Dorsal Raphe Research Articles

Adolescent social stress alters the role of orexin innervation in the hindbrain in male hamsters.

Juvenile male hamsters exposed to chronic social stress eat more, gain weight, and have larger fat pads. The purpose of the present study was to address possible changes in food hoarding and orexin/hypocretin innervation in response to social stress. Male hamsters in early adolescence were exposed to a resident-intruder social stress paradigm or control condition daily for 2 weeks. Metabolism-related physiological measures and behaviors were tracked, and brains were immunocytochemically labeled for orexin-A. Our data confirm our previous observations on appetite, weight gain, and obesity, and showed a strong trend toward enhanced food hoarding as in prior studies. In addition, there were no statistically significant differences in orexin innervation in any brain area analyzed. However, unique correlation patterns were observed between orexin innervation and appetite or metabolic outcome. In particular, opposite correlations were observed between groups within the dorsal raphe nucleus, lateral parabrachial nucleus, and nucleus of the solitary tract. These opposite patterns of correlations suggest chronic social stress causes site-specific alterations in synaptic activity in relation with these behaviors.

Solitude and serotonin: juvenile isolation alters the covariation between social behavior and cFos expression by serotonergic neurons

Variation in the mutual responsiveness of social partners to each other can be reflected in behavioral suites that covary with neural activity in ways that track the salience or valence of interactions. Juvenile social isolation alters social behavior and neural activity during social interaction, but whether and how it alters the covariation between behavior and neural activity has not been as well explored. To address this issue, four classes of experimental subjects: isolated males, socially housed males, isolated females, and socially housed females, were paired with an opposite-sex social partner that had been socially housed. Social behaviors and c-Fos expression in the serotonergic dorsal raphe nucleus (DRN) were then measured in subjects following the social interactions. Relative to social housing, postweaning isolation led to a decrease in the density of neurons double-labeled for tryptophan hydroxylase and c-Fos in the dorsomedial subdivision of the DRN, regardless of sex. Vocal and non-vocal behaviors were also affected by isolation. In interactions with isolated males, both ultrasonic vocalization (USVs) and broadband vocalizations (squeaks) increased in conjunction with greater male investigation of females. Neural and behavioral measures also correlated with each other. In the isolated male group, the density of double-labeled neurons in the dorsomedial DRN was negatively correlated with USV production and positively correlated with a principal component of non-vocal behavior corresponding to greater defensive kicking by females and less investigation and mounting behavior. This correlation was reversed in direction for socially housed males, and for isolated males versus isolated females. These findings confirm that the dynamics of social interactions are reflected in c-Fos activation in the dorsomedial DRN, and suggest an altered responsiveness of serotonergic neurons to social interaction following social isolation in males, in parallel with an altered male response to female cues.

Alzheimer's Disease Protein Targets: Comprehensive Review and Future Directions.

Alzheimer's disease (AD) is a gradual degenerative ailment of the nervous system that is marked by the buildup of amyloid-β plaques and neurofibrillary tangles. This accumulation causes problems with the connections between nerve cells and the loss of these cells. This review paper explores the complex pathophysiology of AD, analyzing the neuronal loss reported in key brain regions like the entorhinal cortex, amygdala, hippocampus, and cortical association areas. The text also examines subcortical nuclei participation, such as the noradrenergic locus coeruleus, serotonergic dorsal raphe, and cholinergic basal nucleus. Also, this review discusses the importance of tau protein hyperphosphorylation, oxidative stress, and metal ion dysregulation in the evolution of AD. Moreover, it explores the cholinergic theory and the influence of the APOE (apolipoprotein E) genotype on the effectiveness of therapy. This article thoroughly summarizes the current knowledge on AD, including its clinical symptoms and possible treatment approaches, by combining several theories and new targets. The study highlights the connection between the degree of tangle development and the severity of dementia, underlining the need for creative methods to tackle the complex difficulties of discovering drugs for AD.

