Signals from the cerebellum guide the pathfinding of inferior olivary axons

Spontaneous cluster activity in the inferior olivary nucleus in brainstem slices from postnatal mice

A distinctive property of the cerebellar system is olivocerebellar modules, where synchronized electrical activity in neurons in the inferior olivary nucleus (IO) evokes organized activity in the cerebellar cortex. However, the exact function of these modules, and how they are developed, is still largely unknown. Here we show that the IO in in vitro slices from postnatal mice spontaneously generates clusters of neurons with synchronous Ca(2+) transients. Neurons in the principal olive (PO), and the vestibular-related dorsomedial cell column (dmcc), showed an age-dependent increase in spontaneous calcium transients. The spatiotemporal activity pattern was occasionally organized in clusters of co-active neighbouring neurons,with regular (16 min-1) and irregular (2-3 min(-1)) repeating cluster activity in the dmcc and PO, respectively. IO clusters had a diameter of 100-170 μm, lasted~1 s, and increased in occurrence from postnatal day P5.5 to P12.5, followed by a sharp drop to near zero at P15.5. IO clusters were overlapping, and comprised nearly identical neurons at some time points, and a varied subset of neurons at others. Some neurons had hub-like properties, being co-active with many other neighbours, and some were co-active with separate clusters at different times. The coherence between calcium transients in IO neurons decreased with Euclidean distance between the cells reaching low values at 100-200 μm distances. Intracellular recordings from IO neurons during cluster formation revealed the presence of spikelet-like potentials, suggesting that electrical coupling between neighbouring IO neurons may serve as a synchronizing mechanism. In conclusion, the IO shows spontaneous cluster activity under in vitro conditions, coinciding with a critical postnatal period in olivocerebellar development. We propose that these clusters may be forerunners of the ensembles of IO neurons shown to be co-active in adult animals spontaneously and during motor acts.

Ephrin A/EphA controls the rostral turning polarity of a lateral commissural tract in chick hindbrain

Most post-crossing commissural axons turn into longitudinal paths to make synaptic connections with their targets. Mechanisms that control their rostrocaudal turning polarity are still poorly understood. We used the hindbrain as a model system to investigate the rostral turning of a laterally located commissural tract, identified as the caudal group of contralateral cerebellar-projecting second-order vestibular neurons (cC-VC). We found that the caudal hindbrain possessed a graded non-permissive/repulsive activity for growing cC-VC axons. This non-permissiveness/repulsion was in part mediated by glycosyl-phosphatidylinositol (GPI)-anchored ephrin A. We further demonstrated that ephrin A2 was distributed in a caudal-high/rostral-low gradient in the caudolateral hindbrain and cC-VC axons expressed EphA receptors. Finally,perturbing ephrin A/EphA signalling both in vitro and in vivo led to rostrocaudal pathfinding errors of post-crossing cC-VC axons. These results suggest that ephrin A/EphA interactions play a key role in regulating the polarity of post-crossing cC-VC axons as they turn into the longitudinal axis.

Naturally occurring neuronal death during the postnatal development of Purkinje cells and their precerebellar afferent projections

Naturally occurring neuronal death plays a substantial developmental role in the building of the neural circuitries. The neuronal death caused by different cerebellar mutations is mostly of an apoptotic nature. Apart from the identity of the intrinsic mechanisms of the mutations, adult cerebellar mutants are a powerful tool to causally study the development of the cerebellar connectivity. Thus, studies on adult cerebellar neuronal cell death occurring in mouse mutants elucidate: (i) the dependence of the postsynaptic neurons on their partners, (ii) the 'en cascade' postsynaptic transneuronal degeneration after target-deprivation, and (iii) the close relationship between the molecular modular organization of the cerebellar cortex and dying Purkinje cells. Neuronal cell death has been extensively studied in developing olivocerebellar system. However, less data are available on the occurrence of naturally occurring neuronal death during the in vivo normal development of the Purkinje cells and the mossy fiber system neurons. The developmental role of neuronal death during the establishment and refinement of the olivocerebellar projection is currently discussed. Moreover, the occurrence of neuronal death during the development of the basilar pontine nuclei and its role in the acquisition of the adult pontocerebellar projection is still poorly understood. In the present review, we correlate the dates of Purkinje cells death with the inferior olivary and basilar pontine neuronal apoptosis, discussing their developmental relationships during the elaboration of the fine-grained maps of the cerebellar afferent connections.