Mu-opioid and GABA(B) receptors modulate different types of Ca2+ currents in rat nodose ganglion neurons

Coexpressed δ-, μ-, and κ-Opioid Receptors Modulate Voltage-Gated Ca2+ Channels in Gastric-Projecting Vagal Afferent Neurons

Opioid analgesics are frequently associated with gastrointestinal side effects, including constipation, nausea, dysphagia, and reduced gastric motility. Though it has been shown that stimulation of opioid receptors expressed in enteric motor neurons contributes to opioid-induced constipation, it remains unclear whether activation of opioid receptors in gastric-projecting nodose ganglia neurons contributes to the reduction in gastric motility and emptying associated with opioid use. In the present study, whole-cell patch-clamp recordings were performed to determine the mechanism underlying opioid receptor-mediated modulation of Ca2+ currents in acutely isolated gastric vagal afferent neurons. Our results demonstrate that CaV2.2 channels provide the majority (71% ± 16%) of Ca2+ currents in gastric vagal afferent neurons. Furthermore, we found that application of oxycodone, U-50488, or deltorphin II on gastric nodose ganglia neurons inhibited Ca2+ currents through a voltage-dependent mechanism by coupling to the Gα i/o family of heterotrimeric G-proteins. Because previous studies have demonstrated that the nodose ganglia expresses low levels of δ-opioid receptors, we also determined the deltorphin II concentration-response relationship and assessed deltorphin-mediated Ca2+ current inhibition following exposure to the δ-opioid receptor antagonist ICI 174,864 (0.3 µM). The peak mean Ca2+ current inhibition following deltorphin II application was 47% ± 24% (EC50 = 302.6 nM), and exposure to ICI 174,864 blocked deltorphin II-mediated Ca2+ current inhibition (4% ± 4% versus 37% ± 20%). Together, our results suggest that analgesics targeting any opioid receptor subtype can modulate gastric vagal circuits. SIGNIFICANCE STATEMENT: This study demonstrated that in gastric nodose ganglia neurons, agonists targeting all three classical opioid receptor subtypes (μ, δ, and κ) inhibit voltage-gated Ca2+ channels in a voltage-dependent mechanism by coupling to Gαi/o. These findings suggest that analgesics targeting any opioid receptor subtype would modulate gastric vagal circuits responsible for regulating gastric reflexes.

Sensitisation of colonic nociceptors by TNFα is dependent on TNFR1 expression and p38 MAPK activity

Abstract

Visceral pain is a leading cause of morbidity in gastrointestinal diseases, which is exacerbated by the gut‐related side‐effects of many analgesics. New treatments are needed and further understanding of the mediators and mechanisms underpinning visceral nociception in disease states is required to facilitate this. The pro‐inflammatory cytokine TNFα is linked to pain in both patients with inflammatory bowel disease and irritable bowel syndrome, and has been shown to sensitize colonic sensory neurons. Somatic, TNFα‐triggered thermal and mechanical hypersensitivity is mediated by TRPV1 signalling and p38 MAPK activity respectively, downstream of TNFR1 receptor activation. We therefore hypothesized that TNFR1‐evoked p38 MAPK activity may also be responsible for TNFα sensitization of colonic afferent responses to the TRPV1 agonist capsaicin, and noxious distension of the bowel. Using Ca²⁺ imaging of dorsal root ganglion sensory neurons, we observed TNFα‐mediated increases in intracellular [Ca²⁺] and sensitization of capsaicin responses. The sensitizing effects of TNFα were dependent on TNFR1 expression and attenuated by p38 MAPK inhibition. Consistent with these findings, ex vivo colonic afferent fibre recordings demonstrated an enhanced response to noxious ramp distention of the bowel and bath application of capsaicin following TNFα pre‐treatment. Responses were reversed by p38 MAPK inhibition and absent in tissue from TNFR1 knockout mice. Our findings demonstrate a contribution of TNFR1, p38 MAPK and TRPV1 to TNFα‐induced sensitization of colonic afferents, highlighting the potential utility of these drug targets for the treatment of visceral pain in gastrointestinal disease.

Key points

The pro‐inflammatory cytokine TNFα is elevated in gastrointestinal disease and sensitizes colonic afferents via modulation of TRPA1 and Na_V1.8 activity. We further develop this understanding by demonstrating a role for p38 MAPK and TRPV1 in TNFα‐mediated colonic afferent sensitization. Specifically, we show that:

TNFα sensitizes sensory neurons and colonic afferents to the TRPV1 agonist capsaicin.

TNFα‐mediated sensitization of sensory neurons and colonic nociceptors is dependent on TNFR1 expression.

TNFα sensitization of sensory neurons and colonic afferents to capsaicin and noxious ramp distension is abolished by inhibition of p38 MAPK.

Collectively these data support the utility of targeting TNFα, TNFR1 and their downstream signalling via p38 MAPK for the treatment of visceral pain in gastrointestinal disease.

Mechanisms of μ-opioid receptor inhibition of NMDA receptor-induced substance P release in the rat spinal cord

The interaction between NMDA receptors and μ-opioid receptors in primary afferent terminals was studied by using NMDA to induce substance P release, measured as neurokinin 1 receptor internalization. In rat spinal cord slices, the μ-opioid receptor agonists morphine, DAMGO and endomorphin-2 inhibited NMDA-induced substance P release, whereas the antagonist CTAP right-shifted the concentration response of DAMGO. In vivo, substance P release induced by intrathecal NMDA after priming with BDNF was inhibited by DAMGO. ω-Conotoxins MVIIC and GVIA inhibited about half of the NMDA-induced substance P release, showing that it was partially mediated by the opening of voltage-gated calcium (Cav) channels. In contrast, DAMGO or ω-conotoxins did not inhibit capsaicin-induced substance P release. In cultured DRG neurons, DAMGO but not ω-conotoxin inhibited NMDA-induced increases in intracellular calcium, indicating that μ-opioid receptors can inhibit NMDA receptor function by mechanisms other than inactivation of Cav channels. Moreover, DAMGO decreased the ω-conotoxin-insensitive component of the substance P release. Potent inhibition by ifenprodil showed that these NMDA receptors have the NR2B subunit. Activators of adenylyl cyclase and protein kinase A (PKA) induced substance P release and this was decreased by the NMDA receptor blocker MK-801 and by DAMGO. Conversely, inhibitors of adenylyl cyclase and PKA, but not of protein kinase C, decreased NMDA-induced substance P release. Hence, these NMDA receptors are positively modulated by the adenylyl cyclase-PKA pathway, which is inhibited by μ-opioid receptors. In conclusion, μ-opioid receptors inhibit NMDA receptor-induced substance P release through Cav channel inactivation and adenylyl cyclase inhibition.

