Functional organization of the dorsal raphe efferent system with special consideration of nitrergic cell groups

Contribution of the serotonergic system to developmental brain abnormalities in autism spectrum disorder

This review highlights a key role of the serotonergic system in brain development and in distortions of normal brain development in early stages of fetal life resulting in cascades of abnormalities, including defects of neurogenesis, neuronal migration, neuronal growth, differentiation, and arborization, as well as defective neuronal circuit formation in the cortex, subcortical structures, brainstem, and cerebellum of autistic subjects. In autism, defects in regulation of neuronal growth are the most frequent and ubiquitous developmental changes associated with impaired neuron differentiation, smaller size, distorted shape, loss of spatial orientation, and distortion of cortex organization. Common developmental defects of the brain in autism include multiregional focal dysplastic changes contributing to local neuronal circuit distortion, epileptogenic activity, and epilepsy. There is a discrepancy between more than 500 reports demonstrating the contribution of the serotonergic system to autism's behavioral anomalies, highlighted by lack of studies of autistic subjects' brainstem raphe nuclei, the center of brain serotonergic innervation, and of the contribution of the serotonergic system to the diagnostic features of autism spectrum disorder (ASD). Discovery of severe fetal brainstem auditory system neuronal deficits and other anomalies leading to a spectrum of hearing deficits contributing to a cascade of behavioral alterations, including deficits of social and verbal communication in individuals with autism, is another argument to intensify postmortem studies of the type and topography of, and the severity of developmental defects in raphe nuclei and their contribution to abnormal brain development and to the broad spectrum of functional deficits and comorbid conditions in ASD.

An Ascending Excitatory Circuit from the Dorsal Raphe for Sensory Modulation of Pain

The dorsal raphe nucleus (DRN) is an important nucleus in pain regulation. However, the underlying neural pathway and the function of specific cell types remain unclear. Here, we report a previously unrecognized ascending facilitation pathway, the DRN to the mesoaccumbal dopamine (DA) circuit, for regulating pain. Chronic pain increased the activity of DRN glutamatergic, but not serotonergic, neurons projecting to the ventral tegmental area (VTA) (DRNGlu-VTA) in male mice. The optogenetic activation of DRNGlu-VTA circuit induced a pain-like response in naive male mice, and its inhibition produced an analgesic effect in male mice with neuropathic pain. Furthermore, we discovered that DRN ascending pathway regulated pain through strengthened excitatory transmission onto the VTA DA neurons projecting to the ventral part of nucleus accumbens medial shell (vNAcMed), thereby activated the mesoaccumbal DA neurons. Correspondingly, optogenetic manipulation of this three-node pathway bilaterally regulated pain behaviors. These findings identified a DRN ascending excitatory pathway that is crucial for pain sensory processing, which can potentially be exploited toward targeting pain disorders.

Dorsal raphe stimulation relays a reward signal to the ventral tegmental area via GluN2C NMDA receptors

BACKGROUND

Glutamate relays a reward signal from the dorsal raphe (DR) to the ventral tegmental area (VTA). However, the role of the different subtypes of N-methyl-D-aspartate (NMDA) receptors is complex and not clearly understood. Therefore, we measured NMDA receptors subunits expression in limbic brain areas. In addition, we studied the effects of VTA down-regulation of GluN2C NMDA receptor on the reward signal that arises from DR electrical stimulation.

METHODS

Using qPCR, we identified the relative composition of the different Grin2a-d subunits of the NMDA receptors in several brain areas. Then, we used fluorescent in situ hybridization (FISH) to evaluate the colocalization of Grin2c and tyrosine hydroxylase (TH) mRNA in VTA neurons. To assess the role of GluN2C in brain stimulation reward, we downregulated this receptor using small interfering RNA (siRNA) in rats self-stimulating for electrical pulses delivered to the DR. To delineate further the specific role of GluN2C in relaying the reward signal, we pharmacologically altered the function of VTA NMDA receptors by bilaterally microinjecting the NMDA receptor antagonist PPPA.

RESULTS

We identified GluN2C as the most abundant subunit of the NMDA receptor expressed in the VTA. FISH revealed that about 50% of TH-positive neurons colocalize with Grin2c transcript. siRNA manipulation produced a selective down-regulation of the GluN2C protein subunit and a significant reduction in brain stimulation reward. Interestingly, PPPA enhanced brain stimulation reward, but only in rats that received the nonactive RNA sequence.

CONCLUSION

The present results suggest that VTA glutamate neurotransmission relays a reward signal initiated by DR stimulation by acting on GluN2C NMDA receptors.

The integrated brain network that controls respiration

Respiration is a brain function on which our lives essentially depend. Control of respiration ensures that the frequency and depth of breathing adapt continuously to metabolic needs. In addition, the respiratory control network of the brain has to organize muscular synergies that integrate ventilation with posture and body movement. Finally, respiration is coupled to cardiovascular function and emotion. Here, we argue that the brain can handle this all by integrating a brainstem central pattern generator circuit in a larger network that also comprises the cerebellum. Although currently not generally recognized as a respiratory control center, the cerebellum is well known for its coordinating and modulating role in motor behavior, as well as for its role in the autonomic nervous system. In this review, we discuss the role of brain regions involved in the control of respiration, and their anatomical and functional interactions. We discuss how sensory feedback can result in adaptation of respiration, and how these mechanisms can be compromised by various neurological and psychological disorders. Finally, we demonstrate how the respiratory pattern generators are part of a larger and integrated network of respiratory brain regions.

Neuronal nitric oxide synthase in dorsal raphe nucleus mediates PTSD-like behaviors induced by single-prolonged stress through inhibiting serotonergic neurons activity

The pathogenesis of post-traumatic stress disorder (PTSD) remains largely unclear. A large body of evidence suggests that the abnormal level of serotonin (5-HT) is closely related to the onset of PTSD. Several reports reveal that nitric oxide (NO) affects extracellular 5-HT levels in various brain regions, but no consistent direction of change was found and the underlying mechanisms remain unknown. The most of serotonergic neurons in dorsal raphe nucleus (DRN), a major source of serotonergic input to the forebrain, co-expresses neuronal nitric oxide synthase (nNOS), a synthase derived nitric oxide (NO) in the central nervous system. Here, we found that the excessive expression of nNOS and thereby the high concentration of NO followed by single-prolonged stress (SPS) caused suppression of the activity of DRN 5-HT neurons, inducing PTSD-like phenotype including increased anxiety-like behaviors, enhanced contextual fear memory, and fear generalization. Our study uncovered an important role of DRN nNOS-NO pathway in the pathology of PTSD, which may contribute to new understanding of the molecular mechanism of PTSD.

Correlation of Decreased Serum Pituitary Adenylate Cyclase-Activating Polypeptide and Vasoactive Intestinal Peptide Levels With Non-motor Symptoms in Patients With Parkinson's Disease

Objective: Pituitary adenylate-cyclase activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) are two neuropeptides that exhibit anti-inflammatory and neuroprotective properties, modulating the production of cytokines and chemokines, and the behavior of immune cells. However, the relationship between PACAP and VIP levels and Parkinson’s disease (PD) are not clear. The aim of the current study was to evaluate serum PACAP and VIP levels in PD patients and to analysis the correlation between neuropeptide levels and non-motor symptoms.

Methods: In this cross-sectional study, we enrolled 72 patients with idiopathic PD and 71 healthy volunteers. Serum PACAP and VIP levels were measured using an enzyme-linked immunosorbent assay (ELISA) kit. Non-motor symptoms were assessed with the Non-Motor Symptoms Scale (NMSS) for PD, including total and single-item scores.

