ROLE OF THALAMIC AND CORTICAL-NEURONS IN AUGMENTING RESPONSES AND SELF-SUSTAINED ACTIVITY - DUAL INTRACELLULAR-RECORDINGS IN-VIVO

Progressively increasing (augmenting) responses are elicited in thalam ocortical systems by repetitive stimuli at similar to 10 Hz. Repeated purse trains at this frequency lead to a form of shortterm plasticity consisting of a persistent increase in depolarizing synaptic responses as well as a prolonged decrease in inhibitory responses. In this stud y, we have investigated the role of thalamocortical (TC) and neocortic al neurons in the initiation of thalamically and cortically evoked aug menting responses. Dual intracellular recordings in anesthetized cats show that thalamically evoked augmenting responses of neocortical neur ons stem from a secondary depolarization (mean onset latency of 11 mse c) that develops in association with a diminution of the early EPSP. T wo nonexclusive mechanisms may underlie the increased secondary depola rization during augmentation: the rebound spike bursts initiated in si multaneously recorded TC cells, which precede by similar to 3 msec the onset of augmenting responses in cortical neurons; and low-threshold responses, uncovered by hyperpolarization in cortical neurons, which m ay follow EPSPs triggered by TC volleys. Thalamic stimulation proved t o be more efficient than cortical stimulation at producing augmenting responses. Stronger augmenting responses in neocortical neurons were f ound in deeply located (<0.8 mm, layers V-VI) regular-spiking and fast rhythmic-bursting neurons than in superficial neurons. Although corti cal augmenting responses are preceded by rebound spike bursts in TC ce lls, the duration of the self-sustained postaugmenting oscillatory act ivity in cortical neurons exceeds that observed in TC neurons. These r esults emphasize the role of interconnected TC and cortical neurons in the production of augmenting responses leading to short-term plastici ty processes.