Multiple opiate receptors: emerging concepts

WIN 44,441: A Stereospecific and Long-Acting Narcotic Antagonist

The opiate antagonist WIN 44,441-3 is a potent, stereospecific antagonist of mu, delta, and kappa opiate receptors. This antagonist activity is of long duration (> 4 h) with no agonist activity being observed. It therefore appears that WIN 44,441-3 will be a useful long-acting opiate antagonist for in vivo studies.

[Is morphine still the analgesic of choice in acute myocardial infarction?]

Chest pain is the most common symptom of patients who present with ischemic heart disease. Morphine has traditionally been the drug of choice for managing chest pain in acute coronary syndrome (ACS) due to its high analgesic potency, though its physiological effects are poorly understood. Routinely used for managing chest pain, morphine is recommended in the 2002 guidelines of the American College of Cardiology/American Heart Association. This recommendation, however, is not based on a high level of scientific evidence but on expert opinion. Studies have found both for and against the use of morphine in ACS, suggesting that its benefits are perhaps not altogether clear. This review examines the pathophysiological effects of morphine and their cardiac implications, with special attention to a possible negative effect on ACS. We reviewed articles in the MEDLINE database from 1982 to 2006.

Opioid receptor subtype antagonists differentially alter GABA agonist-induced feeding elicited from either the nucleus accumbens shell or ventral tegmental area regions in rats

Food intake is significantly increased by administration of either GABAA (e.g., muscimol) or GABAB (e.g., baclofen) agonists into either the shell region of the nucleus accumbens (NAC) or the ventral tegmental area (VTA); these responses are selectively blocked by pretreatment with corresponding GABAA and GABAB antagonists. Previous studies found that a single dose (5 microg) of the general opioid antagonist, naltrexone reduced feeding elicited by muscimol, but not baclofen in the NAC shell, and reduced feeding elicited by baclofen, but not muscimol in the VTA. The present study compared feeding responses elicited by either muscimol or baclofen in either the VTA and NAC shell following pretreatment with equimolar doses of selective mu (0.4, 4 microg), delta (0.4, 4 microg), or kappa (0.6, 6 microg) opioid receptor subtype antagonists. Muscimol (25 ng) and baclofen (200 microg) each significantly and equi-effectively increased food intake over 4 h following VTA or NAC shell microinjections. Muscimol-induced feeding elicited from the VTA was significantly enhanced by mu or delta antagonists, and was significantly reduced by kappa antagonists. Baclofen-induced feeding elicited from the VTA was significantly reduced by mu or kappa, but not delta antagonists. Muscimol-induced feeding elicited from the NAC was significantly reduced by either mu, kappa or delta antagonists. Baclofen-induced feeding elicited from the NAC was significantly reduced by kappa or delta, but not mu antagonists. These data indicate differential opioid receptor subtype antagonist-induced mediation of GABA receptor subtype agonist-induced feeding elicited from the VTA and NAC shell. This is consistent with previously demonstrated differential GABA receptor subtype antagonist-induced mediation of opioid-induced feeding elicited from these same sites. Thus, functional relationships exist for the elaborate anatomical and physiological interactions between these two neurochemical systems in the VTA and NAC shell.

Cardioprotective action of fentanyl in a model of central sympathetic overactivity in rabbits: antiarrhythmic and anti-ischemic effects

BACKGROUND

Sympathetic overactivity resulting from perioperative noxious stimuli elicits hyperdynamic cardiovascular responses that may lead to myocardial ischemia and/or ventricular arrhythmia, especially in patients presenting with coronary artery disease. In the present study we investigated the cardioprotective effects of clinically relevant doses of fentanyl in an experimental model of sympathetic overactivity associated with myocardial ischemia in anesthetized rabbits.

METHODS

Central sympathetic stimulation was achieved through intracerebroventricular (i.c.v.) injection of L-glutamate (10 micro mol), with simultaneous inhibition of nitric oxide (NO) synthesis through i.v. administration of N(omega)-nitro-l-arginine methyl ester (L-NAME; 40 mg kg(-1)).

RESULTS

L-glutamate triggered ventricular arrhythmia and electrocardiographic alterations indicative of myocardial ischemia. The intravenous administration of fentanyl (5, 10 or 50 micro g kg(-1)) reduced the incidence of ST-segment shift (70, 20 and 10%, respectively, vs. 66.7% in controls) as well as of T-wave inversion from 58.3% to 30, 20 and 10%, respectively. The total number of ventricular premature complexes per minute fell from 65.2 +/- 16 in the control group to 6.8 +/- 3, 3.5 +/- 2 and 2.6 +/- 1.5, respectively. The occurrence of ventricular tachycardia and bigeminy was completely abolished by fentanyl. Finally, the i.v. administration of fentanyl did not induce significant hemodynamic effects (except for dP/dt(max) in the 50 micro g kg(-1)-dose).

CONCLUSION

Fentanyl elicits significant cardioprotective effects in a model of arrhythmia resulting from the association of central sympathetic overactivity with myocardial ischemia in rabbits, independently from its systemic hemodynamic actions.

