Morphine enhances and depresses Ca2+-dependent responses in visceral primary afferent neurons

Activation of opioid μ-receptors, but not δ- or κ-receptors, switches pulmonary C-fiber-mediated rapid shallow breathing into an apnea in anesthetized rats

Rapid shallow breathing (RSB) is mainly mediated by bronchopulmonary C-fibers (PCFs). We asked whether this RSB could be modulated by opioids. In anesthetized rats right atrial bolus injection of phenylbiguanide (PBG) to evoke RSB was repeated after: (1) intravenously giving fentanyl (μ-receptor agonist), DPDPE (δ-receptor agonist), or U-50488H (κ-receptor agonist); (2) fentanyl (iv) following naloxone methiodide, a peripheral opioid receptor antagonist; (3) bilateral microinjection of fentanyl into the nodose ganglia; (4) fentanyl (iv) with pre-blocking histamine H(1) and H(2) receptors by diphenhydramine and ranitidine. Systemic fentanyl challenge, but not DPDPE or U-50488H, switched the PBG-induced RSB to a long lasting apnea. This switch was blocked by naloxone methiodide rather than diphenhydramine and ranitidine. After microinjecting fentanyl into the nodose ganglia, PBG also produced an apnea. Our results suggest that activating μ-receptors is capable of turning the PCF-mediated RSB into an apnea, at least partly, via facilitating PCFs' activity and this switching effect appears independent of the released histamine.

Is withdrawal hyperalgesia in morphine-dependent mice a direct effect of a low concentration of the residual drug?

Withdrawal of opioid drugs leads to a cluster of unpleasant symptoms in dependent subjects. These symptoms are stimulatory in nature and oppose the acute, inhibitory effects of opiates. The conventional theory that explains the opioid withdrawal syndrome assumes that chronic usage of opioid drugs activates compensatory mechanisms whose stimulatory effects are revealed upon elimination of the inhibitory opioid drug from the body. Based on previous studies that show a dose-dependent dual activity of opiates, including pain perception, we present here an alternative explanation to the phenomenon of withdrawal-induced hyperalgesia. According to this explanation, the residual low concentration of the drug that remains after cessation of its administration elicits the stimulatory withdrawal hyperalgesia. The goal of the present study was to test this hypothesis. In the present study we rendered mice dependent on morphine by a daily administration of the drug. Cessation of morphine application elicited withdrawal hyperalgesia that was completely blocked by a high dose of the opiate antagonist naloxone (100 mg/kg). Similarly, naloxone (2 mg/kg)-induced withdrawal hyperalgesia was also blocked by 100 mg/kg of naloxone. The blockage of withdrawal hyperalgesia by naloxone suggested the involvement of opioid receptors in the phenomenon and indicated that withdrawal hyperalgesia is a direct effect of a residual, low concentration of morphine. Acute experiments that show morphine- and naloxone-induced hyperalgesia further verified our hypothesis. Our findings offer a novel, alternative approach to opiate detoxifications that may prevent withdrawal symptoms by a complete blockage of the opioid receptors using a high dose of the opioid antagonist.

Opiates: biphasic dose responses

It was shown that biphasic responses are commonly reported for opiates with respect to a broad range of animal models and endpoints. These endpoints include such diverse functions as blood pressure, muscle tension, breathing rates, hCG production, HIV production, neutrophil migration, ACTH production, protein binding, and neuronal functioning. Quantitative features of the dose-response relationships indicated that the maximum stimulatory responses were < or = 3-fold greater than the controls with most being between 10 to 70% greater than the controls. In contrast to the striking similarity in the maximum stimulatory response, there was marked variation with respect to the dose range of the stimulatory responses that varied from 10(1) to 10(10). Mechanistic assessments were conducted for most biphasic dose-response relationships and are addressed in detail.

Opioid tolerance and the emergence of new opioid receptor-coupled signaling

Multiple cellular adaptations are elicited by chronic exposure to opioids. These include diminution of spare opioid receptors, decreased opioid receptor density, and G-protein content and coupling thereof. All imply that opioid tolefance is a manifestation of a loss of opioid function, i.e., desensitization. Recent observations challenge the exclusiveness of this formulation and indicate that opioid tolerance also results from qualitative changes in opioid signaling. In this article, Gintzler and Chakrabarti discuss the evidence that suggests that opioid tolerance results not only from impaired opioid receptor functionality, but also from altered consequences of coupling. Underlying the latter are fundamental changes in the nature of effectors that are coupled to the opioid receptor/G-protein signaling pathway. These molecular changes include the upregulation of adenylyl cyclase isoforms of the type II family as well as a substantial increase in their phosphorylation state. As a result, there is a shift in opioid receptor/G-protein signaling from predominantly Gialpha inhibitory to Gbetagamma stimulatory following chronic in vivo morphine exposure. These adaptations to chronic morphine indicate the plasticity of opioid-signal transduction mechanisms and the ability of chronic morphine to augment new signaling strategies.

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.

