Enkephalinergic System Research Articles

Pathogenesis of Functional Biliary Pain and Pharmacology of Trimebutin

Aim: to provide data on the pathogenesis of functional biliary pain and provide rationale for the use of trimebutine for this indication.Key points. Biliary pain is one of the most frequent reasons for patients to see a doctor. The diagnosis of functional disorder of the gallbladder and Oddi's sphincter is legitimate only after the exclusion of organic causes from both the gastrointestinal tract and other organs and systems. The initial appeal to surgeons with a complaint of pain in the epigastrium or right hypochondrium may lead to unjustified surgical intervention that does not bring relief to the patient's suffering. The consequences of cholecystectomy also have an anatomical and physiological justification for the occurrence or preservation of biliary pain. Currently, two main hypotheses are being considered to explain its cause: increased intraluminal pressure due to morphological and functional obstacles to bile outflow and visceral hypersensitivity. In the multilevel system of regulation of the gallbladder and sphincter apparatus, the opioid system occupies a special place. The agonist of peripheral receptors of the enkephalinergic system, trimebutine, in clinical studies led to the relief of biliary pain in more than 80 % of patients with functional biliary disorders, while a significant decrease in the severity of diarrhea, dyspeptic, and constipation syndromes was revealed.Conclusion. The prescription of the peripheral receptor agonist of the enkephalinergic system, trimebutine, is pathogenetically justified for functional biliary pain.

The role of enkephalinergic systems in substance use disorders.

Enkephalin, an endogenous opioid peptide, is highly expressed in the reward pathway and may modulate neurotransmission to regulate reward-related behaviors, such as drug-taking and drug-seeking behaviors. Drugs of abuse also directly increase enkephalin in this pathway, yet it is unknown whether or not changes in the enkephalinergic system after drug administration mediate any specific behaviors. The use of animal models of substance use disorders (SUDs) concurrently with pharmacological, genetic, and molecular tools has allowed researchers to directly investigate the role of enkephalin in promoting these behaviors. In this review, we explore neurochemical mechanisms by which enkephalin levels and enkephalin-mediated signaling are altered by drug administration and interrogate the contribution of enkephalin systems to SUDs. Studies manipulating the receptors that enkephalin targets (e.g., mu and delta opioid receptors mainly) implicate the endogenous opioid peptide in drug-induced neuroadaptations and reward-related behaviors; however, further studies will need to confirm the role of enkephalin directly. Overall, these findings suggest that the enkephalinergic system is involved in multiple aspects of SUDs, such as the primary reinforcing properties of drugs, conditioned reinforcing effects, and sensitization. The idea of dopaminergic-opioidergic interactions in these behaviors remains relatively novel and warrants further research. Continuing work to elucidate the role of enkephalin in mediating neurotransmission in reward circuitry driving behaviors related to SUDs remains crucial.

Neuroanatomical distribution of the enkephalinergic and tachykininergic systems in the alpaca brainstem: an immunohistochemical study.

A recent study has shown a close neuroanatomical relationship between the enkephalinergic (methionine-enkephalin) and tachykininergic (substance P) systems in the alpaca diencephalon. In this study, our aim is to show this relationship in the alpaca brainstem. Using an immunohistochemical technique, the distribution of immunoreactive (Ir) fibers and cell bodies containing substance P (SP) or methionine-enkephalin (MET) has been studied in the alpaca brainstem. Five adult males were used; brain tissue was fixed and processed by standard methods. SP- and MET-Ir fibers showed a widespread and similar distribution in the mesencephalon, pons and medulla oblongata. The co-localization of fibers containing SP or MET was found in most of the nuclei/tracts of the alpaca brainstem. This close neuroanatomical relationship suggests multiple physiological interactions between both neuropeptides. The distribution of the cell bodies containing SP was very restricted (cell bodies were only observed in a few nuclei located in the mesencephalon and medulla oblongata), whereas MET-Ir perikarya showed a moderately widespread distribution in the mesencephalon, pons and medulla oblongata. This study increases the knowledge on the neuroanatomical distribution/relationship of the tachykininergic (SP) and enkephalinergic (MET) systems in the alpaca central nervous system.

