Neuronal migration is an essential process in the development of the central nervous system. After migration, neurons arrive at the internal granule cell layer and achieve terminal differentiation. Position-specific changes in migratory behavior of cerebellar granule cells require coordinated activity of multiple molecules and signaling systems, such as brain derived neurotrophic factor, somatostatin, EphB2 receptor, and ion channels. To date, a few reports have provided direct evidence for the link between ion channels and cell migration in the cerebellum. Cerebellar granule cells grown in culture possess potassium (K+) currents which include fast transient and delayed rectifier outward currents. K+ channels in cerebellar granule neurons are responsible for determining the neuronal action potential frequency, controlling the strength of synaptic contacts between neurons, contributing to the neuronal resting membrane potential, and regulating excitability of individual neurons. We showed in an earlier study that apoptosis of cerebellar granule neurons induced in low K+ medium was associated with increased current and amplitudes. Melatonin (MT) can modulate ion channels, such as K+, calcium or chloride, and protect granule neurons against apoptosis by receptor or non-receptor mechanisms. MT influences a number of physiological functions by working as a neuromodulator through associated MT receptors. Alterations of the levels of MT have been described in several psychiatric and neurological disorders. The action of MT can be ascribed to its interaction with three receptors that have now been identified as MT1R, MT2R and MT3R and characterized in the hippocampus, cerebral and cerebellar cortex and retina. ? ? In the present study, we examined the effect of MT on granule cell migration. MT increased delayed rectifier outward K+ current amplitude and migration of granule cells, whereas TEA, a K+ channel blocker, decreased the delayed rectifier outward K+ current and slowed migration of the granule cells. Our experiments revealed the existence of TEA-sensitive K+ channel-dependent migration of cerebellar granule cells, and showed that MT stimulates granule cell migration by an increase in K+ channel current, which follows cAMP signal transduction. Although studies on the role of ion channels and ion transporters in cell migration are at an early stage, there is increasing evidence that membrane ion channels are involved in cell locomotion. K+ channels function substantially in the modulation of cell migration suggesting that the activation of K+ channels is responsible for the intracellular K+ loss, which might consequently cause intra-signal transduction leading to cell migration. Incubation of cells with TEA inhibited the K+ outward current and suppressed cell migration, as measured by transwell analysis, and guides the migration of granule cells from the external granule cell layer to the inner granule cell layer in slice cultures. Furthermore, the increasing effect of MT on K+ outward current on cell migration is mediated by MT2R. We found that the reversal effect of TEA on MT-induced migration is incomplete. This reveals, although indirectly, some mechanisms for the facilitory effect of MT on migration other than through modulating the TEA-sensitive K+ channel to promote migration. Among the possible mechanisms underlying the potentiation by MT of K+ currents, cAMP is a major intracellular second messenger that mediates MT regulation of cell functions. In a variety of cells, MT does not alter the basal level of cAMP but inhibits the intracellular accumulation of cAMP. For cyclic nucleotide signaling, cAMP signaling acts as a brake on granule movement. We show that although the application of TEA and cAMP completely block MT-induced K+ current potentiation. MT retains part of the stimulatory effect on cell migration. These data suggest that the MT-induced effect on cerebellar granule cell migration involves an additional signaling pathway associated with MTR. This proposed model illustrates the signaling cascade involved in the neurotrophic action of MT on cerebellar granule cells acting through MT2R, decreases intracellular cAMP and stimulates cell migration.
