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

Morphine actions in the rat forebrain: role of protein kinase C

Acute administration of morphine induces expression of the immediate-early gene (IEG) c-Fos in dorsomedial striatum, portions of cerebral cortex, and in several midline-intralaminar thalamic nuclei, partly via a trans-synaptic mechanism that involves activation of glutamate receptors. Because activation of protein kinase C (PKC) may occur following the activation of glutamate receptors, we determined whether pharmacological inhibition of PKC would attenuate morphine-induced c-Fos expression, and whether acute administration of morphine would induce translocation of PKC. The selective PKC antagonist NPC 15437 given 30 min prior to morphine significantly decreased morphine-induced c-Fos expression in striatum and cingulate cortex, but not in centrolateral thalamus. In another experiment, rats were given an acute dose of morphine, and immunocytochemical analysis was performed for the betaI and betaII isoforms of PKC. Morphine induced a rapid and transient translocation of PKC betaII, but not betaI, from perinuclear spots to plasma membrane in numerous cortical and striatal neurons. Prior administration of naloxone blocked this response. Ultrastructural studies confirmed translocation from Golgi apparatus to plasma membrane 15 min after morphine injection. Double immunocytochemistry at the light microscopic level demonstrated co-localization of translocated PKC betaII and c-Fos in some cortical neurons 90 min after morphine injection. These results support a role for PKC, especially PKC betaII, in the rapid effects of morphine on the brain.

Kappa opioid receptors in rat spinal cord: sex-linked distribution differences

Activation of kappa opioid receptors (KORs) in the spinal cord can diminish nociception. Humans and rodents show sex differences in the analgesia produced by KOR agonists, and female rats show fluctuations in KOR density and sensitivity across the estrous cycle. However, it is unclear whether there are sex differences in the amount and/or distribution of spinal KORs. In the present study, immunocytochemically labeled KORs were examined in laminae I and II of the lumbosacral spinal dorsal horn of male and normally cycling female Sprague-Dawley rats. The basic pattern of KOR labeling was determined in both sexes using qualitative electron microscopy (EM), and sex-linked differences in the density and subcellular distribution of KOR immunoreactivity were determined with quantitative EM and light microscopy. KOR labeling was visualized with immunoperoxidase for optimally sensitive detection, or with immunogold for precise subcellular localization. By EM, the general pattern of KOR immunoreactivity was similar in males and females. KOR immunoreactivity was common in dendrites, axons, and axon terminals, and was in a few glia and neuronal somata. Most KOR-immunoreactive (-ir) axons were fine-diameter and unmyelinated. Most KOR-ir terminals were small or medium-sized, and a minority formed asymmetric or symmetric synapses with unlabeled dendrites. KOR immunoreactivity was associated both with the plasma membrane and with cytoplasmic organelles, notably including dense core vesicles in terminals. Light microscopic densitometry revealed that KOR immunoreactivity was significantly denser in estrus and proestrus females than in males. By EM, the distribution of KOR-immunogold labeling within axon terminals differed, with a greater proportion of cytoplasmic KOR labeling in estrus females compared with males. In contrast, the abundance and types of KOR-immunoperoxidase-labeled profiles did not show sex-linked differences. We conclude that in both sexes, KORs are positioned to influence both pre- and postsynaptic neurotransmission and are present in morphologically heterogeneous neuron populations. These findings are consistent with complex consequences of KOR activation in the spinal cord. In addition, the presence of increased KOR density and proportionally elevated intracellular KORs in proestrus/estrus females suggests a basis for sex-linked differences in KOR-mediated antinociception.

Antinociceptive and nociceptive actions of opioids

Although the opioids are the principal treatment options for moderate to severe pain, their use is also associated with the development of tolerance, defined as the progressive need for higher doses to achieve a constant analgesic effect. The mechanisms which underlie this phenomenon remain unclear. Recent studies revealed that cholecystokinin (CCK) is upregulated in the rostral ventromedial medulla (RVM) during persistent opioid exposure. CCK is both antiopioid and pronociceptive, and activates descending pain facilitation mechanisms from the RVM enhancing nociceptive transmission at the spinal cord and promoting hyperalgesia. The neuroplastic changes elicited by opioid exposure reflect adaptive changes to promote increased pain transmission and consequent diminished antinociception (i.e., tolerance).

Neuronal types expressing μ- and δ-opioid receptor mRNA in the rat hippocampal formation

Opioids are thought to control the excitability of hippocampal principal neurons indirectly by inhibiting GABAergic interneurons. However, direct inhibition of hippocampal principal neurons by opioids has also been reported. To understand better the neuromodulatory role of opioids in rat hippocampal circuits, we analyzed types of micro- and delta-opioid receptor (MOR, DOR)-expressing hippocampal neurons. Most MOR-immunoreactive neurons in the granular and pyramidal cell layers exhibited multipolar morphologies characteristic of GABAergic neurons. Virtually all neurons in the hippocampal formation expressing high MOR mRNA levels cocontained the mRNA for glutamic acid decarboxylase (GAD). Most parvalbumin-, several calretinin-, and several pre-proenkephalin-containing neurons expressed the MOR gene in the hippocampal formation. Expression of high DOR mRNA levels was restricted to GAD-positive neurons in the principal cell layers, oriens layer and hilus. More than 90% of the parvalbumin-positive neurons in the hippocampal formation strongly expressed the DOR gene. Granule cells expressing vesicular glutamate transporter 1 (VGLUT1) mRNA contained very low MOR and DOR transcript levels. In VGLUT1-positive pyramidal cells, weak DOR but no MOR gene expression was detected. Whereas most somatostatinergic hilar neurons were negative for MOR and DOR mRNA, somatostatinergic oriens layer neurons frequently expressed these receptors. Taken together, weak expression of MOR and DOR genes in hippocampal principal cells is in concordance with direct opioid-mediated inhibition of principal cells. However, strong expression of the MOR and DOR genes in the hippocampus is restricted to gamma-aminobutyric acid (GABA)ergic neurons, with DORs being selectively expressed in the parvalbumin- and somatostatin-containing subpopulations. Activation of MOR and/or DOR in parvalbumin- and somatostatin-containing neurons, which provide GABAergic inhibition to the perisomatic and distal dendritic regions of principal cells, respectively, is likely to facilitate principal cell excitation.

Effect of testosterone and season on proenkephalin messenger RNA expression in the preoptic area of the hypothalamus in the ram

Enkephalin appears to exert an inhibitory action on LH secretion, but whether testosterone regulates enkephalin gene expression is unknown. This study tested the hypothesis that testosterone and/or season modulate preproenkephalin mRNA expression in specific areas of the hypothalamus. Romney Marsh rams were castrated (wethers) either during the breeding season or nonbreeding season and received intramuscular injections of either oil or testosterone propionate (five/group). Blood samples were taken for the assay of plasma LH and testosterone. Preproenkephalin mRNA expression was quantified in hypothalamic sections by in situ hybridization. Mean plasma LH concentrations were reduced and the interpulse interval for LH pulses was greater in testosterone propionate-treated wethers compared with oil-treated wethers, with no change in LH pulse amplitude. Testosterone propionate treatment reduced proenkephalin expression in the diagonal band of Broca, the caudal preoptic area, and the bed nucleus of the stria terminalis. Seasonal differences in proenkephalin expression were observed in the bed nucleus of the stria terminalis, lateral septum, periventricular nucleus, and paraventricular nucleus. No differences were observed between treatments in seven other regions examined. We conclude that testosterone and season regulate proenkephalin mRNA levels in the preoptic area/hypothalamus in the ram in a region-specific manner.

Modulation of cellular activity and synaptic transmission in the ventral tegmental area

The mesolimbic dopamine system, of which the cell bodies are located in the ventral tegmental area, has been implicated in the physiology of reward and the related pathophysiology of drug abuse. This area has been a site of significant interest to study the effects of drugs of abuse and neurotransmitter systems implicated in the rewarding effects of these compounds. One important aspect of synaptic transmission is the ability of synapses to strengthen or weaken their connection as a consequence of synaptic activity. Recently, it has become apparent that this phenomenon is also present in the ventral tegmental area and that this may bear important functional consequences for the ways in which drugs of abuse assert their effect. Here, we will review the effects of neurotransmitter systems and drugs of abuse on cellular activity and synaptic transmission in the ventral tegmental area.

Convergent effects of oxytocin and a δ-opioid agonist in the bed nuclei of the stria terminalis of the peripartum rat

In the bed nuclei of the stria terminalis (BST), opioids may suppress the facilitatory effect of oxytocin on its own release pre-partum. In vitro electrophysiological recording showed that in virgin, late-pregnant, and lactating rats, a delta-opioid agonist inhibited a high proportion of BST neurons, many of which were also oxytocin responsive. Response magnitude did not differ significantly between groups, suggesting that the postulated pre-partum increase in opioid tone does not involve postsynaptic changes in neuronal sensitivity.

