Synaptogenesis in the Adult Nervous System

Whereas synapses are formed and eliminated during development, synaptogenesis is believed to be more limited in the adult nervous system. However, recent evidence has shown a greater degree of synaptic turnover than previously believed. Cyclic synaptic turnover in the hippocampus and hypothalamus during the estrous cycle of a female rat shows a high degree of specificity: e.g., in hypothalamus neurons of the VMN show cyclic synaptogenesis directed by ovarian steroids; in CA1 pyramidal neurons, estrogen-induced synaptogenesis occurs on dendritic spines and not on shafts, and there are no estrogen effects on dendritic length or branching; moreover, as far as we can tell, such synaptic plasticity is extremely specific and does not occur on CA3 pyramidal neurons or dentate gyrus granule neurons. The discreteness and specificity of this synapse formation implies that molecular markers may be very specific or subtle and that the mechanism may involve changes in a limited number of cellular events, including transcription of discrete structural genes and posttranscriptional events such as translation of mRNAs for structural proteins. Moreover, local regulation, as via afferent input or interneurons, may be very important.

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