Management of functional gastrointestinal disorders in children. Focus on restoring intestinal microenvironment and motility

Functional gastrointestinal disorders in children are one of the most common reasons why parents visit a pediatrician. Functional gastrointestinal disorders include various persistent combinations of chronic or recurrent gastrointestinal symptoms in the absence of any organic pathology in the gastrointestinal tract (structural abnormalities, inflammatory changes, infections, or tumours) and metabolic disturbances. The article presents details on the mechanisms of development, clinical manifestations, diagnostic criteria and approaches to the treatment of functional gastrointestinal disorders. Clinical manifestations of functional gastrointestinal disorders vary depending on age and characteristics of physical and intellectual development. In more than half of children, they are observed in various combinations, less often as one isolated symptom. The drug therapy should consider the main components of pathogenesis and facilitate the solution of the following issues: improvement of digestion; restoration of intestinal microenvironment and motility; reduction of visceral hypersensitivity. Special attention is paid to the management of motor disorders and restoration of intestinal microenvironment using symbiotics. When choosing a supplement, it is advisable to focus on the composition of probiotic strains, which should comply with the WGO global guidelines (Bifidobacterium bifidum, Lactobacillus acidophilus, etc.).Prescription of peripheral δ-, μ- и κ-receptor agonists – trimebutine and pre- and probiotics combinations to children with functional gastrointestinal disorders results in decreased signs of functional disorders of the digestive tract. By acting on the enkephalinergic system, trimebutine regulates the bowel activity, improves motility, and reduces visceral hypersensitivity. Trimebutine is included in the Russian guidelines for the diagnosis and treatment of functional disorders of the digestive tract in children, as well as in the clinical guidelines for the treatment of irritable bowel syndrome, functional dyspepsia, and functional abdominal pain.

A close neuroanatomical relationship between the enkephalinergic (methionine-enkephalin) and tachykininergic (substance P) systems in the alpaca diencephalon.

In the alpaca diencephalon, the distribution of immunoreactive cell bodies and fibers containing methionine-enkephalin (MET) or substance P (SP) has been studied. The immunohistochemical study was performed by standard method on the diencephalon of four male alpacas that lived at sea level. Nerve fibers containing MET or SP were widely distributed in the thalamus and hypothalamus. METand SP-immunoreactive fibers showed a similar distribution in the whole diencephalon. Immunoreactive cell bodies containing MET or SP were only observed in the hypothalamus. The distribution of MET-immunoreactive cell bodies was more widespread than that observed for cell bodies containing SP. A close neuroanatomical relationship between the tachykininergic (SP) and enkephalinergic (MET) systems was observed in the whole diencephalon suggestive of the existence of multiple physiological interactions between both systems.

Do we understand voiding dysfunction in women? Current understanding and future perspectives: ICI-RS 2017.

Urinary retention in women is poorly understood, compared to the equivalent condition in men, and was the subject of a dedicated session organized at the International Consultation on Incontinence Research Society (ICI-RS) in Bristol, United Kingdom, 2017. The current understanding of the pathophysiological correlates of idiopathic voiding dysfunction in women, the role of urodynamics, neurophysiology, and non-invasive tests in characterizing functional bladder outlet obstruction due to a non-relaxing urethral sphincter, and poorly defined diagnosis of detrusor underactivity were reviewed. Putative factors underlying the pathogenesis of urinary retention following urinary tract infections were discussed. Further research is required to explore the association between bladder wall inflammation and alterations in detrusor contractile functions. The complex interrelationship between urinary retention and psychological co-morbidities and sexual trauma were explored, and the overlap between these and psychological co-morbidities and functional neurological problems with functional urological problems were recognized. Understanding the mechanism of action of sacral neuromodulation, often a successful treatment for urinary retention, would provide insight into the underlying factors that may be responsible for urinary retention. Specifically, there is a need to understand the role of the endogenous enkephalinergic system in the pathogenesis of urinary retention and to identify predictors of successful treatment with sacral neuromodulation. Based on current understanding and potential directions of research in the future, an algorithm for the evaluation of women with voiding dysfunction was proposed. In the future, recommendations for research should lead to a better understanding of urinary retention in women and its treatment.

