Glutamatergic Projections from the Entorhinal Cortex to Dorsal Dentate Gyrus Mediate Context-Induced Reinstatement of Heroin Seeking

Frontostriatal regulation of brain circuits contributes to flexible decision making

Deficits in behavioral or cognitive flexibility that are linked to altered activity in both cortical and subcortical brain regions, are often observed across multiple neuropsychiatric disorders. The medial prefrontal cortex (mPFC)-nucleus accumbens (NAc) pathway in rats plays a critical role in flexible control of behavior. However, the modulation of this pathway on activity and functional connectivity with the rest of the brain remains unclear. In this study, we first confirmed the role of the mPFC-NAc pathway in behavioral flexibility using a set-shifting task in rats and then evaluated the causal effects of mPFC-NAc activation induced by chemogenetic stimulation of the terminal axons of the NAc with DREADD expression on whole-brain activity and functional connectivity measured by functional MRI. mPFC-NAc activation improved performance on the set-shifting task by reducing perseverative errors. Additionally, stimulation of this pathway increased activity in a set of brain regions within the basal ganglia-thalamus-cortical loop network including NAc, thalamus, hypothalamus and various connected cortical regions, while also decreased functional connectivity strength of NAc-mPFC, NAc-secondary motor cortex (M2), and various cortical circuits. Moreover, performance on the set-shifting task was related to the functional connectivity strength of the above frontostriatal and cortical circuits. These findings provide insights into the link between specific frontostriatal circuits on decision making flexibility, which may inform potential future interventions for behavioral flexibility deficits.

μ-Opioid Receptor Modulation of the Glutamatergic/GABAergic Midbrain Inputs to the Mouse Dorsal Hippocampus

We used virus-mediated anterograde and retrograde tracing, optogenetic modulation, immunostaining, in situ hybridization, and patch-clamp recordings in acute brain slices to study the release mechanism and μ-opioid modulation of the dual glutamatergic/GABAergic inputs from the ventral tegmental area and supramammillary nucleus to the granule cells of the dorsal hippocampus of male and female mice. In keeping with previous reports showing that the two transmitters are released by separate active zones within the same terminals, we found that the short-term plasticity and pharmacological modulation of the glutamatergic and GABAergic currents are indistinguishable. We further found that glutamate and GABA release at these synapses are both virtually completely mediated by N- and P/Q-type calcium channels. We then investigated μ-opioid modulation of these synapses and found that activation of μ-opioid receptors (MORs) strongly inhibits the glutamate and GABA release, mostly through inhibition of presynaptic N-type channels. However, the modulation by MORs of these dual synapses is complex, as it likely includes also a disinhibition due to downmodulation of local GABAergic interneurons which make direct axo-axonic contacts with the dual glutamatergic/GABAergic terminals. We discuss how this opioid modulation may enhance LTP at the perforant path inputs, potentially contributing to reinforce memories of drug-associated contexts.

Lateral entorhinal cortex neurons that project to nucleus accumbens mediate contextual associative memory

The lateral entorhinal cortex (LEC) contains glutamatergic projections that innervate the nucleus accumbens (NAc) and may be involved in the encoding of contextual associations with both positive and negative valences, such as those encountered in drug cues or fear conditioning. To determine whether LEC-NAc neurons are activated by the encoding and recall of contexts associated with cocaine or footshock, we measured c-fos expression in these neurons and found that LEC-NAc neurons are activated in both contexts. Specifically, activation patterns of the LEC-NAc were observed in a novel context and reexposure to the same context, highlighting the specific role for LEC-NAc neurons in encoding rather than the valence of a specific event-related memory. Using a combination of circuit-specific chemogenetic tools and behavioral assays, we selectively inactivated LEC-NAc neurons in mice during the encoding and retrieval of memories of contexts associated with cocaine or footshock. Chemogenetic inactivation of LEC-NAc neurons impaired the formation of both positive and negative context-associated memories without affecting the retrieval of an established memory. This finding suggests a critical role for this circuit in the initial encoding of contextual associations. In summary, LEC-NAc neurons facilitate the encoding of contextual information, guiding motivational behaviors without directly mediating the hedonic or aversive properties of these associations.

Chemogenetic Tools and their Use in Studies of Neuropsychiatric Disorders

Chemogenetics is a newly developed set of tools that allow for selective manipulation of cell activity. They consist of a receptor mutated irresponsive to endogenous ligands and a synthetic ligand that does not interact with the wild-type receptors. Many different types of these receptors and their respective ligands for inhibiting or excitating neuronal subpopulations were designed in the past few decades. It has been mainly the G-protein coupled receptors (GPCRs) selectively responding to clozapine-N-oxide (CNO), namely Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), that have been employed in research. Chemogenetics offers great possibilities since the activity of the receptors is reversible, inducible on demand by the ligand, and non-invasive. Also, specific groups or types of neurons can be selectively manipulated thanks to the delivery by viral vectors. The effect of the chemogenetic receptors on neurons lasts longer, and even chronic activation can be achieved. That can be useful for behavioral testing. The great advantage of chemogenetic tools is especially apparent in research on brain diseases since they can manipulate whole neuronal circuits and connections between different brain areas. Many psychiatric or other brain diseases revolve around the dysfunction of specific brain networks. Therefore, chemogenetics presents a powerful tool for investigating the underlying mechanisms causing the disease and revealing the link between the circuit dysfunction and the behavioral or cognitive symptoms observed in patients. It could also contribute to the development of more effective treatments.

