Effects of endomorphin on substantia gelatinosa neurons in rat spinal cord slices

Morphological investigations of endomorphin-2 and spinoparabrachial projection neurons in the spinal dorsal horn of the rat

It has been proved that endomorphin-2 (EM2) produced obvious analgesic effects in the spinal dorsal horn (SDH), which existed in our human bodies with remarkable affinity and selectivity for the μ-opioid receptor (MOR). Our previous study has demonstrated that EM2 made synapses with the spinoparabrachial projection neurons (PNs) in the SDH and inhibited their activities by reducing presynaptic glutamate release. However, the morphological features of EM2 and the spinoparabrachial PNs in the SDH have not been completely investigated. Here, we examined the morphological features of EM2 and the spinoparabrachial PNs by using triple fluorescence and electron microscopic immunohistochemistry. EM2-immunoreactive (-ir) afferents directly contacted with the spinoparabrachial PNs in lamina I of the SDH. Immunoelectron microscopy (IEM) were used to confirm that these contacts were synaptic connections. It was also observed that EM2-ir axon terminals contacting with spinoparabrachial PNs in lamina I contained MOR, substance P (SP) and vesicular glutamate transporter 2 (VGLUT2). In lamina II, MOR-ir neurons were observed to receive direct contacts from EM2-ir varicosities. The synaptic connections among EM2, MOR, SP, VGLUT2, and the spinoparabrachial PNs were also confirmed by IEM. In sum, our results supply morphological evidences for the analgesic effects of EM2 on the spinoparabrachial PNs in the SDH.

Kappa opioids inhibit the GABA/glycine terminals of rostral ventromedial medulla projections in the superficial dorsal horn of the spinal cord

Descending projections from neurons in the rostral ventromedial medulla (RVM) make synapses within the superficial dorsal horn (SDH) of the spinal cord that are involved in the modulation of nociception, the development of chronic pain and itch, and an important analgesic target for opioids. This projection is primarily inhibitory, but the relative contribution of GABAergic and glycinergic transmission is unknown and there is limited knowledge about the SDH neurons targeted. Additionally, the details of how spinal opioids mediate analgesia remain unclear, and no study has investigated the opioid modulation of this synapse. We address this using ex vivo optogenetic stimulation of RVM fibres in conjunction with whole-cell patch-clamp recordings from the SDH in spinal cord slices. We demonstrate that both GABAergic and glycinergic neurotransmission is employed and show that SDH target neurons have diverse morphological and electrical properties, consistent with both inhibitory and excitatory interneurons. Then, we describe a subtype of SDH neurons that have a glycine-dominant input, indicating that the quality of descending inhibition across cells is not uniform. Finally, we discovered that the kappa-opioid receptor agonist U69593 presynaptically suppressed most RVM-SDH synapses. By contrast, the mu-opioid receptor agonist DAMGO acted both pre- and post-synaptically at a subset of synapses, and the delta-opioid receptor agonist deltorphin II had little effect. These data provide important mechanistic information about a descending control pathway that regulates spinal circuits. This information is necessary to understand how sensory inputs are shaped and develop more reliable and effective alternatives to current opioid analgesics. Abstract figure legend We combined ex vivo optogenetic stimulation of RVM fibres with whole cell electrophysiology of SDH neurons to investigate the final synapse in a key descending pain modulatory pathway. We demonstrate that both glycine and GABA mediate signalling at the RVM-SDH synapse, that the SDH targets of RVM projections have diverse electrical and morphological characteristics, and that presynaptic inhibition is directly and consistently achieved by kappa opioid agonists. Opioid receptors shown are sized relative to the proportion of neurons that responded to its specific agonists (81 and 84percent of DF and non-DF neurons responded to kappa opioid receptor agonists, respectively. Responses that occurred in <255 percentage of neurons are not indicated here). This article is protected by copyright. All rights reserved.

Chemokine CCL2 prevents opioid-induced inhibition of nociceptive synaptic transmission in spinal cord dorsal horn

Background

Opioid analgesics remain widely used for pain treatment despite the related serious side effects. Some of those, such as opioid tolerance and opioid-induced hyperalgesia may be at least partially due to modulation of opioid receptors (OR) function at nociceptive synapses in the spinal cord dorsal horn. It was suggested that increased release of different chemokines under pathological conditions may play a role in this process. The goal of this study was to investigate the crosstalk between the µOR, transient receptor potential vanilloid 1 (TRPV1) receptor and C–C motif ligand 2 (CCL2) chemokine and the involvement of spinal microglia in the modulation of opioid analgesia.

