Analgesic activity of intraspinally administered dynorphin and ethylketocyclazocine

Dynorphin and stress-related peptides in rat locus coeruleus: contribution of amygdalar efferents

The interaction between the stress axis and endogenous opioid systems has gained substantial attention, because it is increasingly recognized that stress alters individual sensitivity to opiates. One site at which opiates and stress substrates may interact to have global effects on behavior is within the locus coeruleus (LC). We have previously described interactions of several opioid peptides [e.g., proopiomelanocortin, enkephalin (ENK)] with the stress-related peptide corticotropin-releasing factor (CRF) in the LC. To examine further the interactions among dynorphin (DYN), ENK, and CRF in the LC, sections were processed for detection of DYN and CRF or DYN and ENK in rat brain. DYN- and CRF-containing axon terminals overlapped noradrenergic dendrites in this region. Dual immunoelectron microscopy showed coexistence of DYN and CRF; 35% of axon terminals containing DYN were also immunoreactive for CRF. In contrast, few axon terminals contained both DYN and ENK. A potential DYN/CRF afferent is the central nucleus of the amygdala (CeA). Dual in situ hybridization showed that, in CeA neurons, 31% of DYN mRNA-positive cells colocalized with CRF mRNA, whereas 53% of CRF mRNA-containing cells colocalized with DYN mRNA. Finally, to determine whether limbic DYN afferents target the LC, the CeA was electrolytically lesioned. Light-level densitometry of DYN labeling in the LC showed a significant decrease in immunoreactivity on the side of the lesion. Taken together, these data indicate that DYN- and CRF-labeled axon terminals, most likely arising from amygdalar sources, are positioned dually to affect LC function, whereas DYN and ENK function in parallel.

Possible involvement of dynorphin A-(1-17) release via mu1-opioid receptors in spinal antinociception by endomorphin-2

The antinociception induced by i.t. or i.c.v. administration of endomorphins is mediated via mu-opioid receptors. However, although endomorphins do not have an appreciable affinity for kappa-opioid receptors, pretreatment with the kappa-opioid receptor antagonist norbinaltorphimine markedly reduces the antinociceptive response to i.c.v. or i.t. administered endomorphin-2 but not endomorphin-1. These results suggest that endomorphin-2 initially stimulates mu-opioid receptors, which subsequently induce the release of dynorphins that act on kappa-opioid receptors to produce antinociception. The present study was performed in mice to determine whether the release of dynorphins by i.t. administered endomorphin-2 is mediated through mu-opioid receptors to produce antinociception. Intrathecal pretreatment with an antiserum against dynorphin A-(1-17), but not against dynorphin B-(1-13) or alpha-neoendorphin, dose-dependently prevented the paw-withdrawal inhibition by endomorphin-2. The pretreatments with these antisera did not affect the endomorphin-1- or [D-Ala(2),MePhe(4),Gly(ol)(5)]enkephalin-induced paw-withdrawal inhibition. The attenuation of endomorphin-2-induced antinociception by i.t. pretreatment with an antiserum against dynorphin A-(1-17) or s.c. pretreatment with norbinaltorphimine was blocked dose-dependently by s.c. pretreatment with the mu-opioid receptor antagonist beta-funaltrexamine or the mu(1)-opioid receptor antagonist naloxonazine at ultra-low doses that are ineffective against mu-opioid receptor agonists. These results suggest that the spinal antinociception induced by endomorphin-2 is mediated through the stimulation of a distinct subtype of mu(1)-opioid receptor that induces the release of the endogenous kappa-opioid peptide dynorphin A-(1-17) in the spinal cord.

Acupuncture and endorphins

Acupuncture and electroacupuncture (EA) as complementary and alternative medicine have been accepted worldwide mainly for the treatment of acute and chronic pain. Studies on the mechanisms of action have revealed that endogenous opioid peptides in the central nervous system play an essential role in mediating the analgesic effect of EA. Further studies have shown that different kinds of neuropeptides are released by EA with different frequencies. For example, EA of 2 Hz accelerates the release of enkephalin, beta-endorphin and endomorphin, while that of 100 Hz selectively increases the release of dynorphin. A combination of the two frequencies produces a simultaneous release of all four opioid peptides, resulting in a maximal therapeutic effect. This finding has been verified in clinical studies in patients with various kinds of chronic pain including low back pain and diabetic neuropathic pain.

A review of the properties of spiradoline: a potent and selective kappa-opioid receptor agonist

The selective kappa-opioid receptor agonist spiradoline mesylate (U62,066E), an arylacetamide, was synthesized with the intention of creating an analgesic that, while still retaining its analgesic properties, would be devoid of the, mainly mu receptor mediated, side effects such as physical dependence and respiratory depression associated with morphine. Spiradoline is highly selective for the kappa receptor with K(i) of 8.6 nM in guinea pig. Examination of the enantiomers of spiradoline, showed the (-)enantiomer to be responsible for the kappa agonist properties. Spiradoline easily penetrates the blood brain barrier, and does not seem to have any significant active metabolites. In preclinical studies, spiradoline has a short duration of action with a peak at around 30 min after administration. The analgesic properties of spiradoline are well documented in mice and rats. Antitussive properties have also been reported in rats. Furthermore, spiradoline was reported to display effects suggestive of neuroprotective properties in animal models of ischemia. In humans, spiradoline is a potent diuretic. It also produces significant sedation presumably due to its antihistamine properties. Preclinical studies have shown that spiradoline reduces blood pressure and heart rate, and has possible antiarrhythmic properties. Clinical studies did not confirm these findings. kappa Receptors inhibit dopaminergic neurotransmission. Spiradoline, given systematically to rats, produces a significant and long lasting decrease in dopamine release, and in locomotor activity. It has also antipsychotic-like effect in animal behavioral tests. At low doses spiradoline was reported to decrease tics in patients with Tourette's syndrome. Although spiradoline had promising effects in animal tests of analgesia, and a reasonably good safety profile in preliminary studies, it did not replace morphine as an analgesic. The available clinical data suggest that spiradoline produces disturbing adverse effects such as diuresis, sedation, and dysphoria at doses lower than those needed for analgesic effects. Thus, future development of spiradoline-like analgesic compounds should preferably focus on reduction of unwanted effects on the central nervous system. Spiradoline, which currently is commercially available for preclinical research, might prove useful in some psychiatric conditions and possibly as a neuroprotective agent.

