Dissociation of experience-dependent and -independent changes in excitatory synaptic transmission during development of barrel cortex.

A fundamental problem in the study of cortical development is the extent to which the formation and refinement of synaptic circuitry depends upon sensory experience. The barrel cortex is a useful model system to study experience-dependent cortical development because there is a simple mapping of individual whiskers to the corresponding barrel columns in the cortex. We investigated experience-dependent and -independent changes in glutamatergic synaptic transmission in the barrel cortex during the second postnatal week by comparing synaptic responses from whisker-intact mice at postnatal day (P) 7 and P14 with those from whisker-deprived mice at P14. alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA)-receptor-mediated excitatory synaptic responses were recorded from layer 2/3 pyramidal cells in vitro during voltage-clamp in response to stimulation in layer 4. We observed that the ratio of synaptic AMPA- to NMDA-receptor-mediated current (A/N ratio) increased with developmental age. The development of the A/N ratio was unchanged by deprivation of the whisker input throughout the second postnatal week. In contrast, the NMDA-receptor current decay and sensitivity to the NMDA receptor 2B subunit-selective antagonist ifenprodil was affected strongly by such deprivation. These results demonstrate a concurrent dissociation between sensory experience-dependent and -independent changes of glutamatergic transmission in the barrel cortex during the second postnatal week. Furthermore, they suggest that the development of subunit composition of synaptic receptors is dependent on sensory experience, whereas maturation of the synaptic A/N ratio is independent of such experience. Thus, different components of synaptic development may be governed by different developmental rules.