Comparative analysis of L-DOPA actions on nociceptive and non-nociceptive spinal reflex pathways in the cat

The actions of L-DOPA (40-100 mg/kg i.v.) on nociceptive and non-nociceptive spinal reflex pathways were investigated in anaemically decapitated high spinal cats. The results revealed a differential pattern of effects of L-DOPA on monosynaptic and oligo-orpolysynaptic nociceptive and non-nociceptive reflexes. (1) L-DOPA depressed monosynaptic reflexes of flexors without affecting those of the extensors. (2) Excitatory pathways from flexor reflex afferents (FRA) were distinctly depressed by L-DOPA, pathways from group II muscle afferents reacted with greater sensitivity than pathways from non-nociceptive cutaneous and joint afferents. (3) Inhibitory FRA pathways were distinctly less affected by L-DOPA than excitatory ones. (4) Transmission in nociceptive excitatory FRA pathways was depressed to the same high degree as that in pathways from group II muscle afferents. (5) Effects on transmission in non-FRA pathways such as the group Ib inhibitory pathway and the excitatory nociceptive pathway from the foot pad to plantaris and intrinsic foot extensors were either minor or absent. (6) L-DOPA increased the delay in the reaction to noxious stimulation. (7) The effects of L-DOPA could not be specifically antagonised by naloxone. Thus, mainly excitatory FRA pathways, irrespective of a nociceptive or non-nociceptive origin, are under strong depressive dopaminergic influences. These effects are similar to those evoked by opioids.