Pharmacology of flavor preference conditioning in sham-feeding rats: effects of dopamine receptor antagonists

Opioid and dopamine systems are both implicated in the response to sweet solutions. Our laboratory previously reported that the opioid antagonist, naltrexone, reduced the intake of sweet solutions, yet had little or no effect on sucrose-conditioned flavor preferences in sham-feeding rats. The present study examined the role of dopamine D(1) and D(2) receptors in the expression of flavor preferences conditioned by the sweet taste of sucrose. All sessions were conducted under sham-feeding conditions to minimize postingestive influences. Training was accomplished by adding a novel flavor (CS+) to a 16% sucrose solution, a different flavor (CS-) to a less-preferred 0.2% saccharin solution in alternating, one-bottle sessions. Preferences were assessed in two-bottle tests with the CS+ and CS- flavors presented in mixed sucrose (8%)-saccharin (0.1%) solutions following systemic doses of 0, 50, 200, 400, or 800 nmol/kg of the D(2) antagonist, raclopride (Experiment 1) or the D(1) antagonist, SCH23390 (Experiment 2) under either food-restricted or unrestricted conditions. Rats significantly preferred the CS+ solutions in vehicle tests, and displayed equipotent and dose-dependent reductions in total intake and CS+ preference following either D(1) or D(2) receptor antagonism. Similar results were obtained with SCH23390 and raclopride in Experiment 3 conducted with water-restricted rats. These data indicate that dopaminergic D(1) and D(2) receptors play pivotal and functionally equivalent roles in the expression of flavor preferences conditioned by the sweet taste of sucrose.

Evaluation of opioid receptor subtype antagonist effects in the ventral tegmental area upon food intake under deprivation, glucoprivic and palatable conditions

Opioid receptor subtype antagonists differentially alter food intake under deprivation (24 h), glucoprivic (2-deoxy-D-glucose, 500 mg/kg, i.p.) or palatable (10% sucrose) conditions with mu (beta-funaltrexamine) and kappa (nor-binaltorphamine), but not delta1 ([D-Ala2,Leu5,Cys6]enkephalin) opioid antagonists reducing each form of intake following ventricular microinjection. Both mu and kappa opioid antagonists microinjected into either the hypothalamic paraventricular nucleus or the nucleus accumbens reduce intake under deprivation and glucoprivic conditions. Palatable intake is reduced by both antagonists in the paraventricular nucleus, but only mu antagonists are active in the accumbens. Food intake is stimulated by mu and delta, but not kappa, opioid agonists microinjected into the ventral tegmental area. The present study examined whether food intake under either deprivation, glucoprivic or palatable conditions was altered by bilateral administration of general (naltrexone), mu, kappa, delta1 or delta2 (naltrindole isothiocyanate) opioid antagonists into the ventral tegmental area. Deprivation (24 h)-induced feeding was significantly reduced by high (50 microg), but not lower (10-20 microg) doses of naltrexone (21%), and by delta2 (4 microg, 19%) antagonism in the ventral tegmental area. 2-Deoxy-D-glucose (500 mg/kg, i.p.)-induced hyperphagia was significantly reduced by high (50 microg), but not lower (20 microg) doses of naltrexone (64%), and by delta2 (4 microg, 27%) antagonism in the ventral tegmental area. Sucrose (10%) intake was significantly reduced by naltrexone (20-50 microg, 25-39%) and delta2 (4 microg, 25%) antagonism in the ventral tegmental area. Neither mu, kappa nor delta1 antagonists were effective in reducing any form of intake following microinjection into the ventral tegmental area. These data indicate that the ventral tegmental area plays a relatively minor role in the elicitation of these forms of food intake, and that delta2, rather than mu, kappa or delta1 opioid receptors appear responsible for mediation of these forms of intake by this nucleus.

Opioid antagonists in the periaqueductal gray inhibit morphine and beta-endorphin analgesia elicited from the amygdala of rats

In addition to brainstem sites of action, analgesia can be elicited following amygdala microinjections of morphine and mu-selective opioid agonists. The present study examined whether opioid analgesia elicited by either morphine or beta-endorphin in the amygdala could be altered by either the general opioid antagonist, naltrexone, the mu-selective antagonist, beta-funaltrexamine (BFNA) or the delta 2 antagonist, naltrindole isothiocyanate (Ntii) in the periaqueductal gray (PAG). Both morphine (2.5-5 micrograms) and beta-endorphin (2.5-5 micrograms) microinjected into either the baso-lateral or central nuclei of the amygdala significantly increased tail-flick latencies and jump thresholds in rats. The increases were far more pronounced on the jump test than on the tail-flick test. Placements dorsal and medial to the amygdala were ineffective. Naltrexone (1-5 micrograms) in the PAG significantly reduced both morphine (tail-flick: 70-75%; jump: 60-81%) and beta-endorphin (tail-flick: 100%; jump: 93%) analgesia elicited from the amygdala, indicating that an opioid synapse in the PAG was integral for the full expression of analgesia elicited from the amygdala by both agonists. Both BFNA (68%) and Ntii (100%) in the PAG significantly reduced morphine, but not beta-endorphin analgesia in the amygdala on the tail-flick test. Ntii in the PAG was more effective in reducing morphine (60%) and beta-endorphin (79%) analgesia in the amygdala on the jump test than BFNA (15-24%). Opioid agonist-induced analgesia in the amygdala was unaffected by opioid antagonists administered into control misplacements in the lateral mesencephalon, and the small hyperalgesia elicited by opioid antagonists in the PAG could not account for the reductions in opioid agonist effects in the amygdala. These data indicate that PAG delta 2, and to a lesser degree, mu opioid receptors are necessary for the full expression of morphine and beta-endorphin analgesia elicited from the amygdala.

