The differential influence of PZM21, a nonrewarding µ-opioid receptor agonist with G protein bias, on behavioural despair and fear response in mice.

A growing body of evidence points out the involvement of the µ-opioid receptors in the modulation of stress-related behaviour. It has been suggested that µ-opioid receptor agonists may attenuate behavioural despair following animals' exposure to an acute, inescapable stressor. Moreover, morphine was shown to ameliorate fear memories caused by a traumatic experience. As typical µ-opioid receptor agonists entail a risk of serious side effects and addiction, novel, possibly safer and less addictive agonists of this receptor are currently under investigation. One of them, PZM21, preferentially acting via the G protein signalling pathway, was previously shown to be analgesic, but less addictive than morphine. Here, we aimed to further test this ligand in stress-related behavioural paradigms in mice. The study has shown that, unlike morphine, PZM21 does not decrease immobility in the forced swimming and tail suspension tests. On the other hand, we observed that both mice treated with PZM21 and those receiving morphine presented a slight attenuation of freezing across the consecutive fear memory retrievals in the fear conditioning test. Therefore, our study implies that at the range of tested doses, PZM21, a nonrewarding representative of G protein-biased µ-opioid receptor agonists, may interfere with fear memory consolidation while having no beneficial effects on behavioural despair in mice.

miR-483-5p offsets functional and behavioural effects of stress in male mice through synapse-targeted repression of Pgap2 in the basolateral amygdala.

Severe psychological trauma triggers genetic, biochemical and morphological changes in amygdala neurons, which underpin the development of stress-induced behavioural abnormalities, such as high levels of anxiety. miRNAs are small, non-coding RNA fragments that orchestrate complex neuronal responses by simultaneous transcriptional/translational repression of multiple target genes. Here we show that miR-483-5p in the amygdala of male mice counterbalances the structural, functional and behavioural consequences of stress to promote a reduction in anxiety-like behaviour. Upon stress, miR-483-5p is upregulated in the synaptic compartment of amygdala neurons and directly represses three stress-associated genes: Pgap2, Gpx3 and Macf1. Upregulation of miR-483-5p leads to selective contraction of distal parts of the dendritic arbour and conversion of immature filopodia into mature, mushroom-like dendritic spines. Consistent with its role in reducing the stress response, upregulation of miR-483-5p in the basolateral amygdala produces a reduction in anxiety-like behaviour. Stress-induced neuromorphological and behavioural effects of miR-483-5p can be recapitulated by shRNA mediated suppression of Pgap2 and prevented by simultaneous overexpression of miR-483-5p-resistant Pgap2. Our results demonstrate that miR-483-5p is sufficient to confer a reduction in anxiety-like behaviour and point to miR-483-5p-mediated repression of Pgap2 as a critical cellular event offsetting the functional and behavioural consequences of psychological stress.

Acute stress modulates noradrenergic signaling in the ventral tegmental area-amygdalar circuit.

Noradrenergic neurotransmission is a critical mediator of stress responses. In turn, exposure to stress induces noradrenergic system adaptations, some of which are implicated in the etiology of stress-related disorders. Adrenergic receptors (ARs) in the ventral tegmental area (VTA) have been demonstrated to regulate phasic dopamine (DA) release in the forebrain, necessary for behavioral responses to conditional cues. However, the impact of stress on noradrenergic modulation of the VTA has not been previously explored. We demonstrate that ARs in the VTA regulate dopaminergic activity in the VTA-BLA (basolateral amygdala) circuit, a key system for processing stress-related stimuli; and that such control is altered by acute stress. We utilized fast-scan cyclic voltammetry to assess the effects of intra-VTA microinfusion of α1 -AR and α2 -AR antagonists (terazosin and RX-821002, respectively), on electrically evoked phasic DA release in the BLA in stress-naïve and stressed (unavoidable electric shocks - UES) anesthetized male Sprague-Dawley rats. In addition, we used western blotting to explore UES-induced alterations in AR protein level in the VTA. Intra-VTA terazosin or RX-821002 dose-dependently attenuated DA release in the BLA. Interestingly, UES decreased the effects of intra-VTA α2 -AR blockade on DA release (24 h but not 7 days after stress), while the effects of terazosin were unchanged. Despite changes in α2 -AR physiological function in the VTA, UES did not alter α2 -AR protein levels in either intracellular or membrane fractions. These findings demonstrate that NA-ergic modulation of the VTA-BLA circuit undergoes significant alterations in response to acute stress, with α2 -AR signaling indicated as a key target.

