Naloxone enhancement of spinal reflexes in the rabbit.

Abstract

SUMMARY1. The effects of intravenous morphine, (-)-naloxone and (+ )-naloxone have been studied on three ipsilateral cutaneo-muscular reflexes in spinal rabbits.2. Morphine, 3 mg/kg caused a slow-onset depression of all three reflexes. This effect was naloxone reversible.3. The ipsilateral extensor reflexes, sural to gastrocnemius medialis and saphenous to vastus lateralis were both enhanced to more than double control size following a 5 sag/kg dose of naloxone given in the absence of morphine. For the suralgastrocnemius reflex, naloxone potentiated the reflex drive from all groups of myelinated afferent fibres.4. The ipsilateral flexion reflex, sural to semitendinosus, was only weakly enhanced by naloxone, the 5 ,ug dose leading to an increase in the size of the reflex to 130 % of control. 5. All observed actions of naloxone were stereospecific as the enantiomer (+ )-naloxone failed to affect any of the reflexes even in a dose of 50 ltg/kg.6. We conclude from these findings that opioid peptides are tonically released in rabbit spinal cord, and that they have differential effects in control of flexion and extension reflexes. It is suggested that the ipsilateral extension reflexes are held under a more powerful opioid-mediated depression than that operating upon the flexion reflexes, and that this difference may be related to the greater inhibitory inflow to extensor motoneurones from ipsilateral skin areas.

The contributions of μ‐, δ‐and κ‐opioid receptors to the actions of endogenous opioids on spinal reflexes in the rabbit

“…(Catley et al, 1983) phonate was dissolved in 5% dextrose at 5mgml-' and diluted from this stock. It was given in doses of 0.5, 1, 2, 5, 10, 20, 50, 100, 200 and 500 yg kg-' (a total dose of 888.5 jig kg-') i.v.…”

“…is an example of the more usual flexor reflex. More detailed discussion of the physiological significance of these reflexes and their differential control by endogenous opioids can be found in publications by Clarke (1982) andCatley et al (1983).…”

“…), which is stereospecifically potentiated by low doses of the opioid antagonist naloxone (Catley et al, 1983). The Martin, 1967;Goldfarb & Hu, 1976;Bell & Martin, 1977;Roppolo et al, 1983; and it appears that the control of activity in reflex pathways is one function of the opioids present in the spinal cord.…”

“…Statistical comparisons were by paired t tests: *P <0.05; **P <0.005; ***P <0.0005, otherwise P > 0.05. (Catley et al, 1983), and it seems that the endogenous opioids which act upon this reflex arc suppress responses evoked by all groups ofmyelinated afferent fibres. This is not consistent with the many reports which show that opioids selectively inhibit inputs to the spinal cord from small diameter or nociceptive afferents (see Duggan & North, 1984 for a review).…”

“…The effects of naloxone given after bremazocine depended upon the final dose of this drug. If the dose of bremazocine was restricted to 100 gg kg-', as it was in 3 animals, then (Sherrington, 1910), it is well established that extensor motoneurones receive an excitatory input from skin areas directly overlying their target muscles (Hagbarth, 1952, Catley et al, 1983. As the gastrocnemius muscle lies beneath skin which is innervated by the sural nerve, the sural-g.m.…”