Origin of projections from the midbrain raphe nuclei to the hypothalamic paraventricular nucleus in the rat: a combined retrograde and anterograde tracing study

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.

Stress Adaptation and the Brainstem with Focus on Corticotropin-Releasing Hormone

Stress adaptation is of utmost importance for the maintenance of homeostasis and, therefore, of life itself. The prevalence of stress-related disorders is increasing, emphasizing the importance of exploratory research on stress adaptation. Two major regulatory pathways exist: the hypothalamic–pituitary–adrenocortical axis and the sympathetic adrenomedullary axis. They act in unison, ensured by the enormous bidirectional connection between their centers, the paraventricular nucleus of the hypothalamus (PVN), and the brainstem monoaminergic cell groups, respectively. PVN and especially their corticotropin-releasing hormone (CRH) producing neurons are considered to be the centrum of stress regulation. However, the brainstem seems to be equally important. Therefore, we aimed to summarize the present knowledge on the role of classical neurotransmitters of the brainstem (GABA, glutamate as well as serotonin, noradrenaline, adrenaline, and dopamine) in stress adaptation. Neuropeptides, including CRH, might be co-localized in the brainstem nuclei. Here we focused on CRH as its role in stress regulation is well-known and widely accepted and other CRH neurons scattered along the brain may also complement the function of the PVN. Although CRH-positive cells are present on some parts of the brainstem, sometimes even in comparable amounts as in the PVN, not much is known about their contribution to stress adaptation. Based on the role of the Barrington’s nucleus in micturition and the inferior olivary complex in the regulation of fine motoric—as the main CRH-containing brainstem areas—we might assume that these areas regulate stress-induced urination and locomotion, respectively. Further studies are necessary for the field.

The regulation of liver cytochrome P450 expression and activity by the brain serotonergic system in different experimental models

Introduction: Cytochrome P450 (CYP) metabolizes vital endogenous (steroids, vitamins) and exogenous (drugs, toxins) substrates. Studies of the last decade have revealed that the brain dopaminergic and noradrenergic systems are involved in the regulation of CYP. Recent research indicates that the brain serotonergic system is also engaged in its regulation.Areas covered: This review focuses on the role of the brain serotonergic system in the regulation of liver CYP expression. It shows the effect of lesion and activation of the serotonergic system after peripheral or intracerebral injections of neurotoxins, serotonin precursor, or serotonin (5-HT) receptor agonists. An opposite role of the hypothalamic paraventricular and arcuate nuclei and 5-HT receptors present therein in the regulation of CYP is described. The engagement of those nuclei in the neuroendocrine regulation of CYP by hypothalamic releasing or inhibiting hormones, pituitary hormones, and peripheral gland hormones are shown.Expert opinion: In general, the brain serotonergic system negatively regulates liver cytochrome P450. However, the effects of serotonergic agents on the enzyme expression depend on their mechanism of action, the route of administration (intracerebral/peripheral), as well as on local intracerebral site of injection and 5-HT receptor-subtypes present therein.

Perinatal exposure to bisphenol A at the intersection of stress, anxiety, and depression

Endocrine-disrupting compounds (EDCs) are common contaminants in our environment that interfere with typical endocrine function. EDCs can act on steroid and nuclear receptors or alter hormone production. One particular EDC of critical concern is bisphenol A (BPA) due to its potential harm during the perinatal period of development. Previous studies suggest that perinatal exposure to BPA alters several neurotransmitter systems and disrupts behaviors associated with depression and anxiety in the rodent offspring later in life. Thus, dysregulation in neurotransmission may translate to behavioral phenotypes observed in mood and arousal. Many of the systems disrupted by BPA also overlap with the stress system, although little evidence exists on the effects of perinatal BPA exposure in relation to stress and behavior. The purpose of this review is to explore studies involved in perinatal BPA exposure and the stress response at neurochemical and behavioral endpoints. Although more research is needed, we suggest that perinatal BPA exposure is likely inducing variations in behavioral phenotypes that modulate their action through dysregulation of neurotransmitter systems sensitive to stress and endocrine disruption.

Stimulation of 5-HT2C serotonin receptor subtype in the hypothalamic arcuate nuclei (ARC) increases the cytochrome P450 activity in the liver

BACKGROUND

Our previous study has demonstrated that activation of the 5-HT2, but not 5-HT1 serotonin receptor type in the hypothalamic arcuate nucleus (ARC) is responsible for the neuroendocrine regulation of liver cytochrome P450. The goal of these studies was to determine whether 5-HT2C serotonin receptor subtype in the ARC is engaged in the regulation of liver cytochrome P450.

METHODS

The 5-HT2C serotonin receptor agonist CP-809,101 was injected into the ARC for 5 days. The liver cytochrome P450 activity and protein level were measured.

RESULTS

In rats receiving an injection of the 5-HT2C serotonin receptor agonist CP-809,101 into the ARC (1 μg/side) for five days, the activities of CYP2B, CYP2C11 and CYP3A significantly increased corresponding with the elevated enzyme protein level.

CONCLUSIONS

The obtained results suggest that the 5-HT2C serotonin receptor subtype in the ARC is involved in the positive neuroendocrine regulation of cytochrome P450. Further studies are in progress to explain the physiological mechanism which is responsible for the observed regulation of cytochrome P450 by 5-HT2C receptor present in the ARC.

