Opioid receptor-dependent sex differences in synaptic plasticity in the hippocampal mossy fiber pathway of the adult rat

Opioid receptor antagonists reduce motivated wheel-running behavior in mice

We hypothesized that opioid receptor antagonists would inhibit motivated behavior produced by a natural reward. To evaluate motivated responses to a natural reward, mice were given access to running wheels for 71.5 h in a multi-configuration testing apparatus. In addition to a running wheel activity, locomotor activity (outside of the wheel), food and water intake, and access to a food container were measured in the apparatus. Mice were also tested separately for novel-object exploration to investigate whether naloxone affects behavior unrelated to natural reward. In untreated mice wheel running increased from day 1 to day 3. The selective µ-opioid receptor antagonist β-funaltrexamine (β-FNA) (5 mg/kg) slightly decreased wheel running, but did not affect the increase in wheel running from day 1 to day 3. The non-selective opioid receptor antagonist naloxone produced a greater reduction in wheel running than β-FNA and eliminated the increase in wheel running that occurred over time in the other groups. Analysis of food access, locomotor behavior, and behavior in the novel-object test suggested that the reduction in wheel running was selective for this highly reinforcing behavior. These results indicate that opioid receptor antagonism reduces responses to the natural rewarding effects of wheel running and that these effects involve multiple opioid receptors since the non-selective opioid receptor antagonist had greater effects than the selective µ-opioid receptor antagonist. It is possible that at the doses employed, other receptor systems than opioid receptors might be involved, at least in part, in the effect of naloxone and β-FNA.

Dentate Gyrus Granule Cells Show Stability of BDNF Protein Expression in Mossy Fiber Axons with Age, and Resistance to Alzheimer's Disease Neuropathology in a Mouse Model

Brain-derived neurotrophic factor (BDNF) is important in the development and maintenance of neurons and their plasticity. Hippocampal BDNF has been implicated in Alzheimer's disease (AD) because hippocampal levels in AD patients and AD animal models are often downregulated, suggesting that reduced BDNF contributes to AD. However, the location where hippocampal BDNF protein is most highly expressed, the mossy fiber (MF) axons of dentate gyrus granule cells (GCs), has been understudied, and not in controlled conditions. Therefore, we evaluated MF BDNF protein in the Tg2576 mouse model of AD. Tg2576 and wild-type (WT) mice of both sexes were examined at 2-3 months of age, when amyloid-β (Aβ) is present in neurons but plaques are absent, and 11-20 months of age, after plaque accumulation. As shown previously, WT mice exhibited high levels of MF BDNF protein. Interestingly, there was no significant decline with age in either the genotype or sex. Notably, MF BDNF protein was correlated with GC ΔFosB, a transcription factor that increases after 1-2 weeks of elevated neuronal activity. We also report the novel finding that Aβ in GCs or the GC layer was minimal even at old ages. The results indicate that MF BDNF is stable in the Tg2576 mouse, and MF BDNF may remain unchanged due to increased GC neuronal activity, since BDNF expression is well known to be activity dependent. The resistance of GCs to long-term Aβ accumulation provides an opportunity to understand how to protect vulnerable neurons from increased Aβ levels and therefore has translational implications.

Estradiol and Mu opioid-mediated reward: The role of estrogen receptors in opioid use

Opioid use and opioid use disorder are characterized by sex and gender differences, and some of these differences may be mediated by differences in the hormonal milieu within and across individuals. This review focuses on the role of ovarian hormones, and particularly estradiol, on the endogenous mu opioid receptor system. There is an abundance of data indicating that estradiol influences the activity of endogenous mu opioid peptides, the activation of mu opioid receptors, and the internalization and desensitization of mu opioid receptors. These effects have functional consequences on behaviors mediated by endogenous mu opioid receptor activity and on sensitivity to mu opioid agonists and antagonists. Recent behavioral data suggest these consequences extend to mu opioid reward, and preclinical studies report that estradiol decreases self-administration of mu opioid receptor agonists across a range of experimental conditions. Data collected in human laboratory studies suggest that estradiol may have functionally similar effects in clinical populations, and thus estrogen receptors may be a potential target in the development of novel therapeutics. This review summarizes data from cellular assays to clinical trials to explore how estradiol influences mu opioid receptor activity, as well as potential ways in which estrogen receptors may be targeted to address the problems of opioid use.

Comprehensive analysis of sex differences in the function and ultrastructure of hippocampal presynaptic terminals

Sex differences in the brain, encompassing variations in specific brain structures, size, cognitive function, and synaptic connections, have been identified across numerous species. While previous research has explored sex differences in postsynaptic structures, synaptic plasticity, and hippocampus-dependent functions, the hippocampal presynaptic terminals remain largely uninvestigated. The hippocampus is a critical structure responsible for multiple brain functions. This study examined presynaptic differences in cultured hippocampal neurons derived from male and female mice using a combination of biochemical assays, functional analyses measuring exocytosis and endocytosis of synaptic vesicle proteins, ultrastructural analyses via electron microscopy, and presynaptic Ca2+-specific optical probes. Our findings revealed that female neurons exhibited a higher number of synaptic vesicles at presynaptic terminals compared to male neurons. However, no significant differences were observed in presynaptic protein expression, presynaptic terminal ultrastructure, synaptic vesicle exocytosis and endocytosis, or presynaptic Ca2+ alterations between male and female neurons.

Sex-dependent differences in animal cognition

The differences in cognitive processes driven by biological sex are the issues that have gotten growing attention recently. Considering the increasing population suffering from various cognitive impairments and the development of therapeutic strategies, it is essential that we recognize the mechanisms responsible for discrepancies observed in male and female learning and memory functions. In this review, we discuss recent reports from preclinical studies on rodents regarding selected cognitive domains to explore the state of knowledge on sex-dependent differences and point to challenges encountered during such research. We focus on spatial, recognition, and emotional memory, as well as on executive functions, such as attention, cognitive flexibility, and working memory. This review will help to acknowledge sex-related differences in cognition and indicate some fields that lack sufficient data.

Stability of dentate gyrus granule cell mossy fiber BDNF protein expression with age and resistance of granule cells to Alzheimer's disease neuropathology in a mouse model

The neurotrophin brain-derived neurotrophic factor (BDNF) is important in development and maintenance of neurons and their plasticity. Hippocampal BDNF has been implicated Alzheimer's disease (AD) because hippocampal levels in AD patients and AD animal models are consistently downregulated, suggesting that reduced BDNF contributes to AD. However, the location where hippocampal BDNF protein is most highly expressed, the mossy fiber (MF) axons of dentate gyrus (DG) granule cells (GCs), has been understudied, and never in controlled in vivo conditions. We examined MF BDNF protein in the Tg2576 mouse model of AD. Tg2576 and wild type (WT) mice of both sexes were examined at 2-3 months of age, when amyloid-β (Aβ) is present in neurons but plaques are absent, and 11-20 months of age, after plaque accumulation. As shown previously, WT mice exhibited high levels of MF BDNF protein. Interestingly, there was no significant decline with age in either genotype or sex. Notably, we found a correlation between MF BDNF protein and GC ΔFosB, a transcription factor that increases after 1-2 weeks of elevated neuronal activity. Remarkably, there was relatively little evidence of Aβ in GCs or the GC layer even at old ages. Results indicate MF BDNF is stable in the Tg2576 mouse, and MF BDNF may remain unchanged due to increased GC neuronal activity, since BDNF expression is well known to be activity-dependent. The resistance of GCs to long-term Aβ accumulation provides an opportunity to understand how to protect other vulnerable neurons from increased Aβ levels and therefore has translational implications.

Estrous cycle impacts on dendritic spine plasticity in rat nucleus accumbens core and shell and caudate-putamen

An important factor that can modulate neuron properties is sex-specific hormone fluctuations, including the human menstrual cycle and rat estrous cycle in adult females. Considering the striatal brain regions, the nucleus accumbens (NAc) core, NAc shell, and caudate-putamen (CPu), the estrous cycle has previously been shown to impact relevant behaviors and disorders, neuromodulator action, and medium spiny neuron (MSN) electrophysiology. Whether the estrous cycle impacts MSN dendritic spine attributes has not yet been examined, even though MSN spines and glutamatergic synapse properties are sensitive to exogenously applied estradiol. Thus, we hypothesized that MSN dendritic spine attributes would differ by estrous cycle phase. To test this hypothesis, brains from adult male rats and female rats in diestrus, proestrus AM, proestrus PM, and estrus were processed for Rapid Golgi-Cox staining. MSN dendritic spine density, size, and type were analyzed in the NAc core, NAc shell, and CPu. Overall spine size differed across estrous cycle phases in female NAc core and NAc shell, and spine length differed across estrous cycle phase in NAc shell and CPu. Consistent with previous work, dendritic spine density was increased in the NAc core compared to the NAc shell and CPu, independent of sex and estrous cycle. Spine attributes in all striatal regions did not differ by sex when estrous cycle was disregarded. These results indicate, for the first time, that estrous cycle phase impacts dendritic spine plasticity in striatal regions, providing a neuroanatomical avenue by which sex-specific hormone fluctuations can impact striatal function and disorders.

