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

Apneic response to fentanyl in adult rats: Role of laryngeal afferents

Intravenous (systemic) bolus injection of fentanyl (FNT) reportedly induces an immediate vagal-mediated apnea; however, the precise origin of vagal afferents responsible for this apnea remains unknown. We tested whether intralaryngeal (local) application of FNT would also trigger an apnea and whether the apneic response to both local and systemic administration of FNT was laryngeal afferent-mediated. Cardiorespiratory responses to FNT were recorded in anesthetized male adult rats with and without bilateral sectioning of the superior laryngeal nerve (SLNx) or peri-SLN capsaicin treatment (SLNcap) to block local C-fiber signal conduction. Opioid mu-receptor (MOR)-immunoreactivity was detected in laryngeal C- and myelinated neurons. We found that local and systemic administration of FNT elicited an immediate apnea. SLNx, rather than SLNcap, abolished the apneic response to local FNT application though MORs were abundantly expressed in both laryngeal C- and myelinated neurons. Importantly, SLNx failed to affect the apneic response to systemic FNT administration. These results lead to the conclusion that laryngeal afferents' MORs are responsible for the apneic response to local, but not systemic, administration of FNT.

Signaling diversity of mu- and delta- opioid receptor ligands: Re-evaluating the benefits of β-arrestin/G protein signaling bias

Opioid analgesics are elective for treating moderate to severe pain but their use is restricted by severe side effects. Signaling bias has been proposed as a viable means for improving this situation. To exploit this opportunity, continuous efforts are devoted to understand how ligand-specific modulations of receptor functions could mediate the different in vivo effects of opioids. Advances in the field have led to the development of biased agonists based on hypotheses that allocated desired and undesired effects to specific signaling pathways. However, the prevalent hypothesis associating β-arrestin to opioid side effects was recently challenged and multiple of the newly developed biased drugs may not display the superior side effects profile that was sought. Moreover, biased agonism at opioid receptors is now known to be time- and cell-dependent, which adds a new layer of complexity for bias estimation. Here, we first review the signaling mechanisms underlying desired and undesired effects of opioids. We then describe biased agonism at opioid receptors and discuss the different perspectives that support the desired and undesired effects of opioids in view of exploiting biased signaling for therapeutic purposes. Finally, we explore how signaling kinetics and cellular background can influence the magnitude and directionality of bias at those receptors.

Ventilation and neurochemical changes during µ-opioid receptor activation or blockade of excitatory receptors in the hypoglossal motor nucleus of goats

Neuromodulator interdependence posits that changes in one or more neuromodulators are compensated by changes in other modulators to maintain stability in the respiratory control network. Herein, we studied compensatory neuromodulation in the hypoglossal motor nucleus (HMN) after chronic implantation of microtubules unilaterally ( n = 5) or bilaterally ( n = 5) into the HMN. After recovery, receptor agonists or antagonists in mock cerebrospinal fluid (mCSF) were dialyzed during the awake and non-rapid eye movement (NREM) sleep states. During day studies, dialysis of the µ-opioid inhibitory receptor agonist [d-Ala², N-MePhe⁴, Gly-ol]enkephalin (DAMGO; 100 µM) decreased pulmonary ventilation (V̇i), breathing frequency ( f), and genioglossus (GG) muscle activity but did not alter neuromodulators measured in the effluent mCSF. However, neither unilateral dialysis of a broad spectrum muscarinic receptor antagonist (atropine; 50 mM) nor unilateral or bilateral dialysis of a mixture of excitatory receptor antagonists altered V̇i or GG activity, but all of these did increase HMN serotonin (5-HT) levels. Finally, during night studies, DAMGO and excitatory receptor antagonist decreased ventilatory variables during NREM sleep but not during wakefulness. These findings contrast with previous dialysis studies in the ventral respiratory column (VRC) where unilateral DAMGO or atropine dialysis had no effects on breathing and bilateral DAMGO or unilateral atropine increased V̇i and f and decreased GABA or increased 5-HT, respectively. Thus we conclude that the mechanisms of compensatory neuromodulation are less robust in the HMN than in the VRC under physiological conditions in adult goats, possibly because of site differences in the underlying mechanisms governing neuromodulator release and consequently neuronal activity, and/or responsiveness of receptors to compensatory neuromodulators. NEW & NOTEWORTHY Activation of inhibitory µ-opioid receptors in the hypoglossal motor nucleus decreased ventilation under physiological conditions and did not affect neurochemicals in effluent dialyzed mock cerebral spinal fluid. These findings contrast with studies in the ventral respiratory column where unilateral [d-Ala², N-MePhe⁴, Gly-ol]enkephalin (DAMGO) had no effects on ventilation and bilateral DAMGO or unilateral atropine increased ventilation and decreased GABA or increased serotonin, respectively. Our data support the hypothesis that mechanisms that govern local compensatory neuromodulation within the brain stem are site specific under physiological conditions.

Intrathecal PLC(β3) oligodeoxynucleotides antisense potentiates acute morphine efficacy and attenuates chronic morphine tolerance

Morphine is a mainstay for chronic pain treatment, but its efficacy has been hampered by physical tolerance. The underlying mechanism for chronic morphine induced tolerance is complicated and not well understood. PLC(β3) is regarded as an important factor in the morphine tolerance signal pathway. In this study, we determined intrathecal (i.t.) administration of an antisense oligodeoxynucleotide (ODN) of PLC(β3) could quicken the on-set antinociceptive efficacy of acute morphine treatment and prolong the maximum effect up to 4h. The antisense could also attenuate the development of morphine-induced tolerance and left shift the ED50 after 7 day of coadministration with morphine. These results probably were contributed by the PLC(β3) antisense ODN as they successfully knocked down protein expression levels and reduced activity of PLC(β3) in spinal cord in rats. The mismatch group had no such effects. The results confirmed the important involvement of PLC(β3) in both acute morphine efficacy and chronic morphine tolerance at spinal level in rats. This study may provide an idea for producing a novel adjuvant for morphine treatment.

Intrathecal PKA-selective siRNA treatment blocks sustained morphine-mediated pain sensitization and antinociceptive tolerance in rats

Sustained morphine treatment has been shown to produce paradoxical pain sensitization (opioid-induced hyperalgesia) and also causes increase in spinal pain neurotransmitter, such as calcitonin gene related peptide (CGRP), concentration in experimental animals. Studies have also shown that cyclic adenosine-monophosphate (cAMP)-dependent protein kinase (PKA) plays a major role in the regulation of presynaptic neurotransmitter (such as CGRP and substance P) synthesis and release. We have previously shown that in cultured primary sensory dorsal root ganglion (DRG) neurons sustained in vitro opioid agonist treatment upregulates cAMP levels (adenylyl cyclase (AC) superactivation) and augments basal and capsaicin evoked CGRP release in a PKA dependent manner. In the present study, we investigated the in vivo role of PKA in sustained morphine-mediated pain sensitization. Our data indicate that selective knock-down of spinal PKA activity by intrathecal (i.th.) pretreatment of rats with a PKA-selective small interference RNA (siRNA) mixture significantly attenuates sustained morphine-mediated augmentation of spinal CGRP immunoreactivity, thermal hyperalgesia, mechanical allodynia and antinociceptive tolerance. The present findings indicate that sustained morphine-mediated activation of spinal cAMP/PKA-dependent signaling may play an important role in opioid induced hyperalgesia.

