Extending the cerebellar Lugaro cell class.

We describe here, in Golgi-impregnated rat cerebellar cortex, a new group of large granular layer neurons. These cells have a globular soma located at variable depths in the granular layer, and three to four long radiating dendrites coursing through the three layers of the cortex. The axon projects more or less directly into the molecular layer, where it expands in a local plexus of oblique and tortuous thick collaterals ascending through the major part of the layer. Interestingly, the axons of several of these cells give off a collateral that courses for a long distance in the transverse direction, just above the Purkinje cell somata, parallel to the parallel fibers. While the granular layer location and the polymorphous somato-dendritic pattern of these cells is reminiscent of that of Golgi cells, their axonal pattern is clearly of the same type as that of another large granular layer interneuron, the Lugaro cell. Moreover, double anti-calretinin and anti-calbindin immunolabellings show that Lugaro cells as well as some globular somata dispersed in the granular layer are both calretinin-positive and in close apposition with numerous calbindin-positive varicosities of Purkinje cell axon recurrent collaterals. These latter are known from previous ultrastructural studies to be pre-synaptic to Lugaro cells. The common granular layer location and calretinin labelling, the striking similarity in axonal projection pattern, and the important common recurrent afferentation by Purkinje cell axons strongly argue in favor of the classification of these globular interneurons as a subgroup of a widened Lugaro cell type.