Endomorphin-2 is released from newborn rat primary sensory neurons in a frequency- and calcium-dependent manner

Endomorphin-2 (Endo2) and substance P (SubP) co-application attenuates SubP-induced excitation and alters frequency plasticity in neonatal rat in vitro preparations

Substance P (SubP) and endomorphin-2 (Endo2) are co-localized presynaptically in vesicles of neurons adjacent to inspiratory rhythm-generating pre-Botzinger Complex (preBotC) neurons but the effects of co-released SubP and Endo2 on respiratory motor control are not known. To address this question, SubP alone or a combination of SubP and Endo2 (SubP/Endo2) were bath-applied in a sustained (15-min) or intermittent (5-min application, 5-min washout, x3) pattern at 10-100 nM to neonatal rat brainstem-spinal cord preparations. During neuropeptide application, SubP/Endo2 co-applications generally attenuated SubP-induced increases in burst frequency and decreases in burst amplitude. With respect to frequency plasticity (long-lasting increase in burst frequency 60 min post-neuropeptide application), SubP-induced frequency plasticity was increased with sustained SubP/Endo2 co-applications at 20 and 100 nM. Intermittent SubP/Endo2 co-applications tended to decrease the level of frequency plasticity induced by intermittent SubP alone applications. SubP/Endo2 co-applications revealed potentially new functions for neurokinin-1 (NK1R) and mu-opioid (MOR) receptors on respiratory rhythm-generating medullary neurons.

Disinhibition does not play a role in endomorphin-2-induced changes in inspiratory motoneuron output produced by in vitro neonatal rat preparations

Low level activation of mu-opioid receptors (MORs) in neonatal rat brainstem-spinal cord preparations increases inspiratory burst amplitude recorded on cervical spinal roots. We tested whether: (1) MOR activation with an endogenous ligand, such as endomorphin-2, increases inspiratory burst amplitude, (2) disinhibition of GABAergic or glycinergic inhibitory synaptic transmission is involved, and (3) inflammation alters endomorphin-2 effects. Using neonatal rat (P0-P3) brainstem-spinal cord preparations, bath-applied endomorphin-2 (10-200 nM) increased inspiratory burst amplitude and decreased burst frequency. Blockade of GABAA receptors (picrotoxin), glycine receptors (strychnine), or both (picrotoxin and strychnine) did not abolish endomorphin-2-induced effects. In preparations isolated from neonatal rats injected 3 h previously with lipopolysaccharide (LPS, 0.1 mg/kg), endomorphin-2 continued to decrease burst frequency but abolished the burst amplitude increase. Collectively, these data indicate that disinhibition of inhibitory synaptic transmission is unlikely to play a role in endomorphin-2-induced changes in inspiratory motor output, and that different mechanisms underlie the endomorphin-2-induced increases in inspiratory burst amplitude and decreases in burst frequency.

A novel endomorphin-2/salmon calcitonin hybrid peptide with enhancing anti-allodynic and anti-anxiety effects

Pain, a worldwide problem with a high incidence and complex pathogenesis, has attracted the attention of pharmaceutical enterprises for the development of safer and more effective drugs. Extensive experimental and clinical evidence has demonstrated the analgesic effects of two endogenous peptides: endomorphin-2 (EM-2) and salmon calcitonin (sCT). However, EM-2 has limitations, such as poor ability to cross the blood-brain barrier (BBB) and little therapeutic effect in chronic pain due to rapid in vivo proteolysis. Herein, we propose the design of a novel hybrid peptide TEM2CT by combining EM-2, sCT16-21, and the cell-penetrating peptide HIV-1 trans-activator protein (TAT) with the aim of enhancing their analgesic effects. TEM2CT treatment attenuated nociceptive behavior in both acute and chronic pain mouse models, exhibiting increased anti-allodynic and anti-anxiety effects compared to sCT treatment. Furthermore, TEM2CT also regulated the excitability of pyramidal neurons in the anterior cingulate cortex (ACC) in spared nerve injury (SNI) model mice. The improved efficacy of this hybrid peptide provides a promising strategy for developing analgesic drugs.

