Anatomy of cerebellar Purkinje cells in the rat determined by a specific immunohistochemical marker

Abstract

In the present study we have used guanosine 3′: 5′-phosphate-dependent protein kinase antiserum, a specific immunohistochemical marker for cerebellar Purkinje cells, [ Lohmann, Walter, Miller, Greengard and De Camilli (1981) Proc. natn. Acad. Sci. U.S.A. 78, 653–657], to carry out a detailed analysis of the architecture and projections of Purkinje cells in the adult rat. We have obtained a novel view of aspects of Purkinje cell morphology that were already known and, in addition, we have provided some new information, in particular on the targets of Purkinje cell axons and their pattern of innervation, and on the morphology and course of Purkinje cell axons. Furthermore, we have found a few cells positive for guanosine 3′: 5′ phosphate-dependent protein kinase which are very similar morphologically to Purkinje cells but are located outside of the cerebellar cortex. A unique feature of Purkinje cells is their peculiar monoplanar shape. Not only do their dendritic arbors lie in planes perpendicular to the major axis of the folia, but their axons, including the collaterals, also travel roughly in the same planes. Thus, Purkinje cells can be imagined as lying in longitudinal sheets radiating from the deep cerebellar nuclei. In these sheets, Purkinje cell axons originating from cells located at different rostrocaudal levels of the cortex converge towards the deep cerebellar nuclei without intersecting each other. It is as a result of this precise organization that Purkinje cell axons reach the deep cerebellar nuclei with a mediolateral and rostrocaudal topology that closely reflects the position of their parent cells in the cerebellar cortex. In the subcortical rays of white matter, Purkinje cell axons are interspersed with other axons, being excluded only from longitudinal strips which correspond to the cerebellar raphes. Upon converging towards the deep cerebellar nuclei they segregate into tracts of white matter that alternate with tracts of white matter from which they are excluded. The great majority of Purkinje cell axons terminate in the deep cerebellar nuclei. Recurrent collaterals terminate in close proximity to the Purkinje cell layer. Dense innervation by these axons is found around large interneurons (Lugaro and Golgi cells) and around the Purkinje cell pinceaux. No direct input of recurrent collaterals to Purkinje cell somata is evident in immunostained material. A substantial number of Purkinje cell axons continue beyond the cerebellar nuclei to innervate nearby regions in the brain stem. The most prominently innervated extracerebellar target region is the dorsal part of the lateral vestibular nucleus, which is as heavily innervated by Purkinje axons as the deep cerebellar nuclei are. All the other major parts of the vestibular formation and some adjacent nuclei (including the parabrachial nuclei, the prepositus hypoglossal nucleus and the nucleus of the solitary tract) are innervated to various degrees by Purkinje cells. In these regions heavily innervated cells are interspersed with cells which receive only a moderate degree of innervation and with cells which apparently lack Purkinje cell inputs. Upon reaching the deep cerebellar nuclei, axons destined to extracerebellar targets deviate from the planes of dendritic arborization of their parent cells. They converge into tight bundles which follow an irregular course and intersect each other to reach their targets. Axons travelling in the same bundle often appear to terminate on the same cell. At all target sites Purkinje axons end as varicose terminals which synapse primarily with the perikarya and proximal dendrites of target cells. On the surface of these cells Purkinje cell terminals are often tightly apposed to form a compact mosaic. Both the course of Purkinje cell axons and their pattern of innervation of target cells are consistent with the possibility that contacts between Purkinje cells and their target neurons in the deep cerebellar nuclei and the brain stem are established early in ontogenesis. Purkinje cell-like cells positive for guanosine 3′:5′-phosphate-dependent protein kinase not located in the cerebellar cortex were found predominantly at the dorsal surface of the brain stem and, in particular, in the cortex of the dorsal cochlear nucleus. Only on exception were they found in the cerebellar medulla or in nearby noncortical regions of the brain stem. While some of these cells might be ectopies, the significance of Purkinje cell-like cells in the dorsal cochlear nucleus, a region strikingly similar in architecture to the cerebellar cortex, remains to be established.