Results: The serum PACAP levels of PD patients were significantly lower than those of healthy controls [(76.02 ± 43.78) pg/ml vs. (154.96 ± 76.54) pg/ml, P < 0.001]; and the serum VIP levels of PD patients were also significantly lower than those of healthy controls [(109.56 ± 15.39) pg/ml vs. (136.46 ± 24.16) pg/ml, P < 0.001]. PACAP levels were inversely correlated only with the score on NMSS item five, assessing Attention/memory (r = −0.276, P < 0.05) and lower serum PACAP levels were detected in the cognitive dysfunction subgroup than in the cognitively intact subgroup [(61.87 ± 32.66) pg/ml vs. (84.51 ± 47.59) pg/ml, P < 0.05]; meanwhile, VIP levels were inversely correlated with the NMSS total score (r = −0.285, P < 0.05) and the single-item scores for item one, assessing Cardiovascular (r = −0.257, P < 0.05) and item three, assessing Mood/cognition (r = −0.373, P < 0.05), and lower serum VIP levels were detected in the anxiety subgroup and depression subgroup than in the non-anxiety subgroup and non-depression subgroup, respectively [(107.45 ± 15.40) pg/ml vs. (116.41 ± 13.67) pg/ml, P < 0.05]; [(104.45 ± 15.26) pg/ml vs. (113.43 ± 14.52) pg/ml, P < 0.05].

Conclusion: The serum PACAP and VIP levels of PD patients were significantly lower than those of healthy controls. The non-motor symptoms significantly negatively correlated with serum PACAP level was cognitive dysfunction, while mood disorder was significantly correlated with serum VIP level.

Prunin modulates the Expression of Cerebral Serotonin Induced by Anxiety-Like Behavior in Mice

Anxiety is a state that becomesa disorder when a person experiences disproportionate levels of anxiety on a regular basis. This disproportion is also accompanied by excessive nervousness and fear. This study aimed to determine the protective effect of prunin using different anxiety models. Three preclinical anxiety models, elevated plus maze, light/dark, and social interaction, were employed in the study. Albino mice were selected and treated with pruninand other drugs for 7 days to determine their anti-anxiety effect. Thereafter, their behavior was examined using the plus maze, light-dark chamber, and other stimulatory parameters, such asimmobility, sniffing, and crawling during experimentation. Two doses (50 mg/kg and 100 mg/kg, p.o.) of prunin were administered to two separate mice groups. Further, fluoxetine (10 mg/kg; p.o.) was administered to one of the mice groups for 7 days. Thereafter, the levels of neurotransmitters, such as serotonin and GABA, in brain samples were determined. Based on the results, prunin significantly improved the behavior and mobility of animals in all three anxiety models. Further, prunin modulated the release of serotonin and GABA, demonstrating the mechanistic approach it employs to interact with cellular receptors to mimic neurotransmission. The mRNA expression levels of tph2 (5-HT synthesizing enzyme) and slc6a4 (5-HT transporter) were also found to be downregulated in both prunin- and fluoxetine-treated mice brains. Collectively, our findings suggest that prunin could be administered to treat anxiety in mice. However, further studies should be carried out to explore its potential for clinical application.

Serotonergic regulation of the dopaminergic system: Implications for reward-related functions

Serotonin is a critical neuromodulator involved in development and behavior. Its role in reward is however still debated. Here, we first review classical studies involving electrical stimulation protocols and pharmacological approaches. Contradictory results on the serotonergic' involvement in reward emerge from these studies. These differences might be ascribable to either the diversity of cellular types within the raphe nuclei or/and the specific projection pathways of serotonergic neurons. We continue to review more recent work, using optogenetic approaches to activate serotonergic cells in the Raphe to VTA pathway. From these studies, it appears that activation of this pathway can lead to reinforcement learning mediated through the excitation of dopaminergic neurons by serotonergic neurons co-transmitting glutamate. Finally, given the importance of serotonin during development on adult emotion, the effect of abnormal early-life levels of serotonin on the dopaminergic system will also be discussed. Understanding the interaction between the serotonergic and dopaminergic systems during development and adulthood is critical to gain insight into the specific facets of neuropsychiatric disorders.

Pten is a key intrinsic factor regulating raphe 5-HT neuronal plasticity and depressive behaviors in mice

Serotonin (5-HT)-based antidepressants, selective serotonin reuptake inhibitors (SSRIs) aim to enhance serotonergic activity by blocking its reuptake. We propose PTEN as a target for an alternative approach for regulating 5-HT neuron activity in the brain and depressive behaviors. We show that PTEN is elevated in central 5-HT neurons in the raphe nucleus by chronic stress in mice, and selective deletion of Pten in the 5-HT neurons induces its structural plasticity shown by increases of dendritic branching and density of PSD95-positive puncta in the dendrites. 5-HT levels are elevated and electrical stimulation of raphe neurons evokes more 5-HT release in the brain of condition knockout (cKO) mice with Pten-deficient 5-HT neurons. In addition, the 5-HT neurons remain normal electrophysiological properties but have increased excitatory synaptic inputs. Single-cell RNA sequencing revealed gene transcript alterations that may underlay morphological and functional changes in Pten-deficient 5-HT neurons. Finally, Pten cKO mice and wild-type mice treated with systemic application of PTEN inhibitor display reduced depression-like behaviors. Thus, PTEN is an intrinsic regulator of 5-HT neuron activity, representing a novel therapeutic strategy for producing antidepressant action.

A single-cell transcriptomic and anatomic atlas of mouse dorsal raphe Pet1 neurons

Among the brainstem raphe nuclei, the dorsal raphe nucleus (DR) contains the greatest number of Pet1-lineage neurons, a predominantly serotonergic group distributed throughout DR subdomains. These neurons collectively regulate diverse physiology and behavior and are often therapeutically targeted to treat affective disorders. Characterizing Pet1 neuron molecular heterogeneity and relating it to anatomy is vital for understanding DR functional organization, with potential to inform therapeutic separability. Here we use high-throughput and DR subdomain-targeted single-cell transcriptomics and intersectional genetic tools to map molecular and anatomical diversity of DR-Pet1 neurons. We describe up to fourteen neuron subtypes, many showing biased cell body distributions across the DR. We further show that P2ry1-Pet1 DR neurons - the most molecularly distinct subtype - possess unique efferent projections and electrophysiological properties. These data complement and extend previous DR characterizations, combining intersectional genetics with multiple transcriptomic modalities to achieve fine-scale molecular and anatomic identification of Pet1 neuron subtypes.

A Discrete Dorsal Raphe to Basal Amygdala 5-HT Circuit Calibrates Aversive Memory

Despite a wealth of clinical and preclinical data implicating the serotonin (5-HT) system in fear-related affective disorders, a precise definition of this neuromodulator's role in fear remains elusive. Using convergent anatomical and functional approaches, we interrogate the contribution to fear of basal amygdala (BA) 5-HT inputs from the dorsal raphe nucleus (DRN). We show the DRN→BA 5-HT pathway is engaged during fear memory formation and retrieval, and activity of these projections facilitates fear and impairs extinction. The DRN→BA 5-HT pathway amplifies fear-associated BA neuronal firing and theta power and phase-locking. Although fear recruits 5-HT and VGluT3 co-expressing DRN neurons, the fear-potentiating influence of the DRN→BA 5-HT pathway requires signaling at BA 5-HT1A/2A receptors. Input-output mapping illustrates how the DRN→BA 5-HT pathway is anatomically distinct and connected with other brain regions that mediate fear. These findings reveal how a discrete 5-HT circuit orchestrates a broader neural network to calibrate aversive memory.

