Opioidergic modulation of spinal reflexes activated by mechanical stimulation of the heel in the decerebrated, spinalized rabbit.

The effects of opioid receptor blockade on reflex responses elicited in medial gastrocnemius and semitendinosus motor nerves by controlled mechanical stimulation of the skin over the heel have been investigated in decerebrated, spinalized rabbits. Pinch forces of 173 mN, 561 mN (light touch), 1642 mN (firm pinch) and 4632 mN (firm to painful) were used. In the control state, background activity was absent from gastrocnemius motoneurones and low in semitendinosus. Pinches of 561 mN and above evoked brisk reflexes in gastrocnemius motoneurones, whereas delayed responses (median latency 1 s) were elicited in semitendinosus by pinches of 1642 mN and 4632 mN. Intravenous administration of the non-selective opioid receptor antagonist (-)-quadazocine (1-781 microg kg(-1)) dose-dependently and stereospecifically increased background activity and enhanced reflex responses to all pinch strengths in both medial gastrocnemius and semitendinosus muscle nerves. Pinches of 173 mN became reflexogenic and higher intensity stimuli evoked significantly larger responses than in the control state. These findings show that tonic opioidergic inhibition in rabbit spinal cord is wide-spread and non-selective, having a powerful influence on transmission of signals to motoneurones from high- and low-threshold cutaneous afferents.