Orexin receptors in the hippocampal dentate gyrus modulated the restraint stress-induced analgesia in the animal model of chronic pain

Previous studies have shown that stressful stimuli induced an adaptive response of reduced nociception, known as stress-induced analgesia (SIA). Since orexin neuropeptides are involved in pain modulation, and orexin neurons, primarily located in the lateral hypothalamus (LH), project to various hippocampal regions, such as the dentate gyrus (DG), the current study aimed to examine the role of orexin receptors within the DG region in the restraint SIA in the animal model of chronic pain. One hundred-thirty adult male Wistar rats (230-250 g) were unilaterally implanted with a cannula above the DG region. Animals were given SB334867 or TCS OX2 29 (1, 3, 10, and 30 nmol, 0.5 µl/rat) into the DG region as orexin-1 receptor (OX1r) and orexin-2 receptor (OX2r) antagonists, respectively, five min before exposure to a 3-hour restraint stress (RS) period. Animals were then undergone the formalin test to assess pain-related behaviors as the animal model of chronic pain. The results showed that RS produces an analgesic response during the early and late phases of the formalin test. However, intra-DG microinjection of OX1r and OX2r antagonists attenuated the restraint SIA. OX2r antagonist was more potent than OX1r antagonist in the early phase of the formalin test, while OX1r antagonist was little more effective in the late phase. Predominantly, it could be concluded that the orexinergic system in the DG region might act as a potential endogenous pain control system and a novel target for treating stress-related disorders.

Behavioral pattern separation and cognitive flexibility are enhanced in a mouse model of increased lateral entorhinal cortex-dentate gyrus circuit activity

Behavioral pattern separation and cognitive flexibility are essential cognitive abilities that are disrupted in many brain disorders. A better understanding of the neural circuitry involved in these abilities will open paths to treatment. In humans and mice, discrimination and adaptation rely on the integrity of the hippocampal dentate gyrus (DG) which receives glutamatergic input from the entorhinal cortex (EC), including the lateral EC (LEC). An inducible increase of EC-DG circuit activity improves simple hippocampal-dependent associative learning and increases DG neurogenesis. Here, we asked if the activity of LEC fan cells that directly project to the DG (LEC → DG neurons) regulates the relatively more complex hippocampal-dependent abilities of behavioral pattern separation or cognitive flexibility. C57BL/6J male mice received bilateral LEC infusions of a virus expressing shRNA TRIP8b, an auxiliary protein of an HCN channel or a control virus (SCR shRNA). Prior work shows that 4 weeks post-surgery, TRIP8b mice have more DG neurogenesis and greater activity of LEC → DG neurons compared to SCR shRNA mice. Here, 4 weeks post-surgery, the mice underwent testing for behavioral pattern separation and reversal learning (touchscreen-based location discrimination reversal [LDR]) and innate fear of open spaces (elevated plus maze [EPM]) followed by quantification of new DG neurons (doublecortin-immunoreactive cells [DCX+] cells). There was no effect of treatment (SCR shRNA vs. TRIP8b) on performance during general touchscreen training, LDR training, or the 1st days of LDR testing. However, in the last days of LDR testing, the TRIP8b shRNA mice had improved pattern separation (reached the first reversal more quickly and had more accurate discrimination) compared to the SCR shRNA mice, specifically when the load on pattern separation was high (lit squares close together or "small separation"). The TRIP8b shRNA mice were also more cognitively flexible (achieved more reversals) compared to the SCR shRNA mice in the last days of LDR testing. Supporting a specific influence on cognitive behavior, the SCR shRNA and TRIP8b shRNA mice did not differ in total distance traveled or in time spent in the closed arms of the EPM. Supporting an inducible increase in LEC-DG activity, DG neurogenesis was increased. These data indicate that the TRIP8b shRNA mice had better pattern separation and reversal learning and more neurogenesis compared to the SCR shRNA mice. This study advances fundamental and translational neuroscience knowledge relevant to two cognitive functions critical for adaptation and survival-behavioral pattern separation and cognitive flexibility-and suggests that the activity of LEC → DG neurons merits exploration as a therapeutic target to normalize dysfunctional DG behavioral output.

Polyoxidovanadates as a pharmacological option against brain aging

The world population is aging rapidly, and chronic diseases associated are cardiometabolic syndrome, cancer, and neurodegenerative diseases. Oxidative stress and inflammation are typical hallmarks in them. Polyoxidovanadates (POVs) have shown interesting pharmacological actions against chronic diseases. This work aimed to evaluate the POV effect on hippocampal neuroinflammation, redox balance, and recognition memory in the aging of rats. Rats 18 months old were administered a daily dose of sodium metavanadate (MV), decavanadate (DV), Metformin (Metf), or MetfDeca for two months. Results showed that short-term and long-term recognition memory improved by 28 % and 16 % (DV), 19 % and 20 % (Metf), and 21 % and 27 % (MetfDeca). In hippocampi, reactive oxygen species, IL-1β, and TNF-α, after DV, Metf, and MetfDeca decreased at similar concentrations to young adult control, while lipid peroxidation substantially ameliorated. Additionally, superoxide dismutase and catalase activity increased by 41 % and 42 % (DV), 39 % and 41 % (Metf), and 75 % and 73 % (MetfDeca). POV treatments reduced Nrf2 and GFAP immunoreactivity in CA1 (70-87.5 %), CA3 (60-80 %), and DG (57-89 %). Metformin treatment showed a minor effect, while MV treatment did not improve any parameters. Although DV, Metf, and MetfDeca treatments showed similar results, POVs doses were 16-fold fewer than Metformin. In conclusion, DV and MetfDeca could be pharmacological options to reduce age-related neuronal damage.

