Elsevier

Neuroscience

Volume 125, Issue 3, 2004, Pages 615-623
Neuroscience

Synapses with a segmented, completely partitioned postsynaptic density express more AMPA receptors than other axospinous synaptic junctions

https://doi.org/10.1016/j.neuroscience.2004.02.025Get rights and content

Abstract

Axospinous perforated synapses of one morphological subtype exhibit multiple transmission zones, each one being formed by an axon terminal protrusion apposing a postsynaptic density (PSD) segment and separated from others by complete spine partitions. Such segmented, completely partitioned (SCP) synapses have been implicated in synaptic plasticity and postulated to be exceptionally efficacious. The present study explored the validity of this supposition. Postembedding immunogold electron microscopy was used for quantifying the postsynaptic AMPA receptor (AMPAR) expression, which is widely regarded as a major determinant of synaptic efficacy. Various subtypes of axospinous synapses were examined in the rat CA1 stratum radiatum. The results showed that the number of immunogold particles for AMPARs in SCP synapses markedly and significantly exceeded that in other perforated subtypes (by 101% on the average) and in nonperforated immunopositive synapses (by 1086%). Moreover, the particle number per single PSD segment, each of which also contained NMDA receptors, was significantly higher than that per nonperforated PSD (by 485%). SCP synapses also exhibited a higher particle density per unit PSD area, as well as a larger overall PSD area as compared with other synaptic subtypes. Analysis of covariance revealed that the high AMPAR expression in SCP synapses was related to the segmented PSD configuration, not only to the PSD size. Moreover, the subpopulations of SCP and other perforated synapses with either overlapping or equal PSD sizes differed in AMPAR content and concentration, with both measures being significantly higher in SCP synapses. Thus, the elevated AMPAR expression in SCP synapses is associated with the presence of separate PSD segments, not only with their large PSD area. These findings are consistent with the idea that SCP synapses have a relatively greater efficacy and may support maximal levels of synaptic enhancement characteristic of certain forms of synaptic plasticity such as the early LTP phase.

Section snippets

Animals

Experimental protocols employed in this study were described in detail earlier (Ganeshina et al., 2004). Briefly, four young adult male rats of Fischer-344×Brown Norway strain were acquired from Harlan (Indianapolis, IN, USA). All experiments were performed in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals and approved by Northwestern University's Animal Care and Use Committee. The number of rats used in the present study was kept to a minimum

Identification of SCP synapses in electron micrographs of serial sections

Examination of serial sections obtained from tissue conventionally prepared for transmission electron microscopy showed that SCP synapses were encountered rarely in the CA1 stratum radiatum where their proportion was estimated to be 2.0% of the entire axospinous synaptic population and 12.8% of all perforated axospinous synapses. Synapses belonging to the SCP subtype could be readily identified in such preparations by a complex of specific ultrastructural features. These included the presence

Discussion

The present study demonstrates that SCP synapses are characterized by an exceptionally high AMPAR immunoreactivity as compared with other axospinous synapses in the rat CA1 stratum radiatum. The number of immunogold particles for AMPARs in SCP synapses markedly and significantly exceeds that in other axospinous synaptic junctions (Table 1). Our data also show that the PSD area is significantly larger in SCP synapses than in other synaptic subtypes (Table 1). Despite this, the density of

Acknowledgements

The authors would like to thank Dr. N. P. Spruston for helpful discussions. This work was supported by a grant from NIH/NIA AG17139.

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