Presynaptic Δ opioid receptors differentially modulate rhythm and pattern generation in the ventral respiratory group of the rat

The specific role of the Delta opioid receptor (DOR), in opioid-induced respiratory depression in the ventral respiratory group (VRG) is largely unknown. Here, we sought to determine (1) the relationship between DOR-immunoreactive (ir) boutons, bulbospinal and functionally identified respiratory neurons in the VRG and (2) the effects of microinjection of the selective DOR agonist, D-Pen 2,5-enkephalin (DPDPE), into different subdivisions of the VRG, on phrenic nerve discharge and mean arterial pressure. Following injections of retrograde tracer into the spinal cord or intracellular labelling of respiratory neurons, in Sprague-Dawley rats, brainstem sections were processed for retrograde or intracellular labelling and DOR-ir. Bulbospinal neurons were apposed by DOR-ir boutons regardless of whether they projected to single (cervical or thoracic ventral horn) or multiple (cervical and thoracic ventral horn) targets in the spinal cord. In the VRG, a total of 24 +/- 5% (67 +/- 13/223 +/- 49) of neurons projecting to the cervical ventral horn, and 37 +/- 3% (96 +/- 22/255 +/- 37) of neurons projecting to the thoracic ventral horn, received close appositions from DOR-ir boutons. Furthermore, DOR-ir boutons closely apposed six of seven intracellularly labelled neurons, whilst the remaining neuron itself possessed boutons that were DOR-ir. DPDPE was microinjected (10 mM, 60 nl, unilateral) into regions of respiratory field activity in the VRG of anaesthetised, vagotomised rats, and the effects on phrenic nerve discharge and mean arterial pressure were recorded. DPDPE depressed phrenic nerve amplitude, with little effect on phrenic nerve frequency in the Bötzinger complex, pre-Bötzinger complex and rVRG, the greatest effects occurring in the Bötzinger complex. The results indicate that the DOR is located on afferent inputs to respiratory neurons in the VRG. Activation of the DOR in the VRG is likely to inhibit the release of neurotransmitters from afferent inputs that modulate the pattern of activity of VRG neurons.

Modulation of voltage-dependent potassium currents by opiates in facial motoneurons of neonatal rats

We examined the modulation of rat facial motoneurons (FMNs) by opiates in a slice preparation (7-15 days old) using whole-cell patch clamp techniques. Although application of methionine enkephalin (ME) did not change the peak value of the transient outward current (A-current, IA), it reduced the persistent voltage-dependent K(+) currents (IKs) in a dose-dependent manner. The reduction was antagonized by naloxone (40 microM). IKs were reduced only by mu-selective agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO, 2-121.6 microM). This reduction was antagonized by naloxone (40 microM) or the mu-selective antagonist, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH(2) (CTOP, 1 microM). Agonists for other opiate receptors (delta- and kappa-opiate receptor) showed no effect on IKs. In accord with the effects on IKs, DAMGO (100 microM) prolonged the duration of the action potential evoked in Ca(2+)-free external solution containing 4-aminopiridine (1mM). These results suggest that the activation of mu-opiate receptors contributes to signal transduction in FMNs primarily by modulating action potential duration.

Phylogenetic changes in the expression of delta opioid receptors in spinal cord and dorsal root ganglia

To assess the validity of rodent models for investigating the role of delta opioid receptors (DOR) in analgesia, the distribution of DOR binding and mRNA were compared between rodent and primate spinal cord and dorsal root ganglia (DRG), using receptor autoradiography and in situ hybridization, respectively. In mouse and rat spinal cord, [(125)I]-deltorphin-labeled DOR binding sites were detected throughout the gray matter. In contrast, in primate and particularly in human spinal cord, DOR binding was mainly present in laminae I-II, with little to no binding in deeper layers. Accordingly, in rodent spinal cord, DOR mRNA was expressed by a large number of neurons distributed throughout the ventral and dorsal horns, whereas in the primate, DOR expression was significantly lower, as evidenced by a moderate number of labeled cells throughout the gray matter in monkey and by only few labeled cells in human, mainly in Clarke's column and lamina IX. Major species differences in DOR expression were also observed in primary afferent cells bodies. In rat DRG, intense DOR mRNA hybridization was primarily observed over large ganglion cells immunopositive for neurofilament 200. In contrast, in monkey and human DRG, DOR mRNA was primarily detected over small and medium-sized ganglion cells. These results demonstrate major differences in the expression and distribution of DOR in the spinal cord and DRG between mammalian species. Specifically, they point to a progressive specialization of DOR toward the regulation of primary somatosensory, namely nociceptive, inputs during phylogeny and suggest that the effects of DOR agonists in rodents may not be entirely predictive of their action in humans.

Selective vulnerability of cerebellar granule neuroblasts and their progeny to drugs with abuse liability

Cerebellar development is shaped by the interplay of genetic and numerous environmental factors. Recent evidence suggests that cerebellar maturation is acutely sensitive to substances with abuse liability including alcohol, opioids, and nicotine. Assuming substance abuse disrupts cerebellar maturation, a central question is: what are the basic mechanisms underlying potential drug-induced developmental defects? Evidence reviewed herein suggests that the maturation of granule neurons and their progeny are intrinsically affected by several classes of substances with abuse liability. Although drug abuse is also likely to target directly other cerebellar neuron and glial types, such as Purkinje cells and Bergmann glia, findings in isolated granule neurons suggest that they are often the principle target for drug actions. Developmental events that are selectively disrupted by drug abuse in granule neurons and/or their neuroblast precursors include proliferation, migration, differentiation (including neurite elaboration and synapse formation), and programmed cell death. Moreover, different classes of drugs act through distinct molecular mechanisms thereby disrupting unique aspects of development. For example, drug-induced perturbations in: (i) neurotransmitter biogenesis; (ii) ligand and ion-gated receptor function and their coupling to intracellular effectors; (iii) neurotrophic factor biogenesis and signaling; and (iv) intercellular adhesion are all likely to have significant effects in shaping developmental outcome. In addition to identifying therapeutic strategies for drug abuse intervention, understanding the mechanisms by which drugs affect cellular maturation is likely to provide a better understanding of the neurochemical events that normally shape central nervous system development.

The opioid receptors in inner ear of different stages of postnatal rats

There is increasing evidence that the opioid system has a role in hearing. To provide further evidence for such a role, the expression of opioid receptor mRNAs and proteins in the inner ear of rats was studied during development from birth (P0) to postnatal day 16 (P16). A semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) was employed to detect changes in the expression of delta- (DOR) kappa- (KOR) and mu- (MOR) opioid receptor mRNAs in rat cochleae at P0, P4, P8 and P16. Expression of DOR mRNA levels steadily increased from P0 to P8 with no further increases by P16. KOR mRNA was expressed at a relatively high level at P0 and P4 followed by a decrease while MOR mRNA was expressed at a low level at P0 and P4 followed by an increase by P8 and P16. Immunocytochemical labelling of inner ear sections revealed unique developmental and distribution patterns of opioid receptors. In the organ of Corti DOR immunoreactivity (DOR-IR) was detected in hair cells from P4. In contrast MOR-IR was present only in supporting cells at P0-P16. In the spiral ganglion all three receptor subtypes were expressed from P0 on nerve cell soma and qualitatively appeared to increase with age. Also DOR-IR and MOR-IR were detected at P8 and P16 in nerve fibers within the spiral ganglion. In the limbus DOR-IR was detected at P8 and P16 on cells proximal to the tectorial membrane while MOR-IR was detected more distally. In general these findings demonstrate that within the inner ear each receptor subtype follows specific temporal and spatial developmental patterns, some of which may be associated to the onset of hearing. The data provide further evidence that the opioid system may play a role in the development and functioning of the inner ear.

The mu opioid receptor: from molecular cloning to functional studies

Opioids have been used and abused by humans for centuries. The mu opioid receptor represents the high affinity binding site for opioid narcotics with high abuse liability such as morphine, codeine and fentanyl. Heroin (diacetylmorphine), a semi-synthetic derivative of morphine, crosses the blood-brain barrier more readily than morphine due to its increased hydrophobicity. Once in the brain heroin is hydrolyzed to morphine, which acts at the mu opioid receptor and results in euphoria, thus conferring the reinforcing properties of heroin. Using molecular biology techniques, the mu opioid receptors from several species have been cloned. This article reviews recent progress in this area, with respect to the two major cellular functions of the mu opioid receptor: reduction of intracellular cAMP concentration by inhibiting adenylyl cyclase activity, and inhibition of neuronal firing by modulating membrane ion channels.

The expression of delta- and kappa-opioid receptor is enhanced during intestinal inflammation in mice

In the gut, mu-, delta-, and kappa-opioid receptors are present in the submucous and myenteric plexi and in enterocytes. Using pharmacological methods, our group has shown that intestinal inflammation enhances the antitransit and antisecretory effects of systemic opioids. The aim of the present study was to evaluate whether the enhanced antisecretory effects of delta and kappa-agonists were associated with an increased transcription and/or expression of these receptors at central (brain and spinal cord) and/or peripheral sites (gut); we also evaluated the expression of delta- and kappa-opioid receptors in dissected sections of the gut containing the myenteric (MP/LM) or submucous (SP/M) plexi. The mRNA and protein levels of both opioid receptors were determined using a reverse-transcriptase polymerase chain reaction and immunoprecipitation/Western blot, respectively. Intestinal inflammation significantly augmented the transcription of delta-opioid receptors in the spinal cord (34%) and in the whole gut (102%). Also increased mRNA and protein levels of delta-opioid receptors in the MP/LM and SP/M preparations. The kappa-opioid receptors gene transcription was not altered by inflammation, whereas kappa-opioid receptors protein levels were significantly enhanced in the SP/M preparation. No changes in gene transcription or protein levels for delta- and kappa-opioid receptors could be demonstrated in the brain. These results suggest that local transcriptional and post-transcriptional changes of the delta- and kappa-opioid receptors genes could be responsible for the enhanced antisecretory effects of delta- and kappa-opioid agonists during intestinal inflammation.

