Douglas S.F. Ling - US grants

DESCRIPTION: (Verbatim from the Applicant's Abstract) Acute seizures develop in up to 80 pecent of cases of moderate to severe head trauma, indicating serious cortical damage. The majority of these individuals will have chronic seizures (epilepsy). Concepts regarding the mechanisms underlying epileptogenesis and cell damage have focused on excitotoxicity. Excitotoxicity consists of a cascade of events triggered by excitatory amino acid transmitters, calcium influx via transmitter-gated and voltage-dependent channels, and intracellular calcium release, which then activate autodestructive processes ending in membrane damage and cell death. Epileptogenesis, signaling the dominance of excitation over inhibition can occur at any stage in the excitotoxic process. Such inhibitory failure following trauma cona only be understood by studying local neuronal circuits. A model of traumatic neocortical injury has been developed in order to investigate the mechanisms of excitotoxicity and epileptogenesis. This model utilizes rat in vitro somatosensory cortical slices, wherein after removal of the superficial third of coronal slices, over half the isolated deep segments express epileptiform activity. Preliminary findings postulate that hyperexcitability results from GABAergic disinhibition owing to physical removal of superficial inhibitory circuits and glutamate-triggered increases in intracellular calcium. Experiments will be performed in order to test this hypotheses with regarded to these issues: 1) Strength of glutamatergic excitation and fast GABAergic inhibition in deep neurons from intact versus damaged preparations, 2) Properties of monosynaptic fast GABAergic inhibition in intact versus damage preparations, 3) Intracellular calcium concentration in pyramidal cells after damage, 4) Whether increased intracellular calcium concentration leads to fast GABAergic disinhibition in deep pyramidal neurons, and 5) Testing whether lowering the intracellular calcium concentration in damaged preparations restores fat inhibition. Experiments will utilize standard intracellular or patch clamp recordings of deep pyramidal cells and videoimaging of calcium-sensitive dyes to allow correlation of inhibitory strength with intracellular calcium concentration.

DESCRIPTION: (Adapted from the Investigator's Abstract) The neocortex is the most complex neural system in the mammalian brain and is responsible for mediating the highest forms of perception and cognition. Central to current views of the physiology of higher cortical function is the concept of synaptic plasticity, or use-dependent modifications of synaptic efficacy. When such activity proceeds in a focused and coherent manner the process is associated with normal behavior, healthy cortical development, and normal learning and memory formation. When the process becomes deranged and goes unchecked, neuropathology results, leading to mental disorder and abnormal behavior. The specific disease entity describing a given abnormality will depend on the etiology and the location of the derangement. Understanding normal cortical function and dysfunction must begin at the level of the local neuronal circuit. While most studies of excitotoxicity concentrate on excitatory mechanisms, we will continue to focus on the system which opposes and thereby regulates excitability, namely GABAergic inhibition. The primary goal of this proposal is to examine the dynamics of excitatory drive onto rat neocortical inhibitory interneurons. We have two hypotheses regarding cortical interneurons mediating fast inhibition: 1) they possess few functional NMDA receptors, and 2) they communicate via electrotonic transmission. We further postulate that modulating the activity of AMPA/kainate glutamate receptors or electrotonic synapses will alter the excitability of interneurons, and thereby GABA-mediated inhibitory currents in pyramidal cells. In vitro experiments will be carried out to test these hypotheses with regard to the following issues: 1) Defining the nature of evoked and spontaneous glutamatergic excitation in interneurons, 2) Examining how NMDA and non-NMDA modulation affect interneuron activity, 3) Specifying how non-NMDA modulators affect fast GABAA inhibition recruitment (and the overall effect on the relative excitation-inhibition balance), 4) Gathering physiological evidence for electrotonic transmission in interneurons, and 5) Testing whether modulation of electrotonic coupling alters fast inhibitory strength in pyramidal cells.