SNAP25 differentially contributes to Gi/o-coupled receptor function at glutamatergic synapses in the nucleus accumbens

The nucleus accumbens (NAc) guides reward-related motivated behavior implicated in pathological behavioral states, including addiction and depression. These behaviors depend on the precise neuromodulatory actions of G_i/o-coupled G-protein-coupled receptors (GPCRs) at glutamatergic synapses onto medium spiny projection neurons (MSNs). Previous work has shown that discrete classes of G_i/o-coupled GPCR mobilize Gβγ to inhibit vesicular neurotransmitter release via t-SNARE protein, SNAP25. However, it remains unknown which Gαi/o systems in the NAc utilize Gβγ-SNARE signaling to dampen glutamatergic transmission. Utilizing patch-clamp electrophysiology and pharmacology in a transgenic mouse line with a C-terminal three-residue deletion of SNAP25 (SNAP25Δ3) weaking the Gβγ-SNARE interaction, we surveyed a broad cohort of G_i/o-coupled GPCRs with robust inhibitory actions at glutamatergic synapses in the NAc. We find that basal presynaptic glutamate release probability is reduced in SNAP25Δ3 mice. While κ opioid, CB1, adenosine A1, group II metabotropic glutamate receptors, and histamine H3 receptors inhibit glutamatergic transmission onto MSNs independent of SNAP25, we report that SNAP25 contributes significantly to the actions of GABA_B, 5-HT1_B/D, and μ opioid receptors. These findings demonstrate that presynaptic G_i/o-coupled GPCRs recruit heterogenous effector mechanisms at glutamatergic synapses in the NAc, with a subset requiring SNA25-dependent Gβγ signaling.

Accumbal Histamine Signaling Engages Discrete Interneuron Microcircuits

BACKGROUND

Central histamine (HA) signaling modulates diverse cortical and subcortical circuits throughout the brain, including the nucleus accumbens (NAc). The NAc, a key striatal subregion directing reward-related behavior, expresses diverse HA receptor subtypes that elicit cellular and synaptic plasticity. However, the neuromodulatory capacity of HA within interneuron microcircuits in the NAc remains unknown.

METHODS

We combined electrophysiology, pharmacology, voltammetry, and optogenetics in male transgenic reporter mice to determine how HA influences microcircuit motifs controlled by parvalbumin-expressing fast-spiking interneurons (PV-INs) and tonically active cholinergic interneurons (CINs) in the NAc shell.

RESULTS

HA enhanced CIN output through an H₂ receptor (H₂R)-dependent effector pathway requiring Ca²⁺-activated small-conductance K⁺ channels, with a small but discernible contribution from H₁Rs and synaptic H₃Rs. While PV-IN excitability was unaffected by HA, presynaptic H₃Rs decreased feedforward drive onto PV-INs via AC-cAMP-PKA (adenylyl cyclase-cyclic adenosine monophosphate-protein kinase A) signaling. H₃R-dependent plasticity was differentially expressed at mediodorsal thalamus and prefrontal cortex synapses onto PV-INs, with mediodorsal thalamus synapses undergoing HA-induced long-term depression. These effects triggered downstream shifts in PV-IN- and CIN-controlled microcircuits, including near-complete collapse of mediodorsal thalamus-evoked feedforward inhibition and increased mesoaccumbens dopamine release.

CONCLUSIONS

HA targets H₁R, H₂R, and H₃Rs in the NAc shell to engage synapse- and cell type-specific mechanisms that bidirectionally regulate PV-IN and CIN microcircuit activity. These findings extend the current conceptual framework of HA signaling and offer critical insight into the modulatory potential of HA in the brain.

Calcium-Permeable AMPA Receptors Promote Endocannabinoid Signaling at Parvalbumin Interneuron Synapses in the Nucleus Accumbens Core

Synaptic plasticity is a key mechanism of learning and memory. Synaptic plasticity mechanisms within the nucleus accumbens (NAc) mediate differential behavioral adaptations. Feedforward inhibition in the NAc occurs when glutamatergic afferents onto medium spiny neurons (MSNs) collateralize onto fast-spiking parvalbumin (PV)-expressing interneurons (PV-INs), which exert GABAergic control over MSN action potential generation. Here, we find that feedforward glutamatergic synapses onto PV-INs in the NAc core selectively express Ca²⁺-permeable AMPA receptors (CP-AMPARs). Ca²⁺ influx by CP-AMPARs on PV-INs triggers long-term depression (LTD) mediated by endocannabinoid (eCB) signaling at presynaptic cannabinoid type-1 (CB₁) receptors (CB₁Rs). Moreover, CP-AMPARs authorize tonic eCB signaling to negatively regulate glutamate release probability. Blockade of CP-AMPARs in the NAc core in vivo is sufficient to disinhibit locomotor output. These findings elucidate mechanisms by which PV-IN-embedded microcircuits in the NAc undergo activity-dependent shifts in synaptic strength.

