Influence of experimentally induced agranularity on the synaptogenesis of serotonin nerve terminals in rat cerebellar cortex.

The serotonin (5-HT) innervation of the posterior vermis was studied by high resolution radioautography in both normal and X-ray-induced agranular rat cerebella, following 3 h topical superfusion with 10(-4) M 3H-5-HT. In the normal cerebellar cortex, 5-HT axonal varicosities are scarce and only rarely exhibit the membrane differentiations characterizing synaptic contacts. In the agranular cerebellum, 5-HT terminals appear to have a much higher density than in normal controls, although their absolute number may not be significantly different when the important reduction in volume of this experimental cerebellum is taken into account. These terminals frequently show typical synaptic contracts. Most of them are established on the branchlet spines of Purkinje cell dendrites, but some are also observed on the shafts of Golgi cell dendrites. The 5-HT innervation of the cerebellar cortex thus undergoes important changes in the absence of granule cells. It is suggested that these modifications may be part of the general reorganization process of the cerebellar circuitry consequent on the early destruction of the external granular layer. This new example of synaptic remodelling could imply that the formation of cerebellar connectivity is modulated, to a certain extent, by the local cellular environment.