Architectonic Subdivisions of Neocortex in the Gray Squirrel (Sciurus carolinensis)

Abstract

Squirrels are highly visual mammals with an expanded cortical visual system and a number of well-differentiated architectonic fields. To describe and delimit cortical fields, subdivisions of cortex were reconstructed from serial brain sections cut in the coronal, sagittal, or horizontal planes. Architectonic characteristics of cortical areas were visualized after brain sections were processed with immunohistochemical and histochemical procedures for revealing parvalbumin, calbindin, neurofilament protein, vesicle glutamate transporter 2, limbic-associated membrane protein, synaptic zinc, cytochrome oxidase, myelin or Nissl substance. In general, these different procedures revealed similar boundaries between areas, suggesting that functionally relevant borders were being detected. The results allowed a more precise demarcation of previously identified areas as well as the identification of areas that had not been previously described. Primary sensory cortical areas were characterized by sparse zinc staining of layer 4, as thalamocortical terminations lack zinc, as well as by layer 4 terminations rich in parvalbumin and vesicle glutamate transporter 2. Primary areas also expressed higher levels of cytochrome oxidase and myelin. Primary motor cortex was associated with large SMI-32 labeled pyramidal cells in layers 3 and 5. Our proposed organization of cortex in gray squirrels includes both similarities and differences to the proposed of cortex in other rodents such as mice and rats. The presence of a number of well-differentiated cortical areas in squirrels may serve as a guide to the identification of homologous fields in other rodents, as well as a useful guide in further studies of cortical organization and function.

Distribution of SMI-32-immunoreactive neurons in the central auditory system of the rat

“…These findings are in agreement with previous reports in which the pattern of SMI-32 immunoreactivity was successfully used for the parcellation of the auditory, visual, somatosensory and prefrontal cortical areas in mammalian species such as gerbils (Budinger et al 2000;Bajo and Moore 2005), hamsters (Boire et al 2005), rats (Van De Werd and Uylings 2008), cats (Van der Gucht et al 2001, Mellott et al 2010, tree shrews (Wong and Kaas 2009) and primates (Baldauf 2005). Interestingly, in agreement with our results obtained in the rat auditory cortex, Baldauf (2005) found in marmosets a higher number of SMI-32-ir neurons in the associative cortical areas in comparison with the primary visual cortex, and Wong and Kaas (2008) observed in gray squirrels larger SMI-32-ir cells in layer V of the secondary visual areas compared to the primary visual cortex. On the other hand, in the majority of reports involving SMI-32 immunostaining, the number of differentiated cortical areas is limited.…”

Areas of cat auditory cortex as defined by neurofilament proteins expressing SMI-32

“…These studies have utilized a wide range of species including humans (Campbell and Morrison, 1989;Hof et al, , 1995aDel Rio and DeFelipe, 1994;Vogt et al, 2001), old-world monkey (Campbell and Morrison, 1989;Hof and Nimchinsky, 1992;Hof and Morrison, 1995;Hof et al, 1995b;Cusick et al, 1995;Nimchinsky et al, 1996;Preuss et al, 1997;Lewis and Van Essen, 2000;Suzuki and Amaral, 2003;Luppino et al, 2005;Saleem et al, 2007), new-world-monkey (Chaudhuri et al, 1996;Duffy and Livingstone, 2003;Baldauf, 2005;Soares et al, 2008), cat (Kaneko et al, 1994;Van der Gucht et al, 2001, 2005, dog , dolphin , grey squirrel (Wong and Kaas, 2008), hamster (Boire et al, 2005), echidna (Hassiotis et al, 2004(Hassiotis et al, , 2005 and gerbil (Budinger et al, 2000). Across all species examined, large and medium size pyramidal cells are by far the most frequently reactive neuronal types.…”

The marmoset monkey as a model for visual neuroscience

“…This difference in anatomical projections is obvious following the effects of area 17 lesions, which in primates causes a condition that approximates complete blindness but in other mammals is much less severe (Killackey et al, 1971, 1972; Funk and Rosa, 1998). Also, the subdivisions within the input layers, and presumed segregation of innervation to sublayers containing different cell classes, varies in other mammals from prominent (Freund et al, 1985; Wong and Kaas, 2008) to weak or absent (Wong and Kaas, 2009). It is notable that in at least one species, the tree shrew, the geniculostriate terminations in layer 4 convey not transient and sustained responses, but rather “on” and “off” responses, which are similarly segregated in the retina and in the LGN (Conley et al, 1984; Van Hooser et al, 2013).…”