Synergistic combinations of the dual enkephalinase inhibitor PL265 given orally with various analgesic compounds acting on different targets, in a murine model of cancer-induced bone pain

BackgroundThe first line pharmacological treatment of cancer pain is morphine and surrogates but a significant pain relief and a reduction of the side-effects of these compounds makes it necessary to combine them with other drugs acting on different targets. The aim of this study was to measure the antinociceptive effect on cancer-induced bone pain resulting from the association of the endogenous opioids enkephalin and non-opioid analgesic drugs. For this purpose, PL265 a new orally active single dual inhibitor of the two degrading enkephalins enzymes, neprilysin (NEP) and aminopeptidase N (APN) was used. It strictly increased the levels of enkephalin at their sites of releases. The selected non-opioid compounds are: gabapentin, A-317491 (P2X3 receptor antagonist), ACEA (CB1 receptor antagonist), AM1241 (CB2 receptor antagonist), JWH-133 (CB2 receptor antagonist), URB937 (FAAH inhibitor), and NAV26 (Nav1.7 channel blocker). MethodsExperiments. Experiments were performed in 5–6 weeks old (26–33g weight) C57BL/6 mice. Cell culture and cell inoculation. B16-F10 melanoma cells were cultured and when preconfluent, treated and detached. Finally related cells were resuspended to obtain a concentration of 2×106 cells/100μL. Then 105 cells were injected into the right tibial medullar cavity. Control mice were treated by killed cells by freezing. Behavioural studies. Thermal withdrawal latencies were measured on a unilatered hot plate (UHP) maintained at 49±0.2°C. Mechanical threshold values were obtained by performing the von Frey test using the “up and down” method. To evaluate the nature (additive or synergistic) of the interactions between PL265 and different drugs, an isobolographic analysis following the method described by Tallarida was performed. ResultsThe results demonstrate the ability of PL265, a DENKI that prevents the degradation of endogenous ENKs, to counteract cancer-induced bone thermal hyperalgesia in mice, by exclusively stimulating peripheral opioid receptors as demonstrated by used of an opioid antagonist unable to enter the brain. The development of such DENKIs, endowed with druggable pharmacokinetic characteristics, such as good absorption by oral route, can be considered as an important step in the development of much needed novel antihyperalgesic drugs. Furthermore, all the tested combinations resulted in synergistic antihyperalgesic effects. As shown here, the greatest synergistic antinociceptive effect (doses could be lowered by 70%) was produced by the combination of PL265 with the P2X3 receptor antagonist (A-317491), cannabinoid CB1 receptor agonist (exogenous, ACEA and endogenous URB937-protected-AEA) and Nav1.7 blocker (NAV26) whose mechanism of action involves the direct activation of the enkephalinergic system. ConclusionsThese multi-target-based antinociceptive strategies using combinations of non-opioid drugs with dual inhibitors of enkephalin degrading enzymes may bring therapeutic advantages in terms of efficacy and safety by allowing the reduction of doses of one of the compounds or of both, which is of the utmost interest in the chronic treatment of cancer pain. ImplicationsThis article presents synergistic antinociceptive effect produced by the combination of PL265 with non-opioid analgesic drugs acting via unrelated mechanisms. These multi-target-based antinociceptive strategies may bring therapeutic advantages by allowing the reduction of doses, which is of great interest in the chronic treatment of cancer pain.

The immune system and its modulation mechanism in scallop.

Scallops are a cosmopolitan family of bivalves, and some of them are highly prized as dominant aquaculture species. In the past decades, there have been increasing studies on the basic biology and immunology of scallops, and this review summarizes the research progresses of immune system and its modulation mechanism in scallop. As invertebrate, scallops lack adaptive immunity and they have evolved an array of sophisticated strategies to recognize and eliminate various invaders by employing a set of molecules and cells. It is evident that basic immune reactions such as immune recognition, signal transduction, and effector synthesis involved in immune response are accomplished in a variety of ways. They rely upon an extensive repertoire of phagocytosis, apoptosis and encapsulation of the circulating hemocytes for eliminating invasive pathogens, as well as the production of immune effectors that are active against a large range of pathogens or sensitive for the environmental stress. Furthermore, the molecular constitutions, metabolic pathways and immunomodulation mechanisms of the primitive catecholaminergic, cholinergic, enkephalinergic system and NO system in scallop are also discussed, which can be taken as an entrance to better understand the origin and evolution of the neuroendocrine-immune regulatory network in lower invertebrates.