Opioid peptides content in the rat brain during the ictal phase and after pentylenetetrazol-kindled rats

We determine the opioid peptide content in the rat brain during the ictal phase and postictal depression after pentylenetetrazol kindling rats. Radioimmunoassays with highly specific antisera risen for Met-enkephalin, Leu-enkephalin and octapeptide, were carried out during the ictal phase, and 15, 30 and 60 min after seizures. We always found an initial IR-Met-enkephalin decrease during the postictal depression content, followed by a reduction in IR-Leu-enkephalin and IR-octapeptide tissular concentration. We suggest a functional and differential release of the opioid peptides, during the postictal depression time-course.

Analysis of central opioid receptor subtype antagonism of hypotonic and hypertonic saline intake in water-deprived rats

Intake of either hypotonic or hypertonic saline solutions is modulated in part by the endogenous opioid system. Morphine and selective mu and delta opioid agonists increase saline intake, while general opioid antagonists reduce saline intake in rats. The present study evaluated whether intracerebroventricular administration of general (naltrexone) and selective mu (beta-funaltrexamine, 5-20 micrograms), mu, (naloxonazine, 50 micrograms), kappa (nor-binaltorphamine, 5-20 micrograms), delta (naltrindole, 20 micrograms), or delta 1 (DALCE, 40 micrograms) opioid receptor subtype antagonists altered water intake and either hypotonic (0.6%) or hypertonic (1.7%) saline intake in water-deprived (24 h) rats over a 3-h time course in a two-bottle choice test. Whereas peripheral naltrexone (0.5-2.5 mg/kg) significantly reduced water intake and hypertonic saline intake, central naltrexone (1-50 micrograms) significantly reduced water intake and hypotonic saline intake. Water intake was significantly reduced following mu and kappa receptor antagonism, but not following mu 1, delta, or delta 1 receptor antagonism. In contrast, neither hypotonic nor hypertonic saline intake was significantly altered by any selective antagonist. These data are discussed in terms of opioid receptor subtype control over saline intake relative to the animal's hydrational state and the roles of palatability and/or salt appetite.

Medullary mu and delta opioid receptors modulate mesencephalic morphine analgesia in rats

Supraspinal opioid analgesia is mediated in part by connections between the midbrain periaqueductal gray (PAG) and rostral ventral medulla (RVM) which includes the nuclei raphe magnus and reticularis gigantocellularis. Serotonergic 5HT2 and 5HT3 receptor subtypes appear to participate in this pathway since general and selective serotonergic antagonists microinjected into the RVM significantly reduced morphine analgesia elicited from the PAG. Since both an enkephalinergic pathway between the PAG and RVM and intrinsic enkephalinergic cells in the RVM exist, the present study evaluated the abilities of general (naltrexone), mu-selective (beta-funaltrexamine: B-FNA) and delta 2-selective (naltrindole) opioid receptor subtype antagonists microinjected into the RVM to alter morphine (2.5 micrograms) analgesia elicited from the PAG as measured by the tail-flick and jump tests. Mesencephalic morphine analgesia was significantly reduced after pretreatment in the RVM with naltrexone (1-10 micrograms), B-FNA (0.5-5 micrograms) or naltrindole (0.5-5 micrograms). Naltrexone in the RVM failed to alter basal nociceptive thresholds and none of the opioid antagonists were effective in reducing mesencephalic morphine analgesia when they were microinjected into placements lateral or dorsal to the RVM. These data indicate that mu and delta 2 opioid receptors in the RVM modulate the transmission of opioid pain-inhibitory signals from the PAG.

Solubilization of high-affinity, guanine nucleotide-sensitive mu-opioid receptors from 7315c cell membranes

High-affinity mu-opioid receptors have been solubilized from 7315c cell membranes. Occupancy of the membrane-associated receptors with morphine before their solubilization in the detergent 3-[(3-cholamidopropyl) dimethyl]-1-propane sulfonate was critical for stabilization of the receptor. The solubilized opioid receptor bound [3H]-etorphine with high affinity (KD = 0.304 +/- 0.06 nM; Bmax = 154 +/- 33 fmol/mg of protein). Of the membrane-associated [3H]etorphine binding sites, 40 +/- 5% were recovered in the solubilized fraction. Both mu-selective and non-selective enkephalins competed with [3H]etorphine for the solubilized binding sites; in contrast, delta- and kappa-opioid enkephalins failed to compete with [3H]etorphine for the solubilized binding sites at concentrations of < 1 microM. The mu-selective ligand [3H][D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin also bound with high affinity (KD = 0.79 nM; Bmax = 108 +/- 17 fmol/mg of protein) to the solubilized material. Of the membrane-associated [3H][D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin binding sites, 43 +/- 3% were recovered in the solubilized material. Guanosine 5'-O-(3-thiotriphosphate), GTP, and guanosine 5'-O-(2-thiodiphosphate), but not adenylylimidodiphosphate, diminished [3H][D-Ala2,N-Me-Phe4,Gly5-ol] enkephalin binding in a concentration-dependent manner. Finally, mu-opioid receptors from rat brain membranes were also solubilized in a high-affinity, guanine nucleotide-sensitive state if membrane-associated receptors were occupied with morphine before and during their solubilization with the detergent 3-[(3-cholamidopropyl)dimethyl]-1-propane sulfonate.

