Robust age, but limited sex, differences in mu-opioid receptors in the rat brain: relevance for reward and drug-seeking behaviors in juveniles

Beyond the binary: Characterizing the relationships between sex and neuropeptide receptor binding density measures in the rat brain

Sex differences exist in numerous parameters of the brain. Yet, sex-related factors are part of a large set of variables that interact to affect many aspects of brain structure and function. This raises questions regarding how to interpret findings of sex differences at the level of single brain measures and the brain as a whole. In the present study, we reanalyzed two datasets consisting of measures of oxytocin, vasopressin V1a, and mu opioid receptor binding densities in multiple brain regions in rats. At the level of single brain measures, we found that sex differences were rarely dimorphic and were largely persistent across estrous stage and parental status but not across age or context. At the level of aggregates of brain measures showing sex differences, we tested whether individual brains are 'mosaics' of female-typical and male-typical measures or are internally consistent, having either only female-typical or only male-typical measures. We found mosaicism for measures showing overlap between females and males. Mosaicism was higher a) with a larger number of measures, b) with smaller effect sizes of the sex difference in these measures, and c) in rats with more diverse life experiences. Together, these results highlight the limitations of the binary framework for interpreting sex effects on the brain and suggest two complementary pathways to studying the contribution of sex to brain function: (1) focusing on measures showing dimorphic and persistent sex differences and (2) exploring the relations between specific brain mosaics and specific endpoints.

A case for seeking sex-specific treatments in Alzheimer's disease

There is no satisfactory explanation for the sex-related differences in the incidence of many diseases and this is also true of Alzheimer's disease (AD), where females have a higher lifetime risk of developing the disease and make up about two thirds of the AD patient population. The importance of understanding the cause(s) that account for this disproportionate distribution cannot be overestimated, and is likely to be a significant factor in the search for therapeutic strategies that will combat the disease and, furthermore, potentially point to a sex-targeted approach to treatment. This review considers the literature in the context of what is known about the impact of sex on processes targeted by drugs that are in clinical trial for AD, and existing knowledge on differing responses of males and females to these drugs. Current knowledge strongly supports the view that trials should make assessing sex-related difference in responses a priority with a focus on exploring the sex-stratified treatments.

Mapping Astrocytic and Neuronal μ-opioid Receptor Expression in Various Brain Regions Using MOR-mCherry Reporter Mouse

The μ-opioid receptor (MOR) is a class of opioid receptors characterized by a high affinity for β-endorphin and morphine. MOR is a G protein-coupled receptor (GPCR) that plays a role in reward and analgesic effects. While expression of MOR has been well established in neurons and microglia, astrocytic MOR expression has been less clear. Recently, we have reported that MOR is expressed in hippocampal astrocytes, and its activation has a critical role in the establishment of conditioned place preference. Despite this critical role, the expression and function of astrocytic MOR from other brain regions are still unknown. Here, we report that MOR is significantly expressed in astrocytes and GABAergic neurons from various brain regions including the hippocampus, nucleus accumbens, periaqueductal gray, amygdala, and arcuate nucleus. Using the MOR-mCherry reporter mice and Imaris analysis, we demonstrate that astrocytic MOR expression exceeded 60% in all tested regions. Also, we observed similar MOR expression of GABAergic neurons as shown in the previous distribution studies and it is noteworthy that MOR expression is particularly in parvalbumin (PV)-positive neurons. Furthermore, consistent with the normal MOR function observed in the MOR-mCherry mouse, our study also demonstrates intact MOR functionality in astrocytes through iGluSnFr-mediated glutamate imaging. Finally, we show the sex-difference in the expression pattern of MOR in PV-positive neurons, but not in the GABAergic neurons and astrocytes. Taken together, our findings highlight a substantial astrocytic MOR presence across various brain regions.