Oxytocin and opioid addiction revisited: old drug, new applications

Opioid addiction has devastating health and socio-economic consequences, and current pharmacotherapy is limited and often accompanied by side effects, thus novel treatment is warranted. Traditionally, the neurohypophyseal peptide oxytocin (OT) is known for its effects on mediating reward, social affiliation and bonding, stress and learning and memory. There is now strong evidence that OT is a possible candidate for the treatment of drug addiction and depression-addiction co-morbidities. This review summarizes and critically discusses the preclinical evidence surrounding the consequences of pharmacological manipulation of the oxytocinergic system on opioid addiction-related processes, as well as the effects of opioids on the OT system at different stages of the addiction cycle. The mechanisms underlying the effects of OT on opioid addiction, including OT' interaction with the monoaminergic, glutamatergic, opioidergic systems and its effect on the amygdala, the hypothalamic-pituitary-adrenal axis and on memory consolidation of traumatic memories, are also reviewed. We also review clinical evidence on the effects of intranasal OT administration on opioid-dependent individuals and discuss the therapeutic potential along with the limitations that accompany OT-based pharmacotherapies. Review of these studies clearly indicates that the OT system is profoundly affected by opioid use and abstinence and points towards the OT system as an important target for developing pharmacotherapies for the treatment of opioid addiction and co-existing affective disorders, thereby preventing relapse. Therefore, there is a clear need for clinical studies assessing the efficacy of OT-based pharmacotherapies in opioid addiction.

LINKED ARTICLES

This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Learned helplessness at fifty: Insights from neuroscience

Learned helplessness, the failure to escape shock induced by uncontrollable aversive events, was discovered half a century ago. Seligman and Maier (1967) theorized that animals learned that outcomes were independent of their responses-that nothing they did mattered-and that this learning undermined trying to escape. The mechanism of learned helplessness is now very well-charted biologically, and the original theory got it backward. Passivity in response to shock is not learned. It is the default, unlearned response to prolonged aversive events and it is mediated by the serotonergic activity of the dorsal raphe nucleus, which in turn inhibits escape. This passivity can be overcome by learning control, with the activity of the medial prefrontal cortex, which subserves the detection of control leading to the automatic inhibition of the dorsal raphe nucleus. So animals learn that they can control aversive events, but the passive failure to learn to escape is an unlearned reaction to prolonged aversive stimulation. In addition, alterations of the ventromedial prefrontal cortex-dorsal raphe pathway can come to subserve the expectation of control. We speculate that default passivity and the compensating detection and expectation of control may have substantial implications for how to treat depression. (PsycINFO Database Record

Glucose concentrations modulate brain-derived neurotrophic factor responsiveness of neurones in the paraventricular nucleus of the hypothalamus

The hypothalamic paraventricular nucleus (PVN) is critical for normal energy balance and has been shown to contain high levels of both brain-derived neurotrophic factor (BDNF) and tropomyosin-receptor kinase B mRNA. Microinjections of BDNF into the PVN increase energy expenditure, suggesting that BDNF plays an important role in energy homeostasis through direct actions in this nucleus. The present study aimed to examine the postsynaptic effects of BDNF on the membrane potential of PVN neurones, and also to determine whether extracellular glucose concentrations modulated these effects. We used hypothalamic PVN slices from male Sprague-Dawley rats to perform whole cell current-clamp recordings from PVN neurones. BDNF was bath applied at a concentration of 2 nmol L^-1 and the effects on membrane potential determined. BDNF caused depolarisations in 54% of neurones (n=25; mean±SEM, 8.9±1.2 mV) and hyperpolarisations in 23% (n=11; -6.7±1.4 mV), whereas the remaining cells were unaffected. These effects were maintained in the presence of tetrodotoxin (n=9; 56% depolarised, 22% hyperpolarised, 22% nonresponders), or the GABA_a antagonist bicuculline (n=12; 42% depolarised, 17% hyperpolarised, 41% nonresponders), supporting the conclusion that these effects on membrane potential were postsynaptic. Current-clamp recordings from PVN neurones next examined the effects of BDNF on these neurones at varying extracellular glucose concentrations. Larger proportions of PVN neurones hyperpolarised in response to BDNF as the glucose concentrations decreased [10 mmol L^-1 glucose 23% (n=11) of neurones hyperpolarised, whereas, at 0.2 mmol L^-1 glucose, 71% showed hyperpolarising effects (n=12)]. Our findings reveal that BDNF has direct GABA_A independent effects on PVN neurones, which are modulated by local glucose concentrations. The latter observation further emphasises the critical importance of using physiologically relevant conditions in an investigation of the central pathways involved in the regulation of energy homeostasis.

A Thalamo-Hypothalamic Pathway That Activates Oxytocin Neurons in Social Contexts in Female Rats

Oxytocin is released from neurons in the paraventricular hypothalamic nucleus (PVN) in mothers upon suckling and during adult social interactions. However, neuronal pathways that activate oxytocin neurons in social contexts are not yet established. Neurons in the posterior intralaminar complex of the thalamus (PIL), which contain tuberoinfundibular peptide 39 (TIP39) and are activated by pup exposure in lactating mothers, provide a candidate projection. Innervation of oxytocin neurons by TIP39 neurons was examined by double labeling in combination with electron microscopy and retrograde tract-tracing. Potential classic neurotransmitters in TIP39 neurons were investigated by in situ hybridization histochemistry. Neurons activated after encounter with a familiar conspecific female in a familiar environment were mapped with the c-Fos technique. PVN and the supraoptic nucleus oxytocin neurons were closely apposed by an average of 2.0 and 0.4 TIP39 terminals, respectively. Asymmetric (presumed excitatory) synapses were found between TIP39 terminals and cell bodies of oxytocin neurons. In lactating rats, PIL TIP39 neurons were retrogradely labeled from the PVN. TIP39 neurons expressed vesicular glutamate transporter 2 but not glutamic acid decarboxylase 67. PIL contained a markedly increased number of c-Fos-positive neurons in response to social encounter with a familiar conspecific female. Furthermore, the PIL received ascending input from the spinal cord and the inferior colliculus. Thus, TIP39 neurons in the PIL may receive sensory input in response to social interactions and project to the PVN to innervate and excite oxytocin neurons, suggesting that the PIL-PVN projection contributes to the activation of oxytocin neurons in social contexts.

