Modulatory effects of serotonin on excitatory amino acid responses and sensory synaptic transmission in the ventrobasal thalamus

Fibromyalgia in Pregnancy: Neuro-Endocrine Fluctuations Provide Insight into Pathophysiology and Neuromodulation Treatment

Fibromyalgia (FM) is a chronic pain disorder with unclear pathophysiological mechanisms, which leads to challenges in patient management. In addition to pain, the disorder presents with a broad range of symptoms, such as sleep disruption, chronic fatigue, brain fog, depression, muscle stiffness, and migraine. FM has a considerable female prevalence, and it has been shown that symptoms are influenced by the menstrual cycle and periods of significant hormonal and immunological changes. There is increasing evidence that females with FM experience an aggravation of symptoms in pregnancy, particularly during the third trimester and after childbirth. In this perspective paper, we focus on the neuro-endocrine interactions that occur between progesterone, allopregnanolone, and cortisol during pregnancy, and propose that they align with our previously proposed model of FM pathogenesis based on GABAergic "weakening" in a thalamocortical neural loop system. Based on our hypothesis, we introduce the possibility of utilizing transcranial direct current stimulation (tDCS) as a non-invasive treatment potentially capable of exerting sex-specific effects on FM patients.

Dopaminergic and Serotonergic Degeneration and Cortical [18 F]Fluorodeoxyglucose Positron Emission Tomography in De Novo Parkinson's Disease

BACKGROUND

Degeneration of the nigrostriatal dopaminergic (DA) and the raphe-thalamic serotonergic (SE) systems is among the earliest changes observed in Parkinson's disease (PD). The consequences of those changes on brain metabolism, especially regarding their impact on the cortex, are poorly understood.

OBJECTIVES

Using multi-tracer molecular imaging, we assessed in a cohort of drug-naive PD patients the association between cortical metabolism and DA and SE system deafferentation of either striatum or thalamus, and we explored whether this association was mediated by either striatum or thalamus metabolism.

METHODS

We recruited 96 drug-naive PD patients (aged 71.9 ± 7.5 years) who underwent [¹²³ I]ioflupane single-photon emission computed tomography ([¹²³ I]FP-CIT-SPECT) and brain [¹⁸ F]fluorodeoxyglucose positron emission tomography ([¹⁸ F]FDG-PET). We used a voxel-wise analysis of [¹⁸ F]FDG-PET images to correlate regional metabolism with striatal DA and thalamic SE innervation as assessed using [¹²³ I]FP-CIT-SPECT.

RESULTS

We found that [¹²³ I]FP-CIT specific to nondisplaceable binding ratio (SBR) and glucose metabolism positively correlated with one another in the deep gray matter (thalamus: P = 0.001, r = 0.541; caudate P = 0.001, r = 0.331; putamen P = 0.001, r = 0.423). We then observed a direct correlation between temporoparietal metabolism and caudate DA innervation, as well as a direct correlation between prefrontal metabolism and thalamus SE innervation. The effect of caudate [¹²³ I]FP-CIT SBR values on temporoparietal metabolism was mediated by caudate metabolic values (percentage mediated: 89%, P-value = 0.008), and the effect of thalamus [¹²³ I]FP-CIT SBR values on prefrontal metabolism was fully mediated by thalamus metabolic values (P < 0.001).

CONCLUSIONS

These data suggest that the impact of deep gray matter monoaminergic deafferentation on cortical function is mediated by striatal and thalamic metabolism in drug-naive PD. © 2021 International Parkinson and Movement Disorder Society.

Neuropeptides and Neurotransmitters That Modulate Thalamo-Cortical Pathways Relevant to Migraine Headache

Dynamic thalamic regulation of sensory signals allows the cortex to adjust better to rapidly changing behavioral, physiological, and environmental demands. To fulfill this role, thalamic neurons must themselves be subjected to constantly changing modulatory inputs that originate in multiple neurochemical pathways involved in autonomic, affective, and cognitive functions. This review defines a chemical framework for thinking about the complexity of factors that modulate the response properties of relay trigeminovascular thalamic neurons. Following the presentation of scientific evidence for monosynaptic connections between thalamic trigeminovascular neurons and axons containing glutamate, GABA, dopamine, noradrenaline, serotonin, histamine, orexin, and melanin-concentrating hormone, this review synthesizes a large body of data to propose that the transmission of headache-related nociceptive signals from the thalamus to the cortex is modulated by potentially opposing forces and that the so-called 'decision' of which system (neuropeptide/neurotransmitter) will dominate the firing of a trigeminovascular thalamic neuron at any given time is determined by the constantly changing physiological (sleep, wakefulness, food intake, body temperature, heart rate, blood pressure), behavioral (addiction, isolation), cognitive (attention, learning, memory use), and affective (stress, anxiety, depression, anger) adjustment needed to keep homeostasis.

