A new form of synaptic plasticity is transiently expressed in the developing rat visual cortex: A modulatory role for visual experience and brain-derived neurotrophic factor

Synaptic plasticity has been implicated in the mechanisms contributing to t he shaping of the cortical circuits responsible for the transmission of the visual input in the rat primary visual cortex. However, the degree of plas ticity of the thalamocortical synapse may change during development, perhap s reflecting the degree of stabilization of the circuitry subserving it. We have chosen the ability of this synapse to be first depressed and then pot entiated as a specific indicator of its plasticity. In this study we have i nvestigated how this parameter changes during development and the factors c ontrolling it. Extracellular field potentials in cortical layers 2/3 were e voked by stimulation of the white matter in rat primary visual cortex slice s prepared at different postnatal ages. Low-frequency stimulation (900 puls es at 1 Hz) of the white matter was used to induce long-term depression of field potential amplitude, whereas long-term potentiation was evoked by hig h-frequency stimulation consisting of three trains at 100 Hz. We provide evidence that while it is possible to potentiate previously depr essed synapses soon after eye opening (postnatal day 17) this synaptic char acteristic decreases rapidly thereafter. The decrease in this form of corti cal synaptic plasticity closely matches the stabilization of the cortical c ircuitry towards an adult pattern of connectivity and function. Depressed c ortical synapses cannot be potentiated in normal rats at postnatal 23, but they can be potentiated in rats reared in the dark from postnatal days 17 t o 29. Moreover, application of brain-derived neurotrophic factor, known to be expressed in an activity-dependent manner, was able to restore the abili ty of synapses to be potentiated after long-term depression, thus indicatin g its important modulatory role in brain development. (C) 1999 IBRO. Publis hed by Elsevier Science Ltd.