TRK-fused Gene Protein Localization Is Prominent in Serotonergic and Noradrenergic Cell Groups, and Some Lower Motor Neurons in the Corticospinal Tract of the Rat Brainstem

The TRK-fused gene (TFG) is reported to be involved in the regulation of cell size, apoptosis, cell growth, ER-Golgi protein secretion, NF-κβ pathway signaling, the ubiquitin-proteasome system, and pancreatic β-cell mass and function. TFG mutations were reported in some neurodegenerative diseases affecting sensory and motor functions. However, the function of TFG in the nervous system and how TFG mutations lead to neurodegeneration remain unclear. In this study, we employed double immunohistochemistry to investigate the details of TFG localization patterns in monoaminergic and cholinergic neurons in the brainstem. Intense TFG immunoreactivity was observed in the dorsal raphe nucleus, the locus coeruleus, and the ventral horn of the spinal cord. TFG immunoreactivity was observed in some serotonergic neurons in all B1–B9 cell groups, and some noradrenergic neurons in all A1–A7 cell groups in the rat brainstem, while no immunoreactivity was observed in the dopaminergic neurons in A8–A10 cell groups. TFG immunoreactivity was observed in all ChAT-positive motor nuclei in the lower corticospinal tract of the rat brainstem.

Neurochemical differences between target-specific populations of rat dorsal raphe projection neurons

Serotonin (5-HT)-containing neurons in the dorsal raphe (DR) nucleus project throughout the forebrain and are implicated in many physiological processes and neuropsychiatric disorders. Diversity among these neurons has been characterized in terms of their neurochemistry and anatomical organization, but a clear sense of whether these attributes align with specific brain functions or terminal fields is lacking. DR 5-HT neurons can co-express additional neuroactive substances, increasing the potential for individualized regulation of target circuits. The goal of this study was to link DR neurons to a specific functional role by characterizing cells according to both their neurotransmitter expression and efferent connectivity; specifically, cells projecting to the medial prefrontal cortex (mPFC), a region implicated in cognition, emotion, and responses to stress. Following retrograde tracer injection, brainstem sections from Sprague-Dawley rats were immunohistochemically stained for markers of serotonin, glutamate, GABA, and nitric oxide (NO). 98% of the mPFC-projecting serotonergic neurons co-expressed the marker for glutamate, while the markers for NO and GABA were observed in 60% and less than 1% of those neurons, respectively. To identify potential target-specific differences in co-transmitter expression, we also characterized DR neurons projecting to a visual sensory structure, the lateral geniculate nucleus (LGN). The proportion of serotonergic neurons co-expressing NO was greater amongst cells targeting the mPFC vs LGN (60% vs 22%). The established role of 5-HT in affective disorders and the emerging role of NO in stress signaling suggest that the impact of 5-HT/NO co-localization in DR neurons that regulate mPFC circuit function may be clinically relevant.

Nitrergic neurons of the dorsal raphe nucleus encode information about stress duration

Nitrergic neurons of the dorsal raphe nucleus (DRN) may play a role in physiological stress responses. The caudal lateral wings (CLW) are unique compared to other rostral-caudal DRN sub-regions because they contain distinct nitric oxide (NO) synthase (NOS) populations that are independent of tryptophan hydroxylase (TPH). NOS neurons in the CLW are also highly activated during acute restraint stress. However, the effects of acute stress duration on NOS activation in the CLW are unclear. Here NADPH-d, an index of NOS activity, is used to show that sub-regions of the DRN have differential NOS activation in response to 6 hours of restraint stress in rats. We report increased NOS activity through 6 hours of restraint in the caudal lateral wings and ventromedial sub-regions. These data suggest that, NOS neurons may play a dynamic role in the response to stress duration.

Toward a multiscale modeling framework for understanding serotonergic function

Despite its importance in regulating emotion and mental wellbeing, the complex structure and function of the serotonergic system present formidable challenges toward understanding its mechanisms. In this paper, we review studies investigating the interactions between serotonergic and related brain systems and their behavior at multiple scales, with a focus on biologically-based computational modeling. We first discuss serotonergic intracellular signaling and neuronal excitability, followed by neuronal circuit and systems levels. At each level of organization, we will discuss the experimental work accompanied by related computational modeling work. We then suggest that a multiscale modeling approach that integrates the various levels of neurobiological organization could potentially transform the way we understand the complex functions associated with serotonin.

Expression of cerebral serotonin related to anxiety-like behaviors in C57BL/6 offspring induced by repeated subcutaneous prenatal exposure to low-dose lipopolysaccharide

Prenatal exposure to lipopolysaccharide (LPS), which likely occurs due to infection or contact with environmental allergens during pregnancy, is a proposed risk factor that induces anxiety- and autism spectrum disorder-like behaviors in offspring. However, the molecular and behavioral changes in offspring after maternal immune activation have not been completely identified. We hypothesized that a subcutaneous injection of LPS in a pregnant mouse would induce changes in cerebral serotonin (5-HT) in parallel to the appearance of anxiety-like behaviors in the dam’s offspring. After LPS injections (total, 100 μg/Kg), the time spent in the central region during the open field test and the number of times that the mice moved between the light and dark boxes and between the open and closed arms on the elevated plus maze test revealed anxiety-like behaviors in offspring at 5, 6 and 9 weeks of age. The mRNA expression levels of tph2 (5-HT synthesizing enzyme) and slc6a4 (5-HT transporter) were down-regulated in both adolescent (5 weeks of age) and adult (8 weeks of age) brains. Immunohistochemistry revealed that the numbers and sizes of tph2-expressing cells were notably decreased in the raphe nuclei of the midbrain of adults. Moreover, compared with controls (phosphate-buffered saline-treated offspring), the cerebral 5-HT concentration at adolescence and adulthood in LPS-induced offspring was significantly decreased. We concluded that maternal immune activation induced by exposure to a low dose of LPS decreased cerebral 5-HT levels in parallel to the down-regulation of the tph2 and slc6a4 genes and in conjunction with anxiety-like behaviors in offspring.

Neuroanatomical Substrates of Rodent Social Behavior: The Medial Prefrontal Cortex and Its Projection Patterns

Social behavior encompasses a number of distinctive and complex constructs that form the core elements of human imitative culture, mainly represented as either affiliative or antagonistic interactions with conspecifics. Traditionally considered in the realm of psychology, social behavior research has benefited from recent advancements in neuroscience that have accelerated identification of the neural systems, circuits, causative genes and molecular mechanisms that underlie distinct social cognitive traits. In this review article, I summarize recent findings regarding the neuroanatomical substrates of key social behaviors, focusing on results from experiments conducted in rodent models. In particular, I will review the role of the medial prefrontal cortex (mPFC) and downstream subcortical structures in controlling social behavior, and discuss pertinent future research perspectives.

Specific Connectivity and Unique Molecular Identity of MET Receptor Tyrosine Kinase Expressing Serotonergic Neurons in the Caudal Dorsal Raphe Nuclei

Molecular characterization of neurons across brain regions has revealed new taxonomies for understanding functional diversity even among classically defined neuronal populations. Neuronal diversity has become evident within the brain serotonin (5-HT) system, which is far more complex than previously appreciated. However, until now it has been difficult to define subpopulations of 5-HT neurons based on molecular phenotypes. We demonstrate that the MET receptor tyrosine kinase (MET) is specifically expressed in a subset of 5-HT neurons within the caudal part of the dorsal raphe nuclei (DRC) that is encompassed by the classic B6 serotonin cell group. Mapping from embryonic day 16 through adulthood reveals that MET is expressed almost exclusively in the DRC as a condensed, paired nucleus, with an additional sparse set of MET+ neurons scattered within the median raphe. Retrograde tracing experiments reveal that MET-expressing 5-HT neurons provide substantial serotonergic input to the ventricular/subventricular region that contains forebrain stem cells, but do not innervate the dorsal hippocampus or entorhinal cortex. Conditional anterograde tracing experiments show that 5-HT neurons in the DRC/B6 target additional forebrain structures such as the medial and lateral septum and the ventral hippocampus. Molecular neuroanatomical analysis identifies 14 genes that are enriched in DRC neurons, including 4 neurotransmitter/neuropeptide receptors and 2 potassium channels. These analyses will lead to future studies determining the specific roles that 5-HT^MET+ neurons contribute to the broader set of functions regulated by the serotonergic system.