A distinct impact of repeated morphine exposure on synaptic plasticity at Schaffer collateral-CA1, temporoammonic-CA1, and perforant pathway-dentate gyrus synapses along the longitudinal axis of the hippocampus

We aimed to study how morphine affects synaptic transmission in the dentate gyrus and CA1 regions along the hippocampal long axis. For this, recording and measuring of field excitatory postsynaptic potentials (fEPSPs) were utilized to test the effects of repeated morphine exposure on paired-pulse evoked responses and long-term potentiation (LTP) at Schaffer collateral-CA1 (Sch-CA1), temporoammonic-CA1 (TA-CA1) and perforant pathway-dentate gyrus (PP-DG) synapses in transverse slices from the dorsal (DH), intermediate (IH), and ventral (VH) hippocampus in adult male rats. After repeated morphine exposure, the expression of opioid receptors and the α1 and α5 GABA_A subunits were also examined. We found that repeated morphine exposure blunt the difference between the DH and the VH in their basal levels of synaptic transmission at Sch-CA1 synapses that were seen in the control groups. Significant paired-pulse facilitation of excitatory synaptic transmission was observed at Sch-CA1 synapses in slices taken from all three hippocampal segments as well as at PP-DG synapses in slices taken from the VH segment in the morphine-treated groups as compared to the control groups. Interestingly, significant paired-pulse inhibition of excitatory synaptic transmission was observed at TA-CA1 synapses in the DH slices from the morphine-treated group as compared to the control group. While primed-burst stimulation (a protocol reflecting normal neuronal firing) induced a robust LTP in hippocampal subfields in all control groups, resulting in a decaying LTP at TA-CA1 synapses in the VH slices and at PP-DG synapses in both the IH and VH slices taken from the morphine-treated rats. In the DH of morphine-treated rats, we found increased levels of the mRNAs encoding the α1 and α5 GABA_A subunits as compared to the control group. Taken together, these findings suggest the potential mechanisms through which repeated morphine exposure causes differential changes in circuit excitability and synaptic plasticity in the dentate gyrus and CA1 regions along the hippocampal long axis.

Exploration of beta-arrestin isoform signaling pathways in delta opioid receptor agonist-induced convulsions

The δ-opioid receptor (δOR) has been considered as a therapeutic target in multiple neurological and neuropsychiatric disorders particularly as δOR agonists are deemed safer alternatives relative to the more abuse-liable µ-opioid receptor drugs. Clinical development of δOR agonists, however, has been challenging in part due to the seizure-inducing effects of certain δOR agonists. Especially agonists that resemble the δOR-selective agonist SNC80 have well-established convulsive activity. Close inspection suggests that many of those seizurogenic δOR agonists efficaciously recruit β-arrestin, yet surprisingly, SNC80 displays enhanced seizure activity in β-arrestin 1 knockout mice. This finding led us to hypothesize that perhaps β-arrestin 1 is protective against, whereas β-arrestin 2 is detrimental for δOR-agonist-induced seizures. To investigate our hypothesis, we characterized three different δOR agonists (SNC80, ADL5859, ARM390) in cellular assays and in vivo in wild-type and β-arrestin 1 and β-arrestin 2 knockout mice for seizure activity. We also investigated downstream kinases associated with β-arrestin-dependent signal transduction. We discovered that δOR agonist-induced seizure activity strongly and positively correlates with β-arrestin 2 efficacy for the agonist, but that indirect inhibition of ERK activation using the MEK inhibitor SL327 did not inhibit seizure potency and duration. Inhibition of the PI3K/AKT/mTOR signaling with honokiol but not PQR530, attenuated SNC80 seizure duration in β-arrestin 1 knockout, but honokiol did not reduce SNC80-induced seizures in wild-type mice. Ultimately, our results indicate that β-arrestin 2 is correlated with δOR agonist-induced seizure intensity, but that global β-arrestin 1 knockout mice are a poor model system to investigate their mechanism of action.

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.

HIV-1 Tat reduces apical dendritic spine density throughout the trisynaptic pathway in the hippocampus of male transgenic mice

Nearly one-third of persons infected with HIV-1 (PWH) develop HIV-associated neurocognitive disorders (HAND), which can be exacerbated by exposure to opioids. The impact of opioids on HIV-induced alterations in neuronal plasticity is less well understood. Both morphine exposure and HIV have been shown to disrupt synaptic growth and stability in the hippocampus suggesting a potential site of convergence for their deleterious effects. In the present study, we examined the density of dendritic spines in CA1 and CA3 pyramidal neurons, and granule neurons within the dentate gyrus representing the hippocampal trisynaptic pathway after short-term exposure to the HIV transactivator of transcription (Tat) protein and morphine. We exposed inducible male, HIV-1 Tat transgenic mice to escalating doses of morphine (10-40 mg/kg, b.i.d.) and examined synaptodendritic structure in Golgi-impregnated hippocampal neurons. HIV-1 Tat, but not morphine, systematically reduced the density of apical, but not basilar, dendrites of CA1 and CA3 pyramidal neurons, and granule neuronal apical dendrites, suggesting the coordinated loss of specific synaptic interconnections throughout the hippocampal trisynaptic pathway.

Sex and chronic stress alter the distribution of glutamate receptors within rat hippocampal CA3 pyramidal cells following oxycodone conditioned place preference

Glutamate receptors have a key role in the neurobiology of opioid addiction. Using electron microscopic immunocytochemical methods, this project elucidates how sex and chronic immobilization stress (CIS) impact the redistribution of GluN1 and GluA1 within rat hippocampal CA3 pyramidal cells following oxycodone (Oxy) conditioned place preference (CPP). Four groups of female and male Sprague-Dawley rats subjected to CPP were used: Saline- (Sal) and Oxy-injected (3 mg/kg, I.P.) naïve rats; and Sal- and Oxy-injected CIS rats. GluN1: In both naive and CIS rats, Sal-females compared to Sal-males had elevated cytoplasmic and total dendritic GluN1. Following Oxy CPP, near plasmalemmal, cytoplasmic, and total GluN1 decreased in CA3 dendrites of unstressed females suggesting reduced pools of GluN1 available for ligand binding. Following CIS, Oxy-males (which did not acquire CPP) had increased GluN1 in all compartments of dendrites and spines of CA3 neurons. GluA1: There were no differences in the distribution GluA1 in any cellular compartments of CA3 dendrites in naïve females and males following either Sal or Oxy CPP. CIS alone increased the percent of GluA1 in CA3 dendritic spines in males compared to females. CIS Oxy-males compared to CIS Sal-males had an increase in cytoplasmic and total dendritic GluA1. Thus, in CIS Oxy-males increased pools of GluN1 and GluA1 are available for ligand binding in CA3 neurons. Together with our prior experiments, these changes in GluN1 and GluA1 following CIS in males may contribute to an increased sensitivity of CA3 neurons to glutamate excitation and a reduced capacity to acquire Oxy CPP.

Sex differences in the rodent hippocampal opioid system following stress and oxycodone associated learning processes

Over the past two decades, opioid abuse has risen especially among women. In both sexes hippocampal neural circuits involved in associative memory formation and encoding of motivational incentives are critically important in the transition from initial drug use to drug abuse/dependence. Opioid circuits, particularly the mossy fiber pathway, are crucial for associative memory processes important for addiction. Our anatomical studies, especially those utilizing electron microscopic immunocytochemistry, have provided unique insight into sex differences in the distribution of opioid peptides and receptors in specific hippocampal circuits and how these distributions are altered following stress and oxycodone-associative learning processes. Here we review the hippocampal opioid system in rodents with respect to ovarian hormones effects and baseline sex differences then sex differences following acute and chronic stress. Next, we review sex differences in the hippocampal opioid system in unstressed and chronically stressed rats following oxycodone conditioned place preference. We show that opioid peptides and receptors are distributed within hippocampal circuits in females with elevated estrogen states in a manner that would enhance sensitivity to endogenous and exogenous opioids. Moreover, chronic stress primes the opioid system in females in a manner that would promote opioid-associative learning processes. In contrast, chronic stress has limited effects on the opioid system in males and reduces its capacity to support opioid-mediated learning processes. Interestingly, acute stress appears to prime males for opioid associative learning. On a broader scale the findings highlighted in this review have important implications in understanding sex differences in opioid drug use and abuse.

