Characterisation of axon terminals in the rat dorsal horn that are immunoreactive for serotonin 5-HT3A receptor subunits

A systematic review on descending serotonergic projections and modulation of spinal nociception in chronic neuropathic pain and after spinal cord stimulation

Chronic neuropathic pain is a debilitating ordeal for patients worldwide and pharmacological treatment efficacy is still limited. As many pharmacological interventions for neuropathic pain often fail, insights into the underlying mechanism and role of identified receptors is of utmost importance. An important target for improving treatment of neuropathic pain is the descending serotonergic system as these projections modulate nociceptive signaling in the dorsal horn. Also with use of last resort treatments like spinal cord stimulation (SCS), the descending serotonergic projections are known to be involved in the pain relieving effect. This systematic review summarizes the involvement of the serotonergic system on nociceptive modulation in the healthy adult rodent and the chronic neuropathic rodent and summarizes all available literature on the serotonergic system in the SCS-treated neuropathic rodent. Medline, Embase and Pubmed databases were used in the search for articles. Descending serotonergic modulation of nociceptive signaling in spinal dorsal horn in normal adult rat is mainly inhibitory and mediated by 5-HT1a, 5-HT1b, 5-HT2c, 5-HT3 and 5-HT4 receptors. Upon injury and in the neuropathic rat, this descending serotonergic modulation becomes facilitatory via activation of the 5-HT2a, 5-HT2b and 5-HT3 receptors. Analgesia due to neuromodulatory intervention like SCS restores the inhibitory function of the descending serotonergic system and involves 5-HT2, 5-HT3 and 5-HT4 receptors. The results of this systematic review provide insights and suggestions for further pharmacological and or neuromodulatory treatment of neuropathic pain based on targeting selected serotonergic receptors related to descending modulation of nociceptive signaling in spinal dorsal horn. With the novel developed SCS paradigms, the descending serotonergic system will be an important target for mechanism-based stimulation induced analgesia.

Nicotinic receptor modulation of primary afferent excitability with selective regulation of Aδ-mediated spinal actions

Somatosensory input strength can be modulated by primary afferent depolarization (PAD) generated predominantly via presynaptic GABA_A receptors on afferent terminals. We investigated whether ionotropic nicotinic acetylcholine receptors (nAChRs) also provide modulatory actions, focusing on myelinated afferent excitability in in vitro murine spinal cord nerve-attached models. Primary afferent stimulation-evoked synaptic transmission was recorded in the deep dorsal horn as extracellular field potentials (EFPs), whereas concurrently recorded dorsal root potentials (DRPs) were used as an indirect measure of PAD. Changes in afferent membrane excitability were simultaneously measured as direct current (DC)-shifts in membrane polarization recorded in dorsal roots or peripheral nerves. The broad nAChR antagonist d-tubocurarine (d-TC) selectively and strongly depressed Aδ-evoked synaptic EFPs (36% of control) coincident with similarly depressed A-fiber DRP (43% of control), whereas afferent electrical excitability remained unchanged. In comparison, acetylcholine (ACh) and the nAChR agonists, epibatidine and nicotine, reduced afferent excitability by generating coincident depolarizing DC-shifts in peripheral axons and intraspinally. Progressive depolarization corresponded temporally with the emergence of spontaneous axonal spiking and reductions in the DRP and all afferent-evoked synaptic actions (31%-37% of control). Loss of evoked response was long-lasting, independent of DC repolarization, and likely due to mechanisms initiated by spontaneous C-fiber activity. DC-shifts were blocked with d-TC but not GABA_A receptor blockers and retained after tetrodotoxin block of voltage-gated Na⁺ channels. Notably, actions tested were comparable between three mouse strains, in rat, and when performed in different labs. Thus, nAChRs can regulate afferent excitability via two distinct mechanisms: by central Aδ-afferent actions, and by transient extrasynaptic axonal activation of high-threshold primary afferents.NEW & NOTEWORTHY Primary afferents express many nicotinic ACh receptor (nAChR) subtypes but whether activation is linked to presynaptic inhibition, facilitation, or more complex and selective activity modulation is unknown. Recordings of afferent-evoked responses in the lumbar spinal cord identified two nAChR-mediated modulatory actions: 1) selective control of Aδ afferent transmission and 2) robust changes in axonal excitability initiated via extrasynaptic shifts in DC polarization. This work broadens the diversity of presynaptic modulation of primary afferents by nAChRs.

Selective Ablation of Descending Serotonin from the Rostral Ventromedial Medulla Unmasks Its Pro-Nociceptive Role in Chemotherapy-Induced Painful Neuropathy

Purpose

Chemotherapy-induced painful neuropathy (CIPN) is a severe adverse effect of many anti-neoplastic drugs that is difficult to manage. Serotonin (5-hydroxytryptamine, 5-HT) is an important neurotransmitter in the rostral ventromedial medulla (RVM), which modulates descending spinal nociceptive transmission. However, the influence of the descending 5-HT from the RVM on CIPN is poorly understood. We investigated the role of 5-HT released from descending RVM neurons in a paclitaxel-induced CIPN rat model.

Methods

CIPN rat model was produced by intraperitoneally injecting of paclitaxel. Pain behavioral assessments included mechanical allodynia and heat hyperalgesia. 5-HT content was analyzed by high-performance liquid chromatography (HPLC). Western blot and immunohistochemistry were used to determine tryptophan hydroxylase (Tph) and c-Fos expression. The inhibitors p-chlorophenylalanine (PCPA) and SB203580 were administrated by stereotaxical RVM microinjection. Ondansetron was injected through intrathecal catheterization.

