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Background: Persons with Alzheimer's disease (AD) present with changes in mood, sleep, and arousal that may precede the clinical manifestation of cognitive decline. These early symptoms can be driven by changes in the serotonergic (5-HT) nuclei of the brainstem, particularly the dorsal raphe nucleus (DRN). It is unclear why all 5-HT neurons do not simultaneously develop AD pathology that progresses at the same rate. Objective: We sought to identify any underlying genetic components associated with susceptibility or resistance of 5-HT neurons to AD pathology. Methods: The Visium Spatial Gene Expression platform was used to identify transcriptomic changes across the DRN in a preclinical model of early AD, human tau-overexpressing mice (htau mice). We further used RNAscope and immunohistochemical assessment to validate findings of primary interest. Results: We find that the DRN of htau mice differentially expresses AD-related genes, including those related to kinase binding, ion channel activity, ligand-receptor interactions, and regulation of serine/threonine kinases. We further find that computational sub-clustering of the DRN is consistent with previous circuitry-driven characterizations, allowing for spatial bounding of distinct subregions within the DRN. Of these, we find the dorsolateral DRN is preferentially impacted by 5-HT neuron loss and development of tau pathology, which coincides with increased expression of the long noncoding RNA Map2k3os. Conclusions: Map2k3os may serve regulatory roles relevant for tau phosphorylation and warrants further investigation to characterize its interactions. Overall, this report demonstrates the power of large-scale spatial transcriptomics technologies, while underscoring the need for convergent-data validation to overcome their limitations. [ABSTRACT FROM AUTHOR]

Background: The dorsal raphe nucleus (DRN) is the primary source of serotonergic projections to supratentorial structures. We and others have shown that it is selectively vulnerable to tau pathology in both human and mouse models of early AD. Although well characterized in mice, the neurochemical anatomy of the human DRN, and in particular the role of Vesicular glutamate transporter‐3 (VGLUT3)‐expressing neocortical projection neurons in tau pathology, remains unclear. Method: Post‐mortem human brain tissue was obtained from previously consented cases stored in the Iowa Neuropathology Resource Laboratory. Next of kin provided consent for research use of tissue. Sections were processed, embedded in paraffin, and sectioned in the usual fashion. Tyrosine hydroxylase (TH), tryptophan hydroxylase 2 (TPH2), and phosphorylated tau (AT8) were visualized by immunofluorescence, while VGLUT3 transcripts were identified by RNA in situ bybridization. Imaging was done using a Cytation5 platform with downstream analysis using MATLAB and R. Result: We found that, similar to mice, the human dorsal raphe has both VGLUT3‐positive and –negative serotonergic neurons. Unlike mice however, in the human DRN, these are not anatomically separate, and the proportion of TPH2 neurons that are VGLUT3 appears to be higher than in mice. In patients with DRN tau aggregates, these appear to selectively affect TPH2+/VGLUT3+ neurons. Conclusion: Serotonergic neurons in the dorsal raphe express high levels of VGLUT3, with an anatomic distribution differing from that seen in mice, and these neurons appear to be selectively vulnerable to tau pathology. [ABSTRACT FROM AUTHOR]

Background: It is increasingly apparent that tau pathology in Alzheimer's disease (AD) begins in the brainstems of middle‐aged patients, decades before the onset of symptoms. Most studies are, however, based on brain‐bank cohorts and focus on patients dying of natural causes. The true incidence of tau pathology in the brainstem thus remains unclear. In this study, we report a systematic characterization of Alzheimer's disease and related dementia (ADRD) pathology in the brainstems of a statewide cohort of patients coming to forensic autopsy. Method: Postmortem human brain samples were obtained from asymptomatic forensic deaths in the state of Iowa (n = 12; 6 male and 6 female). Formalin‐fixed, paraffin embedded tissue samples were stained by immunohistochemistry (IHC) on the dorsal raphe (DR), locus coeruleus (LC), and hippocampus (HPC) to assess the presence of tau (AT8), α‐synuclein, and beta amyloid pathology. All stains were independently evaluated by two experienced neuropathologists, with discrepancies resolved by consensus. Result: We identified a total of 12 cases in our initial cohort (median age 51, range 25‐74). Six of the 12 cases contained tau pathology in the HPC, five of 12 contained tau in the DR, and five of 12 contained tau in the LC. Additionally, one case contained beta amyloid in neocortex and alpha synuclein in the DR. All cases positive for pathology were greater than 50 years of age. Conclusion: Our data suggests that incidental ADRD pathology is extremely rare in younger adults, with alpha‐synuclein and beta‐amyloid only seen in a single 74‐year‐old patient. These findings suggest such pathology is less common than previously thought and emphasizes the unique advantage of studying it in forensic cohorts dying of non‐natural causes, which better represents the population as a whole. [ABSTRACT FROM AUTHOR]

