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

Spinal opioid analgesia: how critical is the regulation of substance P signaling?

Although opioids can reduce stimulus-evoked efflux of Substance P (SP) from nociceptive primary afferents, the consequences of this reduction on spinal cord nociceptive processing has not been studied. Rather than assaying SP release, in the present study we examined the effect of opioids on two postsynaptic measures of SP release, Fos expression and neurokinin-1 (NK-1) receptor internalization, in the rat. The functional significance of the latter was first established in in vitro studies that showed that SP-induced Ca(2+) mobilization is highly correlated with the magnitude of SP-induced NK-1 receptor internalization in dorsal horn neurons. Using an in vivo analysis, we found that morphine had little effect on noxious stimulus-evoked internalization of the NK-1 receptor in lamina I neurons. However, internalization was reduced when we coadministered morphine with a dose of an NK-1 receptor antagonist that by itself was without effect. Thus, although opioids may modulate SP release, the residual release is sufficient to exert maximal effects on the target NK-1 receptors. Morphine significantly reduced noxious stimulus-induced Fos expression in lamina I, but the Fos inhibition was less pronounced in neurons that expressed the NK-1 receptor. Taken together, these results suggest that opioid analgesia predominantly involves postsynaptic inhibitory mechanisms and/or presynaptic control of non-SP-containing primary afferent nociceptors.

NMDA and opioid receptors: their interactions in antinociception, tolerance and neuroplasticity

Over the last several years, significant progress has been made in our understanding of interactions between the N-methyl-D-aspartate (NMDA) and opioid receptors. Such interactions have been demonstrated at two distinct sites: (1) modulation of NMDA receptor-mediated electrophysiological events by opioids; and (2) intracellular events involving interactions between NMDA and opioid receptors. Furthermore, a considerable number of studies have shown the involvement of such interactions in neural mechanisms of nociceptive transmission, antinociception in acute and chronic pain states, opioid tolerance/dependence, and neuroplasticity. Importantly, emerging evidence indicates that activation of NMDA receptors may differentially modulate functions mediated by distinct opioid receptor subtypes, namely mu, delta, and kappa receptors. These studies have greatly enriched our knowledge regarding both NMDA and opioid receptor systems and have shed light on neurobiology of both acute and chronic pain. The advancement of such knowledge also promotes new strategies for better clinical management of pain patients.

Role of lipid type on morphine-stimulated diet selection in rats

Administration of morphine is said to increase fat consumption among rats allowed to self-select nutrients. However, fats represent a diverse group of molecules, differing in metabolic and sensory properties. Despite this, lipid has yet to be manipulated as a variable in drug-stimulated nutrient selection studies. To determine whether lipid source can impact daily and morphine-stimulated (1, 3, and 10 mg/kg) diet intake, rats were provided with a choice between a high-fat and high-carbohydrate diet in three regimens in which the source of fat was varied between vegetable shortening, lard, or corn oil. Daily and morphine-stimulated diet selections were determined under all conditions. Under daily feeding conditions, rats ate more of the high-lipid diet compared with the high-carbohydrate diet when vegetable shortening or lard was the main lipid alternative, but lipid and carbohydrate intake did not differ when corn oil was the main lipid alternative. When rats were stimulated with morphine, the percentage of lipid increased relative to baseline intake only when the lipid diets were the preferred alternatives (i.e., vegetable shortening or lard). When preference between lipid and carbohydrate diets was neutral (i.e., corn oil condition), morphine did not enhance lipid consumption. These results indicate that morphine increases consumption of total energy or preferred diets and not lipid per se.

Selective opioid delta agonists elicit antinociceptive supraspinal/spinal synergy in the rat

A multiplicative antinociceptive interaction of morphine activity at supraspinal and spinal sites has been clearly established and is thought to be responsible, in part, for the clinical utility of this compound in normal dose-ranges. While synergistic actions of mu-opioid receptor agonists have been shown, it is unclear whether a similar interaction exists for opioid agonists acting via delta-opioid receptors. Responses to acute nociception were determined with the 52 degrees C hot plate, 52 degrees C warm-water tail-flick and the Hargreaves paw-withdrawal tests. The peptidic opioid delta(1) agonist [D-Pen(2),D-Pen(5)]enkephalin (DPDPE) or delta(2) agonist [D-Ala(2),Glu(4)]deltorphin (DELT) were given into the rostral-ventral medulla (RVM), intrathecally (i.th.) or simultaneously into both the RVM and i.th. (1:1 fixed ratio). Both of the opioid delta agonists produced dose-dependent antinociception in all tests. With the exception of DPDPE in the hot plate test, isobolographic analysis revealed that the supraspinal/spinal antinociceptive interaction for both DPDPE and DELT were synergistic in all nociceptive tests. These data suggest that opioid delta agonists exert a multiplicative antinociceptive interaction between supraspinal and spinal sites to acute noxious stimuli and suggest possibility that compounds acting through delta-opioid receptors may have sufficient potency for eventual clinical application.

The role of opioid-dopamine interactions in the induction and maintenance of ethanol consumption

1. Alcohol is one of the most widely used recreational drugs, but also one of the most widely abused, causing vast economic, social and personal damage. 2. Several animal models are available to study the reinforcing mechanisms that are the basis of the abuse liability of ethanol. Innate differences in opioid or dopamine neurotransmission may enhance the abuse liability of ethanol, as indicated by animal and human studies. 3. Opioid antagonists have been shown to be effective, both experimentally and clinically, in decreasing ethanol consumption, presumably since ethanol induces the release of endogenous opioid peptides in vivo. However, ethanol may also stimulate the formation of opiate-like compounds, which could interact with opioid (or dopamine) receptors. Ethanol may cause changes in neurotransmission mediated via opioid receptors that determines whether alcohol abuse is more or less likely. 4. Ethanol appears to facilitate dopamine release by increasing opioidergic activity, disinhibiting dopaminergic neurons (by inhibition of GABAergic neurotransmission) via mu-opioid receptors in the ventral tegmental area (VTA) and delta-opioid receptors in the nucleus accumbens (NAcc). The effects of ethanol would be antagonised by presynaptic kappa-opioid receptors present on dopaminergic terminals in the NAcc. 5. Mesolimbic dopamine release induced by ethanol consumption seems to indicate ethanol-related stimuli are important, focussing attention on and enabling learning of the stimuli. However, studies indicate that there are redundant pathways, and neural pathways 'downstream' of the mesolimbic dopamine system, which also enable the reinforcing properties of ethanol to be mediated.

Opioids and food intake: distributed functional neural pathways?

Agonists of the mu, delta, kappa and ORL(1)opioid receptors increase food intake while opioid receptor blockade decreases food intake. The majority of the collected data related to opioids and feeding has led to the speculation that opioids are involved in meal maintenance and orosensory reward; however, some data suggest that opioids may impact feeding associated with energy needs. Based on the wide distribution of CNS opioid receptors and the presence of other neuropeptides in the vicinity of opioidergic pathways, it seems likely that opioids affect multiple feeding systems. For example, opioids in the hindbrain might be involved in both sensory and metabolic aspects of food intake, those in the amygdala in processing of 'emotional' properties of foods, and those in the hypothalamus in energy needs. In this review we present data which support functional diversity of opioids in feeding behavior.

