Mechanisms by which diazepam, muscimol, and other drugs change the content of cGMP in cerebellar cortex.

THE CEREBELLUM CONSISTS OF TWO PARTS: the cerebellar nuclei whose connections to the various parts of the central nervous system coordinate muscle movements, and the cerebellar cortex which exerts an inhibitory influence on the cerebellar nuclei through the release of gamma-aminobutyric acid (gammaAbu) from Purkinje cells. The activity of Purkinje cells is regulated by two excitatory inputs to the cerebellar cortex-the climbing and mossy fibers-and by a neuronal network within the cortex which inhibits the activity of Purkinje cells through the release of gammaAbu from interneurons. The net activity of Purkinje cells is related to their content of guanosine 3':5'-cyclic monophosphate (cGMP) which increases or decreases according to changes in the activity of climbing and mossy fibers as well as to changes in the activation of gammaAbu receptors. When these receptors are activated, the cGMP of Purkinje cells decreases; when they are inhibited, the cGMP increases.The cGMP content of the cerebellar cortex is altered by drugs that change either the excitatory input of climbing or mossy fibers or the inhibitory input mediated by the activation of gammaAbu receptors. Mechanisms by which various drugs alter the cerebellar content of cGMP were investigated. By using various experimental designs, it was shown that diazepam and muscimol lowered the cGMP content by activating gammaAbu receptors. In contrast, morphine and haloperidol lowered the cerebellar cortex cGMP by decreasing the excitation of mossy fibers whereas harmaline increased the cGMP by increasing the excitation of the climbing fibers.