Estrogen and progesterone activate spinal kappa-opiate receptor analgesic mechanisms

Rats and humans manifest elevated response thresholds to aversive stimuli during gestation and parturition. This pregnancy-associated antinociception is mediated, in part, by a spinal cord dynorphin/kappa antinociceptive system. Simulating the maternal pregnancy blood concentration profile (in non-pregnant animals) of 17-beta-estradiol (E2) and progesterone (P) produces an opioid antinociception which closely approximates that of actual pregnancy. The current study was initiated in order to determine whether sex steroid-induced antinociception involves a spinal cord kappa-opiate receptor-coupled system (as does the antinociception of actual gestation). Additionally, sex steroid modulation of the intrathecal (i.t.) antinociceptive effectiveness of a kappa agonist was investigated. The opioid antinociception associated with simulating the pregnancy blood concentration profile of E2 and P (hormone-simulated pregnancy, HSP) is significantly antagonized by i.t. administration of nor-binaltorphimine, an antagonist highly specific for the kappa-opiate receptor. This indicates that exposure (of non-pregnant animals) to the pregnancy blood profile of E2 and P activates a spinal cord kappa-opiate receptor analgesic system, as occurs during actual gestation. Furthermore, during HSP, antinociceptive responsiveness to i.t. U50,488H (kappa-selective) is significantly enhanced (approximately 40%). This effect is abolished in animals treated concomitantly with steroid hormones and systemic naltrexone or i.t. nor-binaltorphimine. In contrast to the effects of steroid treatment on antinociceptive responsiveness to i.t. U50,488H, no alteration in antinociceptive responsiveness to i.t. sufentanil was observed on day 19 of HSP over all doses tested (0.1-1 nmol). Thus, during HSP (and actual gestation), a less robust constituent of intrinsic opioid pain-attenuating systems in the spinal cord is recruited.

Estrogen and progesterone activate spinal kappa-opiate receptor analgesic mechanisms

Rats and humans manifest elevated response thresholds to aversive stimuli during gestation and parturition. This pregnancy-associated antinociception is mediated, in part, by a spinal cord dynorphin/kappa antinociceptive system. Simulating the maternal pregnancy blood concentration profile (in non-pregnant animals) of 17-beta-estradiol (E2) and progesterone (P) produces an opioid antinociception which closely approximates that of actual pregnancy. The current study was initiated in order to determine whether sex steroid-induced antinociception involves a spinal cord kappa-opiate receptor-coupled system (as does the antinociception of actual gestation). Additionally, sex steroid modulation of the intrathecal (i.t.) antinociceptive effectiveness of a kappa agonist was investigated. The opioid antinociception associated with simulating the pregnancy blood concentration profile of E2 and P (hormone-stimulated pregnancy, HSP) is significantly antagonized by i.t. administration of nor-binaltorphimine, an antagonist highly specific for the kappa-opiate receptor. This indicates that exposure (of non-pregnant animals) to the pregnancy blood profile of E2 and P activates a spinal cord kappa-opiate receptor analgesic system, as occurs during actual gestation. Furthermore, during HSP, antinociceptive responsiveness to i.t. U50,488H (kappa-selective) is significantly enhanced (approximately 40%). This effect is abolished in animals treated concomitantly with steroid hormones and systemic naltrexone or i.t. nor-binaltorphimine. In contrast to the effects of steroid treatment on antinociceptive responsiveness to i.t. U50,488H, no alteration in antinociceptive responsiveness to i.t. sufentanil was observed on day 19 of HSP over all doses tested (0.1-1 nmol). Thus, during HSP (and actual gestation), a less robust constituent of intrinsic opioid pain-attenuating systems in the spinal cord is recruited. pF to mediate, at least in part, the maternal antinociception of gestation. pF, positive modulation of the spinal cord kappa analgesic system occurs post-synaptically. This laboratory previously reported that simulating the pregnancy blood concentration profile of E2 and P also positively modulates spinal dynorphin content and the processing of its precursor, suggesting a presynaptic loci of action. Thus, female rats possess a spinal dynorphin/kappa analgesic system that can be positively modulated, pre-synaptically as well as post-synaptically, by circulating sex steroids.

Change in opioid peptide level in the heart and blood plasma during acute myocardial ischaemia complicated by ventricular fibrillation

1. The influence of acute myocardial ischaemia (AMI) complicated by ventricular fibrillation (VF) on opioid peptide level in myocardium and blood plasma of rats has been studied. 2. Leu-enkephalin level in myocardium of rats with AMI and VF has been found to be significantly lower than in animals with AMI but without VF. 3. Met-enkephalin level has been found to be significantly increased in both animals with VF and without it. We have not found a significant difference in met-enkephalin level in myocardium of animals with VF and without it. 4. AMI was induced to increase the enkephalin and beta-endorphin level in blood plasma of all the animals, whether VF had occurred or not. 5. Preliminary administration of D-Ala2,Leu5,Arg6-enkephalin, a synthetic analogue of leu-enkephalin, has prevented a decrease of ventricular fibrillation threshold in experimental coronary occlusion. 6. The obtained results allow us to conclude that enkephalins of myocardium but not opioid peptides of blood plasma play an important role in VF occurrence.

Spinal opioid systems in inflammation

Until recently, basic science studies, both behavioural and electrophysiological, have concentrated on the antinociceptive actions of opioids primarily gauged against acute nociceptive responses. However, of more relevance to clinical situations are the actions of opioids in more persistent/prolonged pain states. This review sets out to examine the central actions of opioids against nociception of inflammatory origins. The first section deals with the response of the endogenous opioid system to the development of an inflammatory state and the second examines the ability of exogenous opioids to modulate inflammatory nociception. There are complex changes in the roles of endogenous opioids, in particular dynorphin, at the spinal level after inflammation although the physiological consequences remain unclear. With regard to exogenous opioids, the effectiveness of spinal morphine is rapidly enhanced after inflammation, likely to be due to changes in the interaction between the peptide cholecystokinin and the mu opioid receptor. The ability of inflammatory processes to alter both endogenous opioids and morphine analgesia at the spinal level illustrates the considerable degree of plasticity observed in opioid function.