Sexual dimorphism in the response to N-methyl-D-aspartate receptor antagonists and morphine on behavior and c-Fos induction in the rat brain

It has been suggested that there are sex differences in the neural response to drugs of abuse. Previous studies have shown that, upon administration of morphine, the immediate early gene c-Fos is induced in the striatum, nucleus accumbens and cortex of the rat brain. This induction of c-Fos is reduced by administration of the N-methyl-D-aspartate receptor antagonist dizocilpine maleate. However, in studies using immunocytochemistry, we found that the pattern of this expression differed markedly between the sexes. In male rats treated with morphine (10 mg/kg, s.c.) and killed 2 h later, there was an induction of c-Fos in the dorsomedial caudate-putamen, the nucleus accumbens and in the intralaminar nuclei of the thalamus. Administration of dizocilpine maleate (0.2 mg/kg, i.p.; 30 min before morphine) partially blocked the response in the caudate-putamen, but not in the thalamus. In females, morphine induced c-Fos in the caudate-putamen, but with more inter-animal variability than in males. In the midline intralaminar thalamic nuclei, female rats showed less induction than males. In male rats, dizocilpine maleate alone caused negligible induction of c-Fos, whereas in female rats, it caused a large induction in the rhomboid, reuniens and central medial nuclei of the thalamus, and in the cortex. Whereas dizocilpine maleate partially blocked the morphine-induced c-Fos expression in the caudate-putamen of males, it completely blocked this response in females. With dizocilpine maleate alone, there was little or no effect on behavior in male rats, whereas in female rats, it caused head bobbing, thrashing, hyperactivity and uncoordinated movements. These behavioral sex differences were not seen on treatment of rats with the competitive N-methyl-D-aspartate receptor antagonist 2R,4R,5S-2-amino-4,5-(1,2-cyclohexyl)-7-phosphoheptanoic acid (NPC-17742; 10 mg/kg, i.p.) and this drug did not induce c-Fos expression in either sex. In the caudate-putamen, morphine-induced c-Fos expression was significantly reduced by NPC-17742 (30 min before morphine) in males and completely blocked in females. These results suggest that the responses to both morphine and N-methyl-D-aspartate receptor antagonists differ between the sexes and emphasize that glutamate is involved in morphine-induced immediate early gene expression in the brain. These studies thus have important implications for gender differences in drug addiction.

Expression of ZFOR1, a delta-opioid receptor, in the central nervous system of the zebrafish (Danio rerio)

Opioid receptors, besides mediating the effects of analgesic compounds, are involved in drug addiction. Although a large amount of work has been done studying these receptors in mammals, little information has been obtained from nonmammalian vertebrates. We have studied the regional distribution in the central nervous system (CNS) of the zebrafish of the recently cloned delta-opioid receptor homologue ZFOR1 using nonradioactive in situ hybridization. Our findings show that different nuclei within the main subdivisions of the brain displayed specific mRNA signal. The expression is widespread throughout the brain, but only specific cells within each nucleus displayed ZFOR1. Stained cells were abundant in the telencephalon, both in the olfactory bulb and telencephalic hemispheres, and in the diencephalon, where expression was observed in all the different subdivisions. In the mesencephalon, expression of ZFOR1 was abundant in the periventricular layer of the optic tectum. In the cerebellum, expression of ZFOR1 was detected in valvula cerebelli, corpus cerebelli, and lobus vestibulolateralis in both granule and Purkinje cells. In the myelencephalon, cells expressing ZFOR1 were also distributed in the octavolateralis area, the reticular formation, and the raphe nuclei, among others. Also, ZFOR1 was detected in cells of the dorsal and ventral horn of the spinal cord. This work presents the first detailed distribution of a delta-opioid receptor in the CNS of zebrafish. Distribution of ZFOR1 expression is compared with that of the delta-opioid receptor described in mammals.

Drugs of abuse and brain gene expression

Addictive drugs like cocaine, ethanol, and morphine activate signal transduction pathways that regulate brain gene expression. Such regulation is modulated by the presence of certain transcription factor proteins present in a given neuron. This article summarizes the effects of several addictive drugs on transcriptional processes contributing to the development of a drug-dependent state. The characterization of drug-induced changes in gene expression shows promise for improving our understanding of drug-addiction phenomena and cellular modes of cocaine, ethanol, and morphine action.

Presynaptic kappa-opioid and muscarinic receptors inhibit the calcium-dependent component of evoked glutamate release from striatal synaptosomes

In addition to cytosolic efflux, reversal of excitatory amino acid (EAA) transporters evokes glutamate exocytosis from the striatum in vivo. Both kappa-opioid and muscarinic receptor agonists suppress this calcium-dependent response. These data led to the hypothesis that the calcium-independent efflux of striatal glutamate evoked by transporter reversal may activate a transsynaptic feedback loop that promotes glutamate exocytosis from thalamo- and/or corticostriatal terminals in vivo and that this activation is inhibited by presynaptic kappa and muscarinic receptors. Corollaries to this hypothesis are the predictions that agonists for these putative presynaptic receptors will selectively inhibit the calcium-dependent component of glutamate released from striatal synaptosomes, whereas the calcium-independent efflux evoked by an EAA transporter blocker, L-trans-pyrrolidine-2,4-dicarboxylic acid (L-trans-PDC), will be insensitive to such receptor ligands. Here we report that a muscarinic agonist, oxotremorine (0.01-10 microM), and a kappa-opioid agonist, U-69593 (0.1-100 microM), suppressed the calcium-dependent release of glutamate that was evoked by exposing striatal synaptosomes to the potassium channel blocker 4-aminopyridine. The presynaptic inhibition produced by these ligands was concentration dependent, blocked by appropriate receptor antagonists, and not mimicked by the delta-opioid agonist [D-Pen2,5]-enkephalin. The finding that glutamate efflux evoked by L-trans-PDC from isolated striatal nerve endings was entirely calcium independent supports the notion that intact basal ganglia circuitry mediates the calcium-dependent effects of this agent on glutamate efflux in vivo. Furthermore, because muscarinic or kappa-opioid receptor activation inhibits calcium-dependent striatal glutamate release in vitro as it does in vivo, it is likely that both muscarinic and kappa receptors are inhibitory presynaptic heteroceptors expressed by striatal glutamatergic terminals.

Transcriptional regulation of mouse delta-opioid receptor gene

Three major types of opioid receptors, mu (MOR), delta (DOR), and kappa (KOR), have been cloned and characterized. Each opioid receptor exhibits a distinct pharmacological profile as well as a distinct pattern of temporal and spatial expression in the brain, suggesting the critical role of transcription regulatory elements and their associated factors. Here, we report the identification of a minimum core promoter, in the 5'-flanking region of the mouse DOR gene, containing an E box and a GC box that are crucial for DOR promoter activity in NS20Y cells, a DOR-expressing mouse neuronal cell line. In vitro protein-DNA binding assays and in vivo transient transfection assays indicated that members of both the upstream stimulatory factor and Sp families of transcription factors bound to and trans-activated the DOR promoter via the E box and GC box, respectively. Furthermore, functional and physical interactions between these factors were critical for the basal as well as maximum promoter activity of the DOR gene. Thus, the distinct developmental emergence and brain regional distribution of the delta opioid receptor appear to be controlled, at least in part, by these two regulatory elements and their associated factors.

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.

