Rapid development of somatic spines in stratum granulosum of the adult hippocampus in vitro.

Somatic spines on granule cells have been observed occasionally in the adult rat dentate gyrus in vivo. Here we evaluate the appearance and formation of somatic spines in s. granulosum from adult rat hippocampal slices immediately after slice preparation, 45 min, 3 h, and 5 h later. Initially somatic spines are extremely rare but, after 3 h in vitro, they are readily apparent. Some of these somatic spines form asymmetric synapses that have a spherical vesicle-containing presynaptic bouton and a postsynaptic density. Other somatic spines lack a postsynaptic density and may also lack an apposed presynaptic bouton (free somatic spines) as observed in single thin sections. Both the number of these free somatic spines and the number of somatic spine synapses increase in a time-dependent manner. No somatic spines were observed on CA1 pyramidal cells in the same hippocampal slices. Electrophysiological observations indicate that the formation of somatic spine synapses on granule cells in the hippocampal slice occurs without any apparent granule cell activation. The trigger event for this very rapid synaptogenesis in the adult dentate gyrus remains to be determined.