Supraspinal Gbetagamma-dependent stimulation of PLCbeta originating from G inhibitory protein-mu opioid receptor-coupling is necessary for morphine induced acute hyperalgesia

Although alterations in micro-opioid receptor (microOR) signaling mediate excitatory effects of opiates in opioid tolerance, the molecular mechanism for the excitatory effect of acute low dose morphine, as it relates to microOR coupling, is presently unknown. A pronounced coupling of microOR to the alpha subunit of G inhibitory protein emerged in periaqueductal gray (PAG) from mice systemically administered with morphine at a dose producing acute thermal hyperalgesia. This coupling was abolished in presence of the selective microOR antagonist d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH(2) administered at the PAG site, showing that the low dose morphine effect is triggered by microOR activated G inhibitory protein at supraspinal level. When Gbetagamma downstream signalling was blocked by intra-PAG co-administration of 2-(3,4,5-trihydroxy-6-oxoxanthen-9-yl)cyclohexane-1-carboxylic acid, a compound that inhibits Gbetagamma dimer-dependent signaling, a complete prevention of low dose morphine induced acute thermal hyperalgesia was obtained. Phospholipase C beta3, an enzyme necessary to morphine hyperalgesia, was revealed to be associated with Gbetagamma in PAG. Although opioid administration induces a shift in microOR-G protein coupling from Gi to Gs after chronic administration, our data support that this condition is not realized in acute treatment providing evidence that a separate molecular mechanism underlies morphine induced acute excitatory effect.

Ultra-low-dose naloxone restores the antinociceptive effect of morphine and suppresses spinal neuroinflammation in PTX-treated rats

The aim of the present study was to examine the effect of ultra-low-dose naloxone on pertussis toxin (PTX)-induced thermal hyperalgesia in rats and its underlying mechanisms. Male Wistar rats, implanted with an intrathecal catheter with or without a microdialysis probe, received a single intrathecal injection of PTX (1 microg in 5 microl saline). Four days after PTX injection, they were randomly given a different dose of naloxone (either 15 microg or 15 ng in 5 microl saline), followed by a morphine injection (10 microg in 5 microl saline) after 30 min. The results found that PTX injection induced thermal hyperalgesia and increasing excitatory amino acid (EAA; L-glutamate and L-aspartate) concentration in the spinal CSF dialysates. Ultra-low-dose naloxone not only preserved the antinociceptive effect of morphine but also suppressed the PTX-evoked EAA release as well. Moreover, ultra-low-dose naloxone plus morphine administration inhibited the downregulation of L-glutamate transporters (GTs) and the L-glutamate-metabolizing enzyme glutamine synthetase (GS), and, moreover, inhibited microglial activation and suppressed cytokine expression in PTX-treated rat spinal cords. These results show that ultra-low-dose naloxone preserves the antinociceptive effect of morphine in PTX-treated rats. The mechanisms include (a) inhibition of pro-inflammatory cytokine expression, (b) attenuation of PTX-evoked EAA release, and (c) reversion of the downregulation of GT expression.

Differential Effects of Opioids on Sacrocaudal Afferent Pathways and Central Pattern Generators in the Neonatal Rat Spinal Cord

The effects of opioids on sacrocaudal afferent (SCA) pathways and the pattern-generating circuitry of the thoracolumbar and sacrocaudal segments of the spinal cord were studied in isolated spinal cord and brain stem-spinal cord preparations of the neonatal rat. The locomotor and tail moving rhythm produced by activation of nociceptive and nonnociceptive sacrocaudal afferents was completely blocked by specific application of the μ-opioid receptor agonist [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin acetate salt (DAMGO) to the sacrocaudal but not the thoracolumbar segments of the spinal cord. The rhythmic activity could be restored after addition of the opioid receptor antagonist naloxone to the experimental chamber. The opioid block of the SCA-induced rhythm is not due to impaired rhythmogenic capacity of the spinal cord because a robust rhythmic activity could be initiated in the thoracolumbar and sacrocaudal segments in the presence of DAMGO, either by stimulation of the ventromedial medulla or by bath application of N-methyl-d-aspartate/serotonin. We suggest that the opioid block of the SCA-induced rhythm involves suppression of synaptic transmission through sacrocaudal interneurons interposed between SCA and the pattern-generating circuitry. The expression of μ opioid receptors in several groups of dorsal, intermediate and ventral horn interneurons in the sacrocaudal segments of the cord, documented in this study, provides an anatomical basis for this suggestion.

Effects of intracerebroventricular administration of ultra low doses of histaminergic drugs on morphine state-dependent memory of passive avoidance in mice

The effects of intracerebroventricular (i.c.v.) administration of ultra low doses (ULDs) of histamine, clobenpropit and pyrilamine are studied on morphine state-dependent (STD) memory in mice. Although pre-test administration of different doses of histamine and clobenpropit showed no effect on impairment of memory induced by pre-training morphine, when the above drugs were co-administered with morphine, they inhibited the restoration of memory by morphine. These effects were opposite to microgram doses of the same drugs.

Discrepancy between effects of milligram and nanogram doses of a COX-2 inhibitor (celecoxib) on morphine state-dependent memory of passive avoidance in mice

This experiment examined and compared the effects of pre-test administration of a selective COX-2 inhibitor (celecoxib), at the doses in the range of mg/kg and ng/kg on morphine state-dependent learning in step-down passive avoidance task in mice. Pre-training administration of 5mg/kg of morphine-impaired memory retrieval tested 24h later, which was restored by pre-test administration of the same dose of the drug. Pre-test administration of celecoxib (12.5, 25 and 50mg/kg), alone or in combination with morphine (1mg/kg) prevents morphine-induced memory impairment. Ultra-low doses (ULDs) of celecoxib (2, 10 and 50 ng/kg) produced no change in morphine-induced memory impairment. However, co-administration of nanogram doses of celecoxib with 5mg/kg of morphine in the test day prevented morphine-induced memory improvement, an action different from mg/kg doses. These findings implicate the involvement of COX-2 in memory retrieval and demonstrate that the effect of celecoxib ULD is different from that of mg/kg doses.

Effects of ultra-low doses of morphine, naloxone and ethanol on morphine state-dependent memory of passive avoidance in mice

This experiment examined and compared the effects of pre-test administration of morphine, naloxone and ethanol, at doses in the range of milligram/kg to those of nanogram/kg, on morphine state-dependent learning in a step-down passive avoidance task in mice. Morphine (5 mg/kg) administered before training impaired retention tested 24 hours later, but when the same dose of morphine was also administered before the test, the retention was significantly restored. Pre-training administration of 10 or 20 ng/kg (i.p.) of morphine had no effect, but when co-administered with the same drug at 5 mg/kg (s.c.), it prevented significantly the memory recall improvement after the administration of morphine (5 mg/kg, s.c.) alone. In a parallel experiment, naloxone (5 mg/kg) prevented the memory recall improvement by morphine. However, the effects of naloxone at doses in the range of ng/kg were opposite to those of milligram doses of the same drug. Pre-test administration of ethanol (1 mg/kg) improved memory recall and mimicked the effects of pre-test morphine administration. At doses in the nanogram range, the effects of ethanol were opposite those of mg/kg of the drug. A review of the literature indicates that, for several drugs and chemicals, the effects of nanogram doses are the opposite of the effects of milligrams, because different doses have different sites as well as mechanisms of actions. In conclusion, from the above results one may suggest that, in determination of the dose-response of at least some drugs, the study of the effects of doses much lower than two orders of magnitude of the minimum effective dose are warranted.

