Control of the serotonergic system by the medial prefrontal cortex: potential role in the etiology of PTSD and depressive disorders

Intranasal delivery of shRNA to knockdown the 5HT-2A receptor enhances memory and alleviates anxiety

Short-hairpin RNAs (shRNA), targeting knockdown of specific genes, hold enormous promise for precision-based therapeutics to treat numerous neurodegenerative disorders. However, whether shRNA constructed molecules can modify neuronal circuits underlying certain behaviors has not been explored. We designed shRNA to knockdown the human HTR2A gene in vitro using iPSC-differentiated neurons. Multi-electrode array (MEA) results showed that the knockdown of the 5HT-2A mRNA and receptor protein led to a decrease in spontaneous electrical activity. In vivo, intranasal delivery of AAV9 vectors containing shRNA resulted in a decrease in anxiety-like behavior in mice and a significant improvement in memory in both mice (104%) and rats (92%) compared to vehicle-treated animals. Our demonstration of a non-invasive shRNA delivery platform that can bypass the blood-brain barrier has broad implications for treating numerous neurological mental disorders. Specifically, targeting the HTR2A gene presents a novel therapeutic approach for treating chronic anxiety and age-related cognitive decline.

The Neurodegenerative Elderly Syndrome (NES) hypothesis: Alzheimer and Parkinson are two faces of the same disease

Increasing evidence suggests that Alzheimer's disease (AD) and Parkinson's disease (PD) share monoamine and alpha-synuclein (αSyn) dysfunctions, often beginning years before clinical manifestations onset. The triggers for these impairments and the causes leading these early neurodegenerative processes to become AD or PD remain unclear. We address these issues by proposing a radically new perspective to frame AD and PD: they are different manifestations of one only disease we call "Neurodegenerative Elderly Syndrome (NES)". NES goes through three phases. The seeding stage, which starts years before clinical signs, and where the part of the brain-body affected by the initial αSyn and monoamine dysfunctions, influences the future possible progression of NES towards PD or AD. The compensatory stage, where the clinical symptoms are still silent thanks to compensatory mechanisms keeping monoamine concentrations homeostasis. The bifurcation stage, where NES becomes AD or PD. We present recent literature supporting NES and discuss how this hypothesis could radically change the comprehension of AD and PD comorbidities and the design of novel system-level diagnostic and therapeutic actions.

Mental Health and the Symptoms of PTSD in People with Depression and Anxiety Disorders during the COVID-19 Pandemic

The purpose of this study was to examine the severity of post-traumatic stress disorder (PTSD) symptoms related to the COVID-19 pandemic in people with no diagnosis of mental illness, as well as in people who were diagnosed with depression or anxiety. Moreover, this study aimed to investigate the interplay between PTSD symptoms and self-assessed mental health associated with well-being. The 210 participants were divided into 3 groups: mentally healthy, participants with diagnosed depression, and participants with anxiety disorders. To evaluate the subjective well-being of the participants, the Polish adaptation of the Mental Health Continuum-Short Form (MHC-SF) was applied. The Impact Event Scale-Revised (IES-R) was used to measure the severity of PTSD symptoms. At least a moderate worsening of PTSD symptoms was observed in participants of all groups. The results were as follows: healthy participants M = 37.35 (SD = 18.46); participants with depression M = 36.05 (SD = 18.02); participants with anxiety M = 44.52 (SD = 18.08). The participants diagnosed with depression showed the lowest level of mental well-being M = 41.58 (SD = 15.02). Conclusion: People diagnosed with depression had both the lowest level of well-being and the lowest severity of symptoms specific to PTSD. In all three groups, lower emotional well-being was linked to greater PTSD symptoms.

Systemic Inhibition of Soluble Tumor Necrosis Factor with XPro1595 Exacerbates a Post-Spinal Cord Injury Depressive Phenotype in Female Rats

Spinal cord injury (SCI) is associated with a three-fold risk of major depressive disorder compared with the general population. Current antidepressant therapy is often not as effective in this patient population, suggesting the need for a more efficacious therapeutic target. The goal of this study was to elucidate the role of inflammatory cytokine tumor necrosis factor (TNF) in the dorsal raphe nucleus (DRN, the principle source of serotonin to the brain) in the development and possible treatment of depression after SCI. A depressive phenotype following moderate T9 contusion was identified in adult female rats using a battery of behavioral tests (forced swim test, sucrose preference test, novel object recognition test, open field locomotion, and social exploration). Data revealed two clusters of injured rats (58%) that exhibit increased immobility in the forced swim test, indicating depressive phenotype or a melancholic-depressive phenotype with concomitant decrease in sucrose preference. ElevatedTNF levels in the DRN of these two clusters correlated with increased immobility in the forced swim test. We then tested the efficacy of soluble TNF inhibition with XPro1595 treatment to prevent the depressive phenotype after SCI. Subcutaneous (s.c.) delivery of XPro1595 caused an exacerbation of depressive phenotype, with all treated clusters exhibiting increased forced swim immobility compared with saline-treated non-depressed rats. Intracerebroventricular (i.c.v.) administration of the drug did not prevent or enhance the development of depression after injury. These results suggest a complex role for TNF-based neuroinflammation in SCI-induced depression that needs to be further explored, perhaps in conjunction with a broader targeting of additional post-SCI inflammatory cytokines.

