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

The development of descending serotonergic modulation of the spinal nociceptive network: a life span perspective

The nociceptive network, responsible for transmission of nociceptive signals that generate the pain experience, is not fully developed at birth. Descending serotonergic modulation of spinal nociception, an important part of the pain network, undergoes substantial postnatal maturation and is suggested to be involved in the altered pain response observed in human newborns. This review summarizes preclinical data of the development of descending serotonergic modulation of the spinal nociceptive network across the life span, providing a comprehensive background to understand human newborn pain experience and treatment. Sprouting of descending serotonergic axons, originating from the rostroventral medulla, as well as changes in receptor function and expression take place in the first postnatal weeks of rodents, corresponding to human neonates in early infancy. Descending serotonergic modulation switches from facilitation in early life to bimodal control in adulthood, masking an already functional 5-HT inhibitory system at early ages. Specifically the 5-HT₃ and 5-HT₇ receptors seem distinctly important for pain facilitation at neonatal and early infancy, while the 5-HT_1a, 5-HT_1b, and 5-HT₂ receptors mediate inhibitory effects at all ages. Analgesic therapy that considers the neurodevelopmental phase is likely to result in a more targeted treatment of neonatal pain and may improve both short- and long-term effects. IMPACT: The descending serotonergic system undergoes anatomical changes from birth to early infancy, as its sprouts and descending projections increase and the dorsal horn innervation pattern changes. Descending serotonergic modulation from the rostral ventral medulla switches from facilitation in early life via the 5-HT₃ and 5-HT₇ receptors to bimodal control in adulthood. A functional inhibitory serotonergic system mainly via 5-HT_1a, 5-HT_1b, and 5-HT_2a receptors at the spinal level exists already at the neonatal phase but is masked by descending facilitation.

Endogenously released 5-HT inhibits A and C fiber-evoked synaptic transmission in the rat spinal cord by the facilitation of GABA/glycine and 5-HT release via 5-HT(2A) and 5-HT(3) receptors

Serotonin (5-HT) released from descending fibers plays important roles in spinal functions such as locomotion and nociception. 5-HT2A and 5-HT3 receptors are suggested to contribute to spinal antinociception, although their activation also contributes to neuronal excitation. In the neonatal spinal cord, DL-p-chloroamphetamine (pCA), a 5-HT releaser, inhibited both A fiber-evoked monosynaptic reflex potential (MSR) and C fiber-evoked slow ventral root potential (sVRP). The pCA-mediated inhibition was reversed by ketanserin (a 5-HT2A receptor antagonist) and tropisetron (a 5-HT3 receptor antagonist). Bath-applied 5-HT also inhibited MSR and sVRP; in this case, the actions of 5-HT were antagonized by ketanserin, but not by tropisetron. The pCA-evoked inhibition of sVRP was reduced by bicuculline (a GABAA receptor antagonist) and strychnine (a glycine receptor antagonist). Furthermore, ketanserin inhibited the pCA-evoked release of gamma-aminobutyric acid (GABA) and glycine, while tropisetron inhibited the pCA-evoked release of 5-HT. These results suggest that 5-HT released by pCA activates 5-HT2A receptors, which in turn stimulates the release of GABA/glycine and thereby blocks the spinal nociceptive pathway. 5-HT3 receptors may be involved in the facilitation of 5-HT release via a positive feedback process.

Stimulation of the lateral hypothalamus produces antinociception mediated by 5-HT1A, 5-HT1B and 5-HT3 receptors in the rat spinal cord dorsal horn

The lateral hypothalamus is part of an efferent system that modifies pain at the spinal cord dorsal horn, but the mechanisms by which lateral hypothalamus-induced antinociception occur are not fully understood. Previous work has shown that antinociception produced from electrical stimulation of the lateral hypothalamus is mediated in part by spinally projecting 5-hydroxytryptamine (5-HT) neurons in the ventromedial medulla. To further examine the role of the lateral hypothalamus in antinociception, the cholinergic agonist carbamylcholine chloride (125 nmol) was microinjected into the lateral hypothalamus of female Sprague-Dawley rats and nociceptive responses measured on the tail-flick and foot-withdrawal tests. Intrathecal injections of the selective 5-HT1A, 5-HT1B, 5-HT3 receptor antagonists, WAY 100135, SB-224289, and tropisetron, respectively, and the non-specific antagonist methysergide, were given. Lateral hypothalamus stimulation with carbamylcholine chloride produced significant antinociception that was blocked by WAY 100135, tropisetron, and SB-224289 on both the tail-flick and foot-withdrawal tests. Methysergide was not different from controls on the tail flick test, but increased foot-withdrawal latencies compared with controls. These results suggest that the lateral hypothalamus modifies nociception in part by activating spinally projecting serotonin neurons that act at 5-HT1A, 5-HT1B, and 5-HT3 receptors in the dorsal horn.

