Age-dependence in the homeostatic upregulation of hippocampal dendritic spine number during blocked synaptic transmission.

Homeostatic regulation of spine number in mature hippocampal neurons results in more dendritic spines when synaptic transmission is blocked, providing a mechanism to compensate for diminished synaptic input. It is unsettled whether blockade of synaptic transmission also elevates spine number during development. To address this question, synaptic transmission was blocked in rat hippocampal slices during critical developmental stages of spine formation at postnatal days (P) 6-P22 and compared to adults. CA1 pyramidal cells were labeled with DiI and maintained for 5 h in one of three conditions, control artificial cerebrospinal fluid (ACSF), block media containing synaptic transmission antagonists in ACSF, or block media containing synaptic transmission antagonists in a nominally calcium-free ACSF with high magnesium. Slices were fixed in mixed aldehydes, sectioned, and the lateral dendrites were imaged in stratum radiatum with confocal microscopy. Dendritic spine density was quantified per unit length of dendrite. At P6-7 there were only a few protrusions emerging from the dendrites, which were predominantly filopodia-like in appearance. At both P11-12 and P15-16 there was a mixture of dendritic spines and filopodia-like structures. By P20-22 dendritic spines predominated and spine density was about 82% of the adult level. Dendritic spine density increased during blocked synaptic transmission at P20-22 as in adults, but was unchanged during blockade at younger ages. When extracellular calcium was nominally zero, dendritic spine density further increased on P20-22 dendrites as in adults. In contrast, spine density decreased along P11-12 dendrites under the nominally zero calcium condition. Under control conditions, dendritic protrusions were longer at P6-7 than at all other ages, which did not differ from one another. When synaptic transmission was blocked, dendritic protrusions further elongated at P6-7 only. Under the nominally zero calcium condition with blocked synaptic transmission, dendritic protrusions shortened at P11-12 only. These findings reveal age-dependent changes in the manifestation of homeostatic control of dendritic spines that could be mediated by maturational changes in mechanisms regulating postsynaptic calcium.