Opioid receptors in the caudate nucleus can mediate EMG-recorded rigidity in rats.

Systemic administrations of opioids are known to induce "catatonia" or "lead pipe rigidity" in rats. The relevance of the caudate nucleus in inducing rigidity was tested. For this purpose, several opioids (or saline) were injected into the head of the caudate nucleus ("intrastriatal" injection) through an implanted cannula, and the electromyographical activity (EMG) was recorded in the gastrocnemius-soleus muscle (GS). Morphine (7.5-3.0 microgram), injected unilaterally, induced a continuous EMG activity in the ipsilateral GS muscle. This effect could be antagonized by systemic administration of naloxone (1 or 2 mg/kg i.p.). D-ala2-met5-enkephalinamide (3 microgram) and levorphanol (22.5 microgram) induced an EMG activity, too, whereas an equimolar dose of dextrorphan was ineffective, indicating that this effect was stereospecific and mediated via opioid receptors in the caudate nucleus. The EMG activity observed after systemic morphine administration (15 mg/kg i.p.) was antagonized by intrastriatal injection of naloxone (5 microgram). From our results, it can be concluded that the striatum--at least the head of the caudate nucleus--plays an important role in mediating the rigidity observed after systemic administration of morphine and other opioids.