Selective reduction of N-methyl-D-aspartate-evoked responses by 1,3-di(2-tolyl)guanidine in mouse and rat cultured hippocampal pyramidal neurones

1. The effects of 1,3-di(2-tolyl)guanidine (DTG) were examined on the responses of cultured hippocampal neurones to the excitatory amino acid analogues N-methyl-D-aspartate (NMDA), kainate, quisqualate and (RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA). 2. In rat hippocampal neurones loaded with the Ca(2+)-sensitive dye Fura-2, DTG (10-100 microM) produced a concentration-dependent depression of the NMDA-evoked rises in intracellular free calcium ([Ca2+]i), an effect that was not modified by changes in the extracellular glycine concentration. DTG (at 50 and 100 microM) also attenuated, although to a lesser extent, the rises in [Ca2+]i evoked by naturally-derived quisqualate. In contrast, 50 and 100 microM DTG did not depress responses evoked by kainate, AMPA and synthetic, glutamate-free (+)-quisqualate although on occasions DTG enhanced kainate- and AMPA-evoked rises in [Ca2+]i. 3. DTG attenuated NMDA-evoked currents recorded from mouse hippocampal neurones under whole-cell voltage-clamp with an IC50 (mean +/- s.e. mean) of 37 +/- 5 microM at a holding potential of -60 mV. The DTG block of NMDA-evoked responses was not competitive in nature and was not dependent on the extracellular glycine or spermine concentration. The block did, however, exhibit both voltage-, and use-, dependency. The steady-state current evoked by naturally-derived quisqualate was also attenuated by DTG whereas those evoked by kainate and AMPA were not. 4. We conclude that DTG, applied at micromolar concentrations, is a selective NMDA antagonist in cultured hippocampal neurones, the block exhibiting both Mg(2+)- and phencyclidine-like characteristics. Given the nanomolar affinity of DTG for sigma binding sites it is unlikely that the antagonism observed here is mediated by sigma-receptors, but the data emphasize the potential danger of ascribing the functional consequences of DTG administration solely to sigma receptor-mediated events.

Activation of kappa-opioid receptors in the nucleus ambiguus does not affect cardiovascular function or outcome following acute hemorrhage in the rat

The central cardiovascular response to hemorrhage is believed to be regulated, in part, by specific brain cardioregulatory nuclei, including the nucleus ambiguus (NA) of the hindbrain. Since endogenous opioid peptides and opiate receptors have been localized to this brain region, activation of endogenous opioid systems in the NA may affect the cardiovascular response to acute hemorrhage. The present study examined the effects of intracerebral microinjection of kappa-receptor agonists into the NA prior to acute fixed-volume hemorrhage in awake rats. 15 min prior to fixed volume hemorrhage (7.5 ml/300 g), male Sprague-Dawley rats (n = 59) received a microinjection of either (1) the synthetic kappa-receptor agonist U-50,488H (10 nM) or (2) U-50,488H (100 nM) or (3) the endogenous kappa-receptor agonist dynorphin 1-17 (1 nM) or (4) Des-Tyr dynorphin 2-17, inactive at opiate receptors (1 nM) or (5) equal volume saline. With the exception of the first 10 min post-hemorrhage, where intracerebral injection of both dynorphin 1-17 and dynorphin 2-17 caused a transient suppression of mean arterial blood pressure (P < 0.05 when compared to saline-treated controls), microinjection of the kappa-agonists dynorphin 1-17 or U-50,488H had no effect on blood pressure, heart rate or mortality when compared to control animals. These results suggest that activation of kappa-opiate receptors in the NA does not markedly influence cardiovascular response to acute hemorrhage.

Novel pharmacologic therapies in the treatment of experimental traumatic brain injury: a review

Delayed or secondary neuronal damage following traumatic injury to the central nervous system (CNS) may result from pathologic changes in the brain's endogenous neurochemical systems. Although the precise mechanisms mediating secondary damage are poorly understood, posttraumatic neurochemical changes may include overactivation of neurotransmitter release or re-uptake, changes in presynaptic or postsynaptic receptor binding, or the pathologic release or synthesis of endogenous "autodestructive" factors. The identification and characterization of these factors and the timing of the neurochemical cascade after CNS injury provides a window of opportunity for treatment with pharmacologic agents that modify synthesis, release, receptor binding, or physiologic activity with subsequent attenuation of neuronal damage and improvement in outcome. Over the past decade, a number of studies have suggested that modification of postinjury events through pharmacologic intervention can promote functional recovery in both a variety of animal models and clinical CNS injury. This article summarizes recent work suggesting that pharmacologic manipulation of endogenous systems by such diverse pharmacologic agents as anticholinergics, excitatory amino acid antagonists, endogenous opioid antagonists, catecholamines, serotonin antagonists, modulators of arachidonic acid, antioxidants and free radical scavengers, steroid and lipid peroxidation inhibitors, platelet activating factor antagonists, anion exchange inhibitors, magnesium, gangliosides, and calcium channel antagonists may improve functional outcome after brain injury.

Ingestive behavior following central [D-Ala2, Leu5, Cys6]-enkephalin (DALCE), a short-acting agonist and long-acting antagonist at the delta opioid receptor

DALCE (1-40 micrograms, ICV), a short-acting agonist and long-acting antagonist at the delta opioid receptor, was examined for its effects upon food intake in rats under spontaneous, deprivation, glucoprivic and palatable conditions. DALCE (10 micrograms) significantly stimulated free feeding for up to 10 h but only minimally decreased (40 micrograms) food intake and body weight after 24-72 h. DALCE, administered prior to food deprivation (24 h), failed to affect subsequent 24-h intake and sporadically decreased intake and body weight change after 48-72 h. 2-Deoxy-D-glucose (650 mg/kg, IP) hyperphagia was transiently (2 h) decreased by long-term DALCE (10 micrograms) pretreatment. Hyperphagia following exposure to a high-fat diet was significantly potentiated by long-term DALCE (1 microgram) pretreatment. DALCE (10 micrograms) hyperphagia (2-10 h) was eliminated by central pretreatment with either naltrexone (20 micrograms) or the kappa antagonist, nor-binaltorphamine (20 micrograms) but was minimally affected by central pretreatment with the mu antagonist, beta-funaltrexamine (20 micrograms) or long-term DALCE (40 micrograms). The general inability of the antagonist actions of DALCE to alter these forms of feeding argues against a role for the delta opioid receptor in these responses.