The sigma-1 receptor curtails endogenous opioid analgesia during sensitization of TRPV1 nociceptors

BACKGROUND AND PURPOSE

Peripheral sensitization contributes to pathological pain. While prostaglandin E2 (PGE2) and nerve growth factor (NGF) sensitize peptidergic C-nociceptors (TRPV1+), glial cell line-derived neurotrophic factor (GDNF) sensitizes non-peptidergic C-neurons (IB4+). The sigma-1 receptor (sigma-1R) is a Ca²⁺ -sensing chaperone known to modulate opoid analgesia. This receptor binds both to TRPV1 and the μ opioid receptor, although the functional repercussions of these physical interactions in peripheral sensitization are unknown.

EXPERIMENTAL APPROACH

We tested the effects of sigma-1 antagonism on PGE2-, NGF-, and GDNF-induced mechanical and heat hyperalgesia in mice. We used immunohistochemistry to determine the presence of endomorphin-2, an endogenous μ receptor agonist, on dorsal root ganglion (DRG) neurons. Recombinant proteins were used to study the interactions between sigma-1R, μ- receptor, and TRPV1. We used calcium imaging to study the effects of sigma-1 antagonism on PGE2-induced sensitization of TRPV1+ nociceptors.

KEY RESULTS

Sigma1 antagonists reversed PGE2- and NGF-induced hyperalgesia but not GDNF-induced hyperalgesia. Endomorphin-2 was detected on TRPV1+ but not on IB4+ neurons. Peripheral opioid receptor antagonism by naloxone methiodide or administration of an anti-endomorphin-2 antibody to a sensitized paw reversed the antihyperalgesia induced by sigma-1 antagonists. Sigma-1 antagonism transfers sigma-1R from TRPV1 to μ receptors, suggesting that sigma-1R participate in TRPV1-μ receptor crosstalk. Moreover, sigma-1 antagonism reversed, in a naloxone-sensitive manner, PGE2-induced sensitization of DRG neurons to the calcium flux elicited by capsaicin, the prototypic TRPV1 agonist.

CONCLUSION AND IMPLICATIONS

Sigma-1 antagonism harnesses endogenous opioids produced by TRPV1+ neurons to reduce hyperalgesia by increasing μ receptor activity.

Brainstem Neuronal Circuitries Controlling Gastric Tonic and Phasic Contractions: A Review

This review is on how current knowledge of brainstem control of gastric mechanical function unfolded over nearly four decades from the perspective of our research group. It describes data from a multitude of different types of studies involving retrograde neuronal tracing, microinjection of drugs, whole-cell recordings from rodent brain slices, receptive relaxation reflex, accommodation reflex, c-Fos experiments, immunohistochemical methods, electron microscopy, transgenic mice, optogenetics, and GABAergic signaling. Data obtained indicate the following: (1) nucleus tractus solitarius (NTS)-dorsal motor nucleus of the vagus (DMV) noradrenergic connection is required for reflex control of the fundus; (2) second-order nitrergic neurons in the NTS are also required for reflex control of the fundus; (3) a NTS GABAergic connection is required for reflex control of the antrum; (4) a single DMV efferent pathway is involved in brainstem control of gastric mechanical function under most experimental conditions excluding the accommodation reflex. Dual-vagal effectors controlling cholinergic and non-adrenergic and non-cholinergic (NANC) input to the stomach may be part of the circuitry of this reflex. (5) GABAergic signaling within the NTS via Sst-GABA interneurons determine the basal (resting) state of gastric tone and phasic contractions. (6) For the vagal-vagal reflex to become operational, an endogenous opioid in the NTS is released and the activity of Sst-GABA interneurons is suppressed. From the data, we suggest that the CNS has the capacity to provide region-specific control over the proximal (fundus) and distal (antrum) stomach through engaging phenotypically different efferent inputs to the DMV.

The sciatic nerve injury model in pre-clinical research

In the pre-clinical view, the study of peripheral nerve repair and regeneration still needs to be carried out in animal models due to the structural complexity of this organ which can be only partly simulated in vitro. The far most used experimental model is based on the injury of the sciatic nerve, the largest nerve trunk in mammals. In this paper, the potential application of the sciatic nerve injury model in pre-clinical research is critically reviewed. This paper is aimed at helping researchers in properly employing this in vivo model for the study of nerve repair and regeneration as well as interpreting the results in a clinical translation perspective.

