Chronic Antidepressant Treatment Causes a Selective Reduction of μ-Opioid Receptor Binding and Functional Coupling to G Proteins in the Amygdala of Fawn-Hooded Rats

Possible Involvement of µ Opioid Receptor in the Antidepressant-Like Effect of Shuyu Formula in Restraint Stress-Induced Depression-Like Rats

Recently μ opioid receptor (MOR) has been shown to be closely associated with depression. Here we investigated the action of Shuyu, a Chinese herbal prescription, on repeated restraint stress induced depression-like rats, with specific attention to the role of MOR and the related signal cascade. Our results showed that repeated restraint stress caused significant depressive-like behaviors, as evidenced by reduced body weight gain, prolonged duration of immobility in forced swimming test, and decreased number of square-crossings and rearings in open field test. The stress-induced depression-like behaviors were relieved by Shuyu, which was accompanied by decreased expression of MOR in hippocampus. Furthermore, Shuyu upregulated BDNF protein expression, restored the activity of CREB, and stimulated MEK and ERK phosphorylation in hippocampus of stressed rats. More importantly, MOR is involved in the effects of Shuyu on these depression-related signals, as they can be strengthened by MOR antagonist CTAP. Collectively, these data indicated that the antidepressant-like properties of Shuyu are associated with MOR and the corresponding CREB, BDNF, MEK, and ERK signal pathway. Our study supports clinical use of Shuyu as an effective treatment of depression and also suggests that MOR might be a target for treatment of depression and developing novel antidepressants.

Schizophrenia is primed for an increased expression of depression through activation of immuno-inflammatory, oxidative and nitrosative stress, and tryptophan catabolite pathways

Schizophrenia and depression are two common and debilitating psychiatric conditions. Up to 61% of schizophrenic patients have comorbid clinical depression, often undiagnosed. Both share significant overlaps in underlying biological processes, which are relevant to the course and treatment of both conditions. Shared processes include changes in cell-mediated immune and inflammatory pathways, e.g. increased levels of pro-inflammatory cytokines and a Th1 response; activation of oxidative and nitrosative stress (O&NS) pathways, e.g. increased lipid peroxidation, damage to proteins and DNA; decreased antioxidant levels, e.g. lowered coenzyme Q10, vitamin E, glutathione and melatonin levels; autoimmune responses; and activation of the tryptophan catabolite (TRYCAT) pathway through induction of indoleamine-2,3-dioxygenase. Both show cognitive and neurostructural evidence of a neuroprogressive process. Here we review the interlinked nature of these biological processes, suggesting that schizophrenia is immunologically primed for an increased expression of depression. Such a conceptualization explains, and incorporates, many of the current perspectives on the nature of schizophrenia and depression, and has implications for the nature of classification and treatment of both disorders. An early developmental etiology to schizophrenia, driven by maternal infection, with subsequent impact on offspring immuno-inflammatory responses, creates alterations in the immune pathways, which although priming for depression, also differentiates the two disorders.

Maternally administered sustained-release naltrexone in rats affects offspring neurochemistry and behaviour in adulthood

Naltrexone is not recommended during pregnancy. However, sustained-release naltrexone implant use in humans has resulted in cases of inadvertent foetal exposure. Here, we used clinically relevant dosing to examine the effects of maternally administered sustained-release naltrexone on the rat brain by examining offspring at birth and in adulthood. Maternal treatment (naltrexone or placebo implant) started before conception and ceased during gestation, birth or weaning. Morphometry was assessed in offspring at birth and adulthood. Adult offspring were evaluated for differences in locomotor behaviour (basal and morphine-induced, 10 mg/kg, s.c.) and opioid neurochemistry, propensity to self-administer morphine and cue-induced drug-seeking after abstinence. Blood analysis confirmed offspring exposure to naltrexone during gestation, birth and weaning. Naltrexone exposure increased litter size and reduced offspring birth-weight but did not alter brain morphometry. Compared to placebo, basal motor activity of naltrexone-exposed adult offspring was lower, yet they showed enhanced development of psychomotor sensitization to morphine. Developmental naltrexone exposure was associated with resistance to morphine-induced down-regulation of striatal preproenkephalin mRNA expression in adulthood. Adult offspring also exhibited greater operant responding for morphine and, in addition, cue-induced drug-seeking was enhanced. Collectively, these data show pronounced effects of developmental naltrexone exposure, some of which persist into adulthood, highlighting the need for follow up of humans that were exposed to naltrexone in utero.

