Serotonergic and non-serotonergic projections from the nucleus raphe dorsalis to the caudate-putamen complex in the rat, studied by a combined immunofluorescence and fluorescent retrograde axonal labeling technique

A circuit from lateral hypothalamic to dorsal hippocampal dentate gyrus modulates behavioral despair in mice

Behavioral despair is one of the clinical manifestations of major depressive disorder and an important cause of disability and death. However, the neural circuit mechanisms underlying behavioral despair are poorly understood. In a well-established chronic behavioral despair (CBD) mouse model, using a combination of viral tracing, in vivo fiber photometry, chemogenetic and optogenetic manipulations, in vitro electrophysiology, pharmacological profiling techniques, and behavioral tests, we investigated the neural circuit mechanisms in regulating behavioral despair. Here, we found that CBD enhanced CaMKIIα neuronal excitability in the dorsal dentate gyrus (dDG) and dDGCaMKIIα neurons involved in regulating behavioral despair in CBD mice. Besides, dDGCaMKIIα neurons received 5-HT inputs from median raphe nucleus (MRN) and were mediated by 5-HT1A receptors, whereas MRN5-HT neurons received CaMKIIα inputs from lateral hypothalamic (LH) and were mediated by AMPA receptors to regulate behavioral despair. Furthermore, fluvoxamine exerted its role in resisting behavioral despair through the LH-MRN-dDG circuit. These findings suggest that a previously unidentified circuit of LHCaMKIIα-MRN5-HT-dDGCaMKIIα mediates behavioral despair induced by CBD. Furthermore, these support the important role of AMPA receptors in MRN and 5-HT1A receptors in dDG that might be the potential targets for treatment of behavioral despair, and explain the neural circuit mechanism of fluvoxamine-resistant behavioral despair.

The Effect of Chronic Treatment with the Inhibitor of Phosphodiesterase 5 (PDE5), Sildenafil, in Combination with L-DOPA on Asymmetric Behavior and Monoamine Catabolism in the Striatum and Substantia Nigra of Unilaterally 6-OHDA-Lesioned Rats

The use of phosphodiesterase inhibitors in the treatment of Parkinson's disease is currently widely discussed. The study aimed to investigate the impact of acute and chronic treatment with the phosphodiesterase 5 inhibitor, sildenafil, at low and moderate doses of 2 mg/kg and 6 mg/kg, and L-DOPA (12.5 mg/kg), alone or in combination, on asymmetric behavior and dopamine (DA) and serotonin metabolism in the striatum and substantia nigra of unilaterally 6-OHDA-lesioned rats. Acute administration of sildenafil at both tested doses jointly with L-DOPA significantly increased the number of contralateral rotations during a 2 h measurement compared to L-DOPA alone. The effect of a lower dose of sildenafil combined with L-DOPA was much greater in the second hour of measurement. However, the acute combined administration of a higher dose of sildenafil with L-DOPA resulted in an immediate and much stronger increase in the number of contralateral rotations compared to L-DOPA alone, already visible in the first hour of measurement. Interestingly, the chronic combined administration of 2 mg/kg of sildenafil and L-DOPA significantly reduced the number of contralateral rotations, especially during the first hour of measurement, compared to the long-term treatment with L-DOPA alone. Such an effect was not observed after the long-term combined treatment of a higher dose of sildenafil and L-DOPA compared to L-DOPA alone. The concentration of DA in the ipsilateral striatum and substantia nigra after the last combined chronic dose of sildenafil (2 or 6 mg/kg) and L-DOPA (12.5 mg/kg) was significantly higher than after L-DOPA alone. In spite of much stronger increases in the DA concentration in the ipsilateral striatum and substantia nigra, the number of contralateral rotations was reduced in the group of rats treated with the combination of 2 mg/kg sildenafil and L-DOPA compared to the group receiving L-DOPA alone. Moreover, the combined treatment with a low dose of sildenafil and L-DOPA had an opposite effect on DA catabolism, as assessed by DOPAC/DA and HVA/DA indexes, and these indexes were reduced in the ipsilateral striatum but increased in the contralateral striatum and substantia nigra compared to the treatment with L-DOPA alone. The results of the present study show that the addition of a low dose of a PDE5 inhibitor to the standard L-DOPA therapy differently modulates rotational behavior, the tissue DA concentration and its catabolism in the striatum and substantia nigra.

2,5-Dimethoxy-4-iodoamphetamine and altanserin induce region-specific shifts in dopamine and serotonin metabolization pathways in the rat brain

PURPOSE

For understanding the neurochemical mechanism of neuropsychiatric conditions associated with cognitive deficits it is of major relevance to elucidate the influence of serotonin (5-HT) agonists and antagonists on memory function as well dopamine (DA) and 5-HT release and metabolism. In the present study, we assessed the effects of the 5-HT2A receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) and the 5-HT2A receptor altanserin (ALT) on object and place recognition memory and cerebral neurotransmitters and metabolites in the rat.

METHODS

Rats underwent a 5-min exploration trial in an open field with two identical objects. After systemic injection of a single dose of either DOI (0.1 mg/kg), ALT (1 mg/kg) or the respectice vehicle (0.9 % NaCl, 50 % DMSO), rats underwent a 5-min test trial with one of the objects replaced by a novel one and the other object transferred to a novel place. Upon the assessment of object exploration and motor/exploratory behaviors, rats were sacrificed. DA, 5-HT and metabolite levels were analyzed in cingulate (CING), caudateputamen (CP), nucleus accumbens (NAC), thalamus (THAL), dorsal (dHIPP) and ventral hippocampus (vHIPP), brainstem and cerebellum with high performance liquid chromatography.

RESULTS

DOI decreased rearing but increased head-shoulder motility relative to vehicle. Memory for object and place after both DOI and ALT was not different from vehicle. Network analyses indicated that DOI inhibited DA metabolization in CING, CP, NAC, and THAL, but facilitated it in dHIPP. Likewise, DOI inhibited 5-HT metabolization in CING, NAC, and THAL. ALT facilitated DA metabolization in the CING, NAC, dHIPP, vHIPP, and CER, but inhibited it in the THAL. Additionally, ALT facilitated 5-HT metabolization in NAC and dHIPP.

CONCLUSIONS

DOI and ALT differentially altered the quantitative relations between the neurotransmitter/metabolite levels in the individual brain regions, by inducing region-specific shifts in the metabolization pathways. Findings are relevant for understanding the neurochemistry underlying DAergic and/or 5-HTergic dysfunction in neurological and psychiatric conditions.