Neurochemical pathways that converge on thalamic trigeminovascular neurons: potential substrate for modulation of migraine by sleep, food intake, stress and anxiety

Dynamic thalamic regulation of sensory signals allows the cortex to adjust better to rapidly changing behavioral, physiological and environmental demands. To fulfill this role, thalamic neurons must themselves be subjected to constantly changing modulatory inputs that originate in multiple neurochemical pathways involved in autonomic, affective and cognitive functions. Our overall goal is to define an anatomical framework for conceptualizing how a ‘decision’ is made on whether a trigeminovascular thalamic neuron fires, for how long, and at what frequency. To begin answering this question, we determine which neuropeptides/neurotransmitters are in a position to modulate thalamic trigeminovascular neurons. Using a combination of in-vivo single-unit recording, juxtacellular labeling with tetramethylrhodamine dextran (TMR) and in-vitro immunohistochemistry, we found that thalamic trigeminovascular neurons were surrounded by high density of axons containing biomarkers of glutamate, GABA, dopamine and serotonin; moderate density of axons containing noradrenaline and histamine; low density of axons containing orexin and melanin concentrating hormone (MCH); but not axons containing CGRP, serotonin 1D receptor, oxytocin or vasopressin. In the context of migraine, the findings suggest that the transmission of headache-related nociceptive signals from the thalamus to the cortex may be modulated by opposing forces (i.e., facilitatory, inhibitory) that are governed by continuous adjustments needed to keep physiological, behavioral, cognitive and emotional homeostasis.

The role of glutamatergic and GABAergic systems on serotonin- induced feeding behavior in chicken

It has been reported that serotonin can modulate glutamate and GABA release in central nervous system (CNS). The present study was designed to examine the role of glutamatergic and GABAergic systems on serotonin- induced feeding behavior in chickens. In Experiment 1 intracerebroventricular (ICV) injection of MK- 801(NMDA receptor antagonist, 15 nmol) performed followed by serotonin (10 μg). In experiments 2, 3, 4, 5, 6 and 7 prior to serotonin injection, chickens received CNQX (AMPA/kainate receptor antagonist, 390 nmol), AIDA (mGluR1 antagonist, 2 nmol), LY341495 (mGluR2 antagonist, 150 nmol), UBP1112 (mGluR3 antagonist, 2 nmol), picrotoxin (GABA A receptor antagonist, 0.5 μg), CGP54626 (GABAB receptor antagonist, 20 ng) respectively. Cumulative food intake was determined at 3 h post injection. The results of this study showed that the hypophagic effect of serotonin was significantly attenuated by pretreatment with MK- 801 and CNQX (p < 0.05) but AIDA, LY341495 and UBP1112 had no effect (p > 0.05). Also, the inhibitory effect of serotonin on food intake was amplified by picrotoxin (p < 0.05) while CGP54626 had no effect (p > 0.05). These results suggest that serotonin as a modulator probably interacts with glutamatergic (via NMDA and AMPA/Kainate receptors) and GABAergic (via GABAA receptor) systems on feeding behavior in chicken.

Interactive responses of a thalamic neuron to formalin induced lasting pain in behaving mice

Thalamocortical (TC) neurons are known to relay incoming sensory information to the cortex via firing in tonic or burst mode. However, it is still unclear how respective firing modes of a single thalamic relay neuron contribute to pain perception under consciousness. Some studies report that bursting could increase pain in hyperalgesic conditions while others suggest the contrary. However, since previous studies were done under either neuropathic pain conditions or often under anesthesia, the mechanism of thalamic pain modulation under awake conditions is not well understood. We therefore characterized the thalamic firing patterns of behaving mice in response to nociceptive pain induced by inflammation. Our results demonstrated that nociceptive pain responses were positively correlated with tonic firing and negatively correlated with burst firing of individual TC neurons. Furthermore, burst properties such as intra-burst-interval (IntraBI) also turned out to be reliably correlated with the changes of nociceptive pain responses. In addition, brain stimulation experiments revealed that only bursts with specific bursting patterns could significantly abolish behavioral nociceptive responses. The results indicate that specific patterns of bursting activity in thalamocortical relay neurons play a critical role in controlling long-lasting inflammatory pain in awake and behaving mice.

Bilateral hyperexcitability of thalamic VPL neurons following unilateral spinal injury in rats

In the present study, we have examined whether spinal hemisection injury induces changes in the electrophysiological properties of thalamic ventral posteriorlateral (VPL) neurons in rats. Male Sprague-Dawley rats were subjected to unilateral spinal cord injury by transverse hemisection at the T13 spinal segment. Four weeks after the T13 spinal hemisection, the injured rats displayed robust allodynic behaviors on both sides of hindpaws compared to sham controls (P < 0.05). Extracellular recordings taken 4 weeks after the hemisection revealed that wide dynamic range (WDR) neurons had significantly increased spontaneous and brush-, pressure-, and pinch-evoked activities, respectively, on both sides of the thalamic VPL regions (P < 0.05). In contrast, low threshold (LT) neurons showed only an increase in the brush-evoked activity compared to sham controls (P < 0.05). However, afterdischarge activity in both types of neurons showed no changes. In addition, both sides of the thalamic VPL regions showed higher incidences of WDR neurons. In conclusion, our data demonstrate that spinal unilateral injury induces bilaterally increased evoked activity in thalamic VPL neurons.