Delta-opioid receptors on astroglial cells in primary culture: mobilization of intracellular free calcium via a pertussis sensitive G protein

Astrocytes in primary culture from rat cerebral cortex were probed concerning the expression of delta-opioid receptors and their coupling to changes in intracellular free calcium concentrations ([Ca2+]i). Fluo-3 or fura-2 based microspectrofluorometry was used for [Ca2+]i measurements on single astrocytes in a mixed astroglial-neuronal culture. Application of the selective delta-opioid receptor agonist, [D-Pen2, D-Pen5]-enkephalin (DPDPE), at concentrations ranging from 10 nM to 100 microM, induced concentration-dependent increases in [Ca2+]i (EC50 = 114 nM). The responses could be divided into two phases, with an initial spike in [Ca2+]i followed by either oscillations or a sustained elevation of [Ca2+]i. These effects were blocked by the selective delta-opioid receptor antagonist ICI 174864 (10 microM). The expression of delta-opioid receptors on astroglial cells was further verified immunohistochemically, using specific antibodies, and by Western blot analyses. Pre-treatment of the cells with pertussis toxin (100 ng/ml, 24 h) blocked the effects of delta-opioid receptor activation, consistent with a Gi- or Go-mediated response. The sustained elevation of [Ca2+]i was not observed in low extracellular Ca2+ and was partly blocked by nifedipine (1 microM), indicating the involvement of L-type Ca2+ channels. Stimulating neurons with DPDPE resulted in a decrease in [Ca2+]i, which may be consistent with the closure of the plasma membrane Ca2+ channels on these cells. The current results suggest a role for astrocytes in the response of the brain to delta-opioid peptides and that these opioid effects in part involve altered astrocytic intracellular Ca2+ homeostasis.

Stimulatory effects of opioids on transmitter release and possible cellular mechanisms: overview and original results

Opiates and opioid peptides carry out their regulatory effects mainly by inhibiting neuronal activity. At the cellular level, opioids block voltage-dependent calcium channels, activate potassium channels and inhibit adenylate cyclase, thus reducing neurotransmitter release. An increasing body of evidence indicates an additional opposite, stimulatory activity of opioids. The present review summarizes the potentiating effects of opioids on transmitter release and the possible cellular events underlying this potentiation: elevation of cytosolic calcium level (by either activating Ca2+ influx or mobilizing intracellular stores), blockage of K+ channels and stimulation of adenylate cyclase. Biochemical, pharmacological and molecular biology studies suggest several molecular mechanisms of the bimodal activity of opioids, including the coupling of opioid receptors to various GTP-binding proteins, the involvement of different subunits of these proteins, and the activation of several intracellular signal transduction pathways. Among the many experimental preparations used to study the bimodal opioid activity, the SK-N-SH neuroblastoma cell line is presented here as a suitable model for studying the complete chain of events leading from binding to receptors down to regulation of transmitter release, and for elucidating the molecular mechanism involved in the stimulatory effects of opioid agonists.

Simultaneous activation of spinal antiopioid system (neuropeptide FF) and pain facilitatory circuitry by stimulation of opioid receptors in rats

Neuropeptide FF (NPFF) is a mammalian FMRFamide-like octapeptide with antiopioid properties that inhibits morphine-induced analgesia but also produces hyperalgesia. In the present study, a series of three experiments was carried out to investigate the interactions between opioid receptor stimulation and antiopioid systems. First, by using in vitro superfusion system with rat spinal cord slices, we showed that morphine stimulated NPFF release in a dose-dependent manner. The stimulating effect which was observed with morphine concentrations as low as 100 fM reached a maximum at 0.1 nM, then decreased and was ineffective at 10 microM. The morphine-induced release of NPFF was abolished by naloxone (1 microM) but unaltered by tetrodotoxin. Second, by an in vivo approach, we showed that a single heroin administration (2.5 mg/kg, s.c.) elicited in 30 min a drastic drop (38%) in spinal NPFF content. In a third experiment, we evaluated the capacity of naloxone in revealing an antiopioid component associated with opioid receptor stimulation. The administration of naloxone (1 mg/kg, s.c..) 25 min following that of heroin (2.5 mg/kg, s.c.) not only abolished the heroin-induced increase of tail-flick latency, but also lowered it under the basal value by 30%. These results indicate that opioid receptor stimulation activates both pain inhibitory and pain facilitatory systems in which NPFF may play a significant role and that opiate-induced analgesia is always partly masked.

Dual regulation by mu, delta and kappa opioid receptor agonists of K+ conductance of DRG neurons and neuroblastoma X DRG neuron hybrid F11 cells

The effects of the mu opioid receptor agonists, morphine and Tyr-D-Ala-Gly-N-methyl-Phe-Gly-ol (DAGO), the delta opioid receptor agonist, Tyr-D-Pen-Gly-Phe-D-penicillamine (DPDPE) and the kappa-opioid receptor agonist, dynorphin A-(1-13) on the whole-cell K+ currents (IK) of cultured mouse DRG neurons and neuroblastoma X DRG neuron hybrid F11 cells were studied. These opioid ligands all elicited dual effects. Low concentrations (< nM) usually elicited a transient increase in IK (within 1 min), followed by a sustained decrease in IK. In contrast, microM concentrations rapidly elicited a sustained increase in IK. After brief treatment with cholera toxin subunit B (CTX-B), the usual sustained decrease in IK evoked by < nM opioid agonists no longer occurred. Low concentrations then elicited only a sustained increase in IK. On the other hand, after chronic treatment with pertussis toxin (PTX), the usual microM opioid-induced increases in IK no longer occurred and more than half of the cells responded with a sustained decrease of IK to microM as well as nM opioids. The results suggest that mu, delta and kappa opioid receptors are each coupled to K+ channels through CTX-B- and PTX-sensitive transduction systems. Both systems have similar threshold concentrations to opioids. Activation of the CTX-B-sensitive opioid receptor/transduction system resulted in a decrease in K+ conductance of the cell which is generally associated with an increase in neuronal excitability. Activation of the other system resulted in an increase in K+ conductance which will, in general, decrease neuronal excitability. The net change in the IK depends upon which effect predominates. The dominance at different opioid concentrations may depend on the relative efficacies of the coupling of these two systems to K+ channels.