5-HT1A and 5-HT2A receptor effects on recognition memory, motor/exploratory behaviors, emotionality and regional dopamine transporter binding in the rat

Both dopamine (DA) and serotonin (5-HT) play key roles in numerous functions including motor control, stress response and learning. So far, there is scarce or conflicting evidence about the effects of 5-HT1A and 5-HT2A receptor (R) agonists and antagonists on recognition memory in the rat. This also holds for their effect on cerebral DA as well as 5-HT release. In the present study, we assessed the effects of the 5-HT1AR agonist 8-OH-DPAT and antagonist WAY100,635 and the 5-HT2AR agonist DOI and antagonist altanserin (ALT) on rat behaviors. Moreover, we investigated their impact on monoamine efflux by measuring monoamine transporter binding in various regions of the rat brain. After injection of either 8-OH-DPAT (3 mg/kg), WAY100,635 (0.4 mg/kg), DOI (0.1 mg/kg), ALT (1 mg/kg) or the respective vehicle (saline, DMSO), rats underwent an object and place recognition memory test in the open field. Upon the assessment of object exploration, motor/exploratory parameters and feces excretion, rats were administered the monoamine transporter radioligand N-o-fluoropropyl-2b-carbomethoxy-3b-(4-[123I]iodophenyl)-nortropane ([123I]-FP-CIT; 8.9 ± 2.6 MBq) into the tail vein. Regional radioactivity accumulations in the rat brain were determined post mortem. Compared vehicle, administration of 8-OH-DPAT impaired memory for place, decreased rearing behavior, and increased ambulation as well as head-shoulder movements. DOI administration led to a reduction in rearing behavior but an increase in head-shoulder motility relative to vehicle. Feces excretion was diminished after ALT relative to vehicle. Dopamine transporter (DAT) binding was increased in the caudateputamen (CP), but decreased in the nucleus accumbens (NAC) after 8-OH-DPAT relative to vehicle. Moreover, DAT binding was decreased in the NAC after ALT relative to vehicle. Findings indicate that 5-HT1AR inhibition and 5-HT2AR activation may impair memory for place. Furthermore, results imply associations not only between recognition memory, motor/exploratory behavior and emotionality but also between the respective parameters and the levels of available DA in CP and NAC.

The 5-HT1A receptor agonist 8-OH-DPAT modulates motor/exploratory activity, recognition memory and dopamine transporter binding in the dorsal and ventral striatum

In the present studies, we assessed the effect of the 5-HT1A receptor (R) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on motor and exploratory behaviors, object and place recognition and dopamine transporter (DAT) and serotonin transporter (SERT) binding in the rat brain. In Experiment I, motor/exploratory behaviors were assessed in an open field after injection of either 8-OH-DPAT (0.1 and 3 mg/kg) or vehicle for 30 min without previous habituation to the open field. In Experiment II, rats underwent a 5-min exploration trial in an open field with two identical objects. After injection of either 8-OH-DPAT (0.1 and 3 mg/kg) or vehicle, rats underwent a 5-min test trial with one of the objects replaced by a novel one and the other object transferred to a novel place. Subsequently, N-o-fluoropropyl-2b-carbomethoxy-3b-(4-[123I]iodophenyl)-nortropane ([123I]FP-CIT; 11 ± 4 MBq) was injected into the tail vein. Regional radioactivity accumulations were determined post mortem with a well counter. In both experiments, 8-OH-DPAT dose-dependently increased ambulation and exploratory head-shoulder motility, whereas rearing was dose-dependently decreased. In the test rial of Experiment II, there were no effects of 8-OH-DPAT on overall activity, sitting and grooming. 8-OH-DPAT dose-dependently impaired recognition of object and place. 8-OH-DPAT (3 mg/kg) increased DAT binding in the dorsal striatum relative to both vehicle and 0.1 mg/kg 8-OH-DPAT. Furthermore, in the ventral striatum, DAT binding was decreased after 3 mg/kg 8-OH-DPAT relative to vehicle. Findings indicate that motor/exploratory behaviors, memory for object and place and regional dopamine function may be modulated by the 5-HT1AR. Since, after 8-OH-DPAT, rats exhibited more horizontal and less (exploratory) vertical motor activity, while overall activity was not different between groups, it may be inferred, that the observed impairment of object recognition was not related to a decrease of motor activity as such, but to a decrease of intrinsic motivation, attention and/or awareness, which are relevant accessories of learning. Furthermore, the present findings on 8-OH-DPAT action indicate associations not only between motor/exploratory behavior and the recognition of object and place but also between the respective parameters and the levels of available DA in dorsal and ventral striatum.

Dorsal Raphe Nucleus Serotoninergic Neurons Mediate Morphine Rewarding Effect and Conditioned Place Preference

Morphine rewarding properties are the main reasons for drug-craving in behaviors occurring during morphine addiction. It has been suggested that morphine addiction relies on signals to the mesolimbic dopamine system, although the mechanisms outside the dopaminergic system are still unclear. Notably, the role of the dorsal raphe nucleus (DRN) serotoninergic (5-hydroxytryptamine, 5-HT) system remains unexplored. Using in vivo electrophysiological and optogenetic approaches, we found that morphine treatment increased DRN 5-TH neurons firing rate and optogenetic activation of DRN 5-HT neurons induced a rewarding effect, indicating that morphine reward is related to DRN 5-HT neurons. Accordingly, optogenetic inhibition of DRN 5-HT neurons following morphine injection reversed conditioned place preference (CPP) during chronic morphine treatment. These findings aid our understanding of the new functions of the DRN 5-HT neurons for morphine rewarding effect and provide a potential approach for the treatment of morphine addiction.

Serotonergic Modulation of Nigrostriatal and Mesolimbic Dopamine and Motor/Exploratory Behaviors in the Rat

Purpose: The 5-HT_2A receptor (R) is known to modulate dopamine (DA) release in the mammalian brain. Altanserin (ALT) and 2,5-dimethoxy-4-iodoamphetamine (DOI) act as 5-HT_2AR antagonist and agonist, respectively. In the present study, we assessed the effects of ALT and DOI on motor and exploratory behaviors and on D_2/3R binding in the rat brain with in vivo imaging methods.

Methods: D_2/3R binding was determined after systemic application of ALT (10 mg/kg) or DOI (0.5 mg/kg) and the respective vehicles [dimethyl sulfoxide (DMSO) and 0.9% saline (SAL)] with [¹²³I]IBZM as a single-photon emission computed tomography (SPECT) radioligand. Anatomical information for the delineation of the target regions was obtained with dedicated small animal MRI. Immediately after 5-HT_2AR antagonistic or agonistic treatment, motor/exploratory behaviors were assessed for 45 (ALT) or 30 min (DOI) in an open field. Additional rats underwent behavioral measurements after injection of DMSO or SAL.

Results: ALT increased D_2/3R binding in the ventral hippocampus relative to vehicle, while DOI augmented D_2/3R binding in caudate putamen, frontal cortex, motor cortex, and ventral hippocampus. The 5-HT_2AR agonist as well as antagonist decreased parameters of motor activity and active exploration. However, ALT, in contrast to DOI, decreased explorative head–shoulder motility and increased sitting.

Conclusions: The regional increases of D_2/3R binding after ALT and DOI (90 and 75 min post-challenge) may be conceived to reflect decreases of synaptic DA. The reductions of motor/exploratory activities (min 1–45 and min 1–30 after challenge with ALT and DOI, respectively) contrast the regional reductions of D_2/3R binding, as they indicate elevated DA levels at the time of behavioral measurements. It may be concluded that ALT and DOI modulate DA in the individual regions of the nigrostriatal and mesolimbocortical pathways differentially and in a time-dependent fashion.

Modeling heritability of temperamental differences, stress reactivity, and risk for anxiety and depression: Relevance to research domain criteria (RDoC)

Animal models provide important tools to study biological and environmental factors that shape brain function and behavior. These models can be effectively leveraged by drawing on concepts from the National Institute of Mental Health Research Domain Criteria (RDoC) Initiative, which aims to delineate molecular pathways and neural circuits that underpin behavioral anomalies that transcend psychiatric conditions. To study factors that contribute to individual differences in emotionality and stress reactivity, our laboratory utilized Sprague-Dawley rats that were selectively bred for differences in novelty exploration. Selective breeding for low versus high locomotor response to novelty produced rat lines that differ in behavioral domains relevant to anxiety and depression, particularly the RDoC Negative Valence domains, including acute threat, potential threat, and loss. Bred Low Novelty Responder (LR) rats, relative to their High Responder (HR) counterparts, display high levels of behavioral inhibition, conditioned and unconditioned fear, avoidance, passive stress coping, anhedonia, and psychomotor retardation. The HR/LR traits are heritable, emerge in the first weeks of life, and appear to be driven by alterations in the developing amygdala and hippocampus. Epigenomic and transcriptomic profiling in the developing and adult HR/LR brain suggest that DNA methylation and microRNAs, as well as differences in monoaminergic transmission (dopamine and serotonin in particular), contribute to their distinct behavioral phenotypes. This work exemplifies ways that animal models such as the HR/LR rats can be effectively used to study neural and molecular factors driving emotional behavior, which may pave the way toward improved understanding the neurobiological mechanisms involved in emotional disorders.

