Regulation of 3', 5'-cyclic guanosine monophosphate content in deep cerebellar nuclei.

The 3',5'-cyclic guanosine monophosphate (cGMP) content of deep cerebellar nuclei is decreased following activation of γ-aminobutyrate (GABA) receptors by muscimol and diazepam and is increased following a reduction of GABA content caused by isoniazid. Since previous work has shown that, in deep cerebellar nuclei, GABA receptor stimulation decreases the firing rate of neurones (Ito, Yoshida, Obata, Kawai &;Udo, 1970), it appears that the cGMP content of deep cerebellar nuclei may be an index of the activity of these neurones. Neither the destruction nor the specific activation of climbing fibers changes the cGMP content of deep cerebellar nuclei. Morphine and haloperidol decrease the cGMP content of deep cerebellar nuclei without changing the activity of enzymes that metabolize cGMP in cell free preparations or by activating GABA receptors or changing GABA content. Thus, it is tempting to conclude that a decrease in mossy fiber activity could mediate the decrease in the cGMP content of deep cerebellar nuclei induced by morphine and haloperidol injected parenterally.