Gene Expression Profiling with Cre-Conditional Pseudorabies Virus Reveals a Subset of Midbrain Neurons That Participate in Reward Circuitry

The mesolimbic dopamine pathway receives inputs from numerous regions of the brain as part of a neural system that detects rewarding stimuli and coordinates a behavioral response. The capacity to simultaneously map and molecularly define the components of this complex multisynaptic circuit would thus advance our understanding of the determinants of motivated behavior. To accomplish this, we have constructed pseudorabies virus (PRV) strains in which viral propagation and fluorophore expression are activated only after exposure to Cre recombinase. Once activated in Cre-expressing neurons, the virus serially labels chains of presynaptic neurons. Dual injection of GFP and mCherry tracing viruses simultaneously illuminates nigrostriatal and mesolimbic circuitry and shows no overlap, demonstrating that PRV transmission is confined to synaptically connected neurons. To molecularly profile mesolimbic dopamine neurons and their presynaptic inputs, we injected Cre-conditional GFP virus into the NAc of (anti-GFP) nanobody-L10 transgenic mice and immunoprecipitated translating ribosomes from neurons infected after retrograde tracing. Analysis of purified RNA revealed an enrichment of transcripts expressed in neurons of the dorsal raphe nuclei and lateral hypothalamus that project to the mesolimbic dopamine circuit. These studies identify important inputs to the mesolimbic dopamine pathway and further show that PRV circuit-directed translating ribosome affinity purification can be broadly applied to identify molecularly defined neurons comprising complex, multisynaptic circuits.SIGNIFICANCE STATEMENT The mesolimbic dopamine circuit integrates signals from key brain regions to detect and respond to rewarding stimuli. To further define this complex multisynaptic circuit, we constructed a panel of Cre recombinase-activated pseudorabies viruses (PRVs) that enabled retrograde tracing of neural inputs that terminate on Cre-expressing neurons. Using these viruses and Retro-TRAP (translating ribosome affinity purification), a previously reported molecular profiling method, we developed a novel technique that provides anatomic as well as molecular information about the neural components of polysynaptic circuits. We refer to this new method as PRV-Circuit-TRAP (PRV circuit-directed TRAP). Using it, we have identified major projections to the mesolimbic dopamine circuit from the lateral hypothalamus and dorsal raphe nucleus and defined a discrete subset of transcripts expressed in these projecting neurons, which will allow further characterization of this important pathway. Moreover, the method we report is general and can be applied to the study of other neural circuits.

Microglia in the dorsal raphe nucleus plays a potential role in both suicide facilitation and prevention in affective disorders

An involvement of the central serotonergic system has constantly been reported in the pathogenesis of suicide. The dorsal raphe nucleus (DRN) is the main source of serotonergic innervation of forebrain limbic structures disturbed in suicidal behaviour, in which an abnormal microglia reaction seems to play a role. In our present study, the density of microglia immunostained for the HLA-DR antigen was evaluated in the DRN. These analyses were carried out on paraffin-embedded brains from 24 suicidal and 21 non-suicidal patients; among them, 27 depressed (15 major depressive disorder and 12 bipolar disorder) and 18 schizophrenia (9 residual and 9 paranoid) patients and 22 matched controls without mental disorders. Only the non-suicidal depressed subgroup revealed significantly lower microglial reaction, i.e., a decreased density of HLA-DR positive microglia versus both depressed suicide victims and controls. The effect was not related to antidepressant or antipsychotic medication, as the former correlated positively with microglial density in non-suicidal depressed patients, and the latter had no effect. Moreover, the comparison of these results with previously published data from our workgroup in the same cohort (Krzyżanowska et al. in Psychiatry Res 241:43-46, 4) suggested a positive impact of microglia on ribosomal DNA transcription in DRN neurons in the non-suicidal depressed subgroup, but not in depressed suicidal cases. Therefore, the interaction between microglia and neurons in the DRN may be potentially involved in opposite ways regarding suicide facilitation and prevention in the tested subgroups of depressed patients.

Gamma-Aminobutyric Acidergic Projections From the Dorsal Raphe to the Nucleus Accumbens Are Regulated by Neuromedin U

BACKGROUND

Neuromedin U (NMU) is a neuropeptide enriched in the nucleus accumbens shell (NAcSh), a brain region associated with reward. While NMU and its receptor, NMU receptor 2 (NMUR2), have been studied for the ability to regulate food reward, NMU has not been studied in the context of drugs of abuse (e.g., cocaine). Furthermore, the neuroanatomical pathways that express NMUR2 and its ultrastructural localization are unknown.

METHODS

Immunohistochemistry was used to determine the synaptic localization of NMUR2 in the NAcSh and characterize which neurons express this receptor (n = 17). The functional outcome of NMU on NMUR2 was examined using microdialysis (n = 16). The behavioral effects of NMU microinjection directly to the NAcSh were investigated using cocaine-evoked locomotion (n = 93). The specific effects of NMUR2 knockdown on cocaine-evoked locomotion were evaluated using viral-mediated RNA interference (n = 40).

RESULTS

NMUR2 is localized to presynaptic gamma-aminobutyric acidergic nerve terminals in the NAcSh originating from the dorsal raphe nucleus. Furthermore, NMU microinjection to the NAcSh decreased local gamma-aminobutyric acid concentrations. Next, we evaluated the effects of NMU microinjection on behavioral sensitization to cocaine. When repeatedly administered throughout the sensitization regimen, NMU attenuated cocaine-evoked hyperactivity. Additionally, small hairpin RNA-mediated knockdown of presynaptic NMUR2 in the NAcSh using a retrograde viral vector potentiated cocaine sensitization.

CONCLUSIONS

Together, these data reveal that NMUR2 modulates a novel gamma-aminobutyric acidergic pathway from the dorsal raphe nucleus to the NAcSh to influence behavioral responses to cocaine.

Treadmill exercise ameliorates intracerebral hemorrhage-induced depression in rats

Intracerebral hemorrhage (ICH) is a severe type of stroke causing neurological dysfunction with high mortality rate. Depression is one of the most common complications of ICH. In the present study, the effects of treadmill exercise on ICH-induced depressive symptoms in relation with apoptosis were investigated using rats. ICH rat model was induced by injection of collagenase into the hippocampus using stereotaxic instrument. Open field test for activity and forced swimming test for depressive symptoms were conducted. Apoptosis in the hippocampus was detected using terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assay, immunohistochemistry for caspase-3, and western blot for Bcl-2 and Bax. Western blot analysis for 5-hydroxy-tryptamine (5-HT, serotonin) and tryptophan hydroxylase (TPH) in the dorsal raphe was also conducted for biomarkers of depression. In the present results, immobility time was increased and climbing time was decreased by induction of ICH and treadmill exercise inhibited immobility time and increased climbing time in ICH rats. DNA fragmentation and caspase-3 expression in the hippocampal dentate gyrus were enhanced by induction of ICH and treadmill exercise suppressed ICH-induced DNA fragmentation and caspase-3 expression. Bax expression in the hippocampus was increased by induction of ICH and treadmill exercise inhibited Bax expression in the ICH rats. Expressions of 5-HT and TPH in the dorsal raphe were decreased by induction of ICH and treadmill exercise increased expressions of 5-HT and TPH in the ICH rats. In the present study, treadmill exercise ameliorated depressive symptoms through inhibiting apoptosis.