Dopamine-opiate interaction in the regulation of neostriatal and pallidal neuronal activity as assessed by opioid precursor peptides and glutamate decarboxylase messenger RNA expression

Neostriatal GABAergic neurons projecting to the globus pallidus synthesize the opioid peptide enkephalin, while those innervating the substantia nigra pars reticulata and the entopeduncular nucleus synthesize dynorphin. The differential control exerted by dopamine on the activity of these two efferent projections concerns also the biosynthesis of these opioid peptides. Using in situ hybridization histochemistry, we investigated the role of opioid co-transmission in the regulation of neostriatal and pallidal activity. The expression of the messenger RNAs encoding glutamate decarboxylase-the biosynthetic enzyme of GABA-and the precursor peptides of enkephalin (preproenkephalin) and dynorphin (preprodynorphin) were measured in rats after a sustained blockade of opioid receptors by naloxone (s.c. implanted osmotic minipump, eight days, 3 mg/kg per h), and/or a subchronic blockade of D2 dopamine receptors by haloperidol (one week, 1.25 mg/kg s.c. twice a day). The density of mu opioid receptors in the neostriatum and globus pallidus was determined by autoradiography. Naloxone treatment resulted in a strong up-regulation of neostriatal and pallidal mu opioid receptors that was not affected by the concurrent administration of haloperidol. Haloperidol alone produced a moderate down-regulation of neostriatal and pallidal micro opioid receptors. Haloperidol strongly stimulated the expression of neostriatal preproenkephalin and preprodynorphin messenger RNAs. This effect was partially attenuated by naloxone, which alone produced moderate increases in preproenkephalin and preprodynorphin messenger RNA levels. In the neostriatum, naloxone did not affect either basal or haloperidol-stimulated glutamate decarboxylase messenger RNA expression. A strong reduction of glutamate decarboxylase messenger RNA expression was detected over pallidal neurons following either naloxone or haloperidol treatment, but concurrent administration of the two antagonists did not result in a further decrease. The amplitude of the variations of mu opioid receptor density and of preproenkephalin and preprodynorphin messenger RNA levels suggests that the regulation of neostriatal and pallidal micro opioid receptors is more susceptible to a direct opioid antagonism, while the biosynthesis of opioid peptides in the neostriatum is more dependent on the dopaminergic transmission. The down-regulation of mu opioid receptors following haloperidol represents probably an adaptive change to increased enkephalin biosynthesis and release. The haloperidol-induced increase in neostriatal preprodynorphin messenger RNA expression might result from an indirect, intermittent stimulation of neostriatal D1 receptors. The haloperidol-induced decrease of pallidal glutamate decarboxylase messenger RNA expression suggests, in keeping with the current functional model of the basal ganglia, that the activation of the striatopallidal projection produced by the interruption of neostriatal dopaminergic transmission reduces the GABAergic output of the globus pallidus. The reduction of pallidal glutamate decarboxylase messenger RNA expression following opioid receptor blockade indicates an indirect, excitatory influence of enkephalin upon globus pallidus neurons and, consequently, a functional antagonism between the two neuroactive substances (GABA and enkephalin) of the striatopallidal projection in the control of globus pallidus output. Through this antagonism enkephalin could partly attenuate the GABA-mediated effects of a dopaminergic denervation on pallidal neuronal activity.

Presynaptic mu and delta opioid receptor modulation of GABAA IPSCs in the rat globus pallidus in vitro

The role of enkephalin and the opioid receptors in modulating GABA release within the rat globus pallidus (GP) was investigated using whole-cell patch recordings made from visually identified neurons. Two major GP neuronal subtypes were classified on the basis of intrinsic membrane properties, action potential characteristics, the presence of the anomalous inward rectifier (Ih), and anode break depolarizations. The mu opioid receptor agonist [D-Ala2-N-Me-Phe4-Glycol5]-enkephalin (DAMGO) (1 microM) reduced GABAA receptor-mediated IPSCs evoked by stimulation within the striatum. DAMGO also increased paired-pulse facilitation, indicative of presynaptic mu opioid receptor modulation of striatopallidal input. In contrast, the delta opioid agonist D-Pen-[D-Pen2, 5]-enkephalin (DPDPE) (1 microM) was without effect. IPSCs evoked by stimulation within the GP were depressed by application of [methionine 5']-enkephalin (met-enkephalin) (30 microM). Met-enkephalin also reduced the frequency, but not the amplitude, of miniature IPSCs (mIPSCs) and increased paired-pulse facilitation of evoked IPSCs, indicative of a presynaptic action. Both DAMGO and DPDPE reduced evoked IPSCs and the frequency, but not amplitude, of mIPSCs. However, spontaneous action potential-driven IPSCs were reduced in frequency by met-enkephalin and DAMGO, whereas DPDPE was without effect. Overall, these results indicate that presynaptic mu opioid receptors are located on striatopallidal terminals and pallidopallidal terminals of spontaneously firing GP neurons, whereas presynaptic delta opioid receptors are preferentially located on terminals of quiescent GP cells. Enkephalin, acting at both of these receptor subtypes, serves to reduce GABA release in the GP and may therefore act as an adaptive mechanism, maintaining the inhibitory function of the GP in basal ganglia circuitry.

Presence of opioid receptor-like (ORL1) receptor mRNA splice variants in peripheral sensory and sympathetic neuronal ganglia

The expression of ORL1 receptor mRNA splice variants is determined in peripheral sensory and sympathetic ganglia and compared to mRNA expression for the three classic opioid receptor subtypes (mu, delta, and kappa) using the method of reverse transcription-polymerase chain reaction. ORL1, mu, delta and kappa receptor subtype mRNAs are present in human dorsal root ganglia (DRG) and trigeminal ganglia and rat DRG. ORL1, mu and delta receptor subtype mRNAs are present in rat superior cervical ganglia and only ORL1 and delta receptor mRNAs are present in rat lumbar sympathetic ganglia. Both the ORL1 mRNA splice variants are present in sensory and sympathetic ganglia, however, expression of the shorter ORL1 receptor mRNA dominates over expression of the longer splice variant in rat brain and DRG, whereas, expression of the longer splice variant is dominant in sympathetic ganglia.

Mu-opioid receptor mRNA expression in proopiomelanocortin neurons of the rat arcuate nucleus

It has been previously demonstrated that the activity of proopiomelanocortin (POMC)-containing neurons in the rat arcuate nucleus is regulated by opiates, but the expression of opioid receptors in POMC neurons has never been reported. In the present study, we have applied a double-labeling in situ hybridization technique to investigate the occurrence of mu-opioid receptor mRNA on POMC neurons. We have found that 20+/-3% of arcuate POMC neurons express mu-opioid receptor mRNA and that the proportion of POMC neurons expressing mu-opioid receptor is higher in the caudal than in the rostral portion of the arcuate nucleus. Our data suggest that POMC neurons might be both auto-regulated by beta-endorphin, and regulated by enkephalins.

Enkephalin regulates acute D2 dopamine receptor antagonist-induced immediate-early gene expression in striatal neurons

Projection neurons of the striatum release opioid peptides in addition to GABA. Our previous studies showed that the opioid peptide dynorphin regulates that subtype of projection neurons which sends axons to the substantia nigra/entopeduncular nucleus, as indicated by an inhibitory action of dynorphin/agonists on D1 dopamine receptor-mediated immediate-early gene induction in these neurons. The other subtype of striatal projection neurons projects to the globus pallidus and contains the opioid peptide enkephalin. Here, we investigated whether enkephalin regulates the function of striatopallidal neurons, by analysing opioid effects on immediate-early gene induction by D2 dopamine receptor blockade that occurs in these neurons. Thus, the effects of systemic and intrastriatal administration of various opioid receptor agonists and antagonists on immediate-early gene expression (c-fos, zif 268) induced by the D2 receptor antagonist eticlopride were examined with in situ hybridization histochemistry. Intrastriatal infusion of enkephalin (delta and mu), but not dynorphin (kappa), receptor agonists suppressed immediate-early gene induction by eticlopride in a dose-dependent manner. This suppression was blocked by the opioid receptor antagonist naloxone, confirming the involvement of opioid receptors. Repeated treatment with D2 receptor antagonists produces increased enkephalin expression and diminished immediate-early gene inducibility in striatopallidal neurons, as well as behavioral effects that are attenuated compared to those of acute treatment (e.g., reduced akinesia). Naloxone reversed such behavioral recovery (i.e. reinstated akinesia), but did not significantly affect suppressed immediate-early gene induction. Our results indicate that enkephalin acts, via mu and delta receptors in the striatum, to inhibit acute effects of D2 receptor blockade in striatopallidal neurons. Moreover, the present findings suggest that increased enkephalin expression after repeated D2 receptor antagonist treatment is an adaptive response that counteracts functional consequences of D2 receptor blockade, but is not involved in suppressed immediate-early gene induction. Together with our earlier findings of the role of dynorphin, these results indicate that opioid peptides in the striatum serve as negative feedback systems to regulate the striatal output pathways in which they are expressed.