Ghrelin suppresses Purkinje neuron P-type Ca(2+) channels via growth hormone secretagogue type 1a receptor, the βγ subunits of Go-protein, and protein kinase a pathway

Although ghrelin receptors have been demonstrated to be widely expressed in the central nervous system and peripheral tissues of mammals, it is still unknown whether ghrelin functions in cerebellar Purkinje neurons. In this study, we identified a novel functional role for ghrelin in modulating P-type Ca(2+) channel (P-type channel) currents (IBa) as well as action-potential firing in rat Purkinje neurons. Our results show that ghrelin at 0.1μM reversibly decreased IBa by ~32.3%. This effect was growth hormone secretagogue receptor 1a (GHS-R1a)-dependent and was associated with a hyperpolarizing shift in the voltage-dependence of inactivation. Intracellular application of GDP-β-S and pretreatment with pertussis toxin abolished the inhibitory effects of ghrelin. Dialysis of cells with the peptide QEHA (but not the scrambled peptide SKEE), and a selective antibody raised against the G-protein αo subunit both blocked the ghrelin-induced response. Ghrelin markedly increased protein kinase A (PKA) activity, and intracellular application of PKI 5-24 as well as pretreatment of the cells with the PKA inhibitor KT-5720 abolished ghrelin-induced IBa decrease, while inhibition of PKC had no such effects. At the cellular level, ghrelin induced a significant increase in action-potential firing, and blockade of GHS-R1a by BIM-28163 abolished the ghrelin-induced hyperexcitability. In summary, these results suggest that ghrelin markedly decreases IBa via the activation of GHS-R1a, which is coupled sequentially to the activities of Go-protein βγ subunits and the downstream PKA pathway. This could contribute to its physiological functions, including the spontaneous firing of action potentials in cerebellar Purkinje neurons.

Activation of μ-opioid receptors inhibits calcium-currents in the vestibular afferent neurons of the rat through a cAMP dependent mechanism

Opioid receptors are expressed in the vestibular endorgans (afferent neurons and hair cells) and are activated by the efferent system, which modulates the discharge of action potentials in vestibular afferent neurons (VANs). In mammals, VANs mainly express the μ opioid-receptor, but the function of this receptors activation and the cellular mechanisms by which they exert their actions in these neurons are poorly studied. To determine the actions of μ opioid receptor (MOR) and cell signaling mechanisms in VANs, we made perforated patch-clamp recordings of VANs that were obtained from postnatal days 7 to 10 (P7–10) rats and then maintained in primary culture. The MOR agonist [D-Ala², N-Me-Phe⁴, Gly⁵-ol]-enkephalin (DAMGO) inhibited the total voltage-gated outward current; this effect was prevented by the perfusion of a Ca²⁺-free extracellular solution. We then studied the voltage-gated calcium current (I_ca) and found that DAMGO Met-enkephalin or endomorphin-1 inhibited the I_Ca in a dose-response fashion. The effects of DAMGO were prevented by the MOR antagonist (CTAP) or by pertussis toxin (PTX). The use of specific calcium channel blockers showed that MOR activation inhibited T-, L- and N-type I_Ca. The use of various enzyme activators and inhibitors and of cAMP analogs allowed us to demonstrate that the MOR acts through a cAMP dependent signaling mechanism. In current clamp experiments, MOR activation increased the duration and decreased the amplitude of the action potentials and modulated the discharge produced by current injection. Pre-incubation with PTX occluded MOR activation effect. We conclude that MOR activation inhibits the T-, L- and N-type I_Ca through activation of a Gα_i/o protein that involves a decrease in AC-cAMP-PKA activity. The modulation of I_Ca may have an impact on the synaptic integration, excitability, and neurotransmitter release from VANs.

Opioids inhibit visceral afferent activation of catecholamine neurons in the solitary tract nucleus

Brainstem A2/C2 catecholamine (CA) neurons within the solitary tract nucleus (NTS) influence many homeostatic functions, including food intake, stress, respiratory and cardiovascular reflexes. They also play a role in both opioid reward and withdrawal. Injections of opioids into the NTS modulate many autonomic functions influenced by catecholamine neurons including food intake and cardiac function. We recently showed that NTS-CA neurons are directly activated by incoming visceral afferent inputs. Here we determined whether opioid agonists modulate afferent activation of NTS-CA neurons using transgenic mice with EGFP expressed under the control of the tyrosine hydroxylase promoter (TH-EGFP) to identify catecholamine neurons. The opioid agonist Met-enkephalin (Met-Enk) significantly attenuated solitary tract-evoked excitatory postsynaptic currents (ST-EPSCs) in NTS TH-EGFP neurons by 80%, an effect reversed by wash or the mu opioid receptor-specific antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP). Met-Enk had a significantly greater effect to inhibit afferent inputs onto TH-EGFP-positive neurons than EGFP-negative neurons, which were only inhibited by 50%. The mu agonist, DAMGO, also inhibited the ST-EPSC in TH-EGFP neurons in a dose-dependent manner. In contrast, neither the delta agonist DPDPE, nor the kappa agonist, U69,593, consistently inhibited the ST-EPSC amplitude. Met-Enk and DAMGO increased the paired pulse ratio, decreased the frequency, but not amplitude, of mini-EPSCs and had no effect on holding current, input resistance or current-voltage relationships in TH-EGFP neurons, suggesting a presynaptic mechanism of action on afferent terminals. Met-Enk significantly reduced both the basal firing rate of NTS TH-EGFP neurons and the ability of afferent stimulation to evoke an action potential. These results suggest that opioids inhibit NTS-CA neurons by reducing an excitatory afferent drive onto these neurons through presynaptic inhibition of glutamate release and elucidate one potential mechanism by which opioids could control autonomic functions and modulate reward and opioid withdrawal symptoms at the level of the NTS.