Acute Delta 9-tetrahydrocannabinol administration differentially alters the hippocampal opioid system in adult female and male rats

Our prior studies demonstrated that the rat hippocampal opioid system can undergo sex-specific adaptations to external stimuli that can influence opioid-associated learning processes. This opioid system extensively overlaps with the cannabinoid system. Moreover, acute administration of Δ⁹ Tetrahydrocannabinoid (THC), the primary psychoactive constituent of cannabis, can alter cognitive behaviors that involve the hippocampus. Here, we use light and electron microscopic immunocytochemical methods to examine the effects of acute THC (5 mg/kg, i.p., 1 h) on mossy fiber Leu-Enkephalin (LEnk) levels and the distribution and phosphorylation levels of delta and mu opioid receptors (DORs and MORs, respectively) in CA3 pyramidal cells and parvalbumin dentate hilar interneurons of adult female and male Sprague-Dawley rats. In females with elevated estrogen states (proestrus/estrus stage), acute THC altered the opioid system so that it resembled that seen in vehicle-injected females with low estrogen states (diestrus) and males: (1) mossy fiber LEnk levels in CA2/3a decreased; (2) phosphorylated-DOR levels in CA2/3a pyramidal cells increased; and (3) phosphorylated-MOR levels increased in most CA3b laminae. In males, acute THC resulted in the internalization of MORs in parvalbumin-containing interneuron dendrites which would decrease disinhibition of granule cells. In both sexes, acute THC redistributed DORs to the near plasma membrane of CA3 pyramidal cell dendrites, however, the dendritic region varied with sex. Additionally, acute THC also resulted in a sex-specific redistribution of DORs within CA3 pyramidal cell dendrites which could differentially promote synaptic plasticity and/or opioid-associated learning processes in both females and males.

Long-term depression of excitatory transmission in the lateral septum

Neurons in the lateral septum (LS) integrate glutamatergic synaptic inputs, primarily from hippocampus, and send inhibitory projections to brain regions involved in reward and the generation of motivated behavior. Motivated learning and drugs of abuse have been shown to induce long-term changes in the strength of glutamatergic synapses in the LS, but the cellular mechanisms underlying long-term synaptic modification in the LS are poorly understood. Here, we examined synaptic transmission and long-term depression (LTD) in brain slices prepared from male and female C57BL/6 mice. No sex differences were observed in whole cell patch-clamp recordings of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R)- and N-methyl-d-aspartate receptor (NMDA-R)-mediated currents. Low-frequency stimulation of the fimbria fiber bundle (1 Hz 15 min) induced LTD of the LS field excitatory postsynaptic potential (fEPSP). Induction of LTD was blocked by the NMDA-R antagonist (d)-2-amino-5-phosphonovaleric acid (APV), but not the selective antagonist of GluN2B-containing NMDA-Rs ifenprodil. These results demonstrate the NMDA-R dependence of LTD in the LS. The LS is a sexually dimorphic structure, and sex differences in glutamatergic transmission have been reported in vivo; our results suggest sex differences observed in vivo result from network activity rather than intrinsic differences in glutamatergic transmission.NEW & NOTEWORTHY The lateral septum (LS) integrates information from hippocampus and other regions to provide context-dependent (top down or higher order) regulation of mood and motivated behavior. Learning and drugs of abuse induce long-term changes in the strength of glutamatergic projections to the LS; however, the cellular mechanisms underlying such changes are poorly understood. Here, we demonstrate there are no apparent sex differences in fast excitatory transmission and that long-term synaptic depression in the LS is NMDA-R dependent.

Chronic stress differentially alters mRNA expression of opioid peptides and receptors in the dorsal hippocampus of female and male rats

Chronic immobilization stress (CIS) results in sex-dependent changes in opioid peptide levels and receptor subcellular distributions within the rat dorsal hippocampus, which are paralleled with an inability for males to acquire conditioned place preference (CPP) to oxycodone. Here, RNAScope in situ hybridization was used to determine the expression of hippocampal opioid peptides and receptors in unstressed (US) and CIS estrus female and male adult (∼2.5 months old ) Sprague Dawley rats. In all groups, dentate granule cells expressed PENK and PDYN; additionally, numerous interneurons expressed PENK. OPRD1 and OPRM1 were primarily expressed in interneurons, and to a lesser extent, in pyramidal and granule cells. OPRK1-was expressed in sparsely distributed interneurons. There were few baseline sex differences: US females compared to US males had more PENK-expressing and fewer OPRD1-expressing granule cells and more OPRM1-expressing CA3b interneurons. Several expression differences emerged after CIS. Both CIS females and males compared to their US counterparts had elevated: (1) PENK-expressing dentate granule cells and interneurons in CA1 and CA2/3a; (2) OPRD1 probe number and cell expression in CA1, CA2/3a and CA3b and the dentate gyrus; and (3) OPRK1-expressing interneurons in the dentate hilus. Also, CIS males compared to US males had elevated: (1) PDYN expression in granule cells; (2) OPRD1 probe and interneuron expression in CA2/3a; (3) OPRM1 in granule cells; and (4) OPRK1 interneuron expression in CA2/3a. The sex-specific changes in hippocampal opioid gene expression may impact network properties and synaptic plasticity processes that may contribute to the attenuation of oxycodone CPP in CIS males.

Bidirectional Regulation of Cognitive and Anxiety-like Behaviors by Dentate Gyrus Mossy Cells in Male and Female Mice

The dentate gyrus (DG) of the hippocampus is important for cognition and behavior. However, the circuits underlying these functions are unclear. DG mossy cells (MCs) are potentially important because of their excitatory synapses on the primary cell type, granule cells (GCs). However, MCs also activate GABAergic neurons, which inhibit GCs. We used viral delivery of designer receptors exclusively activated by designer drugs (DREADDs) in mice to implement a gain- and loss-of-function study of MCs in diverse behaviors. Using this approach, manipulations of MCs could bidirectionally regulate behavior. The results suggest that inhibiting MCs can reduce anxiety-like behavior and improve cognitive performance. However, not all cognitive or anxiety-related behaviors were influenced, suggesting specific roles of MCs in some, but not all, types of cognition and anxiety. Notably, several behaviors showed sex-specific effects, with females often showing more pronounced effects than the males. We also used the immediate early gene c-Fos to address whether DREADDs bidirectionally regulated MC or GC activity. We confirmed excitatory DREADDs increased MC c-Fos. However, there was no change in GC c-Fos, consistent with MC activation leading to GABAergic inhibition of GCs. In contrast, inhibitory DREADDs led to a large increase in GC c-Fos, consistent with a reduction in MC excitation of GABAergic neurons, and reduced inhibition of GCs. Together, these results suggest that MCs regulate anxiety and cognition in specific ways. We also raise the possibility that cognitive performance may be improved by reducing anxiety.SIGNIFICANCE STATEMENT The dentate gyrus (DG) has many important cognitive roles as well as being associated with affective behavior. This study addressed how a glutamatergic DG cell type called mossy cells (MCs) contributes to diverse behaviors, which is timely because it is known that MCs regulate the activity of the primary DG cell type, granule cells (GCs), but how MC activity influences behavior is unclear. We show, surprisingly, that activating MCs can lead to adverse behavioral outcomes, and inhibiting MCs have an opposite effect. Importantly, the results appeared to be task-dependent and showed that testing both sexes was important. Additional experiments indicated what MC and GC circuitry was involved. Together, the results suggest how MCs influence behaviors that involve the DG.