Intermittent severe hypoxia induces plasticity within serotonergic and catecholaminergic neurons in the neonatal rat ventrolateral medulla

5-HT neurons contribute to autoresuscitation and survival during intermittent severe hypoxia (IsH). In adults, catecholaminergic neurons in the ventrolateral medulla (VLM) contribute to the autonomic response to hypoxia. We hypothesized that 1) catecholaminergic neurons in the neonatal VLM are activated following IsH, 2) this activation is compromised following an acute loss of brain stem 5-HT, and 3) IsH induces cellular and/or transcriptomic plasticity within catecholaminergic and serotonergic neurons that are within or project to the VLM, respectively. To test these hypotheses, we treated rat pups with 6-fluorotryptophan, a tryptophan hydroxylase (TPH) inhibitor, and then exposed treated and vehicle controls to IsH or air. Along with immunohistochemistry to detect tyrosine hydroxylase (TH)- or Fos-positive neurons, we used RNA sequencing to resolve the effects of IsH and 5-HT deficiency on the expression of serotonergic and catecholaminergic system genes in the VLM. 5-HT deficiency compromised autoresuscitation and survival. IsH significantly increased the number of identifiable TH-positive VLM neurons, an effect enhanced by 5-HT deficiency (P = 0.003). Contrary to our hypothesis, 5-HT-deficient pups had significantly more Fos-positive neurons following IsH (P = 0.008) and more activated TH-positive neurons following IsH or air (P = 0.04). In both groups the expression of the 5-HT transporter and TPH2 was increased following IsH. In 5-HT-deficient pups, the expression of the inhibitory 5-HT1A receptor was decreased following IsH, while the expression of DOPA decarboxylase was increased. These data show that the serotonergic and catecholaminergic systems in the VLM of the neonatal rat are dynamically upregulated by IsH, potentially adapting cardiorespiratory responses to severe hypoxia.

5-Hydroxytryptamine 1A receptors in the dorsomedial hypothalamus connected to dorsal raphe nucleus inputs modulate defensive behaviours and mediate innate fear-induced antinociception

The dorsal raphe nucleus (DRN) is an important brainstem source of 5-hydroxytryptamine (5-HT), and 5-HT plays a key role in the regulation of panic attacks. The aim of the present study was to determine whether 5-HT1A receptor-containing neurons in the medial hypothalamus (MH) receive neural projections from DRN and to then determine the role of this neural substrate in defensive responses. The neurotracer biotinylated dextran amine (BDA) was iontophoretically microinjected into the DRN, and immunohistochemical approaches were then used to identify 5HT1A receptor-labelled neurons in the MH. Moreover, the effects of pre-treatment of the dorsomedial hypothalamus (DMH) with 8-OH-DPAT and WAY-100635, a 5-HT1A receptor agonist and antagonist, respectively, followed by local microinjections of bicuculline, a GABAA receptor antagonist, were investigated. We found that there are many projections from the DRN to the perifornical lateral hypothalamus (PeFLH) but also to DMH and ventromedial (VMH) nuclei, reaching 5HT1A receptor-labelled perikarya. DMH GABAA receptor blockade elicited defensive responses that were followed by antinociception. DMH treatment with 8-OH-DPAT decreased escape responses, which strongly suggests that the 5-HT1A receptor modulates the defensive responses. However, DMH treatment with WAY-100635 failed to alter bicuculline-induced defensive responses, suggesting that 5-HT exerts a phasic influence on 5-HT1A DMH neurons. The activation of the inhibitory 5-HT1A receptor had no effect on antinociception. However, blockade of the 5-HT1A receptor decreased fear-induced antinociception. The present data suggest that the ascending pathways from the DRN to the DMH modulate panic-like defensive behaviours and mediate antinociceptive phenomenon by recruiting 5-HT1A receptor in the MH.

Sex differences in the HPA axis

The hypothalamic-pituitary-adrenal (HPA) axis is a major component of the systems that respond to stress, by coordinating the neuroendocrine and autonomic responses. Tightly controlled regulation of HPA responses is critical for maintaining mental and physical health, as hyper- and hypo-activity have been linked to disease states. A long history of research has revealed sex differences in numerous components of the HPA stress system and its responses, which may partially form the basis for sex disparities in disease development. Despite this, many studies use male subjects exclusively, while fewer reports involve females or provide direct sex comparisons. The purpose of this article is to present sex comparisons in the functional and molecular aspects of the HPA axis, through various phases of activity, including basal, acute stress, and chronic stress conditions. The HPA axis in females initiates more rapidly and produces a greater output of stress hormones. This review focuses on the interactions between the gonadal hormone system and the HPA axis as the key mediators of these sex differences, whereby androgens increase and estrogens decrease HPA activity in adulthood. In addition to the effects of gonadal hormones on the adult response, morphological impacts of hormone exposure during development are also involved in mediating sex differences. Additional systems impinging on the HPA axis that contribute to sex differences include the monoamine neurotransmitters norepinephrine and serotonin. Diverse signals originating from the brain and periphery are integrated to determine the level of HPA axis activity, and these signals are, in many cases, sex-specific.

Tryptophan promotes charitable donating

The link between serotonin (5-HT) and one of the most important elements of prosocial behavior, charity, has remained largely uninvestigated. In the present study, we tested whether charitable donating can be promoted by administering the food supplement L-Tryptophan (TRP), the biochemical precursor of 5-HT. Participants were compared with respect to the amount of money they donated when given the opportunity to make a charitable donation. As expected, compared to a neutral placebo, TRP appears to increase the participants’ willingness to donate money to a charity. This result supports the idea that the food we eat may act as a cognitive enhancer modulating the way we think and perceive the world and others.

Transneuronal circuit analysis with pseudorabies viruses

Our ability to understand the function of the nervous system is dependent upon defining the connections of its constituent neurons. Development of methods to define connections within neural networks has always been a growth industry in the neurosciences. Transneuronal spread of neurotropic viruses currently represents the best means of defining synaptic connections within neural networks. The method exploits the ability of viruses to invade neurons, replicate, and spread through the intimate synaptic connections that enable communication among neurons. Since the method was first introduced in the 1970s, it has benefited from an increased understanding of the virus life cycle, the function of viral genome, and the ability to manipulate the viral genome in support of directional spread of virus and the expression of transgenes. In this unit, we review these advances in viral tracing technology and the way in which they may be applied for functional dissection of neural networks.

