Thalamus as a relay station for catalepsy and rigidity

The aim of the study was to determine to what extent catalepsy and tonic rigidity of muscles induced by muscimol administration into the ventral thalamic nuclei disturb the motor activity of rats. This study also aimed to test whether the ventromedial thalamic nucleus (Vm) was involved in transmitting effects evoked by the systemic injection of neuroleptics or opioids. For this purpose muscimol and/or picrotoxin was injected into the ventral thalamic nuclei and the behaviour of the animals was assessed in a series of test situations. It was found that muscimol administration to the Vm disturbs not only the initiation and performance of voluntary movements but also the occurrence of avoidance when the animal's life is endangered. Postural reflexes remained, however, undisturbed. Those effects seemed to be GABA- and site-specific to Vm. The haloperidol catalepsy was strongly inhibited by administration of picrotoxin to the Vm while the morphine catalepsy remained unchanged after picrotoxin. The Vm plays a crucial role in the motor behaviour and transmission of cataleptogenic effects of haloperidol, whereas similar effects produced by morphine appear to by-pass the investigated thalamic region.