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

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.

Sensory Activation of Command Cells for Locomotion and Modulatory Mechanisms: Lessons from Lampreys

Sensorimotor transformation is one of the most fundamental and ubiquitous functions of the central nervous system (CNS). Although the general organization of the locomotor neural circuitry is relatively well understood, less is known about its activation by sensory inputs and its modulation. Utilizing the lamprey model, a detailed understanding of sensorimotor integration in vertebrates is emerging. In this article, we explore how the vertebrate CNS integrates sensory signals to generate motor behavior by examining the pathways and neural mechanisms involved in the transformation of cutaneous and olfactory inputs into motor output in the lamprey. We then review how 5-hydroxytryptamine (5-HT) acts on these systems by modulating both sensory inputs and motor output. A comprehensive review of this fundamental topic should provide a useful framework in the fields of motor control, sensorimotor integration and neuromodulation.

Role of spinal 5-HT5A, and 5-HT1A/1B/1D, receptors in neuropathic pain induced by spinal nerve ligation in rats

Serotonin (5-HT) participates in pain modulation by interacting with different 5-HT receptors. The role of 5-HT5A receptor in neuropathic pain has not previously studied. The purpose of this study was to investigate: A) the role of 5-HT5A receptors in rats subjected to spinal nerve injury; B) the expression of 5-HT5A receptors in dorsal spinal cord and dorsal root ganglia (DRG). Neuropathic pain was induced by L5/L6 spinal nerve ligation. Tactile allodynia in neuropathic rats was assessed with von Frey filaments. Western blot methodology was used to determine 5-HT5A receptor protein expression. Intrathecal administration (on day 14th) of 5-HT (10-100 nmol) or 5-carboxamidotryptamine (5-CT, 0.03-0.3 nmol) reversed nerve injury-induced tactile allodynia. Intrathecal non-selective (methiothepin, 0.1-0.8 nmol) and selective (SB-699551, 1-10 nmol) 5-HT5A receptor antagonists reduced, by ~60% and ~25%, respectively, the antiallodynic effect of 5-HT (100 nmol) or 5-CT (0.3 nmol). Moreover, both selective 5-HT1A and 5-HT1B/1D receptor antagonists, WAY-100635 (0.3-1 nmol) and GR-127935 (0.3-1 nmol), respectively, partially diminished the antiallodynic effect of 5-HT or 5-CT by about 30%. Injection of antagonists, by themselves, did not affect allodynia. 5-HT5A receptors were expressed in the ipsilateral dorsal lumbar spinal cord and DRG and L5/L6 spinal nerve ligation did not modify 5-HT5A receptor protein expression in those sites. Results suggest that 5-HT5A receptors reduce pain processing in the spinal cord and that 5-HT and 5-CT reduce neuropathic pain through activation of 5-HT5A and 5-HT1A/1B/1D receptors. These receptors could be an important part of the descending pain inhibitory system.

Central amplification and fibromyalgia: disorder of pain processing

Fibromyalgia (FM), a complex chronic pain disorder affecting a heterogeneous patient population, is an area of active basic and clinical research. Although diagnostic criteria for FM have been available for 2 decades, there remains no definitive diagnostic and no consensus regarding its etiology. Accumulating evidence suggests the underlying cause of FM pain results from abnormal pain processing particularly in the central nervous system rather than from dysfunction in peripheral tissues where pain is perceived. In this review, we examine recent studies investigating abnormalities in central pain processing as a component of FM in both preclinical models of generalized muscle hypersensitivity and clinical research in patients with FM. We focus our discussion on two areas where strong evidence exists for abnormalities in sensory signaling: the reduction of descending control, including suppression of descending inhibitory pathways and/or enhancement of descending facilitatory pathways, and changes in key neurotransmitters associated with central sensitization. Finally, we discuss currently available pharmacological treatments indicated for the management of pain in FM patients, based on their proposed mechanism of action and efficacy.

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.