Decreased ribosomal DNA transcription in dorsal raphe nucleus neurons differentiates between suicidal and non-suicidal death

An involvement of the central serotonergic system has been implicated in the pathogenesis of suicide. The dorsal raphe nucleus (DRN) is the main source of serotonergic innervation of forebrain limbic structures disturbed in suicidal behaviour. The study was carried out on paraffin-embedded brainstem blocks containing the DRN obtained from 27 suicide completers (predominantly violent) with unknown psychiatric diagnosis and 30 non-suicidal controls. The transcriptional activity of ribosomal DNA (rDNA) in DRN neurons as a surrogate marker of protein biosynthesis was evaluated by the AgNOR silver staining method. Significant decreases in AgNOR parameters suggestive of attenuated rDNA activity were found in the cumulative analysis of all DRN subnuclei in suicide victims versus controls (U test P values < 0.00001). Our findings suggest that the decreased activity of rDNA transcription in DRN neurons plays an important role in suicide pathogenesis. The method accuracy represented by the area under receiver operating characteristic curve (>80 %) suggests a diagnostic value of the observed effect. However, the possible application of the method in forensic differentiation diagnostics between suicidal and non-suicidal death needs further research.

Effects of cocaine history on postsynaptic GABA receptors on dorsal raphe serotonin neurons in a stress-induced relapse model in rats

The serotonin (5-hydroxytryptamine, 5-HT) system plays an important role in stress-related psychiatric disorders and substance abuse. Stressors and stress hormones can inhibit the dorsal raphe nucleus (DRN)-5-HT system, which composes the majority of forebrain-projecting 5-HT. This inhibition is mediated via stimulation of GABA synaptic activity at DRN-5-HT neurons. Using swim stress-induced reinstatement of morphine conditioned place-preference, recent data from our laboratory indicate that morphine history sensitizes DRN-5-HT neurons to GABAergic inhibitory effects of stress. Moreover, GABAA receptor-mediated inhibition of the serotonergic DRN is required for this reinstatement. In our current experiment, we tested the hypothesis that GABAergic sensitization of DRN-5-HT neurons is a neuroadaptation elicited by multiple classes of abused drugs across multiple models of stress-induced relapse by applying a chemical stressor (yohimbine) to induce reinstatement of previously extinguished cocaine self-administration in Sprague-Dawley rats. Whole-cell patch-clamp recordings of GABA synaptic activity in DRN-5-HT neurons were conducted after the reinstatement. Behavioral data indicate that yohimbine triggered reinstatement of cocaine self-administration. Electrophysiology data indicate that 5-HT neurons in the cocaine group exposed to yohimbine had increased amplitude of inhibitory postsynaptic currents compared to yoked-saline controls exposed to yohimbine or unstressed animals in both drug groups. These data, together with previous findings, indicate that interaction between psychostimulant or opioid history and chemical or physical stressors may increase postsynaptic GABA receptor density and/or sensitivity in DRN-5-HT neurons. Such mechanisms may result in serotonergic hypofunction and consequent dysphoric mood states which confer vulnerability to stress-induced drug reinstatement.

Mechanisms of 5-HT1A receptor-mediated transmission in dorsal raphe serotonin neurons

KEY POINTS

In the dorsal raphe nucleus, it is known that serotonin release activates metabotropic 5-HT1A autoreceptors located on serotonin neurons that leads to an inhibition of firing through the activation of G-protein-coupled inwardly rectifying potassium channels. We found that in mouse brain slices evoked serotonin release produced a 5-HT1A receptor-mediated inhibitory postsynaptic current (IPSC) that resulted in only a transient pause in firing. While spillover activation of receptors contributed to evoked IPSCs, serotonin reuptake transporters prevented pooling of serotonin in the extrasynaptic space from activating 5-HT1A -IPSCs. As a result, the decay of 5-HT1A -IPSCs was independent of the intensity of stimulation or the probability of transmitter release. These results indicate that evoked serotonin transmission in the dorsal raphe nucleus mediated by metabotropic 5-HT1A autoreceptors may occur via point-to-point synapses rather than by paracrine mechanisms.

ABSTRACT

In the dorsal raphe nucleus (DRN), feedback activation by Gαi/o -coupled 5-HT1A autoreceptors reduces the excitability of serotoninergic neurons, which decreases serotonin release both locally within the DRN and in projection regions. Serotonin transmission within the DRN is thought to occur via transmitter spillover and paracrine activation of extrasynaptic receptors. Here, we tested the volume transmission hypothesis in mouse DRN brain slices by recording 5-HT1A receptor-mediated inhibitory postsynaptic currents (5-HT1A -IPSCs) generated by the activation of G-protein-coupled inwardly rectifying potassium channels (GIRKs). We found that in the DRN of ePET1-EYFP mice, which selectively express enhanced yellow fluorescent protein in serontonergic neurons, the local release of serotonin generated 5-HT1A -IPSCs in serotonin neurons that rose and fell within a second. The transient activation of 5-HT1A autoreceptors resulted in brief pauses in neuron firing that did not alter the overall firing rate. The duration of 5-HT1A -IPSCs was primarily shaped by receptor deactivation due to clearance via serotonin reuptake transporters. Slowing diffusion with dextran prolonged the rise and reduced the amplitude the IPSCs and the effects were potentiated when uptake was inhibited. By examining the decay kinetics of IPSCs, we found that while spillover may allow for the activation of extrasynaptic receptors, efficient uptake by serotonin reuptake transporters (SERTs) prevented the pooling of serotonin from prolonging the duration of transmission when multiple inputs were active. Together the results suggest that the activation of 5-HT1A receptors in the DRN results from the local release of serotonin rather than the extended diffusion throughout the extracellular space.

Ribosomal DNA transcription in dorsal raphe nucleus neurons is increased in residual schizophrenia compared to depressed patients with affective disorders

The central serotonergic system is implicated differentially in the pathogenesis of depression and schizophrenia. The dorsal raphe nucleus (DRN) is the main source of serotonergic innervation of forebrain limbic structures disturbed in both disorders. The study was carried out on paraffin-embedded brains from 27 depressed (15 major depressive disorder, MDD and 12 bipolar disorder, BD) and 17 schizophrenia (9 residual and 8 paranoid) patients and 28 matched controls without mental disorders. The transcriptional activity of ribosomal DNA (rDNA) in DRN neurons was evaluated by the AgNOR silver staining method. A significant effect of diagnosis on rDNA activity was found in the cumulative analysis of all DRN subnuclei. Further analysis revealed an increase in this activity in residual (but not paranoid) schizophrenia compared to depressed (both MDD and BD) patients. The effect was most probably neither confounded by suicide nor related to antidepressant and antipsychotic medication. Our findings suggest that increased activity of rDNA in DRN neurons is a distinct phenomenon in residual schizophrenia, related presumably to differentially disturbed inputs to the DRN and/or their local transformation compared with depressive episodes in patients with affective disorders.

Ongoing behavioral state information signaled in the lateral habenula guides choice flexibility in freely moving rats

The lateral habenula (LHb) plays a role in a wide variety of behaviors ranging from maternal care, to sleep, to various forms of cognition. One prominent theory with ample supporting evidence is that the LHb serves to relay basal ganglia and limbic signals about negative outcomes to midbrain monoaminergic systems. This makes it likely that the LHb is critically involved in behavioral flexibility as all of these systems have been shown to contribute when flexible behavior is required. Behavioral flexibility is commonly examined across species and is impaired in various neuropsychiatric conditions including autism, depression, addiction, and schizophrenia; conditions in which the LHb is thought to play a role. Therefore, a thorough examination of the role of the LHb in behavioral flexibility serves multiple functions including understanding possible connections with neuropsychiatric illnesses and additional insight into its role in cognition in general. Here, we assess the LHb’s role in behavioral flexibility through comparisons of the roles its afferent and efferent pathways are known to play. Additionally, we provide new evidence supporting the LHb contributions to behavioral flexibility through organization of specific goal directed actions under cognitively demanding conditions. Specifically, in the first experiment, a majority of neurons recorded from the LHb were found to correlate with velocity on a spatial navigation task and did not change significantly when reward outcomes were manipulated. Additionally, measurements of local field potential (LFP) in the theta band revealed significant changes in power relative to velocity and reward location. In a second set of experiments, inactivation of the LHb with the gamma-aminobutyric acid (GABA) agonists baclofen and muscimol led to an impairment in a spatial/response based repeated probabilistic reversal learning task. Control experiments revealed that this impairment was likely due to the demands of repeated switching behaviors as rats were unimpaired on initial discrimination acquisition or retention of probabilistic learning. Taken together, these novel findings compliment other work discussed supporting a role for the LHb in action selection when cognitive or emotional demands are increased. Finally, we discuss future mechanisms by which a superior understanding of the LHb can be obtained through additional examination of behavioral flexibility tasks.

