The distributions of acetylcholinesterase (AChE) and zinc-containing boutons and their cells of origin in the visual cortex of the house mouse (Mus museulus domestieus) are described. The primary visual area is defined by both acetylcholinesterase and zinc staining. The AChE staining pattern is dark in upper layer I and layers IV and VI. It is light in layers II/III and V. The lack of a densely stained layer IV in the secondary visual cortices defines the borders between primary and secondary areas. Large, multipolar AChE-positive neurons are located throughout the cortical layers, but preferentially in layer VI. Dense zinc-positive neuropil in the primary visual cortex is apparent in layer lb, upper layer II/III, and layers V and VI. Neurons that give rise to zinc-containing boutons are situated in layers II/111 and VI. The medial and lateral borders can be distinguished by a bold contrast of staining in lower layer II/III; the secondary areas have more zinc-positive neurons, and the neuropil stains darker. A surprising observation of this study is the disparity between the mouse and rat visual cortex of the AChE staining pattern. Layer V is very light in the mouse, whereas a dark stain has been described in layer V of the rat. Layer VI stains heavily in the mouse while less AChE activity has been observed in layer VI of the rat.
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