Neurokinin-3 receptor distribution in rat and human brain: an immunohistochemical study

Autoradiographic and immunohistochemical studies have shown that the neurokinin-3 receptor is widely distributed in the rodent CNS. Expression of the neurokinin-3 receptor in human brain, however, has been debated. These conflicting findings, as well as the poor resolution of autoradiographic images, prompted us to develop a polyclonal antibody against an oligopeptide derived from the carboxy-terminus consensus sequence of both the rat and human neurokinin-3 receptor ([C]ASTTSSFISSPYTSVDEYS, amino acids 434-452 of the rat neurokinin-3 receptor). Western blot analysis of both human and rat brain tissue revealed a major band in the molecular weight range 65,000-67,000, the proposed molecular weight of the neurokinin-3 receptor based on its amino acid sequence and presumed glycosylation state. The distribution of selective high affinity neurokinin-3 receptor agonist [3H]senktide binding and neurokinin-3 receptor immunoreactivity were virtually identical in the brains of male Fischer 344 rats. The highest concentrations of neurokinin-3 receptors were observed in cortical layers IV-V; the basolateral amygdaloid nucleus; the hypothalamic paraventricular, perifornical and supraoptic nuclei; the zona incerta; and the entopeduncular and interpeduncular nuclei. [3H]senktide binding and neurokinin-3 receptor immunoreactivity were compared in homologous cortical areas of the human and rat brain. In contrast to the rat, autoradiographic analysis of normal control human brains (35-75 years) revealed a distinct and predominant superficial cortical labeling in the glia limitans and the cortical layer I. However, neurokinin-3 receptor immunoreactivity could be found not only in the superficial cortical layers, but also on pyramidal neurons and astrocytes in the neuropil and white matter. These findings suggest species differences in both the cellular and anatomical distribution of the neurokinin-3 receptor.

Effects of sciatic nerve injuries on delta -opioid receptor and substance P immunoreactivities in the superficial dorsal horn of the rat

The aim of the present study was to examine the effects of transection combined with tight ligation, and crush of the sciatic nerve on delta -opioid receptor and substance P immunoreactivities in the superficial spinal dorsal horn at different time points after injury. Both the delta -opioid receptor and substance P are primarily localised to primary afferent fibres and terminals. Seven days following transection and ligation, a slight decrease in both delta -opioid receptor and substance P levels was seen in laminae I and II. The maximal reduction appeared to take place around 4 weeks. Restoration of immunoreactivity was observed by 32 weeks, and by 1 year the levels were almost back to normal. Regarding crush injury, the reduction in both delta -opioid receptor and substance P immunoreactivities were less pronounced and recovery was faster than after transection injury. Already by 16 weeks, the levels were almost back to normal.These results show that peripheral nerve injuries dramatically reduce the levels of delta -opioid receptor and substance P immunoreactivities in the superficial dorsal horn after short survivals and demonstrate recovery after long survivals. Whether the marked reduction of delta -opioid receptors in the dorsal horn is involved in the decreased ability of opioid analgesics to alleviate neuropathic pain remains to be studied. Copyright 1999 European Federation of Chapters of the International Association for the Study of Pain.

Expression of mu, kappa, and delta opioid receptor messenger RNA in the human CNS: a 33P in situ hybridization study

The existence of at least three opioid receptor types, referred to as mu, kappa, and delta, is well established. Complementary DNAs corresponding to the pharmacologically defined mu, kappa, and delta opioid receptors have been isolated in various species including man. The expression patterns of opioid receptor transcripts in human brain has not been established with a cellular resolution, in part because of the low apparent abundance of opioid receptor messenger RNAs in human brain. To visualize opioid receptor messenger RNAs we developed a sensitive in situ hybridization histochemistry method using 33P-labelled RNA probes. In the present study we report the regional and cellular expression of mu, kappa, and delta opioid receptor messenger RNAs in selected areas of the human brain. Hybridization of the different opioid receptor probes resulted in distinct labelling patterns. For the mu and kappa opioid receptor probes, the most intense regional signals were observed in striatum, thalamus, hypothalamus, cerebral cortex, cerebellum and certain brainstem areas as well as the spinal cord. The most intense signals for the delta opioid receptor probe were found in cerebral cortex. Expression of opioid receptor transcripts was restricted to subpopulations of neurons within most regions studied demonstrating differences in the cellular expression patterns of mu, kappa, and delta opioid receptor messenger RNAs in numerous brain regions. The messenger RNA distribution patterns for each opioid receptor corresponded in general to the distribution of opioid receptor binding sites as visualized by receptor autoradiography. However, some mismatches, for instance between mu opioid receptor receptor binding and mu opioid receptor messenger RNA expression in the anterior striatum, were observed. A comparison of the distribution patterns of opioid receptor messenger RNAs in the human brain and that reported for the rat suggests a homologous expression pattern in many regions. However, in the human brain, kappa opioid receptor messenger RNA expression was more widely distributed than in rodents. The differential and region specific expression of opioid receptors may help to identify targets for receptor specific compounds in neuronal circuits involved in a variety of physiological functions including pain perception, neuroendocrine regulation, motor control and reward.

Localization of mu-opioid receptors on amygdaloid projection neurons in the parabrachial nucleus of the rat

The parabrachial nucleus (PB) is a major relay of noxious and non-noxious visceral sensory information from the nucleus of the solitary tract, spinal cord, and spinal trigeminal nucleus to the forebrain. The nucleus of the solitary tract, spinal cord, and trigeminal dorsal horns contain many enkephalin- and dynorphin-immunoreactive neurons that project to the PB. To study the role of mu-opioid receptors in relaying these inputs, we examined the distribution of mu-opioid receptor immunoreactivity in the PB. The most intense staining was in the external lateral parabrachial subnucleus (PBel), including dendrites extending from the PBel into the lateral crescent subnucleus. Because the Pbel is a major source of projections to the amygdala, we combined retrograde tracing from the central nucleus of the amygdala with immunohistochemistry for mu-opioid receptors. These experiments showed that mu-opioid receptors are expressed by Pbel neurons that project to the amygdala, including those Pbel neurons whose dendrites extend into the lateral crescent subnucleus. These results indicate that mu-opioid receptors in the PB may mediate or modulate nociceptive information relayed to the amygdala from medullary or spinal cord neurons that terminate not only in the Pbel, but also in the adjacent lateral crescent parabrachial subnucleus.

[3H]ethylketocyclazocine binding to brain opioid receptor subtypes in alcohol-preferring AA and alcohol-avoiding ANA rats

We measured brain regional patterns of [3H]ethylketocyclazocine binding to brain opioid receptors in ethanol-naive alcohol-preferring Alko, Alcohol (AA) and alcohol-avoiding Alko, Non-Alcohol (ANA) rats, by using quantitative autoradiography. This agonist ligand labels all opioid receptor subtypes. The proportions of mu- and delta-opioid receptor binding were evaluated by displacing the mu- and delta-opioid receptor components by the peptides Tyr-D-Ala-Gly-N(Me)Phe-Gly-ol (DAMGO, 100 nM) and Tyr-D-Pen-Gly-Phe-D-Pen (DPDPE, 100nM), respectively, the K-component being the naltrexone-sensitive binding left after removal of the above two components. The labeling patterns in the brains of the AA and ANA rats were consistent with the well-known distributions of the opioid receptor subtypes in nonselected rat strains and there was no major difference between the lines. The mu-opioid receptor binding was greater in the AA than ANA rats in several brain regions, most interestingly in the substantia nigra pars reticulata and striatal clusters with elevated shell/core ratios in the nucleus accumbens. The delta-opioid receptor binding did not differ between the lines, whereas the AA rats had more K-opioid receptors than the ANA rats in several brain regions, including limbic areas and basal ganglia. The observed results might indicate altered action of the opioidergic system on dopaminergic pathways in rats with differential alcohol preference.

