Opiate receptor binding sites in human spinal cord

The histology, physiology, neurochemistry and circuitry of the substantia gelatinosa Rolandi (lamina II) in mammalian spinal cord

The substantia gelatinosa Rolandi (SGR) was first described about two centuries ago. In the following decades an enormous amount of information has permitted us to understand - at least in part - its role in the initial processing of pain and itch. Here, I will first provide a comprehensive picture of the histology, physiology, and neurochemistry of the normal SGR. Then, I will analytically discuss the SGR circuits that have been directly demonstrated or deductively envisaged in the course of the intensive research on this area of the spinal cord, with particular emphasis on the pathways connecting the primary afferent fibers and the intrinsic neurons. The perspective existence of neurochemically-defined sets of primary afferent neurons giving rise to these circuits will be also discussed, with the proposition that a cross-talk between different subsets of peptidergic fibers may be the structural and functional substrate of additional gating mechanisms in SGR. Finally, I highlight the role played by slow acting high molecular weight modulators in these gating mechanisms.

The effect of µ-opioid receptor activation on GABAergic neurons in the spinal dorsal horn

The superficial dorsal horn of the spinal cord plays an important role in pain transmission and opioid activity. Several studies have demonstrated that opioids modulate pain transmission, and the activation of µ-opioid receptors (MORs) by opioids contributes to analgesic effects in the spinal cord. However, the effect of the activation of MORs on GABAergic interneurons and the contribution to the analgesic effect are much less clear. In this study, using transgenic mice, which allow the identification of GABAergic interneurons, we investigated how the activation of MORs affects the excitability of GABAergic interneurons and synaptic transmission between primary nociceptive afferent and GABAergic interneurons. We found that a selective µ-opioid agonist, [D-Ala², NMe-Phe⁴, Gly-ol]-enkephanlin (DAMGO), induced an outward current mediated by K⁺ channels in GABAergic interneurons. In addition, DAMGO reduced the amplitude of evoked excitatory postsynaptic currents (EPSCs) of GABAergic interneurons which receive monosynaptic inputs from primary nociceptive C fibers. Taken together, we found that DAMGO reduced the excitability of GABAergic interneurons and synaptic transmission between primary nociceptive C fibers and GABAergic interneurons. These results suggest one possibility that suppression of GABAergic interneurons by DMAGO may reduce the inhibition on secondary GABAergic interneurons, which increase the inhibition of the secondary GABAergic interneurons to excitatory neurons in the spinal dorsal horn. In this circumstance, the sum of excitation of the entire spinal network will control the pain transmission.

UMB-3, a novel rabbit monoclonal antibody, for assessing μ-opioid receptor expression in mouse, rat and human formalin-fixed and paraffin-embedded tissues

BACKGROUND

The immunohistochemical localization of the μ-opioid receptor (MOR, MOP) has been studied in detail in mouse and rat brain using a variety of polyclonal antibodies. However, biochemical analysis of the MOR signaling complex in vivo has been hampered by the lack of suitable monoclonal antibodies for efficient immunoprecipitation of the receptor protein from native sources. Moreover, previous immunohistochemical investigations were restricted to frozen sections from perfusion-fixed rodent brain, largely due to the limited availability of MOR antibodies that effectively stain paraffin-embedded tissues.

METHODS

Here, we extensively characterized the novel rabbit monoclonal anti-MOR antibody UMB-3 using transfected cells and MOR-deficient mice. UMB-3 was also subjected to a comparative immunohistochemical study of formalin-fixed, paraffin-embedded mouse and rat organ samples as well as human normal and neoplastic tissues.

RESULTS

Specificity of UMB-3 was demonstrated by detection of a broad band migrating at M(r) 70,000-80,000 in immunoprecipitates from crude brain homogenates of MOR+/+ mice but not of MOR⁻/⁻ mice; cell surface staining of MOR-transfected cells; translocation of MOR receptor immunostaining after agonist exposure; distinct immunostaining of neuronal cell bodies and fibers in MOR-expressing brain regions; absence of staining in MOR-deficient mice; and abolition of tissue immunostaining by preadsorption of UMB-3 with its immunizing peptide.

CONCLUSIONS

The rabbit monoclonal antibody UMB-3 is an excellent tool for immunoprecipitation of MOR from native sources as well as for immunohistochemical staining of MOR in paraffin-embedded tissue samples of rodent and human origin.

Is the cutaneous silent period an opiate-sensitive nociceptive reflex?

In humans, high-intensity electrical stimuli delivered to the fingers induce an inhibitory effect on C7-T1 motoneurons. This inhibitory reflex, called the cutaneous silent period (CSP) is considered a defense response specific for the human upper limbs. It is not clear whether the CSP-like other defense responses such as the corneal reflex and the R III reflex-is an opiate-sensitive nociceptive reflex. Because opiates suppress some, but not all, nociceptive reflexes, we studied the effect of the narcotic-analgesic drug fentanyl on the CSP and the R III reflex. The CSP was recorded from the first dorsal interosseous (FDI) muscle in seven normal subjects during voluntary contraction, before and 10 and 20 min after fentanyl injection. To assess possible fentanyl-induced changes, we also tested the effect of finger stimulation on motor evoked potentials (MEPs) elicited in the FDI muscle by transcranial magnetic stimulation before and after fentanyl injection. Fentanyl-induced changes were also studied on the R III reflex recorded from the biceps femoris muscle. Fentanyl, as expected, suppressed the R III reflex but failed to change the inhibitory effect of finger stimulation on FDI motoneurons. Finger stimulation reduced the size of MEPs in the FDI, and fentanyl injection left this inhibitory effect unchanged. The differential fentanyl-induced modulation of the CSP and R III reflex provides evidence that the CSP circuit is devoid of mu-opiate receptors and is therefore an opiate-insensitive nociceptive reflex, which may be useful in the assessment of central-acting, non-opioid drugs.

Differential antinociceptive effects of spinal opioids on foot withdrawal responses evoked by C fibre or A delta nociceptor activation

1. Intrathecal application of mu, delta, and kappa opioids attenuate responses on several tests of animal nociception. However, the potency of these opioids differ depending on which tests were used. One factor contributing to these discrepancies is that different types of noxious stimuli activate different sets of nociceptor types, which may be differentially sensitive to opiate inhibition. To examine this hypothesis, we used a recently developed behavioural test which allows for differential assessment of nociception evoked by the activation of myelinated (A delta) and unmyelinated C thermonociceptors. 2. Administration of a kappa-selective agonist was ineffective on either type of response. Delta1 drugs were slightly more potent on C fibre-mediated responses than on A delta-mediated responses. 3. Intrathecal mu and delta2 drugs were antinociceptive on both A delta and C nociceptor-mediated responses. However, unlike the delta1 effects, the dose-response curves for mu and delta2 drugs were significantly more steep for A delta than for C fibre-mediated responses, potentially indicating differences in the mechanisms by which the drugs act on these 2 response types.