Ultra-low dose naltrexone potentiates the anticonvulsant effect of low dose morphine on clonic seizures

Significant potentiation of analgesic effects of opioids can be achieved through selective blockade of their stimulatory effects on intracellular signaling pathways by ultra-low doses of opioid receptor antagonists. However, the generality and specificity of this interaction is not well understood. The bimodal modulation of pentylenetetrazole-induced seizure threshold by opioids provide a model to assess the potential usefulness of this approach in seizure disorders and to examine the differential mechanisms involved in opioid anti- (morphine at 0.5-3 mg/kg) versus pro-convulsant (20-100 mg/kg) effects. Systemic administration of ultra-low doses of naltrexone (100 fg/kg-10 ng/kg) significantly potentiated the anticonvulsant effect of morphine at 0.5 mg/kg while higher degrees of opioid receptor antagonism blocked this effect. Moreover, inhibition of opioid-induced excitatory signaling by naltrexone (1 ng/kg) unmasked a strong anticonvulsant effect for very low doses of morphine (1 ng/kg-100 microg/kg), suggesting that a presumed inhibitory component of opioid receptor signaling can exert strong seizure-protective effects even at very low levels of opioid receptor activation. However, ultra-low dose naltrexone could not increase the maximal anticonvulsant effect of morphine (1-3 mg/kg), possibly due to a ceiling effect. The proconvulsant effects of morphine on seizure threshold were minimally altered by ultra-low doses of naltrexone while being completely blocked by a higher dose (1 mg/kg) of the antagonist. The present data suggest that ultra-low doses of opioid receptor antagonists may provide a potent strategy to modulate seizure susceptibility, especially in conjunction with very low doses of opioids.

Paradoxical Effects of Opioid Antagonist Naloxone on SSRI-Induced Analgesia and Tolerance in Mice

Selective serotonin reuptake inhibitors (SSRIs) have been used clinically as co-analgesics in various devastating painful conditions. Upon chronic treatment tolerance develops to their analgesic effect, which is often refractory to increasing dose. Although modulation of serotonergic pathways considerably explains their clinical efficacy, numerous reports nevertheless indicate the direct/indirect role of the opioidergic pathway in SSRI-induced analgesia. The present study was designed to investigate the effect, if any, of the opioid antagonist naloxone on SSRIs-induced analgesia and tolerance employing acetic acid-induced writhing assay. Two SSRIs, fluoxetine (FLX), and citalopram (CTP) were used in the study. Acute systemic (5-40 mg kg(-1) i.p.), or intrathecal (5-40 microg per mouse, i.t.) administration of fluoxetine or citalopram exhibited a dose-dependent and significant (p < 0.05) antinociceptive effect. Single systemic (2-5 mg kg(-1) i.p.) or intrathecal (1 microg per mouse, i.t.) administration of opioid antagonist naloxone blocked where as systemic ultra-low dose (10 ng/kg) or intrathecal (0.05 ng) naloxone potentiated the acute antinociceptive effect of both SSRIs (10 mg kg(-1) i.p. and 10 microg i.t.). Animals treated chronically over a 7-day period with SSRIs developed tolerance to their antinociceptive effect. Further, chronic administration of ultra-low dose of naloxone intrathecal (0.05 ng per mouse, i.t.) or systemic (10 ng kg(-1) i.p.) with fluoxetine or citalopram (10 microg i.t.; 5 mg kg(-1) i.p.) over a 7-day period reversed the tolerance to the antinociceptive effect of SSRIs. Thus, in ultra-low doses, naloxone paradoxically enhances SSRIs-induced analgesia and reverse tolerance through spinal and peripheral action. These effects of opioid antagonist naloxone on SSRIs-induced antinociception may have an implication in refractory cases upon chronic use of SSRIs as co-analgesics.

Effects of a kappa-opioid agonist, asimadoline, on satiation and GI motor and sensory functions in humans

To compare the effects of the kappa-opioid agonist asimadoline and placebo on visceral sensation and gastrointestinal (GI) motor functions in humans, 91 healthy participants were randomized in a double-blind fashion to 0.15, 0.5, or 1.5 mg of asimadoline or placebo orally twice a day for 9 days. We assessed satiation (nutrient drink test), colonic compliance, tone, perception of colonic distension (barostat), and whole gut transit (scintigraphy). Treatment effect was assessed by analysis of covariance. Asimadoline increased nutrient drink intake (P = 0.03). Asimadoline decreased colonic tone during fasting (P = 0.03) without affecting postprandial colonic contraction, compliance, or transit. Gas scores in response to colonic distension were decreased with 0.5 mg of asimadoline at low levels (8 mmHg above operating pressure) of distension (P = 0.04) but not at higher levels of distension. Asimadoline at 1.5 mg increased gas scores at 16 mmHg of distension (P = 0.03) and pain scores at distensions of 8 and 16 mmHg (P = 0.003 and 0.03, respectively) but not at higher levels of distension. Further studies of this compound in diseases with altered satiation or visceral sensation are warranted.

Paradoxical effects of the opioid antagonist naltrexone on morphine analgesia, tolerance, and reward in rats

Opioid agonists such as morphine have been found to exert excitatory and inhibitory receptor-mediated effects at low and high doses, respectively. Ultra-low doses of opioid antagonists (naloxone and naltrexone), which selectively inhibit the excitatory effects, have been reported to augment systemic morphine analgesia and inhibit the development of tolerance/physical dependence. This study investigated the site of action of the paradoxical effects of naltrexone and the generality of this effect. The potential of ultra-low doses of naltrexone to influence morphine-induced analgesia was investigated in tests of nociception. Administration of intrathecal (0.05 and 0.1 ng) or systemic (10 ng/kg i.p.) naltrexone augmented the antinociception produced by an acute submaximal dose of intrathecal (5 microg) or systemic (7.5 mg/kg i.p.) morphine in the tail-flick test. Chronic intrathecal (0.005 and 0.05 ng) or systemic (10 ng/kg) naltrexone combined with morphine (15 microg i.t.; 15 mg/kg i.p.) over a 7-day period inhibited the decline in morphine antinociception and prevented the loss of morphine potency. In animals rendered tolerant to intrathecal (15 microg) or systemic (15 mg/kg) morphine, administration of naltrexone (0.05 ng i.t.; 10 and 50 ng/kg i.p.) significantly restored the antinociceptive effect and potency of morphine. Thus, in ultra-low doses, naltrexone paradoxically enhances morphine analgesia and inhibits or reverses tolerance through a spinal action. The potential of naltrexone to influence morphine-induced reward was also investigated using a place preference paradigm. Systemic administration of ultra-low doses of naltrexone (16.7, 20.0, and 25.0 ng/kg) with morphine (1.0 mg/kg) extended the duration of the morphine-induced conditioned place preference. These effects of naltrexone on morphine-induced reward may have implications for chronic treatment with agonist-antagonist combinations.

