The cellular localization of 5-HT2A receptors in the spinal cord and spinal ganglia of the adult rat

Expression Pattern of 5-HT (Serotonin) Receptors during Normal Development of the Human Spinal Cord and Ganglia and in Fetus with Cervical Spina Bifida

The expression of 5-HT (serotonin) receptors (sr) was analyzed in the spinal cord and ganglia of 15 human conceptuses (5–10-weeks), and in the 9-week fetus with spina bifida. We used immunohistochemical method to detect sr-positive, apoptotic (caspase-3) and proliferating (Ki-67) cells, double immunofluorescence for co-localization with protein gene peptide (pgp) 9.5 and GFAP, as well as semiquantification and statistical measurements. Following the neurulation process, moderate (sr1 and sr2) and mild (sr3) expression characterized neuroblasts in the spinal cord and ganglia. During further development, sr1 expression gradually increased in the motoneurons, autonomic and sensory neurons, while sr2 and sr3 increased strongly in floor and roof plates. In the ganglia, sr3 expression increased during limited developmental period, while sr1 and sr2 increased throughout the investigated period. Co-expression of sr/pgp 9.5 characterized developing neurons, while sr/GFAP co-localized in the roof plate. In the spinal cord and ganglia of malformed fetus, weaker sr1 and sr2 and stronger sr3 expression accompanied morphological abnormalities. Anomalous roof plate morphology showed an excess of apoptotic and proliferating cells and increased sr3 expression. Our results indicate a human-species specific sr expression pattern, and the importance of sr1 in neuronal differentiation, and sr2 and sr3 in the control of the roof plate morphogenesis in normal and disturbed development.

Serotonin-induced vascular permeability is mediated by transient receptor potential vanilloid 4 in the airways and upper gastrointestinal tract of mice

Endothelial and epithelial cells form physical barriers that modulate the exchange of fluid and molecules. The integrity of these barriers can be influenced by signaling through G protein-coupled receptors (GPCRs) and ion channels. Serotonin (5-HT) is an important vasoactive mediator of tissue edema and inflammation. However, the mechanisms that drive 5-HT-induced plasma extravasation are poorly defined. The Transient Receptor Potential Vanilloid 4 (TRPV4) ion channel is an established enhancer of signaling by GPCRs that promote inflammation and endothelial barrier disruption. Here, we investigated the role of TRPV4 in 5-HT-induced plasma extravasation using pharmacological and genetic approaches. Activation of either TRPV4 or 5-HT receptors promoted significant plasma extravasation in the airway and upper gastrointestinal tract of mice. 5-HT-mediated extravasation was significantly reduced by pharmacological inhibition of the 5-HT2A receptor subtype, or with antagonism or deletion of TRPV4, consistent with functional interaction between 5-HT receptors and TRPV4. Inhibition of receptors for the neuropeptides substance P (SP) or calcitonin gene-related peptide (CGRP) diminished 5-HT-induced plasma extravasation. Supporting studies assessing treatment of HUVEC with 5-HT, CGRP, or SP was associated with ERK phosphorylation. Exposure to the TRPV4 activator GSK1016790A, but not 5-HT, increased intracellular Ca2+ in these cells. However, 5-HT pre-treatment enhanced GSK1016790A-mediated Ca2+ signaling, consistent with sensitization of TRPV4. The functional interaction was further characterized in HEK293 cells expressing 5-HT2A to reveal that TRPV4 enhances the duration of 5-HT-evoked Ca2+ signaling through a PLA2 and PKC-dependent mechanism. In summary, this study demonstrates that TRPV4 contributes to 5-HT2A-induced plasma extravasation in the airways and upper GI tract, with evidence supporting a mechanism of action involving SP and CGRP release.

P11 deficiency increases stress reactivity along with HPA axis and autonomic hyperresponsiveness

Patients suffering from mood disorders and anxiety commonly exhibit hypothalamic-pituitary-adrenocortical (HPA) axis and autonomic hyperresponsiveness. A wealth of data using preclinical animal models and human patient samples indicate that p11 deficiency is implicated in depression-like phenotypes. In the present study, we used p11-deficient (p11KO) mice to study potential roles of p11 in stress responsiveness. We measured stress response using behavioral, endocrine, and physiological readouts across early postnatal and adult life. Our data show that p11KO pups respond more strongly to maternal separation than wild-type pups, even though their mothers show no deficits in maternal behavior. Adult p11KO mice display hyperactivity of the HPA axis, which is paralleled by depression- and anxiety-like behaviors. p11 was found to be highly enriched in vasopressinergic cells of the paraventricular nucleus and regulates HPA hyperactivity in a V_1B receptor-dependent manner. Moreover, p11KO mice display sympathetic-adrenal-medullary (SAM) axis hyperactivity, with elevated adrenal norepinephrine and epinephrine levels. Using conditional p11KO mice, we demonstrate that this SAM hyperactivity is partially regulated by the loss of p11 in serotonergic neurons of the raphe nuclei. Telemetric electrocardiogram measurements show delayed heart rate recovery in p11KO mice in response to novelty exposure and during expression of fear following auditory trace fear conditioning. Furthermore, p11KO mice have elevated basal heart rate in fear conditioning tests indicating increased autonomic responsiveness. This set of experiments provide strong and versatile evidence that p11 deficiency leads to HPA and SAM axes hyperresponsiveness along with increased stress reactivity.

Serotonergic Modulation of Nociceptive Circuits in Spinal Cord Dorsal Horn

Background:

Despite the extensive number of studies performed in the last 50 years, aimed at describing the role of serotonin and its receptors in pain modulation at the spinal cord level, several aspects are still not entirely understood. The interpretation of these results is often complicated by the use of different pain models and animal species, together with the lack of highly selective agonists and antagonists binding to serotonin receptors.

Method:

In this review, a search has been conducted on studies investigating the modulatory action exerted by serotonin on specific neurons and circuits in the spinal cord dorsal horn. Particular attention has been paid to studies employing electro-physiological techniques, both in vivo and in vitro.

Conclusion:

The effects of serotonin on pain transmission in dorsal horn depend on several factors, including the type of re-ceptors activated and the populations of neurons involved. Recently, studies performed by activating and/or recording from identified neurons have importantly contributed to the understanding of serotonergic modulation on dorsal horn circuits.

Grafting Embryonic Raphe Neurons Reestablishes Serotonergic Regulation of Sympathetic Activity to Improve Cardiovascular Function after Spinal Cord Injury

Cardiovascular dysfunction often occurs after high-level spinal cord injury. Disrupting supraspinal vasomotor pathways affects basal hemodynamics and contributes to the development of autonomic dysreflexia (AD). Transplantation of early-stage neurons to the injured cord may reconstruct the descending projections to enhance cardiovascular performance. To determine the specific role of reestablishing serotonergic regulation of hemodynamics, we implanted serotonergic (5-HT⁺) neuron-enriched embryonic raphe nucleus-derived neural stem cells/progenitors (RN-NSCs) into a complete spinal cord transection lesion site in adult female rats. Grafting embryonic spinal cord-derived NSCs or injury alone served as 2 controls. Ten weeks after injury/grafting, histological analysis revealed well-survived grafts and partial integration with host tissues in the lesion site. Numerous graft-derived serotonergic axons topographically projected to the caudal autonomic regions. Neuronal tracing showed that host supraspinal vasomotor pathways regenerated into the graft, and 5-HT⁺ neurons within graft and host brainstem neurons were transsynaptically labeled by injecting pseudorabies virus (PRV-614) into the kidney, indicating reconnected serotonergic circuits regulating autonomic activity. Using an implanted telemeter to record cardiovascular parameters, grafting RN-NSCs restored resting mean arterial pressure to normal levels and remarkably alleviated naturally occurring and colorectal distension-induced AD. Subsequent pharmacological blockade of 5-HT_2A receptors with ketanserin in RN-NSC-grafted rats reduced resting mean arterial pressure and increased heart rate in all but 2 controls. Furthermore, spinal cord retransection below RN-NSC grafts partially eliminated the recovery in AD. Collectively, these data indicate that RN-NSCs grafted into a spinal cord injury site relay supraspinal control of serotonergic regulation for sympathetic activity to improve cardiovascular function.SIGNIFICANCE STATEMENT Disruption of supraspinal vasomotor pathways results in cardiovascular dysfunction following high-level spinal cord injury. To reestablish the descending regulation of autonomic function, we transplanted serotonergic neuron enriched embryonic raphe nucleus-derived neural stem cells/progenitors into the lesion site of completely transected rat spinal cord. Consequently, grafted raphe nucleus-derived neural stem cells/progenitors acted as a neuronal relay to reconnect supraspinal center and spinal sympathetic neurons below the injury. The reconstituted serotonergic regulation of sympathetic activity led to the improvement of hemodynamic parameters and mitigated autonomic dysreflexia. Based on morphological and physiological results, this study validates the effectiveness of transplanting early-stage serotonergic neurons into the spinal cord for cardiovascular functional recovery after spinal cord injury.