A role for the periaqueductal grey in opioidergic inhibition of maternal behaviour

Opiates are known to be involved in the regulation of various events surrounding parturition and lactation, such as maternal behaviour in rats. The onset of this behaviour has been closely linked to opiate action in the medial pre-optic area, where administration of morphine disrupts maternal behaviour during lactation. By combining the use of Fos protein immunohistochemical detection and pharmacological manipulations, in the present paper we show that the periaqueductal grey (PAG) is another region critically involved in the opioidergic blockade of maternal behaviour. According to our observations, a critical level of morphine-induced activation of the rostral lateral PAG appears to be required to inhibit maternal behaviour in lactating rats. This hypothesis was further confirmed in experiments showing that morphine's inhibitory effect on maternal responsiveness was blocked by unilateral naloxone injection into the rostral PAG, but not into nearby regions of the mesencephalic reticular nucleus. Therefore, only a partial inhibition of the opiate's effect on the rostral PAG was needed to block the inhibitory effect of morphine on maternal behaviour. Further studies are needed to ascertain whether the rostral lateral PAG plays a role in the natural onset of maternal behaviour, playing a complementary role to the medial pre-optic area, or merely inhibits maternal behaviour in response to this specific pharmacological challenge. Conversely, the present findings may well reflect a more general role of the PAG, seemingly providing an important piece of information for proposing a hitherto unexplored concept of the PAG as an important centre for the selection of adaptive behavioural responses.

Expression of opioid peptides and receptors in striatum and substantia nigra during rat brain development

We have used highly sensitive in situ hybridization to determine opioid receptor and peptide expression in embryonic and postnatal rat striatum, to follow the compartmentalization into patch and matrix structures, and have examined their developmental expression in the dopaminergic cell group of the substantia nigra (SN). Furthermore, opioid receptor binding sites were characterized in adjacent sections using highly selective ligands for the opioid receptor subtypes. The major findings of the study are: (1) striatal patches were first delineated by prodynorphin mRNA followed by mu opioid receptor mRNA expression at embryonic days 19 and 21, respectively; (2) in neonates, prodynorphin, mu and kappa opioid receptor mRNAs were transiently co-distributed within patches; (3) prodynorphin mRNA was co-expressed with mu but not kappa, receptor mRNA in neonatal patch neurons; (4) in the SN, kappa receptor and prodynorphin mRNAs were detected as early as embryonic days 15 and 19, respectively; (5) kappa receptor, but not prodynorphin, mRNA was expressed in dopaminergic neurons in the SN. The anatomical results are in agreement with the hypothesis that the endogenous opioid system has a trophic role during the development of striatal patch and matrix compartments and suggest the early regulation of dopamine release by kappa opioid receptors.

Involvement of endogenous beta-endorphin in antinociception in the arcuate nucleus of hypothalamus in rats with inflammation

Although exogenous administration of beta-endorphin to the arcuate nucleus of hypothalamus (ARC) had been shown to produce antinociception, the role of endogenous beta-endorphin of the ARC in nociceptive processing has not been studied directly. The aim of the present study was to investigate the effect of endogenous beta-endorphin in the ARC on nociception in rats with carrageenan-induced inflammation. The hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation was assessed by the hot-plate test and the Randall Selitto Test. Intra-ARC injection of naloxone had no significant influence on the HWL to thermal and mechanical stimulation in intact rats. The HWL decreased significantly after intra-ARC injection of 1 or 10 microg of naloxone in rats with inflammation, but not with 0.1 microg of naloxone. Furthermore, intra-ARC administration of the selective mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA) decreased the nociceptive response latencies to both stimulation in a dose-dependent manner in rats with inflammation, while intra-ARC administration of the selective delta-opioid receptor antagonist naltrindole or the selective kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI) showed no influences on the nociceptive response latency. The antiserum against beta-endorphin, administered to the ARC, also dose-dependently reduced the HWL in rats with inflammation. The results indicate that endogenous beta-endorphin in the ARC plays an important role in the endogenous antinociceptive system in rats with inflammation, and that its effect is predominantly mediated by the mu-opioid receptor.

Dynorphin and the hypothalamo-pituitary-adrenal axis during fetal development

Although dynorphin has long been considered an endogenous opioid peptide with high affinity for the kappa-opioid receptor, its biological function remains uncertain. The high concentration of dynorphin peptides and kappa-opioid receptors in the hypothalamus suggest a possible role for dynorphin in neuroendocrine regulation. This review will summarize evidence that support a role for dynorphin in regulation of the developing hypothalamo-pituitary-adrenal (HPA) axis. Dynorphin can exert dual actions on adrenocorticotropin (ACTH) release: (i) via activation of hypothalamic kappa-opioid receptors leading to release of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP), and (ii) via a non-opioid mechanism that involves N-methyl-D-aspartate (NMDA) receptors and prostaglandins, and which is not dependent on CRH or AVP. The primary site of action of dynorphin and NMDA appears to be the fetal hypothalamus or a supra-hypothalamic site. The non-opioid mechanism does not mature until a few days prior to parturition and is active for only the brief perinatal period. In contrast, the opioid mechanism behaves as a constitutive system with sustained activity from prenatal to postnatal life. It is likely that the two mechanisms may respond to different stress stimuli and play a different role during development.

Anti-inflammatory properties of the mu opioid receptor support its use in the treatment of colon inflammation

The physiologic role of the mu opioid receptor (MOR) in gut nociception, motility, and secretion is well established. To evaluate whether MOR may also be involved in controlling gut inflammation, we first showed that subcutaneous administration of selective peripheral MOR agonists, named DALDA and DAMGO, significantly reduces inflammation in two experimental models of colitis induced by administration of 2,4,6-trinitrobenzene sulfonic acid (TNBS) or peripheral expansion of CD4(+) T cells in mice. This therapeutic effect was almost completely abolished by concomitant administration of the opioid antagonist naloxone. Evidence of a genetic role for MOR in the control of gut inflammation was provided by showing that MOR-deficient mice were highly susceptible to colon inflammation, with a 50% mortality rate occurring 3 days after TNBS administration. The mechanistic basis of these observations suggests that the anti-inflammatory effects of MOR in the colon are mediated through the regulation of cytokine production and T cell proliferation, two important immunologic events required for the development of colon inflammation in mice and patients with inflammatory bowel disease (IBD). These data provide evidence that MOR plays a role in the control of gut inflammation and suggest that MOR agonists might be new therapeutic molecules in IBD.

Morphine withdrawal precipitated by specific mu, delta or kappa opioid receptor antagonists: a c-Fos protein study in the rat central nervous system

We have recently shown concurrent changes in behavioural responses and c-Fos protein expression in the central nervous system in both naive and morphine-dependent rats after systemic administration of the opioid antagonist naloxone. However, because naloxone acts on the three major types of opioid receptors, the present study aimed at determining, in the same animals, both changes in behaviour and c-Fos-like immunoreactivity after intravenous injection of selective opioid antagonists, such as mu (beta-funaltrexamine, 10 mg/kg), delta (naltrindole, 4 mg/kg) or kappa (nor-binaltorphimine, 5 mg/kg) opioid receptor antagonists, in naive or morphine-dependent rats. In a first experimental series, only beta-funaltrexamine increased c-Fos expression in the eight central nervous system structures examined, whereas no effect was seen after naltrindole or nor-binaltorphimine administration in naive rats. These results suggest a tonic activity in the endogenous opioid peptides acting on mu opioid receptors in normal rats. A second experimental series in morphine-dependent rats showed that beta-funaltrexamine had the highest potency in the induction of classical signs of morphine withdrawal syndrome, as well as the increase in c-Fos expression in the 22 central nervous system structures studied, suggesting a major role of mu opioid receptors in opioid dependence. However, our results also demonstrated that naltrindole and, to a lesser extent, nor-binaltorphimine were able to induce moderate signs of morphine withdrawal and relatively weak c-Fos protein expression in restricted central nervous system structures. Therefore, delta and kappa opioid receptors may also contribute slightly to opioid dependence.

Anti-inflammatory properties of the mu opioid receptor support its use in the treatment of colon inflammation

The physiologic role of the mu opioid receptor (MOR) in gut nociception, motility, and secretion is well established. To evaluate whether MOR may also be involved in controlling gut inflammation, we first showed that subcutaneous administration of selective peripheral MOR agonists, named DALDA and DAMGO, significantly reduces inflammation in two experimental models of colitis induced by administration of 2,4,6-trinitrobenzene sulfonic acid (TNBS) or peripheral expansion of CD4(+) T cells in mice. This therapeutic effect was almost completely abolished by concomitant administration of the opioid antagonist naloxone. Evidence of a genetic role for MOR in the control of gut inflammation was provided by showing that MOR-deficient mice were highly susceptible to colon inflammation, with a 50% mortality rate occurring 3 days after TNBS administration. The mechanistic basis of these observations suggests that the anti-inflammatory effects of MOR in the colon are mediated through the regulation of cytokine production and T cell proliferation, two important immunologic events required for the development of colon inflammation in mice and patients with inflammatory bowel disease (IBD). These data provide evidence that MOR plays a role in the control of gut inflammation and suggest that MOR agonists might be new therapeutic molecules in IBD.