Dorsal raphe nucleus projecting retinal ganglion cells: Why Y cells?

Retinal ganglion Y (alpha) cells are found in retinas ranging from frogs to mice to primates. The highly conserved nature of the large, fast conducting retinal Y cell is a testament to its fundamental task, although precisely what this task is remained ill-defined. The recent discovery that Y-alpha retinal ganglion cells send axon collaterals to the serotonergic dorsal raphe nucleus (DRN) in addition to the lateral geniculate nucleus (LGN), medial interlaminar nucleus (MIN), pretectum and the superior colliculus (SC) has offered new insights into the important survival tasks performed by these cells with highly branched axons. We propose that in addition to its role in visual perception, the Y-alpha retinal ganglion cell provides concurrent signals via axon collaterals to the DRN, the major source of serotonergic afferents to the forebrain, to dramatically inhibit 5-HT activity during orientation or alerting/escape responses, which dis-facilitates ongoing tonic motor activity while dis-inhibiting sensory information processing throughout the visual system. The new data provide a fresh view of these evolutionarily old retinal ganglion cells.

Top-Down Control of Serotonin Systems by the Prefrontal Cortex: A Path toward Restored Socioemotional Function in Depression

Social withdrawal, increased threat perception, and exaggerated reassurance seeking behaviors are prominent interpersonal symptoms in major depressive disorder (MDD). Altered serotonin (5-HT) systems and corticolimbic dysconnectivity have long been suspected to contribute to these symptomatic facets; however, the underlying circuits and intrinsic cellular mechanisms that control 5-HT output during socioemotional interactions remain poorly understood. We review literature that implicates a direct pathway between the ventromedial prefrontal cortex (vmPFC) and dorsal raphe nucleus (DRN) in the adaptive and pathological control of social approach-avoidance behaviors. Imaging and neuromodulation during approach-avoidance tasks in humans point to the cortical control of brainstem circuits as an essential regulator of socioemotional decisions and actions. Parallel rodent studies using viral-based connectomics and optogenetics are beginning to provide a cellular blueprint of the underlying circuitry. In these studies, manipulations of vmPFC synaptic inputs to the DRN have revealed bidirectional influences on socioaffective behaviors via direct monosynaptic excitation and indirect disynaptic inhibition of 5-HT neurons. Additionally, adverse social experiences that result in permanent avoidance biases, such as social defeat, drive long-lasting plasticity in this microcircuit, potentiating the indirect inhibition of 5-HT output. Conversely, neuromodulation of the vmPFC via deep brain stimulation (DBS) attenuates avoidance biases by restoring the direct excitatory drive of 5-HT neurons and strengthening a key subset of forebrain 5-HT projections. Better understanding the cellular organization of the vmPFC-DRN pathway and identifying molecular determinants of its neuroplasticity can open fundamentally novel avenues for the treatment of affective disorders.

Genetic Variations in the Serotonergic System Mediate a Combined, Weakened Response to SSRI Treatment: A Proposed Model

Individuals with the short (S) allele in the promoter region of the serotonin transporter gene (5-HTTLPR) show a less favorable response to selective serotonin reuptake inhibitor (SSRI) treatment than individuals with the long (L) allele. Similarly, individuals with the C(-1019)G allele for the mutation found in the promoter region of the serotonin 1A receptor gene (5-HTR1A) have shown blunted responses to SSRI treatment when compared with individuals lacking this polymorphism. While these findings have been replicated across multiple studies, only two studies to date have reported data for a gene-gene interaction associated with response to SSRI treatment. Both of these studies reported a combined effect for these genotypes, with individuals homozygous for the L allele and the C allele (5-HTT(L/L)-1A(C/C)) reporting the most favorable response to SSRI treatment, and individuals homozygous for the S allele and the G allele (5-HTT(S/S)-1A(G/G)) reporting the least favorable response to SSRI treatment. Additionally, no neural mechanisms have been proposed to explain why this gene-gene interaction has been observed. To that end, this article provides a review of the relevant literature associated with these polymorphisms and proposes a feasible model that describes a genotype-dependent modulation of postsynaptic serotonin signaling associated with the 5-HTT and 5-HTR1A genes.

Two major network domains in the dorsal raphe nucleus

Serotonin neurons in the dorsal and median raphe nuclei (DR and MR) are clustered into heterogeneous groups that give rise to topographically organized forebrain projections. However, a compelling definition of the key subgroups of serotonin neurons within these areas has remained elusive. In order to be functionally distinct, neurons must participate in distinct networks. Therefore, we analyzed subregions of the DR and MR by their afferent input. Clustering methods and principal component analysis were applied in mouse to anterograde tract-tracing experiments available from the Allen Mouse Brain Connectivity Atlas. The results revealed a major break in the networks of the DR such that the caudal third of the DR was more similar in afferent innervation to the MR than it was to the rostral two-thirds of the DR. The rostral part of the DR is associated with networks controlling motor and motivated behavior, while the caudal DR is more closely aligned with regions that regulate rhythmic hippocampal activity. Thus, a major source of heterogeneity within the DR is inclusion of the caudal component, which may be more accurately viewed as a dorsal extension of the MR.

Behavioral training reverses global cortical network dysfunction induced by perinatal antidepressant exposure

Abnormal cortical circuitry and function as well as distortions in the modulatory neurological processes controlling cortical plasticity have been argued to underlie the origin of autism. Here, we chemically distorted those processes using an antidepressant drug-exposure model to generate developmental neurological distortions like those characteristics expressed in autism, and then intensively trained altered young rodents to evaluate the potential for neuroplasticity-driven renormalization. We found that young rats that were injected s.c. with the antidepressant citalopram from postnatal d 1-10 displayed impaired neuronal repetition-rate following capacity in the primary auditory cortex (A1). With a focus on recovering grossly degraded auditory system processing in this model, we showed that targeted temporal processing deficits induced by early-life antidepressant exposure within the A1 were almost completely reversed through implementation of a simple behavioral training strategy (i.e., a modified go/no-go repetition-rate discrimination task). Degraded parvalbumin inhibitory GABAergic neurons and the fast inhibitory actions that they control were also renormalized by training. Importantly, antidepressant-induced degradation of serotonergic and dopaminergic neuromodulatory systems regulating cortical neuroplasticity was sharply reversed. These findings bear important implications for neuroplasticity-based therapeutics in autistic patients.

Ribosomal DNA transcription in the dorsal raphe nucleus is increased in residual but not in paranoid schizophrenia

The central serotonergic system is implicated in the pathogenesis of schizophrenia, where the imbalance between dopamine, serotonin and glutamate plays a key pathophysiological role. The dorsal raphe nucleus (DRN) is the main source of serotonergic innervation of forebrain limbic structures disturbed in schizophrenia patients. The study was carried out on paraffin-embedded brains from 17 (8 paranoid and 9 residual) schizophrenia patients and 28 matched controls without mental disorders. The transcriptional activity of ribosomal DNA (rDNA) in DRN neurons was evaluated by the AgNOR silver-staining method. An increased rDNA transcriptional activity was found in schizophrenia patients in the cumulative analysis of all DRN subnuclei (t test, P = 0.02). Further subgroup analysis revealed that it was an effect specific for residual schizophrenia versus paranoid schizophrenia or control groups (ANOVA, P = 0.002). This effect was confounded neither by suicide nor by antipsychotic medication. Our findings suggest that increased activity of rDNA in DRN neurons is a distinct phenomenon in schizophrenia, particularly in residual patients. An activation of the rDNA transcription in DRN neurons may represent a compensatory mechanism to overcome the previously described prefrontal serotonergic hypofunction in this diagnostic subgroup.