Gender effects and central opioid analgesia

Central morphine analgesia is significantly greater in male than in female rats. Since mu and delta opioid receptor subtypes have been implicated in supraspinal analgesia, the present study evaluated whether gender or adult gonadectomy altered (a) analgesia on the tail-flick and jump tests following central administration of the mu-selective agonist, [D-Ala2, Me-Phe4, Gly(ol)5] enkephalin (DAMGO) and the delta-selective agonist, [D-Ser2,Leu5] enkephalin-Thr6 (DSLET) and (b) mu1, mu2 and delta opioid receptor binding. Sham-operated male rats displayed significantly greater magnitudes of peak and total analgesia than sham-operated females on the tail-flick test following DAMGO, but not DSLET. Gender differences were not observed for DAMGO and DSLET analgesia on the jump test. Gonadectomy failed to consistently affect either DAMGO or DSLET analgesia. Regression analyses failed to reflect significant shifts in the dose-response functions for either agonist on either measure. Gender differences were not observed for mu1, mu2, or delta binding in hypothalamus or cortex. These data are compared with analgesic responses sensitive to gender differences.

Pharmacological interaction of opiates with various classes of centrally acting dopaminergic drugs

The comparative analgesic and sedative (narcosis potentiating) efficacy of mu and kappa opioids was studied as a function of time in rats and mice. The mu agonists, morphine and fentanyl, produced antinociceptive actions against both heat and chemical noxious agents, but the half-lives of their ED50s were longer in the writhing than in the hot plate test. The kappa agonist drugs, bremazocine, ethylketocyclazocine and pentazocine, proved to be inactive against heat nociception, and produced a potent, long-lasting analgesia in the acetic acid writhing test, similar to mu agonists. The combination of two mu agonists resulted in a synergistic interaction and a remarkable prolongation of antinociceptive action. When the kappa-drug bremazocine was coadministered with morphine, there was a significant prolongation of the duration of analgesic action, without any influence on the potency. The interactions of mu and kappa opioids with agonists and antagonists at dopamine receptors were also studied in narcosis. The time course of the naloxone-morphine antagonism in analgesiometric assays revealed similarities, when apparent pA2 values were estimated at the peak of agonist and antagonist activity, but it was different in the writhing test when the pA2 was determined 84 minutes after morphine administration (EDt1/2, half-life belonging to the ED50) while naloxone was given at its peak effect.

Reduction by central beta-funaltrexamine of food intake in rats under freely-feeding, deprivation and glucoprivic conditions

The present study evaluated the central effects of beta-funaltrexamine (B-FNA), a non-equilibrium antagonist of mu-opioid receptors and a reversible agonist of kappa-opioid receptors upon food intake in rats under freely-feeding, deprivation and glucoprivic conditions. B-FNA elicited distinct short-term and long-term actions, consistent with binding studies demonstrating its reversible kappa agonist actions and its irreversible mu receptor blockade. Whereas B-FNA (1-20 micrograms, i.c.v.) significantly stimulated free feeding for up to 6 h, B-FNA (10-20 micrograms) significantly inhibited (35-41%) free feeding at 24, 48 and 72 h after injection, a pattern temporally similar to its biochemical opioid effects. Pretreatment (24 h) with B-FNA (10-20 micrograms) significantly inhibited (33-49%) the increased intake following 24 h of food deprivation. Pretreatment (24 h) with B-FNA (10-20 micrograms) also significantly inhibited (75-100%) the increased glucoprivic intake induced by 2-deoxy-D-glucose. The short-term stimulation of food intake by central B-FNA was antagonized by the selective kappa antagonist, nor-binaltorphamine, but was unaffected by pretreatment 24 h earlier with the mu antagonist, B-FNA. Significant reductions in striatal (89%) and hypothalamic (46%) mu-opioid binding occurred in rats pretreated (24 h) with B-FNA; the low levels of delta binding in these structures precluded interpretation of B-FNA effects. These data indicate the importance of the mu-opioid receptor in the modulation of different forms of feeding behavior, and underscores the ability of selective opioid antagonists to delineate precise functional roles for different opioid receptor subtypes.

Opiatergic modulation of preparatory and consummatory components of feeding and drinking

We present data here indicating that stimulation of kappa but not mu opiate receptors influences motivational and consummatory aspects of feeding and drinking. To differentiate mu and kappa mechanisms controlling preparatory (appetitive) and consummatory components of ingestive behavior, the effects of morphine (MORPH), compound U50488H (U50) and naloxone (NAL) were studied in rats trained to negotiate a straight runway using food or water as a reinforcer. At doses that increase feeding and drinking in conditions of free access to food and water (i.e., 1-2 mg/kg IP), MORPH affected neither food- nor water-maintained runway performance. Since 1 mg/kg of NAL is also devoid of effects, mu-opiate mechanisms are probably not involved in food- or water-maintained behavior. Pharmacological manipulation of kappa-opiate mechanisms had complex effects. At 5 mg/kg, NAL accelerated satiation, depressing food intake, without affecting running. U50 did not increase food intake, but accelerated running for food, an effect that was antagonized by a high dose of NAL (5 mg/kg). These findings suggest that motivational and consummatory components of food-maintained runway performance are both activated by kappa-opiate mechanisms. NAL also reduced water intake but had minimal influences on running. In contrast, U50 depressed both water intake and runway performance; rather than being antagonized, these effects were slightly enhanced by NAL. The combined antidipsic and diuretic effects of U50 suggest that kappa-opiate mechanisms play a dissipatory role in water balance. However, the similar antidipsic effects of U50 and NAL, and the fact that NAL did not antagonize the antidipsic effects of U50, suggest that U50 may reduce drinking by mechanisms other than kappa-opiate agonism.