The social brain: neurobiological basis of affiliative behaviours and psychological well-being

The social brain hypothesis proposes that the demands of the social environment provided the evolutionary pressure that led to the expansion of the primate brain. Consistent with this notion, that functioning in the social world is crucial to our survival, while close supportive relationships are known to enhance well-being, a range of social stressors such as abuse, discrimination and dysfunctional relationships can increase the risk of psychiatric disorders. The centrality of the social world to our everyday lives is further exemplified by the fact that abnormality in social behaviour is a salient feature of a range of neurodevelopmental and psychiatric disorders. This paper aims to provide a selective overview of current knowledge of the neurobiological basis of our ability to form and maintain close personal relationships, and of the benefits these relationships confer on our health. Focusing on neurochemical and neuroendocrine interactions within affective and motivational neural circuits, it highlights the specific importance of cutaneous somatosensation in affiliative behaviours and psychological well-being and reviews evidence, in support of the hypothesis, that a class of cutaneous unmyelinated, low threshold mechanosensitive nerves, named c-tactile afferents, have a direct and specific role in processing affiliative tactile stimuli.

Serotonergic neuron regulation informed by in vivo single-cell transcriptomics

Despite the recognized importance of the dorsal raphe (DR) serotonergic (5-HT) nuclei in the pathophysiology of depression and anxiety, the molecular components/putative drug targets expressed by these neurons are poorly characterized. Utilizing the promoter of an ETS domain transcription factor that is a stable marker of 5-HT neurons (Pet-1) to drive 5-HT neuronal expression of YFP, we identified 5-HT neurons in live acute slices. We isolated RNA from single 5-HT neurons in the ventromedial and lateral wings of the DR and performed single-cell RNA-Seq analysis identifying >500 G-protein coupled receptors (GPCRs) including receptors for classical transmitters, lipid signals, and peptides as well as dozens of orphan-GPCRs. Using these data to inform our selection of receptors to assess, we found that oxytocin and lysophosphatidic acid 1 receptors are translated and active in costimulating, with the α1-adrenergic receptor, the firing of DR 5-HT neurons, while the effects of histamine are inhibitory and exerted at H3 histamine receptors. The inhibitory histamine response provides evidence for tonic in vivo histamine inhibition of 5-HT neurons. This study illustrates that unbiased single-cell transcriptomics coupled with functional analyses provides novel insights into how neurons and neuronal systems are regulated.

Sensitization of restraint-induced corticosterone secretion after chronic restraint in rats: involvement of 5-HT₇ receptors

Serotonin (5-HT) modulates the hypothalamic-pituitary-adrenal (HPA) axis response to stress. We examined the effect of chronic restraint stress (CRS; 20 min/day) as compared to control (CTRL) conditions for 14 days, on: 1) restraint-induced ACTH and corticosterone (CORT) secretion in rats pretreated with vehicle or SB-656104 (a 5-HT₇ receptor antagonist); 2) 5-HT₇ receptor-like immunoreactivity (5-HT₇-LI) and protein in the hypothalamic paraventricular nucleus (PVN) and adrenal glands (AG); 3) baseline levels of 5-HT and 5-hydroxyindolacetic acid (5-HIAA), and 5-HIAA/5-HT ratio in PVN and AG; and 4) 5-HT-like immunoreactivity (5-HT-LI) in AG and tryptophan hydroxylase (TPH) protein in PVN and AG. On day 15, animals were subdivided into Treatment and No treatment groups. Treatment animals received an i.p. injection of vehicle or SB-656104; No Treatment animals received no injection. Sixty min later, Treatment animals were either decapitated with no further stress (0 min) or submitted to acute restraint (10, 30, 60 or 120 min); hormone serum levels were measured. No Treatment animals were employed for the rest of measurements. CRS decreased body weight gain and increased adrenal weight. In CTRL animals, acute restraint increased ACTH and CORT secretion in a time of restraint-dependent manner; both responses were inhibited by SB-656104. Exposure to CRS abolished ACTH but magnified CORT responses to restraint as compared to CTRL conditions; SB-656104 had no effect on ACTH levels but significantly inhibited sensitized CORT responses. In CTRL animals, 5-HT₇-LI was detected in magnocellular and parvocellular subdivisions of PVN and sparsely in adrenal cortex. Exposure to CRS decreased 5-HT₇-LI and protein in the PVN, but increased 5-HT₇-LI in the adrenal cortex and protein in whole AG. Higher 5-HT and 5-HIAA levels were detected in PVN and AG from CRS animals but 5-HIAA/5-HT ratio increased in AG only. Finally, whereas 5-HT-LI was sparsely observed in the adrenal cortex of CTRL animals, it strongly increased in the adrenal cortex of CRS animals. No TPH protein was detected in AG from both animal groups. Results suggest that CRS promotes endocrine disruption involving decreased ACTH and sensitized CORT responses to acute restraint. This phenomenon may be associated with increased function and expression of 5-HT₇ receptors as well as 5-HT turnover in AG.

