Activation of mu-opioid receptors in the ventrolateral orbital cortex inhibits the GABAergic miniature inhibitory postsynaptic currents in rats

Ethnicity-dependent effect of rs1799971 polymorphism on OPRM1 expression in the postmortem brain and responsiveness to antipsychotics

Schizophrenia is associated with aberration of inhibitory neurons. Although the mu-opioid receptor (MOR) is an essential modulator of inhibitory neurons, the effect of rs1799971 polymorphism in the MOR gene on risk of schizophrenia is controversial. Moreover, the disturbance of opioids systems in patients with schizophrenia has not been fully examined. We firstly conducted preliminary meta-analyses integrating Asian and European populations separately over 12,000 subjects to assess the effect of rs1799971 on risk of schizophrenia. Based on the above result, we also investigated the effect on the expression levels of MOR mRNA in the prefrontal cortex (PFC) and caudate nucleus of 41 postmortem brains. In addition, we determined whether these levels were related to antemortem schizophrenia symptoms and pharmacotherapeutic effects. The rs1799971 G-allele reduced the risk of schizophrenia in Asian populations (OR: 0.56, 95%CI: 0.32-0.98, p = 0.042) but increased it in European populations (OR: 1.66, 95%CI: 1.08-2.56, p = 0.022). It decreased MOR mRNA levels in PFC in the Japanese population (p = 0.031). Increased MOR mRNA level in PFC correlated with higher total score of antemortem schizophrenia symptoms (p = 0.017). Furthermore, the pharmacotherapeutic effect of first-generation antipsychotics was higher for genotype AA than AG/GG of rs1799971 (p = 0.036). The rs1799971 affects risk of schizophrenia and MOR mRNA expression and the effect varies according to ethnicity. Overexpression of MOR might induce severe schizophrenia symptoms. Therefore, MOR modulation may be the key clue for treating antipsychotics-resistant schizophrenia, and genotyping rs1799971 may provide a better pharmacotherapeutic strategy.

Shared Mechanisms of GABAergic and Opioidergic Transmission Regulate Corticolimbic Reward Systems and Cognitive Aspects of Motivational Behaviors

The functional interplay between the corticolimbic GABAergic and opioidergic systems plays a crucial role in regulating the reward system and cognitive aspects of motivational behaviors leading to the development of addictive behaviors and disorders. This review provides a summary of the shared mechanisms of GABAergic and opioidergic transmission, which modulate the activity of dopaminergic neurons located in the ventral tegmental area (VTA), the central hub of the reward mechanisms. This review comprehensively covers the neuroanatomical and neurobiological aspects of corticolimbic inhibitory neurons that express opioid receptors, which act as modulators of corticolimbic GABAergic transmission. The presence of opioid and GABA receptors on the same neurons allows for the modulation of the activity of dopaminergic neurons in the ventral tegmental area, which plays a key role in the reward mechanisms of the brain. This colocalization of receptors and their immunochemical markers can provide a comprehensive understanding for clinicians and researchers, revealing the neuronal circuits that contribute to the reward system. Moreover, this review highlights the importance of GABAergic transmission-induced neuroplasticity under the modulation of opioid receptors. It discusses their interactive role in reinforcement learning, network oscillation, aversive behaviors, and local feedback or feedforward inhibitions in reward mechanisms. Understanding the shared mechanisms of these systems may lead to the development of new therapeutic approaches for addiction, reward-related disorders, and drug-induced cognitive impairment.

Opioid Receptor-Mediated Regulation of Neurotransmission in the Brain

Opioids mediate their effects via opioid receptors: mu, delta, and kappa. At the neuronal level, opioid receptors are generally inhibitory, presynaptically reducing neurotransmitter release and postsynaptically hyperpolarizing neurons. However, opioid receptor-mediated regulation of neuronal function and synaptic transmission is not uniform in expression pattern and mechanism across the brain. The localization of receptors within specific cell types and neurocircuits determine the effects that endogenous and exogenous opioids have on brain function. In this review we will explore the similarities and differences in opioid receptor-mediated regulation of neurotransmission across different brain regions. We discuss how future studies can consider potential cell-type, regional, and neural pathway-specific effects of opioid receptors in order to better understand how opioid receptors modulate brain function.