Serotonin Regulation of the Prefrontal Cortex: Cognitive Relevance and the Impact of Developmental Perturbation

The prefrontal cortex is essential for both executive function and emotional regulation. The interrelationships among these behavioral domains are increasingly recognized, as well as their sensitivity to serotonin (5-hydroxytryptamine, 5-HT). Prefrontal cortex receives serotonergic inputs from the dorsal and median raphe nuclei and is modulated by multiple subtypes of 5-HT receptor across its layers and cell types. Extremes of serotonergic modulation alter mood regulation in vulnerable individuals, yet the impact of serotonin under more typical physiological parameters remains unclear. In this regard, new tools are permitting a closer examination of the behavioral impact of the serotonin system. Optogenetic and chemogenetic manipulations of dorsal raphe 5-HT neurons reveal that serotonin has a greater impact on executive function than previously appreciated. Domains that appear sensitive to fluctuations in 5-HT neuronal excitability include patience and cognitive flexibility. This work is broadly consistent with ex vivo research investigating how 5-HT regulates prefrontal cortex and its output projections. A growing literature suggests 5-HT modulation of these prefrontal circuits is unexpectedly flexible to alteration during development by genetic, behavioral, environmental or pharmacological manipulations, with lasting repercussions for cognition and emotional regulation. Here, we review the cellular and circuit mechanisms of prefrontal serotonergic modulation, investigate recent research into the cognitive consequences of the serotonergic system, and probe the lasting consequences of developmental perturbations. Understanding both the complexity of the prefrontal serotonin system and its sensitivity during development are essential to learn more about the vulnerabilities of this system in mood and anxiety disorders and the underappreciated cognitive consequences of these disorders and their treatment.

Serotonin and motherhood: From molecules to mood

Emerging research points to a valuable role of the monoamine neurotransmitter, serotonin, in the display of maternal behaviors and reproduction-associated plasticity in the maternal brain. Serotonin is also implicated in the pathophysiology of numerous affective disorders and likely plays an important role in the pathophysiology of maternal mental illness. Therefore, the main goals of this review are to detail: (1) how the serotonin system of the female brain changes across pregnancy and postpartum; (2) the role of the central serotonergic system in maternal caregiving and maternal aggression; and (3) how the serotonin system and selective serotonin reuptake inhibitor medications (SSRIs) are involved in the treatment of maternal mental illness. Although there is much work to be done, studying the central serotonin system's multifaceted role in the maternal brain is vital to our understanding of the processes governing matrescence and the maintenance of motherhood.

Stress-Induced Functional Alterations in Amygdala: Implications for Neuropsychiatric Diseases

The amygdala plays a major role in the processing of physiologic and behavioral responses to stress and is characterized by gamma-aminobutyric acid (GABA)-mediated high inhibitory tone under resting state. Human and animal studies showed that stress lead to a hyperactivity of amygdala, which was accompanied by the removal of inhibitory control. However, the contribution of hyperactivity of amygdala to stress-induced neuropsychiatric diseases, such as anxiety and mood disorders, is still dubious. In this review, we will summarize stress-induced various structural and functional alterations in amygdala, including the GABA receptors expression, GABAergic transmission and synaptic plasticity. It may provide new insight on the neuropathologic and neurophysiological mechanisms of neuropsychiatric diseases.

Serotonin 5-HT4 receptors modulate the development of glutamatergic input to the dorsal raphe nucleus

The dorsal raphe nucleus (DRN) is a major serotonin (5-hydroxytryptamine, 5-HT)-producing region in the central nervous system. It receives glutamatergic inputs from several brain regions, which are reciprocally modulated by serotonergic signals. We investigated whether serotonin 5-HT₄ receptors (5-HT₄Rs) play a role in the development of glutamatergic control of the DRN, with an emphasis on cortical inputs. Double-label immunohistochemistry and confocal microscopy were used to quantify vesicular glutamate transporter 1 (vGluT1)-immunoreactive terminals in the DRN of mice with a null-mutation in the 5-HT₄R gene. We found no significant change in the overall density of vGluT1-positive terminals in homozygous and heterozygous mice, but heterozygous mice had a significantly higher density of vGluT1-positive terminals contacting serotonergic neurons. These results suggest that altered 5-HT₄R expression may affect the development of cortical glutamatergic control of the DRN.

Long-Term Citalopram Treatment Alters the Stress Responses of the Cortical Dopamine and Noradrenaline Systems: the Role of Cortical 5-HT1A Receptors

BACKGROUND

Cortical dopamine and noradrenaline are involved in the stress response. Citalopram, a selective serotonin reuptake inhibitor, has direct and indirect effects on the serotonergic system. Furthermore, long-term treatment with citalopram affects the dopamine and noradrenaline systems, which could contribute to the therapeutic action of antidepressants.

METHODS

The effects of long-term treatment with citalopram on the responses of the dopamine and noradrenaline systems in the rat prefrontal cortex to acute handling stress were evaluated using in vivo microdialysis.

