A comparison of the actions of some drugs on decerebrate rigidity, muscle spindle activity and alpha-adrenoceptors.

1 The relative potencies of methotrimeprazine, (+)-methotrimeprazine, (+/-)-10-(3-dimethylamino-2-methylpropyl)-2-valeroyl phenothiazine hydrochloride (M & B 18,706) and (+)-M & B 18,706 in reducing the pressor action of noradrenaline in the spinal cat, reducing intercollicular decerebrate rigidity, and muscle spindle afferent activity have been studied.2 Methotrimeprazine was eight times as potent as (+)-methotrimeprazine in reducing the pressor action of noradrenaline and six times as potent in reducing decerebrate rigidity. M & B 18,706 was also eight times as potent as (+)-M & B 18,706 in reducing the pressor action of noradrenaline and six times as potent in reducing decerebrate rigidity.3 For the above compounds and chlorpromazine there was a significant correlation between the effective doses for the inhibition of the pressor action of noradrenaline and for the reduction of decerebrate rigidity.4 The doses which reduced decerebrate rigidity were similar to those that reduced muscle spindle afferent discharge. It is likely that these drugs reduce decerebrate rigidity by inhibiting fusimotor activity.5 Desipramine increased decerebrate rigidity and increased spindle afferent discharge.6 It is thought that the phenothiazine derivatives studied reduce decerebrate rigidity and spindle afferent discharge by inhibiting receptors for noradrenaline in the central nervous system.