Sex differences in morphine-induced trafficking of mu-opioid and corticotropin-releasing factor receptors in locus coeruleus neurons

Heterogeneous organization of Locus coeruleus: An intrinsic mechanism for functional complexity

Locus coeruleus (LC) is a small nucleus located deep in the brainstem that contains the majority of central noradrenergic neurons, which provide the primary source of noradrenaline (NA) throughout the entire central nervous system (CNS).The release of neurotransmitter NA is considered to modulate arousal, sensory processing, attention, aversive and adaptive stress responses as well as high-order cognitive function and memory, with the highly ramified axonal arborizations of LC-NA neurons sending wide projections to the targeted brain areas. For over 30 years, LC was thought to be a homogeneous nucleus in structure and function due to the widespread uniform release of NA by LC-NA neurons and simultaneous action in several CNS regions, such as the prefrontal cortex, hippocampus, cerebellum, and spinal cord. However, recent advances in neuroscience tools have revealed that LC is probably not so homogeneous as we previous thought and exhibits heterogeneity in various aspects. Accumulating studies have shown that the functional complexity of LC may be attributed to its heterogeneity in developmental origin, projection patterns, topography distribution, morphology and molecular organization, electrophysiological properties and sex differences. This review will highlight the heterogeneity of LC and its critical role in modulating diverse behavioral outcomes.

Central metabolism as a potential origin of sex differences in morphine antinociception but not induction of antinociceptive tolerance in mice

BACKGROUND AND PURPOSE

In rodents, morphine antinociception is influenced by sex. However, conflicting results have been reported regarding the interaction between sex and morphine antinociceptive tolerance. Morphine is metabolised in the liver and brain into morphine-3-glucuronide (M3G). Sex differences in morphine metabolism and differential metabolic adaptations during tolerance development might contribute to behavioural discrepancies. This article investigates the differences in peripheral and central morphine metabolism after acute and chronic morphine treatment in male and female mice.

EXPERIMENTAL APPROACH

Sex differences in morphine antinociception and tolerance were assessed using the tail-immersion test. After acute and chronic morphine treatment, morphine and M3G metabolic kinetics in the blood were evaluated using LC-MS/MS. They were also quantified in several CNS regions. Finally, the blood-brain barrier (BBB) permeability of M3G was assessed in male and female mice.

KEY RESULTS

This study demonstrated that female mice showed weaker morphine antinociception and faster induction of tolerance than males. Additionally, female mice showed higher levels of M3G in the blood and in several pain-related CNS regions than male mice, whereas lower levels of morphine were observed in these regions. M3G brain/blood ratios after injection of M3G indicated no sex differences in M3G BBB permeability, and these ratios were lower than those obtained after injection of morphine.

CONCLUSION

These differences are attributable mainly to morphine central metabolism, which differed between males and females in pain-related CNS regions, consistent with weaker morphine antinociceptive effects in females. However, the role of morphine metabolism in antinociceptive tolerance seemed limited.

LINKED ARTICLES

This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Anterior cingulate cortex and its projections to the ventral tegmental area regulate opioid withdrawal, the formation of opioid context associations and context-induced drug seeking

Clinical evidence suggests that there are correlations between activity within the anterior cingulate cortex (ACC) following re-exposure to drug-associated contexts and drug craving. However, there are limited data contributing to our understanding of ACC function at the cellular level during re-exposure to drug-context associations as well as whether the ACC is directly related to context-induced drug seeking. Here, we addressed this issue by employing our novel behavioral procedure capable of measuring the formation of drug-context associations as well as context-induced drug-seeking behavior in male mice (8–12 weeks of age) that orally self-administered oxycodone. We found that mice escalated oxycodone intake during the long-access training sessions and that conditioning with oxycodone was sufficient to evoke conditioned place preference (CPP) and drug-seeking behaviors. Additionally, we found that thick-tufted, but not thin-tufted pyramidal neurons (PyNs) in the ACC as well as ventral tegmental area (VTA)-projecting ACC neurons had increased intrinsic membrane excitability in mice that self-administered oxycodone compared to controls. Moreover, we found that global inhibition of the ACC or inhibition of VTA-projecting ACC neurons was sufficient to significantly reduce oxycodone-induced CPP, drug seeking, and spontaneous opioid withdrawal. These results demonstrate a direct role of ACC activity in mediating context-induced opioid seeking among other behaviors, including withdrawal, that are associated with the DSM-V criteria of opioid use disorder.

