The N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor is essential for neuronal differentiation and establishment or elimination of synapses in a developing brain. The activity of the NMDA receptor has now been shown to also regulate the migration of granule cells in slice preparations of the developing mouse cerebellum. First, blockade of NMDA receptors by specific antagonists resulted in the curtailment of cell migration. Second, enhancement of NMDA receptor activity by the removal of magnesium or by the application of glycine increased the rate of cell movement. Third, increase of endogenous extracellular glutamate by inhibition of its uptake accelerated the rate of cell migration. These results suggest that NMDA receptors may play an early role in the regulation of calcium-dependent cell migration before neurons reach their targets and form synaptic contacts.

Transient elevations of intracellular Ca2+ levels play critical roles in neuronal development, but such elevations have not been demonstrated in migrating neurons. Here, we show that the amplitude and frequency components of Ca2+ fluctuations are correlated positively with the rate of granule cell movement in cerebellar microexplant cultures. Moreover, depression of the amplitude and frequency components of Ca2+ fluctuations by blockade of Ca2+ influx across the plasma membrane results in a reversible retardation of cell movement. These results indicate that the combination of amplitude and frequency components of intracellular Ca2+ fluctuations may provide an intracellular signal controlling the rate of neuronal cell migration.

Analysis of neuronal migration in mouse cerebellar slice preparations by a laser scanning confocal microscope revealed that postmitotic granule cells initiate their migration only after the expression of N-type calcium channels on their plasmalemmal surface. Furthermore, selective blockade of these channels by addition of omega-conotoxin to the incubation medium curtailed cell movement. In contrast, inhibitors of L- and T-type calcium channels, as well as those of sodium and potassium channels, had no effect on the rate of granule cell migration. These results suggest that N-type calcium channels, which have been predominantly associated with neurotransmitter release in adult brain, also play a transient but specific developmental role in directed migration of immature neurons before the establishment of their synaptic circuits.

In the mammalian CNS, oligodendrocyte precursor cells (OPCs) express most neurotransmitter receptors, but their function remains unclear. The current studies suggest a physiological role for glutamate (AMPA and/or kainate) receptors in OPC migration. AMPA stimulated ␣ v integrin-mediated OPC migration by increasing both the rate of cell movement and the frequency of Ca 2ϩ transients. A protein complex containing the myelin proteolipid protein (PLP) and ␣ v integrin modulated the AMPA-stimulated migration, and stimulation of OPC AMPA receptors resulted in increased association of the AMPA receptor subunits themselves with the ␣ v integrin/PLP complex. Thus, after AMPA receptor stimulation, an ␣ v integrin/PLP/neurotransmitter receptor protein complex forms that reduces binding to the extracellular matrix and enhances OPC migration. To assess the extent to which PLP was involved in the AMPA-stimulated migration, OPCs from the myelin-deficient (MD) rat, which has a PLP gene mutation, were analyzed. OPCs from the MD rat had a normal basal migration rate, but AMPA did not stimulate the migration of these cells, suggesting that the PLP/␣ v integrin complex was important for the AMPA-mediated induction. AMPA-induced modulation of OPC migration was abolished by pertussis toxin, although baseline migration was normal. Thus, G-protein-dependent signaling is crucial for AMPA-stimulated migration of OPCs but not for basal OPC migration. Other signaling pathways involved in this AMPA-stimulated OPC migration were also determined. These studies highlight novel signaling determinants of OPC migration and suggest that glutamate could play a pivotal role in regulating integrin-mediated OPC migration.

After their final mitosis, cerebellar granule cells remain in the external granular layer (EGL) for 20-48 hr before initiating their radial migration across the molecular layer (ML), but the significance of this latent period is not well understood. In the present study, we used a confocal microscope to examine morphogenetic changes and behavior of postmitotic granule cells restricted to the EGL in slice preparations of the postnatal mouse cerebellum. We found that, coincident with the extension of two uneven horizontal processes oriented parallel to the longitudinal axis of the folium, postmitotic granule cells start to migrate tangentially in the direction of the larger process. Interestingly, their morphology and the speed of cell movement change systematically with their position within the EGL. The rate of tangential cell movement is fastest (ϳ14.8 m/hr) in the middle of the EGL, when cells have two short horizontal processes. As granule cells elongate their somata and extend longer horizontal processes at the bottom of the EGL, they move at a reduced rate (ϳ12.6 m/hr). At the interface of the EGL and ML where cells migrate tangentially at the slowest rate (ϳ4.1 m/hr), their somata round and then begin to extend couples of the descending processes into the ML. After the stationary period, granule cells abruptly extend a single vertical process and initiate the transition from tangential to radial migration, reshaping their rounded somata into a vertically elongated spindle. These observations suggest that tangential migration of granule cells within the EGL may provide the developmental mechanisms for their appropriate allocation across parasagittal compartments of the expanding cerebellar cortex.