Serotonergic modulation of sacral dorsal root stimulation-induced locomotor output in newborn rat

Descending neuromodulators from the brainstem play a major role in the development and regulation of spinal sensorimotor functions. Here, the contribution of serotonergic signaling in the lumbar spinal cord was investigated in the context of the generation of locomotor activity. Experiments were performed on in vitro spinal cord preparations from newborn rats (0-5 days). Rhythmic locomotor episodes (fictive locomotion) triggered by tonic electrical stimulations (2Hz, 30s) of a single sacral dorsal root were recorded from bilateral flexor-dominated (L2) and extensor-dominated (L5) ventral roots. We found that the activity pattern induced by sacral stimulation evolves over the 5 post-natal (P) day period. Although alternating rhythmic flexor-like motor bursts were expressed at all ages, the locomotor pattern of extensor-like bursting was progressively lost from P1 to P5. At later stages, serotonin (5-HT) and quipazine (5-HT2A receptor agonist) at concentrations sub-threshold for direct locomotor network activation promoted sacral stimulation-induced fictive locomotion. The 5-HT2A receptor antagonist ketanserin could reverse the agonist's action but was ineffective when fictive locomotion was already expressed in the absence of 5-HT (mainly before P2). Although inhibiting 5-HT7 receptors with SB266990 did not affect locomotor pattern organization, activating 5-HT1A receptors with 8-OH-DPAT specifically deteriorated extensor phase motor burst activity. We conclude that during the first 5 post-natal days in rat, serotonergic signaling in the lumbar cord becomes increasingly critical for the expression of fictive locomotion. Our findings therefore further underline the importance of both descending serotonergic and sensory afferent pathways in shaping locomotor activity during postnatal development. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Cervical spinal 5-HT2A and 5-HT2B receptors are both necessary for moderate acute intermittent hypoxia-induced phrenic long-term facilitation

Serotonin (5-HT) is a key regulator of spinal respiratory motor plasticity. For example, spinal 5-HT receptor activation is necessary for the induction of phrenic long-term facilitation (pLTF), a form of respiratory motor plasticity triggered by moderate acute intermittent hypoxia (mAIH). mAIH-induced pLTF is blocked by cervical spinal application of the broad-spectrum 5-HT-receptor antagonist, methysergide. However, methysergide does not allow distinctions between the relative contributions of different 5-HT receptor subtypes. Intravenous administration of the Gq protein-coupled 5-HT_2A/2C receptor antagonist ketanserin blocks mAIH-induced pLTF when administered before, but not after, mAIH; thus, 5-HT₂ receptor activation is necessary for the induction but not maintenance of mAIH-induced pLTF. However, systemic ketanserin administration does not identify the site of the relevant 5-HT_2A/2C receptors. Furthermore, this approach does not differentiate between the roles of 5-HT_2A versus 5-HT_2C receptors, nor does it preclude involvement of other Gq protein-coupled metabotropic 5-HT receptors capable of eliciting long-lasting phrenic motor facilitation, such as 5-HT_2B receptors. Here we tested the hypothesis that mAIH-induced pLTF requires cervical spinal 5-HT₂ receptor activation and determined which 5-HT₂ receptor subtypes are involved. Anesthetized, paralyzed, and ventilated adult male Sprague Dawley rats were pretreated intrathecally with cervical (~C3-C5) spinal injections of subtype selective 5-HT_2A/2C, 5-HT_2B, or 5-HT_2C receptor antagonists before mAIH. Whereas cervical spinal 5-HT_2C receptor inhibition had no impact on mAIH-induced pLTF, pLTF was no longer observed after pretreatment with either 5-HT_2A/2C or 5-HT_2B receptor antagonists. Furthermore, spinal pretreatment with an MEK/ERK MAPK inhibitor blocked phrenic motor facilitation elicited by intrathecal injections of 5-HT_2A but not 5-HT_2B receptor agonists. Thus, mAIH-induced pLTF requires concurrent cervical spinal activation of both 5-HT_2A and 5-HT_2B receptors. However, these distinct receptor subtypes contribute to phrenic motor facilitation via distinct downstream signaling cascades that differ in their requirement for ERK MAPK signaling. The demonstration that both 5-HT_2A and 5-HT_2B receptors make unique contributions to mAIH-induced pLTF advances our understanding of mechanisms that underlie 5-HT-induced phrenic motor plasticity.NEW & NOTEWORTHY Moderate acute intermittent hypoxia (mAIH) triggers a persistent enhancement in phrenic motor output, an effect termed phrenic long-term facilitation (pLTF). mAIH-induced pLTF is blocked by cervical spinal application of the broad-spectrum serotonin (5-HT) receptor antagonist methysergide, demonstrating the need for spinal 5-HT receptor activation. However, the exact type of 5-HT receptors required for initiation of pLTF remains unknown. To the best of out knowledge, the present study is the first to demonstrate that 1) spinal coactivation of two distinct Gq protein-coupled 5-HT₂ receptor subtypes is necessary for mAIH-induced pLTF, and 2) these receptors contribute to pLTF via cascades that differ in their requirement for ERK MAPK signaling.

Role of spinal 5-HT2 receptors subtypes in formalin-induced long-lasting hypersensitivity

BACKGROUND

The purpose of this study was to determine the role of spinal 5-HT2A, 5-HT2B and 5-HT2C receptors in the development and maintenance of formalin-induced long-lasting secondary allodynia and hyperalgesia in rats, as well as their expression in the dorsal root ganglia (DRG) during this process.

METHODS

0.5-1% formalin was used to produce long-lasting secondary allodynia and hyperalgesia in rats. Western blot was used to determine 5-HT2 receptors expression in DRG.

RESULTS

Formalin (0.5-1%) injection produced long-lasting (1-12 days) secondary allodynia and hyperalgesia in both ipsilateral and contralateral hind paws. Intrathecal pre-treatment or post-treatment with the 5-HT2 receptor agonist, DOI (1-10nmol), increased 0.5% formalin-induced secondary allodynia and hyperalgesia in both paws. In contrast, intrathecal pre-treatment with the selective 5-HT2A (ketanserin 1-100nmol), 5-HT2B (RS 127445 1-100nmol) or 5-HT2C (RS 102221 1-100nmol) receptor antagonists prevented and reversed, respectively, 1% formalin-induced secondary allodynia and hyperalgesia in both paws. Likewise, the pronociceptive effect of DOI (10nmol) was blocked by ketanserin, RS 127445 or RS 102221 (0.01nmol). 5-HT2A/2B/2C receptors were expressed in DRG of naïve rats. Formalin injection (1%) increased bilaterally 5-HT2A/2B receptors expression in DRG. In contrast, formalin injection decreased 5-HT2C receptors expression bilaterally in DRG.

CONCLUSION

Data suggest that spinal 5-HT2A/2B/2C receptors have pronociceptive effects and participate in the development and maintenance of formalin-induced long-lasting hypersensitivity. These receptors are expressed in DRG and their expression is modulated by formalin.

Effects of serotonergic agents on respiratory recovery after cervical spinal injury

Unilateral cervical spinal cord hemisection (i.e., C2Hx) usually interrupts the bulbospinal respiratory pathways and results in respiratory impairment. It has been demonstrated that activation of the serotonin system can promote locomotor recovery after spinal cord injury. The present study was designed to investigate whether serotonergic activation can improve respiratory function during the chronic injury state. Bilateral diaphragm electromyogram and tidal volume were measured in anesthetized and spontaneously breathing adult rats at 8 wk post-C2Hx or C2laminectomy. A bolus intravenous injection of a serotonin precursor [5-hydroxytryptophan (5-HTP), 10 mg/kg], a serotonin reuptake inhibitor (fluoxetine, 10 mg/kg), or a potent agonist for serotonin 2A receptors (TCB-2, 0.05 mg/kg) was used to activate the serotonergic system. Present results demonstrated that 5-HTP and TCB-2, but not fluoxetine, significantly increased the inspiratory activity of the diaphragm electromyogram ipsilateral to the lesion for at least 30 min in C2Hx animals, but not in animals that received sham surgery. However, the tidal volume was not increased after administration of 5-HTP or TCB-2, indicating that the enhancement of ipsilateral diaphragm activity is not associated with improvement of the tidal volume. These results suggest that exogenous activation of the serotonergic system can specifically enhance the ipsilateral diaphragmatic motor outputs, but this approach may not be sufficient to improve respiratory functional recovery following chronic cervical spinal injury.

5-HT₂ and 5-HT₇ receptor agonists facilitate plantar stepping in chronic spinal rats through actions on different populations of spinal neurons

There is considerable evidence from research in neonatal and adult rat and mouse preparations to warrant the conclusion that activation of 5-HT₂ and 5-HT_1A/7 receptors leads to activation of the spinal cord circuitry for locomotion. These receptors are involved in control of locomotor movements, but it is not clear how they are implicated in the responses to 5-HT agonists observed after spinal cord injury. Here we used agonists that are efficient in promoting locomotor recovery in paraplegic rats, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OHDPAT) (acting on 5-HT_1A/7 receptors) and quipazine (acting on 5-HT₂ receptors), to examine this issue. Analysis of intra- and interlimb coordination confirmed that the locomotor performance was significantly improved by either drug, but the data revealed marked differences in their mode of action. Interlimb coordination was significantly better after 8-OHDPAT application, and the activity of the extensor soleus muscle was significantly longer during the stance phase of locomotor movements enhanced by quipazine. Our results show that activation of both receptors facilitates locomotion, but their effects are likely exerted on different populations of spinal neurons. Activation of 5-HT₂ receptors facilitates the output stage of the locomotor system, in part by directly activating motoneurons, and also through activation of interneurons of the locomotor central pattern generator (CPG). Activation of 5-HT_7/1A receptors facilitates the activity of the locomotor CPG, without direct actions on the output components of the locomotor system, including motoneurons. Although our findings show that the combined use of these two drugs results in production of well-coordinated weight supported locomotion with a reduced need for exteroceptive stimulation, they also indicate that there might be some limitations to the utility of combined treatment. Sensory feedback and some intraspinal circuitry recruited by the drugs can conflict with the locomotor activation.