Lateral habenula and the rostromedial tegmental nucleus innervate neurochemically distinct subdivisions of the dorsal raphe nucleus in the rat

The lateral habenula (LHb) is an epithalamic structure differentiated in a medial (LHbM) and a lateral division (LHbL). Together with the rostromedial tegmental nucleus (RMTg), the LHb has been implicated in the processing of aversive stimuli and inhibitory control of monoamine nuclei. The inhibitory LHb influence on midbrain dopamine neurons has been shown to be mainly mediated by the RMTg, a mostly GABAergic nucleus that receives a dominant input from the LHbL. Interestingly, the RMTg also projects to the dorsal raphe nucleus (DR), which also receives direct LHb projections. To compare the organization and transmitter phenotype of LHb projections to the DR, direct and indirect via the RMTg, we first placed injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin into the LHb or the RMTg. We then confirmed our findings by retrograde tracing and investigated a possible GABAergic phenotype of DR-projecting RMTg neurons by combining retrograde tracing with in situ hybridization for GAD67. We found only moderate direct LHb projections to the DR, which mainly emerged from the LHbM and were predominantly directed to the serotonin-rich caudal DR. In contrast, RMTg projections to the DR were more robust, emerged from RMTg neurons enriched in GAD67 mRNA, and were focally directed to a distinctive DR subdivision immunohistochemically characterized as poor in serotonin and enriched in presumptive glutamatergic neurons. Thus, besides its well-acknowledged role as a GABAergic control center for the ventral tegmental area (VTA)-nigra complex, our findings indicate that the RMTg is also a major GABAergic relay between the LHb and the DR.

Conditional anterograde tracing reveals distinct targeting of individual serotonin cell groups (B5-B9) to the forebrain and brainstem

Serotoninergic innervation of the central nervous system is provided by hindbrain raphe nuclei (B1–B9). The extent to which each raphe subdivision has distinct topographic organization of their projections is still unclear. We provide a comprehensive description of the main targets of the rostral serotonin (5-HT) raphe subgroups (B5–B9) in the mouse brain. Adeno-associated viruses that conditionally express GFP under the control of the 5-HT transporter promoter were used to label small groups of 5-HT neurons in the dorsal (B7d), ventral (B7v), lateral (B7l), and caudal (B6) subcomponents of the dorsal raphe (DR) nucleus as well as in the rostral and caudal parts of the median raphe (MR) nucleus (B8 and B5, respectively), and in the supralemniscal (B9) cell group. We illustrate the distinctive and largely non-overlapping projection areas of these cell groups: for instance, DR (B7) projects to basal parts of the forebrain, such as the amygdala, whereas MR (B8) is the main 5-HT source to the hippocampus, septum, and mesopontine tegmental nuclei. Distinct subsets of B7 have preferential brain targets: B7v is the main source of 5-HT for the cortex and amygdala while B7d innervates the hypothalamus. We reveal for the first time the target areas of the B9 cell group, demonstrating projections to the caudate, prefrontal cortex, substantia nigra, locus coeruleus and to the raphe cell groups. The broad topographic organization of the different raphe subnuclei is likely to underlie the different functional roles in which 5-HT has been implicated in the brain. The present mapping study could serve as the basis for genetically driven specific targeting of the different subcomponents of the mouse raphe system.

Serotonergic versus nonserotonergic dorsal raphe projection neurons: differential participation in reward circuitry

The dorsal raphe nucleus (DRN) contains the largest group of serotonin-producing neurons in the brain and projects to regions controlling reward. Although pharmacological studies suggest that serotonin inhibits reward seeking, electrical stimulation of the DRN strongly reinforces instrumental behavior. Here, we provide a targeted assessment of the behavioral, anatomical, and electrophysiological contributions of serotonergic and nonserotonergic DRN neurons to reward processes. To explore DRN heterogeneity, we used a simultaneous two-vector knockout/optogenetic stimulation strategy, as well as cre-induced and cre-silenced vectors in several cre-expressing transgenic mouse lines. We found that the DRN is capable of reinforcing behavior primarily via nonserotonergic neurons, for which the main projection target is the ventral tegmental area (VTA). Furthermore, these nonserotonergic projections provide glutamatergic excitation of VTA dopamine neurons and account for a large majority of the DRN-VTA pathway. These findings help to resolve apparent discrepancies between the roles of serotonin versus the DRN in behavioral reinforcement.

Computational modeling of spike generation in serotonergic neurons of the dorsal raphe nucleus

Serotonergic neurons of the dorsal raphe nucleus, with their extensive innervation of limbic and higher brain regions and interactions with the endocrine system have important modulatory or regulatory effects on many cognitive, emotional and physiological processes. They have been strongly implicated in responses to stress and in the occurrence of major depressive disorder and other psychiatric disorders. In order to quantify some of these effects, detailed mathematical models of the activity of such cells are required which describe their complex neurochemistry and neurophysiology. We consider here a single-compartment model of these neurons which is capable of describing many of the known features of spike generation, particularly the slow rhythmic pacemaking activity often observed in these cells in a variety of species. Included in the model are 11 kinds of ion channels: a fast sodium current INa, a delayed rectifier potassium current IKDR, a transient potassium current IA, a slow non-inactivating potassium current IM, a low-threshold calcium current IT, two high threshold calcium currents IL and IN, small and large conductance potassium currents ISK and IBK, a hyperpolarization-activated cation current IH and a leak current ILeak. In Sections 3-8, each current type is considered in detail and parameters estimated from voltage clamp data where possible. Three kinds of model are considered for the BK current and two for the leak current. Intracellular calcium ion concentration Cai is an additional component and calcium dynamics along with buffering and pumping is discussed in Section 9. The remainder of the article contains descriptions of computed solutions which reveal both spontaneous and driven spiking with several parameter sets. Attention is focused on the properties usually associated with these neurons, particularly long duration of action potential, steep upslope on the leading edge of spikes, pacemaker-like spiking, long-lasting afterhyperpolarization and the ramp-like return to threshold after a spike. In some cases the membrane potential trajectories display doublets or have humps or notches as have been reported in some experimental studies. The computed time courses of IA and IT during the interspike interval support the generally held view of a competition between them in influencing the frequency of spiking. Spontaneous activity was facilitated by the presence of IH which has been found in these neurons by some investigators. For reasonable sets of parameters spike frequencies between about 0.6Hz and 1.2Hz are obtained, but frequencies as high as 6Hz could be obtained with special parameter choices. Topics investigated and compared with experiment include shoulders, notches, anodal break phenomena, the effects of noradrenergic input, frequency versus current curves, depolarization block, effects of cell size and the effects of IM. The inhibitory effects of activating 5-HT1A autoreceptors are also investigated. There is a considerable discussion of in vitro versus in vivo firing behavior, with focus on the roles of noradrenergic input, corticotropin-releasing factor and orexinergic inputs. Location of cells within the nucleus is probably a major factor, along with the state of the animal.