RESULTS

Acute handling stress increased dopamine and noradrenaline levels in the prefrontal cortex. The dopamine and noradrenaline responses were suppressed by local infusion of a 5-HT1A receptor agonist, 7-(Dipropylamino)-5,6,7,8-tetrahydronaphthalen-1-ol;hydrobromide, into the prefrontal cortex. The dopamine response was abolished by long-term treatment with citalopram, and the abolished dopamine response was reversed by local infusion of a 5-HT1A receptor antagonist, (Z)-but-2-enedioic acid;N-[2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl]-N-pyridin-2-ylcyclohexanecarboxamide into the prefrontal cortex. On the other hand, long-term treatment with citalopram reduced the basal noradrenaline levels (approximately 40% of the controls), but not the basal dopamine levels. The noradrenaline response was maintained despite the low basal noradrenaline levels. Signaling from the 5-HT1A receptors and α2-adrenoceptors was not involved in the decrease in the basal noradrenaline levels but partially affected the noradrenaline response.

CONCLUSIONS

Chronic citalopram treatment differentially suppresses the dopamine and noradrenaline systems in the prefrontal cortex, and the dopamine stress response was preferentially controlled by upregulating 5-HT1A receptor signaling. Our findings provide insight into how antidepressants modulate the dopamine and noradrenaline systems to overcome acute stress.

Serotonin 2a Receptor and Serotonin 1a Receptor Interact Within the Medial Prefrontal Cortex During Recognition Memory in Mice

Episodic memory, can be defined as the memory for unique events. The serotonergic system one of the main neuromodulatory systems in the brain appears to play a role in it. The serotonin 2a receptor (5-HT2aR) one of the principal post-synaptic receptors for 5-HT in the brain, is involved in neuropsychiatric and neurological disorders associated with memory deficits. Recognition memory can be defined as the ability to recognize if a particular event or item was previously encountered and is thus considered, under certain conditions, a form of episodic memory. As human data suggest that a constitutively decrease of 5-HT2A signaling might affect episodic memory performance we decided to compare the performance of mice with disrupted 5-HT2aR signaling (htr2a^−/−) with wild type (htr2a^+/+) littermates in different recognition memory and working memory tasks that differed in the level of proactive interference. We found that ablation of 5-HT2aR signaling throughout development produces a deficit in tasks that cannot be solved by single item strategy suggesting that 5-HT2aR signaling is involved in interference resolution. We also found that in the absence of 5-HT2aR signaling serotonin has a deleterious effect on recognition memory retrieval through the activation of 5-HT1aR in the medial prefrontal cortex.

Adult AMPA GLUA1 receptor subunit loss in 5-HT neurons results in a specific anxiety-phenotype with evidence for dysregulation of 5-HT neuronal activity

Both the glutamatergic and serotonergic (5-HT) systems are implicated in the modulation of mood and anxiety. Descending cortical glutamatergic neurons regulate 5-HT neuronal activity in the midbrain raphe nuclei through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors. To analyze the functional role of GLUA1-containing AMPA receptors in serotonergic neurons, we used the Cre-ERT2/loxP-system for the conditional inactivation of the GLUA1-encoding Gria1 gene selectively in 5-HT neurons of adult mice. These Gria1(5-HT-/-) mice exhibited a distinct anxiety phenotype but showed no alterations in locomotion, depression-like behavior, or learning and memory. Increased anxiety-related behavior was associated with significant decreases in tryptophan hydroxylase 2 (TPH2) expression and activity, and subsequent reductions in tissue levels of 5-HT, its metabolite 5-hydroxyindoleacetic acid (5-HIAA), and norepinephrine in the raphe nuclei. However, TPH2 expression and activity as well as monoamine levels were unchanged in the projection areas of 5-HT neurons. Extracellular electrophysiological recordings of 5-HT neurons revealed that, while α1-adrenoceptor-mediated excitation was unchanged, excitatory responses to AMPA were enhanced and the 5-HT1A autoreceptor-mediated inhibitory response to 5-HT was attenuated in Gria1(5-HT-/-) mice. Our data show that a loss of GLUA1 protein in 5-HT neurons enhances AMPA receptor function and leads to multiple local molecular and neurochemical changes in the raphe nuclei that dysregulate 5-HT neuronal activity and induce anxiety-like behavior.

A pilot study to investigate the induction and manipulation of learned helplessness in healthy adults

Eliminating the controllability of a noxious stimulus may induce a learned helplessness (LH) that resembles aspects of depression and post-traumatic stress disorder (PTSD). This study examined whether repetitive transcranial magnetic stimulation (rTMS) of the left dorsolateral prefrontal cortex (DLPFC) promotes resilience in an aversive stimulus model of LH. All 55 participants were told that an undisclosed sequence of button presses would terminate an aversive stimulus on their forearm. In truth, only half had control (+C). The other half had no control (-C). All participants received real (R) or sham (S) left DLPFC rTMS during the paradigm (+C/R, -C/S,+C/S,-C/R). We evaluated the cognitive effects of LH using an anagram task. The LH paradigm successfully reduced perceived control in the -C groups. As predicted, the +C/R and +C/S groups tended to give up less quickly and take less time to solve each anagram than did the -C/S group. Superior anagram performance in the -C/R group approached statistical significance. Our preliminary results suggest that manipulating the controllability of an aversive stimulus may induce an LH effect that manifests as impaired anagram performance. Further research is needed to refine this model and determine if DLPFC rTMS mitigates any LH effects.