Behavioral arrest and a characteristic slow waveform are hallmark responses to selective 5-HT2A receptor activation

Perception, emotion, and mood are powerfully modulated by serotonin receptor (5-HTR) agonists including hallucinogens. The 5-HT_2AR subtype has been shown to be central to hallucinogen action, yet the precise mechanisms mediating the response to 5-HT_2AR activation remain unclear. Hallucinogens induce the head twitch response (HTR) in rodents, which is the most commonly used behavioral readout of hallucinogen pharmacology. While the HTR provides a key behavioral signature, less is known about the meso level changes that are induced by 5-HT_2AR activation. In response to administration of the potent and highly selective 5-HT_2AR agonist 25I-NBOH in mice, we observe a disorganization of behavior which includes frequent episodes of behavioral arrest that consistently precede the HTR by a precise interval. By combining behavioral analysis with electroencephalogram (EEG) recordings we describe a characteristic pattern composed of two distinctive EEG waveforms, Phase 1 and Phase 2, that map onto behavioral arrest and the HTR respectively, with the same temporal separation. Phase 1, which underlies behavioral arrest, is a 3.5-4.5 Hz waveform, while Phase 2 is slower at 2.5-3.2 Hz. Nicotine pretreatment, considered an integral component of ritualistic hallucinogen practices, attenuates 25I-NBOH induced HTR and Phase 2 waveforms, yet increases behavioral arrest and Phase 1 waveforms. Our results suggest that in addition to the HTR, behavioral arrest and characteristic meso level slow waveforms are key hallmarks of the response to 5-HT_2AR activation. Increased understanding of the response to serotonergic hallucinogens may provide mechanistic insights into perception and hallucinations, as well as regulation of mood.

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.

Serotonergic neurons in the dorsal raphe nucleus mediate the arousal-promoting effect of orexin during isoflurane anesthesia in male rats

Previous studies have demonstrated that the activation of orexinergic neurons facilitates the recovery of animals from general anesthesia. Moreover, serotonergic neurons that receive projections from orexin neurons have also been shown to participate in sleep-wakefulness regulation. In the present study, we aimed to explore whether orexinergic neurons facilitate emergence from isoflurane anesthesia in rats by activating serotonergic neurons. Orexin A (30 or 100 pmol), orexin B (30 or 100 pmol), and their respective antagonists SB-334867 and TCS-OX2-29 (5 or 20 μg) were microinjected into the dorsal raphe nucleus (DRN) of rats, and their effects on induction and emergence times were analyzed. Electroencephalogram (EEG) changes were also recorded and analyzed to illuminate the effect of orexin injection into the DRN on cortical excitability under isoflurane anesthesia. Activation of serotonergic neurons was detected via immunohistochemical analysis of c-Fos expression following orexin administration. Our results indicated that injection of neither orexins nor orexin antagonists into the rat DRN exerted an impact on induction time, whereas orexin-A injection (100 pmol) enhanced arousal when compared with the saline group. In contrast, administration of orexin receptor type 1 antagonist SB-334867 (20 μg) prolonged emergence time from isoflurane anesthesia. Microinjection of orexin-A induced an arousal pattern on EEG, and decreased the burst suppression ratio under isoflurane anesthesia. Isoflurane anesthesia inhibited the activity of serotonergic neurons, as shown by decrease in the number of c-Fos-immunoreactive serotonergic neurons when compared with the sham group. This inhibitory effect was partially reversed by administration of orexin-A. Taken together, our findings suggest that orexinergic signals facilitate emergence from isoflurane anesthesia, at least partially, by reversing the effects of isoflurane on serotonergic neurons of the DRN.

Interactions of the tricyclic antidepressant drug amitriptyline with L-DOPA in the striatum and substantia nigra of unilaterally 6-OHDA-lesioned rats. Relevance to motor dysfunction in Parkinson's disease

Antidepressant drugs are recommended for the treatment of Parkinson's disease (PD)-associated depression but their role in the modulation of L-DOPA-induced behavioral and neurochemical markers is poorly explored. The aim of the present study was to examine the impact of the tricyclic antidepressant amitriptyline and L-DOPA, administered chronically alone or in combination, on rotational behavior, monoamine levels and binding of radioligands to their transporters in the dopaminergic brain structures of unilaterally 6-OHDA-lesioned rats. Binding of [³H]nisoxetine to noradrenaline transporter (NET), [³H]GBR 12,935 to dopamine transporter (DAT) and [³H]citalopram to serotonin transporter (SERT) were analyzed by autoradiography. Amitriptyline administered alone did not induce rotational behavior but in combination with L-DOPA increased the number of contralateral rotations much more strongly than L-DOPA alone. The combined treatment also significantly increased the tissue dopamine (DA) content in the ipsilateral striatum and substantia nigra (SN) vs. L-DOPA alone. 6-OHDA-mediated lesion of nigrostriatal DA neurons drastically reduced DAT and NET bindings in the ipsilateral striatum. In the ipsilateral SN, DAT binding decreased while NET binding rose. SERT binding increased significantly mainly in the SN. Amitriptyline administered alone or jointly with L-DOPA had no effect on DAT binding on the lesioned side, significantly decreased SERT binding in the striatum and SN while NET binding only in the SN. Since in the DA-denervated striatum, SERT is mainly responsible for reuptake of L-DOPA-derived DA while in the SN, SERT and NET are involved, the inhibition of these transporters by antidepressant drugs may improve dopaminergic transmission and consequently motor behavior.

5-HT release in nucleus accumbens rescues social deficits in mouse autism model

Dysfunction in prosocial interactions is a core symptom of autism spectrum disorder. However, the neural mechanisms that underlie sociability are poorly understood, limiting the rational development of therapies to treat social deficits. Here we show in mice that bidirectional modulation of the release of serotonin (5-HT) from dorsal raphe neurons in the nucleus accumbens bidirectionally modifies sociability. In a mouse model of a common genetic cause of autism spectrum disorder-a copy number variation on chromosome 16p11.2-genetic deletion of the syntenic region from 5-HT neurons induces deficits in social behaviour and decreases dorsal raphe 5-HT neuronal activity. These sociability deficits can be rescued by optogenetic activation of dorsal raphe 5-HT neurons, an effect requiring and mimicked by activation of 5-HT1b receptors in the nucleus accumbens. These results demonstrate an unexpected role for 5-HT action in the nucleus accumbens in social behaviours, and suggest that targeting this mechanism may prove therapeutically beneficial.

Analysis of neuroanatomical differences in mice with genetically modified serotonin transporters assessed by structural magnetic resonance imaging

Background

The serotonin (5-HT) system has long been implicated in autism spectrum disorder (ASD) as indicated by elevated whole blood and platelet 5-HT, altered platelet and brain receptor and transporter binding, and genetic linkage and association findings. Based upon work in genetically modified mice, 5-HT is known to influence several aspects of brain development, but systematic neuroimaging studies have not previously been reported. In particular, the 5-HT transporter (serotonin transporter, SERT; 5-HTT) gene, Slc6a4, has been extensively studied.