Serotonin receptors modulate trigeminovascular responses in ventroposteromedial nucleus of thalamus: a migraine target?

Triptans, serotonin 5-HT(1B/1D), receptor agonists, which are so effective in acute migraine, are considered to act directly on the trigeminovascular system. Using an in vivo model of trigeminovascular nociception, we report a potentially novel action for the triptans within the somatosensory thalamus. Both microiontophoretically applied and intravenous naratriptans potently and reversibly modulate nociceptive neurotransmission by trigeminovascular thalamic neurons in the ventroposteromedial nucleus (VPM) driven by stimulation of the superior sagittal sinus. Naratriptan also suppresses l-glutamate activated trigeminovascular VPM neurons. Co-ejection of naratriptan with the 5-HT(1B/1D) receptor antagonist GR127935 antagonized this effect. (S)-WAY 100135 the 5-HT(1A) receptor antagonist also partially inhibited the effect of naratriptan in the VPM when co-ejected with it. Taken together, the new data suggest a potential effect of triptans in the VPM nucleus of the thalamus acting through 5-HT(1A/1B/1D) mechanisms, and offer an entirely new direction for the development of and understanding of the effects of anti-migraine medicines.

Serotonin as a modulator of glutamate- and GABA-mediated neurotransmission: implications in physiological functions and in pathology

The neurotransmitter serotonin (5-HT), widely distributed in the central nervous system (CNS), is involved in a large variety of physiological functions. In several brain regions 5-HT is diffusely released by volume transmission and behaves as a neuromodulator rather than as a "classical" neurotransmitter. In some cases 5-HT is co-localized in the same nerve terminal with other neurotransmitters and reciprocal interactions take place. This review will focus on the modulatory action of 5-HT on the effects of glutamate and gamma-amino-butyric acid (GABA), which are the principal neurotransmitters mediating respectively excitatory and inhibitory signals in the CNS. Examples of interaction at pre-and/or post-synaptic levels will be illustrated, as well as the receptors involved and their mechanisms of action. Finally, the physiological meaning of neuromodulatory effects of 5-HT will be briefly discussed with respect to pathologies deriving from malfunctioning of serotonin system.

Reduced basal release of serotonin from the ventrobasal thalamus of the rat in a model of neuropathic pain

Drugs that inhibit reuptake of monoamines are frequently used to treat pain syndromes, e.g. neuropathy or fibromyalgia, where mechanical allodynia is present. Several lines of evidence suggest the involvement of supraspinal sites of action of these drugs. However, a direct study of supraspinal serotonin (5-HT) or norepinephrine (NE) release in an animal model in which allodynia is expressed, e.g. neuropathy, has not been done. The ventrobasal (VB) thalamus and the hypothalamus are major supraspinal projection regions for spinal neurons that transmit nociceptive information and are innervated by monoaminergic fibers. This study determined if peripheral neuropathy would induce changes in extracellular monoamines in VB thalamus and hypothalamus. Male Sprague-Dawley rats had spinal nerve roots L5 and L6 tightly ligated (neuropathic rats; NP) or sham (SHAM) surgery; contralateral and ipsilateral VB thalamus and contralateral hypothalamus were dialyzed with modified artificial cerebral spinal fluid (aCSF), with and without fluoxetine. NP rats had significantly decreased 5-HT content in dialysates of the contralateral VB thalamus compared with SHAM rats with (82% decrease) or without (63% decrease) fluoxetine in the perfusion medium over the 180 min of the study. There were no differences in the ipsilateral VB thalamus. In contrast, release of 5-HT was unchanged in the hypothalamic dialysates of SHAM vs. NP rats. NE release was not different in dialysates of either the VB thalamus or hypothalamus of SHAM vs. NP rats. Synthesis of 5-HT, as assessed by accumulation of 5-hydroxytrytophan after treatment with an L-amino acid decarboxylase inhibitor, was not different between NP and SHAM rats in VB thalamic and hypothalamic brain tissue. This study is the first to demonstrate changes in monoamine release supraspinally in NP rats. The differential effect between VB thalamus and hypothalamus suggests that a terminal field change may be involved. Putative mechanisms for mediating this change include alterations of GABA-ergic systems and/or plasticity related to alterations in N-methyl-D-aspartate receptor activation and nitric oxide release related to afferent hyperactivity induced by neuropathic pain.