Identification of 5-HT receptor subtypes enhancing inhibitory transmission in the rat spinal dorsal horn in vitro

Background

5-hydroxytryptamine (5-HT) is one of the major neurotransmitters widely distributed in the CNS. Several 5-HT receptor subtypes have been identified in the spinal dorsal horn which act on both pre- and postsynaptic sites of excitatory and inhibitory neurons. However, the receptor subtypes and sites of actions as well as underlying mechanism are not clarified rigorously. Several electrophysiological studies have been performed to investigate the effects of 5-HT on excitatory transmission in substantia gelatinosa (SG) of the spinal cord. In the present study, to understand the effects of 5-HT on the inhibitory synaptic transmission and to identify receptor subtypes, the blind whole cell recordings were performed from SG neurons of rat spinal cord slices.

Results

Bath applied 5-HT (50 μM) increased the frequency but not amplitudes of spontaneous inhibitory postsynaptic currents (sIPSCs) in 58% of neurons, and both amplitude and frequency in 23% of neurons. The frequencies of GABAergic and glycinergic mIPSCs were both enhanced. TTX (0.5 μM) had no effect on the increasing frequency, while the enhancement of amplitude of IPSCs was eliminated. Evoked-IPSCs (eIPSCs) induced by focal stimulation near the recording neurons in the presence of CNQX and APV were enhanced in amplitude by 5-HT. In the presence of Ba²⁺ (1 mM), a potassium channel blocker, 5-HT had no effect on both frequency and amplitude. A 5-HT_2A receptor agonist, TCB-2 mimicked the 5-HT effect, and ketanserin, an antagonist of 5-HT_2A receptor, inhibited the effect of 5-HT partially and TCB-2 almost completely. A 5-HT_2C receptor agonist WAY 161503 mimicked the 5-HT effect and this effect was blocked by a 5-HT_2C receptor antagonist, N-desmethylclozapine. The amplitudes of sIPSCs were unaffected by 5-HT_2A or 5-HT_2C agonists. A 5-HT₃ receptor agonist mCPBG enhanced both amplitude and frequency of sIPSCs. This effect was blocked by a 5-HT₃ receptor antagonist ICS-205,930. The perfusion of 5-HT_2B receptor agonist had no effect on sIPSCs.

Conclusions

Our results demonstrated that 5-HT modulated the inhibitory transmission in SG by the activation of 5-HT_2A and 5-HT_2C receptors subtypes located predominantly at inhibitory interneuron terminals, and 5-HT₃ receptors located at inhibitory interneuron terminals and soma-dendrites, consequently enhanced both frequency and amplitude of IPSCs.

Selective impairment of spinal mu-opioid receptor mechanism by plasticity of serotonergic facilitation mediated by 5-HT2A and 5-HT2B receptors

Opioid analgesia is compromised by intracellular mediators such as protein kinase C (PKC). The phosphatidylinositol hydrolysis-coupled serotonin receptor 5-HT2 is ideally suited to promote PKC activation. We test the hypothesis that 5-HT2A and 5-HT2B receptors, which have been previously shown to become pro-excitatory after spinal nerve ligation (SNL), can negatively influence the ability of opioids to depress spinal excitation evoked by noxious input. Spinal superfusion with (100 nM) mu-opioid receptor (MOR)-agonist DAMGO significantly depressed C fiber-evoked spinal field potentials. Simultaneous administration of subclinical 5-HT2AR antagonist 4F 4PP (100 nM) or 5-HT2BR antagonist SB 204741 (100 nM) significantly reduced the IC50 value for DAMGO in nerve-ligated rats (97.56 nM ± 1.51 and 1.20 nM ± 1.28 respectively, relative to 104 nM ± 1.08 at the baseline condition), but not in sham-operated rats. Both antagonists failed to alter depression induced by delta-opioid receptor (DOR)-agonist D-ala2-deltorphin II after SNL as well as in the sham condition. Western blot analysis of dorsal horn homogenates revealed bilateral upregulation of 5-HT2AR and 5-HT2BR protein band densities after SNL. As assessed from double immunofluorescence labeling for confocal laser scanning microscopy, scarce dorsal horn cell processes showed co-localization color overlay for 5-HT2AR/MOR, 5-HT2BR/MOR, 5-HT2AR/DOR, or 5-HT2BR/DOR in sham-operated rats. Intensity correlation-based analyses showed significant increases in 5-HT2AR/MOR and 5-HT2BR/MOR co-localizations after SNL. These results indicate that plasticity of spinal serotonergic neurotransmission can selectively reduce spinal MOR mechanisms via 5-HT2A and 5-HT2B receptors, including upregulation of the latter and increased expression in dorsal horn neurons containing MOR.

5-HT 2A receptor activation of the external urethral sphincter and 5-HT 2C receptor inhibition of micturition: a study based on pharmacokinetics in the anaesthetized female rat

Central and peripheral 5-hydroxytryptamine (5-HT) receptors play a critical role in the regulation of micturition. Bolus doses of 5-HT(2A/2C) receptor agonists have been shown to activate the external urethral sphincter (EUS) and to inhibit micturition. This study was designed to determine the contribution of these two 5-HT receptor subtypes to activation of the EUS and inhibition of micturition utilising pharmacokinetic knowledge to better control drug exposure. Recordings of urethral and bladder pressure, EUS-Electromyogram (EMG), the micturition reflex induced by bladder filling, blood pressure and heart rate were made in anaesthetized female rats. The effects of intravenous (i.v.) infusions of the 5-HT(2) receptor agonist (2S)-1-(6-chloro-5-fluoroindol-1-yl)propan-2-amine fumarate (Ro 60-0175) in the absence or presence of the selective 5-HT(2C) receptor antagonist 6-chloro-5-methyl-N-[6-(2-methylpyridin-3-yl)oxypyridin-3-yl]-2,3-dihydroindole-1-carboxamide dihydrochloride (SB 242084) or 5-HT(2A) receptor antagonist (R)-(2,3-dimethoxyphenyl)-[1-[2-(4-fluorophenyl)ethyl]piperidin-4-yl]methanol (MDL-100,907) were studied on these variables. Continuous infusion of increasing concentrations of Ro 60-0175 only evoked EUS-EMG activity at the highest concentration, which was blocked by co-infusion of MDL-100,907 but not SB 242084. Urethral pressure was unaffected by any drug infusion. Ro 60-0175 at the lowest concentration inhibited the micturition reflex but as the concentration increased this was reversed to facilitation. SB 242084 blocked the inhibition while MDL-100,907 blocked the excitation. Activation of 5-HT(2A) not 5-HT(2C) receptors evoked EUS-EMG activity. In conclusion, 5-HT(2A) receptor activation facilitated the micturition reflex and evoked EUS-EMG while 5-HT(2C) receptor activation only inhibited the micturition reflex.

Inhibition of SNL-induced upregulation of CGRP and NPY in the spinal cord and dorsal root ganglia by the 5-HT(2A) receptor antagonist ketanserin in rats

Our previous study has demonstrated that topical and systemic administration of the 5-HT(2A) receptor antagonist ketanserin attenuates neuropathic pain. To explore the mechanisms involved, we examined whether ketanserin reversed the plasticity changes associated with calcitonin gene-related peptides (CGRP) and neuropeptide Y (NPY) which may reflect distinct mechanisms: involvement and compensatory protection. Behavioral responses to thermal and tactile stimuli after spinal nerve ligation (SNL) at L5 demonstrated neuropathic pain and its attenuation in the vehicle- and ketanserin-treated groups, respectively. SNL surgery induced an increase in CGRP and NPY immunoreactivity (IR) in laminae I-II of the spinal cord. L5 SNL produced an expression of NPY-IR in large, medium and small diameter neurons in dorsal root ganglion (DRG) only at L5, but not adjacent L4 and L6. Daily injection of ketanserin (0.3 mg/kg, s.c.) for two weeks suppressed the increase in CGRP-IR and NPY-IR in the spinal cord or DRG. The present study demonstrated that: (1) the expression of CGRP was enhanced in the spinal dorsal horn and NPY was expressed in the DRG containing injured neurons, but not in the adjacent DRG containing intact neurons, following L5 SNL; (2) the maladaptive changes in CGRP and NPY expression in the spinal cord and DRG mediated the bioactivity of 5-HT/5-HT(2A) receptors in neuropathic pain and (3) the blockade of 5-HT(2A) receptors by ketanserin reversed the evoked upregulation of both CGRP and NPY in the spinal cord and DRG contributing to the inhibition of neuropathic pain.