An extension of hypotheses regarding rapid-acting, treatment-refractory, and conventional antidepressant activity of dextromethorphan and dextrorphan

It was previously hypothesized that dextromethorphan (DM) and dextrorphan (DX) may possess antidepressant properties, including rapid and conventional onsets of action and utility in treatment-refractory depression, based on pharmacodynamic similarities to ketamine. These similarities included sigma-1 (σ(1)) agonist and NMDA antagonist properties, calcium channel blockade, muscarinic binding, serotonin transporter (5HTT) inhibition, and μ receptor potentiation. Here, six specific hypotheses are developed in light of additional mechanisms and evidence. Comparable potencies to ketamine for DM and DX are detailed for σ(1) (DX>DM>ketamine), NMDA PCP site (DX>ketamine>DM), and muscarinic (DX>ketamine>>>>DM) receptors, 5HTT (DM>DX≫ketamine), and NMDA antagonist potentiation of μ receptor stimulation (DM>ketamine). Rapid acting antidepressant properties of DM include NMDA high-affinity site, NMDR-2A, and functional NMDR-2B receptor antagonism, σ(1) stimulation, putative mTOR activation (by σ(1) stimulation, μ potentiation, and 5HTT inhibition), putative AMPA receptor trafficking (by mTOR activation, PCP antagonism, σ(1) stimulation, μ potentiation, and 5HTT inhibition), and dendritogenesis, spinogenesis, synaptogenesis, and neuronal survival by NMDA antagonism and σ(1) and mTOR signaling. Those for dextrorphan include NMDA high-affinity site and NMDR-2A antagonism, σ(1) stimulation, putative mTOR activation (by σ(1) stimulation and ß adrenoreceptor stimulation), putative AMPA receptor trafficking (by mTOR activation, PCP antagonism, σ(1) stimulation, ß stimulation, and μ antagonism), and dendritogenesis, spinogenesis, synaptogenesis, and neuronal survival by NMDA antagonism and σ(1) and mTOR signaling. Conventional antidepressant properties for dextromethorphan and dextrorphan include 5HTT and norepinephrine transporter inhibition, σ(1) stimulation, NMDA and PCP antagonism, and possible serotonin 5HT1b/d receptor stimulation. Additional properties for dextromethorphan include possible presynaptic α(2) adrenoreceptor antagonism or postsynaptic α(2) stimulation and, for dextrorphan, ß stimulation and possible muscarinic and μ antagonism. Treatment-refractory depression properties include increased serotonin and norepinephrine availability, PCP, NMDR-2B, presynaptic alpha-2 antagonism, and the multiplicity of other antidepressant receptor mechanisms. Suggestions for clinical trials are provided for oral high-dose dextromethorphan and Nuedexta (dextromethorphan combined with quinidine to block metabolism to dextrorphan, thereby increasing dextromethorphan plasma concentrations). Suggestions include exclusionary criteria, oral dosing, observation periods, dose-response approaches, and safety and tolerability are considered. Although oral dextromethorphan may be somewhat more likely to show efficacy through complementary antidepressant mechanisms of dextrorphan, a clinical trial will be more logistically complex than one of Nuedexta due to high doses and plasma level variability. Clinical trials may increase our therapeutic armamentarium and our pharmacological understanding of treatment-refractory depression and antidepressant onset of action.

Association of mu-opioid receptor variants and response to citalopram treatment in major depressive disorder

OBJECTIVE

Because previous preclinical and clinical studies have implicated the endogenous opioid system in major depression and in the neurochemical action of antidepressants, the authors examined how DNA variation in the mu-opioid receptor gene may influence population variation in response to citalopram treatment.

METHOD

A total of 1,953 individuals from the Sequenced Treatment Alternatives to Relieve Depression (STAR*D) study were treated with citalopram and genotyped for 53 single nucleotide polymorphisms (SNPs) in a 100-kb region of the OPRM1 gene. The sample consisted of Non-Hispanic Caucasians, Hispanic Caucasians, and African Americans. Population stratification was corrected using 119 ancestry informative markers and principal components analysis. Markers were tested for association with phenotypes for general and specific citalopram response as well as remission.

RESULTS

Association between one SNP and specific citalopram response was observed. After Bonferroni correction, the strongest finding was the association between the rs540825 SNP and specific response. The rs540825 polymorphism is a nonsynonymous SNP in the final exon of the mu-opioid receptor-1X isoform of the OPRM1 gene, resulting in a histidine to glutamine change in the intracellular domain of the receptor. When Hispanic and Non-Hispanic Caucasians were analyzed separately, similar results in the population-corrected analyses were detected.

CONCLUSIONS

These results suggest that rates of response to antidepressants and consequent remission from major depressive disorder are influenced by variation in the mu-opioid receptor gene as a result of either an effect on placebo response or true pharmacologic response.