Rational design of oligonucleotide probes to avoid optimization steps in in situ hybridization

In situ hybridization (ISH) is a widely used technique in neuroscience since it allows a relatively straightforward determination of gene expression in the brain, in respect to distribution as well as in respect to quantification. It is based upon the hybridization of a nucleic acid probe with the mRNA under investigation and does not require the creation of specific antibodies as in immunohistochemistry. However, a major drawback of ISH is the fact that all standard protocols available include time consuming optimization steps of several critical parameters such as tissue fixation, hybridization conditions and washing procedures. Therefore, the aim of our investigation was a rational design of oligonucleotide probes which were adapted to our standard ISH protocol and which could therefore be used without changing any parameter. This approach also worked well for the detection of rare gene products such as neuropeptide receptor mRNAs. To adapt the probes to our standard procedure, sequence, calculated melting temperature, length and secondary structures of the oligonucleotides were considered according to certain constraints as outlined in the following.

5-HT2A receptor or alpha1-adrenoceptor activation induces excitatory postsynaptic currents in layer V pyramidal cells of the medial prefrontal cortex

We compared 5-hydroxytryptamine (5-HT), norepinephrine and dopamine for their efficacy at increasing excitatory postsynaptic current frequency in layer V pyramidal cells from rat medial prefrontal cortical slices. 5-HT, norepinephrine and dopamine increased the excitatory postsynaptic current frequency by 15.9-, 4.5- and 1.7-fold, respectively. Similar to previous results with 5-HT-induced excitatory postsynaptic currents, blockade of mu-opioid receptors, of alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) receptors and fast Na+ channels suppressed the norepinephrine-induced excitatory postsynaptic currents. The norepinephrine-induced, and in most cases, the dopamine-induced increase in excitatory postsynaptic current frequency was blocked by the alpha1-adrenoceptor antagonist prazosin while the alpha2-adrenoceptor antagonist yohimbine did not block either the norepinephrine- or the 5-HT-induced increase in excitatory postsynaptic currents frequency. The potency of three 5-HT2 receptor antagonists with varying selectivity for 5-HT2A/2B/2C receptors tested against the 5-HT-induced increase in excitatory postsynaptic current frequency are in agreement with the affinity of these drugs for the 5-HT2A receptor. These findings suggest that 5-HT2A receptor or alpha1-adrenoceptor activation enhance neurotransmitter release from a similar subset of glutamate terminals that innervate apical dendrites of layer V pyramidal cells.

Quantitative autoradiography of mu-,delta- and kappa1 opioid receptors in kappa-opioid receptor knockout mice

Mice deficient in the kappa-opioid receptor (KOR) gene have recently been developed by the technique of homologous recombination and shown to lack behavioural responses to the selective kappa1-receptor agonist U-50,488H. We have carried out quantitative autoradiography of mu-, delta- and kappa1 receptors in the brains of wild-type (+/+), heterozygous (+/-) and homozygous (-/-) KOR knockout mice to determine if there is any compensatory expression of mu- and delta-receptor subtypes in mutant animals. Adjacent coronal sections were cut from the brains of +/+, +/- and -/- mice for the determination of binding of [3H]CI-977, [3H]DAMGO (D-Ala2-MePhe4-Gly-ol5 enkephalin) or [3H]DELT-I (D-Ala2 deltorphin I) to kappa1-, mu- and delta-receptors, respectively. In +/- mice there was a decrease in [3H]CI-977 binding of approximately 50% whilst no kappa1-receptors could be detected in any brain region of homozygous animals confirming the successful disruption of the KOR gene. There were no major changes in the number or distribution of mu- or delta-receptors in any brain region of mutant mice. There were, however some non-cortical regions where a small up-regulation of delta-receptors was observed in contrast to an opposing down-regulation of delta-receptors evident in mu-knockout brains. This effect was most notable in the nucleus accumbens and the vertical limb of the diagonal band, and suggests there may be functional interactions between mu- and delta-receptors and kappa1- and delta-receptors in mouse brain.

Mu- and delta-opioid receptor mRNAs are expressed in spinally projecting serotonergic and nonserotonergic neurons of the rostral ventromedial medulla

The rostral ventromedial medulla (RVM) is an important mediator of the supraspinal component of opioid antinociception. Previous studies have suggested that activation of the cloned mu- and delta-opioid receptors (MOR1 and DOR1 respectively) in the RVM produces the antinociception mediated by spinally projecting neurons. In the present study, we investigated the expression of mRNA encoding either MOR1 or DOR1 in the RVM of rats. In addition, we examined quantitatively the expression of MOR1 and DOR1 mRNAs in spinally projecting RVM neurons including serotonergic (5HT) cells by using in situ hybridization, immunocytochemistry, retrograde tract-tracing, and the physical disector. Brainstem neurons were labeled in 14 male Sprague-Dawley rats by applying Fluoro-Gold (FG) topically to the dorsal surface of the lumbosacral spinal cord. Five-micrometer-thick cryostat sections were cut and in situ hybridization was performed by using full-length cRNA probes labeled with 35S-UTP. We found that 43% of RVM projection neurons expressed MOR1 mRNA and 83% of RVM projection neurons expressed DOR1 mRNA. Of 192 retrogradely labeled cells in the RVM, 51 cells (27%) were immunoreactive for 5HT. Of this population, half appeared to be labeled for the mRNA encoding MOR1 and over three-fourths appeared to be labeled for the mRNA encoding DOR1. Thus, we conclude that bulbospinal neurons express MOR1 and DOR1; moreover, MOR1 and DOR1 are expressed by significant proportions of 5HT neurons projecting to or through the dorsal spinal cord.

Chronic morphine exposure and spontaneous withdrawal are associated with modifications of dopamine receptor and neuropeptide gene expression in the rat striatum

The influence of chronic morphine and spontaneous withdrawal on the expression of dopamine receptors and neuropeptide genes in the rat striatum was investigated. Morphine dependence was induced by subcutaneous implantation of two morphine pellets for 6 days. Rats were made abstinent by removal of the pellets 1, 2 or 3 days before they were killed. The mRNA levels coding for D1- and D2-dopamine receptors, dynorphin, preproenkephalin A and substance P were determined by quantitative in situ hybridization. The caudate putamen and the nucleus accumbens showed equivalent modifications in dopamine receptor and neuropeptide gene expression. After 6 days of morphine, a decrease in D2-dopamine receptor and neuropeptide mRNA levels was observed (-30%), but there was no change in D1-dopamine receptor mRNA. In abstinent rats, both D1- and D2-dopamine receptor mRNA levels were decreased 1 day after withdrawal (-30% compared with chronic morphine). In contrast, neuropeptide mRNA levels were unaffected when compared with those observed after 6 days of morphine. During the second and third day of withdrawal, there was a gradual return to the levels seen in the placebo-treated group, for both dopamine receptor and neuropeptide mRNAs. Phenotypical characterization of striatal neurons expressing mu and kappa opioid receptor mRNAs showed that, in striatonigral neurons, both mRNAs were colocalized with D1-receptor and Dyn mRNAs. Our results suggest that during morphine dependence, dopamine and morphine exert opposite effects on striatonigral neurons, and that effects occurring on striatopallidal neurons are under dopaminergic control. We also show that withdrawal is associated with a down regulation of the postsynaptic D1 and D2 receptors.