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.

WITHDRAWN: Enkephalin- and dynorphin-release produced by electrical stimulation of different frequencies in rat brain slices

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Sciatic nerve transection triggers release and intercellular transfer of a genetically expressed macromolecular tracer in dorsal root ganglia

We recently developed a genetic transneuronal tracing approach that allows for the study of circuits that are altered by nerve injury. We generated transgenic (ZW-X) mice in which expression of a transneuronal tracer, wheat germ agglutinin (WGA), is induced in primary sensory neurons, but only after transection of their peripheral axon. By following the transneuronal transport of the tracer into the central nervous system (CNS) we can label the circuits that are engaged by the WGA-expressing damaged neurons. Here we used the ZW-X mouse line to analyze dorsal root ganglia (DRG) for intraganglionic connections between injured sensory neurons and their neighboring "intact" neurons. Because neuropeptide Y (NPY) expression is strongly induced in DRG neurons after peripheral axotomy, we crossed the ZW-X mouse line with a mouse that expresses Cre recombinase under the influence of the NPY promoter. As expected, sciatic nerve transection triggered WGA expression in NPY-positive DRG neurons, most of which are of large diameter. As expected, double labeling for ATF-3, a marker of cell bodies with damaged axons, showed that the tracer predominated in injured (i.e., axotomized) neurons. However, we also found the WGA tracer in DRG cell bodies of uninjured sensory neurons. Importantly, in the absence of nerve injury there was no intraganglionic transfer of WGA. Our results demonstrate that intraganglionic, cell-to-cell communication, via transfer of large molecules, occurs between the cell bodies of injured and neighboring noninjured primary afferent neurons.

micro-Opioid receptor stimulation in the medial subnucleus of the tractus solitarius inhibits gastric tone and motility by reducing local GABA activity

We examined the effects of altering mu-opioid receptor (MOR) activity in the medial subnucleus of the tractus solitarius (mNTS) on several gastric end points including intragastric pressure (IGP), fundus tone, and the receptive relaxation reflex (RRR). Microinjection of the MOR agonist [d-Ala(2),MePhe(4),Gly(ol)(5)]enkephalin (DAMGO; 1-10 fmol) into the mNTS produced dose-dependent decreases in IGP. Microinjection of the endogenous MOR agonists endomorphin-1 and endomorphin-2 (20 fmol) into the mNTS mimicked the effects of 10 fmol DAMGO. Microinjection of 1 and 100 pmol DAMGO into the mNTS produced a triphasic response consisting of an initial decrease, a transient increase, and a persistent decrease in IGP. The increase in IGP appeared to be due to diffusion to the dorsal motor nucleus of the vagus. The effects of 10 fmol DAMGO in the mNTS were blocked by vagotomy and by blockade of MORs, GABA(A) receptors, and ionotropic glutamate receptors in the mNTS. The RRR response was abolished by bilateral microinjection of the opioid receptor antagonist naltrexone into the mNTS and reduced by intravenous administration of naltrexone. Our data demonstrate that 1) activation of MORs in the mNTS with femtomole doses of agonist inhibits gastric motility, 2) the mechanism of MOR effects in the mNTS is through suppression of local GABA activity, and 3) blockade of MORs in the mNTS prevents the RRR response. These data suggest that opioids play an important role in mediating a vagovagal reflex through release of an endogenous opioid in the mNTS, which, in turn, inhibits ongoing local GABA activity and allows vagal sensory input to excite second-order mNTS neurons.