Results

The results demonstrated that Tph, the rate-limiting enzyme in 5-HT synthesis, was significantly upregulated in the RVM, and that spinal 5-HT release was increased in CIPN rats. Intra-RVM microinjection of Tph inhibitor PCPA significantly attenuated mechanical and thermal pain behavior through Tph downregulation and decreased spinal 5-HT. Intra-RVM administration of p38 mitogen-activated protein kinase (p38 MAPK) inhibitor SB203580 alleviated paclitaxel-induced pain in a similar manner to PCPA. Intrathecal injection of ondansetron, a 5-HT3 receptor antagonist, partially reversed paclitaxel-induced pain, indicating that 5-HT3 receptors were involved in descending serotoninergic modulation of spinal pain processing.

Conclusion

The results suggest that activation of the p38 MAPK pathway in the RVM leads to increased RVM Tph expression and descending serotoninergic projection to the spinal dorsal horn and contributes to the persistence of CIPN via spinal 5-HT3 receptors.

The developmental emergence of differential brainstem serotonergic control of the sensory spinal cord

Descending connections from brainstem nuclei are known to exert powerful control of spinal nociception and pain behaviours in adult mammals. Here we present evidence that descending serotonergic fibres not only inhibit nociceptive activity, but also facilitate non-noxious tactile activity in the healthy adult rat spinal dorsal horn via activation of spinal 5-HT₃ receptors (5-HT₃Rs). We further show that this differential serotonergic control in the adult emerges from a non-modality selective system in young rats. Serotonergic fibres exert background 5-HT₃R mediated facilitation of both tactile and nociceptive spinal activity in the first three postnatal weeks. Thus, differential descending serotonergic control of spinal touch and pain processing emerges in late postnatal life to allow flexible and context-dependent brain control of somatosensation.

5HT3 receptors: Target for new antidepressant drugs

5HT3 receptors (5HT3Rs) have long been identified as a potential target for antidepressants. Several studies have reported that antagonism of 5HT3Rs produces antidepressant-like effects. However, the exact role of 5HT3Rs and the mode of antidepressant action of 5HT3R antagonists still remain a mystery. Here, we provide a comprehensive overview of 5HT3Rs: (a) regional and subcellular distribution of 5HT3Rs in discrete brain regions, (b) preclinical and clinical evidence supporting the antidepressant effect of 5HT3R antagonists, and (c) neurochemical, biological and neurocellular signaling pathways associated with the antidepressant action of 5HT3R antagonists. 5HT3Rs located on the serotonergic and other neurotransmitter interneuronal projections control their release and affect mood and emotional behavior; however, new evidence suggests that apart from modulating the neurotransmitter functions, 5HT3R antagonists have protective effects in the pathogenic events including hypothalamic-pituitary-adrenal-axis hyperactivity, brain oxidative stress and impaired neuronal plasticity, pointing to hereby unknown and novel mechanisms of their antidepressant action. Nonetheless, further investigations are warranted to establish the exact role of 5HT3Rs in depression and antidepressant action of 5HT3R antagonists.

Inhibition of temporomandibular joint input to medullary dorsal horn neurons by 5HT3 receptor antagonist in female rats

Repeated forced swim (FS) conditioning enhances nociceptive responses to temporomandibular joint (TMJ) stimulation in female rats. The basis for FS-induced TMJ hyperalgesia remains unclear. To test the hypothesis that serotonin 3 receptor (5HT3R) mechanisms contribute to enhanced TMJ nociception after FS, ovariectomized female rats were treated with estradiol and subjected to FS for three days. On day 4, rats were anesthetized with isoflurane and TMJ-responsive neurons were recorded from superficial and deep laminae at the trigeminal subnucleus caudalis/upper cervical (Vc/C1-2) region and electromyographic (EMG) activity was recorded from the masseter muscle. Only Vc/C1-2 neurons activated by intra-TMJ injections of ATP were included for further analysis. Although neurons in both superficial and deep laminae were activated by ATP, only neurons in deep laminae displayed enhanced responses after FS. Local application of the 5HT3R antagonist, ondansetron (OND), at the Vc/C1-2 region reduced the ATP-evoked responses of neurons in superficial and deep laminae and reduced the EMG response in both sham and FS rats. OND also decreased the spontaneous firing rate of neurons in deep laminae and reduced the high-threshold convergent cutaneous receptive field area of neurons in superficial and deep laminae in both sham and FS rats. These results revealed that central application of a 5HT3R antagonist, had widespread effects on the properties of TMJ-responsive neurons at the Vc/C1-2 region and on jaw muscle reflexes under sham and FS conditions. It is concluded that 5HT3R does not play a unique role in mediating stress-induced hyperalgesia related to TMJ nociception.

Tensor tympani motoneurons receive mostly excitatory synaptic inputs

The tensor tympani is a middle ear muscle that contracts in two different situations: in response to sound or during voluntary movements. To gain insight into the inputs and neural regulation of the tensor tympani, we examined the ultrastructure of synaptic terminals on labeled tensor tympani motoneurons (TTMNs) using transmission electron microscopy. Our sample of six TTMNs received 79 synaptic terminals that formed 126 synpases. Two types of synapses are associated with round vesicles and form asymmetric junctions (excitatory morphology). One of these types has vesicles that are large and round (Lg Rnd) and the other has vesicles that are smaller and round (Sm Rnd) and also contains at least one dense core vesicle. A third synapse type has inhibitory morphology because it forms symmetric synapses with pleomorphic vesicles (Pleo). These synaptic terminals can be associated with TTMN spines. Two other types of synapse are found on TTMNs but they are uncommon. Synaptic terminals of all types form multiple synapses but those from a single terminal are always the same type. Terminals with Lg Rnd vesicles formed the most synpases per terminal (avg. 2.73). Together, the synaptic terminals with Lg Rnd and Sm Rnd vesicles account for 62% of the terminals on TTMNs, and they likely represent the pathways driving the contractions in response to sound or during voluntary movements. Having a high proportion of excitatory inputs, the TTMN innervation is like that of stapedius motoneurons but proportionately different from other types of motoneurons.