Noradrenergic circuits and signaling in substance use disorders

The central noradrenergic system innervates almost all regions of the brain and, as such, is well positioned to modulate many neural circuits implicated in behaviors and physiology underlying substance use disorders. Ample pharmacological evidence demonstrates that α1, α2, and β adrenergic receptors may serve as therapeutic targets to reduce drug -seeking behavior and drug withdrawal symptoms. Further, norepinephrine is a key modulator of the stress response, and stress has been heavily implicated in reinstatement of drug taking. In this review, we discuss recent advances in our understanding of noradrenergic circuitry and noradrenergic receptor signaling in the context of opioid, alcohol, and psychostimulant use disorders.

Weight-gain propensity and morphine withdrawal alters locomotor behavior and regional norepinephrine-related gene expression in male and female mice

Interactions between obesity and opioid use are poorly understood. The objective of this study was to determine whether phenotypic differences in diet-induced weight gain altered morphine withdrawal responses. Male and female C57BL/6J mice were characterized as obese prone (OP) or obese resistant (OR) based on median split in body weights following exposure to high-fat diet (45% fat). After classification into OP or OR, all mice were fed a low-fat diet (10% fat) for the remainder of the study (≥5 weeks) to remain weight matched. Mice were treated with a 7-day escalating dosing scheme of morphine (20-100 mg/kg; IP) or saline and underwent a spontaneous withdrawal. Morphine-induced weight loss was restored by withdrawal day 7. On withdrawal day 8, male OP demonstrated less total time mobile in the open field test (OFT). In females, OR-morphine traveled less distance than OR-saline, and OR-morphine spent less time mobile compared with all other groups in the OFT. Female OP also increased time spent in the center of the apparatus, regardless of treatment. On withdrawal day 8, relative gene expression was measured by qPCR. For males, expression of dopamine beta-hydroxylase (dbh), alpha-adrenergic receptor 2 a (adra2a), and orexin receptor 1 (orx1) were increased in the locus coeruleus (LC) region of OP mice, regardless of treatment. In comparison, in females, dbh and adra2a were decreased in the LC region of OP mice, regardless of treatment. Also, in the LC region of females, OP-morphine had lower expression of alpha-adrenergic receptor 1 a (adra1a) than OR-morphine and OP-saline. In the hypothalamic paraventricular nucleus (PVN) of females, adra2a was increased in OP-morphine compared with OP-saline and OR-morphine. Our findings suggest morphine withdrawal responses and regional expression of noradrenergic-related genes are differentially influenced by weight gain propensity.

Chronic opioid exposure differentially modulates oxycodone self-administration in male and female rats

Withdrawal from opioid painkillers can produce short‐lived physical symptoms and protracted psychological symptoms including anxiety and depressive‐like states that often lead to opioid misuse and opioid use disorder (OUD). Studies testing the hypothesis that opioid withdrawal potentiates the reinforcing effects of opioid self‐administration (SA) are largely inconclusive and have focused on males. Although some clinical evidence indicates that women are more likely than men to misuse opioids to self‐medicate, preclinical studies in both sexes are lacking. Based on clinical reports, we hypothesized that withdrawal from escalating‐dose morphine injections that approximates a prescription painkiller regimen would lead to increased oxycodone SA to a greater extent in female compared to male rats. After escalating‐dose morphine (5–30 mg/kg or vehicle, twice/day for 12 days), rats underwent a 2‐week abstinence period during which withdrawal signs were measured. The impact of this treatment was assessed on oxycodone SA acquisition, maintenance, dose response, and progressive ratio responding, with additional analyses to compare sexes. We found that both sexes expressed somatic withdrawal, whereas only males demonstrated hyperalgesia in the warm water tail flick assay. During SA acquisition, males with prior morphine exposure took significantly more oxycodone than females. Finally, females with prior morphine exposure demonstrated the lowest motivation to SA oxycodone in the progressive ratio test. Contrary to our initial hypothesis, our findings suggest that prior opioid exposure increases vulnerability to initiate misuse more in males and decreases the reinforcing efficacy of oxycodone in females.

