Differential expression of synapsins I and II among rat retinal synapses

Abstract

The synapsins are a family of synaptic vesicle-associated phosphoproteins thought to regulate the availability of vesicles for neurotransmitter release. In order to assess variability of synapsin isoform expression, we compared the localization of synapsins Ia, Ib, IIa, and IIb in the inner plexiform layer of the rat retina. Double labeling in conjunction with confocal fluorescence and electron microscopy allowed imaging of synapsin I and II immunoreactivity within single presynaptic terminals. No qualitative differences were observed between expression of the a and b isoforms of synapsin I in individual terminals; likewise, the a and b isoforms of synapsin II were identically distributed. In contrast, marked differences were seen upon comparison of synapsin I and synapsin II expression in single terminals. Our results indicate the existence of three classes of presumed amacrine cell synaptic terminals: synapsin I+/synapsin II-, synapsin I-/synapsin II+, and synapsin I+/synapsin II+. Each class of synapse has a different distribution among five IPL sublayers, suggesting that they represent different subpopulations of amacrine cells. Double labeling with an antibody to choline acetyltransferase indicates that synapsin I-/II+ terminals may be those of cholinergic amacrine cells. Furthermore, all synapsin II+ terminals appear to be distinct from those expressing the GABA synthetic enzyme glutamic acid decarboxylase. The observed variations in synapsin content suggest the existence of presynaptic terminal heterogeneity that is not apparent from conventional morphological studies.

Neuroligin 2 Controls the Maturation of GABAergic Synapses and Information Processing in the Retina

“…3B). Similarly, in the IPL, conventional and bipolar ribbon synapses [respectively labeled with synapsin (Mandell et al, 1992), and VGLUT1 (Johnson et al, 2003)] were present at normal densities (Fig. 3C).…”

Restoration of visual function by expression of a light-gated mammalian ion channel in retinal ganglion cells or ON-bipolar cells

“…S2A). Due to the limited retinal penetration of the AAV2/2 serotype (15) and the lack of syn-1 expression in choline acetyltransferasepositive amacrine cells (28), these transduced cells are likely to be predominantly RGCs. LiGluR expression was panretinal and localized to soma and dendrites of both ON-and OFF-RGCs, as seen by stratification of the dendritic terminals in both on-and offsublayers of the inner plexiform layer (IPL) (Fig.…”

Synaptic transmission at retinal ribbon synapses

“…The functional consequences of this rod/ cone difference are not yet fully understood but it is worth noting that SV2 proteins interact with synaptotagmin (Lazzell et al, 2004). The vesicle protein, synapsin 1, appears to be absent from all ribbon synapses (Mandell et al, 1990(Mandell et al, , 1992Von Kriegstein et al, 1999). Synapsin 1 has been proposed to promote clustering of vesicles (Greengard et al, 1994; but see Sudhof, 2004) but at the ribbon, this function may be replaced by proteins tethering vesicles to the ribbon.…”

“…The orderly array of ribbon-associated vesicles contrasts with the clustering of synaptic vesicles observed at conventional synapses. The absence of clusters is partly due to the physical separation of neighboring vesicles created by the tethers, but another contributing factor may be that photoreceptors and bipolar cells lack the protein, synapsin 1 (Mandell et al, 1990(Mandell et al, , 1992, that has been proposed to promote clustering (Greengard et al, 1994;but see Sudhof, 2004).…”