Recruitment of inhibitory neuronal pathways regulating dopaminergic activity for the control of cocaine seeking.

Drug seeking is associated with the ventral tegmental area (VTA) dopaminergic (DA) activity. Previously, we have shown that brief optogenetic inhibition of VTA DA neurons with 1 s pulses delivered every 9 s attenuates cocaine seeking under extinction conditions in rats without producing overt signs of dysphoria or locomotor sedation. Whether recruitment of neuronal pathways inhibiting VTA neuronal activity would suppress drug seeking remains unknown. Here, we asked if optogenetic stimulation of the lateral habenula (LHb) efferents in the rostromedial tegmental nucleus (RMTg) as well as RMTg efferents in VTA would reduce drug seeking. To investigate this, we measured how recruitment of elements of this inhibitory pathway affects cocaine seeking in male rats under extinction conditions. The effectiveness of brief optogenetic manipulations was confirmed electrophysiologically at the level of electrical activity of VTA DA neurons. Real-time conditioned place aversion (RT-CPA) and open field tests were performed to control for potential dysphoric/sedating effects of brief optogenetic stimulation of LHb-RMTg-VTA circuitry. Optogenetic stimulation of either RMTg or LHb inhibited VTA DAergic neuron firing, whereas similar stimulation of RMTg efferents in VTA or LHb efferents in RMTg reduced cocaine seeking under extinction conditions. Moreover, stimulation of LHb-RMTg efferents produced an effect that was maintained 24 h later, during cocaine seeking test without stimulation. This effect was specific, as brief optogenetic stimulation did not affect locomotor activity and was not aversive. Our results indicate that defined inhibitory pathways can be recruited to inhibit cocaine seeking, providing potential new targets for non-pharmacological treatment of drug craving.

Alpha1-adrenergic receptor blockade in the ventral tegmental area attenuates acquisition of cocaine-induced pavlovian associative learning.

Activity of the alpha1-adrenergic receptor (α1-AR) in the ventral tegmental area (VTA) modulates dopaminergic activity, implying its modulatory role in the behavioral functions of the dopamine (DA) system. Indeed, intra-VTA α1-AR blockade attenuates conditioned stimulus dependent behaviors such as drug seeking responses signifying a role of the noradrenergic signaling in the VTA in conditioned behaviors. Importantly, the role of the VTA α1-AR activity in Pavlovian associative learning with positive outcomes remains unknown. Here, we aimed to examine how intra-VTA α1-AR blockade affects acquisition of cocaine-induced Pavlovian associative learning in the conditioned place preference (CPP) paradigm. The impact of α1-AR blockade on cocaine-reinforced operant responding and cocaine-evoked ultrasonic vocalizations (USVs) was also studied. In addition, both α1-AR immunoreactivity in the VTA and its role in phasic DA release in the nucleus accumbens (NAc) were assessed. We demonstrated cellular localization of α1-AR expression in the VTA, providing a neuroanatomical substrate for the α1-AR mechanism. We showed that prazosin (α1-AR selective antagonist; 1 µg/0.5 µl) microinfusion attenuated electrically evoked DA transients in the NAc and dose-dependently (0.1-1 µg/0.5 µl) prevented the acquisition of cocaine CPP but did not affect cocaine-reinforced operant responding nor cocaine-induced positive affective state (measured as USVs). We propose that the VTA α1-AR signaling is necessary for the acquisition of Pavlovian associative learning but does not encode hedonic value. Thus, α1-AR signaling in the VTA might underlie salience encoding of environmental stimuli and reflect an ability of alerting/orienting functions, originating from bottom-up information processing to guide behaviors.

Regulation of cocaine seeking behavior by locus coeruleus noradrenergic activity in the ventral tegmental area is time- and contingency-dependent.

