Abstract
Inhibitory and excitatory neurons exhibit distinct patterns of development in the mammalian cerebral cortex. The morphological development of inhibitory and excitatory neurons derived from fetal rat cerebral cortex has now been compared in vitro. Inhibitory neurons were identified by immunofluorescence staining with antibodies to gamma-aminobutyric acid, and axon formation was detected by staining with antibodies to phosphorylated neurofilaments. In chemically defined, glia-free and low-density cultures, excitatory neurons formed axons within three days of plating. By contrast, inhibitory neurons required more than six days to form axons. Time-lapse analysis over six days revealed that most inhibitory neurons were bipolar and that their two processes exhibited alternate growth and retraction without giving rise to axons. Movement of the cell body towards the growing process was apparent in about one-half of inhibitory neurons, whereas such movement was never seen in excitatory neurons. The migratory behavior of neurons was further investigated by culture on a glial cell monolayer. Inhibitory neurons migrated over substantially larger distances than did excitatory neurons. The centrosome of inhibitory neurons translocated to the base of the newly emerging leading process, suggesting the existence of a force that pulls intracellular organelles towards the leading process. Centrosome translocation was not detected in excitatory neurons. These observations suggest that the developmental programs of excitatory and inhibitory neurons differ. Inhibitory neurons thus possess a more effective cytoskeletal machinery for migration than excitatory neurons and they form axons later.
Highlights
The morphogenesis of projection neurons precedes that of local circuit neurons during development, and this is a basic feature of maturation of the mammalian central nervous system. (Jacobson, 1991)
Given that almost all nonpyramidal neurons in the cerebral cortex contain γaminobutyric acid (GABA) (Meinecke and Peters, 1987), the later development of these cells is responsible for the delayed formation of inhibitory systems in the cortex, providing a window in which enhanced excitation confers neuronal plasticity in young animals (Hensch et al, 1998; Huang et al, 1999)
On the basis of the pattern of staining with anti-GABA and TUJ1, we classified cultured cells as GABA-positive cells (14.4% of a total of 167 cells examined), GABA-negative and TUJ1-positive cells (79%), or TUJ1-negative cells (6.6%). These categories correspond to inhibitory neurons, excitatory neurons and nonneuronal cells, respectively
Summary
The morphogenesis of projection neurons precedes that of local circuit neurons during development, and this is a basic feature of maturation of the mammalian central nervous system. (Jacobson, 1991). Pyramidal neurons begin to form axons while they are still migrating, whereas axons of nonpyramidal neurons develop after these cells have completed their migration (Shoukimas and Hinds, 1978; Parnavelas and Lieberman, 1979; Miller, 1986). Synaptogenesis by the axons of nonpyramidal neurons begins later than that by pyramidal neuron axons (Blue and Parnavelas, 1983; Miller, 1986). Given that almost all nonpyramidal neurons in the cerebral cortex contain γaminobutyric acid (GABA) (Meinecke and Peters, 1987), the later development of these cells is responsible for the delayed formation of inhibitory systems in the cortex, providing a window in which enhanced excitation confers neuronal plasticity in young animals (Hensch et al, 1998; Huang et al, 1999)
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