TDCS Cranial Nerve Co-Stimulation: Unveiling Brainstem Pathways Involved in Trigeminal Nerve Direct Current Stimulation in Rats.

The effects of transcranial direct current stimulation (tDCS) are typically attributed to the polarization of cortical neurons by the weak electric fields it generates in the cortex. However, emerging evidence indicates that certain tDCS effects may be mediated through the co-stimulation of peripheral or cranial nerves, particularly the trigeminal nerve (TN), which projects to critical brainstem nuclei that regulate the release of various neurotransmitters throughout the central nervous system. Despite this, the specific pathways involved remain inadequately characterized. In this study, we examined the effects of acute transcutaneous TN direct current stimulation (TN-DCS) on tonic (i.e. mean spike rate and spike rate over time) and phasic (number of bursts, spike rate per burst, burst duration, and inter-burst interval) activities while simultaneously recording single-neuron activity across three brainstem nuclei in rats: the locus coeruleus (LC), dorsal raphe nucleus (DRN), and median raphe nucleus (MnRN). We found that TN-DCS significantly modulated tonic activity in the LC, with notable interactions between stimulation amplitude, polarity, and time epoch affecting mean spike rates. Similar effects were observed in the DRN regarding tonic activity. Further, phasic activity in the LC was influenced by TN-DCS, with changes in burst number, duration, and inter-burst intervals linked to stimulation parameters. Conversely, MnRN tonic activity following TN-DCS remained unchanged. Importantly, xylocaine administration to block TN abolished the effects on tonic activities in both the LC and DRN. These results suggest that tDCS effects may partially arise from indirect modulation of the TN, leading to altered neuronal activity in DRN and LC. Besides, the differential changes in tonic and phasic LC activities underscore their complementary roles in mediating TN-DCS effects on higher cortical regions. This research bears significant translational implications, providing mechanistic insights that could enhance the efficacy of tDCS applications and deepen our understanding of its neurophysiological effects.

Dorsal raphe dopaminergic neurons target CaMKII+ neurons in dorsal bed nucleus of the stria terminalis for mediating depression-related behaviors

Dopamine (DA) neurons play a crucial role in the development and manifestation of depression, as well as in response to antidepressant treatments. While the function of the predominantly distributed DA neurons in the ventral tegmental area (VTA) is well established, the contribution of a small fraction of DA neurons in the dorsal raphe nucleus (DRN) during depression remains unclear. In this study, we found that chronic unpredictable stress (CUS) induces depression-related behaviors and decreases spontaneous firing rates, excitatory and inhibitory postsynaptic currents of DA neurons in the DRN associated with reduced excitatory synaptic transmission in male and female mice. The chemogenetic inhibition of DA neurons in the DRN produces depressive phenotypes. Conversely, their activation completely reversed the anhedonic and despair behaviors induced by CUS. Furthermore, we showed that a DRN dopaminergic projecting to the dorsal bed nucleus of the stria terminalis (dBNST) selectively controls depressive behaviors by influencing the neural activity and N-methyl-D-aspartate receptor (NMDAR) mediating EPSC of calcium/calmodulin-dependent protein kinase II+ (CaMKII+) target neurons by regulating dopamine neurotransmitter and dopamine receptor 2 (DR2) in the dBNST. Overall, these findings highlight the essential role of the DRNDA → dBNSTCaMKII+ neural circuit in bi-directionally mediating stress-induced depression-related behaviors. Our findings indicate that DRN DA neurons are a key component of the neural circuitry involved in regulating depression-related behaviors, making them a potential therapeutic target for depression.