The anti-botulism triterpenoid toosendanin elicits calcium increase and exocytosis in rat sensory neurons

Toosendanin, a triterpenoid from Melia toosendan Sieb et Zucc, has been found before to be an effective anti-botulism agent, with a bi-phasic effect at both motor nerve endings and central synapse: an initial facilitation followed by prolonged depression. Initial facilitation may be due to activation of voltage-dependent calcium channels plus inhibition of potassium channels, but the depression is not fully understood. Toosendanin has no effect on intracellular calcium or secretion in the non-excitable pancreatic acinar cells, ruling out general toosendanin inhibition of exocytosis. In this study, toosendanin effects on sensory neurons isolated from rat nodose ganglia were investigated. It was found that toosendanin stimulated increases in cytosolic calcium and neuronal exocytosis dose dependently. Experiments with membrane potential indicator bis-(1,3-dibutylbarbituric acid)trimethine oxonol found that toosendanin hyperpolarized capsaicin-insensitive but depolarized capsaicin-sensitive neurons; high potassium-induced calcium increase was much smaller in hyperpolarizing neurons than in depolarizing neurons, whereas no difference was found for potassium-induced depolarization in these two types of neurons. In neurons showing spontaneous calcium oscillations, toosendanin increased the oscillatory amplitude but not frequency. Toosendanin-induced calcium increase was decreased in calcium-free buffer, by nifedipine, and by transient receptor potential vanilloid 1 (TRPV1) antagonist capsazepine. Simultaneous measurements of cytosolic and endoplasmic reticulum (ER) calcium showed an increase in cytosolic but a decrease in ER calcium, indicating that toosendanin triggered ER calcium release. These data together indicate that toosendanin modulates sensory neurons, but had opposite effects on membrane potential depending on the presence or absence of capsaicin receptor/TRPV 1 channel.

Src family kinases mediate the inhibition of substance P release in the rat spinal cord by μ-opioid receptors and GABA(B) receptors, but not α2 adrenergic receptors

GABA(B) , μ-opioid and adrenergic α(2) receptors inhibit substance P release from primary afferent terminals in the dorsal horn. Studies in cell expression systems suggest that μ-opioid and GABA(B) receptors inhibit transmitter release from primary afferents by activating Src family kinases (SFKs), which then phosphorylate and inhibit voltage-gated calcium channels. This study investigated whether SFKs mediate the inhibition of substance P release by these three receptors. Substance P release was measured as neurokinin 1 receptor (NK1R) internalization in spinal cord slices and in vivo. In slices, NK1R internalization induced by high-frequency dorsal root stimulation was inhibited by the μ-opioid agonist DAMGO and the GABA(B) agonist baclofen. This inhibition was reversed by the SFK inhibitor PP1. NK1R internalization induced by low-frequency stimulation was also inhibited by DAMGO, but PP1 did not reverse this effect. In vivo, NK1R internalization induced by noxious mechanical stimulation of the hind paw was inhibited by intrathecal DAMGO and baclofen. This inhibition was reversed by intrathecal PP1, but not by the inactive PP1 analog PP3. PP1 produced no effect by itself. The α(2) adrenergic agonists medetomidine and guanfacine produced a small but statistically significant inhibition of NK1R internalization induced by low-frequency dorsal root stimulation. PP1 did not reverse the inhibition by guanfacine. These results show that SFKs mediate the inhibition of substance P release by μ-opioid and GABA(B) receptors, but not by α(2) receptors, which is probably mediated by the binding of G protein βγ subunits to calcium channels.

NMDA receptors in primary afferents require phosphorylation by Src family kinases to induce substance P release in the rat spinal cord

The function of N-methyl-d-aspartate (NMDA) receptors in primary afferents remains controversial, in particular regarding their ability to evoke substance P release in the spinal cord. The objective of this study was, first, to confirm that substance P release evoked by NMDA is mediated by NMDA receptors in primary afferent terminals. Second, we investigated whether these NMDA receptors are inactivated in some conditions, which would explain why their effect on substance P release was not observed in some studies. Substance P release was induced in spinal cord slices and measured as neurokinin 1 (NK1) receptor internalization in lamina I neurons. NMDA (combined with d-serine) induced NK1 receptor internalization with a half of the effective concentration (EC50) of 258 nM. NMDA-induced NK1 receptor internalization was abolished by the NK1 receptor antagonist L-703,606, confirming that is was caused by substance P release, by NMDA receptor antagonists (MK1801 and ifenprodil), showing that it was mediated by NMDA receptors containing the NR2B subunit, and by preincubating the slices with capsaicin, showing that the substance P release was from primary afferents. However, it was not affected by lidocaine and omega-conotoxin MVIIA, which block Na+ channels and voltage-dependent Ca2+ channels, respectively. Therefore, NMDA-induced substance P release does not require firing of primary afferents or the opening of Ca2+ channels, which is consistent with the idea that NMDA receptors induce substance P directly by letting Ca2+ into primary afferent terminals. Importantly, NMDA-induced substance P release was eliminated by preincubating the slices for 1 h with the Src family kinase inhibitors PP1 and dasatinib, and was substantially increased by the protein tyrosine phosphatase inhibitor BVT948. In contrast, PP1 did not affect NK1 receptor internalization induced by capsaicin. These results show that tyrosine-phosphorylation of these NMDA receptors is regulated by the opposite actions of Src family kinases and protein tyrosine phosphatases, and is required to induce substance P release.

GABAB receptors as drug targets to treat gastroesophageal reflux disease

For many years, acid-suppressive therapy has been at the forefront of treating gastroesophageal reflux disease (GERD), yet despite the advent of the proton pump inhibitors (PPIs) some patients continue to experience persistent GERD symptoms. Therapeutic (non-surgical) options for such patients are currently limited. To tackle this clinical issue, research efforts have begun to focus on 'reflux inhibition' as a potential therapeutic target - i.e. inhibition of transient lower esophageal relaxations (TLESRs), the predominant mechanism of gastroesophageal reflux. Preclinical research has identified a number of drug targets through which TLESRs can be modulated, and the gamma-aminobutyric acid (GABA) type B (GABA(B)) receptor has emerged as one of the most promising. Studies with baclofen, a well-known agonist of this receptor, have demonstrated that reflux inhibition is a valid concept in the clinical setting in that reducing the incidence of TLESRs improves GERD symptoms. But baclofen is associated with significant central nervous system (CNS) side effects, rendering it undesirable for use as a treatment for GERD. Further development work has yielded a number of novel GABA(B) receptor agonists with reduced CNS side effect profiles, and clinical trials are currently being performed with several agents. Compounds that target TLESRs may therefore present a new add-on treatment for patients with persistent GERD symptoms despite PPI therapy.

