EXCITATORY SYNAPTIC MECHANISMS IN THE CNS

The central role of excitatory amino acids as neurotransmitters,
neuromodulators and as mediators of neuropathological processes
is now widely accepted. Specifically, receptors and ion channels
activated by L-glutamate and related endogenous excitatory
amino acids may play a role in synaptic plasticity in the
hippocampus and integration of both sensory and motor
information in the spinal cord; current hypotheses of the
pathogenesis of neuronal cell death in stroke, Huntington's
disease, Alzheimer's, spinocerebellar degenerations and seizure-
related brain damage also rely in part of 'excessive' activation of
these same receptors. The broad objective of this project is to explore the cellular and
molecular mechanisms by which excitatory amino acids exert
their effects with a particular focus on the role of the N-methyl-
D-aspartate (NMDA) receptor subtype in synaptic transmission.
This unique agonist-gated channel has two features - voltage-
dependence and calcium permeability - which make this channel
an excellent candidate as a modulator of neuronal excitability.
The general strategy will be to explore, using electrophysiological
methods, the properties of L-glutamate activated ion channels
which are likely to contribute to excitatory synaptic efficacy. In
particular the contribution of second messenger activation,
desensitization and agonist-gated transmembrane calcium influx
on agonist-evoked responses and on single excitatory synapses will
be examined. The relevance to synaptic transmission and drug
action of three modulatory binding sites (Mg, Zn and glycine) on
the NMDA receptor channel will also be examined. These three
potent endogenous modulators have the capability to regulate ion
flux through NMDA-receptor channels, and underscore the
potential for pharmacological action at NMDA receptors. The
experimental approach will use voltage and patch clamp methods
on primary dissociated neurons in cultures prepared from rodent
central nervous system, primarily hippocampus. Significant effort
will be directed to preparations allowing study of identified cells
types, and microisland cultures to allow study of single excitatory
synapses.