Expression of tryptophan hydroxylase in rat adrenal glands: Upregulation of TPH2 by chronic stress

It has been shown that chronic restraint stress (CRS) increases adrenal 5-HT levels and turnover through a mechanism that appears unrelated to tryptophan hydroxylase (TPH). In the present study we re-analyzed the effects of CRS (20 min/day) for 14 days relative to control (CTRL) conditions on TPH expression, distribution, and activity in rat adrenal glands. On day 15, adrenal glands were collected for TPH1 and TPH2 immunohistochemistry, Western blot, and RT-PCR; TPH activity was estimated by quantification of 5-hydroxytryptophan (5-HTP) and, indirectly, through measurement of 5-HT and 5-hydroxindolacetic acid (5-HIAA) levels and turnover (5-HIAA/5-HT ratio) by HPLC. TPH expression and activity in the dorsal raphe nucleus (DRN) were also determined for comparison. TPH1 and TPH2 immunostaining was observed in the adrenal medulla, and measurable levels of TPH1 and TPH2 protein and mRNA were detected in rat adrenal glands from CTRL animals. CRS exposure noticeably increased TPH2- but not THP1-immunostaining in the medulla and the outer adrenocortical areas of left (LAG) but not of right adrenal glands (RAG). In addition, CRS exposure increased TPH2 protein and mRNA levels in LAG; however, both measures decreased in DRN. Finally, CRS treatment produced an increase and a decrease of TPH activity and 5-HT turnover in LAG and DRN, respectively. Results indicate that TPH is indeed expressed in rat adrenal glands. Exposure to CRS upregulates TPH2 in LAG, while inducing downregulation of it in the DRN. Then, the increased levels of 5-HT in LAG from CRS-exposed animals likely results from TPH2-mediated synthesis.

S-(+)-mecamylamine increases the firing rate of serotonin neurons and diminishes depressive-like behaviors in an animal model of stress

Mecamylamine, a noncompetitive blocker of nicotinic acetylcholine receptors (nAChRs), is the racemic mixture of two stereoisomers: S-(+)-mecamylamine (S-mec) and R-(−)-mecamylamine (R-mec), with distinct interactions with α4β2 nAChRs. It has been shown that mecamylamine increases glutamate release and excites serotonergic (5-HT) neurons in the dorsal raphe nucleus (DRN). In this study, we separately evaluated the effects of S-mec and R-mec on 5-HT neuron excitability. S-mec (3 μM) increased firing frequency by 40 %, while R-mec (3 μM) raised it by only 22 %. S-mec acts as a positive allosteric modulator on high-sensitivity (HS) α4β2 nAChRs at glutamate terminals, enhancing spontaneous excitatory postsynaptic currents (sEPSCs) in 5-HT neurons. Conversely, R-mec decreased sEPSCs by blocking HS α4β2 nAChRs and reduced GABA-mediated inhibitory currents (sIPSCs) by blocking α7 nAChRs at GABAergic terminals. These mechanisms make S-mec more effective than R-mec in enhancing 5-HT neuron firing. Moreover, combining S-mec with TC-2559, a selective agonist of HS α4β2 nAChRs, increased firing frequency by 65 %, exceeding the effect of S-mec alone. To validate these findings, we evaluated the antidepressant effects of S-mec (1 mg/kg) combined with TC-2559 or RJR-2403, another α4β2 nAChR agonist. This combination successfully reduced depression-like behaviors, suggesting a potential treatment strategy for patients resistant to conventional antidepressants.

Role of the Dorsal Raphe Nucleus in Pain Processing.

The dorsal raphe nucleus (DRN) has gained attention owing to its involvement in various physiological functions, such as sleep-awake, feeding, and emotion, with its analgesic role being particularly significant. It is described as the "pain inhibitory nucleus" in the brain. The DRN has diverse projections from hypothalamus, midbrain, and pons. In turn, the DRN is a major source of projections to diverse cortex, limbic forebrain thalamus, and the midbrain and contains highly heterogeneous neuronal subtypes. The activation of DRN neurons in mice prevents the establishment of neuropathic, chronic pain symptoms. Chemogenetic or optogenetic inhibition neurons in the DRN are sufficient to establish pain phenotypes, including long-lasting tactile allodynia, that scale with the extent of stimulation, thereby promoting nociplastic pain. Recent progress has been made in identifying the neural circuits and cellular mechanisms in the DRN that are responsible for sensory modulation. However, there is still a lack of comprehensive review addressing the specific neuron types in the DRN involved in pain modulation. This review summarizes the function of specific cell types within DRN in the pain regulation, and aims to improve understanding of the mechanisms underlying pain regulation in the DRN, ultimately offering insights for further exploration.