Cholera toxin-B subunit blocks excitatory opioid receptor-mediated hyperalgesic effects in mice, thereby unmasking potent opioid analgesia and attenuating opioid tolerance/dependence

In a previous study we demonstrated that injection (i.p.) of low doses of GM1 ganglioside in mice rapidly attenuates morphine's analgesic effects. This result is consonant with our electrophysiologic studies in nociceptive types of dorsal root ganglion (DRG) neurons in culture, which showed that exogenous GM1 rapidly increased the efficacy of excitatory (Gs-coupled) opioid receptor functions. By contrast, treatment of DRG neurons with the non-toxic B-subunit of cholera toxin (CTX-B) which binds selectively to GM1, blocked the excitatory, but not inhibitory, effects of morphine and other bimodally-acting opioid agonists, thereby resulting in a net increase in inhibitory opioid potency. The present study provides more direct evidence that endogenous GM1 plays a physiologic role in regulating excitatory opioid receptor functions in vivo by demonstrating that cotreatment with remarkably low doses of CTX-B (10 ng/kg, s.c.) selectively blocks hyperalgesic effects elicited by morphine or by a kappa opioid agonist, thereby unmasking potent opioid analgesia. These results are comparable to the effects of cotreatment of mice with morphine plus an ultra-low dose of the opioid antagonist, naltrexone (NTX) which blocks opioid-induced hyperalgesic effects, unmasking potent opioid analgesia. Low-dose NTX selectively blocks excitatory opioid receptors at their recognition site, whereas CTX-B binds to, and interferes with, a putative allosteric GM1 regulatory site on excitatory opioid receptors. Furthermore, chronic cotreatment of mice with morphine plus CTX-B attenuates development of opioid tolerance and physical dependence, as previously shown to occur during cotreatment with low-dose NTX.

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.

Opposite modulation of capsaicin-evoked substance P release by glutamate receptors

Substance P and glutamate are present in primary afferent C-fibers and play important roles in persistent inflammatory and neuropathic pain. In the present study, we have examined whether activation of different glutamate receptor subtypes modulates the release of substance P evoked by the C-fiber selective stimulant capsaicin (1 microM) from rat trigeminal nucleus slices. The selective NMDA glutamate receptor agonist L-CCG-IV (1-10 microM) enhanced capsaicin-evoked substance P release about 100%. This facilitatory effect was blocked by 0.3 microM MK-801, a selective NMDA receptor antagonist. The metabotropic glutamate receptor agonists L-AP4 (group III) and DHPG (group I) (30-100 microM) inhibited capsaicin-evoked substance P release by approximately 60%. These inhibitory effects were blocked by the selective metabotropic glutamate receptor antagonist (+/-)-MCPG (5 microM). On the other hand, AMPA and kainate (0.1-10 microM), did not significantly affect capsaicin-evoked substance P release. Thus, substance P release from non-myelinated primary afferents, and possibly nociception, may be under the functional antagonistic control of some metabotropic and ionotropic glutamate receptor subtypes.

Opioid-activated postsynaptic, inward rectifying potassium currents in whole cell recordings in substantia gelatinosa neurons

Opioid-activated postsynaptic, inward rectifying potassium currents in whole cell recordings in substantia gelatinosa neurons. J. Neurophysiol. 80: 2954-2962, 1998. Using tight-seal, whole cell recordings from isolated transverse slices of hamster and rat spinal cord, we investigated the effects of the mu-opioid agonist (-Ala2, N-Me-Phe4,Gly5-ol)-enkephalin (DAMGO) on the membrane potential and conductance of substantia gelatinosa (SG) neurons. We observed that bath application of 1-5 microM DAMGO caused a robust and repeatable hyperpolarization in membrane potential (Vm) and decrease in neuronal input resistance (RN) in 60% (27/45) of hamster neurons and 39% (9/23) of rat neurons, but significantly only when ATP (2 mM) and guanosine 5'-triphosphate (GTP; 100 microM) were included in the patch pipette internal solution. An ED50 of 50 nM was observed for the hyperpolarization in rat SG neurons. Because G-protein mediation of opioid effects has been shown in other systems, we tested if the nucleotide requirement for opioid hyperpolarization in SG neurons was due to G-protein activation. GTP was replaced with the nonhydrolyzable GTP analogue guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S; 100 microM), which enabled DAMGO to activate a nonreversible membrane hyperpolarization. Further, intracellular application of guanosine-5'-O-(2-thiodiphosphate) (GDP-beta-S; 500 microM), which blocks G-protein activation, abolished the effects of DAMGO. We conclude that spinal SG neurons are particularly susceptible to dialysis of GTP by whole cell recording techniques. Moreover, the depletion of GTP leads to the inactivation of G-proteins that mediate mu-opioid activation of an inward-rectifying, potassium conductance in these neurons. These results explain the discrepancy between the opioid-activated hyperpolarization in SG neurons observed in previous sharp electrode experiments and the more recent failures to observe these effects with whole cell patch techniques.

GM1 ganglioside-induced modulation of opioid receptor-mediated functions

Electrophysiologic studies of dorsal-root ganglion (DRG) neurons in culture have demonstrated both excitatory (Gs-coupled) as well as inhibitory (Gi/Go-coupled) opioid receptor-mediated actions. Brief treatment of DRG neurons with cholera toxin-beta which binds specifically to GM1 sites on neuronal membranes, selectively blocks opioid excitatory but not inhibitory effects. Conversely, after brief treatment of DRG neurons with GM1, but not with GM2, GM3, or other related gangliosides, the threshold concentration of opioid agonists for eliciting excitatory effects is markedly decreased from nM to pM-fM levels and opioid antagonists, for example, naloxone (NLX), at low concentrations paradoxically elicit excitatory effects. These studies suggest that the excitatory opioid supersensitivity of GM1-treated DRG neurons is due primarily to increased efficacy of excitatory opioid-receptor activation of Gs. Recent studies of cloned delta opioid receptors transfected into CHO cells suggest that this supersensitivity of GM1-treated DRG neurons may be further augmented by rapid conversion of many opioid receptors from a Gi/Go-coupled inhibitory mode to a Gs-coupled excitatory mode. The opioid excitatory supersensitivity elicited in DRG neurons by acute elevation of exogenous GM1 provides novel insights into mechanisms underlying opioid tolerance and dependence, since remarkably similar supersensitivity occurs in DRG and other neurons after chronic treatment with morphine or other opioid agonists that upregulate endogenous GM1.