Author(s): Catherine A. Marcinkiewcz [1]; Christopher M. Mazzone [1, 2]; Giuseppe DAgostino [3]; Lindsay R. Halladay [4]; J. Andrew Hardaway [1]; Jeffrey F. DiBerto [1]; Montserrat Navarro [5]; Nathan Burnham [...]

Alcoholism is a pervasive disorder perpetuated in part to relieve negative mood states like anxiety experienced during alcohol withdrawal. Emerging evidence demonstrates a role for the serotonin-rich dorsal raphe (DR) in anxiety following ethanol withdrawal. The current study examined the effects of chronic ethanol vapor exposure on the DR using slice electrophysiology in male DBA2/J mice. We found that chronic ethanol exposure resulted in deficits in social approach indicative of increased anxiety-like behavior at both 24 h and 7 days post-ethanol exposure. At 24 h post-ethanol exposure, we observed increased excitability and decreased spontaneous inhibitory transmission (inhibitory postsynaptic currents, IPSCs) in the DR. At 7 days post-ethanol exposure, we observed increased spontaneous and miniature excitatory transmission (excitatory postsynaptic currents, EPSCs). Because acute ethanol alters GABA transmission in other brain regions, we assessed the effects of ex vivo ethanol (50 mM) on miniature IPSCs (mIPSCs) in the DR 24-h post-ethanol exposure. Bath application of ethanol enhanced the amplitude of mIPSCs in cells from ethanol-naive and chronic intermittent ethanol-exposed (CIE) mice, but significantly enhanced the frequency of mIPSCs only in cells from CIE mice, suggesting that DR neurons are more sensitive to the inhibitory effects of acute ethanol following CIE. On the basis of these findings, we hypothesize that net excitation of DR neurons following chronic ethanol exposure contributes to enhanced anxiety during ethanol withdrawal, and that increased sensitivity of DR neurons to subsequent ethanol exposure may mediate acute ethanol's ability to relieve anxiety during ethanol withdrawal. [ABSTRACT FROM AUTHOR]

Schizophrenia is marked by poor social functioning that can have a severe impact on quality of life and independence, but the underlying neural circuity is not well understood. Here we used a translational model of subanesthetic ketamine in mice to delineate neural pathways in the brain linked to social deficits in schizophrenia. Mice treated with chronic ketamine (30 mg/kg/day for 10 days) exhibit profound social and sensorimotor deficits as previously reported. Using three- dimensional c-Fos immunolabeling and volume imaging (iDISCO), we show that ketamine treatment resulted in hypoactivation of the lateral septum (LS) in response to social stimuli. Chemogenetic activation of the LS rescued social deficits after ketamine treatment, while chemogenetic inhibition of previously active populations in the LS (i.e. social engram neurons) recapitulated social deficits in ketamine-naïve mice. We then examined the translatome of LS social engram neurons and found that ketamine treatment dysregulated genes implicated in neuronal excitability and apoptosis, which may contribute to LS hypoactivation. We also identified 38 differentially expressed genes (DEGs) in common with human schizophrenia, including those involved in mitochondrial function, apoptosis, and neuroinflammatory pathways. Chemogenetic activation of LS social engram neurons induced downstream activity in the ventral part of the basolateral amygdala, subparafascicular nucleus of the thalamus, intercalated amygdalar nucleus, olfactory areas, and dentate gyrus, and it also reduces connectivity of the LS with the piriform cortex and caudate-putamen. In sum, schizophrenia-like social deficits may emerge via changes in the intrinsic excitability of a discrete subpopulation of LS neurons that serve as a central hub to coordinate social behavior via downstream projections to reward, fear extinction, motor and sensory processing regions of the brain.