Direct agonist activity of tricyclic antidepressants at distinct opioid receptor subtypes

Tricyclic antidepressants (TCAs) have been reported to interact with the opioid system, but their pharmacological activity at opioid receptors has not yet been elucidated. In the present study, we investigated the actions of amoxapine, amitriptyline, nortriptyline, desipramine, and imipramine at distinct cloned and native opioid receptors. In Chinese hamster ovary (CHO) cells expressing delta-opioid receptors (CHO/DOR), TCAs displaced [3H]naltrindole binding and stimulated guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding at micromolar concentrations with amoxapine displaying the highest potency and efficacy. Amoxapine and amitriptyline inhibited cyclic AMP formation and induced the phosphorylation of signaling molecules along the extracellular signal-regulated kinase 1/2 (ERK1/2) and phosphatidylinositol-3 kinase pathways. Amoxapine also activated delta-opioid receptors in rat dorsal striatum and nucleus accumbens and human frontal cortex. In CHO cells expressing kappa-opioid receptors (CHO/KOR), TCAs, but not amoxapine, exhibited higher receptor affinity and more potent stimulation of [(35)S]GTPgammaS binding than in CHO/DOR and effectively inhibited cyclic AMP accumulation. Amitriptyline regulated ERK1/2 phosphorylation and activity in CHO/KOR and C6 glioma cells endogenously expressing kappa-opioid receptors, and this effect was attenuated by the kappa-opioid antagonist nor-binaltorphimine. In rat nucleus accumbens, amitriptyline slightly inhibited adenylyl cyclase activity and counteracted the inhibitory effect of the full kappa agonist trans-(-)-3,4dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide (U50,488). At the cloned mu-opioid receptor, TCAs showed low affinity and no significant agonist activity. These results show that TCAs differentially regulate opioid receptors with a preferential agonist activity on either delta or kappa subtypes and suggest that this property may contribute to their therapeutic and/or side effects.

Long-term administration of cocaine or serotonin reuptake inhibitors results in anatomical and neurochemical changes in noradrenergic, dopaminergic, and serotonin pathways

The catechol and indole pathways are important components underlying plasticity in the frontal cortex and basal ganglia. This study demonstrates that administering rats either cocaine or a selective serotonin (or 5-hydroxytryptamine; 5-HT) reuptake inhibitor (SSRI) for 16 weeks results in reduced density of dopaminergic and noradrenergic terminals in the striatum and olfactory bulb, respectively, reflecting pruning of the terminal arbor of ventral midbrain dopaminergic and locus coeruleus noradrenergic neurones. In the striatum of cocaine-treated animals, basal dopamine levels, as well as cocaine-induced dopamine release, is diminished compared with controls. In contrast, serotonergic fibers, projecting from the raphe, sprout and have increased terminal density in the lateral septal nucleus and frontal cortex, following long-term cocaine or SSRI treatment. This is associated with elevated basal 5-HT and enhanced cocaine-induced 5-HT release in the frontal cortex. The anatomical and neurochemical changes in serotonergic fibers following cocaine or SSRI treatment may be explained by attenuated 5-HT(1A) autoreceptor function in the raphe. This study demonstrates extensive plasticity in the morphology and neurochemistry of the catechol and indole pathways that contribute to drug-induced plasticity of the corticostriatal (and other) projections. Moreover, our data suggest that drug-induced plastic adaptation is anatomically widespread and consequently, likely to have multiple and complex consequences.

Differential brain opioid receptor availability in central and peripheral neuropathic pain

This study used positron emission tomography (PET) and [11C]diprenorphine to compare the in vivo distribution abnormalities of brain opioid receptors (OR) in patients with peripheral (n=7) and central post-stroke pain (CPSP, n=8), matched for intensity and duration. Compared with age- and sex-matched controls, peripheral neuropathic pain (NP) patients showed bilateral and symmetrical OR binding decrease, while in CPSP binding decrease predominated in the hemisphere contralateral to pain. In CPSP patients, interhemispheric comparison demonstrated a significant decrease in opioid binding in posterior midbrain, medial thalamus and the insular, temporal and prefrontal cortices contralateral to the painful side. Peripheral NP patients did not show any lateralised decrease in opioid binding. Direct comparison between the central and peripheral groups confirmed a significant OR decrease in CPSP, contralateral to pain. While bilateral binding decrease in both NP groups may reflect endogenous opioid release secondary to chronic pain, the more important and lateralised decrease specific to CPSP suggests opioid receptor loss or inactivation in receptor-bearing neurons. Opioid binding decrease was much more extensive than brain anatomical lesions, and was not co-localised with them; metabolic depression (diaschisis) and/or degeneration of OR neurons-bearing secondary to central lesions appears therefore as a likely mechanism. Central and peripheral forms of NP may differ in distribution of brain opioid system changes and this in turn might underlie their different sensitivity to opiates.