Serotonin in the inferior colliculus

It has been recognized for some time that serotonin fibers originating in raphe nuclei are present in the inferior colliculi of all mammalian species studied. More recently, serotonin has been found to modulate the responses of single inferior colliculus neurons to many types of auditory stimuli, ranging from simple tone bursts to complex species-specific vocalizations. The effects of serotonin are often quite strong, and for some neurons are also highly specific. A dramatic illustration of this is that serotonin can change the selectivity of some neurons for sounds, including species-specific vocalizations. These results are discussed in light of several theories on the function of serotonin in the IC, and of outstanding issues that remain to be addressed.

Influence of serotonin on the glutamate-induced excitations of secondary vestibular neurons in the rat

The excitatory responses evoked by glutamate and its agonists in secondary vestibular neurons of the rat were studied during microiontophoretic application of 5-hydroxytryptamine (5-HT). Ejection of 5-HT modified neuronal responsiveness to glutamate in 86% of the studied units, the effect being a depression of the excitatory responses in two-thirds of cases and an enhancement in the remaining third. 5-HT was also effective in modifying 94% of the responses evoked by N-methyl-d-aspartate (NMDA), inducing a depressive effect in 76% of cases and an enhancement in the remaining ones. Quisqualate-evoked effects were depressed and enhanced by 5-HT in about the same number of cases; in contrast, kainate-evoked responses were enhanced. The depressive action of 5-HT was mimicked by application of alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT), a 5-HT(2) receptor agonist, whereas the enhancing effect could be evoked by application of 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT(1A) receptor agonist. The 5-HT(2) receptor antagonist ketanserin was able to reduce, but not to block totally, the depressive action of 5-HT on glutamate- or NMDA-evoked responses. No significant difference was detected between neuronal responses in the lateral and the superior vestibular nucleus. These results indicate that 5-HT is able to modulate the responsiveness of secondary vestibular neurons to excitatory amino acids. Its action is mostly depressive, involves 5-HT(2) receptors, and is exerted on NMDA receptors. A minor involvement of other 5-HT receptors (at least 5-HT(1A)) and other glutamate receptors (for quisqualate and kainate) in the modulatory action of 5-HT is plausible.

Long term depletion of serotonin leads to selective changes in glutamate receptor subunits

The present study was carried out to clarify possible modulation mechanism of serotonin (5-HT) on glutamatergic neurotransmission in the rat cerebral cortex. 5-HT was depleted by a 5-HT metabolite blocker (para-chlorophenylalanine; pCPA) for a week. Receptor binding experiments using (S)-[(3)H]alpha-amino-3-hydroxy-5-methylisoxazol-4-propionic acid (AMPA) showed a considerable increase in B(max) value of the membrane samples prepared from the cerebral cortex of rats compared with that of control animals received saline. In contrast, B(max) value of the [(3)H]MK-801 binding experiments for NMDA receptor was not changed by pCPA-treatment. Changes in the density of each AMPA receptor subtype were examined in the cerebral cortex by immunoblot analyses using antibodies against AMPA receptor subunits. The density of immunoreactive bands with receptor subtype specific antibodies against GluR2/3 and GluR2 receptors was increased, whereas that of GluR1 receptors was decreased. Considering GluR2 receptor subtype inhibits Ca(2+) influx into neurons, the present study suggests that 5-HT appears to modulate synaptic plasticity by regulating the density of each AMPA receptor subtype.

Serotonergic modulation of the responses to excitatory amino acids of rat dorsal horn neurons in vitro: implications for somatosensory transmission

Serotonin (5-HT) is one of the major transmitters involved in supraspinal control of somatic sensation and nociception. The aim of the present study was to investigate if the 5-HT-induced modulation of sensory transmission in the dorsal horn could be due to regulation of neuronal responses to excitatory amino acids. Experiments were performed in an in vitro preparation of the young rat spinal cord. Responses to dorsal root stimulation (DR-EPSP) and to droplet application of N-methyl-D-aspartic acid (NMDA) and alpha-amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazolepropanoic acid (AMPA) were obtained by means of intracellular recordings of dorsal horn neurons. Bath applications of 5-HT (50 microM) generally caused reductions in amplitude and integrated area of DR-EPSPs and of responses to NMDA but the responses to AMPA were unaltered. A linear correlation was found between the effects of 5-HT on the DR-EPSP and on the NMDA response measured as percentage change in amplitude (r2 = 0.45; P < or = 0.01) and integrated area (r2 = 0.77; P < or = 0.001). The NMDA receptor antagonist d-AP5 (50 microM) completely abolished NMDA responses and caused a depression of the DR-EPSP similar to that of 5-HT. The 5-HT1 receptor agonist 5-carboxamidotryptamine (5-CT; 1 microM) mimicked the depressant effects of 5-HT but had a stronger depressant action on the DR-EPSP than 5-HT. The depression of NMDA responses induced by 5-HT and 5-CT was tetrodotoxin (1 microM) resistant. It is concluded that 5-HT-induced depression of NMDA responses explains partially the depressant action of 5-HT on dorsal horn synaptic transmission activating a postsynaptic site sensitive to 5-CT. The possible activation of coadjuvant mechanisms is discussed.

