Abstract
Purpose::
. The lateral geniculate nucleus (LGN) is the thalamic relay for the visual pathway to the neocortex. At birth in the mouse, axonal terminal arbors from the ipsilateral and contralateral eyes overlap in the LGN but then progressively segregate into sharply defined fields by postnatal day 8 (P8) . The mechanisms of retraction and axonal branch pruning involved in the establishment of the eye-specific innervation of the LGN remains ill-understood. Previously, we observed that the myelin-associated inhibitor for axonal regeneration Nogo-A was not only produced by oligodendrocytes after myelinogenesis but was also highly present in RGC axons during postnatal maturation . We therefore analysed if neuronal Nogo-A could participate to axonal arbor refinement in the LGN.
Methods::
We anterogradely labeled ipsi- and contralateral retinal axons at P8, P10, P15 in wild type (WT) and Nogo-A knock out (Nogo-A-/-) mice. To do this, mouse pups were injected with 1 µl of 0.5% cholera toxin beta subunit (CTB) coupled to alexa-488 (CTB-488) or alexa-594 (CTB-594) in the right and left eye respectively. Two days after CTB injections, mice were perfused intracardially and RGC axon projections were visualized on 40 µm-thick brain coronal sections by fluorescent microscopy. The proportion of overlap between the ipsi- and contralateral eyes was calculated for 3-4 sections in the right and left LGN (7-8 measurements per animal) with NIH software.
Results::
The observations of RGC projections in the LGN revealed that the overlap of the territories of the ipsi- and contralateral eye was higher in the Nogo-A-/- mice (N=5) at P10 compared with age-matched WT animals (N=4). On average, Nogo-A-/- animals presented 14% more overlap than WT pups (t-test, P<0.05). In contrast, in adult Nogo-A-/- mice, eye-specific projections appeared completely separated in the LGN similarly to WT mice.
Conclusions::
Our data suggest that Nogo-A is involved in the process of axonal arbor maturation in the LGN. Ongoing experiments should help to determine if Nogo-A exerts its effects during the early phase of segregation controlled by spontaneous retinal activity or later during the maintenance phase influenced by visual experience.
Keywords: thalamus/lateral geniculate nucleus • development • optic nerve