Barium elicits reversal of low-concentration etorphine-induced decrease of potassium conductance in cultures of dissociated dorsal root ganglion neurons

Etorphine is an non-selective opioid receptor agonist with very potent analgesic effect. Low concentrations (< nM) of most opioid receptor agonists decrease the K+ conductance (gK) in cultures of dissociated mouse dorsal root ganglion neurons regardless of the presence of Ba2+ However, low concentrations of etorphine, in contrast to all other opioids tested, decreased gK only in the absence of Ba2+. In the presence of Ba2+, pM-nM etorphine elicited dose-dependent increases, instead of decreases in gK. Higher concentrations of etorphine (> nM) not only increased gK but, in addition, appreciably increased a delayed-onset inward Ca2+ current during pulsed depolarization regardless of the presence of Ba2+.

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.

Cross-tolerance to acute administration of mu and kappa opioid agonists at the spinal cord level in the rat

Development of tolerance and cross-tolerance after acute administration of the mu agonist morphine and the kappa agonist U-50,488H was assessed in rats, through recording of a C-fiber-evoked spinal nociceptive reflex. Rats rendered tolerant to morphine (a single dose of 1 mg/kg i.p.) showed, after a 5-hour period, tolerance to morphine and cross-tolerance to the kappa-opioid receptor agonist U-50,488H, as revealed by depressed C-reflex responsiveness. In contrast, pretreatment with U-50,488H (a single dose of 1 mg/kg i.p.) rendered tolerant the rats to U-50,488H, but the animals did not develop cross-tolerance to morphine. Results indicate that acute administration of mu and kappa ligands leads to development of unidirectional cross-tolerance in rat spinal cord. This points to limitations in using alternated mu and kappa opioid agonists to bypass the problem of development of opioid tolerance in chronic pain complaints.

Modulatory effects of Gs-coupled excitatory opioid receptor functions on opioid analgesia, tolerance, and dependence

Electrophysiologic studies of opioid effects on nociceptive types of dorsal root ganglion (DRG) neurons in organotypic cultures have shown that morphine and most mu, delta, and kappa opioid agonists can elicit bimodal excitatory as well as inhibitory modulation of the action potential duration (APD) of these cells. Excitatory opioid effects have been shown to be mediated by opioid receptors that are coupled via Gs to cyclic AMP-dependent ionic conductances that prolong the APD, whereas inhibitory opioid effects are mediated by opioid receptors coupled via Gi/Go to ionic conductances that shorten the APD. Selective blockade of excitatory opioid receptor functions by low (ca. pM) concentrations of naloxone, naltrexone, etorphine and other specific agents markedly increases the inhibitory potency of morphine or other bimodally acting agonists and attenuates development of tolerance/dependence. These in vitro studies have been confirmed by tail-flick assays showing that acute co-treatment of mice with morphine plus ultra-low-dose naltrexone or etorphine remarkably enhances the antinociceptive potency of morphine whereas chronic co-treatment attenuates development of tolerance and naloxone-precipitated withdrawal-jumping symptoms.

Chronic opioid treatment of neuroblastoma x dorsal root ganglion neuron hybrid F11 cells results in elevated GM1 ganglioside and cyclic adenosine monophosphate levels and onset of naloxone-evoked decreases in membrane K+ currents

Prolongation of the action potential duration of dorsal root ganglion (DRG) neurons by low (nM) concentrations of opioids occurs through activation of excitatory opioid receptors that are positively coupled via Gs regulatory protein to adenylate cyclase. Previous results suggested GM1 ganglioside to have an essential role in regulating this excitatory response, but not the inhibitory (APD-shortening) response to higher (microM) opioid concentrations. Furthermore, it was proposed that synthesis of GM1 is upregulated by prolonged activation of excitatory opioid receptor functions. To explore this possibility we have utilized cultures of hybrid F11 cells to carry out closely correlated electrophysiological and biochemical analyses of the effects of chronic opioid treatment on a homogeneous population of clonal cells which express many functions characteristic of DRG neurons. We show that chronic opioid exposure of F11 cells does, in fact, result in elevated levels of GM1 as well as cyclic adenosine monophosphate (AMP), concomitant with the onset of opioid excitatory supersensitivity as manifested by naloxone-evoked decreases in voltage-dependent membrane K+ currents. Such elevation of GM1 would be expected to enhance the efficacy of excitatory opioid receptor activation of the Gs/adenylate cyclase/cyclic AMP system, thereby providing a positive feedback mechanism that may account for the remarkable supersensitivity of chronic opioid-treated neurons to the excitatory effects of opioid agonists as well as antagonists. These in vitro findings may provide novel insights into the mechanisms underlying naloxone-precipitated withdrawal syndromes and opioid-induced hyperalgesia after chronic opiate addiction in vivo.

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.

Chronic morphine-treated sensory ganglion neurons remain supersensitive to the excitatory effects of naloxone for months after return to normal culture medium: an in vitro model of 'protracted opioid dependence'

Chronic morphine-treated dorsal-root ganglion (DRG) neurons in DRG/spinal cord explant cultures were previously shown to become supersensitive to the excitatory effects of remarkably low concentrations of the opioid agonists, morphine and dynorphin, and the opioid antagonist, naloxone. The present study demonstrates that this opioid excitatory supersensitivity of chronic morphine-treated DRG neurons (1 microM for > 1 week) is retained for periods > 3 months after return to control culture medium. Acute application of femtomolar dynorphin, as well as nanomolar naloxone, to the treated neurons after months in control medium evoked characteristic prolongation of the action potential duration (APD), as occurs in cells tested during or shortly after chronic opioid exposure. The threshold concentrations for eliciting these excitatory effects in naive DRG neurons are > 1000-fold higher. Furthermore, treatment of micromolar morphine-sensitized neurons with 1 nM etorphine (which is a potent excitatory opioid receptor antagonist) for I week prior to return to control medium blocked further expression of opioid excitatory supersensitivity when tested after an additional 1-7 weeks in culture. These results provide a unique in vitro model system for analyses of some of the cellular mechanisms underlying protracted opioid dependence in vivo.

Noradrenaline effects on cultured rat sensory neurones exposed to morphine in utero or in vitro

Dorsal root ganglion (DRG) neurones cultured from newborn rats were chronically exposed to 1 microM morphine. Untreated cells and in vitro morphine treated cells were cultured for 1 or 2 weeks from control offspring while in utero morphine treated cells were cultured for 1 week from offspring of morphine dependent rats. Noradrenaline (1 microM) was applied by pressure ejection. Intracellular recordings measured the change in action potential duration (APD). Noradrenaline significantly decreased the APD of 1-week-old untreated cells while the 2-week-old untreated cells and the groups of morphine treated cells showed no significant change of APD following the drug application. Thus, newborn rat DRG neurones demonstrated a different sensitivity to noradrenaline with the culture duration or with morphine exposure.