Serotoninergic modulation of sensory transmission to brainstem reticulospinal cells

Sensory inputs are subjected to modulation by central neural networks involved in controlling movements. It has been shown that serotonin (5-HT) modulates sensory transmission. This study examines in lampreys the effects of 5-HT on sensory transmission to brainstem reticulospinal (RS) neurons and the distribution of 5-HT cells that innervate RS cells. Cells were recorded intracellularly in the in vitro isolated brainstem of larval lampreys. Trigeminal nerve stimulation elicited disynaptic excitatory responses in RS neurons, and bath application of 5-HT reduced the response amplitude with maximum effect at 10 mum. Local ejection of 5-HT either onto the RS cells or onto the relay cells decreased sensory-evoked excitatory postsynaptic potentials (EPSPs) in RS cells. The monosynaptic EPSPs elicited from stimulation of the relay cells were also reduced by 5-HT. The reduction was maintained after blocking either N-methyl-d-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors. The local ejection of glutamate over RS cells elicited excitatory responses that were only slightly depressed by 5-HT. In addition, 5-HT increased the threshold for eliciting sustained depolarizations in response to trigeminal nerve stimulation but did not prevent them. Combined 5-HT immunofluorescence with axonal tracing revealed that the 5-HT innervation of RS neurons of the middle rhombencephalic reticular nucleus comes mainly from neurons in the isthmic region, but also from neurons located in the pretectum and caudal rhombencephalon. Our results indicate that 5-HT modulates sensory transmission to lamprey brainstem RS cells.

Serotonin concentrations in the lumbosacral spinal cord of the adult rat following microinjection or dorsal surface application

Application of neuroactive substances, including monoamines, is common in studies examining the spinal mechanisms of sensation and behavior. However, affected regions and time courses of transmitter activity are uncertain. We measured the spatial and temporal distribution of serotonin [5-hydroxytryptamine (5-HT)] in the lumbosacral spinal cord of halothane-anesthetized adult rats, following its intraspinal microinjection or surface application. Carbon fiber microelectrodes (CFMEs) were positioned at various locations in the spinal cord and oxidation currents corresponding to extracellular 5-HT were measured by fast cyclic voltammetry. Intraspinal microinjection of 5-HT (100 microM, 1-3 microl) produced responses that were most pronounced at CFMEs positioned <or=800 microm from the drug micropipette: 5-HT concentration was significantly higher (1.43 vs. <0.28% of initial concentration) and response latency was shorter (67.1 vs. 598.2 s) compared with more distantly positioned CFMEs. Treatment with the selective 5-HT reuptake inhibitor clomipramine only slightly affected the spread of microinjected 5-HT. Surface application over several segments led to a transient rise in concentration that was usually apparent within 30 s and was dramatically attenuated with increasing depth: 0.25% of initial concentration (1 mM) within 400 microm of the dorsal surface and <0.001% between 1,170 and 2,000 microm. This initial response to superfusion was sometimes followed by a gradual increase to a new concentration plateau. In sum, compared with bath application, microinjection can deliver about tenfold higher transmitter concentrations, but to much more restricted areas of the spinal cord.

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.

Dynamic sensorimotor interactions in locomotion

Locomotion results from intricate dynamic interactions between a central program and feedback mechanisms. The central program relies fundamentally on a genetically determined spinal circuitry (central pattern generator) capable of generating the basic locomotor pattern and on various descending pathways that can trigger, stop, and steer locomotion. The feedback originates from muscles and skin afferents as well as from special senses (vision, audition, vestibular) and dynamically adapts the locomotor pattern to the requirements of the environment. The dynamic interactions are ensured by modulating transmission in locomotor pathways in a state- and phase-dependent manner. For instance, proprioceptive inputs from extensors can, during stance, adjust the timing and amplitude of muscle activities of the limbs to the speed of locomotion but be silenced during the opposite phase of the cycle. Similarly, skin afferents participate predominantly in the correction of limb and foot placement during stance on uneven terrain, but skin stimuli can evoke different types of responses depending on when they occur within the step cycle. Similarly, stimulation of descending pathways may affect the locomotor pattern in only certain phases of the step cycle. Section ii reviews dynamic sensorimotor interactions mainly through spinal pathways. Section iii describes how similar sensory inputs from the spinal or supraspinal levels can modify locomotion through descending pathways. The sensorimotor interactions occur obviously at several levels of the nervous system. Section iv summarizes presynaptic, interneuronal, and motoneuronal mechanisms that are common at these various levels. Together these mechanisms contribute to the continuous dynamic adjustment of sensorimotor interactions, ensuring that the central program and feedback mechanisms are congruous during locomotion.