Wash-resistant inhibition of phencyclidine- and haloperidol-sensitive sigma receptor sites in guinea pig brain by putative affinity ligands: determination of selectivity

Several putative affinity ligands, based on the structures of phencyclidine etoxadrol, 5-methyl-10,11-dihydro-5H-dibenzo[a,d] cycloheptene-5,10-imine (MK801) and 1,3-di-(2-methylphenyl)guanidine (DTG) were evaluated in vitro for their ability to produce a wash-resistant inhibition of phencyclidine and sigma receptor sites in homogenates of the brain of the guinea pig. All the phencyclidine-based ligands, including 1-[1-(3-isothiocyanatophenyl)cyclohexyl]piperidine (Metaphit) and (+/-)-N-(2-isothiocyanatoethyl) MK801 [(+/-)-MK801-NCS], produced a wash-resistant inhibition of binding sites for phencyclidine, labelled by [3H]-1-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) and sigma binding sites, labelled by [3H]DTG. The DTG-based ligands, 1-(4-isothiocyanato-2-methylphenyl)-3-(2-methylphenyl)guanidine (DIGIT) and 1-(4-[2-(2-isothiocyanatoethoxy)ethoxy]-2-methyl-phenyl)-3-(2- methylphenyl)guanidine (DIGIE), produced a wash-resistant inhibition of sigma sites, at concentrations as small as 1 microM and also inhibited binding sites for phencyclidine at larger concentrations (100 microM). Both 1-(3-isothiocyanatophenyl)-1-ethyl-4-(2-piperidyl)-1,3-dioxolane (ETOX-NCS) and 1-[1-(3-bromoacetyloxyphenyl)cyclohexyl]-1,2,3,6-tetrahydropyri din e (Bromoacetyl-PCP) were the most potent and selective inhibitors of the binding of [3H]TCP, while DIGIT was the most selective inhibitor of the binding of [3H]DTG. Future studies will examine the selectivity of these agents in vivo after intracerebroventricular administration.

Specificity of phencyclidine-like drugs and benzomorphan opiates for two high affinity phencyclidine binding sites in guinea pig brain

Recently, the presence of two high affinity binding sites for phencyclidine were described in guinea pig brain, with one site coupled to the glutamate excitatory amino acid receptor, specifically activated by N-methyl-D-aspartate (NMDA) (site 1) and the other site associated with the dopamine (DA) reuptake carrier (site 2). Phencyclidine and its analogs, as well as the benzomorphan opiates, are known to interact with binding sites for phencyclidine. In this study, the equilibrium dissociation constants (Kd) of these compounds for the two binding sites for phencyclidine were determined. Phencyclidine and 1-[1-(2-thienyl)cyclohexyl]piperidine (TCP), an analog of PCP, were essentially non-selective between the two sites and also were the two drugs of the group observed to have the highest affinity for site 2. (+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cycloheptene-5,10-imine [(+)MK801] was the most selective agent for site 1, while none of the drugs tested showed selectivity for site 2. In humans, phencyclidine produces psychotomimetic effects, while (+)MK801 has been reported to produce minimal, if any, psychotomimetic effects, at doses sufficient to reduce seizures. These clinical observations, in conjunction with the present biochemical binding data, suggest that (+)MK801 may serve as a "marker" for site 1 and that the psychotomimetic effects of phencyclidine might be mediated by site 2.

Differential modulation by cations of sigma and phencyclidine binding sites in rat brain

The present investigation attempted to differentiate haloperidol-sensitive sigma sites (sigma H) from phencyclidine (PCP) binding sites in rat brain membranes. We studied the effects of several cations at physiologically relevant concentrations on the binding of radioligands selective for sigma H sites ([3H]haloperidol, [3H](+)3-PPP**), and [3H](+)SKF10,047), or for PCP sites ([3H]PCP and [3H]TCP). The PCP sites displayed a markedly greater sensitivity to cations than sigma H sites. This property was reflected by a greater extent of inhibition of the binding of PCP-selective relative to sigma H-selective ligands at a given cation concentration, as well as by lower IC50's and by steeper slopes of the cation dose-response curves. Divalent cations were approximately 100 times more potent than monovalent cations. All cations were inhibitory, except Sr2+ and Ba2+ which, at micromolar concentrations, enhanced PCP binding but not sigma H binding. Thus, PCP-selective sites appeared to be distinct from sigma H sites with regards to several aspects of cation modulation. This is consistent with the view that PCP and sigma H sites are distinct molecular entities. Further, the marked cation sensitivity of the PCP site is consistent with the current hypothesis according to which the PCP site is linked to the N-methyl-D-aspartate (NMDA) receptor-cation channel complex.

