Synaptic depression in visual cortex tissue slices: an in vitro model for cortical neuron adaptation.

Synaptic depression was assessed from intracellular recordings in cortical tissue slices. Evoked postsynaptic potentials exhibited synaptic depression with an exponential or double exponential decrease (time constants: < 1-30 s) in amplitude during repetitive afferent stimulation by short trains of suprathreshold stimuli. Depressed synaptic responses with an exponential time course (time constants: 10 s-8 min) during presentation of similar short trains of stimuli every 5 or 10 s. Cortical cells recorded extracellularly in cat visual cortex show similar time constants of response decrement during adaptation to moving stripes. Postsynaptic voltage- or ion-regulated conductances and chloride conductances do not appear to be involved in synaptic depression. Input resistance changes and effects of injection of chloride indicate a lack of GABAA receptor-mediated effects. Hyperpolarizing or depolarizing neurons, and pairing polarization with afferent stimulation, also did not affect synaptic depression. This distinguishes these processes from long-term depression and long-term potentiation. Our results suggest that the most likely mechanisms of synaptic depression and adaptation in cortical cells are presynaptic decrease in transmitter release and/or receptor desensitization. Short-term postsynaptic changes may also occur after synaptic depression.