Characterization of the effects of opiorphin and sialorphin and their analogs substituted in position 1 with pyroglutamic acid on motility in the mouse ileum

Opiorphin and sialorphin are two recently discovered endogenous enkephalin-degrading enzyme inhibitors. Our aim was to characterize their effect on the mouse ileum motility and to investigate the role of glutamine in position 1. Opiorphin, sialorphin, and their analogs substituted in position 1 with pyroglutamic acid (pGlu) were synthesized by the solid-phase method using Fmoc chemistry. The effect of peptides on gastrointestinal (GI) motility was characterized using in vitro assays and in mouse model of upper GI transit. Opiorphin and sialorphin, but not their analogs, significantly increased electrical field-stimulated contractions in the mouse ileum in a δ-opioid receptor-dependent manner. Opiorphin, sialorphin, and their analogs did not influence the effect of [Met(5)]enkephalin on smooth muscle contractility in the mouse ileum in vitro. [Met(5)]enkephalin and sialorphin, but not opiorphin injected intravenously (1 mg/kg), significantly inhibited the upper GI transit. The intraperitoneal administration of peptides (3 mg/kg) did not change the mouse upper GI transit. In conclusion, this is the first study investigating the effect of opiorphin and sialorphin on the mouse ileum motility and demonstrating that glutamine in position 1 is crucial for their pharmacological action. Our results may be important for further structure-activity relationship studies on opiorphin and sialorphin and future development of potent clinical therapeutics aiming at the enkephalinergic system.

Cannabinoids, Opioids and MDMA: Neuropsychological Interactions Related to Addiction

3,4-methylenedioxymethamphetamine (MDMA, "ecstasy") is an amphetamine derivative with psychostimulant properties. This substance is widely used around the world by young adults in recreational settings. One of the most remarkable characteristic of ecstasy users is the concurrent consumption of several other drugs of abuse including psychostimulants, alcohol, tobacco, LSD, cannabis and opioids. This poly-drug pattern of use is now prompting research towards understanding how the combination of MDMA with cannabis and opioids could affect neuropsychobiological processes related to addiction. As with other drugs of abuse, behavioural evidence has been presented supporting the role of the endocannabinoid system as a modulator of the rewarding/reinforcing properties of MDMA. On the other hand, the neurochemical substrate for the complex interactions between the endocannabinoid system and MDMA is poorly understood. MDMA also modulates the activity of the dynorphinergic and enkephalinergic systems in several brain structures related to addiction, as it has been shown for other psychostimulants. The work regarding the contribution of micro- and delta-opioid receptors in the rewarding effects of MDMA shows differential results in pharmacological studies in rats, with respect to studies using knock-out mice. The present review describes the behavioural and neurochemical interactions between MDMA, cannabinoids and opioids with respect to addiction processes.

CS10-02 Maternal deprivation in rat: Model of vulnerability to opiate addiciton

Aims:The concept of individual vulnerability applied to mental pathology involves genetic background and/or environmental context. We studied the impact of environmental factors such as maternal deprivation on the appearance of a vulnerability to addiction. Maternal deprivation represents an early stress, a lack of nursing occurring during a period of intense neuronal development and then could lead to long term brain functioning abnormalities. It has been recently shown that > 900 genes are stably regulated by maternal care in rat.Methods:Pups were separated from their mother and littermate the day after birth 3h/day for 14 days. Place preference and oral auto-administration tests were performed at the adulthood. Opioidergic and dopaminergic systems were studied.Results:We showed that maternal deprivation leads in adult male rats to a hypersensitivity to the rewarding and reinforcing effects of morphine correlated with a basal hypoactivity of the striatal enkephalinergic system. Maternal deprivation did not change oral self-administration behavior of cocaine and alcohol. Adolescent chronic tetrahydrocannabinol exposure suppressed morphine dependence in adult deprived rats.Conclusion:Maternal deprivation represents a highly valuable model to study environmentally triggered inter-individual vulnerability to opiate addiction. The suppression of morphine dependence in adult deprived rats after adolescent chronic tetrahydrocannabinol exposure raise the question of the beneficial effects of cannabis in opiate dependence vulnerability context.