Changes of opioid binding density in the rat spinal cord following unilateral dorsal rhizotomy

Mu, delta and kappa opioid receptors in the vertebrate spinal cord mediate the potent antinociceptive effects of opioid agonists administered onto the spinal cord. The present experiments were conducted to determine the effect of unilateral dorsal rhizotomy on mu, delta and kappa spinal opioid binding sites. Measurements of opioid binding were made at 1, 2, 4 or 8 days after rhizotomy and comparisons were made to intact animals. The changes in mu, delta and kappa opioid binding sites were determined by receptor autoradiography using the highly selective radioligands [3H]sufentanil, [3H]DPDPE and [3H]U69593, respectively. Within autoradiograms of each spinal cord, three regions on each side of the spinal cord were targeted for densitometric analysis: laminae I-II (medial), V (lateral) and X. When effects of unilateral rhizotomy within animals were assessed by comparison of the density of binding on the side ipsilateral to the rhizotomy to the contralateral side, decreases in the binding of all three radioligands were observed in laminae I-II on the side of the spinal cord ipsilateral to the rhizotomy at 2-8 days postlesion. A significant reduction in binding was also noted for mu and delta sites in lamina V after 8 days and for delta binding in lamina X at 2 and 4 days on the side ipsilateral to the rhizotomy. However, when densities of binding sites were compared with the corresponding regions in control, it was clear that dorsal rhizotomy resulted in significant changes in opioid binding on both sides of the spinal cord; changes differed for each type of opioid binding site. On the contralateral side of the spinal cord, rhizotomy caused a significant decrease of mu opioid sites 1 day after the lesion and showed partial recovery by day 8. Delta opioid sites were also significantly decreased as early as 1 day postlesion, but did not recover. Kappa opioid sites did not change at 1 day after the rhizotomy but increased on day 2, decreased on day 4 and fully recovered 8 days after rhizotomy. The present results support the hypothesis that a significant proportion of spinal mu, delta and kappa opioid binding sites are present on the central terminations of primary afferents. Finally the present data are the first to report a contralateral effect of the unilateral rhizotomy on spinal opioid binding sites. The contralateral changes in binding were specific to the type of opioid site examined, time after the surgery and region of the spinal cord examined.

3H-naloxone binding in brain regions of normotensive Wistar, spontaneously hypertensive and renal hypertensive rats

In order to study the role of opioid receptors in two models of experimental hypertension the binding of 3H-naloxone to membranes prepared from discrete brain regions and spinal cord was determined. Renal hypertensive rats (RHR) were found to have a greater density of 3H-naloxone binding sites in the hippocampus and hypothalamus when compared to spontaneously hypertensive rats (SHR) and normotensive Wistar rats (NR). The apparent dissociation constant (Kd) for 3H-naloxone binding did not differ between groups.

Autoradiographic localization of receptors for neuropeptide FF, FLFQPQRFamide, in human spinal sensory system

The regional distribution of FLFQPQRFamide binding sites on fresh unfixed cryostate sections from post mortem specimens of human spinal cord and lower medulla oblongata was studied by quantitative autoradiographic methods using [125I]YLFQPQRFamide as ligand. Samples were taken from five cases who had died with no history of neurological disease at ages ranging from 5 months to 66 years. The biochemical and pharmacological characteristics of [125I]YLFQPQRFamide binding to mounted tissue sections were comparable to those reported for the rat in a previous study. [125I]YLFQPQRFamide appeared to interact reversibly with high affinity binding sites (Kd = 0.06 nM), distinct from opiate receptors. Sites labelled with [125I]YLFQPQRFamide were distributed unevenly within the human spinal cord and lower medulla oblongata, with the highest density in the superficial layers of the dorsal horn and the spinal trigeminal nucleus. Although moderate labelling was observed in the ventral part of spinal grey matter, dense labelling appeared in the gracile and cuneate nuclei. No binding sites were detected in white matter. These results show that, as in the rat, FLFQPQRFamide receptors in the human spinal cord and lower medulla oblongata, are mainly concentrated within spinal areas implicated in the analgesic action of opiates. The possible role of these receptors in modulating spinal nociceptive information is discussed with respect to the pharmacological effects of substances acting on FLFQPQRFamide receptors in animals.

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.

The presence and actions of opioid receptors in bovine pineal gland

The mammalian pineal gland and its main hormone, melatonin, working in conjunction with the hypothalamic suprachiasmatic nuclei, synchronize circadian rhythm and hence refine numerous physiological and biochemical parameters. An interaction among melatonin, opioids, and analgesia has been suspected for many years, since during nighttime, when the level of melatonin is high, the mammals are less sensitive to pain. In studying this phenomenon further, we have identified a single population of opioid receptors in the bovine pineal gland using [3H]-diprenorphine and other ligands. The receptors have a dissociation equilibrium constant (Kd) of 1.36 +/- 0.31 nM and a density (Bmax) of 17.93 +/- 5.22 fmol/mg protein. In competitive experiments, the concentration of drugs required to inhibit 50% of the [3H]-diprenorphine binding (IC50) in descending order of potency was found to be naltrexone > fentanyl > naloxone > nalbuphine > morphine > nalorphine > DAGO > dynorphin > metenkephalin. In order to delineate the function of the opioid system in the pineal gland, the effects of both opioid receptor agonists and antagonists on the basal activity of N-acetyltransferase were examined in the bovine pineal explants in culture. Morphine, an opioid receptor agonist, increased significantly the activity of N-acetyltransferase in a dose-dependent fashion. In addition, the stimulatory effect of morphine was inhibited by naloxone, an opioid receptor antagonist. The results of these studies indicate the existence of pineal opioid receptors, which play a pivotal role in the synthesis of melatonin and its action in synchronizing pineal events.

N-methyl-D-aspartate (NMDA) and opioid receptors mediate dynorphin-induced spinal cord injury: behavioral and histological studies

Both N-methyl-D-aspartate (NMDA) and opioid receptors have been implicated in the pathophysiology of traumatic spinal cord injury and dynorphin-induced paralysis. The present studies compared the effects of the non-competitive NMDA antagonist dextrorphan (Dex) and the kappa-selective opioid antagonist nor-binaltorphimine (nor-BNI) on the acute motor deficits and chronic neuropathological alterations caused by intrathecally administered dynorphin A-(1-17) (Dyn A). Infusion of Dyn A into the rat lower thoracic spinal subarachnoid space produced acute, reversible hindlimb paresis. Histological evaluations of spinal cord sections from these animals at 2 weeks post-infusion revealed ventral grey matter necrosis, neuronal loss and gliosis as well as axonal loss in adjacent white matter; however, there was minimal alteration in serotonin immunocytochemistry caudal to the injury zone. Dex or non-BNI pretreatment each significantly (P less than 0.05) reduced, and to a similar degree, the acute motor deficits and certain histological changes associated with Dyn A administration. These findings further support the hypothesis that dynorphin-induced spinal cord injury involves both NMDA receptors and opioid receptors.