Spinal 5-HT receptors that contribute to the pain-relieving effects of spinal cord stimulation in a rat model of neuropathy

Spinal cord stimulation (SCS) is extensively employed in the management of neuropathic pain, but the underlying mechanisms are only partially understood. Recently, we demonstrated that the pain-relieving effect of SCS appears to involve the spinal serotonin system, and the present study aimed at identifying the types of the spinal serotonin receptors involved. Experiments were performed on rats with neuropathy produced by partial ligation of the sciatic nerve. Tactile sensitivity was assessed using von Frey filaments, and cold and heat sensitivity with cold spray and radiant heat, respectively. Selective 5-HT receptor antagonists, methiothepin (5-HT(1,6,7)), ketanserin tartrate (5-HT(2A)), TICM (5-HT(3)), SDZ-205,557 (5-HT(4)), as well as receptor agonists, α-m-5-HT (5-HT(2)), m-CPBG (5-HT(3)) in per se ineffective doses, or vehicle, were administrated intrathecally 5 minutes prior to the application of SCS. Ketanserin and SDZ-205,557 significantly attenuated the suppressive effect of SCS on tactile hypersensitivity, while methiothepin and TICM were ineffective. The suppressive effect on cold hypersensitivity of SCS was counteracted by ketanserin only. None of the 5-HT receptor antagonists attenuated the suppressive effect on heat hyperalgesia of SCS. Subeffective doses of α-m-5-HT and m-CPBG enhanced the suppressive effect of SCS on tactile hypersensitivity. The enhancing effect of m-CPBG was abolished by a γ-aminobutyric acid (GABA)(A) or GABA(B) antagonist intrathecally. These results suggest that the activation of 5-HT(2A), 5-HT(3), and 5-HT(4) receptors plays an important role in SCS-induced relief of neuropathic pain. The activation of 5-HT(3) receptors appears to operate via spinal GABAergic interneurons.

Spinal cord mechanisms mediating behavioral hyperalgesia induced by neurokinin-1 tachykinin receptor activation in the rostral ventromedial medulla

Hyperalgesia in animal injury models is linked to activation of descending raphespinal modulatory circuits originating in the rostral ventromedial medulla (RVM). A neurokinin-1 (NK-1) receptor antagonist microinjected into the RVM before or after inflammation produced by complete Freund's adjuvant (CFA) resulted in an attenuation of thermal hyperalgesia. A transient (acute) or a continuous infusion of Substance P (SP) microinjected into the RVM of non-inflamed animals led to similar pain hypersensitivity. Intrathecal pretreatment or post-treatment of a 5-HT3 receptor antagonist (Y-25130 or ondansetron) blocked the SP-induced hyperalgesia. The SP-induced hyperalgesia was both GABA(A) and NMDA receptor-dependent after pre- and post-treatment with selective antagonists at the spinal level. A microinjection of SP into the RVM also led to increased NMDA NR1 receptor subunit phosphorylation in spinal cord tissue. The GABA(A) receptor-mediated hyperalgesia involved a shift in the anionic gradient in dorsal horn nociceptive neurons and an increase in phosphorylated NKCC1 protein (isoform of the Na-K-Cl cotransporter). Following a low dose of SP infused into the RVM, intrathecal muscimol (GABA(A) agonist) increased SP-induced thermal hyperalgesia, phosphorylated NKCC1 protein expression, and NMDA NR1 subunit phosphorylation in the spinal cord. The thermal hyperalgesia was blocked by intrathecal gabazine, the GABA(A) receptor antagonist, and MK-801, the NMDA receptor channel blocker. These findings indicate that NK-1 receptors in the RVM are involved in SP-induced thermal hyperalgesia, this hyperalgesia is 5-HT3-receptor dependent at the spinal level, and involves the functional interaction of spinal GABA(A) and NMDA receptors.

Lack of analgesic efficacy of spinal ondansetron on thermal and mechanical hypersensitivity following spinal nerve ligation in the rat

The balance between descending inhibition and facilitation is thought to be disturbed in chronic pain states. Increased facilitation by spinally released serotonin has been suggested by demonstration that mechanically evoked neuronal responses of wide dynamic range neurons are inhibited by 5-HT3 receptor antagonists in rats following spinal nerve ligation (SNL) but not sham operation. Despite these physiologic data, the effects of spinal 5-HT3 receptor blockade on behavioral hypersensitivity and neurochemical alterations in spinal serotonergic system have not been thoroughly investigated following spinal nerve ligation in the rat. To test this, we acutely injected intrathecal ondansetron in rats between 14 and 30 days after SNL and assessed effects on thermal and mechanical hypersensitivity. We also determined the density of serotonergic nerve fibers, serotonin content and the levels of 5-HT3 receptors within the spinal cord at this time point. Intrathecal ondansetron (1, 3, 10, 30, and 100microg) produced no effect on behavioral measures of thermal or mechanical hypersensitivity whereas intrathecal morphine (1microg) and gabapentin (200microg) partially reversed thermal and mechanical hypersensitivity following SNL. In addition, SNL did not alter the density of serotonergic fibers or 5-HT3 receptor immunoreactivity or spinal tissue content of 5-HT within the dorsal horn. These results do not support anatomic plasticity of descending serotonergic pathways or tonic 5-HT3 receptor activity in maintaining hypersensitivity after nerve injury and in contrast to previous studies fail to demonstrate an anti-hypersensitivity effect of intrathecal injection of the 5-HT3 receptor antagonist ondansetron. Importantly, behavioral measures of mechanical hypersensitivity assess threshold responses whereas physiological studies of mechanically evoked neuronal responses involve application of suprathreshold stimuli. Thus, suprathreshold or more intense stimuli may be necessary to recruit descending serotonergic facilitatory drive required to observe the inhibitory effects of ondansetron on spinal neuronal excitability and behavioral hypersensitivity.