Delayed estrogen actions diminish food consumption without changing food approach, motor activity, or hypothalamic activation elicited by corticostriatal µ-opioid signaling

Mu-opioid receptor (μ-OR) signaling in forebrain sites including nucleus accumbens (Acb) and ventromedial prefrontal cortex (vmPFC) modulates reward-driven feeding and may play a role in the pathophysiology of disordered eating. In preclinical models, intra-Acb or intra-vmPFC μ-OR stimulation causes overeating and vigorous responding for food rewards. These effects have been studied mainly in male animals, despite demonstrated sex differences and estrogen modulation of central reward systems. Hence, the present study investigated sex differences and estrogen modulation of intra-Acb and intra-vmPFC μ-OR-driven feeding behaviors. First, the dose-related effects of intra-Acb and intra-vmPFC infusions of the μ-OR-selective agonist, DAMGO, were compared among intact female, ovariectomized (OVX) female, and intact male rats. The DAMGO feeding dose-effect function was flattened in intact females relative to the robust, dose-dependent effects observed in OVX females and intact males. Thus, in intact females, intra-Acb DAMGO failed to elevate food intake relative to vehicle, while intra-vmPFC DAMGO elevated food intake, but to a smaller degree compared to males and OVX females. Next, to explore the possible role of estrogen in mediating the diminished DAMGO response observed in intact females, OVX rats were given intra-Acb or intra-vmPFC infusions of DAMGO either immediately after a subcutaneous injection of 17-beta-estradiol 3-benzoate (EB; 5 μg/0.1 mL) or 24 h after EB injection. Intra-Acb DAMGO effects were not changed at the immediate post-EB time point. At the delayed post-EB timepoint, significant lordosis was noted and the duration of intra-Acb DAMGO-driven feeding bouts was significantly reduced, with no change in the number of bouts initiated, locomotor hyperactivity, or Fos immunoreactivity in hypothalamic feeding and arousal systems. Similarly, EB failed to alter the motor-activational effects of intra-vmPFC DAMGO while reducing feeding. These findings indicate that delayed, presumably genomically mediated estrogen actions modulate the μ-OR-generated motivational state by reducing consummatory activity while sparing goal-approach and general arousal/activity. The results additionally suggest that EB regulation of consummatory activity occurs outside of forebrain-μ-OR control of hypothalamic systems.

Effects of prenatal opioid exposure on synaptic adaptations and behaviors across development

In this review, we focus on prenatal opioid exposure (POE) given the significant concern for the mental health outcomes of children with parents affected by opioid use disorder (OUD) in the view of the current opioid crisis. We highlight some of the less explored interactions between developmental age and sex on synaptic plasticity and associated behavioral outcomes in preclinical POE research. We begin with an overview of the rich literature on hippocampal related behaviors and plasticity across POE exposure paradigms. We then discuss recent work on reward circuit dysregulation following POE. Additional risk factors such as early life stress (ELS) could further influence synaptic and behavioral outcomes of POE. Therefore, we include an overview on the use of preclinical ELS models where ELS exposure during key critical developmental periods confers considerable vulnerability to addiction and stress psychopathology. Here, we hope to highlight the similarity between POE and ELS on development and maintenance of opioid-induced plasticity and altered opioid-related behaviors where similar enduring plasticity in reward circuits may occur. We conclude the review with some of the limitations that should be considered in future investigations. This article is part of the Special Issue on 'Opioid-induced addiction'.

SEX-DEPENDENT IMPACT OF MICROBIOTA STATUS ON CEREBRAL μ -OPIOID RECEPTOR DENSITY IN FISCHER RATS

μ‐opioid receptors (MOPr) play a critical role in social play, reward and pain, in a sex‐ and age‐dependent manner. There is evidence to suggest that sex and age differences in brain MOPr density may be responsible for this variability; however, little is known about the factors driving these differences in cerebral MOPr density. Emerging evidence highlights gut microbiota's critical influence and its bidirectional interaction with the brain on neurodevelopment. Therefore, we aimed to determine the impact of gut microbiota on MOPr density in male and female brains at different developmental stages. Quantitative [³H]DAMGO autoradiographic binding was carried out in the forebrain of male and female conventional (CON) and germ‐free (GF) rats at postnatal days (PND) 8, 22 and 116–150. Significant ‘microbiota status X sex’, ‘age X brain region’ interactions and microbiota status‐ and age‐dependent effects on MOPr binding were uncovered. Microbiota status influenced MOPr levels in males but not females, with higher MOPr levels observed in GF versus CON rats overall regions and age groups. In contrast, no overall sex differences were observed in GF or CON rats. Interestingly, within‐age planned comparison analysis conducted in frontal cortical and brain regions associated with reward revealed that this microbiota effect was restricted only to PND22 rats. Thus, this pilot study uncovers the critical sex‐dependent role of gut microbiota in regulating cerebral MOPr density, which is restricted to the sensitive developmental period of weaning. This may have implications in understanding the importance of microbiota during early development on opioid signalling and associated behaviours.

