Anatomy of the CNS opioid systems

Neurochemistry of the mammillary body

The mammillary body (MB) is a component of the extended hippocampal system and many studies have shown that its functions are vital for mnemonic processes. Together with other subcortical structures, such as the anterior thalamic nuclei and tegmental nuclei of Gudden, the MB plays a crucial role in the processing of spatial and working memory, as well as navigation in rats. The aim of this paper is to review the distribution of various substances in the MB of the rat, with a description of their possible physiological roles. The following groups of substances are reviewed: (1) classical neurotransmitters (glutamate and other excitatory transmitters, gamma-aminobutyric acid, acetylcholine, serotonin, and dopamine), (2) neuropeptides (enkephalins, substance P, cocaine- and amphetamine-regulated transcript, neurotensin, neuropeptide Y, somatostatin, orexins, and galanin), and (3) other substances (calcium-binding proteins and calcium sensor proteins). This detailed description of the chemical parcellation may facilitate a better understanding of the MB functions and its complex relations with other structures of the extended hippocampal system.

Ethanol consumption activates a subset of arcuate nucleus pro-opiomelanocortin (POMC)-producing neurons: a c-fos immunohistochemistry study

Ethanol activates various opioid peptide-containing circuits within the brain that may underlie its motivational and rewarding effects. One component of this circuitry consists of neurons located in the arcuate nucleus (ArcN) of the hypothalamus which express pro-opiomelanocortin (POMC), an opioid precursor peptide that is cleaved to form bioactive fragments including β-endorphin and α-melanocyte stimulating hormone. In this study, we sought to determine if ethanol intake activates ArcN POMC neurons as measured by expression of the immediate early gene c-fos. Male and female POMC-EGFP mice underwent drinking-in-the-dark (DID) procedures for 3 consecutive days (2 h/day) and were allowed to consume either ethanol (20% v/v), saccharin (0.2% w/v), or water. On the fourth day of DID procedures, animals were allowed to consume their respective solutions for 20 min, and 1 h following the session brains were harvested and processed for c-fos immunohistochemistry and co-localization with EGFP. Our results indicate that ethanol intake activates a subset (~15-20%) of ArcN POMC neurons, whereas saccharin or water intake activates significantly fewer (~5-12%) of these neurons. The percent of activated POMC neurons did not correlate with blood ethanol levels at the time of tissue collection, and activation appeared to be distributed throughout the rostrocaudal axis of the ArcN. No sex differences were observed in the degree of neuronal activation across drinking solutions. These findings indicate a preferential activation of ArcN POMC neurons by ethanol consumption, strengthening the notion that ethanol activates endogenous opioid systems in the brain which may underlie its motivational properties.

Elimination of Kalrn expression in POMC cells reduces anxiety-like behavior and contextual fear learning

Kalirin, a Rho GDP/GTP exchange factor for Rac1 and RhoG, is known to play an essential role in the formation and maintenance of excitatory synapses and in the secretion of neuropeptides. Mice unable to express any of the isoforms of Kalrn in cells that produce POMC at any time during development (POMC cells) exhibited reduced anxiety-like behavior and reduced acquisition of passive avoidance behavior, along with sex-specific alteration in the corticosterone response to restraint stress. Strikingly, lack of Kalrn expression in POMC cells closely mimicked the effects of global Kalrn knockout on anxiety-like behavior and passive avoidance conditioning without causing the other deficits noted in Kalrn knockout mice. Our data suggest that deficits in excitatory inputs onto POMC neurons are responsible for the behavioral phenotypes observed.

The behavioral, anatomical and pharmacological parallels between social attachment, love and addiction

RATIONALE

Love has long been referred to as an addiction in literature and poetry. Scientists have often made comparisons between social attachment processes and drug addiction, and it has been suggested that the two may share a common neurobiological mechanism. Brain systems that evolved to govern attachments between parents and children and between monogamous partners may be the targets of drugs of abuse and serve as the basis for addiction processes.

OBJECTIVES

Here, we review research on drug addiction in parallel with research on social attachments, including parent-offspring attachments and social bonds between mating partners. This review focuses on the brain regions and neurochemicals with the greatest overlap between addiction and attachment and, in particular, the mesolimbic dopamine (DA) pathway.

RESULTS

Significant overlap exists between these two behavioral processes. In addition to conceptual overlap in symptomatology, there is a strong commonality between the two domains regarding the roles and sites of action of DA, opioids, and corticotropin-releasing factor. The neuropeptides oxytocin and vasopressin are hypothesized to integrate social information into attachment processes that is not present in drug addiction.

CONCLUSIONS

Social attachment may be understood as a behavioral addiction, whereby the subject becomes addicted to another individual and the cues that predict social reward. Understandings from both fields may enlighten future research on addiction and attachment processes.

Volume transmission of beta-endorphin via the cerebrospinal fluid; a review

There is increasing evidence that non-synaptic communication by volume transmission in the flowing CSF plays an important role in neural mechanisms, especially for extending the duration of behavioral effects. In the present review, we explore the mechanisms involved in the behavioral and physiological effects of β-endorphin (β-END), especially those involving the cerebrospinal fluid (CSF), as a message transport system to reach distant brain areas. The major source of β-END are the pro-opio-melano-cortin (POMC) neurons, located in the arcuate hypothalamic nucleus (ARH), bordering the 3^rd ventricle. In addition, numerous varicose β-END-immunoreactive fibers are situated close to the ventricular surfaces. In the present paper we surveyed the evidence that volume transmission via the CSF can be considered as an option for messages to reach remote brain areas. Some of the points discussed in the present review are: release mechanisms of β-END, independence of peripheral versus central levels, central β-END migration over considerable distances, behavioral effects of β-END depend on location of ventricular administration, and abundance of mu and delta opioid receptors in the periventricular regions of the brain.

Evolution of gnathostome prodynorphin and proenkephalin: characterization of a shark proenkephalin and prodynorphin cDNAs

Analyses of prodynorphin and proenkephalin cDNAs cloned from the central nervous system of the shark, Heterodontus portusjacksoni, provided additional evidence that these two opioid precursor-coding genes were most likely directly derived from a common ancestral gene. The two cDNAs could be aligned by inserting only seven gaps. The prodynorphin cDNA encodes five opioid sequences which could be aligned to opioid positions B through F in the proenkephalin cDNA. The sequence identity within the opioid positions was 59% at the amino acid level. Shark α-neo-endorphin, dynorphin A, and dynorphin B have amino acid motifs in common with shark met-enkephalin-8, and shark proenkephalin opioid positions E and F, respectively, which have not been observed in other gnathostome prodynorphin and proenkephalin precursor sequences. Shark prodynorphin encodes both kappa (α-neo-endorphin, dynorphin A, and dynorphin B) and delta (met-enkephalin and leu-enkephalin) opioid sequences. Mixed function prodynorphin precursors (encoding both enkephalins and dynorphins) are also found in representatives of the teleost fishes, lungfishes, and amphibians. It appears that only mammals evolved a prodynorphin precursor that exclusively encodes kappa opioid agonists (dynorphins).