Methods

Patch-clamp recordings of miniature excitatory postsynaptic currents (mEPSCs) and dorsal root evoked currents (eEPSC) in spinal cord slices superficial dorsal horn neurons were used to evaluate the effect of µOR agonist [D-Ala², N-Me-Phe⁴, Gly⁵-ol]-enkephalin (DAMGO), CCL2, TRPV1 antagonist SB366791 and minocycline. Paw withdrawal test to thermal stimuli was combined with intrathecal (i.t.) delivery of CCL2 and DAMGO to investigate the modulation in vivo.

Results

Application of DAMGO induced a rapid decrease of mEPSC frequency and eEPSC amplitude, followed by a delayed increase of the eESPC amplitude, which was prevented by SB366791. Chemokine CCL2 treatment significantly diminished all the DAMGO-induced changes. Minocycline treatment prevented the CCL2 effects on the DAMGO-induced eEPSC depression, while mEPSC changes were unaffected. In behavioral experiments, i.t. injection of CCL2 completely blocked DAMGO-induced thermal hypoalgesia and intraperitoneal pre-treatment with minocycline prevented the CCL2 effect.

Conclusions

Our results indicate that opioid-induced inhibition of the excitatory synaptic transmission could be severely attenuated by increased CCL2 levels most likely through a microglia activation-dependent mechanism. Delayed potentiation of neurotransmission after µOR activation is dependent on TRPV1 receptors activation. Targeting CCL2 and its receptors and TRPV1 receptors in combination with opioid therapy could significantly improve the analgesic properties of opioids, especially during pathological states.

Peripheral endomorphins drive mechanical alloknesis under the enzymatic control of CD26/DPPIV

BACKGROUND

Mechanical alloknesis (or innocuous mechanical stimuli-evoked itch) often occurs in dry skin-based disorders such as atopic dermatitis and psoriasis. However, the molecular and cellular mechanisms underlying mechanical alloknesis remain unclear. We recently reported the involvement of CD26 in the regulation of psoriatic itch. This molecule exhibits dipeptidyl peptidase IV (DPPIV) enzyme activity and exerts its biologic effects by processing various substances, including neuropeptides.

OBJECTIVE

The aim of the present study was to investigate the peripheral mechanisms of mechanical alloknesis by using CD26/DPPIV knockout (CD26KO) mice.

METHODS

We applied innocuous mechanical stimuli to CD26KO or wild-type mice. The total number of scratching responses was counted as the alloknesis score. Immunohistochemical and behavioral pharmacologic analyses were then performed to examine the physiologic activities of CD26/DPPIV or endomorphins (EMs), endogenous agonists of μ-opioid receptors.

RESULTS

Mechanical alloknesis was more frequent in CD26KO mice than in wild-type mice. The alloknesis score in CD26KO mice was significantly reduced by the intradermal administration of recombinant DPPIV or naloxone methiodide, a peripheral μ-opioid receptor antagonist, but not by that of mutant DPPIV without enzyme activity. EMs (EM-1 and EM-2), selective ligands for μ-opioid receptors, are substrates for DPPIV. Immunohistochemically, EMs were located in keratinocytes, fibroblasts, and peripheral sensory nerves. Behavioral analyses revealed that EMs preferentially provoked mechanical alloknesis over chemical itch. DPPIV-digested forms of EMs did not induce mechanical alloknesis.

CONCLUSION

The present results suggest that EMs induce mechanical alloknesis at the periphery under the enzymatic control of CD26/DPPIV.

Cellular Mechanisms for Antinociception Produced by Oxytocin and Orexins in the Rat Spinal Lamina II-Comparison with Those of Other Endogenous Pain Modulators

