Serotonin preferentially hyperpolarizes capsaicin-sensitive C type sensory neurons by activating 5-HT1A receptors

Involvement of Serotonergic System in Oxaliplatin-Induced Neuropathic Pain

Oxaliplatin is a chemotherapeutic agent widely used against colorectal and breast cancers; however, it can also induce peripheral neuropathy that can rapidly occur even after a single infusion in up to 80–90% of treated patients. Numerous efforts have been made to understand the underlying mechanism and find an effective therapeutic agent that could diminish pain without damaging its anti-tumor effect. However, its mechanism is not yet clearly understood. The serotonergic system, as part of the descending pain inhibitory system, has been reported to be involved in different types of pain. The malfunction of serotonin (5-hydroxytryptamine; 5-HT) or its receptors has been associated with the development and maintenance of pain. However, its role in oxaliplatin-induced neuropathy has not been clearly elucidated. In this review, 16 in vivo studies focused on the role of the serotonergic system in oxaliplatin-induced neuropathic pain were analyzed. Five studies analyzed the involvement of 5-HT, while fourteen studies observed the role of its receptors in oxaliplatin-induced allodynia. The results show that 5-HT is not involved in the development of oxaliplatin-induced allodynia, but increasing the activity of the 5-HT_1A, 5-HT_2A, and 5-HT₃ receptors and decreasing the action of 5-HT_2C and 5-HT₆ receptors may help inhibit pain.

Localisation of cannabinoid and cannabinoid-related receptors in the equine dorsal root ganglia

BACKGROUND

Growing evidence recognises cannabinoid receptors as potential therapeutic targets for pain. Consequently, there is increasing interest in developing cannabinoid receptor agonists for treating pain. As a general rule, to better understand the actions of a drug, it would be of extreme importance to know the cellular distribution of its specific receptors. The localisation of cannabinoid receptors in the dorsal root ganglia of the horse has not yet been investigated.

OBJECTIVES

To localise the cellular distribution of canonical and putative cannabinoid receptors in the equine cervical dorsal root ganglia.

STUDY DESIGN

Qualitative and quantitative immunohistochemical study.

METHODS

Cervical (C6-C8) dorsal root ganglia were collected from six horses (1.5 years of age) at the slaughterhouse. The tissues were fixed and processed to obtain cryosections which were used to investigate the immunoreactivity of canonical cannabinoid receptors 1 (CB1R) and 2 (CB2R), and for three putative cannabinoid-related receptors: nuclear peroxisome proliferator-activated receptor alpha (PPARα), transient receptor potential ankyrin 1 (TRPA1) and serotonin 5-HT1a receptor (5-HT1aR).

RESULTS

The neurons showed immunoreactivity for CB1R (100%), CB2R (80% ± 13%), PPARα (100%), TRPA1 (74% ± 10%) and 5-HT1aR (84% ± 6%). The neuronal satellite glial cells showed immunoreactivity for CB2R, PPARα, TRPA1 and 5-HT1aR.

MAIN LIMITATIONS

The low number of horses included in the study.

CONCLUSIONS

This study highlighted the expression of cannabinoid receptors in the sensory neurons and glial cells of the dorsal root ganglia. These findings could be of particular relevance for future functional studies assessing the effects of cannabinoids in horses to manage pain.

Role of 5-HT1A and 5-HT3 receptors in serotonergic activation of sensory neurons in relation to itch and pain behavior in the rat

Serotonin (5-hydroxytryptamine, 5-HT) released by platelets, mast cells, and immunocytes is a potent inflammatory mediator which modulates pain and itch sensing in the peripheral nervous system. The serotonergic receptors expressed by primary afferent neurons involved in these sensory functions are not fully identified and appear to be to a large extent species dependent. Moreover, the mechanisms through which 5-HT receptor activation is coupled to changes in neuronal excitability have not been completely revealed. Using a combination of in vitro (calcium and voltage imaging and patch-clamp) and in vivo behavioral methods, we used both male and female Wistar rats to provide evidence for the involvement of two 5-HT receptor subtypes, 5-HT1A and 5-HT3, in mediating the sustained and transient effects, respectively, of 5-HT on rat primary afferent neurons involved in pain and itch processing. In addition, our results are consistent with a model in which sustained serotonergic responses triggered via the 5-HT1A receptor are due to closure of background potassium channels, followed by membrane depolarization and action potentials, during which the activation of voltage-gated calcium channels leads to calcium entry. Our results may provide a better understanding of mammalian serotonergic itch signaling.

Chronic pain and psychedelics: a review and proposed mechanism of action

The development of chronic pain is a complex mechanism that is still not fully understood. Multiple somatic and visceral afferent pain signals, when experienced over time, cause a strengthening of certain neural circuitry through peripheral and central sensitization, resulting in the physical and emotional perceptual chronic pain experience. The mind-altering qualities of psychedelics have been attributed, through serotonin 2A (5-HT2A) receptor agonism, to ‘reset’ areas of functional connectivity (FC) in the brain that play prominent roles in many central neuropathic states. Psychedelic substances have a generally favorable safety profile, especially when compared with opioid analgesics. Clinical evidence to date for their use for chronic pain is limited; however, several studies and reports over the past 50 years have shown potential analgesic benefit in cancer pain, phantom limb pain and cluster headache. While the mechanisms by which the classic psychedelics may provide analgesia are not clear, several possibilities exist given the similarity between 5-HT2A activation pathways of psychedelics and the nociceptive modulation pathways in humans. Additionally, the alterations in FC seen with psychedelic use suggest a way that these agents could help reverse the changes in neural connections seen in chronic pain states. Given the current state of the opioid epidemic and limited efficacy of non-opioid analgesics, it is time to consider further research on psychedelics as analgesics in order to improve the lives of patients with chronic pain conditions.

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.

An Association of Serotonin with Pain Disorders and Its Modulation by Estrogens

Ovarian hormones play an important role in pain perception, and are responsible, at least in part, for the pain threshold differences between the sexes. Modulation of pain and its perception are mediated by neurochemical changes in several pathways, affecting both the central and peripheral nervous systems. One of the most studied neurotransmitters related to pain disorders is serotonin. Estrogen can modify serotonin synthesis and metabolism, promoting a general increase in its tonic effects. Studies evaluating the relationship between serotonin and disorders such as irritable bowel syndrome, fibromyalgia, migraine, and other types of headache suggest a clear impact of this neurotransmitter, thereby increasing the interest in serotonin as a possible future therapeutic target. This literature review describes the importance of substances such as serotonin and ovarian hormones in pain perception and illustrates the relationship between those two, and their direct influence on the presentation of the aforementioned pain-related conditions. Additionally, we review the pathways and receptors implicated in each disorder. Finally, the objective was to stimulate future pharmacological research to experimentally evaluate the potential of serotonin modulators and ovarian hormones as therapeutic agents to regulate pain in specific subpopulations.