Pronociceptive role of peripheral and spinal 5-HT7 receptors in the formalin test

The possible pronociceptive role of peripheral and spinal 5-HT7 receptors in the formalin test was assessed. Local administration of 5-HT7 (SB-269970, 2.5-77.1 nmol/paw), but not 5-HT(1A) (WAY-100635, 1-60 nmol/paw), receptor antagonist significantly reduced formalin-induced flinching. Local 5-hydroxytryptamine (5-HT, 3-100 nmol/paw) or 5-carboxamidotryptamine (5-CT, 0.3-3 nmol/paw) (a 5-HT7/1A receptor agonist) augmented, in a dose-dependent manner, 0.5% formalin-induced nociceptive behavior. The local pronociceptive effect of 5-HT or 5-CT was significantly reduced by SB-269970 (25 and 77.1 nmol/paw), but not by WAY-100635 (10 nmol/paw). 5-HT7 receptors were observed in myelinated and unmyelinated axons of the digital nerves in rat hindpaw. Intrathecal SB-269970 (2.5-77.1 nmol/rat) or WAY-100635 (1-50 nmol/rat) did not modify 1% formalin-induced nociceptive behavior. Spinal 5-HT (25-200 nmol/rat) significantly reduced formalin-induced flinching behavior during phase 2. At lower doses (0.1-3 nmol/rat) intrathecal 5-CT dose-dependently increased flinching during phase 2. In contrast, higher doses (10-30 nmol/rat) of 5-CT reduced formalin-induced nociceptive behavior during both phases. The spinal pronociceptive effect of 5-CT was reduced by SB-269970 (7.7-77 nmol/rat), but not by WAY-100635 (10 nmol/rat). In addition, the spinal antinociceptive effect of 5-CT was partially reversed by WAY-100635 (10 nmol/rat). The spinal antinociceptive effect of 5-HT was unaffected either by SB-269970 (77 nmol/rat) or WAY-100635 (10 nmol/rat). Data suggest that 5-HT7, but not 5-HT1A, receptors play a pronociceptive role in peripheral and spinal sites in the rat formalin test.

Conversion of the modulatory actions of dopamine on spinal reflexes from depression to facilitation in D3 receptor knock-out mice

Descending monoaminergic systems modulate spinal cord function, yet spinal dopaminergic actions are poorly understood. Using the in vitro lumbar cord, we studied the effects of dopamine and D2-like receptor ligands on spinal reflexes in wild-type (WT) and D3-receptor knock-out mice (D3KO). Low dopamine levels (1 microM) decreased the monosynaptic "stretch" reflex (MSR) amplitude in WT animals and increased it in D3KO animals. Higher dopamine concentrations (10-100 microM) decreased MSR amplitudes in both groups, but always more strongly in WT. Like low dopamine, the D3 receptor agonists pergolide and PD 128907 reduced MSR amplitude in WT but not D3KO mice. Conversely, D3 receptor antagonists (GR 103691 and nafadotride) increased the MSR in WT but not in D3KO mice. In comparison, D2-preferring agonists bromocriptine and quinpirole depressed the MSR in both groups. Low dopamine (1-5 microM) also depressed longer-latency (presumably polysynaptic) reflexes in WT but facilitated responses in D3KO mice. Additionally, in some experiments (e.g., during 10 microM dopamine or pergolide in WT), polysynaptic reflexes were facilitated in parallel to MSR depression, demonstrating differential modulatory control of these reflex circuits. Thus, low dopamine activates D3 receptors to limit reflex excitability. Moreover, in D3 ligand-insensitive mice, excitatory actions are unmasked, functionally converting the modulatory action of dopamine from depression to facilitation. Restless legs syndrome (RLS) is a CNS disorder involving abnormal limb sensations. Because RLS symptoms peak at night when dopamine levels are lowest, are relieved by D3 agonists, and likely involve increased reflex excitability, the D3KO mouse putatively explains how impaired D3 activity could contribute to this sleep disorder.