Much evidence indicates that hypothalamus-derived neuropeptides, oxytocin, orexins A and B, inhibit nociceptive transmission in the rat spinal dorsal horn. In order to unveil cellular mechanisms for this antinociception, the effects of the neuropeptides on synaptic transmission were examined in spinal lamina II neurons that play a crucial role in antinociception produced by various analgesics by using the whole-cell patch-clamp technique and adult rat spinal cord slices. Oxytocin had no effect on glutamatergic excitatory transmission while producing a membrane depolarization, γ-aminobutyric acid (GABA)-ergic and glycinergic spontaneous inhibitory transmission enhancement. On the other hand, orexins A and B produced a membrane depolarization and/or a presynaptic spontaneous excitatory transmission enhancement. Like oxytocin, orexin A enhanced both GABAergic and glycinergic transmission, whereas orexin B facilitated glycinergic but not GABAergic transmission. These inhibitory transmission enhancements were due to action potential production. Oxytocin, orexins A and B activities were mediated by oxytocin, orexin-1 and orexin-2 receptors, respectively. This review article will mention cellular mechanisms for antinociception produced by oxytocin, orexins A and B, and discuss similarity and difference in antinociceptive mechanisms among the hypothalamic neuropeptides and other endogenous pain modulators (opioids, nociceptin, adenosine, adenosine 5’-triphosphate (ATP), noradrenaline, serotonin, dopamine, somatostatin, cannabinoids, galanin, substance P, bradykinin, neuropeptide Y and acetylcholine) exhibiting a change in membrane potential, excitatory or inhibitory transmission in the spinal lamina II neurons.

The effect of µ-opioid receptor activation on GABAergic neurons in the spinal dorsal horn

The superficial dorsal horn of the spinal cord plays an important role in pain transmission and opioid activity. Several studies have demonstrated that opioids modulate pain transmission, and the activation of µ-opioid receptors (MORs) by opioids contributes to analgesic effects in the spinal cord. However, the effect of the activation of MORs on GABAergic interneurons and the contribution to the analgesic effect are much less clear. In this study, using transgenic mice, which allow the identification of GABAergic interneurons, we investigated how the activation of MORs affects the excitability of GABAergic interneurons and synaptic transmission between primary nociceptive afferent and GABAergic interneurons. We found that a selective µ-opioid agonist, [D-Ala², NMe-Phe⁴, Gly-ol]-enkephanlin (DAMGO), induced an outward current mediated by K⁺ channels in GABAergic interneurons. In addition, DAMGO reduced the amplitude of evoked excitatory postsynaptic currents (EPSCs) of GABAergic interneurons which receive monosynaptic inputs from primary nociceptive C fibers. Taken together, we found that DAMGO reduced the excitability of GABAergic interneurons and synaptic transmission between primary nociceptive C fibers and GABAergic interneurons. These results suggest one possibility that suppression of GABAergic interneurons by DMAGO may reduce the inhibition on secondary GABAergic interneurons, which increase the inhibition of the secondary GABAergic interneurons to excitatory neurons in the spinal dorsal horn. In this circumstance, the sum of excitation of the entire spinal network will control the pain transmission.

Endomorphin-2 Inhibition of Substance P Signaling within Lamina I of the Spinal Cord Is Impaired in Diabetic Neuropathic Pain Rats

Opiate analgesia in the spinal cord is impaired in diabetic neuropathic pain (DNP), but until now the reason is unknown. We hypothesized that it resulted from a decreased inhibition of substance P (SP) signaling within the dorsal horn of the spinal cord. To investigate this possibility, we evaluated the effects of endomorphin-2 (EM2), an endogenous ligand of the μ-opioid receptor (MOR), on SP release within lamina I of the spinal dorsal horn (SDH) in rats with DNP. We established the DNP rat model and compared the analgesic efficacy of EM2 between inflammation pain and DNP rat models. Behavioral results suggested that the analgesic efficacy of EM2 was compromised in the condition of painful diabetic neuropathy. Then, we measured presynaptic SP release induced by different stimulating modalities via neurokinin-1 receptor (NK1R) internalization. Although there was no significant change in basal and evoked SP release between control and DNP rats, EM2 failed to inhibit SP release by noxious mechanical and thermal stimuli in DNP but not in control and inflammation pain model. We also observed that EM2 decreased the number of FOS-positive neurons within lamina I of the SDH but did not change the amount of FOS/NK1R double-labeled neurons. Finally, we identified a remarkable decrease in MORs within the primary afferent fibers and dorsal root ganglion (DRG) neurons by Western blot (WB) and immunohistochemistry (IHC). Taken together, these data suggest that reduced presynaptic MOR expression might account for the loss of the inhibitory effect of EM2 on SP signaling, which might be one of the neurobiological foundations for decreased opioid efficacy in the treatment of DNP.