In 2020, stay-at-home orders were implemented to stem the spread of SARS-CoV-2 worldwide. Social isolation can be particularly harmful to children and adolescents-during the pandemic, the prevalence of obesity increased by ∼37% in persons aged 2-19. Obesity is often comorbid with type 2 diabetes, which was not assessed in this human pandemic cohort. Here, we investigated whether male mice isolated throughout adolescence develop type 2 diabetes in a manner consistent with human obesity-induced diabetes, and explored neural changes that may underlie such an interaction. We find that isolating C57BL/6J mice throughout adolescence is sufficient to induce type 2 diabetes. We observed fasted hyperglycemia, diminished glucose clearance in response to an insulin tolerance test, decreased insulin signaling in skeletal muscle, decreased insulin staining of pancreatic islets, increased nociception, and diminished plasma cortisol levels compared to group-housed control mice. Using Promethion metabolic phenotyping chambers, we observed dysregulation of sleep and eating behaviors, as well as a time-dependent shift in respiratory exchange ratio of the adolescent-isolation mice. We profiled changes in neural gene transcription from several brain areas and found that a neural circuit between serotonin-producing and GLP-1-producing neurons is affected by this isolation paradigm. Overall, spatial transcription data suggest decreased serotonin neuron activity (via decreased GLP-1-mediated excitation) and increased GLP-1 neuron activity (via decreased serotonin-mediated inhibition). This circuit may represent an intersectional target to further investigate the relationship between social isolation and type 2 diabetes, as well as a pharmacologically-relevant circuit to explore the effects of serotonin and GLP-1 receptor agonists., Article Highlights: Isolating C57BL/6J mice throughout adolescence is sufficient to induce type 2 diabetes, presenting with fasted hyperglycemia.Adolescent-isolation mice have deficits in insulin responsiveness, impaired peripheral insulin signaling, and decreased pancreatic insulin production.Transcriptional changes across the brain include the endocannabinoid, serotonin, and GLP-1 neurotransmitters and associated receptors. The neural serotonin/GLP-1 circuit may represent an intersectional target to further investigate the relationship between social isolation and type 2 diabetes. Serotonin-producing neurons of adolescent-isolation mice produce fewer transcripts for the GLP-1 receptor, and GLP-1 neurons produce fewer transcripts for the 5-HT 1A serotonin receptor.

Social interaction is a core component of motivational behavior that is perturbed across multiple neuropsychiatric disorders, including alcohol use disorder (AUD). Positive social bonds are neuroprotective and enhance recovery from stress, so reduced social interaction in AUD may delay recovery and lead to alcohol relapse. We report that chronic intermittent ethanol (CIE) induces social avoidance in a sex-dependent manner and is associated with hyperactivity of serotonin (5-HT) neurons in the dorsal raphe nucleus (DRN). While 5-HT DRN neurons are generally thought to enhance social behavior, recent evidence suggests that specific 5-HT pathways can be aversive. Using chemogenetic iDISCO, the nucleus accumbens (NAcc) was identified as one of 5 regions that were activated by 5-HT DRN stimulation. We then employed an array of molecular genetic tools in transgenic mice to show that 5-HT DRN inputs to NAcc dynorphin neurons drive social avoidance in male mice after CIE by activating 5-HT 2C receptors. NAcc dynorphin neurons also inhibit dopamine release during social interaction, reducing the motivational drive to engage with social partners. This study reveals that excessive serotonergic drive after chronic alcohol can promote social aversion by inhibiting accumbal dopamine release. Drugs that boost brain serotonin levels may be contraindicated for individuals with AUD.