Spinal Motor Actions of the μ-opioid Receptor Agonist DAMGO in the Cat

For further evaluation of opioidergic spinal motor functions the action of the μ-opioid receptor agonist DAMGO was tested on transmission in different non-nociceptive and nociceptive spinal reflex pathways from flexor reflex afferents (FRA), and in non-FRA reflex pathways in spinal cats. The action of DAMGO was complex, not following a simple pattern with selective depression of nociceptive pathways compared to non-nociceptive ones. Monosynaptic reflexes of the flexor posterior biceps semitendinosus (PBSt) and transmission in nociceptive as well as non-nociceptive excitatory FRA pathways to PBSt were depressed, while the specific excitatory nociceptive non-FRA pathway from the central foot pad to foot extensors was mainly not depressed but rather facilitated by DAMGO. DAMGO caused a facilitation of monosynaptic reflexes to the extensor gastrocnemius soleus (GS) and partly a reversal of inhibitory to excitatory conditioning effects from cutaneous afferents to GS. FRA interneurones could show either an increase or a cessation of their spontaneous activity, but responsiveness to nociceptive and non-nociceptive afferent activation was blocked by DAMGO. The main DAMGO action is generated via interneuronal systems rather than on motoneurones themselves. The results indicate that opioidergic spinal functions are extensively involved in spinal motor control exceeding a mere suppression of nociceptive motor withdrawal reactions.