Downregulation of spinal endomorphin-2 correlates with mechanical allodynia in a rat model of tibia cancer

The endogenous tetrapeptide endomorphin-2 (EM2) participates in pain modulation by binding to pre- and/or post-synaptic μ opioid receptor (MOR). In the present study, pathological expression and antinociceptive effects of EM2 at the spinal level were investigated in a rat model of bone cancer pain. The model was established by introducing Walker 256 mammary gland carcinoma cells into the tibia medullary cavity. Immunohistochemical staining for EM2 showed a markedly reduced EM2-immunoreactivity in the ipsilateral spinal dorsal horn on days 6, 12 and 18 post Walker 256 inoculation (p < 0.05). Intrathecal injection (i.t.) of EM2 significantly attenuated cancer-induced mechanical allodynia (p < 0.05) which could be blocked by β-funaltrexamine (β-FNA), the μ receptor antagonist (p < 0.05). Furthermore, topical application of EM2 dose-dependently inhibited the electrically evoked C-fiber responses and postdischarge of wide dynamic range (WDR) neurons within the spinal cord (p < 0.05), and pretreatment with β-FNA abolished the hyperactivity of these neurons. Compared with the antinociception of morphine which took effect from 40 min to 100 min post application, the analgesic action of EM2 was characterized by quick onset and short-lived efficacy (p < 0.05), being most potent at 10 min and lasting about 20 min. These findings indicate that the down-regulated spinal EM2 is an important contributor to the neuropathological process of bone cancer pain and enhancing activation of EM2/μ receptor signaling might provide a therapeutic alternative to optimizing the treatment of cancer-induced bone pain.

Opioid system and Alzheimer's disease

The opioid system may be involved in the pathogenesis of AD, including cognitive impairment, hyperphosphorylated tau, Aβ production, and neuroinflammation. Opioid receptors influence the regulation of neurotransmitters such as acetylcholine, norepinephrine, GABA, glutamate, and serotonin which have been implicated in the pathogenesis of AD. Opioid system has a close relation with Aβ generation since dysfunction of opioid receptors retards the endocytosis and degradation of BACE1 and γ-secretase and upregulates BACE1 and γ-secretase, and subsequently, the production of Aβ. Conversely, activation of opioid receptors increases the endocytosis of BACE1 and γ-secretase and downregulates BACE1 and γ-secretase, limiting the production of Aβ. The dysfunction of opioid system (opioid receptors and opioid peptides) may contribute to hyperphosphorylation of tau and neuroinflammation, and accounts for the degeneration of cholinergic neurons and cognitive impairment. Thus, the opioid system is potentially related to AD pathology and may be a very attractive drug target for novel pharmacotherapies of AD.

Presynaptic enhancement by eugenol of spontaneous excitatory transmission in rat spinal substantia gelatinosa neurons is mediated by transient receptor potential A1 channels

Eugenol, which is contained in several plants including clove, has been widely used as an analgesic and anti-inflammatory drug in the dental clinic. Eugenol also has anesthetic effects and produces sedation and the reduction of convulsion threshold. These benefits have been partly attributed to the effects of eugenol on neural tissues, such as inhibition of voltage-gated ion channels. As expected from the fact that eugenol is a vanilloid compound, this drug activates transient receptor potential (TRP) V1 channels in the peripheral nervous system (PNS). Although eugenol affects synaptic transmission in the central nervous system (CNS), this has not yet been fully examined. We investigated how eugenol affects spontaneous glutamatergic excitatory transmission in substantia gelatinosa (SG; lamina II of Rexed) neurons of adult rat spinal cord slices by use of the blind whole-cell patch-clamp technique. Bath-applied eugenol reversibly enhanced spontaneous excitatory transmission and produced an outward current at -70 mV in SG neurons. The former action was due to a large increase in the frequency of spontaneous excitatory postsynaptic current (sEPSC) with a small increase in the amplitude. These actions of eugenol were seen by its repeated application and resistant to a voltage-gated Na(+) channel blocker tetrodotoxin. The effect of eugenol on sEPSC frequency was concentration-dependent with an EC(50) value of 3.8 mM and unaffected by a TRPV1 antagonist capsazepine, whereas inhibited by a nonspecific TRP antagonist ruthenium red and a TRPA1 antagonist HC-030031. On the other hand, the eugenol-induced outward current was not affected by these TRP antagonists. It is concluded that eugenol activates TRPA1 channels in the SG, leading to an increase in the spontaneous release of L-glutamate to SG neurons, and that eugenol also produces a membrane hyperpolarization that is not mediated by TRP channels. Eugenol is suggested to activate different types of TRP channel between the PNS and CNS.