New insights into the neuroanatomical distribution and phylogeny of opioids and POMC-derived peptides in fish

This review re-evaluates the use of immunological probes to map enkephalinergic, dynorphinergic, and endorphinergic circuits in the CNS of lobe-finned fishes, ray-finned fishes, and cartilaginous fishes in light of the characterization of proenkephalin, prodynorphin, and POMC sequences from representatives of these groups of fish over the past 20 years. The use of α-MSH specific antisera is a reliable method for detecting POMC immunopositive cell bodies and fibers. Since α-MSH and β-endorphin are co-localized in the same neurons, these studies also reveal the distribution of endorphinergic networks. Met-enkephalin specific antisera can be used to detect enkephalinergic circuits in the CNS of gnathostomes because of the ubiquitous presence of this pentapeptide in the proenkephalin sequences of gnathostomes. However, the use of leu-enkephalin specific antisera to detect enkephalinergic networks is more problematic. While this immunological probe is appropriate for analyzing enkephalinergic networks in mammals and perhaps teleosts, for the lungfishes and cartilaginous fishes this probe is more likely able to detect dynorphinergic circuits. In this regard, there is a need to re-examine dynorphinergic networks in non-mammalian gnathostomes by using species specific antisera directed against dynorphin end-products.

Lateralized response of dynorphin a peptide levels after traumatic brain injury

Traumatic brain injury (TBI) induces a cascade of primary and secondary events resulting in impairment of neuronal networks that eventually determines clinical outcome. The dynorphins, endogenous opioid peptides, have been implicated in secondary injury and neurodegeneration in rodent and human brain. To gain insight into the role of dynorphins in the brain's response to trauma, we analyzed short-term (1-day) and long-term (7-day) changes in dynorphin A (Dyn A) levels in the frontal cortex, hippocampus, and striatum, induced by unilateral left-side or right-side cortical TBI in mice. The effects of TBI were significantly different from those of sham surgery (Sham), while the sham surgery also produced noticeable effects. Both sham and TBI induced short-term changes and long-term changes in all three regions. Two types of responses were generally observed. In the hippocampus, Dyn A levels were predominantly altered ipsilateral to the injury. In the striatum and frontal cortex, injury to the right (R) hemisphere affected Dyn A levels to a greater extent than that seen in the left (L) hemisphere. The R-TBI but not L-TBI produced Dyn A changes in the striatum and frontal cortex at 7 days after injury. Effects of the R-side injury were similar in the two hemispheres. In naive animals, Dyn A was symmetrically distributed between the two hemispheres. Thus, trauma may reveal a lateralization in the mechanism mediating the response of Dyn A-expressing neuronal networks in the brain. These networks may differentially mediate effects of left and right brain injury on lateralized brain functions.

6β-N-heterocyclic substituted naltrexamine derivative NAP as a potential lead to develop peripheral mu opioid receptor selective antagonists

A 6β-N-heterocyclic substituted naltrexamine derivative, NAP, was proposed as a peripheral mu opioid receptor (MOR) selective antagonist based on the in vitro and in vivo pharmacological and pharmacokinetic studies. To further validate this notion, several functional assays were carried out to fully characterize this compound. In the charcoal gavage and intestinal motility assay in morphine-pelleted mice, when administered 0.3 mg/kg or higher doses up to 3 mg/kg subcutaneously, NAP significantly increased the intestinal motility compared to the saline treatment. The comparative opioid withdrawal precipitation study and the lower locomotor assay demonstrated that NAP showed only marginal intrinsic effect in the central nervous system either given subcutaneously or intravenously: no jumps were witnessed for the tested animals even given up to a dose of 50 mg/kg, while similar noticeable wet-dog shakes only occurred at the dose 50 times of those for naloxone or naltrexone, and significant reduction of the hyper-locomotion only happened at the dose as high as 32 mg/kg. Collectively, these results suggested that NAP may serve as a novel lead to develop peripheral MOR selective antagonist which might possess therapeutic potential for opioid-induced bowel dysfunction (OBD), such as opioid-induced constipation (OIC).

Which cue to 'want'? Opioid stimulation of central amygdala makes goal-trackers show stronger goal-tracking, just as sign-trackers show stronger sign-tracking

Pavlovian cues that have been paired with reward can gain incentive salience. Drug addicts find drug cues motivationally attractive and binge eaters are attracted by food cues. But the level of incentive salience elicited by a cue re-encounter still varies across time and brain states. In an animal model, cues become attractive and 'wanted' in an 'autoshaping' paradigm, where different targets of incentive salience emerge for different individuals. Some individuals (sign-trackers) find a predictive discrete cue attractive while others find a reward contiguous goal cue more attractive (location where reward arrives: goal-trackers). Here we assessed whether central amygdala mu opioid receptor stimulation enhances the phasic incentive salience of the goal-cue for goal-trackers during moments of predictive cue presence (expressed in both approach and consummatory behaviors to goal cue), just as it enhances the attractiveness of the predictive cue target for sign-trackers. Using detailed video analysis we measured the approaches, nibbles, sniffs, and bites directed at their preferred target for both sign-trackers and goal-trackers. We report that DAMGO microinjections in central amygdala made goal-trackers, like sign-trackers, show phasic increases in appetitive nibbles and sniffs directed at the goal-cue expressed selectively whenever the predictive cue was present. This indicates enhancement of incentive salience attributed by both goal trackers and sign-trackers, but attributed in different directions: each to their own target cue. For both phenotypes, amygdala opioid stimulation makes the individual's prepotent cue into a stronger motivational magnet at phasic moments triggered by a CS that predicts the reward UCS.