Facilitatory actions of serotonin type 3 receptors on GABAergic inhibitory synaptic transmission in the spinal superficial dorsal horn

Analgesic effects of serotonin (5-hydroxytryptamine [5-HT]) type 3 (5-HT3) receptors may involve the release of gamma-aminobutyric acid (GABA) in the spinal dorsal horn. However, the precise synaptic mechanisms for 5-HT3 receptor-mediated spinal analgesia are not clear. In this study, we investigated whether GABAergic neurons in the superficial dorsal horn (SDH) express functional 5-HT3 receptors and how these 5-HT3 receptors affect GABAergic inhibitory synaptic transmission in the SDH, by using slice preparations from adult glutamate decarboxylase 67-green fluorescent protein (GAD67-GFP) knock-in mice. Tight-seal whole cell recordings from GFP-positive and -negative neurons showed that 5-HT3 receptor-specific agonist 2-methyl-serotonin (2-Me-5-HT) induced inward currents in a substantial population of both GFP-positive and -negative neurons. Additionally, we confirmed expression of 5-HT3 receptors in both types of neurons by single-cell reverse transcription-polymerase chain reaction (RT-PCR) analysis. Further, GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs)-both those evoked by electrical stimulation and those occurring spontaneously in tetrodotoxin (i.e., miniature IPSCs [mIPSCs])-were recorded from GFP-negative neurons. 2-Me-5-HT increased the amplitude of the evoked IPSCs and the frequency of mIPSCs. The amplitude of mIPSCs was not affected by 2-Me-5-HT, suggesting that 5-HT augments GABAergic synaptic transmission via presynaptic mechanisms. The present observations indicate that 5-HT3 receptors are expressed on both somadendritic regions and presynaptic terminals of GABAergic neurons and regulate GABAA receptor-mediated inhibitory synaptic transmission in the SDH. Taken together, these results provide clues for the underlying mechanisms of the antinociceptive actions of 5-HT3 receptors in the spinal dorsal horn.

Responses to 5-HT in morphologically identified neurons in the rat substantia gelatinosa in vitro

Bath application of 5-HT (1-1000 muM) induced a tetrodotoxin (TTX)-resistant outward current at the holding membrane potential (V(H)) of -50 mV in 104/162 (64.2%) of substantia gelatinosa (SG) neurons from the rat spinal cord in vitro. The 5-HT-induced outward current was suppressed by an external solution containing Ba(2+), or a pipette solution containing Cs(2)SO(4) and tetraethylammonium. It was reversed near the equilibrium potential of the K(+) channel. The response to 5-HT was abolished 30 min after patch formation with a pipette solution containing guanosine-5-O-(2-thiodiphosphate)-S. The 5-HT-induced outward current was mimicked by a 5-HT(1A) agonist, (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide, and suppressed by a 5-HT(1A) antagonist, WAY100635, suggesting the 5HT(1A) receptor-mediated activation of K(+) channels in the outward current. In 11/162 (6.8%) SG neurons, 5-HT produced an inward current, which was mimicked by a 5-HT(3) agonist, 1-(m-chlorophenyl)-biguanide (mCPBG). The 5-HT-induced outward currents were observed in vertical cells (21/34) and small islet cells (11/34), while inward currents were induced in islet cells (1/5) and small islet (4/5) cells, but not in vertical cells. It is known that most vertical cells and islet cells in the SG are excitatory (glutamatergic) and inhibitory interneurons, respectively, while small islet cells consist of both excitatory and inhibitory neurons. Bath application of 5-HT or mCPBG increased the amplitude and the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs), but no neurons showed a decrease in sIPSC. Furthermore, frequency, but not amplitude, of miniature IPSCs increased with perfusion with 5-HT in the presence of TTX. These findings, taken together, suggest that 5-HT induces outward currents through 5-HT(1A) receptors in excitatory SG neurons. These findings also suggest that the inward currents are post- and presynaptically evoked through 5-HT(3) receptors, probably in inhibitory neurons.

Descending serotonergic controls regulate inflammation-induced mechanical sensitivity and methyl-CpG-binding protein 2 phosphorylation in the rat superficial dorsal horn

BACKGROUND

Regulation of pain states is, in part, dependent upon plastic changes in neurones within the superficial dorsal horn. There is also compelling evidence that pain states are under the control of descending projections from the brainstem. While a number of transcription factors including Methyl-CpG-binding protein 2 (MeCP2), Zif268 and Fos have been implicated in the regulation of dorsal horn neurone sensitization following injury, modulation of their activity by descending controls has not been investigated.

RESULTS

Here, we describe how descending controls regulate MeCP2 phosphorylation (P-MeCP2), known to relieve transcriptional repression by MeCP2, and Zif268 and Fos expression in the rat superficial dorsal horn, after CFA injection into the hind paw. First, we report that CFA significantly increased P-MeCP2 in Lamina I and II, from 30 min post injection, with a maximum reached after 1 h. The increase in P-MeCP2 paralleled that of Zif268 and Fos, and P-MeCP2 was expressed in large sub-populations of Zif268 and Fos expressing neurones. Serotonergic depletion of the lumbar spinal cord with 5,7 di-hydroxytryptamine creatinine sulphate (5,7-DHT) reduced the inflammation evoked P-MeCP2 in the superficial dorsal horn by 57%, and that of Zif268 and Fos by 37.5% and 30% respectively. Although 5,7-DHT did not change primary thermal hyperalgesia, it significantly attenuated mechanical sensitivity seen in the first 24 h after CFA.