Acetylcholine and norepinephrine mediate GABAergic but not glycinergic transmission enhancement by melittin in adult rat substantia gelatinosa neurons

GABAergic and glycinergic inhibitory synaptic transmissions in substantia gelatinosa (SG; lamina II of Rexed) neurons of the spinal dorsal horn play an important role in regulating nociceptive transmission from the periphery. It has not yet been well known whether each of the inhibitory transmissions plays a distinct role in the regulation. We report an involvement of neurotransmitters in GABAergic but not glycinergic transmission enhancement produced by the PLA2 activator melittin, where the whole-cell patch-clamp technique is applied to the SG neurons of adult rat spinal cord slices. Glycinergic but not GABAergic spontaneous inhibitory postsynaptic current (sIPSC) was increased in frequency and amplitude by melittin in the presence of nicotinic, muscarinic acetylcholine, and α1-adrenergic receptor antagonists (mecamylamine, atropine, and WB-4101, respectively). GABAergic transmission enhancement produced by melittin was unaffected by the 5-hydroxytryptamine 3 receptor and P2X receptor antagonists (ICS-205,930 and pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid, respectively). Nicotinic and muscarinic acetylcholine receptor agonists [(−)-nicotine and carbamoylcholine, respectively] and norepinephrine, as well as melittin, increased GABAergic sIPSC frequency and amplitude. A repeated application of (−)-nicotine, carbamoylcholine, and norepinephrine, but not melittin, at an interval of 30 min produced a similar transmission enhancement. These results indicate that melittin produces the release of acetylcholine and norepinephrine, which activate (nicotinic and muscarinic) acetylcholine and α1-adrenergic receptors, respectively, resulting in GABAergic but not glycinergic transmission enhancement in SG neurons. The desensitization of a system leading to the acetylcholine and norepinephrine release is slow in recovery. This distinction in modulation between GABAergic and glycinergic transmissions may play a role in regulating nociceptive transmission.

Biphasic modulation by galanin of excitatory synaptic transmission in substantia gelatinosa neurons of adult rat spinal cord slices

Although intrathecally administrated galanin modulates nociceptive transmission in a biphasic manner, this has not been fully examined previously. In the present study, the action of galanin on synaptic transmission in the substantia gelatinosa (SG) neurons of adult rat spinal cord slices was examined, using the whole cell patch-clamp technique. Galanin concentration-dependently increased the frequency of spontaneous excitatory postsynaptic current (EPSC; EC50 = 2.0 nM) without changing the amplitude, indicating a presynaptic effect. This effect was reduced in a Ca2+-free, or voltage-gated Ca2+ channel blocker La3+-containing Krebs solution and was produced by a galanin type-2/3 receptor (GalR2/R3) agonist, galanin 2–11, but not by a galanin type-1 receptor (GalR1) agonist, M617. Galanin also concentration-dependently produced an outward current at −70 mV (EC50 = 44 nM), although this appeared to be contaminated by a small inward current. This outward current was mimicked by M617, but not by galanin 2–11. Moreover, galanin reduced monosynaptic Aδ-fiber- and C-fiber-evoked EPSC amplitude; the former reduction was larger than the latter. A similar action was produced by galanin 2–11, but not by M617. Spontaneous and focally evoked inhibitory (GABAergic and glycinergic) transmission was unaffected by galanin. These findings indicate that galanin at lower concentrations enhances the spontaneous release of l-glutamate from nerve terminals by Ca2+ entry from the external solution following GalR2/R3 activation, whereas galanin at higher concentrations also produces a membrane hyperpolarization by activating GalR1. Moreover, galanin reduces l-glutamate release onto SG neurons from primary afferent fibers by activating GalR2/R3. These effects could partially contribute to the behavioral effect of galanin.

Origins of endomorphin-2 immunopositive fibers and terminals in the spinal dorsal horn of the rat