Photoactivatable neuropeptides for spatiotemporally precise delivery of opioids in neural tissue

Neuropeptides activate G protein-coupled receptors to acutely modulate cellular excitability and synaptic transmission. However, due to the lack of reagents for precise delivery of peptides within dense brain tissue, the spatiotemporal scale over which neuropeptides act is unknown. To achieve rapid and spatially delimited delivery of neuropeptides in mammalian brain tissue, we developed photoactivatable analogs of two opioids: [Leu⁵]-enkephalin (LE) and the 8 amino acid form of Dynorphin A (Dyn-8). These peptides are functionally inactive prior to photolysis, and exposure to ultraviolet (UV) light causes clean release of LE and Dyn-8. Recordings from acute slices of rat locus coeruleus (LC) demonstrated that photorelease of LE activates mu opioid receptor-coupled K+ channels with kinetics that approach the limits imposed by G protein-mediated signaling. Temporally precise and spatially delimited photorelease revealed the kinetics and ionic nature of the mu opioid response and the mechanisms that determine the spatial profile of enkephalinergic volume transmission in LC.

Estrogen signaling in hypothalamic circuits controlling reproduction

It is well known that many of the actions of 17β-estradiol (E2) in the central nervous system are mediated via intracellular receptor/transcription factors that interact with steroid response elements on target genes. However, there is compelling evidence for membrane steroid receptors for estrogen in hypothalamic and other brain neurons. Yet, it is not well understood how estrogen signals via membrane receptors and how these signals impact not only membrane excitability but also gene transcription in neurons that modulate GnRH neuronal excitability. Indeed, it has been known for some time that E2 can rapidly alter neuronal activity within seconds, indicating that some cellular effects can occur via membrane delimited events. In addition, E2 can affect second messenger systems including calcium mobilization and a plethora of kinases to alter cell signaling. Therefore, this review will consider our current knowledge of rapid membrane-initiated and intracellular signaling by E2 in hypothalamic neurons critical for reproductive function.

[The endogenous opioid system and drug addiction]

Drug addiction is a chronic brain disorder leading to complex adaptive changes within the brain reward circuits. Several neurotransmitters, including the endogenous opioid system are involved in these changes. The opioid system plays a pivotal role in different aspects of addiction. Thus, opioid receptors and endogenous opioid peptides are largely distributed in the mesolimbic system and modulate dopaminergic activity within the reward circuits. Opioid receptors and peptides are selectively involved in several components of the addictive processes induced by opioids, cannabinoids, psychostimulants, alcohol and nicotine. This review is focused on the contribution of each component of the endogenous opioid system in the addictive properties of the different drugs of abuse.

Ethanol injected into the hypothalamic arcuate nucleus induces behavioral stimulation in rats: an effect prevented by catalase inhibition and naltrexone

It is suggested that some of the behavioral effects of ethanol, including its psychomotor properties, are mediated by beta-endorphin and opioid receptors. Ethanol-induced increases in the release of hypothalamic beta-endorphin depend on the catalasemic conversion of ethanol to acetaldehyde. Here, we evaluated the locomotor activity in rats microinjected with ethanol directly into the hypothalamic arcuate nucleus (ArcN), the main site of beta-endorphin synthesis in the brain and a region with high levels of catalase expression. Intra-ArcN ethanol-induced changes in motor activity were also investigated in rats pretreated with the opioid receptor antagonist, naltrexone (0-2 mg/kg) or the catalase inhibitor 3-amino-1,2,4-triazole (AT; 0-1 g/kg). We found that ethanol microinjections of 64 or 128, but not 256 microg, produced locomotor stimulation. Intra-ArcN ethanol (128 microg)-induced activation was prevented by naltrexone and AT, whereas these compounds did not affect spontaneous activity. The present results support earlier evidence indicating that the ArcN and the beta-endorphinic neurons of this nucleus are necessary for ethanol to induce stimulation. In addition, our data suggest that brain structures that, as the ArcN, are rich in catalase may support the formation of ethanol-derived pharmacologically relevant concentrations of acetaldehyde and, thus be of particular importance for the behavioral effects of ethanol.

A role for brain stress systems in addiction

Drug addiction is a chronically relapsing disorder characterized by compulsion to seek and take drugs and has been linked to dysregulation of brain regions that mediate reward and stress. Activation of brain stress systems is hypothesized to be key to the negative emotional state produced by dependence that drives drug seeking through negative reinforcement mechanisms. This review explores the role of brain stress systems (corticotropin-releasing factor, norepinephrine, orexin [hypocretin], vasopressin, dynorphin) and brain antistress systems (neuropeptide Y, nociceptin [orphanin FQ]) in drug dependence, with emphasis on the neuropharmacological function of extrahypothalamic systems in the extended amygdala. The brain stress and antistress systems may play a key role in the transition to and maintenance of drug dependence once initiated. Understanding the role of brain stress and antistress systems in addiction provides novel targets for treatment and prevention of addiction and insights into the organization and function of basic brain emotional circuitry.

Hindbrain neuroglucopenia elicits site-specific transcriptional activation of glutamate decarboxylase-immunopositive neurons in the septopreoptic area of female rat brain

Recent studies implicate the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), in septopreoptic (SPO) mechanisms that suppress preovulatory pituitary luteinizing hormone (LH) secretion during neuroglucopenia. Since Fos immunolabeling of the SPO of rats treated by caudal fourth ventricular (CV4) administration of the glucose antimetabolite, 5-thioglucose (5TG), parallels the distribution of GABA neuronal perikarya, the current studies investigated the genomic responsiveness of neuroanatomically-defined populations of glutamate decarboxylase (GAD)-immunoreactive (-ir) neurons in this region of the brain to hindbrain glucoprivation. In lieu of reports that CV4 5TG enhances SPO GABA turnover via mu opioid receptor (mu-R)-dependent mechanisms and evidence that GAD- and mu-R-ir are codistributed within the SPO, patterns of cellular colocalization of these antigens were also evaluated here. Neural tissue was obtained from groups of steroid-primed ovariectomized female rats 2 h after CV4 injection of vehicle or 5TG. Neuronal cell bodies in the lateral and medial septum, medial (MPN) and median preoptic nuclei (MEPO), and rostral medial preoptic area (rMPO) were immunostained for cytoplasmic GAD-ir, but only GAD-reactive neurons in the rMPO and MEPO exhibited robust nuclear colabeling for Fos in response to 5TG. SPO GABA neurons in the vehicle-treated controls were uniformly Fos-ir-negative. Dual immunolabeling for GAD- and mu-R revealed approximately 52% and 36% colabeling of this phenotype in the MEPO and MPN, and colocalization of lesser magnitude (18%) in the rMPO. These results demonstrate site-specific genomic activation of GABAergic neurons in the female rat SPO by CV4 glucose antimetabolite administration, and implicate MEPO and rMPO GABA cell populations in neural pathways that mediate regulatory effects of hindbrain glucoprivic signaling on CNS functions, including inhibition of the steroid positive feedback-activated gonadotropin-releasing hormone/LH neuroendocrine axis. The current studies also support the view that a proportion of neuroglucoprivic-sensitive GABA neurons in the MEPO and rMPO may be direct substrates for mu-R ligand modulatory actions during this state of central substrate imbalance.