Monosynaptic inputs to ventral tegmental area glutamate and GABA co-transmitting neurons.

A unique population of ventral tegmental area (VTA) neurons co-transmits glutamate and GABA. However, the circuit inputs to VTA VGluT2+VGaT+ neurons are unknown, limiting our understanding of their functional capabilities. By coupling monosynaptic rabies tracing with intersectional genetic targeting in male and female mice, we found that VTA VGluT2+VGaT+ neurons received diverse brain-wide inputs. The largest numbers of monosynaptic inputs to VTA VGluT2+VGaT+ neurons were from superior colliculus, lateral hypothalamus, midbrain reticular nucleus, and periaqueductal gray, whereas the densest inputs relative to brain region volume were from dorsal raphe nucleus, lateral habenula, and ventral tegmental area. Based on these and prior data, we hypothesized that lateral hypothalamus and superior colliculus inputs were glutamatergic neurons. Optical activation of glutamatergic lateral hypothalamus neurons activated VTA VGluT2+VGaT+ neurons regardless of stimulation frequency and resulted in flee-like ambulatory behavior. In contrast, optical activation of glutamatergic superior colliculus neurons activated VTA VGluT2+VGaT+ neurons for a brief period of time at high stimulation frequency and resulted in head rotation and arrested ambulatory behavior (freezing). Stimulation of glutamatergic lateral hypothalamus neurons, but not glutamatergic superior colliculus neurons, was associated with VTA VGluT2+VGaT+ footshock-induced activity. In addition, inhibition of lateral hypothalamus glutamatergic neurons disrupted VTA VGluT2+VGaT+ tailshock-induced activity. We interpret these results such that inputs to VTA VGluT2+VGaT+ neurons may integrate diverse signals related to the detection and processing of motivationally-salient outcomes.Significance Statement VTA glutamate neurons have roles in motivated behavior and unique neurotransmission capabilities. A specific VTA glutamate neuron subtype, those that co-transmit glutamate and GABA, have unique outcome signaling properties compared to other VTA cell-types. However, the circuits that regulate these neurons are unclear. We identified the whole-brain inputs to VTA glutamate and GABA co-transmitting neurons. We also identify two distinct glutamatergic inputs that activate VTA glutamate and GABA co-transmitting neurons and result in different behavioral repertoires suggestive of threat processing. Together, these results provide novel insights into the circuit and cell-type specific influences on VTA glutamate and GABA co-transmitting neuronal activity as integrators of motivationally salient outcomes.

The differential effect of optogenetic serotonergic manipulation on sustained motor actions and waiting for future rewards in mice.

Serotonin is an essential neuromodulator that affects behavioral and cognitive functions. Previous studies have shown that activation of serotonergic neurons in the dorsal raphe nucleus (DRN) promotes patience to wait for future rewards. However, it is still unclear whether serotonergic neurons also regulate persistence to act for future rewards. Here we used optogenetic activation and inhibition of DRN serotonergic neurons to examine their effects on sustained motor actions for future rewards. We trained mice to perform waiting and repeated lever-pressing tasks with variable reward delays and tested effects of optogenetic activation and inhibition of DRN serotonergic neurons on task performance. Interestingly, in the lever-pressing task, mice tolerated longer delays as they repeatedly pressed a lever than in the waiting task, suggesting that lever-pressing actions may not simply be costly, but may also be subjectively rewarding. Optogenetic activation of DRN serotonergic neurons prolonged waiting duration in the waiting task, consistent with previous studies. However, its effect on lever presses was nuanced, and was detected only by focusing on the period before premature reward check and by subtracting the trends within and across sessions using generalized linear model. While optogenetic inhibition decreased waiting, it did not affect lever pressing time or numbers. These results revealed that the necessity of motor actions may increase motivation for delayed rewards and that DRN serotonergic neurons more significantly promote waiting rather than persistent motor actions for future rewards.