Influence of opioids on substance P release evoked by antidromic stimulation of primary afferent fibers in the hind instep of rats

The effect of opioids on the release of immunoreactive substance P (iSP) following simultaneous electrical stimulation of the sectioned sciatic and saphenous nerves was examined by perfusion of the subcutaneous space in the rat instep. Antidromic stimulation of both the nerves caused an increase in iSP release, which was dependent on the intensity of stimulation, and an approx. 200% increase in Evans blue extravasation. Stimulation-induced iSP release and extravasation were suppressed by pretreatment with capsaicin (50 mg/kg s.c.) and spantide (10 mumol/kg i.p.), respectively. Intra-arterial infusion of morphine (30 mumol/kg) or ethylketocyclazocine (30 mumol/kg) or [D-Ala2,D-Leu5]enkephalin (30 mumol/kg) inhibited the increase in iSP release evoked by antidromic stimulation at 10 V. This inhibitory effect of morphine was antagonized by pretreatment with naloxone (2 mg/kg, i.p.). These results suggest existence of multiple types of opioid receptor on the peripheral endings of primary afferent fibers, that regulate SP release from the peripheral nerve endings into the extravascular space.

Morphine blocks descending pain inhibitory controls in humans

In man, heterotopic painful thermal conditioning stimuli induce parallel decreases in the spinal nociceptive flexion (RIII) reflex and the concurrent sensation of pain elicited by electrical stimulation of the sural nerve at the ankle. Such phenomena may be related to the diffuse noxious inhibitory controls (DNIC) which were initially described in the rat and subsequently documented in humans. In 9 subjects in the present study, a 2 min application of a moderately noxious temperature (46 degrees C) to the contralateral hand strongly depressed the RIII reflex elicited in the biceps femoris muscle by electrical stimulation of the sural nerve at 1.2 times the reflex threshold. These depressive effects were maximal during the second min of the conditioning period, showing a 80% inhibition of the RIII reflex which gradually recovered to its baseline value 7 min after the end of the conditioning period. Such inhibitory effects were completely blocked 15-26 min after administration of a low dose of morphine hydrochloride (0.05 mg/kg, i.v.). The lifting of the inhibitions was compatible with an action at the opioid receptors since the inhibitions were re-observed 5-16 min after naloxone injection (0.006 mg/kg, i.v.). During all the experimental sessions, heart and respiratory rates remained stable at their control levels. Since it has been shown previously that such a dose of morphine could not have a direct effect within the spinal cord (Willer 1985), it is concluded that this opiate blocks, in a naloxone-reversible fashion, those bulbo-spinal controls which are triggered by heterotopic nociceptive events. Possible implications for hypoalgesia based on the principles of counter-irritation are discussed.

Characterization of opioid binding sites in rat spinal cord

Binding sites were characterized in rat whole spinal cord crude membrane preparations using selective labelling techniques with multiple methods of mathematical analysis of experimental curves. Mathematical analysis of single [3H]-[D-Ala2,MePhe4,Gly-ol5] enkephalin (DAGO) saturation curves suggested binding of the [3H]-ligand at one site, while displacement curves of low concentrations of [3H]-DAGO with selective mu-ligands indicated the presence of high- and low-affinity sites. All the [3H]-DAGO curves processed simultaneously by LIGAND analysis showed the presence of high (27%) and low (73%) affinity components, with a total Bmax of 3.19 pmol/g tissue. Eighty percent of [3H]-[D-Ala2,D-Leu5] enkephalin (DADLE) binding was displaced by DAGO with high affinity, indicating that a high percentage of [3H]-DADLE binding was at mu-sites. Saturation curves of [3H]-DADLE after inhibition of mu-sites by unlabelled DAGO (delta-sites) were monophasic with non-linear fitting analysis and the Bmax was 0.90 pmol/g tissue. Most mathematical analysis of single saturation curves of [3H]-(-)-bremazocine indicated binding at more than one site. DAGO, DADLE, U-69,593 and PD 117302 displaced 0.15 nM of [3H]-(-)-bremazocine biphasically: the percentages of displacement calculated with the non-linear fitting program were respectively 50 (mu-sites), 64 ((mu + delta)-sites), 18 and 25 (kappa-sites). Haloperidol displaced [3H]-(-)-bremazocine only at microM concentrations. suggesting no binding at sigma-sites. In the presence of 225 nM of DAGO, DADLE displaced only 21% of [3H]-(-)-bremazocine 0.15 nM binding (delta-sites). Most mathematical analysis of saturation curves of [3H]-(-)-bremazocine, after inhibition of binding at mu- and delta-sites by DAGO and DADLE, still indicated binding at more than one site and the selective kappa-ligands U-69,593 and PD 117302 displaced [3H]-(-)-bremazocine in these experimental conditions. LIGAND analysis of saturation and inhibition curves of [3H]-(-)-bremazocine by U-69,593 and PD 117302 showed the presence of high (43%) and low (57%) affinity components, with a total Bmax of 2,73 pmol/g tissue. Thus in rat spinal cord there are at least two mu-sites bound by [3H]-DAGO which amount together to approximately 47% of total opioid sites, delta-sites bound by [3H]-DADLE amounting to approximately 13%, kappa-sites and other unknown sites (possibly a kappa-subtype) bound by [3H]-(-)-bremazocine, which together are approximately 40% of total opioid sites.

Central analgesic mechanisms: a review of opioid receptor physiopharmacology and related antinociceptive systems

Clinical applications of these principles, based on the increased understanding of central analgetic mechanisms, are already being undertaken. Not only does the use of intrathecal and epidural opioids have the potential to decrease pain and related morbidity after surgical procedures, but there is at least one study that demonstrates a significant reduction in both major morbidity and mortality in high-risk surgical patients in whom epidural anesthesia and analgesia were used. These principles are also useful for the management of patients undergoing cardiac surgery. Currently, high-dose narcotic anesthesia is the technique of choice for such patients because of the greater hemodynamic stability this anesthetic technique provides. However, breakthrough hypertension and tachycardia still occur, and prolonged postoperative ventilation is a necessary consequence due to the high doses of narcotics that are required. In one study of patients undergoing coronary artery surgery, preoperative administration of clonidine, 5 micrograms/kg, orally, was demonstrated to decrease fentanyl requirements by 45% (110 to 61 micrograms/kg) while producing a similar degree of hemodynamic stability as seen with high-dose fentanyl. Extubation times were not compared, but the significantly lower dosage of fentanyl in the clonidine-treated group would be expected to lead to an earlier extubation. Whether similar potentiation of narcotic effects would be seen with dexmedetomidine, which may also prevent narcotic-induced rigidity, has not been determined, but the clinical application of such synergistic and complementary agents is another consequence of the greater understanding of central analgesic mechanisms, and augurs well for the future.