Endomorphin 2 (EM2) plays essential roles in regulating nociceptive transmission within the spinal dorsal horn, where EM2-immunopositive (EM2-IP) fibers and terminals are densely encountered. However, the origins of these EM2-IP structures are still obscure. Unilateral primary sensory afferents disruption (lumbar 3-6 dorsal roots rhizotomy) significantly decreased the density of EM2-IP fibers and terminals in the superficial laminae (laminae I and II) on the ipsilateral but not contralateral lumbar dorsal horn (LDH). Spinal hemisection at the 7th thoracic (T7) segment down-regulated bilateral EM2 expression, with a higher influence on the ipsilateral side of the LDH. Unilateral L3-6 dorsal roots rhizotomy combined with spinal transection but not with hemisection at T7 level completely obliterated EM2-IP fibers and terminals on the rhizotomized-side of the LDH. Disruption of bilateral (exposure to the primary afferent neurotoxin, capsaicin) primary sensory afferents combined with spinal hemisection at T7 decreased the EM2-IP density bilaterally but could obliterate it on neither side of the LDH. While in capsaicin plus transection rats, EM2 was depleted symmetrically and completely. In the colchicine treated rats, no EM2-IP neuronal cell bodies could be detected in the spinal gray matter. After injecting tetramethyl rhodamine dextran-amine (TMR) into the LDH, some of the TMR retrogradely labeled neurons in the nucleus tractus solitarii (NTS) showed EM2-immunoreactivities. The present results indicate that EM2-IP fibers and terminals in the spinal dorsal horn originate from the ipsilateral primary afferents and bilateral descending fibers from NTS.

Neurochemical phenotypes of endomorphin-2-containing neurons in vagal nodose neurons of the adult rat

It has been shown that endomorphin-2-like immunoreactive (EM2-LI) neurons in dorsal root ganglion play important roles in regulating somatic information transmission. Although EM2-ergic neurons have been found in nodose ganglion (NG) which is mainly involved in transmitting visceral information into the nucleus tractus solitarii (NTS), the neurochemical phenotypes of EM2-ergic neurons have not yet been investigated. In the present study, immunofluorescent histochemical staining showed that 43.5% of the NG neurons contained EM2 and these neurons were small to medium in size. 15.2%, 27.8%, 74.4% and 25.2% of the EM2-LI NG neurons expressed substance P (SP), calcitonin gene-related peptide (CGRP), nitric oxide synthase (NOS) and vasoactive intestinal peptide (VIP), respectively. In addition, about 90.8% of EM2-LI NG neurons also contained mu-opioid receptor (MOR). EM2/MOR and EM2/SP double-labeled peripheral axons were observed in the vagal trunk. Anterograde tracing combined with immunofluorescent staining showed EM2/MOR and EM2/SP double-labeled vagal afferents in the NTS. EM2/MOR/SP and EM2/MOR/CGRP triple-labeled neurons and axons were observed in the NG. Importantly, at the ultrastructrual level, post-embedding electron microscopy revealed that EM2-LI and SP-LI gold particles coexisted in the same large dense-cored synaptic vesicles in the pre-synaptic button, while MOR-LI gold particles existed on both pre- and post-synaptic membranes in the NTS. These results suggest that EM2 in axon terminals of NG neurons might be involved in visceral information transmission and homeostatic control through modulating the release of other neurotransmitters (such as SP, CGRP, NO, VIP) via pre-synaptic MOR and through post-synaptic mechanisms in the NTS.

Proteinase-activated receptor-1 activation presynaptically enhances spontaneous glutamatergic excitatory transmission in adult rat substantia gelatinosa neurons

Proteinase-activated receptors (PARs) have a unique activation mechanism in that a proteolytically exposed N-terminal region acts as a tethered ligand. A potential impact of PAR on sensory processing has not been fully examined yet. Here we report that synthetic peptides with sequences corresponding to PAR ligands enhance glutamatergic excitatory transmission in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by using the whole cell patch-clamp technique. The frequency of spontaneous excitatory postsynaptic current (EPSC) was increased by PAR-1 agonist SFLLRN-NH2 (by 47% at 1 microM) with small increases by PAR-2 and -4 agonists (SLIGKV-NH2 and GYPGQV-OH, respectively; at >3 microM); there was no change in its amplitude or in holding current at -70 mV. The PAR-1 peptide action was inhibited by PAR-1 antagonist YFLLRNP-OH. TFLLR-NH2, an agonist which is more selective to PAR-1 than SFLLRN-NH2, dose-dependently increased spontaneous EPSC frequency (EC50=0.32 microM). A similar presynaptic effect was produced by PAR-1 activating proteinase thrombin in a manner sensitive to YFLLRNP-OH. The PAR-1 peptide action was resistant to tetrodotoxin and inhibited in Ca2+-free solution. Primary-afferent monosynaptically evoked EPSC amplitudes were unaffected by PAR-1 agonist. These results indicate that PAR-1 activation increases the spontaneous release of L-glutamate onto SG neurons from nerve terminals in a manner dependent on extracellular Ca2+. Considering that sensory processing within the SG plays a pivotal role in regulating nociceptive transmission to the spinal dorsal horn, the PAR-1-mediated glutamatergic transmission enhancement could be involved in a positive modulation of nociceptive transmission.