Central neural pathways for thermoregulation

Central neural circuits orchestrate a homeostatic repertoire to maintain body temperature during environmental temperature challenges and to alter body temperature during the inflammatory response. This review summarizes the functional organization of the neural pathways through which cutaneous thermal receptors alter thermoregulatory effectors: the cutaneous circulation for heat loss, the brown adipose tissue, skeletal muscle and heart for thermogenesis and species-dependent mechanisms (sweating, panting and saliva spreading) for evaporative heat loss. These effectors are regulated by parallel but distinct, effector-specific neural pathways that share a common peripheral thermal sensory input. The thermal afferent circuits include cutaneous thermal receptors, spinal dorsal horn neurons and lateral parabrachial nucleus neurons projecting to the preoptic area to influence warm-sensitive, inhibitory output neurons which control thermogenesis-promoting neurons in the dorsomedial hypothalamus that project to premotor neurons in the rostral ventromedial medulla, including the raphe pallidus, that descend to provide the excitation necessary to drive thermogenic thermal effectors. A distinct population of warm-sensitive preoptic neurons controls heat loss through an inhibitory input to raphe pallidus neurons controlling cutaneous vasoconstriction.

Subtype-specific changes in 5-HT receptor-mediated modulation of C fibre-evoked spinal field potentials are triggered by peripheral nerve injury

Neurotransmitter serotonin (5-HT) released from descending pain modulation pathways to the dorsal horn is crucial to spinal nociception processing. This study sought to gain insight into the modulatory roles of specific serotonin receptor subtypes in experimentally induced neuropathic pain. In rats subjected to spinal nerve ligation (SNL) surgery, we recorded field potentials evoked in the spinal dorsal horn by C fibre-input, during spinal superfusion with subtype-selective drugs. In neuropathic rats, subtype 5-HT1A agonist 8-OH-DPAT (100 nM) was found to potently depress evoked field potentials, as opposed to 5-HT2A or 5-HT2B subtype agonists TCB-2 (100 nM) or BW 723C86 (1 microM), respectively, which consistently enhanced evoked potentials. All three failed to alter spinal field potentials in sham operated rats. CP 94253 (1 microM), WAY 161503 (1 mM) or SR 57227 (at 1 microM in SNL rats, and 100 microM in sham rats), selective agonists for 5-HT1B, 5-HT2C and 5-HT3 receptors, respectively, significantly depressed evoked field potentials in both animal groups. The 5-HT4 agonist RS 67333 (1 microM) was depressant only in sham operated animals. Only after SNL, spinal superfusion with 5-HT1A- or 5-HT1B receptor-antagonists (S)-WAY 100135 (100 microM) or SB 224289 (100 microM), respectively, disinhibited C fibre-evoked potentials, whereas 5-HT2A or 5-HT2B receptor-antagonists 4F 4PP (100 microM) or SB 204741 (100 microM) depressed evoked potentials, suggesting tonic activity of all four subtypes as a consequence of experimental nerve injury. The present findings reveal profound subtype-specific changes in the functional modulatory activities of spinal serotonin receptors following peripheral nerve injury. In particular, spinal hyperexcitation promoted by receptors 5-HT2A and 5-HT2B is suggested as a novel pathogenic pathway contributing to neuropathic pain.

Serotonin receptors are involved in the spinal mediation of descending facilitation of surgical incision-induced increase of Fos-like immunoreactivity in rats

Background

Descending pronociceptive pathways may be implicated in states of persistent pain. Paw skin incision is a well-established postoperative pain model that causes behavioral nociceptive responses and enhanced excitability of spinal dorsal horn neurons. The number of spinal c-Fos positive neurons of rats treated intrathecally with serotonin, noradrenaline or acetylcholine antagonists where evaluated to study the descending pathways activated by a surgical paw incision.

Results

The number of c-Fos positive neurons in laminae I/II ipsilateral, lamina V bilateral to the incised paw, and in lamina X significantly increased after the incision. These changes: remained unchanged in phenoxybenzamine-treated rats; were increased in the contralateral lamina V of atropine-treated rats; were inhibited in the ipsilateral lamina I/II by 5-HT_1/2B/2C (methysergide), 5-HT_2A (ketanserin) or 5-HT_{1/2A/2C/5/6/7} (methiothepin) receptors antagonists, in the ipsilateral lamina V by methysergide or methiothepin, in the contralateral lamina V by all the serotonergic antagonists and in the lamina X by LY 278,584, ketanserin or methiothepin.

Conclusions

We conclude: (1) muscarinic cholinergic mechanisms reduce incision-induced response of spinal neurons inputs from the contralateral paw; (2) 5-HT_1/2A/2C/3 receptors-mediate mechanisms increase the activity of descending pathways that facilitates the response of spinal neurons to noxious inputs from the contralateral paw; (3) 5-HT_1/2A/2C and 5-HT_1/2C receptors increases the descending facilitation mechanisms induced by incision in the ipsilateral paw; (4) 5-HT_2A/3 receptors contribute to descending pronociceptive pathways conveyed by lamina X spinal neurons; (5) α-adrenergic receptors are unlikely to participate in the incision-induced facilitation of the spinal neurons.

Endogenous activation of spinal 5-hydroxytryptamine (5-HT) receptors contributes to the thermoregulatory activation of brown adipose tissue

Neurons in the rostral raphe pallidus (RPa) play an essential role in the regulation of sympathetically mediated metabolism and thermogenesis in brown adipose tissue (BAT). The presence of serotonergic neurons in the RPa that are retrogradely labeled following pseudorabies virus injections into BAT suggests that these neurons play a role in the regulation of BAT. In urethane/chloralose-anesthetized rats, whole body cooling decreased skin (-5.7 +/- 2.3 degrees C) and core (-1.3 +/- 0.2 degrees C) temperatures and resulted in an increase in BAT sympathetic nerve activity (SNA; +1,026 +/- 344% of baseline activity). Serial microinjections of the 5-hydroxytryptamine (5-HT) receptor antagonist, methysergide (1.2 nmol/site), but not saline vehicle, into the intermediolateral cell column (IML) in spinal segments T2-T5 markedly attenuated the cooling-evoked increase in BAT SNA (remaining area under the curve, AUC: 36 +/- 9% of naive cooling response). Microinjections of the 5-HT(1A) receptor antagonist, WAY-100635 (1.2 nmol/site), or the 5-HT(7) receptor antagonist, SB-269970 (1.2 nmol/site), into the T2-T5 IML also attenuated the cold-evoked increase in BAT SNA (remaining activity at peak inhibition: 47 +/- 8% and 39 +/- 12% of the initial cold-evoked response, respectively). The increases in BAT SNA evoked by microinjection of N-methyl-d-aspartate (NMDA) (12 pmol) or bicuculline (30 pmol) into the RPa were attenuated following microinjections of methysergide, but not saline vehicle, into the T2-T5 IML (NMDA remaining AUC, 64 +/- 13% of naive response; bicuculline remaining AUC, 52 +/- 5% of naive response). These results are consistent with our earlier demonstration of a potentiating effect of 5-HT within the IML on BAT SNA and indicate that activation of 5-HT(1A) and 5-HT(7) receptors in the spinal cord contributes to increases in BAT SNA and thermogenesis.

Endogenous extracellular serotonin modulates the spinal locomotor network of the neonatal mouse

Serotonin (5-HT) can potently activate and modulate spinal locomotor circuits in a variety of species. Many of these findings have been obtained by applying serotonin exogenously to the isolated spinal cord of in vitro preparations, which has the drawback of indiscriminately activating extrasynaptic receptors and neurons. To investigate the role of endogenously released serotonin in modulating locomotor networks, the selective serotonin reuptake inhibitor citalopram was used. Fictive locomotion was elicited by either electrical stimulation of the brainstem or the sacral 4 (S4) dorsal root. The addition of 20 microm of citalopram caudal to thoracic segment 5 (T5) had an overall inhibitory effect on the lumbar central pattern generator (CPG). Left-right and flexor-extensor coupling were significantly decreased, and there was also a phase shift in the flexor-extensor relationship. In addition, there was a significant decrease in burst amplitude. These effects were observed during both afferent and brainstem evoked fictive locomotion. When citalopram was added in the presence of 5-HT(1A) and 5-HT(1B) antagonists, the inhibitory effects were largely reversed. The remaining excitatory effects were mediated by 5-HT(7) and 5-HT(2) receptors. These results suggest that endogenous 5-HT release can modulate locomotor-like activity early in neonatal development.