Distribution of mu-opioid receptors in rat visceral premotor neurons

Agonists of the mu-opioid receptor (MOR) can modulate the activity of visceral premotor neurons, including cardiac premotor neurons. Neurons in brainstem regions containing these premotor neurons also contain dense concentrations of the MOR1. This study examined the distribution of MOR1 within two populations of visceral premotor neurons: one located in the dorsal motor nucleus of the vagus and the other in the nucleus ambiguus. Visceral premotor neurons contained the retrograde tracer Fluoro-Gold following injections of the tracer into the pericardiac region of the thoracic cavity. MOR1 was localized using immunogold detection of an anti-peptide antibody. Visceral premotor neurons in both regions contained MOR1 at somatic and dendritic sites, although smaller dendrites were less likely to contain the receptor than larger dendrites, suggesting there may be selective trafficking of MOR1 within these neurons. MOR1 labeling in nucleus ambiguus neurons was more likely to be localized to plasma membrane sites, suggesting that ambiguus neurons may be more responsive to opioid ligands than neurons in the dorsal motor nucleus of the vagus. In addition, many of the dendrites of visceral premotor neurons were in direct apposition to other dendrites. MOR1 was often detected at these dendro-dendritic appositions that may be gap junctions. Together these findings indicate that the activity of individual visceral premotor neurons, as well as the coupling between neurons, may be regulated by ligands of the MOR.

Differential involvement of the mu and kappa opioid receptors in spatial learning

In order to test the role of mu and kappa opioid receptors (Mu opioid receptor (MOR) and Kappa opioid receptor (KOR)) in hippocampal-dependent spatial learning, we analyzed genetically engineered null mutant mice missing the functional MOR or KOR gene. Compared to wild-type mice, the homozygous MOR null mutants exhibited an impairment in the ultimate level of spatial learning as shown in two distinct tasks, the 8-arm radial-maze and the Morris water-maze. Control behaviors were normal. The learning impairment could be associated with the impairment we found in the maintenance of long-term potentiation in mossy fibers in CA3. In comparison, there was no impairment in spatial learning in our KOR mutants or in mossy fibers (mf) in CA3 region long-term potentiation (LTP). Our work suggests that the MOR may play a positive role in learning and memory by increasing LTP in CA3 neurons.

The effect of endomorphins on the release of 3H-norepinephrine from rat nucleus tractus solitarii slices

We used two, 3-min field stimulation cycles 30 min apart (S1, S2) in 3H-norepinephrine-loaded, superfused rat nucleus tractus solitarii-dorsal motor vagal nucleus (NTS-DVN) slices. The stimulation-induced release was expressed as the area above the baseline. Drugs were introduced 12 min before S2 and drug actions were characterized in terms of alterations of S2/S1 ratios. The S2/S1 ratio was 1.047 (0.946-1.159, n = 4, geometric mean and 95% confidence interval) in controls and 0.336 (0.230-0.490, n = 3), 0.726 (0.590-0.892, n = 4), 0.613 (0.594-0.683, n = 4) and 0.665 (0.500-0.886, n = 4) in the presence of 10(-6) M clonidine, D-Ala(2),MePhe(4),Gly(5)-ol-enkephalin (DAMGO), endomorphin-1 (Tyr-Pro-Trp-Phe-NH(2), EM-1) and -2 (Tyr-Pro-Phe-Phe-NH(2), EM-2) [the latter two in the presence of 10(-4) M diprotin A, an inhibitor of dipeptidyl-aminopeptidase IV (DAP-IV) enzyme]. The effect of DAMGO at 10(-5) M was significantly higher than at 10(-6) M, whereas the effect of endomorphins did not differ at the two concentration levels. Diprotin A potentiated only very modestly the action of endomorphins. These data (a) confirm the presence of functional mu-opioid receptors in the vagal complex, (b) render it likely that the enzymic degradation of endomorphins is not a highly effective process in brain slices and (c) may suggest that the apparent ceiling in the effect of endomorphins might be related to their partial agonist property.

Opioid receptor subtypes involved in the regulation of prolactin secretion during pregnancy and lactation

Afferent endogenous opioid neuronal systems facilitate prolactin secretion in a number of physiological conditions including pregnancy and lactation, by decreasing tuberoinfundibular dopamine (TIDA) inhibitory tone. The aim of this study was to investigate the opioid receptor subtypes involved in regulating TIDA neuronal activity and therefore facilitating prolactin secretion during early pregnancy, late pregnancy and lactation in rats. Selective opioid receptor antagonists nor-binaltorphimine (kappa-receptor antagonist, 15 micro g/5 micro l), beta funaltrexamine (mu-receptor antagonist, 5 microg/5 microl) and naltrindole (delta-receptor antagonist, 5 microg/5 microl) or saline were administered intracerebroventricularly (i.c.v.) on day 8 of pregnancy during a nocturnal prolactin surge, on day 21 of pregnancy during the ante partum prolactin surge or on day 7 of lactation before the onset of a suckling stimulus. Serial blood samples were collected at regular time intervals, via chronic indwelling jugular cannulae, before and after drug administration and plasma prolactin was determined by radioimmunoassay. TIDA neuronal activity was measured using the 3,4-dihydroxyphenylacetic acid (DOPAC) : dopamine ratio in the median eminence 2 h 30 min after i.c.v. drug injection. In each experimental condition, plasma prolactin was significantly inhibited by both kappa- and mu-receptor antagonists, whereas the delta-receptor antagonist had no effect compared to saline-injected controls. Similarly, nor-binaltorphimine and beta funaltrexamine significantly increased the median eminence DOPAC : dopamine ratio during early and late pregnancy, and lactation whereas naltrindole had no effect compared to saline-injected controls. These data suggest that TIDA neuronal activity, and subsequent prolactin secretion, is regulated by endogenous opioid peptides acting at both kappa- and mu-opioid receptors during prolactin surges of early pregnancy, late pregnancy and lactation.

Up-regulation and trafficking of delta opioid receptor in a model of chronic inflammation: implications for pain control

Pharmacological and physiological evidence supports a role for delta (delta) opioid receptors in the nociceptive mechanisms of inflammation. However, few data exist regarding delta opioid receptor expression and localization in such conditions. In this study, we have assessed the distribution and function of delta opioid receptors in the rat spinal cord following induction of chronic inflammation by intraplantar injection of complete Freund's adjuvant (CFA). Intrathecal administration of the selective delta opioid receptor agonist, D-[Ala(2), Glu(4)] deltorphin, dose-dependently reversed thermal hyperalgesia induced by CFA. In situ hybridization and Western blotting experiments revealed an increase in delta opioid receptor mRNA and protein levels, respectively, in the dorsal lumbar spinal cord ipsilateral to the CFA injection site compared to the contralateral side and sham-injected controls. By electron microscopy, immunopositive delta opioid receptors were evident in neuronal perikarya, dendrites, unmyelinated axons and axon terminals. Quantification of immunopositive signal in dendrites revealed a twofold increase in the number of immunogold particles in the ipsilateral dorsal spinal cord of CFA-injected rats compared to the contralateral side and to sham-injected rats. Moreover, the relative frequency of immunogold particles associated with or in close proximity to the plasma membrane was increased in the ipsilateral dorsal spinal cord, indicating a more efficient targeting of delta opioid receptors to neuronal plasma membranes. These data demonstrate that CFA induces an up-regulation and increased membrane targeting of delta opioid receptors in the dorsal spinal cord which may account for the enhanced antinociceptive effects of delta opioid receptor agonists in chronic inflammatory pain models.

Lack of effect of kappa-opioid receptor agonism on long-term methamphetamine-induced neurotoxicity in rats

High-dose methamphetamine treatment induces long-term deficits in central monoamine systems. However, the mechanisms underlying these effects are unknown. Previous work has shown that the Kappa-opioid receptor agonist U-69593 [(+)-(5alpha,7alpha,8b)-(+)-N-methyl-N[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl] benzeneacetamide] attenuates the neurotoxic effects of methamphetamine on extracellular dopamine levels in mice, suggesting that endogenous Kappa-opioid receptor ligands, such as dynorphin, may protect against methamphetamine-induced toxicity and play a role in mediating the long-term consequences of methamphetamine. To further examine the role that dynorphin systems play in methamphetamine-induced neurotoxicity, we administered to male rats a total of four injections of methamphetamine (7.5 mg/kg, s.c.), with a 2-h interval between each dose. Rats were pretreated with either the Kappa-agonist U-69593 (0.32 mg/kg, s.c.) or vehicle, 15 min prior to the first and third methamphetamine injection. Furthermore, cages containing the U-69593 + methamphetamine-treated rats were placed on heating pads for 30 min after the first U-69593 injection to prevent the drug from blocking methamphetamine-induced hyperthermia. Rats were sacrificed 7 days after treatment. Striatal dopamine and serotonin contents were decreased approximately 75% and 55%, respectively, in the methamphetamine-treated rats and approximately 88% and 65%, respectively, in rats receiving the U-69593 + methamphetamine combination. There was a approximately 20% mortality rate in the rats treated with methamphetamine compared to approximately 75% mortality rate in rats treated with both U-69593 and methamphetamine. A similar rate of mortality was observed when combining a different Kappa-agonist, U-50488 [trans-(-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamine], with methamphetamine. These data suggest that Kappa-agonists do not protect against methamphetamine-induced toxicity to monoamines in rats, and may potentiate mortality when co-administered with methamphetamine.

