Neurons from the cerebellar cortex of cats were examined in electron microscopic preparations after intracellular recording and pressure injections of horseradish peroxidase (HRP). Neurons that contained HRP reaction product within dendrites were identified as either Purkinje or Golgi II cells. This identification was based on specific ultrastructural criteria that included the presence of synapses on the surfaces of dendritic shafts or spines, the identification of the presynaptic component of these synapses, and the presence of certain visible intracellular organelles. In addition, we examined specimens that contained two types of labeled dendrites after a single HRP injection. These dendrites were identified as arising from Purkinje and Golgi II cells and were shown to interdigitate with each other in a dendritic glomerulus. Dendritic appendages or sheet-like spines emanated from the Purkinje cell dendrite and sent small finger-like protrusions that surrounded and invaginated the Golgi II cell dendrite. In this glomerulus, the dendrites were shown to approach each other, and preliminary results suggest the presence of a gap junction at this site of direct apposition. This finding supports physiologic data which suggest electrical coupling between Purkinje and Golgi II cells. In addition, the results of this study demonstrate the usefulness of combining intracellular electrophysiology with HRP staining for the ultrastructural identification of recorded neurons. © 1980.

Intracellular recordings were obtained from 161 neurons of the cerebellar cortex of 20 awake cats. Seventeen neurons marked by intracellular pressure injection of horseradish peroxidase were morphologically identified as Purkinje cells. Seven other neurons were recovered with a morphology characteristic of Golgi II cells. In addition to intrinsic action potentials typical of cerebellar and other mammalian cortical recordings, both types of cells revealed a fast spike-like component that was relatively insensitive to injected current and could not be attributed to regenerative Na+ or Ca2+ conductances intrinsic to the penetrated neuron. In Purkinje cells, the fast spike often appeared at the initial portion of the potential configuration normally called the climbing fiber response. In the other class of cells, the fast, current-insensitive spike followed the intrinsic cell discharge. The results demonstrate distinctive patterns of electrical activity in two different types of morphologically identified cerebellar neurons. The fast spikes could reflect electrical coupling between the two types of cells. © 1980.