TRKB Signaling Controls the Developmental Expression of NMDAR Subunits in Rat and Mouse Visual Cortex

NMDA receptors (NMDARs) are multimeric proteins whose biological and functional characteristics depend on differential subunits assembly during postnatal development. In the present work we deal 'Nith the question as to whether the expression of the different NMDAR subunits is influenced by neurotrophins in developing visual cortex. We used a soluble form of TrkB receptor engineered as an immunoadhesin (TrkB-IgG) in order to block TrkB ligands in the rat visual cortex. TrkB-IgG was released through a cannula implanted in the occipital pole and connected to a mini-osmotic pump. TrkB-IgG was continuously released from postnatal day 20-21 (P20-21) to P36-37. These two post-natal ages were chosen for the beginning and the end of the treatment because they represent the peak and the end of the critical period in the visual cortex respectively. In a different group of animals used as controls osmotic pumps were filled with saline. Different antibodies were used to stain neurons expressing NRl, NR2A and NR2B. We counted the number of neurons stained for NR2A and NR2B subunits and expressed it as percentage with respect to the total number of cresyl-violet stained neurons in each cortical layer. In the visual cortex of TrkB-IgG treated rats the percentage of neurons expressing NR2A was significantly increased in all cortical layers. Concerning NR2B subunit, the percentage of stained neurons was not significantly different between TrkBIgG treated and control rats. The intensity of staining for both NR2A and NR2B was reduced in all cortical layers in TrkB-IgG treated animals with respect to controls. In accordance with these results, the endogenous levels of NR2A and NR2B subunits were reduced in TrkB-IgG treated animals as shown by western blot. Application of TrkB-IgG presented the disadvantage to block at the same time both TrkB ligands: BDNF and NT-4/5. In order to study separately the effects of deprivation of these neurotrophins we acquired knock-out (ko) mice for BDNF and NT-4/5. We considered 3 post-natal ages (P12-14, before eye-opening; P21-23, peak of critical period; adulthood) and we compared the levels of expression of NMDAR subunits NRl, NR2A and NR2B by immunocyology and western blot, in wild type and in ko animals. The absence of BDNF determines a reduction in the level of NR2A at an early stage of post-natal development, while NT-4/5 more likely exerts a modulatory action on both subunits. Thus, TrkB signalling controls the cellular expression of NMDAR subunits in rat and mouse visual cortical neurons during postnatal development and its action is specific for NR2A and NR2B subunits without affecting NRl, whose development is almost over at the time we started our analysis.