Mu-Opioids Suppress GABAergic Synaptic Transmission onto Orbitofrontal Cortex Pyramidal Neurons with Subregional Selectivity

The orbitofrontal cortex (OFC) plays a critical role in evaluating outcomes in a changing environment. Administering opioids to the OFC can alter the hedonic reaction to food rewards and increase their consumption in a subregion-specific manner. However, it is unknown how mu-opioid signaling influences synaptic transmission in the OFC. Thus, we investigated the cellular actions of mu-opioids within distinct subregions of the OFC. Using in vitro patch-clamp electrophysiology in brain slices containing the OFC, we found that the mu-opioid agonist DAMGO produced a concentration-dependent inhibition of GABAergic synaptic transmission onto medial OFC (mOFC), but not lateral OFC (lOFC) neurons. This effect was mediated by presynaptic mu-opioid receptor activation of local parvalbumin (PV⁺)-expressing interneurons. The DAMGO-induced suppression of inhibition was long lasting and not reversed on washout of DAMGO or by application of the mu-opioid receptor antagonist CTAP, suggesting an inhibitory long-term depression (LTD) induced by an exogenous mu-opioid. We show that LTD at inhibitory synapses is dependent on downstream cAMP/protein kinase A (PKA) signaling, which differs between the mOFC and lOFC. Finally, we demonstrate that endogenous opioid release triggered via moderate physiological stimulation can induce LTD. Together, these results suggest that presynaptic mu-opioid stimulation of local PV⁺ interneurons induces a long-lasting suppression of GABAergic synaptic transmission, which depends on subregional differences in mu-opioid receptor coupling to the downstream cAMP/PKA intracellular cascade. These findings provide mechanistic insight into the opposing functional effects produced by mu-opioids within the OFC.SIGNIFICANCE STATEMENT Considering that both the orbitofrontal cortex (OFC) and the opioid system regulate reward, motivation, and food intake, understanding the role of opioid signaling within the OFC is fundamental for a mechanistic understanding of the sequelae for several psychiatric disorders. This study makes several novel observations. First, mu-opioids induce a long-lasting suppression of inhibitory synaptic transmission onto OFC pyramidal neurons in a regionally selective manner. Second, mu-opioids recruit parvalbumin inputs to suppress inhibitory synaptic transmission in the mOFC. Third, the regional selectivity of mu-opioid action of endogenous opioids is due to the efficacy of mu-opioid receptor coupling to the downstream cAMP/PKA intracellular cascades. These experiments are the first to reveal a cellular mechanism of opioid action within the OFC.

Opioid and chemokine regulation of cortical synaptodendritic damage in HIV-associated neurocognitive disorders

Human immunodeficiency virus (HIV)-associated neurocognitive disorders (HAND) persist despite effective antiretroviral therapies (ART). Evidence suggests that modern HAND is driven by subtle synaptodendritic damage in select brain regions, as ART-treated patients do not display overt neuronal death in postmortem brain studies. HAND symptoms are also aggravated by drug abuse, particularly with injection opioids. Opioid use produces region-specific synaptodendritic damage in similar brain regions, suggesting a convergent mechanism that may enhance HAND progression in opioid-using patients. Importantly, studies indicate that synaptodendritic damage and cognitive impairment in HAND may be reversible. Activation of the homeostatic chemokine receptor CXCR4 by its natural ligand CXCL12 positively regulates neuronal survival and dendritic spine density in cortical neurons, reducing functional deficits. However, the molecular mechanisms that underlie CXCR4, as well as opioid-mediated regulation of dendritic spines are not completely defined. Here, we will consolidate studies that describe the region-specific synaptodendritic damage in the cerebral cortex of patients and animal models of HAND, describe the pathways by which opioids may contribute to cortical synaptodendritic damage, and discuss the prospects of using the CXCR4 signaling pathway to identify new approaches to reverse dendritic spine deficits. Additionally, we will discuss novel research questions that have emerged from recent studies of CXCR4 and µ-opioid actions in the cortex. Understanding the pathways that underlie synaptodendritic damage and rescue are necessary for developing novel, effective therapeutics for this growing patient population.

Cortical endogenous opioids and their role in facilitating repetitive behaviors in deer mice