Methods

Using a 7-T MRI and deformation-based morphometry, we assessed neuroanatomical differences in an Slc6a4 knockout mouse on a C57BL/6 genetic background, along with an Slc6a4 Ala56 knockin mouse on two different genetic backgrounds (129S and C57BL/6).

Results

Individually (same sex, same background, same genotype), the only differences found were in the female Slc6a4 knockout mouse; all the others had no significant differences. However, an analysis of variance across the whole study sample revealed a significant effect of Slc6a4 on the amygdala, thalamus, dorsal raphe nucleus, and lateral and frontal cortices.

Conclusions

This work shows that an increase or decrease in SERT function has a significant effect on the neuroanatomy in 5-HT relevant regions, particularly the raphe nuclei. Notably, the Slc6a4 Ala56 knockin alone appears to have an insignificant, but suggestive, effect compared to the KO, which is consistent with Slc6a4 function. Despite the small number of 5-HT neurons and their localization to the brainstem, it is clear that 5-HT plays an important role in neuroanatomical organization.

Electronic supplementary material

The online version of this article (10.1186/s13229-018-0210-z) contains supplementary material, which is available to authorized users.

Monoaminergic neuropathology in Alzheimer's disease

None of the proposed mechanisms of Alzheimer's disease (AD) fully explains the distribution patterns of the neuropathological changes at the cellular and regional levels, and their clinical correlates. One aspect of this problem lies in the complex genetic, epigenetic, and environmental landscape of AD: early-onset AD is often familial with autosomal dominant inheritance, while the vast majority of AD cases are late-onset, with the ε4 variant of the gene encoding apolipoprotein E (APOE) known to confer a 5-20 fold increased risk with partial penetrance. Mechanisms by which genetic variants and environmental factors influence the development of AD pathological changes, especially neurofibrillary degeneration, are not yet known. Here we review current knowledge of the involvement of the monoaminergic systems in AD. The changes in the serotonergic, noradrenergic, dopaminergic, histaminergic, and melatonergic systems in AD are briefly described. We also summarize the possibilities for monoamine-based treatment in AD. Besides neuropathologic AD criteria that include the noradrenergic locus coeruleus (LC), special emphasis is given to the serotonergic dorsal raphe nucleus (DRN). Both of these brainstem nuclei are among the first to be affected by tau protein abnormalities in the course of sporadic AD, causing behavioral and cognitive symptoms of variable severity. The possibility that most of the tangle-bearing neurons of the LC and DRN may release amyloid β as well as soluble monomeric or oligomeric tau protein trans-synaptically by their diffuse projections to the cerebral cortex emphasizes their selective vulnerability and warrants further investigations of the monoaminergic systems in AD.

Differential stress induced c-Fos expression and identification of region-specific miRNA-mRNA networks in the dorsal raphe and amygdala of high-responder/low-responder rats

Chronic stress triggers a variety of physical and mental health problems, and how individuals cope with stress influences risk for emotional disorders. To investigate molecular mechanisms underlying distinct stress coping styles, we utilized rats that were selectively-bred for differences in emotionality and stress reactivity. We show that high novelty responding (HR) rats readily bury a shock probe in the defensive burying test, a measure of proactive stress coping behavior, while low novelty responding (LR) rats exhibit enhanced immobility, a measure of reactive coping. Shock exposure in the defensive burying test elicited greater activation of HR rats' caudal dorsal raphe serotonergic cells compared to LRs, but lead to more pronounced activation throughout LRs' amygdala (lateral, basolateral, central, and basomedial nuclei) compared to HRs. RNA-sequencing revealed 271 mRNA transcripts and 33 microRNA species that were differentially expressed in HR/LR raphe and amygdala. We mapped potential microRNA-mRNA networks by correlating and clustering mRNA and microRNA expression and identified networks that differed in either the HR/LR dorsal raphe or amygdala. A dorsal raphe network linked three microRNAs which were down-regulated in LRs (miR-206-3p, miR-3559-5p, and miR-378a-3p) to repression of genes related to microglia and immune response (Cd74, Cyth4, Nckap1l, and Rac2), the genes themselves were up-regulated in LR dorsal raphe. In the amygdala, another network linked miR-124-5p, miR-146a-5p, miR-3068-3p, miR-380-5p, miR-539-3p, and miR-7a-1-3p with repression of chromatin remodeling-related genes (Cenpk, Cenpq, Itgb3bp, and Mis18a). Overall this work highlights potential drivers of gene-networks and downstream molecular pathways within the raphe and amygdala that contribute to individual differences in stress coping styles and stress vulnerabilities.

Effect of MDMA-Induced Axotomy on the Dorsal Raphe Forebrain Tract in Rats: An In Vivo Manganese-Enhanced Magnetic Resonance Imaging Study

3,4-Methylenedioxymethamphetamine (MDMA), also known as “Ecstasy”, is a common recreational drug of abuse. Several previous studies have attributed the central serotonergic neurotoxicity of MDMA to distal axotomy, since only fine serotonergic axons ascending from the raphe nucleus are lost without apparent damage to their cell bodies. However, this axotomy has never been visualized directly in vivo. The present study examined the axonal integrity of the efferent projections from the midbrain raphe nucleus after MDMA exposure using in vivo manganese-enhanced magnetic resonance imaging (MEMRI). Rats were injected subcutaneously six times with MDMA (5 mg/kg) or saline once daily. Eight days after the last injection, manganese ions (Mn²⁺) were injected stereotactically into the raphe nucleus, and a series of MEMRI images was acquired over a period of 38 h to monitor the evolution of Mn²⁺-induced signal enhancement across the ventral tegmental area, the medial forebrain bundle (MFB), and the striatum. The MDMA-induced loss of serotonin transporters was clearly evidenced by immunohistological staining consistent with the Mn²⁺-induced signal enhancement observed across the MFB and striatum. MEMRI successfully revealed the disruption of the serotonergic raphe-striatal projections and the variable effect of MDMA on the kinetics of Mn²⁺ accumulation in the MFB and striatum.

Functional connectivity of the dorsal and median raphe nuclei at rest

Serotonin (5-HT) is a neurotransmitter critically involved in a broad range of brain functions and implicated in the pathophysiology of neuropsychiatric illnesses including major depression, anxiety and sleep disorders. Despite being widely distributed throughout the brain, there is limited knowledge on the contribution of 5-HT to intrinsic brain activity. The dorsal raphe (DR) and median raphe (MR) nuclei are the source of most serotonergic neurons projecting throughout the brain and thus provide a compelling target for a seed-based probe of resting-state activity related to 5-HT. Here we implemented a novel multimodal neuroimaging approach for investigating resting-state functional connectivity (FC) between DR and MR and cortical, subcortical and cerebellar target areas. Using [(11)C]DASB positron emission tomography (PET) images of the brain serotonin transporter (5-HTT) combined with structural MRI from 49 healthy volunteers, we delineated DR and MR and performed a seed-based resting-state FC analysis. The DR and MR seeds produced largely similar FC maps: significant positive FC with brain regions involved in cognitive and emotion processing including anterior cingulate, amygdala, insula, hippocampus, thalamus, basal ganglia and cerebellum. Significant negative FC was observed within pre- and postcentral gyri for the DR but not for the MR seed. We observed a significant association between DR and MR FC and regional 5-HTT binding. Our results provide evidence for a resting-state network related to DR and MR and comprising regions receiving serotonergic innervation and centrally involved in 5-HT related behaviors including emotion, cognition and reward processing. These findings provide a novel advance in estimating resting-state FC related to 5-HT signaling, which can benefit our understanding of its role in behavior and neuropsychiatric illnesses.