Role of serotonin neurons in the dorsal raphe nucleus in heroin self-administration and punishment.

One hallmark of substance use disorder is continued drug use despite negative consequences. When drug-taking behavior is punished with aversive stimuli, i.e. footshock, rats can also be categorized into punishment-resistant or compulsive vs. punishment-sensitive or non-compulsive phenotypes. The serotonin (5-hydroxytryptamine, 5-HT) system modulates responses to both reward and punishment. The goal of the current study was to examine punishment phenotypes in heroin self-administration and to determine the role of dorsal raphe nucleus (DRN) 5-HT neurons in both basal and punished heroin self-administration. First, rats were exposed to punished heroin self-administration and neuronal excitability of DRN 5-HT neuronswas compared between punishment-resistant and punishment-sensitive phenotypes using ex vivo electrophysiology. Second, DRN 5-HT neuronal activity was manipulated in vivo during basal and punished heroin self-administration using chemogenetic tools in a Tph2-iCre rat line. While rats separated into punishment-resistant and punishment-sensitive phenotypes for punished heroin self-administration, DRN 5-HT neuronal excitability did not differ between the phenotypes. While chemogenetic inhibition of DRN 5-HT neurons was without effect, chemogenetic activation of DRN 5-HT neurons increased both basal and punished heroin self-administration selectively in punishment-resistant animals. Additionally, the responsiveness to chemogenetic activation of DRN 5-HT neurons in basal self-administration and motivation for heroin in progressive ratio each predicted resistance to punishment. Therefore, our data support the role for the DRN 5-HT system in compulsive heroin self-administration.

Estrous cycle-dependent modulation of sexual receptivity in female mice by estrogen receptor beta-expressing cells in the dorsal raphe nucleus

The sexual receptivity of female mice, shown as lordosis response, is mainly regulated by estradiol action on estrogen receptor alpha (ERα) and beta (ERβ), depending on the day of the estrous cycle. Previous studies revealed that ERα in the ventromedial nucleus of the hypothalamus (VMH) plays an essential role in the induction of lordosis on the day of estrus (Day 1). However, the mechanisms of the transition to non-receptive states on the day after estrus (Day 2) are not completely understood. In the present study, we investigated the possible role of ERβ, which is highly expressed in the dorsal raphe nucleus (DRN), in lordosis expression. We found that ERβ-Cre female mice, which were ovariectomized and primed with estradiol and progesterone to mimic the estrous cycle, showed high levels of lordosis on Day 2 when ERβ-expressing DRN (DRN-ERβ+) neuronal activity was chemogenetically suppressed. This finding suggests that excitation of DRN-ERβ+ neurons is necessary for the decline of lordosis on Day 2. Fiber photometry recordings during female-male behavioral interactions revealed that DRN-ERβ+ neuronal activation in response to male intromission was significantly more prolonged on Day 2 compared to Day 1. Chemogenetic over-stimulation of DRN-ERβ+ neurons induced c-Fos expression in brain areas known to be inhibitory for lordosis expression, even though they did not express anterogradely labeled fibers of DRN-ERβ+ cells. These findings collectively suggest that DRN-ERβ+ neuronal excitation serves as an inhibitory modulator and is responsible for the decline in receptivity during non-estrus phases.Significance StatementIn females, switching from a sexually receptive state to a non-receptive phase during the estrous cycle is essential for effective behavioral interactions with males and successful reproduction. Here, we delineate the possible brain regions that regulate the decline in receptive behavioral responses, such as lordosis, from those shown on the day of estrus to those on the day after estrus. We found that excitation of neurons expressing estrogen receptor (ER) β in the midbrain dorsal raphe nucleus is crucial for the suppression of lordosis during the day after estrus. This is contrasted with the facilitatory action of ERα, another type of ER, and provides new insights for understanding the neural basis of the adaptive expression of female reproductive behavior.

