Spinal pharmacology of antinociception produced by microinjection of mu or delta opioid receptor agonists in the ventromedial medulla of the rat

Identification and characterization of rostral ventromedial medulla neurons synaptically connected to the urinary bladder afferents in female rats with or without neonatal cystitis

The neurons in the rostral ventromedial medulla (RVM) play a major role in pain modulation. We have previously shown that early-life noxious bladder stimuli in rats resulted in an overall spinal GABAergic disinhibition and a long-lasting bladder/colon sensitization when tested in adulthood. However, the neuromolecular alterations within RVM neurons in the pathophysiology of early life bladder inflammation have not been elucidated. In this study, we have identified and characterized RVM neurons that are synaptically linked to the bladder and colon and examined the effect of neonatal bladder inflammation on molecular expressions of these neurons. A transient bladder inflammation was induced by intravesicular instillation of protamine sulfate and zymosan during postnatal days 14 through 16 (P14-16) followed by pseudorabies virus PRV-152 and PRV-614 injections into the bladder and colon, respectively, on postnatal day P60. Tissues were examined 96 h postinoculation for serotonergic, GABAergic, and enkephalinergic expressions using in situ hybridization and/or immunohistochemistry techniques. The results revealed that > 50% of RVM neurons that are synaptically connected to the bladder (i.e., PRV-152+) were GABAergic, 40% enkephalinergic, and about 14% expressing serotonergic marker tryptophan hydroxylase 2 (TpH2). Neonatal cystitis resulted in a significant increase in converging neurons in RVM receiving dual synaptic inputs from the bladder and colon. In addition, neonatal cystitis significantly downregulated vesicular GABA transporter (VGAT) with a concomitant increase in TpH2 expression in bladder-linked RVM neurons, suggesting an alteration in supraspinal signaling. These alterations of synaptic connectivity and GABAergic/serotonergic expressions in RVM neurons may contribute to bladder pain modulation and cross-organ visceral sensitivity.

G protein-coupled estrogen receptor in the rostral ventromedial medulla contributes to the chronification of postoperative pain

Aims

Chronification of postoperative pain is a common clinical phenomenon following surgical operation, and it perplexes a great number of patients. Estrogen and its membrane receptor (G protein‐coupled estrogen receptor, GPER) play a crucial role in pain regulation. Here, we explored the role of GPER in the rostral ventromedial medulla (RVM) during chronic postoperative pain and search for the possible mechanism.

Methods and Results

Postoperative pain was induced in mice or rats via a plantar incision surgery. Behavioral tests were conducted to detect both thermal and mechanical pain, showing a small part (16.2%) of mice developed into pain persisting state with consistent low pain threshold on 14 days after incision surgery compared with the pain recovery mice. Immunofluorescent staining assay revealed that the GPER‐positive neurons in the RVM were significantly activated in pain persisting rats. In addition, RT‐PCR and immunoblot analyses showed that the levels of GPER and phosphorylated μ‐type opioid receptor (p‐MOR) in the RVM of pain persisting mice were apparently increased on 14 days after incision surgery. Furthermore, chemogenetic activation of GPER‐positive neurons in the RVM of Gper‐Cre mice could reverse the pain threshold of pain recovery mice. Conversely, chemogenetic inhibition of GPER‐positive neurons in the RVM could prevent mice from being in the pain persistent state.

Conclusion

Our findings demonstrated that the GPER in the RVM was responsible for the chronification of postoperative pain and the downstream pathway might be involved in MOR phosphorylation.

Wrist-ankle acupuncture attenuates cancer-induced bone pain by regulating descending pain-modulating system in a rat model

Background

Cancer-induced bone pain (CIBP) presents a multiple-mechanism of chronic pain involving both inflammatory and neuropathic pain, and its pathogenesis is closely related to endogenous descending system of pain control. However, the action mechanism underlying the effects of wrist–ankle acupuncture (WAA) versus electroacupuncture (EA) on CIBP remains unknown.

Methods

Thirty-two Wistar rats were divided into sham, CIBP, EA-treated and WAA-treated groups. CIBP was induced in rats of the latter three groups. Time courses of weight and mechanical hyperalgesia threshold (MHT) were evaluated. After 6 days of EA or WAA treatment, the expressions of 5-hydroxytryotamine type 3A receptor (5-HT_3AR) and μ-opioid receptor (MOR) in rostral ventromedial medulla (RVM) and/or spinal cord, as well as the levels of 5-HT, β-endorphin, endomorphin-1 and endomorphin-2 in RVM and spinal cord, were detected.

Results

Injection of cancer cells caused decreased MHT, which was attenuated by EA or WAA (P < 0.05). WAA had a quicker analgesic effect than EA (P < 0.05). No significant difference of MOR in RVM was found among the four groups. EA or WAA counteracted the cancer-driven upregulation of 5-HT_3AR and downregulation of MOR in spinal cord (P < 0.05), and upregulation of 5-HT and downregulation of endomorphin-1 in both RVM and spinal cord (P < 0.05). β-endorphin and endomorphin-2 in RVM and spinal cord decreased in CIBP group compared with sham group (P < 0.05), but EA or WAA showed no significant effect on them, although a tendency of increasing effect was observed.

Conclusion

WAA, similar to EA, alleviated mechanical hyperalgesia in CIBP rats by suppressing the expressions of 5-HT and 5-HT_3AR, and increasing the expressions of MOR and endomorphin-1 in RVM-spinal cord pathway of the descending pain-modulating system. However, WAA produced a quicker analgesic effect than EA, the mechanisms of which need further investigation.

Comparison of Spinal Anesthesia Quality Between Patients Addicted and Not Addicted to Opium

PURPOSE

This study aimed to compare the quality of spinal anesthesia between opium-dependent and opium-naive patients.

DESIGN

A case-control study.

METHODS

Two groups of subjects including opium-dependent and opium-naive patients (30 per group) were enrolled. Spinal anesthesia was performed using 15 mg of bupivacaine. The level of anesthesia was assessed every minute for 10 min and then every 10 min for 180 min. Motor block was recorded at 10, 60, 120, and 180 minutes. The duration of anesthesia was recorded.

FINDINGS

The mean duration of spinal anesthesia was significantly shorter in opium-dependent patients (101.45 ± 28.670) than in opium-naive patients (126.14 ± 24.206) (P = .003). The mean onset of sensory block was not significantly different between the two groups (4.14 ± 1.62 vs 3.69 ± 1.36, P = .259).

CONCLUSIONS

Substance abuse affects the duration of spinal anesthesia, so it is recommended to use intravenous medications or higher doses of spinal marcaine for these patients.

Electroacupuncture Attenuates Visceral Hypersensitivity by Inhibiting JAK2/STAT3 Signaling Pathway in the Descending Pain Modulation System

Electroacupuncture (EA) has been used for treating visceral hypersensitivity (VH). However, the underlying molecular mechanism remains unclear. This study was aim to testify the effect of EA on ileitis-provoked VH, and to confirm whether EA attenuates VH through Janus kinase 2 (JAK2)/signal transducers and activators of transcription 3 (STAT3) signaling pathway in the periaqueductal gray (PAG)-the rostral ventromedial medulla (RVM)-the spinal cord dorsal horn (SCDH) axis.