The potential role of serotonergic mechanisms in the spinal oxytocin-induced antinociception

The role of oxytocin (OXT) in pain modulation has been suggested. Indeed, hypothalamic paraventricular nuclei (PVN) electrical stimuli reduce the nociceptive neuronal activity (i.e., neuronal discharge associated with activation of Aδ- and C-fibers) of the spinal dorsal horn wide dynamic range (WDR) cells and nociceptive behavior. Furthermore, raphe magnus nuclei lesion reduces the PVN-induced antinociception, suggesting a functional interaction between the OXT and the serotoninergic system. The present study investigated in Wistar rats the potential role of spinal serotonergic mechanisms in the OXT- and PVN-induced antinociception. In long-term secondary mechanical allodynia and hyperalgesia induced by formalin or extracellular unitary recordings of the WDR cells we evaluated the role of 5-hydroxytryptamine (5-HT) effect on the OXT-induced antinociception. All drugs were given intrathecally (i.t.). OXT (1×10^-5-1×10^-4nmol) or 5-HT (1×10^-3-1×10^-1nmol) prevented the formalin-induced sensitization, an effect mimicked by PVN stimulation. Moreover, administration of OXT (1×10^-5nmol) plus 5-HT (1×10^-3nmol) at ineffective doses, produced antinociception. This effect was antagonized by: (i) d(CH₂)₅[Tyr(Me)²,Thr⁴,Tyr-NH₂⁹]OVT (oxytocin receptor antagonist; 2×10^-2nmol); or (ii) methiothepin (a non-specific 5-HT_1/2/5/6/7 receptor antagonist; 80nmol). Similar results were obtained with PVN stimulation plus 5-HT (5×10^-5nmol). In WDR cell recordings, the PVN-induced antinociception was enhanced by i.t. 5-HT and partly blocked when the spinal cord was pre-treated with methiothepin (80nmol). Taken together, these results suggest that serotonergic mechanisms at the spinal cord level are partly involved in the OXT-induced antinociception.

Sex differences in the expression of serotonin-synthesizing enzymes in mouse trigeminal ganglia

Migraine headaches are more prevalent in women and often occur during the early phases of the menstrual cycle, implying a link between migraine and ovarian steroids. Serotonin (5-HT) and its receptors have been proposed to play a key role in the pathophysiology of migraine. The trigeminal ganglion (TG) has been proposed as a site for 5-HT synthesis based on the expression of the rate limiting enzyme in peripheral 5-HT synthesis, tryptophan hydroxylase 1 (TPH1), in female rodent trigeminal ganglia. Tryptophan hydroxylase levels vary over the estrus cycle, however, the expression and potential regulation of other enzymes involved in 5-HT synthesis has not been reported in this tissue. C57/BL6 mice of both sexes expressed TPH1 and aromatic amino acid decarboxylase (AADC), the key enzymes involved in 5-HT synthesis. Levels of both enzymes were significantly higher in juvenile males compared with females. In naturally cycling females TPH1 and AADC expression was highest during proestrus when compared with the other phases of the cycle, and this regulation was mirrored at the mRNA level. In situ hybridization experiments detected TPH1 and AADC mRNA in presumptive neurons in the trigeminal ganglion. Both key enzymes involved in the synthesis of 5-HT are expressed in mouse trigeminal ganglion and are localized to neurons. The levels of these enzymes are dependent on gender and estrus cycle stage, suggesting that ovarian steroids might play a role in the regulation of sensory neuron 5-HT synthesis.

Taltirelin, a thyrotropin-releasing hormone analog, alleviates mechanical allodynia through activation of descending monoaminergic neurons in persistent inflammatory pain

Thyrotropin-releasing hormone (TRH) and its analogs have been reported to modulate descending monoaminergic inhibitory neurons, resulting in antinociception. However, it remains unknown whether TRH exerts an antiallodynic effect during persistent pain. Here, we investigated the action of taltirelin, a stable TRH analog, on mechanical allodynia in mice with inflammatory persistent pain induced by an injection of complete Freund's adjuvant into the hindpaw. Systemic administration of 1.0 mg/kg taltirelin markedly reduced mechanical allodynia. This effect was abolished by the 6-hydroxydopamine (6-OHDA)-induced depletion of central noradrenaline. While intraperitoneal injection of the α₁-adrenoceptor antagonist prazosin had no effect, intraperitoneal and intrathecal administration of the α₂-adrenoceptor antagonist yohimbine prevented the antiallodynic action of taltirelin. In addition, DL-p-chlorophenylalanine (PCPA)-induced depletion of serotonin (5-HT) and intraperitoneal and intrathecal injection of the 5-HT(1A) receptor antagonist WAY-100635 blocked the effect of taltirelin on allodynia. These findings suggest that taltirelin alleviates mechanical allodynia in inflammatory persistent pain by modulating the descending noradrenergic and serotonergic neuronal pathways via indirect activation of spinal α₂-adrenergic and 5-HT(1A) receptors.

Reserpine causes biphasic nociceptive sensitivity alteration in conjunction with brain biogenic amine tones in rats

The present study investigated the precise relationship between brain biogenic amine (dopamine, noradrenaline, and serotonin) tones and nociception. Nociceptive sensitivities to multimodal (muscle pressure, tactile, cold, and heat) stimuli were assessed in acute phase (up to 24 h after reserpine or tetrabenazine injection) and chronic phase (on day 2 or later) in rats. A single injection of reserpine (3 mg/kg s.c.) significantly decreased biogenic amines in the spinal cord (SC), thalamus (THA), and prefrontal cortex (PFC) in both acute and chronic phases, but significantly increased a dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the SC and a serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the SC and THA in acute phase. The content of all biogenic amine metabolites was at low level in chronic phase. Animals exhibited hypersensitivities to tactile and heat stimuli and hyposensitivity to muscle pressure stimulus in acute phase. In chronic phase, they manifested hypersensitivities to all modes of stimuli. Tetrabenazine (20 mg/kg i.p.) significantly decreased brain biogenic amines for a short time, although it did not significantly affect the nociceptive sensitivities. In conclusion, a single injection of reserpine causes a biphasic alteration of nociceptive sensitivities, which is in conjunction with the dynamic change of brain biogenic amine tones, in rats. Cold and heat hypersensitivities in addition to mechanical ones are induced by the reserpine treatment. Sustained modification of brain biogenic amine tones would be critical to induce a robust change in nociceptive sensitivities based on the different effects between reserpine and tetrabenazine.

