CELLULAR AND SUBCELLULAR-LOCALIZATION OF SYNTAXIN-LIKE IMMUNOREACTIVITY IN THE RAT STRIATUM AND CORTEX

Syntaxin is a synapse-specific protein previously localized to the pla sma membrane of axon terminals. Biochemical and molecular biological s tudies indicate a prominent role for syntaxin 1A and 1B in synaptic ve sicle docking and/or fusion, suggesting that these proteins are locali zed to active zone regions of most terminal varicosities in the centra l nervous system. We sought to test this hypothesis by examining the c ellular and subcellular immunocytochemical localization of syntaxin 1 proteins in the striatum and frontal cortex of rats. Using either a po lyclonal anti-syntaxin antibody, or a monoclonal antibody directed aga inst the identical protein, HPC-1, immunoperoxidase reaction product w as localized to preterminal axons and terminal varicosities that made almost exclusively Type I (asymmetric) synapses on dendritic spines or distal shafts. Immunoreactive terminals forming Type II (symmetric) s ynapses were observed rarefy and only in tissue that was pretreated by rapid freeze-thaw to enhance antibody penetration. From a semi-quanti tative analysis, it was estimated that at least 48-62% of all vesicle- tilled varicosities and 67-69% of all terminals forming Type I synapse s were immunoreactive for syntaxin or HPC-1, respectively. Using a pre -embedding immunogold-silver technique to provide a non-diffusible mar ker for subcellular localization, gold-silver particles for syntaxin o r HPC-1 were localized to the cytoplasmic surface of non-synaptic port ions of the plasma membrane of preterminal axons and terminal varicosi ties. Enrichment of presynaptic active zone regions was not observed w ith immunogold-silver staining. These findings suggest that syntaxin i s primarily contained in a subpopulation of terminals that are associa ted with excitatory amino acid transmitters, but appears not to be ubi quitously expressed in all terminal classes. The results further indic ate that syntaxin is localized to non-synaptic regions of axon and ter minal membranes, but may not be enriched in presynaptic active zones. The apparent inconsistency between the subcellular localization of syn taxin and its proposed role in vesicle exocytosis is discussed in term s of possible technical limitations and alternative functions for synt axin.