Injection of the reagents, such as virus vectors, anterograde and retrograde tracers, antagonists and agonists, into the brain is an important method in neuroscience. Since the injection site of solutions is generally restricted to a target region, such as thalamic nuclei and cortical areas, the functions, connections and morphology of the target regions can be examined by the injections of reagents.
The neocortical layer 1 contains a small number of neurons and is composed predominantly of dendritic and axonal connections<sup>1</sup>, which receive feedback inputs from the cortex and inputs from the thalamic nuclei and other subcortical regions<sup>2-5</sup>. Moreover, the layer 1 would exert a direct and concerted effect on the firing properties of pyramidal cells in the deep layers<sup>6,7</sup>. Thus, the layer 1 is an important region, in which the feedforward and feedback information from higher cortical areas as well as from other lower cortical areas and subcortical regions, could be associated. Although the research on the layer 1 will become more important in the future, the injection of the regents to the layer 1 was not yet established. In this paper, we introduce the microinjection of virus vectors into the shallow neocortical layer 1.