Substance use disorder is linked to impairments in the ventral tegmental area (VTA) dopamine (DA) reward system. Noradrenergic (NA) inputs from locus coeruleus (LC) into VTA have been shown to modulate VTA neuronal activity, and are implicated in psychostimulant effects. Phasic LC activity controls time- and context-sensitive processes: decision making, cognitive flexibility, motivation and attention. However, it is not yet known how such temporally-distinct LC activity contributes to cocaine seeking. In a previous study we demonstrated that pharmacological inhibition of NA signaling in VTA specifically attenuates cocaine-seeking. Here, we used virally-delivered opsins to target LC neurons for inhibition or excitation, delivered onto afferents in VTA of male rats seeking cocaine under extinction conditions. Optogenetic stimulation or inhibition was delivered in distinct conditions: upon active lever press, contingently with discreet cues; or non-contingently, i.e., throughout the cocaine seeking session. Non-contingent inhibition of LC noradrenergic terminals in VTA attenuated cocaine seeking under extinction conditions. In contrast, contingent inhibition increased, while contingent stimulation reduced cocaine seeking. These findings were specific for cocaine, but not natural reward (food) seeking. Our results show that NA release in VTA drives behavior depending on timing and contingency between stimuli - context, discreet conditioned cues and reinforcer availability. We show that, depending on those factors, noradrenergic signaling in VTA has opposing roles, either driving CS-induced drug seeking, or contributing to behavioral flexibility and thus extinction.

The frequency and risk factors for surgery dissatisfaction in patients undergoing lumbar or cervical surgery for degenerative spinal conditions.

This study evaluated the frequency and risk factors for surgery dissatisfaction in patients undergoing lumbar or cervical surgery for degenerative spinal conditions. Based on the Patient Satisfaction Index (PSI) at 6 months after surgery, we divided patients into two groups: a satisfied and a dissatisfied group. We evaluated the association between patient dissatisfaction and five categories of variables:1) sociodemographic; 2) preoperative pain and disability [pain duration, level of surgery, previous spinal surgeries, pain scores as measured by the Short Form McGill Pain Questionnaire (SF-MPQ), numerical rating of average pain (NRS), disability as measured by the Oswestry Disability Index (ODI)]; 3) preoperative psychological status [depression, anxiety, and overall distress as measured by the Hospital Anxiety and Depression Scale (HADS), life satisfaction as measured by the Satisfaction With Life Scale (SWLS), and surgery expectations (SE) as measured by a Likert scale]; 4) postoperative improvements in pain and disability [improvements in SF-MPQ, improvement in ODI] and 5) postoperative psychological status [HADS, SWLS]. Results showed that 17.8% patients were dissatisfied with surgery. In the multivariate logistic analysis, more negative surgery expectations, smaller improvement in ODI scores, and a greater postoperative overall distress were significant risk factors associated with patient dissatisfaction with surgery.

Astroglial Knockout of Glucocorticoid Receptor Attenuates Morphine Withdrawal Symptoms, but Not Antinociception and Tolerance in Mice.

The development of tolerance and drug dependence limit the clinical application of opioids for the treatment of severe pain. Glucocorticoid receptors (GRs) are among molecular substrates involved in these processes. Most studies focus on the role of neuronal GR, while the involvement of GR on glial cells is not fully understood. To address this issue, we used a transgenic model of conditional GR knockout mice, targeted to connexin 30-expressing astrocytes, treated with repeated doses of morphine. We observed no difference between control mice and astrocytic GR knockouts in the development of antinociceptive tolerance. Nevertheless, when animals were subjected to precipitated withdrawal, knockouts presented some attenuated symptoms, including jumping. Taken together, our data suggest that hippocampal and spinal astrocytic GRs appear to be involved in opioid withdrawal, and drugs targeting the GR may relieve some symptoms of morphine withdrawal without influencing its antinociceptive properties.

Influence of G protein-biased agonists of µ-opioid receptor on addiction-related behaviors.