Subdiaphragmatic vagal afferent nerves modulate visceral pain

Activation of the vagal afferents by noxious gastrointestinal stimuli suggests that vagal afferents may play a complex role in visceral pain processes. The contribution of the vagus nerve to visceral pain remains unresolved. Previous studies reported that patients following chronic vagotomy have lower pain thresholds. The patient with irritable bowel syndrome has been shown alteration of vagal function. We hypothesize that vagal afferent nerves modulate visceral pain. Visceromotor responses (VMR) to graded colorectal distension (CRD) were recorded from the abdominal muscles in conscious rats. Chronic subdiaphragmatic vagus nerve sections induced 470, 106, 51, and 54% increases in VMR to CRD at 20, 40, 60 and 80 mmHg, respectively. Similarly, at light level of anesthesia, topical application of lidocaine to the subdiaphragmatic vagus nerve in rats increased VMR to CRD. Vagal afferent neuronal responses to low or high-intensity electrical vagal stimulation (EVS) of vagal afferent Adelta or C fibers were distinguished by calculating their conduction velocity. Low-intensity EVS of Adelta fibers (40 microA, 20 Hz, 0.5 ms for 30 s) reduced VMR to CRD at 40, 60, and 80 mmHg by 41, 52, and 58%, respectively. In contrast, high-intensity EVS of C fibers (400 microA, 1 Hz, 0.5 ms for 30 s) had no effect on VMR to CRD. In conclusion, we demonstrated that vagal afferent nerves modulate visceral pain. Low-intensity EVS that activates vagal afferent Adelta fibers reduced visceral pain. Thus EVS may potentially have a role in the treatment of chronic visceral pain.

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

Recent evidence indicates that endomorphins, endogenous mu-opioid receptor (MOR) agonists, modulate synaptic transmission in both somatic and visceral sensory pathways. Here we show that endomorphin-2 (END-2) is expressed in newborn rat dorsal root ganglion (DRG) and nodose-petrosal ganglion complex (NPG) neurons, and rarely co-localizes with brain-derived neurotrophic factor (BDNF). In order to examine activity-dependent release of END-2 from neurons, we established a model using dispersed cultures of DRG and NPG cells activated by patterned electrical field stimulation. To detect release of END-2, we developed a novel rapid capture enzyme-linked immunosorbent assay (ELISA), in which END-2 capture antibody was added to neuronal cultures shortly before their electrical stimulation. The conventional assay was effective at reliably detecting END-2 only when the cells were stimulated in the presence of CTAP, a MOR-selective antagonist. This suggests that the strength of the novel assay is related primarily to rapid capture of released END-2 before it binds to endogenous MORs. Using the rapid capture ELISA, we found that stimulation protocols known to induce plastic changes at sensory synapses were highly effective at releasing END-2. Removal of extracellular calcium or blocking voltage-activated calcium channels significantly reduced the release. Together, our data provide the first evidence that END-2 is expressed by newborn DRG neurons of all sizes found in this age group, and can be released from these, as well as from NPG neurons, in an activity-dependent manner. These results point to END-2 as a likely mediator of activity-dependent plasticity in sensory pathways.

Opioid modulation of ferret vagal afferent mechanosensitivity

Despite universal use of opioids in the clinic to inhibit pain, there is relatively little known of their peripheral actions on sensory nerve endings, where in fact they may be better targeted with more widespread applications. Here we show differential effects of mu-, kappa-, and delta-opioids on mechanosensitive ferret esophageal vagal afferent endings investigated in vitro. The effects of selective agonists [d-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (DAMGO), 2-(3, 4-dichlorophenyl)-N-methyl-N-[(1S)-1phenyl-2-(1-pyrrolidinyl) ethyl] acetamide hydrochlorine (ICI 199441), and (+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC-80), respectively, on mechanosensory stimulus-response functions were quantified. DAMGO (10(-7) to 10(-5) M) reduced the responses of tension receptors to circumferential tension (1-5 g) by up to 50%, and the responses of mucosal receptors to mucosal stroking (10-1,000 mg von Frey hair) by >50%. DAMGO effects were reversed by naloxone (10(-5) M). Tension/mucosal (TM) receptor responses to tension and stroking were unaffected by DAMGO. ICI 199441 (10(-6) to 10(-5) M) potently inhibited all responses except TM receptor responses to tension, and SNC-80 (10(-5) to 10(-3) M) had no effect other than a minor inhibition of mucosal receptor responses to intense stimuli at 10(-3) M. We conclude that mu- and kappa-opioids have potent and selective peripheral effects on esophageal vagal afferents that may have applications in treatment of disorders of visceral sensation.

Central and peripheral mechanisms of narcotic antitussives: codeine-sensitive and -resistant coughs

Narcotic antitussives such as codeine reveal the antitussive effect primarily via the mu-opioid receptor in the central nervous system (CNS). The kappa-opioid receptor also seems to contribute partly to the production of the antitussive effect of the drugs. There is controversy as to whether delta-receptors are involved in promoting an antitussive effect. Peripheral opioid receptors seem to have certain limited roles. Although narcotic antitussives are the most potent antitussives at present, certain types of coughs, such as chronic cough, are particularly difficult to suppress even with codeine. In guinea pigs, coughs elicited by mechanical stimulation of the bifurcation of the trachea were not able to be suppressed by codeine. In gupigs with sub-acute bronchitis caused by SO2 gas exposure, coughing is difficult to inhibit with centrally acting antitussives such as codeine. Some studies suggest that neurokinins are involved in the development of codeine-resistant coughs. However, evidence supporting this claim is still insufficient. It is very important to characterize opiate-resistant coughs in experimental animals, and to determine which experimentally induced coughs correspond to which types of cough in humans. In this review, we describe the mechanisms of antitussive effects of narcotic antitussives, addressing codeine-sensitive and -resistant coughs, and including our own results.

Chemosensitivity of unmyelinated axons in isolated human gastric vagus nerve

Vagal afferent neurons from the stomach may be activated not only by chemical stimuli in the mucosa but also by circulating factors. In the present study, we have used electrophysiological techniques to characterize functional activity of several receptors for chemical mediators on unmyelinated axons in isolated fascicles of human gastric vagus nerve. Application of agonists at the nicotinic acetylcholine receptor (nAChR), 5-HT(3) subtype of serotonin receptor, and the transient receptor potential vanilloid receptor-1 (TRPV1) resulted in a change in the height and/or threshold of the C-fiber compound action potential. These effects were blocked by specific antagonists of nAChR (mecamylamine), 5-HT(3) (Y-25130), and TRPV1 (capsazepine). We conclude that the chemosensitivity of unmyelinated vagal axons can be studied using isolated segments of human gastric vagus nerve. The presence of receptors indicates that circulating factors may modify vagal afferent neurons also by effects on the axonal membrane.