Methods: Goats were anesthetized and laparotomized for injecting 2,4,6-trinitro-benzene-sulfonic acid (TNBS)-ethanol solution (30mg TNBS dissolved in 40% ethanol) into the ileal wall to induce VH. EA was treated for 30min from day 7, then every 3 days for six times. VH was assessed by visceromotor response (VMR) and pain behavior response to 20, 40, 60, 80, and 100 mmHg colorectal distension pressures at day 7, 10, 13, 16, 19, and 22. The spinal cord in the eleventh thoracic vertebra and the brain were collected at day 22. The protein and mRNA levels of IL-6, JAK2, and STAT3 in the SCDH were detected with western blot and qPCR, respectively. The distribution of these substances was observed with immunohistochemistry in the ventrolateral PAG (vlPAG), RVM (mainly the nucleus raphe magnus, NRM), SCDH, the nucleus tractus solitaries (NTS) and the dorsal motor nucleus of vagi (DMV).

Results: Goats administered with TNBS-ethanol solution showed diarrhea, enhanced VMR and pain behavior response, and increased IL-6, phosphorylated JAK2 and STAT3 (pJAK2 and pSTAT3) in the vlPAG, NRM, NTS and DMV, and their protein and mRNA levels in the SCDH. EA relieved diarrhea, VMR and pain behavior response, decreased IL-6, pJAK2 and pSTAT3 levels in the vlPAG, NRM, SCDH, NTS, and DMV except for pSTAT3 in the DMV, but did not affect mRNA level of these three substances in the SCDH.

Conclusion: EA attenuates VH probably through inhibiting JAK2/STAT3 signaling pathway in the PAG-RVM-SCDH axis.

Long-Term Effects of Chronic Buspirone during Adolescence Reduce the Adverse Influences of Neonatal Inflammatory Pain and Stress on Adaptive Behavior in Adult Male Rats

Neonatal pain and stress induce long-term changes in pain sensitivity and behavior. Previously we found alterations in pain sensitivity in adolescent rats exposed to early-life adverse events. We tested whether these alterations have long-lasting effects and if those effects can be improved by the 5-hydroxytryptamine 1A (5-HT1A) receptor agonist buspirone injected chronically during the adolescent period. This study investigates: (1) effects of inflammatory pain (the injection of formalin into the pad of a hind paw) or stress (short maternal deprivation-isolation, MI), or their combination in 1-2-day-old rats on the adult basal pain, formalin-induced pain, anxiety and depression; (2) effects of adolescent buspirone in adult rats that experienced similar early-life insults. Changes in nociceptive thresholds were evaluated using the hot plate (HP) and formalin tests; levels of anxiety and depression were assessed with the elevated plus maze and forced swim tests respectively. Both neonatal painful and stressful treatments induced long-term alterations in the forced swim test. Other changes in adult behavioral responses were dependent on the type of neonatal treatment. There was a notable lack of long-term effects of the combination of early inflammatory pain and stress of MI on the pain responses, anxiety levels or on the effects of adolescent buspirone. This study provides the first evidence that chronic injection of buspirone in adolescent rats alters antinociceptive and anxiolytic effects limited to adult rats that showed behavioral alterations induced by early-life adverse treatments. These data highlight the role of 5-HT1A receptors in long-term effects of neonatal inflammatory pain and stress of short MI on adaptive behavior and possibility of correction of the pain and psychoemotional behavior that were altered by adverse pain/stress intervention using buspirone during critical adolescent period.

Entanglement between thermoregulation and nociception in the rat: the case of morphine

In thermoneutral conditions, rats display cyclic variations of the vasomotion of the tail and paws, the most widely used target organs in current acute or chronic animal models of pain. Systemic morphine elicits their vasoconstriction followed by hyperthermia in a naloxone-reversible and dose-dependent fashion. The dose-response curves were steep with ED50 in the 0.5-1 mg/kg range. Given the pivotal functional role of the rostral ventromedial medulla (RVM) in nociception and the rostral medullary raphe (rMR) in thermoregulation, two largely overlapping brain regions, the RVM/rMR was blocked by muscimol: it suppressed the effects of morphine. "On-" and "off-" neurons recorded in the RVM/rMR are activated and inhibited by thermal nociceptive stimuli, respectively. They are also implicated in regulating the cyclic variations of the vasomotion of the tail and paws seen in thermoneutral conditions. Morphine elicited abrupt inhibition and activation of the firing of on- and off-cells recorded in the RVM/rMR. By using a model that takes into account the power of the radiant heat source, initial skin temperature, core body temperature, and peripheral nerve conduction distance, one can argue that the morphine-induced increase of reaction time is mainly related to the morphine-induced vasoconstriction. This statement was confirmed by analyzing in psychophysical terms the tail-flick response to random variations of noxious radiant heat. Although the increase of a reaction time to radiant heat is generally interpreted in terms of analgesia, the present data question the validity of using such an approach to build a pain index.

Analgesic Neural Circuits Are Activated by Electroacupuncture at Two Sets of Acupoints

To investigate analgesic neural circuits activated by electroacupuncture (EA) at different sets of acupoints in the brain, goats were stimulated by EA at set of Baihui-Santai acupoints or set of Housanli acupoints for 30 min. The pain threshold was measured using the potassium iontophoresis method. The levels of c-Fos were determined with Streptavidin-Biotin Complex immunohistochemistry. The results showed pain threshold induced by EA at set of Baihui-Santai acupoints was 44.74% ± 4.56% higher than that by EA at set of Housanli acupoints (32.64% ± 5.04%). Compared with blank control, EA at two sets of acupoints increased c-Fos expression in the medial septal nucleus (MSN), the arcuate nucleus (ARC), the nucleus amygdala basalis (AB), the lateral habenula nucleus (HL), the ventrolateral periaqueductal grey (vlPAG), the locus coeruleus (LC), the nucleus raphe magnus (NRM), the pituitary gland, and spinal cord dorsal horn (SDH). Compared with EA at set of Housanli points, EA at set of Baihui-Santai points induced increased c-Fos expression in AB but decrease in MSN, the paraventricular nucleus of the hypothalamus, HL, and SDH. It suggests that ARC-PAG-NRM/LC-SDH and the hypothalamus-pituitary may be the common activated neural pathways taking part in EA-induced analgesia at the two sets of acupoints.

Effects of tramadol on substantia gelatinosa neurons in the rat spinal cord: an in vivo patch-clamp analysis

Tramadol is thought to modulate synaptic transmissions in the spinal dorsal horn mainly by activating µ-opioid receptors and by inhibiting the reuptake of monoamines in the CNS. However, the precise mode of modulation remains unclear. We used an in vivo patch clamp technique in urethane-anesthetized rats to determine the antinociceptive mechanism of tramadol. In vivo whole-cell recordings of spontaneous inhibitory postsynaptic currents (sIPSCs) and spontaneous excitatory postsynaptic currents (sEPSCs) were made from substantia gelatinosa (SG) neurons (lamina II) at holding potentials of 0 mV and -70 mV, respectively. The effects of intravenous administration (0.5, 5, 15 mg/kg) of tramadol were evaluated. The effects of superfusion of tramadol on the surface of the spinal cord and of a tramadol metabolite (M1) were further analyzed. Intravenous administration of tramadol at doses >5 mg/kg decreased the sEPSCs and increased the sIPSCs in SG neurons. These effects were not observed following naloxone pretreatment. Tramadol superfusion at a clinically relevant concentration (10 µM) had no effect, but when administered at a very high concentration (100 µM), tramadol decreased sEPSCs, produced outward currents, and enhanced sIPSCs. The effects of M1 (1, 5 mg/kg intravenously) on sEPSCs and sIPSCs were similar to those of tramadol at a corresponding dose (5, 15 mg/kg). The present study demonstrated that systemically administered tramadol indirectly inhibited glutamatergic transmission, and enhanced GABAergic and glycinergic transmissions in SG neurons. These effects were mediated primarily by the activation of μ-opioid receptors. M1 may play a key role in the antinociceptive mechanisms of tramadol.