Serotoninergic-mediated inhibition of substance P sensitive deep dorsal horn neurons: a combined electrophysiological and morphological study in vitro

Dorsal horn neurons that express the neurokinin 1 receptor (NK-1R) play an important role in nociceptive processing. The targetting of NK-1R neurons by serotoninergic (5-hydroxytryptamine, 5-HT) axons would provide a straightforward means to exert an inhibitory analgesic effect at spinal level. This study used single cell electrophysiology to analyse and correlate the responses of rat deep DH neurons in vitro to both 5-HT and the NK-1R agonist [Sar9,Met(O2)11]-substance P (SP). Subsequently a combination of immunocytochemistry and confocal imaging was applied to biocytin-filled laminae III-VI neurons to reveal putative 5-HT innervation in this neuronal sample. A population of neurons was identified in which 5-HT (50 microM) significantly attenuated the dorsal root-evoked excitatory postsynaptic potential and [Sar9,Met(O2)11]-SP (2 microM) induced a direct tetrodotoxin-resistant depolarisation. Immunolabelling revealed that all of these neurons were inhibited by 5-HT, including those that were excited by [Sar9,Met(O2)11]-SP, were overlaid by a plexus of 5-HT immunoreactive fibres and in some instances, closely apposed putative contacts with somata and proximal dendrites identified although their incidence was low. Inhibition by 5-HT of deep DH neurons directly responsive to SP may account at least in part for monoamine-induced modulation of nociceptive processing in the spinal cord.

Stimulation of spinal 5-HT(2A/2C) receptors potentiates the capsaicin-induced in vivo release of substance P-like immunoreactivity in the rat dorsal horn

Stimulation of spinal serotonin (5-HT)(2A/2C) receptors has previously been reported to lead to either a pro-nociceptive or an anti-nociceptive response. Behavioral data have indicated that the pro-nociceptive effect is related to the release of substance P (SP). The aim of this in vivo microdialysis study was to investigate if stimulation of spinal 5-HT(2A/2C) receptors by the selective agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) induces spontaneous or capsaicin-evoked increase in the release of SP-like immunoreactivity (SP-LI) in the rat dorsal horn. A dose of capsaicin (25 microM in the perfusion medium administered for 30 min), which did not lead to a significant release of SP-LI on its own, induced a significant increase of greater than 4-fold of the SP-LI level following spinal application of 50 nmol DOI. Higher (500 nmol) or lower (5 nmol) doses of DOI failed to induce a similar effect. In rats with a peripheral inflammation, induced by carrageenan, capsaicin (25 microM) induced a non-significant increase of SP-LI. A significant 8-fold increase of the SP-LI level was detected following administration of 50 nmol DOI in combination with capsaicin. The effect of DOI, which was completely prevented by co-administration of the 5-HT(2A) receptor antagonist ketanserin in control animals without peripheral inflammation, was only partly blocked in animals with carrageenan induced peripheral inflammation. In conclusion, stimulation of 5-HT(2A/2C) receptors facilitates the capsaicin-evoked release of SP-LI in the dorsal horn in both animals with and without carrageenan-induced unilateral inflammation. The observation that the highest dose of DOI failed to induce SP-LI release may be due to an inhibitory postsynaptic action at this dose.

