The mu-opioid receptor (MOR1) is mainly restricted to neurons that do not contain GABA or glycine in the superficial dorsal horn of the rat spinal cord

CB1-cannabinoid and mu-opioid receptor co-localization on postsynaptic target in the rat dorsal horn

Cannabinoids and opioids interact in the control of nociception at the spinal level. Likely, several mechanisms are involved, with one of them being co-localization of cannabinoid and opioid receptors. In order to validate this hypothesis, a double labeling study of CB1 cannabinoid receptors and mu-opioid receptors in the dorsal horn of the rat spinal cord was performed. A strong co-localization of CB1 and mu-opioid receptors was observed in lamina II interneurons at the ultrastructural level. The physiological consequences of the co-localization are discussed.

Effect of morphine on cholecystokinin and mu-opioid receptor-like immunoreactivities in rat spinal dorsal horn neurons after peripheral axotomy and inflammation

In order to further investigate the interaction between the octapeptide cholecystokinin and opioid analgesia in the spinal cord we used double-colour immunofluorescence to examine the anatomical distribution of cholecystokinin and mu-opioid receptors in the dorsal horn, as well as the effect of morphine on cholecystokinin- and mu-opioid receptor-like immunoreactivities following peripheral nerve injury and inflammation. Mu-opioid receptor-like immunoreactivity was present in 65.6% of cholecystokinin-positive neurons in laminae I and II of rat spinal cord. Conversely, 40.4% of mu-opioid receptor-positive neurons contained cholecystokinin-like immunoreactivity. Systemic application of morphine (1, 3 or 10 mg/kg; i.v.) after sciatic nerve section significantly, but reversibly, decreased mu-Opioid receptor-like immunoreactivity in the medial half of lamina II in segment L5 of the ipsilateral dorsal horn, and cholecystokinin-like immunoreactivity was also markedly reduced in the same region. These effects were dose- and time-dependent and could be prevented by naloxone preadministration. In contrast, no significant change in the pattern of distribution or intensity of mu-opioid receptor- and cholecystokinin-like immunoreactivities was observed in intact rats or during peripheral inflammation. These results provide a cellular basis for the interaction of mu-opioid receptors and cholecystokinin at the spinal level by showing a high degree of co-existence of these two molecules in local interneurons, and also show that morphine can induce rapid and short lasting effects on mu-opioid receptors after peripheral nerve injury. The results contribute to our understanding of how endogenous cholecystokinin reduces the analgesic effect of morphine.

Pharmacological characterization of morphine-induced in vivo release of cholecystokinin in rat dorsal horn: effects of ion channel blockers

Previous studies indicate that an increased release of cholecystokinin (CCK) in response to morphine administration may counteract opioid-induced analgesia at the spinal level. In the present study we used in vivo microdialysis to demonstrate that systemic administration of antinociceptive doses of morphine (1-5 mg/kg, s.c.) induces a dose-dependent and naloxone-reversible release of CCK-like immunoreactivity (CCK-LI) in the dorsal horn of the spinal cord. A similar response could also be observed following perfusion of the dialysis probe for 60 min with 100 microM but not with 1 microM morphine. The CCK-LI release induced by morphine (5 mg/kg, s.c.) was found to be calcium-dependent and tetrodotoxin-sensitive (1 microM in the perfusion medium). Topical application of either the L-type calcium channel blocker verapamil (50 microg) or the N-type calcium channel blocker omega-conotoxin GVIA (0.4 microg) onto the dorsal spinal cord completely prevented the CCK-LI release induced by morphine (5 mg/kg, s.c.). Our data indicate that activation of L- and N-type calcium channels is of importance for morphine-induced CCK release, even though the precise site of action of morphine in the dorsal horn remains unclear. The present findings also suggest a mechanism for the potentiation of opioid analgesia by L- and N-type calcium channel blocking agents.