Effects of opium addiction on level of sensory block in spinal anesthesia with bupivacaine for lower abdomen and limb surgery: a case-control study

BACKGROUND

In clinical practice, the level of sensory block in spinal anesthesia in opium abusers is lower than that in non-abusers because of adaptive changes caused by opium use.

OBJECTIVES

The aim of this study was to investigate the level of sensory block resulting from the intrathecal administration of 0.5% bupivacaine in opium abuser patients undergoing lower extremity and lower abdominal surgeries.

PATIENTS AND METHODS

A total of 100 patients who were candidates of elective lower extremity orthopedic and lower abdominal surgeries were recruited and assigned to two groups based on their history of opium addiction (Case or control). Both groups underwent the same anesthesia procedure and pinprick test was used to assess the level of anesthesia.

RESULTS

No statistically significant difference was observed between groups regarding age, duration of the surgery, and type of surgery. The frequency of addiction was higher in males than in females. The level of sensory block at three minutes was significantly lower in the opium abuser group (P = 0.006). The mean time to achieve T10 sensory block was 10.33 ± 5.79 minutes in the opium abusers and 6.89 ± 3.88 minutes in the controls (P = 0.001). The level of the highest sensory block was lower in the opium abuser group (P = 0.002).

CONCLUSIONS

The findings of this study suggested that after induction of spinal anesthesia with intrathecal administration of bupivacaine, chronic opium abusers would have a lower level of sensory block in comparison with patients without a history of opium abuse.

Effectiveness of local anesthesia with lidocaine in chronic opium abusers

CONTEXT

It had been demonstrated that chronic opium abusers have lower pain thresholds compared with than non-abusers.

AIMS

This study aimed to compare the required dose of lidocaine, a common local anesthetic drug, with regard to the onset of anesthesia in opium abusers with that in non-abusers undergoing similar suturing of minor hand lacerations.

MATERIALS AND METHODS

In this study, 109 opium abusers and 91 non-abusers underwent similar suturing for hand lacerations known as digital block. We explained the patients that their case records would remain confidential and admitting to any history of opioid abuse could assist in alleviating any pain associated with the procedure.

RESULTS

There was no statistically significant difference between the two groups with regard to age, gender, and cause of injury (P > 0.05). We found a significantly longer duration of onset of analgesia in nonopioid abusers (5.42 ± 1.93 min) than that in abusers (10.30 ± 1.79 min) (P < 0.001). In addition, the lidocaine dose administered to abusers (6.67 ± 1.21 ml) was higher (4.07 ± 1.26) as well as statistically significant (P < 0.001) than that in non-abusers.

CONCLUSION

The findings from this study suggest a longer duration of onset of anesthesia and requirement of a higher lidocaine dose in opium abusers compared with nonopioid abuser. Moreover, chronic opium abusers (4-5 years) experienced significantly higher time of onset of anesthesia compared with nonchronic abusers. This outcome may be useful in selecting a more suitable local anesthetic protocol in such patients.

GABA pharmacology: the search for analgesics

Decades of research have been devoted to defining the role of GABAergic transmission in nociceptive processing. Much of this work was performed using rigid, orthosteric GABA analogs created by Povl Krogsgaard-Larsen and his associates. A relationship between GABA and pain is suggested by the anatomical distribution of GABA receptors and the ability of some GABA agonists to alter nociceptive responsiveness. Outlined in this report are data supporting this proposition, with particular emphasis on the anatomical localization and function of GABA-containing neurons and the molecular and pharmacological properties of GABAA and GABAB receptor subtypes. Reference is made to changes in overall GABAergic tone, GABA receptor expression and activity as a function of the duration and intensity of a painful stimulus or exposure to GABAergic agents. Evidence is presented that the plasticity of this receptor system may be responsible for the variability in the antinociceptive effectiveness of compounds that influence GABA transmission. These findings demonstrate that at least some types of persistent pain are associated with a regionally selective decline in GABAergic tone, highlighting the need for agents that enhance GABA activity in the affected regions without compromising GABA function over the long-term. As subtype selective positive allosteric modulators may accomplish these goals, such compounds might represent a new class of analgesic drugs.

Loss of neurons in rostral ventromedial medulla that express neurokinin-1 receptors decreases the development of hyperalgesia

It is well known that neurons in the rostral ventromedial medulla (RVM) are involved in descending modulation of nociceptive transmission in the spinal cord. It has been shown that activation of neurokinin-1 receptors (NK-1Rs) in the RVM, which are presumably located on pain facilitating ON cells, produces hyperalgesia whereas blockade of NK-1Rs attenuates hyperalgesia. To obtain a better understanding of the functions of NK-1R expressing neurons in the RVM, we selectively ablated these neurons by injecting the stable analog of substance P (SP), Sar(9),Met(O2)(11)-Substance P, conjugated to the ribosomal toxin saporin (SSP-SAP) into the RVM. Rats received injections of SSP-SAP (1 μM) or an equal volume of 1 μM of saporin conjugated to artificial peptide (Blank-SAP). Stereological analysis of NK-1R- and NeuN-labeled neurons in the RVM was determined 21-24 days after treatment. Withdrawal responses to mechanical and heat stimuli applied to the plantar hindpaw were determined 5-28 days after treatment. Withdrawal responses were also determined before and after intraplantar injection of capsaicin (acute hyperalgesia) or complete Freund's adjuvant (CFA) (prolonged hyperalgesia). The proportion of NK-1R-labeled neurons in the RVM was 8.8 ± 1.3% in naïve rats and 8.1 ± 0.8% in rats treated with Blank-SAP. However, injection of SSP-SAP into the RVM resulted in a 90% decrease in NK-1R-labeled neurons. SSP-SAP did not alter withdrawal responses to mechanical or heat stimuli under normal conditions, and did not alter analgesia produced by morphine administered into the RVM. In contrast, the duration of nocifensive behaviors produced by capsaicin and mechanical and heat hyperalgesia produced by capsaicin and CFA were decreased in rats pretreated with SSP-SAP as compared to those that received Blank-SAP. These data support our earlier studies using NK-1R antagonists in the RVM and demonstrate that RVM neurons that possess the NK-1R do not play a significant role in modulating acute pain or morphine analgesia, but rather are involved in pain facilitation and the development and maintenance of hyperalgesia.

The effect of adjuvant midazolam compared with fentanyl on the duration of spinal anesthesia with 0.5% bupivacaine in opium abusers

Background

There are a number of adjuvants to be used for local anesthetics in spinal block. The aim of this study was to demonstrate the possible effect of intrathecal midazolam compared with bupivacaine as adjuvants in spinal anesthesia with bupivacaine in chronic opium abuses.