Serotonergic signaling inhibits hyperalgesia induced by spinal cord damage

Although dysesthesia is one of the most serious problems in patients with spinal cord injury, most of them being unresponsive to conventional treatments. In this study, we established a rat thoracic spinal cord mild-compression model that revealed thermal hyperalgesia in the hind limb. The thoracic spinal cord was compressed gently, using a 20 g weight for 20 min. The withdrawal latency of the thermal stimulation of the bilateral hind-limb was monitored using Hargreaves' Plantar test apparatus. In this model, thermal-hyperalgesia was observed for 1 week after the injury. The spinal cord injury-induced thermal-hyperalgesia was mimicked by the intrathecal application of metergoline, a non-selective 5-HT antagonist, 1-(2-methoxyphenyl)-4-[4-(2-phthalimido) butyl]-piperazine hydrobromide (NAN190), a selective 5-HT1 antagonist, and 3-tropanyl-3,5-dichlorobenzoate (MDL72222), a selective 5-HT3 antagonist. Intraperitoneal application of fluvoxamine maleate, a selective serotonin reuptake inhibitor, reduced the intensity of hyperalgesia induced by spinal cord injury. The inhibitory effect of fluvoxamine maleate on thermal hyperalgesia was prevented by the application of the aforementioned nonselective or selective 5-HT receptor antagonists. Intrathecal application of fluvoxamine maleate and selective 5-HT receptor agonists, i.e., 8-hydroxy-2-(di-n-proplyamino)-tetralin hydrobromide (8-OH-DPAT: 5HT-1 selective) and 2-methyl-5-hydroxytryptamine maleate (2-m-5-HT: 5HT-3 selective), inhibited the spinal cord injury-induced hyperalgesia. These results suggest that the change in the descending serotonergic signal plays an important role in hyperalgesia after the spinal cord injury, and that the application of selective serotonin reuptake inhibitors will be one of the candidates for new therapeutic methods against post-spinal cord injury dysesthesia.

Interactions between cutaneous afferent inputs to a withdrawal reflex in the decerebrated rabbit and their control by descending and segmental systems

Previous studies have suggested that activation of nociceptive afferents from the heel recruits a supraspinal mechanism, which is modulated by adrenergic descending inhibition, that augments withdrawal reflexes in medial gastrocnemius (MG) motoneurones. To test this idea, we have studied the temporal evolution of reflexes evoked in MG by electrical stimulation of sural nerve A(beta)-, A(delta)- and C-fibre axons at 1 Hz, in decerebrated rabbits. Reflexes were analysed in three time bands, estimated to accord to afferent drive from A(beta)- (phase 1), A(delta)- (phase 2) and C-fibre (phase 3) inputs. Stimulation of A(delta)- and C-fibres gave significant temporal summation of all reflexes. The alpha(2)-adrenoceptor antagonist RX 821002 ((2-(2,3-dihydro-2-methoxy-1,4-benzodioxin-2-yl)-4,5-dihydro-1-H-imidazole)-HCl) (100 microg intrathecal (i.t.)) potentiated, and the alpha(2)-agonist dexmedetomidine (1-30 microg i.t.) depressed all reflexes per se, but the effects of these drugs on temporal summation were secondary to changes in baseline excitability. When C-fibres were stimulated, the N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (1 mg i.t.) reduced temporal summation of phase 2 and 3 but not phase 1 reflexes. Spinalisation at L1 in the absence of drugs increased phase 2 and 3 reflexes but had no effect on phase 1, whereas spinalisation after RX 821002 resulted in decreased phase 1 responses with no significant change in later phases. Spinalisation in the presence of dizocilpine resulted in small reductions in phase 3 reflexes only. In all cases spinalisation virtually abolished temporal summation. In spinalised animals, dizocilpine selectively reduced late reflexes, and the opioid antagonist naloxone (100 microg i.t.) augmented all reflexes but gave rise to temporal subtraction of reflexes when C-fibres were stimulated.The present experiments have revealed a number of novel and important features of the sural-MG reflex pathway: (i) activity in fine afferent axons augments the reflexogenic potential of all subsequent afferent input, thereby allowing all afferent drive from the sural field to contribute to withdrawal of the heel; (ii) endogenous adrenergic control of this reflex pathway is completely non-selective; (iii) there is a non-adrenergic element of descending inhibition that is selective for the late components of MG reflex responses, and this element is directed particularly against transmission through NMDA receptors; (iv) temporal summation in this reflex is dependent on NMDA receptor-dependent and -independent mechanisms; and (v) this temporal summation is in some way dependent on the integrity of descending pathways.