The role of peripheral 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E and 5-HT1F serotonergic receptors in the reduction of nociception in rats

This study assessed the possible antinociceptive role of peripheral 5-HT(1) receptor subtypes in the rat formalin test. Rats were injected into the dorsum of the hind paw with 50 microl of diluted formalin (1%). Nociceptive behavior was quantified as the number of flinches of the injected paw. Reduction of flinching was considered as antinociception. Ipsilateral, but not contralateral, peripheral administration of the 5-HT(1) receptor agonists R(+)-UH-301 (5-HT(1A); 0.1-3 microg/paw), CGS-12066A (5-HT(1B); 0.01-0.3 microg/paw), GR46611 (5-HT(1B/1D); 0.3-10 microg/paw), BRL54443 (5-HT(1E/1F); 3-300 microg/paw) or LY344864 (5-HT(1F); 3-300 microg/paw) significantly reduced formalin-induced flinching. The corresponding vehicle was devoid of any effect by itself. The local antinociceptive effect of R(+)-UH-301 (0.3 microg/paw) was significantly reduced by WAY-100635 (30-100 microg/paw; a 5-HT(1A) receptor antagonist). Moreover, the antagonists GR55562 (30-100 microg/paw; 5-HT(1B/D)) or SB224289 (30-100 microg/paw; 5-HT(1B)) dose-dependently reduced the antinociceptive effect of CGS-12066A (0.3 microg/paw) whereas GR55562 (30-100 microg/paw) or BRL15572 (30-100 microg/paw, 5-HT(1D)) reduced the antinociceptive effect of GR46611 (0.3 microg/paw). Interestingly, the effects of BRL54443 and LY344864 (300 microg/paw each) were partially reduced by methiothepin, but not by the highest doses of WAY-100635, SB224289 or BRL15572. The above antagonists did not produce any effect by themselves. These results suggest that peripheral activation of the 5-HT(1A,) 5-HT(1B), 5-HT(1D), 5-HT(1F) and, probably, 5-HT(1E) receptor subtypes leads to antinociception in the rat formalin test. Thus, the use of selective 5-HT(1) receptor agonists could be a therapeutic strategy to reduce inflammatory pain.

The endogenous redox agent L-cysteine induces T-type Ca2+ channel-dependent sensitization of a novel subpopulation of rat peripheral nociceptors

Recent studies have demonstrated a previously unrecognized contribution of T-type Ca2+ channels in peripheral sensory neurons to pain sensation (nociception). However, the cellular mechanisms underlying the functions of these channels in nociception are not known. Here, in both acutely dissociated and intact rat dorsal root ganglion neurons, we characterize a novel subpopulation of capsaicin- and isolectin B4-positive nociceptors that also expresses a high density of T-type Ca2+ currents. Using these "T-rich" cells as a model, we demonstrate that the endogenous reducing agent L-cysteine lowers the threshold for nociceptor excitability and induces burst firing by increasing the amplitude of T-type currents and shifting the gating parameters of T-type channels. These findings, which provide the first direct evidence of T-type Ca2+ channel involvement in the control of nociceptor excitability, suggest that endogenous T-type channel agonists may sensitize a unique subpopulation of peripheral nociceptors, consequently influencing pain processing under normal or pathological conditions.

Suppression of bladder reflex activity in chronic spinal cord injured cats by activation of serotonin 5-HT1A receptors

The effects of 8-OH-DPAT (5-HT1A receptor agonist) and WAY100635 (5-HT1A receptor antagonist) on reflex bladder activity were investigated in alpha-chloralose anesthetized or conscious chronic spinal cord injured cats. The results were similar in both anesthetized and conscious animals. Cystometrograms revealed that 8-OH-DPAT (0.5 mg/kg, s.c.) significantly increased the bladder volume threshold for eliciting a large amplitude micturition contraction, but only slightly reduced the amplitude of the contractions and did not alter the small amplitude pre-micturition contractions. 8-OH-DPAT also reduced the amplitude of isovolumetric bladder contractions. The inhibitory effect of 8-OH-DPAT was reversed by WAY100635 (0.5 mg/kg) or blocked by pre-treatment with WAY100635. Reflex bladder contractions evoked by tactile stimulation of the perigenital region were not altered by 8-OH-DPAT. These results suggest that the inhibitory effect of 8-OH-DPAT is mediated by an action on interneuronal pathways in the spinal cord or on the C-fiber afferent limb of the spinal micturition reflex and not on bladder smooth muscle or the efferent limb of the reflex pathway. Drugs that activate 5-HT1A receptors might be useful in treating detrusor overactivity after spinal cord injury.

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.

Effects of the high-efficacy 5-HT1A receptor agonist, F 13640 in the formalin pain model: a c-Fos study

We studied the effects of the high-efficacy 5-hydroxytryptamine1A (5-HT1A) receptor agonist, F 13640 on both formalin-induced spinal cord c-Fos protein expression and pain behaviours in the rat. Replicating earlier data, F 13640 (0.63 mg/kg, i.p.; t(-15 min)) completely inhibited the elevation and licking of the formalin-injected paw. In the same animals, and in spite of the agent as in earlier data increasing the number of c-Fos labelled nuclei when it was administered alone, F 13640 markedly reduced the number of formalin-induced c-Fos labelled nuclei. This was found in both the superficial (I-II) and deep (V-VI) dorsal horn laminae (2 h post-injection: 72+/-2% and 92+/-1% of reduction, respectively; P<0.001 in either case), spinal areas that contain neurons responsive to nociceptive stimulation. Co-operation occurred so that after the co-administration of F 13640 and formalin, c-Fos expression was inferior to that induced when either stimulation was administered alone. The data provide initial evidence for the agent's inhibitory effects on noxiously evoked c-Fos expression. The results indicate that co-operation between 5-HT1A receptor activation and nociceptive stimulation powerfully inhibits responses to severe, tonic nociception.

