Segeth: A model of effective diffusion and tortuosity in the extracellular space of rat brain

Brain function is based on communication between individual cells. This communication utilizes small neurotransmitter and neuromodulator molecules, which are transported through extracellular space (ECS). Substances released into the ECS spread predominantly by diffusion because the ECS lacks any active transport mechanism. Because its structure exerts a direct influence on the intercellular signaling and also on transport of nutrients, metabolites and therapeutic agents Diffusion can be exploited experimentally to quantify two major structural parameters of the ECS, volume fraction and diffusion permeability Activation of noradrenergic system is known to cause wakefulness, increase vigilance and facilitate the transition of cortical activity from the sleep state to the awake state [5]. We measured ECS parameters in a visual region of the rat neocortical slices under control conditions and after activation of noradrenergic system by the noradrenergic agonist isoproterenol. Using the Real-Time Iontophoretic (RTI) method [1], we found that α decreased from 0.22 to 0.18 when isoproterenol was applied, suggesting that activation of noradrenergic receptors mimics the awake state in a brain slice. Next, we utilized this experimental paradigm as an in vitro model of sleep and awake brain states to study diffusive spread of molecules with small and high molecular weights in these states. To this end, we quantified diffusion of a small cation tetramethylammonium (MW 74) with the RTI method and fluorescently-labeled macromolecule dextran (MW 3000) with Integrative Optical Imaging method [6] in the visual neocortex both under the control conditions and after application of isoproterenol. Our preliminary results show that θ TMA remained constant (0.376 vs. 0.381) while θ dextran decreased (0.346 vs. 0.289) after the application of isoproterenol. In conclusion, our pilot study suggests that the diffusive spread of macromolecules in brain ECS is more restricted during the awake-like state than during the sleep-like state. This result has important implications for transport of growth factors, proteins and macromolecular therapeutics in brain ECS. References [1] C. Nicholson, J. Phillips: Ion diffusion modified by tortuosity and volume fraction in the extracellular microenvironment of the rat cerebellum. J. Physiol. 321, 225-257 (1981)

Determining the neurotransmitter concentration profile at active synapses

Establishing the temporal and concentration profiles of neurotransmitters during synaptic release is an essential step towards understanding the basic properties of inter-neuronal communication in the central nervous system. A variety of ingenious attempts has been made to gain insights into this process, but the general inaccessibility of central synapses, intrinsic limitations of the techniques used, and natural variety of different synaptic environments have hindered a comprehensive description of this fundamental phenomenon. Here, we describe a number of experimental and theoretical findings that has been instrumental for advancing our knowledge of various features of neurotransmitter release, as well as newly developed tools that could overcome some limits of traditional pharmacological approaches and bring new impetus to the description of the complex mechanisms of synaptic transmission

Molecular Constraints on Synaptic Tagging and Maintenance of Long-Term Potentiation: A Predictive Model

Protein synthesis-dependent, late long-term potentiation (LTP) and depression (LTD) at glutamatergic hippocampal synapses are well characterized examples of long-term synaptic plasticity. Persistent increased activity of the enzyme protein kinase M (PKM) is thought essential for maintaining LTP. Additional spatial and temporal features that govern LTP and LTD induction are embodied in the synaptic tagging and capture (STC) and cross capture hypotheses. Only synapses that have been "tagged" by an stimulus sufficient for LTP and learning can "capture" PKM. A model was developed to simulate the dynamics of key molecules required for LTP and LTD. The model concisely represents relationships between tagging, capture, LTD, and LTP maintenance. The model successfully simulated LTP maintained by persistent synaptic PKM, STC, LTD, and cross capture, and makes testable predictions concerning the dynamics of PKM. The maintenance of LTP, and consequently of at least some forms of long-term memory, is predicted to require continual positive feedback in which PKM enhances its own synthesis only at potentiated synapses. This feedback underlies bistability in the activity of PKM. Second, cross capture requires the induction of LTD to induce dendritic PKM synthesis, although this may require tagging of a nearby synapse for LTP. The model also simulates the effects of PKM inhibition, and makes additional predictions for the dynamics of CaM kinases. Experiments testing the above predictions would significantly advance the understanding of memory maintenance.Comment: v3. Minor text edits to reflect published versio

Multiple roles of GluN2B-containing NMDA receptors in synaptic plasticity in juvenile hippocampus

AbstractIn the CA1 area of the hippocampus N-methyl-d-aspartate receptors (NMDARs) mediate the induction of long-term depression (LTD), short-term potentiation (STP) and long-term potentiation (LTP). All of these forms of synaptic plasticity can be readily studied in juvenile hippocampal slices but the involvement of particular NMDAR subunits in the induction of these different forms of synaptic plasticity is currently unclear. Here, using NVP-AAM077, Ro 25-6981 and UBP145 to target GluN2A-, 2B- and 2D-containing NMDARs respectively, we show that GluN2B-containing NMDARs (GluN2B) are involved in the induction of LTD, STP and LTP in slices prepared from P14 rat hippocampus. A concentration of Ro (1 μM) that selectively blocks GluN2B-containing diheteromers is able to block LTD. It also inhibits a component of STP without affecting LTP. A higher concentration of Ro (10 μM), that also inhibits GluN2A/B triheteromers, blocks LTP. UBP145 selectively inhibits the Ro-sensitive component of STP whereas NVP inhibits LTP. These data are consistent with a role of GluN2B diheretomers in LTD, a role of both GluN2B- and GluN2D- containing NMDARs in STP and a role of GluN2A/B triheteromers in LTP.This article is part of the Special Issue entitled ‘Ionotropic glutamate receptors’

Dynamic extracellular space alters spatiotemporal distribution of chemical signals in brain

Brain can be considered as a porous medium. The brain cells form a solid phase while the liquid-filled extracellular space (ECS) forms a porous phase that surrounds each individual cell. Brain ECS is of a fundamental importance for brain function [1]. It serves as a reservoir for ions and a channel for diffusion-mediated transport of biologically significant molecules and therapeutics. ECS volume is the main factor governing the extracellular concentrations of these substances. Any ECS volume change may lead to a change in concentration of ions and transported substances, and this has consequences for brain function

Differential downregulation of protein kinase C isoforms in spreading depression

Spreading depression (SD) is a propagating depolarization of populations of neurons induced by intense electrical, chemical, or mechanical stimulation, which has been proposed to be an important mechanism in the aura of migraine. SD is characterized by a transient loss of synaptic transmission and thus may involve signal transduction mechanisms known to modulate synaptic strength. To examine the underlying pathophysiological molecular mechanisms of SD, we analyzed the regulation of eight protein kinase C (PKC) isoforms by immunoblot during SD induced by a high-intensity stimulus of synaptic afferents in the CA1 region of hippocampal slices. We observed a downregulation of the conventional (α, βI, βII, γ) and the novel (δ, ε, η) PKC isoforms in SD, but no change in the atypical isozyme (ζ). The coordinate downregulation of multiple PKC isoforms may be important in the functional depression of neuronal activity in SD. In contrast, the atypical ζ, and its constitutively active fragment PKMζ, is a specific PKC isozyme that has been implicated in the maintenance of long-term potentiation (LTP) and long-term depression (LTD), widely studied models for the mechanism of memory. The stability of PKCζ and PKMζ in SD indicates that a molecular mechanism for the maintenance of LTP/LTD is relatively resistant to alterations that occur during pathophysiologically large ionic fluxes. This result could help to explain the retention of information stored in the cortex despite the massive release of excitatory neurotransmitter and neuronal depolarization that may occur during the migrainous aura