Critical assessment of the involvement of perforations, spinules, and spine branching in hippocampal synapse formation

Abstract

Several studies propose that long-term enhancement of synaptic transmission between neurons results from the enlargement, perforation, and splitting of synapses and dendritic spines. Unbiased analyses through serial electron microscopy were used to assess the morphological basis for synapse spilitting in hippocampal area CA1. Few perforated synapses and almost no split (i.e., branched) spines occurred at postnatal day 15, an age of high synaptogenesis; thus, synapse splitting is unlikely to be important during development. The synapse splitting hypothesis predicts an intermediate stage of branched spines with both heads sharing the same presynaptic bouton. Ninety-one branched dendritic spines were traced through serial sections, and the different branches never synapsed with the same presynaptic bouton. Projections from spines, called "spinules," have been thought to extend from perforations in the postsynaptic density (PSD), thereby dividing the presynaptic bouton. Forty-six spinules were traced, and only 13% emerged from perforations in the PSD. Most spinules emerged from the edges of nonperforated PSDs, or from spine necks, where they extended into boutons that were not presynaptic to the spine. In summary, these morphological characteristics are inconsistent with synapse and spine splitting. An alternative is discussed whereby perforated synapses and spinules are transient components of synaptic activation, and branched spines appear from synapses forming in close proximity to one another.

Experience-driven brain plasticity: beyond the synapse

“…Initially, the synaptic junction was thought to enlarge, then develop a perforation, then split into two separate synaptic junctions within a single synaptic terminal and, finally, the spine itself would divide into two spines, each containing one synaptic junction. However, when Kristin Harris' group examined synapses carefully in CA1 of hippocampus using unbiased stereological techniques as applied to electron microscopy, they failed to observe a single branched ('multiple-headed' spine) with the different 'heads' in synaptic contact with the same presynaptic bouton (Sorra et al, 1998), an intermediate stage that is predicted by the splitting hypothesis. Subsequently, they examined the issue of spine splitting directly by serially reconstructing synapses on hippocampal dendrites and the surrounding neuropil across development and in response to hippocampal LTP (Fiala et al, 2002).…”

Trans-Endocytosis via Spinules in Adult Rat Hippocampus

“…Coated pits occur in another mode of internalization characterized by double-membrane invaginations from dendritic spines into presynaptic axons (Westrum and Blackstad, 1962;Andres, 1964;Routtenberg, 1977, 1979;Boyne and Tarrant, 1982;Sorra et al, 1998). This structure has been referred to as the "synaptic spinule" Routtenberg, 1977, 1979) or "spinule complex" (Westrum and Blackstad, 1962).…”

Three-dimensional reconstruction of synapses and dendritic spines in the rat and ground squirrel hippocampus: New structural-functional paradigms for synaptic function

“…Data have been obtained [61,62] showing that these "spinules" can be present both in the area of the postsynaptic densities (PSD) and around the periphery of the heads of spines. The possible functions of "spinules" are usually assessed in terms of their contribution to the efficiency of synaptic transmission due to increases in the areas of contacts between the pre-and postsynaptic membranes, especially when new spines form [23,25], though experimental data supporting this role for "spinules" in synaptic transmission have yet to be obtained [61]. We note that in the state of cold-induced torpor in the ground squirrel, when brain electrical activity is minimal, "spinules" penetrating into the presynaptic bouton can also be found on the surfaces of dendritic stems [2,3].…”

“…Both in organotypic cultures of the hippocampus and in cultures of isolated neurons, de novo synaptogenesis and formation of filopodia occurs virtually constantly, and they may reach lengths of up to 10 µm [15,60,77]. Data have been obtained showing that the hippocampus slices usually used also show synaptogenesis [28,38,55,56,[61][62][63]76]. As demonstrated, comparative analysis of dendritic spines of cultured neurons and intact hippocampus showed that about half the excitatory synapses in vitro are located on dendrites, while in vivo most are located on spines [11].…”

“…Unfortunately, quantitative analysis and modeling of various subcellular structures on the basis of single sections lead to misunderstandings because they introduce large errors [14]. A number of studies [14,19,28,[61][62][63]73] have provided detailed grounds for the importance of both an unbiased quantitative stereological analysis and the need for 3D reconstruction. Recent years have seen the development of relatively simple and thus available programs for 3D reconstruction using IBM personal computers, which can be used to produce three-dimensional models along with simultaneous assessment of their quantitative parameters, such as the surface areas and volumes of study structures [19].…”