Degradation Kinetics of Methionine5-Enkephalin by Select Brain Areas From Patients With Chronic Schizophrenia

After 10-minute incubation of [3H]-tyrosine methionine-enkephalin (MET) with 100,000 x g supernatant from select brain regions of patients with chronic schizophrenia (n = 3), essentially all of the labeled tyrosine was recovered as the free amino acid. Initial velocity and half-life of MET degradation obtained from different brain areas (limbic system, thalamus, basal ganglia, cerebellum, and cortex) of individual brains or from equivalent sections from different brains were scattered and considerable spread out (brains A, B, and C: 21.7-60.2 and 2.1-14.3, 25.6-88.7 and 1.6-14.1, 24.5-56.1 and 2.6-14.3 pg MET/mg brain tissue/min and min, respectively; brains A-C range, 21.7-88.7 pg MET/mg brain tissue/min and 1.6-14.3 min, respectively). These results failed to identify consistent differences in peptide degradation kinetics between the various brains areas studied from the same individual or from equivalent section from different subjects. MET metabolic rate was pH and temperature-dependent (optimum 7.4 degrees C and 37 degrees C), reduced by the aminopeptidase inhibitors puromycin, bacitracin, and bestatin, and to a lesser extent by thioridazine. However, peptide metabolism was not significantly affected by differences in tissue storage time or repeated freezing and thawing; by preincubation with N-carboxymethyl phenyl leucine, captopril, or thiorphan (dipeptidyl peptidase[s] or peptidyl dipeptidase[s] inhibitors, respectively); or by the many different drugs used by the patients with chronic schizophrenia. Our findings, although of a preliminary nature and generally similar to those recently reported for comparable studies on nonneuropsychiatric patients, provide a much needed understanding of the mechanisms regulating brain MET metabolism. Whether these results may contribute to the rational design of pharmacologic strategies for the treatment of pathologies associated with alterations in the enkephalinergic system needs further research.

Differential Regulation of Morphine Antinociceptive Effects by Endogenous Enkephalinergic System in the Forebrain of Mice

BackgroundMice lacking the preproenkephalin (ppENK) gene are hyperalgesic and show more anxiety and aggression than wild-type (WT) mice. The marked behavioral changes in ppENK knock-out (KO) mice appeared to occur in supraspinal response to painful stimuli. However the functional role of enkephalins in the supraspinal nociceptive processing and their underlying mechanism is not clear. The aim of present study was to compare supraspinal nociceptive and morphine antinociceptive responses between WT and ppENK KO mice.ResultsThe genotypes of bred KO mice were confirmed by PCR. Met-enkephalin immunoreactive neurons were labeled in the caudate-putamen, intermediated part of lateral septum, lateral globus pallidus, intermediated part of lateral septum, hypothalamus, and amygdala of WT mice. Met-enkephalin immunoreactive neurons were not found in the same brain areas in KO mice. Tail withdrawal and von Frey test results did not differ between WT and KO mice. KO mice had shorter latency to start paw licking than WT mice in the hot plate test. The maximal percent effect of morphine treatments (5 mg/kg and 10 mg/kg, i.p.) differed between WT and KO mice in hot plate test. The current source density (CSD) profiles evoked by peripheral noxious stimuli in the primary somatosenstory cortex (S1) and anterior cingulate cortex (ACC) were similar in WT and KO mice. After morphine injection, the amplitude of the laser-evoked sink currents was decreased in S1 while the amplitude of electrical-evoked sink currents was increased in the ACC. These differential morphine effects in S1 and ACC were enhanced in KO mice. Facilitation of synaptic currents in the ACC is mediated by GABA inhibitory interneurons in the local circuitry. Percent increases in opioid receptor binding in S1 and ACC were 5.1% and 5.8%, respectively.ConclusionThe present results indicate that the endogenous enkephalin system is not involved in acute nociceptive transmission in the spinal cord, S1, and ACC. However, morphine preferentially suppressed supraspinal related nociceptive behavior in KO mice. This effect was reflected in the potentiated differential effects of morphine in the S1 and ACC in KO mice. This potentiation may be due to an up-regulation of opioid receptors. Thus these findings strongly suggest an antagonistic interaction between the endogenous enkephalinergic system and exogenous opioid analgesic actions in the supraspinal brain structures.