Mu-opioids activate phospholipase C in SH-SY5Y human neuroblastoma cells via calcium-channel opening

We have recently reported that, in SH-SY5Y cells, mu-opioid receptor occupancy activates phospholipase C via a pertussis toxin-sensitive G-protein. In the present study we have further characterized the mechanisms involved in this process. Fentanyl (0.1 microM) caused a monophasic increase in inositol 1,4,5-trisphosphate mass formation, with a peak (20.5 +/- 3.6 pmol/mg of protein) at 15 s. Incubation in Ca(2+)-free buffer abolished this response, while Ca2+ replacement 1 min later restored the stimulation of inositol 1,4,5-trisphosphate formation (20.1 +/- 0.6 pmol/mg of protein). In addition, nifedipine (1 nM-0.1 mM), an L-type Ca(2+)-channel antagonist, caused a dose-dependent inhibition of inositol 1,4,5-trisphosphate formation, with an IC50 of 60.3 +/- 1.1 nM. Elevation of endogenous beta/gamma subunits by selective activation of delta-opioid and alpha 2 adrenoceptors failed to stimulate phospholipase C. Fentanyl also caused a dose-dependent (EC50 of 16.2 +/- 1.0 nM), additive enhancement of carbachol-induced inositol 1,4,5-trisphosphate formation. In summary, we have demonstrated that in SH-SY5Y cells activation of the mu-opioid receptor allows Ca2+ influx to activate phospholipase C. However, the possible role of this mechanism in the process of analgesia remains to be elucidated.

mu-Opioid receptor stimulation of inositol (1,4,5)trisphosphate formation via a pertussis toxin-sensitive G protein

The cellular mechanisms underlying opioid action remain to be fully determined, although there is now growing indirect evidence that some opioid receptors may be coupled to phospholipase C. Using SH-SY5Y human neuroblastoma cells (expressing both mu- and delta-opioid receptors), we demonstrated that fentanyl, a mu-preferring opioid, caused a dose-dependent (EC50 = 16 nM) monophasic increase in inositol (1,4,5)trisphosphate mass formation that peaked at 15 s and returned to basal within 1-2 min. This response was of similar magnitude (25.4 +/- 0.8 pmol/mg of protein for 0.1 microM fentanyl) to that found in the plateau phase (5 min) following stimulation with 1 mM carbachol (18.3 +/- 1.4 pmol/mg of protein), and was naloxone-, but not naltrindole- (a delta antagonist), reversible. Further studies using [D-Ala2, MePhe4, Gly(ol)5]enkephalin and [D-Pen2,5]enkephalin confirmed that the response was specific for the mu receptor. Incubation with Ni2+ (2.5 mM) or in Ca(2+)-free buffer abolished the response, as did pretreatment (100 ng/ml for 24 h) with pertussis toxin (control plus 0.1 microM fentanyl, 26.9 +/- 1.5 pmol/mg of protein; pertussis-treated plus 0.1 microM fentanyl, 5.1 +/- 1.3 pmol/mg of protein). In summary, we have demonstrated a mu-opioid receptor-mediated activation of phospholipase C, via a pertussis toxin-sensitive G protein, that is Ca(2+)-dependent. This stimulatory effect of opioids on phospholipase C, and the potential inositol (1,4,5)trisphosphate-mediated rises in intracellular Ca2+, could play a part in the cellular mechanisms of opioid action.

Stimulus frequency and intensity: critical determinants of opioid enhancement or inhibition of evoked methionine-enkephalin release

Responses to opioids can be bimodal depending on the concentration of opioid used. For example, low concentrations (nM) enhance whereas higher concentrations (10-100 nM) inhibit the electrically evoked release of enkephalin from the myenteric plexus. The nature of the responsiveness of the enkephalin release process to opioids is also dependent on the intracellular and/or extracellular milieu of enkephalin-containing neurons or other neurons of this plexus. Intracellular levels of cAMP, availability of pertussis toxin- and cholera toxin-sensitive guanine nucleotide binding proteins and intracellular calcium concentration have all been shown to be important determinants of opioid excitatory versus inhibitory actions. The present data indicate that the inhibition of enkephalin release produced by U50,488H or sufentanil is no longer observed when the applied voltage is increased 3- or 2-fold, respectively. Under this condition, a previously inhibitory concentration of opioid produces an enhancement of stimulated enkephalin release. Increasing the frequency of the applied stimulation from 5 to 60 Hz (at a constant voltage) also reverses the sufentanil-induced inhibition to a facilitation of enkephalin release. These data indicate that the intensity (voltage) or frequency of the stimulation that is used to release enkephalin is a critical determinant of the nature of its modulation by opioids. The possible relevance of these findings to known differences in opioid sensitivity between different types of pain is discussed.

Brief treatment of sensory ganglion neurons with GM1 ganglioside enhances the efficacy of opioid excitatory effects on the action potential

In previous studies, we showed that low (nM) concentrations of opioid prolong the action potential duration (APD) of many mouse dorsal root ganglion (DRG) neurons via Gs-linked excitatory opioid receptors, whereas micromolar opioid levels shorten the APD via Gi/Go-linked inhibitory receptors. In addition, cholera toxin-B subunit (CTX-B) selectively blocks opioid- but not forskolin-induced prolongation of the APD in DRG neurons. Since CTX-B binds with selective high affinity to GM1 ganglioside located on the cell surface, the results suggest that GM1 plays an essential role in regulating excitatory opioid receptor functions. This hypothesis was tested by treating DRG neurons in mouse DRG-cord explants with exogenous gangliosides and determining whether the efficacy of opioid agonists in prolonging the APD is enhanced. The threshold concentration of the opioids, dynorphin(1-13) and morphine required to prolong the APD in many DRG neurons was markedly decreased from nM to fM levels after bath exposure to 10 nM to 1 microM GM1 ganglioside for less than 5 min. In contrast, GM2 and GM3 gangliosides and asialo-GM1 ganglioside were ineffective, even when DRG neurons were exposed to high concentrations (1-10 microM) for periods greater than 1 h. Although GD1a, GD1b and GQ1b gangliosides appeared to be as effective as GM1 when tested at microM concentrations for 15 min, tests at lower concentrations, shorter periods, and/or at lower temperature (24 degrees vs 34 degrees C), showed that they were significantly less effective than GM1.(ABSTRACT TRUNCATED AT 250 WORDS)

