Exp Brain Res (1991) 86:518-526 Experimental Brain Research 9 Springer-Verlag 1991 Chemoarchitectonic organization of the cat primary auditory cortex M.N. Wallace*, L.M. Kitzes, and E.G. Jones Department of Anatomy and Neurobiology,Universityof California, Irvine, CA 92717, USA Received December 12, 1990 / Accepted April 17, 1991 Summary. Acetylcholinesterase (ACHE) activity, demon- strated histochemically, defines an area of cortex on the middle ectosylvian gyrus that appears to correspond to the cytoarchitectonically defined area 41 and the physio- logically defined primary auditory area (AI). In this area there are high levels of AChE in layers III, IV and VI while in the surrounding areas there are comparatively low levels of enzyme in these layers. The monoclonal antibody CAT 301, which was raised against a cell sur- face proteoglycan, also defines this area. There are high levels of CAT 301 immunoreactivity in cell bodies and the neuropil of layer Ill and an absence of very large immunoreactive neurons in layer V. Furthermore there are higher levels of the calcium binding protein, parval- bumin and the metabolic enzyme, cytochrome oxidase, in layers III and IV of AI, than in most of the surround- ing cortex. By contrast the distribution of the calcium binding protein, calbindin and the distribution of mye- linated fibers are similar in area 41 and the surrounding areas. Key words: Architectonic organization Acetylcholines- terase CAT 301 - Calbindin - Parvalbumin - Cat Introduction Rose and Woolsey (1949) originally defined the primary auditory cortex (AI) in terms of: 1. evoked responses following cochlear nerve stimulation; 2. cytoarchitecton- ic organization; 3. thalamic degeneration following cor- tical ablation. However subsequent work has shown that even when the medial geniculate body of the thalamus is divided into subnuclei, the different subnuclei project to two or more cortical areas. Thus AI receives a major projection from most of the rostral half of the ventral * Present address: School of Biomedical Sciences, Division of Anatomy, Universityof Aberdeen, AberdeenAB9 1 AS, UK Offprint requests to ." E.G. Jones (address see above) subnucleus, but also from part of the posterior division and the magnocellular portion of the medial division. Each of these subnuclei also has a minor projection to other auditory fields and a small proportion of neurons are double labeled by the injection of retrograde tracers into two auditory fields (Morel and Imig 1987). This makes it very difficult to define AI in terms of thalamic connections. It is also difficult to define AI by cytoarchi- tectonic criteria (Rose 1949; Sousa-Pinto 1973) because it is bordered on three sides by sulci where the laminar pattern of cells is disrupted by the cortical folding and because the border with AII does not shown any sharp transition (Wirier 1984a). AI is best defined by physio- logical criteria because it has a clear tonotopic organiza- tion (Merzenich et al. 1973) and the anterior and posteri- or borders are defined by the reversal of the frequency gradient shown by the sequence of best frequencies at threshold stimulation for single units (Reale and Imig 1980). The dorsal border is more difficult to delineate because the frequency gradient becomes distorted (Mid- dlebrooks and Zook 1983). The ventral border does not have a sharp boundary as there is a transition zone of 0.5-1 mm betwen AI and AII (Schreiner and Cynader 1984). In the present study we sought histochemical markers that would provide reliable markers for the borders of AI, as has been found for the visual and somatosensory areas in the cat and several other species (e.g. Mesulam et al. 1984; Bear et al. 1985; Kristt 1987). The first staining pattern studied was that obtained with the monoclonal antibody CAT-301 (McKay and Hockfield 1982) which recognizes a cell-surface proteo- glycan (Zaremba and Hockfield 1989). Dramatic changes in the laminar staining pattern, shown with the antibody, occur at the area 17/18 border of cat visual cortex (Hendry et al. 1988). Similar changes were shown to occur in the auditory cortex and may have marked the architectonic border of AI. AChE was also studied because the distribution of AChE activity in the brain shows marked regional variations which respect cortical cytoarchitectonic boundaries both in the adult monkey (Mesulam et al. 1984) and in the neonatal rat (Robertson