Optogenetic modulation of descending prefrontocortical inputs to the dorsal raphe bidirectionally bias socioaffective choices after social defeat

It has been well established that modulating serotonin (5-HT) levels in humans and animals affects perception and response to social threats, however the circuit mechanisms that control 5-HT output during social interaction are not well understood. A better understanding of these systems could provide groundwork for more precise and efficient therapeutic interventions. Here we examined the organization and plasticity of microcircuits implicated in top-down control of 5-HT neurons in the dorsal raphe nucleus (DRN) by excitatory inputs from the ventromedial prefrontal cortex (vmPFC) and their role in social approach-avoidance decisions. We did this in the context of a social defeat model that induces a long lasting form of social aversion that is reversible by antidepressants. We first used viral tracing and Cre-dependent genetic identification of vmPFC glutamatergic synapses in the DRN to determine their topographic distribution in relation to 5-HT and GABAergic subregions and found that excitatory vmPFC projections primarily localized to GABA-rich areas of the DRN. We then used optogenetics in combination with cFos mapping and slice electrophysiology to establish the functional effects of repeatedly driving vmPFC inputs in DRN. We provide the first direct evidence that vmPFC axons drive synaptic activity and immediate early gene expression in genetically identified DRN GABA neurons through an AMPA receptor-dependent mechanism. In contrast, we did not detect vmPFC-driven synaptic activity in 5-HT neurons and cFos induction in 5-HT neurons was limited. Finally we show that optogenetically increasing or decreasing excitatory vmPFC input to the DRN during sensory exposure to an aggressor's cues enhances or diminishes avoidance bias, respectively. These results clarify the functional organization of vmPFC-DRN pathways and identify GABAergic neurons as a key cellular element filtering top-down vmPFC influences on affect-regulating 5-HT output.

Functional associations among G protein-coupled neurotransmitter receptors in the human brain

Background

The activity of neurons is controlled by groups of neurotransmitter receptors rather than by individual receptors. Experimental studies have investigated some receptor interactions, but currently little information is available about transcriptional associations among receptors at the whole-brain level.

Results

A total of 4950 correlations between 100 G protein-coupled neurotransmitter receptors were examined across 169 brain regions in the human brain using expression data published in the Allen Human Brain Atlas. A large number of highly significant correlations were found, many of which have not been investigated in hypothesis-driven studies. The highest positive and negative correlations of each receptor are reported, which can facilitate the construction of receptor sets likely to be affected by altered transcription of one receptor (such sets always exist, but their members are difficult to predict). A graph analysis isolated two large receptor communities, within each of which receptor mRNA levels were strongly cross-correlated.

Conclusions

The presented systematic analysis shows that the mRNA levels of many G protein-coupled receptors are interdependent. This finding is not unexpected, since the brain is a highly integrated complex system. However, the analysis also revealed two novel properties of global brain structure. First, receptor correlations are described by a simple statistical distribution, which suggests that receptor interactions may be guided by qualitatively similar processes. Second, receptors appear to form two large functional communities, which might be differentially affected in brain disorders.

Prefrontal/amygdalar system determines stress coping behavior through 5-HT/GABA connection

Coping is defined as the behavioral and physiological effort made to master stressful situations. The ability to cope with stress leads either to healthy or to pathogenic outcomes. The medial prefrontal cortex (mpFC) and amygdala are acknowledged as having a major role in stress-related behaviors, and mpFC has a critical role in the regulation of amygdala-mediated arousal in response to emotionally salient stimuli. Prefrontal cortical serotonin (5-hydroxytryptamine (5-HT)) is involved in corticolimbic circuitry, and GABA has a major role in amygdala functioning. Here, using mice, it was assessed whether amygdalar GABA regulation by prefrontal 5-HT is involved in processing stressful experiences and in determining coping outcomes. First (experiment 1), bilateral selective 5-HT depletion in mpFC of mice reduced GABA release induced by stress in basolateral amygdala (BLA) and passive coping in the Forced Swimming Test (FST) (experiment 2). Moreover, prefrontal-amygdala disconnection procedure that combined a selective unilateral 5-HT depletion of mpFC and infusion of an inhibitor of GABA synthesis into the contralateral BLA, thereby to disrupt prefrontal-amygdalar serial connectivity bilaterally, showed that disconnection selectively decreases immobility in the FST. These results point to prefrontal/amygdala connectivity mediated by 5-HT and GABA transmission as a critical neural mechanism in stress-induced behavior.