Opioid analgesics remain a gold standard for the treatment of moderate to severe pain. However, their clinical utility is seriously limited by a range of adverse effects. Among them, their high-addictive potential appears as very important, especially in the context of the opioid epidemic. Therefore, the development of safer opioid analgesics with low abuse potential appears as a challenging problem for opioid research. Among the last few decades, different approaches to the discovery of novel opioid drugs have been assessed. One of the most promising is the development of G protein-biased opioid agonists, which can activate only selected intracellular signaling pathways. To date, discoveries of several biased agonists acting via µ-opioid receptor were reported. According to the experimental data, such ligands may be devoid of at least some of the opioid side effects, such as respiratory depression or constipation. Nevertheless, most data regarding the addictive properties of biased µ-opioid receptor agonists are inconsistent. A global problem connected with opioid abuse also requires the search for effective pharmacotherapy for opioid addiction, which is another potential application of biased compounds. This review discusses the state-of-the-art on addictive properties of G protein-biased µ-opioid receptor agonists as well as we analyze whether these compounds can diminish any symptoms of opioid addiction. Finally, we provide a critical view on recent data connected with biased signaling and its implications to in vivo manifestations of addiction.

Comparison of an Addictive Potential of µ-Opioid Receptor Agonists with G Protein Bias: Behavioral and Molecular Modeling Studies.

Among different approaches to the search for novel-safer and less addictive-opioid analgesics, biased agonism has received the most attention in recent years. Some µ-opioid receptor agonists with G protein bias, including SR compounds, were proposed to induce diminished side effects. However, in many aspects, behavioral effects of those compounds, as well as the mechanisms underlying differences in their action, remain unexplored. Here, we aimed to evaluate the effects of SR-14968 and SR-17018, highly G protein-biased opioid agonists, on antinociception, motor activity and addiction-like behaviors in C57BL/6J mice. The obtained results showed that the compounds induce strong and dose-dependent antinociception. SR-14968 causes high, and SR-17018 much lower, locomotor activity. Both agonists develop reward-associated behavior and physical dependence. The compounds also cause antinociceptive tolerance, however, developing more slowly when compared to morphine. Interestingly, SR compounds, in particular SR-17018, slow down the development of antinociceptive tolerance to morphine and inhibit some symptoms of morphine withdrawal. Therefore, our results indicate that SR agonists possess rewarding and addictive properties, but can positively modulate some symptoms of morphine dependence. Next, we have compared behavioral effects of SR-compounds and PZM21 and searched for a relationship to the substantial differences in molecular interactions that these compounds form with the µ-opioid receptor.

Knockdown of the astrocytic glucocorticoid receptor in the central nucleus of the amygdala diminishes conditioned fear expression and anxiety.

The amygdala is a key structure involved in both physiological and behavioural effects of fearful and stressful stimuli. The central stress response is controlled by the activity of the hypothalamic-pituitary-adrenal (HPA) axis via glucocorticoid hormones, acting mainly through glucocorticoid receptors (GR), widely expressed among different brain regions, including the central nucleus of the amygdala (CeA). Although to date, neuronal GR was postulated to be involved in the mediating stress effects, increasing evidence points to the vital role of glial GR. Here, we aimed to evaluate the role of astrocytic GR in CeA in various aspects of the stress response. We used a lentiviral vector to disrupt an astrocytic GR in the CeA of Aldh1l1-Cre transgenic mice. Astrocytic GR knockdown mice (GR KD) exhibited an attenuated expression of fear-related memory in the fear conditioning paradigm. Interestingly, the consolidation of non-stressful memory in the novel object recognition test remained unchanged. Moreover, GR KD group presented reduced anxiety, measured in the open field test. However, knockdown of astrocytic GR in the CeA did not affect an acute response to stress in the tail suspension test. Taken together, obtained results suggest that astrocytic GR in the CeA promotes aversive memory consolidation and some aspects of anxiety behaviour.

Molecular Modeling of µ Opioid Receptor Ligands with Various Functional Properties: PZM21, SR-17018, Morphine, and Fentanyl-Simulated Interaction Patterns Confronted with Experimental Data.

Molecular modeling approaches are an indispensable part of the drug design process. They not only support the process of searching for new ligands of a given receptor, but they also play an important role in explaining particular activity pathways of a compound. In this study, a comprehensive molecular modeling protocol was developed to explain the observed activity profiles of selected µ opioid receptor agents: two G protein-biased µ opioid receptor agonists(PZM21 and SR-17018), unbiased morphine, and the ß-arrestin-2-biased agonist,fentanyl. The study involved docking and molecular dynamics simulations carried out for three crystal structures of the target at a microsecond scale, followed by the statistical analysis of ligand-protein contacts. The interaction frequency between the modeled compounds and the subsequent residues of a protein during the simulation was also correlated with the output of in vitro and in vivo tests, resulting in the set of amino acids with the highest Pearson correlation coefficient values. Such indicated positions may serve as a guide for designing new G protein-biased ligands of the µ opioid receptor.