Methods

In a double blind, randomized clinical trial, 90 opium abuser patients undergoing lower limb orthopedic surgery were selected and randomly assigned into 3 groups (30 cases each). The patients received 15 mg plain bupivacaine, or 15 mg bupivacaine plus 25 mcg fentanyl or 15 mg bupivacaine plus 1 mg midazolam, intrathecally.

Results

The duration of anesthesia was much longer in the bupivacaine-midazolam group than the bupivacaine-fentanyl group; both were longer than the plain bupivacaine group (P < 0.05).

Conclusions

Subarachnoid injection of adjuvant midazolam or fentanyl with plain 0.5% bupivacaine in opium abusers in lower limb orthopedic surgery increases the duration of sensory block. Therefore midazolam is more effective than fentanyl in such cases.

Changes in response properties of rostral ventromedial medulla neurons during prolonged inflammation: modulation by neurokinin-1 receptors

Activation of neurokinin-1 (NK-1) receptors in the rostral ventromedial medulla (RVM) can facilitate pain transmission in conditions such as inflammation, and thereby contribute to hyperalgesia. Since blockade of NK-1 receptors in the RVM can attenuate hyperalgesia produced by prolonged inflammation, we examined the role of NK-1 receptors in changes of response properties of RVM neurons following four days of hind paw inflammation with complete Freund's adjuvant. Recordings were made from functionally identified ON, OFF and NEUTRAL cells in the RVM. Spontaneous activity and responses evoked by a series of mechanical (10, 15, 26, 60, 100, and 180 g) and heat (34-50 °C) stimuli applied to the inflamed and non-inflamed hind paws were determined before and at 15 and 60 min after injection of the NK-1-antagonist L-733,060 or vehicle into the RVM. Prolonged inflammation did not alter the proportions of functionally-identified ON, OFF and NEUTRAL cells. ON cells exhibited enhanced responses to mechanical (60-100g) and heat (48-50 °C) stimuli applied to the inflamed paw, which were attenuated by L-733,060 but not by vehicle. Inhibitory responses of OFF cells evoked by mechanical stimuli applied to the inflamed paw were also inhibited by L-733,060, but responses evoked by stimulation of the contralateral paw were increased. Heat-evoked responses of OFF cells were not altered by L-733,060. Also, neither L-733,060 nor vehicle altered spontaneous ongoing discharge rate of RVM neurons. These data indicate that NK-1 receptors modulate excitability of ON cells which contribute to both mechanical and heat hyperalgesia, whereas NK-1 modulation of OFF cells contributes to mechanical hyperalgesia during prolonged inflammation.

High novelty-seeking predicts aggression and gene expression differences within defined serotonergic cell groups

Aggression frequently coincides with specific dimensions of emotionality, such as impulsivity, risk-taking, and drug abuse. Serotonergic (5-HTergic) neurotransmission contributes to the regulation of numerous neurobiological functions, and is thought to play a key role in modulating aggressive responses. The current study uses selectively-bred High (bHR) and Low (bLR) Responder rats that exhibit differences in emotionality and behavioral control, with bHRs exhibiting heightened novelty-induced exploration, impulsivity, and increased sensitivity to drugs of abuse, and with bLRs characterized by exaggerated depressive- and anxiety-like behaviors. Based on this behavioral profile we hypothesized that bHR rats exhibit increased aggression along with changes in testosterone and corticosterone secretion characteristic of aggression, and that these changes are accompanied by alterations in the expression of key genes that regulate 5-HTergic neurotransmission (Tph2 and Sert) as well as in the activation of 5-HTergic cell groups following aggressive encounter. Our data demonstrate that when compared to bLR rats, bHRs express increased baseline Tph2 and Sert in select brainstem nuclei, and when tested on the resident-intruder test they exhibited: 1) increased aggressive behavior; 2) potentiated corticosterone and testosterone secretion; and 3) diminished intrusion-induced c-fos expression in select 5-HTergic brainstem cell groups. The most prominent gene expression differences occurred in the B9 cell group, pontomesencephalic reticular formation, median raphe, and the gigantocellular nucleus pars α. These data are consistent with the notion that altered 5-HT neurotransmission contributes to bHRs' heightened aggression. Furthermore, they indicate that a specific subset of brainstem 5-HTergic cell groups contributes to the regulation of intrusion-elicited behavioral responses.

Opioid receptor modulation of GABAergic and serotonergic spinally projecting neurons of the rostral ventromedial medulla in mice

The rostral ventromedial medulla (RVM) is an important site of opioid actions and forms part of an analgesic pathway that projects to the spinal cord. The neuronal mechanisms by which opioids act within this brain region remain unclear, particularly in relation to the neurotransmitters GABA and serotonin. In the present study, we examined serotonergic and GABAergic immunoreactivity, identified using immunohistochemistry for tryptophan hydroxylase (TPH) and glutamate decarboxylase (GAD), in combination with in vitro whole cell patch clamping to investigate the role of opioids on the mouse RVM with identified projections to the spinal cord. Tyr-d-Ala-Gly-N-Me-Phe-Gly-ol enkephalin (DAMGO) produced μ-opioid receptor-mediated outward currents in virtually all TPH-immunoreactive projecting neurons and GAD-immunoreactive nonprojecting neurons (87% and 86%). The other groups of RVM neurons displayed mixed responsiveness to DAMGO (40-68%). Deltorphin II and U-69593 produced δ- and κ-opioid receptor-mediated outward currents in smaller subpopulations of RVM neurons, with many of the δ-opioid responders forming a subpopulation of μ-opioid-sensitive GABAergic nonprojecting neurons. These findings are consistent with prior electrophysiological and anatomic studies in the rat RVM and indicate that both serotonergic and GABAergic RVM neurons mediate the actions of μ-opioids. Specifically, μ-opioids have a direct postsynaptic inhibitory influence over both GABAergic and serotonergic neurons, including those that project to the dorsal spinal cord.

Rostral ventromedial medulla μ, but not κ, opioid receptors are involved in electroacupuncture anti-hyperalgesia in an inflammatory pain rat model

It has been reported that intracerebroventricular injection of a μ receptor antagonist blocked 2 but not 100Hz electroacupuncture (EA)-produced analgesia in an uninjured animal model. Because persistent pain changes neural response to external stimulation, we hypothesized that the mechanisms of EA anti-hyperalgesia may be different in persistent pain than in health. Hyperalgesia, decreased paw withdrawal latency (PWL) to a noxious thermal stimulus, was induced by subcutaneously injecting complete Freund's adjuvant (CFA) into the hind paws of rats. Selective antagonists against μ (CTOP: D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-ThrNH2, 6.25 nmol) and κ (Nor-BIN: nor-binaltorphimine, 10 nmol) opioid receptors were infused into the rostral ventromedial medulla (RVM) 10 min before a 30-min EA treatment at acupoint Huantiao (GB30) 1h 30 min post-CFA. PWL was measured before and 2.5 post-CFA. Both 10 Hz and 100 Hz EA-produced anti-hyperalgesia were blocked by intra-RVM μ, but not κ, receptor antagonists. Double immunofluorescence staining demonstrated that μ receptor-containing neurons were GABAnergic and that GABAa receptor-containing neurons were serotonergic in the RVM. The results demonstrated an involvement of RVM μ, but not κ, receptors in EA-produced anti-hyperalgesia. In summary, EA may induce release of endogenous endomorphins that activate μ opioid receptors in GABAnergic neurons to suppress the release of GABA. This removes the tonic inhibition of GABA on serotonergic neurons in the RVM, and activation of these serotonergic neurons inhibits pain. EA may be used as complementary treatment for inflammatory pain.