Actions of opioids on excitatory and inhibitory transmission in substantia gelatinosa of adult rat spinal cord

1. The actions of opioid receptor agonists on synaptic transmission in substantia gelatinosa (SG) neurones in adult (6- to 10-week-old) rat spinal cord slices were examined by use of the blind whole-cell patch-clamp technique. 2. Both the mu-receptor agonist DAMGO (1 microM) and the delta-receptor agonist DPDPE (1 microM) reduced the amplitude of glutamatergic excitatory postsynaptic currents (EPSCs) which were monosynaptically evoked by stimulating Adelta afferent fibres. Both also decreased the frequency of miniature EPSCs without affecting their amplitude. 3. In contrast, the kappa-receptor agonist U-69593 (1 microM) had little effect on the evoked and miniature EPSCs. 4. The effects of DAMGO and DPDPE were not seen in the presence of the mu-receptor antagonist CTAP (1 microM) and the delta-receptor antagonist naltrindole (1 microM), respectively. 5. Neither DAMGO nor DPDPE at 1 microM affected the responses of SG neurones to bath-applied AMPA (10 microM). 6. Evoked and miniature inhibitory postsynaptic currents (IPSCs), mediated by either the GABAA or the glycine receptor, were unaffected by the mu-, delta- and kappa-receptor agonists. Similar results were also obtained in SG neurones in young adult (3- to 4-week-old) rat spinal cord slices. 7. These results indicate that opioids suppress excitatory but not inhibitory synaptic transmission, possibly through the activation of mu- and delta- but not kappa-receptors in adult rat spinal cord SG neurones; these actions are presynaptic in origin. Such an action of opioids may be a possible mechanism for the antinociception produced by their intrathecal administration.

Expression of mu-, kappa- and delta-opioid receptors in P19 mouse embryonal carcinoma cells

P19 embryonal carcinoma cells are pluripotential and able to differentiate into a variety of cell types, including neurons, glia and fibroblast-like cells, upon retinoic acid treatment and cellular aggregation. Using reverse transcription-polymerase chain reaction, ligand binding and immunocytochemical methods, kappa- and delta-opioid receptors were detected in undifferentiated P19 cells. The mu-opioid receptor was not observed until one day after plating, following one essential step of differentiation, but increased in number in the four days after plating. Several different expression patterns were detected in these differentiated cells. Some cells exhibited mu- and delta-opioid receptors co-expressed, with or without K-opioid receptor; whereas some of the cells expressed only K-opioid receptor. All three opioid receptors are detected on aggregated cells which are postmitotic and also expressing neurofilaments, indicating neuronal characteristics. Furthermore, those cells expressing mu and delta-opioid receptors also expressed glutamate decarboxylase, characteristic of the GABAergic phenotype. Based on these findings, we propose that P19 cells may serve as a model system to study the developmental regulation of opioid receptors, and in particular their relationship with GABA.

Mu opioid receptors in developing human spinal cord

The distribution of mu opioid receptors was studied in human fetal spinal cords between 12-13 and 24-25 wk gestational ages. Autoradiographic localisation using [3H] DAMGO revealed the presence of mu receptors in the dorsal horn at all age groups with a higher density in the superficial laminae (I-II). A biphasic expression was noted. Receptor density increased in the dorsal horn, including the superficial laminae, between 12-13 and 16-17 wk. This could be associated with a spurt in neurogenesis. The density increased again at 24-25 wk in laminae I-II which resembled the adult pattern of distribution. A dramatic proliferation of cells was noted from the region of the ventricular zone between 16-17 and 24-25 wk. These were considered to be glial cells from their histological features. Mu receptor expression was noted over a large area of the spinal cord including the lateral funiculus at 24-25 wk. This may be due to receptor expression by glial cells. The study presents evidence of mu receptor expression by both neurons and glia during early development of human spinal cord.