Different G proteins mediate the opioid inhibition or enhancement of evoked [5-methionine]enkephalin release

This laboratory has previously demonstrated that there is an opiate receptor-mediated, concentration-dependent modulation of the electrically stimulated release of enkephalin from the guinea pig myenteric plexus. Low doses of opioids (nanomolar) enhance release, whereas higher concentrations (10-100 nM) inhibit release. We now demonstrate that the in vivo i.p. administration of the islet-activating protein from pertussis toxin (PTX; 50 micrograms/500 g of body weight) markedly diminishes the potency of mu, delta, or kappa-selective opioids to inhibit the evoked release of enkephalin. In contrast, PTX is without effect on the enhancement of enkephalin release observed after treatment with nanomolar concentrations of the above opioids. Conversely, pretreatment with cholera toxin (CTX; 0.01 nM for 3 hr in vitro) has no effect on the mu, delta, or kappa opioid inhibition of evoked enkephalin release but abolishes the ability of nanomolar concentrations of these agonists to enhance stimulated enkephalin release. These data indicate that different classes of guanine nucleotide-binding proteins (G proteins) appear to mediate the opioid enhancement or inhibition of stimulated enkephalin release. Furthermore, they suggest that a PTX-sensitive G protein (Gi or Go) and a CTX-sensitive G protein (Gs) are integral components of the mechanism that mediates opioid inhibition and opioid enhancement, respectively, of evoked enkephalin release. To our knowledge, this report represents the first demonstration that Gs-coupled opiate receptors (in addition to those that are coupled to Gi) can modulate transmitter release.

Cholera toxin-A subunit blocks opioid excitatory effects on sensory neuron action potentials indicating mediation by Gs-linked opioid receptors

Our previous studies indicated that opioid-induced prolongation of the Ca2+ component of the action potential duration (APD) in dorsal root ganglion (DRG) neurons is mediated by excitatory opioid receptors that are coupled to cyclic AMP-dependent voltage-sensitive ionic conductances. In the present study, DRG neurons were treated with cholera toxin (CTX), or with the A subunit of CTX, in order to determine if these excitatory opioid receptors are positively coupled via the GTP-binding protein Gs to the adenylate cyclase/cyclic AMP system. In contrast, inhibitory opioid receptors have been shown to be linked to pertussis toxin-sensitive Gi/Go regulatory proteins that mediate APD shortening responses. After pretreatment of DRG-spinal cord explants with remarkably low concentrations of CTX-A (1 pg/ml-1 ng/ml; greater than 15 min) or whole toxin (1 pg/ml-1 microgram/ml) the APD prolongation elicited in DRG neurons by 1-10 nM delta/mu (DADLE) or kappa (U-50,488H) opioids was blocked (29 out of 30 cells), whereas APD shortening by microM opioid concentrations was unaffected. Opioid-induced APD prolongation was blocked even when the initial treatment with CTX or CTX-A alone did not prolong the APD. The blocking effects of CTX and CTX-A were reversed in tests made 2 h after return to control medium. The mechanisms underlying the unusually potent blocking effects of CTX and CTX-A on opioid excitatory modulation of the APD of DRG neurons require correlative biochemical analyses.(ABSTRACT TRUNCATED AT 250 WORDS)

Opioids can evoke direct receptor-mediated excitatory effects on sensory neurons

Activation of opioid receptors has generally been considered to produce inhibitory effects on neuronal activity. However, recent studies indicate that specific mu-, delta- and kappa-opioid receptor agonists can elicit excitatory, as well as inhibitory, modulation of the action potentials of sensory neurons isolated in culture. Stanley Crain and Ke-Fei Shen review the evidence for mediation of these direct excitatory effects by naloxone-reversible opioid receptors. They propose that this dual modulatory mechanism may help to account for previously unexplained enhancement by opioids of transmitter release, paradoxical hyperalgesic and aversive effects of opioids, and some aspects of opioid tolerance and addiction.

Opioids can enhance and inhibit the electrically evoked release of methionine-enkephalin

The stimulated (40 Hz) release of enkephalin from the myenteric plexus can be modulated by multiple types of opiate receptor. The direction of the modulation is not fixed but is bimodal. Both an inhibition and an enhancement of evoked release is observed depending on the concentration of opioid agonist that is used. Each of these effects can be antagonized by naloxone. Following pretreatment of guinea pig myenteric plexus in vitro with forskolin (0.5 microM) or the lipid soluble cAMP analog 8-(4-chlorphenylthio)-cAMP (8-CPT-cAMP; 100 microM) the inhibition of stimulated Met-enkephalin release that is produced by sufentanil (10(-8) M), [D-Pen2-D-Pen5]enkephalin (DPDPE, 10(-8) M) or dynorphin (10(-7) M) is no longer observed. On the contrary, in forskolin- or 8-CPT-cAMP treated myenteric plexus a previously inhibitory concentration of the above opioids now produces an enhancement of the magnitude of the stimulated Met-enkephalin release. Excitatory responses (enhanced release) to lower concentrations of sufentanil (1 nM) or DPDPE (5 nM) are not affected by pretreatment with the same concentration of forskolin or 8-CPT-cAMP. These data suggest that the ability of opioids to enhance or inhibit evoked enkephalin release is mediated via different biochemical processes (separate second messenger systems). This could imply that the opioid enhancement of enkephalin release is due to a direct facilitation of release and not to disinhibition. The ability of opioids to enhance the release of at least some neurotransmitters should be taken into account when attempting to explain the physiological sequelae of the acute and chronic effects of narcotics.