Cardiovascular and respiratory effects of an opioid kappa agonist ethylketazocine and sigma agonist N-allylnormetazocine in acutely decerebrated dogs

Effects of opioid kappa agonist ethylketazocine (EKC), sigma agonist (+-)-N-allylnormetazocine (NANM), and naloxone alone and in combination on mean blood pressure (MBP), heart rate (HR), respiratory rate (RR) and minute volume (MV) were studied in acutely decerebrated dogs. EKC (0.5 mg/kg) decreased HR, MBP, RR and MV. Post-EKC NLX increased RR and MV and reversed the bradycardia and hypotension produced by EKC. NANM (1 mg/kg) produced respiratory depression and tachycardia without changing MBP. Post-NANM NLX antagonized tachycardia, increased MBP, however did not significantly change RR and MV. When decerebrate dogs were spinalized at the C-1 level, EKC decreased MBP and HR. These effects were antagonized by NLX. NANM did not change HR but raised MBP in spinalized decerebrate dogs. Since EKC- and NANM-induced cardiovascular and respiratory depression were not observed in conscious intact or chronic spinal dog, it is suggested that: 1) kappaergic system rostral to mesencephalon may play a role in counteracting these depressant effects of EKC; 2) sigma receptor-mediated tachypnea and tachycardia are dissociable; the tachypneic effect may be mediated through higher center while the medulla oblongata is involved in producing tachycardia. These results also suggest that (+-)-NANM probably has several mechanisms of action at several brain sites in producing its effects on respiration and cardiovascular function.

Effects of beta-endorphin, met-enkephalin, and dynorphin A on basal and stimulated insulin secretion in the mouse

Since opioid peptides and opiate receptors have been demonstrated in the pancreatic islets, we investigated the effects of beta-endorphin, met-enkephalin, and dynorphin A, on basal and stimulated insulin secretion in the mouse. Each of the three opioid peptides was injected intravenously (0.06-64 nmol/kg) alone or together with each of the three insulin releasing agents glucose (2.8 mmol/kg), carbachol (cholinergic agonist, 0.16 mumol/kg), or terbutaline (beta 2-adrenoceptor agonist, 3.6 mumol/kg). It was found that beta-endorphin, met-enkephalin, and dynorphin A were all without effect on basal plasma insulin levels, except a slight elevation by beta-endorphin induced at 2 min after its injection at 64 nmol/kg (to 41 +/- 2 microU/mL vs 28 +/- 4 microU/mL in controls; p less than 0.05). Glucose- and terbutaline-induced insulin secretion were inhibited by beta-endorphin at the lower dose levels of 0.25 (p less than 0.01) and 1 nmol/kg (p less than 0.05). This effect was counteracted by the opiate receptor antagonist naloxone (500 micrograms/kg). In contrast, beta-endorphin at the high dose levels of 16 and 64 nmol/kg augmented the glucose- and terbutaline-induced insulin secretion (p less than 0.05). Carbachol-induced insulin secretion was not affected by beta-endorphin at the lower dose levels but augmented by the peptide at 64 nmol/kg (p less than 0.01). Met-enkephalin inhibited glucose- (p less than 0.01) and terbutaline- (p less than 0.05) induced insulin secretion at the high dose rates of 16 and 64 nmol/kg, but the peptide was without effect on carbachol-induced insulin secretion. The inhibitory effects were counteracted by naloxone. Dynorphin A did not affect stimulated insulin secretion at any of the dose levels tested. In summary, in the mouse 1. beta-Endorphin at low dose levels inhibits and at high dose levels augments stimulated insulin secretion; 2. Met-enkephalin inhibits stimulated insulin secretion; and 3. Dynorphin A does not affect insulin secretion. It is suggested that the main influence of beta-endorphin and met-enkephalin under in vivo conditions in the mouse is to inhibit stimulated insulin secretion.

Regulation of sigma-receptors: high- and low-affinity agonist states, GTP shifts, and up-regulation by rimcazole and 1,3-Di(2-tolyl)guanidine

The regulation of the central sigma-binding site was investigated using both in vitro and in vivo manipulations in conjunction with radioligand binding. The displacement of the binding of R(+)-[3H]3-[3-hydroxyphenyl]-N-(1-propyl)piperidine [R(+)-[3H]3-PPP] to cortical homogenates by a range of drugs was consistent with the site labelled being a sigma-receptor. (+)-SKF 10,047, (-)-SKF 10,047, (+/-)-cyclazocine, phencyclidine, and dexoxadrol displaced R(+)-[3H]3-PPP with pseudo-Hill coefficients of less than 1. Further analysis employing nonlinear curve fitting techniques demonstrated that displacement data for these compounds were described better by a model whereby R(+)-[3H]3-PPP was displaced from two discrete sites; approximately 65% of the total sites were in the high-affinity state. In the presence of 10 mM Mg2+ and 0.3 mM GTP, displacement curves for (+)-SKF 10,047 and (+/-)-cyclazocine were shifted to the right. These findings were due to the shift of some 15% of the high-affinity binding sites to a low-affinity state. Saturation experiments revealed that 0.3 mM GTP acted competitively to decrease the affinity of R(+)-[3H]3-PPP for the sigma sites. The sigma-binding site was thus likely to be linked to a guanine nucleotide regulatory (G) protein. Thus sigma drugs could be subdivided on the basis of their GTP sensitivity and pseudo-Hill coefficients, and by analogy with other receptors R(+)-3-PPP, (+)-SKF 10,047, and (+/-)-cyclazocine, may be putative sigma-agonists. 1,3-Di(2-tolyl)guanidine (DTG), rimcazole, and haloperidol displaced R(+)-[3H]3-PPP with pseudo-Hill coefficients of approximately unity and thus may be sigma-antagonists. Subchronic treatment with rimcazole was characterized by slight sedation and a concomitant up-regulation, with a decrease in the affinity, of sigma-binding sites. The schedule of rimcazole also increased dopamine turnover in the nucleus accumbens; both the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) and the DOPAC/dopamine ratio were elevated. DTG produced similar alterations to the binding parameters of the sigma-binding site; however, changes were not observed in general behavior or accumbal dopamine turnover. sigma-Receptors are likely to be linked to a G protein and are functionally involved in the CNS.