Maternal deprivation effect on morphine-induced CPP is related to changes in Opioid receptors in selected rat brain regions (hippocampus, prefrontal cortex, and nucleus accumbens)

Early-life environmental conditions affect offspring's development. Maternal deprivation (MD) can induce persistent changes that give rise to neuropsychiatric diseases including substance abuse disorders. However, long-lasting mechanisms that determine vulnerability to drug addiction remain unknown. We hypothesized that MD could induce changes in Opioid system, HPA (hypothalamic-pituitary-adrenal) axis, and BDNF (brain-derived neurotrophic factor), so may be involved in the drug abuse in later life. Male offspring of Wistar rats (n=8 per group) were subjected to 3h of daily MD during postnatal days 1-14. In adulthood, morphine-induced CPP (conditioned place preference) was investigated using two doses of morphine (3 and 5mg/kg). Serum corticosterone level was measured by ELISA method. The expression level of genes in selected brain regions (hippocampus, prefrontal cortex, and nucleus accumbens) was determined by qPCR (quantitative PCR). A greater morphine-induced CPP was observed in MD rats with 3 and 5mg/kg morphine compared to controls. MD group had a higher corticosterone level. A significant decrease was observed in the expression of BDNF gene (in all of the selected brain regions) and GR (glucocorticoid receptor) gene (in the hippocampus and nucleus accumbens) in MD rats. Also, a significant increase in the expression of μ Opioid receptor (in all of the selected brain regions) and κ Opioid receptor (in the prefrontal cortex and nucleus accumbens) was observed in MD rats. Our results suggest that MD induces alterations in the HPA axis function, BDNF level, and Opioid receptors system that enhance vulnerability to morphine at adulthood.

Emerging roles for microglia and microbiota in the development of social circuits

Social withdrawal is a core component of the behavioral response to infection. This fact points to a deep evolutionary and biologic relationship between the immune system and the social brain. Indeed, a large body of literature supports such an intimate connection. In particular, immune activation during the perinatal period has been shown to have long-lasting consequences for social behavior, but the neuroimmune mechanisms by which this occurs are only partially understood. Microglia, the resident immune cells of the brain, influence the formation of neural circuits by phagocytosing synaptic and cellular elements, as well as by releasing chemokines and cytokines. Intriguingly, microbiota, especially those that reside within the gut, may also influence brain development via the release of metabolites that travel to the brain, by influencing vagal nerve signaling, or by modulating the host immune system. Here, I will review the work suggesting important roles for microglia and microbiota in social circuit formation during development. I will then highlight avenues for future work in this area, as well as technological advances that extend our capacity to ask mechanistic questions about the relationships between microglia, microbiota, and the social brain.

Morphine exposure alters Fos expression in a sex-, age-, and brain region-specific manner during adolescence

Data in both humans and preclinical animal models clearly indicate drug exposure during adolescence, when the "reward" circuitry of the brain develops, increases the risk of substance use and other mental health disorders later in life. Human data indicate that different neural and behavioral sequelae can be observed in early versus late adolescence. However, most studies with rodent models examine a single adolescent age compared to a mature adult age, and often only in males. Herein, we sought to determine whether the acute response to the opioid morphine would also differ across adolescence, and by sex. By quantifying Fos positive cells, a proxy for neural activity, at different stages during adolescence (pre-, early, mid-, and late adolescence) and in multiple reward regions (prefrontal cortex, nucleus accumbens, caudate/putamen), we determined that the neural response to acute morphine is highly dependent on adolescent age, sex, and brain region. These data suggest that heterogeneity in the consequences of adolescent opioid exposure may be due to age- and sex-specific developmental profiles in individual reward processing regions. In future studies, it will be important to add age within adolescence as an independent variable for a holistic view of healthy or abnormal reward-related neural development.