Presence of pro-opiomelanocortin mRNA in the rat medial prefrontal cortex, nucleus accumbens and ventral tegmental area: studies by RT-PCR and in situ hybridization techniques

Pro-opiomelanocortin (POMC) is a large proteic precursor which originates several biologically actives neuropeptides, such as beta-lipotropin (beta-LPH), beta-endorphin (beta-END), adenocorticotropic hormone (ACTH) and alpha-melanocyte-stimulating hormone (alpha-MSH). The arcuate nucleus of the hypothalamus is the main POMC producing cell group in brain and innervates several areas of the limbic system and brainstem. POMC-derived neuropeptides have been related to several motivated and rewarding behaviours, including sexual facilitation, feeding, and drug addiction. However, POMC mRNA has not been detected in regions of the dopaminergic mesocorticolimbic system, which represents the most important reward pathway. The aim of this work was to investigate if POMC mRNA is expressed in the medial prefrontal cortex (mPFC), the nucleus accumbens (NAcc) and the ventral tegmental area (VTA) of the rat. We used the reverse transcriptase reaction coupled to the polymerase chain reaction (RT-PCR). We also used the in situ hybridization technique to study the regional distribution of POMC mRNA in the same regions. We report that RT-PCR amplification of extracted RNA with two different pairs of primers generates the predicted 94bp and 678bp POMC-PCR products. Both the amplification of RNA obtained from the rat glial C-6 cell line (which does not express POMC mRNA) and the omission of reverse transcriptase from the RT reaction of rat brain samples showed no amplification products. We have shown for the first time that the rat medial prefrontal cortex, the nucleus accumbens and the ventral tegmental area contain POMC mRNA. This mRNA is in low concentration, ranging from 21% to 31% with respect to the hypothalamus. In situ hybridization experiments showed that POMC mRNA is homogeneously distributed in these areas. The presence of POMC mRNA in regions of the mesocorticolimbic system could have functional implications in motivated behaviours.

The opioid system in alcohol and drug dependence: family-based association study

Opioid receptors and their endogenous peptide ligands play important roles in neurotransmission and neuromodulation in response to addictive drugs such as heroin, cocaine, and alcohol. In an earlier study, we reported that variation in the genes encoding the kappa-opioid receptor (OPRK1) and its peptide ligand (PDYN) were associated with the risk for alcoholism. We continued our investigation of the role of the opioid system in alcohol dependence by analyzing the genes encoding the micro- and delta-opioid receptors and their peptide ligands. We analyzed 18 OPRM1 SNPs, 18 OPRD1 SNPs, 7 PENK SNPs, and 7 POMC SNPs in a sample of 1923 European Americans from 219 multiplex alcohol dependent families. Employing a family-based test of association, we found no evidence that these four genes were significantly associated with alcohol dependence. We also did not find association between these genes and illicit drug dependence. Secondary analyses employing the narrower phenotype of opioid dependence (83 affected individuals) demonstrated association with SNPs in PENK and POMC, but not in OPRM1 or OPRD1. Haplotype analyses provided further support for the association of PENK and POMC with opioid dependence. Therefore, our data provide no support for the idea that variations in OPRM1, OPRD1, PENK and POMC are associated with alcohol dependence or general illicit drug dependence, but variations in PENK and POMC appear to be associated with the narrower phenotype of opioid dependence in these families.

Presynaptic modulation of 5-HT release in the rat septal region

5-HT released from serotonergic axon terminals in the septal nuclei modulates the activity of septal output neurons (e.g. septohippocampal cholinergic neurons) bearing somatodendritic 5-HT receptors. Therefore, we studied the mechanisms involved in the presynaptic modulation of 5-HT release in the lateral (LS) and medial septum (MS), and the diagonal band of Broca (DB). HPLC analysis showed that tissue concentrations of noradrenaline, dopamine and 5-HT were highest in DB (DB>MS>LS). Slices prepared from LS, MS and DB regions were preincubated with [(3)H]5-HT, superfused in the presence of 6-nitro-2-(1-piperazinyl)-quinoline (6-nitroquipazine) and electrically stimulated up to three times (first electrical stimulation period (S(1)), S(2), S(3); 360 pulses, 3 Hz, 2 ms, 26-28 mA). In all septal regions the Ca(2+)-dependent and tetrodotoxin-sensitive electrically-evoked overflow of [(3)H] was inhibited by the 5-HT(1B) agonist CP-93,129 and the alpha(2)-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline tartrate (UK-14,304). Also the mu- and kappa-opioid receptor agonists (d-Ala(2), N-Me-Phe(4), glycinol(5))-enkephalin (DAMGO) and [trans-(1S,2S(-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzenacetamide hydro-chloride] (U-50,488H), respectively, acted inhibitory (although less potently), whereas the delta-opioid receptor agonist (d-Pen(2), d-Pen(5))-enkephalin (DPDPE), the dopamine D(2) receptor agonist quinpirole and the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine were all ineffective; the GABA(B) receptor agonist baclofen had weak effects. All inhibitory effects of the agonists were antagonized by the corresponding antagonists (3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride (GR-55,562), idazoxan, naloxone, nor-binaltorphimine), which also significantly enhanced the evoked release of 5-HT at S(1). It is concluded that 5-HT release in septal nuclei of the rat is modulated by presynaptic 5-HT(1B) autoreceptors, as well as by alpha(2)-, mu- and kappa-opioid heteroreceptors. All of these receptors seem to be under a tonic inhibitory influence of the corresponding endogenous agonists and show qualitatively comparable modulatory properties along the dorso-ventral distribution of the 5-HT terminals.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.

