SYNAPTIC DEPRESSION IN VISUAL-CORTEX TISSUE-SLICES - AN IN-VITRO MODEL FOR CORTICAL NEURON ADAPTATION

Synaptic depression was assessed from intracellular recordings in cort ical tissue slices. Evoked postsynaptic potentials exhibited synaptic depression with an exponential or double exponential decrease (time co nstants: <1-30 s) in amplitude during repetitive afferent stimulation by short trains of suprathreshold stimuli. Depressed synaptic response s recovered with an exponential time course (time constants: 10 s-8 mi n) during presentation of similar short trains of stimuli every 5 or 1 0 s. Cortical cells recorded extracellularly in cat visual cortex show similar time constants of response decrement during adaptation to mov ing stripes. Postsynaptic voltage- or ion-regulated conductances and c hloride conductances do not appear to be involved in synaptic depressi on. Input resistance changes and effects of injection of chloride indi cate a lack of GABA(A) receptor-mediated effects. Hyperpolarizing or d epolarizing neurons, and pairing polarization with afferent stimulatio n, also did not affect synaptic depression. This distinguishes these p rocesses from long-term depression and long-term potentiation. Our res ults suggest that the most likely mechanisms of synaptic depression an d adaptation in cortical cells are presynaptic decrease in transmitter release and/or receptor desensitization. Short-term postsynaptic chan ges may also occur after synaptic depression.