Behavioral pattern separation and cognitive flexibility are enhanced in a mouse model of increased lateral entorhinal cortex-dentate gyrus circuit activity

Behavioral pattern separation and cognitive flexibility are essential cognitive abilities which are disrupted in many brain disorders. Better understanding of the neural circuitry involved in these abilities will open paths to treatment. In humans and mice, discrimination and adaptation rely on integrity of the hippocampal dentate gyrus (DG) which both receive glutamatergic input from the entorhinal cortex (EC), including the lateral EC (LEC). Inducible increase of EC-DG circuit activity improves simple hippocampal-dependent associative learning and increases DG neurogenesis. Here we asked if the activity of LEC fan cells that directly project to the DG (LEC➔DG neurons) regulates behavioral pattern separation or cognitive flexibility. C57BL6/J male mice received bilateral LEC infusions of a virus expressing shRNA TRIP8b, an auxiliary protein of an HCN channel or a control virus (SCR shRNA); this approach increases the activity of LEC➔DG neurons. Four weeks later, mice underwent testing for behavioral pattern separation and reversal learning (touchscreen-based Location Discrimination Reversal [LDR] task) and innate fear of open spaces (elevated plus maze [EPM]) followed by counting of new DG neurons (doublecortin-immunoreactive cells [DCX+] cells). TRIP8b and SCR shRNA mice performed similarly in general touchscreen training and LDR training. However, in late LDR testing, TRIP8b shRNA mice reached the first reversal more quickly and had more accurate discrimination vs. SCR shRNA mice, specifically when pattern separation was challenging (lit squares close together or "small separation"). Also, TRIP8b shRNA mice achieved more reversals in late LDR testing vs. SCR shRNA mice. Supporting a specific influence on cognitive behavior, SCR shRNA and TRIP8b shRNA mice did not differ in total distance traveled or in time spent in the closed arms of the EPM. Supporting an inducible increase in LEC-DG activity, DG neurogenesis was increased. These data indicate TRIP8b shRNA mice had better pattern separation and reversal learning and more neurogenesis vs. SCR shRNA mice. This work advances fundamental and translational neuroscience knowledge relevant to two cognitive functions critical for adaptation and survival - behavioral pattern separation and cognitive flexibility - and suggests the activity of LEC➔DG neurons merits exploration as a therapeutic target to normalize dysfunctional DG behavioral output.

Neural circuit dynamics of drug-context associative learning in the mouse hippocampus

The environmental context associated with previous drug consumption is a potent trigger for drug relapse. However, the mechanism by which neural representations of context are modified to incorporate information associated with drugs of abuse remains unknown. Using longitudinal calcium imaging in freely behaving mice, we find that unlike the associative learning of natural reward, drug-context associations for psychostimulants and opioids are encoded in a specific subset of hippocampal neurons. After drug conditioning, these neurons weakened their spatial coding for the non-drug paired context, resulting in an orthogonal representation for the drug versus non-drug context that was predictive of drug-seeking behavior. Furthermore, these neurons were selected based on drug-spatial experience and were exclusively tuned to animals' allocentric position. Together, this work reveals how drugs of abuse alter the hippocampal circuit to encode drug-context associations and points to the possibility of targeting drug-associated memory in the hippocampus.

Pathway-specific inhibition of critical projections from the mediodorsal thalamus to the frontal cortex controls kindled seizures

There is a large unmet need for improved treatment for temporal lobe epilepsy (TLE); circuit-specific manipulation that disrupts the initiation and propagation of seizures is promising in this regard. The midline thalamus, including the mediodorsal nucleus (MD) is a critical distributor of seizure activity, but its afferent and efferent pathways that mediate seizure activity are unknown. Here, we used chemogenetics to silence input and output projections of the MD to discrete regions of the frontal cortex in the kindling model of TLE in rats. Chemogenetic inhibition of the projection from the amygdala to the MD abolished seizures, an effect that was replicated using optogenetic inhibition. Chemogenetic inhibition of projections from the MD to the prelimbic cortex likewise abolished seizures. By contrast, inhibition of projections from the MD to other frontal regions produced partial (orbitofrontal cortex, infralimbic cortex) or no (cingulate, insular cortex) attenuation of behavioral or electrographic seizure activity. These results highlight the particular importance of projections from MD to prelimbic cortex in the propagation of amygdala-kindled seizures.

Neuroimaging reveals distinct brain glucose metabolism patterns associated with morphine consumption in Lewis and Fischer 344 rat strains

Vulnerability to addiction may be given by the individual's risk of developing an addiction during their lifetime. A challenge in the neurobiology of drug addiction is understanding why some people become addicted to drugs. Here, we used positron emission tomography (PET) and statistical parametric mapping (SPM) to evaluate changes in brain glucose metabolism in response to chronic morphine self-administration (MSA) in two rat strains with different vulnerability to drug abuse, Lewis (LEW) and Fischer 344 (F344). Four groups of animals were trained to self-administer morphine or saline for 15 days. 2-deoxy-2-[¹⁸F]-fluoro-d-glucose (FDG)-PET studies were performed on the last day of MSA (acquisition phase) and after 15 days of withdrawal. PET data were analyzed using SPM12. LEW-animals self-administered more morphine injections per session than F344-animals. We found significant brain metabolic differences between LEW and F344 strains in the cortex, hypothalamus, brainstem, and cerebellum. In addition, the different brain metabolic patterns observed after the MSA study between these rat strains indicate differences in the efficiency of neural substrates to translate the drug effects, which could explain the differences in predisposition to morphine abuse between one individual and another. These findings have important implications for the use of these rat strains in translational morphine and opiate research.

A Baldwin interpretation of adult hippocampal neurogenesis: from functional relevance to physiopathology

Hippocampal adult neurogenesis has been associated to many cognitive, emotional, and behavioral functions and dysfunctions, and its status as a selected effect or an "appendix of the brain" has been debated. In this review, we propose to understand hippocampal neurogenesis as the process underlying the "Baldwin effect", a particular situation in evolution where fitness does not rely on the natural selection of genetic traits, but on "ontogenetic adaptation" to a changing environment. This supports the view that a strong distinction between developmental and adult hippocampal neurogenesis is made. We propose that their functions are the constitution and the lifelong adaptation, respectively, of a basic repertoire of cognitive and emotional behaviors. This lifelong adaptation occurs through new forms of binding, i.e., association or dissociation of more basic elements. This distinction further suggests that a difference is made between developmental vulnerability (or resilience), stemming from dysfunctional (or highly functional) developmental hippocampal neurogenesis, and adult vulnerability (or resilience), stemming from dysfunctional (or highly functional) adult hippocampal neurogenesis. According to this hypothesis, developmental and adult vulnerability are distinct risk factors for various mental disorders in adults. This framework suggests new avenues for research on hippocampal neurogenesis and its implication in mental disorders.

