Light experience induces differential asymmetry pattern of GABA- and parvalbumin-positive cells in the pigeon's...

The formation of functional and morphological asymmetries within the pigeon's tectofugal system depends on left-right differences in visual input during embryonic development. This asymmetric stimulation presumably affects activity-dependent differentiation processes within the optic tectum. Behavioral studies reveal that prehatch light stimulation asymmetry influences both left- and right-hemispheric processes in a differential way. Thus, we have to assume divergent effects on both hemispheres. This study represents an attempt to test the hypothesis that embryonic light asymmetry induces different, cell-type-specific effects in the left and the right optic midbrain. Since it is likely that inhibitory interneurons play a critical role in the establishment of asymmetries, we examined in both sides of the brain the soma sizes of GABA- and parvalbumin- (PV) immunoreactive (ir) cells of the tectum and the magnocellular isthmic nucleus in controls and in dark-incubated animals. No cell size asymmetries of magnocellular isthmic neurons were found in either dark-incubated or control birds. Dark-incubation also prevented the establishment of lateralized differences in GABAergic and PV-positive tectal cells. However, in control birds GABAergic cells displayed larger somata in the left tectum, whereas PV-ir neurons were enlarged within the right tectum. This complementary asymmetry pattern suggests that PV- and GABA-ir tectal cells represent different cellular populations which react differently to visual input. Thus, our data show that visual lateralization does not result from a mere growth promoting effect that enhances differentiation within the behaviorally dominant left side, but is constituted by different cell type-specific circuits which are divergently adjusted in the left and in the right tectum.