Guanylate cyclase activities in enriched preparations of neurones, astroglia and a synaptic complex isolated from rat cerebellum.

Guanylate cyclase activity was measured in homogenates of a number of freshly isolated and cultured cell preparations derived from the 8-day old rat cerebellum. The freshly isolated fractions enriched in astrocyte perikarya, exhibited twice the basal activity of the Purkinje neurone enriched fraction. In contrast, cultured granule neurones and astrocytes displayed approximately equivalent activities. In freshly isolated cells guanylate cyclase activity could be stimulated approx 15-fold by 1.0 mM sodium nitroprusside. Cultured neurones and astroglia were relatively insensitive to this activator. The excitatory amino acid analogue N- methyl- d -aspartate produced a 150-fold increase in the cGMP content of intact freshly isolated cells, suggesting an enzyme stimulation of a similar order of magnitude to that produced by sodium nitroprusside. Significantly, the cGMP content of intact cultured cells did not increase in the presence of excitatory amino acids or potassium stimulation. These studies demonstrate the presence of high levels of guanylate cyclase activity within astrocytes, and suggest that these cells may make a major contribution to the cGMP synthesizing capacity of the cerebellum. Activation of guanylate cyclase in freshly isolated cells by excitatory amino acids or sodium nitroprusside may involve a mechanism which is absent or ineffective in cultured cells.