Serotonergic control of cerebellar mossy fiber activity by modulation of signal transfer by rat pontine nuclei neurons

Serotonergic modulation of precerebellar nuclei may be crucial for the function of the entire cerebellar system. To study the effects of serotonin (5-HT) on neurons located within the pontine nuclei (PN), the main source of cerebellar mossy fibers, we performed standard intracellular recordings from PN neurons in a slice preparation of the rat pontine brain stem. Application of 5 microM 5-HT significantly altered several intrinsic membrane properties of PN neurons. First, it depolarized the somatic membrane potential by 6.5 +/- 3.5 mV and increased the apparent input resistance from 49.5 +/- 14.6 to 62.7 +/- 21.1 MOmega. Second, 5-HT altered the I-V relationship of PN neurons: it decreased the inward rectification in hyperpolarizing direction, but increased it when depolarizing currents were applied. Third, it decreased the rheobase from 0.32 +/- 0.14 to 0.24 +/- 0.14 nA without affecting the firing threshold. Finally, the amplitude of medium-duration after hyperpolarizations was reduced from -14.9 +/- 2.0 to -12.3 +/- 2.4 mV. Together, these 5-HT effects on the intrinsic membrane properties result in an increase in excitability and instantaneous firing rate. In addition, application of 5 microM 5-HT also modulated postsynaptic potentials (PSPs) evoked by electric stimulations within the cerebral peduncle. The amplitude, maximal slope, and integral of these PSPs were reduced to 46.2 +/- 23.4%, 45.7 +/- 23.7%, and 61.4 +/- 28.4% of the control value, respectively. In contrast, we found no change in the decay and voltage dependence of PSPs. To test modulatory effects on short-term synaptic facilitation, we applied pairs of electrical stimuli at intervals between 10 and 1,000 ms. 5-HT selectively enhanced the paired-pulse facilitation for interstimulus-intervals >20 ms. The alteration of paired-pulse facilitation points to a presynaptic site of action for 5-HT effects on synaptic transmission. Pharmacological experiments suggested that pre- and postsynaptic effects of 5-HT were mediated by two different kinds of 5-HT receptors: changes in intrinsic membrane properties were blocked by the 5-HT(2) receptor antagonist cinanserin while the reduction of PSPs was prevented by the 5-HT(1) receptor antagonist cyanopindolol. In conclusion, 5-HT increases the excitability of PN neurons but decreases the synaptic transmission on them. The selective enhancement of synaptic facilitation may, however, allow high-frequency inputs to effectively drive PN neurons, thus the PN may act as a high-pass filter during periods of 5-HT release.

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.

5-HT(1B) but not 5-HT(6) or 5-HT(7) receptors mediate depression of spinal nociceptive reflexes in vitro

1. The identity of the serotonin (5-HT) receptors modulating the transmission of segmental C-fibre mediated signals was studied using an in vitro preparation of the hemisected spinal cord from rat pups. 2. Responses to trains of stimuli delivered to a lumbar dorsal root were recorded from the corresponding ventral root. The resulting cumulative depolarization (CD) mediated by unmyelinated fibres was quantified in terms of integrated area. The amplitude of the mono-synaptic reflex was also measured. Serotonergic agents were superfused at known concentrations and their effects on the reflexes evaluated. 3. 5-HT had depressant effects on the CD (EC(50) 34 microM). The rank order of potency of agonists for the depression of the CD was 5-carboxamidotryptamine (5-CT)>alpha-methylserotonin (alpha-met-5-HT) approximately 5-HT>42-methylserotonin (2-met-5-HT)approximately 8-OH-DPAT. 4. All the agonists including 2-met-5-HT and 8-OH-DPAT had strong depressant effects on the mono-synaptic reflex with the following order of potency: 5-CT>48-OH-DPAT>4alpha-met-5-HT approximate5-HTapproximate2-met-5-HT. 5. The inhibitory effects of 5-HT, alpha-met-5-HT and 5-CT were attenuated by the non-specific 5-HT antagonist methiothepin (1 microM) and by the 5-HT(1A/1B) antagonist SDZ 21009 (100 nM) but not by the selective 5-HT(1A) antagonist WAY 100135 (1 microM). 6. Other antagonists known to block 5-HT(2), 5-HT(6) and/or 5-HT(7) receptors (ketanserin, RO 04-6790, ritanserin and clozapine) did not change the effect of the agonists. 7. The data suggest an important contribution of 5-HT(1B) receptors to the inhibition of spinal C-fibre mediated nociceptive reflexes but no experimental support was found for the intervention of 5-HT(2), 5-HT(6) or 5-HT(7) receptors in this in vitro model.

