The action of K-agonists on the nociceptive responses of neurones in the medullary dorsal horn of the anaesthetized rat

Neurophysiological evidence for increased kappa opioidergic control of spinal cord neurons in rats with unilateral inflammation at the ankle

The role of the endogenous kappa opioid system in the control of neuronal activity has been studied in the spinal cord of normal rats and in rats with Freund's adjuvant induced unilateral inflammation of the ankle under barbiturate anaesthesia. During recordings from neurons with ankle input the kappa receptor agonist U50,488H and/or the kappa antagonist nor-binaltorphamine were administered ionophoretically using multibarrel electrodes. In most neurons tested U50,488H reduced the responses evoked by pressure applied across the ankle whereas smaller proportions of neurons showed increased activity or were not affected. The kappa opioid antagonist nor-binaltorphamine affected more neurons in rats with inflammation than in control rats. Ongoing activity was increased in 7 of 19 (37%) neurons in control rats, in 16 of 24 (67%) neurons in the acute phase of inflammation (2 days post inoculation) and in 15 of 23 (65%) neurons in the chronic phase of inflammation (16-20 days post inoculation). During application of nor-binaltorphamine in control rats, the responses to pressure were increased in 9 cells (36%), reduced in 7 cells (28%) and unaffected in 9 cells (36%). In the acute phase of inflammation significantly more neurons (11 of 15, 73%) showed enhanced responses to pressure during ionophoresis of nor-binaltorphamine but not in the chronic phase. These results show that spinal cord neurons with ankle input are influenced by the endogenous kappa opioid system particularly under inflammatory conditions. The upregulation of this system under inflammatory conditions may serve to counteract inflammation-induced hyperexcitability.

The effects of sham and full spinalization on the systemic potency of mu- and kappa-opioids on spinal nociceptive reflexes in rats

1. Flexor withdrawal reflexes to noxious mechanical pinch stimuli were recorded as single motor unit activity in alpha-chloralose anaesthetized rats, by means of tungsten bipolar electrodes inserted percutaneously into hindlimb flexor muscles. The relative spinal and supraspinal contributions to mu- and kappa-opioid agonists in inhibiting these spinal reflexes, together with possible potency changes elicited by surgical trauma, were examined by comparing their relative potencies in spinally unoperated, sham spinalized and spinalized rats. 2. The noxious stimuli, which were of comparable intensity in the three groups, elicited similar mean firing rates of the motor units in all groups. This indicates that the excitability levels in the reflex pathway were not greatly affected by either sham or actual spinalization. 3. The mu-agonists morphine and fentanyl, and the kappa-agonist U-50,488H, inhibited the reflexes in a dose-dependent manner, when administered intravenously in a log2 cumulative dose regime. 4. The surgery of sham spinalization had little effect on the potency of morphine and fentanyl, whereas it doubled the potency of U-50,488. 5. Spinalization did not affect the potency of morphine. In contrast it decreased the potency of fentanyl 2-4 fold and that of U-50,488 approximately 6 fold. 6. The effects of all agonists were antagonized by naloxone. Dose-dependence studies indicating that antagonism of U-50,488H required about 5 times the dose of naloxone that antagonized morphine. 7. The data suggest that surgical trauma to the spinal column and/or dura mater triggers supraspinal mechanisms that significantly enhance the potency of kappa- but not mu-agonists. 8. It is concluded that most of the effects of systemic morphine on spinal reflexes are mediated, under all three conditions tested, by direct effects in the spinal cord. In contrast, the inhibition of reflexes by U-50,488H is mediated at both spinal and supraspinal levels, the latter being enhanced in the presence of surgical trauma. The differences between morphine and fentanyl remain unexplained.