MicroRNA-132 is involved in morphine dependence via modifying the structural plasticity of the dentate gyrus neurons in rats

Repeated morphine exposure has been shown to induce neuronal plasticity in reward-related areas of the brain. miR-132, a CREB-induced and activation-dependent microRNA, has been suggested to be involved in the neuronal plasticity by increasing neuronal dendritic branches and spinogenesis. However, it is still unclear whether miR-132 is related to morphine dependence. Here, we investigate whether miR-132 is involved in morphine dependence and whether it is related to the structural plasticity of the dentate gyrus (DG) neurons. Sprague-Dawley rats are treated with increasing doses of morphine injection for six consecutive days to develop morphine dependence. Our results show that dendritic branching and spinogenesis of the DG neurons of morphine dependent rats are increased. Morphine treatment (24 h) promotes the differentiation of N2a cells stably expressing μ-opioid receptor by up-regulating miR-132 expression. Moreover, inhibiting miR-132 3p (but not 5p) of the DG neurons can reverse the structural plasticity and disrupt the formation of morphine dependence in rats. These findings indicate that miR-132 in the DG neurons is involved in morphine dependence via modifying the neuronal plasticity.

Optogenetic stimulation of entorhinal cortex reveals the implication of insulin signaling in adult rat's hippocampal neurogenesis

Adult neurogenesis in the hippocampal dentate gyrus plays a critical role in learning and memory. Projections originating from entorhinal cortex, known as the perforant pathway, provide the main input to the dentate gyrus and promote neurogenesis. However, neuromodulators and molecular changes mediating neurogenic effects of this pathway are not yet fully understood. Here, by means of an optogenetic approach, we investigated neurogenesis and synaptic plasticity in the hippocampus of adult rats induced by stimulation of the perforant pathway. The lentiviruses carrying hChR2 (H134R)-mCherry gene under the control of the CaMKII promoter were injected into the medial entorhinal cortex region of adult rats. After 21 days, the entorhinal cortex region was exposed to the blue laser (473 nm) for five consecutive days (30 min/day). The expression of synaptic plasticity and neurogenesis markers in the hippocampus were evaluated using molecular and histological approaches. In parallel, the changes in the gene expression of insulin and its signaling pathway, trophic factors, and components of mitochondrial biogenesis were assessed. Our results showed that optogenetic stimulation of the entorhinal cortex promotes hippocampal neurogenesis and synaptic plasticity concomitant with the increased levels of insulin mRNA and its signaling markers, neurotrophic factors, and activation of mitochondrial biogenesis. These findings suggest that effects of perforant pathway stimulation on the hippocampus, at least in part, are mediated by insulin increase in the dentate gyrus and subsequently activation of its downstream signaling pathway.

Sexually dimorphic muscarinic acetylcholine receptor modulation of contextual fear learning in the dentate gyrus

Contextual fear conditioning, where the prevailing situational cues become associated with an aversive unconditional stimulus such as electric shock, is sexually dimorphic. Males typically show higher levels of fear than females. There are two components to contextual fear conditioning. First the multiple cues that encompass the context must be integrated into a coherent representation, a process that requires the hippocampus. The second is that representation must be communicated to the basolateral amygdala where it can be associated with shock. If there is inadequate time for forming the representation prior to shock poor conditioning results and this is called the immediate shock deficit. One can isolate the contextual processing component, as well as alleviate the deficit, by providing an opportunity to explore the context without shock prior to the conditioning session. The purpose of the present study was to determine the extent to which cholinergic processes within the dentate gyrus of the hippocampus during contextual processing contribute to the sexual dimorphism. Clozapine-n-oxide (CNO) is a putatively inactive compound that acts only upon synthetic genetically engineered receptors. However, we found that CNO infused into the dentate gyrus prior to exploration eliminated the sexual dimorphism by selectively decreasing freezing in males to the level of females. Biological activity of CNO is usually attributed to metabolism of CNO to clozapine and we found that clozapine, and the muscarinic cholinergic antagonist, scopolamine, produced results similar to CNO, preferentially affecting males. On the other hand, the muscarinic agonist oxotremorine selectively impaired conditioning in females. Overall, the current experiments reveal significant off-target effects of CNO and implicate muscarinic cholinergic receptors in the dentate gyrus as a significant mediator of the sexual dimorphism in contextual fear conditioning.

Chemogenetic inhibition of prefrontal projection neurons constrains top-down control of attention in young but not aged rats

The prefrontal cortex (PFC) governs top-down control of attention and is known to be vulnerable in aging. Cortical reorganization with increased PFC recruitment is suggested to account for functional compensation. Here, we hypothesized that reduced PFC output would exert differential effects on attentional capacities in young and aged rats, with the latter exhibiting a more robust decline in performance. A chemogenetic approach involving designer receptors exclusively activated by designer drugs was utilized to determine the impact of silencing PFC projection neurons in rats performing an operant attention task. Visual distractors were presented in all behavioral testing sessions to tax attentional resources. Under control conditions, aged rats exhibited impairments in discriminating signals with the shortest duration from non-signal events. Surprisingly, chemogenetic inhibition of PFC output neurons did not worsen performance amongst aged animals. Conversely, significant impairments in attentional capacities were observed in young subjects following such manipulation. Given the involvement of PFC-projecting basal forebrain cholinergic neurons in top-down regulation of attention, amperometric recordings were conducted to measure alterations in prefrontal cholinergic transmission in a separate cohort of young and aged rats. While PFC silencing resulted in a robust attenuation of tonic cholinergic signaling across age groups, the capacity to generate phasic cholinergic transients was impaired only amongst young animals. Collectively, our findings suggest a reduced efficiency of PFC-mediated top-down control of attention and cholinergic system in aging, and that activity of PFC output neurons does not reflect compensation in aged rats, at least in the attention domain.