Delta-opioid mediated inhibitions of acute and prolonged noxious-evoked responses in rat dorsal horn neurones

1. The effects of a selective delta-opioid agonist Tyr-D-Ser(Otbu)-Gly-Phe-Leu-Thr (DSTBULET) were examined on the C- and A beta-evoked responses of convergent dorsal horn neurones in the halothane anaesthetized, intact rat. 2. Intrathecal DSTBULET produced selective dose-dependent inhibitions of electrically-evoked C fibre responses of both superficial and deep neurones. A near-complete inhibition of 83 +/- 5% followed 100 micrograms of DSTBULET and the ED50 was 9 micrograms (13.5 nmol). Inhibitions were antagonised by intrathecal naloxone and ICI 174,864 but were not antagonised by pretreatment with intrathecal beta-funaltrexamine at a dose that blocked mu-opioid effects. By contrast, DSTBULET produced excitations of electrically-evoked responses of cells recorded in a zone intermediate between the superficial and deep neurones. 3. DSTBULET (50 micrograms) was also tested on the more prolonged noxious neuronal response produced by subcutaneous formalin (5%, 50 microliters) into the receptive field. DSTBULET profoundly inhibited the response to formalin. Pretreatment with ICI 174,864 before DSTBULET antagonised the effects of the delta-agonist on the formalin response. 4. The full peptidase inhibitor kelatorphan, known to protect endogenous enkephalins, was also tested on the formalin response. The intrathecal administration of 50 micrograms kelatorphan has previously been shown to inhibit electrically-evoked C fibre resonses of dorsal horn neurones and to be antagonised by ICI 174,864. The same dose of kelatorphan inhibited the formalin response in the present study. 5. From this study it appears that the delta-opioid agonist DSTBULET can produce profound inhibitions of the responses of convergent neurones to nociceptive afferent inputs. Furthermore, activation of delta-opioid receptors either by DSTBULET, or by protection of endogenous enkephalins with kelatorphan, can inhibit a more prolonged chemically-evoked nociceptive input onto these dorsal horn neurones.

Dual opioid modulation of the action potential duration of mouse dorsal root ganglion neurons in culture

Multiple modulatory effects of opioids on the duration of the calcium component of the action potential (APD) of dorsal-root ganglion (DRG) neurons of mouse spinal cord-ganglion explants were studied. The APD of DRG neuron perikarya has been previously shown to be shortened by exposure to high concentrations of opioids (ca. 0.1-1 microM) in about 1/2 of the cells tested. The present study demonstrates that in addition to these inhibitory modulatory effects of opioids, lower concentrations (1-10 nM) of present study demonstrates that in addition to these inhibitory modulatory effects of opioids, lower concentration (1-10 nM) of delta- mu, and kappa-opioid agonists elicit excitatory modulatory effects, i.e. prolongation of the APD, in about 2/3 of the sensory neurons tested. APD prolongation as well as shortening elicited by delta, mu, and kappa agonists were prevented by coperfusion with the opioid antagonists, naloxone or diprenorphine (10 nM). APD prolongation induced by the delta-agonist [D-Ala2-D-Leu5]enkephalin (DADLE) was prevented in the presence of multiple K+ channel blockers, whereas excitatory modulation by the specific kappa-agonist, U-50,488H was not attenuated under these conditions. After treatment of DRG neurons with pertussis toxin (1 micrograms/ml for several days) or forskolin (50 muM for less than 15 min), a much smaller fraction of cells showed opioid-induced APD shortening; moreover, a much larger fraction of cells showed opioid-induced APD prolongation, even when tested with high concentrations of DADLE (1-10 muM). These data indicate that opioid-induced APD prolongation is not mediated by pertussis toxin-sensitive G proteins (which have been shown to regulate opioid inhibitory effects) and suggest that elevation of cyclic AMP levels may enhance opioid excitatory responsiveness. Furthermore, our analyses indicate that mu-, delta- and kappa-subtypes of excitatory as well as inhibitory opioid receptors may be expressed on the same DRG neuron perikaryon under in vitro conditions. If dual opioid modulation of the APD of DRG perikarya also occurs in central DRG terminals this may play a significant role both in nociceptive signal transmission as well as tolerance to opioid analgesia.

Calcium-dependent slow outward current in visceral primary afferent neurones of the rabbit

Slow outward currents were recorded from voltage-clamped neurones in nodose ganglia excised from rabbits. In the majority of Type C neurones, a short depolarizing command pulse evoked a slow outward tail current (ISAH) with a decay time constant ranging from 0.5 to 2 s. The ISAH was due to an increase in membrane conductance to K+ because its reversal potential was approximately equal to the Nernst potential for K+. The ISAH was reversibly blocked by removal of external Ca2+ or by Ca2+ antagonists. A Ca2+ ionophore, A23187, produced an outward current which was similar to the ISAH. The ISAH was resistant to tetraethylammonium and depressed by Ba2+, whereas it was not affected by Cs+ and 4-aminopyridine. The ISAH was initially augmented and subsequently depressed by apamin (1-10 nM) and (+)-tubocurarine (100-600 microM). It is concluded that the ISAH in visceral primary neurones may be due to a long-lasting increase in K+ conductance caused by an increase in the concentration of intracellular Ca2+, resulting from Ca2+ entry during the depolarizing command pulse.