Functional interaction between α2-adrenoceptors, μ- and κ-opioid receptors in the guinea pig myenteric plexus: Effect of chronic desipramine treatment

The existence of a functional interplay between alpha(2)-adrenoceptor and opioid receptor inhibitory pathways modulating neurotransmitter release has been demonstrated in the enteric nervous system by development of sensitivity changes to alpha(2)-adrenoceptor, mu- and kappa-opioid receptor agents on enteric cholinergic neurons after chronic sympathetic denervation. In the present study, to further examine this hypothesis we evaluated whether manipulation of alpha(2)-adrenoceptor pathways by chronic treatment with the antidepressant drug, desipramine (10 mg/kg i.p. daily, for 21 days), could entail changes in enteric mu- and kappa-opioid receptor pathways in the myenteric plexus of the guinea pig distal colon. In this region, subsensitivity to the inhibitory effect of both UK14,304 and U69,593, respectively alpha(2A)-adrenoceptor and kappa-opioid receptor agonist, on the peristaltic reflex developed after chronic desipramine treatment. On opposite, in these experimental conditions, supersensitivity developed to the inhibitory effect of [D-Ala, N-Me-Phe4-Gly-ol5]-enkephalin (DAMGO), mu-opioid receptor agonist, on propulsion velocity. Immunoreactive expression levels of alpha(2A)-adrenoceptors, mu- and kappa-opioid receptors significantly decreased in the myenteric plexus of the guinea pig colon after chronic desipramine treatment. In these experimental conditions, mRNA levels of alpha(2A)-adrenoceptors, mu- and kappa-opioid receptors significantly increased, excluding a direct involvement of transcription mechanisms in the regulation of receptor expression. Levels of G protein-coupled receptor kinase 2/3 and of inhibitory G(i/o) proteins were significantly reduced in the myenteric plexus after chronic treatment with desipramine. Such changes might represent possible molecular mechanisms involved in the development of subsensitivity to UK14,304 and U69,593 on the efficiency of peristalsis. Alternative molecular mechanisms, including a higher efficiency in the coupling between receptor activation and downstream intracellular effector systems, possibly independent from inhibitory G(i/o) proteins, may be accounted for the development of supersensitivity to DAMGO. Increased sensitivity to the mu-opioid agonist might compensate for the development of alpha(2A)-adrenoceptor and kappa-opioid receptor subsensitivity. On the whole, the present data further strengthen the concept that, manipulation of alpha(2)-adrenergic inhibitory receptor pathways in the enteric nervous system entails changes in opioid inhibitory receptor pathways, which might be involved in maintaining homeostasis as suggested for mu-opioid, but not for kappa-opioid receptors.

Modulation of high alcohol drinking in the inbred Fawn-Hooded (FH/Wjd) rat strain: implications for treatment

The Fawn-Hooded rat (FH/Wjd) is an inbred alcohol-preferring rat strain, unlike most of the other strains that were selectively bred for high alcohol intake and preference. It was chosen for study some 16 years ago because of a reported mutation that disrupted platelet serotonin function. Although the FH/Wjd rat has high alcohol intake (>5 g/kg/day) and preference (>65%), interbreeding with an alcohol-non-preferring inbred strain suggested that these measures are unrelated to the serotonin abnormality. Similarly, the exaggerated immobility of the FH/Wjd rats in the forced swim test did not correlate with the high alcohol intake. Many compounds have been tested in the FH/Wjd rats after both acute and chronic treatment and a substantial number of them have proved effective. However, as the case with opiate antagonists, tolerance to the effects of the drug can develop. An up-regulation of opioid receptors accompanied the chronic treatment and this mechanism may account for the development of tolerance. Tolerance to opiate antagonists has also been demonstrated in two of the selectively bred alcohol-preferring rat lines, but it is unknown whether this process may contribute to the relapses seen in individuals being treated with naltrexone. Other drugs that reliably decrease alcohol intake in the FH/Wjd rats include the 5-hydroxytryptamine-2A receptor antagonist, amperozide, the mGlu5 receptor antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) and herbal derivatives such as ibogaine, St. John's wort and kudzu extract. Thus, studies in the FH/Wjd rat have led to the discovery of a wide variety of targets for the development of novel agents to treat alcoholism. The fact that several of these drugs were shown to reduce alcohol intake in some of the selectively bred alcohol-preferring rat lines and/or alcohol-preferring vervet monkeys increases our confidence that they are good candidates for further development.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.