SHORT-TERM SYNAPTIC PLASTICITY IN THE VISUAL-CORTEX DURING DEVELOPMENT

The maturation of short-term synaptic plasticity was studied in slices of the visual cortex obtained from rats during the first 47 days of p ostnatal life. Responses of cortical neurons to repetitive stimulation of the white matter at frequencies > 5 Hz were examined by recording intracellularly at the resting membrane potential level. Paired-pulse facilitation, an increase in the excitatory intracellular response fol lowing an initial response, was present in similar to 40% of the neuro ns studied from postnatal day 5 (P5) to P10. Most of the remaining neu rons studied at these ages did not reveal paired-pulse interactions. T here was a progressive, age-related increase in the proportion of cell s displaying paired-pulse depression, a decrease in the second excitat ory response relative to the first, and a concomitant decrease in the proportion of cells displaying paired pulse facilitation. Thus, at P31 -P47 approximately half of the neurons revealed depression of synaptic transmission following an initial stimulus, while most of the other n eurons displayed a lack of temporal interactions. At these later ages, inhibitory potentials also displayed paired-pulse interactions. Matur ation of paired pulse depression of the excitatory response is tempora lly correlated with the development of intracortical inhibitory mechan isms and may reflect subtractive or shunting inhibition in the postsyn aptic neuron as well as presynaptic inhibitory mechanisms. Consistent with a role of GABAergic inhibition, application of GABA receptor anta gonists produced reversible blockade of paired-pulse depression, In co nclusion, cortical neurons display substantial maturation in short-ter m synaptic plasticity during the first postnatal month. Temporal facil itation may be important in enhancing excitatory neurotransmission at a time when synapses are very immature. In the mature cortex, suppress ive temporal interactions could provide an important substrate for neu ronal processing of visual information.