Effects of iontophoretically applied serotonin on three classes of physiologically characterized putative pain modulating neurons in the rostral ventromedial medulla of lightly anesthetized rats

The importance of the rostral ventromedial medulla (RVM) in nociceptive modulation is well documented, and several lines of evidence point to a role for serotonin (5HT) in regulating the activity of pain modulating neurons in this region. The aim of the present study was to examine the effect of iontophoretically applied 5HT upon the firing of three physiologically defined classes of RVM neurons with distinct roles in pain modulation. The predominant effect across all classes was a facilitation of ongoing or evoked activity. A minority of cells within each class were inhibited by 5HT itself, but agonists at 5HT1 receptor types inhibited the majority of cells tested. The results thus indicate that the behavioral effects of manipulating 5HT within the RVM cannot be attributed to a selective influence on a particular cell class.

Evidence that excitatory amino acids mediate afferent input to the primate somatosensory thalamus

Neurons in and posterior to the primate sensory nucleus of the thalamus (VPL) were tested for excitation by iontophoretically applied excitatory amino acid agonists. In addition, in several cells the effects of iontophoretically applied excitatory amino acid antagonists on the responses to cutaneous stimuli were examined. All neurons showed responses to all agonists. The responses of several neurons to cutaneous stimuli were attenuated or blocked by local administration of receptor antagonists. These results provide initial evidence that somatosensory responses of neurons in the primate thalamus are mediated by excitatory amino acids.

Functions of ionotropic and metabotropic glutamate receptors in sensory transmission in the mammalian thalamus

The thalamic relay nuclei play a pivotal role in gating and processing sensory information en route to the cerebral cortex. The major ascending sensory afferents and the descending cortico-fugal afferents to the thalamus almost certainly use the excitatory amino acid L-glutamate as their transmitter. This paper reviews the nature of this transmission in terms of the receptor types which may be used (NMDA, AMPA, kainate and metabotropic glutamate receptors), their electrophysiological and pharmacological properties, and their differential location in the thalamus on neurones, terminals and glial elements. Whilst AMPA receptors, probably of more than one variety, are likely to mediate fast transmission in the thalamus, the contributions of NMDA receptors and metabotropic glutamate receptors to sensory responses under different stimulus conditions may be more varied. This is discussed in the context of the possible functional significance of the interplay of L-glutamate-gated currents with intrinsic membrane currents of thalamic neurones. The interaction of L-glutamate transmission with other modulators (acetylcholine, noradrenaline, serotonin, glycine, D-serine, nitric oxide, arginine, redox agents) is considered.

Effects of 5-HT and 5-HT1A receptor agonists and antagonists on dorsal vagal preganglionic neurones in anaesthetized rats: an ionophoretic study

1. Effects of ionophoretic administration of 5-hydroxytryptamine (5-HT) and selective 5-HT1A receptor agonists and antagonists on identified dorsal vagal preganglionic and dorsal raphe neurones were studied in pentobarbitone sodium or chloral hydrate-anaesthetized rats, respectively. 2. Extracellular recordings were made from 176 preganglionic neurones in the dorsal vagal nucleus (DVN). Application of 5-HT at low currents (< or = 10 nA) increased the activity of these neurones. However, at increased currents (10-60 nA), it had a predominantly depressant effect. Application of selective 5-HT1A receptor antagonists, (+/-)-pindolol or WAY-100635, attenuated the excitatory responses evoked by 5-HT. 3. Ionophoresis of the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (5-30 nA) increased the firing rate of 19 and decreased that of 67 of the 104 vagal neurones tested. Other 5-HT1A receptor agonists, flesinoxan and N,N-di-n-propyl-5-carboxamidotryptamine (DP-5-CT) also had predominantly depressant effects. 4. (+/-)-Pindolol attenuated excitations but not inhibitions evoked by 8-OH-DPAT. Surprisingly, WAY-100635 and 8-OH-DPAT produced the same effect on these neurones and when applied together, WAY-100635 failed to attenuate the 8-OH-DPAT responses. 5. Dorsal raphe neurones were identified by their low, regular firing rate and their subsequent histological localization. 8-OH-DPAT reversibly reduced the activity in all 7 neurones tested and this was antagonized by WAY-100635 in all 3 neurones tested. 6. In conclusion, 5-HT applied to vagal preganglionic neurones evokes excitatory and inhibitory responses. The excitatory, but not the inhibitory responses may be mediated, at least in part, by activation of 5-HT1A receptors.