Cellular profile of the dorsal raphe lateral wing sub-region: relationship to the lateral dorsal tegmental nucleus

As one of the main serotonergic (5HT) projections to the forebrain, the dorsal raphe nucleus (DRN) has been implicated in disorders of anxiety and depression. Although the nucleus contains the densest population of 5HT neurons in the brain, at least 50% of cells within this structure are non-serotonergic, including a large population of nitric oxide synthase (NOS) containing neurons. The DRN has a unique topographical efferent organization and can also be divided into sub-regions based on rostro-caudal and medio-lateral dimensions. NOS is co-localized with 5HT in the midline DRN but NOS-positive cells in the lateral wing (LW) of the nucleus do not express 5HT. Interestingly, the NOS LW neuronal population is immediately rostral to and in line with the cholinergic lateral dorsal tegmental nucleus (LDT). We used immunohistochemical methods to investigate the potential serotonergic regulation of NOS LW neurons and also the association of this cell grouping to the LDT. Our results indicate that >75% of NOS LW neurons express the inhibitory 5HT1A receptor and are cholinergic (>90%). The findings suggest this assembly of cells is a rostral extension of the LDT, one that it is subject to regulation by 5HT release. As such the present study suggests a link between 5HT signaling, activation of cholinergic/NOS neurons, and the stress response including the pathophysiology underlying anxiety and depression.

The median raphe nucleus in anxiety revisited

Although the role of the median raphe nucleus (MRN) in the regulation of anxiety has received less attention than that of the dorsal raphe nucleus (DRN) there is substantial evidence supporting this function. Reported results with different animal models of anxiety in rats show that whereas inactivation of serotonergic neurons in the MRN causes anxiolysis, the stimulation of the same neurons is anxiogenic. In particular, studies using the elevated T-maze comparing serotonergic interventions in the MRN and in the DRN indicate that the former affect only the inhibitory avoidance task, which has been related to generalized anxiety. In contrast, similar operations in the DRN change both the inhibitory avoidance and the one-way escape task, the latter being representative of panic disorder. Simultaneous injections of 5-HT-acting drugs in the MRN and in the dorsal hippocampus (DH) suggest that the MRN-DH pathway mediates the regulatory function of the MRN in anxiety. Overall, the results discussed in this review point to a relevant role of the MRN in the regulation of anxiety, but not panic, through the 5-HT pathway that innervates the DH.

Functional topography of serotonergic systems supports the Deakin/Graeff hypothesis of anxiety and affective disorders

Over 20 years ago, Deakin and Graeff hypothesized about the role of different serotonergic pathways in controlling the behavioral and physiologic responses to aversive stimuli, and how compromise of these pathways could lead to specific symptoms of anxiety and affective disorders. A growing body of evidence suggests these serotonergic pathways arise from topographically organized subpopulations of serotonergic neurons located in the dorsal and median raphe nuclei. We argue that serotonergic neurons in the dorsal/caudal parts of the dorsal raphe nucleus project to forebrain limbic regions involved in stress/conflict anxiety-related processes, which may be relevant for anxiety and affective disorders. Serotonergic neurons in the "lateral wings" of the dorsal raphe nucleus provide inhibitory control over structures controlling fight-or-flight responses. Dysfunction of this pathway could be relevant for panic disorder. Finally, serotonergic neurons in the median raphe nucleus, and the developmentally and functionally-related interfascicular part of the dorsal raphe nucleus, give rise to forebrain limbic projections that are involved in tolerance and coping with aversive stimuli, which could be important for affective disorders like depression. Elucidating the mechanisms through which stress activates these topographically and functionally distinct serotonergic pathways, and how dysfunction of these pathways leads to symptoms of neuropsychiatric disorders, may lead to the development of novel approaches to both the prevention and treatment of anxiety and affective disorders.

Prenatal nicotine exposure enhances the trigeminocardiac reflex via serotonin receptor facilitation in brainstem pathways

In this study we used a rat model for prenatal nicotine exposure to test whether clinically relevant concentrations of brain nicotine and cotinine are passed from dams exposed to nicotine to her pups, whether this changes the trigeminocardiac reflex (TCR), and whether serotonergic function in the TCR brainstem circuitry is altered. Pregnant Sprague-Dawley dams were exposed to 6 mg·kg(-1)·day(-1) of nicotine via osmotic minipumps for the duration of pregnancy. Following birth dams and pups were killed, blood was collected, and brain nicotine and cotinine levels were measured. A separate group of prenatal nicotine-exposed pups was used for electrophysiological recordings. A horizontal brainstem slice was obtained by carefully preserving the trigeminal nerve with fluorescent identification of cardiac vagal neurons (CVNs) in the nucleus ambiguus. Stimulation of the trigeminal nerve evoked excitatory postsynaptic current in CVNs. Our data demonstrate that prenatal nicotine exposure significantly exaggerates both the TCR-evoked changes in heart rate in conscious unrestrained pups, and the excitatory neurotransmission to CVNs upon trigeminal afferent nerve stimulation within this brainstem reflex circuit. Application of the 5-HT1A receptor antagonist WAY 100635 (100 μM) and 5-HT2A/C receptor antagonist ketanserin (10 μM)significantly decreased neurotransmission, indicating an increased facilitation of 5-HT function in prenatal nicotine-exposed animals. Prenatal nicotine exposure enhances activation of 5-HT receptors and exaggerates the trigeminocardiac reflex.

Forebrain GABAergic projections from the dorsal raphe nucleus identified by using GAD67-GFP knock-in mice

The dorsal raphe nucleus (DR) contains serotonergic (5-HT) neurons that project widely throughout the forebrain. These forebrain regions also receive innervation from non-5-HT neurons in the DR. One of the main groups of non-5-HT neurons in the DR is γ-aminobutyric acid (GABA)ergic, but their projections are poorly understood due to the difficulty of labeling these neurons immunohistochemically. To identify GABAergic projection neurons within the DR in the current study, we used a knock-in mouse line in which expression of green fluorescent protein (GFP) is controlled by the glutamic acid decarboxylase (GAD)67 promotor. Projections of GAD67-GFP neurons to the prefrontal cortex (PFC), nucleus accumbens (NAC), and lateral hypothalamus (LH) were evaluated by using retrograde tract tracing. The location of GAD67-GFP neurons projecting to each of these areas was mapped by rostrocaudal and dorsoventral location within the DR. Overall, 16% of DR neurons projecting to either the PFC or NAC were identified as GAD67-GFP neurons. GAD67-GFP neurons projecting to the PFC were most commonly found ventrally, in the rostral two-thirds of the DR. NAC-projecting GAD67-GFP neurons had an overlapping distribution that extended dorsally. GAD67-GFP neurons made a larger contribution to the projection of the DR to the LH, accounting for 36% of retrogradely labeled neurons, and were widespread throughout the DR. The current data indicate that DR GABAergic neurons not only may have the capacity to influence local network activity, but also make a notable contribution to DR output to multiple forebrain targets. J. Comp. Neurol. 520:4157-4167, 2012. © 2012 Wiley Periodicals, Inc.

Rethinking 5-HT1A receptors: emerging modes of inhibitory feedback of relevance to emotion-related behavior

The complexities of the involvement of the serotonin transmitter system in numerous biological processes and psychiatric disorders is, to a substantial degree, attributable to the large number of serotonin receptor families and subtypes that have been identified and characterized for over four decades. Of these, the 5-HT(1A) receptor subtype, which was the first to be cloned and characterized, has received considerable attention based on its purported role in the etiology and treatment of mood and anxiety disorders. 5-HT(1A) receptors function both at presynaptic (autoreceptor) and postsynaptic (heteroreceptor) sites. Recent research has implicated distinct roles for these two populations of receptors in mediating emotion-related behavior. New concepts as to how 5-HT(1A) receptors function to control serotonergic tone throughout life were highlights of the proceedings of the 2012 Serotonin Club Meeting in Montpellier, France. Here, we review recent findings and current perspectives on functional aspects of 5-HT(1A) auto- and heteroreceptors with particular regard to their involvement in altered anxiety and mood states.