The spinal antinociceptive actions of morphine metabolites morphine-6-glucuronide and normorphine in the rat

The profound and prolonged effects of morphine in patients with renal dysfunction have been associated with high plasma levels of the opiate metabolites morphine-6-glucuronide (M6G) and morphine-3-glucuronide (M3G) rather than an increased concentration of morphine. We present here electrophysiological evidence to suggest that potent spinal antinociception can be produced by both M6G and normorphine, another metabolite of morphine. Extracellular recordings of A beta- and C-fibre-evoked responses of convergent dorsal horn neurones were made in the halothane anaesthetised rat. M6G elicited dose-dependent, naloxone-reversible inhibitions of C-fibre-evoked responses which were completely suppressed (8% of control) by 2 micrograms M6G whereas A beta-fibre-evoked responses were only reduced to 57% of controls. The ED50 for the effects of M6G on C-fibre-evoked activity was calculated to be 0.53 micrograms. Systemic administration of M6G (2 mg/kg) also profoundly reduced noxious evoked neuronal activity. Intrathecal normorphine was less potent than M6G but complete selective inhibitions of C-fibre-evoked response could be elicited by 25 micrograms and the ED50 was calculated to be 2.68 micrograms. No such inhibitions were observed following administration of M3G. A comparison with intrathecal morphine in the same preparation reveals that normorphine is equipotent with morphine whereas M6G is 13-fold more potent. These results therefore confirm that M6G and normorphine might be significant contributers to opiate analgesia after administration of morphine.

Inhibitor of cyclic AMP-dependent protein kinase blocks opioid-induced prolongation of the action potential of mouse sensory ganglion neurons in dissociated cell cultures

The duration of the calcium component of the action potential (APD) of dorsal root ganglion (DRG) neurons in mouse spinal cord-ganglion explants has been shown to be dually modulated via excitatory and inhibitory opioid receptors. In order to determine if opioid-induced APD prolongation is modulated by receptors that are positively coupled to the adenylate cyclase (AC)/cyclic AMP second messenger system, whole-cell recordings were made from mouse DRG neurons grown in dissociated cell cultures. Tests for opioid responsivity were carried out after intracellular dialysis of an inhibitor of cAMP-dependent protein kinase (PKI). In control recordings, both DADLE-induced APD prolongation as well as shortening were prevented by co-perfusion with the opioid antagonist, diprenorphine (10 nM). Intracellular dialysis of PKI in these neurons completely blocked opioid-induced APD prolongation but did not attenuate APD shortening generally elicited by higher opioid concentrations. Bath perfusion of 10 nM DADLE elicited APD prolongation in 59% of the DRG neurons (n = 34) tested with control solution in the recording pipette, whereas none showed APD prolongation when the pipette contained PKI (n = 18). In control tests with 1 microM DADLE, the APD was prolonged in 37% of the cells and shortened in 26% (n = 19); in contrast, a matched group of PKI-treated cells showed no APD prolongation, whereas 42% showed APD shortening (n = 26). The results support the hypothesis that opioid-induced APD prolongation in DRG neurons is mediated by opioid receptor subtypes that are positively coupled via Gs to AC/cAMP-dependent voltage-sensitive ionic conductances.

Opioids excite rather than inhibit sensory neurons after chronic opioid exposure of spinal cord-ganglion cultures

Tests were carried out to determine if the tolerance that develops in dorsal-horn network responses of mouse dorsal root ganglion (DRG)-spinal cord explants after chronic exposure to opioids could be accounted for by alterations in the excitability and pharmacologic properties of the afferent DRG cells. Intracellular recordings were made from DRG neurons in organotypic DRG-cord explants after chronic treatment with 1 microM D-Ala2-D-Leu5-enkephalin (DADLE) for greater than 4 days in vitro. Acute application of 10 microM DADLE shortened the duration of the Ca2+ component of the somatic action potential (APD) in only 5% of the treated neurons (4 out of 79 cells), in contrast to about 50% of the cells in naive explants (36 out of 74). Thus many DRG neuron perikarya became tolerant to the APD-shortening effects of DADLE. Furthermore, 77% of the treated DRG cells (61 out of 79) showed prolongation of the APD in response to an acute increase in DADLE concentration vs 34% in naive explants (25 out of 74). However, when the DADLE responsivity tests were carried out in the presence of multiple K+ channel blockers, only 20% of the treated DRG neurons showed APD prolongation (3 out of 15 cells), whereas 73% showed APD-shortening responses (11 out of 15 cells). The results suggest that: (1) DADLE-induced APD prolongation of the treated DRG neurons is mediated by opioid receptor subtypes that decrease a voltage-sensitive K+ conductance; (2) the DADLE-induced APD-shortening effects which are unmasked during more complete K+ channel blockade are mediated by opioid-receptor subtypes in the same neuron that reduce a voltage-sensitive Ca2+ conductance (resembling kappa receptors). DRG neurons did not become tolerant to either of these two opioid effects after chronic exposure to DADLE. Opioid shortening of the APD of DRG neuron perikarya has been generally accepted to be a model of opioid inhibition of calcium influx and transmitter release at presynaptic DRG terminals6,52,53,65,75,76. It is postulated that the opioid-induced APD prolongation observed in the present study provides evidence that opioids can also evoke direct excitatory effects on neurons. The enhancement of DADLE-induced excitatory responses and attenuation of DADLE-induced inhibitory responses of DRG neurons after chronic exposure to this opioid show striking similarities to the effects of forskolin or pertussis toxin treatment. These in vitro studies may provide clues to compensatory mechanisms underlying physiologic expression of tolerance to opioid analgesic effects in primary afferent synaptic networks.