Subcellular distribution of delta-opioid receptors in the rat spinal cord: an approach using a three-dimensional reconstruction of confocal series of immunolabelled neurons

Using a specific rat monoclonal anti-idiotypic antibody we have examined the subcellular distribution of -opioid receptors in various neuronal subtypes of the rat spinal cord. The immunofluorescence was detected with a confocal microscope and in some cases serial images were processed for a three-dimensional (3-D) reconstruction of the neurons. Immunolabelling was found to be distributed throughout the spinal cord grey matter specially in the most superficial layers of the dorsal horn, around the central canal and in the region of motoneurons of the ventral horn. The 3-D reconstruction made on large neurons of lamina IX in the ventral horn and on neurons of lamina X around the central canal allowed the visualization of 5 -opioid receptors in the cytoplasm of the soma and proximal neurites of immunofluorescent neurons. Some immunolabelled receptors were also detected at the level of the plasma membrane of the cell bodies and in the nuclear matrix. Interestingly, a particular arrangement of delta-opioid receptors organized along parallel alignments was observed on the plasma membrane of some neurons. This study emphasizes the potential usefulness of a 3-D reconstruction in the study of the spatial arrangement of cellular components.

Area- and lamina-specific organization of a neuronal subpopulation defined by expression of latexin in the rat cerebral cortex

The aim of the present study was to investigate the density, laminar distribution, size, morphology, and neurotransmitter phenotype of rat cortical neurons expressing latexin, an inhibitor of carboxypeptidase A. Immunohistochemical analyses established that latexin-immunoreactive neurons are restricted essentially to the infragranular layers of lateral cortical areas in the rat. The overall density, laminar or sublaminar localization, and cell size distribution of latexin-positive neurons differed substantially across cytoarchitectonic areas within lateral cortex. Numerous latexin-positive neurons had the morphology of modified pyramidal cells especially of layer VI. The vast majority of latexin-positive neurons were glutamate-immunoreactive in the six lateral neocortical areas examined, while neurons immunoreactive for both latexin and GABA were virtually absent. Thus the majority of latexin-positive neurons are likely to be excitatory projection neurons. The area- and lamina-specific distribution of the latexin-expressing subpopulation of glutamate-immunoreactive neurons is a distinctive feature that may contribute to the functional specialization of the lateral cortical areas.

The opioid peptide analog D-Ala2-Met-enkephalinamide decreases bile flow by a central mechanism

The existence of an opioid central pathway that may regulate bile secretion was explored by studying the effect of the intracisternal (i.c.) administration of the opiate D-Ala2-Met-enkephalinamide (DAME) on bile secretion in anesthetized male rats. The i.c. administration of DAME was associated with a dose-related decrease in bile flow that ranged from 12% to 41%, which was prevented by the opiate antagonist naloxone. Bicarbonate secretion into bile decreased significantly after i.c. DAME. Chemical adrenergic denervation and cholinergic pharmacological blockade with atropine did not prevent the DAME-induced decrease in bile flow. The data support the existence of an opioid-mediated pathway that starts in the brain and that contributes to the regulation of bile secretion.

The electrophysiology of prefrontal serotonin systems: therapeutic implications for mood and psychosis

A newly described synaptic action of serotonin (5-HT) in the cerebral cortex is reviewed, and implications for mood and psychosis are discussed. Recordings in brain slices show that 5-HT induces a rapid increase in excitatory postsynaptic potentials/currents (EPSPs/EPSCs) in virtually all layer V pyramidal cells of neocortex. This effect is mediated by the 5-HT2A receptor, which has been linked to the action of hallucinogenic and atypical antipsychotic drugs. The increase in EPSCs is seen most prominently in medial prefrontal cortex and other frontal regions where 5-HT2A receptors are enriched. The induction of EPSCs by 5-HT appears to occur through a novel mechanism that does not depend on the activation of afferent impulse flow. Instead, 5-HT appears to act presynaptically, directly or indirectly, to induce a focal release of glutamate from a subpopulation of glutamatergic terminals impinging upon the apical (but not basilar) dendrites of layer V pyramidal cells; a working hypothesis of the transduction pathway (involving asynchronous transmitter release) for this process is presented. Consistent with a focal action upon glutamatergic nerve terminals, the 5-HT-induced EPSPs can be suppressed by presynaptic inhibitory modulators such as mu-opiate or group II/III metabotropic agonists. We suggest that the suppression of 5-HT-induced EPSCs by 5-HT2A antagonists and mu-opiate agonists may underlie certain shared clinical effects of 5-HT2A antagonists and mu-opiate agonists. We suggest further that since presynaptic group II/III metabotropic glutamate agonists suppress 5-HT-induced EPSCs, metabotropic glutamate agonists may also possess antidepressant and/or antipsychotic properties.

ORL-1 and mu opioid receptor antisera label different fibers in areas involved in pain processing

Mu opioid receptors (MOR) mediate the analgesic effects of opioid drugs such as morphine. The opioid receptor-like (ORL-1) receptor is structurally related to opioid receptors and the ORL-1 receptor agonist, orphanin FQ/nociceptin, induces analgesia at the spinal level, but appears to recruit different circuitry than that used by mu opioids. When administered intracerebroventricularly, orphanin FQ/nociceptin produces hyperalgesia and/or reverses opioid analgesia. The functionally distinct actions elicited by MOR and ORL-1 receptors, which activate similar intracellular signaling systems and show similar regional distributions, could be explained by their differential cellular localization. By using double label immunohistochemistry and confocal microscopy, the present study investigates the distribution of MOR and ORL-1 receptors in regions of the rat nervous system that are involved with nociceptive processing. In general co-localization of MOR and ORL-1 receptor immunoreactivity was not observed in either perikarya or neuropil in the dorsal root ganglia, nor in the Lissauer's tract and superficial laminae of the spinal cord. Likewise, there was no evidence for co-localization of these receptors within the periaqueductal gray, the nucleus raphe magnus, the gigantocellular reticular nucleus, and the nucleus of the solitary tract. These observations indicate that MOR and ORL-1 receptors are expressed predominantly on different fiber systems in these regions. This differential distribution is consistent with the distinct pharmacology of ORL-1 and MOR receptor agonists and suggests that the antisera to MOR and ORL-1 receptors may provide useful markers for further investigations of analgesic and counteranalgesic pathways modulating pain perception.

Spinal mu-, delta- and kappa-opioid receptors mediate intense stimulation-elicited inhibition of a nociceptive reflex in the rat

Intense electrical stimulation of meridian points in the rat inhibits the nociceptive tail withdrawal reflex. The objective of the present study was to determine whether spinal opioid receptors mediate this inhibition. Electrical stimulation was applied with 2 ms square pulses, at 4 Hz for 20 min at 20 times the threshold, to previously defined meridian points in the hindlimb. Threshold was the minimum current required to elicit muscle twitch. In lightly anaesthetized intact rats (n = 8) stimulation inhibited tail withdrawal during and for greater than one hour after the end of stimulation. In unanaesthetized spinal rats (n = 12) this inhibition was less and the post-stimulation effect lasted for 15 min. In control anaesthetized intact (n = 28) and unanaesthetized spinal rats (n = 14) placement of electrodes without stimulation had no effect. In spinal rats, preadministration of naloxone (25 mg/kg, i.p.) blocked the evoked inhibition (n = 11). In intact animals both naloxone (n = 8) and the mu-opioid receptor antagonist, beta-funaltrexamine (10 nmol; n = 9), given via a chronic intrathecal catheter, attenuated inhibitions during and after the end of stimulation by 50-60%. The delta-opioid receptor antagonist H-Tyr-tic psi[CH2NH]Phe-Phe-OH (TIPP[psi]; 10 nmol; n = 7) and the kappa-opioid receptor antagonist nor-binaltorphimine (10 nmol; n = 13) given by lumbar puncture attenuated the inhibition during the stimulation by 30% and 56%, respectively; both antagonists blocked the post-stimulation effect and even facilitated the withdrawal. The data suggest that spinal mu-, delta- and kappa-opioid receptors each contribute to the evoked inhibition.

