FORMATION OF NEW SYNAPTIC CONTACTS BY PURKINJE AXON COLLATERALS IN THE GRANULAR LAYER OF DEAFFERENTED CEREBELLAR CORTEX OF ADULT-RAT

Abstract

In addition to mossy terminals, Golgi axons, granule cell dendrites and, occasionally, Golgi cell dendrites, a 3rd axonal profile identified by morphological criteria as the collateral of Purkinje axons, was found in 2% of all cerebellar glomeruli. These infrequent components of a few glomeruli were never seen in normal cerebellar cortex to establish specialized synaptic contact with glomerular dendrites. After surgical isolation of the cerebellar cortex i.e., following the destruction of efferent and afferent fibers at .apprx. 2-4 wk, the number of glomeruli containing (hypertrophic) axonal branches of Purkinje cells increased to 13% of all surveyed glomeruli. The Purkinje axon terminals in the mossy fiber-deprived glomeruli were observed to establish numerous Gray II-type synaptic contacts with surrounding granule cell dendrites. The development of heterologous synapses between hypertrophic, or even intact, Purkinje axon collaterals and the mossy fiber-vacated granule cell dendrites is a compensatory, reactive process to the synaptic desaturation of granule neurons, which demonstrate a dormant potential of Purkinje cells to form new synaptic contacts in the adult cerebellum.