Opioid receptor-induced GTPgamma35S binding during mouse development

Although a large superfamily of G-protein-coupled receptors serves multiple functions, little is known about their functional activation during ontogeny. To examine the functional activation of the mu-opioid receptor (MOR) and the delta-opioid receptor (DOR) during development, sections of mouse embryos and fetuses from e11.5 until birth were treated with DAMGO and DPDPE, respectively, and the ability of these drugs to induce G-protein coupling was assessed by using GTPgamma(35)S binding autoradiography. MOR activation was first detected in the caudate-putamen (CPU) at e12.5, and by e15.5, activity had not only increased in this region but also expanded to include the midbrain, medial habenula, hypothalamus, pons, and medulla. DOR activity first appeared at e17.5 in the hypothalamus, pons, medial habenula, and medulla and at p1 in the CPU at levels noticeably less than those of the MOR. In general, MOR and DOR activation lagged only slightly behind the appearance of MOR-1 and DOR-1 mRNA but delayed activation was particularly pronounced in the trigeminal ganglia, where MOR-1 gene expression was first detected at e13.5, but MOR activity was not observed even at birth. Thus, the data demonstrate temporal and often region-specific differences in the appearance and magnitude of functional activity in cell groups expressing either the MOR-1 or DOR-1 genes, suggesting that interaction between the opioid receptors, G-proteins, and other signaling cofactors is developmentally regulated.

Cocaine alters mu but not delta or kappa opioid receptor-stimulated in situ [35S]GTPgammaS binding in rat brain

Chronic cocaine administration produces alterations in mu and kappa opioid receptor density as well as striatal and accumbens opioid-regulated adenylyl cyclase activity, suggesting a psychostimulant responsive interaction between opioidergic and dopaminergic systems. Stimulation of G-protein-coupled opioid receptors inhibits adenylyl cyclase production of cyclic AMP. The present study employed in situ [(35)S]GTPgammaS binding to measure opioid receptor-stimulated activation of G-proteins in response to acute and chronic cocaine exposure. Male Fischer rats received acute (1 or 3 days) or chronic (14 days) binge pattern cocaine administration. Three and 14 days of cocaine injections resulted in greater increases in the ability of the mu receptor agonist DAMGO to stimulate [(35)S]GTPgammaS binding in both the core and the shell of the nucleus accumbens, all regions of the caudate putamen and the cingulate cortex compared with saline-matched controls. The greatest increases in DAMGO-stimulated [(35)S]GTPgammaS binding were observed in the dorsal areas of the caudate putamen in animals that received 14 days of cocaine. No significant changes in delta (DPDPE), or kappa (dynorphin A(1-17)) receptor-stimulated [(35)S]GTPgammaS binding were found in any brain region in response to cocaine administration. These results demonstrate that binge pattern cocaine administration induce changes in mu but not delta or kappa opioid receptor-mediated G-protein activity. This study provides support for the hypothesis that the addictive properties of both psychostimulants and opiates may share common neurochemical signaling substrates.

Localization of mu-opioid receptor 1A on sensory nerve fibers in human skin

Opioid peptides are endogenous neuromodulators that play a major role in the nociceptive pathway by interacting with opioid receptors. So far, four opioid receptors (micro-, delta-, kappa-, orphan-receptor) have been cloned with a wide distribution in the central and peripheral nervous system. In the present study, we give first evidence for the presence of the micro-opioid receptor (MOR) isoform 1A in nerve fibers of human skin. Immunohistochemical analysis revealed MOR immunoreactivity to be present in dermal and epidermal nerve fibers. Double-immunofluorescence staining revealed that MOR is present on calcitonin gene-related protein (CGRP)-positive sensory nerve fibers, while autonomic nerves of blood vessels, hair follicles, or skin glands were negative. In diseased skin such as psoriasis vulgaris, atopic dermatitis, and prurigo nodularis, distribution of MOR 1A immunoreactivity was similar to that of normal skin. These findings expand our knowledge about a direct regulatory role of cutaneous opioid receptors in the skin. Thus, peripheral cutaneous opioid receptors may be involved in the transmission of pain and pruritus, respectively. This is supported by previous observation that opioid receptor antagonists may significantly diminish experimentally evoked histamine-induced itch of the skin. Together, our findings contribute to the understanding of the high antipruritic potency of opioid receptor antagonists in various skin and systemic diseases.

Basic transcription element binding protein is a thyroid hormone-regulated transcription factor expressed during metamorphosis in Xenopus laevis

Basic transcription element binding protein (BTEB) is a member of the Krüppel family of zinc finger transcription factors. It has been shown that BTEB plays a role in promoting neuronal process formation during postembryonic development. In the present study, the biochemical properties, transactivation function, and the developmental and hormone-regulated expression of BTEB in Xenopus laevis (xBTEB) are described. xBTEB binds the GC-rich basic transcription element (BTE) with high affinity and functions as a transcriptional activator on promoters containing multiple or single GC boxes. xBTEB mRNA levels increase in the tadpole brain, intestine and tail during metamorphosis, and are correlated with tissue-specific morphological and biochemical transformations. xBTEB mRNA expression can be induced precociously in premetamorphic tadpole tissues by treatment with thyroid hormone. In situ hybridization histochemistry showed that thyroid hormone upregulates xBTEB mRNA throughout the brain of premetamorphic tadpoles, with the highest expression found in the subventricular zones of the telencephalon, diencephalon, optic tectum, cerebellum and spinal cord. xBTEB protein parallels changes in its mRNA, and it was found that xBTEB is not expressed in mitotic cells in the developing brain, but is expressed just distal to the proliferative zone, supporting the hypothesis that this protein plays a role in neural cell differentiation.

Opioid receptor gene expression in dopamine transporter knock-out mice in adult and during development

Dopamine transporter knock-out mice display locomotor hyperactivity due to increased extracellular striatal levels of dopamine. Hyperdopaminergic activity within this mesolimbic pathway is involved in the rewarding properties of morphine which are also increased in these mice. Due to the hyperdopaminergia, profound alterations in gene expression for dopamine receptors and neuropeptides are observed in the caudate putamen and nucleus accumbens. Here we investigated (1) the levels of mu-, delta- and kappa-opioid receptors mRNAs in normal mice from gestational day 13 (G13) to adult, and (2) the adaptive changes in the expression of these receptors in mice lacking the dopamine transporter. Our results show that, in wild-type mice, mu-opioid receptor mRNA expression appears early during development (G13) with a homogeneous distribution that evolves towards a patchy distribution in adult. Delta-opioid receptor mRNA appears only at G17 and kappa-opioid receptor mRNA is not observed before adulthood. The levels of delta-opioid receptor mRNA are not modified during development in knock-out mice compared to wild-type, but are increased in adult caudate putamen (+39%, P<0.05) and nucleus accumbens (+66%, P<0.05) at a time when these receptors are believed to be functional. The mu- and kappa-opioid receptors mRNA levels are not modified in the adult knock-out mice. In addition, we observed no differences in any opioid receptor mRNA level in dopamine transporter knock-out mice during prenatal ontogeny compared to wild-type. Our results constitute a detailed neuroanatomical description of opioid receptor mRNA expression from the time of their appearance during prenatal development until adulthood. Furthermore, we show here that chronic constitutive hyperdopaminergia only affects delta-opioid receptor mRNA levels in adult. Even if the propensity of knock-out mice to show increased rewarding properties to morphine seems to be mainly due to the substantial and further increase in hyperdopaminergic activity following drug treatment, the involvement of increased delta-opioid receptor mRNA levels in this behavior remains to be elucidated.

Differential promoter usage of mouse mu-opioid receptor gene during development

Previously, we demonstrated that mouse mu-opioid receptor (MOR) gene expression is regulated by both distal and proximal promoters, with the latter playing a major role in controlling MOR transcription in the adult mouse brain. Here, we report studies of the relative usages of the mouse MOR dual promoters during murine development. We used the reverse transcription-polymerase chain reaction (RT-PCR) method, which gave results similar to those using binding assays or in situ hybridization. However, due to the greater sensitivity of RT-PCR method, we were able to detect the emergence of MOR as early as at embryonic day 8.5 (E8.5). We found that both proximal and distal promoters were active at E8.5. The proximal promoter initiated approximately two-thirds of total MOR transcripts at E8.5, with the distal promoter directing transcription of the remaining one-third. This is the greatest relative contribution of the distal promoter to MOR transcription we have observed during any time in development. Thereafter, the percentage of transcripts directed by the distal promoter gradually declined, and remained at a low but detectable level (approximately 5% of total MOR transcripts) throughout development and adulthood. Conversely, a progressive increase of the contribution of the proximal promoter to MOR transcription was observed during development, reaching its maximum in the adult. In summary, our results demonstrated the pivotal role of the proximal promoter in directing MOR transcription during murine development.

