title: Synaptotagmin-2 is a reliable marker for parvalbumin positive inhibitory boutons in the mouse visual cortex.
abstract: BACKGROUND
  Inhibitory innervation by parvalbumin (PV) expressing interneurons has been implicated in the onset of the sensitive period of visual plasticity. Immunohistochemical analysis of the development and plasticity of these inhibitory inputs is difficult because PV expression is low in young animals and strongly influenced by neuronal activity. Moreover, the synaptic boutons that PV neurons form onto each other cannot be distinguished from the innervated cell bodies by immunostaining for this protein because it is present throughout the cells. These problems call for the availability of a synaptic, activity-independent marker for PV+ inhibitory boutons that is expressed before sensitive period onset. We investigated whether synaptotagmin-2 (Syt2) fulfills these properties in the visual cortex. Syt2 is a synaptic vesicle protein involved in fast Ca(2+) dependent neurotransmitter release. Its mRNA expression follows a pattern similar to that of PV throughout the brain and is present in 30-40% of hippocampal PV expressing basket cells. Up to now, no quantitative analyses of Syt2 expression in the visual cortex have been carried out.
  
  
  RESULTS
  We used immunohistochemistry to analyze colocalization of Syt2 with multiple interneuron markers including vesicular GABA transporter VGAT, calbindin, calretinin, somatostatin and PV in the primary visual cortex of mice during development and after dark-rearing.
  
  
  CONCLUSIONS
  We show that in the adult visual cortex Syt2 is only found in inhibitory, VGAT positive boutons. Practically all Syt2 positive boutons also contain PV and vice versa. During development, Syt2 expression can be detected in synaptic boutons prior to PV and in contrast to PV expression, Syt2 is not down-regulated by dark-rearing. These properties of Syt2 make it an excellent marker for analyzing the development and plasticity of perisomatic inhibitory innervations onto both excitatory and inhibitory neurons in the visual cortex.
authors.affiliation: Department of Molecular Visual Neuroscience, Netherlands Institute for Neuroscience, an institute of the Royal Netherlands Academy of Arts and Sciences, Amsterdam, The Netherlands.
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Sommeijer
authors.forename: Christiaan N
authors.initials: CN
authors.lastname: Levelt
mesh_terms: D000818:Animals; D064032:Calbindin 2; D064026:Calbindins; D005260:Female; D007150:Immunohistochemistry; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D010320:Parvalbumins / Q000378:metabolism*; D017729:Presynaptic Terminals / Q000378:metabolism; D064030:S100 Calcium Binding Protein G / Q000032:analysis / Q000378:metabolism; D013004:Somatostatin / Q000032:analysis / Q000378:metabolism; D050861:Synaptotagmin II / Q000032:analysis / Q000378:metabolism*; D050495:Vesicular Inhibitory Amino Acid Transport Proteins / Q000032:analysis / Q000378:metabolism; D014793:Visual Cortex / Q000378:metabolism*
chemical_list: C576511:Calb2 protein, mouse; D064032:Calbindin 2; D064026:Calbindins; D010320:Parvalbumins; D064030:S100 Calcium Binding Protein G; D050861:Synaptotagmin II; D050495:Vesicular Inhibitory Amino Acid Transport Proteins; C492244:Viaat protein, mouse; D013004:Somatostatin