Protective role of neuronal and lymphoid cannabinoid CB2 receptors in neuropathic pain.

Cannabinoid CB2 receptor (CB2) agonists are potential analgesics void of psychotropic effects. Peripheral immune cells, neurons and glia express CB2; however, the involvement of CB2 from these cells in neuropathic pain remains unresolved. We explored spontaneous neuropathic pain through on-demand self-administration of the selective CB2 agonist JWH133 in wild-type and knockout mice lacking CB2 in neurons, monocytes or constitutively. Operant self-administration reflected drug-taking to alleviate spontaneous pain, nociceptive and affective manifestations. While constitutive deletion of CB2 disrupted JWH133-taking behavior, this behavior was not modified in monocyte-specific CB2 knockouts and was increased in mice defective in neuronal CB2 knockouts suggestive of increased spontaneous pain. Interestingly, CB2-positive lymphocytes infiltrated the injured nerve and possible CB2transfer from immune cells to neurons was found. Lymphocyte CB2depletion also exacerbated JWH133 self-administration and inhibited antinociception. This work identifies a simultaneous activity of neuronal and lymphoid CB2that protects against spontaneous and evoked neuropathic pain.

Neuropathic Pain Dysregulates Gene Expression of the Forebrain Opioid and Dopamine Systems.

Disturbances in the function of the mesostriatal dopamine system may contribute to the development and maintenance of chronic pain, including its sensory and emotional/cognitive aspects. In the present study, we assessed the influence of chronic constriction injury (CCI) of the sciatic nerve on the expression of genes coding for dopamine and opioid receptors as well as opioid propeptides in the mouse mesostriatal system, particularly in the nucleus accumbens. We demonstrated bilateral increases in mRNA levels of the dopamine D1 and D2 receptors (the latter accompanied by elevated protein level), opioid propeptides proenkephalin and prodynorphin, as well as delta and kappa (but not mu) opioid receptors in the nucleus accumbens at 7 to 14 days after CCI. These results show that CCI-induced neuropathic pain is accompanied by a major transcriptional dysregulation of molecules involved in dopaminergic and opioidergic signaling in the striatum/nucleus accumbens. Possible functional consequences of these changes include opposite effects of upregulated enkephalin/delta opioid receptor signaling vs. dynorphin/kappa opioid receptor signaling, with the former most likely having an analgesic effect and the latter exacerbating pain and contributing to pain-related negative emotional states.

Effects of brief inhibition of the ventral tegmental area dopamine neurons on the cocaine seeking during abstinence.

Preclinical studies strongly suggest that cocaine seeking depends on the neuronal activity of the ventral tegmental area (VTA) and phasic dopaminergic (DA) signaling. Notably, VTA pharmacological inactivation or dopamine receptor blockade in the forebrain may induce behavioral inhibition in general and acute aversive states in particular, thus reducing cocaine seeking indirectly. Such artifacts hinder successful translation of these findings in clinical studies and practice. Here, we aimed to evaluate if dynamic VTA manipulations effectively reduce cocaine seeking. We used male tyrosine hydroxylase (TH) IRES-Cre+ rats along with optogenetic tools to inhibit directly and briefly VTA DA neurons during conditioned stimulus (CS)-induced cocaine seeking under extinction conditions. The behavioral effects of optogenetic inhibition were also assessed in the real-time dynamic place aversion, conditioned place aversion, and CS-induced food-seeking tests. We found that brief and nondysphoric/nonsedative pulses of VTA photo-inhibition (1 s every 9 s, ie, for 10% of time) attenuated CS-induced cocaine seeking under extinction conditions in rats expressing archaerhodopsin selectively on the TH+ neurons. Furthermore, direct inhibition of the VTA DA activity reduced CS-induced cocaine seeking 24 hours after photo-modulation. Importantly, such effect appears to be selective for cocaine seeking as similar inhibition of the VTA DA activity had no effect on CS-induced food seeking. Thus, briefly inhibiting VTA DA activity during CS-induced cocaine seeking drastically and selectively reduces seeking without behavioral artifacts such as sedation or dysphoria. Our results point to the therapeutic possibilities of coupling nonpharmacologic treatments with extinction training in reducing cocaine addiction.