Distribution of prepro-nociceptin/orphanin FQ mRNA and its receptor mRNA in developing and adult mouse central nervous systems

Nociceptin/orphanin FQ (N/OFQ) and its receptor share similarities to opioids and their receptors in terms of the molecular structure and signaling pathway, but the two systems exhibit different actions in vivo. To understand the mechanism of N/OFQ-system actions, we examined, by in situ hybridization analysis, the distribution of preproN/OFQ and N/OFQ receptor mRNAs in the developing and adult mouse central nervous systems (CNS). In most neural regions, preproN/OFQ mRNA was mainly expressed in a small population of middle-sized neurons. These neurons were scattered between large projection-type neurons or within the neuropil, suggestive of interneurons. In some other nuclei (lateral septum, bed nucleus of the stria terminalis, reticular thalamic nucleus, inferior colliculus, and rostral periolivery nucleus), preproN/OFQ mRNA was expressed in a number of large projection-type neurons. By contrast, N/OFQ receptor mRNA was evenly expressed in most neurons of the adult CNS. Considering the inhibitory actions of N/OFQ, the distinct cellular expression pattern of the N/OFQ system suggests that the release of N/OFQ from interneurons may lower neuronal and synaptic activities of neighboring neurons, leading to integration or modulation of local circuits. Furthermore, the cellular expression pattern, distinct from that of the opioid system, may provide a possible molecular/cellular basis for the different in vivo actions of N/OFQ and opioids. In embryonic stages, both preproN/OFQ and N/OFQ receptor mRNAs were highly and widely expressed in the mantle zone, suggesting the possible importance of N/OFQ signaling in CNS development.

5-Hydroxytryptamine-induced excitatory postsynaptic currents in neocortical layer V pyramidal cells: suppression by mu-opiate receptor activation

Activation of 5-hydroxytryptamine-2A receptors increases the frequency of excitatory postsynaptic currents through a focal action at apical, but not basilar, dendrites of neocortical layer V pyramidal cells. Since mu-, delta- and kappa-opiate receptors are known to inhibit depolarization-induced glutamate release in cerebrocortical slices, we examined the opiate receptor subtype(s) that suppress(es) 5-hydroxytryptamine-induced excitatory postsynaptic currents in the medial prefrontal cortex and whether this suppression was occurring through a presynaptic or a postsynaptic mechanism. Only opioid agonists that act upon mu-receptors (i.e. [D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin, the endogenous mu-selective agonist endomorphin-1 and the non-selective opioid agonist [Met]enkephalin) suppressed 5-hydroxytryptamine-induced excitatory postsynaptic currents. The delta-agonist [D-phen(2,5)]enkephalin and the kappa-agonist U50,488 were ineffective. Only the selective mu-antagonist CTOP blocked the suppressant effect of enkephalin, while the selective delta-antagonist naltrindole and the selective kappa-antagonist norbinaltorphimine were ineffective. Since the 5-hydroxytryptamine-induced excitatory postsynaptic currents are mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-type excitatory amino acid receptors, the failure of mu-agonists to either block postsynaptic AMPA responses or induce outward currents in layer V pyramidal cells suggest that mu-agonists are acting at a presynaptic site to block 5-hydroxytryptamine-induced excitatory postsynaptic currents. Strikingly, a regional selectivity in the suppressant effect of mu-receptor activation on 5-hydroxytryptamine-induced excitatory postsynaptic currents exists, as 300 nM [D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin suppressed 5-hydroxytryptamine-induced excitatory postsynaptic currents in the medial prefrontal cortex by nearly 100%, while in the frontoparietal cortex 1 microM [D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin suppressed 5-hydroxytryptamine-induced excitatory postsynaptic currents by only 58%. This is the first demonstration of a previously unsuspected physiological interaction between 5-hydroxytryptamine-2A and mu-opiate receptors and may be relevant to the relationship between these receptors and both mood and psychotic disorders.

Latexin expression in smaller diameter primary sensory neurons in the rat

Most of the smaller diameter neurons of dorsal root and trigeminal ganglia in adult rats expressed latexin, which has the inhibitor activity of carboxypeptidase A. Most of the dorsal root ganglion (DRG) neurons containing either calcitonin gene-related peptide (CGRP), substance P (SP) or somatostatin (SST) coexpressed latexin. Latexin was widely distributed in the cytoplasm of the cell body and in axonal fibers of cultured DRG neurons which were sensitive to capsaicin. In addition, latexin-immunoreactivity was observed throughout lamina II of the spinal cord in normal animals, but was lost following sciatic nerve-axotomy, suggesting the presence of latexin-immunoreactive axonal fibers and/or terminals from DRG neurons. Immunoelectron microscopy indeed revealed latexin-immunoreactive axonal terminals and thinly myelinated and unmyelinated axonal fibers within the dorsal horn. These observations suggest that latexin may be involved in nociceptive information transmission or its modulation.

Opioid receptor gene expression in the rat brain during ontogeny, with special reference to the mesostriatal system: an in situ hybridization study

The three main types of opioid receptors micro, delta and kappa are found in the central nervous system and periphery. In situ hybridization study was undertaken to determine the expression of mu, delta, kappa-opioid receptors mRNAs in the brain during pre- and postnatal development, especially in the mesostriatal system. By G13, mu and kappa-opioid receptor mRNA were detectable in the telencephalon; mu-opioid receptor mRNA was found in the striatal neuroepithelium and cortical plate and kappa-opioid receptor mRNA in the corroidal fissure. By G15, kappa-opioid receptor mRNA was detectable in the nucleus accumbens and dorsal striatum, and in the substantia nigra and ventral tegmental area, suggesting an early expression of the corresponding receptor on dopaminergic terminal fibers. For the mu-opioid receptor mRNA in the striatum, patches appeared at G20. Delta-opioid receptor mRNA was first detected at G21, in many areas including the accumbens nucleus and the dorsal striatum. At P8, delta-opioid receptor mRNA was detected in large-sized cells of the striatum, possibly cholinergic, suggesting a possible modulation by opioids of the striatal cholinergic neurons. Our results demonstrate the early appearance of mu and kappa-opioid receptor mRNA (G13) and the relatively late development of delta-opioid receptor mRNA (G21) in the brain. We also show a distinct pattern of expression for mu, delta and kappa-opioid receptor mRNAs in the mesostriatal system during the development.