Biochemical characterization and regional quantification of mu, delta and kappa opioid binding sites in rat spinal cord

The spinal cord contains mu, delta and kappa opioid receptors which mediate the antinociceptive effects of opioid agonists administered onto the spinal cord. In this study, we characterized the binding sites for highly-selective mu, delta and kappa opioid radioligands and quantified the distribution of opioid binding sites in rat lumbosacral spinal cord using autoradiography. In sections of rat brain mounted on glass slides, the mu ligand, [3H]sufentanil, bound with high affinity with an apparent Kd of 0.46 nM. The delta ligand, [3H]DPDPE [( D-Pen2.5]-enkephalin), bound with a Kd of 4.31 nM, and the kappa-ligand, [3H]U69593, bound with a Kd of 2.27 nM. Three regions of the spinal gray were targeted for quantification of binding sites by autoradiography. The data indicate that when considered as a percentage of the total opioid binding capacity within a region, the contribution of mu sites in laminae I-II was about 90%, with delta and kappa sites 7% and 3%, respectively. In lamina V, the mu sites comprised about 70% of the total opioid sites, with delta and kappa sites comprising 28% and 2%, respectively. In the area adjacent to the central canal, mu sites contributed about 65% of the total opioid sites followed by delta sites at 33% and kappa sites at 2% of total opioid sites. These results demonstrate a differential distribution of mu, delta and kappa binding sites with respect to the organization of the spinal gray matter. The preferential occurrence of all 3 opioid binding sites in the superficial dorsal horn is noteworthy since many fine caliber primary afferent fibers mediating nociception establish synaptic contact in this region.

Autoradiographic distribution of mu, delta and kappa opioid binding sites in the superficial dorsal horn, over the rostrocaudal axis of the rat spinal cord

The purpose of this study was to use [3H]DAMGO, [3H]DTLET and [3H]EKC in the presence of 100 nM DAMGO and 100 nM DTLET, combined with a quantitative autoradiography to analyse the different proportions and the rostrocaudal distribution of mu, delta and kappa opioid binding sites in the superficial layers (laminae I and II) of the cervical (C6-C8), thoracic (T5-T7), lumbar (L3-L5) and sacral (S2-S3) dorsal horn of the rat. The proportions of the three main types of opioid binding sites, assessed by autoradiography in laminae I and II, were found homogeneous at each segmental level considered: 70.4-74.3%, 18.4-20.3% and 7.3-9.5% for mu, delta, kappa sites, respectively. The physiological relevance of these data is discussed.

Pharmacology of spinal peptides affecting sensory and motor functions: dynorphins, somatostatins and tachykinins

In recent years, the pharmacological activity of dynorphins and somatostatins on spinal sensory transmission has been intensively investigated with a view to developing new agents for pain control. Similarly, a series of tachykinin-related peptides with apparent receptor antagonist activity on endogenous substance P and neurokinins has been investigated. However, a number of observations suggest that these peptides, injected intrathecally in laboratory animals, not only exert a direct effect on nociceptive transmission but also affect a broader range of spinal somatomotor and autonomic functions and may cause peculiar neurotoxic effects that are not elicited by a large number of peptides affecting spinal neurotransmission. This article makes a critical review of their pharmacological activity on spinal sensory and motor functions and briefly touches on their anatomical and functional organization in the spinal cord.

High resolution radioautographic localization of [125I]FK-33-824-labelled mu opioid receptors in the spinal cord of normal and deafferented rats

Recent data have shown that [125I]D-Ala2, MePhe4, Met(o)ol5-enkephalin (FK-33-824) is a highly selective and specific mu opioid receptor ligand [Moyse et al. (1986) Peptides 7, 351-355]. This probe was used here to investigate the detailed radioautographic distribution of mu sites at various levels of the spinal cord. [125I]FK-33-824 binding sites were localized by both tritium-sensitive film and liquid emulsion radioautography in the spinal cord of naive and deafferented rats. In naive animals, high densities of mu sites were apparent within laminae I-II at all levels of the dorsal horn, with higher levels of labelling seen in layer IIi as compared to IIo in the lumbar segment. Laminae III-IV contained about half the quantities of binding observed in superficial layers. Relatively high densities of sites were also seen over lamina VI in the upper cervical cord and throughout Clarke's column. Within the latter, [125I]FK-33-824 binding clearly spared the large perikarya of the spinocerebellar neurons. In the ventral horn, [125I]FK-33-824 binding was mainly concentrated in layer IX, at the level of cervical and lumbar enlargements. Labelled sites were confined to the neuropil, mostly sparing the soma of motoneurons. Significant decreases in [125I]FK-33-824 binding in laminae I-II (55%) and III-IV (28%) were detected four days following cervical (C3-C7) or lumbar (L1-L6) rhizotomies. These decrements were most evident at seven days post-lesion at C3-C7 levels (93 and 76% in laminae I-II and III-IV, respectively) and recovered slightly thereafter up to 28 days post-lesion. In contrast, dorsal rhizotomies did not influence mu labelling in either the ventral horn or Clarke's column. These results confirm the association of mu opioid binding sites with dorsal primary afferent fibres and demonstrate the presence of mu sites in Clarke's column and lamina IX of the ventral horn. These findings suggest that endogenous opioids in the spinal cord play a role in sensory motor integration as well as in the modulation of primary nociceptive inputs.

Pre- and postsynaptic distribution of mu, delta and kappa opioid receptors in the superficial layers of the cervical dorsal horn of the rat spinal cord

Highly selective tritiated ligands and quantitative autoradiography have been used to study mu, delta and kappa binding sites in the dorsal horn of the rat spinal cord. We have measured the proportions of the 3 main types of opioid binding sites in the superficial layers of the cervical dorsal horn (laminae I and II). The proportions of mu, delta and kappa sites were 70 +/- 4%, 23 +/- 2% and 7 +/- 1%, respectively, over the whole C4-T2 extent. Similar percentages were encountered at the level of each individual segment from C4 to T2. Eight days after a unilateral dorsal rhizotomy C4-T2, dramatic decreases were seen on the ipsilateral side to the lesion by comparison to the intact side. In the C7 segment, these decreases were 76 +/- 1%, 61 +/- 1% and 53 +/- 3% for mu, delta and kappa binding sites, respectively. The C7 segment can be considered as completely deafferented, so we attribute the residual values to postsynaptic binding whereas the decrease can be attributed to a loss of the presynaptic sites. These results are discussed with respect to the contribution of pre- and postsynaptic depressive effects of opiates on the transmission of noxious messages at the level of the dorsal horn.

Pharmacology of spinal opioids

This review presents the pharmacology of spinal opioid receptor systems which are primarily involved in pain processing. The major areas upon which we will focus are: the structure and cellular functioning of the opioid receptor systems; the physiologic effects induced by spinally administered opioids, particularly in pain modulation; and pharmacokinetic and dynamic considerations, with special attention to the problem of opioid tolerance development.