CONCLUSION

We conclude that descending serotonergic pathways play a crucial role in regulating gene expression in the dorsal horn and mechanical sensitivity associated with an inflammatory pain state.

Diverse synaptic terminals on rat stapedius motoneurons

Stapedius motoneurons (SMN) mediate the contraction of the stapedius muscle, which protects the inner ear from injury and reduces the masking effects of background noise. A variety of inputs to SMNs are known to exist, but their terminal ultrastructure has not been investigated. We characterized the synaptic terminals on retrogradely labeled SMNs found just ventromedial to the facial motor nucleus. About 80% of the terminals contained round synaptic vesicles. One type (Sm Rnd) had small, round vesicles filling the terminal with occasional dense core vesicles and formed an asymmetric synapse. Sm Rnd terminals were small with lengths of apposition to the SMN less than 3 microm. Partial reconstructions from serial sections demonstrated that these terminals formed up to three synapses per terminal. Another terminal type (Lg Rnd) had large, round vesicles and asymmetric synapses. Most Lg Rnd terminals were small but some were extensive, e.g., abutting the SMN for up to 10 microm. One of these terminals formed at least seven synapses. Another terminal type (Pleo) had pleomorphic vesicles and symmetric active zones that, in some cases, were invaginated by spines from the SMN. A fourth uncommon terminal type (Het Rnd) had round vesicles of heterogeneous sizes and asymmetric synapses. A fifth rare terminal type (Cist) had large, round vesicles and an accompanying subsurface cistern in the SMN. These were generally the same kinds of terminals found on other motoneurons, but the high proportion of round vesicle synapses indicate that SMNs receive mostly excitatory inputs.

Differential modulation by monoamine membrane receptor agonists of reticulospinal input to lamina VIII feline spinal commissural interneurons

Noradrenaline and serotonin have previously been demonstrated to facilitate the transmission between descending reticulospinal tracts fibres and commissural interneurons coordinating left-right hindlimb muscle activity. The aim of the present study was to investigate the contribution of subclasses of monoaminergic membrane receptors to this facilitation. The neurons were located in Rexed lamina VIII in midlumbar segments and identified by their projections to the contralateral gastrocnemius-soleus motor nuclei and by lack of projections rostral to the lumbosacral enlargement. The effects of ionophoretically applied membrane receptor agonists [phenylephrine (noradrenergic alpha(1)), clonidine (noradrenergic alpha(2)), 8-OH-DPAT (5-HT(1A), 5-HT(7)), 2-me-5-HT (5-HT(3)), 5-me-5-HT (5-HT(2)) and alpha-me-5-HT (5-HT(2))] were examined on extracellularly recorded spikes evoked monosynaptically by electric stimulation of descending reticulospinal fibres in the medial longitudinal fascicle. Application of alpha(1) and 5-HT(2) agonists resulted in a facilitation of responses in all investigated neurons while application of alpha(2), 5-HT(1A/7) and 5-HT(3) agonists resulted in a depression. These opposite modulatory effects of different agonists suggest that the facilitatory actions of noradrenaline and serotonin on responses of commissural interneurons reported previously following ionophoretic application are the net outcome of the activation of different subclasses of monoaminergic membrane receptors. As these receptors may be distributed predominantly, or even selectively, at either pre- or postsynaptic sites their differential modulatory actions could be compatible with a presynaptically induced depression and a postsynaptically evoked enhancement of synaptic transmission between reticulospinal neurons and commissural interneurons.

Anti-convulsants and anti-depressants

Damage to a nerve should only lead to sensory loss. While this is common, the incidence of spontaneous pain, allodynia and hyperalgesia indicate marked changes in the nervous system that are possible compensations for the loss of normal function that arises from the sensory loss. Neuropathic pain arises from changes in the damaged nerve which then alter function in the spinal cord and the brain and lead to plasticity in areas adjacent to those directly influenced by the neuropathy. The peripheral changes drive central compensations so that the mechanisms involved are multiple and located at a number of sites. Nerve damage increases the excitability of both the damaged and undamaged nerve fibres, neuromas and the cell bodies in the dorsal root ganglion. These peripheral changes are substrates for the ongoing pain and the efficacy of excitability blockers such as carbamazepine, lamotrigine and mexiletine, all anti-convulsants. A better understanding of ion channels at the sites of injury has shown important roles of particular sodium, potassium and calcium channels in the genesis of neuropathic pain. Within the spinal cord, increases in the activity of calcium channels and the receptors for glutamate, especially the N-methyl-D-aspartate (NMDA) receptor, trigger wind-up and central hyperexcitability. Increases in transmitter release, neuronal excitability and receptive field size result from the damage to the peripheral nerves. Ketamine and gabapentin/pregabalin, again with anti-convulsant activity, may interact with these mechanisms. Ketamine acts on central spinal mechanisms of excitability whereas gabapentin acts on a subunit of calcium channels that is responsible for the release of pain transmitters into the spinal cord. In addition to these spinal mechanisms of hyperexcitability, spinal cells participate in a spinal-supraspinal loop that involves parts of the brain involved in affective responses to pain but also engages descending excitatory and inhibitory systems that use the monoamines. These pathways become more active after nerve injury and are the site of action of anti-depressants. This chapter reviews the evidence and mechanisms of drugs, both anti-depressants and anti-convulsants, that are believed to be effective in pain control, with a major emphasis on the neuropathic state.

