Inhibition in the release of immunoreactive beta-endorphin from the periaqueductal grey during isometric contractions of cat hind-limb muscles: the effects of clonidine.

Glass microelectrodes, coated with antibodies specific for beta-endorphin, were inserted into the right periaqueductal grey (PAG) (at PO.5-1.0 mm, LR 2.0 mm and 6.0 mm below the dorsal surface of the colliculi) of cats anesthetized with alpha-chloralose to determine whether immunoreactive beta-endorphins (ir-END) were released in response to fatiguing isometric contractions of the hind-limb muscles. Probes were inserted into the PAG prior to, during and following muscle contraction in the absence or presence of clonidine. ir-END was released from the PAG up to 3 h after surgery was completed while cats remained at rest. In the absence of clonidine, mean arterial pressure increased by 65 +/- 12 mmHg during contractions and the release of ir-END was inhibited during the contraction periods compared to resting periods. Clonidine abolished the pressor response to muscular contraction when injected into the PAG, but did not cause the release of ir-END during fatiguing isometric contractions. These data suggest that isometric contractions of skeletal muscle do not induce the release of ir-END-like substances from the PAG and clonidine does not attenuate the muscle pressor response by causing the release of ir-END from this level in the PAG.