Differential effects of granule cell transplantation on two sprouted axonal systems in granuloprival coeruleocerebellar cultures

The changeable nervous system: studies on neuroplasticity in cerebellar cultures

Circuit reorganization after injury was studied in a cerebellar culture model. When cerebellar cultures derived from newborn mice were exposed at explantation to a preparation of cytosine arabinoside that destroyed granule cells and oligodendrocytes and compromised astrocytes, Purkinje cells surviving in greater than usual numbers were unensheathed by astrocytic processes and received twice the control number of inhibitory axosomatic synapses. Purkinje cell axon collaterals sprouted and many of their terminals formed heterotypical synapses with other Purkinje cell dendritic spines. The resulting circuit reorganization preserved inhibition in the cerebellar cortex. Following this reorganization, replacement of the missing granule cells and glia was followed by a restitution of the normal circuitry. Most of these developmental and reconstructive changes were not dependent on neuronal activity, the major exception being inhibitory synaptogenesis. The full complement of inhibitory synapses did not develop in the absence of neuronal activity, which could be mitigated by application of exogenous TrkB receptor ligands. Inhibitory synaptogenesis could also be promoted by activity-induced release of endogenous TrkB receptor ligands or by antibody activation of the TrkB receptor.

Plastic changes in Ara C-treated and "transplanted" coeruleocerebellar cultures

Locus coeruleus axons project to cerebellar cortex in coeruleocerebellar cultures, where they make functional contacts, and also appear as fine fibers in the outgrowth zones. The predominant catecholamine of locus coeruleus neurons in culture is dopamine. When coeruleocerebellar cultures are exposed to cytosine arabinoside to destroy cerebellar granule cells and functionally compromise glia, there is a resultant increase of Purkinje cell survival and a sprouting of Purkinje cell recurrent axon collaterals, plus an increase of catecholaminergic axons accompanied by a doubling of tissue dopamine content. If such reorganized cultures are transplanted with granule cells and glia, a second round of plastic changes ensues in which the Purkinje cell population and the recurrent axon collaterals are reduced to control levels, but catecholaminergic axons and dopamine content remain increased. The maintenance of catecholaminergic axons does not appear to depend on the persistence of target neurons.