Potentiation of 5-HT3 receptor function by the activation of coexistent 5-HT2 receptors in trigeminal ganglion neurons of rats

5-HT receptor subtypes are widely expressed in primary sensory neurons, yet so far little is known about the interaction among them. This study aimed to investigate whether the activation of 5-HT2 and 5-HT1 receptors could modulate 5-HT3 receptor mediated current in rat trigeminal ganglion (TG) neurons using whole-cell patch clamp technique. The majority of TG neurons examined responded to 5-HT (10(-7)-10(-3) M) with a fast activating and rapid desensitizing inward current (77.2%, 71/92). This 5-HT activated current (I(5-HT)) was blocked by ICS 205-930 and mimicked by 2-methyl-5-HT, indicating that it was mediated by 5-HT3 receptor. With alpha-methyl-5-HT applied prior to 5-HT application, I(5-HT) was potentiated in a concentration-dependent manner, with the maximal modulatory effect at 10(-9) M of alpha-methyl-5-HT. The concentration-response curve for I(5-HT) pretreated with alpha-methyl-5-HT shifts upwards compared with that for I(5-HT) without alpha-methyl-5-HT pretreatment, the maximal I(5-HT) value having increased by (60.3 +/- 5.7)% of its control while the EC50 values of the two curves being very close, i.e. (2.0 +/- 0.3) x 10(-5) M vs (1.7 +/- 0.2) x 10(-5) M, respectively. The alpha-methyl-5-HT potentiation of I(5-HT) was removed by intracellular dialysis of either GDP-beta-S, a non-hydrolyzable GDP analog, or GF109203X, a selective PKC inhibitor, almost completely. Preapplication of R-(+)-UH-301, a selective agonist of 5-HT(1A) receptor, had no modulatory effect on I(5-HT). These results suggest that in the membrane of TG neurons, the activation of 5-HT2 receptors can exert an enhancing effect on the function of coexistent 5-HT3 receptors while that of 5-HT(1A) receptors cannot.

5beta-reduced neuroactive steroids are novel voltage-dependent blockers of T-type Ca2+ channels in rat sensory neurons in vitro and potent peripheral analgesics in vivo

T-type Ca(2+) channels are believed to play an important role in pain perception, and anesthetic steroids such as alphaxalone and allopregnanolone, which have a 5alpha-configuration at the steroid A, B ring fusion, are known to inhibit T-type Ca(2+) channels and cause analgesia in a thermal nociceptive model (Soc Neurosci Abstr 29:657.9, 2003). To define further the structure-activity relationships for steroid analgesia, we synthesized and examined a series of 5beta-reduced steroids for their ability to induce thermal antinociception in rats when injected locally into the peripheral receptive fields of the nociceptors and studied their effects on T-type Ca(2+) channel function in vitro. We found that most of the steroids completely blocked T-type Ca(2+) currents in vitro with IC(50) values at a holding potential of -90 mV ranging from 2.8 to 40 microM. T current blockade exhibited mild voltage-dependence, suggesting that 5beta-reduced neuroactive steroids stabilize inactive states of the channel. For the most potent steroids, we found that other voltage-gated currents were not significantly affected at concentrations that produce nearly maximal blockade of T currents. All tested compounds induced dose-dependent analgesia in thermal nociceptive testing; the most potent effect (ED(50), 30 ng/100 microl) obtained with a compound [(3beta,5beta,17beta)-3-hydroxyandrostane-17-carbonitrile] that was also the most effective blocker of T currents. Compared with previously studied 5alpha-reduced steroids, these 5beta-reduced steroids are more efficacious blockers of neuronal T-type Ca(2+) channels and are potentially useful as new experimental reagents for understanding the role of neuronal T-type Ca(2+) channels in peripheral pain pathways.

Immediate plasticity in the motor pathways after spinal cord hemisection: implications for transcranial magnetic motor-evoked potentials

The present study evaluates motor functional recovery after C2 spinal cord hemisection with or without contralateral brachial root transection, which causes a condition that is similar to the crossed phrenic phenomenon on rats. Descending motor pathways, including the reticulospinal extrapyramidal tract and corticospinal pyramidal tracts, were evaluated by transcranial magnetic motor-evoked potentials (mMEPs) and direct cortical electrical motor-evoked potentials (eMEP), respectively. All MEPs recorded from the left forelimb were abolished immediately after the left C2 hemisection. Left mMEPs recovered dramatically immediately after contralateral right brachial root transection. Corticospinal eMEPs never recovered, regardless of transection. The facilitation of mMEPs in animals that had undergone combined contralateral root transection was well correlated with open-field behavioral motor performance. Both electrophysiological and neurological facilitations were significantly attenuated by the selective serotonin synthesis inhibitor para-chlorophenylalanine (p-CPA). These results suggest that serotonergic reticulospinal fibers located contralateral to hemisection contribute to the behavioral and electrophysiological improvement that immediately follows spinal cord injury (SCI).

The inflammatory mediators serotonin, prostaglandin E2 and bradykinin evoke calcium influx in rat sensory neurons

The inflammatory mediators bradykinin, prostaglandin E(2) and serotonin interact to excite and sensitize nociceptive neurons. All three mediators are coupled to signaling pathways that potentially induce rises in intracellular calcium concentration in other models. The aim of this study was therefore to investigate if the three mediators cause calcium rises in isolated rat sensory neurons that may explain their sensitizing action. Neurons exposed to serotonin, bradykinin, and prostaglandin E(2) exhibited reversible increases in intracellular calcium concentration, which were absent in calcium-free solution. The calcium increase induced by serotonin was preserved in the presence of extracellular cadmium suggesting calcium influx potentially through the serotonin receptor ion channel 5-HT(3). The bradykinin-induced calcium response was slower, showed pronounced tachyphylaxis and was absent in the presence of extracellular cadmium ions. Similar results were obtained for prostaglandin E(2) although the calcium rises were fast and not prone to tachyphylaxis. This suggests that prostaglandin E(2) as well as bradykinin via activation of G protein-coupled receptors seem to couple to calcium-permeant ion channels possibly the heat-transducing vanilloid receptor type 1 or related ion channels. The three mediators, however, did not cooperate to induce supra-additive calcium responses when applied simultaneously. In summary, our results suggest that the inflammatory mediators serotonin, prostaglandin E(2) and bradykinin induce calcium influx in sensory neurons. However, they do not utilize a calcium-dependent cooperative mechanism to facilitate proton-induced currents.

Perikaryal surface specializations of neurons in sensory ganglia

Slender projections, similar to microvilli, are the main specialization of the perikaryal surface of sensory ganglion neurons. The extent of these projections correlates closely with the volume of the corresponding nerve cell body. It is likely that the role of perikaryal projections of sensory ganglion neurons, which lack dendrites, is to maintain the surface-to-volume ratio of the nerve cell body above some critical level for adequate metabolic exchange. Satellite cells probably have the ability to promote, or provide a permissive environment for, the outgrowth of these projections. It is not yet known whether the effect of satellite cells is mediated by molecules associated with their plasma membrane or by diffusible factors. Furthermore, receptor molecules for numerous chemical agonists are located on the nerve cell body surface, but it is not known whether certain molecules are located exclusively on perikaryal projections or are also present on the smooth surface between these projections. Further study of the nerve cell body surface and of the influence that satellite cells exert on it will improve our understanding of the interactions between sensory ganglion neurons and satellite neuroglial cells.

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.