Signaling mechanisms of μ-opioid receptor (MOR) in the hippocampus: disinhibition versus astrocytic glutamate regulation

μ-opioid receptor (MOR) is a class of opioid receptors that is critical for analgesia, reward, and euphoria. MOR is distributed in various brain regions, including the hippocampus, where traditionally, it is believed to be localized mainly at the presynaptic terminals of the GABAergic inhibitory interneurons to exert a strong disinhibitory effect on excitatory pyramidal neurons. However, recent intensive research has uncovered the existence of MOR in hippocampal astrocytes, shedding light on how astrocytic MOR participates in opioid signaling via glia-neuron interaction in the hippocampus. Activation of astrocytic MOR has shown to cause glutamate release from hippocampal astrocytes and increase the excitability of presynaptic axon fibers to enhance the release of glutamate at the Schaffer Collateral-CA1 synapses, thereby, intensifying the synaptic strength and plasticity. This novel mechanism involving astrocytic MOR has been shown to participate in hippocampus-dependent conditioned place preference. Furthermore, the signaling of hippocampal MOR, whose action is sexually dimorphic, is engaged in adult neurogenesis, seizure, and stress-induced memory impairment. In this review, we focus on the two profoundly different hippocampal opioid signaling pathways through either GABAergic interneuronal or astrocytic MOR. We further compare and contrast their molecular and cellular mechanisms and their possible roles in opioid-associated conditioned place preference and other hippocampus-dependent behaviors.

Sex and chronic stress alter delta opioid receptor distribution within rat hippocampal CA1 pyramidal cells following behavioral challenges

Following oxycodone (Oxy) conditioned place preference (CPP), delta opioid receptors (DORs) differentially redistribute in hippocampal CA3 pyramidal cells in female and male rats in a manner that would promote plasticity and opioid-associative learning processes. However, following chronic immobilization stress (CIS), males do not acquire Oxy-CPP and the trafficking of DORs in CA3 pyramidal neurons is attenuated. Here, we examined the subcellular distribution of DORs in CA1 pyramidal cells using electron microscopy in these same cohorts.

CPP

Saline (Sal)-females compared to Sal-males have more cytoplasmic and total DORs in dendrites and more DOR-labeled spines. Following Oxy-CPP, DORs redistribute from near-plasmalemma pools in dendrites to spines in males.

CIS

Control females compared to control males have more near-plasmalemmal dendritic DORs. Following CIS, dendritic DORs are elevated in the cytoplasm in females and near-plasmalemma in males.

CIS plus CPP

CIS Sal-females compared to CIS Sal-males have more DORs on the plasmalemma of dendrites and in spines. After Oxy, the distribution of DORs does not change in either females or males.

Conclusion

Following Oxy-CPP, DORs within CA1 pyramidal cells remain positioned in naïve female rats to enhance sensitivity to DOR agonists and traffic to dendritic spines in naïve males where they can promote plasticity processes. Following CIS plus behavioral enrichment, DORs are redistributed within CA1 pyramidal cells in females in a manner that could enhance sensitivity to DOR agonists. Conversely, CIS plus behavioral enrichment does not alter DORs in CA1 pyramidal cells in males, which may contribute to their diminished capacity to acquire Oxy-CPP.

Oxycodone injections not paired with conditioned place preference have little effect on the hippocampal opioid system in female and male rats

Oxycodone (Oxy) conditioned place preference (CPP) in Sprague Dawley rats results in sex-specific alterations in hippocampal opioid circuits in a manner that facilitates opioid-associative learning processes, particularly in females. Here, we examined if Oxy (3 mg/kg, I.P.) or saline (Sal) injections not paired with behavioral testing similarly affect the hippocampal opioid system. Sal-injected females compared to Sal-injected males had: (1) higher densities of cytoplasmic delta opioid receptors (DOR) in GABAergic hilar dendrites suggesting higher baseline reserve DOR pools and (2) elevated phosphorylated DOR levels, but lower phosphorylated mu opioid receptor (MOR) levels in CA3a suggesting that the baseline pools of activated opioid receptors vary in females and males. In contrast to CPP studies, Oxy-injections in the absence of behavioral tests resulted in few changes in the hippocampal opioid system in either females or males. Specifically, Oxy-injected males compared to Sal-injected males had fewer DORs near the plasma membrane of CA3 pyramidal cell dendrites and in CA3 dendritic spines contacted by mossy fibers, and lower pMOR levels in CA3a. Oxy-injected females compared to Sal-injected females had higher total DORs in GABAergic dendrites and lower total MORs in parvalbumin-containing dendrites. Thus, unlike Oxy CPP, Oxy-injections redistributed opioid receptors in hippocampal neurons in a manner that would either decrease (males) or not alter (females) excitability and plasticity processes. These results indicate that the majority of changes within hippocampal opioid circuits that would promote opioid-associative learning processes in both females and males do not occur with Oxy administration alone, and instead must be paired with CPP.

Alzheimer's Disease, a Lipid Story: Involvement of Peroxisome Proliferator-Activated Receptor α

Alzheimer's disease (AD) is the leading cause of dementia in the elderly. Mutations in genes encoding proteins involved in amyloid-β peptide (Aβ) production are responsible for inherited AD cases. The amyloid cascade hypothesis was proposed to explain the pathogeny. Despite the fact that Aβ is considered as the main culprit of the pathology, most clinical trials focusing on Aβ failed and suggested that earlier interventions are needed to influence the course of AD. Therefore, identifying risk factors that predispose to AD is crucial. Among them, the epsilon 4 allele of the apolipoprotein E gene that encodes the major brain lipid carrier and metabolic disorders such as obesity and type 2 diabetes were identified as AD risk factors, suggesting that abnormal lipid metabolism could influence the progression of the disease. Among lipids, fatty acids (FAs) play a fundamental role in proper brain function, including memory. Peroxisome proliferator-activated receptor α (PPARα) is a master metabolic regulator that regulates the catabolism of FA. Several studies report an essential role of PPARα in neuronal function governing synaptic plasticity and cognition. In this review, we explore the implication of lipid metabolism in AD, with a special focus on PPARα and its potential role in AD therapy.

Female and male rats readily consume and prefer oxycodone to water in a chronic, continuous access, two-bottle oral voluntary paradigm

The increasing abuse of opioids - such as oxycodone - poses major challenges for health and socioeconomic systems. Human prescription opioid abuse is marked by chronic, voluntary, oral intake and sex differences. To develop interventions, the field would benefit from a preclinical paradigm that similarly provides rodents with chronic, continuous, oral, voluntary and free-choice access to oxycodone. Here we show female and male rats voluntarily ingest and choose oxycodone over water and show both dependence and motivation to take oxycodone during a chronic oral voluntary, two-bottle choice, continuous access paradigm. Adult female and male Long-Evans rats were given unlimited, continuous homecage access to two bottles containing water (Control) or one bottle of water and one bottle of oxycodone dissolved in water (Experimental). Virtually all experimental rats voluntarily drank oxycodone (~10 mg/kg/day) and escalated their intake over 22 weeks. Females self-administered twice as much oxycodone by body weight (leading to higher blood levels of oxycodone) and engaged in more gnawing behavior of wooden blocks relative to males. Precipitated withdrawal revealed high levels of dependence in both sexes. Reflecting motivation to drink oxycodone, ascending concentrations of citric acid suppressed the intake of oxycodone (Experimental) and the intake of water (Control); however, Experimental rats returned to pre-citric acid preference levels whereas Controls rats did not. Pre-screening behaviors of rats on open field exploration predicted oxycodone intake. Thus, rats consumed and preferred oxycodone over time in this chronic two-bottle oral choice paradigm and both sexes displayed many features of human oxycodone abuse.

Sex and chronic stress differentially alter phosphorylated mu and delta opioid receptor levels in the rat hippocampus following oxycodone conditioned place preference

Following oxycodone conditioned place preference (CPP) in naïve female and male Sprague Dawley rats, delta- and mu-opioid receptors (DORs and MORs) redistribute in hippocampal CA3 pyramidal cells and GABAergic interneurons in a manner that would promote opioid-associative learning processes, particularly in females. MORs and DORs similarly redistribute in CA3 and hilar neurons following chronic immobilization stress (CIS) in females, but not males, essentially "priming" the opioid system for oxycodone-associative learning. Following CIS, only females acquire oxycodone CPP. The present study determined whether sex and CIS differentially affect the levels of phosphorylated MORs and DORs (pMORs and pDORs) in the hippocampus following oxycodone CPP as phosphorylation is important for opioid receptor internationalization and trafficking. In naïve oxycodone-injected (Oxy) female rats, the density of pMOR-immunoreactivity (ir) was increased in CA1 stratum oriens and CA3a,b strata lucidum and radiatum compared to saline-injected (Sal)-females. Additionally, the density of pDOR-ir increased in the pyramidal cell layer and stratum radiatum of CA2/3a in Oxy-males compared to Sal-males. In CIS females that acquire CPP, pDOR-ir levels were increased in the CA2/3a. These findings indicate only rats that acquire oxycodone CPP have activated MORs and DORs in the hippocampus but that the subregion containing activated opioid receptors differs in females and males. These results are consistent with previously observed sex differences in the hippocampal opioid system following Oxy-CPP.