Delta-opioid receptor binding sites in rodent spinal cord

1. The delta-opioid receptor agonist [D-Pen2,D-Pen5]enkephalin showed an antinociceptive effect in the mouse tail-flick test, following intrathecal administration. This action was reversed by naloxone and by the selective delta-opioid receptor antagonist ICI 174864. 2. High affinity, saturable binding of [3H]-[D-Pen2,D-Pen5]enkephalin has been demonstrated in spinal cord homogenates from guinea-pig, hamster, rat and both adult and young (18-20 g) mice. The binding was shown by autoradiography to be concentrated in the superficial laminae of the dorsal horn. 3. Competition studies confirmed that the binding of [3H]-[D-Pen2,D-Pen5]enkephalin was to the delta-opioid site. However, anomalies were seen with displacement assays using mu-ligands, which may suggest some common high affinity site for delta- and mu-opioid receptor agonists in the spinal cord. 4. The results add further evidence for a role of the delta-opioid receptor in spinally-mediated antinociception.

Characterization of kappa opiate receptors in rat spinal cord-dorsal root ganglion cocultures and their regulation by chronic opiate treatment

We have investigated the expression and regulation of kappa opiate receptors in rat spinal cord-dorsal root ganglion primary cocultures. The density of opiate receptors increased markedly during the differentiation of the cultures; after 10 days in vitro the number of [3H]diprenorphine binding sites reached 244 +/- 47 fmol/mg protein. Most of the binding sites were of the kappa type, representing about 65-80% of total opiate receptors, while mu sites were expressed at a lower density (ca. 20% of total opiate sites). Following this period of development, the number of kappa and mu receptors did not change significantly. No detectable delta sites were observed at any time of culture (up to 4 weeks in vitro). Chronic opiate agonist treatment (24 h) of the cultured cells with either 10 microM U50488 (a selective kappa agonist), or 1 microM etorphine (a nonselective opiate agonist), did not change the number of kappa receptors and their binding affinity to [3H]diprenorphine. On the other hand, 50% of the mu receptor sites down-regulated following 24 h treatment with 1 microM etorphine. Chronic antagonist exposure (5 days) with 10 microM naloxone, markedly up-regulated the mu receptors (261% of control), whereas kappa sites exhibited a much weaker upregulation (164% of control). These data demonstrate that kappa opiate receptors are expressed at high concentration in spinal cord-dorsal root ganglion cocultures and that contrary to mu sites, kappa receptor density is less susceptible to modulation by chronic opiate treatment. The results also suggest that postreceptor components are important in regulating the kappa receptor function following prolonged opiate exposure.

Pre- and postnatal development of opiate receptor subtypes in rat spinal cord

We have studied the developmental expression of opiate binding sites in the rat spinal cord at various prenatal and postnatal stages. For each developmental stage, we have compared the expression pattern of kappa receptors with that of mu and delta receptor subtypes. Both mu and kappa receptors appear relatively early during spinal cord ontogeny (from the 15th prenatal day), while delta sites are expressed later at the postnatal period (starting at the 1st postnatal day). The number of kappa sites predominates throughout the development (55-80% of total opiate sites) with two peaks of binding activity: one at the 20th gestational day, and the other around the 7th postnatal day. mu sites represent 20-38% of the total opiate receptor population with one peak of binding activity appearing at the 1st postnatal day. The densities of mu and kappa receptors at the adult stage are lower by 40-50% than the peak values observed at the early postnatal periods. The relative amounts of delta sites remain low throughout the ontogeny (4-8% of the total opiate sites). The binding properties of neonatal (1 day after birth) kappa sites (ligand binding affinities, regulation of agonist binding by guanosine triphosphate and various cations) are similar to those displayed by kappa receptors in adult spinal cord.

Opioid and nonopioid mechanisms may contribute to dynorphin's pathophysiological actions in spinal cord injury

It has been suggested that the opioid dynorphin, an endogenous agonist for kappa-opiate receptors, contributes to secondary tissue damage after spinal cord injury. To evaluate this hypothesis further, effects of intrathecally administered dynorphin (Dyn) A-(1-17), dynorphin antiserum, or the kappa-selective opiate antagonist nor-binaltorphimine (nor-BNI) were studied in rats subjected to standardized impact trauma to the thoracic spinal cord. Effects of intrathecal Dyn A-(1-17) were also compared to those of Dyn A-(2-17), which is inactive at opiate receptors, in uninjured and injured animals. Both Dyn A-(1-17) and Dyn A-(2-17) produced motor dysfunction in uninjured rats, but Dyn A-(1-17) was approximately 2.5 times more potent. At lower doses of Dyn A-(1-17), paraparesis was markedly attenuated by nor-BNI; nor-BNI was less effective at higher doses of Dyn A-(1-17) and did not modify the motor dysfunction produced by Dyn A-(2-17). Treatment with dynorphin antiserum significantly improved outcome after trauma as compared to control treatment with normal rabbit serum or leucine-enkephalin antiserum. Dyn A-(1-17), but not Dyn A-(2-17) at similar doses, exacerbated neurological dysfunction after spinal cord injury. Pretreatment with nor-BNI attenuated neurological dysfunction after traumatic spinal cord injury to a similar degree in rats administered saline or Dyn A-(1-17). These observations support the hypothesis that dynorphin contributes to certain pathophysiological changes after traumatic spinal cord injury through both opiate-receptor (kappa-receptor)-mediated and nonopioid mechanisms.

Characterisation of kappa-opioid binding sites in rat and guinea-pig spinal cord

The binding of radiolabelled ligands with high affinity for kappa-opioid binding sites has been studied in homogenates of lumbo-sacral spinal cord from the rat. The unselective ligands [3H]bremazocine and [3H]diprenorphine labelled a large number of sites which could not be fully resolved in terms of mu-, delta- and kappa-types by displacement assays. In particular binding at the kappa-site appeared anomalous in that sites which could be identified as high affinity kappa-type represented only 40% of total kappa-binding, defined using the unselective [3H]ligands. This was confirmed by the low levels of binding seen with the kappa-agonists [3H]dynorphin A(1-9) and [3H]U-69593. In guinea-pig cord, under conditions in which binding to mu- and delta-sites was suppressed, [3H]dynorphin A(1-9) and [3H]U-69593 labelled only 60% of the kappa population, defined by the [3H]unselective ligands. The reasons for the observed discrepancies are discussed.

