Differential expression of synapsins I and II among rat retinal synapses

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.