Molecular depletion of descending serotonin unmasks its novel facilitatory role in the development of persistent pain

Recent studies indicate that persistent pain after tissue or nerve injury is accompanied by an enhanced net descending facilitatory drive that contributes to an amplification and spread of pain. Although 5-HT-containing neurons in the rostral ventromedial medulla (RVM) provide the major descending serotonergic projection to the spinal cord, it is not clear whether the neurotransmitter 5-HT itself released from RVM-spinal neurons contributes to descending pain modulation. In the present study, we determined the role of the descending 5-HT in rat nocifensive behaviors after persistent pain by selectively depleting functional phenotypes of 5-HT in RVM neurons with regional shRNA interference (RNAi) of tryptophan hydroxylase-2 (Tph-2), the rate-limiting enzyme in the synthesis of neuronal 5-HT. Compared to negative control shRNA, Tph-2 shRNA induced significantly prolonged downregulation of Tph-2 in the RVM and 5-HT in spinal dorsal horn. The 5-HT-depleted rats showed normal pain sensitivity in responses to acute noxious stimulation. However, the same RNAi treatment attenuated formalin-induced spontaneous nocifensive responses and tissue or nerve injury-induced allodynia and hyperalgesia. Furthermore, in control shRNA-treated animals, intra-RVM microinjection of brain-derived neurotrophic factor produced a reversible hyperalgesia, which was completely prevented by Tph-2 RNAi pretreatment. Descending inhibition induced by intra-RVM electrical stimulation, but not microinjection of the mu- or kappa-opioid receptor agonists in control shRNA-treated animals was eliminated in 5-HT-depleted rats. These results indicate that the descending 5-HT from the RVM is an important contributor to pain facilitation during the development of persistent pain, and may not mediate opioid-induced descending inhibition in acute pain.

The role of amygdalar mu-opioid receptors in anxiety-related responses in two rat models

Amygdala opioids such as enkephalin appear to play some role in the control of anxiety and the anxiolytic effects of benzodiazepines, although the opioid receptor subtypes mediating such effects are unclear. This study compared the influences of mu-opioid receptor (MOR) activation in the central nucleus of the amygdala (CEA) on unconditioned fear or anxiety-like responses in two models, the elevated plus maze, and the defensive burying test. The role of MORs in the anxiolytic actions of the benzodiazepine agonist diazepam was also examined using both models. Either the MOR agonist [D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO), or the MOR antagonists Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP) or beta-funaltrexamine (FNA) were bilaterally infused into the CEA of rats before testing. The results show that microinjection of DAMGO in the CEA decreased open-arm time in the plus maze, whereas CTAP increased open-arm behaviors. In contrast, DAMGO injections in the CEA reduced burying behaviors and increased rearing following exposure to a predator odor, suggesting a shift in the behavioral response in this context. Amygdala injections of the MOR agonist DAMGO or the MOR antagonist CTAP failed to change the anxiolytic effects of diazepam in either test. Our results demonstrate that MOR activation in the central amygdala exerts distinctive effects in two different models of unconditioned fear or anxiety-like responses, and suggest that opioids may exert context-specific regulation of amygdalar output circuits and behavioral responses during exposure to potential threats (open arms of the maze) vs discrete threats (predator odor).

Periaqueductal gray neurons project to spinally projecting GABAergic neurons in the rostral ventromedial medulla

The analgesic effects of morphine are mediated, in part, by periaqueductal gray (PAG) neurons that project to the rostral ventromedial medulla (RVM). Although much of the neural circuitry within the RVM has been described, the relationship between RVM neurons and PAG input and spinal output is not known. The objective of this study was to determine whether GABAergic output neurons from the PAG target RVM reticulospinal neurons. Immunocytochemistry and confocal microscopy revealed that PAG neurons project extensively to RVM neurons projecting to the spinal cord, and two-thirds of these reticulospinal neurons appear to be GABAergic (contain GAD67 immunoreactivity). The majority (71%) of PAG fibers that contact RVM reticulospinal GAD67-immunoreactive neurons also contained GAD67 immunoreactivity. Thus, there is an inhibitory projection from PAG to inhibitory RVM reticulospinal neurons. However, there were also PAG projections to the RVM that did not contain GAD67 immunoreactivity. Additional experiments were conducted to determine whether the heterogeneity in this projection can be explained by the electrophysiological character of the RVM target neurons. PAG projections to electrophysiologically defined and juxtacellularly filled ON, OFF, and Neutral cells in the RVM were examined. Similar to the pattern reported above, both GAD67- and non-GAD67-immunoreactive PAG neurons project to RVM ON, OFF, and Neutral cells in the RVM. These inputs include a GAD67-immunoreactive projection to a GAD67-immunoreactive ON cell and non-GAD67 projections to GAD67-immunoreactive OFF cells. This pattern is consistent with PAG neurons producing antinociception by direct excitation of RVM OFF cells and inhibition of ON cells.

Participation of mu-opioid, GABA(B), and NK1 receptors of major pain control medullary areas in pathways targeting the rat spinal cord: implications for descending modulation of nociceptive transmission

Several brain areas modulate pain transmission through direct projections to the spinal cord. The descending modulation is exerted by neurotransmitters acting both at spinally projecting neurons and at interneurons that target the projection neurons. We analyzed the expression of mu-opioid (MOR), gamma-aminobutyric acid GABA(B), and NK1 receptors in spinally projecting neurons of major medullary pain control areas of the rat: rostroventromedial medulla (RVM), dorsal reticular nucleus (DRt), nucleus of the solitary tract, ventral reticular nucleus, and lateralmost part of the caudal ventrolateral medulla. The retrograde tracer cholera toxin subunit B (CTb) was injected into the spinal dorsal horn, and medullary sections were processed by double immunocytochemistry for CTb and each receptor. The RVM contained the majority of double-labeled neurons followed by the DRt. In general, high percentages of MOR- and NK1-expressing neurons were retrogradely labeled, whereas GABA(B) receptors were mainly expressed in neurons that were not labeled from the cord. The results suggest that MOR and NK1 receptors play an important role in direct and indirect control of descending modulation. The co-localization of MOR and GABA(B) in DRt neurons also demonstrated by the present study suggests that the pronociceptive effects of this nucleus may be controlled by local opoidergic and GABAergic inhibition of the pronociception increased during chronic pain.