Spinal antinociceptive actions and naloxone reversibility of intravenous mu- and kappa-opioids in spinalized rats: potency mismatch with values reported for spinal administration

1. The relative spinal effectiveness of mu- and kappa-opioids has been assessed by their intravenous potencies on nociceptive responses (heat and/or pinch) of single motoneurones recorded in alpha-chloralose anaesthetized, spinalized rats. 2. The depressant actions of both mu- and kappa-opioids were reversed by low intravenous doses of naloxone (10 to 100 micrograms kg-1). When tested at a dose of 1 microgram kg-1 i.v., naloxone antagonized the effects of the mu-agonist morphine but had no effect on the kappa-opioid U-50,488. This provides further support for the theory that the actions of mu- and kappa-ligands were mediated at different subclasses of opioid receptor but highlights the difficulties in using antagonists with poor receptor selectivity to differentiate between mu- and kappa-receptor-mediated effects in vivo. 3. The molar potency rations of fentanyl: morphine:U-50,488: tifluadom for thermal and mechanical nociceptive responses were 620: 1.0:0.74:5.7 and 520:1.0:0.56:7.7 respectively. These potency ratios, as well as the absolute potencies, agree well with those reported in several behavioural studies in which systemic administration of agonists was used in non-thermal tests. 4. The agonist potency values obtained in this study contrast with those reported for local spinal administration. By this route, the potency of lipophilic opioids (e.g. fentanyl, U-50,488 and tifluadom) relative to hydrophilic opioids (e.g. morphine) is much reduced, implying that activity of intrathecally administered opioids is more dependent on the physico-chemical properties of the agonists used than on the relative abundance in the spinal cord of functional opioid receptors of the mu- and kappa-subtypes. This conclusion indicates that the results with locally applied opioids should not be used to assess spinal opioid receptor function.

Effects of cervical spinal hemisection on dihydromorphine binding in brainstem and spinal cord in cat

Cats were sacrificed 1-3 weeks after cervical (C1-C2) hemisection and receptor binding experiments were carried out with 4.0 and 0.6 nM concentrations of [3H]dihydromorphine [( 3H]DHM); these concentrations were shown by Scatchard analysis to represent the approximate Kd values of high and low affinity dihydromorphine binding sites in brain homogenates. Unilateral cervical hemisection produced significant (P less than 0.05), bilateral, reductions in the levels of [3H]DHM binding in the periaqueductal gray (PAG; 35-40%) and mesencephalic reticular formation (MRF; 47-51%), medial pons (40-56%) and medial medulla (30-37%). In paramedial pons and medulla, numerical reductions in [3H]DHM binding were observed (18 and 28%) which did not achieve statistical significance. In spinal cord, significant reductions were observed in the dorsal (45%) and ventral (29%) ipsilateral but not contralateral quadrants. We believe that these results in the brainstem and spinal cord reflect in part the loss of opiate binding on spinobulbar terminals and bulbospinal terminals, respectively, following orthograde degeneration. These observations support the hypothesis that the analgetic effects of opiates in the brainstem may in part be mediated by the direct inhibition of transmission through spinobulbar terminals.

Monoclonal antibodies against the S2-serotonin receptor from rat brain that cross-react with dopamine and opiate receptors

Balb/c mice were immunized with rat striatal integral membrane proteins. After hybridization of splenocytes with myeloma cells, hybridoma lines secreting antibodies against serotonin, dopamine and opiate receptors were detected by inhibition of ligand binding to brain membrane preparations. Antibodies from two positive lines, Mab/a9 and Mab/a18, were able to inhibit ligand binding to the S2-serotonin (Kd range: 10-100 nM), the mu-opiate (Kd range: 0.4-3 microM) and the delta-opiate receptors (Kd range: 0.7-1.1 microM), while Mab/a9 was also found to inhibit ligand binding to the D2/D4-dopamine receptor (Kd approximately 50 nM). An apparent molecular mass of 60 kDa could be ascribed to the delta-opiate receptor and apparent molecular masses of 29 and 36 kDa to the mu-opiate receptor by ligand elution from immuno-precipitates.

Induction of the gene encoding pro-dynorphin by experimentally induced arthritis enhances staining for dynorphin in the spinal cord of rats

The response of dynorphinergic neurons in the lumbosacral spinal cord of the rat to chronic arthritic inflammation was studied by the combined use of biochemical and immunohistochemical procedures. In polyarthritic rats, in which all four limbs showed a swelling, inflammation and hyperalgesia, a pronounced elevation was seen in the level of messenger ribonucleic acid encoding prodynorphin (pro-enkephalin B) in the lumbosacral spinal cord. In addition, the levels of immunoreactive dynorphin A1-17, a primary gene product of this precursor, were greatly increased. This activation was reflected in a striking intensification of the immunohistochemical staining of both dynorphin and alpha/beta-neo-endorphin, a further major product of pro-dynorphin. In control animals perikarya were stained exceedingly rarely and encountered only in laminae I and II. Stained fibres and varicosities were seen throughout the dorsal and ventral gray matter, being most concentrated in laminae I, II, IV and V of the dorsal horn and dorsolateral to the central canal. In polyarthritic rats, fibres and varicosities were much more intensely stained throughout the cord, particularly in laminae I/II, IV and V and dorsolateral to the central canal. Many strongly-stained perikarya could be seen: these comprised many small diameter cells in laminae I and II, and some large diameter marginal neurons and large diameter cells, heterogenous in appearance, in the deeper laminae IV and V. Monolaterally inflamed rats injected in the right hind-paw showed pathological changes only in this limb. Correspondingly, in unilateral inflammation, an elevation in immunoreactive dynorphin was seen exclusively in the right dorsal horn and the above-described intensification of staining for dynorphin and neo-endorphin was seen only in this quadrant. This reveals the neuroanatomical specificity of the response. Thus, in the lumbosacral cord of the rat, pro-dynorphin neurons are most preponderant in laminae I, II, IV and V. A pronounced intensification of the immunohistochemical staining of these neurons is seen in chronic arthritis. Furthermore, there is a parallel elevation in the levels of messenger ribonucleic acid encoding pro-dynorphin and of its primary products dynorphin and neo-endorphin. These findings demonstrate an enhancement in the functional activity of spinal cord localized dynorphin neurons in the response to chronic arthritic inflammation.

Endorphins in male impotence: evidence for naltrexone stimulation of erectile activity in patient therapy

In the present study we evaluated whether naltrexone administration could stimulate sexual function in 30 male patients, ages 25 to 50 years, with idiopathic impotence of at least one year's duration and not of organic etiology. The patients received naltrexone (50 mg/day) or placebo, on a random basis for two weeks. Sexual performance, expressed as the number of full coitus/week, was assessed before (time 0) and during (on days 7 and 15) each treatment. The naltrexone therapy significantly increased the number of successful coitus compared to placebo after 7 and 15 days of treatment: improvement of sexual performance was evident in 11 out of the 15 treated patients. All the patients experienced a significant increase in morning and spontaneous full penile erections/week. No significant side effects were reported. Endocrine studies revealed no significant modification of plasma LH, FSH or testosterone by naltrexone, suggesting that the positive effect of the drug on sexual behavior was exerted at a central level. A two-month follow-up, at which time patients were off treatment, erectile capacity had returned to baseline in 10 patients, while five reported complete recovery of their sexual ability. We hypothesize that an alteration in central opioid tone is present in idiopathic impotence and is involved in the impairment of sexual behavior.