A whole-brain atlas of inputs to serotonergic neurons of the dorsal and median raphe nuclei

The serotonin system is proposed to regulate physiology and behavior and to underlie mood disorders; nevertheless, the circuitry controlling serotonergic neurons remains uncharacterized. We therefore generated a comprehensive whole-brain atlas defining the monosynaptic inputs onto forebrain-projecting serotonergic neurons of dorsal versus median raphe based on a genetically restricted transsynaptic retrograde tracing strategy. We identified discrete inputs onto serotonergic neurons from forebrain and brainstem neurons, with specific inputs from hypothalamus, cortex, basal ganglia, and midbrain, displaying a greater than anticipated complexity and diversity in cell-type-specific connectivity. We identified and functionally confirmed monosynaptic glutamatergic inputs from prefrontal cortex and lateral habenula onto serotonergic neurons as well as a direct GABAergic input from striatal projection neurons. In summary, our findings emphasize the role of hyperdirect inputs to serotonergic neurons. Cell-type-specific classification of connectivity patterns will allow for further functional analysis of the diverse but specific inputs that control serotonergic neurons during behavior.

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.

Corticotropin-releasing factor-related peptides, serotonergic systems, and emotional behavior

Corticotropin-releasing factor (CRF) is a 41-amino acid neuropeptide that is involved in stress-related physiology and behavior, including control of the hypothalamic-pituitary-adrenal (HPA) axis. Members of the CRF family of neuropeptides, including urocortin 1 (UCN 1), UCN 2, and UCN 3, bind to the G protein-coupled receptors, CRF type 1 (CRF₁) and CRF₂ receptors. In addition, CRF binding protein (CRFBP) binds both CRF and UCN 1 and can modulate their activities. There are multiple mechanisms through which CRF-related peptides may influence emotional behavior, one of which is through altering the activity of brainstem neuromodulatory systems, including serotonergic systems. CRF and CRF-related peptides act within the dorsal raphe nucleus (DR), the major source for serotonin (5-HT) in the brain, to alter the neuronal activity of specific subsets of serotonergic neurons and to influence stress-related behavior. CRF-containing axonal fibers innervate the DR in a topographically organized manner, which may contribute to the ability of CRF to alter the activity of specific subsets of serotonergic neurons. CRF and CRF-related peptides can either increase or decrease serotonergic neuronal firing rates and serotonin release, depending on their concentrations and on the specific CRF receptor subtype(s) involved. This review aims to describe the interactions between CRF-related peptides and serotonergic systems, the consequences for stress-related behavior, and implications for vulnerability to anxiety and affective disorders.

Serotonin transporter gene, stress and raphe-raphe interactions: a molecular mechanism of depression

Reports of gene-environment interactions (GxE) between the serotonin transporter gene and stress on risk of depression have generated both excitement and controversy. The controversy persists in part because a mechanistic account of this GxE on serotonergic neurotransmission and risk of depression has been lacking. In this Opinion, we draw on recent discoveries in the functional neuroanatomy of the serotonergic dorsal raphe nucleus (DR) to propose such a mechanistic account. We argue that genetically produced variability in serotonin reuptake during stressor-induced raphe-raphe interactions alters the balance in the amygdala-ventromedial prefrontal cortex (VMPFC)-DR circuitry underlying stressor reactivity and emotion regulation. In particular, the recently characterized stressor-responsive serotonergic interneurons originating from the dorsolateral DR may hold a key to unlocking the GxE mechanism of depression.

In vivo electrochemical evidence for simultaneous 5-HT and histamine release in the rat substantia nigra pars reticulata following medial forebrain bundle stimulation

Exploring the mechanisms of serotonin [5-hydroxytryptamine (5-HT)] in the brain requires an in vivo method that combines fast temporal resolution with chemical selectivity. Fast-scan cyclic voltammetry is a technique with sufficient temporal and chemical resolution for probing dynamic 5-HT neurotransmission events; however, traditionally it has not been possible to probe in vivo 5-HT mechanisms. Recently, we optimized fast-scan cyclic voltammetry for measuring 5-HT release and uptake in vivo in the substantia nigra pars reticulata (SNR) with electrical stimulation of the dorsal raphe nucleus (DRN) in the rat brain. Here, we address technical challenges associated with rat DRN surgery by electrically stimulating 5-HT projections in the medial forebrain bundle (MFB), a more accessible anatomical location. MFB stimulation elicits 5-HT in the SNR; furthermore, we find simultaneous release of an additional species. We use electrochemical and pharmacological methods and describe physiological, anatomical and independent chemical analyses to identify this species as histamine. We also show pharmacologically that increasing the lifetime of extracellular histamine significantly decreases 5-HT release, most likely because of increased activation of histamine H-3 receptors that inhibit 5-HT release. Despite this, under physiological conditions, we find by kinetic comparisons of DRN and MFB stimulations that the simultaneous release of histamine does not interfere with the quantitative 5-HT concentration profile. We therefore present a novel and robust electrical stimulation of the MFB that is technically less challenging than DRN stimulation to study 5-HT and histamine release in the SNR.

The serotonergic system in Parkinson's disease

Although the cardinal manifestations of Parkinson's disease (PD) are attributed to a decline in dopamine levels in the striatum, a breadth of non-motor features and treatment-related complications in which the serotonergic system plays a pivotal role are increasingly recognised. Serotonin (5-HT)-mediated neurotransmission is altered in PD and the roles of the different 5-HT receptor subtypes in disease manifestations have been investigated. The aims of this article are to summarise and discuss all published preclinical and clinical studies that have investigated the serotonergic system in PD and related animal models, in order to recapitulate the state of the current knowledge and to identify areas that need further research and understanding.

Collateralized dorsal raphe nucleus projections: a mechanism for the integration of diverse functions during stress

The midbrain dorsal raphe nucleus (DR) is the origin of the central serotonin (5-HT) system, a key neurotransmitter system that has been implicated in the expression of normal behaviors and in diverse psychiatric disorders, particularly affective disorders such as depression and anxiety. One link between the DR-5-HT system and affective disorders is exposure to stressors. Stress is a major risk factor for affective disorders, and stressors alter activity of DR neurons in an anatomically specific manner. Stress-induced changes in DR neuronal activity are transmitted to targets of the DR via ascending serotonergic projections, many of which collateralize to innervate multiple brain regions. Indeed, the collateralization of DR efferents allows for the coordination of diverse components of the stress response. This review will summarize our current understanding of the organization of the ascending DR system and its collateral projections. Using the neuropeptide corticotropin-releasing factor (CRF) system as an example of a stress-related initiator of DR activity, we will discuss how topographic specificity of afferent regulation of ascending DR circuits serves to coordinate activity in functionally diverse target regions under appropriate conditions.

