Differential effects of ocular BDNF-injections onto the development of tectal cells characterized by calcium-binding proteins in pigeons

The optic tectum of vertebrates bears a set of visual neurons which can be differentiated by the expression of distinct calcium-binding proteins (CaBPs). Using immunohistochemistry, we mapped the distribution of the CaBPs calbindin (CB) and parvalbumin (PV) in the pigeon's optic tectum and examined if their differentiation is affected by retinal brain-derived neurotrophic factor (BDNF)-injections. CB-immunoreactive (ir) and PV-ir cells displayed a lamination pattern which differed from other birds. While PV-ir cells were present in several retinorecipient tectal laminae, CB-ir cells were confined to layer 3 and 5 and - as a specialization of pigeons - were also detected in a subpopulation of layer 13 neurons. Comparison of saline- and BDNF-injected animals revealed that this general expression pattern was not affected by ocular BDNF-injections. In contrast, the size of tectal cells was differentially modulated. While CB-ir cells in layers 3 and 13 were unaffected by retinal BDNF, cells in layer 5 developed enlarged cell bodies. The PV-ir cells displayed smaller soma sizes within both tectal hemispheres suggesting also an indirect effect of retinal BDNF. These data indicate a differential sensitivity of tectal cell types to retinal BDNF, which might be one mechanism by which retinal input modulates tectal circuitries.