Contribution of peripheral 5-HT2A or 5-HT3 receptors to Fos expression in the trigeminal spinal nucleus produced by acute injury to the masseter muscle during persistent temporomandibular joint inflammation in rats

We investigated the contribution of peripheral 5-HT2A or 5-HT3 receptors to Fos expression in the trigeminal spinal nucleus (VSP) following acute masseter muscle injury in male rats with or without temporomandibular joint (TMJ) inflammation persisting for 7 days. TMJ inflammation was evoked by an injection of complete Freund's adjuvant (CFA). Two hours after formalin injection into the masseter muscle produced Fos-like immunoreactivity (Fos-LI) in several regions of the VSP and upper cervical spinal cord (C2), such as ventrolateral (vl) area of the trigeminal subnucleus caudalis (Vc)/subnucleus interpolaris (Vi) transition (vl-Vi/Vc), paratrigeminal nucleus (dPa5), middle portion of the Vc (mid-Vc) and Vc/C2 transition (Vc/C2) regions in both groups. Significant increases in the number of Fos-LI were observed in these areas in CFA group compared with non-CFA group. TMJ inflammation alone did not induce a significant level of Fos-LI in the VSP. In order to assess the effect of antagonizing 5-HT2A or 5-HT3 receptors on formalin-induced Fos-LI, rats were pre-treated with local (masseter muscle) administration of ketanserin or tropisetron (0.01, 0.1 mg/rat) 20 min prior to formalin injection. In CFA group, these antagonists given locally reduced the Fos-LI response in the laminae I-II at the mid-Vc and Vc/C2 regions. These antagonists reduced the Fos-LI response in the dPa5, but not in the vl-Vi/Vc region. The Fos-LI response was not affected by i.v. administration of ketanserin (0.01, 0.1 mg/rat) or tropisetron (0.01 mg/rat). In non-CFA group, these antagonists given locally did not reduce the Fos-LI response. These results suggest that peripheral 5-HT2A and 5-HT3 receptors contribute to nociceptive processing in the masseter muscle in TMJ inflammatory conditions.

Descending serotonergic facilitation of spinal ERK activation and pain behavior

Serotonin (5-HT) derived from bulbo-spinal projection is released by nociceptive input into the spinal dorsal horn. Here we report that formalin injection in the paw produced pain behavior (flinching) and phosphorylation of spinal ERK1/2 (P-ERK1/2, indicating activation) in rats. Depletion of spinal 5-HT by intrathecal (IT) 5,7-DHT, a serotonergic neurotoxin, profoundly reduced formalin evoked flinching and the increase in P-ERK1/2. Ondansetron (a 5-HT3 receptor antagonist) at IT doses that inhibited flinching also attenuated spinal ERK activation. These findings reveal that primary afferent-evoked activation of spinal ERK requires the input from an excitatory 5-HT descending pathway.

Spatial and temporal patterns of serotonin release in the rat's lumbar spinal cord following electrical stimulation of the nucleus raphe magnus

The monoamine neurotransmitter serotonin is released from spinal terminals of nucleus raphe magnus (NRM) neurons and important in sensory and motor control, but its pattern of release has remained unclear. Serotonin was measured by the high-resolution method of fast cyclic voltammetry (2 Hz) with carbon-fiber microelectrodes in lumbar segments (L3-L6) of halothane-anesthetized rats during electrical stimulation of the NRM. Because sites of serotonin release are often histologically remote from membrane transporters and receptors, rapid emergence into aggregate extracellular space was expected. Increased monoamine oxidation currents were found in 94% of trials of 50-Hz, 20-s NRM stimulation across all laminae. The estimated peak serotonin concentration averaged 37.8 nM (maximum 287 nM), and was greater in dorsal and ventral laminae (I-III and VIII-IX) than in intermediate laminae (IV-VI). When measured near NRM-evoked changes, basal monoamine levels (relative to dorsal white matter) were highest in intermediate laminae, while changes in norepinephrine level produced by locus ceruleus (LC) stimulation were lowest in laminae II/III and VII. The NRM-evoked monoamine peak was linearly proportional to stimulus frequency (10-100 Hz). The peak often occurred before the stimulus ended (mean 15.6 s at 50 Hz, range 4-35 s) regardless of frequency, suggesting that release per impulse was constant during the rise but fell later. The latency from stimulus onset to electrochemical signal detection (mean 4.2 s, range 1-23 s) was inversely correlated with peak amplitude and directly correlated with time-to-peak. Quantitative modeling suggested that shorter latencies mostly reflected the time below detection threshold (5-10 nM), so that extrasynaptic serotonin was significantly elevated well within 1 s. Longer latencies (>5 s), which were confined to intermediate laminae, appeared mainly to be due to diffusion from distant sources. In conclusion, except possibly in intermediate laminae, serotonergic volume transmission is a significant mode of spinal control by the NRM.

Spinal-supraspinal serotonergic circuits regulating neuropathic pain and its treatment with gabapentin

Not all neuropathic pain patients gain relief from current therapies that include the anticonvulsant, gabapentin, thought to modulate calcium channel function. We report a neural circuit that is permissive for the effectiveness of gabapentin. Substance P-saporin (SP-SAP) was used to selectively ablate superficial dorsal horn neurons expressing the neurokinin-1 receptor for substance P. These neurons project to the brain as shown by retrograde labelling and engage descending brainstem serotonergic influences that enhance spinal excitability via a facilitatory action on 5HT(3) receptors. We show the integrity of this pathway following nerve injury contributes to the behavioural allodynia, neuronal plasticity of deep dorsal horn neurons and the injury-specific actions of gabapentin. Thus SP-SAP attenuated the tactile and cold hypersensitivity and abnormal neuronal coding (including spontaneous activity, expansion of receptive field size) seen after spinal nerve ligation. Furthermore the powerful actions of gabapentin after neuropathy were blocked by either ablation of NK-1 expressing neurones or 5HT(3) receptor antagonism using ondansetron. Remarkably, 5HT(3) receptor activation provided a state-dependency (independent of that produced by neuropathy) allowing GBP to powerfully inhibit in normal uninjured animals. This circuit is therefore a crucial determinant of the abnormal neuronal and behavioural manifestations of neuropathy and importantly, the efficacy of gabapentin. As this spino-bulbo-spinal circuit contacts areas of the brain implicated in the affective components of pain, this loop may represent a route by which emotions can influence the degree of pain in a patient, as well as the effectiveness of the drug treatment. These hypotheses are testable in patients.