Lateral hypothalamic-induced antinociception may be mediated by a substance P connection with the rostral ventromedial medulla

Stimulation of the lateral hypothalamus (LH) produces antinociception modified by intrathecal serotonergic receptor antagonists. Spinally-projecting serotonergic neurons in the LH have not been identified, suggesting that the LH innervates brainstem serotonergic neurons in the rostral ventromedial medulla (RVM), known to modify nociception in the spinal cord dorsal horn. To determine whether substance P (SP) plays a role in LH-induced antinociception mediated by the RVM, we conducted an anatomical experiment using retrograde tract tracing combined with double label immunocytochemistry and found that neuron profiles immunoreactive for SP in the LH project to the RVM. To further identify a functional connection between SP neurons in the LH and the RVM, the cholinergic agonist carbachol (125 nmol) was microinjected into the LH of female Sprague-Dawley rats (250-350 g) and antinociception was obtained on the tail flick or foot withdrawal tests. Cobalt chloride (100 nM) was then microinjected in the RVM to block synaptic activation of spinally-projecting RVM neurons. Within 5 min of the cobalt chloride injection, the antinociceptive effect of carbachol stimulation was blocked. In another set of experiments, the specific NK1 receptor antagonist L-703,606 (5 microg) was microinjected in the RVM following LH stimulation with carbachol and abolished LH-induced antinociception as well. Microinjection of cobalt chloride or L-703,606 in the absence of LH stimulation had no effect. These anatomical and behavioral experiments provide converging evidence to support the hypothesis that antinociception produced by activating neurons in the LH is mediated in part by the subsequent activation of spinally-projecting neurons in the RVM.

Trafficking of central opioid receptors and descending pain inhibition

The delta-opioid receptor (DOR) belongs to the superfamily of G-protein-coupled receptors (GPCRs) with seven transmembrane domains, and its membrane trafficking is regulated by intracellular sorting processes involving its C-tail motifs, intracellular sorting proteins, and several intracellular signaling pathways. In the quiescent state, DOR is generally located in the intracellular compartments in central neurons. However, chronic stimulation, such as chronic pain and sustained opioid exposure, may induce membrane trafficking of DOR and its translocation to surface membrane. The emerged functional DOR on cell membrane is actively involved in pain modulation and opioid analgesia. This article reviews current understanding of the mechanisms underlying GPCRs and DOR membrane trafficking, and the analgesic function of emerged DOR through membrane trafficking under certain pathophysiological circumstances.

The duration of spinal anesthesia with 5% lidocaine in chronic opium abusers compared with nonabusers

BACKGROUND

It has been demonstrated that chronic opium abusers have lower thresholds for pain. In this study we sought to determine whether chronic opium abuse has any effect on the duration of spinal block by local anesthetics.

METHODS

In a case-controlled study, 50 opium abusers and 50 nonabusers undergoing lower abdomen operations were selected from among the patients admitted to a university hospital for elective surgery. All patients received 100 mg hyperbaric preservative-free 5% lidocaine in dextrose, intrathecally.

RESULTS

The duration of anesthesia was much shorter in the opium abusers (60 +/- 7 min) than in the nonabusers (83 +/- 10 min) (P < 0.0001).

CONCLUSION

The study documents a shortened duration of spinal block in opium abusers.

Mechanisms responsible for the enhanced antinociceptive effects of micro-opioid receptor agonists in the rostral ventromedial medulla of male rats with persistent inflammatory pain

This study investigated three possible mechanisms by which the antinociceptive effects of the mu-opioid receptor (MOR) agonist [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) and the delta-opioid receptor (DOR) agonist [d-Ala(2),Glu(4)]-deltorphin (deltorphin II) (DELT), microinjected into the rostral ventromedial medulla (RVM), are enhanced in rats with persistent inflammatory injury. Radioligand binding determined that neither the B(max) nor the K(d) values of [(3)H]DAMGO differed in RVM membranes from rats that received an intraplantar injection of saline or complete Freund's adjuvant (CFA) in one hindpaw 4 h, 4 days, or 2 weeks earlier. Likewise, neither the EC(50) nor the E(max) value for DAMGO-induced stimulation of guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding differed in the RVM of saline- or CFA-treated rats at any time point. Microinjection of fixed dose combinations of DAMGO and DELT in the RVM of naive rats indicated that these agonists interact synergistically to produce antinociception when DAMGO is present in equal or greater amounts than DELT and, additively, when DELT is the predominant component. Thus, unlike the periphery or spinal cord, potentiation of MOR-mediated antinociception does not entail an increase in MOR number, affinity, or coupling. Rather, the data are concordant with our proposal that potentiation results from a synergistic interaction of exogenous MOR agonist with DOR-preferring enkephalins whose levels are increased in CFA-treated rats (J Neurosci 21:2536-2545, 2001). Virtually no specific [(3)H]DELT binding nor stimulation of [(35)S]GTPgammaS binding by DELT was obtained in RVM membranes from CFA- or saline-treated rats at any time point. The mechanisms responsible for the potentiation of DELT-mediated antinociception remain to be elucidated.

Roles of 5-hydroxytryptamine (5-HT) receptor subtypes in the inhibitory effects of 5-HT on C-fiber responses of spinal wide dynamic range neurons in rats

5-Hydroxytryptamine (5-HT; serotonin) plays an important role in the descending control of nociception. 5-HT and its receptors have been extensively studied in the modulation of nociceptive transmission at the spinal level using behavioral tests that may be affected by the effects of 5-HT on motor performance and skin temperature. Using electrophysiological methods, the present study aimed to systematically investigate the roles of 5-HT receptor subtypes on the inhibitory effects of 5-HT on responses of the spinal wide dynamic range (WDR) neurons to C-fiber inputs in rats. Under basal conditions, topical application of 5-HT to the spinal cord inhibited the C-fiber responses of WDR neurons dose-dependently, whereas antagonists of 5-HT(1A) [WAY 100635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-cyclohexanecarboxamide maleate salt]], 5-HT(1B) [GR 55562 [3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyrid-dinyl)phenyl]benzamide dihydrochloride]], 5-HT(2A) [ketanserin [3-[2-[4-(fluorobenzoyl)-1-piperidinyl]ethyl]-2,4[1H,3H]-quinazolinedione tartrate]], 5-HT(2C) [RS 102221 [8-[5-(2,4-dimethoxy-5-(4-trifluoromethylphenylsulfonamido)phenyl-5-oxopentyl]-1,3,8-triazaspiro[4.5]decane-2,4-dione hydrochloride]], 5-HT(3) [MDL 72222 [3-tropanyl-3,5-dichlorobenzoate]], and 5-HT(4) [GR 113808 ([1-[2-[(methylsulfonyl)-amino]ethyl]-4-piperidinyl]methyl 1-methyl-1H-indole-3-carboxylate)] had no effect on their own. The inhibitory effects of 5-HT were reversed by antagonists of 5-HT(1B) (GR 55562), 5-HT(2A) (ketanserin), 5-HT(2C) (RS 102221), 5-HT(3) (MDL 72222), and 5-HT(4) (GR 113808) but not by 5-HT(1A) (WAY 100635) receptor antagonists. Topical administration of agonists of 5-HT(1A) [(2R)-(+)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide], 5-HT(1B) [CGS 12066 [7-trifluoromethyl-4-(4-methyl-1-piperazinyl)pyrrolo-[1,2-a]quinoxaline maleate salt]], 5-HT(2A) (alpha-methyl-5-hydroxytryptamine maleate), 5-HT(2C) [MK 212 [6-chloro-2-(1-piperazinyl)pyrazine hydrochloride]], 5-HT(3) [1-(3-chlorophenyl)biguanide hydrochloride], and 5-HT(4) [2-[1-(4-piperonyl)piperazinyl]benzothiazole] also inhibited the C-responses. These results suggest that, under basal conditions, there is no tonic serotonergic inhibition on the C-responses of dorsal horn neurons, and multiple 5-HT receptor subtypes including 1B, 2A, 2C, 3, and 4 may be involved in mediating the inhibitory effects of 5-HT.

