The source of corticocollicular and corticobulbar projections in area Te1 of the rat

Abstract

Cortical area Te1 in the rat commonly is associated with primary auditory cortex. It is the source of direct projections to the inferior colliculus (IC), superior olivary complex (SOC), and the cochlear nucleus (CN). A question that arises is whether these descending pathways derive from a common source or separate populations of cortical neurons. We addressed this question in seven rats by injecting either Diamidino yellow (DiY) or Fast blue (FB) into the IC and injecting the other tracer into the CN (n=4) or SOC (n=3). All injections were made on the left side of the brain. In a sample of sections through area Te1 in both hemispheres, we counted single- and double-labeled cells. We estimate that IC-projecting cells outnumber those projecting to the CN or SOC by at least a factor of ten. The source of corticofugal pathways to the left IC was heavily biased towards the same side of the brain (ipsi/contra ratio 8 +/- 2.5), whereas it was more equally distributed between the two hemispheres for the left CN and SOC (ipsi/contra ratios ranged from 0.7-2.3). Finally, we observed that only 10-20% of those cells filled with a tracer injection in the CN or SOC also contained the tracer injected into the IC. In a previous study, we observed a similarly small percentage of double labeled cells when FB and DiY were injected into the CN and SOC, respectively. Combined with the distinct laminar distribution of IC-, SOC-, and CN-projecting neurons within layer V, the results suggest that these three pathways largely derive from different populations of cortical neurons.

Long‐term regulation in calretinin staining in the rat inferior colliculus after unilateral auditory cortical ablation

“…The AC to IC projection has been widely studied (Diamond et al, 1969;FitzPatrick and Imig, 1978;Beyerl, 1978;Andersen et al, 1980;Syka, 1984a,b, 2001;Faye-Lund and Osen, 1985;Coleman and Clerici, 1987;Games and Winer, 1988;Herbert et al, 1991;Saldaña et al, 1996;Winer et al, 1998;Prieto and Winer, 1999;Winer and Prieto, 2001;Doucet et al, 2003;Bajo and Moore, 2005;Bajo et al, 2007;Schofield, 2009), and its distribution in the different IC subdivisions is well known. In the rat, anterograde tract tracing with Phaseolus vulgaris leucoagglutinin in the AC suggests higher terminal field density in the DC and LC relative to the CN of the IC (Saldaña et al, 1996).…”

Multiple topographically organized projections connect the central nucleus of the inferior colliculus to the ventral division of the medial geniculate nucleus in the gerbil, Meriones unguiculatus

“…Projections to the thalamus arise in the superficial and deep parts of cortical layer V and throughout layer VI (Winer and Prieto, 2001). In contrast, projections to the IC arise mainly in the middle of layer V (Games and Winer, 1988;Prieto and Winer, 1999;Doucet et al, 2003;Bajo and Moore, 2005;Bajo et al, 2006), where corticothalamic cells are not found (cf. Winer, 2006).…”

Sound-Evoked Olivocochlear Activation in Unanesthetized Mice

“…Interactions between top-down modulatory systems and the MOC efferent system are particularly intriguing, yet poorly understood, perhaps because of the frequent use of anesthesia in animal studies. A small percentage of deep layer cortical projection neurons directly target MOC neurons in the ventral nucleus of the trapezoid body (Coomes and Schofield 2004; Feliciano and Potashner 1995), but the more prominent routes from the auditory cortex to the MOC neurons are mediated by polysynaptic projections via the inferior colliculus (Doucet et al 2003;Thompson and Thompson 1993;Vetter et al 1993) or cochlear nucleus (Schofield and Coomes 2005). Direct electrical stimulation of the auditory cortex in awake bats can modulate the cochlear microphonic in a frequencyspecific fashion, suggesting that the anatomical links between the corticofugal and olivocochlear control systems have a functional counterpart as well (Suga et al 2002).…”