Signaling mechanisms of angiotensin II-induced attenuation of GABAergic input to hypothalamic presympathetic neurons

The hypothalamic paraventricular nucleus (PVN) is an important site for the regulation of sympathetic outflow. Angiotensin II (Ang II) can activate AT(1) receptors to stimulate PVN presympathetic neurons through inhibition of GABAergic input. However, little is known about the downstream pathway involved in this presynaptic action of Ang II in the PVN. In this study, using whole cell recording from retrogradely labeled PVN neurons in rat brain slices, we determined the signaling mechanisms responsible for the effect of Ang II on synaptic GABA release to spinally projecting PVN neurons. Bath application of Ang II reproducibly decreased the frequency of GABAergic miniature postsynaptic inhibitory currents (mIPSCs) in fluorescence-labeled PVN neurons. Ang II failed to change the frequency of mIPSCs in labeled PVN neurons treated with pertussis toxin. However, Ang II-induced inhibition of mIPSCs persisted in the presence of either CdCl(2), a voltage-gated Ca(2+) channel blocker, or 4-aminopyridine, a blocker of voltage-gated K(+) channels. Interestingly, inhibition of superoxide with superoxide dismutase or Mn(III) tetrakis (4-benzoic acid) prophyrin completely blocked Ang II-induced decrease in mIPSCs. By contrast, inhibition of hydroxyl radical formation with the ion chelator deferoxamine did not significantly alter the effect of Ang II. These findings suggest that the presynaptic action of Ang II on synaptic GABA release in the PVN is mediated by the pertussis toxin-sensitive G(i/o) proteins but not by voltage-gated Ca(2+) and K(+) channels. Ang II attenuates GABAergic input to PVN presympathetic neurons through reactive oxygen species, especially superoxide anions.

Blocker-resistant presynaptic voltage-dependent Ca2+ channels underlying glutamate release in mice nucleus tractus solitarii

The visceral sensory information from the internal organs is conveyed via the vagus and glossopharyngeal primary afferent fibers and transmitted to the second-order neurons in the nucleus of the solitary tract (NTS). The glutamate release from the solitary tract (TS) axons to the second-order NTS neurons remains even in the presence of toxins that block N- and P/Q-type voltage-dependent Ca(2+) channels (VDCCs). The presynaptic VDCC playing the major role at this synapse remains unidentified. To address this issue, we examined two hypotheses in this study. First, we examined whether the remaining large component occurs through activation of a omega-conotoxin GVIA (omega-CgTX)-insensitive variant of N-type VDCC by using the mice genetically lacking its pore-forming subunit alpha(1B). Second, we examined whether R-type VDCCs are involved in transmitter release at the TS-NTS synapse. The EPSCs evoked by stimulation of the TS were recorded in medullary slices from young mice. omega-Agatoxin IVA (omega-AgaIVA; 200 nM) did not significantly affect the EPSC amplitude in the mice genetically lacking N-type VDCC. SNX-482 (500 nM) and Ni(2+) (100 microM) did not significantly reduce EPSC amplitude in ICR mice. These results indicate that, unlike in most of the brain synapses identified to date, the largest part of the glutamate release at the TS-NTS synapse in mice occurs through activation of non-L, non-P/Q, non-R, non-T and non-N (including its posttranslational variants) VDCCs at least according to their pharmacological properties identified to date.

Inhibition of mechanosensitivity in visceral primary afferents by GABAB receptors involves calcium and potassium channels

GABA(B) receptors inhibit mechanosensitivity of visceral afferents. This is associated with reduced triggering of events that lead to gastro-esophageal reflux, with important therapeutic consequences. In other neuronal systems, GABA(B) receptor activation may be linked via G-proteins to reduced N-type Ca(2+) channel opening, increased inward rectifier K(+) channel opening, plus effects on a number of intracellular messengers. Here we aimed to determine the role of Ca(2+) and K(+) channels in the inhibition of vagal afferent mechanoreceptor function by the GABA(B) receptor agonist baclofen. The responses of three types of ferret gastro-esophageal vagal afferents (mucosal, tension and tension mucosal receptors) to graded mechanical stimuli were investigated in vitro. The effects of baclofen (200 microM) alone on these responses were quantified, and the effects of baclofen in the presence of the G-protein-coupled inward rectifier potassium channel blocker Rb(+) (4.7 mM) and/or the N-type calcium channel blocker omega-conotoxin GVIA (0.1 microM). Baclofen inhibition of mucosal receptor mechanosensitivity was abolished by both blockers. Its inhibitory effect on tension mucosal receptors was partly reduced by both. The inhibitory effect of baclofen on tension receptors was unaffected. The data indicate that the inhibitory action of GABA(B) receptors is mediated via different pathways in mucosal, tension and tension mucosal receptors via mechanisms involving both N-type Ca(2+) channels and inwardly rectifying K(+) channels and others.

Activation of protein kinase C antagonizes the opioid inhibition of calcium current in rat spinal dorsal horn neurons

Spinal dorsal horn (SDH) is one of important regions in both nociceptive transmission and antinociception. Opioid peptides produce analgesia via regulation of neurotransmitter release through modulation of voltage-dependent Ca(2+) channel (VDCC) in neuronal tissues. The modulatory effect of micro-opioid receptor (MOR) activation on VDCC was investigated in acutely isolated rat SDH neurons under the conventional whole-cell patch-clamp recording mode. The Ba(2+) current passing through VDCC was reversibly inhibited by a MOR agonist, [D-Ala(2),N-MePhe(4),Gly(5)-ol]-enkephalin (DAMGO, 1 microM). Among 108 SDH neurons tested, VDCC of 39 neurons (36%) were inhibited by MOR activation, while other 69 neurons (64%) were not affected. The L-, N-, P/Q-, and R-type VDCC components shared 58.4+/-18.9%, 29.3+/-12.1%, 8.7+/-7.2%, and 3.4+/-4.8% of the total VDCC, respectively. Among VDCC subtypes inhibited by MOR activation, L- and N-types were 61.4+/-12.8% and 30.7+/-14.4%, respectively, while both P/Q- and R-types were 7.9+/-11.8%. A depolarizing pre-pulse increased the amplitude of VDCC and suppressed most of the inhibitory effect of MOR activation. Application of 1 microM phorbol-12-myristate-13-acetate completely antagonized the inhibitory effect of MOR activation without any alteration of basal VDCC amplitude. In contrast, the response of MOR activation was not altered by application of 4-alpha-phorbol (1 microM), 2-[3-Dimethylaminopropyl]indol-3-yl]-3-(indol-3-yl) maleimide (GF109203X, 1 microM), forskolin (1 microM), N-(2-[p-Bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide hydrochloride (H-89, 1 microM). These results indicate that activation of MOR coupled to G-proteins inhibits VDCC, and that this G-protein-mediated inhibition is antagonized by PKC-dependent phosphorylation.