Treatment with ketanserin produces opioid-mediated hypoalgesia in the late phase of carrageenan-induced inflammatory hyperalgesia in rats

Both pro-nociceptive and antinociceptive mediators are released in the tissues during inflammation. Balance of these two types of mediators determines the induction and maintenance of pain or hypernociception. This study was designed to explore whether 5-HT(2A) receptors in the periphery contributed to the maintenance of carrageenan-evoked hyperalgesia. Intraplantar (i.pl.) injection of carrageenan evoked hyperalgesia detected by noxious heat stimulus. The 5-HT(2A) receptor antagonist ketanserin administered i.pl. 1 h after carrageenan dose-dependently (2-20 microg) prolonged paw withdrawal latency (PWL) during the late phase (24 h) of carrageenan-evoked inflammation. Following treatments with carrageenan and ketanserin, i.pl. injection of formalin (1%) produced significantly fewer nocifensive behaviors and expression of c-Fos protein in the spinal dorsal horn, confirming the hypoalgesic status in the inflamed site. However, injection of ketanserin in naive site failed to produce hypoalgesia. The hypoalgesia was completely abolished by local or systemic injection of naloxone methiodide. The present study suggests that 5-HT(2A) receptors were involved in the maintenance of inflammatory pain, and that 5-HT suppressed inflammation-associated endogenous opioid analgesia contributing to its pro-nociceptive actions in the periphery. It implied a possible therapeutic benefit of blockade of local 5-HT(2A) receptors in the treatment of inflammatory pain.

SEROTONERGIC MECHANISMS IN AMYOTROPHIC LATERAL SCLEROSIS

Serotonin (5-HT) has been intimately linked with global regulation of motor behavior, local control of motoneuron excitability, functional recovery of spinal motoneurons as well as neuronal maturation and aging. Selective degeneration of motoneurons is the pathological hallmark of amyotrophic lateral sclerosis (ALS). Motoneurons that are preferentially affected in ALS are also densely innervated by 5-HT neurons (e.g., trigeminal, facial, ambiguus, and hypoglossal brainstem nuclei as well as ventral horn and motor cortex). Conversely, motoneuron groups that appear more resistant to the process of neurodegeneration in ALS (e.g., oculomotor, trochlear, and abducens nuclei) as well as the cerebellum receive only sparse 5-HT input. The glutamate excitotoxicity theory maintains that in ALS degeneration of motoneurons is caused by excessive glutamate neurotransmission, which is neurotoxic. Because of its facilitatory effects on glutaminergic motoneuron excitation, 5-HT may be pivotal to the pathogenesis and therapy of ALS. 5-HT levels as well as the concentrations 5-hydroxyindole acetic acid (5-HIAA), the major metabolite of 5-HT, are reduced in postmortem spinal cord tissue of ALS patients indicating decreased 5-HT release. Furthermore, cerebrospinal fluid levels of tryptophan, a precursor of 5-HT, are decreased in patients with ALS and plasma concentrations of tryptophan are also decreased with the lowest levels found in the most severely affected patients. In ALS progressive degeneration of 5-HT neurons would result in a compensatory increase in glutamate excitation of motoneurons. Additionally, because 5-HT, acting through presynaptic 5-HT1B receptors, inhibits glutamatergic synaptic transmission, lowered 5-HT activity would lead to increased synaptic glutamate release. Furthermore, 5-HT is a precursor of melatonin, which inhibits glutamate release and glutamate-induced neurotoxicity. Thus, progressive degeneration of 5-HT neurons affecting motoneuron activity constitutes the prime mover of the disease and its progression and treatment of ALS needs to be focused primarily on boosting 5-HT functions (e.g., pharmacologically via its precursors, reuptake inhibitors, selective 5-HT1A receptor agonists/5-HT2 receptor antagonists, and electrically through transcranial administration of AC pulsed picotesla electromagnetic fields) to prevent excessive glutamate activity in the motoneurons. In fact, 5HT1A and 5HT2 receptor agonists have been shown to prevent glutamate-induced neurotoxicity in primary cortical cell cultures and the 5-HT precursor 5-hydroxytryptophan (5-HTP) improved locomotor function and survival of transgenic SOD1 G93A mice, an animal model of ALS.

Locomotor-activated neurons of the cat. I. Serotonergic innervation and co-localization of 5-HT7, 5-HT2A, and 5-HT1A receptors in the thoraco-lumbar spinal cord

Monoamines are strong modulators and/or activators of spinal locomotor networks. Thus monoaminergic fibers likely contact neurons involved in generating locomotion. The aim of the present study was to investigate the serotonergic innervation of locomotor-activated neurons within the thoraco-lumbar spinal cord following induction of hindlimb locomotion. This was determined by immunohistochemical co-localization of serotonin (5-HT) fibers or 5-HT(7)/5-HT2A/5-HT1A receptors with cells expressing the activity-dependent marker c-fos. Experiments were performed on paralyzed, decerebrate cats in which locomotion was induced by electrical stimulation of the mesencephalic locomotor region. Abundant c-fos immunoreactive cells were observed in laminae VII and VIII throughout the thoraco-lumbar segments of locomotor animals. Control sections from the same segments showed significantly fewer labeled neurons, mostly within the dorsal horn. Multiple serotonergic boutons were found in close apposition to the majority (80-100%) of locomotor cells, which were most abundant in lumbar segments L3-7. 5-HT7 receptor immunoreactivity was observed on cells across the thoraco-lumbar segments (T7-L7), in a dorsoventral gradient. Most locomotor-activated cells co-localized with 5-HT7, 5-HT2A, and 5-HT1A receptors, with largest numbers in laminae VII and VIII. Co-localization of c-fos and 5-HT7 receptor was highest in the L5-L7 segments (>90%) and decreased rostrally (to approximately 50%) due to the absence of receptors on cells within the intermediolateral nucleus. In contrast, 60-80 and 35-80% of c-fos immunoreactive cells stained positive for 5-HT2A and 5-HT1A receptors, respectively, with no rostrocaudal gradient. These results indicate that serotonergic modulation of locomotion likely involves 5-HT(7)/5-HT2A/5-HT1A receptors located on the soma and proximal dendrites of serotonergic-innervated locomotor-activated neurons within laminae VII and VIII of thoraco-lumbar segments.

Participation of 5-hydroxytryptamine in pain-related behavior induced by nucleus pulposus applied on the nerve root in rats

STUDY DESIGN

The role of 5-hydroxytryptamine (5-HT) in sciatica in lumbar disc herniation (LDH) in rats was investigated.

OBJECTIVE

We evaluated the effects of exogenous 5-HT applied on the nerve root on pain-related behavior, the release of endogenous 5-HT in plasma, and the expression of 5-HT2A receptors in dorsal root ganglion (DRG) in a rat LDH model.

SUMMARY OF BACKGROUND DATA

In previous studies, 5-HT2A receptor antagonists improved sciatica in patients with LDH and attenuated pain-related behavior induced by nucleus pulposus applied to the nerve root in rats.

METHODS

Adult female Sprague-Dawley rats were divided into four experimental groups [control group; low-dose (10 microg) 5-HT-group; high-dose (30 microg) 5-HT-group; and autologous nucleus pulposus (NP) and saline group] and each drug was applied to the L5 nerve root. Von Frey tests were used for pain-behavior testing. To assess levels of endogenous 5-HT released in capillaries surrounding inflamed nerve roots, we measured 5-hydroxyindole acetic acid (5-HIAA), a metabolite of 5-HT. Expression of 5-HT2A receptors in the left L5 DRG was examined by immunohistochemical and immunoblotting analyses in the control and NP groups.

RESULTS

Mechanical withdrawal thresholds of the high-dose 5-HT and the NP groups were significantly decreased after surgery compared with the control group and recovered after 14 days in the high-dose 5-HT group. 5-HIAA in plasma was increased by nucleus pulposus applied on the nerve root for 7 days after surgery. The expression of 5-HT2A receptors was enhanced in a time-dependent manner by nucleus pulposus.

CONCLUSION

Exogenous 5-HT to the nerve root induced pain-related behavior with short-lasting effects compared with the nucleus pulposus application. 5-HIAA content in plasma and expression of 5-HT2A receptors in DRG neurons increased early time points after the nucleus pulposus application. These results suggest that 5-HT plays a role in the early phase of the chemical pathogenesis of sciatica in LDH in rats.

Central control of thermogenesis in mammals

Thermogenesis, the production of heat energy, is an essential component of the homeostatic repertoire to maintain body temperature in mammals and birds during the challenge of low environmental temperature and plays a key role in elevating body temperature during the febrile response to infection. The primary sources of neurally regulated metabolic heat production are mitochondrial oxidation in brown adipose tissue, increases in heart rate and shivering in skeletal muscle. Thermogenesis is regulated in each of these tissues by parallel networks in the central nervous system, which respond to feedforward afferent signals from cutaneous and core body thermoreceptors and to feedback signals from brain thermosensitive neurons to activate the appropriate sympathetic and somatic efferents. This review summarizes the research leading to a model of the feedforward reflex pathway through which environmental cold stimulates thermogenesis and discusses the influence on this thermoregulatory network of the pyrogenic mediator, prostaglandin E(2), to increase body temperature. The cold thermal afferent circuit from cutaneous thermal receptors ascends via second-order thermosensory neurons in the dorsal horn of the spinal cord to activate neurons in the lateral parabrachial nucleus, which drive GABAergic interneurons in the preoptic area to inhibit warm-sensitive, inhibitory output neurons of the preoptic area. The resulting disinhibition of thermogenesis-promoting neurons in the dorsomedial hypothalamus and possibly of sympathetic and somatic premotor neurons in the rostral ventromedial medulla, including the raphe pallidus, activates excitatory inputs to spinal sympathetic and somatic motor circuits to drive thermogenesis.

