AFFERENT INNERVATION INFLUENCES THE DEVELOPMENT OF DENDRITIC BRANCHESAND SPINES VIA BOTH ACTIVITY-DEPENDENT AND NON-ACTIVITY-DEPENDENT MECHANISMS

The present investigation uses an in vitro co-culture system to study the role of afferent innervation in early development and differentiat ion of hippocampal neurons. Our experiments indicate that the formatio n of two key morphological features, dendritic branches and dendritic spines, is induced by afferent innervation, Hippocampal neurons develo p multiple dendritic branches and spines only when extensively innerva ted by living axonal afferents. No morphological changes occurred when hippocampal neurons were plated on other cell surfaces such as fixed axons or astrocytes. Furthermore, afferents exerted their effect local ly on individual dendrites that they contacted. When one portion of th e dendritic arbor of a neuron was contacted by afferents and the other portion was not, morphological effects were restricted to the innerva ted dendrites, Innervation of some of the dendrites on a neuron did no t produce global effects throughout the neuron. Afferent-induced dendr itic branching is independent of activity, since branch induction was unaffected by chronic application of TTX or glutamate receptor blocker s. In contrast, the formation of dendritic spines is influenced by act ivity, The number of developing spines was reduced when TTX or a cockt ail of three glutamate receptor blockers was applied, Blockade of indi vidual AMPA, NMDA, or metabotropic glutamate receptors did not affect the number of spines, These results, taken together, demonstrate that afferents can have a prominent influence on the development of postsyn aptic target cells via both activity-dependent and non-activity-depend ent mechanisms, indicating the presence of multiple signals. According ly, this suggests an important interplay between pre-and postsynaptic elements early in development.