A Dynamic Memory Systems Framework for Sex Differences in Fear Memory

Emerging research demonstrates that a pattern of overlapping but distinct molecular and circuit mechanisms are engaged by males and females during memory tasks. Importantly, sex differences in neural mechanisms and behavioral strategies are evident even when performance on a memory task is similar between females and males. We propose that sex differences in memory may be best understood within a dynamic memory systems framework. Specifically, sex differences in hormonal influences and neural circuit development result in biases in the circuits engaged and the information preferentially stored or retrieved in males and females. By using animal models to understand the neural networks and molecular mechanisms required for memory in both sexes, we can gain crucial insights into sex and gender biases in disorders including post-traumatic stress disorder (PTSD) in humans.

Comparative study of hypoglycaemia induced by opioids. Is it a class effect?

Objective: Drug-induced hypoglycaemia has been described with the use of tramadol and methadone. The authors aimed to determine if drug-induced hypoglycaemia could be a class effect for opioids. Methods: The authors performed a disproportionality analysis in VigiBase®, the WHO global individual case safety report database with nine opioids (codeine, fentanyl, hydromorphone, methadone, morphine, oxycodone, tramadol, buprenorphine and nalbuphine) using the broad Standardised MedDRA Query for hypoglycaemia. The authors also carried out a descriptive study of opioid-induced hypoglycaemia in the French PharmacoVigilance DataBase (FPVDB) using the MedDRA Preferred Term 'hypoglycaemia'. Results: The global adjusted Reporting Odds Ratio (aROR) value for the 9 opioids was 1.53 (95% CI 1.52-1.54). The aROR ranged from 1.09 to 1.97 depending on the opioid, but all were statistically significant. A sex ratio of 0.74 was found for the reports of opioid-induced hypoglycaemia in Vigibase®. The authors also found 133 reports of hypoglycaemia in the FPVDB related to opioids. Among the reports, 55 were glycaemic imbalances in diabetics occurring shortly after the start of opioid treatment. Conclusion: This work highlighted a significant association between all opioids and hypoglycaemia, thereby indicating that opioid-induced hypoglycaemia is probably a class effect. Women and/or diabetics seem to be more at risk for developing opioid-induced hypoglycaemia.

Sex Differences in Neuroplasticity- and Stress-Related Gene Expression and Protein Levels in the Rat Hippocampus Following Oxycodone Conditioned Place Preference

Prescription opioid abuse is a serious public health issue. Recently, we showed that female and male Sprague-Dawley rats acquire conditioned place preference (CPP) to the mu opioid receptor agonist oxycodone. Anatomical analysis of the hippocampus from these rats unveiled sex differences in the opioid system in a way that would support excitation and opiate associative learning processes especially in females. In this study, we examined the expression and protein densities of opioid, plasticity, stress and related kinase and signaling molecules in the hippocampus of female and male rats following oxycodone CPP. Oxycodone CPP females have: a) increases in ARC (activity regulated cytoskeletal-associated protein)-immunoreactivity (ir) in CA3 pyramidal cells; b) decreases in Npy (neuropeptide Y) gene expression in the medial hippocampus but higher numbers of NPY-containing hilar interneurons compared to males; c) increases in Crhr2 (corticotropin releasing factor receptor 2) expression in CA2/3; d) increases in Akt1 (AKT serine/threonine kinase 1) expression in medial hippocampus; and e) decreases in phosphorylated MAPK (mitogen activated protein kinase)-ir in CA1 and dentate gyrus. Oxycodone CPP males have: a) increases in Bdnf (brain derived-neurotrophic factor) expression, which is known to be produced in granule cells, relative to females; b) elevated Mapk1 expression and pMAPK-ir in the dentate hilus which harbors newly generated granule cells; and c) increases in CRHR1-ir in CA3 pyramidal cell soma. These sex-specific changes in plasticity, stress and kinase markers in hippocampal circuitry parallel previously observed sex differences in the opioid system after oxycodone CPP.

Sex Differences in the Behavioral and Synaptic Consequences of a Single in vivo Exposure to the Synthetic Cannabimimetic WIN55,212-2 at Puberty and Adulthood

Heavy cannabis consumption among adolescents is associated with significant and lasting neurobiological, psychological and health consequences that depend on the age of first use. Chronic exposure to cannabinoid agonists during the perinatal period or adolescence alters social behavior and prefrontal cortex (PFC) activity in adult rats. However, sex differences on social behavior as well as PFC synaptic plasticity after acute cannabinoid activation remain poorly explored. Here, we determined that the consequences of a single in vivo exposure to the synthetic cannabimimetic WIN55,212-2 differently affected PFC neuronal and synaptic functions after 24 h in male and female rats during the pubertal and adulthood periods. During puberty, single cannabinoid exposure (SCE) reduced play behavior in females but not males. In contrast, the same treatment impaired sociability in both sexes at adulthood. General exploration and memory recognition remained normal at both ages and both sexes. At the synaptic level, SCE ablated endocannabinoid-mediated synaptic plasticity in the PFC of females of both ages and heightened excitability of PFC pyramidal neurons at adulthood, while males were spared. In contrast, cannabinoid exposure was associated with impaired long-term potentiation (LTP) specifically in adult males. Together, these data indicate behavioral and synaptic sex differences in response to a single in vivo exposure to cannabinoid at puberty and adulthood.

Chronic immobilization stress primes the hippocampal opioid system for oxycodone-associated learning in female but not male rats

In adult female, but not male, Sprague Dawley rats, chronic immobilization stress (CIS) increases mossy fiber (MF) Leu-Enkephalin levels and redistributes delta- and mu-opioid receptors (DORs and MORs) in hippocampal CA3 pyramidal cells and GABAergic interneurons to promote excitation and learning processes following subsequent opioid exposure. Here, we demonstrate that CIS females, but not males, acquire conditioned place preference (CPP) to oxycodone and that CIS "primes" the hippocampal opioid system in females for oxycodone-associated learning. In CA3b, oxycodone-injected (Oxy) CIS females relative to saline-injected (Sal) CIS females exhibited an increase in the cytoplasmic and total densities of DORs in pyramidal cell dendrites so that they were similar to Sal- and Oxy-CIS males. Consistent with our earlier studies, Sal- and Oxy-CIS females but not CIS males had elevated DOR densities in MF-CA3 dendritic spines, which we have previously shown are important for opioid-mediated long-term potentiation. In the dentate gyrus, Oxy-CIS females had more DOR-labeled interneurons than Sal-CIS females. Moreover, Sal- and Oxy-CIS females compared to both groups of CIS males had elevated levels of DORs and MORs in GABAergic interneuron dendrites, suggesting capacity for greater synthesis or storage of these receptors in circuits important for opioid-mediated disinhibition. However, more plasmalemmal MORs were on large parvalbumin-containing dendrites of Oxy-CIS males compared to Sal-CIS males, suggesting a limited ability for increased granule cell disinhibition. These results suggest that low levels of DORs in MF-CA3 synapses and hilar GABAergic interneurons may contribute to the attenuation of oxycodone CPP in males exposed to CIS.

Sex differences in hippocampal cognition and neurogenesis

Sex differences are reported in hippocampal plasticity, cognition, and in a number of disorders that target the integrity of the hippocampus. For example, meta-analyses reveal that males outperform females on hippocampus-dependent tasks in rodents and in humans, furthermore women are more likely to experience greater cognitive decline in Alzheimer's disease and depression, both diseases characterized by hippocampal dysfunction. The hippocampus is a highly plastic structure, important for processing higher order information and is sensitive to the environmental factors such as stress. The structure retains the ability to produce new neurons and this process plays an important role in pattern separation, proactive interference, and cognitive flexibility. Intriguingly, there are prominent sex differences in the level of neurogenesis and the activation of new neurons in response to hippocampus-dependent cognitive tasks in rodents. However, sex differences in spatial performance can be nuanced as animal studies have demonstrated that there are task, and strategy choice dependent sex differences in performance, as well as sex differences in the subregions of the hippocampus influenced by learning. This review discusses sex differences in pattern separation, pattern completion, spatial learning, and links between adult neurogenesis and these cognitive functions of the hippocampus. We emphasize the importance of including both sexes when studying genomic, cellular, and structural mechanisms of the hippocampal function.

