GLUTAMATE-RECEPTOR ACTIVITY IS REQUIRED FOR NORMAL DEVELOPMENT OF TECTAL CELLS DENDRITES IN-VIVO

Glutamatergic retinotectal inputs mediated principally by NMDA recepto rs can be recorded from optic tectal neurons early during their morpho logical development in Xenopus tadpoles. As tectal cell dendrites elab orate, retinotectal synaptic responses acquire an AMPA receptor-mediat ed synaptic component, in addition to the NMDA component. Here, we tes ted whether glutamatergic activity was required for the elaboration of dendritic arbors in Xenopus optic tectal neurons. In vivo time-lapse imaging of single Dil-labeled neurons shows that the NMDA receptor ant agonist APV (100 mu M) blocked the early development of the tectal cel l dendritic arbor, whereas the AMPA receptor antagonist CNQX (20 mu M) or the sodium channel blocker TTX (1 mu M) did not. The decreased den dritic development is attributable to failure to add new branches and extend preexisling branches. These observations indicate that NMDA-typ e glutamatergic activity promotes the initial development of the dendr itic arbor. At later stages of tectal neuron development when AMPA rec eptor-mediated synaptic transmission is strong, both APV and CNQX decr ease dendritic arbor branch length, consistent with a role for glutama tergic synaptic transmission in maintaining dendritic arbor structure, These results indicate that AMPA and NMDA receptors can differentiall y influence dendritic growth at different stages of neuronal developme nt, in correlation with changes in the relative contribution of the re ceptor subtype to synaptic transmission.