Coexpression of prodynorphin and corticotrophin-releasing hormone in the rat central amygdala: Evidence of two distinct endogenous opioid systems in the lateral division

The lateral subdivision of the central nucleus of the amygdala (CeA) comprises two groups of gamma-aminobutyric acid (GABA) neurons that express corticotrophin-releasing hormone (CRH) and enkephalin. Regulation of the expression and release of these neuropeptides by glucocorticoids and other factors has been suggested to have a regulatory function on the diverse somatic, autonomic, and neuroendocrine responses that are coordinated by the CeA. Because another opioid peptide, dynorphin, has been reported to be also expressed by neurons in the lateral CeA, this study examined the neuronal expression of this kappa-opioid (KOP) receptor-preferring ligand by using immunohistochemistry for the precursor peptide prodynorphin. Prodynorphin neurons in the extended amygdala were observed mostly in the medial and central regions of the lateral CeA and the oval of the bed nucleus of the stria terminalis (BST). About one-third of the prodynorphin neurons in the CeA coexpressed CRH, whereas no coexpression with CRH was detected in the BST. Prodynorphin was not expressed by calbindin neurons in the medial part of the lateral CeA, and indirect evidence suggested that it was not expressed by enkephalin neurons. Coexpression of prodynorphin in extrahypothalamic CRH neurons in the CeA could provide an anatomical basis for regulation of the stress responses and other CRH-related functions by the brain dynorphin/KOP receptor system.

Role of beta-endorphin, corticotropin-releasing hormone, and autonomic nervous system in mediation of the effect of chronic ethanol on natural killer cell cytolytic activity

BACKGROUND

We have recently shown that alcohol feeding suppresses natural killer (NK) cell cytolytic activity partly by decreasing the function of hypothalamic beta-endorphin (beta-EP) neurons. The neuronal mechanism by which hypothalamic beta-EP communicates with the spleen to regulate the action of ethanol on NK cells is not known. In the present study, we evaluated the roles of beta-EP neurons, corticotropin releasing hormone (CRH) neurons, and the autonomic nervous system (ANS) in regulation of the ethanol effect on splenic NK cell cytolytic function.

METHODS

Male rats were fed an ethanol-containing liquid diet or control diets. These rats were used to determine the hormone release from the paraventricular nuclei (PVN) of the hypothalamus or used to determine the splenic NK cell cytolytic function after PVN administration of CRH or intraperitoneal (i.p.) administration of a ganglionic blocker chlorisondamine. The release of hormones from the PVN was measured using the push-pull perfusion method. Splenic cytolytic activity was determined using the 4-hour (51)Cr release assay against YAC-1 lymphoma target cells.

RESULTS

Alcohol feeding decreased the amount of beta-EP but increased the amount of CRH in the push-pull perfusate (PPP) samples collected from the PVN. When exogenous beta-EP was perfused into the PVN, it suppressed the release of endogenous CRH found in PPP samples of the PVN. Conversely, perfusion of an opiate antagonist naltrexone into the PVN increased the levels of endogenous CRH in PPP samples of the PVN. In addition, administration of exogenous beta-EP in the PVN stimulated the cytolytic function of NK cells, an action that was antagonized by CRH as well as by ethanol. Corticotropin-releasing hormone and ethanol alone also had an inhibitory action on NK cells. Finally, the ganglionic blocker used prevented the effect that ethanol, beta-EP, and CRH had on NK cells. These data suggest that ethanol inhibits the function of NK cells partly by suppressing the influence of the beta-EP-CRH-ANS signal to the spleen.

Atypical anxiolytic-like response to naloxone in benzodiazepine-resistant 129S2/SvHsd mice: role of opioid receptor subtypes

RATIONALE

Mice of many 129 substrains respond to environmental novelty with behavioural suppression and high levels of anxiety-like behaviour. Although resistant to conventional anxiolytics, this behavioural phenotype may involve stress-induced release of endogenous opioids.

OBJECTIVES

To assess the effects of opioid receptor blockade on behavioural reactions to novelty stress in a chlordiazepoxide-resistant 129 substrain.

MATERIALS AND METHODS

Experiment 1 contrasted the effects of the broad-spectrum opioid receptor antagonist naloxone (1.0-10.0 mg/kg) in C57BL/6JOlaHsd and 129S2/SvHsd mice exposed to the elevated plus-maze. Experiments 2-4 examined the responses of 129S2/SvHsd mice to the mu-selective opioid receptor antagonist beta-funaltrexamine (2.5-10.0 mg/kg), the delta-selective antagonist naltrindole (2.5-10.0 mg/kg) and the kappa-selective antagonist nor-binaltorphimine (2.5-5.0 mg/kg).

RESULTS

129 mice displayed higher levels of anxiety-like behaviour and lower levels of general exploration relative to their C57 counterparts. Although naloxone failed to alter the behaviour of C57 mice, both doses of this antagonist produced behaviourally selective reductions in open-arm avoidance in 129 mice. Surprisingly, none of the more selective opioid receptor antagonists replicated this effect of naloxone: beta-funaltrexamine was devoid of behavioural activity, naltrindole suppressed rearing (all doses) and increased immobility (10 mg/kg), while nor-binaltorphimine (5 mg/kg) nonspecifically increased percent open arm entries.

CONCLUSIONS

Recent evidence suggests differential involvement of opioid receptor subtypes in the anxiolytic efficacy of diverse compounds including conventional benzodiazepines. The insensitivity of 129 mice to the anxiolytic action of chlordiazepoxide, coupled with their atypical anxiolytic response to naloxone (but not more selective opioid receptor antagonists), suggests an abnormality in anxiety-related neurocircuitry involving opioid-GABA interactions.

The role of opioid receptor subtypes in the development of behavioral sensitization to ethanol

Nonspecific blockade of opioid receptors has been found to prevent development of behavioral sensitization to ethanol. Whether this effect is achieved through a specific opioid receptor subtype, however, is not clear. The present study investigated, for the first time, the role of specific opioid receptor subtypes in the development of ethanol-(2.5 g/kg/day; six sessions) induced locomotor sensitization in mice. We confirmed previous results showing that the nonspecific antagonism of opioid receptors (naltrexone; 0-2 mg/kg) prevented the development of behavioral sensitization to ethanol, an effect attained at doses presumed to occupy only mu opioid receptors. This was confirmed by using the selective mu opioid receptor antagonist CTOP (0-1.5 mg/kg), which also blocked sensitization to ethanol. The selective delta receptor antagonist, naltrindole (0-10 mg/kg), however, did not alter sensitization. We further assessed the role of mu opioid receptors in sensitization to ethanol by exploring the involvement of mu(1), mu(1+2), and mu(3) opioid receptor subtypes. Results of these experiments revealed that the blockade of mu(1) (naloxonazine; 0-30 mg/kg) or mu(3) opioid receptors (3-methoxynaltrexone; 0-6 mg/kg) did not prevent locomotor sensitization to ethanol. Using naloxonazine under treatment conditions that block mu(1+2) opioid receptor subtypes we observed a retarded sensitization. The present data suggest that the concurrent inactivation of all mu opioid receptor subtypes may be required to prevent the neural adaptations underlying the development of behavioral sensitization to ethanol. In addition, these results support previous data suggesting a putative role for the mu opioid receptor endogenous ligand, beta-endorphin, and the hypothalamic arcuate nucleus in ethanol sensitization.

