Pleiotrophin, nitric oxide and glutamate AMPA receptors in chick cerebellum morphogenesis.

Avian cerebellum, a highly conserved, laminated and foliated structure, provides an excellent model for developmental studies. During the intermediate embryonic stages, granule cell progenitor proliferation and the inwards migration of post-mitotic granule cells have been implicated in the morphogenesis of cerebellar cortex cytoarchitecture and foliation. The present study questioned the spatio-temporal expression pattern of pleiotrophin, an extracellular matrix growth factor, during the morphogenesis of embryonic cerebellum and the roles of ionotropic AMPA glutamate receptors and the diffusible neuromodulator nitric oxide (NO) in the proliferation pattern of EGL granule cell progenitors. For this, the density of proliferating cells in the developing embryonic external granule layer (EGL) was determined following acute treatment with AMPA receptor antagonist CNQX or NO synthase inhibitor L-NAME, at embryonic stages HH38-41 (E12-E15 days), by means of BrdU immunohistochemistry and double immunofluorescence. Importantly, at earlier stages pleiotrophin-like immunoreactivity showed high expression levels in the EGL that gradually decreased, persisting within the growing folia apices, later in development. Interestingly, blockage of AMPA receptors had no effect; while NOS inhibition resulted in transient age- and region-specific increases of EGL granule progenitor cell proliferation at earlier stages, but decreased the post-mitotic granule cells at folia apices, at a later stage HH41 (E15 day). Taken all together, NO had a transient anti-proliferative effect in EGL similar to mammalian cerebellum, acting as a modulator of the EGL function at different stages, suggesting its possible implication in complex processes guiding cerebellar cytoarchitecture and folia formation.