Prenatal exposure to valproic acid disturbs the enkephalinergic system functioning, basal hedonic tone, and emotional responses in an animal model of autism

It has been suggested that behavioral aberrations observed in autism could be the result of dysfunction of the neuroregulatory role performed by the endogenous opioid peptides. Many of those aberrations have been recently modeled in rats exposed to valproic acid (VPA) on the 12th day of gestation (VPA rats). The aim of the present study was to elucidate functioning of the enkephalinergic system, one of the endogenous opioid peptide systems strongly involved in emotional responses, in VPA rats using both biochemical and behavioral methods. In situ hybridization was used to measure proenkephalin mRNA expression in adult VPA rats' central nucleus of the amygdala, the dorsal striatum, and the nucleus accumbens. Additional groups of animals were examined in a conditioned place aversion to naloxone, the elevated plus maze, and object recognition tests to assess their basal hedonic tone, anxiety, learning and memory, respectively. Prenatal exposure to VPA decreased proenkephalin mRNA expression in the dorsal striatum and the nucleus accumbens but not in the central nucleus of the amygdala. It also increased anxiety and attenuated conditioned place aversion to naloxone but had no impact on learning and memory. The present results suggest that prenatal exposure to VPA may lead to the decreased activity of the striatal enkephalinergic system and in consequence to increased anxiety and disregulated basal hedonic tone observed in VPA rats. Presented results are discussed in light of interactions between enkephalinergic, GABAergic, and dopaminergic systems in the striatum and mesolimbic areas of the brain.

Changes in Proenkephalin mRNA expression in forebrain areas after amphetamine-induced behavioural sensitization

Acute and repeated psychostimulant administration induces a long-lasting enhanced behavioural response to a subsequent drug challenge, known as behavioural sensitization. This phenomenon involves persistent neurophysiological adaptations, which may lead to drug addiction. Brain dopaminergic pathways have been implicated as the main neurobiological substrates of behavioural sensitization, although other neurotransmitters and neuromodulators may also participate. In order to investigate a possible involvement of opioid systems in amphetamine (AMPH) behavioural sensitization, we studied the AMPH-induced changes in Proenkephalin (Pro-Enk) mRNA expression in forebrain areas in both drug-naïve and AMPH-sensitized rats. Male Sprague–Dawley rats were sensitized to AMPH by means of a single AMPH (1 mg/kg s.c.) injection and the same dose was injected 7 days later to assess the expression of sensitization. Pro-Enk mRNA levels were evaluated by in situ hybridization in coronal brain sections. AMPH injection induced an increase in Pro-Enk mRNA expression in the nucleus accumbens and the medial–posterior caudate-putamen in drug-naïve rats. Challenge with AMPH to rats injected 1 week earlier with AMPH induced motor sensitization and increased and decreased Pro-Enk mRNA expression in the prefrontal cortex and the anterior medial caudate-putamen, respectively. Our results suggest that alterations in cortical and striatal enkephalinergic systems could contribute to the expression of AMPH behavioural sensitization.

The selective κ-opioid receptor agonist U50,488 reduces l-dopa-induced dyskinesias but worsens parkinsonism in MPTP-treated primates

Several lines of evidence demonstrate that the striatal enkephalinergic system may be involved in the development of LIDs. Preproenkephalin-B (PPE-B) transcript levels are elevated with LIDs and there are also declines in κ-opioid and other opioid receptors in different regions of the basal ganglia. If reduced κ-opioid receptors are linked to LIDs, it is possible that drugs that stimulate this subtype may decrease dyskinesias. We therefore initiated experiments to investigate the effect of κ-opioid receptor activation on LIDs. We first tested the selective κ-agonist U50,488 in rats with unilateral lesions of the nigrostriatal pathway. Chronic l-dopa treatment induced abnormal involuntary movements, including axial, orolingual and forelimb dyskinesias contralateral to the lesion. U50,488 administration prior to l-dopa treatment reduced these movements by 70%, suggesting that U50,488 has potential as an anti-dyskinetic treatment. We next tested its effect in a parkinsonian nonhuman primate model, which offers the advantage that parkinsonism and LIDs can clearly be differentiated and that the dyskinesias are similar to those in parkinsonian patients. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys were treated with l-dopa (5 mg/kg p.o.) twice daily for 3 weeks to induce dyskinesias. As in the rodent model, U50,488 (0.1–1.0 mg/kg i.m.) decreased LIDs in a dose-dependent fashion. However, the anti-parkinsonian effect of l-dopa was similarly reduced, and side effects developed, including sedation and vomiting. These data suggest that κ-opioid agonists such as U50,488 may not be clinically useful antidyskinetic agents because they also reverse the anti-parkinsonian effect of l-dopa.