Reproduction phase-related expression of beta-endorphin-like immunoreactivity in the nucleus lateralis tuberis of the female Indian major carp Cirrhinus mrigala: correlation with the luteinising hormone cells-ovary axis

The present study aimed to determine whether beta-endorphin immunoreactivity (bEP-ir) in the neurones of the nucleus lateralis tuberis (NLT) is linked to the seasonal cycle and shows correlation with the number of luteinising hormone (LH) cells in the pituitary gland and ovaries in the teleost, Cirrhinus mrigala. Although LH cells were moderately immunostained during the resting phase (December to January), the morphological profile suggested increased synthetic and secretory activity during the preparatory (February to April) and prespawning (May to June) phases. However, LH immunoreactivity was greatly reduced (P < 0.001) in the spawning (July to August) phase, suggesting massive discharge of the hormone; this pool was partly replenished in the postspawning (September to November) phase. The ovaries grew rapidly in the preparatory and prespawning phases; maximal size was attained during spawning, when ovulation occurred. Thereafter, the ovaries regressed. The NLT of C. mrigala is divisible into the pars lateralis (NLTl) and medialis (NLTm). During the postspawning and resting phases, bEP-ir was readily detectable in the NLTm as well as NLTl neurones. However, a steady reduction in the immunoreactivity was observed in the NLTm neurones during the preparatory through spawning phases (P < 0.001), suggesting a negative correlation with the LH cells-ovary axis. Thus, the inhibitory influence of beta-endorphin on the gonadotrophin-releasing hormone (GnRH)-LH axis appears to be attenuated during the preparatory through spawning phases. This may be necessary for the rapid stimulation of the axis culminating in spawning. Neurones of the NLTl also showed a gradual reduction in bEP-ir during the preparatory and prespawning phases (P < 0.01) and may therefore play a similar role. However, significant augmentation of the immunoreactivity was noticed in these neurones during the spawning phase (P < 0.001), the physiological significance of which is unknown. Although the present study demonstrated a temporal correlation between the beta-endorphin in the NLT, LH cells and the ovary, we suggest that the peptide in the NLTl and NLTm may show functional duality during the spawning phase.

Trends in the Evolution of the Proenkephalin and Prodynorphin Genes in Gnathostomes

The opioid/orphanin gene family provides a model system for analyzing the outcomes of genome duplication events. Recent studies on the proenkephalin gene provide additional evidence that the organizational plan for this gene has been conserved throughout the extensive radiation of the gnathostome vertebrates. However, an analysis of the amino acid sequence of proenkephalin from the zebrafish, Danio rerio, suggests that novel forms of this opioid precursor may be evolving in teleosts. Analyses of sarcopterygian prodynorphin sequences revealed a proenkephalin signature in prodynorphin. Current studies on the opioid/orphanin gene family point to the duplication events that shaped this family occurring prior to the radiation of the gnathostomes.

Estrogen differentially modulates the cannabinoid- induced presynaptic inhibition of amino acid neurotransmission in proopiomelanocortin neurons of the arcuate nucleus

The present study sought to determine whether cannabinoids inhibit glutamatergic and GABAergic synaptic input onto neurons of the hypothalamic arcuate nucleus (ARC), and whether estrogen modulates this process. Whole-cell patch clamp recordings were performed in hypothalamic slices prepared from ovariectomized female guinea pigs. CB1 receptor activation reduced the amplitude of excitatory postsynaptic currents (EPSCs) evoked by electrical stimulation that were sensitive to ionotropic glutamate receptor antagonists. The CB1 receptor antagonist AM251 increased evoked EPSC (eEPSC) amplitude, and reversed the agonist-induced decrease. CB1 receptor activation similarly decreased the amplitude of evoked inhibitory postsynaptic currents (eIPSCs). The cannabinoid-induced reduction in eEPSC and eIPSC amplitude correlated with a decrease in the frequency of miniature EPSCs (mEPSCs) and IPSCs (mIPSCs) that were abolished by ionotropic glutamate and GABA(A) receptor antagonists, respectively. AM251 increased mEPSC frequency, and antagonized the agonist-induced decrease. Compared to neurons obtained from vehicle-treated controls, estradiol benzoate (25 mug; s.c.) given 24 h prior to experimentation increased mEPSC frequency, and markedly decreased the potency of CB1 receptor agonists to decrease mEPSC frequency. Conversely, the steroid potentiated the cannabinoid-induced decrease in mIPSC frequency. These effects were observed in neurons subsequently identified as proopiomelanocortin (POMC) neurons. These data reveal that ARC neurons, including POMC neurons, receive glutamatergic and GABAergic synaptic inputs that are presynaptically inhibited by cannabinoids, and differentially modulated by estrogen. These opposing effects of estrogen on the cannabinoid regulation of amino acid neurotransmission excite POMC neurons, and lend additional insight into the mechanisms underlying estrogen-induced anorexia and negative feedback of the reproductive axis.

A Microdialysis Profile of Met-Enkephalin Release in the Rat Nucleus Accumbens Following Alcohol Administration

BACKGROUND

Pharmacological studies have implicated the endogenous opioid system in mediating alcohol intake. Other evidence has shown that alcohol administration can influence opioid activity. In this regard, the majority of studies have concentrated on endorphinergic systems, whereas other opioid systems have been granted comparably less attention. This is the case despite some compelling evidence that has implicated enkephalinergic peptide systems, particularly Met-enkephalin, in mediating alcohol preference. The aim of the present study was to investigate the effect of alcohol administration on extracellular levels of Met-enkephalin in the rat nucleus accumbens--a brain region that plays a significant role in the processes underlying reinforcement and stress.

METHODS

Male Sprague-Dawley rats were implanted with a microdialysis probe aimed at the shell region of the nucleus accumbens. Artificial cerebrospinal fluid was pumped at a rate of 1.75 mul/min in awake and freely moving rats and dialysates were collected at 30-minute intervals. After several baseline collections, rats were injected intraperitoneally with either physiological saline or one of four doses of alcohol: 0.8, 1.6, 2.4, or 3.2 g/kg ethanol body weight. The levels of Met-enkephalin in the dialysates were analyzed with solid-phase radioimmunoassay.