A link between oxytocin and serotonin in humans: supporting evidence from peripheral markers

Pharmacological studies indicate a functional interaction between the serotonergic and oxytocinergic systems. In particular, some selective serotonin (5-HT) reuptake inhibitors, such as citalopram and fluvoxamine, seem to exert part of their antidepressant effects through oxytocin (OT) release. Further, the administration of fenfluramine, a serotonergic agonist, to healthy subjects increases plasma OT levels. Interestingly, immunocytochemical and double-immunofluorescent techniques revealed a high degree of overlap between 5-HT transporter (SERT)-labeled fibers and OT-containing cells in the paraventricular and supraoptic nuclei of primate hypothalamus. These findings suggest that the influence of 5-HT on OT system might be mediated by SERT. In this study, we explored the possible existence of a link between OT and SERT in human subjects, by means of two peripheral markers, the platelet SERT, as measured by [³H]-paroxetine ([³H]-Par) binding, and plasma OT levels. As far as [³H]-Par binding parameters are concerned, the Bmax (mean ± SD, fmol/mg protein) was 1155 + 130 and the Kd (mean ± SD, nM) was 1.31 ± 0.61. The OT plasma levels (mean ± SD, pg/ml) were 1.14 ± 1.07. A significant and positive correlation was found between plasma OT levels and Kd values (correlation coefficient: r: 0.466, p = .038). This result represents the first evidence of an interaction between OT and SERT, as measured by [³H]-Par binding, at peripheral levels in humans. Given the several activities mediated by both OT and 5-HT, such a relationship might provide new perspectives and insights into psychiatric disorders and/or social relationship disturbances, as well as novel treatment strategies overcoming and/or integrating the serotonergic paradigm.

Architecture of the hypothalamo-posthypophyseal complex is controlled by monoamines

The hypothalamo-neurohypophyseal system displays significant plasticity when subjected to physiological stimuli, such as dehydration, parturition, or lactation. This plasticity arises at the neurochemical and electrophysiological levels but also at a structural level. Several studies have demonstrated the role of monoaminergic afferents in controlling neurochemical and electrophysiological plasticity of the supraoptic nucleus (SON) and of the neurohypophysis (NH), but little is known about how the changes in structural plasticity are triggered. We used Tg8 mice, disrupted for the monoamine oxidase A gene, to study monamine involvement in the architecture of the SON and of the NH. SON astrocytes in Tg8 mice displayed an active status, characterized by an increase in S100β expression and a significant decrease in vimentin expression, with no modification in glial fibrillary acidic protein (GFAP) levels. Astrocytes showed a decrease in glutamate dehydrogenase (GDH) levels, whereas glutamine synthetase (GS) levels remained constant, suggesting a reduction in astrocyte glutamate catabolism. Tenascin C and polysialic acid-neural cell adhesion molecule (PSA-NCAM) expressions were also elevated in the SON of Tg8 mice, suggesting an increased capacity for structural remodelling in the SON. In the NH, similar date were obtained with a stability in GFAP expression and an increase in PSA-NCAM immunostaining. These results establish monoamine (serotonin and noradrenaline) involvement in SON and NH structural arrangement. Monoamines therefore appear to be crucial for the coordination of the neurochemical and structural aspects of neuroendocrine plasticity, allowing the hypothalamo-neurohypopyseal system to respond appropriately when stimulated.

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.

Androgenic influence on serotonergic activation of the HPA stress axis

The higher incidence of stress-mediated affective disorders in women may be a function of gonadal hormone influence on complex interactions between serotonin and neural circuits that mediate the hypothalamic-pituitary-adrenal (HPA) stress axis. The paraventricular nucleus of the hypothalamus (PVN) receives serotonergic innervation, and selective serotonin reuptake inhibitors such as citalopram activate the HPA axis independent of stress. We have previously demonstrated that the magnitude of this serotonergic activation was greater in females and was attenuated by testosterone administration; however, the potential central sites of action where androgens reduce these serotonergic effects have not been determined. Therefore, we examined a time course of corticosterone production and used central c-Fos protein levels to assay neuronal activation in stress-related brain regions in female, male, and gonadectomized male mice after an acute citalopram injection (15 mg/kg). In the hippocampus, c-Fos-immunoreactivity was greater in males than in females or gonadectomized males. This same pattern emerged in the lateral septum after vehicle and gonadectomy reversed the effect of citalopram. These regions are important for inhibitory influences on the PVN, and accordingly, hippocampal c-Fos levels were negatively correlated with corticosterone production. No sex differences in c-Fos were detected in the PVN, cingulate cortex, or paraventricular thalamus in response to vehicle or citalopram. These data support brain region-specific regulation of the HPA axis where sex differences may be mediated partly through androgen enhancement of signaling in inhibitory regions.

Developmental gene x environment interactions affecting systems regulating energy homeostasis and obesity

Most human obesity is inherited as a polygenic trait which is largely refractory to medical therapy because obese individuals avidly defend their elevated body weight set-point. This set-point is mediated by an integrated neural network that controls energy homeostasis. Epidemiological studies suggest that perinatal and pre-pubertal environmental factors can promote offspring obesity. Rodent studies demonstrate the important interactions between genetic predisposition and environmental factors in promoting obesity. This review covers issues of development and function of neural systems involved in the regulation of energy homeostasis and the roles of leptin and insulin in these processes, the ways in which interventions at various phases from gestation, lactation and pre-pubertal stages of development can favorably and unfavorably alter the development of obesity n offspring. These studies suggest that early identification of obesity-prone humans and of the factors that can prevent them from becoming obese could provide an effective strategy for preventing the world-wide epidemic of obesity.

Sex differences in the serotonergic influence on the hypothalamic-pituitary-adrenal stress axis

Appropriate interactions between serotonin (5-HT) and stress pathways are critical for maintaining homeostasis. Dysregulation of hypothalamic-pituitary-adrenal (HPA) stress axis is a common feature in affective disorders in which an involvement of 5-HT neurocircuitry has been implicated in disease vulnerability and treatment responsiveness. Because there is a greater prevalence of affective disorders in women, sex differences in the 5-HTergic influence on stress pathways may contribute to disease disparity. Therefore, our studies compared stress or citalopram-induced corticosterone levels in male and female mice. To determine whether sex-dependent HPA axis responsiveness was mediated by the difference in testosterone levels, testosterone-treated females were also examined. Gene expression patterns in 5-HTergic and stress neurocircuitry were analyzed to determine sites of potential sex differences and mechanisms of testosterone action. As expected, restraint stress corticosterone levels were higher in intact females and were masculinized by testosterone. Interestingly, citalopram administration independent of stress resulted in a greater corticosterone response in females, which was also masculinized by testosterone. Analyses along the 5-HT-HPA axis revealed sex differences including greater pituitary 5-HT receptors and adrenal weights in females. Moreover, in stress-regulatory regions, we found sex differences in glucocorticoid receptor and glutamic acid decarboxylase expression supportive of greater inhibitory modulation and feedback potential in males. Taken together, these data suggest that multiple sites related to 5-HTergic stimulation, corticosterone production, and negative feedback of HPA neurocircuitry combine to produce higher female stress responsiveness. These studies support a potential for sex-specific involvement of 5-HT and stress pathways in the etiology of affective disorders.