Nitric oxide regulates BDNF release from nodose ganglion neurons in a pattern-dependent and cGMP-independent manner

Activity of arterial baroreceptors is modulated by neurohumoral factors, including nitric oxide (NO), released from endothelial cells. Baroreceptor reflex responses can also be modulated by NO signaling in the brainstem nucleus tractus solitarius (NTS), the primary central target of cardiovascular afferents. Our recent studies indicate that brain-derived neurotrophic factor (BDNF) is abundantly expressed by developing and adult baroreceptor afferents in vivo, and released from cultured nodose ganglion (NG) neurons by patterns of baroreceptor activity. Using electrical field stimulation and ELISA in situ, we show that exogenous NO nearly abolishes BDNF release from newborn rat NG neurons in vitro stimulated with single pulses delivered at 6 Hz, but not 2-pulse bursts delivered at the same 6-Hz frequency, that corresponds to a rat heart rate. Application of L-NAME, a specific inhibitor of endogenous NO synthases, does not have any significant effect on activity-dependent BDNF release, but leads to upregulation of BDNF expression in an activity-dependent manner. The latter effect suggests a novel mechanism of homeostatic regulation of activity-dependent BDNF expression with endogenous NO as a key player. The exogenous NO-mediated effect does not involve the cGMP-protein kinase G (PKG) pathway, but is largely inhibited by N-ethylmaleimide and TEMPOL that are known to prevent S-nitrosylation. Together, our current data identify previously unknown mechanisms regulating BDNF availability, and point to NO as a likely regulator of BDNF at baroafferent synapses in the NTS.

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.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Intrathecally injected Ile-Pro-Ile, an inhibitor of membrane ectoenzyme dipeptidyl peptidase IV, is antihyperalgesic in rats by switching the enzyme from hydrolase to synthase functional mode to generate endomorphin 2

We have found recently that membrane-bound dipeptidyl peptidase IV (DPP-IV) generated extracellularly immunoreactive endomorphin-2 from Tyr-Pro precursor in a depolarisation-sensitive manner in rat isolated L4,5 dorsal root ganglia when the enzyme was switched to synthase mode by the hydrolase inhibitor Ile-Pro-Ile. Presently, we induced hyperalgesia in rats by injecting carrageenan into the right hindpaw and measured the reduction in nociceptive threshold (hyperalgesia) to pressure (Randall-Selitto test). The hyperalgesia, peaking at 180 min after injection, was fully reversed by intrathecal administration of 30 nmol/rat Ile-Pro-Ile. The antihyperalgesic action was antagonized by s.c. naloxone (1 mg/kg) and intrathecally injected specific antiserum to endomorphin-2 indicating that the opioid receptor-mediated effect was produced by an endogenously generated endomorphin-2-like immunoreactive substance. Intrathecal Ile-Pro-Ile was ineffective as an analgesic in the acute nociceptive test such as the rat tail-flick, whereas endomorphin-2 (EC(50)=13.3 nmol/rat), endomorphin-1 (6.8 nmol/rat), morphine (0.11 nmol/rat) and DAMGO (0.0059 nmol/rat) exerted opioid receptor-mediated analgesia given by the same route. We concluded that carrageenan-induced C-fiber barrage (wind-up) may create ideal conditions for the de novo synthesis of endomorphin-2 in rat spinal cord dorsal horns if the DPP-IV enzyme is switched to the synthase functional mode by Ile-Pro-Ile.

Immunoreactive endomorphin 2 is generated extracellularly in rat isolated L4,5 dorsal root ganglia by DPP-IV

BACKGROUND AND AIMS

The gene(s) encoding for endomorphin precursor(s) is/are still unknown. We have raised the possibility of and did find some evidence for a potential de novo biosynthetic route starting from Tyr-Pro precursor. To pursue further this possibility we measured the generation of immunoreactive endomorphin-2 (E2-IR) in adult rat isolated L4,5 dorsal root ganglia.

RESULTS AND CONCLUSIONS

In rat isolated dorsal root ganglia the combination of presumed biosynthetic precursor of endomorphin 2 (E2), Tyr-Pro with the dipeptidyl peptidase IV (DPP-IV) inhibitor Ile-Pro-Ile generated 1.60+/-0.37 pg/mg Wet Tissue Weight_30 min E2-IR in the bathing fluid (n=4) with an 8-fold increase upon depolarization whereas the tissue content was low (0.50+/-0.08 pg/mg_WTW). Substance P, as determined by ELISA in the pilot experiments, was found almost exclusively within the tissues. It is concluded that E2-IR was generated extracellularly by a membrane-bound DPP-IV, which was switched to "synthase" mode by the hydrolase inhibitor Ile-Pro-Ile. DPP-IV was depolarization-sensitive in "synthase" functional mode.

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.