic Neurotransmission

PURPOSE: Microglia represent the primary resident immune cells in the CNS, and have been implicated in the pathology of neurodegenerative diseases. Under basal or "resting" conditions, microglia possess ramified morphologies and exhibit dynamic surveying movements in their processes. Despite the prominence of this phenomenon, the function and regulation of microglial morphology and dynamic behavior are incompletely understood. We investigate here whether and how neurotransmission regulates "resting" microglial morphology and behavior. METHODS: We employed an ex vivo mouse retinal explant system in which endogenous neurotransmission and dynamic microglial behavior are present. We utilized live-cell time-lapse confocal imaging to study the morphology and behavior of GFP-labeled retinal microglia in response to neurotransmitter agonists and antagonists. Patch clamp electrophysiology and immunohistochemical localization of glutamate receptors were also used to investigate direct-versus-indirect effects of neurotransmission by microglia. RESULTS: Retinal microglial morphology and dynamic behavior were not cell-autonomously regulated but are instead modulated by endogenous neurotransmission. Morphological parameters and process motility were differentially regulated by different modes of neurotransmission and were increased by ionotropic glutamatergic neurotransmission and decreased by ionotropic GABAergic neurotransmission. These neurotransmitter influences on retinal microglia were however unlikely to be directly mediated; local applications of neurotransmitters were unable to elicit electrical responses on microglia patch-clamp recordings and ionotropic glutamatergic receptors were not located on microglial cell bodies or processes by immunofluorescent labeling. Instead, these influences were mediated indirectly via extracellular ATP, released in response to glutamatergic neurotransmission through probenecid-sensitive pannexin hemichannels. CONCLUSIONS: Our results demonstrate that neurotransmission plays an endogenous role in regulating the morphology and behavior of "resting" microglia in the retina. These findings illustrate a mode of constitutive signaling between the neural and immune compartments of the CNS through which immune cells may be regulated in concert with levels of neural activity

Inhibitory Plasticity: From Molecules to Computation and Beyond

Synaptic plasticity is the cellular and molecular counterpart of learning and memory and, since its first discovery, the analysis of the mechanisms underlying long-term changes of synaptic strength has been almost exclusively focused on excitatory connections. Conversely, inhibition was considered as a fixed controller of circuit excitability. Only recently, inhibitory networks were shown to be finely regulated by a wide number of mechanisms residing in their synaptic connections. Here, we review recent findings on the forms of inhibitory plasticity (IP) that have been discovered and characterized in different brain areas. In particular, we focus our attention on the molecular pathways involved in the induction and expression mechanisms leading to changes in synaptic efficacy, and we discuss, from the computational perspective, how IP can contribute to the emergence of functional properties of brain circuits

Proliferation, differentiation, and glutamatergic synapses: Communication between neurons and oligodendrocyte precursor cells

The healthy physiological functioning of the mammalian central nervous system relies on the precise communication between many different cell types. The most general communication channel between neurons is the chemical synapse. Fascinatingly, oligodendrocyte precursor cells also receive synaptic input from glutamatergic neurons. Oligodendrocyte precursor cells are responsible for forming the myelin sheaths by differentiating into oligodendrocytes, and the myelination itself seems to be regulated by neuronal firing patterns. The differentiation and proliferation of oligodendrocyte precursor cells are influenced by transient changes of neuronal firing. Therefore the questions arise: Can oligodendrocyte precursor cells discriminate dissimilar patterns of neuronal firing? How exactly those different patterns would influence their proliferation and differentiation? How the physiological properties of synaptic signaling would influence the cellular behavior of oligodendrocyte precursors? In my doctoral thesis I analyzed in details the synaptic responses of oligodendrocyte precursors to different, repetitive axonal stimulation patterns; and found that the postsynaptic responses are very diverse upon the various patterns of axonal activation. I showed in vivo that these distinct patterns do influence the proliferation and differentiation of oligodendrocyte precursors in a dissimilar way, even though the sum activity and transmitted charge through their synaptic receptors were similar in the applied paradigms. I also demonstrated that the quantal parameters at the axon – OPC synapses are sensitive to the method by which the quantal synaptic events had been triggered. Therefore the different approaches to trigger quantal events are not interchangeable or substitutable with each other. Lastly, with my colleagues we established that the exact physiological parameters of the glutamatergic synaptic transmission matter greatly to these cells: modifications of the AMPA receptors on oligodendrocyte precursors considerably altered their proliferation and differentiation. The results discussed in this thesis not only show how millisecond-scale events, such as synaptic currents, can influence slow biological processes as cell cycle or cell differentiation; but also carry key implications for various myelin-related diseases, for instance multiple sclerosis

Roles of Presynaptic NMDA Receptors in Neurotransmission and Plasticity

Presynaptic NMDA receptors (preNMDARs) play pivotal roles in excitatory neurotransmission and synaptic plasticity. They facilitate presynaptic neurotransmitter release and modulate mechanisms controlling synaptic maturation and plasticity during formative periods of brain development. There is an increasing understanding of the roles of preNMDARs in experience-dependent synaptic and circuit-specific computation. In this review, we summarize the latest understanding of compartment-specific expression and function of preNMDARs, and how they contribute to synapse-specific and circuit-level information processing

Kainate receptor function in rodent subcortical visual processing.