Prefrontal cortical anandamide signaling coordinates coping responses to stress through a serotonergic pathway

The endocannabinoid system has recently emerged as a vital component of the stress response and is an appealing target for the treatment of mood and anxiety disorders. Additionally, corticolimbic endocannabinoid signaling is important for stress-induced regulation of emotional behavior. However, the mechanism by which this occurs remains elusive. Combining biochemical and behavioral analyses within the forced swim test, we examined whether stress-induced regulation of endocannabinoid signaling in the medial prefrontal cortex contributes to behavioral responses to stress, and whether these responses are dependent on serotonergic neurotransmission. Forced swim stress produced a rapid and pronounced reduction in medial prefrontal anandamide content, but had no effect on 2-arachidonoylglycerol content within this region. Local administration of the anandamide hydrolysis inhibitor URB597 (0.01μg) into the ventromedial region of the prefrontal cortex decreased passive coping responses and increased active behavioral strategies, a phenomenon which was blocked by local antagonism of the CB(1) receptor. Furthermore, local inhibition of anandamide hydrolysis within the medial PFC increased the firing rate of serotonergic neurons within the dorsal raphe, suggesting that prefrontal cortical endocannabinoid signaling may modulate stress coping behaviors through a regulation of serotonergic neurotransmission. Accordingly, serotonin depletion prevented the ability of inhibition of anandamide hydrolysis within the medial PFC to promote active stress coping responses. Collectively, these data argue that stress-induced changes in endocannabinoid signaling within the medial PFC modulate stress-coping behaviors through a regulation of serotonergic neurotransmission and provide a neuroanatomical framework by which we may understand the mechanisms subserving the antidepressant potential of the endocannabinoid system.

Reduced mismatch negativity in posttraumatic stress disorder: a compensatory mechanism for chronic hyperarousal?

Mismatch Negativity (MMN) is yet poorly understood in the context of Posttraumatic Stress Disorder (PTSD, e.g. [Morgan 3rd, C.A., Grillon, C., 1999. Abnormal mismatch negativity in women with sexual assault-related posttraumatic stress disorder. Biol. Psychiatry 45, 827-832.]). PTSD symptoms like hyperarousal, emotional pressure and avoidance may interfere with pre-attentive sensory processing. We tested this in an optimized MMN design [Näätänen, R., Pakarinen, S., Rinne, T., Takegata, R. (2004) The mismatch negativity (MMN): towards the optimal paradigm. Clin. Neurophysiol. 115: 140-144.] with PTSD victims and a control group without PTSD. A group of PTSD subjects was compared with gender and age-matched, healthy comparison subjects without PTSD. A "memory trace" was elicited by frequently presented "standard" auditory stimuli (50% occurrence) of 1 kHz, 75 ms duration, intermittently with 8 rare "deviants", which differed in frequency (higher/lower), intensity (louder/softer), duration (shorter), direction (left/right) or by the presence of a gap in the sound. During presentation of tones a silent film was shown. Psychometric data were collected by SCID, BSI, Attentiveness Inventory, Edinburgh Handedness Questionnaire, and the PTSD Screening Scale by Breslau et al. [Breslau, N., Peterson, E.L., Kessler, R.C., Schultz, L.R. (1999) Short screening scale for DSM-IV posttraumatic stress disorder. Am. J. Psychiatry 156: 908-911.]. Group comparisons of the MMN were performed for left/right-frontal/temporal, and for midline electrode sites. A good differentiation of both groups was found in psychometric and electrophysiological data. The PTSD group revealed on most BSI scales enhanced values of psychic aberration. The amplitude of the MMN was significantly reduced in the PTSD compared to non-PTSD subjects. MMN was significantly correlated with the total PTSD score. The data suggest a reduction in pre-attentive auditory sensory memory in PTSD due to specific symptom variables such as hyperarousal, sleeplessness, impaired concentration and a general enhanced excitation of the nervous system. This protective inhibition is thought to be a fine-tuning process in PTSD in order to prevent arousal overload.

Lateral habenula lesions improve the behavioral response in depressed rats via increasing the serotonin level in dorsal raphe nucleus

The dorsal raphe nucleus (DRN)-serotonin (5-HT) system plays a key role in stress-related psychiatric disorders such as anxiety and depression. The habenular nucleus (Hb) is closely connected with the DRN both morphologically and functionally. Here, we used two types of depressive animal models by exposing rats to chronic mild stress (CMS) and by chronically administering the tricyclic antidepressant clomipramine (CLI) in the rat during the neonatal state of life to produce adult depressed rats. We investigated the effects of lateral habenular nucleus (LHb) lesions on the behavioral response and on the level of 5-HT in DRN in the depressed rats. Forced-swimming test (FST) showed that the immobility time decreased, and the climbing time increased after lesioning LHb of depressed rats. Microdialysis results indicated that the 5-HT level in DRN in depressed rats was lower than that of the control group. Lesion of the LHb was followed by an increased 5-HT turnover in the DRN. Our results suggested that the lesion of the LHb could improve the behavioral response of the depressed rats and the 5-HT level of the DRN increased by LHb lesions could be involved in the effects.