Antagonists at excitatory opioid receptors on sensory neurons in culture increase potency and specificity of opiate analgesics and attenuate development of tolerance/dependence

At low (< nM) concentrations, mu, delta or kappa opioid peptides as well as morphine and other opioid alkaloids elicit dose-dependent excitatory prolongation of the calcium-dependent component of the action potential duration (APD) of many mouse sensory dorsal root ganglion (DRG) neurons, whereas application of the same opioids at higher (uM) concentrations results in inhibitory shortening of the APD. These bimodal opioid excitatory/inhibitory effects on DRG neurons are blocked by naloxone. In contrast to bimodally acting opioids, the opioid alkaloids, etorphine and dihydroetorphine (thebaine-oripavine derivatives) uniquely elicited only dose-dependent, naloxone-reversible inhibitory effects on sensory neurons in DRG-spinal cord explants, even at concentrations as low as 1 pM, and showed no excitatory effects at lower concentrations. These remarkably potent inhibitory opioid receptor agonists also act as antagonists at excitatory opioid receptors since pretreatment of DRG neurons with subthreshold concentrations (< pM) blocked excitatory APD prolongation by nM morphine (or other opioids) and unmasked inhibitory APD shortening which generally requires much higher concentrations. Furthermore, acute application of pM-nM etorphine to chronic microM morphine- or D-Ala2-D-Leu5 enkephalin (DADLE)-treated DRG neurons blocked the nM naloxone-precipitated APD prolongation that generally occurs in DRG cells sensitized by bimodally acting opioids. In the presence of pM etorphine, chronic treatment of DRG neurons with microM morphine or DADLE no longer resulted in development of tolerance/dependence effects, as previously observed after similar chronic opioid treatment in the presence of cholera toxin-B subunit. These in vitro studies may clarify the mechanisms underlying the potent analgesic effects of etorphine and dihydroetorphine in vivo and to guide the use of these and other excitatory opioid receptor antagonists in attenuating development of opiate dependence/addiction.

Inhibition of the antianalgesic action of dynorphin A in mice by cholera toxin

Dynorphin A-(1-17) (Dyn A) administered intrathecally (IT) or released spinally in the mouse produces an antianalgesic action. The present experiments indicate that IT administration of cholera toxin inhibited the antianalgesic action of Dyn A. When clonidine was administered intracerebroventricularly (ICV) to release spinal Dyn A, IT cholera toxin inhibited the antianalgesic action of Dyn A so that the analgesic component of action of clonidine became evident. Dyn A given IT inhibited the analgesic action of morphine given ICV. Cholera toxin given IT eliminated the antagonistic action of Dyn A. These results, in addition to others, indicate that IT cholera toxin antagonized the action of Dyn A. When the antianalgesic action of Dyn A was attenuated by pretreatment with dynorphin antiserum or by pretreatment that produced desensitization to Dyn A, cholera toxin had no effect. These results suggested that the antianalgesic action of Dyn A is mediated by activation of opioid receptors that are positively coupled to adenylate cyclase through a Gs regulatory protein.

Morphine activates omega-conotoxin-sensitive Ca2+ channels to release adenosine from spinal cord synaptosomes

Morphine-induced release of adenosine from the spinal cord is believed to contribute to spinal antinociception. Although this release is Ca2+ dependent, little is known of the nature of this dependence. In this study, the effects of the dihydropyridine L-type Ca2+ channel agonist Bay K 8644 and the antagonist nifedipine, the N-type Ca2+ channel antagonist omega-conotoxin, and ruthenium red, a blocker of Ca2+ influx induced by capsaicin, on release of adenosine evoked by morphine were determined. The effect of partial depolarization with a minimally effective concentration of K+ on morphine-evoked release of adenosine also was examined. Morphine 10(-5)-10(-4) M produced a dose-dependent enhancement of adenosine release from dorsal spinal cord synaptosomes. Following the addition of 6 mM K+ (total K+ concentration of 10.7 mM), 10(-6) M morphine also enhanced release, and an additional component of action at 10(-8) M was revealed. Release was Ca(2+)-dependent as it was not observed in the absence of Ca2+ and presence of EGTA. Bay K 8644 (10 nM) and nifedipine (100 nM) had no effect on the release of adenosine evoked by morphine, but omega-conotoxin (100 nM) markedly reduced such release in both the absence and the presence of the additional 6 mM K+. Morphine-evoked adenosine release was not altered in the presence of a partially effective dose of capsaicin, nor by ruthenium red.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic selective activation of excitatory opioid receptor functions in sensory neurons results in opioid 'dependence' without tolerance

We previously showed that mouse sensory dorsal root ganglion (DRG) neurons chronically exposed to 1 microM D-ala2-D-leu5-enkephalin (DADLE) or morphine for > 2-3 days in culture become tolerant to the usual opioid inhibitory receptor-mediated effects, i.e. shortening of the duration of the calcium-dependent component of the action potential (APD), and supersensitive to opioid excitatory APD-prolonging effects elicited by low opioid concentrations. Whereas nanomolar concentrations of dynorphin(1-13) or morphine are generally required to prolong the APD of naive DRG neurons (by activating excitatory opioid receptors), femtomolar levels become effective after chronic opioid treatment. Whereas 1-30 nM naloxone or diprenorphine prevent both excitatory and inhibitory opioid effects but do not alter the APD of native DRG neurons, both opioid antagonists unexpectedly prolong the APD of most of the chronic opioid-treated cells. In the present study, chronic exposure of DRG neurons to 1 microM DADLE together with cholera toxin-B subunit (which selectively blocks GM1 ganglioside-regulated opioid excitatory, but not inhibitory, receptor functions) prevented the development of opioid excitatory supersensitivity and markedly attenuated tolerance to opioid inhibitory effects. Conversely, sustained exposure of DRG neurons to 1 nM DADLE, which selectively activates excitatory opioid receptor functions, resulted in characteristic opioid excitatory supersensitivity but no tolerance. These results suggest that 'dependence'-like properties can be induced in chronic opioid-treated sensory neurons in the absence of tolerance. On the other hand, development of some components of tolerance in these cells may require sustained activation of both excitatory, as well as inhibitory, opioid receptor functions.

Cyclic adenosine 3'5'-monophosphate potentiates excitatory amino acid and synaptic responses of rat spinal dorsal horn neurons

Intracellular recordings were made from rat dorsal horn neurons in the in vitro slice preparation to study the actions of cyclic adenosine 3',5'-monophosphate (cyclic AMP). In the presence of TTX, bath application of the membrane permeable analogue of cyclic AMP, 8-Br cyclic AMP (25-100 microM) caused a small depolarization of the resting membrane potential accompanied by a variable change in membrane input resistance. In addition, 8-Br cyclic AMP caused a long-lasting increase in the spontaneous synaptic activity and the amplitude of presumed monosynaptic excitatory postsynaptic potentials evoked in the substantia gelatinosa neurons by orthodromic stimulation of a lumbar dorsal root. When the fast voltage-sensitive Na conductance was blocked by TTX, 8-Br cyclic AMP enhanced in a reversible manner, the depolarizing responses of a proportion of dorsal horn neurons to N-methyl-D-aspartic acid (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), quisqualic acid (QA) and kainic acid (KA). The effects of 8-Br cyclic AMP on the resting membrane potential and the NMDA response of dorsal horn neurons were mimicked by reducing phosphodiesterase activity with bath application of 3-isobutyl-1-methylxanthine, but not by cyclic AMP applied extracellularly. Moreover, we have found that intracellular application of a protein inhibitor of cyclic AMP-dependent protein kinase (PKI) into dorsal horn neurons prevents the 8-Br cyclic AMP-induced potentiation of the NMDA response of these cells. These results suggest that in the rat spinal dorsal horn the activation of the adenylate cyclase-cyclic AMP-dependent protein kinase system may be involved in the enhancement of the sensitivity of postsynaptic excitatory amino acid (NMDA, AMPA, KA) receptors and modulation of primary afferent neurotransmission, including nociception.