Astrocytes determine conditioned response to morphine via glucocorticoid receptor-dependent regulation of lactate release.

To date, neurons have been the primary focus of research on the role of glucocorticoids in the regulation of brain function and pathological behaviors, such as addiction. Astrocytes, which are also glucocorticoid-responsive, have been recently implicated in the development of drug abuse, albeit through as yet undefined mechanisms. Here, using a spectrum of tools (whole-transcriptome profiling, viral-mediated RNA interference in vitro and in vivo, behavioral pharmacology and electrophysiology), we demonstrate that astrocytes in the nucleus accumbens (NAc) are an important locus of glucocorticoid receptor (GR)-dependent transcriptional changes that regulate rewarding effects of morphine. Specifically, we show that targeted knockdown of the GR in the NAc astrocytes enhanced conditioned responses to morphine, with a concomitant inhibition of morphine-induced neuronal excitability and plasticity. Interestingly, GR knockdown did not influence sensitivity to cocaine. Further analyses revealed GR-dependent regulation of astroglial metabolism. Notably, GR knockdown inhibited induced by glucocorticoids lactate release in astrocytes. Finally, lactate administration outbalanced conditioned responses to morphine in astroglial GR knockdown mice. These findings demonstrate a role of GR-dependent regulation of astrocytic metabolism in the NAc and a key role of GR-expressing astrocytes in opioid reward processing.

Mu and delta opioid receptors play opposite nociceptive and behavioural roles on nerve-injured mice.

BACKGROUND AND PURPOSE: Mu and delta opioid receptors(MOP, DOP) contribution to the manifestations of pathological pain is not understood. We used genetic approaches to investigate the opioid mechanisms modulating neuropathic pain and its comorbid manifestations. EXPERIMENTAL APPROACH: We generated conditional knockout mice with MOP or DOP deletion in sensoryNav1.8-positive neurons (Nav1.8), in GABAergic forebrain neurons (DLX5/6) orconstitutively (CMV). Mutant mice and wild-type littermates were subjected topartial sciatic nerve ligation (PSNL) or sham surgery and their nociception wascompared. Anxiety-, depressivelike behaviour and cognitive performance were also measured. Opioid receptor mRNA expression, microgliosis and astrocytosis were assessed in the dorsalroot ganglia (DRG) and/or the spinal cord (SC). KEY RESULTS: Constitutive CMV-MOP knockouts after PSNL displayed reduced mechanical allodynia and enhanced heat hyperalgesia. This phenotype was accompanied by increased DOP expression in DRG and SC, and reduced microgliosis and astrocytosis in deep dorsal horn laminae. Conditional MOP knockouts and control mice developed similar hypersensitivity after PSNL, except for anenhanced heat hyperalgesia by DLX5/6-MOP male mice. Neuropathic pain-induced anxiety was aggravated in CMV-MOP and DLX5/6-MOP knockouts. Nerve-injured CMV-DOP mice showed increased mechanical allodynia, whereas Nav1.8-DOP and DLX5/8-DOP mice had partial nociceptive enhancement. CMV-DOP and DLX5/6-DOP mutants showed increased depressive-like behaviour after PSNL. CONCLUSIONS AND IMPLICATIONS: MOP activity after nerve injury increased anxiety-like responses involving forebrain GABAergic neurons and enhanced mechanical pain sensitivity along with repression of DOP expression and spinal cord gliosis. In contrast, DOP shows a protective function limiting nociceptive and affective manifestations of neuropathic pain.

Functional characterization of a novel opioid, PZM21, and its effects on the behavioural responses to morphine.