5-HT7 receptors are involved in mediating 5-HT-induced activation of rat primary afferent neurons

The purpose of this study was to determine whether the 5-hydroxytryptamine7 (5-HT7) receptor is expressed by nociceptor-like neurons in the rat PNS and whether 5-HT activates these nociceptors via the 5-HT7 receptor subtype. Using a polyclonal antibody and the method of immunofluorescence staining, we demonstrated that the 5-HT7 receptor appears predominately on "nociceptor-like" neurons of the rat lumbar dorsal root ganglia. Using immunocytochemical methods, we showed that the immunoreactivity of the 5-HT7 receptor antibody complex is localized in the superficial layers of the spinal cord dorsal horn, which corresponds with laminae I, IIouter and IIinner. Furthermore, we demonstrated that noxious stimulation produced by knee injection of 5-HT or a 5-HT7 agonist dose-dependently increases c-Fos production of the rat spinal cord dorsal horn. This effect was significantly inhibited by the preinjection of a 5-HT7 antagonist. We conclude that the 5-HT7 receptor is expressed by rat primary afferent nociceptors which terminate in the superficial layers of the spinal cord dorsal horn and that the 5-HT7 receptor subtype is involved in nociceptor activation by 5-HT.

Redox modulation of T-type calcium channels in rat peripheral nociceptors

Although T-type calcium channels were first described in sensory neurons, their function in sensory processing remains unclear. In isolated rat sensory neurons, we show that redox agents modulate T currents but not other voltage- and ligand-gated channels thought to mediate pain sensitivity. Similarly, redox agents modulate currents through Ca(v)3.2 recombinant channels. When injected into peripheral receptive fields, reducing agents, including the endogenous amino acid L-cysteine, induce thermal hyperalgesia. This hyperalgesia is blocked by the oxidizing agent 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) and the T channel antagonist mibefradil. DTNB alone and in combination with mibefradil induces thermal analgesia. Likewise, L-cysteine induces mechanical DTNB-sensitive hyperalgesia in peripheral receptive fields. These data strongly suggest a role for T channels in peripheral nociception. Redox sites on T channels in peripheral nociceptors could be important targets for agents that modify pain perception.

5-HT(1A) and 5-HT(1B/1D) receptors are involved in the modulation of the trigeminovascular system of the cat: a microiontophoretic study

Electrical stimulation of the superior sagittal sinus in the cat activated neurones in the trigeminal nucleus caudalis. The mean latency of these responses (10.1 ms) was consistent with activation of Adelta-fibres. Microiontophoretic ejection of either the selective serotonin(1A) (5-HT(1A)) agonist (+)8-OH-DPAT or the 5-HT(1B/1D) agonist alniditan resulted in the reversible suppression of the response to superior sagittal sinus stimulation of 29/46 and 18/20 trigeminal neurones, respectively. The response to sagittal sinus stimulation was suppressed by 39+/-5% (n=46) by (+)8-OH-DPAT and 65+/-5% (n=20) by alniditan. Microiontophoretic ejection of the selective 5-HT(1A) receptor antagonist WAY-100635 significantly antagonised the effect of (+)8-OH-DPAT (effect reduced by 30%, P<0.05). The ejection of GR-127935, a selective 5-HT(1B/1D), antagonist, significantly antagonised the effect of alniditan (effect reduced by 52%, P<0.02). In eight neurones the response to convergent facial receptive field stimulation was also tested in the presence of alniditan. Only 4/8 receptive field responses were suppressed by alniditan (compared to 8/8 sagittal sinus responses) and alniditan had significantly less quantitative effect on the response to receptive field stimulation than on the response to sagittal sinus stimulation in the same neurones (mean reduction 36+/-14% and 66+/-8%, respectively, P<0.05). These results suggest that pharmacological modulation of the trigeminovascular system can occur at the first central synapse and that, in addition to 5-HT(1B/1D) receptors, 5-HT(1A) receptors may be involved in the modulation of sensory neurotransmission in the trigeminovascular system.

Lack of efficacy of topical capsaicin in serotonin-induced itch

Capsaicin (CAP) has been demonstrated to be an effective topical inhibitor of cutaneous vasodilatation, pain and pruritus induced by a variety of chemical and physical stimuli. In a previous study, we showed a significantly inhibitory effect of topical CAP treatment on histamine-induced itch and cutaneous vascular reactions in healthy subjects compared to atopic eczema patients. As serotonin is proposed to play a pathophysiological role in some types of pruritus (e.g. uremic and hepatic pruritus) and CAP has been described to be successful in hemodialysis-related pruritus, we investigated the antipruritic effect of topical CAP on serotonin-induced reactions in 10 healthy volunteers in comparison to untreated skin (UPS) and placebo substance (vehicle)-treated skin (VS). On the first day, serotonin iontophoresis was performed in untreated skin. One week later, the treatments started, using either CAP 0.05% liniment or a placebo liniment (vehicle) 3 times daily over a 5-day period. On day 6, serotonin was applied by iontophoresis within the pretreated skin. After another 1-week break, the treatments were performed vice versa on the corresponding infrascapular region. Weal and flare areas were planimetrically evaluated. Itch sensations were documentated by the volunteer on a scale over a 24-min follow-up period. The examination also comprised alloknesis, which stands for induction of perifocal sensations by usually non-itching stimuli. In CAP-treated skin, serotonin-induced wheals were significantly larger post-application compared to non-pretreated skin. Wheals were significantly larger in VS than in UPS. Comparison of serotonin-induced flares in the different study arms did not reveal any significant differences. Itch sensations were not significantly reduced by topical CAP application. The areas of alloknesis were smaller in capsaicin-treated skin compared to VS and UPS, but did not reach significant value. In conclusion, topical CAP application is not effective in serotonin-induced itching in healthy volunteers. Serotonin is most unlikely to play a role in the mechanism of action of CAP.

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

The localization of serotonin2A (5-HT2A) receptors in the adult rat spinal cord and dorsal root ganglia was examined by using a polyclonal antibody that recognizes the C-terminus peptides of the mouse 5-HT2A receptor. Positive cell bodies of 5-HT2A receptor were found in several regions of the spinal cord. Generally, large-to-intermediate sized neuronal cell bodies were intensely immunolabeled. Motoneurons in the ventral horn were the most intensely labeled. Dot-like immunoreactive profiles were located beneath the cell membrane of motoneurons. Neuronal somata in the intermediolateral nucleus of the thoracic spinal cord were moderately labeled. The immunoreactivity in the dorsal horn was weak. A considerable number of glial cell bodies in the white matter were immunostained. The majority of both small and large sized neurons were 5-HT2A immunopositive in the dorsal root ganglion.