The Mesencephalic Trigeminal Nucleus Controls Food Intake and Body Weight via Hindbrain POMC Projections

The mesencephalic trigeminal nucleus (Mes5) processes oral sensory–motor information, but its role in the control of energy balance remains unexplored. Here, using fluorescent in situ hybridization, we show that the Mes5 expresses the melanocortin-4 receptor. Consistent with MC4R activation in other areas of the brain, we found that Mes5 microinjection of the MC4R agonist melanotan-II (MTII) suppresses food intake and body weight in the mouse. Furthermore, NTS POMC-projecting neurons to the Mes5 can be chemogenetically activated to drive a suppression in food intake. Taken together, these findings highlight the Mes5 as a novel target of melanocortinergic control of food intake and body weight regulation, although elucidating the endogenous role of this circuit requires future study. While we observed the sufficiency of Mes5 MC4Rs for food intake and body weight suppression, these receptors do not appear to be necessary for food intake or body weight control. Collectively, the data presented here support the functional relevance of the NTS POMC to Mes5 projection pathway as a novel circuit that can be targeted to modulate food intake and body weight.

Phosphorylation of GluN2B subunits of N-methyl-d-aspartate receptors in the frontal association cortex involved in morphine-induced conditioned place preference in mice

Morphine is one of the most abused drugs in the world, which has resulted in serious social problems. The frontal association cortex (FrA) has been shown to play a key role in memory formation and drug addiction. N-Methyl-d-aspartate receptors (NMDARs) are abundant in the prefrontal cortex (PFc) and much evidence indicates that GluN2B-containing NMDARs are involved in morphine-induced conditioned place preference (CPP). However, the function of GluN2B in the FrA during morphine-induced CPP has yet to be fully investigated. In the present work, a CPP animal model was employed to measure the expression of phosphorylated (p-) GluN2B (Serine; Ser 1303) in the FrA and NAc in different phases of morphine-induced CPP. We found that p-GluN2B (Ser 1303) was increased in the FrA during the development and reinstatement phases but unchanged in the extinction phase. The use of ifenprodil, a GluN2B-specific antagonist, to block the activity of GluN2B in the two phases attenuated morphine-induced CPP and reinstatement. Furthermore, ifenprodil also blocked morphine-induced upregulation of p-GluN2B (Ser 1303) in the FrA in both phases. These results indicate that GluN2B-containing NMDARs in the FrA may be involved in the regulation of morphine-induced CPP and reinstatement.

Gray matter alteration in heroin-dependent men: An atlas-based magnetic resonance imaging study

Previous imaging studies on heroin addiction have reported brain morphological alterations. However, the effects of heroin exposure on gray matter volume varied among different studies due to different factors such as substitution treatment or mandatory abstinence. Meanwhile, the relationship between gray matter and heroin use history remains unknown. Thirty-three male heroin-dependent (HD) individuals who are not under any substitution treatment or mandatory abstinence and 40 male healthy controls (HC) were included in this structural magnetic resonance imaging study. With an atlas-based approach, gray matter structures up to individual functional area were delineated, and the differences in their volumes between the HD and HC groups were analyzed. In addition, the relationship between gray matter volume and duration of heroin use was explored. The HD group demonstrated significantly lower cortical volume mainly in the prefrontal cortex and mesolimbic dopaminergic regions across different parcellation levels, whereas several visual and somatosensory cortical regions in the HD group had greater volume relative to the HC group at a more detailed parcellation level. The duration of heroin use was negatively correlated with the gray matter volume of prefrontal cortex. These findings suggest that heroin addiction be related to gray matter alteration and might be related to damage/maladaption of the inhibitory control, reward, visual, and somatosensory functions of the brain, although cognitive correlates are warranted in future study. In addition, the atlas-based morphology analysis is a potential tool to help researchers search biomarkers of heroin addiction.

Expression of a heroin contextually conditioned immune effect in male rats requires CaMKIIα-expressing neurons in dorsal, but not ventral, subiculum and hippocampal CA1

The physiological and motivational effects of heroin and other abused drugs become associated with environmental (contextual) stimuli during repeated drug use. As a result, these contextual stimuli gain the ability to elicit drug-like conditioned effects. For example, after context-heroin pairings, exposure to the heroin-paired context alone produces similar effects on peripheral immune function as heroin itself. Conditioned immune effects can significantly exacerbate the adverse health consequences of heroin use. Our laboratory has shown that exposure to a heroin-paired context suppresses lipopolysaccharide (LPS)-induced splenic nitric oxide (NO) production in male rats, and this effect is mediated in part by the dorsal hippocampus (dHpc). However, specific dHpc output regions, whose efferents might mediate conditioned immune effects, have not been identified, nor has the contribution of ventral hippocampus (vHpc) been investigated. Here, we evaluated the role of CaMKIIα-expressing neurons in the dHpc and vHpc main output regions by expressing G_i-coupled designer receptors exclusively activated by designer drugs (DREADDs) under a CaMKIIα promoter in the dorsal subiculum and CA1 (dSub, dCA1) or ventral subiculum and CA1 (vSub, vCA1). After context-heroin conditioning, clozapine-N-oxide (CNO, DREADD agonist) or vehicle was administered systemically prior to heroin-paired context (or home-cage control) exposure and LPS immune challenge. Chemogenetic inhibition of CaMKIIα-expressing neurons in dHpc, but not vHpc, output regions attenuated the expression of conditioned splenic NO suppression. These results establish that the main dHpc output regions, the dSub and dCA1, are critical for this context-heroin conditioned immune effect.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Anterior Insular Cortex is Critical for the Propensity to Relapse Following Punishment-Imposed Abstinence of Alcohol Seeking