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

The serotonin (5HT) system of the brain is involved in many CNS functions including sensory perception, stress responses and psychological disorders such as anxiety and depression. Of the nine 5HT nuclei located in the mammalian brain, the dorsal raphe nucleus (DRN) has the most extensive forebrain connectivity and is implicated in the manifestation of stress-related psychological disturbances. Initial investigations of DRN efferent connections failed to acknowledge the rostrocaudal and mediolateral organization of the nucleus or its neurochemical heterogeneity. More recent studies have focused on the non-5HT contingent of DRN cells and have revealed an intrinsic intranuclear organization of the DRN which has specific implications for sensory signal processing and stress responses. Of particular interest are spatially segregated subsets of nitric oxide producing neurons that are activated by stressors and that have unique efferent projection fields. In this regard, both the midline and lateral wing subregions of the DRN have emerged as prominent loci for future investigation of nitric oxide function and modulation of sensory- and stressor-related signals in the DRN and coinciding terminal fields.

Topographical distribution of corticotropin-releasing factor type 2 receptor-like immunoreactivity in the rat dorsal raphe nucleus: co-localization with tryptophan hydroxylase

Corticotropin-releasing factor (CRF) and CRF-related neuropeptides are involved in the regulation of stress-related physiology and behavior. Members of the CRF family of neuropeptides bind to two known receptors, the CRF type 1 (CRF₁) receptor, and the CRF type 2 (CRF₂) receptor. Although the distribution of CRF₂ receptor mRNA expression has been extensively studied, the distribution of CRF₂ receptor protein has not been characterized. An area of the brain known to contain high levels of CRF₂ receptor mRNA expression and CRF₂ receptor binding is the dorsal raphe nucleus (DR). In the present study we investigated in detail the distribution of CRF₂ receptor immunoreactivity throughout the rostrocaudal extent of the DR. CRF₂ receptor-immunoreactive perikarya were observed throughout the DR, with the highest number and density in the mid-rostrocaudal DR. Dual immunofluorescence revealed that CRF₂ receptor immunoreactivity was frequently co-localized with tryptophan hydroxylase, a marker of serotonergic neurons. This study provides evidence that CRF₂ receptor protein is expressed in the DR, and that CRF₂ receptors are expressed in topographically organized subpopulations of cells in the DR, including serotonergic neurons. Furthermore, these data are consistent with the hypothesis that CRF₂ receptors play an important role in the regulation of stress-related physiology and behavior through actions on serotonergic and non-serotonergic neurons within the DR.

Coexpression of serotonin and nitric oxide in the raphe complex: cortical versus subcortical circuit

Several lines of evidence have implicated a direct reciprocal interaction between serotonin and nitric oxide (NO). The goal of this investigation was, therefore, to examine the coexpression of tryptophan hydroxylase (TPH; the rate limiting enzyme for the synthesis of serotonin) and neuronal NO synthase (nNOS) in the ascending cortical projecting raphe nuclei (B6-B9 subgroups), when compared with the descending spinal cord projecting raphe nuclei (B1-B3 subgroups). Our data demonstrated that: (1) a significant number of raphe-cortical projecting neurons was identified not only in the midline subgroup of dorsal raphe (B6, 7) but also in the median raphe (B8), as well as in the supralemniscal nucleus (B9); (2) serotonergic cortical projecting neurons from these three raphe nuclei exhibited a high (>80%) percentage of coexpression with nNOS immunoreactivity; (3) similarly, serotonin transporter immunoreactive fibers in the medial prefrontal cortex were also double-labeled with nNOS immunoreactivity; (4) in contrast, the descending spinal cord projecting raphe nuclei revealed only TPH but not nNOS immunoreactivity. Our present findings suggest the existence of a direct interaction between serotonin and NO in the ascending cortical projecting raphe system. In contrast, a different strategy appears to operate the descending spinal cord projecting raphe system.

Functional topography of midbrain and pontine serotonergic systems: implications for synaptic regulation of serotonergic circuits

RATIONALE

Dysfunction of serotonergic systems is thought to play an important role in a number of neurological and psychiatric disorders. Recent studies suggest that there is anatomical and functional diversity among serotonergic systems innervating forebrain systems involved in the control of physiologic and behavioral responses, including the control of emotional states.

OBJECTIVE

Here, we highlight the methods that have been used to investigate the heterogeneity of serotonergic systems and review the evidence for the unique anatomical, hodological, and functional properties of topographically organized subpopulations of serotonergic neurons in the midbrain and pontine raphe complex.

CONCLUSION

The emerging understanding of the topographically organized synaptic regulation of brainstem serotonergic systems, the topography of the efferent projections of these systems, and their functional properties, should enable identification of novel therapeutic approaches to treatment of neurological and psychiatric conditions that are associated with dysregulation of serotonergic systems.

Investigation of a central nucleus of the amygdala/dorsal raphe nucleus serotonergic circuit implicated in fear-potentiated startle

Serotonergic systems are thought to play an important role in control of motor activity and emotional states. We used a fear-potentiated startle paradigm to investigate the effects of a motor-eliciting stimulus in the presence or absence of induction of an acute fear state on serotonergic neurons in the dorsal raphe nucleus (DR) and cells in subdivisions of the central amygdaloid nucleus (CE), a structure that plays an important role in fear responses, using induction of the protein product of the immediate-early gene, c-Fos. In Experiment 1 we investigated the effects of fear conditioning training, by training rats to associate a light cue (conditioned stimulus, CS; 1000 lx, 2 s) with foot shock (0.5 s, 0.5 mA) in a single session. In Experiment 2 rats were given two training sessions identical to Experiment 1 on days 1 and 2, then tested in one of four conditions on day 3: (1) placement in the training context without exposure to either the CS or acoustic startle (AS), (2) exposure to 10 trials of the 2 s CS, (3) exposure to 40 110 dB AS trials, or (4) exposure to 40 110 dB AS trials with 10 of the trials preceded by and co-terminating with the CS. All treatments were conducted during a 20 min session. Fear conditioning training, by itself, increased c-Fos expression in multiple subdivisions of the CE and throughout the DR. In contrast, fear-potentiated startle selectively increased c-Fos expression in the medial subdivision of the CE and in serotonergic neurons in the dorsal part of the dorsal raphe nucleus (DRD). These data are consistent with previous studies demonstrating that fear-related stimuli selectively activate DRD serotonergic neurons. Further studies of this mesolimbocortical serotonergic system could have important implications for understanding mechanisms underlying vulnerability to stress-related psychiatric disorders, including anxiety and affective disorders.

Corticotropin-releasing factor increases GABA synaptic activity and induces inward current in 5-hydroxytryptamine dorsal raphe neurons

Stress-related psychiatric disorders such as anxiety and depression involve dysfunction of the serotonin [5-hydroxytryptamine (5-HT)] system. Previous studies have found that the stress neurohormone corticotropin-releasing factor (CRF) inhibits 5-HT neurons in the dorsal raphe nucleus (DRN) in vivo. The goals of the present study were to characterize the CRF receptor subtypes (CRF-R1 and -R2) and cellular mechanisms underlying CRF-5-HT interactions. Visualized whole-cell patch-clamp recording techniques in brain slices were used to measure spontaneous or evoked GABA synaptic activity in DRN neurons of rats and CRF effects on these measures. CRF-R1 and -R2-selective agonists were bath applied alone or in combination with receptor-selective antagonists. CRF increased presynaptic GABA release selectively onto 5-HT neurons, an effect mediated by the CRF-R1 receptor. CRF increased postsynaptic GABA receptor sensitivity selectively in 5-HT neurons, an effect to which both receptor subtypes contributed. CRF also had direct effects on DRN neurons, eliciting an inward current in 5-HT neurons mediated by the CRF-R2 receptor and in non-5-HT neurons mediated by the CRF-R1 receptor. These results indicate that CRF has direct membrane effects on 5-HT DRN neurons as well as indirect effects on GABAergic synaptic transmission that are mediated by distinct receptor subtypes. The inhibition of 5-HT DRN neurons by CRF in vivo may therefore be primarily an indirect effect via stimulation of inhibitory GABA synaptic transmission. These results regarding the cellular mechanisms underlying the complex interaction between CRF, 5-HT, and GABA systems could contribute to the development of novel treatments for stress-related psychiatric disorders.