Acute noise stress reduces [3H]5-hydroxytryptamine uptake in rat brain synaptosomes: protective effects of buspirone and tianeptine

Acute noise stress decreased [3H]5-hydroxytryptamine ([3H]5-HT) uptake in synaptosomes from rat hypothalamus, hippocampus and cerebral cortex. The decrease was due to the maximum rate of [3H]5-HT uptake, which peaked 30 min after stress and partly returned to resting values within 4 h, with no changes in affinity (Km values). No changes in [3H]paroxetine binding and basal [3H]5-HT release were found in stressed rats. Tianeptine, given at the dose of 10 mg/kg 1 h before stress, counteracted the noise-induced decrease of 5-HT uptake, since it increased [3H]5-HT uptake in both resting and stressed animals, but did not prevent the rise in plasma corticosterone of stressed rats. Buspirone pretreatment had no effect on [3H]5-HT uptake in resting rats but prevented the noise-induced decrease in [3H]-HT uptake. Diazepam did not modify either the basal or the noise-induced reduction in [3H]5-HT uptake. The evidence that treatments reducing extrasynaptic 5-HT, by increasing its reuptake (tianeptine) or reducing its release (buspirone) in innervated regions are able to modify the stress-induced decrease in 5-HT uptake, further confirms the importance of serotonin in the mechanisms mediating neurochemical responses to stress.

Serotonin differentially modulates responses mediated by specific excitatory amino acid receptors in the rat locus coeruleus

Microiontophoretic application of selective agonists for the three major excitatory amino acid receptors, N-methyl-D-aspartate (NMDA), quisqualate and kainate, increased the discharge rate of noradrenergic locus coeruleus (LC) neurons in vivo. NMDA activation was selectively attenuated by iontophoretic application of 2-amino-5-phosphonopentanoate (AP5), an antagonist at NMDA receptors, whereas kainate- and quisqualate-evoked responses were attenuated by both NMDA and non-NMDA antagonists iontophoresis. NMDA- and quisqualate-evoked responses were significantly decreased by co-iontophoresis of serotonin (5-HT). When the NMDA receptor-mediated component of the response to kainate was blocked with AP5 iontophoresis, 5-HT increased the response of LC neurons to kainate. These results revealed that 5-HT differentially modulates the responsiveness of LC neurons to excitatory amino acids, depending on the receptor subtypes responsible for the neuronal activation.

Serotonin depresses excitatory amino acid-induced excitation of cerebellar Purkinje cells in the adult rat in vivo

The modulatory effects of serotonin (5-HT) on excitatory amino acid (EAA)-induced excitations of Purkinje cells (PCs) were examined in urethane-anesthetized adult male rats using microiontophoresis and extracellular recordings. Application of 5-HT had minimal effects on the spontaneous firing rates of PCs but depressed excitations elicited by glutamate (Glu), aspartate (Asp), kainate (KA), and quisqualate (QA), and to a lesser extent those of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA). Excitations induced by the metabotropic EAA agonist, (+-)-1-aminocyclopentane-trans-1,3-dicarboxylate (t-ACPD), were unaffected by 5-HT. In summary, 5-HT depressed EAA-mediated excitations with the following rank order of effectiveness: Glu = Asp = KA = QA > AMPA >> t-ACPD. These findings suggest that 5-HT shows some selectivity in its modulation of EAA-mediated excitations of PCs and thus may serve an important neuromodulatory role in the cerebellum.

Serotoninergic innervation of the dorsal column nuclei and its relation to cytoarchitectonic subdivisions: an immunohistochemical study in cats and monkeys (Aotus trivirgatus)

The serotoninergic innervation of the dorsal column nuclei (DCN) was investigated in cats and owl monkeys (Aotus trivirgatus) with immunohistochemical methods. A dense network of serotonin-immunoreactive fibers was present in the reticular regions of DCN in cats, and in the pars triangularis of the cuneate nucleus and the peripheral and caudal regions of the gracile nucleus in owl monkeys. The cat's cluster regions and the monkey's rotund regions were more sparsely innervated. Electron microscopic examination showed that the labeled fibers were thin and unmyelinated. Vesicle-containing, terminal-like structures were small. They were in contact with dendrites, other terminals and cell bodies, but synapses were rare. The results demonstrate that the serotoninergic projection to the DCN in both cats and owl monkeys is heterogeneously distributed in a pattern that is faithfully related to the cytoarchitectonic subdivisions of the DCN. The densely innervated reticular regions in the DCN of cats and the corresponding regions in monkeys are predominantly involved in the processing of sensory information to the cerebellum, either directly, or indirectly through projections to the inferior olive, pontine gray, tectum, pretectum, red nucleus, or zona incerta. Thus, the present findings suggest that the serotoninergic innervation of the DCN is primarily related to the DCN's involvement in motor functions.

Tianeptine attenuates stress-induced morphological changes in the hippocampus

Repeated 6-h daily restraint stress over 21 days reduces length and number of branch points of hippocampal CA3c pyramidal dendrites in the hippocampal formation of adult male rats. This effect is mimicked by daily injections of 40 mg/kg corticosterone. Daily treatment with tianeptine (15 mg/kg) prior to stress sessions or the corticosterone treatment prevented these effects of stress or corticosterone, respectively. Tianeptine treatment did not prevent the effects of stress to increase adrenal/body weight ratio, nor did it prevent the effects of stress to decrease body weight gain, indicating that its actions are not mediated solely by effects on stress-induced secretion of corticosterone. Because tianeptine is known to enhance neural uptake of serotonin, these results suggest that the serotonergic system may be involved in modulating stress and corticosterone effects on dendritic morphology.