5-HT2A receptor subtype is involved in the thermal hyperalgesic mechanism of serotonin in the periphery

The present study was designed to investigate which subtypes of 5-HT receptors are involved in 5-HT-induced hyperalgesia using behavioral assessment of hyperalgesia. 5-HT and various putative agonists for 5-HT receptor subtypes (5-HT(1A, 2, 3)) were intradermally injected into the rat ipsilateral hindpaw. Paw-withdrawal latency to radiant heat stimulation was examined every 15 min for 2 h. Injection of 5-HT (30 microg) and 5-HT2A receptor agonist (alpha-methyl 5-HT; 0.86 mg/kg) significantly reduced the paw-withdrawal latency. On the other hand, injection of 5-HT3 receptor agonists (2-methyl 5-HT; 0.86 mg/kg, m-CPG; 8 mg/kg) did not produce hyperalgesia. Furthermore, pretreatment with 5-HT2A receptor antagonist (ketanserin), but not with 5-HT3 receptor antagonist (tropisetron), attenuated the behavioral response after the injection of 5-HT. These findings strongly suggest that the 5-HT2A receptor subtype, but not the 5-HT3 subtype, is involved in 5-HT-induced hyperalgesia in acute injury and inflammation in the rat. In situ hybridization histochemistry revealed the presence of 5-HT2 receptor mRNA in a subpopulation of both large and small neurons in the rat dorsal root ganglia.

Serotonin-induced population primary afferent depolarisation in vitro: the effects of neonatal capsaicin treatment

The effect of 5-hydroxytryptamine (5-HT) on population primary afferent depolarisation (PAD) has been studied using in vitro spinal cord preparations from normal and capsaicin pre-treated (neonatal subcutaneous injection; 75 mg kg-1) rats aged 10-14 days. In preparations from untreated rats, the concentration-response curve for PAD in response to 0.1-100 microM 5-HT was bell-shaped but in the capsaicin pre-treated group, a non-saturating 5-HT-induced PAD concentration-response curve was generated. Quantitatively, the mean PAD responses to 0.1-10 microM 5-HT were of a greater amplitude in the control group compared to the capsaicin pre-treated group (p</=0. 05). For the highest 5-HT concentration of 100 microM, PAD values were significantly greater in the capsaicin pre-treated group (p</=0. 05). These data indicate that control of sensory afferent polarity may involve two 5-HT receptor types and that nociceptive and non-nociceptive afferents may be targets for released 5-HT.

5-Hydroxytryptamine receptor subtype messenger RNAs in human dorsal root ganglia: a polymerase chain reaction study

Although serotonin has been shown to play an important role in peripheral pain mechanisms, the specific subtypes of serotonin receptors involved in pain and hyperalgesia remain poorly understood. To date, no previous study has attempted to determine the presence of any serotonin receptor subtype in human dorsal root ganglia. In this study, the presence of messenger RNA for eight human serotonin receptor subtypes in lumbar dorsal root ganglia was detected using the method of polymerase chain reaction. Dorsal root ganglia were excised post mortem from four patients. Oligonucleotide primers were chosen based on unique regions of complimentary DNA sequence for eight cloned human serotonin receptor subtypes (i.e. 5-HT1A, 5-HT1D alpha, 5-HT1D beta, 5-HT1E, 5-HT1F, 5-HT2A, 5-HT2C and 5-HT7). The presence of 5-HT1D alpha, 5-HT1D beta, 5-HT1E, 5-HT1F, 5-HT2A and 5-HT7 receptor subtype messenger RNA was detected in dorsal root ganglia from three of the four subjects. 5-HT1A receptor subtype messenger RNA was detected in one of the four subjects. No 5-HT2C receptor subtype messenger RNA could be detected. Findings from this study may direct further efforts to determine the role of serotonin receptors in the peripheral nervous system.

Electrophysiological studies on rat dorsal root ganglion neurons after peripheral axotomy: changes in responses to neuropeptides

The effect of three peptides, galanin, sulfated cholecystokinin octapeptide, and neurotensin (NT), was studied on acutely extirpated rat dorsal root ganglia (DRGs) in vitro with intracellular recording techniques. Both normal and peripherally axotomized DRGs were analyzed, and recordings were made from C-type (small) and A-type (large) neurons. Galanin and sulfated cholecystokinin octapeptide, with one exception, had no effect on normal C- and A-type neurons but caused an inward current in both types of neurons after sciatic nerve cut. In normal rats, NT caused an outward current in C-type neurons and an inward current in A-type neurons. After sciatic nerve cut, NT only caused an inward current in both C- and A-type neurons. These results suggest that (i) normal DRG neurons express receptors on their soma for some but not all peptides studied, (ii) C- and A-type neurons can have different types of receptors, and (iii) peripheral nerve injury can change the receptor phenotype of both C- and A-type neurons and may have differential effects on these neuron types.

5HT4 receptors couple positively to tetrodotoxin-insensitive sodium channels in a subpopulation of capsaicin-sensitive rat sensory neurons

The distribution of tetrodotoxin (TTX)-sensitive and -insensitive Na+ currents and their modulation by serotonin (5HT) and prostaglandin E2 (PGE2) was studied in four different types of dorsal root ganglion (DRG) cell bodies (types 1, 2, 3, and 4), which were previously identified on the basis of differences in membrane properties (). Types 1 and 2 DRG cells expressed TTX-insensitive Na+ currents, whereas types 3 and 4 DRG cells exclusively expressed TTX-sensitive Na+ currents. Application of 5HT (1-10 microM) increased TTX-insensitive Na+ currents in type 2 DRG cells but did not affect Na+ currents in type 1, 3, or 4 DRG cells. The 5HT receptor involved resembled the 5HT4 subtype. It was activated by 5-methoxy-N,N-dimethyltryptamine (10 microM) but not by 5-carboxyamidotryptamine (1 microM), (+)-8-hydroxydipropylaminotetralin (10 microM), or 2-methyl-5HT (10 microM), and was blocked by ICS 205-930 with an EC50 of approximately 2 microM but not by ketanserin (1 microM). PGE2 (4 or 10 microM) also increased Na+ currents in varying portions of cells in all four groups. The effect of 5HT and PGE2 on Na+ currents was delayed for 20-30 sec after exposure to 5HT, suggesting the involvement of a cytosolic diffusible component in the signaling pathway. The agonist-mediated increase in Na+ current, however, was not mimicked by 8-chlorophenylthio-cAMP (200 microM), suggesting the possibility that cAMP was not involved. The data suggest that the 5HT- and PGE2-mediated increase in Na+ current may be involved in hyperesthesia in different but overlapping subpopulations of nociceptors.

