ABSTRACT
Axonal fasciculation is a mechanism deployed by growing axons to reach their targets during development of the nervous system. Published data have suggested the involvement of neuronal cell adhesion molecules (NCAM) in axonal fasciculation. We have characterized the formation of axonal fascicles in serum-free, primary cultures of cortical neurons from embryonic rat brains. Unlike the published data, axonal fascicles in our system have a unique morphology: they are waveform, are rarely thicker than 20 µm, and can reach up to several millimeters in length. We observed an age and time dependence in the formation of fascicles. They formed only in cultures from embryonic day 15-17 brain and only between 4 days in vitro (DIV) and 11 DIV. Electron microscopy showed that the fascicles consisted of mostly axonal processes. Immunocytochemical staining confirmed that the fascicles were positive for the 66-kDa neurofilament protein, NF66, but they contained few, if any, microtubule-associated protein-2-positive or glial fibrillary acidic protein-positive processes. Polysialic acids appeared to be critical in the formation of fascicles. Neuraminidase treatment prevented the formation of fascicles when added before 5 DIV. Addition of a specific inhibitor blocked the effect of neuraminidase. The cortical neurons in our model shared several important features with axon fasciculation in vivo and may provide a unique system for studying the molecular mechanisms involved in the formation of axonal tracts in the brain.
Subject(s)
Axons/ultrastructure , Cerebral Cortex/embryology , Neural Cell Adhesion Molecule L1/metabolism , Neurogenesis/physiology , Sialic Acids/metabolism , Animals , Axons/metabolism , Cells, Cultured , Cerebral Cortex/metabolism , Cerebral Cortex/ultrastructure , Embryo, Mammalian , Microscopy, Confocal , Microscopy, Electron, Scanning , Microscopy, Electron, Transmission , RatsABSTRACT
Schizophrenia has been treated effectively with atypical neuroleptics without serious side effects. We have shown previously that long-term treatment with atypical neuroleptics is correlated with an improvement of cognition in adult rats. We report here that atypical neuroleptics stimulate a 2- to 3-fold increase in newly divided cells in the subventricular zone in the rat and that some of these new cells in the subventricular zone and hippocampus also express a neuronal marker. We used bromodeoxyuridine (BrdU) to identify newly divided cells and confirmed the observation with antibody to a cell-cycle-specific, endogenous proliferating cell nuclear antigen (PCNA). Identification of BrdU-positive cells in the anterior subventricular zone (SVZa) particularly in rats treated with the atypical neuroleptics but not in those in the haloperidol-treated and control rats, suggests increased rostral migratory stream (RMS) cell traffic to replenish neurons in the olfactory bulb. Expression of a neuronal marker, NeuN, in BrdU-positive cells in rats treated with atypical neuroleptics, also suggests that these compounds may modulate in vivo differentiation of neuronal progenitor cells even within a day of BrdU injection. Our results indicate that atypical neuroleptics have a mechanism of action other than the previously proposed mechanisms, which might explain their role in improved cognition in animal and in schizophrenic patients. If substantiated by future studies, our findings may lead to an expanded use of atypical neuroleptics in other neurodegenerative diseases to stimulate neuronal replacement and repair.
