Plasticity of the Retinogeniculate Synapse
The precise relay of information from one area of the brain to another is important for normal neurological function. Disruption of this communication can lead to cognitive disorders such as mental retardation and behavioral abnormalities. We use the murine visual system to identify and characterize mechanisms important in the relay of visual information from the retina to the visual cortex via the retinogeniculate synapse in the lateral geniculate nucleus.
One major objective of our research is to understand how synapses develop and mature in the central nervous system. Our previous studies at the retinogeniculate synapse have found that the properties of the synapse changes dramatically over development as the strength of retinal ganglion cell inputs to postsynaptic relay neurons increase more than 20-fold over a 3 week period. We found that developmental synaptic plasticity occurs over a much longer time scale than previously recognized at this subcortical visual synapse Our studies show two robust phases of synaptic remodeling, the first dependent on spontaneous activity, the other, previously unrecognized, on vision (Hooks and Chen, 2006). Surprisingly, the onset of the vision-dependent phase occurs much later after eye-opening, after about a week of visual experience. Over the last year, we have characterized a developmental period of sensitivity to sensory experience at the retinogeniculate synapse, similar to that of the critical period described in the visual cortex. Our studies showed that several days of vision following eye-opening is necessary for triggering experience-dependent plasticity. Shorter periods of visual experience do not permit similar experience-dependent synaptic reorganization. Furthermore, connectivity is rapidly reversible simply by restoring normal vision. However, this plasticity did not last throughout adult life. Shifting the onset of sensory deprivation to 25 days after birth did not elicit the same rewiring of retinogeniculate connections. While synaptic strength still weakened, recruitment of additional retinal inputs no longer occurred. Therefore, there is a critical period in the visual thalamus during which synaptic circuits are malleable late in development–– after the time when normal synapse elimination and pruning has occurred. This thalamic sensitive period corresponds temporally with experience-dependent changes in the cortex, suggesting that subcortical plasticity may influence cortical responses to sensory experience. These findings raises the hypothesis that is an opportunity for synaptic connections to rewire connections in the thalamus during this critical/sensitive period. A manuscript summarizing this work is in press in the Journal of Neuroscience.