Paravalbumin-Containing Interneuron Connectivity and Critical Period Modulation

Local inhibitory interneurons in cortex comprise a diverse group of cells whose properties may lend themselves to distinct roles within cortical microcircuits. Yet the position of these cells with respect to incoming sensory input, and the development of their specific laminar connections have not been established. Fast-spiking parvalbumin-containing (PV) cells comprise the largest subset of GABAergic interneurons in visual cortex, and have been implicated in developmental plasticity. To explore the position of these cells with respect to sensory input, I determined the synaptic contribution of thalamic input onto PV interneurons in layer 4 of ferret visual cortex at early postnatal and adult ages. Utilizing dual and triple-labeling immuno-electron microscopy to visualize thalamic arbors, PV interneurons and GABAergic cells, I determined that ~100f all thalamocortical targets in adult layer 4 were PV-positive, and that every GABAergic target was immunoreactive for parvalbumin. This selective thalamocortical drive onto parvalbumin fast-spiking cells may account for the total thalamic innervation to GABAergic interneurons in adult layer 4. To investigate how PV interneuron inhibition impinges upon the subsequent cortical circuit, I conducted a laminar developmental analysis of PV terminal synapses with GABAergic and non-GABAergic targets using preand post-embedding electron microscopy. Moreover, I determined the contribution of inhibitory and excitatory synaptic input to PV interneurons within the same sections. This developmental analysis was performed in a line of transgenic mice in which the parvalbumin promoter drives enhanced green fluorescent protein expression. I found that PV 3 interneuron input to other GABAergic cells is nearly lost in layers 2/3 and 4, at an age commensurate with the peak for anatomical plasticity in visual cortex. Yet by the end of this critical period, parvalbumin containing interneurons have established a strong reciprocal inhibitory circuit with other GABAergic cells. These results are discussed in relation to the role that PV interneurons play in the generation of cortical response properties, and their possible modulation of a period for reduced anatomical and physiological malleability in visual cortex.

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