Central diabetes insipidus and pain medications - a risky combination

Background

Central Diabetes Insipidus (CDI) results from decreased production of antidiuretic hormone (ADH) leading to an inability to concentrate urine. CDI is treated with desmopressin (DDAVP). Rarely reported in the literature, opioids and non-steroidal anti-inflammatories (NSAIDs) can induce hyponatremia in individuals treated for CDI.

Case presentation

A 10-year-old boy with septo-optic dysplasia and CDI was treated with DDAVP 1.6 mg orally TID maintaining normal sodium levels. Post admission for a femur fracture, he was discharged on ibuprofen and hydromorphone. Sodium was 136 mmol/l two days before discharge.

He returned to the ED after having a seizure at home. He was euvolemic and mildly lethargic. Sodium was low at 108 mmol/l. DDAVP and hydromorphone were held and he was fluid restricted, but the sodium remained low. Sodium began to rise when Ibuprofen was stopped. Intermittent small doses of DDAVP were given to facilitate gradual correction of hyponatremia. At discharge, sodium had normalized.

Conclusion

Hyponatremia has occasionally been described as a side effect of opioids and rarely of NSAIDs in patients with CDI. Stimulation of the thirst centre may play a role with opioids while a decrease in urine output may be the mechanism with NSAIDs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40842-021-00124-9.

Chronic developmental lead exposure increases μ-opiate receptor levels in the adolescent rat brain

Childhood lead (Pb2+) intoxication is a global public health problem best known for producing deficits in learning and poor school performance. Human and preclinical studies have suggested an association between childhood Pb2+ intoxication and proclivity to substance abuse and delinquent behavior. While environmental factors have been implicated in opioid addiction, less is known about the role of exposure to environmental pollutants on the brain opioid system. Opioid receptors are involved in the biological effects of opioids and other drugs of abuse. In this study, we examine the effect of chronic developmental Pb2+ exposure (1500 ppm in the diet) on μ-opioid receptor (MOR) levels in the rat brain using [3H]-d-Ala2-MePhe4-Gly-ol5 enkephalin ([3H]-DAMGO) quantitative receptor autoradiography at different developmental stages (juvenile, early-adolescent, late adolescent and adult) in male and female rats. Our results indicate that chronic developmental Pb2+ exposure increases the levels of [3H]-DAMGO specific binding to MOR in juvenile and early adolescent Pb2+-exposed male and female rat brain with no changes in late-adolescent (PN50) and minor changes in Pb2+-exposed adult male rats (PN120). Specifically, at PN14, Pb2+-exposed males had an increase in MOR binding in the lateral posthalamic nuclei (LPTN), and Pb2+-exposed females had increased MOR binding in LPTN, medial thalamus, and hypothalamus. At PN28, Pb2+-exposed males had increased MOR levels in the striatum, stria medullaris of the thalamus, LPTN, medial thalamus, and basolateral amygdala, while Pb2+-exposed females showed an increase in nucleus accumbens core, LPTN, and medial thalamus. No changes were detected in any brain region of male and female rats at PN50, and at PN120 there was a decrease in MOR binding of Pb2+-exposed males in the medial thalamus. Our findings demonstrate age and gender specific effects of MOR levels in the rat brain as a result of chronic developmental Pb2+ exposure. These results indicate that the major changes in brain MOR levels were during pre-adolescence and early adolescence, a developmental period in which there is higher engagement in reward and drug-seeking behaviors in humans. In summary, we show that chronic exposure to Pb2+, an ubiquitous and well-known environmental contaminant and neurotoxicant, alters MOR levels in brain regions associated with addiction circuits in the adolescent period, these findings have important implications for opioid drug use and abuse.