Opioid and cannabinoid receptor inhibition of adenylyl cyclase in brain

Both opioids and cannabinoids bind to G-protein-coupled receptors to inhibit adenylyl cyclase in neurons. These reactions were assayed in brain membranes, where maximal inhibitory activity occurred in the following regions: mu-opioid inhibition in rat thalamus, delta-opioid inhibition in rat striatum, kappa-opioid inhibition in guinea pig cerebellum, and cannabinoid inhibition in cerebellum. The inhibition of adenylyl cyclase by both cannabinoid and opioid agonists was typical of G-protein-linked receptors: they required GTP, they were not supported by non-hydrolyzable GTP analogs, and they were abolished (in primary neuronal cell culture) by pertussis toxin treatment. The immediate targets of this system were determined by assaying protein phosphorylation in the presence of receptor agonists and App(NH)p, a substrate for adenylyl cyclase. In striatal membranes, opioid agonists inhibited the phosphorylation of at least two bands of MW 85 and 63 kDa, which may be synapsins I and II, respectively. Other experiments determined the long-term effects of this second messenger system. In primary neuronal cultures, opioid-inhibited adenylyl cyclase attenuated forskolin-stimulated pro-enkephalin mRNA levels, thus providing a feedback regulation of opioid synthesis. Finally, in cerebellar granule cells, both cannabinoid and opioid receptors may exist on the same cells. In these cells, agonists which bind to different receptor types may produce similar biological responses.

Morphine produces a multiphasic effect on the release of substance P from rat trigeminal nucleus slices by activating different opioid receptor subtypes

Morphine (MOR) produces a concentration-dependent multiphasic effect (inhibitions and facilitations) on K(+)-evoked substance P (SP) release from rat trigeminal nucleus slices. In this study, we tested the action of selective opioid receptor antagonists on this multiphasic effect of MOR. 1 nM MOR produced an inhibition of K(+)-evoked release of SP that was affected only by the selective mu 1-opioid receptor antagonist naloxonazine (1 nM). MOR at 100 nM elicited an increase in SP release which was abolished selectively by the mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA; 20 nM) and attenuated by the delta-opioid receptor antagonist, ICI 174,864 (0.3 microM). 3 microM MOR produced an inhibition of SP release that was reversed only by ICI 174,864 (0.3 microM). MOR at even higher concentrations (30 microM) produced an enhancement of SP release that was reversed selectively by 3 nM n-binaltorphimine (n-BNI; 3 nM), a kappa-opioid receptor antagonist. In slices pretreated with 20 nM beta-FNA and in the presence of 0.3 microM ICI 174,864 (mu- and delta-opioid receptor blockade), both 100 nM and 3 microM MOR elicited a strong facilitation of K(+)-evoked SP release which was sensitive to 3 nM n-BNI. Thus, the increase in SP release produced by 100 nM may be mediated by the simultaneous stimulation of beta-FNA-sensitive mu- and excitatory delta-opioid receptors whereas the facilitation of SP release induced by 30 microM MOR could be due to the activation of kappa-opioid receptors. 1 nM and 3 microM MOR may inhibit SP release by stimulating naloxonazine-sensitive mu 1- and inhibitory delta-opioid receptors, respectively.

Multiphasic effect of morphine on the release of substance P from rat trigeminal nucleus slices

It is generally accepted that morphine acts presynaptically to inhibit substance P (SP) release from afferent terminals in the trigeminal nucleus. Recent studies, however, provide evidence that opioids produce both inhibitory and excitatory effects on SP release which are concentration- and receptor subtype-dependent. In the present study, we have examined a wide range of morphine concentrations on K(+)-evoked SP release from rat trigeminal nucleus caudalis slices. Immunoreactive SP was measured in perfusates. Morphine produced multiphasic effects on K(+)-evoked SP release without affecting basal release. A very low nanomolar concentration (1 nM) suppressed release, higher nanomolar concentrations (100-300 nM) facilitated release, a low micromolar concentration (3 microM) suppressed release, and a higher micromolar concentration (30 microM) facilitated release. These effects were abolished by opioid receptor blockade with naloxone (30 nM). Thus, morphine produces a complex bi-directional modulation of SP release from TNC which is concentration- and possibly receptor subtype-dependent.

After chronic opioid exposure sensory neurons become supersensitive to the excitatory effects of opioid agonists and antagonists as occurs after acute elevation of GM1 ganglioside

Mouse sensory dorsal-root ganglion (DRG) neurons chronically exposed to 1 microM D-Ala2-D-Leu5-enkephalin (DADLE) for greater than 1 week in culture become tolerant to opioid inhibitory effects, i.e. shortening of the duration of the calcium-dependent component of the action potential (APD). Acute application of higher concentrations of DADLE (ca. 10 microM) to these treated neurons not only fails to shorten the APD but, instead, generally elicits excitatory effects, i.e. prolongation of the APD. The present study shows that chronic DADLE- or morphine-treated DRG neurons also become supersensitive to the excitatory effects of opioids. Whereas nM concentrations of dynorphin(1-13) are generally required to prolong the APD of naive DRG neurons, fM levels become effective after chronic opioid treatment. Whereas 1-30 nM naloxone or diprenorphine do not alter the APD of naive DRG neurons, both opioid antagonists unexpectedly prolong the APD of most of the treated cells. Similar supersensitivity to the excitatory effects of opioid agonists and antagonists was previously observed after acute treatment of naive DRG neurons with GM1 ganglioside. Our results suggest that both chronic opioid and acute GM1 treatments of DRG neurons greatly enhance the efficacy of opioid excitatory receptor functions so that even the extremely weak agonist properties of naloxone and diprenorphine become effective in prolonging the APD of these treated cells when tested at low concentrations, whereas their antagonist properties at inhibitory opioid receptors do not appear to be altered. Furthermore, whereas cholera toxin-B subunit (CTX-B; 1-10 nM) blocks opioid-induced APD prolongation in naive DRG neurons (presumably by interfering with endogenous GM1 modulation of excitatory opioid receptors functions), even much higher concentrations of CTX-B were ineffective in chronic opioid-treated as well as acute GM1-elevated neurons. These and related data suggest that opioid excitatory supersensitivity in chronic opioid-treated DRG neurons may be due to a cyclic AMP-dependent increase in GM1 ganglioside levels. Our results may clarify mechanisms of opioid dependence and the paradoxical supersensitivity to naloxone which triggers withdrawal symptoms after opiate addiction.

