INFLUENCE OF OPIOIDS AND NALOXONE ON RHYTHMIC MOTOR-ACTIVITY IN SPINAL CATS

The effects of L-DOPA, naloxone, and the opioids (D-Ala(2),N-Me-Phe(4) ,Gly(5)-ol)-enkephalin (DAGO) and D-Ser(2)-Leu-enkephalnin-Thr(6) (DSL ET) on spinal motor rhythm generation were compared in anemically deca pitated high spinal cats. After premedication with nialamide, DOPA cau sed the well-known, slow rhythmic motor activity with a locomotor patt ern. The cycle duration of the evoked rhythm was usually between 3.9 a nd 5.0 s. The opioids DAGO and DSLET, injected intravenously (1.2-2 mg /kg) or suffused over the lumbar spinal cord (10(-3)-10(-4) M in Ringe r's solution), severely depressed the DOPA-induced rhythmic activity, sometimes completely abolishing efferent motor activity. Naloxone (0.5 -1 mg/kg i.v.) exerted different rhythm-facilitating effects, dependin g on the experimental condition. In the acute phase after spinalizatio n, without paralysis and without nialamide and DOPA, naloxone induced rhythmic movements with a main frequency of 1.2-2 Hz. In the same prep aration with paralysis, naloxone induced a rhythmic motor activity wit h a distinctly higher frequency (main range 4.3-5.8 Hz). After premedi cation with nialamide and DOPA, naloxone facilitated or, if a rhythm w as absent, induced the slow-frequency DOPA type of rhythm. Given after i.v. or topical opioid application, naloxone antagonized the rhythm-d epressing action of the opioid and caused an additional facilitation o f rhythmic activity. Dopa and naloxone facilitated the long-latency, s egmental reflex pathways from flexor reflex afferents (FRA), while the opioids depressed them. The short-latency FRA pathways were depressed by DOPA and opioids but were facilitated by naloxone. The influence o f the different drugs on spinal motor rhythm generation is discussed i n relation to their influence on short- and long-latency segmental pat hways from FRA. If the rhythm generation induced by DOPA is based on t he release of the long-latency FRA pathways, as has been proposed befo re, the rhythm-depressing action of opioids may be due to the suppress ion of these pathways, and the particular rhythm-generating function o f naloxone may be related to its facilitation of short- and long-laten cy FRA pathways.