Nerve injury alters profile of receptor-mediated Ca2+ channel modulation in vagal afferent neurons of rat nodose ganglia

Although nerve injury is known to up- and down-regulate some metabotropic receptors in vagal afferent neurons of the nodose ganglia (NG), the functional significance has not been elucidated. In the present study, thus, we examined whether nerve injury affected receptor-mediated Ca2+ channel modulation in the NG neurons. In this regard, unilateral vagotomy was performed using male Sprague-Dawley rats. One week after vagotomy, Ca2+ currents were recorded using the whole-cell variant of patch-clamp technique in enzymatically dissociated NG neurons. In sham controls, norepinephrine (NE)-induced Ca2+ current inhibition was negligible. Following vagotomy, however, the NE responses were dramatically increased. This phenomenon was in accordance with up-regulation of alpha2A/B-adrenergic receptor mRNAs as quantified using real-time RT-PCR analysis. In addition, neuropeptide Y (NPY) and prostaglandin E2 responses were moderately augmented in vagotomized NG neurons. The altered NPY response appears to be caused by up-regulation of Y2 receptors negatively coupled to Ca2+ channels. In contrast, nerve injury significantly suppressed opioid (tested with DAMGO)-induced Ca2+ current inhibition with down-regulation of micro-receptors. Taken together, these results demonstrated for the first time that the profile of neurotransmitter-induced Ca2+ channel modulation is significantly altered in the NG neurons under pathophysiological state of nerve injury.

The presynaptic modulation of corticostriatal afferents by mu-opioids is mediated by K+ conductances

Population spikes associated with the paired pulse ratio protocol were used to measure the presynaptic inhibition of corticostriatal transmission caused by mu-opioid receptor activation. A 1 microM of [D-Ala(2), N-MePhe(4), Gly-ol(5)]-enkephalin (DAMGO), a selective mu-opioid receptor agonist, enhanced paired pulse facilitation by 44+/-8%. This effect was completely blocked by 2 nM of the selective mu-receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-NH (CTOP). Antagonists of N- and P/Q-type Ca(2+) channels inhibited, whereas antagonists of potassium channels enhanced, synaptic transmission. A 1 microM of omega-conotoxin GVIA, a blocker of N-type Ca(2+) channels, had no effect on the action of DAMGO, but 400 nM omega-agatoxin TK, a blocker of P/Q-type Ca(2+)-channels, partially blocked the action of this opioid. However, 5 mM Cs(2+) and 400 microM Ba(2+), unselective antagonists of potassium conductances, completely prevented the action of DAMGO on corticostriatal transmission. These data suggest that presynaptic inhibition of corticostriatal afferents by mu-opioids is mediated by the modulation of K(+) conductances in corticostriatal afferents.

Functional GABAB receptors are present in guinea pig nodose ganglion cell bodies but not in peripheral mechanosensitive endings

The effects of the GABAB-selective agonist baclofen were studied on guinea pig nodose ganglion neurones using grease gap and intracellular recording techniques, and on peripheral mechanosensitive endings in the guinea pig oesophagus and stomach with extracellular recordings. GABA dose-dependently reduced the amplitude of the compound action potential of C-type neurones (C spikes, EC50 = 30.9 microM), which was prevented by the GABAA antagonist bicuculline (10 microM). The GABAB agonist baclofen (1-300 microM) did not produce any significant effect on the amplitude of C spikes. In microelectrode studies, baclofen (100 microM) evoked hyperpolarisation (by 2.53 +/- 0.51 mV, n = 6, N = 5) in a subset of nodose neurones (6 out of 26, N = 18). In seven out of eight neurones (N = 8) with a slow after-hyperpolarisation following action potentials, baclofen significantly inhibited its amplitude by 19 +/- 4% (n = 7, p < 0.05). GABA (100 microM) evoked a depolarisation of 9.3 +/- 2.4 mV (10 nodose neurones, N = 9, p < 0.05) associated with a decrease in input impedance of 49 +/- 12% (N = 4, p < 0.05). Baclofen (100-200 microM) did not affect either spontaneous or stretch-evoked firing of distension-sensitive vagal mechanoreceptors of the guinea pig oesophagus and stomach but did inhibit mechanoreceptors in the ferret oesophagus. Antibodies to GABAB receptor 1a splice variants labelled most of the neurones and numerous fibres in the guinea pig nodose ganglion while antibodies to GABAB receptor 1b splice variants stained only nerve cell bodies. There were numerous nerve fibres showing GABAB receptor 1a- and 1b-like immunoreactivity in the myenteric plexus in the guinea pig oesophagus and stomach but not in anterogradely labelled extrinsic vagal nerve fibres. The result indicates that most guinea pig C-type nodose ganglion neurones have GABAB receptors on their cell bodies but their density on distension-sensitive peripheral endings is too low to allow modulation of mechanotransduction. There is a significant species-dependent difference in the expression of GABAB receptors on peripheral vagal mechanosensitive endings.