Serotonin 5-HT2 receptors induce a long-lasting facilitation of spinal reflexes independent of ionotropic receptor activity

Dorsal root-evoked stimulation of sensory afferents in the hemisected in vitro rat spinal cord produces reflex output, recorded on the ventral roots. Transient spinal 5-HT(2C) receptor activation induces a long-lasting facilitation of these reflexes (LLFR) by largely unknown mechanisms. Two Sprague-Dawley substrains were used to characterize network properties involved in this serotonin (5-HT) receptor-mediated reflex plasticity. Serotonin more easily produced LLFR in one substrain and a long-lasting depression of reflexes (LLDR) in the other. Interestingly, LLFR and LLDR were bidirectionally interconvertible using 5-HT(2A/2C) and 5-HT(1A) receptor agonists, respectively, regardless of substrain. LLFR was predominantly Abeta afferent fiber mediated, consistent with prominent 5-HT(2C) receptor expression in the Abeta fiber projection territories (deeper spinal laminae). Reflex facilitation involved an unmasking of polysynaptic pathways and an increased receptive field size. LLFR emerged even when reflexes were evoked three to five times/h, indicating an activity independent induction. Both the NMDA and AMPA/kainate receptor-mediated components of the reflex could be facilitated, and facilitation was dependent on 5-HT receptor activation alone, not on coincident reflex activation in the presence of 5-HT. Selective blockade of GABA(A) and/or glycine receptors also did not prevent reflex amplification and so are not required for LLFR. Indeed, a more robust response was seen after blockade of spinal inhibition, indicating that inhibitory processes serve to limit reflex amplification. Overall we demonstrate that the serotonergic system has the capacity to induce long-lasting bidirectional changes in reflex strength in a manner that is nonassociative and independent of evoked activity or activation of ionotropic excitatory and inhibitory receptors.

M-current modulators alter rat spinal nociceptive transmission: an electrophysiological study in vitro

M-currents constitute a unique effector system to control neuronal excitability due to their voltage and ligand sensitivities. Here we have used retigabine, an M-current agonist, and XE-991, an M-current antagonist, to study the possible involvement of these currents in the processing of spinal sensory and motor processing of nociceptive information in normal, untreated rats. Experiments were performed in a hemisected spinal cord preparation from rat pups using extracellular recordings. Responses to activation of nociceptive and non-nociceptive afferent fibres were recorded. M-current modulators were bath applied to the entire cord or applied locally by pressure ejection. Retigabine and XE-991 produced long-lasting and concentration-dependent effects on nociceptive reflexes showing only minor effects on non-nociceptive reflexes. Retigabine depressed responses to repetitive stimulation of the dorsal root recorded from motor neurones and dorsal horn neurones, whereas XE-991 showed the opposite potentiatory effect and reversed effects of retigabine. Local application of the modulators close by motor nuclei produced changes in reflex responses similar to those caused by bath application. These results constitute a clear indication of the existence of functional M-currents in dorsal and ventral horn elements of the mammalian spinal cord where they may serve to regulate early sensory and motor processing of nociceptive information. The weak effect of modulators on non-nociceptive reflexes suggest that M-currents constitute a promising novel target for analgesics.

Serotonin receptor mRNA expression in rat dorsal root ganglion neurons

In the present study, we have used in situ hybridization to examine the distribution of serotonin (5-HT) receptors in rat dorsal root ganglion (DRG) neurons. Within DRG neurons, mRNAs for 5-HT1B, 5-HT1D, 5-HT2A, 5-HT2B, 5-HT3B and 5-HT4 receptors were readily detected in small (<25 microm), medium (25-45 microm) and large (>45 microm) diameter neurons. In contrast mRNAs for 5-HT1A, 5-HT1E, 5-HT2C, 5-HT5A, 5-HT5B, 5-HT6 and 5-HT7 receptors were undetectable in these neurons. The present study provides an insight into the molecular profile of 5-HT receptor subtypes in neurons responsible for modulating sensory information.

Role of serotonin1A receptors on the modulation of rat spinal mono-synaptic reflexes in vitro

The present study aimed at determining the role of the serotonin(1A) (5-HT(1A)) receptor subtype on the modulation of the mono-synaptic reflex (MSR) elicited by dorsal root stimulation and recorded from ventral roots in the hemisected spinal cord obtained from rat pups. Serotonin and 5-carboxamidotryptamine (5-CT) depressed both the MSR and the cumulative depolarisation (CD) produced by repetitive dorsal root stimulation, whereas the specific 5-HT(1A) receptor agonist (R)-(+)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) applied at 1 microM showed a selective depressant effect on the MSR. Superfusion of the 5-HT(1A) receptor antagonist (+)-N-tert-butyl-3(4(2-methoxyphenyl)-piperazin-1-yl)-2-phenylprpanamide ((+)WAY 100135) produced a complete blockade of 8-OH-DPAT effects. In addition (+)WAY 100135 blocked partially the effects of 5-HT and 5-CT on the MSR but not the effects of these compounds on the CD. The results are consistent with an intervention of 5-HT(1A) receptors in the modulation of non-nociceptive reflexes but do not support a prominent role for this receptor in the modulation of nociceptive reflexes.