Excitatory effects of human immunodeficiency virus 1 Tat on cultured rat cerebral cortical neurons

Human immunodeficiency virus 1 (HIV-1) Tat protein is one of the neurotoxins involved in the pathogenesis of HIV-1-associated neuronal disorders. Combined electrophysiological and optical imaging experiments were undertaken to investigate whether HIV-1 Tat30-86, herein referred to as Tat30-86, acted directly or indirectly via the release of glutamate or both and to test its effect on the properties of spontaneous quantal events in cultured cortical neurons. Whole-cell patch recordings were made from cultured rat cortical neurons in either current- or voltage-clamp mode. Tat30-86 (50-1000 nM) induced in a population of cortical neurons a long-lasting depolarization, which was accompanied by a decrease of membrane resistance and persisted in a Krebs solution containing tetrodotoxin (TTX, 0.5 microM). Depolarizations were slightly reduced by pretreatment with glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (10 microM) and d-2-amino-5-phosphonovaleric acid (AP-5) (50 microM), and were markedly reduced in a Ca(2+)-free Krebs solution; the differences were statistically significant. Tat30-86-induced inward currents had a reversal potential between -30 and 0 mV. While not causing a noticeable depolarization, lower concentrations of Tat30-86 (10 nM) increased membrane excitability, as indicated by increased numbers of neuronal discharge in response to a step depolarizing pulse. Tat30-86 (10 nM) increased the frequency of spontaneous miniature excitatory postsynaptic currents (mEPSCs), while not significantly affecting their amplitude. Tat30-86 (10 nM) moderately increased the frequency as well as the amplitude of spontaneous miniature inhibitory postsynaptic currents (mIPSCs). Ratiometric Ca(2+) imaging studies showed that Tat30-86 produced three types of Ca(2+) responses: 1) a fast and transitory increase, 2) Ca(2+) oscillations, and 3) a fast increase followed by a plateau; the glutamate receptor antagonists eliminated the late component of Ca(2+) response. The result suggests that Tat30-86 is an active fragment and that it excites cortical neurons directly and indirectly via releasing glutamate from adjacent neurons.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.

Inhibition by endomorphin-1 and endomorphin-2 of excitatory transmission in adult rat substantia gelatinosa neurons

Intrathecally-administered endomorphin-1 and endomorphin-2 produce antinociceptive effects which are different from each other. In order to elucidate a cellular basis for this result, we examined the effects of endomorphin-1 and endomorphin-2 on holding currents and spontaneous glutamatergic excitatory transmission in substantia gelatinosa neurons of adult rat spinal cord slices by use of the whole-cell patch-clamp technique. In about half of the neurons examined, endomorphin-1 and endomorphin-2 produced an outward current having a similar amplitude (25-27 pA at 1 microM) at -70 mV with almost the same value of effective concentration producing half-maximal response (0.19-0.21 microM). Both of them reversed at a potential close to the equilibrium potential for K+, and had the slope conductance that was larger at negative (-120 to -140 mV) than positive potentials (-60 to -90 mV). The endomorphin-1 and endomorphin-2 currents were reduced in amplitude by K+-channel inhibitors, Ba2+ (100 microM) and 4-aminopyridine (1 mM), and also by mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (1 microM) to a similar extent. The endomorphin-2 but not endomorphin-1 current amplitude was increased by dipeptidyl peptidase IV inhibitor diprotin A (30 microM). One micromolar endomorphin-1 and endomorphin-2 reduced the frequency of spontaneous excitatory postsynaptic current with a similar time course and extent without altering its amplitude; these actions were not in the presence of D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (1 microM). We conclude that endomorphin-1 and endomorphin-2 hyperpolarize membranes by opening inwardly-rectifying K+ channels and attenuate the spontaneous release of L-glutamate from nerve terminals in the substantia gelatinosa, both of which are mediated by mu-opioid receptors, in a manner quantitatively similar to each other. The difference in antinociceptive effects between endomorphin-1 and endomorphin-2 could not be attributed to a distinction in their effects on excitatory transmission in substantia gelatinosa neurons, and may be explained by a difference in their enzymatic degradation.