The role of catecholamines in modulating responses to stress: Sex-specific patterns, implications, and therapeutic potential for post-traumatic stress disorder and opiate withdrawal

Emotional arousal is one of several factors that determine the strength of a memory and how efficiently it may be retrieved. The systems at play are multifaceted; on one hand, the dopaminergic mesocorticolimbic system evaluates the rewarding or reinforcing potential of a stimulus, while on the other, the noradrenergic stress response system evaluates the risk of threat, commanding attention, and engaging emotional and physical behavioral responses. Sex-specific patterns in the anatomy and function of the arousal system suggest that sexually divergent therapeutic approaches may be advantageous for neurological disorders involving arousal, learning, and memory. From the lens of the triple network model of psychopathology, we argue that post-traumatic stress disorder and opiate substance use disorder arise from maladaptive learning responses that are perpetuated by hyperarousal of the salience network. We present evidence that catecholamine-modulated learning and stress-responsive circuitry exerts substantial influence over the salience network and its dysfunction in stress-related psychiatric disorders, and between the sexes. We discuss the therapeutic potential of targeting the endogenous cannabinoid system; a ubiquitous neuromodulator that influences learning, memory, and responsivity to stress by influencing catecholamine, excitatory, and inhibitory synaptic transmission. Relevant preclinical data in male and female rodents are integrated with clinical data in men and women in an effort to understand how ideal treatment modalities between the sexes may be different.

Neurochemically distinct circuitry regulates locus coeruleus activity during female social stress depending on coping style

Stress-related psychiatric diseases are nearly twice as prevalent in women compared to men. We recently showed in male rats that the resident-intruder model of social stress differentially engages stress-related circuitry that regulates norepinephrine-containing neurons of the locus coeruleus (LC) depending on coping strategy as determined by the latency to assume a defeat posture. Here, we determined whether this social stress had similar effects in female rats. LC afferents were retrogradely labeled with Fluorogold (FG) and rats had one or five daily exposures to an aggressive resident. Sections through the nucleus paragigantocellularis (PGi), a source of enkephalin (ENK) afferents to the LC, and central nucleus of the amygdala (CeA), a source of corticotropin-releasing factor (CRF) afferents to the LC, were processed for immunocytochemical detection of c-fos, a marker of neuronal activity, FG and ENK or CRF. Like male rats, female rats defeated with a relatively short latency (SL) in response to a single resident-intruder exposure and showed significant c-fos activation of LC neurons, PGi-ENK LC afferents, and CeA-CRF-LC afferents. With repeated exposure, some rats exhibited a long latency to defeat (LL). LC neurons and CeA-CRF-LC afferents were activated in SL rats compared to control and LL, whereas PGi-ENK LC afferents were not. Conversely, in LL rats, PGi-ENK LC and CeA-CRF-LC afferents were activated compared to controls but not LC neurons. CRF type 1 receptor (CRF1) and µ-opioid receptor (MOR) expression levels in LC were decreased in LL rats. Finally, electron microscopy showed a relative increase in MOR on the plasma membrane of LL rats and a relative increase in CRF1 on the plasma membrane of SL rats. Together, these results suggest that as is the case for males, social stress engages divergent circuitry to regulate the LC in female rats depending on coping strategy, with a bias towards CRF influence in more subordinate rats and opioid influence in less subordinate rats.