Effects of intravenous mu and kappa opioid receptor agonists on sensory responses of convergent neurones in the dorsal horn of spinalized rats

1. Electrophysiological experiments have been performed to assess the effects of intravenously administered mu and kappa opioid agonists on the responses to noxious thermal and mechanical and non-noxious tactile stimuli of single convergent neurones in laminae III-VI of the dorsal horn of spinalized rats anaesthetized with alpha-chloralose. 2. The mu receptor agonists tested were fentanyl (1-16 micrograms kg-1) and morphine (0.5-16 mg kg-1) and the kappa-receptor agonists U-50,488 (1-16 mg kg-1) and tifluadom (0.1-1.6 mg kg-1). Multiple drug tests were made on each cell so that compounds could be compared under closely comparable conditions. 3. In one protocol, thermal and mechanical nociceptive responses of matched amplitudes were elicited alternately. Both mu and kappa agonists dose-dependently reduce the neuronal responses. Thermal nociceptive responses were as sensitive to the kappa agents as were the mechanical nociceptive responses; the mu agonists similarly reduced both types of response in parallel. 4. In another protocol, nociceptive and non-nociceptive responses were elicited alternately to permit the degree of selective antinociception to be assessed. The mu agonists were scarcely selective, fentanyl reducing nociceptive only slightly (but significantly at 4-16 micrograms kg-1) more than non-nociceptive responses. The kappa-opioid agonist U50,488 reduced tactile responses somewhat more than nociceptive responses. 5. The spontaneous discharge of these cells with ongoing activity was reduced to a significantly greater degree than the evoked responses; this is likely to have contributed to the non-selectivity of the reduction of the evoked responses. 6. The results are discussed with respect firstly to previous reports that K opioids are ineffective in tests of thermal nociception, and secondly to the likely spinal mechanisms by which opioid receptor agonists mediate antinociception.

Kappa-opioid receptors and analgesia

Over the past decade, opioids have attracted great attention. One important reason for this is the need for novel, strong analgesics free of the abuse potential and side-effects of narcotics such as morphine. Because morphine acts at mu-opioid receptors, efforts have been made to characterize analgesia mediated by non-mu sites, in particular kappa-opioid receptors. There is now good evidence that kappa-receptors do indeed mediate analgesia. However, kappa-agonists display properties that could curtail their therapeutic exploitation. Since the first selective kappa-agonists are now entering clinical trials, this is an opportune moment for Mark Millan to review the pharmacology of drugs of this type in the control of nociception and their therapeutic potential as analgesics.

Correlation of ontogeny with function of [3H]U69593 labelled kappa opioid binding sites in the rat spinal cord

In this study, we have used a variety of in vitro and in vivo techniques to demonstrate the presence, and examine the function, of [3H]U69593 binding sites in the spinal cord of the 9-16-day-old rat in comparison to the adult. Equilibrium binding of [3H]U69593 to homogenates of adult rat spinal cord revealed a single population of non-interacting sites with a maximum binding capacity of 10.4 +/- 1.4 fmol/mg protein and an apparent equilibrium dissociation constant of 2.31 +/- 0.47 nM while in 9-16-day-old cord these parameters were 57.0 +/- 9.4 fmol/mg protein and 2.28 +/- 0.22 nM, respectively. The total binding capacity per cord was 95.8 +/- 8.3 and 121.8 +/- 7.7 fmol/cord for adult and immature rat, respectively. Competition studies using receptor-selective opioid ligands showed that these sites were kappa opioid in nature. Autoradiographical techniques demonstrated a uniform distribution of these sites over transverse sections of 9-16-day-old rat cord. In vitro electrophysiology was performed on spinal cord slice preparations from the 9-16-day-old rat. U69593 (100 nM-1 microM) had no effect on passive membrane properties but produced a naloxone-reversible depression of both spontaneous and electrically evoked activity in dorsal horn neurones. Direct intrathecal injection of U69593 (0.3-10.0 micrograms/animal) into 9-16-day-old rats produced a dose-dependent, naloxone-reversible, antinociception when measured using the paw-pressure test. In conclusion, we have shown that, in contrast to the adult, the spinal cord of the 9-16-day-old rat has a significantly higher concentration of [3H]U69593 binding sites which have functional in vitro and in vivo correlates.

kappa-Opioid agonists produce antinociception after i.v. and i.c.v. but not intrathecal administration in the rat