Maternal deprivation specifically enhances vulnerability to opiate dependence

Maternal deprivation has been shown to increase vulnerability to morphine dependence and to disturb the enkephalinergic system in adulthood. To study whether or not this vulnerability to opiates is a specific feature, we examined oral self-administration behaviour of various reinforcing substances. Experiments were performed with morphine (25 mg/l), ethanol (10%), amphetamine (25 mg/l) and cocaine (100 mg/l). Drugs were available in a continuous free choice paradigm during 3 months. Cocaine and ethanol consumption and preference were similar in both deprived and control rats. Deprived rats greatly increased their morphine consumption and 78% of them showed a progressive decrease in morphine aversion. Only a slight, but significant, increase in oral amphetamine consumption was observed in deprived rats when compared with control rats. The difference in amphetamine self-administration in control and deprived rats cannot be explained by a modification of dopamine transporter expression measured by immunoautoradiography. Altogether, we conclude that maternal deprivation worsens inherent susceptibility to dependence, specifically for opiates, and therefore represents a highly valuable model to study environmentally triggered interindividual vulnerability to opiate addiction.

Schild (apparent pA2) analysis of a κ-opioid antagonist in Planaria

Previous investigators have provided radioimmunological and immunocytochemical evidence for an enkephalinergic (opioid) system in Planaria and described naloxone-sensitive qualitative behavioral responses to κ-opioid receptor agonists. We report the application of Schild-analysis to the antagonism of a selective κ agonist (U-50,488H) by a selective κ antagonist ( nor-BNI) in a quantitative in vivo endpoint. The results provide further evidence of a κ-opioid-like receptor in planarians.

Antisera against endogenous opioids increase the nocifensive response to formalin: demonstration of inhibitory β-endorphinergic control

The roles of endogenous opioid peptides in the brain in the modulation of nocifensive responses to formalin in ICR mice were studied. Mice were pretreated intracerebroventricularly (i.c.v.) with rabbit antiserum against β-endorphin, [Leu 5]enkephalin, [Met 5]enkephalin or dynorphin A-(1–17) 1 h prior to intraplantar injection of formalin (0.5%, 25 μl) and the nocifensive licking responses were then observed. Pretreatment of mice with antiserum against β-endorphin enhanced the second phase, but not the first phase of the nocifensive responses to formalin. Pretreatment with antiserum against [Leu 5]enkephalin also caused a small but statistically significant enhancement of the second phase, but not the first phase of nocifensive responses to formalin. On the other hand, pretreatment with antiserum against [Met 5]enkephalin or dynorphin A-(1–17) did not affect the nocifensive response to formalin. Our results indicate that β-endorphinergic, and to a lesser extent, [Leu 5]enkephalinergic systems are activated at the supraspinal sites to attenuate the nocifensive responses to formalin stimulation.

Cannabinoid Withdrawal Syndrome Is Reduced in Pre-Proenkephalin Knock-Out Mice

The functional interactions between the endogenous cannabinoid and opioid systems were evaluated in pre-proenkephalin-deficient mice. Antinociception induced in the tail-immersion test by acute Delta9-tetrahydrocannabinol was reduced in mutant mice, whereas no difference between genotypes was observed in the effects induced on body temperature, locomotion, or ring catalepsy. During a chronic treatment with Delta9-tetrahydrocannabinol, the development of tolerance to the analgesic responses induced by this compound was slower in mice lacking enkephalin. In addition, cannabinoid withdrawal syndrome, precipitated in Delta9-tetrahydrocannabinol-dependent mice by the injection of SR141716A, was significantly attenuated in mutant mice. These results indicate that the endogenous enkephalinergic system is involved in the antinociceptive responses of Delta9-tetrahydrocannabinol and participates in the expression of cannabinoid abstinence.