Brown adipose tissue sympathetic nerve activity is potentiated by activation of 5-hydroxytryptamine (5-HT)1A/5-HT7 receptors in the rat spinal cord

In urethane-chloralose anesthetized, neuromuscularly blocked, ventilated rats, microinjection of NMDA (12 pmol) into the right fourth thoracic segment (T4) spinal intermediolateral nucleus (IML) immediately increased ipsilateral brown adipose tissue (BAT) sympathetic nerve activity (SNA; peak +492% of control), expired CO2 (+0.1%) heart rate (+48 beats min(-1)) and arterial pressure (+8 mmHg). The increase in BAT SNA evoked by T4 IML microinjection of NMDA was potentiated when it was administered immediately following a T4 IML microinjection of 5-hydroxytryptamine (5-HT, 100 pmol) or the 5-HT1A/5-HT7 receptor agonist, 8-OH-DPAT (600 pmol), (area under the curve: 184%, and 259% of the NMDA-only response, respectively). In contrast, T4 IML microinjection of the 5-HT2 receptor agonist, DOI (28 pmol) did not potentiate the NMDA-evoked increase in BAT SNA (101% of NMDA-only response). Microinjection into the T4 IML of the selective 5-HT1A antagonist, WAY-100635 (500 pmol), plus the 5-HT7 antagonist, SB-269970 (500 pmol), prevented the 5-HT-induced potentiation of the NMDA-evoked increase in BAT SNA. When administered separately, WAY-100635 (800 pmol) and SB-269970 (800 pmol) attenuated the 8-OH-DPAT-induced potentiation of the NMDA-evoked increase in BAT SNA through effects on the amplitude and duration of the response, respectively. The selective 5-HT2 receptor antagonist, ketanserin (100 pmol), did not attenuate the potentiations of the NMDA-evoked increase in BAT SNA induced by either 5-HT or 8-OH-DPAT. These results demonstrate that activation of 5-HT1A/5-HT7 receptors can act synergistically with NMDA receptor activation within the IML to markedly increase BAT SNA.

Activation of central 5HT2A receptors reduces the craniofacial nociception of rats

We assessed the contribution of central 5HT2A receptors to the craniofacial tissue nociception in naïve male rats. First, we tested whether activation of central 5HT2A receptors affected nociceptive neural activities recorded from superficial laminae of the trigeminal subnucleus caudalis (Vc)/upper cervical spinal cord junction (Vc/C2) region. Two types of units, such as deep-nociceptive or skin-wide dynamic range (WDR) units were identified from extracellular recordings. Topical administration of 5HT2A receptor agonist, (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) onto the Vc/C2 region significantly reduced deep-nociceptive unit discharges evoked by formalin injection into the masseter muscle. Noxious pinch stimulation to the facial skin-evoked skin-WDR unit discharges was significantly reduced by topical administration of 0.1 mg/rat DOI onto the Vc/C2 region. Second, we tested whether i.c.v. administration of DOI affected Fos-like immunoreactivity (-LI) evoked by formalin injection into the masseter muscle. Fos-LI was significantly induced mainly at the ventrolateral (vl) area of trigeminal subnucleus interpolaris (Vi)/Vc junction (vl-Vi/Vc) region and Vc/C2 region in vehicle-treated rats. Formalin-evoked Fos-LI was significantly reduced in laminae I-II of the Vc/C2, but not vl-Vi/Vc region after i.c.v. administration of DOI. Finally, orofacial nocifensive behavioral activities evoked by formalin injection into the masseter muscle were significantly reduced by intracisternal administration of DOI. These results suggest that 5HT2A receptors in the Vc/C2 region mediate antinociceptive effects in the craniofacial nociception.

Serotonin potentiates sympathetic responses evoked by spinal NMDA

In urethane-chloralose anaesthetized, neuromuscularly blocked, ventilated rats, we examined the effects on sympathetic outflow to brown adipose tissue (BAT) of separate and simultaneous spinal microinjections of NMDA and serotonin. Microinjection of NMDA (12 pmol) into the right T4 spinal intermediolateral nucleus (IML) immediately increased ipsilateral brown adipose tissue (BAT) sympathetic nerve activity (SNA; peak: +546% of control), BAT thermogenesis (+0.8 degrees C) and heart rate (+53 beats min-1), whereas microinjection of a lower dose of NMDA (1.2 pmol) did not change any of the recorded variables. Microinjection of 5-hydroxytryptamine (5-HT, 2 nmol) into the T4 IML increased BAT SNA (peak: +342% of control) at a long latency (mean onset: 23 min). The long latency 5-HT-evoked increase in BAT SNA was prevented by microinjection of methysergide (600 pmol) into the T4 IML. The increases in BAT SNA evoked by T4 IML microinjections of NMDA (12 pmol) were significantly potentiated (two to three times larger than the response to NMDA alone) following T4 IML microinjections of 5-HT (100 pmol to 2 nmol, but not 20 pmol). Also, microinjection of 5-HT (200 pmol) converted the subthreshold dose of NMDA (1.2 pmol) into an effective dose for increasing BAT SNA and heart rate. The 5-HT-mediated potentiation of the increase in BAT SNA evoked by microinjection of NMDA into the T4 IML was reversed by microinjection of methysergide (600 pmol) into the T4 IML. These results demonstrate that BAT SNA and thermogenesis can be driven by activation of spinal excitatory amino acid or 5-HT receptors and that concomitant activation of spinal NMDA and 5-HT receptors can act synergistically to markedly increase BAT SNA and thermogenesis.

Localization of L-type calcium channel Ca(V)1.3 in cat lumbar spinal cord--with emphasis on motoneurons

Voltage-dependent persistent inward currents (PICs) which underlie the plateau potentials are an important intrinsic property of spinal motoneurons. Electrophysiological experiments have indicated that a subtype of the low threshold L-type calcium channel, Ca(V)1.3, mediates this current. In mouse and turtle lumbar spinal cord it has been shown that these channel proteins are mainly found on motoneuron dendrites. In the present study we have used immunohistochemistry to locate these channels in lumbar spinal neurons, especially motoneurons, of the cat. The results indicate that Ca(V)1.3 immunoreactivity was unevenly distributed among the laminae of the spinal grey matter. The small neurons in superficial dorsal horn (laminae I-III) were sparsely and weakly labelled, while large neurons in ventral horn were frequently and densely labelled. Groups of motoneurons in lamina IX that were immunoreactive to choline acetyltransferase also co-expressed Ca(V)1.3. The immunoreactivity was mainly associated with neuronal somata and proximal dendrites. Double staining with antibodies against Ca(V)1.3 and MAP2 (a dendritic marker) showed that some fine fibres, which may include distal dendrites, were also labelled. These results in the cat spinal cord show some differences from studies in mouse and turtle motoneurons where the immunoreactivity against this channel was mainly localized to the dendrites.

Topical ketanserin attenuates hyperalgesia and inflammation in arthritis in rats

We investigated effects of topical application of ketanserin, a 5-HT2A receptor antagonist, on hyperalgesia and edema in the arthritic rat, a chronic pain model with inflammation. Unilateral, but not bilateral, arthritis was induced with intra-articular injection of a mixture of kaolin and carrageenan in one side, as indicated by the shortened paw withdrawal latency and an increase in the circumference of the knee joint. Topical application of ketanserin onto skin over the arthritic joint delivered in a mixture of gelatin, glycerol and kaolin produced dose-dependent attenuation of nociceptive and inflammatory effects resulting from intra-articularly injected kaolin/carrageenan. One and 3% ketanserin produced significant or even complete anti-hyperalgesia, as well as a remarkable anti-inflammatory effect (50-70% reduction of edema) while 0.3% ketanserin and placebo failed to produce any effect. Moreover, the effects of ketanserin were maintained for 13 days without decline. In contrast, 3% ketanserin applied to skin of the knee joint on the non-inflamed side for 2 weeks did not alter nociceptive thresholds of the paw and the size of the knee joint in both the inflamed and non-inflamed limbs. These results indicate that 5-HT2A receptors in the periphery play a significant role in the maintenance and/or development of inflammatory pain. The present study suggests that topical ketanserin is a promising direction for potential clinical exploration to relieve established hyperalgesia and inflammation in arthritis without adverse effects and tolerance.

Long-lasting recovery of locomotor function in chronic spinal rat following chronic combined pharmacological stimulation of serotonergic receptors with 8-OHDPAT and quipazine

In chronic spinal rats, long-term stimulation of 5-HT receptors with quipazine or 8-OHDPAT by means of daily injection, promotes robust locomotor recovery. The question of a possible potentiation between treatments when applied together was addressed. Daily injections of both 8-OHDPAT and quipazine, were performed for a month in spinal animals. Animals were placed on a treadmill and the bipedal hindlimb locomotion was tested. Motor performances (behavioural test) and locomotor parameters (EMG and kinematic) were analysed weekly during the treatment. Furthermore, the locomotor performances were evaluated during two supplemental months following the end of the treatment. Our results suggest that association of both agonists induced long-lasting positive effects on locomotor function. Motor performances were significantly better after combined injection of both drugs than when the agonists were used separately. But, the most significant and new result is that the locomotor scores did not decrease during the weeks that followed the end of the treatment. These results suggests a long-lasting and 5-HT-dependent reorganisation of spinal networks.