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.

Modulatory actions of serotonin, norepinephrine, dopamine, and acetylcholine in spinal cord deep dorsal horn neurons

The deep dorsal horn represents a major site for the integration of spinal sensory information. The bulbospinal monoamine transmitters, released from serotonergic, noradrenergic, and dopaminergic systems, exert modulatory control over spinal sensory systems as does acetylcholine, an intrinsic spinal cord biogenic amine transmitter. Whole cell recordings of deep dorsal horn neurons in the rat spinal cord slice preparation were used to compare the cellular actions of serotonin, norepinephrine, dopamine, and acetylcholine on dorsal root stimulation-evoked afferent input and membrane cellular properties. In the majority of neurons, evoked excitatory postsynaptic potentials were depressed by the bulbospinal transmitters serotonin, norepinephrine, and dopamine. Although, the three descending transmitters could evoke common actions, in some neurons, individual transmitters evoked opposing actions. In comparison, acetylcholine generally facilitated the evoked responses, particularly the late, presumably N-methyl-D-aspartate receptor-mediated component. None of the transmitters modified neuronal passive membrane properties. In contrast, in response to depolarizing current steps, the biogenic amines significantly increased the number of spikes in 14/19 neurons that originally fired phasically (P < 0.01). Together, these results demonstrate that even though the deep dorsal horn contains many functionally distinct subpopulations of neurons, the bulbospinal monoamine transmitters can act at both synaptic and cellular sites to alter neuronal sensory integrative properties in a rather predictable manner, and clearly distinct from the actions of acetylcholine.

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.

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.

Effects of monoamines on interneurons in four spinal reflex pathways from group I and/or group II muscle afferents

Effects of locally applied serotonin (5-HT) and noradrenaline (NA) were tested on extracellularly recorded responses of single spinal interneurons in deeply anaesthetized cats. These effects were tested on: (i) interneurons mediating reciprocal inhibition from group Ia afferents; (ii) interneurons mediating non-reciprocal inhibition from group Ia and Ib afferents; (iii) intermediate zone interneurons co-excited by group I and II afferents; and (iv) dorsal horn interneurons excited by group II afferents. Effects of monoamines were tested on responses evoked at latencies compatible with monosynaptic coupling. Responses evoked by group Ia and/or Ib muscle afferents were facilitated in all of the tested interneurons both by NA and 5-HT. Responses evoked by group II muscle afferents were depressed in the majority of the interneurons but were facilitated in some of them. 5-HT depressed these responses in all dorsal horn interneurons and in one subpopulation of intermediate zone interneurons, while it facilitated them in another subpopulation of intermediate zone interneurons. NA depressed them in all intermediate zone interneurons and in one subpopulation of dorsal horn interneurons, while it facilitated them in another subpopulation of dorsal horn interneurons. The results of this study lead to the conclusions that: (i) modulation of synaptic actions of muscle spindle and tendon organ afferents on spinal interneurons by 5-HT and NA is related to both the type of the afferent and the functional type of the interneuron; and that (ii) 5-HT and NA counteract each others' actions on some interneuronal types but mutually enhance them on the others.