Medullary raphe neurons facilitate brown adipose tissue activation

Recent evidence suggests that neurons in the medullary raphe are critical to the activation of brown adipose tissue (BAT), the major source of nonshivering heat production in the rat. Yet it is unclear which medullary raphe cells participate in cold defense and how participating cells contribute to BAT activation. Therefore, we recorded extracellularly from raphe cells during three thermoregulatory challenges that evoked an increase in BAT temperature in anesthetized rats: central cold, ambient cold, or intracerebroventricular prostaglandin E2 (PGE2) injection. Physiologically identified serotonergic (p5HT) cell discharge increased in response to cold or PGE2 administration and was positively correlated with BAT temperature. However, none of the 147 physiologically identified non-serotonergic (non-p5HT) cells recorded responded to thermoregulatory challenges that evoked an increase in BAT temperature. To test for modulation of BAT activation by non-p5HT cells that are either excited (ON cells) or inhibited (OFF cells) by noxious cutaneous stimulation, noxious stimuli were applied during evoked BAT temperature increases. Noxious stimulation suppressed BAT activation, suggesting that cells inhibited by noxious stimulation facilitate spinal circuits controlling BAT. To test whether medullary OFF cells modulate BAT activity, the mu-opiate receptor agonist (d-Ala2, N-Me-Phe4, Gly-ol5)-enkephalin (DAMGO) was microinjected into the raphe magnus, a manipulation that selectively activates OFF cells. DAMGO microinjection blocked noxious stimulation-evoked suppression of PGE2-induced BAT temperature increases. Thus, both p5HT and non-p5HT OFF cells in the medullary raphe facilitate BAT activation in response to cold challenge or pyrogen.

Emergence of functional delta-opioid receptors induced by long-term treatment with morphine

Opioid analgesics remain the choice for the treatment of moderate to severe pain. Recent research has established that the mu-opioid receptor is predominantly responsible for mediating many opioid actions, including analgesia and opioid tolerance. However, the function of delta-opioid receptors is rather puzzling at present, with inconsistent reports of system effects by agonists of delta-opioid receptors. The functional interaction between mu-opioid receptors and delta-opioid receptors is also poorly understood. In this study, we demonstrated that in a brainstem site critically involved in opioid analgesia, agonists of delta-opioid receptors, ineffective in opioid naive rats, significantly inhibit presynaptic GABA release in the brainstem neurons from morphine-tolerant rats. In membrane preparation from control brainstem tissues, Western blot detected no proteins of delta-opioid receptors, but consistent delta-opioid receptor proteins were expressed in membrane preparation from morphine-tolerant rats. Immunohistochemical studies revealed that long-term morphine treatment significantly increases the number of delta-opioid receptor-immunoreactive varicosities that appose the postsynaptic membrane of these neurons. The colocalization of delta-opioid receptor-immunoreactive varicosities with the labeling of the GABA-synthesizing enzyme glutamic acid decarboxylase is also significantly increased. From a behavioral perspective, activation of delta-opioid receptors in the brainstem nucleus, lacking an effect in opioid naive rats, became analgesic in morphine-tolerant rats and significantly reduced morphine tolerance. These findings indicate that long-term morphine treatment induces the emergence of functional delta-opioid receptors and delta-opioid receptor-mediated analgesia, probably through receptor translocation to surface membrane in GABAergic terminals. They also suggest that opioid drugs with preference for delta-opioid receptors may have better therapeutic effect in a mu-opioid-tolerant state.

The Role of GABA in the Mediation and Perception of Pain

A great deal of effort has been expended in attempting to define the role of GABA in mediating the transmission and perception of pain. Pursuit of this question has been stimulated by the fact that GABAergic neurons are widely distributed throughout the central nervous system, including regions of the spinal cord dorsal horn known to be important for transmitting pain impulses to the brain. In addition, GABA neurons and receptors are found in supraspinal sites known to coordinate the perception and response to painful stimuli and this neurotransmitter system has been shown to regulate control of sensory information processing in the spinal cord. The discovery that GABA receptor agonists display antinociceptive properties in a variety of animal models of pain has provided an impetus for developing such agents for this purpose. It has been shown that GABA receptor agonists, as well as inhibitors of GABA uptake or metabolism, are clinically effective in treating this symptom. However, even with an enhanced understanding of the relationship between GABAergic transmission and pain, it has proven difficult to exploit these findings in designing novel analgesics that can be employed for the routine management of pain. Work in this area has revealed a host of reasons why GABAergic drugs have, to date, been of limited utility in the management of pain. Chief among these are the side effects associated with such agents, in particular sedation. These limitations are likely due to the simultaneous activation of GABA receptors throughout the neuraxis, most of which are not involved in the transmission or perception of pain. This makes it difficult to fully exploit the antinociceptive properties of GABAergic drugs before untoward effects intervene. The discovery of molecularly and pharmacologically distinct GABAA receptors may open the way to developing subtype selective agents that target those receptors most intimately involved in the transmission and perception of pain. The more limited repertoire of GABAB receptor subunits makes it more difficult to develop subtype selective agents for this site. Nonetheless, a GABAB agonist, CGP 35024, has been identified that induces antinociceptive responses at doses well below those that cause sedation (Patel et al., 2001). It has also been reported that, unlike baclofen, tolerance to antinociceptive responses is not observed with CGP 44532, a more potent GABAB receptor agonist (Enna et al., 1998). While the reasons for these differences in responses to members of the same class remain unknown, these findings suggest it may be possible to design a GABAB agonist with a superior clinical profile than existing agents. Besides the challenges associated with identifying subtype selective GABAA and GABAB receptor agonists, the development of GABA analgesics has been hindered by the fact that the responsiveness of these receptor systems appear to vary with the type and duration of pain being treated and the mode of drug administration. Further studies are necessary to more precisely define the types of pain most amenable to treatment with GABAergic drugs. Inasmuch as the antinociceptive responses to these agents in laboratory animals are mediated, at least in part, through activation or inhibition of other neurotransmitter and neuromodulator systems, it is conceivable that GABA agonists will be most efficacious as analgesics when administered in combination with other agents. The results of anatomical, biochemical, molecular, and pharmacological studies support the notion that generalized activation of GABA receptor systems dampens the response to painful stimuli. The data leave little doubt that, under certain circumstances, stimulation of neuroanatomically discreet GABA receptor sites could be of benefit in the management of pain. Continued research in this area is warranted given the limited choices, and clinical difficulties, associated with conventional analgesics.