Opioid antinociception in amphibians

Systemic and spinal administration of opioids produces a behaviorally defined antinociception in a variety of mammalian models. Although endogenous opioid peptides and opioid binding sites are ubiquitous throughout phylogeny, little attention has been paid to the function of endogenous opioid system(s) or development of nociceptive models in nonmammalian species. Recent work has shown that the amphibian, Rana pipiens, provides an appropriate model for assessment of opioid antinociception and that endogenous opioid systems may likewise modulate the central processing of noxious information in amphibians as well as mammals.

Effects of opioids on the heat stimulus-evoked substance P release and thermal edema in the rat hind paw

We examined the effect of opioids on the heat stimulus-evoked release of substance P (SP) into the subcutaneous space and the formation of edema in the rat hind paw. Immersion of the rat hind paw for 30 min into hot water adjusted to 47 degrees C led to a marked increase in the release of SP into the subcutaneous perfusate with the formation of thermal edema. Intra-arterial infusion of morphine (10-100 mumol/kg) or ethylketocyclazocine (30-100 mumol/kg) inhibited dose dependently the heat stimulus-evoked increase in SP release and the thermal edema and the inhibitory effects were antagonized by pretreatment with N-methyl levallorphan (10 mg/kg i.p.) and Win 44,441-3 (10 mg/kg i.p.). The heat stimulus-evoked release of SP was reduced significantly during the intra-arterial infusion of [D-Ala2,Met5] enkephalinamide (100 mumol/kg). These results suggest that the opioid-induced inhibition of heat-induced edema could result from inhibition of the release of SP from peripheral sensory nerve endings.

Modulation of adenylate cyclase activity of mouse spinal cord-ganglion explants by opioids, serotonin and pertussis toxin

Organotypic cultures of fetal mouse spinal cord-ganglion explants (2-4 weeks in vitro) contain forskolin-stimulated adenylate cyclase (AC) activity that is inhibited by levorphanol and other opioid agonists in a dose-dependent manner. Inhibition by levorphanol no longer occurs if sodium is omitted from the incubation and the levorphanol inhibition is blocked by the opioid antagonist, naloxone. These findings together with the ineffectiveness of dextrorphan indicate that the opioid inhibition of forskolin-stimulated AC is receptor mediated. Both the delta- and kappa-receptor subtypes appear to be involved since the selective delta-opioid agonist, [D-Pen2, D-Pen5]enkephalin, and the selective kappa-opioid agonist, t-3,4-dichloro-N-methyl-N[2-(1-pyrrolidinyl)cyclohexyl]-benzene acetamide (U-50,488H) are both effective at nanomolar concentrations. In contrast, the selective mu-opioid agonist, Tyr-D-Ala-Gly-N-MePhe-Gly-ol, has no significant effect even at micromolar concentrations. Both cord and ganglion components of the explants contain opioid-sensitive AC. Forskolin-stimulated AC of the explants is also inhibited by serotonin and carbachol. The serotonin effect appears to be mediated by 5-HT1A receptors, based on relative agonist and antagonist selectivity. Chronic exposure of cultures to morphine results in enhanced basal and forskolin-stimulated AC as well as attenuation of opioid-inhibition of AC assayed in the presence of forskolin; treatment of explants with pertussis toxin causes similar changes in the AC system. The inhibitory effect of serotonin is also attenuated by the pertussis toxin treatment. Basal AC activity of the explants (assayed without forskolin present) is stimulated to a small but significant extent by opioids and by serotonin. The opioid stimulatory effect is markedly enhanced following either morphine or pertussis toxin treatment of the explants. The attenuation of opioid- and serotonin-inhibition of AC produced by chronic exposure to pertussis toxin and the attenuation of opioid inhibition produced by exposure to morphine are consonant with the attenuation of opioid and monoaminergic depression of sensory evoked dorsal horn network responses after similar chronic treatments. It is proposed that the inhibitory effects of opioids and serotonin on these neurons are mediated by receptors that are negatively coupled via a pertussis toxin sensitive Gi protein to AC. Furthermore, alterations of AC with chronic morphine treatment may be involved in the development of physiologic tolerance to opioids.

CSF morphine levels after lumbar intrathecal administration of isobaric and hyperbaric solutions for cancer pain

The objectives of this study were to compare the pharmacokinetic properties and the duration of analgesia following intrathecal administration (L5-S1) of 2 mg morphine in 2 forms: (1) an isobaric (NaCl 0.9%) and (2) a hyperbaric solution (7% dextrose). The study was carried out on 5 cancer patients with severe, intractable pain in the lower half of the body. Samples of CSF were collected at the level of the 10th thoracic vertebra at regular intervals for 15 h after administration. Morphine concentrations were determined by HPLC. The pharmacokinetic properties of the solutions (I and II) were quite different. Peak levels (I) were reached in 5-15 min (30 and 60 micrograms/ml); they then fell rapidly during the 1st hour (7 and 11 micrograms/ml) with an elimination half-life of 10 and 15 min, followed by a change in slope (elimination half-life of 108 and 140 min). Peak levels (II) were reached in 4-5 h (0.8-3.3 micrograms/ml); they then fell progressively according to a single exponential function (elimination half-life: 144-246 min). The duration of analgesia for a dose of 2 mg was 30 h for solution 2 and 24 h for solution 1. The hyperbaric solution, which produced the same degree of analgesia as the isobaric solution, limited the cephalad diffusion of morphine and reduced or abolished the central depressant effects of the drug.

Distribution of opioid binding sites in spinal cord

Opioid binding sites have been demonstrated in the spinal cords of four species (rat, guinea-pig, cow, pig). Similar numbers of sites, as measured with the unselective ligand [3H] bremazocine, are seen along the length of the rat spinal cord. Binding in the dorsal horn is three times greater than in ventral horn. The distribution of mu, delta and kappa sites is similar in all regions. The results are discussed in terms of reported distributions of peptides.

Subtraction autoradiography of opiate receptor subtypes in human brain

Opiate binding sites in sections of human brain were labelled with [3H]etorphine. Many brain areas contained high levels of [3H]etorphine binding sites although the globus pallidus was an exception. Subtraction autoradiography was performed using selective displacement of [3H]etorphine with opioid agonists to visualise mu-, delta- and kappa-opiate receptor subtypes. mu-Receptors were most abundant in the thalamus, caudate nucleus, putamen and the superficial layers of the cerebral cortex. Kappa receptors were concentrated in the deep layers of the cortex, the claustrum and the caudate nucleus. Human brain contained very few delta-receptors. Some brain areas with high concentrations of endogenous opioid peptides have many receptors, but in other areas, for example the globus pallidus, there is a mismatch between peptide concentration and receptor density.

