Limitedly selective action of a delta-agonistic leu-enkephalin on the transmission in spinal motor reflex pathways in cats.

1. The influence of the delta-opioid receptor agonist (D-Ser2)-leu-enkephalin (Thr6) (DSLET) on different spinal reflex pathways was investigated in anaemically decapitated, high spinal cats. Monosynaptic reflexes were tested to analyse excitatory and inhibitory flexor reflex afferent (FRA) pathways from nociceptive (from the skin of the central pad) and non-nociceptive (from skin, joint or group II muscle) afferents, as well as an excitatory nociceptive non-FRA pathway from the central pad to plantaris and intrinsic foot extensors and the inhibitory pathway from Ib muscle afferents. 2. DSLET suffused over the spinal cord (concentration 10(-3)-10(-6) M) caused a concentration-dependent depression of transmission in nociceptive and non-nociceptive FRA pathways. The excitatory FRA pathways including those from group II muscle afferents were more sensitive than the inhibitory ones. The nociceptive non-FRA pathway from the central pad to plantaris and intrinsic foot extensors was less affected than the FRA pathways. The inhibitory pathway from Ib muscle afferents remained almost unaffected. 3. Intravenous injection of DSLET (0.5-3.6 mg/kg) induced dose-dependent effects similar to those from local spinal application. The main difference was that I.V. injection more readily caused depression of the inhibitory FRA pathways to extensors. 4. The effects of local spinal application and of I.V. injection of DSLET were antagonized by I.V. injection of naloxone (0.1-1 mg/kg). 5. The effects of DSLET on spinal reflex pathways in many respects resemble that of monoamines. Possibly there is an interaction and a co-operation of enkephalins and monoamines in motor control.