Emergence of cyclic guanosine 3':5'-monophosphate-dependent protein kinase immunoreactivity in developing rhesus monkey cerebellum: correlative immunocytochemical and electron microscopic analysis.

The appearance of cyclic guanosine 3':5'-monophosphate-dependent protein kinase (cGK), an enzyme that may be involved in the regulation of various aspects of neuronal function and that is highly concentrated in cerebellar Purkinje cells, was studied in the developing and adult monkey cerebellum by indirect immunofluorescent staining. The appearance and distribution of cGK immunoreactivity were then correlated with the stages of Purkinje cell differentiation and with the establishment of synaptic inputs to Purkinje cells as revealed by electron microscopy and by the presence of synapsin I, a specific nerve terminal marker. In the adult monkey, in analogy with previous observations in the adult rat, cGK immunoreactivity was detected throughout the cytoplasm of all Purkinje cells and was not seen in other neurons. During ontogenesis, cGK immunoreactivity appeared for the first time in Purkinje cells at the 97th embryonic day (E97). On this day it was detectable in Purkinje cells situated in the posterior lobe concurrently with the emergence of synapsin I immunoreactivity surrounding their somata. The cGK-positive cells had entered the phase of rapid dendritic growth and had begun establishing axosomatic synapses. By E102, Purkinje cells in the posterior lobe and in most of the anterior lobe were cGK positive. By E125, all Purkinje cells had received synaptic contacts and had become cGK positive. In addition to typical Purkinje cells situated in the cortex, we found another population of cGK-positive neurons present transiently in the prospective cerebellar white matter. These neurons, which were observed only during the second half of gestation, had morphological similarities to Purkinje cells. The emergence of cGK in these neurons also coincided with their dendritic proliferation and with the appearance of synapsin I immunoreactivity around their cell bodies. Subcortical cGK-positive cells were not observed in postnatal animals. Such neurons may be Purkinje cells which, failing to reach the cortical plate, subsequently degenerate. The close temporal correlation between appearance of cGK immunoreactivity, onset of synaptic inputs, and dendritic proliferation, both in typical Purkinje cells and in the Purkinje cell-like cells in subcortical areas, suggests that expression of high levels of cGK may be an important aspect of the neuronal differentiation of these cells.