Endogenous Opiates and Behavior: 2018

This paper is the forty-first consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2018 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (2), the roles of these opioid peptides and receptors in pain and analgesia in animals (3) and humans (4), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (5), opioid peptide and receptor involvement in tolerance and dependence (6), stress and social status (7), learning and memory (8), eating and drinking (9), drug abuse and alcohol (10), sexual activity and hormones, pregnancy, development and endocrinology (11), mental illness and mood (12), seizures and neurologic disorders (13), electrical-related activity and neurophysiology (14), general activity and locomotion (15), gastrointestinal, renal and hepatic functions (16), cardiovascular responses (17), respiration and thermoregulation (18), and immunological responses (19).

Potential role of primed microglia during obesity on the mesocorticolimbic circuit in autism spectrum disorder

Autism spectrum disorder (ASD) is a complex neurodevelopmental disease which involves functional and structural defects in selective central nervous system (CNS) regions that harm function and individual ability to process and respond to external stimuli. Individuals with ASD spend less time engaging in social interaction compared to non-affected subjects. Studies employing structural and functional magnetic resonance imaging reported morphological and functional abnormalities in the connectivity of the mesocorticolimbic reward pathway between the nucleus accumbens and the ventral tegmental area (VTA) in response to social stimuli, as well as diminished medial prefrontal cortex in response to visual cues, whereas stronger reward system responses for the non-social realm (e.g., video games) than social rewards (e.g., approval), associated with caudate nucleus responsiveness in ASD children. Defects in the mesocorticolimbic reward pathway have been modulated in transgenic murine models using D2 dopamine receptor heterozygous (D2+/-) or dopamine transporter knockout mice, which exhibit sociability deficits and repetitive behaviors observed in ASD phenotypes. Notably, the mesocorticolimbic reward pathway is modulated by systemic and central inflammation, such as primed microglia, which occurs during obesity or maternal overnutrition. Therefore, we propose that a positive energy balance during obesity/maternal overnutrition coordinates a systemic and central inflammatory crosstalk that modulates the dopaminergic neurotransmission in selective brain areas of the mesocorticolimbic reward pathway. Here, we will describe how obesity/maternal overnutrition may prime microglia, causing abnormalities in dopamine neurotransmission of the mesocorticolimbic reward pathway, postulating a possible immune role in the development of ASD.

Deficits in opioid receptor-mediated prediction error contribute to impaired fear extinction during adolescence

Adolescent-onset anxiety disorders are more common and costly than those that emerge later in life. Unfortunately, nearly half of adolescents undergoing cognitive behavioural therapies, including exposure therapies, show significant symptom relapse. Such poor treatment outcomes are consistent with preclinical work examining fear extinction, in which adolescents show persistent fear to extinguished cues. Both extinction and exposure are dependent on the generation of prediction error (i.e., the difference between the expected and actual outcome of a cue presentation), a process which involves the opioid system. We investigated the contribution of prediction error signalling to extinction during adolescence using the opioid receptor antagonist naloxone. We demonstrated that unlike in juvenile and adult rats, fear expression during extinction training and test in adolescent rats was unaffected by naloxone, suggesting that adolescent rats are impaired in using prediction error signalling to extinguish fear under typical conditions. However, in two circumstances where adolescents exhibit good extinction retention, opioid receptor blockade impaired extinction retention, suggesting that the recruitment of prediction error signalling mechanisms promotes extinction in this age group, just as it does in adults. Importantly, additional extinction training may be required to enable prediction error mechanisms to be recruited during adolescence.

Chronic aerobic exercise: Autoradiographic assessment of GABA(a) and mu-opioid receptor binding in adult rats

Exercise programs have shown great potential for both the prevention and treatment of substance use disorder (SUD). As exercise has been shown to have potent effects on physical and psychological health, it is reasonable to examine the mechanism of how exercise can be used as an adjunct treatment for addiction. The present study examined the effects of chronic aerobic (treadmill) exercise on both GABA(a) and mu-opioid receptor levels in the brains of male and female rats. GABA(a) receptor binding, measured by [³H] Flunitrazepam, was increased in the cingulate cortex following exercise, but only in females. Mu-opioid receptor expression, measured by [³H] ([D-Ala², N-MePhe⁴, Gly-ol]-enkephalin) (DAMGO), showed no effect of exercise while showing an effect of sex, with increased [³H] DAMGO binding in the brains of sedentary males compared to that of sedentary females. Our findings support the potential role for GABA(a) signaling in the cingulate cortex as part of the mechanism of action of aerobic exercise. These data, along with prior reports, aid our understanding of the neurochemical impact and mechanism of chronic aerobic exercise on neuropsychiatric disease, particularly regarding addiction.