Humans with alcohol use disorder typically abstain because of the negative consequences associated with excessive drinking, and exposure to contexts previously associated with alcohol use can trigger relapse. We used a rat model that captures a characteristic of this human condition: namely voluntary abstinence from alcohol use because of contingent punishment. There is substantial variability in the propensity to relapse following extended periods of abstinence, and this is a critical feature preventing the successful treatment of alcohol use disorder. Here we examined relapse following acute or prolonged abstinence. In male alcohol preferring P rats, we found an increased propensity to relapse in Context B, the punishment context after prolonged abstinence. Next, we found that neither alcohol intake history nor the motivational strength of alcohol predicted the propensity to relapse. We next examined the putative circuitry of context-induced relapse to alcohol seeking following prolonged abstinence using Fos as a marker of neuronal activation. The anterior insular cortex (AI) was the only brain region examined where Fos expression correlated with alcohol seeking behavior in Context B after prolonged abstinence. Finally, we used local infusion of GABAA and GABAB receptor agonists (muscimol + baclofen) to show a causal role of the AI in context-induced relapse in Context B, the punishment context after prolonged abstinence. Our results show that there is substantial individual variability in the propensity to relapse in the punishment-associated context after prolonged abstinence, and this is mediated by activity in the AI.SIGNIFICANCE STATEMENT A key feature of alcohol use disorder is that sufferers show an enduring propensity to relapse throughout their lifetime. Relapse typically occurs despite the knowledge of adverse consequences including health complications or relationship breakdowns. Here we use a recently developed rodent model that recapitulates this behavior. After an extended period of abstinence, relapse propensity is markedly increased in the "adverse consequence" environment, akin to humans with alcohol use disorder relapsing in the face of adversity. From a circuitry perspective, we demonstrate a causal role of the anterior insular cortex in relapse to alcohol seeking after extended abstinence following punishment imposed voluntary cessation of alcohol use.

MicroRNA-132 in the Adult Dentate Gyrus is Involved in Opioid Addiction Via Modifying the Differentiation of Neural Stem Cells

MicroRNA-132 (miR-132), a small RNA that regulates gene expression, is known to promote neurogenesis in the embryonic nervous system and adult brain. Although exposure to psychoactive substances can increase miR-132 expression in cultured neural stem cells (NSCs) and the adult brain of rodents, little is known about its role in opioid addiction. So, we set out to determine the effect of miR-132 on differentiation of the NSCs and whether this effect is involved in opioid addiction using the rat morphine self-administration (MSA) model. We found that miR-132 overexpression enhanced the differentiation of NSCs in vivo and in vitro. Similarly, specific overexpression of miR-132 in NSCs of the adult hippocampal dentate gyrus (DG) during the acquisition stage of MSA potentiated morphine-seeking behavior. These findings indicate that miR-132 is involved in opioid addiction, probably by promoting the differentiation of NSCs in the adult DG.

Relapse to opioid seeking in rat models: behavior, pharmacology and circuits

Lifetime relapse rates remain a major obstacle in addressing the current opioid crisis. Relapse to opioid use can be modeled in rodent studies where drug self-administration is followed by a period of abstinence and a subsequent test for drug seeking. Abstinence can be achieved through extinction training, forced abstinence, or voluntary abstinence. Voluntary abstinence can be accomplished by introducing adverse consequences of continued drug self-administration (e.g., punishment or electric barrier) or by introducing an alternative nondrug reward in a discrete choice procedure (drug versus palatable food or social interaction). In this review, we first discuss pharmacological and circuit mechanisms of opioid seeking, as assessed in the classical extinction-reinstatement model, where reinstatement is induced by reexposure to the self-administered drug (drug priming), discrete cues, discriminative cues, drug-associated contexts, different forms of stress, or withdrawal states. Next, we discuss pharmacological and circuit mechanisms of relapse after forced or voluntary abstinence, including the phenomenon of "incubation of heroin craving" (the time-dependent increases in heroin seeking during abstinence). We conclude by discussing future directions of preclinical relapse-related studies using opioid drugs.

Common neurocircuitry mediating drug and fear relapse in preclinical models

BACKGROUND

Comorbidity of anxiety disorders, stressor- and trauma-related disorders, and substance use disorders is extremely common. Moreover, therapies that reduce pathological fear and anxiety on the one hand, and drug-seeking on the other, often prove short-lived and are susceptible to relapse. Considerable advances have been made in the study of the neurobiology of both aversive and appetitive extinction, and this work reveals shared neural circuits that contribute to both the suppression and relapse of conditioned responses associated with trauma or drug use.

OBJECTIVES

The goal of this review is to identify common neural circuits and mechanisms underlying relapse across domains of addiction biology and aversive learning in preclinical animal models. We focus primarily on neural circuits engaged during the expression of relapse.

KEY FINDINGS

After extinction, brain circuits involving the medial prefrontal cortex and hippocampus come to regulate the expression of conditioned responses by the amygdala, bed nucleus of the stria terminalis, and nucleus accumbens. During relapse, hippocampal projections to the prefrontal cortex inhibit the retrieval of extinction memories resulting in a loss of inhibitory control over fear- and drug-associated conditional responding.

CONCLUSIONS

The overlapping brain systems for both fear and drug memories may explain the co-occurrence of fear and drug-seeking behaviors.

Chemogenetic Manipulations of Ventral Tegmental Area Dopamine Neurons Reveal Multifaceted Roles in Cocaine Abuse

Ventral tegmental area (VTA) dopamine (DA) neurons perform diverse functions in motivation and cognition, but their precise roles in addiction-related behaviors are still debated. Here, we targeted VTA DA neurons for bidirectional chemogenetic modulation during specific tests of cocaine reinforcement, demand, and relapse-related behaviors in male rats, querying the roles of DA neuron inhibitory and excitatory G-protein signaling in these processes. Designer receptor stimulation of G_q signaling, but not G_s signaling, in DA neurons enhanced cocaine seeking via functionally distinct projections to forebrain limbic regions. In contrast, engaging inhibitory G_i/o signaling in DA neurons blunted the reinforcing and priming effects of cocaine, reduced stress-potentiated reinstatement, and altered behavioral strategies for cocaine seeking and taking. Results demonstrate that DA neurons play several distinct roles in cocaine seeking, depending on behavioral context, G-protein-signaling cascades, and DA neuron efferent targets, highlighting their multifaceted roles in addiction.SIGNIFICANCE STATEMENT G-protein-coupled receptors are crucial modulators of ventral tegmental area (VTA) dopamine neuron activity, but how this metabotropic signaling impacts the complex roles of dopamine in reward and addiction is poorly understood. Here, we bidirectionally modulate dopamine neuron G-protein signaling with DREADDs (designer receptors exclusively activated by designer drugs) during a variety of cocaine-seeking behaviors, revealing nuanced, pathway-specific roles in cocaine reward, effortful seeking, and relapse-like behaviors. G_q and G_s stimulation activated dopamine neurons, but only G_q stimulation robustly enhanced cocaine seeking. G_i/o inhibitory signaling reduced some, but not all, types of cocaine seeking. Results show that VTA dopamine neurons modulate numerous distinct aspects of cocaine addiction- and relapse-related behaviors, and point to potential new approaches for intervening in these processes to treat addiction.