Inhibitory effects of opioids on voltage-dependent Ca(2+) channels and catecholamine secretion in cultured porcine adrenal chromaffin cells

The inhibitory effects of opioids on voltage-dependent calcium channels (VDCCs) were investigated in cultured porcine adrenal chromaffin cells using whole-cell patch clamp technique. The effects of the opioid on [Ca(2+)](i) increase and catecholamine secretion induced by high K(+) were also examined in single cells by fura-2 microfluorimetry and amperometry. A depolarizing pulse to 0 mV (test pulse) from a holding potential of -80 mV evoked an inward barium current (I(Ba)), which was reversibly inhibited by methionine-enkephalin. This inhibitory effect of methionine-enkephalin was abolished by naloxone. Selective agonists of opioid receptor subtypes (DAMGO: mu, DPDPE: delta, U50488: kappa) dose-dependently inhibited I(Ba). In inhibitory potency, the order was DAMGO>U50488>DPDPE. These agonists applied sequentially produced a reversible I(Ba) inhibition in the same cells. The inhibitory effect of DAMGO on I(Ba) almost disappeared in the presence of omega-conotoxin GVIA but not omega-agatoxin IVA plus nifedipine. Application of a conditioning prepulse to +100 mV prior to the test pulse partly retrieved the I(Ba) inhibition by DAMGO, suggesting the involvement of voltage-sensitive components in opioid-induced VDCC inhibition. Intracellular application of GDPbetaS or GTPgammaS as well as pretreatment with pertussis toxin significantly reduced the extent of I(Ba) inhibition induced by DAMGO. DAMGO reversibly inhibited the [Ca(2+)](i) increase and catecholamine release induced by high K(+). RT-PCR revealed the expression of mu-, delta- and kappa-opioid receptor mRNAs in cultured adrenal chromaffin cells. These results suggest that porcine adrenal chromaffin cells possess mu-, delta- and kappa-opioid receptors and activation of opioid receptors mainly inhibits N-type VDCCs via pertussis toxin-sensitive G-proteins.

Antagonizing effect of protein kinase C activation on the mu-opioid agonist-induced inhibition of high voltage-activated calcium current in rat periaqueductal gray neuron

Opioids have been thought to induce analgesia by activating the descending pain control system, especially at the level of periaqueductal gray, and regulate the neurotransmitter release through the inhibition of calcium channel. In the present study, the modulatory effects of protein kinase C and protein kinase A on the mu-opioid agonist-induced inhibition of the high-voltage activated calcium current were examined in the acutely dissociated rat periaqueductal gray neurons with the nystatin-perforated patch-clamp technique. Among 505 neurons tested, the barium current passing through the high-voltage activated calcium channels of 172 neurons (34%) were inhibited by 32+/-3% with the application of an mu-opioid agonist, [D-Ala(2),N-MePhe(4),Gly(5)-ol]-enkephalin (DAMGO, 1 microM). The barium currents itself and the DAMGO-induced inhibitory effects were not affected by the application of either an adenylate cyclase activator (forskolin, 1 microM) or a protein kinase inhibitor (staurosporin, 10 nM) for 2 min. The DAMGO inhibition was completely and irreversibly antagonized by the application of a protein kinase C activator, phorbol-12-myristate-13-acetate (PMA, 1 microM) for 2 min without any alteration of the barium current itself. However, the antagonizing effect of PMA was completely abolished by the application of 10 nM staurosporin for 2 min. After then, PMA did not show the antagonizing effect any more. Inversely, when staurosporin was applied before PMA, the antagonizing effect of PMA was also not shown. These results demonstrate that the mu-opioid agonist-induced inhibition of the periaqueductal gray neuronal high-voltage activated calcium current can be antagonized by protein kinase C activation. This finding may provide us a significant clue to understand the action mechanism of opioid-induced analgesia in the periaqueductal gray.

Gastrointestinal afferents as targets of novel drugs for the treatment of functional bowel disorders and visceral pain

An intricate surveillance network consisting of enteroendocrine cells, immune cells and sensory nerve fibres monitors the luminal and interstitial environment in the alimentary canal. Functional bowel disorders are characterized by persistent alterations in digestive regulation and gastrointestinal discomfort and pain. Visceral hyperalgesia may arise from an exaggerated sensitivity of peripheral afferent nerve fibres and/or a distorted processing and representation of gut signals in the brain. Novel strategies to treat these sensory bowel disorders are therefore targeted at primary afferent nerve fibres. These neurons express a number of molecular traits including transmitters, receptors and ion channels that are specific to them and whose number and/or behaviour may be altered in chronic visceral pain. The targets under consideration comprise vanilloid receptor ion channels, acid-sensing ion channels, sensory neuron-specific Na(+) channels, P2X(3) purinoceptors, 5-hydroxytryptamine (5-HT), 5-HT(3) and 5-HT(4) receptors, cholecystokinin CCK(1) receptors, bradykinin and prostaglandin receptors, glutamate receptors, tachykinin and calcitonin gene-related peptide receptors as well as peripheral opioid and cannabinoid receptors. The utility of sensory neuron-targeting drugs in functional bowel disorders will critically depend on the compounds' selectivity of action for afferent versus enteric or central neurons.

Receptors and transmission in the brain-gut axis: potential for novel therapies. IV. GABA(B) receptors in the brain-gastroesophageal axis

GABA(B) receptors are inhibitory G protein-coupled receptors that are commonly associated with presynaptic inhibition of transmitter release in the central nervous system. In the brain-gastroesophageal axis, a role has recently been demonstrated for GABA(B) receptors on extrinsic afferent endings within the stomach and esophagus, where they reduce mechanosensitivity. This action is compounded by inhibition of communication centrally from these afferents in the brain stem and within central circuits. There is a final peripheral action on the motor pathway where GABA(B) receptors reduce output of acetylcholine from vagal preganglionic motoneurons. These potent, multiple actions of GABA(B) receptors may have therapeutic benefit by reducing the triggering of transient lower esophageal relaxations, which are the major cause of gastroesophageal reflux. An important clinical application is therefore emerging for this recent discovery.

Mechanism of action of baclofen in rat dorsal motor nucleus of the vagus

Using whole cell patch-clamp recordings, we investigated the effects of the GABA(B) receptor agonist baclofen in thin slices of rat brain stem containing identified gastric- or intestinal-projecting dorsal motor nucleus of the vagus (DMV) neurons. Perfusion with baclofen (0.1-100 microM) induced a concentration-dependent outward current (EC(50), 3 microM) in 54% of DMV neurons with no apparent differences between gastric- and intestinal-projecting neurons. The outward current was attenuated by pretreatment with the selective GABA(B) antagonists saclofen and 2-hydroxysaclofen, but not by the synaptic blocker TTX, indicating a direct effect at GABA(B) receptors on DMV neurons. Using the selective ion channel blockers barium, nifedipine, and apamin, we showed that the outward current was due to effects on potassium and calcium currents as well as calcium-dependent potassium currents. The calcium-mediated components of the outward current were more prominent in intestinal-projecting neurons than in gastric-projecting neurons. These data indicate that although baclofen inhibits both intestinal- and gastric-projecting neurons in the rat DMV, its mechanism of action differs among the neuronal subpopulations.

