Influence of dark-rearing and subsequent light exposure on spine dynamics of primary visual cortical neurons in vivo.

Daniela Tropea*, Ania Majewska and Mriganka Sur.

PCLM and Brain and Cognitive Sciences, MIT, Cambridge MA , 02139 , USA .

Dendritic spines are subcellular structures that constitute the postsynaptic sites of most excitatory synapses in the CNS. Changes in spine density, morphology and motility have been described both in vivo during normal development and with sensory deprivation, and in vitro following protocols that elicit changes in synaptic strength. Dark rearing has been shown to perturb the normal physiological properties of neurons in primary visual cortex, V1 and in particular to delay the maturation of visual cortex circuitry. Re-exposure to light rapidly restores orientation selectivity and responsiveness and the expression of activity-dependent molecules. In this study, we measured spine dynamics in V1 of mice dark reared from birth. Experiments were performed at postnatal day 28 (P28), which corresponds to the peak of the critical period. Animals were anesthetized in the dark prior to imaging. We used time-lapse two-photon laser scanning microscopy to assay motility of spines on V1 neurons in vivo, in mice expressing GFP in a subset of cortical layer 5 neurons. Compared to controls, dark-rearing caused a small but significant (p<0.001) increase in the motility of different types of spines: mushroom 20%, stubby 31%, thin 37%, filopodia 13%. The total increase in motility, considering all spines, was ~20%. Interestingly, the types of spines present in our sample was affected by dark rearing: there was a decrease in the percentage of stubby spines (from 26% to 15%) and an increase in the percentage of thin spines (from 11% to 21%) and filopodia (from 2% to 4%). Since thin spines and filopodia are generally the most motile protrusions and are also thought to form transient synapses, it is likely that dark rearing destabilizes existing spines and increases the number of transient connections in order to counteract decreased activity in visual cortex. Surprisingly, re-exposure to light for two days was not sufficient to restore either spine class composition or spine motility levels to those observed in control animals.

Supported by: NIH grants EY15068 and 14134; Burroughs-Wellcome Fund