Methionine enkephalin presynaptically facilitates and inhibits bullfrog sympathetic ganglionic transmission

The effects of [Met5]enkephalin (ME) on the fast excitatory postsynaptic potential (EPSP) in bullfrog sympathetic ganglion cells were studied with intracellular recording in vitro. The variance and failure methods were used to calculate quantal content and quantal size of the fast EPSP in a low Ca2+/high Mg2+ solution. High concentrations of ME (1 and 10 microM) decreased the amplitude and the mean quantal content of the fast EPSP, whereas low concentrations (10 pM to 10 nM) of the peptide increased EPSP amplitude and quantal content. The mean quantal size of the EPSP was not changed by ME. The inhibitory effect of ME at high concentration (10 microM) was reversibly antagonized by the same concentration of naloxone; the facilitatory effect of ME at low concentration (1 nM) was not affected by 10 times higher concentration of naloxone (10 nM), but inhibited by 10 microM naloxone. A low concentration of naloxone (100 pM) itself increased the amplitude and the mean quantal content of the fast EPSP without changing the mean quantal size. The other concentrations of naloxone used in this study (1 pM to 10 microM) caused no significant change in the fast EPSP. ME (100 fM to 10 microM) and naloxone (1 pM to 10 microM) did not change the resting membrane potential or input resistance, the amplitude and duration of action potentials, and the sensitivity to the acetylcholine applied by iontophoresis. These results indicate that ME may act on preganglionic nerve terminals either to facilitate or to depress transmitter release; the inhibitory action is naloxone sensitive, whereas the facilitatory action is less sensitive to naloxone.

Ouabain augments calcium-dependent potassium conductance in visceral primary afferent neurones of the rabbit

1. The effects of ouabain (1 nM-100 microM) on the membrane properties of rabbit visceral primary afferent neurones (nodose ganglion cells) were studied with intracellular recordings and voltage-clamp techniques in vitro. 2. Ouabain (greater than or equal to 1 microM) often produced a membrane hyperpolarization associated with a fall of membrane resistance in type C neurones. The ouabain-induced hyperpolarization reversed in polarity at about -90 mV. These suggest that the ouabain-induced hyperpolarization is due to an increase in potassium conductance. 3. Both the peak amplitude and the duration of the after-hyperpolarization following an action potential were reversibly increased with increasing concentration of ouabain. In tetraethylammonium (TEA, 10-20 mM) and tetrodotoxin (TTX, 1-10 microM), the duration of both the calcium-dependent action potential and the after-hyperpolarization following the action potential was prolonged by ouabain (greater than or equal to 10 nM). 4. A depolarizing command pulse evoked a slow outward current in TEA (10-20 mM) and TTX (1-10 microM). This was increased in amplitude and prolonged in duration by ouabain (100 nM-1 microM). Such augmentation of the slow outward current by ouabain was usually associated with an increase in a slow inward current during the period of the depolarizing command pulse. 5. An outward current produced by the calcium ionophore A23187 was reversibly augmented by ouabain (greater than or equal to 10 nM). 6. An outward current caused by exchanging a potassium-free superfusion solution for one containing 4.7 or 10 mM-potassium was completely abolished by ouabain (greater than or equal to 10 nM). 7. The hyperpolarization elicited by intracellular injection of calcium was reversibly prolonged by either ouabain (1 microM) or caffeine (10 nM). 8. These results suggest that ouabain augments the after-hyperpolarization both by an increase in calcium influx across the cellular membrane and by an increase in intracellular calcium concentration.

Prostaglandins block a Ca2+-dependent slow spike afterhyperpolarization independent of effects on Ca2+ influx in visceral afferent neurons

The blockade of a slow Ca2+-activated K+-dependent afterhyperpolarization (AHPs) in rabbit visceral sensory neurons by the prostaglandins, PGE1 and PGD2, was investigated to determine whether the blockade was indirectly due to a reduction in Ca2+ influx. The prostaglandins (PGs) could block the AHPs in the absence of any change in Ca2+-dependent spikes elicited in the presence of tetrodotoxin and tetraethylammonium bromide. A PG-induced decrease in Ca2+-dependent spike width observed in some neurons was temporally dissociated from the PG-induced block of the AHPs. In addition, a slow afterhyperpolarization produced by the application of the Ca2+ ionophore, A23187, was blocked by the PGs. It is concluded that a reduction in Ca2+ influx is not responsible for the PG-induced blockade of the AHPs.

Two calcium-sensitive spike after-hyperpolarizations in visceral sensory neurones of the rabbit

Intracellular recordings were made from rabbit nodose neurones in vitro. Two temporally distinct spike after-hyperpolarizations (a.h.p.s) were identified in a subpopulation of C-type neurones. The fast a.h.p. after a single spike lasted no longer than 500 ms, while the slow a.h.p. persisted for seconds. Both a.h.p.s. were increased in amplitude in low K+ (0.56 mM) solutions and decreased in amplitude in high K+ (11.2 mM) solutions, and both were reversed at hyperpolarized membrane potentials. The slow a.h.p. was reduced in low Ca2+ (0.22 mM), in the presence of Ca2+ antagonists (Ni2+, 1 mM; Cd2+, 100 microM; or Co2+, 1 mM) and was enhanced in tetraethylammonium (5 mM). In approximately half of the cells tested, the fast a.h.p. was reduced in low Ca2+ and in the presence of the Ca2+ antagonists. In the remaining cells the fast a.h.p. was insensitive to these procedures. Prostaglandin (PGE1, 1-10 micrograms/ml) reduced the slow a.h.p. in all cells tested. Neither the Ca2+-sensitive nor the Ca2+-insensitive fast a.h.p. was affected by the prostaglandin. It is concluded that there is a subpopulation of C-type nodose neurones possessing a slow a.h.p. which is due to a Ca2+-dependent K+ current. This subpopulation of neurones can further be divided on the basis of the presence of a Ca2+-sensitive fast a.h.p. Furthermore, PGE1 pharmacologically separates the fast and slow a.h.p.s by selectively blocking the slow one. The blockage by the PGE1 is most probably not due to a reduction in Ca2+ influx.