Oxycodone Decreases Dendritic Complexity in Female but not Male Rat Striatal Neurons In Vitro

The use of oxycodone in the past two decades has dramatically risen, yet the amount of research regarding how it impacts neuronal health is lacking. As prescription use and misuse in women of reproductive age increases there has been a corresponding increase in the number of infants who have been exposed to oxycodone in utero. Given the critical role of the striatum in motor control and reward regulation, the aim of the current study was to examine the effects of oxycodone on developing rat striatal neurons. Sex-specific effects of oxycodone on neuronal cytoarchitecture were examined in cultured rat striatal neurons with a primary focus on dendritic arborization. Neurons were extracted from either male or female embryonic day 18 rat striata and cultured and exposed to varying concentrations of oxycodone over a ten-day period. Dendritic complexity of the neurons was measured using Sholl analysis. Results indicate that oxycodone inhibits dendritic complexity in a dose-dependent manner in female but not male striatal neurons. Additional analysis indicated the number of non-primary dendrites in female striatal neurons significantly decreased with increasing concentrations of oxycodone, while the number of primary dendrites as well as the length of primary and non-primary dendrites was unaffected by oxycodone treatment in both sexes. These in vitro findings demonstrate sex-specific effects of oxycodone on the development of striatal dendritic architecture which may be important for understanding the effects of oxycodone exposure in utero.

Opioids differentially modulate two synapses important for pain processing in the amygdala

BACKGROUND AND PURPOSE

Pain is a subjective experience involving sensory discriminative and emotionally aversive components. Consistent with its role in pain processing and emotions, the amygdala modulates the aversive component of pain. The laterocapsular region of the central nucleus of the amygdala (CeLC) receives nociceptive information from the parabrachial nucleus (PB) and polymodal, including nociceptive, inputs from the basolateral nucleus of the amygdala (BLA). Opioids are strong analgesics and reduce both the sensory discriminative and the affective component of pain. However, it is unknown whether opioids regulate activity at the two nociceptive inputs to the amygdala.

EXPERIMENTAL APPROACH

Using whole-cell electrophysiology, optogenetics, and immunohistochemistry, we investigated whether opioids inhibit the rat PB-CeLC and BLA-CeLC synapses.

KEY RESULTS

Opioids inhibited glutamate release at the PB-CeLC and BLA-CeLC synapses. Opioid inhibition is via the μ-receptor at the PB-CeLC synapse, while at the BLA-CeLC synapse, inhibition is via μ-receptors in all neurons and via δ-receptors and κ-receptors in a subset of neurons.

CONCLUSIONS AND IMPLICATIONS

Agonists of μ-receptors inhibited two of the synaptic inputs carrying nociceptive information into the laterocapsular amygdala. Therefore, μ-receptor agonists, such as morphine, will inhibit glutamate release from PB and BLA in the CeLC, and this could serve as a mechanism through which opioids reduce the affective component of pain and pain-induced associative learning. The lower than expected regulation of BLA synaptic outputs by δ-receptors does not support the proposal that opioid receptor subtypes segregate into subnuclei of brain regions.

Psychotropic Drugs for the Management of Chronic Pain and Itch

Clinical observations have shown that patients with chronic neuropathic pain or itch exhibit symptoms of increased anxiety, depression and cognitive impairment. Such patients need corrective therapy with antidepressants, antipsychotics or anticonvulsants. It is known that some psychotropic drugs are also effective for the treatment of neuropathic pain and pruritus syndromes due to interaction with the secondary molecular targets. Our own clinical studies have identified antipruritic and/or analgesic efficacy of the following compounds: tianeptine (atypical tricyclic antidepressant), citalopram (selective serotonin reuptake inhibitor), mianserin (tetracyclic antidepressant), carbamazepine (anticonvulsant), trazodone (serotonin antagonist and reuptake inhibitor), and chlorprothixene (antipsychotic). Venlafaxine (serotonin-norepinephrine reuptake inhibitor) is known to have an analgesic effect too. The mechanism of such effect of these drugs is not fully understood. Herein we review and correlate the literature data on analgesic/antipruritic activity with pharmacological profile of these compounds.