Comparative study of the analgesic and paralytic effects induced by intrathecal dynorphin a in rats

Intrathecal injection of dynorphin A produced dual effects on sensory and motor functions in the spinal cord of the rat. At a dose of 5 nmol, dynorphin A produced an increase in tail flick latency (TFL) as well as a reversible motor paralysis as assessed by change in the angle of inclined plane. At a dose of 10 or 20 nmol, dynorphin produced a motor paralysis lasting for up to 24 hours. The effect of dynorphin A on the sensory function of the spinal cord was shown by an increase in the vocalization threshold induced by electrical stimulation of the tail, at dose range of 1.25-10 nmol, with a quick onset (5 min) and relatively short duration (within 60 min). Unlike tail flick reaction which involves spinal motor function, tail stimulation-induced vocalization threshold is a relatively pure index for spinal nociceptive activities. The differential effect of dynorphin on sensory and motor function was supported by the evidence that (1) dynorphin-induced analgesic effect (increase in vocalization threshold) was naloxone reversible, whereas dynorphin-induced motor paralysis was naloxone resistant. (2) Nor-BNI, a specific antagonist for kappa opioid receptor, blocked the sensory effect of dynorphin, but had no influence on motor effect of dynorphin. It is thus concluded that dynorphin has both analgesic and paralytic effects in spinal cord. The analgesia shown by an increase of vocalization threshold is an opioid effect, most probably mediated by kappa opioid receptor; the paralytic effect, however, is a non-opioid effect. The increase of TFL induced by dynorphin involves both sensory (analgesia) and motor (paralysis) effects.

Mu, delta, and kappa opioid receptor mRNA expression in the rat CNS: an in situ hybridization study

The mu, delta, and kappa opioid receptors are the three main types of opioid receptors found in the central nervous system (CNS) and periphery. These receptors and the peptides with which they interact are important in a number of physiological functions, including analgesia, respiration, and hormonal regulation. This study examines the expression of mu, delta, and kappa receptor mRNAs in the rat brain and spinal cord using in situ hybridization techniques. Tissue sections were hybridized with 35S-labeled cRNA probes to the rat mu (744-1,064 b), delta (304-1,287 b), and kappa (1,351-2,124 b) receptors. Each mRNA demonstrates a distinct anatomical distribution that corresponds well to known receptor binding distributions. Cells expressing mu receptor mRNA are localized in such regions as the olfactory bulb, caudate-putamen, nucleus accumbens, lateral and medial septum, diagonal band of Broca, bed nucleus of the stria terminalis, most thalamic nuclei, hippocampus, amygdala, medial preoptic area, superior and inferior colliculi, central gray, dorsal and median raphe, raphe magnus, locus coeruleus, parabrachial nucleus, pontine and medullary reticular nuclei, nucleus ambiguus, nucleus of the solitary tract, nucleus gracilis and cuneatus, dorsal motor nucleus of vagus, spinal cord, and dorsal root ganglia. Cellular localization of delta receptor mRNA varied from mu or kappa, with expression in such regions as the olfactory bulb, allo- and neocortex, caudate-putamen, nucleus accumbens, olfactory tubercle, ventromedial hypothalamus, hippocampus, amygdala, red nucleus, pontine nuclei, reticulotegmental nucleus, motor and spinal trigeminal, linear nucleus of the medulla, lateral reticular nucleus, spinal cord, and dorsal root ganglia. Cells expressing kappa receptor mRNA demonstrate a third pattern of expression, with cells localized in regions such as the claustrum, endopiriform nucleus, nucleus accumbens, olfactory tubercle, medial preoptic area, bed nucleus of the stria terminalis, amygdala, most hypothalamic nuclei, median eminence, infundibulum, substantia nigra, ventral tegmental area, raphe nuclei, paratrigeminal and spinal trigeminal, nucleus of the solitary tract, spinal cord, and dorsal root ganglia. These findings are discussed in relation to the physiological functions associated with the opioid receptors.

Non-opioid effects of dynorphins: possible role of the NMDA receptor

Dynorphin A (dynA) and related opioid peptides produce moderate analgesic effects with restricted types of pain stimuli that are often accompanied by a large variety of naloxone-insensitive biochemical and behavioural effects. In binding assays in vitro, dynA possesses a high affinity for mu-, delta- and kappa- opioid receptors with some selectivity for kappa sites, but it also binds to specific non-opioid sites. The involvement of the NMDA receptor has been suggested to explain some of the non-opioid effects of dynA and related peptides. In this article, Vijay Shukla and Simon Lemaire review the experimental evidence that suggests a role for the NMDA receptor in some of the pharmacological effects of dynA and related peptides.

Effects of a mixture of peptidase inhibitors (amastatin, captopril and phosphoramidon) on Met-enkephalin-, beta-endorphin-, dynorphin-(1-13)- and electroacupuncture-induced antinociception in rats

The effects of a mixture of three peptidase inhibitors (PIs), amastatin, captopril and phosphoramidon, on methionine-enkephalin (Met-enk)-, beta-endorphin (beta-end)-, dynorphin-(1-13) (Dyn)- and electroacupuncture (EA)-induced antinociception were compared in rats. EA was performed by passing electric pulses (3 Hz, 0.1-msec duration, for 45 min) through acupuncture needles inserted into the Hoku-point. The antinociceptive effect was estimated by the hind paw pressure test. The antinociceptive effects of Met-enk and beta-end injected i.c.v. or i.t. and of Dyn injected i.t. were clearly potentiated by the PIs pretreated by the same administration routes as used for the injection of opioid peptides. The antinociceptive effects of Met-enk, beta-end and Dyn injected i.c.v. were also potentiated significantly by i.t.-PIs. PIs injected into the periaqueductal gray (PAG) potentiated EA antinociception. However, the EA effect was not affected by i.t.-PIs and was rather attenuated by i.c.v.-PIs. These results suggest that: i) Met-enk hydrolyzing enzymes are involved in the degradation of not only Met-enk but also beta-end and Dyn in the rat central nervous system; ii) Met-enk and beta-end act on both supraspinal and spinal sites, while Dyn acts only on the spinal site; iii) EA antinociception is mediated by supraspinal Met-enk and/or beta-end; and iv) an anti-opiate peptide system may be activated by EA stimulation, being susceptible to Met-enk hydrolyzing enzymes.