Simultaneous Demonstration of Serotonin-immunoreactive Terminals and GABAergic Neurons in the VPL Nucleus of the Cat Thalamus

Pre-embedding immunoperoxidase (for serotonin) and postembedding immunogold (for gamma-aminobutyric acid; GABA) labelling were combined at light and electron microscopic levels to demonstrate the neuronal targets of serotonin (5-HT) afferents in the ventral posterior lateral nucleus (VPL) of the cat thalamus. 5-HT-immunoreactive fibres and terminal varicosities were found in close proximity to GABA-immunoreactive interneurons and non-GABAergic relay neurons. Ultrastructurally, the vast majority of 5-HT terminals made close membrane contacts without overt membrane specializations with GABAergic axon terminals, GABAergic presynaptic dendrites and GABAergic somata. A very small number of 5-HT terminals formed typical asymmetrical synapses with GABAergic presynaptic dendrites and with dendritic shafts of relay cells. Some 5-HT terminals participated with the presynaptic dendrites in triadic synaptic arrangements. These findings suggest a dual innervation pattern by 5-HT afferents in VPL and the release of 5-HT in large part at sites not associated with morphologically detectable synapses.

Effects of selective neurotoxic lesion of lumbosacral serotonergic and noradrenergic systems on autotomy behaviour in rats

Male rats underwent unilateral ligation and transection of the sciatic and saphenous nerves 2, 7 or 14 days after being injected intrathecally (at the thoracolumbar junction) with 6-hydroxydopamine (6-OHDA), 5,6-dihydroxytryptamine (5,6-DHT) or vehicle, and the development of autotomy was monitored. The effects of both neurotoxins on cervicothoracic (C5-T1) and lumbosacral (L1-S1) norepinephrine (NE), dopamine (DA) and 5-hydroxytryptamine (5-HT) spinal cord levels were analysed by HPLC in separate groups of rats. 6-OHDA treatment (20 micrograms/10 microliters) produced a rapid (from day 2) and significant (90-95%) fall in NE content only at L1-S1. 5,6-DHT administration (20 micrograms/10 microliters) produced a gradual (68%, 90% and 94%, at 2, 7 and 14 days, respectively) and selective depletion of 5-HT only at L1-S1. DA levels remained essentially unchanged after both neurotoxins. No differences in monoamine levels were detected among groups injected with vehicle. The main effects of neurotoxins on autotomy were: (1) a significant delay in the onset of autotomy in the rats injected with 6-OHDA 2 days before neurectomy; (2) a trend to autotomize earlier and more severely in the rats injected with 5,6-DHT 7 days before neurectomy and (3) an almost complete suppression of autotomy in the rats injected with 5,6-DHT 14 days before neurectomy. These results revealed that the expression of autotomy in rats can be modulated by interfering with spinal cord serotonergic activity and suggest new possible avenues for the treatment of certain specific pain diseases, such a phantom limb pain, by using selective agents capable of activating or blocking spinal cord serotonergic receptor subtypes.

Serotonin1A facilitation of frog motoneuron responses to afferent stimuli and to N-methyl-D-aspartate

The effects of serotonin and excitatory amino acids on motoneurons were examined by sucrose gap recordings from the ventral root of the isolated, hemisected frog spinal cord superfused with magnesium-free, carbonate-buffered Ringer solution. Low concentrations of serotonin (0.1 microM) and the serotonin1A agonist 8-hydroxy-2-(n-dipropylamino)tetralin (8-OH-DPAT; 0.01 microM) significantly increased the duration and amplitude of the polysynaptic components of ventral root potentials produced by dorsal root stimulation. The facilitations of the ventral root potentials were blocked by the serotonin1A antagonist spiroxatrine, but were unaffected by the serotonin2 antagonist ketanserin or the serotonin3 antagonist 1 alpha H,3 alpha,5 alpha H-tropan-3-yl-3,-dichlorobenzoate (MDL 72222). The actions of 0.1 microM serotonin on motoneuron depolarizations evoked by the putative excitatory amino acid transmitters L-glutamate and L-aspartate were quite variable, but in the presence of ketanserin (20 microM), small consistent increases in amino acid-induced motoneuron depolarizations were observed. 8-OH-DPAT significantly enhanced motoneuron depolarizations elicited by the selective excitatory amino acid agonist N-methyl-D-aspartate in both normal and tetrodotoxin-containing Ringer solution. Quisqualate-induced motoneuron depolarizations were also facilitated by 8-OH-DPAT in normal Ringer solution, but these increases were eliminated by addition of either tetrodotoxin or the N-methyl-D-aspartate antagonist D(-)-2-amino-5-phosphonovalerate to the Ringer superfusate. Kainate-depolarizations were not altered by low concentrations of serotonin or 8-OH-DPAT. Prior exposure of the cord to spiperone, but not ketanserin or MDL 72222 blocked the enhancement of N-methyl-D-aspartate-induced motoneuron depolarizations by 8-OH-DPAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in motor performance and rubral single unit activity in cats after microinjections of serotonin into the red nucleus area