Juvenile social experience regulates central neuropeptides relevant to emotional and social behaviors

Stressful social experiences during early-life can increase the risk of developing neuropsychiatric disorders associated with anxiety, mood, and personality. Early neglect also alters peripheral arginine vasopression (AVP) and oxytocin (OXT). We hypothesized that a lack of social stimuli should adversely affect developmental AVP and OXT systems. To test this idea, we examined changes of central AVP- and OXT-immunoreactive (ir) cell number as well as its related behaviors in socially isolated rats. Animals were weaned at 23 days of age, divided into group- or isolation-reared conditions, and maintained for at least 2 weeks. At 38-48 days of age, animals were sacrificed for immunohistochemistry, or used for two behavioral tests: elevated plus-maze test and social recognition test. The results from immunohistochemistry showed that isolation-reared males have decreased AVP-ir cells in the paraventricular nucleus hypothalamus (PVH), medial parvicellular part, ventral zone, and that isolation-reared females have decreased OXT-ir cells in the PVH, medial parvicellular part, dorsal zone, when compared with group-reared counterparts. The results from behavioral assessment showed that isolation-reared animals have difficulty with social recognition, and that isolation-reared males, but not females, have anxiogenic profile. The present study demonstrates that post-weaning social isolation results in decrease of male AVP-ir cells and female OXT-ir cells in the PVH parvocellular divisions, and supports the idea that juvenile social environment may play a critical role in neuronal and behavioral development.

Serotonin inhibits GABA synaptic transmission in presympathetic paraventricular nucleus neurons

Activation of serotonin (5-hydroxytryptamine, 5-HT) receptors produces various autonomic and neuroendocrine responses in the hypothalamic paraventricular nucleus (PVN), including increased blood pressure and heart rate. However, the role(s) of 5-HT on the local GABA synaptic circuit have not been well understood in the PVN, where the inhibitory neurotransmitter GABA plays a key role in the modulation of sympathoexcitatory outflow. In the present study, we examined the effects of 5-HT on GABA synaptic transmission in presympathetic PVN neurons projecting to spinal cord using patch-clamp electrophysiology combined with tract-tracing techniques. Bath application of 5-HT (0.01-100 microM) reversibly decreased the frequency of spontaneous GABAergic inhibitory postsynaptic currents (sIPSC) in a concentration dependent manner (IC50, 0.07 microM), with no significant changes in the amplitudes and decay kinetics of sIPSC. The sIPSC inhibition of 5-HT was mimicked by 5-HT1A agonist, 8-OH-DPAT (8-hydroxy-2(di-n-propylamino)tetralin, 10 microM), and blocked by 5-HT1A antagonist WAY-100635 but not by 5-HT1B antagonist SB224289. 5-HT also reduced the frequency of miniature IPSC (mIPSC) (2.59+/-0.51 Hz, control vs. 1.25+/-0.31 Hz, 5-HT, n=16) in similar extent with 5-HT induced reduction of sIPSC frequency (sIPSCs, 55.8+/-6.2%, n=11 vs. mIPSCs, 52.30+/-5.85%, n=16; p>0.5). All together, our results indicate that 5-HT can inhibit presynaptic GABA release via presynaptic 5-HT1A receptors in presympathetic PVN neurons projecting to spinal cord.

Androgen receptor expressing neurons that project to the paraventricular nucleus of the hypothalamus in the male rat

Androgen receptors are distributed throughout the central nervous system and are contained by a variety of nuclei that are known to project to or regulate the paraventricular nucleus (PVN) of the hypothalamus, the final common pathway by which the brain regulates the hypothalamic-pituitary-adrenal (HPA) response to homeostatic threat. Here we characterized androgen receptor staining within cells identified as projecting to the PVN in male rats bearing iontophoretic or crystalline injections of the retrograde tracer FluoroGold aimed at the caudal two-thirds of the nucleus, where corticotropin-releasing hormone-expressing neurons are amassed. Androgen receptor (AR) and FluoroGold (FG) double labeling was revealed throughout the limbic forebrain, including scattered numbers of cells within the anterior and posterior subdivisions of the bed nuclei of the stria terminalis; the medial zone of the hypothalamus, including large numbers of AR-FG-positive cells within the anteroventral periventricular and medial preoptic cell groups. Strong and consistent colabeling was also revealed throughout the hindbrain, predominantly within the periaqueductal gray and the lateral parabrachial nucleus, and within various medullary cell groups identified as catecholaminergic, predominantly C1 and A1 neurons of the ventral medulla. These connectional data predict that androgens can act on a large assortment of multimodal inputs to the PVN, including those involved with the processing of various types of sensory and limbic information, and provide an anatomical framework for understanding how gonadal status could contribute to individual differences in HPA function.

