Significance of dopamine receptor antagonists in human postural control

The purpose of this study was to investigate the effect of different dopamine receptor antagonists on the function of proprioceptive reflex mechanisms involved in the regulation of stance and gait. Haloperidol as preferential D2 and the 'pure' dopamine D2 receptor antagonist sulpiride significantly reduced the level of the reflex response in the gastrocnemius muscle following backward perturbation. Flupentixol, however, which in addition blocks D1 receptors, had no effect on compensatory gastrocnemius EMG responses. Furthermore, neither the peripherally acting dopamine D2 receptor antagonist domperidon nor the sedative diazepam had any significant influence on this functionally essential reflex mechanism. None of the drugs tested had a significant effect on the tibialis anterior responses elicited following forward perturbation. It is concluded that the reduced stretch sensitivity of the gastrocnemius (but not that of the tibialis anterior) during perturbations of stance described for parkinsonian patients corresponds to these results and arises from an impaired function of central dopamine D2 receptors.