1. Nociceptive thresholds to noxious mechanical (paw pressure) and thermal (tail flick) stimuli were recorded in conscious rats. The effects of three selective kappa-opioid receptor agonists on the responses to these stimuli were determined following intravenous, intracerebroventricular or intrathecal administration. Results were compared with those obtained with morphine. 2. Following intravenous administration PD117302, U50488, U69593 and morphine produced steep parallel dose-response curves indicating antinociceptive activity when evaluated in the paw pressure test. When U50488 and U69593 were tested at a single dose of 3.3 mg kg-1 no effect was seen in the tail flick test. 3. When given by the intrathecal route only morphine was effective at increasing the nociceptive threshold. PD117302, U50488 and U69593 were without effect in either the paw pressure or tail flick tests when tested at doses up to 100 micrograms per rat. PD117302 caused flaccid paralysis of the hindlimbs following intrathecal administration at the top dose tested. This effect was not reversible by naloxone. 4. All three kappa-opioid receptor agonists produced naloxone-reversible antinociception in the paw pressure test, and to a lesser extent in the tail flick test, when injected directly into the third cerebral ventricle with the maximum effect occurring between 5 and 10 min after administration and declining back to control levels by 60 min. Morphine had a much slower onset of action with the peak effect being observed 30 min after dosing. 5. It is concluded that, under our experimental conditions in the rat, the antinociceptive effects of kappa-agonists are likely to be operated via an action at a supraspinal rather than a spinal site.

Effects of selective and non-selective kappa-opioid receptor agonists on cutaneous C-fibre-evoked responses of rat dorsal horn neurones

We have studied the effects of 3 putative kappa-opioid receptor agonists, U50488H, ethylketocyclazocine (EKC) and dynorphin A1-13 (DYN) on the processing of nociceptive information in the dorsal horn of the rat under halothane anaesthesia. Extracellular single unit recordings were made from convergent or multireceptive lumbar dorsal horn neurones, which could be excited by impulses in A beta and C fibre afferents following transcutaneous electrical stimulation of their ipsilateral hind paw receptive fields and also by noxious and innocuous natural stimuli. Agonists were applied directly onto the surface of the spinal cord. DYN and U50488H consistently produced both a facilitation and inhibition of the C-fibre evoked nociceptive responses of individual cells, these dual effects being relatively insensitive to naloxone antagonism and cancelled each other for the whole population of cells. A beta fibre-evoked responses were little altered. In contrast, EKC consistently depressed C-fibre transmission in a dose-dependent, naloxone reversible manner, analogous to, but considerably less potent than intrathecal morphine under identical experimental conditions. Agonist-induced effects on neuronal responses to natural stimulation (noxious pinch and innocuous prod) were consistent with the changes observed with the electrically evoked responses. The present results therefore indicate that EKC probably exerts its spinal antinociceptive activity in the rat spinal cord in a manner akin to mu-receptor activation. Results with U50488H and DYN indicate that -opioids can excite and inhibit individual neurones but produce no overall change on the whole population, so differing from effects mediated by the other opiate receptors.

Anatomical relationship between opioid peptides and receptors in rhesus monkey brain

To determine whether opioid peptide-receptor pharmacological association found in vitro (e.g., enkephalin-delta, dynorphin-kappa) predict anatomical relationships in situ, immunocytochemical and receptor autoradiographic studies were carried out on adjacent sections from the same brains of formaldehyde-perfused rhesus monkeys. Apparent mu and kappa opioid receptors (labeled, respectively, by [3H] naloxone and [3H]bremazocine under different incubation conditions), but not delta opioid receptors (labeled by [3H]D-Ala2, D-Leu5-enkephalin), survived the fixation procedure, and were found to be colocalized throughout the brain. We have observed complex associations between these binding sites and one, two, or all three opioid peptide systems (i.e., proopiomelanocortin, proenkephalin, and prodynorphin) in different brain regions. These multiple opioid peptide-receptor subtype associations are apparent, for example, in neural systems involved in the processing of pain stimuli, and may be important for mediating different types of analgesia. Since differential processing of proenkephalin and prodynorphin can give rise to opioids of varying receptor selectivities, the colocalization of opioid receptor subtypes may signify that such processing is a key regulatory event in determining which receptor subtype is activated and, thus, the physiological consequences of opioid neurotransmission.