Inhibitory cerebellar circuits: GAD immunoreactivity in the frog cerebellum

The presence of GABAergic structures in the frog cerebellum with simple nervous circuits was investigated with an immune serum raised against glutamic acid decarboxylase (GAD). GAD immunoreactivity is detectable in all the cerebellar layers. The most obvious difference in comparison with mammals is the scarcity of GAD-positive terminal boutons on Purkinje cells and this is consistent with neurophysiological data indicating that the inhibition on frog Purkinje cells is weaker than in mammals. However, there are numerous GAD synapses on cerebellar and vestibular nuclei. A large number of GAD immunoprecipitates are present in the molecular and granular layers, though their morphological organization is different from that in mammals. In the upper part of the molecular layer GAD immunoreactive cells and fibres form a plexus-like structure and in the internal granule layer immunoreactive varicosities are not arranged in the ring-like assemblies of mammals.