The serotonergic control exerted on the red nucleus (RN) was studied in unrestrained cats during the performance of a simple reaction time task which consisted of releasing a lever in response to an auditory go-signal. The effects of microinjections of serotonin-oxalate salt into the rubral area on the motor activity and on the firing of neurons recorded concomitantly in the red nucleus were investigated. Injections of serotonin (5-HT) (200-400 ng) into the red nucleus or its dorsal border induced subtle alterations in the conditioned motor performances but had no major effects on the spontaneous motor behavior. The changes in the conditioned motor output (an increase in the static force exerted on the lever and a speeding up of the lever release) are reminiscent of the facilitatory influence of serotonin on various motor reflexes previously reported. Changes in the neuronal activity were observed concomitantly with the effects on the motor output: 5-HT either enhanced or reduced the firing rate of the rubral neurons. These effects were apparently dependent on the discharge pattern of the neurons during the static motor activity. The results suggest that the serotonergic input to the red nucleus may participate in motor control by exerting a dual modulatory action on the activity of rubral neurons.

Serotoninergic innervation of the thalamus in the primate: an immunohistochemical study

Little is known of the serotoninergic innervation of the thalamus in primates; therefore, we undertook a detailed study of the distribution of 5-hydroxytryptamine (5-HT)-immunoreactive neuronal profiles in the thalamus of the squirrel monkey (Saimiri sciureus) with a specific antibody directly raised against 5-HT. All thalamic nuclei in the squirrel monkey displayed 5-HT-immunoreactive fibers, but none contained immunopositive cell bodies. The 5-HT innervation of the thalamus derived from extrinsic fibers arising mostly from the midbrain raphe nuclei and forming the transtegmental system. Most of the fibers destined to the thalamus collected into a major bundle that swept dorsoventrally within the midbrain tegmentum and coursed beneath the thalamus along its entire caudorostral extent. Several fiber fascicles broke off from this main bundle at different levels and ascended dorsally to innervate the various thalamic nuclei. Overall, the 5-HT innervation of the thalamus in the squirrel monkey was more massive than would have been expected from earlier studies in nonprimate species. Marked differences in the regional density of innervation were noted both between the various nuclei and within single nuclei. The most densely innervated nuclei were those delineating the principal subdivisions of the thalamic mass, that is, the midline, rostral intralaminar, limitans, and reticular nuclei, where very dense fields of isolated axonal varicosities occurred. In contrast to the rostral intralaminar nuclei, which were rather uniformly innervated, the centre médian/parafascicular complex contained immunoreactive fibers and isolated varicosities distributed according to a mediolateral gradient. The habenula and the ventral anterior nucleus were among the most weakly innervated nuclei. In the latter nucleus, as well as in more densely innervated nuclei, thin varicose fibers formed numerous pericellular contacts on cell bodies and proximal dendrites of thalamic neurons. The 5-HT innervation of the lateral nuclear group as well as that of the medial and lateral geniculate nuclei ranged from very weak to dense. The mediodorsal nucleus displayed a highly heterogeneous 5-HT innervation that varied from weak in its central portion to moderate or dense in its medial and lateral borders. A moderate 5-HT innervation was observed in the anterior nuclear group. The surprisingly dense and heterogeneous 5-HT innervation of the thalamus noted in the present study suggests that serotonin may be involved in several specific functions of the thalamus in primates.

Modulation of [3H]-glutamate binding by serotonin in the rat hippocampus: an autoradiographic study

Serotonin (5-HT) added in vitro (10 microM) increased [3H]-glutamate specific binding in the rat hippocampus, reaching statistical significance in layers rich in N-Methyl-D-Aspartate sensitive glutamate receptors. This effect was explained by a significant increase in the apparent affinity of [3H]-glutamate when 5-HT is added in vitro. Two days after lesion of serotonergic afferents to the hippocampus with 5,7-Dihydroxytryptamine [3H]-glutamate binding was significantly decreased in the CA3 region and stratum lacunosum moleculare of the hippocampus, this reduction being reversed by in vitro addition of 10 microM 5-HT. The decrease observed is due to a significant reduction of quisqualate-insensitive (radiatum CA3) and kainate receptors (strata oriens, radiatum, pyramidal of CA3). Five days after lesion [3H]-glutamate binding increased significantly in the CA3 region of the hippocampus but was not different from sham animals in the other hippocampal layers. Two weeks after lesion [3H]-glutamate binding to quisqualate-insensitive receptors was increased in all the hippocampal layers, while kainate and quisqualate-sensitive receptors were not affected. These data are consistent with the possibility that 5-HT is a direct positive modulator of glutamate receptor subtypes.