Intradermal 5-HT induces Fos expression in rat dorsal horn neurons not via 5-HT3 but via 5-HT2A receptors

We investigated the effects of peripherally administered 5-HT on the secondary neurons in the spinal cord of rats using Fos-like immunoreactivity (FLI) as a marker of neuronal activation. The intradermal administration of 5-HT (30, 60 microg) induced a large number of FLI neurons in the ipsilateral dorsal horn. In animals given 5-HT2A receptor agonists (DOI: 0.28 to 2.8 micromol/kg, alpha-methyl 5-HT: 0.28 to 2.8 micromol/kg) intradermally, immunoreactive neurons were evoked in the same manner as those given 5-HT. Other agonists, including 5-HT3 receptor agonists (m-CPG: 16 to 32 micromol/kg, 2-methyl 5-HT: 0.0028 to 2.8 micromol/kg), did not induce FLI neurons at any dose examined. Furthermore, 5-HT2A receptor antagonist (ketanserin: 1 mg/kg, i.p.) suppressed the expression of FLI in the dorsal horn caused by peripheral 5-HT, but 5-HT3 receptor antagonist (tropisetron: 1 mg/kg, i.p.) did not. These findings suggest that the 5-HT-induced nociceptive response is mediated by 5-HT2A receptors in the periphery.

Pre- and post-synaptic actions of 5-hydroxytryptamine in the rat lumbar dorsal horn in vitro: implications for somatosensory transmission

Since the relative contribution of pre- versus post-synaptic actions of 5-hydroxytryptamine (5-HT) to modulation of somatosensory processing in the dorsal hom is not known, recordings from primary afferents and dorsal hom neurons from in vitro rat spinal cord were used to address this issue. 5-HT produced a depression of spontaneous dorsal root potentials and a slow primary afferent depolarization (PAD): the PAD versus 5-HT concentration-response curve was bell shaped (maximum at 5 microM; 250 +/- 41.5 microV). In 28/40 dorsal horn neurons, 5-HT elicited a slow depolarization not clearly associated with a specific input resistance change. Excitatory synaptic transmission from primary afferents to dorsal horn neurons was depressed by 5-HT in 40/45 neurons. 5-HT > or = 5 microM significantly (P < or = 0.05) decreased the amplitude, shortened the total duration and half-decay time of the excitatory post-synaptic potential (EPSP). A dominant effect of 5-HT on longer latency EPSP components was evident. There was not direct relationship between the magnitude of PAD and the reduction of the EPSP by 5-HT. 5-Carboxamidotryptamine, an agonist for 5-HT1 receptors, mimicked the depression of neurotransmission in the dorsal horn without producing PAD. A sample of dorsal horn neurons (n = 8) was injected with biocytin and their morphology described. All had somata within laminae III-VI. In five of these neurons 5-HT depressed the EPSP but in one interneuron-like and one unclassed neuron the EPSP was potentiated. These data suggest that whilst depression of synaptic transmission is the predominant effect of 5-HT in the deep dorsal horn, this is not easily related to PAD or cellular actions of 5-HT on dorsal horn neurons.

Capsaicin preferentially affects small-diameter acutely isolated rat dorsal root ganglion cell bodies

The effects of capsaicin were tested on 221 acutely isolated dorsal root ganglion neurons of the rat, which ranged in diameter from 15 to 55 microns. In a sub-population of these cells, ranging in diameter from 17.5 to 33 microns (n = 117), capsaicin (1 microM) produced an inward shift in holding current that was associated with an increase in membrane conductance in most cells (114 of 117). These effects of capsaicin were reversible upon washout of the drug. Other cells ranging in diameter from 15 to 52.5 microns (n = 104) were unaffected in this manner by the 1 micron concentration of capsaicin. Capsaicin-sensitive cells had, on average, significantly longer duration action potentials and expressed significantly less IH than capsaicin-insensitive cells. The relatively long duration action potentials and/or small cell body diameter and paucity of IH observed in most of the capsaicin-sensitive cells is consistent with their representing C- or A delta-type sensory neurons.

Dual effect of serotonin on formalin-induced nociception in the rat spinal cord

To examine the role of the descending serotonergic system in the regulation of spinal nociceptive processing, the effects of serotonin (5-HT) and selective ligands for 5-HT receptor subtypes on persistent nociception were investigated. Formalin (5% formaldehyde) injected into the plantar region of the rat hindpaw induced two phases of aversive responses such as licking and biting. Intrathecal administration of selective 5-HT3 receptor antagonists, granisetron (0.1-100 pmol/rat) and ondansetron (1-1000 pmol/rat), reduced the second phase of the formalin-induced aversive responses without affecting the first one. The antinociceptive effect of granisetron (100 pmol/rat) was abolished when 5-HT was depleted from the lumbar cord by pretreatment with 5,7-dihydroxytryptamine (5,7-DHT). In the 5,7-DHT-treated rats, intrathecal administration of 1-(m-chlorophenyl)-biguanide, a selective 5-HT3 receptor agonist, facilitated the aversive responses in the second phase whereas that of 8-OH-DPAT, a selective 5-HT1A receptor agonist, suppressed them. Intrathecal administration of 5-HT showed a dual effect on the second phase of the aversive responses in the 5,7-DHT-treated rats; 5-HT inhibited the aversive responses when administered at a low dose (0.1 nmol/rat) but facilitated them at a high dose (1 nmol/rat). In addition, the inhibitory and facilitatory effects of intrathecal 5-HT were blocked by its co-administration with NAN190, a 5-HT1A receptor antagonist, and granisetron, respectively. These results suggest that 5-HT suppresses formalin-induced nociception in the spinal cord via the 5-HT1A receptor and facilitates it via the 5-HT3 receptor.

Endogenous monoamines inhibit glutamate transmission in the spinal trigeminal nucleus of the guinea-pig

1. With the use of whole-cell patch clamp recordings in slices of guinea-pig substantia gelatinosa (SG), we studied the serotonin (5-HT)- and noradrenaline (NA)-mediated inhibition of glutamate-mediated EPSCs evoked from primary afferent stimulation. 2. The frequency of spontaneous EPSPs was reduced by 5-HT and NA. 3. The inhibition of EPSCs caused by 5-HT was mediated by the 5-HT1D receptor subtype, since the 5-HT1D agonist, sumatriptan (1 microM), was effective. 4. NA and the alpha 2-agonist, 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK 14304), decreased the EPSCs and this inhibition was blocked by the alpha 2-antagonists, idazoxan (1 microM) and yohimbine (1 microM). 5. The 5-HT-releasing agent, fenfluramine (10 microM), and the Na-releasing agent, amphetamine (1 microM), also depressed EPSCs. Pretreatment of slices with the 5-HT-depleting agent, p-chloro-amphetamine (10 microM), attenuated the inhibition of fenfluramine but failed to antagonize the effects of exogenously applied 5-HT. 6. These in vitro results suggest that presynaptic inhibition of glutamate release from primary afferents can provide another mechanism to explain the antinociceptive effects of 5-HT and NA obtained in vivo.