A Motivational and Neuropeptidergic Hub: Anatomical and Functional Diversity within the Nucleus Accumbens Shell

The mesocorticolimbic pathway is canonically known as the "reward pathway." Embedded within the center of this circuit is the striatum, a massive and complex network hub that synthesizes motivation, affect, learning, cognition, stress, and sensorimotor information. Although striatal subregions collectively share many anatomical and functional similarities, it has become increasingly clear that it is an extraordinarily heterogeneous region. In particular, the nucleus accumbens (NAc) medial shell has repeatedly demonstrated that the rules dictated by more dorsal aspects of the striatum do not apply or are even reversed in functional logic. These discrepancies are perhaps most easily captured when isolating the functions of various neuromodulatory peptide systems within the striatum. Endogenous peptides are thought to play a critical role in modulating striatal signals to either amplify or dampen evoked behaviors. Here we describe the anatomical-functional backdrop upon which several neuropeptides act within the NAc to modulate behavior, with a specific emphasis on nucleus accumbens medial shell and stress responsivity. Additionally, we propose that, as the field continues to dissect fast neurotransmitter systems within the NAc, we must also provide considerable contextual weight to the roles local peptides play in modulating these circuits to more comprehensively understand how this important subregion gates motivated behaviors.

Neuro-Immune Mechanisms Regulating Social Behavior: Dopamine as Mediator?

Social interactions are fundamental to survival and overall health. The mechanisms underlying social behavior are complex, but we now know that immune signaling plays a fundamental role in the regulation of social interactions. Prolonged or exaggerated alterations in social behavior often accompany altered immune signaling and function in pathological states. Thus, unraveling the link between social behavior and immune signaling is a fundamental challenge, not only to advance our understanding of human health and development, but for the design of comprehensive therapeutic approaches for neural disorders. In this review, we synthesize literature demonstrating the bidirectional relationship between social behavior and immune signaling and highlight recent work linking social behavior, immune function, and dopaminergic signaling in adolescent neural and behavioral development.

Opioid modulation of social play reward in juvenile rats

Social play behaviour is a vigorous form of social interaction abundant during the juvenile and adolescent phases of life in many mammalian species, including rats and humans. Social play is thought to be important for social, emotional and cognitive development. Being a rewarding activity, the expression of social play depends on its pleasurable and motivational properties. Since opioids have been widely implicated in reward processes, in the present study we investigated the role of opioids in the pleasurable and motivational properties of social play behaviour in rats. To assess social play motivation, an operant conditioning setup was used in which rats responded for social play under a progressive ratio schedule of reinforcement. Treatment with the opioid receptor agonist morphine reduced responding for social play at the highest dose tested, likely due to its rate-limiting effects. Morphine treatment increased the expression of social play behaviour during reinforced periods. The acquisition of social play-induced conditioned place preference (CPP) in a subeffective conditioning protocol was enhanced by treatment with morphine. Morphine treatment alone also induced CPP. In contrast, antagonizing opioid receptors with naloxone reduced responding for social play, the expression of social play and blocked the development of social play-induced CPP. These data implicate opioid neurotransmission in both the pleasurable and the motivational aspects of social play behaviour in rats. This article is part of the Special Issue entitled 'The neuropharmacology of social behavior: from bench to bedside'.

Connecting the dots: an association between opioids and acute hippocampal injury

Acute hippocampal injury represents a relatively rare cause of amnesia. Interestingly however, between 2012 and 2017, 18 patients were reported at hospitals in Massachusetts with sudden-onset amnesia in the setting of complete diffusion-weighted hyperintensity of both hippocampi on magnetic resonance imaging. Notably, 17 of the 18 patients tested positive for opioids or had a recorded history of opioid use. This observation suggests an association between opioids and acute hippocampal injury. With particular attention to the Massachusetts cluster and data on fentanyl and its congeners, the epidemiological and pathophysiological evidence that supports this hypothesis is presented, as are potential underlying mechanisms.