Effect of ICI197067, a kappa-opioid receptor agonist, spinally on A delta and C reflexes and intracerebrally on respiration

Intrathecal (i.t.) injection of a kappa-opioid receptor agonist, ICI197067, caused a similar dose dependent depression of A delta and C fibre mediated nociceptive reflexes in renal sympathetic nerves due to supramaximal electrical stimulation of tibial nerves in anaesthetized dogs. A total dose of 8 mg i.t. abolished these reflexes. When administered into the 4th ventricle (i.c.v.) in a total dose range from 0.1-2.5 mg ICI197067 caused no respiratory depression; a total dose of 10 mg i.c.v. abolished both phrenic nerve activity and spontaneous respiration. The ED50 ratio of ICI197067 for depression of respiration (i.c.v.) and somatosympathetic reflexes (i.t.) is approximately 1.5:1 compared with 0.3:1 for fentanyl. ICI197067 i.c.v. caused a similar reduction in arterial pressure compare to fentanyl without comparable changes in heart rate. Thus in terms of cardiorespiratory depression and blockade of A delta and C fibre pathways kappa-opioid receptor agonists may be safer and more effective for producing spinal analgesia than mu-opioid receptor agonists.

Endogenous dynorphin modulates calcium-mediated antinociception in mice

We previously reported that calcium administered IT produces antinociception by stimulating spinal Met-enkephalin release. However, at times the antinociceptive effects of calcium in the tail-flick test are greatly diminished. The results of this study indicates that during these periods calcium also stimulates endogenous dynorphin release. Dynorphin has been reported to block opiate-induced antinociception. Calcium-injected mice (150-600 nmol, IT) pretreated with vehicle IP displayed a poor degree of antinociception. Alternatively, pretreating mice with pentobarbital (45 mg/kg, IP) restored the antinociceptive effects of calcium. Low doses of naloxone and norbinaltorphimine (BNI) did not produce antinociception but restored the antinociceptive effects of calcium. Dynorphin (1-17) (Dyn 1-17), and Dyn (1-13), but not Dyn (1-8), blocked the antinociceptive effects of calcium restored with pentobarbital. These results indicate that calcium-mediated antinociception was sensitive to injected dynorphins. In additional experiments, antiserum to Dyn (1-13) was found to restore the antinociceptive effects of calcium, presumably by binding dynorphin released by calcium.

Involvement of spinal kappa opioid receptors in a type of footshock induced analgesia in mice

We have studied the effects of several opioid antagonists on a type of footshock stress-induced analgesia (FSIA) measured by the tail-flick test in male mice. Naloxone injected either subcutaneously (0.1-10 mg/kg) or intrathecally (1-20 micrograms) antagonized FSIA at higher doses than those that blocked a similar degree of analgesia induced by morphine. Intracerebroventricular (i.c.v.) naloxone (1-20 micrograms) did not modify the FSIA while antagonizing the i.c.v. morphine-induced analgesia. As a consequence, the antagonism of the FSIA by naloxone probably occurs at the level of the spinal cord and through receptors different than mu. The delta selective antagonist naltrindole (0.1-3 mg/kg s.c.) did not antagonize the analgesic effects of the stress. Nor-binaltorphimine, a kappa selective antagonist, blocked the FSIA when administered systemically (1-4 mg/kg i.p.) or locally (0.1-1 microgram i.t.). These results strongly suggest that spinal kappa opioid receptors are responsible for this type of endogenous analgesia.

Unmyelinated primary afferent fiber stimulation depletes dynorphin A (1-8) immunoreactivity in rat ventral horn

The present study demonstrates many dynorphin (DYN)-immunoreactive fibers and presumed presynaptic terminals in rat lumbar ventral horn. The fibers and terminals seem to arise largely from DYN-containing intrinsic neurons in the dorsal horn. The majority of the presumed terminals closely surround a subpopulation of motoneurons that tend to be located in flexor motoneuron columns. Acute C fiber, but not A fiber, primary afferent stimulation depletes the ventral horn DYN immunostaining. We interpret these findings to indicate that the spinal DYN neurons are well positioned to serve both as modulators of nociceptive input and as interneurons in motor reflexes. We further hypothesize that the depletion of DYN-immunoreactivity that follows either acute C fiber stimulation or intense nociceptive stimuli may be the trigger for the upregulation in spinal cord DYN that occurs in models of chronic pain states.

Involvement of spinal kappa opioid receptors in the antagonistic effect of dynorphins on morphine antinociception

The modulatory effects of intrathecally (i.t.) administered dynorphin A(1-17) and dynorphin A(1-13) on morphine antinociception have been studied previously in rats by other investigators. However, both potentiating and attenuating effects have been reported. In this study, the modulatory effects of i.t. administered dynorphin A(1-17) as well as the smaller fragment, dynorphin A(1-8), were studied in mice. In addition, nor-binaltorphimine (nor-BNI), a highly selective kappa opioid receptor antagonist, and naltrindole (NTI), a highly selective delta opioid receptor antagonist, were used to characterize the possible involvement of spinal kappa and delta opioid receptors in the modulatory effects of the dynorphins. Dynorphin A(1-17) and dynorphin A(1-8) administered i.t. at doses that did not alter tail-flick latencies, were both able to antagonize in a dose-dependent manner, the antinociceptive action of s.c. administered morphine sulfate. The antinociceptive ED50 of morphine sulfate was increased 3.9- and 5.3-fold by 0.4 nmol/mouse of dynorphin A(1-17) and dynorphin A(1-8), respectively. Injections of 0.4 and 0.8 nmol/mouse of nor-BNI i.t., but not its inactive enantiomer (+)-1-nor-BNI, inhibited dose-dependently the antagonistic effects of the dynorphins. These doses of nor-BNI alone did not affect the antinociceptive action of morphine sulfate. Intrathecal administration of 5 nmol/mouse of NTI also did not affect the modulatory effects of dynorphins. These observations that dynorphins exert their antagonistic effects on morphine-induced antinociception stereoselectively through spinal kappa opioid receptors may suggest a coupling between spinal kappa and mu opioid receptors.

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.

