The effects of opioids in the periaqueductal grey region of rat brain on hind-limb muscle tone.

A mechanical apparatus was used to measure hind limb resistance to flexion in rats injected with opioid peptides in the brain periaqueductal grey region (PAG). (D-Ala2, D-Leu5) enkephalin (delta agonist) and dynorphin1-8 and ethylketocyclazocine (kappa agonists) had no effects on hind limb tone. The mu agonist (D-Ala2, MePhe4, Gly-ol5) enkephalin induced limb rigidity when injected into the PAG. beta-Endorphin caused a dose-related limb rigidity that persisted for up to 2h. beta-Endorphin rigidity was prevented by pretreatment of rats with the serotonin depletor 5,7-dihydroxytryptamine. It is concluded that mu receptors and serotonin mechanisms in the PAG can mediate opiate rigidity.