Sex Differences in the Rat Hippocampal Opioid System After Oxycodone Conditioned Place Preference

Although opioid addiction has risen dramatically, the role of gender in addiction has been difficult to elucidate. We previously found sex-dependent differences in the hippocampal opioid system of Sprague-Dawley rats that may promote associative learning relevant to drug abuse. The present studies show that although female and male rats acquired conditioned place preference (CPP) to the mu-opioid receptor (MOR) agonist oxycodone (3 mg/kg, I.P.), hippocampal opioid circuits were differentially altered. In CA3, Leu-Enkephalin-containing mossy fibers had elevated levels in oxycodone CPP (Oxy) males comparable to those in females and sprouted in Oxy-females, suggesting different mechanisms for enhancing opioid sensitivity. Electron microscopy revealed that in Oxy-males delta opioid receptors (DORs) redistributed to mossy fiber-CA3 synapses in a manner resembling females that we previously showed is important for opioid-mediated long-term potentiation. Moreover, in Oxy-females DORs redistributed to CA3 pyramidal cell spines, suggesting the potential for enhanced plasticity processes. In Saline-injected (Sal) females, dentate hilar parvalbumin-containing basket interneuron dendrites had fewer MORs, however plasmalemmal and total MORs increased in Oxy-females. In dentate hilar GABAergic dendrites that contain neuropeptide Y, Sal-females compared to Sal-males had higher plasmalemmal DORs, and near-plasmalemmal DORs increased in Oxy-females. This redistribution of MORs and DORs within hilar interneurons in Oxy-females would potentially enhance disinhibition of granule cells via two different circuits. Together, these results indicate that oxycodone CPP induces sex-dependent redistributions of opioid receptors in hippocampal circuits in a manner facilitating opioid-associative learning processes and may help explain the increased susceptibility of females to opioid addiction acquisition and relapse.

Systematic review of sex-based differences in opioid-based effects

There are several biological factors that might play a role in physiological response to opioids and/or the onset of problematic opioid use; however, sex-based differences in non-analgesic opioid-based effects are poorly understood. The goal of this review is to provide a current analysis of the pre-clinical literature on sex-based differences in response to endogenous and exogenous opioids, including the interplay between sex hormones and opioid receptor-mediated neuronal activity and associated behaviours. A systematic search was performed on the following terms within PubMed between March and April 2018: 'opioid oestrogen', 'opioid progesterone', 'opioid oestradiol', and 'opioid testosterone'. Pre-clinical research on the non-analgesic, sex-based effects of opioids is disparate, both in terms of methodology and outcomes, which prohibits a cohesive summary of the results. Themes from the pre-clinical literature suggest that opioid receptor binding, coupling, and density vary as a function of hormone exposure. Findings also suggest that interactions between endogenous opioid and stress systems may differ between males and females as a function of ovarian hormones. Given the current opioid-related public health crisis, there is a pressing need to increase systematic pre-clinical and clinical research on sex-based differences in opioid-effects and opioid use disorder.

Sex Differences in Synaptic Plasticity: Hormones and Beyond

Notable sex-differences exist between neural structures that regulate sexually dimorphic behaviors such as reproduction and parenting. While anatomical differences have been well-characterized, advancements in neuroimaging and pharmacology techniques have allowed researchers to identify differences between males and females down to the level of the synapse. Disparate mechanisms at the synaptic level contribute to sex-specific neuroplasticity that is reflected in sex-dependent behaviors. Many of these synaptic differences are driven by the endocrine system and its impact on molecular signaling and physiology. While sex-dependent modifications exist at baseline, further differences emerge in response to stimuli such as stressors. While some of these mechanisms are unifying between sexes, they often have directly opposing consequences in males and females. This variability is tied to gonadal steroids and their interactions with intra- and extra-cellular signaling mechanisms. This review article focuses on the various mechanisms by which sex can alter synaptic plasticity, both directly and indirectly, through steroid hormones such as estrogen and testosterone. That sex can drive neuroplasticity throughout the brain, highlights the importance of understanding sex-dependent neural mechanisms of the changing brain to enhance interpretation of results regarding males and females. As mood and stress responsivity are characterized by significant sex-differences, understanding the molecular mechanisms that may be altering structure and function can improve our understanding of these behavioral and mental characteristics.

Sex Differences in the Subcellular Distribution of Corticotropin-Releasing Factor Receptor 1 in the Rat Hippocampus following Chronic Immobilization Stress

Corticotropin-releasing factor receptors (CRFR1) contribute to stress-induced adaptations in hippocampal structure and function that can affect learning and memory processes. Our prior studies showed that female rats with elevated estrogens compared to males have more plasmalemmal CRFR1 in CA1 pyramidal cells, suggesting a greater sensitivity to stress. Here, we examined the distribution of hippocampal CRFR1 following chronic immobilization stress (CIS) in female and male rats using immuno-electron microscopy. Without stress, total CRFR1 dendritic levels were higher in females in CA1 and in males in the hilus; moreover, plasmalemmal CRFR1 was elevated in pyramidal cell dendrites in CA1 in females and in CA3 in males. Following CIS, near-plasmalemmal CRFR1 increased in CA1 pyramidal cell dendrites in males but not to levels of control or CIS females. In CA3 and the hilus, CIS decreased cytoplasmic and total CRFR1 in dendrites in males only. These results suggest that in naive rats, CRF could induce a greater activation of CA1 pyramidal cells in females than males. Moreover, after CIS, which leads to even greater sex differences in CRFR1 by trafficking it to different subcellular compartments, CRF could enhance activation of CA1 pyramidal cells in males but to a lesser extent than either unstressed or CIS females. Additionally, CA3 pyramidal cells and inhibitory interneurons in males have heightened sensitivity to CRF, regardless of stress state. These sex differences in CRFR1 distribution and trafficking in the hippocampus may contribute to reported sex differences in hippocampus-dependent learning processes in baseline conditions and following chronic stress.

Morphine-potentiated cognitive deficits correlate to suppressed hippocampal iNOS RNA expression and an absent type 1 interferon response in LP-BM5 murine AIDS

Opioid use accelerates neurocognitive impairment in HIV/AIDS patients. We assessed the effect of chronic morphine treatment and LP-BM5/murine AIDS (MAIDS) infection on cognition, cytokine production, and type 1 interferon (IFN) expression in the murine CNS. Morphine treatment decreased expression of pro-inflammatory factors (CCL5, iNOS) and reduced cognitive performance in LP-BM5-infected mice, correlating to increased hippocampal viral load and a blunted type 1 IFN response. In the striatum, morphine reduced viral load while increasing IFN-α RNA expression. Our results suggest that differentially regulated type 1 IFN responses may contribute to distinct regional outcomes in the hippocampus and striatum in LP-BM5/MAIDS.

Sex differences after chronic stress in the expression of opioid-, stress- and neuroplasticity-related genes in the rat hippocampus

Opioid peptides and their receptors re-organize within hippocampal neurons of female, but not male, rats following chronic immobilization stress (CIS) in a manner that promotes drug-related learning. This study was conducted to determine if there are also sex differences in gene expression in the hippocampus following CIS. Adult female and male rats were subjected to CIS (30 min/day) for 10 days. Twenty-four hours after the last stressor, the rats were euthanized, the brains were harvested and the medial (dentate gyrus/CA1) and lateral (CA2/CA3) dorsal hippocampus were isolated. Following total RNA isolation, cDNA was prepared for gene expression analysis using a RT² Profiler PCR expression array. This custom designed qPCR expression array contained genes for opioid peptides and receptors, as well as genes involved in stress-responses and candidate genes involved in synaptic plasticity, including those upregulated following oxycodone self-administration in mice. Few sex differences are seen in hippocampal gene expression in control (unstressed) rats. In response to CIS, gene expression in the hippocampus was altered in males but not females. In males, opioid, stress, plasticity and kinase/signaling genes were all down-regulated following CIS, except for the gene that codes for corticotropin releasing hormone, which was upregulated. Changes in opioid gene expression following chronic stress were limited to the CA2 and CA3 regions (lateral sample). In conclusion, modest sex- and regional-differences are seen in expression of the opioid receptor genes, as well as genes involved in stress and plasticity responses in the hippocampus following CIS.

