Control of spine formation by electrical activity in the adult rat cerebellum

Dendritic spines are a key structure in neuronal plasticity. Enhanced activ ity is commonly associated with an increase in spine size and density. Purk inje cell dendrites are characterized by a proximal and a distal compartmen t on which climbing fibers and parallel fibers, respectively, impinge. The proximal region has a very low spine density, whereas the distal region has a high density. Previous experiments showed that after climbing fiber dele tion, Purkinje cells become hyperactive, and a large number of spines devel op on the proximal dendrites, Here we show that the same hyperspiny transfo rmation occurs in the proximal dendrites of adult Purkinje cells by depress ing electrical activity with tetrodotoxin. Thus, spines in different dendri tic compartments are created or maintained independently from the level of Purkinje cell-firing rate and when the afferent activity is blocked. This c onclusion supports the view that spinogenesis is the expression of an intri nsic program and the two regions of the dendritic tree respond differently to activity block because of differences in the inputs that they receive. O n tetrodotoxin treatment, climbing fibers become atrophic and may sprout th in collateral ramifications directed mainly toward the granular layer, All changes are reversible on tetrodotoxin removal. Therefore, Purkinje cells p rovide a model where spines in different compartments of the same neuron ar e differently regulated by the activity of their local afferents. In additi on, electrical activity is also essential to maintain the full climbing fib er innervation.