Spinal G-protein-gated potassium channels contribute in a dose-dependent manner to the analgesic effect of mu- and delta- but not kappa-opioids

Opioids can evoke analgesia by inhibiting neuronal targets in either the brain or spinal cord, and multiple presynaptic and postsynaptic inhibitory mechanisms have been implicated. The relative significance of presynaptic and postsynaptic inhibition to opioid analgesia is essentially unknown, as are the identities and relevant locations of effectors mediating opioid actions. Here, we examined the distribution of G-protein-gated potassium (GIRK) channels in the mouse spinal cord and measured their contribution to the analgesia evoked by spinal administration of opioid receptor-selective agonists. We found that the GIRK channel subunits GIRK1 and GIRK2 were concentrated in the outer layer of the substantia gelatinosa of the dorsal horn. GIRK1 and GIRK2 were found almost exclusively in postsynaptic membranes of putative excitatory synapses, and a significant degree of overlap with the mu-opioid receptor was observed. Although most GIRK subunit labeling was perisynaptic or extrasynaptic, GIRK2 was found occasionally within the synaptic specialization. Genetic ablation or pharmacologic inhibition of spinal GIRK channels selectively blunted the analgesic effect of high but not lower doses of the mu-opioid receptor-selective agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin. Dose-dependent contributions of GIRK channels to the analgesic effects of the -opioid receptor-selective agonists Tyr-D-Ala-Phe-Glu-Val-Val-Gly amide and [D-Pen(2,5)]-enkephalin were also observed. In contrast, the analgesic effect of the agonist (trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl] benzeneacetamide methanesulfonate hydrate was preserved despite the absence of GIRK channels. We conclude that the activation of postsynaptic GIRK1 and/or GIRK2-containing channels in the spinal cord dorsal horn represents a powerful, albeit relatively insensitive, means by which intrathecal mu- and -selective opioid agonists evoke analgesia.

Endomorphin-1 is more potent than endomorphin-2 in inhibition of synaptic transmission in substantia gelatinosa of adult rat spinal cord

Effects of endomorphin-1 (EM-1) and endomorphin-2 (EM-2) on synaptic trans-mission were investigated on neurons in substantia gelatinosa (SG) of the spinal dorsal horn. Both EM-1 (1 microM) and EM-2 (1 microM) remarkably reduced the frequency but not the amplitude of miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs). These effects were antagonized by beta-funaltrexamine (beta-FNA, 10 microM), a selective mu-opioid receptor antagonist. Noticeably, EM-1 showed higher potency in decreasing the frequency of mEPSCs and mIPSCs than that of EM-2. These results indicate that EMs suppress both excitatory and inhibitory synaptic transmission by activating presynaptic mu-opioid receptors in the SG and EM-1, compared with EM-2, might be a more potent endogenous analgesic at the spinal cord level.

Synaptic input of rat spinal lamina I projection and unidentified neurones in vitro

Spinal lamina I projection neurones that transmit nociceptive information to the brain play a pivotal role in hyperalgesia in various animal models of inflammatory and neuropathic pain. Consistently, activity-dependent long-term potentiation can be induced at synapses between primary afferent C-fibres and lamina I projection neurones but not unidentified neurones in lamina I. The specific properties that enable projection neurones to undergo long-term potentiation and mediate hyperalgesia are not fully understood. Here, we have tested whether lamina I projection neurones differ from unidentified neurones in types or strength of primary afferent input and/or action potential-independent excitatory and inhibitory input. We used the whole-cell patch-clamp technique to record synaptic currents in projection and unidentified lamina I neurones in a transverse lumbar spinal cord slice preparation from rats between postnatal day 18 and 37. Lamina I neurones with a projection to the parabrachial area or the periaqueductal grey were identified by retrograde labelling with a fluorescent tracer. The relative contribution of NMDA receptors versus AMPA/kainate receptors to C-fibre-evoked excitatory postsynaptic currents of lamina I neurones significantly decreased with age between postnatal day 18 and 27, but was independent of the supraspinal projection of the neurones. We did not find a significant contribution of kainate receptors to C-fibre-evoked excitatory postsynaptic currents. Lamina I projection and unidentified neurones possessed functional GABAA and glycine receptors but received scarce action potential-independent spontaneous GABAergic and glycinergic inhibitory input as measured by miniature inhibitory postsynaptic currents. The miniature excitatory postsynaptic current frequencies were five times higher in projection than in unidentified neurones. The predominance of excitatory synaptic input to projection neurones, taken together with the previous finding that their membranes are more easily excitable than those of unidentified neurones, may facilitate the induction of synaptic long-term potentiation.