Prefrontal cortex modulates firing pattern in the nucleus reuniens of the midline thalamus via distinct corticothalamic pathways

The thalamus has long been recognized for its role in relaying sensory information from the periphery, a function accomplished by its "first-order" nuclei. However, a second category of thalamic nuclei, termed "higher-order" nuclei, have been shown instead to mediate communication between cortical areas. The nucleus reuniens of the midline thalamus (RE) is a higher-order nucleus known to act as a conduit of reciprocal communication between the medial prefrontal cortex (mPFC) and hippocampus. While anatomical and behavioural studies of RE are numerous, circuit-based electrophysiological studies, particularly those examining the impact of cortical input and the thalamic reticular nucleus (TRN) on RE neuron firing, are sparse. To characterize RE neuron firing properties and dissect the circuit dynamics of the infralimbic subdivision of the mPFC (ilPFC), the TRN and RE, we used in vivo, extracellular, single-unit recordings in male Sprague Dawley rats and manipulated neural activity using targeted pharmacological manipulations, electrical stimulation and a projection-specific implementation of designer receptors exclusively activated by designer drugs (DREADDs). We show that ilPFC inhibition reduces multiple burst firing parameters in RE, whereas ilPFC stimulation drives burst firing and dampens tonic firing. In addition, TRN inhibition reduces the number of spontaneously active neurons in RE. Finally, inhibition of ilPFC terminals in RE selectively enhances a subset of burst firing parameters. These findings demonstrate that ilPFC input, both via direct projections and via the TRN, can modulate RE neuron firing pattern in nuanced and complex ways. They also highlight the ilPFC-TRN-RE circuit as a likely critical component of prefrontal-hippocampal interactions.

Normal extinction and reinstatement of morphine-induced conditioned place preference in the GluA1-KO mouse line

Extinction and reinstatement of morphine-induced conditioned place preference were studied in glutamate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-receptor GluA1 subunit-deficient mice (global GluA1-KO mice). In line with previous findings, both acquisition and expression of conditioned place preference to morphine (20 mg/kg, subcutaneously) were fully functional in GluA1 KO mice compared with wild-type littermate controls (GluA1-WT), thus enabling the study of extinction. With a 10-session extinction paradigm, the GluA1 KO mice showed complete extinction similar to that of the GluA1-WT mice. Morphine-induced reinstatement (10 mg/kg, subcutaneously) was detected in both mouse lines. GluA1 KO mice moved more during all the phases of the experiment, including the place conditioning trials, extinction sessions, and place preference tests. The results suggest that the GluA1 subunit may be dispensable or prone to compensation at the neural circuitries delineating extinction and reinstatement. The GluA1 KO mice show altered long-term between-session habituation, which extends longer than previously anticipated.

Dorsal hippocampal neural immune signaling regulates heroin-conditioned immunomodulation but not heroin-conditioned place preference

Repeated pairings of heroin and a context results in Pavlovian associations which manifest as heroin-conditioned appetitive responses and peripheral immunomodulation upon re-exposure to heroin-paired conditioned stimuli (CS). The dorsal hippocampus (DH) plays a key role in the neurocircuitry governing these context-heroin associations. Within the DH, expression of the pro-inflammatory cytokine interleukin-1β (IL-1β) is required for heroin-conditioned peripheral immunomodulation to occur. However, the role of signaling via IL-1 receptor type 1 (IL-1R1) has not been examined. Furthermore, it has not been evaluated whether the involvement of IL-1 in associative learning extends to classically conditioned appetitive behaviors, such as conditioned place preference (CPP). The first set of experiments investigated whether DH IL-1R1 signaling during CS re-exposure modulates heroin-conditioned immunomodulation and heroin-CPP. The second set of experiments employed chemogenetic techniques to examine whether DH astroglial signaling during CS re-exposure alters the same Pavlovian responses. This line of investigation is based on previous research indicating that astrocytes support hippocampal-dependent learning and memory through the expression of IL-1β protein and IL-1R1. Interestingly, IL-1R1 antagonism disrupted heroin-conditioned suppression of peripheral immune parameters but failed to alter heroin-CPP. Similarly, chemogenetic stimulation of Gi-signaling in DH astrocytes attenuated heroin-conditioned peripheral immunomodulation but failed to alter heroin-CPP. Collectively our data show that both IL-1R1 stimulation and astrocyte signaling in the DH are critically involved in the expression of heroin-conditioned immunomodulation but not heroin-CPP. As such these findings strongly suggest hippocampal neuroimmune signaling differentially regulates Pavlovian immunomodulatory and appetitive behaviors.