Deer mice provide a non-pharmacologically induced model for the study of repetitive behaviors. In captivity, these animals develop frequent jumping and rearing that resemble clinical symptoms of obsessive-compulsive behavior (OCB), autism spectrum disorder (ASD), complex motor stereotypies (CMS), and Tourette's syndrome (TS). In this study, we pursue the mechanism of repetitive behaviors by performing stereological analyses and liquid chromatography/ mass spectrometry (LC-MS/MS) measurements of glutamate (Glut), GABA, 3,4-dihydroxyphenylacetic acid (DOPAC), dopamine (DA), leu-enkephalin (leu-enk), and dynorphin-A (dyn-A) in frontal cortex (FC), prefrontal cortex (PFC), and basal ganglia. The only significant stereological alteration was a negative correlation between repetitive behaviors and the cell count in the ventromedial striatum (VMS). Neurochemical analyses demonstrated a significant negative correlation between repetitive behaviors and endogenous opioids (leu-enk and dyn-A) in the FC - the site of origin of habitual behaviors and cortical projections to striatal MSNs participating in direct and indirect pathways. The precise neurochemical process by which endogenous opioids influence synaptic neurotransmission is unknown. One postulated cortical mechanism, supported by our findings, is an opioid effect on cortical interneuron GABA release and a consequent effect on glutamatergic cortical pyramidal cells. Anatomical changes in the VMS could have a role in repetitive behaviors, recognizing that this region influences goal-directed and habitual behaviors.

μ-Opioid Receptor Activation Directly Modulates Intrinsically Photosensitive Retinal Ganglion Cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) encode light intensity and trigger reflexive responses to changes in environmental illumination. In addition to functioning as photoreceptors, ipRGCs are post-synaptic neurons in the inner retina, and there is increasing evidence that their output can be influenced by retinal neuromodulators. Here we show that opioids can modulate light-evoked ipRGC signaling, and we demonstrate that the M1, M2 and M3 types of ipRGCs are immunoreactive for μ-opioid receptors (MORs) in both mouse and rat. In the rat retina, application of the MOR-selective agonist DAMGO attenuated light-evoked firing ipRGCs in a dose-dependent manner (IC50 < 40 nM), and this effect was reversed or prevented by co-application of the MOR-selective antagonists CTOP or CTAP. Recordings from solitary ipRGCs, enzymatically dissociated from retinas obtained from melanopsin-driven fluorescent reporter mice, confirmed that DAMGO exerts its effect directly through MORs expressed by ipRGCs. Reduced ipRGC excitability occurred via modulation of voltage-gated potassium and calcium currents. These findings suggest a potential new role for endogenous opioids in the mammalian retina and identify a novel site of action-MORs on ipRGCs-through which opioids might exert effects on reflexive responses to environmental light.

Prefrontal Cortical Opioids and Dysregulated Motivation: A Network Hypothesis

Loss of inhibitory control over appetitively motivated behavior occurs in multiple psychiatric disorders, including drug abuse, behavioral addictions, and eating disorders with binge features. In this opinion article, novel actions of μ-opioid peptides in the prefrontal cortex (PFC) that could contribute to inhibitory control deficits will be discussed. Evidence has accrued to suggest that excessive intra-PFC μ-opioid receptor (μ-OR) signaling alters the PFC response to excitatory drive, resulting in supernormal and incoherent recruitment of multiple PFC output pathways. Affected pathways include functionally opposed PFC→hypothalamus 'appetitive driver' and PFC→striatum 'appetitive limiter' projections. This network perturbation engenders disorganized, impulsive appetitive responses. Evidence supporting this hypothesis from human imaging and animal studies will be discussed, and combinatorial drug treatments targeting μ-ORs and specific PFC subcortical targets will be explored.

Endogenous opioidergic dysregulation of pain in fibromyalgia: a PET and fMRI study

Endogenous opioid system dysfunction potentially contributes to chronic pain in fibromyalgia (FM), but it is unknown if this dysfunction is related to established neurobiological markers of hyperalgesia. We previously reported that µ-opioid receptor (MOR) availability was reduced in patients with FM as compared with healthy controls in several pain-processing brain regions. In the present study, we compared pain-evoked functional magnetic resonance imaging with endogenous MOR binding and clinical pain ratings in female opioid-naive patients with FM (n = 18) using whole-brain analyses and regions of interest from our previous research. Within antinociceptive brain regions, including the dorsolateral prefrontal cortex (r = 0.81, P < 0.001) and multiple regions of the anterior cingulate cortex (all r > 0.67; all P < 0.02), reduced MOR availability was associated with decreased pain-evoked neural activity. Additionally, reduced MOR availability was associated with lower brain activation in the nucleus accumbens (r = 0.47, P = 0.050). In many of these regions, pain-evoked activity and MOR binding potential were also associated with lower clinical affective pain ratings. These findings are the first to link endogenous opioid system tone to regional pain-evoked brain activity in a clinical pain population. Our data suggest that dysregulation of the endogenous opioid system in FM could lead to less excitation in antinociceptive brain regions by incoming noxious stimulation, resulting in the hyperalgesia and allodynia commonly observed in this population. We propose a conceptual model of affective pain dysregulation in FM.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 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, drug abuse and alcohol, 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.