Electrophysiological heterogeneity of spinally projecting serotonergic and nonserotonergic neurons in the rostral ventromedial medulla

This study examined the passive membrane and action potential properties of serotonergic and nonserotonergic neurons in the rostral ventromedial medulla (RVM) of the rat using whole cell patch-clamp recording techniques in the slice. Serotonergic neurons were identified by immunoreactivity for tryptophan hydroxylase (TrpH). Spinally projecting neurons were retrogradely labeled with 1'-dioactadecyl-3,3,3',3'-tetramethylindocarbodyanine perchlorate (DiI). Three types of neurons were identified within both spinally projecting serotonergic and nonserotonergic populations. Type 1 neurons exhibited irregular or sporadic spontaneous activity interspersed with periods of quiescence. Type 2 neurons were not spontaneously active and were additionally discriminated by a more negative resting membrane potential and a larger-amplitude action potential. Type 3 neurons fired repetitively without pause. Serotonergic neurons had a higher membrane resistance and greater action potential half-width than their nonserotonergic counterparts and rarely exhibited a fast afterhyperpolarization. Serotonergic type 3 neurons also fired more slowly and regularly than nonserotonergic type 3 neurons. Comparison of electrophysiological and immunohistochemical characteristics suggested that the smallest type 3 serotonergic neurons had an increased risk of immunohistochemical "misclassification" due to failure to detect TrpH, possibly due to more complete dialysis of intracellular contents during lengthy recordings. This risk was minimal for type 1 or 2 serotonergic neurons. The three different types of spinally projecting serotonergic neurons also differed markedly in their responsiveness to the mu opioid receptor agonist D-Ala2, NMePhe4, Gly5-ol]enkephalin. These results provide important new electrophysiological and pharmacological evidence for a significant heterogeneity among spinally projecting serotonergic RVM neurons. They may also provide a basis for resolving the controversy concerning the role of serotonergic RVM neurons in opioid analgesia.

The endogenous opioid system and clinical pain management

The endogenous opioid system is one of the most studied innate pain-relieving systems. This system consists of widely scattered neurons that produce three opioids: beta-endorphin, the met- and leu-enkephalins, and the dynorphins. These opioids act as neurotransmitters and neuromodulators at three major classes of receptors, termed mu, delta, and kappa, and produce analgesia. Like their endogenous counterparts, the opioid drugs, or opiates, act at these same receptors to produce both analgesia and undesirable side effects. This article examines some of the recent findings about the opioid system, including interactions with other neurotransmitters, the location and existence of receptor subtypes, and how this information drives the search for better analgesics. We also consider how an understanding of the opioid system affects clinical responses to opiate administration and what the future may hold for improved pain relief. The goal of this article is to assist clinicians to develop pharmacological interventions that better meet their patient's analgesic needs.

The monoamine-mediated antiallodynic effects of intrathecally administered milnacipran, a serotonin noradrenaline reuptake inhibitor, in a rat model of neuropathic pain

Antidepressants are often used to treat neuropathic pain. In the present study, we determined the antiallodynic effects of selective monoamine reuptake inhibitors in the spinal cord in a rat model of neuropathic pain. Mechanical allodynia was produced by tight ligation of the left L5 and L6 spinal nerves and determined by applying von Frey filaments to the left hindpaw. A serotonin noradrenaline reuptake inhibitor, milnacipran, a selective serotonin reuptake inhibitor, paroxetine, or a selective noradrenaline reuptake inhibitor, maprotiline, was administered intrathecally via a chronically implanted catheter. Milnacipran produced dose-dependent antiallodynic effects at doses between 3 microg and 100 microg. The effect lasted for 7 h after injection of 100 microg (P < 0.05). The antiallodynic effect of 30 microg of milnacipran was attenuated by intrathecal coadministration of 30 microg of yohimbine, an alpha(2)-adrenoceptor antagonist, 30 microg of methysergide, a serotonin receptor antagonist, or 30 microg of atropine, a muscarinic receptor antagonist (P < 0.01, respectively). Intraperitoneal administration of milnacipran had no antiallodynic effects at doses of 3 to 30 mg/kg. Antiallodynic effects were not produced by intrathecal administration of paroxetine (10 to 100 microg) or maprotiline (10 to 100 microg). These findings suggest that simultaneous inhibition of serotonin and noradrenaline reuptake in the spinal cord is essential to mediate antiallodynic effects. Milnacipran might be effective for suppression of neuropathic pain.

A parametric study of electroacupuncture on persistent hyperalgesia and Fos protein expression in rats

We previously reported the anti-hyperalgesia of electroacupuncture (EA) on persistent inflammatory pain in an unrestrained, unsedated, and conscious rat model. Using this model, induced by injecting complete Freund's adjuvant (CFA) into one hind paw, we systematically evaluated the anti-hyperalgesia of EA stimulation parameters (frequency, intensity, treatment duration, and pulse width). We assessed hyperalgesia by paw withdrawal latency (PWL) to a noxious thermal stimulus and found that 10- and 100-Hz EA frequencies at a current intensity of 3 mA produced the greatest anti-hyperalgesia, when compared to other parameters. Both frequencies significantly increased PWL in the early phases of hyperalgesia (2.5 and 24 h; p < 0.05), and 10 Hz EA also significantly increased PWL in later phases (5 to 7 days; p < 0.05). A sufficient but tolerable intensity of 3 mA was more effective than lower intensities (1-2 mA). A 20-min treatment produced better anti-hyperalgesia than longer and shorter (10 and 30 min) treatments. Acupoint specificity study demonstrated that GB30 produced significant EA anti-hyperalgesia, while Waiguan (TE5) and sham points, an abdominal point and a point at the opposite aspect of GB30, did not. The spinal Fos protein expression study demonstrated that the optimal EA selectively suppressed Fos expression in superficial laminae (I/II) and activated it in deeper laminae (III/IV) of the spinal dorsal horn. The results suggest that the EA anti-hyperalgesia is parameter-dependent and point-specific, and they provide important information for designing further clinical acupuncture research on persistent inflammatory pain.

5-HT receptors involved in opioid-activated descending inhibition of spinal withdrawal reflexes in the decerebrated rabbit

The role of 5-HT(1B/1D), 5-HT(2) and 5-HT(3) receptors in mediating descending inhibition of spinal reflexes activated by application of fentanyl to the fourth ventricle has been studied in rabbits decerebrated under N(2)O/isoflurane anaesthesia. In the control state, intraventricular fentanyl (3-30 microg kg(-1)) depressed, to an equal extent, short- and long-latency reflexes in the medial gastrocnemius muscle nerve evoked by electrical stimulation of all sural nerve afferents. Inhibition of reflexes resulted from a decreased base line excitability in the reflex pathway accompanied by a reduction in the rate of temporal summation of responses. Fentanyl-induced suppression of short- and long-latency reflexes was significantly reduced after intrathecal administration of the selective 5-HT(2)-receptor antagonist ICI 170,809 (300 microg). The same dose of the selective 5-HT(1B/1D) blocker GR 127,935 reduced inhibition from intraventricular fentanyl only for long-latency reflexes (i.e. those parts of the response for which the afferent drive is provided mainly by Adelta and C-fibre afferents). The 5-HT(3) antagonist tropisetron (also 300 microg intrathecal) did not significantly alter the descending inhibition of reflexes evoked by fentanyl. Both GR 127,935 and tropisetron reduced temporal summation of reflexes per se, effects that were reversed by intraventricular fentanyl. These data suggest that the descending pathway(s) activated by intraventricular fentanyl liberate 5-HT in the spinal cord to inhibit withdrawal reflexes by acting at 5-HT(2) and 5-HT(1B/1D), but not 5-HT(3) receptors. 5-HT(1B/1D), and to a lesser extent 5-HT(3) receptors also appear to have a role in modulating temporal summation of reflexes evoked by repetitive stimuli.