Amelioration of social impairments in autism: Possible role of vagal afferent stimulation in modification of the prefrontal-amygdala connectivity

Impairment in social interactions is a prominent feature of autism spectrum disorder (ASD). The present hypothesis explains the possible role of vagal afferent stimulation in the modification of prefrontal-amygdala connectivity in the amelioration of social impairments in ASD. Currently, there is no definitive treatment for ASD. However, there is documented evidence showing that interventions that increase vagal tone lead to the improvement of autism-related social symptoms. However, the exact mechanism that explains the correlation between the elevation of vagal tone and the amelioration of autism-related symptoms has not been elucidated. In the present hypothesis, it is proposed that the increase in vagal tone affects the nucleus tractus solitarius (NTS) in the brainstem. The NTS has strong connections with other brainstem nuclei, such as the locus coeruleus and dorsal raphe nucleus. The aforementioned nuclei are the main sources of norepinephrine and serotonin, with extensive projections to various brain areas, especially the prefrontal cortex (PFC) and amygdala. PFC-amygdala functional connectivity plays an important role in social behavior and emotion regulation. Therefore, visceral stimulation, which subsequently increases the vagal tone, possibly leads to the improvement of autistic social deficits through the enhancement of PFC-amygdala functional connectivity. Further investigations are needed to evaluate the function of the mentioned neural pathways during visceral stimulation in individuals with ASD.

Region of interest determination algorithm of lensless calcium imaging datasets.

Advances in fluorescence imaging technology have been crucial to the progress of neuroscience. Whether it was specific expression of indicator proteins, detection of neurotransmitters, or miniaturization of fluorescence microscopes, fluorescence imaging has improved upon electrophysiology, the gold standard for monitoring brain activity, and enabled novel methods to sense activity in the brain. Hence, we developed a lightweight and compact implantable CMOS-based lensless Ca2+ imaging device for freely moving transgenic G-CaMP mouse experiments. However, without a lens system, determination of regions of interest (ROI) has proven challenging. Localization of fluorescence activity and separation of signal from noise are difficult. In this study, we report an ROI selection method using a series of adaptive binarizations with a gaussian method and morphological image processing. The parameters for each operation such as the kernel size, sigma and footprint size were optimized. We then validated the utility of the algorithm with simulated data and freely moving nociception experiments using the lensless devices. The device was implanted in the dorsal raphe nucleus to observe pain-related brain activity following a formalin test to stimulate pain. We observed significant increases in fluorescence activity after formalin injection compared to the control group when using the ROI determination algorithm.

Periaqueductal gray connectivity in spinal cord injury-induced neuropathic pain.

Neuropathic pain (NP) is a debilitating condition following spinal cord injury (SCI). The role of periaqueductal gray (PAG) in NP development following SCI remains underexplored. Using resting-state functional MRI (rsfMRI), our study aimed to demonstrate the alterations in functional connectivity (FC) of PAG in NP following SCI. Ten SCI patients (SCI+NP, n=7, and SCI-NP, n=3), alongside 10 healthy controls (HCs), were enrolled. rsfMRI was conducted followed by seed-to-voxel analysis using PAG as the seed region and then group-based analysis comprising three groups (SCI+NP, SCI-NP, and HC). Age and gender were considered as confounding variables. Compared to HCs, SCI+NP demonstrated decreased FC between PAG and right insula, right frontal orbital cortex, right pallidum, dorsal raphe nucleus (DRN), red nuclei (RN), substantia nigra (SN), and ventral posterolateral (VPL) thalamic nuclei. Compared to SCI-NP, SCI+NP demonstrated increased FC between PAG and posterior cingulate cortex (PCC), hippocampus, cerebellar vermis lobules IV and V, and thalamic structures (posterior and lateral pulvinar, the mediodorsal nuclei, and the ventral lateral nuclei). Additionally, decreased FC between the PAG and VPL, geniculate bodies, intralaminar nuclei of thalamus, DRN, RN, SN, and prefrontal cortex was observed in this comparison. Altered FC between PAG and right anterior insula, VPL, DRN, RN, SN, cerebellar vermis lobules IV and V, frontal cortex, and PCC was associated with NP sequelae of SCI. Additionally, SCI was independently associated with decreased FC between PAG and right posterior insula, cerebellar lobules IV and V, and cerebellar vermis lobules III, IV, and V.