Serotonin increases the excitability of the hypothalamic paraventricular nucleus magnocellular neurons

Recent evidence that 5-hydroxytryptamine (5-HT or serotonin) enhances the release and the gene expression of vasopressin and oxytocin in the hypothalamic paraventricular nucleus (PVN) suggests that 5-HT can excite the PVN magnocellular neurons. The objective of this study was to examine the underlying mechanisms for such excitatory action in the electrophysiologically identified hypothalamic PVN magnocellular neurons in rats using whole-cell patch-clamp. We found that 5-HT weakly depolarizes 33.3% of PVN magnocellular neurons in the presence of tetrodotoxin. A minuscule inward current was produced by 5-HT in 48% of the cells, which was attenuated when the 5-HT(4) antagonist GR113808 or the 5-HT(7) antagonist SB269970 was added. In addition, 5-HT reduced the frequency of miniature inhibitory postsynaptic currents in a dose-dependent manner. This inhibition was mimicked by the 5-HT(1B) agonist CP93129, and reversed in the presence of 5-HT(1B) antagonists cyanopindolol and SB224289. Besides, 5-HT induced a biphasic effect on the frequency of miniature excitatory postsynaptic currents, comprising a transient inhibition and a delayed concentration-dependent excitation (onset latency approximately 5 min). The facilitation was mimicked by the 5-HT(2A/2C) agonist DOI and abolished in the presence of the 5-HT(2C) antagonist RS102221. Our findings reveal that 5-HT directly increases the excitability of the PVN magnocellular neurons via multiple receptor subtypes and mechanisms. This may help understanding the regulation of 5-HT-induced hormone release and feeding behavior in the PVN.

Sertraline, a selective serotonin reuptake inhibitor, affects thirst, salt appetite and plasma levels of oxytocin and vasopressin in rats

We investigated the effects of chronic administration of sertraline (SERT; approximately 20 mg kg(-1) day(-1) in drinking water), a selective serotonin reuptake inhibitor, on water and sodium intake and on plasma levels of oxytocin (OT) and vasopressin (AVP) in basal and stimulated conditions. Basal water intake was reduced in SERT-treated rats. After 24 h of water deprivation, rats treated with SERT for 21 days ingested less water than the control rats (9.7 +/- 0.5 versus 20.0 +/- 0.9 ml, respectively, at 300 min after water presentation, P < 0.0001). Subcutaneous injection of 2 m NaCl or isoproterenol evoked a lower dipsogenic response in rats treated with SERT for 21 days. Fluid and food deprivation also induced a weaker dipsogenic response in SERT-treated rats (1.6 +/- 0.5 versus 10.2 +/- 1.2 ml, at 300 min, P < 0.0001) but had no effect on saline intake. Sodium depletion induced a higher natriorexigenic response in the SERT group (5.6 +/- 1.3 versus 1.2 +/- 0.3 ml, at 300 min, P < 0.0002). Higher urinary density and lower plasma sodium levels were observed after SERT treatment. Sertraline also increased plasma levels of vasopressin and oxytocin (AVP, 2.65 +/- 0.36 versus 1.31 +/- 0.16 pg ml(-1), P < 0.005; OT, 17.16 +/- 1.06 versus 11.3 +/- 1.03 pg ml(-1), P < 0.0009, at the third week post-treatment). These data constitute the first evidence that chronic SERT treatment affects water and sodium intake in rats. These effects seem to be related to the hyponatraemia caused by the higher plasma levels of AVP and OT.

Effect of an increase in brain serotonin on the osmoregulatory response to a hypo- or hyperosmotic load in Wistar and vasopressin-deficient Brattleboro rats

Serotonin and its receptor agonists stimulate the release of arginine vasopressin (AVP) into peripheral blood under intraventricular injection. To test the hypothesis that brain serotonin can modulate the development of natural osmoregulatory responses, the effect of an increase in endogenous brain serotonin on the response to an intragastric hypo- or hyperosmotic loading was studied in Wistar and AVP-deficient Brattleboro rats. 5-Hydroxytryptophan (5-HTP), the rate-limiting serotonin biosynthesis precursor known to increase the brain level of serotonin, was injected intraperitoneally (5 mg/100 g body weight). The renal functional parameters (glomerular filtration rate [GFR], free water reabsorption, and urine flow rate) were monitored during the 4 h after intragastric infusion of water or a 2% NaCl solution (5% of body weight). Plasma AVP was measured by radioimmunoassay. In Wistar rats, intraperitoneal injection of 5-HTP at the same time as water loading prevented the development of the renal diuretic response: there was no increase in urine flow rate and GFR, and free water reabsorption remained at the high level. In AVP-deficient Brattleboro rats, unlike Wistar rats, 5-HTP treatment was without effect on the renal function parameters. In Wistar rats, injection of 5-HTP at the peak of water diuresis produced an abrogation of the diuretic response to water loading due to the increase in free water reabsorption. Plasma AVP increased from 1.2 +/- 0.4 to 4.2 +/- 1.6 pg/ml (n = 8 in each group, p < 0.01). Hyperosmotic treatment of Wistar rats with a 2% NaCl solution stimulated AVP secretion compared to baseline (from 3.2 +/- 0.1, n = 7 to 5.6 +/- 0.9, n = 7, p < 0.01), and the saluretic response developed on the background of high free water reabsorption. When injected concomitantly with NaCl solution, 5-HTP revealed no additive effect on plasma AVP and on free water reabsorption. We conclude that the 5-HTP-caused increase in brain serotonin contributed significantly to the dynamics of changes in the osmoregulatory response to the hypo-osmotic challenge due to stimulation of AVP secretion. 5-HTP had no additive effect on the osmoregulatory response to hyperosmotic loading. Peripherally injected 5-HTP had no effect on the renal function, being absent in AVP-deficient Brattleboro rats.

Oxytocin Involvement in SSRI-Induced Delayed Ejaculation: A Review of Animal Studies

INTRODUCTION

Selective serotonin reuptake inhibitors (SSRIs) differ in the severity of induced ejaculation delay. Various studies indicate that oxytocin is involved in sexual behavior.

AIM

To review and evaluate the involvement of oxytocin in SSRI-induced ejaculation delay.

MAIN OUTCOME MEASURES

Oxytocine release, 5-hydroxytryptamine (5-HT) neurotransmission, and desensitization of 5-HT(1A) receptors.

