CELLULAR MECHANISMS OF THE AUGMENTING RESPONSE - SHORT-TERM PLASTICITY IN A THALAMOCORTICAL PATHWAY

Some thalamocortical pathways display an ''augmenting response'' when stimuli are delivered at frequencies between 7 and 14 Hz. Cortical res ponses to the first three stimuli of a series increase progressively i n amplitude and are relatively stable thereafter. We have investigated the cellular mechanisms of the augmenting response using extracellula r and intracellular recordings in vivo and in slices of the sensorimot or neocortex of the rat. Single stimuli to the ventrolateral (VL) nucl eus of the thalamus generate EPSPs followed by feedforward IPSPs that hyperpolarize cells in layer V. A long-latency depolarization interrup ts the IPSP with a peak at similar to 200 msec. A second VL stimulus d elivered during the hyperpolarization and before the peak of the long- latency depolarization yields an augmenting response. The shortest lat ency for augmenting responses occurs in cells of layer V, and they app ear in dendrites and somata recorded in upper layers similar to 5 msec later. Recordings in vitro show that some layer V cells have hyperpol arization-activated and deinactivated conductances that may serve to i ncrease their excitability after IPSPs. Also in vitro, cells from laye r V, but not from layer III, generated augmenting responses at the sam e stimulation frequencies that were effective in vivo. Control experim ents indicated that neither paired-pulse depression of IPSPs nor presy naptically mediated facilitation can account for the augmenting respon se. Active dendritic conductances contribute to the spread of augmenti ng responses into upper layers by way of back-propagating fast spikes, which attenuate with repetition, and long-lasting spikes, which enhan ce in parallel with the augmenting response. In conclusion, we propose that the initiation of augmenting responses depends on an interaction between inhibition, intrinsic membrane properties, and synaptic inter connections of layer V pyramidal neurons.