Expression of serotonin2A receptors in Purkinje cells of the developing rat cerebellum

Previous physiological and pharmacological studies have shown that the serotonin2A (5-HT2A) receptor is involved in cerebellar functions. However, the expression of 5-HT2A receptors in the developing cerebellum has not been elucidated to date. In the present immunohistochemical study, we examined developmental changes of the distribution of 5-HT2A receptors in Purkinje cells of the rat cerebellum from embryonic day 18 (E18) to postnatal day 21 (P21). The weak immunoreaction to 5-HT2A receptors was found in the deep cerebellar nuclei on E19. In the cerebellar cortex of the hemisphere and the posterior vermis, somata of Purkinje cells became weakly immunoreactive on P0. With the dendritic elongation and arborization, the immunoreaction appeared in the proximal parts of Purkinje cell dendrites. Distal parts of the dendrites became immunoreactive after P12, and were strongly immunolabeled by P21. The present study may provide a structural basis to investigate the roles of 5-HT2A receptors during the cerebellar development.

Postnatal appearance of 5-HT2A receptors on fast flexor and slow extensor rat motor neurons

Motor neurons to the slowly contracting extensor soleus muscle in behaving rats begin to fire tonically in the 2nd week after birth. In the adult, tonic firing becomes predominant and appears to arise from plateau potentials under monoaminergic control. In the present work, motor neurons to slowly contracting extensor soleus and rapidly contracting extensor digitorum longus, a physiological flexor muscle, were retrogradely labeled with fluorescent dextran and examined for immunoreactivity to 5-HT(2A) receptors in 1 and 2 week old and adult rats. No reactivity was detected at 1 week. At 2 weeks, reactivity was detected on 67% slowly contracting extensor soleus (16 of 24) and 19% extensor digitorum longus (11 of 57) motor neurons. In the adult, the intensity of staining was higher and the percentage of labeled motor neurons 79 for slowly contracting extensor soleus (34 of 43) and 31 for extensor digitorum longus (11 of 35). On slowly contracting extensor soleus motor neurons, labeling appeared more often on soma and dendrites than on dendrites only, whereas on extensor digitorum longus motor neurons, labeling appeared more often on dendrites only. These results are consistent with the hypothesis that serotonergic innervation contributes to the appearance and subsequent increase in tonic firing of rat slowly contracting extensor soleus motor neurons in postnatal development.

The contribution of peripheral 5-hydroxytryptamine2A receptor to carrageenan-evoked hyperalgesia, inflammation and spinal Fos protein expression in the rat

The present study was conducted to test the hypothesis that the peripheral 5-hydroxytryptamine (5-HT)2A receptor is involved in inflammatory hyperalgesia and production of noxious stimulus-induced neuronal activity at the level of the spinal cord dorsal horn. Intraplantar (i.pl.) injection of carrageenan dramatically reduced paw withdrawal latency to noxious heat (47 degrees C) and caused paw swelling. Pretreatment with ketanserin, a selective antagonist of 5-HT2A receptor, in the hindpaw produced dose-dependent inhibition of the hyperalgesia (0.5, 3 and 5 mug; i.pl.) with full relief at 5 mug. The drug also moderately reduced carrageenan-induced paw swelling in a dose-dependent manner. Carrageenan induced conspicuous expression of c-fos-like immunoreactivity (FLI) in the spinal dorsal horn of segments L4-5. Ketanserin (5 mug) markedly reduced carrageenan-induced FLI in all laminae of the dorsal horn. However, blockade of peripheral 5-HT1A receptors by (N-2-[4-(2-methoxyphenyl-1-piperazinyl] ethyl]-N-2-pyridinylcyclohexanecarboxamide at maximally effective doses (30 and 100 mug; i.pl.) did not alter carrageenan-induced hyperalgesia, edema or expression of FLI. The present study provided evidence at cellular level that the peripheral 5-HT2A receptor is preferentially involved in the development of thermal hyperalgesia in the carrageenan model of inflammation.

Antiallodynic effects of intrathecally administered 5-HT(2C) receptor agonists in rats with nerve injury

Intrathecal administration of serotonin type 2 (5-HT(2)) receptor agonists, alpha-methyl-5-hydroxytryptamine maleate (alpha-m-5-HT) or (+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI), produces antiallodynic effects in a rat model of neuropathic pain. In the present study, we examined the antiallodynic effects of intrathecally administered agents which are selective for 5-HT(2C) receptors. Allodynia was produced by tight ligation of the left L5 and L6 spinal nerves, and was measured by applying von Frey filaments to the left hindpaw. Administration of the 5-HT(2C) receptor agonist, 6-chloro-2-(1-piperazinyl)-pyrazine (MK212; 3-100 microg), 1-(m-chlorophenyl)-piperazine (mCPP; 30-300 microg), or 1-(m-trifluoromethylphenyl)-piperazine (TFMPP; 30-300 microg), produced antiallodynic effects in a dose-dependent manner with no associated motor weakness. The ED(50) values of MK212, mCPP, and TFMPP were 39.2, 119.9, and 191.9 microg, respectively. Intrathecal pretreatment with the selective 5-HT(2C) receptor antagonist RS-102221 (30 microg) diminished the effects of the highest doses of 5-HT(2C) receptor agonists. The preferential 5-HT(2A) receptor antagonist ketanserin (30 microg) did not reverse the effects. In contrast to 5-HT(2C) receptor agonists, the antiallodynic effects of intrathecally administered alpha-m-5-HT (30 microg) and DOI (100 microg) were reversed by ketanserin, but not by RS-102221. These results indicate that 5-HT(2C) receptors have a role in spinal inhibition of neuropathic pain, and the effects produced by intrathecal administration of 5-HT(2C) receptor agonists are mediated by a mechanism different from that of alpha-m-5-HT or DOI, which seem to produce their effects through 5-HT(2A) receptors.

The 5-HT2A receptor is widely distributed in the rat spinal cord and mainly localized at the plasma membrane of postsynaptic neurons

Serotonin (5-HT) plays a major role at the spinal level by modulating most spinal functions through several receptor subtypes including the 5-HT2A receptor. To gain further insight into the cellular role of this receptor, we performed an immunocytochemical study of 5-HT2A receptors in the rat spinal cord, at light and electron microscope levels. The results showed that 5-HT2A receptors were widely distributed in the spinal cord at all segmental levels. Immunolabeling was particularly dense in lamina IX and in the dorsal horn lamina IIi. Immunoreactive cell bodies were numerous in lamina IX, where many but not all motoneurons were labeled, as shown by double labeling with choline acetyltransferase antibodies. Stained cell bodies were also observed in the gray matter. The study at the ultrastructural level focused on the lumbar dorsal horn (laminae I-II) and ventral horn (lamina IX). At both levels, 5-HT2A immunoreactivity was mainly postsynaptic on dendrites and cell bodies. However, a little presynaptic labeling was also observed in axon and axon terminals, some of them containing large granular vesicles attesting to their peptidergic nature. The main result of our study was the "nonsynaptic" plasma membrane localization of 5-HT2A receptors covering a large surface of cell bodies and dendrites, suggesting a paracrine form of action of serotonin. These observations are consistent with a double role (pre- and postsynaptic) for serotonin on these receptors on various cellular targets.

5-HT2A receptor subtype in the peripheral branch of sensory fibers is involved in the potentiation of inflammatory pain in rats

One of the major serotonin (5-HT) receptor subtypes expressed in the rat dorsal root ganglion (DRG) neurons is the 5-HT2A receptor. We have previously shown that 5-HT2A receptors in the peripheral sensory terminals are responsible for 5-HT-induced pain and hyperalgesia. In the present study, we characterized neurons expressing 5-HT2A receptors in the rat DRG neurons by means of in situ hybridization, immunohistochemistry, reverse transcription-polymerase chain reaction (RT-PCR) and behavioral tests. In situ hybridization on consecutive sections revealed that 5-HT2A receptor mRNA is colocalized with calcitonin-gene related peptide (CGRP) mRNA (100/104; 96.2%) but not with c-Ret mRNA (1/115; 0.9%). Signals for 5-HT2A receptor mRNA were found in 9.4 +/- 2.2% of normal DRG (L5) neurons, most of which were small to medium in size. Four days of complete Freund's adjuvant-induced inflammation of the hindpaw doubled the incidence of 5-HT2A receptor mRNA-expressing neurons to 19.3 +/- 2.8%. The level of 5-HT2A receptor mRNA in DRGs of normal and various pathological conditions was then determined by RT-PCR. The level was up-regulated by peripheral inflammation, but not by axotomy or chronic constriction of the peripheral nerve. Systemic administration of 5-HT2A receptor antagonist (Sarpogrelate HCI) produced analgesic effects on thermal hyperalgesia caused by peripheral inflammation, but failed to attenuate thermal hyperalgesia in chronic constriction injury model. These findings suggest that 5-HT2A receptors are mainly expressed in CGRP-synthesizing small DRG neurons and may be involved in the potentiation of inflammatory pain in the periphery.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Activation of spinal serotonin(2A/2C) receptors augments nociceptive responses in the rat

The role of spinal 5-HT(2A/2C) receptors in the regulation of spinal nociceptive transmission was studied. The 5-HT(2A/2C) agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the antagonist ketanserin tartrate were administered intrathecally immediately before the formalin test. Activation of spinal 5-HT(2A/2C) receptors increased the pain-like behavioural response in both the early and late phases. The findings support the hypothesis that spinal 5-HT(2A/2C) receptors augment the spinal afferent nociceptive impulses induced by peripheral inflammation.