Cellular effects of swim stress in the dorsal raphe nucleus

Swim stress regulates forebrain 5-hydroxytryptamine (5-HT) release in a complex manner and its effects are initiated in the serotonergic dorsal raphe nucleus (DRN). The purpose of this study was to examine the effects of swim stress on the physiology of DRN neurons in conjunction with 5-HT immunohistochemistry. Basic membrane properties, 5-HT(1A) and 5-HT(1B) receptor-mediated responses and glutamatergic excitatory postsynaptic currents (EPSCs) were measured using whole-cell patch clamp techniques. Rats were forced to swim for 15min and 24h later DRN brain slices were prepared for electrophysiology. Swim stress altered the resting membrane potential, input resistance and action potential duration of DRN neurons in a neurochemical-specific manner. Swim stress selectively elevated glutamate EPSC frequency in 5-HT DRN neurons. Swim stress non-selectively reduced EPSC amplitude in all DRN cells. Swim stress elevated the 5-HT(1B) receptor-mediated inhibition of glutamatergic synaptic activity that selectively targeted 5-HT cells. Non-5-HT DRN neurons appeared to be particularly responsive to the effects of a milder handling stress. Handling elevated EPSC frequency, reduced EPSC decay time and enhanced a 5-HT(1B) receptor-mediated inhibition of mEPSC frequency selectively in non-5-HT DRN cells. These results indicate that swim stress has both direct, i.e., changes in membrane characteristics, and indirect effects, i.e., via glutamatergic afferents, on DRN neurons. These results also indicate that there are distinct local glutamatergic afferents to neurochemically specific populations of DRN neurons, and furthermore that these distinct afferents are differentially regulated by swim stress. These cellular changes may contribute to the complex effects of swim stress on 5-HT neurotransmission and/or the behavioral changes underlying the forced swimming test model of depression.

AMPA and NMDA receptor regulation of firing activity in 5-HT neurons of the dorsal and median raphe nuclei

The glutamatergic regulation of 5-hydroxytryptamine (5-HT) neuronal activity has not been extensively studied. Here, we used extracellular single unit recording in midbrain slices to examine glutamate receptor mediated effects on 5-HT neuronal activity in the dorsal raphe nucleus (DRN) and the median raphe nucleus (MRN). Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA; 1 and 3 microm) concentration-dependently increased firing in 5-HT neurons in both the DRN and the MRN. The response to AMPA was blocked by the AMPA receptor antagonist, 6,7-dinitroquinoxaline-2,3(1H-4H)-dione (DNQX; 10 microm) but not the N-methyl-d-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonopentanoic acid (AP-5; 50 microm). NMDA (10-100 microm) also increased 5-HT neuronal firing in a concentration-dependent manner in both the DRN and MRN; a response that was blocked by AP-5 (50 microm). In some DRN neurons the NMDA response was partially antagonized by DNQX (10 microm) suggesting that NMDA, as well as directly activating 5-HT neurons, evokes local release of glutamate, which indirectly activates AMPA receptors on 5-HT neurons. Responses of DRN 5-HT neurons to AMPA and NMDA were enhanced by the gamma-amino-butyric acid (GABA)(A) receptor antagonist, bicuculline (50 microm), suggesting that both AMPA and NMDA increase local release of GABA. Finally in the DRN the 5-HT(1A) receptor antagonist, WAY100635 (100 nm), failed to enhance the response of 5-HT neurons to AMPA and caused only a small increase in the excitatory response to NMDA suggesting a low degree of tonic activation of 5-HT(1A) autoreceptors even when 5-HT neuronal firing rate is high.

Reduced serotonin-1A receptor binding in social anxiety disorder

BACKGROUND

Results from studies in serotonin-1A (5-HT1A) knockout mice and previous positron emission tomography (PET) studies in humans imply a role for 5-HT1A receptors in normal state anxiety as well as in certain anxiety disorders. The objective of this study was to investigate 5-HT1A receptor binding potential (BP) in social anxiety disorder (SAD).

METHODS

Using PET and [carbonyl-11C]WAY-100635, we compared a homogeneous group of 12 unmedicated, male SAD patients with 18 healthy control subjects (HC). A multivariate ANOVA with all regional BP values as dependent variables, age and four radiochemical variables as covariates was performed.

RESULTS

We found a significantly lower 5-HT1A BP in several limbic and paralimbic areas but not in the hippocampus (p = .234) of SAD patients. The difference in 5-HT1A binding was most significant in the amygdala (-21.4%; p = .003). There was also a more than 20% lower 5-HT(1A) BP of SAD patients in the anterior cingulate cortex (p = .004), insula (p = .003), and dorsal raphe nuclei (p = .030).

CONCLUSIONS

The lower 5-HT1A binding in the amygdala and mesiofrontal areas of SAD patients is consistent with 1) preclinical findings of elevated anxiety in 5-HT1A knockout mice, 2) a previous PET study in healthy volunteers showing an inverse correlation between 5-HT1A BP and state anxiety, and 3) another human PET study in patients with panic disorder showing reduced 5-HT1A binding, thus corroborating the potential validity of 5-HT1A receptors as targets in the treatment of human anxiety disorders.