mu-opioid receptors are present in vagal afferents and their dendritic targets in the medial nucleus tractus solitarius

Ligands of the mu-opiate receptor (MOR) are known to influence many functions that involve vagal afferent input to the nucleus tractus solitarius (NTS), including cardiopulmonary responses, gastrointestinal activity, and cortical arousal. The current study sought to determine whether a cellular substrate exists for direct modulation of vagal afferents and/or their neuronal targets in the NTS by ligands of the MOR. Anterograde tracing of vagal afferents arising from the nodose ganglion was achieved with biotinylated dextran amine (BDA), and the MOR was detected by using antipeptide MOR antiserum. The medial subdivision of the intermediate NTS was examined by electron microscopy for the presence of peroxidase-labeled, BDA-containing vagal afferents and immunogold MOR labeling. MOR was present in both presynaptic axon terminals and at postsynaptic sites, primarily dendrites. In dendrites, MOR immunogold particles usually were located along extrasynaptic portions of the plasma membrane. Of 173 observed BDA-labeled vagal afferent axon terminals, 33% contained immunogold labeling for MOR within the axon terminal. Many of these BDA-labeled terminals formed asymmetric, excitatory-type synapses with dendrites, some of which contained MOR immunogold labeling. MORs were present in 19% of the dendrites contacted by BDA-labeled terminals but were present rarely in both the vagal afferent and its dendritic target. Together, these results suggest that MOR ligands modulate either the presynaptic release from or the postsynaptic responses to largely separate populations of vagal afferents in the intermediate NTS. These results provide a cellular substrate for direct actions of MOR ligands on primary visceral afferents and their second-order neuronal targets in NTS.

Activation of the GABA(B) receptor inhibits transient lower esophageal sphincter relaxations in dogs

BACKGROUND & AIMS

Transient lower esophageal sphincter relaxation (TLESR) appears to be the most frequent motor event responsible for gastroesophageal reflux. Because TLESRs are considered to be triggered by activation of gastric mechanoreceptors, and because the gamma-aminobutyric acid type B (GABA(B))-receptor agonist baclofen is known to inhibit transmitter release from mechanosensitive afferents, the effects of baclofen on TLESRs in the dog were assessed.

METHODS

A total of 183 recordings of the pharyngeal, esophageal, lower esophageal sphincter, and gastric pressures as well as measurement of esophageal pH were performed in 15 awake dogs. Racemic baclofen, its enantiomers, and the GABA(B)-receptor antagonist CGP36742 were administered before stimulation of TLESRs by a liquid meal and air insufflation. The pharmacodynamics of baclofen were compared with its pharmacokinetics.

RESULTS

Baclofen dose-dependently inhibited TLESRs, with a 50% effective dose (ED(50)) of 1.0 micromol/kg after intravenous administration. The maximal inhibition amounted to approximately 80%. Intragastric baclofen was almost equally effective (ED(50), 1.8 micromol/kg), compatible with the complete oral availability of the drug (100%). The inhibitory effect of baclofen resided in the pharmacologically active R enantiomer, and CGP36742 reduced some of the effects of baclofen.

CONCLUSIONS

Baclofen is a potent and efficacious inhibitor of TLESRs and reflux in the dog. Activation of the GABA(B) receptor may be a new approach to the treatment of reflux disease.

Opioid modulation of calcium current in cultured sensory neurons: mu-modulation of baroreceptor input

We used the whole cell open-patch or perforated-patch technique to characterize mu-opioid modulation of Ca(2+) current (I(Ca)) in nodose sensory neurons and in a specific subpopulation of nodose cells, aortic baroreceptor neurons. The mu-opiate receptor agonist Tyr-D-Ala-Gly-MePhe-Gly-ol enkephalin (DAGO) inhibited I(Ca) in 95% of neonatal [postnatal day (P)1-P3] nodose neurons. To the contrary, only 64% of juvenile cells (P20-P35) and 61% of adult cells (P60-P110) responded to DAGO. DAGO-mediated inhibition of I(Ca) was naloxone sensitive, irreversible in the presence of guanosine 5'-O-(3-thiotriphosphate), absent with guanosine 5'-O-(2-thiodiphosphate), and eliminated with pertussis toxin; DAGO's inhibition of I(Ca) was G protein mediated. Incubation of neurons with omega-conotoxin GVIA eliminated the effect of DAGO in neonatal but not in juvenile cells. In the latter, DAGO reduced 37% of the current remaining in the presence of omega-conotoxin. In the subset of nodose neurons, aortic baroafferents, the effect of DAGO was concentration dependent, with an IC(50) of 1.82 x 10(-8) M. DAGO slowed activation of I(Ca), but activation curves constructed from tail currents were the same with and without DAGO (100 nM). In summary, mu-opiate modulation of I(Ca) in nodose neurons was demonstrated in three age groups, including specifically labeled baroafferents. The demonstration of a mechanism of action of mu-opioids on baroreceptor afferents provides a basis for the attenuation of the baroreflex that occurs at the level of the nucleus tractus solitarii.

Interactions between cholecystokinin and opioids in the isolated guinea-pig ileum

1. Although cholecystokinin octapeptide sulphate (CCK-8) activates the opioid system of isolated guinea-pig ileum (GPI) whether it activates the mu- or kappa-system, or both, remains unclear. Neither is it known whether CCK-8 influences the withdrawal responses in GPI preparations briefly exposed to opioid agonists. This study was designed to clarify whether CCK-8 activates mu- or kappa-opioid systems or both; and to investigate its effect on the withdrawal contractures in GPI exposed to mu- or kappa-agonists and on the development of tolerance to the withdrawal response. 2. In GPI exposed to CCK-8, the selective kappa-antagonist nor-binaltorphimine elicited contractile responses that were concentration-related to CCK-8 whereas the selective mu-antagonist cyprodime did not. 3. In GPI preparations briefly exposed to the selective mu-agonist, dermorphin, or the selective kappa-agonist, U-50, 488H, and then challenged with naloxone, CCK-8 strongly enhanced the withdrawal contractures. 4. During repeated opioid agonist/CCK-8/opioid antagonist tests tolerance to opioid-induced withdrawal responses did not develop. 5. These results show that CCK-8 preferentially activates the GPI kappa-opioid system and antagonizes the mechanism(s) that control the expression of acute dependence in the GPI.