Depression of some drug-induced in vivo changes of cerebellar guanosine 3',5'-monophosphate by control of motor and respiratory responses

Paralysis of rats with d-tubocurarine and maintenance of normal arterial pH, CO₂ and O₂ tensions reduced cerebellar cGMP to values only one fourth to one third that of spontaneously roving animals. Since administration of d-tubocurarine intracisternally elevated rather than decreased cerebellar cGMP, the decrease in nucleotide content may be a result of decreased motor behavior and subsequent cerebellar afferent stimulation. In paralyzed rats an increasing mechanical distortion of the chest by increasing tidal volumes raised cerebellar cGMP when arterial gas tensions were held constant. The relative decreases in cerebellar cGMP produced by pentobarbital, ethanol, and halothane in spontaneously active rats were sharply reduced in paralyzed rats. The magnitude of cGMP elevation produced by some drugs (thyrotropin releasing hormone, apomorphine) is also reduced when secondary motor and respiratory effects of these drugs are prevented, whereas the increase produced by harmaline is not altered. Systematic variations of arterial CO₂ and O₂ tensions revealed a negative correlation between the log cGMP content with arterial CO₂ tension and a positive correlation of the log arterial O₂ tension with cerebellar cGMP content in hypercarbic rats. It is concluded that a significant portion of the drug-induced changes in cerebellar cGMP content produced in vivo may be secondary to altered motor and/or respiratory actions of these drugs.