The types of neuron which contain protein kinase C gamma in rat spinal cord

Protein kinase C (PKC) is thought to have a role in sensitization of dorsal horn neurons in certain pain states, and a recent study has reported that mice which lack the gamma isoform (PKCgamma) show reduced neuropathic pain after peripheral nerve injury. Although PKCgamma is present at high levels in the ventral part of lamina II we have limited information concerning the types of neuron in which it is located. In this study we have used immunocytochemistry to characterise the neurons which contain PKCgamma. Immunoreactive neurons were concentrated in ventral lamina II, but were also present in lamina III. Some weakly-immunoreactive neurons were located in the dorsal part of lamina II and in lamina I. The great majority (92%) of cells with PKCgamma were not GABA-immunoreactive, and these cells are likely to be excitatory interneurons. Dual-immunofluorescence labelling showed that PKCgamma was not randomly distributed amongst non-GABAergic neurons, since it was present in 76% of cells with neurotensin and 45% of those with somatostatin, but only 5% of those with the mu-opioid receptor (MOR-1). Cells with the neurokinin 1 receptor are found in lamina I and lamina III, and PKCgamma was present in 22% and 37% of these populations, respectively. These results suggest that excitatory interneurons in laminae II and III which lack the micro-opioid receptor may have a significant role in generating neuropathic pain.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

A quantitative study of neurons which express neurokinin-1 or somatostatin sst2a receptor in rat spinal dorsal horn

The neurokinin-1 and somatostatin sst2a receptors have both been identified on spinal cord neurons. In this study we have estimated the proportions of neurons in different parts of the spinal cord which express these receptors, by using a monoclonal antibody against a neuronal nuclear protein named NeuN and combining the optical disector method with confocal microscopy. The NeuN antibody was initially tested on over 3200 neurons identified with antisera against a variety of compounds, including neuropeptides, enzymes and receptors, and also on astrocytes and oligodendrocytes. All of the neurons, but none of the glial cells that were examined possessed NeuN-immunoreactivity, which suggests that NeuN is a reliable marker for all spinal cord neurons. We found that approximately 45% of neurons in lamina I, 23-29% of those in laminae IV-VI and 18% in lamina X possessed the neurokinin-1 receptor, while the receptor was present on a smaller proportion of neurons in laminae II and III (6% and 11%, respectively). Thirteen percent of lamina I neurons and 15% of those in lamina II expressed the sst2a receptor. To provide further information about the types of neuron which possess the sst2a receptor, we searched for possible co-existence with the neurokinin-1 receptor as well as with GABA and glycine. sst2a and neurokinin-1 receptors were not co-localized on neurons in laminae I and II. All of the sst2a-immunoreactive neurons examined were also GABA-immunoreactive, and 83.5% were glycine-immunoreactive, indicating that the receptor is located on inhibitory neurons in the superficial dorsal horn. These results demonstrate the proportions of neurons in each region of the spinal cord which can be directly activated by substance P or somatostatin acting through these receptors. Levels of receptors can change in pathological states, and this method could be used to determine whether or not these changes involve alterations in the number of neurons which express receptors. In addition, the method can be used to estimate the sizes of neurochemically-defined populations of spinal cord neurons.

Differential effects of morphine on corneal-responsive neurons in rostral versus caudal regions of spinal trigeminal nucleus in the rat