The influence of opioid receptor subtypes on the processing of nociceptive inputs in the spinal dorsal horn of the cat

Extracellular recordings were made of the cutaneous sensory responses of spinocervical tract (SCT) neurones in the lumbar dorsal horn of anaesthetised and paralysed cats. All of the neurones studied were multireceptive, showing excitatory responses to both innocuous and noxious (thermal and when tested, mechanical) stimuli applied to their cutaneous receptive fields on the ipsilateral hindlimb. The effects of iontophoretically applied opioids were studied on a regular cycle of responses to these cutaneous stimuli and also to D.L-homocysteic acid (DLH). In the first series of experiments, drugs were applied in the vicinity of the SCT neurones. The kappa-receptor agonists dynorphin A(1-13) and U50488H, but not dynorphin A(2-13), the mu-agonist DAGO, or the delta-agonist DADL, caused a selective reduction of the nociceptive responses of the neurones. The corresponding responses to innocuous stimuli or to DLH, and spontaneous activity were unaffected. In the second series of experiments, drugs were applied from a second electrode placed in the region of the substantia gelatinosa directly dorsal to the tip of the recording electrode. Under these conditions, the mu-receptor agonist DAGO, but not the kappa-agonist dynorphin A(1-13) or the delta-agonists DADL, DSLET or DLPEN, showed a selective antinociceptive effect. In both series, the antinociceptive effects of the opioids were readily reversed by iontophoretically applied naloxone. The effect of dynorphin A(1-13) applied close to SCT neurones, but not that of DAGO applied in the region of the substantia gelatinosa, was reversed by the alpha 2-adrenoceptor antagonist, idazoxan. The results indicate that both mu- and kappa-opioid receptors (at anatomically distinct sites) can participate in the selective antinociceptive influence that opioids can exert over somatosensory information ascending to supraspinal levels. The antagonism of kappa-receptor-mediated antinociception by idazoxan is consistent with an interaction of opioid and noradrenaline influences at the level of the dorsal horn.

Unidirectional analgesic cross-tolerance between morphine and levorphanol in the rat

Clinically, patients often demonstrate incomplete cross-tolerance between opiate analgesics. Although dispositional and pharmacokinetic factors may be a factor, our results suggest that differences in selectivity of various opioids for those opioid receptor subtypes involved in analgesia, mu 1, kappa and delta, also play an important role. In binding studies, levorphanol potently labelled all 3 classes whereas morphine was relatively selective for mu sites. Levorphanol infusions yielded tolerance to both morphine and levorphanol while morphine infusions selectively produced tolerance to morphine. This unidirectional tolerance might be due to the selectivity of morphine for mu receptors compared to levorphanol's ability to interact more potently with other relevant receptor subtypes. These observations raise the possibility that the order in which different opioid analgesics are administered may be of clinical significance.

In situ hybridization histochemistry and immunocytochemistry reveal an increase in spinal dynorphin biosynthesis in a rat model of peripheral inflammation and hyperalgesia

Dynorphin, an opioid peptide, is thought to play an important role in the modulation of nociceptive neural circuits at the level of the spinal cord. In a model of peripheral inflammation and hyperalgesia, an oligodeoxyribonucleotide probe complementary to a portion of preprodynorphin mRNA and antisera to dynorphin A-(1-8) were used to localize changes in dynorphin mRNA and peptide to individual spinal cord neurons. Intraplantar injection in rats of complete Freund's adjuvant resulted in edema and hyperalgesia to radiant heat stimulation of the injected hind paw that reached a peak at 4 days. At the same time, in situ hybridization histochemistry and immunocytochemistry identified an increase in transcription of preprodynorphin mRNA that was paralleled by an increase in dynorphin peptide. These changes were seen in spinal neurons in the medial two-thirds of laminae I and II and in laminae V and VI of lumbar segments receiving innervation from the inflamed paw. Since neurons demonstrating the increase in dynorphin biosynthesis are located in both the superficial and deep dorsal horn laminae, our data provide evidence for opioid modulation of nociceptive neural circuits in these two distinct spinal locations.

Intrathecal dynorphin A1-13 and dynorphin A3-13 reduce rat spinal cord blood flow by non-opioid mechanisms

Radiolabeled microspheres were used to examine the effects of paralytic intrathecal doses of dynorphin A (Dyn A1-13) and Dyn A3-13 on rat brain and spinal cord blood flows and cardiac output. Dyn A1-13 produced significant dose-related reductions in blood flow to lumbosacral and thoracic spinal cord without altering cardiac output and blood flow to brain and cervical spinal cord. Naloxone failed to block these effects. Dyn A3-13, which lacks opioid activity, also significantly reduced blood flow in lumbosacral spinal cord. Thus, the paralytic effects of Dyn A in the rat may involve reductions in spinal cord resulting from non-opioid actions of Dyn A.

Intrathecal dynorphin(1-13) results in an irreversible loss of the tail-flick reflex in rats

Intrathecal injection of dynorphin produced a loss of the tail-flick reflex that lasted throughout the 14-day experimental period whereas, the inclined plane test of motor function and tail-shock vocalization recovered within an hour. An important aspect of the loss of the tail-flick reflex was that it was an all-or-none event. At any dose of tail-flick latency either remained unchanged when compared with pre-injection latencies or the latency was elevated to the cut off time of 14 s. The ED50's +/- S.E.M. for tail-flick, inclined plane and tail-shock vocalization were 65.4 +/- 5.0, 67.7 +/- 5.0 and 68.0 +/- 3.9 nmol respectively. Results from the hot-plate test revealed no statistical difference between saline and dynorphin injected animals one day following the injection. Animals injected with morphine sulphate s.c. lost the tail-flick reflex but completely recovered by 24 h. Histology of the spinal cord of animals treated with dynorphin 24 h prior to sacrifice revealed dead neurons primarily in the ventral horn with little or no damage in the dorsal horn. These data demonstrate that dynorphin(1-13) injected intrathecally results in a rather specific neurotoxic action in the spinal cord.

Nerve fibers surrounding intracranial and extracranial vessels from human and other species contain dynorphin-like immunoreactivity

Dynorphin B(20-32) was visualized by immunohistochemistry in guinea-pig and rat perivascular nerve fibers and was measured by radioimmunoassay within the walls of feline, canine, bovine and human cephalic and systemic arteries and veins. Canine vessels contained the highest levels. When human blood vessels or trigeminal ganglia were subjected to reverse-phase high-performance liquid chromatography, dynorphin immunoreactivity exhibited a retention time identical to that of synthetic dynorphin B. No differences in dynorphin-like immunoreactivity were measurable between feline systemic arteries and veins, or between cephalic and systemic vessels. The highest amounts were present in leptomeninges devoid of large pial arteries. Relatively high levels were also measured in feline and human trigeminal ganglia and feline superior cervical and sphenopalatine ganglia, three sources of projecting perivascular axons. Levels did not diminish, however, in ipsilateral feline cephalic vessels following either unilateral trigeminal or superior cervical ganglionectomies. Hence, dynorphin-containing fibers may project from parasympathetic cell bodies or perhaps from intrinsic brain sources. Previously published reports indicate that the kappa agonist dynorphin does not modify vessel tone when added in vitro but does inhibit release of neurotransmitters from afferent and sympathetic axons via prejunctional receptors. These observations suggest a pharmacological role for dynorphin on sensory and autonomic functions of the vasculature.