Neurobiological mechanisms for the regulation of mammalian sleep-wake behavior: reinterpretation of historical evidence and inclusion of contemporary cellular and molecular evidence

At its most basic level, the function of mammalian sleep can be described as a restorative process of the brain and body; recently, however, progressive research has revealed a host of vital functions to which sleep is essential. Although many excellent reviews on sleep behavior have been published, none have incorporated contemporary studies examining the molecular mechanisms that govern the various stages of sleep. Utilizing a holistic approach, this review is focused on the basic mechanisms involved in the transition from wakefulness, initiation of sleep and the subsequent generation of slow-wave sleep and rapid eye movement (REM) sleep. Additionally, using recent molecular studies and experimental evidence that provides a direct link to sleep as a behavior, we have developed a new model, the cellular-molecular-network model, explaining the mechanisms responsible for regulating REM sleep. By analyzing the fundamental neurobiological mechanisms responsible for the generation and maintenance of sleep-wake behavior in mammals, we intend to provide a broader understanding of our present knowledge in the field of sleep research.

Differential projections of dorsal raphe nucleus neurons to the lateral septum and striatum

The dorsal raphe nucleus (DRN)-serotonin (5-HT) system has been implicated in acute responses to stress and stress-related psychiatric disorders such as anxiety and depression. Stress alters serotonin (5-HT) release in a regionally specific manner. For example, swim stress increases extracellular levels of 5-HT in the striatum and decreases levels in the lateral septum. This finding suggests that the 5-HT efferents to the striatum and lateral septum arise from distinct populations of DRN neurons that are differentially affected by swim stress. To further examine this, retrograde axonal transport of fluorescent RetroBeads was used to identify the distribution of DRN neurons projecting to the lateral septum and striatum in the rat brain. Retrograde labeling from the lateral septum was observed primarily within the more caudal portions of the DRN, while labeling from the striatum was observed in neurons located in the more rostral regions of the DRN. Few cell bodies were observed that were labeled from both the striatum and lateral septum suggesting that DRN neurons do not send collateralized projections to the septal region and striatum. Many septal- and striatal-projecting neurons in the DRN exhibited 5-HT, and collateralized projections, when observed, were immunoreactive for 5-HT. Taken together with previous microdialysis studies, these results support the existence of distinct DRN-5-HT-forebrain projections that are differentially regulated by stress.

Functional mapping of the cardiorespiratory effects of dorsal and median raphe nuclei in the rat

The dorsal (DRN) and median (MRN) raphe nuclei are important sources of serotonergic innervation to the forebrain, projecting to sites involved in cardiovascular regulation. These nuclei have been mapped using electrical stimulation, which has the limitation of stimulating fibers of passage. The present study maps these areas with chemical stimulation, investigating their influence on cardiorespiratory parameters. Urethane-anesthetized (1.2 g/kg, iv) male Wistar rats (280-300 g) were instrumented for pulsatile and mean blood pressure (MBP), heart rate, renal nerve activity, and respiratory frequency recordings. Microinjections of L-glutamate (0.18 M, 50-100 nl with 1% Pontamine Sky Blue) were performed within the DRN or the MRN with glass micropipettes. At the end of the experiments the sites of microinjection were identified. The majority of sites within the MRN (86.1%) and DRN (85.4%) evoked pressor responses when stimulated (DRN: DeltaMBP = +14.7 +/- 1.2; MRN: DeltaMBP = +13.6 +/- 1.3 mmHg). The changes in renal nerve activity and respiratory rate caused by L-glutamate were +45 +/- 11 and +42 +/- 9% (DRN; P < 0.05%), +40 +/- 10 and +29 +/- 7% (MRN, P < 0.05), respectively. No significant changes were observed in saline-microinjected animals. This study shows that: a) the blood pressure increases previously observed by electrical stimulation within the raphe are due to activation of local neurons, b) this pressor effect is due to sympathoexcitation because the stimulation increased renal sympathetic activity but did not produce tachycardia, and c) the stimulation of cell bodies in these nuclei also increases the respiratory rate.

Firing of 5-HT neurones in the dorsal and median raphe nucleus in vitro shows differential alpha1-adrenoceptor and 5-HT1A receptor modulation

The median raphe nucleus and dorsal raphe nucleus together are the major source of ascending 5-HT projections. Here, using in vitro extracellular single unit electrophysiology we examined the responses of individual neurones in the rat median raphe nucleus and dorsal raphe nucleus to alpha(1)-adrenoceptor and 5-HT(1A) receptor activation and made comparisons between the two nuclei. In the presence of the alpha(1)-adrenoceptor agonist phenylephrine (1microM) all spontaneously active neurones recorded in the median and dorsal raphe nuclei fired slowly (<5Hz) and regularly. Most were inhibited by 5-HT (10-50microM), although a few were excited by 5-HT. 5-HT-induced inhibition was attenuated by the 5-HT(1A) receptor antagonist, WAY100635 (100nM). Compared to those in the dorsal raphe nucleus, the neurones in the median raphe nucleus which were inhibited by 5-HT had: (1) lower basal firing rates in the continuous presence of phenylephrine (1microM), (2) smaller excitatory responses to higher concentrations of phenylephrine (3-10microM), (3) smaller excitatory responses to brief application of norepinephrine (10-100microM) and (4) smaller inhibitory responses to 5-HT (10-50microM). The lower sensitivity of median raphe neurones to alpha(1)-adrenoceptor excitation and 5-HT(1A) receptor inhibition will have consequences for 5-HT neurotransmission in forebrain regions innervated by the two nuclei.

Sensorimotor-related discharge of simultaneously recorded, single neurons in the dorsal raphe nucleus of the awake, unrestrained rat

Multi-channel, multi-neuron recording procedures were used to monitor simultaneously the spike train activity of single neurons (n=7-16 cells/animal) in the dorsal raphe (DR) nucleus of the awake, freely moving rat. Putative serotonergic and non-serotonergic neurons were distinguished from one another on the basis of established criteria, i.e. waveform shape and duration, firing pattern and firing frequency. As a group, presumed serotonergic neurons exhibited low tonic discharge rates, depressed firing after serotonin (5HT)-1a agonist administration, and, except for the transition from sleep to waking, a general insensitivity to specific sensory or motor events. By contrast, non-serotonergic cells in midline and lateral wing sub-regions of the nucleus displayed responses to a variety of sensorimotor events including locomotion, grooming, head movement, chewing, auditory stimuli, and whisker movement (both passive and active). However, within this latter group, the sensorimotor response repertoire of individual cells was not uniform. Likewise, non-5HT cells with diverse response profiles were identified in both medial and lateral sub-regions of the nucleus. Cells categorized as non-serotonergic also had varied responses to 5HT1a agonist administration. These results emphasize the diverse input/output relationships of individual DR neurons and underscore the need for a more comprehensive analysis of such properties under waking conditions in order to obtain a better understanding of the role of the DR nucleus in brain function.