Feedforward inhibition of stress by brainstem neuropeptide Y neurons

Resistance to stress is a key determinant for mammalian functioning. While many studies have revealed neural circuits and substrates responsible for initiating and mediating stress responses, little is known about how the brain resists to stress and prevents overreactions. Here, we identified a previously uncharacterized neuropeptide Y (NPY) neuronal population in the dorsal raphe nucleus and ventrolateral periaqueductal gray region (DRN/vlPAG) with anxiolytic effects in male mice. NPYDRN/vlPAG neurons are rapidly activated by various stressful stimuli. Inhibiting these neurons exacerbated hypophagic and anxiety responses during stress, while activation significantly ameliorates acute stress-induced hypophagia and anxiety levels and transmits positive valence. Furthermore, NPYDRN/vlPAG neurons exert differential but synergic anxiolytic effects via inhibitory projections to the paraventricular thalamic nucleus (PVT) and the lateral hypothalamic area (LH). Together, our findings reveal a feedforward inhibition neural mechanism underlying stress resistance and suggest NPYDRN/vlPAG neurons as a potential therapeutic target for stress-related disorders.

Uncovering the correlation between neurotransmitter-specific functional connectivity and multidimensional anxiety in a non-clinical cohort.

Research on anxiety faces challenges due to the wide range of symptoms, making it difficult to determine if different aspects of anxiety are linked to distinct neurobiological processes. Both alterations in functional brain connectivity (FC) and monoaminergic neurotransmitter systems are implicated as potential neural bases of anxiety. We aimed to investigate whole-brain FC involving monoaminergic nuclei and its association with anxiety dimensions in 178 non-clinical participants. Nine anxiety-related scales were used, encompassing trait and state anxiety scores, along with measures of cost-probability, hypervigilance, reward-punishment sensitivity, uncertainty, and trait worry. Resting-state functional magnetic resonance imaging data were acquired, focusing on seven brainstem regions representing serotonergic, dopaminergic, and noradrenergic nuclei, with their FC patterns voxel-wise correlated with the scales. All models underwent family-wise-error correction for multiple comparisons. We observed intriguing relationships: trait and state anxiety scores exhibited opposing correlations in FC between the dorsal raphe nucleus and the paracingulate gyrus. Additionally, we identified shared neural correlates, such as a negative correlation between the locus coeruleus and the frontal pole. This connection was significantly associated with scores on measures of probability, hypervigilance, reward sensitivity, and trait worry. These findings underscore the intricate interplay between anxiety dimensions and subcortico-cortical FC patterns, shedding light on the underlying neural mechanisms governing anxiety.

Rapid reorganization of serotonin projections and antidepressant response to 5-HT1A-biased agonist NLX-101 in fluoxetine-resistant cF1ko mice