The initial processing of corneal sensory input in the rat occurs in two distinct regions of the spinal trigeminal nucleus, at the subnucleus interpolaris/caudalis transition (Vi/Vc) and in laminae I-II at the subnucleus caudalis/spinal cord transition (Vc/C1). Extracellular recording was used to compare the effects of morphine on the evoked activity of corneal-responsive neurons located in these two regions. Neurons also were characterized by cutaneous receptive field properties and parabrachial area (PBA) projection status. Electrical corneal stimulation-evoked activity of most (10/13) neurons at the Vi/Vc transition region was increased [146 +/- 16% (mean +/- SE) of control, P < 0.025] after systemic morphine and reduced after naloxone. None of the Vi/Vc corneal units were inhibited by morphine. By contrast, all corneal neurons recorded at the Vc/C1 transition region displayed a naloxone-reversible decrease (55 +/- 10% of control, P < 0.001) in evoked activity after morphine. None of 13 Vi/Vc corneal units and 7 of 8 Vc/C1 corneal units tested projected to the PBA. To determine if the Vc/C1 transition acted as a relay for the effect of intravenous morphine on corneal stimulation-evoked activity of Vi/Vc units, morphine was applied topically to the dorsal brain stem surface overlying the Vc/C1 transition. Local microinjection of morphine at the Vc/C1 transition increased the evoked activity of 4 Vi/Vc neurons, inhibited that of 2 neurons, and did not affect the remaining 12 corneal neurons tested. In conclusion, the distinctive effects of morphine on Vi/Vc and Vc/C1 neurons support the hypothesis that these two neuronal groups contribute to different aspects of corneal sensory processing such as pain sensation, autonomic reflex responses, and recruitment of descending controls.

GluR1 and GluR2/3 subunits of the AMPA-type glutamate receptor are associated with particular types of neurone in laminae I-III of the spinal dorsal horn of the rat

GluR1 and GluR2 subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor are expressed at high levels by neurones in laminae I-III of rat spinal dorsal horn, an area which contains numerous, densely packed small neurones. In order to determine whether these subunits are expressed by inhibitory or excitatory neurones, we combined pre-embedding immunocytochemistry with antibodies that recognize either GluR1, or an epitope common to GluR2 and 3, with postembedding detection of gamma-aminobutyric acid (GABA) and glycine. Most (78%) of the neurones with GluR1-immunoreactivity were GABA-immunoreactive, and some of these were also glycine-immunoreactive, whereas nearly all (97%) of the GluR2/3-immunoreactive neurones were not GABA- or glycine-immunoreactive. We carried out double-immunofluorescence and confocal microscopy to provide further information on the neurochemistry of cells that express these subunits. As expected, all neurotensin- and virtually all somatostatin-immunoreactive cells (which are thought to be excitatory interneurones) were GluR2/3- but not GluR1-immunoreactive, whereas parvalbumin-containing cells (most of which are GABAergic) possessed GluR1-, but usually not GluR2/3-immunoreactivity. Neurones that contained nitric oxide synthase (most of which are GABAergic) were more variable, with 57% GluR1-immunoreactive and 41% GluR2/3-immunoreactive. Cholinergic neurones in lamina III (which are also GABAergic) invariably showed each type of GluR-immunoreactivity. These results suggest that neuronal populations in laminae I-III have characteristic patterns of GluR expression: GluR1 is particularly associated with inhibitory neurones, and GluR2 with excitatory neurones. This makes it likely that some of the AMPA receptors present on the inhibitory interneurones lack the GluR2 subunit, and may therefore have significant Ca2+-permeability.

GABAergic synapses on mu-opioid receptor-expressing neurons in the superficial dorsal horn: an electron microscope study in the cat spinal cord

A double-immunocytochemical electron microscope study was performed in the cat to examine whether GABAergic axons might be in synaptic contact with spinal neurons expressing mu-opioid receptor (MOR) in laminae I and II of the spinal dorsal horn at the lumbar cord segments. Structures showing MOR-like immunoreactivity (-LI) and those showing GABA-LI were labeled, respectively, with diaminobenzidine/peroxidase-reaction products and immunogold particles. Approximately one-third of dendritic profiles with MOR-LI in laminae I and II were postsynaptic to axon terminals with GABA-LI; about one-fourth of somatic profiles with MOR-LI were also postsynaptic to axon terminals with GABA-LI. The results suggest that activation of MOR on postsynaptic neurons may modulate effects which are induced by GABA released from presynaptic neurons.