Further characterization of preproenkephalin mRNA-containing cells in the rodent globus pallidus

The globus pallidus (external pallidum of primates) is an essential nucleus within basal ganglia circuitry, in part because it receives at least one-half of striatal efferent projections. Neurons of the globus pallidus can be divided into subpopulations based on anatomical, physiological, and chemical features. Globus pallidus neurons project to several structures (the striatum, subthalamic nucleus, entopeduncular nucleus, and substantia nigra pars reticulata), have one of two alternative waveforms (positive/negative versus negative/positive), contain either the calcium binding protein parvalbumin or the neuropeptide precursor preproenkephalin mRNA and show differential immediate early gene responses to dopamine receptor agonists and antagonists. The objective of the present study was to characterize in greater detail the preproenkephalin mRNA-containing pallidal neurons using Sprague-Dawley rats. In situ hybridization for preproenkephalin mRNA was combined with immunocytochemical detection of: (i) the neuron-specific nuclear protein, NeuN, (ii) FluoroGold-labeled pallidostriatal and pallidosubthalamic cells, or (iii) Fos induced by either systemic combined D1-class/D2-class dopamine receptor agonists or a D2-class receptor antagonist. These experiments demonstrated that a substantial population (42%) of globus pallidus neurons contains preproenkephalin mRNA, and that globus pallidus neurons retrogradely labeled after FluoroGold injections into the striatum are more frequently preproenkephalinergic, compared to the population of pallidosubthalamic neurons. Furthermore, systemic administration of a D2 receptor antagonist, eticlopride, induced Fos immunoreactivity predominantly in globus pallidus neurons expressing preproenkephalin mRNA, while combined administration of D1 and D2 receptor agonists induced Fos predominantly in pallidal neurons lacking preproenkephalin mRNA.These results support the conclusion that preproenkephalin mRNA identifies one of the two major subpopulations of pallidal neurons. This preproenkephalin mRNA-expressing pallidal subpopulation preferentially targets the striatum and is more readily activated in its immediate early gene expression by D2 receptor antagonists than by dopamine receptor agonists. This projection provides a pallidal substrate for the dopaminergic regulation of striatal information processing.

8-Carboxamidocyclazocine: a long-acting, novel benzomorphan

To obtain benzomorphans with a longer duration of action that may be potential therapeutics for treating cocaine abuse, 8-carboxamidocyclazocine was synthesized. The pharmacological properties of 8-carboxamidocyclazocine were compared with the parent compound cyclazocine. Changing the 8-hydroxyl group on cyclazocine to an 8-carboxamido group resulted in only a 2-fold decrease in the affinity of the compound for the kappa-receptor, and no change in the affinity for the mu-opioid receptor, with both compounds having K(i) values of less than 1 nM, based on radioligand binding assays. In the guanosine 5'-O -(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding assay, the two compounds produced moderate stimulation of GTP binding to the human kappa- and mu-receptors. When given by i.c.v. injection, the compounds produced less than 60% antinociception in the mouse 55 degrees C warm-water tail-flick test. However, in the mouse writhing test, the compounds had high potency in producing antinociception. Antinociception induced by either 8-carboxamidocyclazocine or cyclazocine was mediated by both kappa- and mu-opioid receptors. Cyclazocine acted as a mu-antagonist in addition to its agonist properties at the mu-receptor, as measured by the inhibition of morphine-induced antinociception. In contrast, 8-carboxamidocyclazocine did not inhibit morphine-induced antinociception, demonstrating that it was not a mu-opioid receptor antagonist in this assay. An i.p. injection of an ED(70) dose of 8-carboxamidocyclazocine produced antinociception that lasted for 15 h in contrast to cyclazocine, which produced antinociception, lasting 2 h. 8-Carboxamidocyclazocine is a novel, long-acting benzomorphan, which possesses pharmacological properties that are distinct from the properties of cyclazocine.

Central sensitization of nociceptive neurons in trigeminal subnucleus oralis depends on integrity of subnucleus caudalis

Our recent studies have shown that application to the tooth pulp of the inflammatory irritant mustard oil (MO) produces a prolonged (>40 min) "central sensitization" reflected in neuroplastic changes in the mechanoreceptive field (RF) and response properties of nociceptive brain stem neurons in subnuclei oralis (Vo) and caudalis (Vc) of the trigeminal spinal tract nucleus. In view of the previously demonstrated ascending modulatory influence of Vc on Vo, our aim was to determine whether the Vo neuroplastic changes induced by MO application to the tooth pulp depend on an ascending influence from Vc. In chloralose/urethan-anesthetized rats, MO application to the pulp produced significant increases in Vo nociceptive neuronal orofacial RF size and responses to mechanical noxious stimuli that lasted as long as 40-60 min. These changes were not affected by vehicle (saline) microinjected into Vc at 20 min after MO application, but 0.3 microl of a 5 mM CoCl(2) solution microinjected into the ipsilateral Vc produced a reversible blockade of the MO-induced Vo neuroplastic changes. A similar volume and concentration of CoCl(2) solution injected into subnucleus interpolaris of the trigeminal spinal tract nucleus did not affect the MO-induced neuroplastic changes in Vo. These findings indicate that inflammatory pulp-induced central sensitization in Vo is dependent on the functional integrity of Vc.

Anti-nociceptive effect of neuropeptide Y in the nucleus accumbens of rats: an involvement of opioid receptors in the effect

The present study investigated the effect of neuropeptide Y on nociception in the nucleus accumbens of rats. Intra-nucleus accumbens administration of neuropeptide Y induced dose-dependent increases in the hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation in rats. There were no significant changes in the HWL to both stimulation during 60 min after the administration of NPY to outside of the nucleus accumbens. The anti-nociceptive effect of NPY was blocked by subsequent intra-nucleus accumbens injection of the Y1 receptor antagonist neuropeptide Y 28-36, indicating that Y1 receptor is involved in the neuropeptide Y-induced anti-nociception in the nucleus accumbens. Furthermore, the anti-nociceptive effect of neuropeptide Y was attenuated by intra-nucleus accumbens administration of the opioid antagonist naloxone, suggesting an involvement of the endogenous opioid system in the neuropeptide Y-induced anti-nociception in the nucleus accumbens of rats. Moreover, the neuropeptide Y-induced anti-nociception was attenuated by following intra-nucleus accumbens injection of the selective opioid antagonists nor-binaltorphimine and beta-funaltrexamine, but not by naltrindole, illustrating that mu- and kappa-opioid receptors, not the delta-opioid receptor, were involved in the neuropeptide Y-induced anti-nociception in the nucleus accumbens of rats.

C1 adrenergic neurons are contacted by presynaptic profiles containing DELTA-opioid receptor immunoreactivity

Ligands of the delta-opioid receptor tonically influence sympathetic outflow. Some of the actions of delta-opioid receptor agonists may be mediated through C1 adrenergic neurons in the rostral ventrolateral medulla. The goal of this study was to determine whether C1 adrenergic neurons or their afferents contain delta-opioid receptors. Single sections through the rostral ventrolateral medulla were labeled for delta-opioid receptor using the immunoperoxidase method and the epinephrine synthesizing enzyme phenylethanolamine N-methyltransferase (PNMT) using the immunogold method, and examined at the light and electron microscopic level. Few ( approximately 5% of 903) profiles dually labeled for PNMT and delta-opioid receptor were detected; most of these were dendrites with diameters < 1.5 microm. delta-Opioid receptor immunoreactivity was affiliated with multivesicular bodies in dually labeled perikarya, whereas delta-opioid receptor immunoperoxidase labeling appeared as isolated clusters within both singly and dually labeled dendrites. The majority ( approximately 83% of 338) of delta-opioid receptor-immunoreactive profiles were axons and axon terminals. delta-Opioid receptor-immunoreactive terminals averaged 0.75 microm in diameter, contained numerous large dense-core vesicles and usually formed appositions or asymmetric (excitatory-type) synapses with their targets. The majority (>50% of 250) of delta-opioid receptor-immunoreactive axons and axon terminals contacted PNMT-immunoreactive profiles. Most of the contacts formed by delta-opioid receptor-immunoreactive profiles ( approximately 75% of 132) were on single-labeled PNMT-immunoreactive dendrites with diameters <1.5 microm. The prominent localization of delta-opioid receptors to dense-core vesicle-rich presynaptic profiles suggests that delta-opioid receptor activation by endogenous or exogenous agonists may modulate neuropeptide release. Furthermore, the presence of delta-opioid receptors on axon terminals that form excitatory-type synapses with PNMT-immunoreactive dendrites suggests that delta-opioid receptor ligands may modulate afferent activity to C1 adrenergic neurons. The observation that some PNMT-immunoreactive neurons contain delta-opioid receptor immunoreactivity associated with multivesicular bodies and other intracellular organelles suggests that some C1 adrenergic neurons may present, endocytose and/or recycle delta-opioid receptors.

mu-1 opioid receptor stimulation decreases body temperature in conscious, unrestrained neonatal rats

The influence of mu-selective opioid agonists on neonatal thermoregulatory mechanisms has received little attention. Opioid treatment in adult subjects can cause either hyper- or hypothermia, depending on the experimental conditions, the strain of rat used, and the dose and route of administration of the drug. The present study assessed the effect of two mu opioid agonists on body temperature in neonatal Wistar rats aged 2 to 13 days. Rat pups were administered either saline or one of the two mu-selective opioid agonists, dermorphin (0.4 mg/kg) or fentanyl (0.06 mg/kg), by subcutaneous injection. Continuous rectal temperatures were measured both prior to and following drug or saline injection in freely moving, conscious animals. Ambient temperature in a plethysmograph chamber was maintained within or close to the thermoneutral zone for pups (32 degrees C). To distinguish between mu-1 and mu-2 effects, all animals received either saline or 10 mg/kg of the irreversible mu-1 antagonist naloxonazine (NALZ) 1 day prior to agonist administration. NALZ on its own had no effect on body temperature. Dermorphin and fentanyl both caused a fall in body temperature in pups of all age groups. The temperature decreases ranged from 0.8 degrees -2.2 degrees C. These opioid-induced changes were inhibited by NALZ pretreatment. Although there was no evidence for endogenous mu-1 opioid activity, this study indicated that stimulation of mu-1 opioid receptors causes a decrease in body temperature in conscious, unrestrained neonatal rats under or close to thermoneutral conditions.