Opioid receptor systems and the endorphins: a review of their spinal organization

A review of the spinal organization of opioid receptor systems and endorphins is presented. The review is a consideration of the physiological mechanisms underlying the effect of spinal opioids, the pharmacology of the opioid receptors that moderate a variety of spinal processing systems, and the endorphin systems that act upon the spinal receptors.

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.

The influence of chronic stress on multiple opioid peptide systems in the rat: pronounced effects upon dynorphin in spinal cord

Recurrent exposure to intermittent electrical foot-shock (30 min, twice daily) for 7 days caused an increase in immunoreactive (ir) dynorphin and ir-alpha-neo-endorphin in lumbar and cervical (but not thoracic) spinal cord as measured 16 h following the final session. At this time the level of ir-Met-enkephalin-Arg6-Gly7-Leu8 (MEAGL) was also increased at the lumbar level. An acute foot-shock depleted spinal cord dynorphin in chronically stressed but not in naive rats. No alterations in levels of ir-dynorphin or ir-MEAGL were seen in discrete brain tissues. In contrast to the brain, where no effects were seen, the levels of beta-endorphin increased in both lobes of the pituitary. This change, however, was not accompanied by an alteration in levels of beta-endorphin in plasma. These data show that chronic foot-shock stress selectively influences particular pools of opioid peptides, predominantly those derived from proenkephalin B in the spinal cord and from proopiomelanocortin in the anterior pituitary. It is suggested that alterations observed in the spinal cord reflect enhanced activity of the proenkephalin B system in response to chronic nociceptive stimulation.

Comparison of met-enkephalin-, dynorphin A-, and neurotensin-immunoreactive neurons in the cat and rat spinal cords: I. Lumbar cord

This study compared the distribution of methionine enkephalin-, dynorphin A 1-8-, and neurotensin-immunoreactive (IR) perikarya in laminae I and IV-VII of selected segments of lumbar spinal cord of cat(L5) and rat(4). Immunoreactive neurons for each peptide were found throughout the dorsal horn and dorsal lamina VII but were quantified only within laminae I and IV-VII. In lamina I, both large (greater than 20 micron) and small (less than 20 micron) IR neurons were identified. Large IR neurons for each peptide in both species resembled Waldeyer neurons studied by Golgi stain and were outnumbered by small IR neurons. Comparison among the laminae of the distribution of met-enkephalin IR neurons showed a similar pattern in the two species with the majority of IR neurons (greater than 65%) in laminae V and VI. Differences in laminar distribution occurred between species for the other peptides. Dynorphin IR neurons were greatest in number in lamina V in rat but greatest in number in laminae I and V in cat. Neurotensin IR neurons occurred predominantly in cat lamina I but were nearly equal in density in rat laminae I and VI. The topographic distribution of each peptide in laminae V and VI was similar between the two species with IR neurons occurring laterally in lamina V and more medially in lamina VI. Comparisons between species of the numbers of IR neurons/segment indicated distinct relationships for each peptide. The number of met-enkephalin IR neurons in laminae of cat L5 was generally two times greater than the number of IR neurons in the same laminae of rat L4, except in laminae I and IV, where the numbers were nearly equal. In contrast, the number of dynorphin IR neurons in cat laminae was generally one-half the number in rat, except in lamina I, where the number in cat was two times greater than rat. A high degree of variability occurred in laminar comparisons of neurotensin IR neurons. Neurotensin IR neurons in lamina I of cat outnumbered those of rat 2:1, but in laminae IV-VII, the ratio of cat to rat IR neurons varied from 1:1 to 1:20. The met-enkephalin, dynorphin, and neurotensin IR neurons quantified in this study may be interneurons or may serve as projection neurons to brainstem and/or thalamic nuclei. The observed differences in distribution may be relevant to differences in spinal cord physiology in the two species.

A dose-ratio comparison of mu and kappa agonists in formalin and thermal pain

The effects of putative mu and kappa agonists, with and without naloxone, were compared in the formalin and tail flick tests in rats. The mu agonist sufentanil was more potent in the tail flick test than the formalin test while the opposite was true for the kappa agonist ethylketocyclazocine (EKC). MR2034 was equipotent in the two tests and in the tail flick test, analgesia decreased at high doses. The naloxone (0.1 mg/kg) dose-ratios (DR) for sufentanil and EKC were 3 to 7 times larger for the tail flick test than the formalin test. From this and other DR studies it is argued that in thermal pain tests, opioid analgesia is mediated primarily by mu receptors while in non thermal tests kappa effects predominate.

Kappa opioid receptors in human lumbo-sacral spinal cord

[3H]Etorphine and [3H]ethylketocyclazocine bind with high affinity (Kd between 0.25-2.0 nM) to a single class of sites in human lumbo-sacral spinal cord. Other ligands such as [3H]morphine, [3H]dihydromorphine and [3H]D-Ala2, D-Leu5-enkephalin (DADLE) did not bind to significant number of sites under our incubation conditions. Ligand selectivity pattern strongly suggests that [3H]etorphine labels kappa opioid binding sites in the human lumbo-sacral spinal cord since benzomorphans and oripavines are much more potent than mu and delta agonists. Furthermore, [3H]etorphine and [3H]ethylketocyclazocine binding is sensitive to high concentrations of DADLE suggesting that these sites are of the kappa 2 sub-type. Finally, the visualization of these sites by receptor autoradiography demonstrates that they are mainly concentrated in lamina II and III of the dorsal horn. Moderate densities of sites are present around the central canal. Thus, it is possible that kappa opioid binding sites could be involved in the control of sensory and autonomic functions in the human lumbo-sacral spinal cord.

Dynorphin-A-(1-13) antagonizes morphine analgesia in the brain and potentiates morphine analgesia in the spinal cord

Intrathecal injection of subanalgesic doses of morphine (7.5 nmol) and dynorphin-A-(1-13) (1.25 nmol) in combination resulted in a marked analgesic effect as assessed by tail flick latency in the rat. The analgesic effect of the composite dynorphin/morphine was dose-dependent in serial dilutions so that a composition of 1/8 of the analgesic dose of dynorphin and 1/3 that of morphine produced an analgesic effect equipotent to full dose of either drug applied separately. The analgesic effect induced by dynorphin/morphine mixture was not accompanied by motor dysfunction and was easily reversed by a small dose (0.5 mg/kg) of naloxone. Contrary to the augmentatory effect of dynorphin on morphine analgesia in the spinal cord, intracerevroventricular (ICV) injection of 20 nmol of dynorphin-A-(1-13) exhibited a marked antagonistic effect on the analgesia produced by morphine (120 nmol, ICV). The theoretical considerations and practical implications of the differential interactions between dynorphin-A-(1-13) and morphine in the brain versus spinal cord are discussed.