RESULTS

Within the first 30 minutes of administration, an alcohol dose of 1.6 g/kg caused a significant and prolonged elevation in the extracellular levels of Met-enkephalin. Alcohol did not have a major effect on the release of Met-enkephalin at any other dose.

CONCLUSIONS

In this experiment, only a moderate dose of alcohol was capable of stimulating Met-enkephalin release in the nucleus accumbens. Enkephalins may modulate local neurotransmitter release by binding to presynaptic Delta-opioid receptors, or, they may inhibit effector cells by binding to postsynaptic Delta- or mu-opioid receptors. This may be one of multiple neurological mechanisms that modulate alcohol-drinking behavior.

Levels and circadian rhythmicity of plasma ACTH, cortisol, and beta-endorphin as a function of family history of alcoholism

RATIONALE

Individuals with a family history of alcoholism may present a dysfunction in the activity of the hypothalamic-pituitary-adrenal (HPA) axis that predates the development of alcoholism.

OBJECTIVE

The present study investigated the hypothesis that this HPA-axis dysfunction is associated with alterations in the pattern of the circadian (24 h) secretions of adrenal corticotropic hormone (ACTH), cortisol, and beta-endorphin.

METHODS

Men with [high risk (HR)] or without [low risk (LR)] family history of alcoholism participated in the study. Blood samples were drawn every 30 min for 24 h for estimation of the plasma hormone levels. Participants ingested meals at predetermined intervals and filled out mood questionnaires prior to the placement of the catheter and 1 h after each meal.

RESULTS

The circadian peaks for beta-endorphin, ACTH, and cortisol occurred between 0800 and 0830 hours in both LR and HR participants. The plasma ACTH and beta-endorphin concentrations were lower in HR than LR participants, while the plasma cortisol concentrations were similar between HR and LR participants. For each hormone, the total 24-h secretion was estimated from the area under the 24-h time-concentration curve (AUC). For ACTH and beta-endorphin, but not the cortisol, AUC were lower in HR than LR participants. LR participants reported being more nervous than HR participants. For the LR participants, but not HR participants, the initial mood ratings of "nervous" were positively correlated with the initial plasma cortisol and beta-endorphin concentrations as well as with the cortisol and beta-endorphin AUC.

CONCLUSIONS

HR participants presented lower plasma concentrations as well as lower AUC for beta-endorphin and ACTH but not for cortisol. This suggests a dysfunction of the HPA-axis in HR participants that predates the development of alcoholism and a dissociation between plasma ACTH and cortisol levels as a function of family history of alcoholism.

Hypothalamic resistin immunoreactivity is reduced by obesity in the mouse: co-localization with alpha-melanostimulating hormone

Resistin is a new adipokine expressed in mouse, rat and human adipose tissue. Resistin may be an important link between obesity and insulin resistance, though this controversial view is complicated by the discovery of multiple sites of resistin expression, including human macrophages, placenta and pancreas. In previous studies we demonstrated that the mouse hypothalamo-pituitary system was also a site of resistin production. Pituitary resistin is developmentally regulated, reduced in the ob/ob mouse and severely down-regulated by food deprivation (24 h). An unexpected finding was that hypothalamic resistin mRNA remained unaffected by fasting. The present experiments examined the localization and possible regulation of hypothalamic resistin protein. Using immunohistochemistry we observed a complex network of resistin+ fibres extending rostrally from the arcuate nucleus of the hypothalamus (ARC) to the preoptic area. Labelled cell bodies occurred only in the ARC and in a periventricular region of the dorsal hypothalamus. Hypothalamic resistin immunoreactivity (ir) was unaffected by fasting (48 h) or by a high fat diet, but the periventricular staining was greatly increased in the lactating mouse. Marked reductions in resistin+ fibres were seen in brain tissue from: (a) ob/ob mice, (b) young mice made underweight for their age by raising them in large litters (20 pups per litter) and (c) mice with hypothalamic lesions induced by monosodium glutamate (MSG) or gold thioglucose (GTG). We speculate that the resistin-ir deficit in genetically obese mice, and in severely underweight mice, could be due to low or absent leptin. In contrast, though MSG- and GTG-treated mice have high levels of circulating leptin, in the presence of excessive visceral fat deposits, we hypothesize that damage to the ARC destroys the resistin+ cell bodies. This latter supposition led us to an additional hypothesis, that resistin-ir would be contained in neurons expressing the proopiomelanocortin (POMC) gene. This proved to be correct. Double label immunofluorescence histochemistry revealed that alpha-MSH-ir, a marker for POMC neurons, was co-localized with resistin-ir. In conclusion, our data reveal a second example of an adipocytokine co-localized with a hypothalamic neuropeptide. We reported previously that leptin was co-localized with oxytocin and vasopressin. RT-PCR analysis confirmed that resistin mRNA is readily detectable in ARC, but further work is required to determine whether the resistin gene is expressed in POMC neurons or if resistin is specifically accumulated by these cells. Nonetheless, our data suggest that the hypothalamus is a target tissue for resistin.

Catalase inhibition in the Arcuate nucleus blocks ethanol effects on the locomotor activity of rats

Previous studies have demonstrated that there is a bidirectional modulation of ethanol-induced locomotion produced by drugs that regulate brain catalase activity. In the present study we have assessed the effect in rats of intraperitoneal, intraventricular or intracraneal administration of the catalase inhibitor sodium azide in the locomotor changes observed after ethanol (1 g/kg) administration. Our results show that sodium azide prevents the effects of ethanol in rats locomotion not only when sodium azide was systemically administered but also when it was intraventricularly injected, then confirming that the interaction between catalase and ethanol takes place in Central Nervous System (CNS). Even more interestingly, the same results were observed when sodium azide administration was restricted to the hypothalamic Arcuate nucleus (ARC), a brain region which has one of the highest levels of expression of catalase. Therefore, the results of the present study not only confirm a role for brain catalase in the mediation of ethanol-induced locomotor changes in rodents but also point to the ARC as a major neuroanatomical location for this interaction. These results are in agreement with our reports showing that ethanol-induced locomotor changes are clearly dependent of the ARC integrity and, especially of the POMc-synthesising neurons of this nucleus. According to these data we propose a model in which ethanol oxidation via catalase could produce acetaldehyde into the ARC and to promote a release of beta-endorphins that would activate opioid receptors to produce locomotion and other ethanol-induced neurobehavioural changes.

