Elsevier

Neuroscience

Volume 141, Issue 4, 2006, Pages 2097-2106
Neuroscience

Sensory system
Dense core vesicles resemble active-zone transport vesicles and are diminished following synaptogenesis in mature hippocampal slices

https://doi.org/10.1016/j.neuroscience.2006.05.033Get rights and content

Abstract

Large dense core vesicles (∼100 nm) contain neuroactive peptides and other co-transmitters. Smaller dense core vesicles (∼80 nm) are known to contain components of the presynaptic active zone and thought to transport and deliver these components during developmental synaptogenesis. It is not known whether excitatory axons in area CA1 contain such dense core vesicles, and whether they contribute to synaptic plasticity of mature hippocampus. Serial section electron microscopy was used to identify dense core vesicles in presynaptic axons in s. radiatum of area CA1 in adult rat hippocampus. Comparisons were made among perfusion-fixed hippocampus and hippocampal slices that undergo synaptogenesis during recovery in vitro. Dense core vesicles occurred in 26.1±3.6% of axonal boutons in perfusion fixed hippocampus, and in only 17.6±4.5% of axonal boutons in hippocampal slices (P<0.01). Most of the dense core vesicle positive boutons contained only one dense core vesicle, and no reconstructed axonal bouton had more than a total of 10 dense core vesicles in either condition. Overall the dense core vesicles had average diameters of 79±11 nm. These small dense core vesicles were usually located near nonsynaptic membranes and rarely occurred near the edge of a presynaptic active zone. Their size, low frequency, locations, and decrease following recuperative synaptogenesis in slices are novel findings that merit further study with respect to small dense core vesicle content and possible contributions to synapse assembly and plasticity in the mature hippocampus.

Section snippets

Experimental procedures

DCVs were quantified in presynaptic boutons through serial section electron micrographs from four mature male rats of the Long-Evans strain, two each from perfusion fixed and hippocampal slices (see Table 1 in, Kirov et al., 1999). Methodological issues relevant to the present analyses are briefly described here; Kirov et al. (1999) should be consulted for the complete details. In addition, updated methods are posted on our website called “SynapseWeb” at synapse-web.org. All experiments

Results

DCVs had similar appearances in both preparations whether 2.5% or 6% glutaraldehyde was used and whether the fixation was facilitated by microwave irradiation to speed diffusion, as required for the immersion fixation in slices (Fig. 1, Fig. 2). Upon viewing through many serial EM sections, DCVs could be distinguished along the axons of perfusion-fixed hippocampus (Fig. 1) and hippocampal slices (Fig. 2). DCVs occurred among the small synaptic vesicles (SSVs) in some presynaptic axonal boutons (

Discussion

These findings provide the first three-dimensional quantification of DCVs in mature hippocampal axons. The DCVs occurred in 20–30% of the axonal boutons, most having just one DCV, and none were observed having more than 10 DCVs per fully reconstructed bouton. DCVs co-mingled with smaller clear vesicles in both synaptic and nonsynaptic boutons. They were occasionally positioned at the edge of an existing active zone, but usually DCVs were located closer to non-synaptic plasma membranes

Conclusions

During the preparation of hippocampal slices, neurons undergo considerable reorganization, synapse loss, and subsequently exuberant synaptogenesis during recovery. This reorganization motivated a search for local presynaptic organelles that could be involved in rapid assembly of the presynaptic active zones. We elucidated characteristics of small DCVs in the presynaptic axons of the CA3 to CA1 synapse that might facilitate rapid synaptogenesis in the mature hippocampus. The relatively small

Acknowledgments

This work was supported by grants NS21184 and NS33574 from the National Institutes of Health and the Human Brain Project grant EB002170 to K.M.H. and NIH KO1MH02000 to S.K. We thank Craig Garner for his encouragement and for reading the manuscript, John Fiala for the Reconstruct software (available at http://synapses.bu.edu; or http://synapses.mcg.edu); Marcia Feinberg for serial sectioning, and Robert Smith and Cameron Slayden for technical assistance on the images.

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