Selective 5-HT receptor inhibition of glutamatergic and GABAergic synaptic activity in the rat dorsal and median raphe

The dorsal (DR) and median (MR) raphe nuclei contain 5-hydroxytryptamine (5-HT) cell bodies that give rise to the majority of the ascending 5-HT projections to the forebrain. The DR and MR have differential roles in mediating stress, anxiety and depression. Glutamate and GABA activity sculpt putative 5-HT neuronal firing and 5-HT release in a seemingly differential manner in the MR and DR, yet isolated glutamate and GABA activity within the DR and MR has not been systematically characterized. Visualized whole-cell voltage-clamp techniques were used to record excitatory and inhibitory postsynaptic currents (EPSC and IPSC) in 5-HT-containing neurons. There was a regional variation in action potential-dependent (spontaneous) and basal [miniature (m)] glutamate and GABAergic activity. mEPSC activity was greater than mIPSC activity in the DR, whereas in the MR the mIPSC activity was greater. These differences in EPSC and IPSC frequency indicate that glutamatergic and GABAergic input have distinct cytoarchitectures in the DR and MR. 5-HT(1B) receptor activation decreased mEPSC frequency in the DR and the MR, but selectively inhibited mIPSC activity only in the MR. This finding, in concert with its previously described function as an autoreceptor, suggests that 5-HT(1B) receptors influence the ascending 5-HT system through multiple mechanisms. The disparity in organization and integration of glutamatergic and GABAergic input to DR and MR neurons and their regulation by 5-HT(1B) receptors may contribute to the distinction in MR and DR regulation of forebrain regions and their differential function in the aetiology and pharmacological treatment of psychiatric disease states.

NMDA receptors trigger neurosecretion of 5-HT within dorsal raphe nucleus of the rat in the absence of action potential firing

Activity and calcium-dependent release of neurotransmitters from the somatodendritic compartment is an important signalling mechanism between neurones throughout the brain. NMDA receptors and vesicles filled with neurotransmitters occur in close proximity in many brain areas. It is unknown whether calcium influx through these receptors can trigger the release of somatodendritic vesicles directly, or whether postsynaptic action potential firing is necessary for release of these vesicles. Here we addressed this question by studying local release of serotonin (5-HT) from dorsal raphé nucleus (DRN) neurones. We performed capacitance measurements to monitor the secretion of vesicles in giant soma patches, in response to short depolarizations and action potential waveforms. Amperometric measurements confirmed that secreted vesicles contained 5-HT. Surprisingly, two-photon imaging of DRN neurones in slices revealed that dendritic calcium concentration changes in response to somatic firing were restricted to proximal dendritic areas. This implied that alternative calcium entry pathways may dominate the induction of vesicle secretion from distal dendrites. In line with this, transient NMDA receptor activation, in the absence of action potential firing, was sufficient to induce capacitance changes. By monitoring GABAergic transmission onto DRN 5-HT neurones in slices, we show that endogenous NMDA receptor activation, in the absence of postsynaptic firing, induced release of 5-HT, which in turn increased the frequency of GABAergic inputs through activation of 5-HT(2) receptors. We propose here that calcium influx through NMDA receptors can directly induce postsynaptic 5-HT release from DRN neurones, which in turn may facilitate GABAergic input onto these cells.

Photostimulation alters c-Fos expression in the dorsal raphe nucleus

Retinal afferents to the dorsal raphe nucleus (DRN) have been described in a number of species, including Mongolian gerbils, but functional correlates of this optic pathway are unknown at present. To determine whether temporally modulated photostimulation can affect c-Fos expression in the gerbil DRN, quantitative analysis of c-Fos-immunoreactive (c-Fos-ir) neurons was conducted following 60-min exposure to pulsed (2 Hz) photostimulation at selected times over the 12:12 h light/dark cycle. For comparison, c-Fos expression was also analyzed in the subnuclei of the lateral geniculate complex and in the suprachiasmatic nucleus (SCN). In the DRN, a substantial reduction was observed in the number of c-Fos immunoreactive (c-Fos-ir) neurons during the light period and early dark period in photostimulated vs. control animals. Similar results were obtained in the intergeniculate leaflet (IGL) and ventral lateral geniculate (VLG). However, no significant changes were observed in the number of c-Fos-ir neurons in the dorsal lateral geniculate nucleus or suprachiasmatic nucleus (SCN) following photostimulation, except for an increase in the middle of the dark period. These findings indicate that photic stimulation can lead to a suppression or down-regulation of c-Fos expression in the DRN that is probably mediated via the direct retinal pathway to the DRN in this species. The similarity between c-Fos expression profiles in the DRN and IGL/VGL suggest that efferent projections from the DRN may modulate c-Fos expression to visual stimulation in these subnuclei of the lateral geniculate complex.

Prefrontal cortex serotonin, stress, and morphine-induced nucleus accumbens dopamine

Uncontrollable, but not controllable, stress produces a persistent potentiation of morphine-induced nucleus accumbens dopamine (DA) efflux and morphine-induced medial prefrontal cortex serotonin (5-HT) efflux. Here we investigate medial prefrontal cortex 5-HT mediation of this potentiation. Male Sprague-Dawley rats received bilateral medial prefrontal cortex microinjections of the neurotoxin 5,7-dihydroxytriptamine (5,7-DHT, 8 microg/microl/side), which selectively depleted medial prefrontal cortex 5-HT, or vehicle (Sham), and cannula implantation in the nucleus accumbens shell. After 2 weeks, rats received either uncontrollable stress or no stress. Microdialysis and morphine (3 mg/kg) treatment were performed the following day. Morphine produced an enhanced increase in DA in the Stress-Sham group that was completely blocked by 5,7-DHT lesions, suggesting that 5-HT in the medial prefrontal cortex mediates this potentiation.