The opioid system in the gastrointestinal tract

Mu-, delta- and kappa-opioid receptors (ORs) mediate the effects of endogenous opioids and opiate drugs. Here we report (1) the distribution of muOR in the guinea-pig and human gastrointestinal tract in relation to endogenous ligands, to functionally distinct structures in the gut and to deltaOR and kappaOR; and (2) the ligand-induced muOR endocytosis in enteric neurones using in vitro and in vivo models. In the guinea pig, muOR immunoreactivity is confined mainly to the myenteric plexus. MuOR myenteric neurones are most numerous in the small intestine, followed by the stomach and the proximal colon. MuOR immunoreactive fibres are dense in the muscle layer and the deep muscular plexus, where they are in close association with interstitial cells of Cajal. This distribution closely matches the pattern of enkephalin. MuOR enteric neurones comprise functionally distinct populations of neurones of the ascending and descending pathways of the peristaltic reflex. In human gut, muOR immunoreactivity is localized to myenteric and submucosal neurones and to immune cells of the lamina propria. DeltaOR immunoreactivity is located in both plexuses where it is predominantly in varicose fibres in the plexuses, muscle and mucosa, whereas kappaOR immunoreactivity appears to be confined to the myenteric plexus and to bundles of fibres in the muscle. MuOR undergoes endocytosis in a concentration-dependent manner, in vitro and in vivo. Pronounced muOR endocytosis is observed in neurones from animals that underwent abdominal surgery that has been shown to induce delay in gastrointestinal transit. We can conclude that all three ORs are localized to the enteric nervous system with differences among species, and that muOR endocytosis can be utilized as a means to visualize enteric neurones activated by opioids and sites of opioid release.

Opposite effects of acute versus chronic naltrexone administration on ethanol-induced locomotion

Several studies have pointed out that the mu opioid receptor (MOR) can play a key role in some of the behavioural effects of ethanol. In the present study, the implication of the MOR in ethanol-induced locomotion in mice was assessed. First, the effects of the administration of different naltrexone doses (0.001-1.000 mg/kg) on the locomotor changes produced by ethanol (2.5 g/kg) were evaluated. In a second set of experiments, the ability of repeated naltrexone (6 mg/kg) administrations to modify the effects of ethanol was also assessed on mice locomotion. The results of the present study revealed that an acute naltrexone administration reduced dose-dependently ethanol-induced locomotion. Conversely, after repeated naltrexone injections, a transient boost of ethanol induced locomotor activity was observed. Thus, the results of the present study revealed that the effects of these naltrexone pretreatments on ethanol-induced locomotion are similar to the previously described changes on MOR activity. Moreover, the same (acute and chronic) naltrexone pretreatments produced similar changes on the locomotion of mice after a challenge with morphine (a MOR agonist), but not after tert-butanol (an alcohol which does not release beta-endorphins) administration. Therefore, our results are discussed in terms of the proved ability of ethanol to promote the release of beta-endorphins and, consequently, to activate the MOR.

The delta agonists DPDPE and deltorphin II recruit predominantly mu receptors to produce thermal analgesia: a parallel study of mu, delta and combinatorial opioid receptor knockout mice

Delta-selective agonists have been developed to produce potent analgesic compounds with limited side-effects. DPDPE and deltorphin II are considered prototypes, but their delta-selectivity in vivo and the true ability of delta receptors to produce analgesia remain to be demonstrated. Here we have performed a parallel analysis of mu, delta and combinatorial opioid receptor knockout mice, in which we found no obvious alteration of G-protein coupling for remaining opioid receptors. We compared behavioural responses in two models of acute thermal pain following DPDPE and deltorphin II administration by intracerebroventricular route. In the tail-immersion test, both compounds were fully analgesic in delta knockout mice and totally inactive in mu knockout mice. In the hotplate test, the two compounds again produced full analgesia in delta knockout mice. In mu knockout mice, there was significant, although much lower, analgesia. Furthermore, DPDPE analgesia in the delta knockout mice was fully reversed by the mu selective antagonist CTOP in both tests. Together, this suggests that mu rather than delta receptors are recruited by the two agonists for the tail withdrawal and the hotplate responses. Finally, deltorphin II slightly prolonged jump latencies in double mu/kappa knockout mice (delta receptors only) and this response was abolished in the triple knockout mice, demonstrating that the activation of delta receptors alone can produce weak but significant mu-independent thermal antinociception.

Minireview: Characterization of influence of central nociceptin/orphanin FQ on consummatory behavior

Nociceptin/orphanin FQ (N/OFQ), a peptide closely related to dynorphin A, is the endogenous agonist of the NOP receptor that moderately increases food intake under various conditions. Its orexigenic properties are mediated by the brain circuitry. In the present review, we focus on discussing the nature of hyperphagic effects of N/OFQ with special emphasis on its function within feeding-related neural networks. Although some of N/OFQ's orexigenic effects resemble those induced by opioids, reward-dependent feeding appears to be affected in a different manner by agonists of the NOP and classical opioid receptors. Also, data suggest that N/OFQ may not only promote feeding initiation, but rather its role may be to inhibit signaling responsible for inhibition of consummatory behavior. Central systems involved in termination of feeding that seem to be influenced by N/OFQ encompass oxytocin, alpha-MSH, and CRH.

Opioids and alcoholism

Although far from conclusive, evidence implicating the endogenous opioid system in the development and maintenance of alcoholism is growing. Currently available data suggest that ethanol increases opioid neurotransmission and that this activation is part of the mechanism responsible for its reinforcing effects. Findings from preclinical research indicate that ethanol consumption and ethanol-induced dopamine (DA) release are both reduced by opioid antagonists. Individual differences in endogenous opioid activity have been linked to inherited risks for alcoholism in studies comparing ethanol-preferring and nonpreferring rats, as well as in studies using targeted gene mutation (knockout) strategies. To a large extent, findings from human studies have paralleled those from the preclinical work. Persons who differ in family history of alcoholism have been shown to also differ in basal beta-endorphin activity, beta-endorphin response to alcohol, and subjective and HPA axis hormonal response to opioid antagonists. Findings from clinical trials indicate that opioid antagonists may reduce ethanol consumption in alcoholics, particularly in persons who have resumed drinking. Nevertheless, many questions remain unanswered about the use of opioid antagonists in alcoholism treatment and about the exact role of the opioid system in ethanol preference and reward. The progression of knowledge in this field suggests that many of these questions are imminently answerable, as our ability to characterize relationships between opioid activity and human behavior continues to develop. This paper summarizes both the progress that has been made and the gaps that remain in our understanding of the interactions between the endogenous opioid system and risk for alcoholism.