Distinct distribution of opioid receptor types in rat lumbar spinal cord

The distribution of opiate binding sites was studied in sections of rat lumbar spinal cord under conditions selective for mu, delta and kappa receptors. While the levels of mu binding sites were highest in the substantia gelatinosa, elevated levels were also observed in laminae III, IV, V and VIII. In contrast, delta binding was notable only in lamina I. The levels of typical kappa sites were low, and were concentrated in the substantia gelatinosa. An additional, atypical site was detected using 3H-diprenorphine in the presence of mu, delta and kappa receptor blocking agents, and this site was also concentrated in the substantia gelatinosa.

Spinal opiates: a review of their effect on spinal function with emphasis on pain processing

Several opiate receptor systems have been identified in the spinal cord. They produce a powerful analgesia when opioid agonists are administered intrathecally in the intact, unanesthetized animal. These effects appear mediated by an action on opioid receptors which are located presynaptically, in the terminals of primary afferents, and postsynaptically on certain dorsal horn neurons. Based on structure-activity relationships in different tests, quantitative studies of naloxone antagonism and selective cross tolerance, it appears that, in the spinal cord, there are three distinguishable populations of opioid receptors: mu, delta and kappa. Aside from the effects on nociception, these receptors are also associated with a variety of spinal mechanisms related to other aspects of sensory, autonomic and motor functions. Though in some cases these represent important side-effects (e.g. inhibition of the micturition reflex), in others, the subtle effects may have important therapeutic benefits (e.g. relieving spasticity in spinal injured patients).

Morphine- and ketocyclazocine-induced analgesia in the developing rat: differences due to type of noxious stimulus and body topography

Patterns of morphine- and ketocyclazocine-induced analgesia in limb withdrawal and tail-flick tests of thermal and mechanical nociception were examined in the preweanling rat. In the forepaw test, morphine was more effective than ketocyclazocine with both thermal and mechanical stimuli. Both drugs first induced analgesia between 3 and 5 days of age. In the tail-flick test, ketocyclazocine-induced analgesia preceded morphine's effects against both thermal and mechanical stimuli by several days. Ketocyclazocine produced robust analgesia between 7 and 10 days of age, while the effects of morphine did not peak until day 14. In the hindpaw, morphine was more effective than ketocyclazocine against a higher intensity mechanical stimulus, while ketocyclazocine was more effective against a lower intensity mechanical stimulus. Morphine-induced analgesia was reversed by lower doses of naloxone than was ketocyclazocine-induced analgesia, regardless of body part tested, against all noxious stimuli. These findings demonstrate differences in morphine- and ketocyclazocine-induced analgesia that are dependent upon age, body topography, stimulus type and intensity and imply different physiologic roles of mu- and chi-opioid receptors in analgesia.

Opioid receptor binding in rat spinal cord

The interaction of various radioligands with spinal opioid receptors has been characterized under variable experimental conditions. Binding to mu, delta, and kappa sites was measured in all (cervical, thoracic, lumbar) segments. The apparent affinity constant (K) of [3H]Ethylketocyclazocine (EKC) was similar in Tris, 2.09 (+/- 1.06) X 10(8) M-1, and phosphate buffer, 2.16 (+/- 0.02) X 10(8) M-1, when its interaction with delta and mu sites was blocked. Without blocking ligands, EKC binding was resolved in two components: K1 = 1.01 (+/- 0.21) X 10(9) M-1 and K2 = 0.95 (+/- 0.61) X 10(7) M-1. Likewise, the binding of [D-Ala2, MePhe4, Gly(ol)5]enkephalin (DAGO) or [D-Ala2, D-Leu5]-enkephalin (DADLE) alone was represented by a 2-site model. By adjusting the radioligand and receptor concentration or by the addition of blocking ligands, binding was represented by a 1-site model for DAGO, K = 4.35 (+/- 1.41) X 10(8) M-1, and DADLE, K = 2.44 (+/- 0.08) X 10(8) M-1.

Opiate receptors in the human spinal cord: a detailed anatomical study comparing the autoradiographic localization of [3H]diprenorphine binding sites with the laminar pattern of substance P, myelin and nissl staining

The anatomical localization of opiate receptors in the human spinal cord has been examined in six cases aged 7-41 years using quantitative autoradiographic methods following the incubation of fresh, unfixed cryostat sections with [3H]diprenorphine. In order to precisely localize the distribution of receptors in the spinal cord, the laminar anatomy of the spinal grey was demonstrated at each spinal level examined using 50-microns sections stained for myelin, Nissl substance and substance P. In all cases, autoradiograms demonstrated that opiate receptors were distributed in a similar fashion in the grey matter of the cervical, thoracic, lumbar, sacral and coccygeal regions of the human spinal cord. At all 25 spinal levels examined, opiate receptors were mainly localized within the upper laminae of the dorsal horn (laminae I-III) and within the tract of Lissauer. The highest density of opiate receptors was localized within the inner segment of lamina II where the receptors formed a very dense band lying immediately dorsal to lamina III. The density of receptors in this inner region of lamina II (33 +/- 2 fmol/mg) was more than two-and-one-half times greater than that in the remaining upper laminae which showed moderate receptor densities: lamina I (12 +/- 4 fmol/mg) and outer lamina II (13 +/- 3 fmol/mg) both showed similar receptor densities which were higher than those in lamina III (10 +/- 3 fmol/mg) The tract of Lissauer (11 +/- 2 fmol/mg) also showed a moderate density of opiate receptors which was intermediate between the densities in laminae I/IIo and the density of lamina III. The density of receptors in the remaining laminae of the spinal cord varied from moderately low to virtually zero. Moderately low densities of receptors were found in laminae V, VI, VIII, IX and X with very low levels within laminae IV and VII. In particular, in lamina VII opiate receptors were unable to be detected above normal background levels in the dorsal nucleus of Clarke. These results show that, as in other mammalian species, opiate receptors in the human spinal cord are mainly concentrated in the upper laminae of the dorsal horn and in the tract of Lissauer. The possible role of these receptors in modulating spinal nociceptive information is discussed with respect to previous findings on the relationship of opiate receptors to primary afferent fibres in the spinal cord.

Pharmacology of delta-opioid receptors in the hamster vas deferens

Electrically evoked contractions of the hamster isolated vas deferens are inhibited only by opioid drugs which have agonist activity at delta-opioid receptors. Opioids which are mu-, kappa- or sigma-selective were either inactive or were antagonists. The compound beta-funaltrexamine, which irreversibly blocks mu- and delta-opioid receptors, caused a flattening of the dose-response curve and a reduced maximum inhibition available to delta-opioid agonists. Analysis of the curves by the double-reciprocal null method enabled the affinity of these agonists at delta-opioid receptors to be calculated.