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Unstressed female rats have less Arc expression in hippocampus than males.

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Chronic immobilization stress (CIS) down-regulates opioid gene expression in males.

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CIS up-regulates Crh but down-regulates other stress genes in male hippocampi.

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CIS down-regulates Arc and other plasticity genes in male hippocampi.

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CIS down-regulates select kinases and signaling molecules in male hippocampi.

Sex differences in hippocampal area CA3 pyramidal cells

Numerous studies have demonstrated differences between males and females in hippocampal structure, function, and plasticity. There also are many studies about the different predisposition of a males and females for disorders where the hippocampus plays an important role. Many of these reports focus on area CA1, but other subfields are also very important, and unlikely to be the same as area CA1 based on what is known. Here we review basic studies of male and female structure, function, and plasticity of area CA3 pyramidal cells of adult rats. The data suggest that the CA3 pyramidal cells of males and females are distinct in structure, function, and plasticity. These sex differences cannot be simply explained by the effects of circulating gonadal hormones. This view agrees with previous studies showing that there are substantial sex differences in the brain that cannot be normalized by removing the gonads and depleting peripheral gonadal hormones. Implications of these comparisons for understanding sex differences in hippocampal function and dysfunction are discussed. © 2016 Wiley Periodicals, Inc.

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.

Understanding the broad influence of sex hormones and sex differences in the brain

Sex hormones act throughout the entire brain of both males and females via both genomic and nongenomic receptors. Sex hormones can act through many cellular and molecular processes that alter structure and function of neural systems and influence behavior as well as providing neuroprotection. Within neurons, sex hormone receptors are found in nuclei and are also located near membranes, where they are associated with presynaptic terminals, mitochondria, spine apparatus, and postsynaptic densities. Sex hormone receptors also are found in glial cells. Hormonal regulation of a variety of signaling pathways as well as direct and indirect effects on gene expression induce spine synapses, up- or downregulate and alter the distribution of neurotransmitter receptors, and regulate neuropeptide expression and cholinergic and GABAergic activity as well as calcium sequestration and oxidative stress. Many neural and behavioral functions are affected, including mood, cognitive function, blood pressure regulation, motor coordination, pain, and opioid sensitivity. Subtle sex differences exist for many of these functions that are developmentally programmed by hormones and by not yet precisely defined genetic factors, including the mitochondrial genome. These sex differences and responses to sex hormones in brain regions, which influence functions not previously regarded as subject to such differences, indicate that we are entering a new era of our ability to understand and appreciate the diversity of gender-related behaviors and brain functions. © 2016 Wiley Periodicals, Inc.

Expansion of mossy fibers and CA3 apical dendritic length accompanies the fall in dendritic spine density after gonadectomy in male, but not female, rats

Androgen loss is an important clinical concern because of its cognitive and behavioral effects. Changes in androgen levels are also suspected to contribute to neurological disease. However, the available data on the effects of androgen deprivation in areas of the brain that are central to cognition, like the hippocampus, are mixed. In this study, morphological analysis of pyramidal cells was used to investigate if structural changes could potentially contribute to the mixed cognitive effects that have been observed after androgen loss in males. Male Sprague-Dawley rats were orchidectomized or sham-operated. Two months later, their brains were Golgi-impregnated for morphological analysis. Morphological endpoints were studied in areas CA3 and CA1, with comparisons to females either intact or 2 months after ovariectomy. CA3 pyramidal neurons of orchidectomized rats exhibited marked increases in apical dendritic arborization. There were increases in mossy fiber afferent density in area CA3, as well as robust enhancements to dendritic structure in area CA3 of orchidectomized males, but not in CA1. Remarkably, apical dendritic length of CA3 pyramidal cells increased, while spine density declined. By contrast, in females overall dendritic structure was minimally affected by ovariectomy, while dendritic spine density was greatly reduced. Sex differences and subfield-specific effects of gonadal hormone deprivation on the hippocampal circuitry may help explain the different behavioral effects reported in males and females after gonadectomy, or other conditions associated with declining gonadal hormone secretion.

Sex differences in subcellular distribution of delta opioid receptors in the rat hippocampus in response to acute and chronic stress

Drug addiction requires associative learning processes that critically involve hippocampal circuits, including the opioid system. We recently found that acute and chronic stress, important regulators of addictive processes, affect hippocampal opioid levels and mu opioid receptor trafficking in a sexually dimorphic manner. Here, we examined whether acute and chronic stress similarly alters the levels and trafficking of hippocampal delta opioid receptors (DORs). Immediately after acute immobilization stress (AIS) or one-day after chronic immobilization stress (CIS), the brains of adult female and male rats were perfusion-fixed with aldehydes. The CA3b region and the dentate hilus of the dorsal hippocampus were quantitatively analyzed by light microscopy using DOR immunoperoxidase or dual label electron microscopy for DOR using silver intensified immunogold particles (SIG) and GABA using immunoperoxidase. At baseline, females compared to males had more DORs near the plasmalemma of pyramidal cell dendrites and about 3 times more DOR-labeled CA3 dendritic spines contacted by mossy fibers. In AIS females, near-plasmalemmal DOR-SIGs decreased in GABAergic hilar dendrites. However, in AIS males, near-plasmalemmal DOR-SIGs increased in CA3 pyramidal cell and hilar GABAergic dendrites and the percentage of CA3 dendritic spines contacted by mossy fibers increased to about half that seen in unstressed females. Conversely, after CIS, near-plasmalemmal DOR-SIGs increased in hilar GABA-labeled dendrites of females whereas in males plasmalemmal DOR-SIGs decreased in CA3 pyramidal cell dendrites and near-plasmalemmal DOR-SIGs decreased hilar GABA-labeled dendrites. As CIS in females, but not males, redistributed DOR-SIGs near the plasmalemmal of hilar GABAergic dendrites, a subsequent experiment examined the acute affect of oxycodone on the redistribution of DOR-SIGs in a separate cohort of CIS females. Plasmalemmal DOR-SIGs were significantly elevated on hilar interneuron dendrites one-hour after oxycodone (3 mg/kg, I.P.) administration compared to saline administration in CIS females. These data indicate that DORs redistribute within CA3 pyramidal cells and dentate hilar GABAergic interneurons in a sexually dimorphic manner that would promote activation and drug related learning in males after AIS and in females after CIS.

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Females have more near-plasmalemmal DORs in pyramidal CA3 dendrites than males.

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Acute stress in males relocates DORs in CA3 & GABA dendrites to promote activation.

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Chronic stress in females relocates DORs in GABA dendrites in females to promote activation.

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Chronic stress in males relocates DORs in GABA dendrites opposite of females.

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DOR-stress relocation may contribute to sexually dimorphic effects on drug related learning.

Sex Differences in Long-Term Potentiation at Temporoammonic-CA1 Synapses: Potential Implications for Memory Consolidation

Sex differences in spatial memory have long been observed in humans, non-human primates and rodents, but the underlying cellular and molecular mechanisms responsible for these differences remain obscure. In the present study we found that adolescent male rats outperformed female rats in 7 d and 28 d retention probes, but not in learning trials and immediate probes, in the Morris water maze task. Male rats also had larger long-term potentiation (LTP) at hippocampal temproammonic-CA1 (TA-CA1) synapses, which have been implicated to play a key role in place field and memory consolidation, when protocols designed to elicit late-stage LTP (LLTP) were used. Interestingly, the ratio of evoked AMPA/NMDA currents was found to be smaller at TA-CA1 synapses in male rats compared to female rats. Protein biotinylation experiments showed that male rats expressed more surface GluN1 receptors in hippocampal CA1 stratum lacunosum-moleculare (SLM) than female rats, although GluA1 expression was also slightly higher in male rats. Taken together, our results suggest that differences in the expression of AMPA and NMDA receptors may affect LTP expression at TA-CA1 synapses in adolescent male and female rats, and thus possibly contribute to the observed sex difference in spatial memory.