Delta opioid inhibition of light-induced phase advances in hamster circadian activity rhythms

A master neuronal pacemaker located within the suprachiasmatic nucleus in the ventral hypothalamus generates circadian activity rhythms in hamsters. The circadian pacemaker receives afferent input from many brain regions, one of which is the intergeniculate leaflet of the thalamus. This thalamic input to the suprachiasmatic nucleus in hamsters contains enkephalins, neuropeptide Y, neurotensin, and GABA. The role of enkephalins in modulating light-induced phase shifts of hamster activity rhythms has not been reported. Therefore, in this study, we examined the ability of enkephalin-mimetic and other opioid compounds to modulate light-induced phase advances in hamster circadian activity rhythms. The delta opioid agonists SNC 80 and BW373U86 both inhibited light-induced phase advances of hamster circadian activity rhythms. Neither the mu opioid agonist morphine, nor the kappa opioid agonist U50488H had any effect on light-induced phase shifts. The antagonists naltrindole, naltrexone, and nor-binaltorphimine, selective for delta, mu, and kappa opioids respectively, were also without effect on light-induced phase advances. Therefore, we found that only delta opioid agonists modulate light-induced phase advances in hamster circadian activity rhythms. These results imply that enkephalins released from the intergeniculate leaflet onto components of the suprachiasmatic pacemaker may be capable of inhibiting the responsiveness of the pacemaker to photic input arriving from the retina. The inability of antagonists to modulate light-induced phase advances suggests that endogenous opiate systems are not tonically active in generating circadian activity rhythms, but rather that enkephalins are probably used by the circadian system to modulate responses only under certain conditions or time of day.

Systemic morphine selectively depresses a thalamic link of widespread nociceptive inputs in the rat

The lateral part of the ventromedial thalamus (VM l) relays nociceptive inputs from the whole body surface to the dorsolateral frontal cortex. The aim of the present study was to investigate the effects of systemic morphine on nociceptive activity evoked in VM l neurones either by thermal (48 degrees C) or by supramaximal percutaneous electrical stimuli. The noxious thermal evoked responses were depressed by 10.8 +/- 10.1%, 48.3 +/- 23.0% and 67.3 +/- 10.1%, 5 min after i.v. injections of 1.0, 1.73 and 3.0 mg/kg of morphine, respectively. Moreover, strong depressive effects on the Adelta- and C-fibre responses were already present 5 min after the injection. The responses were significantly reduced by 7.2 +/- 5.9%, 32.5 +/ 11.1% and 37.2 +/- 11.8% for Adelta fibres after i.v. injections of 1.0, 1.73 and 3.0 mg/kg of morphine, respectively. The corresponding values for C-fibre evoked responses were 16.3 +/- 16.2%, 57.0 +/- 12.0% and 69.0 +/- 8.2%. The dose of morphine that reduced VM l neuronal nociceptive responses by 50% (1.73 mg/kg) was around 3.5 times lower than that necessary to inhibit the responses of its spinal or medullary relays under similar experimental conditions. These results, added to the data of the literature, suggest that supraspinal effects of morphine are primarily mediated at the thalamic level. It is tempting to speculate that morphine-induced reductions of attentional or psychomotor responses related to pain may be mediated by its action on VM l.

Mu- and delta-opioid receptor mRNAs are expressed in periaqueductal gray neurons projecting to the rostral ventromedial medulla

Opioid antinociception appears to be mediated at least in part by a pathway that projects from the periaqueductal gray (PAG) to the rostral ventromedial medulla (RVM), but the relationship between opioid receptors and PAG-RVM projection neurons is unclear. Previous electrophysiological studies have suggested that opioids act directly on some PAG neurons projecting to the RVM. However, immunoreactivity for neither the cloned mu-opioid receptor (MOR1) nor the cloned delta-opioid receptor (DOR1) has been observed in PAG cells retrogradely labeled from the RVM. In the present study, we examined the expression of DOR1 and MOR1 mRNAs in PAG neurons projecting to RVM using quantitative in situ hybridization and retrograde tract-tracing. Mesencephalic neurons were labeled in three male Sprague-Dawley rats by microinjection of Fluoro-Gold into the RVM. Five micrometer cryostat sections were cut and in situ hybridization was performed using full-length cRNA probes labeled with 35S-UTP. Retrogradely labeled neurons that were also labeled for MOR1 or DOR1 mRNA were observed in the dorsomedial, lateral, and ventrolateral portions of the PAG. Quantification was performed in the dorsomedial and ventrolateral PAG using the physical disector. We found that of 219 retrogradely labeled neurons, 50 +/- 14% expressed DOR1 mRNA. In a second set of 120 Fluoro-Gold-labeled neurons, 27 +/- 8% expressed MOR1 mRNA. Significantly more PAG-RVM projection neurons were labeled for MOR1 mRNA in the ventrolateral subregion of the PAG than in the dorsomedial subregion. However, no significant difference was observed in the proportions of retrogradely labeled neurons labeled for DOR1 mRNA in the ventrolateral subregion compared to the dorsomedial subregion. We conclude that opioids are likely to exert direct effects on PAG-RVM projection neurons through both delta- and mu-opioid receptors. In addition, direct effects on PAG-RVM projection neurons from activation of MOR1 appear more likely to be exerted in the ventrolateral PAG than in the dorsomedial PAG.

Delta- and kappa-opioid receptors in the caudal midline medulla mediate haemorrhage-evoked hypotension

In mammals blood loss can trigger, shock, an abrupt, life-threatening hypotension and bradycardia. In the halothane-anaesthetised rat this response is blocked by inactivation of a discrete, vasodepressor area in the caudal midline medulla (CMM). Haemorrhagic shock is blocked also by systemic or ventricular injections of the opioid antagonist, naloxone. This study investigated, in the halothane anaesthetised rat, the contribution of delta-, kappa- and mu-opioid receptors in the CMM vasodepressor region to haemorrhage-evoked shock (i.e. hypotension and bradycardia) and its recovery. It was found that microinjections into the CMM of the delta-opioid receptor antagonist, naltrindole delayed and attenuated the hypotension and bradycardia evoked by haemorrhage, but did not promote recompensation. In contrast, CMM microinjections of the kappa-opioid receptor antagonist, nor-binaltorphamine, although it did not alter haemorrhage-evoked hypotension and bradycardia, did lead to a rapid restoration of AP, but not HR. CMM microinjections of the mu-opioid receptor antagonist, CTAP had no effect on haemorrhage-evoked shock or recompensation. These data indicate that delta- and kappa- (but not mu-) opioid receptor-mediated events within the CMM contribute to the hypotension and bradycardia evoked by haemorrhage and the effectiveness of naloxone in reversing shock.

Microinjection of morphine into various amygdaloid nuclei differentially affects nociceptive responsiveness and RVM neuronal activity

The goal of the present study was to identify nuclei of the amygdala in which opioid-sensitive systems can act to recruit nociceptive modulatory circuitry in the rostral ventromedial medulla (RVM) and affect nociceptive responsiveness. In lightly anesthetized rats, 10 microg of morphine was bilaterally microinjected into basolateral, cortical, medial, central, and lateral nuclei of the amygdala to determine the relative influence on the activity of identified ON, OFF and NEUTRAL cells in the RVM and on the latency of the tail flick reflex evoked by noxious radiant heat. Infusions of morphine into the basolateral nuclei resulted in a substantial, naloxone-reversible increase in tail flick latency, and significantly increased ongoing firing of OFF cells and depressed that of ON cells. The reflex-related changes in cell firing were also attenuated. Morphine infusions into the cortical nuclei resulted in a small (approximately 1 s) but significant increase in tail flick latency. As with basolateral microinjections, ongoing activity of the OFF cells was increased, and although the ongoing firing of ON cells was not significantly changed, the reflex-related burst that characterizes these neurons was reduced. Microinjections in the medial nuclei again altered ongoing activity of both ON cells and OFF cells. However, the duration of the OFF cell pause and tail flick latency were unchanged. NEUTRAL cells were not affected by morphine at any site. Morphine applied within the central, medial lateral and dorsal lateral nuclei had no effect on RVM neurons or on the tail flick. Thus, focal application of morphine within the basolateral nucleus of the amygdala produced hypoalgesia and influenced RVM ON and OFF cells in a manner similar to that seen following systemic or RVM opioid administration. Opioid action within the medial and cortical nuclei also influenced RVM cell activity, but did not prevent the reflex-related OFF cell pause, and failed to alter the tail flick substantially. These observations, plus the lack of an opioid-activated influence from the central and lateral nuclei, demonstrate fundamental differences among systems linking the different amygdalar nuclei with the RVM. One way in which the modulatory circuitry of the RVM might be engaged physiologically in behaving animals is via opioid-mediated activation of the basolateral nucleus.