Intrathecal administration of beta-endorphin and dynorphin-(1-13) for the treatment of intractable pain

Seven cases of chronic pain were treated by intrathecal administration of 30 micrograms of beta-endorphin and dynorphin-(1-13). Compared with saline, both peptides were able to suppress pain for periods up to 4.5 and 7 hours on the average, respectively. No significant side reactions were noticed during the entire investigation.

Autoradiographic localization of mu, delta and kappa opioid receptor binding sites in rat and guinea pig spinal cord

The autoradiographic distribution of mu, delta and kappa opioid binding sites was evaluated in various segments of the rat and guinea pig spinal cord. Mu opioid receptor binding sites are highly concentrated in the superficial layers of the dorsal horn (laminae II and III) in both species, without any marked gradient along the cord. Delta binding sites are somewhat concentrated in the superficial layers of the dorsal horn. However, delta binding sites are also present and evenly distributed in other areas of the gray matter. The highest density of delta sites is found in the cervical segment with only low levels in the lumbo-sacral region of the rat and guinea pig spinal cord. Kappa opioid binding sites are highly concentrated in the superficial layers of the dorsal horn of the spinal cord. Lower levels are seen in the rest of the gray matter with some enrichment in lamina X. Moreover, the lumbo-sacral portion of the spinal cord is enriched in kappa sites as compared to the cervical and thoracic segments. These data demonstrate the differential laminar distribution of mu, delta and kappa opioid binding sites in rat and guinea pig spinal cord.

Failure of ES 52, a highly potent enkephalinase inhibitor, to affect nociceptive transmission by rat dorsal horn convergent neurones

The effects of ES 52, a highly potent derivative of the enkephalinase (enkephalin-dipeptidylcarboxypeptidase) inhibitor thiorphan, were studied on nociceptive activities of dorsal horn convergent neurones in the anaesthetized rat. Neither the C-fibre component of the responses elicited by supramaximal electrical stimulation of the hindpaw excitatory receptive fields nor diffuse noxious inhibitory controls triggered by immersion of the tail in 46-48 degrees C waterbaths, were affected by ES 52. Thus we conclude that, in our experimental conditions, modulations of the transmission of nociceptive messages at the spinal level are not greatly modified by specifically blocking the degradation of enkephalins. If a major role for enkephalinase (vs aminopeptidase) in the catabolism of enkephalins at the spinal level can be confirmed, then comparison of the present data with our previous results obtained using the opioid antagonist naloxone, might suggest a predominant role for proenkephalin B products (i.e. dynorphins and/or alpha-neo-endorphin) in modulating nociceptive transmission in the spinal cord.

Characterization of dynorphin A-induced antinociception at spinal level

Dynorphin A (DYN A) injected intrathecally in the rat produced a significant elevation of the nociceptive threshold, measured by the tail flick test. The highest dose of DYN A (25 nmol) produced maximal elevation of tail flick latency to radiant heat together with hindlimb paralysis and tail flaccidity lasting several hours, thus confirming several previous reports. A lower dose of DYN A (12.5 nmol) produced only a smaller, not constant, short-lasting change in the nociceptive threshold. The vocalization test (electrical stimulation of the tail) gave a different result: the time course curve showed that the antinociceptive effect had worn off 60 min after DYN A 25 nmol. Thus it can be assumed that the prolonged depression of the tail flick reflex was related to motor dysfunction and did not completely reflect the animal's response to painful stimuli. Tolerance to the antinociceptive and motor effects developed after the chronic intrathecal infusion of DYN A with osmotic minipumps. Intrathecal MR 1452 (30 nmol), a purported kappa-receptor blocker, fully prevented the effects of DYN A but not morphine-induced antinociception. Naloxone antagonized DYN A only at a 4 fold higher dose. MR 1452 (90 nmol) administered after DYN A reversed the elevation of the vocalization threshold while tail flick latency remained unmodified. Analysis by high performance liquid chromatography of intrathecally injected radiolabelled DYN A revealed that DYN A was largely broken down about 10 min after its administration. Our results seem to indicate that DYN A in the spinal cord causes alterations in nociception and motor function, clearly distinguishable in time and both mediated by an opioid receptor, probably of the kappa type. However, different mechanism(s), possibly non-opioid in nature, may contribute to the prolonged depression of the tail flick.

Multiple opioid ligands and receptors in the control of nociception

This paper summarizes the results of recent data characterizing the role of endogenous opioid peptides and opioid receptors in nociception. In addition, evidence is given that antinociception induced by intracerebroventricular injection of opioids into mouse brain is mediated by receptors resembling those mediating the inhibitory action of these substances on the rat vas deferens (putative epsilon-receptors). The endogenous ligands for these receptor are beta-endorphin and the peptides deriving from proenkephalin A.

Analgesic effects of dynorphin-A and morphine in mice

To investigate whether or not dynorphin-A is analgesic, the effect of this peptide was tested in comparison with that of morphine in mice. Dynorphin-A produced a potent analgesic effect in the acetic acid writhing and tail pinch tests, but a weak effect in the tail flick test when given by intracerebroventricular injection. In contrast, morphine caused a potent analgesia in all the tests. Dynorphin-A was more effective when given by intrathecal injection than by intracerebroventricular injection, whereas morphine was equipotent by both injection routes. The results suggest that dynorphin-A is analgesic and that its analgesia may be differentiated from that of morphine.

Dynorphin: important mediator for electroacupuncture analgesia in the spinal cord of the rabbit

Intrathecal injection of 12 nmol of dynorphin elicited marked analgesia as measured by tail flick latency, the effect being about 20 times more potent than with morphine. This analgesic effect could be reversed by naloxone at a dose 1.5-fold higher than that needed to reverse morphine analgesia. Intrathecal injection of anti-dynorphin antibody blocked electroacupuncture (EA) analgesia by 77%, the effect lasting for at least 4 h. In rabbits made tolerant to EA analgesia by long-term EA stimulation, intrathecal injection of dynorphin no longer exhibited an analgesic effect. No analgesia was noticed when dynorphin (10 nmol) was injected into the periaqueductal grey (PAG) of the rabbit, nor was EA analgesia blocked by anti-dynorphin antibody injected into PAG. These results suggest that dynorphin reduces nocifensive responses in the spinal cord and may play an important role in mediating EA analgesia at the spinal level.