Divergence of the feeding and thermogenic pathways influenced by NPY in the hypothalamic PVN of the rat

Neuropeptide Y (NPY) injected into the paraventricular nucleus (PVN) increases feeding and decreases brown adipose tissue (BAT) uncoupling protein (UCP) and lipoprotein lipase (LPL) mRNA. Previously we reported that the feeding and BAT effects induced by NPY in the PVN are blocked by 50 microg naltrexone (NTX) in the rostral nucleus of the solitary tract (rNTS). We sought to determine whether the effect of rNTS NTX on PVN NPY-induced alterations in energy metabolism occurred at lower doses of NTX. Male Sprague-Dawley rats were fitted with cannulas into two sites: PVN and rNTS. Feeding response, BAT UCP, and LPL mRNA were measured after injection of 0, 5, 10, and 25 microg NTX in the rNTS +/- 1 microg NPY in the PVN. One-hour feeding response to PVN NPY was significantly and dose dependently decreased by 10 and 25 microg rNTS NTX (-23 and -31%, respectively). However, rNTS NTX did not block the PVN NPY-induced decrease in BAT UCP or LPL mRNA. BAT beta-actin mRNA (as a measure of overall changes in gene expression) was unchanged among treatment groups. These results indicate a possible divergence in the PVN NPY feeding-stimulatory/BAT-inhibitory pathway, such that PVN NPY feeding effects may be routed through the rNTS whereas BAT effects may be due to alterations at another neural site.

A leu-enkephalin depresses transmission from muscle and skin non-nociceptors to first-order feline spinal neurones

1. The effects of an opioid (D-Ser-Leu-enkephalin-Thr; DSLET) were tested on synaptic actions of non-nociceptive afferents: group I and II muscle afferents and low-threshold skin afferents. They were tested on population EPSPs (field potentials) evoked in the dorsal horn and the intermediate zone of mid-lumbar segments, and on monosynaptically evoked responses of single interneurones at the same location. DSLET was applied locally (ionophoretically) at locations at which the field potentials were maximal and close to the selected neurones. 2. DSLET potently depressed transmission from group II muscle afferents and from low-threshold skin afferents. Transmission to neurones located in the dorsal horn or in the intermediate zone was depressed to a similar extent. The depression was readily antagonized by naloxone. Transmission from group Ia or Ib muscle afferents to neurones located in the intermediate zone was not affected, or was facilitated by DSLET. 3. The results show that DSLET has similar depressive actions on spinal neurones to monoamines, but its actions are more widespread. Like monoamines it affects transmission from nociceptors and group II muscle afferents, but in addition it gates transmission from low-threshold cutaneous afferents. Furthermore its effects do not appear to be restricted to interneurones at particular locations since it depressed responses of dorsal horn interneurones (gated by serotonin) as well as intermediate zone interneurones (gated by noradrenaline).

The effect of fentanyl on electrophysiologic recovery of CA 1 pyramidal cells from anoxia in the rat hippocampal slice

Fentanyl is widely used in conditions in which the brain is at risk of ischemic or anoxic injury. We evaluated the effect of fentanyl on anoxic injury to CA 1 pyramidal cells in the rat hippocampus. These neurons are extremely sensitive to anoxic injury and are densely populated with opioid receptors. We prepared hippocampal slices from adult Sprague-Dawley rats and evoked a postsynaptic population spike in the CA 1 pyramidal cell region by stimulating the Schaffer collateral pathway. The amplitude of this response was used to evaluate the effect of fentanyl on anoxic injury. Pretreatment with fentanyl (50 or 500 ng/mL) did not alter the amplitude of the CA 1 population spike before anoxia, nor did it alter the recovery of this response after 5,6, or 7 min of anoxia. After 5 min of anoxia, the population spike recovered to 76% of its preanoxic level in the control group and to 87% in the group treated with 500 ng/mL of fentanyl. After 6 min of anoxia, recovery was 45% in the control group, 57% in the group treated with 50 ng/mL of fentanyl, and 58% in the group treated with 500 ng/mL of fentanyl. After 7 min of anoxia, recovery was 5% in the control group and 4% in the group treated with 50 ng/mL of fentanyl. We conclude that fentanyl does not affect the recovery of the electrophysiological response in rat hippocampal neurons subjected to an anoxic insult.

IMPLICATIONS

Because fentanyl is used in large doses during surgical procedures in which the brain is at increased risk of ischemic or anoxic injury, it is important to determine its effect on such injury. Using the rat hippocampal slice model, we found fentanyl to be neither neurotoxic nor protective against anoxic injury to neurons when used in concentrations comparable to those produced in clinical practice.

Pentylenetetrazole (PTZ)-induced c-fos expression in the hippocampus of kindled rats is suppressed by concomitant treatment with naloxone

Rats were kindled by repeated injections of pentylenetetrazole (PTZ; 37.5 mg/kg; i.p.) in the presence or absence of the opioid receptor antagonist naloxone. Naloxone (10 mg/kg; i.p.) applied 30 min before each PTZ application had no major effect on the seizure development, although a slight decrease in the seizure expression of fully kindled animals could be observed. In the kindled animals, a pronounced but transient increase in c-fos mRNA level was observed in several brain areas after the injection of PTZ. The magnitude of c-fos induction was related to the seizure stage reached. Detectable c-fos mRNA levels in the cortex were observed in rats showing stage four seizures, whereas the expression of c-fos in the hippocampus required stage five kindled seizures. The induction of c-fos expression in the hippocampus of stage five kindled rats but not in other brain areas was prevented by treatment of naloxone prior to each PTZ application. In contrast, a single injection of naloxone to kindled rats was not sufficient to prevent c-fos mRNA expression in the hippocampus. In addition, a single PTZ application (at the higher dose of 45 mg/kg) to rats that were not kindled also caused c-fos expression in several brain regions, but this was not influenced by naloxone. Assuming that c-fos expression reflects neuronal activity our findings suggest a functional role of endogenous opioid peptides in the development of kindling-induced neuronal excitation in the hippocampus. In addition, the excitation of the hippocampus does not appear to be involved in the seizure activity but may be responsible for the impairment of learning in PTZ-kindled rats which can be prevented by pretreatment with naloxone [A. Becker, G. Grecksch, M. Brosz, Naloxone ameliorates the learning deficit induced by pentylenetetrazole kindling in rats, Eur. J. Neurosci. 6 (1994) 1512-1515].

Antinociception induced by stimulating amygdaloid nuclei in rats: changes produced by systemically administered antagonists

The antinociceptive effects of stimulating the medial (ME) and central (CE) nuclei of the amygdala in rats were evaluated by the changes in the latency for the tail withdrawal reflex to noxious heating of the skin. A 30-s period of sine-wave stimulation of the ME or CE produced a significant and short increase in the duration of tail flick latency. A 15-s period of stimulation was ineffective. Repeated stimulation of these nuclei at 48-h intervals produced progressively smaller effects. The antinociception evoked from the ME was significantly reduced by the previous systemic administration of naloxone, methysergide, atropine, phenoxybenzamine, and propranolol, but not by mecamylamine, all given at the dose of 1.0 mg/kg. Previous systemic administration of naloxone, atropine, and propranolol, but not methysergide, phenoxybenzamine, or mecamylamine, was effective against the effects of stimulating the CE. We conclude that the antinociceptive effects of stimulating the ME involve at least opioid, serotonergic, adrenergic, and muscarinic cholinergic descending mechanisms. The effects of stimulating the CE involve at least opioid, beta-adrenergic, and muscarinic cholinergic descending mechanisms.

Catecholaminergic CATH.a cells express predominantly delta-opioid receptors

CATH.a cells are a catecholaminergic cell line of neuronal origin. The opioid receptor complement expressed by CATH.a cells was defined pharmacologically and by reverse transcription-polymerase chain reaction (RT-PCR). CATH.a cells were found to express mRNA encoding all three of the major subtypes of opioid receptors. The relative abundance of CATH.a cell opioid receptor transcripts was delta > kappa> mu. Pharmacological and functional data were in agreement with the results of RT-PCR inasmuch as delta-opioid receptor was identified as the most abundant opioid receptor subtype expressed by CATH.a cells. In addition, at least one of the opioid signalling pathways, inhibition of adenylyl cyclase activity, was found to be operant in this cell line. CATH.a cells should be of general utility for the study of opioid receptor signalling mechanisms in the context of catecholaminergic neurons.