DEVELOPMENTAL DOWN-REGULATION OF LTD IN CORTICAL LAYER-IV AND ITS INDEPENDENCE OF MODULATION BY INHIBITION

For in vitro LTD to remain viable as a model for synaptic weakening in visual cortical plasticity, it is crucial that it display a critical period for its induction within layer IV. A complicating factor, howev er, is that LTD in layer IV is modulated by inhibitory postsynaptic po tentials (IPSPs); postsynaptic responses characterized as containing I PSPs do not depress in response to 1 Hz afferent stimulation. By block ing IPSPs intracellularly, we find that the ability to induce LTD in l ayer IV neurons is restored in juvenile, but not in mature animals. Th is developmental down-regulation of LTD induction is specific for laye r IV when compared with LTD induction in layers II/III. These data are consistent with the hypothesis that an LTD-like phenomenon is involve d in critical period plasticity and is apparently independent of devel opmental changes in inhibitory circuitry.