Manz et al. show that CP-AMPARs are expressed at glutamatergic synapses onto PV-INs but not D1- or D2-expressing MSNs in the NAc core. Ca²⁺ influx through CP-AMPARs triggers endocannabinoid-dependent tone and synaptic plasticity. Intra-NAc blockade of CP-AMPARs in vivo increases basal locomotion.

Patch-clamp and multi-electrode array electrophysiological analysis in acute mouse brain slices

Patch-clamp and multi-electrode array electrophysiology techniques are used to measure dynamic functional properties of neurons. Whole-cell and cell-attached patch-clamp recordings in brain slices can be performed in voltage-clamp and current-clamp configuration to reveal cell-type-specific synaptic and cellular parameters governing neurotransmission. Multi-electrode array electrophysiology can provide spike activity recordings from multiple neurons, enabling larger sample sizes, and long-term recordings. We provide our guide to preparing acute rodent brain slices with example experiments and analyses intended for novice and expert electrophysiologists.

For complete details on the use and execution of this protocol, please refer to Manz et al. (2020b).

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Viable and efficient preparation of mouse brain tissue

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Comprehensive material source for acute brain slice electrophysiology

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Detailed approach to whole-cell patch-clamp recording of neurons

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Utility and application of multi-electrode array electrophysiology in mouse brain

Cannabinoid type 1 receptors in A2a neurons contribute to cocaine-environment association

RATIONALE

Cannabinoid type 1 receptors (CB1Rs) are widely expressed within the brain's reward circuits and are implicated in regulating drug induced behavioral adaptations. Understanding how CB1R signaling in discrete circuits and cell types contributes to drug-related behavior provides further insight into the pathology of substance use disorders.

OBJECTIVE AND METHODS

We sought to determine how cell type-specific expression of CB1Rs within striatal circuits contributes to cocaine-induced behavioral plasticity, hypothesizing that CB1R function in distinct striatal neuron populations would differentially impact behavioral outcomes. We crossed conditional Cnr1^fl/fl mice and striatal output pathway cre lines (Drd1a -cre; D1, Adora2a -cre; A2a) to generate cell type-specific CB1R knockout mice and assessed their performance in cocaine locomotor and associative behavioral assays.

RESULTS

Both knockout lines retained typical locomotor activity at baseline. D1-Cre x Cnr1^fl/fl mice did not display hyperlocomotion in response to acute cocaine dosing, and both knockout lines exhibited blunted locomotor activity across repeated cocaine doses. A2a-cre Cnr1^fl/fl, mice did not express a preference for cocaine paired environments in a two-choice place preference task.

CONCLUSIONS

This study aids in mapping CB1R-dependent cocaine-induced behavioral adaptations onto distinct striatal neuron subtypes. A reduction of cocaine-induced locomotor activation in the D1- and A2a-Cnr1 knockout mice supports a role for CB1R function in the motor circuit. Furthermore, a lack of preference for cocaine-associated context in A2a-Cnr1 mice suggests that CB1Rs on A2a-neuron inhibitory terminals are necessary for either reward perception, memory consolidation, or recall. These results direct future investigations into CB1R-dependent adaptations underlying the development and persistence of substance use disorders.

Histamine H3 Receptor Function Biases Excitatory Gain in the Nucleus Accumbens

BACKGROUND

Histamine (HA), a wake-promoting monoamine implicated in stress-related arousal states, is synthesized in histidine decarboxylase-expressing hypothalamic neurons of the tuberomammillary nucleus. Histidine decarboxylase-containing varicosities diffusely innervate striatal and mesolimbic networks, including the nucleus accumbens (NAc). The NAc integrates diverse monoaminergic inputs to coordinate motivated behavior. While the NAc expresses various HA receptor subtypes, mechanisms by which HA modulates NAc circuit dynamics are undefined.

METHODS

Using male D1tdTomato transgenic reporter mice, whole-cell patch-clamp electrophysiology, and input-specific optogenetics, we employed a targeted pharmacological approach to interrogate synaptic mechanisms recruited by HA signaling at glutamatergic synapses in the NAc. We incorporated an immobilization stress protocol to assess whether acute stress engages these mechanisms at glutamatergic synapses onto D₁ receptor-expressing [D1(+)] medium spiny neurons (MSNs) in the NAc core.

RESULTS

HA negatively regulates excitatory gain onto D1(+)-MSNs via presynaptic H₃ receptor-dependent long-term depression that requires G_βγ-directed Akt-GSK3β signaling. Furthermore, HA asymmetrically regulates glutamatergic transmission from the prefrontal cortex and mediodorsal thalamus, with inputs from the prefrontal cortex undergoing robust HA-induced long-term depression. Finally, we report that acute immobilization stress attenuates this long-term depression by recruiting endogenous H₃ receptor signaling in the NAc at glutamatergic synapses onto D1(+)-MSNs.