5-Hydroxytryptamine receptor subtype messenger RNAs in rat peripheral sensory and sympathetic ganglia: a polymerase chain reaction study

While serotonin has been shown to play an important role in peripheral pain mechanisms, the specific subtypes of receptors involved and their differential distribution between the sensory and sympathetic nervous system remains poorly understood. In this study, the presence of messenger RNA for rat serotonin receptor subtypes in peripheral sensory and sympathetic ganglia was detected using the method of polymerase chain reaction. Lumbar dorsal root ganglia, superior cervical sympathetic ganglia and lumbar sympathetic ganglia were excised from anesthetized Sprague-Dawley rats. Oligonucleotide primers were chosen based on unique regions of complementary DNA sequence for each of the 12 cloned rat serotonin receptor subtypes (i.e. 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1F, 5-HT2A, 5-HT2B, 5-HT2C, 5-HT3, 5-HT5A, 5-HT5B, 5-HT6 and 5-HT7) and high stringency conditions were used during polymerase chain reaction. Within lumbar dorsal root ganglia, the presence of the 5-HT1B, 5-HT1D, 5-HT2A, 5-HT2C, 5-HT3 and 5-HT7 receptor subtype messenger RNAs was detected. Within superior cervical ganglia, the presence of messenger RNA for 5-HT1A, 5-HT1B, 5-HT1D, 5-HT2A, 5-HT3, 5-HT6, and 5-HT7 receptor subtypes was detected. Lumbar sympathetic ganglia displayed banding identical to the superior cervical ganglia with the exception of the 5-HT6 receptor which was not detected in the lumbar sympathetic ganglia. The polymerase chain reaction product from each positively-detected receptor subtype was subcloned and sequenced and found to correspond to published complementary DNA sequences. Findings from this study may direct further efforts to determine the role of 5-hydroxytryptamine receptors in the peripheral nervous system.

Sensory afferent processing in multi-responsive DRG neurons

The recent advance in molecular and neurobiological techniques disclosed the multi-responsive nature of DRG neurons. The survival, phenotype expression and electrical properties of these neurons are under the control of a variety of substances through their specific receptors. In pathological conditions, such as tissue inflammation or nerve injury, DRG neurons change their responsiveness through the dynamic reconstruction of their receptor system. This reconstruction is initiated by environmental stimuli. Thus the properties of polymodal nociceptors can be altered according to the environmental conditions. The whole story of this mechanism is not disclosed yet. In order to understand this mechanism, it is basically important to identify various receptor mRNAs in DRG neurons, precise localization of receptor proteins, site of synthesis and route of supply of ligands for these receptors.

Effects of dorsal rhizotomy and selective lesion of serotonergic and noradrenergic systems on 5-HT1A, 5-HT1B, and 5-HT3 receptors in the rat spinal cord

Autoradiographic studies were performed in combination with dorsal rhizotomy or selective lesion of descending serotonergic or noradrenergic systems in an attempt to identify the neuronal cell types endowed with the serotonin 5-HT1A, 5-HT1B and 5-HT3 receptors in the rat spinal cord. Unilateral sectioning of seven dorsal roots (C4-T2) at the cervical level produced a marked decrease (approximately-75%, 10 days after the surgery) in the binding of [125I]iodozacopride to 5-HT3 receptors in the superficial layers of the ipsilateral dorsal horn, further confirming the preferential location of these receptors on primary afferent fibres. In addition, a significant decrease (approximately 20%) in the binding of [3H]8-OH-DPAT to 5-HT1A receptors and of [125I]GTI to 5-HT1B receptors was also observed in the same spinal area in rhizotomized rats, suggesting that a small proportion of these receptors are also located on primary afferent fibres. The labelling of 5-HT1B receptors was significantly decreased (-12%) in the dorsal horn at the cervical (but not the lumbar) level, and that of 5-HT3 receptors was unchanged in the whole spinal cord in rats whose descending serotonergic projections had been destroyed by 5,7-dihydroxytryptamine. Conversely, the labelling of 5-HT1A receptors was significantly increased in the cervical (+13%) and lumbar (+42%) dorsal horn in 5,7-dihydroxytryptamine-lesioned rats. Similarly, [3H]8-OH-DPAT binding to 5-HT1A receptors significantly increased (+26%) in the lumbar (but not the cervical) dorsal horn in rats whose noradrenergic systems had been lesioned by DSP-4. The labelling of 5-HT1B receptors was also increased (+31% at the cervical level; +17% at the lumbar level), whereas that of 5-HT3 receptors remained unchanged in these animals. These data indicate that complex adaptive changes in the expression of 5-HT1A and 5-HT1B receptors occurred in the rat spinal cord following the lesion of descending monoaminergic systems.

Functional properties of cells in rat trigeminal subnucleus interpolaris following local serotonergic deafferentation

We have previously demonstrated increases in serotonin (5-HT) content and immunoreactivity within spinal trigeminal subnucleus interpolaris (SpVi) that are correlated with the functional changes observed in this subnucleus following adult infraorbital nerve (ION) transection. To assess the possible functional significance of this change, we have examined the influence of 5-HT afference upon the normal response properties of cells in SpVi. We employed local depletion of the transmitter, using 5,7-dihydroxtryptamine (5,7-DHT), in combination with extracellular single-cell recording. Chromatographic methods revealed a 97.6% depletion of 5-HT 24 hr after neurotoxin injection. Immunocytochemical procedures revealed depletion of 5-HT throughout SpVi. Physiological recordings were made from 403 SpVi cells in 5,7-DHT-injected rats and 387 cells in vehicle-injected rats. All recordings were made 19-27 hr after injection. Horseradish peroxidase (HRP) deposits from the recording electrode were used to mark recording tracks. 5-HT depletion did not influence receptive field (RF) location, size, or continuity, or the dynamic response characteristics of SpVi cells. It did, however, (1) alter the probability that certain types of somatosensory receptor surfaces would activate local-circuit neurons, and (2) influence the rate of firing of spontaneously active SpVi cells. There was a significant increase in the proportion of vibrissa-sensitive cells with infraorbital RF components, and a concurrent decrease in the proportion of guard-hair-sensitive cells. It therefore appears that 5-HT input to SpVi is necessary for some mechanoreceptive features of the normal functional organization of this area. These functional changes were interesting in that they were opposite to those found following adult ION transection, which increases 5-HT within SpVi. Thus, changes in 5-HT central afference to SpVi that follow ION damage may be responsible for at least one type of functional change observed following this peripheral lesion.