The role of biochemically defined systems in cerebellar motor control

THE ROLE OF BIOCHEMICALLY DEFINED SYSTEMS IN CEREBELLAR MOTOR CONTROL 

The processes responsible for the selection of the appropriate output for a particular mossy fiber-parallel fiber input may function at the level of the synapses of the mossy fibers with granule cells, unipolar brush cells, or Golgi cells and at the level of the parallel fiber-Purkinje cell synapse. Several systems may be involved in this selection process. Nitric oxide, that is synthetized by subsets of granule cells (Schilling et ah, 1994 Hawkes and Turner, 1994) and in stellate and basket cells (Bredt et ah, 1990) (Section 

) has been shown to be involved in the production of long-term depression (LTD) of transmission in synapses of the parallel fibers with the Purkinje cell dendritic spines (see Section 3.1.6.), but may also function at the level of the granular layer. Parallel fiber-Purkinje cell transmission also can be blocked by adenosine, acting on the Al-adenosine receptors on the parallel fibers (Section 3.5.). The production of adenosine and the levels of the enzyme 5 -nucleotidase in the molecular layer that degrades adenosine monophosphate to adenosine, both are under the influence of the climbing fibers, and, in certain species, 5 -nucleotidase is distributed in bands. 

A major drawback of many studies of the chemical neuroanatomy is that they were conducted in only one species, the rat. There is extensive evidence for species differences in the distribution of the synthetizing enzyme of acetylcholine (ChAT), muscarinic cholinergic receptors and acetylcholinesterase (see Section 3.10.), and there is reason to assume that a similar interspecies variability exists for other transmitter systems. The expression of Zebrin by certain subpopulations of Purkinje cells, and the zonal patterns in the distribution of 5 -nucleotidase, only occur in certain species. It is a fortunate coincidence for the experimental neuroscientist that the Zebrin zonal pattern is expressed in rats, but in other species like the cat or macaque monkeys all Purkinje cells are Zebrin-immunoreactive. Many species-differences in the chemical neuroanatomy of the cerebellum may be due to the selectivity of the antibodies employed in the im-munocytochemical techniques, but other differences may be real and may reflect true variations in structure or in the transmission and second messenger systems of the cerebellum. 

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