BACKGROUND AND PURPOSE: The concept of opioid ligands biased towards the G protein pathway with minimal recruitment of ß-arrestin-2 is a promising approach for the development of novel, efficient, and potentially nonaddictive opioid therapeutics. A recently discovered biased µ-opioid receptor agonist, PZM21, showed analgesic effects with reduced side effects. Here, we aimed to further investigate the behavioural and biochemical properties of PZM21. EXPERIMENT APPROACH: We evaluated antinociceptive effects of systemic and intrathecal PZM21 administration. Its addiction-like properties were determined using several behavioural approaches: conditioned place preference, locomotor sensitization, precipitated withdrawal, and self-administration. Also, effects of PZM21 on morphine-induced antinociception, tolerance, and reward were assessed. Effects of PZM21 on striatal release of monoamines were evaluated using brain microdialysis. KEY RESULTS: PZM21 caused long-lasting dose-dependent antinociception. It did not induce reward- and reinforcement-related behaviour; however, its repeated administration led to antinociceptive tolerance and naloxone-precipitated withdrawal symptoms. Pretreatment with PZM21 enhanced morphine-induced antinociception and attenuated the expression of morphine reward. In comparison to morphine, PZM21 administration induced a moderate release of dopamine and a robust release of 5-HT in the striatum. CONCLUSIONS AND IMPLICATIONS: PZM21 exhibited antinociceptive efficacy, without rewarding or reinforcing properties. However, its clinical application may be restricted, as it induces tolerance and withdrawal symptoms. Notably, its ability to diminish morphine reward implies that PZM21 may be useful in treatment of opioid use disorders.

Alpha1-adrenergic receptor blockade in the ventral tegmental area modulates conditional stimulus-induced cocaine seeking.

Exposure to drug-associated cues evokes drug-craving and upregulates noradrenaline (NA) and dopamine (DA) system activity. Importantly, conditional stimulus-induced drug-seeking behavior depends particularly on phasic DA signaling downstream from the ventral tegmental area (VTA), a midbrain structure key for the regulation of cocaine seeking. In particular, the activity of the alpha1-adrenergic receptor (α1-AR), which has recently been hypothesized to modulate salience encoding, is capable of bidirectional regulation of VTA dopaminergic activity. Thus, the impact of the conditional stimuli (CSs) on drug-seeking behavior might involve α1-AR signaling in the VTA. To date, the role of VTA α1-ARs in regulating CS-induced cocaine seeking has not been studied. In male Sprague-Dawley rats, we found that intra-VTA terazosin, a selective α1-AR antagonist, attenuated CS-induced cocaine seeking in a novel context and under extinction conditions, as well as CS-induced reinstatement of cocaine seeking. In contrast, terazosin microinfusion in a dose that attenuated CS-induced cocaine seeking had no effects on CS-induced food seeking or stress (2 mg/kg yohimbine)-evoked reinstatement of cocaine seeking. The potential nonspecific effects (sedative, anxiogenic) of α1-AR blockade of the VTA were also measured in the open-field test. Finally, using immunostaining, we demonstrated dopamine ß-hydroxylase (DBH)-positive afferents in the VTA of cocaine-abstinent rats, providing a neuroanatomical substrate for the α1-AR mechanism. These results demonstrated for the first time that NAergic signaling via VTA α1-ARs potently and selectively regulates CS-induced cocaine seeking. Our findings provide new neuronal mechanisms that regulate cocaine craving.

Antagonism of µ-opioid receptors reduces sensation seeking-like behavior in mice.

Novelty- and sensation-seeking behaviors induce activity of the brain reward system and are associated with increased susceptibility to drug abuse. Endogenous opioids have been implicated in reward-related behavior; however, the involvement of specific opioid receptors in the mechanism of sensation seeking is unknown. Here, we show that selective inhibition of opioid receptors reduce operant sensation seeking in mice. Administration of naltrexone (a nonselective opioid antagonist) reduced instrumental responding for sensory stimuli at one of the tested doses (2 mg/kg). More robust effects were observed in the case of cyprodime, a selective µ opioid receptor antagonist, which reduced instrumental responses by ∼50% at doses of 0.5 mg/kg and larger. Conversely, selective δ and κ receptor antagonists (naltrindole and nor-binaltorphimine, respectively) had no effect on sensation-seeking behavior. Importantly, while naltrexone produces aversion in the conditioned place preference test, cyprodime had no such effect. Therefore, reduced instrumental responding was not correlated with aversive effects of the opioid antagonists. In conclusion, our results revealed a novel mechanism of action of selective opioid receptors antagonists, which may have relevance for their efficacy in the treatment of drug abuse.