GROWTH OF PYRAMIDAL, BUT NOT NONPYRAMIDAL, DENDRITES IN LONG-TERM ORGANOTYPIC EXPLANTS OF NEONATAL RAT NEOCORTEX CHRONICALLY EXPOSED TO NEUROTROPHIN-3

The present study was undertaken to determine the effects of neurotrop hin-3 (NT3) and spontaneous bioelectric activity (SEA) on dendritic el ongation and branching in long-term isolated organotypic explants of r at neocortex. Viral vector-directed expression of NT3 was used as an e ffective means to ensure a continuous, local production of the neurotr ophic factor. Quantitative light microscopic measurement of dendritic branching patterns was carried out on Golgi-stained materials. Explant s were exposed to an adenoviral vector encoding the genetic sequence f or neurotrophin-3 (Ad-NT3), or to exogenous additions of the neuropept ide NT3. In order to test for activity-dependent growth effects under control and experimental conditions, explants were exposed to glutamat ergic blockade using a cocktail of APV and DNQX. Both Ad-NT3 and NT3 p eptide potently promoted apical and basal dendritic growth (elongation and branching) in pyramidal neurons. This growth was observed to be s ignificant in layers II-IV and V. These growth effects were also not a ctivity dependent, inasmuch as they were elicited from explants in whi ch spontaneous bioelectric activity had been suppressed. Non-pyramidal neurons, throughout the neocortical slice, showed no significant dend ritic responses to the prolonged presence of NT3. These findings show that pyramidal dendritic growth in long-term neocortical explants resp onds to at least one neurotrophic growth factor, NT3, and is independe nt of intrinsic bioelectric activity. The use of viral vectors in deli vering a continuous high level of neurotrophic factor within developin g neural tissues demonstrates its potential application to in vivo tis sues during development, or in the stimulation of neuritogenesis and n euroregeneration following injuries.