Amino acids as transmitters of synaptic excitation in neocortical sensory processes

Few synaptic transmitters are known to exist that are not represented in some region or another, or at some layer or other, in the cerebral cortex of mammalian brain. The more difficult job than mere identification of which substances are present, is that of the assignment of particular functional role(s) of such substances, and as well, of determining upon exactly which element(s) of the known synaptic circuitry of neocortex, such transmitters operate. Current wisdom subscribes to the view that the excitatory amino acids, most likely L-glutamate, and L-aspartate but perhaps also L-cysteate, L-homocysteate, L-cysteine sulfinate or even (although much less likely) the endogenous dipeptide substance, N-acetyl-L-aspartyl-L-glutamate, are the major excitatory synaptic transmitters of intracortical (associational) fibres, of corticofugal projections, and, as this article will attest, of thalamocortical inputs, as well. What particular limits, or restrictions, are imposed upon these generalizations, such as whether the data pertain only to primary sensory areas or follow some other yet to be determined rule, remains to be discovered in future experiments. This paper first presents an overview of the advances in understanding that have come about during the past few decades concerning the synaptic roles of amino acid transmitters. Next, an experimental section presents new evidence based on release studies and the microiontophoretic approach, which supports the view that the amino acids, glutamate and aspartate, interact with specific, pharmacologically identified subtypes of receptors in neocortex as transmitters of synaptic excitation released from thalamic afferent terminals.