Glutamate is found throughout the central nervous system and has been shown to be an important excitatory neurotransmitter in the visual system. There are two subdivisions of receptor on which this ubiquitous neurotransmitter acts, metabotropic (mGluR) and ionotropic (GluR) glutamate receptors. There are eight sub types of mGluR falling into three groups, and fifteen GluR subunits also divided into three groups. Kainate receptors (KARs) comprise one group of the ionotropic glutamate receptor subdivision. Relay cells of the lateral geniculate nucleus (LGN) are driven and modulated by a variety of NMDA, AMPA and metabotropic receptors. In addition, investigation into the involvement of mGluR, AMPA and NMDA receptor function in the synaptic processing of the superior colliculus (SC) has been well documented. It has been difficult, however, to establish specific KAR function in these brain structures due to lack of pharmacological agents acting solely at kainate receptors. In recent years such agents have become available, thus enabling the present study of GluR5 involvement in visual processing within the SC and LGN. The purpose of this body of work has been to assess the involvement of GluR5-containing Kainate receptors (KARs) in synaptic transmission between retinal ganglion cells (RGCs) and subcortical brain structures involved in the processing of visual information namely the superficial superior colliculus (SSC) and the lateral geniculate nucleus (LGN). The majority of the work focused on the function of KARs in the SSC. To elucidate the involvement of KARs in visual processing, both in vivo and in vitro methods were utilised. In vivo electrophysiology was used for extracellular recording of evoked activity of both SSC and LGN neurons in response to visual stimuli. This was carried out during intravenous injection of GluR5 antagonist. In vivo recording twinned with iontophoretic administration of GluR5-specific pharmacological compounds was also employed to investigate KAR participation in direct synaptic transmission between RGCs and the SSC neurons. The same technique was used to study KAR involvement in the phenomenon of response habituation exhibited by these neurons. To parallel in vivo protocols, in vitro SSC slice experiments were performed to study the effect of GluR5 agonists and antagonists on evoked postsynaptic currents. This enabled the administration of drugs at concentrations specific for GluR5 subunits whilst investigating GluR5 involvement in direct synaptic transmission between RGC input and SSC neurons. In addition, a paired pulse protocol was employed to propose a presynaptic location of GluR5-containing KARs at retinal input into the SSC. Furthermore, the use of GluR5- specific and GABAR-specific compounds during evoked current recording indicated the involvement of GluR5-containing receptors in the direct modulation of excitatory but not inhibitory input into the SSC. In summary, therefore, both in vivo and in vitro electrophysiology techniques were used to indicate a location and function for GluR5 KARs in the subcortical visual system of the rat. GluR5-containing receptors were found to modulate visual processing of both the LGN and SSC. It was unclear whether these receptors were located in the LGN itself due to the use of systemic injection protocols, however, iontophoresis of GluR5-selective drugs demonstrated a role in modulating visual responses within the SSC. The mechanism by which GluR5 receptors modulated responses in the SSC was further elucidated by a series of whole cell patch-clamp experiments which revealed that GluR5-containing receptors reduced synaptic transmission at excitatory inputs directly onto recorded cells and those connections with the intrinsic inhibitory circuitry of the SSC. In addition a paired-pulse protocol was used to determine that the decrease in excitatory transmission was caused by the presynaptic reduction of glutamatergic transmission

Cholinergic Control of Cortical Circuit Activity

Cholinergic neurons of the basal forebrain send extensive projections to all regions of the neocortex and are critically involved in a diverse array of cognitive functions, including sensation, attention and learning. Cholinergic signaling also plays a crucial role in the moment-to-moment control of ongoing cortical state transitions that occur during periods of wakefulness. Yet, the underlying circuit mechanisms of synaptic cholinergic function in the neocortex remain unclear. Moreover, acetylcholine continues to be widely viewed as a slow and diffuse neuromodulator, despite the preponderance of in vivo evidence demonstrating rapid cholinergic function. In this study, we used a combination of optogenetics and in vitro electrophysiology to examine spatiotemporally precise control of cortical network activity by endogenous acetylcholine. We show that even brief activation of cholinergic afferents could powerfully suppress evoked cortical recurrent activity for several seconds. This suppression was reliant on the engagement of both nicotinic and muscarinic acetylcholine receptors. Nicotinic receptors mediated transient suppression by acting in the superficial cortical layers, while muscarinic receptors mediated prolonged suppression in layer 4. In agreement, we found nicotinic-mediated excitation of inhibitory neurons in the supragranular layers, and muscarinic-mediated hyperpolarization of excitatory cells in layer 4. Together, these findings present novel circuit mechanisms for fast and robust cholinergic signaling in neocortex