Differential coupling of opioid binding sites to guanosine triphosphate binding regulatory proteins in subcellular fractions of rat brain

In this study, we present evidence for the occurrence of mu, delta, and kappa opioid binding sites in synaptic plasma membranes (SPM) and microsomes of rat brain. Binding to all three opioid classes was inhibited by 5'-guanylylimidodiphosphate (Gpp[NH]p) in SPM, while microsomal sites proved to be insensitive to this GTP analog. Sensitivity was restored upon solubilization of microsomes with digitonin, suggesting that opioid receptors are physically separated from G proteins in this fraction. Modulation of microsomal binding by Na+ and Mn++ was greater than that of SPM. Pertussis toxin-catalyzed adenosine diphosphate (ADP) ribosylation revealed the presence of G proteins with alpha-subunit molecular weights of 40 kDa in both subcellular fractions. Basal low Km GTPase activity in SPM was greater than in microsomes. Etorphine elicited a concentration-dependent stimulation of guanosine triphosphatase (GTPase) activity in SPMs but not in microsomes, indicating functional coupling of opioid receptors to G protein in the former and an uncoupling in the latter. Microsomes from 3-day-old rat brain contained more mu opioid sites and they were more sensitive to Gpp(NH)p inhibition than those in adults. These results are consistent with the hypothesis that opioid binding sites in adult microsomes are internalized and G protein uncoupled, while those in neonates are newly synthesized, coupled receptors.

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)

Opioids at low concentration decrease openings of K+ channels in sensory ganglion neurons

Previous studies showed that low concentrations of opioids prolong the calcium-dependent component of the action potential duration (APD) of dorsal root ganglion (DRG) neurons, whereas higher concentrations shorten the APD. In the present study whole-cell voltage-clamp, as well as cell-attached membrane-patch voltage-clamp, recordings demonstrate that application of picomolar to nanomolar concentrations of mu, delta or kappa opioid agonists (DAGO, DPDPE or dynorphin) to DRG neurons in dissociated cell cultures reversibly decreased the activities of voltage-sensitive K+ channels. Pretreatment of DRG neurons with the opioid receptor antagonists, naloxone (30 nM) or diprenorphine (1 nM) prevented mu/delta or kappa opioid-induced decreases in K+ channel activities, respectively. Since opioids added to the bath solution decreased the activities of K+ channels in the membrane patch sealed off by the pipette tip, our results provide strong evidence that some modes of excitatory modulation of the action potential of DRG neurons are mediated by diffusible second messengers. The data are consonant with our previous studies indicating that opioids can elicit excitatory effects on sensory neurons via cholera toxin-sensitive Gs-linked excitatory opioid receptors coupled to cyclic AMP-dependent ionic channels.

Pacemaker activity of locus coeruleus neurons: whole-cell recordings in brain slices show dependence on cAMP and protein kinase A

Noradrenergic neurons of the rat locus coeruleus (LC) are endogenous pacemakers that exhibit slow, tonic firing even in the complete absence of synaptic inputs. In the present study a time-dependent decline in LC spontaneous firing activity was found on intracellular dialysis during whole-cell recording with low-resistance patch electrodes; this decline was accentuated by a specific inhibitor of cAMP-dependent protein kinase (PKI5-24). Conversely, the inclusion of cAMP, 8-Br-cAMP, or the catalytic subunit of cAMP-dependent protein kinase (PKAcat) in the patch pipettes dose-dependently increased firing rate; intracellular PKI5-24 blocked both 8-Br-cAMP and PKAcat-induced firing in LC neurons. These results indicate that endogenous cAMP, via a phosphorylation-dependent route, drives tonic pacemaker activity in LC neurons.

Opioid receptor-coupled second messenger systems

Although pharmacological data provide strong evidence for different types of opioid receptors (e.g., mu, delta, and kappa), they share many common properties in their ability to couple to second messenger systems. All opioid receptor types are coupled to G-proteins, since agonist binding is diminished by guanine nucleotides and agonist-stimulated GTPase activity has been identified in several preparations. Moreover, all three types inhibit adenylyl cyclase. This second messenger system has been identified for opioid receptors in both isolated brain membranes and in transformed cell culture. Studies with chronic treatment with opioid agonists suggest that the coupling of receptors with G-proteins and second messenger effectors may play important roles in development of opioid tolerance.

Cholera toxin-B subunit blocks excitatory effects of opioids on sensory neuron action potentials indicating that GM1 ganglioside may regulate Gs-linked opioid receptor functions

In a previous study, we demonstrated that cholera toxin-A subunit, as well as the whole toxin, selectively blocks opioid-induced prolongation of the Ca2+ component of the action potential duration (APD) in dorsal root ganglion (DRG) neurons, indicating mediation of this excitatory effect by Gs-linked opioid receptors. The present study shows that pretreatment of DRG neurons with the B subunit of cholera toxin (1-10 ng/ml; greater than 15 min) can also block mu/delta and kappa opioid-induced APD prolongation, but not shortening. Since the B subunit binds selectively to GM1 ganglioside located on the cell surface, these results suggest that this ganglioside may regulate Gs-linked excitatory opioid receptor functions in DRG neurons. Possible contamination of purified B subunit preparations of cholera toxin with traces of the more potent A subunit was eliminated by heating the stock solution to 56 degrees C for 20 min. Exposure of DRG neurons to an affinity-purified anti-GM1 antiserum also blocked opioid-induced APD prolongation, providing further evidence that GM1 ganglioside may play an essential role in excitatory opioid modulation of the action potential of these cells. The blockade by cholera toxin-B subunit and anti-GM1 antibodies of opioid-induced APD prolongation is best accounted for by the following hypothesis: CTX-B interferes with an endogenous GM1 ganglioside component of the excitatory, but not inhibitory, opioid receptor complex on DRG neurons that may allosterically regulate coupling of the receptors via Gs to adenylate cyclase/cyclic adenosine monophosphate-dependent ionic conductances.

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.

Neuropeptides as positive or negative neuronal growth regulatory factors: effects of ACTH and leu-enkephalin on cultured serotonergic neurons

In summary, we have presented evidence that neuropeptides can function as either positive or negative growth regulatory factors during development. The ACTH family of peptides appear to act predominantly as a positive growth regulatory factor-enhancing neurite outgrowth, cell survival, biochemical maturation and behavioral expression. These effects of ACTH are most pronounced prior to the time the afferent cell has reached its target. Thus, ACTH may act as a low level general neurotrophic growth regulatory factor. The opioids have the opposite effect. These neuropeptides inhibit neurite extension, cell survival, and biochemical maturation. The effects of these negative growth regulatory factors are observed even when the afferents have reached their targets. The action of the opioids is thought to occur through specific receptors and known second messenger systems. Thus, CNS neuropeptide levels can have important actions in regulating the development of a variety of CNS systems, and permanently influencing the structure and function of the brain.

Opioids and the developing organism: a comprehensive bibliography, 1984-1988

A comprehensive bibliography of the literature concerned with opioids and the developing organism for 1984-1988 is presented. Utilized with companion papers (Neurosci. Biobehav. Rev. 6:439-479; 1982; 8:387-403; 1984), these articles cover the clinical and laboratory references beginning in 1875. For the years 1984, 1985, 1986, 1987, and 1988, a total of 877 citations were recorded. A series of indexes accompanies the citations in order to make the literature more accessible. These indexes are divided into clinical and laboratory topics, and subdivided into such topics as the type of opioid explored and the general area of biological interest (e.g., physiology).