Abstract
BackgroundInhibitory 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 Ca2+ 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.Methodology/Principal FindingsWe 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/SignificanceWe 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.
Highlights
Ocular dominance plasticity is a classical model for studying experience-dependent plasticity in the neocortex
We found that the expression patterns were strikingly similar (Fig. 1A, B, C). mRNA expression patterns of both genes in the neocortex, the hippocampal areas (HA) and subiculum (S) were sparse and scattered, typical for interneuron-specific expression
The activity-dependent development of synapses formed by PV+ interneurons is believed to be one of the crucial regulators of sensitive period onset in the visual cortex
Summary
Ocular dominance plasticity is a classical model for studying experience-dependent plasticity in the neocortex. PV+ interneurons form networks connected through reciprocal chemical and electrical synapses [6,7] which allows them to coordinate the firing of large sets of cortical excitatory neurons This underlies the appearance of gamma-band oscillations (30–100 Hz) [8] believed to be involved in the processing of sensory input [9], attention and experience-dependent plasticity [10,11]. 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. No quantitative analyses of Syt expression in the visual cortex have been carried out
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