METHODS

A review and critical analysis of animal studies investigating the interaction of serotonergic and oxytocinergic neurotransmission in relation to the ejaculation process.

RESULTS

Although acute treatment with the SSRIs fluoxetine and paroxetine immediately causes increased serotonin levels, delayed ejaculation does not occur. The increased serotonin levels induce oxytocin release via activation of 5-HT(1A) receptors, and this might compensate for the inhibitory actions of serotonin on sexual behavior. Chronic treatment with fluoxetine and paroxetine desensitizes 5-HT(1A) receptors on oxytocin neurons, and that might in part determine the onset of delayed ejaculation. Desensitization of 5-HT(1A) receptors is less strong following chronic treatment with the SSRIs fluvoxamine or citalopram, which may attenuate the degree of delayed ejaculation.

CONCLUSIONS

Preliminary data suggest that the severity of chronic SSRI treatment-induced delayed ejaculation and the differences between the various SSRIs in inducing ejaculation delay is related to gradual desensitization of 5-HT(1A) receptors on oxytocin neurons.

Anterograde tracing of projections from the dorsal raphe nucleus to the vestibular nuclei

This study used the anterograde transport of biotinylated dextran amine (BDA) to identify the course and terminal distribution of projections from the dorsal raphe nucleus (DRN) to the vestibular nuclei in rats. After iontophoretic injection of BDA into the medial and lateral regions of DRN, anterogradely labeled fibers descend within the medial longitudinal fasciculus and the ventricular fiber plexus to terminate within two discrete regions of the vestibular nuclear complex. One terminal field was located primarily ipsilateral to the injection site and involved rostrodorsal aspects of the vestibular nuclei, including superior vestibular nucleus and rostral portions of the medial vestibular nucleus (MVN) and lateral vestibular nucleus (LVN). The other terminal field involved caudoventral aspects of both ipsilateral and contralateral MVN and LVN and was less heavily innervated. These findings confirm that the vestibular nuclei are targeted by a regionally-selective projection from the DRN. The segregation of DRN terminals into anatomically distinct fields indicates that the DRN-vestibular nucleus projections are organized to selectively modulate processing within specific functional domains of the vestibular nuclear complex. In particular, these terminal fields may be organized to modulate vestibular regions involved in eye movement-related velocity storage, coordination of vestibular and affective responses, and the bilateral coordination of horizontal eye movement reflexes.

Circadian and light regulation of oxytocin and parvalbumin protein levels in the ciliated ependymal layer of the third ventricle in the C57 mouse

The walls of the third ventricle have been proposed to serve as a bidirectional conduit for exchanges between the neural parenchyma and the cerebrospinal fluid. In immunohistochemical studies of mice, we observed that light exposure and circadian phase affected peptide staining surrounding the third ventricle at the level of the suprachiasmatic nuclei. Under high magnification, we observed robust staining for the neurohormone oxytocin and the calcium-binding protein parvalbumin associated with cilia extending into the third ventricle from the surrounding ventricular wall; no similar staining was observed for vasopressin or calbindin. Retinal illumination had opposite effects on levels of parvalbumin and oxytocin in the cilia: light exposure during late subjective night increased oxytocin staining, but decreased parvalbumin staining in the cilia. Preventing cellular transport with colchicine eliminated immunohistochemical staining for oxytocin in the cilia. There was also a significant daily rhythm of oxytocin immunostaining in the third ventricle wall, and in magnocellular neurons in the anterior hypothalamus. The results suggest that environmental lighting and circadian rhythms regulate levels of oxytocin in the cerebrospinal fluid, possibly by regulating movement of oxytocin through the third ventricle wall.

Local inhibition of organic cation transporters increases extracellular serotonin in the medial hypothalamus

In the rat dorsomedial hypothalamus (DMH), serotonin (5-HT) concentrations are altered rapidly in response to acute stressors. The mechanism for rapid changes in 5-HT concentrations in the DMH is not clear. We hypothesize that the mechanism involves corticosteroid-induced alterations in the uptake of 5-HT from extracellular fluid through the action of corticosterone-sensitive organic cation transporters (OCTs). To determine if OCTs affect the clearance of 5-HT from the extracellular fluid compartment within the medial hypothalamus (MH), the OCT blocker, decynium 22 (0, 10, 30, or 100 microM), was perfused into the MH via a microdialysis probe, and dialysate 5-HT concentrations were measured at 20 min intervals. In addition, home cage behavior was measured both before and after drug administration. Inhibition of OCTs in the MH resulted in a reversible dose-dependent increase in extracellular 5-HT concentration. Increases in extracellular 5-HT concentrations were associated with increases in grooming behavior in rats treated with the highest concentration of decynium 22. No other behavioral responses were observed following administration of any concentration of decynium 22. These findings are consistent with the hypothesis that OCTs in the MH play an important role in the regulation of serotonergic neurotransmission and specific behavioral responses. Because the MH plays an important role in the neuroendocrine, autonomic, and behavioral responses to stress-related stimuli, these data lead to new questions regarding the role of interactions between corticosterone and corticosterone-sensitive OCTs in stress-induced 5-HT accumulation within the MH as well as the physiological and behavioral consequences of these interactions.

Noradrenergic mechanisms in the pathophysiology of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is a serious psychiatric illness that may develop in individuals after exposure to a traumatic event. Recent data suggest that trauma and/or long-term stressors can cause alterations in the functioning of neuroanatomical structures and neural networks throughout the central nervous system. Specifically, dysregulation in central and perhaps, peripheral noradrenergic neural networks has been implicated as the cause of specific symptom clusters in the pathophysiology of PTSD. In this review, both clinical and preclinical data are presented to highlight types of noradrenergic dysfunction observed in individuals with PTSD. Additionally, the role of noradrenaline dysregulation in the acquisition/initiation, and maintenance of hyperarousal and reexperiencing symptom clusters in PTSD will be addressed.