The role of Kv1.2-containing potassium channels in serotonin-induced glutamate release from thalamocortical terminals in rat frontal cortex

Serotonin 5-HT(2A) receptors have been implicated in psychiatric illness and the psychotomimetic effects of hallucinogens. In brain slices, focal stimulation of 5-HT(2A) receptors in rat prefrontal cortex results in dramatically increased glutamate release onto layer V pyramidal neurons, as measured by an increase in "spontaneous" (nonelectrically evoked) EPSCs. This glutamate release is blocked by tetrodotoxin (TTX) and is thought to involve local spiking in thalamocortical axon terminals; however, the detailed mechanism has remained unclear. Here, we investigate parallels in EPSCs induced by either serotonin or the potassium channel blockers 4-aminopyridine (4-AP) or alpha-dendrotoxin (DTX). DTX, a selective blocker of Kv1.1-, Kv1.2-, and Kv1.6-containing potassium channels, has been shown to release glutamate in cortical synaptosomes, presumably by inhibiting a subthreshold-activated, slowly inactivating potassium conductance. By comparing DTX with other potassium channel blockers, we found that the ability to induce EPSCs in cortical pyramidal neurons depends on affinity for Kv1.2 subunits. DTX-induced EPSCs are similar to 5-HT-induced EPSCs in terms of sensitivity to TTX and omega-agatoxin-IVA (a blocker of P-type calcium channels) and laminar selectivity. The involvement of thalamocortical terminals in DTX-induced EPSCs was confirmed by suppression of these EPSCs by micro-opiates and thalamic lesions. More directly, DTX-induced EPSCs substantially occlude those induced by 5-HT, suggesting a common mechanism of action. No occlusion by DTX was seen when EPSCs were induced by a nicotinic mechanism. These results indicate that blockade of Kv1.2-containing potassium channels is part of the mechanism underlying 5-HT-induced glutamate release from thalamocortical terminals.

A major role for thalamocortical afferents in serotonergic hallucinogen receptor function in the rat neocortex

Activation of 5-hydroxytryptamine(2A) (5-HT(2A)) receptors by hallucinogenic drugs is thought to mediate many psychotomimetic effects including changes in affect, cognition and perception. Conversely, blockade of 5-HT(2A) receptors may mediate therapeutic effects of many atypical antidepressant and antipsychotic drugs. The purpose of the present study was to determine the source of subcortical glutamatergic afferents, which would project widely throughout the anterior-posterior axis of the rat brain to the apical dendrites of layer V pyramidal cells of the medial prefrontal cortex, from which serotonin induces transmitter release via activation of 5-HT(2A) receptors. Fiber-sparing chemical lesions of the medial thalamus selectively decreased the frequency of serotonin-induced excitatory postsynaptic currents recorded from layer V pyramidal cells in the prelimbic region of the medial prefrontal cortex by 60%. In contrast, large bilateral lesions of the amygdala did not alter the serotonin response. These thalamic lesions significantly decreased the amount of binding to either mu-opioid or metabotropic glutamate 2/3 receptors in the prelimbic region of the medial prefrontal cortex as expected from previous evidence that these agonists for these receptors suppress serotonin-induced excitatory postsynaptic currents by a presynaptic mechanism. Surprisingly, the amount of specific binding to cortical 5-HT(2A) receptors was significantly increased by the medial thalamic lesions. Thus, these experiments demonstrate that activation of cortical 5-HT(2A) receptors modulates transmitter release from thalamocortical terminals. Unexpectedly, lesioning the thalamocortical terminals also alters 5-HT(2A) receptor binding in the prefrontal cortex. These findings are of interest with respect to understanding therapeutic effects of antidepressant/antipsychotic drugs and the known behavioral effects of thalamic lesions in humans.

Immunohistochemical distribution of delta opioid receptors in the rat central nervous system: evidence for somatodendritic labeling and antigen-specific cellular compartmentalization

Many studies have reported on the distribution of delta opioid receptors (delta OR) in the mammalian central nervous system (CNS) by using a variety of techniques. However, no general consensus has emerged with regards to the localization of this receptor due to inconsistencies in the immunohistochemical literature. In the present study, we analyzed the cellular and subcellular distribution of immunoreactive delta OR in the rat CNS using two different antibodies (directed against a sequence in the C-terminus or N-terminus of the rat delta OR). By using Western blotting, these two antibodies recognized similar forms of the delta OR in COS-7 cells transfected with this receptor, but distinct forms in membranes from the rat spinal cord. By using light microscopic immunohistochemistry, both antibodies recognized identical populations of nerve cell bodies throughout the CNS; the distribution of these cell bodies conformed to that of delta OR mRNA-expressing cells detected by in situ hybridization. However, whereas the C-terminus-directed antibody recognized predominantly perikarya and proximal dendrites, the N-terminus-directed antibody also labeled extensively dendritic and terminal arbors. Furthermore, by using electron microscopy, the two antibodies were found not only to label differentially somatodendritic versus axonal compartments, but also plasma membrane versus cytoplasmic ones, suggesting that distinct immunological forms of the receptor are being targeted preferentially to different cellular and subcellular domains.

Endogenous opioid regulation of oxytocin and ACTH secretion during pregnancy and parturition

Progress of parturition in the rat is optimal when there is increased oxytocin secretion, thus ensuring quick birth and otherwise risking adverse neonatal health. To ensure that the mechanisms for this are available, oxytocin neurons adapt in pregnancy and this includes development of a tonic inhibition by endogenous opioids. Endogenous opioid inhibition of oxytocin secretion increases in pregnancy, initially acting on the nerve terminals in the posterior pituitary and later on oxytocin cell bodies and their inputs. This inhibition enhances stores of oxytocin and enables restraint of oxytocin neuron responsiveness to selected excitatory inputs. The hypothalamic neurons which mediate stress also adapt in late pregnancy so that hypothalamo-pituitary-adrenal axis and oxytocin secretory responses to stressor exposure are attenuated. This is also partly due to endogenous opioid inhibition. Thus, in pregnancy oxytocin and hypothalamo-pituitary-adrenal axis secretion in response to stimulation is restrained, protecting the unborn fetus(es) from premature delivery and glucocorticoid exposure and preparing the oxytocin neurons for their important secretory role during parturition. In parturition itself, endogenous opioids continue to inhibit these neurons. Stress exposure during parturition delays births, probably due to endogenous opioid inhibition of pulsatile oxytocin secretion. On the other hand, basal ACTH and corticosterone secretion are reduced in parturition through inhibition by endogenous opioids. So, opioids continue to regulate the activity of oxytocin and hypothalamo-pituitary-adrenal mechanisms in labor; inhibition of oxytocin neurons at this time may control the spacing of pup births.

Expression of orphanin FQ and the opioid receptor-like (ORL1) receptor in the developing human and rat brain

The orphanin peptide system, although structurally similar to the endogenous opioid family of peptides and receptors, has been established as a distinct neurochemical entity. The distribution of the opioid receptor-like (ORL1) receptor and its endogenous ligand orphanin FQ (OFQ) in the central nervous system of the adult rat has been recently reported, and although diffusely disseminated throughout the brain, this neuropeptide system is particularly expressed within stress and pain circuitry. Little is known concerning the normal expression of the orphanin system during gestation, nor how opiate or stress exposure may influence its development. Using in situ hybridization techniques, the present study was undertaken to determine the normal pattern of expression of ORL1 mRNA in the human and rat brain at various developmental stages. Rat embryos, postnatal rat brains and postmortem human brains were collected, frozen and cut into 15 microm coronal sections. In situ hybridization was performed using riboprobes generated from cDNA containing representative human and rat ORL1 and OFQ sequences. Both ORL1 and OFQ mRNA is detected as early as E12 in the cortical plate, basal forebrain, brainstem and spinal cord. Expression for both ORL1 and OFQ is strongest during the early postnatal period, remaining strong in the spinal cord, brainstem, ventral forebrain, and neocortex into the adult. Human ORL1 and OFQ expression is observed at 16 weeks gestation, remaining relatively unchanged up to 36 weeks. The influence of early orphanin expression on maturation of stress and pain circuitry in the developing brain remains unknown.

Kappa opioid receptor stimulation decreases amphetamine-induced behavior and neuropeptide mRNA expression in the striatum

The purpose of this study was to investigate the role that kappa opioid receptor stimulation has upon stimulant-induced behavior and neuropeptide gene expression in the striatum. Acute administration of amphetamine (2.5 mg/kg i.p.) caused an increase in behavioral activity and preprodynorphin, substance P, and preproenkephalin mRNA expression. When amphetamine-treated rats were pretreated with U69593, a kappa agonist (0.16 or 0.32 mg/kg s.c.), there was a significant decrease in behavioral activity. Quantitative in situ hybridization histochemistry revealed that 0.32 mg/kg U69593 significantly decreased amphetamine-induced mRNA expression of all three neuropeptides; however, only the induction of preproenkephalin mRNA was decreased by 0.16 mg/kg. These data suggest that stimulation of kappa receptors decreases acute amphetamine-induced behavior and mRNA expression of neuropeptides in the rat striatum.