Central serotonin 3 receptors play an important role in the modulation of nociceptive neural activity of trigeminal subnucleus caudalis and nocifensive orofacial behavior in rats with persistent temporomandibular joint inflammation

The role of central serotonin 3 receptors on neural activities recorded from superficial laminae of trigeminal subnucleus caudalis/upper cervical spinal cord junction region was investigated using rats with (Complete Freund's Adjuvant day 7 group) or without (non-Complete Freund's Adjuvant group) persistent temporomandibular joint inflammation evoked by Complete Freund's Adjuvant for 7 days. We identified two types of units, Deep-wide dynamic range units and Skin-wide dynamic range units from extracellular recordings. Deep-wide dynamic range units have mechanoreceptive fields in the deep craniofacial tissues including masseter muscle but do not have cutaneous mechanoreceptive fields. Deep-wide dynamic range unit discharges evoked by the formalin injection into masseter muscle were significantly enhanced in the late phase in Complete Freund's Adjuvant day 7 group. Discharges of Skin-wide dynamic range units evoked by the noxious pinch stimulation to facial skin in Complete Freund's Adjuvant day 7 group were significantly enhanced compared with those in non-Complete Freund's Adjuvant group. Topical administration of central serotonin 3 receptor antagonist, tropisetron, onto trigeminal subnucleus caudalis/upper cervical spinal cord junction region significantly reduced both formalin-evoked Deep-wide dynamic range unit and pinch-evoked Skin-wide dynamic range unit discharges in non-Complete Freund's Adjuvant and Complete Freund's Adjuvant day 7 groups significantly. The inhibitory effects of tropisetron on pinch-evoked Skin-wide dynamic range unit discharges were prolonged in Complete Freund's Adjuvant day 7 group compared with those in non-Complete Freund's Adjuvant group. The role of central serotonin 3 receptors in trigeminal subnucleus caudalis/upper cervical spinal cord junction region was also tested by orofacial formalin test in Complete Freund's Adjuvant day 7 group. Intracisternal administration of tropisetron decreased the orofacial nocifensive behavior in the late phase evoked by the injection of formalin into the masseter muscle. These results suggest that central serotonin 3 receptors in trigeminal subnucleus caudalis/upper cervical spinal cord junction region are involved in mediating pronociceptive effects in both superficial and deep craniofacial tissues nociception during persistent temporomandibular joint inflammation.

Descending pathways modulating the spinal circuitry for ejaculation: effects of chronic spinal cord injury

Sexual dysfunction is a common complication in men with chronic spinal cord injury. In particular, ejaculation is severely compromised or absent and the resulting infertility issues are important to this group of predominantly young men. To investigate the neural circuits and descending spinal pathways involved in ejaculation, animal models have been developed in normal and spinal cord-injured preparations. Primarily through studies in rats, spinal ejaculatory circuits have been described including (i) autonomic circuits at the thoracolumbar and lumbosacral levels mediating the emission phase of ejaculation, (ii) somatic circuits at the lumbosacral level controlling the expulsion phase of ejaculation through sequential and rhythmic contraction of perineal striated muscles (e.g. bulbospongiosus), and (iii) a proposed ejaculatory pattern generator in the lumbar cord. Midthoracic incomplete chronic spinal cord injury has revealed the dependency of spinal ejaculatory circuits on bilateral spinal pathways from the brainstem via modulation of pudendal motor neuron reflexes and pudendal nerve autonomic fibers. Accordingly, sensory input from the dorsal nerve of the penis, required to trigger the ejaculatory response in animals and humans, is no longer inhibited from the lateral paragigantocellularis nucleus in the ventrolateral medulla. This inhibitory effect, likely presynaptic through a serotonergic pathway, is thought to be necessary to provide the rhythmic, bursting, and sequential contractions of the perineal muscles during ejaculation. Chronic lateral hemisection injury, which severs half of the descending lateral funiculus-located pathways, results in new functional connections of the pudendal reflex inhibitory and pudendal sympathetic activation pathways across the midline, above and below the lesion, respectively. Clinical correlations in spinal cord-injured men have demonstrated the validity of the rodent animal for the study of ejaculatory dysfunction after chronic injury.