Generation of silent synapses in dentate gyrus correlates with development of alcohol addiction

The brain circuits and synaptic processes that underlie alcohol addiction are currently the subject of intensive research. Here we focus on hippocampal circuitry and show that chemogenetic inhibition of dentate gyrus (DG) during presentation of alcohol-associated cues has long-lasting effects on mice behavior. DG inhibition enhances alcohol seeking and drinking, suggesting that DG regulates addiction-related behaviors. To test this hypothesis, we perform whole-cell patch-clamp recordings from the granule cells of DG and look for electrophysiological correlates of alcohol addiction. We observe that presentation of alcohol-associated cue light that induces relapse to alcohol-seeking results in generation of silent synapses, that lack functional AMPA receptors. Furthermore, using human criteria of addiction, we differentiate mice controlling their alcohol consumption from those that undergo transition to addiction to discover that the levels of silent synapses induced by alcohol cues are specifically increased in the addicted mice. As the total level of dendritic spines that harbor synapses is constant at this time point, our data indicate that synapses of perforant path to DG are weakened during cue relapse. Finally we demonstrate that, acamprosate, a drug that limits alcohol drinking and seeking in addicts, prevents generation of silent synapses in DG upon presentation of alcohol-associated cues. Altogether, our data suggest that weakening of DG synapses upon cue relapse contributes to persistent alcohol addiction-related behaviors.

Role of Glutamatergic Projections from the Ventral CA1 to Infralimbic Cortex in Context-Induced Reinstatement of Heroin Seeking

The prelimbic cortex (PL) and infralimbic cortex (IL) play a role in context-induced reinstatement of heroin seeking in an animal model of drug relapse. Both the PL and IL receive direct glutamatergic projections from the ventral CA1 (vCA1), which is also involved in context-induced reinstatement of cocaine and heroin seeking. Here we studied the role of vCA1-PL and vCA1-IL projections in context-induced reinstatement of heroin seeking by using electrophysiological, neuropharmacological, chemogenetic, and molecular methods. We showed that context-induced reinstatement of heroin seeking caused selective activation of the vCA1-IL but not vCA1-PL glutamatergic projections, decreased synaptosomal GluA2 expression in the IL, impaired basal synaptic transmission, and facilitation of long-term depression (LTD) in the vCA1-IL pathway. Additionally, chemogenetic inactivation of the vCA1-IL but not vCA1-PL pathway decreased context-induced reinstatement of heroin seeking. Inactivation of the vCA1-IL pathway also reversed synaptosomal GluA2 downregulation and basal transmission reduction, and blocked LTD induction. Taken together, our results demonstrate a critical role of the vCA1-IL glutamatergic projection in context-induced reinstatement of heroin seeking in a rat model of drug relapse.

The use of chemogenetics in behavioural neuroscience: receptor variants, targeting approaches and caveats

The last decade has seen major advances in neuroscience tools allowing us to selectively modulate cellular pathways in freely moving animals. Chemogenetic approaches such as designer receptors exclusively activated by designer drugs (DREADDs) permit the remote control of neuronal function by systemic drug administration. These approaches have dramatically advanced our understanding of the neural control of behaviour. Here, we review the different techniques and genetic approaches available for the restriction of chemogenetic receptors to defined neuronal populations. We highlight the use of a dual virus approach to target specific circuitries and the effectiveness of different routes of administration of designer drugs. Finally, we discuss the potential caveats associated with DREADDs including off-target effects of designer drugs, the effects of chronic chemogenetic receptor activation and the issue of collateral projections associated with DREADD activation and inhibition.

The Winding Road to Relapse: Forging a New Understanding of Cue-Induced Reinstatement Models and Their Associated Neural Mechanisms

In drug addiction, cues previously associated with drug use can produce craving and frequently trigger the resumption of drug taking in individuals vulnerable to relapse. Environmental stimuli associated with drugs or natural reinforcers can become reliably conditioned to increase behavior that was previously reinforced. In preclinical models of addiction, these cues enhance both drug self-administration and reinstatement of drug seeking. In this review, we will dissociate the roles of conditioned stimuli as reinforcers from their modulatory or discriminative functions in producing drug-seeking behavior. As well, we will examine possible differences in neurobiological encoding underlying these functional differences. Specifically, we will discuss how models of drug addiction and relapse should more systematically evaluate these different types of stimuli to better understand the neurobiology underlying craving and relapse. In this way, behavioral and pharmacotherapeutic interventions may be better tailored to promote drug use cessation outcomes and long-term abstinence.

Decrease of cocaine, but not heroin, self-administration and relapse by the tyrosine kinase inhibitor masitinib in male Sprague Dawley rats

RATIONALE

Accumulating evidence shows that cocaine, and also heroin, influence several tyrosine kinases, expressed in neurons and in non-neuronal populations such as microglia, astrocytes and mast-cells. Drug-induced activation of mast cells both triggers inflammatory processes in the brain mediated by the glial cells they activate, and facilitates histamine release which may directly influence the dopamine system. Thus, by triggering the activation and degranulation of mast cells dependent on the tyrosine kinase c-kit and Fyn, the latter being also involved in NMDA-dependent synaptic plasticity, cocaine and heroin may indirectly influence the neural mechanisms that mediate their reinforcing properties. Masitinib, a novel tyrosine kinase inhibitor with high selectivity for c-Kit, Fyn and Lyn, may alter the aberrant consequences of the activation of these tyrosine kinases by cocaine and heroin.

OBJECTIVE

We investigated in rats the effect of a chronic oral treatment with masitinib (20 mg/kg) on the reinforcing and motivational properties of self-administered cocaine (250 μg/infusion) and heroin (40 μg/infusion).

METHODS

Three different cohorts of rats were trained instrumentally to respond for cocaine, heroin or food under continuous reinforcement. In each group, we assessed the influence of chronic daily treatment with masitinib on the maintenance of instrumental responding and intake and the motivation for the reinforcer. Thus, masitinib and vehicle-treated rats were challenged to adapt to high behavioural demand, to respond under a progressive ratio schedule of reinforcement and to reinstate instrumental responding after extinction and/or abstinence.

RESULTS

Masitinib selectively decreased cocaine intake, the motivation for cocaine and the subsequent propensity to respond for cocaine under extinction, while having no effect on instrumental responding for heroin or food.

CONCLUSION

The present findings suggest masitinib, a drug with proven efficacy in CNS disorders, could represent a novel treatment for cocaine addiction provided its influence on the reinforcing and incentive properties of the drug is confirmed.