Pain, nociception and spinal opioid receptors

Opioid peptides derived from proenkephalin and prodynorphin are differentially distributed in the spinal cord. Proenkephalin peptides are preferentially located in the sacral portion of the cord while prodynorphin peptides are concentrated in the cervical spinal cord. Mu opioid receptor are highly concentrated in superficial layers of the dorsal horn in all the spinal cord. Delta opioid receptor are more diffusely distributed in the gray matter of the spinal cord. These sites are principally located in cervical and thoracic portions of the spinal cord. Kappa opioid receptors are highly concentrated in the superficial layers of the lumbo-sacral spinal cord. Its density decreased in the upper levels of the spinal cord. It appears that mu opioid receptors are indifferentially activated by thermal, pressure and visceral nociceptive inputs. Delta receptors are more likely to be involved in thermal nociception while kappa opioid binding sites are associated to visceral pain nociceptive inputs.

Characterization and localization of immunoreactive dynorphin, alpha-neo-endorphin, Met-enkephalin and substance P in human spinal cord

By use of specific antisera, the distributions of immunoreactive dynorphin (ir-DYN), alpha-neo-endorphin (ir-alpha-NEO), Met-enkephalin (ir-MET) and substance P (ir-SP) were evaluated in discrete regions of human spinal cord and spinal ganglia. The relative concentrations of immunoreactive peptides in particular regions were as follows: sacral greater than lumbar greater than cervical greater than thoracic. Concentrations of ir-DYN, ir-alpha-NEO and ir-SP were 2-10-fold, but of ir-MET 1-2-fold, higher in the dorsal as compared to the ventral parts of cervical, lumbar and sacral cord. The concentrations of all peptides (when examined in discrete areas of thoracic cord) were found to be highest in the substantia gelatinosa. All peptides were present in the gray matter but only ir-MET was found in white matter. Gel-permeation chromatography of dorsal sacral spinal cord extracts revealed two major ir-DYN peaks. The smaller molecular weight peak, eluted at the position of synthetic dynorphin1-17. ir-alpha-NEO and ir-SP comigrated exactly with their respective synthetic marker peptides. Substantial amounts of ir-SP and also, as confirmed by high pressure liquid chromatography, ir-MET, were found in the dorsal and ventral roots and spinal ganglia, and very low concentrations of ir-DYN or ir-alpha-NEO were also detected in these tissue. These results suggest that dynorphin and alpha-neo-endorphin, in addition to enkephalins, may be involved in transmission of somatosensory information in the human spinal cord.

Studies in vivo with dynorphin-(1-9): analgesia but not gastrointestinal effects following intrathecal administration to mice

Direct administration of dynorphin-(1-9) (10-100 micrograms) into the spinal subarachnoid space of mice produced a dose-related analgesic effect at 5 and 10, but not 35 min, in the tail-flick test using hot water as the nociceptive stimulus. By contrast, the same doses did not affect gastrointestinal transit at 5, 10, 15 or 35 min after injection. These results suggest that dynorphin-(1-9) is similar in this endpoint to ketazocine-type kappa opioid agonists in naive animals while confirming that intrathecal administration of dynorphin is analgesic in mice.

Sites of analgesic action of dynorphin

Analgesic effects of dynorphin and dynorphin-(1-13) injected into various regions of the CNS of rats were investigated using the tail pinch method. Dynorphin and dynorphin-(1-13) produced a dose-dependent analgesic effect when injected into the third ventricle (3rd vent.) or the nuclei reticularis gigantocellularis and paragigantocellularis (NRGC-NRPG) and the analgesic activity of dynorphin was 3 to 5 times more potent than that of dynorphin-(1-13). These peptides in higher doses elicited transient "barrel-rolling" and rigidity in behavior. On the other hand, when injected intrathecally, dynorphin exerted an analgesia accompanied by paralysis of hindlimbs which were long lasting. But its analgesic activity was less effective as compared with the 3rd vent. and NRGC-NRPG injections.

Mixed opioid/nonopioid effects of dynorphin and dynorphin related peptides after their intrathecal injection in rats

Dynorphin dose-dependently increased the tail flick latency of rats to radiant heat following its intrathecal injection. This effect was accompanied by an alteration in motor function that was characterized by a flaccid extension of the hindlimbs and flaccidity of the tail. Naloxone (10 but not 1 mg/kg) blocked the antinociceptive effect and motor disturbance produced by dynorphin. The non-opioid analogue des-Tyr1-dynorphin(1-13) also increased tail flick latency and produced paralysis. Dynorphin(1-8) significantly elevated tail flick latency without affecting motor function. Furthermore, the effect of dynorphin(1-8) was blocked by 1 mg/kg naloxone. These data suggest a possible physiological role of dynorphin in influencing motor function in the spinal cord and a role of dynorphin(1-8) in modulating pain transmission. Another finding of the present study was that dynorphin was approximately ten times more potent in producing its effects when injected one day after surgery compared to when it when it was injected one week or more after surgery.

Analgesic effects of mu-, delta- and kappa-opiate agonists and, in particular, dynorphin at the spinal level

To investigate the role of multiple opiate receptors in spinal mechanisms of antinociception the activity of various opiate receptor agonists was determined against different nociceptive stimuli in the mouse and rat. Intrathecal administration of the putative kappa-receptor agonists dynorphin A (DYN), bremazocine, and ethylketocyclazocine, as well as the delta-agonist D-Ala2,D-Leu5-enkephalin and the mu-agonists, Tyr-D-Ala-Gly-MePhe-NH-(CH2)2OH and morphine resulted in a dose-dependent antinociceptive effect. The order of potencies was similar for visceral and thermal pain. Inhibition of writhing in mice was much more strongly antagonized by MR 2266 than by naloxone, and des-Tyr1-DYN1-13 was 50 times less potent than DYN in this test suggesting that DYN acts through the kappa-receptor. It is concluded that mu-, delta- and kappa-opiate receptors are involved in spinally mediated antinociception related to kinds of noxious stimuli.