SHORT-TERM PLASTICITY DURING INTRATHALAMIC AUGMENTING RESPONSES IN DECORTICATED CATS

The intrathalamic mechanisms of frequency-dependent augmenting respons es were investigated in decorticated cats by means of intracellular re cordings from thalamocortical (TC) neurons in ventrolateral (VL) nucle us, including simultaneous impalements from two TC neurons. Pulse trai ns (10 Hz) applied to VL nucleus elicited two types of augmenting resp onses: (1) in 68% of cells, the incremental responses occurred on a pr ogressive depolarization associated with the decrease in IPSPs produce d by preceding stimuli in the train; (2) in the remaining cells, progr essively growing low-threshold (LT) responses resulted from the enhanc ement of Cl--dependent IPSPs, giving rise to postinhibitory rebound bu rsts, followed by a self-sustained sequence of spindle waves. Although in some TC cells the augmenting responses developed from LT responses once the latter reached a given level of depolarization, other neuron s displayed augmenting responses immediately after the early antidromi c spike that depolarized the neuron to the required level, without an intermediate step of LT rebound. Repeated pulse trains led to a progre ssive and persistent increase in slow depolarizing responses of TC cel ls, as well as to a persistent and prolonged decrease in the amplitude s of the IPSPs. On the basis of parallel experiments, we propose that the two types of augmentation in TC cells are a result of contrasting responses of thalamic reticular neurons evoked by repetitive thalamic stimuli: decremental responses, which may account for disinhibition le ading to depolarizing responses in TC cells, and incremental responses , explaining the progressive hyperpolarization of TC cells. These data demonstrate that frequency-dependent changes in neuronal excitability are present in the thalamus of a decorticated hemisphere and suggest that short-term plasticity processes in the gateway to the cerebral co rtex may decisively influence cortical excitability during repetitive responses.