Selective serotonin (5-HT) reuptake inhibitors (SSRIs) like fluoxetine remain a first-line treatment for major depression, but are effective in less than half of patients and can take 4–8 weeks to show results. In this study, we examined cF1ko mice with genetically induced upregulation of 5-HT1A autoreceptors that reduces 5-HT neuronal activity. These mice display anxiety- and depression-related behaviors that did not respond to chronic fluoxetine treatment. We examined treatment with NLX-101, a biased agonist that preferentially targets 5-HT1A heteroreceptors. By testing different doses of NLX-101, we found that a dose of 0.2 mg/kg was effective in reducing depression-related behavior in cF1ko mice without causing hypothermia, a 5-HT1A autoreceptor-mediated response. After 1 h, this dose activated dorsal raphe 5-HT neurons and cells in the medial prefrontal cortex (mPFC), increasing nuclear c-fos labelling in cF1ko mice. In cF1ko mice but not wild-type littermates, 0.2 mg/kg NLX-101 administered 1 h prior to each behavioral test for two weeks reduced depressive behavior in the forced swim test, but increased anxiety-related behaviors in the open field, elevated plus maze, and novelty suppressed feeding tests. During this treatment, NLX-101 induced widespread increases in the density of 5-HT axons, varicosities, and especially synaptic and triadic structures, particularly in depression-related brain regions including mPFC, hippocampal CA1 and CA2/3, amygdala and nucleus accumbens of cF1ko mice. Overall, NLX-101 was rapid and effective in reducing depressive behavior in SSRI-resistant mice, but also induced anxiety-related behaviors. The increase in serotonin innervation induced by intermittent NLX-101 may contribute to its behavioral actions.

The use of serotonin type 7 receptor antagonists as a pharmacological intervention in chronic stress. Insights from animal studies

This mini-review presents our current understanding of serotonin type 7 receptor research focusing on the possible network mechanisms underlying the behavioral action of receptor antagonists. The serotonin type 7 receptor is expressed widely throughout the nervous system and known to be involved in various cognitive and physiological mechanisms. It became a clinically significant target after the discovery that its selective antagonist SB 269970 can exert rapid-onset antidepressant effects either alone or in combination with lower doses of conventional antidepressant drugs. Further research has shown that administration of SB 269970 can effectively counteract negative neurobiological outcomes in various chronic stress paradigms. The authors hope they can introduce a wider scientific audience to this promising pharmacological target which, if successful, could in time lead to more discoveries and a better understanding of the underlying serotonin receptor biology as well as its clinical potential. Highlights•The 5-HT7 receptor is widely distributed throughout the nervous system.•5-HT7 receptor antagonist SB 269970 exerts rapid-onset antidepressant effects.•Antidepressant effects of SB 269970 occur via changes in dorsal raphe activity and subsequent cortical serotonin release.

Leptin Activation of Dorsal Raphe Neurons Inhibits Feeding Behavior.

Leptin is a homeostatic regulatory element that signals the presence of adipocyte energy stores, reduces food intake, and increases energy expenditure. Similarly, serotonin (5- HT), a signaling molecule found in both the central and peripheral nervous systems, also controls food intake. Using neuronal tract-tracing, pharmacological and optogenetics approaches, and in vivo microdialysis, combined with behavioral endpoints, we tested the hypothesis that leptin controls food intake not only by activating hypothalamic leptin receptors (LepRs), but also through activation of LepRs expressed by serotonergic raphe neurons that send projections to the arcuate (ARC). We show that microinjection of leptin directly into the dorsal raphe nucleus (DRN) reduces food intake in rats. This effect is mediated by LepR expressing neurons in the DRN as selective optogenetic activation of these neurons at either their DRN cell bodies or their ARC terminals reduces food intake. Anatomically, we identified a unique population of serotonergic raphe neurons expressing LepRs that send projections to the ARC. Finally, by utilizing in vivo microdialysis, we show that leptin administration to the DRN increases 5-HT efflux into the ARC, and specific antagonism of the 5-HT2C receptors in the ARC diminishes the leptin anorectic effect. Overall, this study identifies a novel circuit for leptin-mediated control of food intake through a DRN-ARC pathway, identifying a new level of interaction between leptin and serotonin to control food intake. Characterization of this new pathway creates opportunities for understanding how the brain controls eating behavior, as well as opens alternative routes for the treatment of eating disorders.