The human raphe nuclei and the serotonergic system

The raphe nuclei are distributed near the midline of the brainstem along its entire rostro-caudal extension. The serotonergic neurons are their main neuronal components, although a proportion of them lie in subdivisions of the lateral reticular formation. They develop from mesopontine and medullary primordia, and the resulting grouping into rostral and caudal clusters is maintained into adulthood, and is reflected in the connectivity. Thus, the mesencephalon and rostral pons, neurons within the rostral raphe complex (caudal linear, dorsal raphe, and median raphe nuclei) project primarily to the forebrain. By contrast, in the caudal pons and medulla oblongata, neurons within the caudal raphe complex (raphe magnus, raphe obscurus, raphe pallidus nuclei and parts of the adjacent lateral reticular formation) project to the brainstem nuclei and to the spinal cord. The median raphe and dorsal raphe nuclei provide parallel and overlapping projections to many forebrain structures with axon fibers exhibiting distinct structural and functional characteristics. The caudal group of the serotonergic system projects to the brainstem, and, by three parallel projections, to the dorsal, intermediate and ventral columns in the spinal cord. The serotonergic axons arborize over large areas comprising functionally diverse targets. Some projections form classical chemical synapses while many do not, thus contributing to the so-called paracrine or volume transmission. The serotonergic projections participate in the regulation of different functional (motor, somatosensory, limbic) systems; and have been associated with a wide range of neuropsychiatric and neurological disorders. Finally, recent experimental data support the role of serotonin in modulating brain development, such that a dysfunction in serotonergic transmission during early life could lead to long lasting structural and functional alterations.

Median and dorsal raphe neurons are not electrophysiologically identical

The dorsal (DR) and median raphe (MR) nuclei contain 5-hydroxytryptamine (serotonin, 5-HT) cell bodies that give rise to the majority of the ascending 5-HT projections to the forebrain limbic areas that control emotional behavior. In the past, the electrophysiological identification of neurochemically identified 5-HT neurons has been limited. Recent technical developments have made it possible to re-examine the electrophysiological characteristics of identified 5-HT- and non-5-HT-containing neurons. Visualized whole cell electrophysiological techniques in combination with fluorescence immunohistochemistry for 5-HT were used. In the DR, both 5-HT- and non-5-HT-containing neurons exhibited similar characteristics that have historically been attributed to putative 5-HT neurons. In contrast, in the MR, the 5-HT-and non-5-HT-containing neurons had very different characteristics. Interestingly, the MR 5-HT-containing neurons had a shorter time constant and larger afterhyperpolarization (AHP) amplitude than DR 5-HT-containing neurons. The 5-HT(1A) receptor-mediated response was also measured. The efficacy of the response elicited by 5-HT(1A) receptor activation was greater in 5-HT-containing neurons in the DR than the MR, whereas the potency was similar, implicating greater autoinhibition in the DR. Non-5-HT-containing neurons in the DR were responsive to 5-HT(1A) receptor activation, whereas the non-5-HT-containing neurons in the MR were not. These differences in the cellular characteristics and 5-HT(1A) receptor-mediated responses between the MR and DR neurons may be extremely important in understanding the role of these two 5-HT circuits in normal physiological processes and in the etiology and treatment of pathophysiological states.

Distinguishing characteristics of serotonin and non-serotonin-containing cells in the dorsal raphe nucleus: electrophysiological and immunohistochemical studies

The membrane properties and receptor-mediated responses of rat dorsal raphe nucleus neurons were measured using intracellular recording techniques in a slice preparation. After each experiment, the recorded neuron was filled with neurobiotin and immunohistochemically identified as 5-hydroxytryptamine (5-HT)-immunopositive or 5-HT-immunonegative. The cellular characteristics of all recorded neurons conformed to previously determined classic properties of serotonergic dorsal raphe nucleus neurons: slow, rhythmic activity in spontaneously active cells, broad action potential and large afterhyperpolarization potential. Two electrophysiological characteristics were identified that distinguished 5-HT from non-5-HT-containing cells in this study. In 5-HT-immunopositive cells, the initial phase of the afterhyperpolarization potential was gradual (tau=7.3+/-1.9) and in 5-HT-immunonegative cells it was abrupt (tau=1.8+/-0.6). In addition, 5-HT-immunopositive cells had a shorter membrane time constant (tau=21.4+/-4.4) than 5-HT-immunonegative cells (tau=33.5+/-4.2). Interestingly, almost all recorded neurons were hyperpolarized in response to stimulation of the inhibitory 5-HT(1A) receptor. These results suggested that 5-HT(1A) receptors are present on non-5-HT as well as 5-HT neurons. This was confirmed by immunohistochemistry showing that although the majority of 5-HT-immunopositive cells in the dorsal raphe nucleus were double-labeled for 5-HT(1A) receptor-IR, a small but significant population of 5-HT-immunonegative cells expressed the 5-HT(1A) receptor. These results underscore the heterogeneous nature of the dorsal raphe nucleus and highlight two membrane properties that may better distinguish 5-HT from non-5-HT cells than those typically reported in the literature. In addition, these results present electrophysiological and anatomical evidence for the presence of 5-HT(1A) receptors on non-5-HT neurons in the dorsal raphe nucleus.

Sibutramine, a serotonin uptake inhibitor, increases dopamine concentrations in rat striatal and hypothalamic extracellular fluid

We measured, using microdialysis, the effects of sibutramine, given intraperitoneally, on brain dopamine and serotonin flux into striatal and hypothalamic dialysates of freely moving rats, and on the uptake of [(3)H]-DA into striatal synaptosomes. For microdialysis experiments, samples collected every 30 min were assayed by high-pressure liquid chromatography, in a single run. Administration of a low dose of sibutramine (2.0 mg/kg, i.p) had no effect on dopamine or serotonin concentrations in striatal dialysates but higher doses increased both: 5 mg/kg increased these concentrations to 196+/-24% (p<0.01) and 221+/-28% (p<0.01) of baseline, respectively; 10 mg/kg increased dopamine to 260+/-66% (p<0.01) and serotonin to 160+/-20% (p<0.05) of baseline. In hypothalamus, the 5 mg/kg sibutramine dose increased the dopamine concentration to 186+/-40% (p<0.05) and that of serotonin to 312+/-86% (p<0.01) of baseline, while the 10 mg/kg (i.p.) dose increased dopamine to 392+/-115% (p<0.01), and serotonin to 329+/-104% (p<0.01) of baseline. In vitro, sibutramine blocked [(3)H]-dopamine uptake into striatal synaptosomes, with an IC(50) value of 3.8 microM. These findings indicate that sibutramine has at least as great an effect on brain extracellular dopamine levels as on brain serotonin, and suggest that the drug's antiobesity action may result from the changes it produces in brain dopamine as well as serotonin metabolism.

Effects of fenfluramine and phentermine (fen-phen) on dopamine and serotonin release in rat striatum: in vivo microdialysis study in conscious animals

We measured the effects of acute or chronic administration of fenfluramine and phentermine, alone or in combination, on brain dopamine and serotonin release into striatal dialysates of freely moving rats. Samples collected every 30 min were assayed in a single run by high-pressure liquid chromatography. Acute or chronic administration of fenfluramine (1 mg/kg, i.p.) did not significantly change dopamine concentrations in rat striatal dialysates, but increased those of serotonin by 182% (acute) and 124% (chronic). Phentermine (2 mg/kg, i.p.), on the other hand, significantly increased dopamine concentrations by 52% (acute) and 80% (chronic) without affecting those of serotonin. Administration of the drugs in combination (fenfluramine 1 mg/kg and phentermine 2 mg/kg) amplified the effects of each, increasing striatal dopamine concentrations by 209% (acute) and serotonin concentrations by 330% (acute) and 299% (chronic).