Modulation of afferent-evoked neurotransmission by 5-HT3 receptors in young rat dorsal horn neurones in vitro: a putative mechanism of 5-HT3 induced anti-nociception

1. The in vitro hemisected spinal cord from young rat was used to investigate the mechanism of serotoninergic modulation of primary afferent-mediated synaptic transmission in the dorsal horn through activation of the 5-HT3 receptor. 2. Dorsal root-evoked excitatory post-synaptic potentials (DR-EPSPs) were recorded intracellularly from dorsal horn neurones. Extracellular recordings of the population primary afferent depolarization (PAD) and the dorsal root-evoked dorsal root reflex (DR-DRR) were made from segmental dorsal roots. 3. 5-Hydroxytryptamine (5-HT) and the selective 5-HT3 receptor agonist 1-(m-chloro-phenyl)-biguanide hydrochloride (m-ChPB) (10 and 50 microM) induced statistically significant reductions of the DR-EPSP amplitude (P<0.001) and duration (P<0.001) in the majority of dorsal horn neurones. The 5-HT3 receptor selective antagonists 3-Tropanyl-indole-3-carboxylate hydrochloride (Tropisetron, 10 microM) and N-(1-Azabicyclo[2.2.2]oct-3-yl)-6-chloro-4-methyl-3-oxo-3,4-dihydro-2H-1 ,4-benzoxazine-8-carboxamide (Y-25130, 10 microM) abolished m-ChPB-induced DR-EPSP attenuation and partially blocked the 5-HT effect. 4. m-ChPB (50 microM)-induced DR-EPSP amplitude and duration attenuation was retained in the presence of the GABA(A) receptor antagonist bicuculline (30 microM), the GABA(B) receptor antagonist saclofen (50 microM) and the opioid receptor antagonist naloxone (50 microM). 5. Both 5-HT and m-ChPB (10 and 50 microM) induced a PAD but the mean peak amplitude of 5-HT-induced PAD was significantly greater than PAD to m-ChPB (98.6+/-12 microV compared to 51.8+/-10 V for 50 microM of agonist, respectively). Tropisetron partially reduced 5-HT-induced PAD and abolished m-ChPB-induced PAD. 5-HT, but not m-ChPB, significantly (P<0.001) reduced the peak amplitude of the DR-DRR and this action of 5-HT was unaffected by Tropisetron or Y-25130. 6. These data provide experimental evidence for a putative cellular mechanism at the level of the dorsal horn for anti-nociceptive effects of 5-HT3 receptor activation. This 5-HT3-mediated modulation of sensory afferent transmission was evidently independent of inhibitory GABA- or opioid-dependent interneuronal pathways. The extent to which the 5-HT3 receptor could be involved in the operation of endogenous analgesia and sensory modulation by descending monoamine bulbo-spinal pathways is discussed.

Serotonin-induced inhibition of locomotor rhythm of the rat isolated spinal cord is mediated by the 5-HT1 receptor class

The neurotransmitter serotonin (5-HT) induces rhythmic motor patterns (fictive locomotion) of the neonatal rat spinal cord in vitro; this is a useful experimental model to study the generation of a motor programme at exclusively spinal level. Nevertheless, 5-HT slows down the fictive locomotion typically elicited by activation of NMDA glutamate receptors, suggesting a complex action of this monoamine. By means of electrophysiological recordings from multiple ventral roots we demonstrated that the decrease caused by 5-HT in NMDA-induced periodicity was dose-dependent, enhanced after pharmacological blocking of 5-HT2 excitatory receptors, and imitated by pharmacological agonists of the 5-HT1 receptor family. Selective blockers of the 5-HT1A or 5-HT1B/D receptor classes, either alone or in combination, largely (but not completely) attenuated this inhibitory action of 5-HT. It is concluded that the principal inhibitory action of 5-HT on the spinal locomotor network was mediated by certain subtypes of the 5-HT1 receptor class, which tends to oppose the 5-HT2 receptor-mediated excitation of the same network.