METABOTROPIC GLUTAMATE RECEPTORS AND VISUAL CORTICAL SYNAPTIC PLASTICITY

Two forms of use-dependent synaptic plasticity, called long-term poten tiation (LTP) and long-term depression (LTD), can be elicited in the v isual cortex following different paradigms of electrophysiological sti mulation. These neurobiological phenomena often are considered as nece ssary components of models for the alteration in function of the nervo us system that must occur at some level for the establishment and (or) maintenance of memory engrams, for learning processes, or for the con solidation of active neural connections and regression of inactive con tacts in thr developing brain. It has been postulated that for LTP and LTD to be produced in the hippocampus, activation of a particular sub type of excitatory amino acid receptor, the metabotropic receptor, is a critical requirement. Only recently has it become possible to test t his hypothesis directly, as a new compound, (+/-)-alpha-methyl-4-carbo xyphenylglycine (MCPG), has been introduced and the suggestion made th at it selectively antagonizes the metabotropic receptor. This substanc e has been tested in the present study on responses recorded from slic es of rat visual cortex and has been found both to block the activatio n of the metabotropic receptor and to interfere selectively with the f orm of synaptic plasticity called LTD. It thus appears from the experi ments reported in this paper as though the metabotropic receptor subty pe that is blocked by MCPG is required for the expression of LTD but n ot for the expression of LTP, in the visual cortex of adult rats.