Abstract
Callosal patches in primary visual cortex of Long Evans rats, normally associated with ocular dominance columns, emerge by postnatal day 10 (P10), but they do not form in rats monocularly enucleated a few days before P10. We investigated whether we could replicate the results of monocular enucleation by using tetrodotoxin (TTX) to block neural activity in one eye, or in primary visual cortex. Animals received daily intravitreal (P6-P9) or intracortical (P7-P9) injections of TTX, and our physiological evaluation of the efficacy of these injections indicated that the blockade induced by a single injection lasted at least 24h. Four weeks later, the patterns of callosal connections in one hemisphere were revealed after multiple injections of horseradish peroxidase in the other hemisphere. We found that in rats receiving either intravitreal or cortical injections of TTX, the patterns of callosal patches analyzed in tangential sections from the flattened cortex were not significantly different from the pattern in normal rats. Our findings, therefore, suggest that the effects of monocular enucleation on the distribution of callosal connections are not due to the resulting imbalance of afferent ganglion cell activity, and that factors other than neural activity are likely involved.
Highlights
Visual callosal connections in several species have proven to be a useful model for investigating the role of sensory input in the development of cortical connections (Cusick & Lund, 1982; Olavarria et al, 1987; reviewed in Olavarria, 2002; Restani & Caleo, 2016)
It was recently reported that callosal connections in primary visual cortex (V1) of Long Evans rats segregate into patches that overlap with ipsilateral ocular dominance columns (ODCs) (Laing et al, 2015)
To investigate whether retinal activity is necessary during this period for the specification and development of callosal patches, we carried out daily (P6–P9) monocular intravitreal injections of tetrodotoxin (TTX), and subsequently examined the pattern of callosal connections in V1 following multiple intracortical injections of the tracer horseradish peroxidase (HRP) in the opposite hemisphere
Summary
Visual callosal connections in several species have proven to be a useful model for investigating the role of sensory input in the development of cortical connections (Cusick & Lund, 1982; Olavarria et al, 1987; reviewed in Olavarria, 2002; Restani & Caleo, 2016) Most of these early studies do not examine the role of neural activity in this process. Evidence that neural activity plays an important role in the development of visual callosal connections comes from studies examining the effect of reducing excitability of callosal neurons via overexpression of Kir2.1, a hyperpolarizing inward-rectifying potassium channel (Mizuno et al, 2007, 2010). These results provide evidence that neither retinal nor cortical activity play a significant role in the specification and development of V1 callosal patches before P10
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