Calcium-dependent after-potentials in visceral afferent neurones of the rabbit

Intracellular recordings were made from neurones in nodose ganglia excised from rabbits. In C neurones, 1-60 action potentials were followed by an after-hyperpolarization with a peak amplitude of 16 mV and a time constant of decay ranging from 3 to 10 s. In A neurones, the action potentials were followed only by a brief (up to 50 ms) after-hyperpolarization. The after-hyperpolarization was associated with an increase in the membrane conductance to potassium ions; it reversed polarity at the potassium equilibrium potential. The increase in conductance following the action potentials was blocked by removal of calcium ions, or addition of cobalt to the extracellular solution. Intracellular injection of ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) abolished the after-hyperpolarization; intracellular injection of calcium mimicked the after-hyperpolarization. It is concluded that calcium entry during the action potential leads to a long-lasting increase in potassium conductance in visceral afferent C neurones.

Generation of an unusual depolarizing response in rabbit primary afferent neurones in the absence of divalent cations

The effects of divalent cations on responses to 5-hydroxytryptamine (5-HT), gamma-aminobutyric acid (GABA) and 1,1-dimethyl-4-phenyl piperazinium (DMPP) were investigated using a sucrose-gap method to record population responses. In Ca-free medium responses to 5-HT were enhanced, those to DMPP depressed and those to GABA unchanged. In Mg-free medium responses to 5-HT were unchanged, while those to DMPP and GABA were depressed. Removal of both Ca and Mg from the superfusion medium caused a small reduction of GABA responses and a large reduction of DMPP responses. Responses to 5-HT were not only greatly potentiated but were changed in character; the depolarizing phase became sigmoid and the dose dependence between quantity of 5-HT and response magnitude was lost as if 5-HT were triggering an all-or-nothing phenomenon. Dose--response relationships for GABA were normal in the large majority of preparations. In about 10% of preparations, supramaximal amounts of GABA or DMPP evoked large responses of a similar character to those evoked by 5-HT. The large responses, generated by an unknown mechanism, were termed X responses. Further reduction in tissue divalent cations by EGTA (1 mM) caused X responses to be generated spontaneously. Ca, Mg, Mn or Co (1 mM) could suppress X responses. DMPP responses, reduced in Ca/Mg-free medium, were largely restored by 1 mM-Ca. Depression of GABA responses in Ca/Mg-free medium could be entirely attributed to the absence of Mg, Mn being able to substitute for Mg. X responses were generated only after equilibration for 1 h with Ca/Mg-free medium. Attempts to manipulate [Ca]i with dinitrophenol or caffeine did not produce the conditions under which X responses were generated. Intracellular records of responses to 5-HT, GABA or DMPP showed that cells with A fibres responded to GABA but not to 5-HT or DMPP. Fifty-four out of sixty-seven cells with C fibre axons (80%) were depolarized by 5-HT, thirty-seven out of forty-nine (76%) by DMPP and forty out of fifty-seven (70%) by GABA. Eighteen out of thirty-eight (47%) C cells were depolarized by all three agents. Some C cells were very sensitive to 5-HT, 10(-6) M evoking a substantial response. In most, responses to 10(-5) M-5-HT had a slower rate of rise than responses to 10(-4) or 10(-3) M-GABA or DMPP, yet lower 5-HT concentrations normally elicited X responses in sucrose-gap experiments whereas GABA or DMPP normally did not.(ABSTRACT TRUNCATED AT 400 WORDS)

Sites and mechanisms of actions of enkephalin in the feline parasympathetic ganglion

Intracellular recordings were made in vitro from neurones of the cat parasympathetic ciliary ganglion with a current- or voltage-clamp technique. (Met5)enkephalin and (leu5)enkephalin (10 nM to 10 microM) were applied by superfusion. Both caused a membrane hyperpolarization which persisted in a calcium-free/high-magnesium solution, and both reduced the amplitude of excitatory post-synaptic potentials (e.p.s.p.s). These actions of enkephalin were antagonized by naloxone. The enkephalin-induced hyperpolarization was associated with an increase in membrane conductance, reversed in polarity at -90 mV and was not altered by changing external sodium and chloride concentrations. This indicates that the enkephalin hyperpolarization is due mainly to activation of potassium conductance. Enkephalin decreased the mean quantal content of e.p.s.p.s recorded in low-calcium/high-magnesium solution, without changing quantal size. Furthermore, the increase in the frequency of miniature e.p.s.p.s after tetanic preganglionic stimulations was inhibited by enkephalin. Acetylcholine potentials were not altered by enkephalin. These findings suggest that enkephalin reduces transmitter release. The experiments suggest that enkephalin may inhibit ganglionic transmission by both pre- and post-synaptic actions in a mammalian parasympathetic ganglion.

Morphine augments calcium-dependent potassium conductance in guinea-pig myenteric neurones

Intracellular recordings were made from myenteric neurones removed from guinea-pig ileum and maintained in vitro. Action potentials were elicited by passing brief depolarizing currents through the recording electrode. In AH cells they were followed by afterhyperpolarizations resulting from an increase in potassium conductance (GK,Ca). Morphine (1 nM - 1 microM), applied by superfusion, increased the duration of the afterhyperpolarization (and the underlying GK,Ca) which followed from 1 to 30 action potentials. Morphine did not change the peak amplitude of the afterhyperpolarization. This action of morphine occurred both in cells which showed no change in resting membrane potential or resistance and in cells which were hyperpolarized. It was prevented by naloxone (10 nM - 1 microM). The possibility is proposed that morphine inhibits one of the mechanisms by which myenteric neurones control their free intracellular calcium concentration close to the plasma membrane.