Enkephalin levels and the number of neuropeptide Y-containing interneurons in the hippocampus are decreased in female cannabinoid-receptor 1 knock-out mice

Drug addiction requires learning and memory processes that are facilitated by activation of cannabinoid-1 (CB1) and opioid receptors in the hippocampus. This involves activity-dependent synaptic plasticity that is partially regulated by endogenous opioid (enkephalin and dynorphin) and non-opioid peptides, specifically cholecystokinin, parvalbumin and neuropeptide Y, the neuropeptides present in inhibitory interneurons that co-express CB1 or selective opioid receptors. We tested the hypothesis that CB1 receptor expression is a determinant of the availability of one or more of these peptide modulators in the hippocampus. This was achieved by quantitatively analyzing the immunoperoxidase labeling for each of these neuropeptide in the dorsal hippocampus of female wild-type (CB1+/+) and cannabinoid receptor 1 knockout (CB1-/-) C57/BL6 mice. The levels of Leu(5)-enkephalin-immunoreactivity were significantly reduced in the hilus of the dentate gyrus and in stratum lucidum of CA3 in CB1-/- mice. Moreover, the numbers of neuropeptide Y-immunoreactive interneurons in the dentate hilus were significantly lower in the CB1-/- compared to wild-type mice. However, CB1+/+ and CB1-/- mice did not significantly differ in expression levels of either dynorphin or cholecystokinin, and showed no differences in numbers of parvalbumin-containing interneurons. These findings suggest that the cannabinoid and opioid systems have a nuanced, regulatory relationship that could affect the balance of excitation and inhibition in the hippocampus and thus processes such as learning that rely on this balance.

Effects of sex and DTNBP1 (dysbindin) null gene mutation on the developmental GluN2B-GluN2A switch in the mouse cortex and hippocampus

BACKGROUND

Neurodevelopmental disorders such as autism spectrum disorders and schizophrenia differentially impact males and females and are highly heritable. The ways in which sex and genetic vulnerability influence the pathogenesis of these disorders are not clearly understood. The n-methyl-d-aspartate (NMDA) receptor pathway has been implicated in schizophrenia and autism spectrum disorders and changes dramatically across postnatal development at the level of the GluN2B-GluN2A subunit "switch" (a shift from reliance on GluN2B-containing receptors to reliance on GluN2A-containing receptors). We investigated whether sex and genetic vulnerability (specifically, null mutation of DTNBP1 [dysbindin; a possible susceptibility gene for schizophrenia]) influence the developmental GluN2B-GluN2A switch.

METHODS

Subcellular fractionation to enrich for postsynaptic density (PSD), together with Western blotting and kinase assay, were used to investigate the GluN2B-GluN2A switch in the cortex and hippocampus of male and female DTNBP1 null mutant mice and their wild-type littermates. Main effects of sex and DTNBP1 genotype, and interactions with age, were assessed using factorial ANOVA.

RESULTS

Sex differences in the GluN2B-GluN2A switch emerged across development at the frontal cortical synapse, in parameters related to GluN2B. Males across genotypes displayed higher GluN2B:GluN2A and GluN2B:GluN1 ratios (p < 0.05 and p < 0.01, respectively), higher GluN2B phosphorylation at Y1472 (p < 0.01), and greater abundance of PLCγ (p < 0.01) and Fyn (p = 0.055) relative to females. In contrast, effects of DTNBP1 were evident exclusively in the hippocampus. The developmental trajectory of GluN2B was disrupted in DTNBP1 null mice (genotype × age interaction p < 0.05), which also displayed an increased synaptic GluN2A:GluN1 ratio (p < 0.05) and decreased PLCγ (p < 0.05) and Fyn (only in females; p < 0.0005) compared to wild-types.

CONCLUSIONS

Sex and DTNBP1 mutation influence the GluN2B-GluN2A switch at the synapse in a brain-region-specific fashion involving pY1472-GluN2B, Fyn, and PLCγ. This highlights the possible mechanisms through which risk factors may mediate their effects on vulnerability to disorders of NMDA receptor dysfunction.

Effect of Genistein on reproductive parameter and serum nitric oxide levels in morphine-treated mice

BACKGROUND

The predominant phytoestrogen in soy and derived products is the isoflavone Genistein. Genistein has antioxidant properties. Morphine is a main psychoactive chemical in opium that can increase the generation of free radicals and therefore it could adversely affects the spermatogenesis.

OBJECTIVE

The main goal was to investigate whether the Genistein could protect morphine adverse effects on sperm cells viability, count, motility, and testis histology and testosterone hormone and nitric oxide in blood serum.

MATERIALS AND METHODS

In this study, various doses of Genistein (0, 1, 2, and 3 mg/kg) and Genistein plus morphine (0, 1, 2, and 3 mg/kg) were administered interaperitoneally to 48 male mice for 30 consequent days. These mice were randomly assigned to 8 groups (n=6) and sperm parameters (sperm cells viability, count, motility and morphology), testis weight and histology, testosterone hormone (ELISA method), FSH and LH hormones (immunoradiometry) and serum nitric oxide (griess assay) were analyzed and compared.

RESULTS

The results indicated that morphine administration significantly decreased testosterone (0.03 ng/mg) LH and FSH level, histological parameters, count, viability (55.3%), morphology and motility of sperm cells (1%), testis weight (0.08 gr) and increase nitric oxide compared to saline group (p=0.00). However, administration of Genistein and Genistein plus morphine significantly boosted motility, morphology, count, viability of sperm cells, seminiferous tubules diameter, germinal thickness, testosterone, LH and FSH while decrease nitric oxide level in all groups compared to morphine group (p<0.025).

CONCLUSION

It seems that Genistein administration could increase the quality of spermatozoa and prevent morphine- induced adverse effects on sperm parameters.

The estrous cycle surpasses sex differences in regulating the transcriptome in the rat medial prefrontal cortex and reveals an underlying role of early growth response 1

Background

Males and females differ in cognitive functions and emotional processing, which in part have been associated with baseline sex differences in gene expression in the medial prefrontal cortex. Nevertheless, a growing body of evidence suggests that sex differences in medial prefrontal cortex-dependent cognitive functions are attenuated by hormonal fluctuations within the menstrual cycle. Despite known genomic effects of ovarian hormones, the interaction of the estrous cycle with sex differences in gene expression in the medial prefrontal cortex remains unclear and warrants further investigations.

Results

We undertake a large-scale characterization of sex differences and their interaction with the estrous cycle in the adult medial prefrontal cortex transcriptome and report that females with high and low ovarian hormone levels exhibited a partly opposed sexually biased transcriptome. The extent of regulation within females vastly exceeds sex differences, and supports a multi-level reorganization of synaptic function across the estrous cycle. Genome-wide analysis of the transcription factor early growth response 1 binding highlights its role in controlling the synapse-related genes varying within females.

Conclusions

We uncover a critical influence of the estrous cycle on the adult rat medial prefrontal cortex transcriptome resulting in partly opposite sex differences in proestrus when compared to diestrus females, and we discovered a direct role for Early Growth Response 1 in this opposite regulation. In addition to illustrating the importance of accounting for the estrous cycle in females, our data set the ground for a better understanding of the female specificities in cognition and emotional processing.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-015-0815-x) contains supplementary material, which is available to authorized users.

Interictal spike frequency varies with ovarian cycle stage in a rat model of epilepsy

In catamenial epilepsy, seizures exhibit a cyclic pattern that parallels the menstrual cycle. Many studies suggest that catamenial seizures are caused by fluctuations in gonadal hormones during the menstrual cycle, but this has been difficult to study in rodent models of epilepsy because the ovarian cycle in rodents, called the estrous cycle, is disrupted by severe seizures. Thus, when epilepsy is severe, estrous cycles become irregular or stop. Therefore, we modified kainic acid (KA)- and pilocarpine-induced status epilepticus (SE) models of epilepsy so that seizures were rare for the first months after SE, and conducted video-EEG during this time. The results showed that interictal spikes (IIS) occurred intermittently. All rats with regular 4-day estrous cycles had IIS that waxed and waned with the estrous cycle. The association between the estrous cycle and IIS was strong: if the estrous cycles became irregular transiently, IIS frequency also became irregular, and when the estrous cycle resumed its 4-day pattern, IIS frequency did also. Furthermore, when rats were ovariectomized, or males were recorded, IIS frequency did not show a 4-day pattern. Systemic administration of an estrogen receptor antagonist stopped the estrous cycle transiently, accompanied by transient irregularity of the IIS pattern. Eventually all animals developed severe, frequent seizures and at that time both the estrous cycle and the IIS became irregular. We conclude that the estrous cycle entrains IIS in the modified KA and pilocarpine SE models of epilepsy. The data suggest that the ovarian cycle influences more aspects of epilepsy than seizure susceptibility.