Resumo

OBJECTIVES: The rat retino-collicular system is a glutamatergic pathway that presents a precise retinotopic organization, which develops, from diffuse projections, within the first two weeks after birth. This precise retinotopic map is generated and maintained by acoplated pre- and post-synaptic activity, which could be related to retrograde signaling. Previously, our group showed that nitric oxide (NO) synthesis blockade induces expansion of uncrossed retino-collicular axons. Nitric oxide has been described as an endogenous modulator of adenosine levels, which in turns, are involved in retino-collicular map development. These axon remodeling phenomena are frequently associated to reorganization of growth cone, and growth associated protein 43 (GAP-43) modulation. In this context, we decided to study the in-vivo effects of nitric oxide synthesis blockade on plasticity related proteins expression, such as AMPA glutamate receptor subunits, adenosine receptors and phopspho-GAP-43 (pGAP-43) in superior colliculus. METHODS: Lister Hodded rats were submitted to daily injections of the nitric oxide synthase inhibitor, L-Narg (50 mg/kg ip), from the post-natal day (PND) 9 through PND 12. Control animals were treated with saline 0,9%. At PND 13, the visual layers of superior colliculus were extracted and submitted to western blot analysis against GluR1, GluR2, A1 and A2a receptors and pGAP-43. RESULTS: Our results show that L-Narg promoted a decline in pGAP-43 levels, when compared to saline injected rats. Moreover, nitric oxide synthesis inhibition enhances A1 expression, while doesn´t modulates A2a expression. Concerning the AMPA receptor subunits, the treatment promotes up-regulation of GluR1, whereas down-regulates GluR2 expression in the superior colliculus. CONCLUSIONS: Summarizing, our results suggests that L-Narg-induced retino-collicular plasticity involves pGAP-43, adenosine and AMPA receptors modulation. The lower levels found for pGAP-43 should be related to a more labile system, since pGAP-43 is involved in connectivity maintenance and potentiation. The increase in inhibitory A1 receptors should be destabilizing pre-formed connections, leading to the sprouting of retino-collicular axons. The increase in GluR1 concentration in comparison to GluR2 indicates that the system is making new functional synapses, since GluR1 containing receptors are the first AMPA composition to appear in newly generated synapses.