Role of Afferent Innervation and Neuronal Activity in Dendritic Development and Spine Maturation of Fascia Dentata Granule Cells

By using slice cultures of hippocampus as a model, we have studied the development of dendritic spines in fascia dentata granule cells. We raised the question as to what extent spine development is dependent on a major afferent input to these neurons, the fibers from the entorhinal cortex and neuronal activity mediated by these axons. Our results can be summarized as follows: (i) the entorhino-hippocampal projection develops in an organotypic manner in co-cultures of entorhinal cortex and hippocampus. Like in vivo, entorhinal fibers, labeled by anterograde tracing with biocytin, terminate in the outer molecular layer of the fascia dentata. (ii) The layer-specific termination of entorhinal fibers is not altered by the blockade of neuronal activity with tetrodotoxin. Likewise, the differentiation of the dendritic arbor of postsynaptic granule cells does not require neuronal activity. Blockade of neuronal activity did not affect the mean spine number of granule cell dendrites in entorhino-hippocampal co-cultures, but led to a relative increase in thin, long filiform spines that are characteristic of immature neurons. (iii) The maturation of the granule cell dendritic arbor is, however, controlled by the afferent fibers from the entorhinal cortex in an activity-independent manner. In single slice cultures of hippocampus lacking entorhinal input, Golgi-impregnated granule cells have much shorter, less branched dendrites when compared with granule cells in entorhino-hippocampal co-cultures. This reduction in dendritic length in granule cells lacking entorhinal input results in a lower mean total number of spines per neuron, but the mean number of spines per microm is not reduced in the absence of entorhinal innervation. These results indicate that innervation by fibers from the entorhinal cortex, but not neuronal activity mediated via these axons, is essential for the normal development of the granule cell dendritic arbor. Neuronal activity is required, however, for the maturation of dendritic spines.