CONCLUSIONS

Stress-evoked HA signaling in the NAc recruits H₃ heteroreceptor signaling to shift thalamocortical input onto D1(+)-MSNs in the NAc. Our findings provide novel insight into an understudied neuromodulatory system within the NAc and implicate HA in stress-associated physiological states.

Synaptic Plasticity in the Nucleus Accumbens: Lessons Learned from Experience

Synaptic plasticity contributes to behavioral adaptations. As a key node in the reward pathway, the nucleus accumbens (NAc) is important for determining motivation-to-action outcomes. Across animal models of motivation including addiction, depression, anxiety, and hedonic feeding, selective recruitment of neuromodulatory signals and plasticity mechanisms have been a focus of physiologists and behaviorists alike. Experience-dependent plasticity mechanisms within the NAc vary depending on the distinct afferents and cell-types over time. A greater understanding of molecular mechanisms determining how these changes in synaptic strength track with behavioral adaptations will provide insight into the process of learning and memory along with identifying maladaptations underlying pathological behavior. Here, we summarize recent findings detailing how changes in NAc synaptic strength and mechanisms of plasticity manifest in various models of motivational disorders.

Diagnostic value of cerebrospinal fluid CXCL13 for acute Lyme neuroborreliosis. A systematic review and meta-analysis

OBJECTIVES

The utility of cerebrospinal fluid (CSF) CXCL13 for diagnosis of acute Lyme neuroborreliosis (LNB) has been debated and the test is not yet routinely performed. This study's aim was to evaluate its overall diagnostic accuracy through meta-analysis.

METHODS

Electronic searches in PubMed MEDLINE and Web of Science were performed to identify relevant articles published before January 2018. A summary receiver operating characteristic curve and an optimal cut-off were estimated modelling multiple cut-offs. Publication bias was evaluated using a funnel plot and the associated regression test.

RESULTS

A total of 18 studies involving 618 individuals with acute LNB and 2326 individuals with other neurological disorders meeting the eligibility criteria were identified. The pooled sensitivity for CSF CXCL13 was 89% (95% CI 85%-93%) and the pooled specificity was 96% (95% CI 92%-98%), using the identified optimal cut-off value of 162 pg/mL. There was marked heterogeneity between studies, caused by differences in the designs of the study populations and age distribution. The optimal cut-off in the seven studies with a cross-sectional design was 91 pg/mL (sensitivity 96%, 95% CI 92%-98%; specificity 94%, 95% CI 86%-97%) and in the 11 case-control studies it was 164 pg/mL (sensitivity 85%, 95% CI 78%-91%; specificity 95%, 95% CI 90%-98%). CSF CXCL13 values above the optimal cut-off level (determined in this meta-analysis) were also detectable in some other central nervous system disorders, namely neurosyphilis and central nervous system lymphoma.

CONCLUSIONS

Our meta-analysis shows that CSF CXCL13 has the potential to become a useful adjunct in the diagnosis of acute LNB.

A novel adolescent chronic social defeat model: reverse-Resident-Intruder Paradigm (rRIP) in male rats

Psychosocial stress is linked to the etiology of several neuropsychiatric disorders, including Major Depressive Disorder and Post-Traumatic-Stress-Disorder. Adolescence is a critical neurobehavioral developmental period wherein the maturing nervous system is sensitive to stress-related psychosocial events. The effects of social defeat stress, an animal model of psychosocial stress, on adolescent neurobehavioral phenomena are not well explored. Using the standard Resident-Intruder-Paradigm (RIP), adolescent Long-Evans (LE, residents, n = 100) and Sprague-Dawley (SD, intruders, n = 100) rats interacted for five days to invoke chronic social stress. Tests of depressive behavior (forced-swim-test (FST)), fear conditioning, and long-term synaptic plasticity are affected in various adult rodent chronic stress models, thus we hypothesized that these phenomena would be similarly affected in adolescent rats. Serendipitously, we observed the Intruders became the dominant rats and the Residents were the defeated/submissive rats. This robust and reliable role-reversal resulted in defeated LE-Residents showing a depressive-like state (increased time spent immobile in the FST), enhanced fear conditioning in both hippocampal-dependent and hippocampal-independent fear paradigms and altered hippocampal long-term synaptic plasticity, measured electrophysiologically in vitro in hippocampal slices. Importantly, SD-Intruders, SD and LE controls did not significantly differ from each other in any of these assessments. This reverse-Resident-Intruder-Paradigm (rRIP) represents a novel animal model to study the effects of stress on adolescent neurobehavioral phenomenon.