Distribution of Serotonin 2A and 2C Receptor mRNA Expression in the Cervical Ventral Horn and Phrenic Motoneurons Following Spinal Cord Hemisection

Cervical spinal cord injury leads to a disruption of bulbospinal innervation from medullary respiratory centers to phrenic motoneurons. Animal models utilizing cervical hemisection result in inhibition of ipsilateral phrenic nerve activity, leading to paralysis of the hemidiaphragm. We have previously demonstrated a role for serotonin (5-HT) as one potential modulator of respiratory recovery following cervical hemisection, a mechanism that likely occurs via 5-HT2A and/or 5-HT2C receptors. The present study was designed to specifically examine if 5-HT2A and/or 5-HT2C receptors are colocalized with phrenic motoneurons in both intact and spinal-hemisected rats. Adult female rats (250-350 g; n = 6 per group) received a left cervical (C2) hemisection and were injected with the fluorescent retrograde neuronal tracer Fluorogold into the left hemidiaphragm. Twenty-four hours later, animals were killed and spinal cords processed for in situ hybridization and immunohistochemistry. Using (35)S-labeled cRNA probes, cervical spinal cords were probed for 5-HT2A and 5-HT2C receptor mRNA expression and double-labeled using an antibody to Fluorogold to detect phrenic motoneurons. Expression of both 5-HT2A and 5-HT2C receptor mRNA was detected in motoneurons of the cervical ventral horn. Despite positive expression of both 5-HT2A and 5-HT2C receptor mRNA-hybridization signal over phrenic motoneurons, only 5-HT2A silver grains achieved a signal-to-noise ratio representative of colocalization. 5-HT2A mRNA levels in identified phrenic motoneurons were not significantly altered following cervical hemisection compared to sham-operated controls. Selective colocalization of 5-HT2A receptor mRNA with phrenic motoneurons may have implications for recently observed 5-HT2A receptor-mediated regulation of respiratory activity and/or recovery in both intact and injury-compromised states.

Serotonin Increases the Incidence of Primary Afferent-Evoked Long-Term Depression in Rat Deep Dorsal Horn Neurons

5-hydroxytryptamine (5-HT) is released in spinal cord by descending systems that modulate somatosensory transmission and can potently depress primary afferent-evoked synaptic responses in dorsal horn neurons. Since primary afferent activity-induced long-term potentiation (LTP) may contribute to central sensitization of nociception, we studied the effects of 5-HT on the expression of sensory-evoked LTP and long-term depression (LTD) in deep dorsal horn (DDH) neurons. Whole cell, predominantly current clamp, recordings were obtained from DDH neurons in transverse slices of neonatal rat lumbar spinal cord. The effect of 5-HT on dorsal-root stimulation-evoked synaptic responses was tested before, during, or after high-frequency conditioning stimulation (CS). In most cells (80%), 5-HT caused a depression of the naı̈ve synaptic response. Even though 5-HT depressed evoked responses, CS in the presence of 5-HT was not only still capable of inducing LTD but also increased its incidence from 54% in controls to 88% ( P < 0.001). Activation of ligands selective for 5-HT1A/1B and 5-HT1B, but not 5-HT2A/2C or 5-HT3receptors, best reproduced these actions. 5-HT also potently depressed postconditioning synaptic responses regardless of whether the induced plasticity was LTP or LTD. Our results demonstrate that in addition to depressing the amplitude of evoked sensory input, 5-HT can also control the direction of its long-term modifiability, favoring the expression of LTD. These findings demonstrate cellular mechanisms that may contribute to the descending serotonergic control of nociception.

Distribution of serotonin 2A, 2C and 3 receptor mRNA in spinal cord and medulla oblongata

It is known that 5-HT receptors have significant roles in nociceptive and motor functions. We have compared the cellular localization of the mRNAs encoding serotonin 5-HT(2A,) 5-HT(2C,) 5-HT(3) receptor subtypes within different levels of the rat spinal cord and medulla. In the spinal cord, 5-HT(2C) receptor mRNA is expressed at high levels in most of the gray matter, except for lamina II. In contrast, 5-HT(2A) receptor mRNA is expressed exclusively in lamina IX. 5-HT(3) receptor mRNA has a low level and diffuse pattern of expression increasing towards the ventral horn. In both gray and white matter, there is a characteristic presence of a few highly stained cells. For each subtype, the expression pattern is similar in all four levels of the spinal cord. In the medulla, 5-HT(2C) receptor mRNA is at high levels in many nuclei including the hypoglossal nucleus, the gigantocellular reticular nucleus alpha and the parvocellular reticular nucleus alpha, the spinal nucleus of the trigeminal tract, the facial, and the dorsal medullary reticular field. Moderate to low levels of expression are seen in the spinal vestibular nucleus, the vagus, the solitary nuclei and the raphe. 5-HT(2A) receptor is expressed at high levels in some nuclei such as the hypoglossal nucleus, the intercalate nucleus, the inferior olive and the lateral reticular nucleus. Moderate to low levels of expression are seen in the facial, the medial vestibular nuclei, the nucleus ambiguous, the vagus, and the gigantocellular reticular nucleus. 5-HT(3) receptor mRNA is present at low levels in most of the nuclei examined, with a few scattered strongly labeled cells. The results show a distinct distribution of the three subtypes of receptors supporting their physiological roles and will help to understand the mechanisms of nociception and motor function.

Pharmacological characterization of serotonin receptor subtypes modulating primary afferent input to deep dorsal horn neurons in the neonatal rat

Spinal cord slices and whole-cell patch clamp recordings were used to investigate the effects of serotonergic receptor ligands on dorsal root-evoked synaptic responses in deep dorsal horn (DDH) neurons of the neonatal rat at postnatal days (P) 3 - 6 and P10 - 14. Bath applied 5-hydroxytryptamine (5-HT) potently depressed synaptic responses in most neurons. Similarly, the 5-HT(1/7) receptor agonist, 5-carboxamidotryptamine (5-CT) depressed synaptic responses. This action was probably mediated by 5-HT(1A) receptor activation, since it occurred in the presence of the 5-HT(7) receptor antagonist clozapine and was not observed in the presence of NAN-190, a 5-HT(1A) receptor antagonist. In the absence of any agonist, 5-HT(1A) receptor antagonists often facilitated synaptic responses, suggesting that there is sufficient endogenous 5-HT to tonically activate 5-HT(1A) receptors. 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), the 5-HT(1A/7) receptor agonist, facilitated synaptic responses, an action probably mediated by 5-HT(7) receptors, since the facilitation could be reversed by subsequent application of the 5-HT(7) receptor antagonist clozapine. Agonists for the 5-HT(1B), 5-HT(2) and 5-HT(3) receptors exerted only modest modulatory actions. A pharmacological analysis of the depression evoked by 5-HT suggested an action partly mediated by 5-HT(1A) receptor activation, since antagonism of the 5-HT(1A) receptor with NAN-190 or WAY-100635 partly reversed 5-HT-evoked depression. In comparison, 5-HT(7) receptor activation could account for much of the 5-HT-evoked facilitation. We conclude that 5-HT is capable of modulating sensory input onto DDH neurons via several receptor subtypes, producing both facilitatory and depressant actions. Also, the actions of most receptor ligands on the evoked responses were similar within the first 2 postnatal weeks.

Serotonin 2A receptor-like immunoreactivity is detected in astrocytes but not in oligodendrocytes of rat spinal cord

The distribution of the serotonin 2A (5-HT2A) receptor in glial cells in the white matter of rat spinal cord was immunohistochemically examined with specific antibodies against the 5-HT2A receptor. 5-HT2A receptor-like immunoreactivity was detected in astrocytes that were identified by an antibody against the glial fibrillary acidic protein. In contrast, 5-HT2A receptor-like immunoreactivity was not observed in oligodendrocytes.

The role of serotonin in reflex modulation and locomotor rhythm production in the mammalian spinal cord

Over the past 40 years, much has been learned about the role of serotonin in spinal cord reflex modulation and locomotor pattern generation. This review presents an historical overview and current perspective of this literature. The primary focus is on the mammalian nervous system. However, where relevant, major insights provided by lower vertebrate models are presented. Recent studies suggest that serotonin-sensitive locomotor network components are distributed throughout the spinal cord and the supralumbar regions are of particular importance. In addition, different serotonin receptor subtypes appear to have different rostrocaudal distributions within the locomotor network. It is speculated that serotonin may influence pattern generation at the cellular level through modulation of plateau properties, an interplay with N-methyl-D-aspartate receptor actions, and afterhyperpolarization regulation. This review also summarizes the origin and maturation of bulbospinal serotonergic projections, serotonin receptor distribution in the spinal cord, the complex actions of serotonin on segmental neurons and reflex pathways, the potential role of serotonergic systems in promoting spinal cord maturation, and evidence suggesting serotonin may influence functional recovery after spinal cord injury.