Membrane receptors involved in modulation of responses of spinal dorsal horn interneurons evoked by feline group II muscle afferents

Modulatory actions of a metabotropic 5-HT1A&7 membrane receptor agonist and antagonist [(+/-)-8-hydroxy-2-(di-n-propylamino)-tetralin; N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane-carboxamide] and an ionotropic 5-HT3 membrane receptor agonist and antagonist [2-methyl-serotonin (2-Me 5-HT); N-(1-azabicyclo[2.2.2]oct-3-yl)-6-chloro-4-methyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-8-carboxamide hydrochloride] were investigated on dorsal horn interneurons mediating reflex actions of group II muscle afferents. All drugs were applied ionophoretically in deeply anesthetized cats. Effects of agonists were tested on extracellularly recorded responses of individual interneurons evoked by electrical stimulation of group II afferents in a muscle nerve. Effects of antagonists were tested against the depression of these responses after stimulation of raphe nuclei. The results show that both 5-HT1A&7 and 5-HT3 membrane receptors are involved in counteracting the activation of dorsal horn interneurons by group II afferents. Because only quantitative differences were found within the sample of the tested neurons, these results suggest that modulatory actions of 5-HT on excitatory and inhibitory interneurons might be similar. The relationship between 5-HT axons and axons immunoreactive for the 5-HT3A receptor subunit, which contact dorsal horn interneurons, was analyzed using immunofluorescence and confocal microscopy. Contacts from both types of axons were found on all interneurons, but their distribution and density varied, and there was no obvious relationship between them. In two of six interneurons, 5-HT3A-immunoreactive axons formed ring-like arrangements around the cell bodies. In previous studies, axons possessing 5-HT3 receptors were found to be excitatory, and as 2-Me 5-HT depressed transmission to dorsal horn interneurons, the results indicate that 5-HT operates at 5-HT3 receptors presynaptic to these neurons to depress excitatory transmission.

Transmitter content, origins and connections of axons in the spinal cord that possess the serotonin (5-hydroxytryptamine) 3 receptor

Recent evidence suggests that serotonin has pronociceptive actions in the spinal cord when it acts through 5-hydroxytryptamine (5-HT)(3) receptors. Cells and axon terminals which are concentrated in the superficial dorsal horn possess this receptor. We performed a series of immunocytochemical studies with an antibody raised against the 5-HT(3A) subunit in order to address the following questions: 1) Are axons that possess 5-HT(3) receptors excitatory? 2) Are 5-HT(3) receptors present on terminals of myelinated primary afferents? 3) What is the chemical nature of dorsal horn cells that possess 5-HT(3) receptors? 4) Do axons that possess 5-HT(3) receptors target lamina I projection cells? Approximately 45% of 5-HT(3A) immunoreactive boutons were immunoreactive for the vesicular glutamate transporter 2 and almost 80% formed synapse-like associations with GluR2 subunits of the AMPA receptor therefore it is principally glutamatergic axons that possess the receptor. Immunoreactivity was not present on myelinated primary afferent axons labeled with the B-subunit of cholera toxin or those containing the vesicular glutamate transporter 1. Calbindin (which is associated with excitatory interneurons) was found in 44% of 5-HT(3A) immunoreactive cells but other markers for inhibitory and excitatory cells were not present. Lamina I projection cells that possessed the neurokinin-1 receptor were associated with 5-HT(3A) axons but the density of contacts on individual neurons varied considerably. The results suggest that 5-HT(3) receptors are present principally on terminals of excitatory axons, and at least some of these originate from dorsal horn interneurons. The relationship between lamina I projection cells and axons possessing the 5-HT(3) receptor indicates that this receptor has an important role in regulation of ascending nociceptive information.

Bad news from the brain: descending 5-HT pathways that control spinal pain processing

The identification of opioid systems led to much of the early work on pain pharmacology being based on understanding inhibitory mechanisms of analgesia. However, hyperalgesia and allodynia are common clinical symptoms and therefore hyperexcitability must be a major component of pain. Thus, the emphasis of current research into pain has shifted to understanding excitatory pathways that underlie neuronal sensitization and potentiation. Although much evidence supports the presence of descending inhibitory mechanisms of pain, reports of facilitatory pathways from the brainstem have been scarce. In this article, we review evidence for facilitatory 5-HT pathways that link spinal cord and brainstem areas involved in mood and emotions. Because pain encompasses affective aspects, we suggest that these 5-HT pathways and other circuits are important in determining the levels of pain, the outcome of drug treatments and provide a mechanism whereby emotions can alter pain perception.

The 5-HT3 receptor facilitates at-level mechanical allodynia following spinal cord injury

Spinal cord injury (SCI) results in the development of mechanical allodynia immediately rostral to the lesion site, within the dermatome border of normal sensation and sensory loss (at-level mechanical allodynia). We propose that an observed threefold increase in serotonergic fibre immunoreactivity within spinal segments corresponding to these allodynic dermatomes facilitates the maintenance of chronic neuropathic pain via activation of the 5-HT(3) receptor (5-HT(3)-R). Serotonin (5-HT), the non-selective 5-HT(1)/5-HT(2) receptor antagonist, methysergide, the 5-HT(3)-R agonist, m-chlorophenylbiguanide (m-CPBG) or the 5-HT(3)-R antagonist, ondansetron were intrathecally administered five weeks following SCI in rats. Ondansetron produced a robust, long-term reduction of at-level mechanical allodynia, while m-CPBG exacerbated allodynia. Exogenous 5-HT transiently reduced at-level mechanical allodynia. This effect was opposed by methysergide, which enhanced mechanical allodynia. Co-administration of 5-HT and ondansetron produced a short-lasting partial summation of effects, further decreasing mechanical allodynia while co-administration of methysergide attenuated the anti-allodynic effect of ondansetron. Depletion of spinal 5-HT via 5,7-dihydroxytryptamine (5,7-DHT) resulted in decreased at-level mechanical allodynia. The reduction of allodynia by ondansetron was lost following 5,7-DHT administration, suggesting that reduced allodynia following intrathecal ondansetron is via blockade of 5-HT-induced excitation of the 5-HT(3)-R. These results suggest that increased 5-HT fibre density immediately rostral to the SCI lesion site could have transient effects to reduce mechanical allodynia via actions at 5-HT(1) and/or 5-HT(2) receptors. However, the more long-lasting effects of this enhanced serotonergic input may facilitate chronic, at-level allodynia via the 5-HT(3)-R.