Frontocortical 5-HT4 receptors exert positive feedback on serotonergic activity: viral transfections, subacute and chronic treatments with 5-HT4 agonists

BACKGROUND

We recently identified a facilitory control exerted by serotonin4 (5-HT4) receptors on the in vivo firing activity of dorsal raphe nucleus (DRN) serotonergic (5-HT) neurons. However, these findings were based on acute administrations of 5-HT4 receptor agonists and antagonists, which were active only in a subpopulation of 5-HT neurons. We had no evidence that this influence was significant when considering the entire DRN, nor if it was persistent after chronic treatments. In addition, the poor distribution of 5-HT4 receptors within the DRN raised the question of the neuroanatomical bases underlying this control.

METHODS AND RESULTS

Here we show that the subacute intraperitoneal (IP) injection of the 5-HT4 receptor agonists prucalopride (2.5 mg/kg) and RS 67333 (1.5 mg/kg) 30 minutes before the beginning of recordings augment the mean firing rate of DRN neurons by 40% and 66%, respectively. These increases remain stable when the compounds are administered continuously during 3 and 21 days; the effects of the 3-day treatment are blocked by the 5-HT4 receptor antagonist GR 125487 (1000 microg/kg, intravenous [i.v.]). In addition, stereotaxic microinjections of herpes simplex viruses, transformed to overexpress 5-HT4 receptors, increase DRN 5-HT neuronal mean activity when performed in the medial prefrontal cortex (mPFC) but not in the striatum or in the hippocampus.

CONCLUSIONS

This finding suggests the existence of a 5-HT(4)-dependent activation of DRN that may involve the mPFC, unveiling the 5-HT4 receptor as a putative player in the physiopathology of several disorders related to central 5-HT dysfunction.

Gene-environment interplay in schizopsychotic disorders

Genetic studies have sought to identify subtypes or endophenotypes of schizophrenia in an effort to improve the reliability of findings. A number of chromosomal regions or genes have now been shown to have had replicated linkage to schizophrenia susceptibility. Molecules involved in neurodevelopment or neurotransmitter function are coded by many of the genes that have been implicated in schizophrenia. Studies of neurotransmitter function have identified, among others, a possible role for GABA, glutamate and dopamine in animal models of schizophrenia. GABA neurons that co-express the calcium binding protein parvalbumin have been implicated as have glutamatergic metabotropic receptors and dopamine D3 receptors. Stress influences glutamate and dopamine providing another environmental factor that may interact with the influence of genes on neurotransmitter function. Neurotransmitter interactions include influences on signaling molecules and these too have been implicated in forms of learning thought to be affected in schizophrenia. Results continue to unravel the interplay of genes and environment in the etiology of schizophrenia and other psychotic disorders.

Neurodevelopmental liabilities in schizophrenia and affective disorders

There is now considerable evidence that both schizophrenia and affective disorders have their origin at least in part in events that occur during early pre- and post-natal development. In the case of schizophrenia, many observations, for example, increased risk for schizophrenia in the offspring of mothers who had influenza A during their second trimester of pregnancy and evidence for abnormal neuronal migration in the cerebral cortex of post mortem tissue from schizophrenic patients, suggest that a second trimester insult may have occurred and that this insult may have increased the risk for the development of schizophrenia in late adolescence or early adulthood. Animal studies have found that rats that undergo exocitotoxic damage to the ventral hippocampus on postnatal day 7 develop exaggerated sensitivity to dopamine-stimulating drugs or to stressful stimuli that becomes apparent after sexual maturity but not before, providing a neurodevelopmental model of schizophrenia. Similarly, post-weaning social isolation leads to enhanced responses to dopaminergic drugs and to stress that emerges after sexual maturity. These animal models are proving to be valuable tools to study the neurobiological mechanisms mediating the influence of early insults to the nervous system on later behavioural functions. In the case of affective disorders, although the evidence is not as strong, a number of the same observations have been made suggesting that an insult during early ontogeny may lead to the development of affective disorders later in life. For example, retrospective studies of people with affective disorders showed that they were more likely to have attained motor milestones at a later age and to have had poorer academic performance as children. There is a wealth of evidence suggesting hyperfunctioning of the hypothalamic-pituitary-adrenal (HPA) axis in affective disorders. Animal studies have shown that early maternal deprivation can lead to lasting changes in the reactivity of the HPA axis to stressful stimuli, providing another link from early experience to adult psychopathology. Continued studies of the effects of pre- and early post-natal events on the development of the nervous system and the relationships of these events to schizophrenia or affective disorder will provide new insights into the mechanisms underlying these common neuropsychiatric illnesses.