The neuroanatomy, neurophysiology, and neurochemistry of pain, stress, and analgesia in newborns and children

Beginning with a brief description of mature anatomic pathways and neurotransmitters in the "pain system," this article details their development in the human fetus, neonate, and child. Special emphasis is given to the basic mechanisms and physiologic effects of opioid analgesia. The clinical implications of these data are described, particularly with regard to the maintenance of cardiovascular stability and hormonal-metabolic homeostasis in newborns and children undergoing surgery or other forms of stress.

Hypothermia elicited by some prodynorphin-derived peptides: opioid and non-opioid actions

Prodynorphin-derived peptides were tested for their effects on body temperature after intracerebroventricular administration to unrestrained male rats. Dynorphin A (Dyn A) (5 and 10 nmol) and Dynorphin A-(1-32) (Dyn A-(1-32) (2.5 and 5 nmol) lowered body temperature with a maximum approximately 30 min after administration. Dyn B (up to 50 nmol) did not induce hypothermia. Lower doses of all peptides did not alter body temperature. The hypothermic effect was significantly, but not completely prevented by MR1452 (30 nmol), a preferential antagonist of the kappa receptor, administered intracerebroventricularly. Naloxone, a mu receptor antagonist, naltrexone, its long acting analog up to doses of 100 nmol, as well as MR1453, the (+)-enantiomer of kappa antagonist MR1452 with no opioid binding properties, did not prevent the hypothermic effect. Moreover, episodic barrel rolling and bizarre postures elicited by Dyn A and Dyn A-(1-32) were reduced in rats pretreated i.c.v. with MR1452 (30 nmol), but not with naloxone (up to 100 nmol). Interestingly, des-Tyr-Dynorphin A (Dyn A-(2-17)), a fragment with virtually no opioid binding potential, was 4 times less potent that Dyn A in inducing hypothermia. These findings are consistent with the hypothesis that prodynorphin-derived peptides effects are not exclusively opioids in nature.

Effect of chronic neonatal morphine and naloxone on sensorimotor and social development of young rats

Chronic morphine treatment of newborn Long-Evans rat pups between 3-26 days of age (thrice daily starting with 0.5 mg/kg, increased daily by 0.5 mg/kg to 10 mg/kg) led to lags of 1 to 3 days in physical development (body weights and eye opening times) and motor coordination (catalepsy test, grasping, swimming). Chronic naloxone treatment (5 mg/kg administered thrice daily from day 3-26), in contrast, led to modest gains in development on a number of measures (body weights, vaginal opening). Morphine animals also lagged behind controls and naloxone-tested animals in social behaviors, such as homing and play. Chronic naloxone did not block or retard social development; in fact naloxone-treated animals exhibited more rapid acquisition of homing behavior than controls.

The involvement of opioid peptides in stress-induced analgesia in the slug Arion ater

Application of tail-pinch stress to the terrestrial slug, Arion ater, produced a significant increase in the response time when tested on the hot-plate for foot-lifting response. The analgesia was completely reversed by injections of the opiate antagonists, naltrexone and ICI 174864, in a dose-dependent manner. Analgesia could also be elicited by the injection into the foot of beta-endorphin and the enkephalin analogues, DAGO and DADLE. No effect was seen with dynorphin A (1-8) or dynorphin A (1-17). The stress-induced analgesia disappeared after 30 minutes but could be maintained for 100 min following the injection of a mixture of bestatin and the enkephalinase inhibitor, N-carboxymethyl-L-phenylalanyl-L-leucine. This work suggests that in the slug, a physical stressor produces an analgesia which may be due to the release of endogenous opiates.

Subchronic administration of MK-801 in the rat decreases cortical binding of [3H]D-AP5, suggesting down-regulation of the cortical N-methyl-D-aspartate receptors

The effects of the subchronic administration of (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine (MK-801) (0.5 mg/kg twice daily, 7 days) on N-methyl-D-aspartate, phencyclidine and sigma binding sites, behaviour and catecholamine turnover were investigated in the rat. Overt behaviours induced by MK-801 on day 7 were significantly altered relative to day 1 with subchronically treated rats not showing head weaving, goss ataxia or loss of hindlimb control: locomotion and sniffing were largely unaffected. The mean intensities of behaviour were 1.8 and 5.4 for days 7 and 1, respectively. Behavioural tolerance was accompanied by a significant reduction in the density of cortical N-methyl-D-aspartate receptors as measured by [3H]D-2-amino-5-phosphonopentanoic acid binding, while affinity was unchanged: the density of binding sites was 3.52 and 1.88 pmol/mg protein for saline- and MK-801-treated rats, respectively. The N-methyl-D-aspartate ion channel as measured by the binding of [3H]N-(1-[2-thienyl]cyclohexyl)piperidine was not affected by the schedule of MK-801. Additionally, changes were not observed to N-methyl-D-aspartate- or glycine-stimulated [3H]N-(1-[2-thienyl]cyclohexyl)piperidine binding or to sigma binding. Catecholamine turnover was unaltered in the nucleus accumbens septi after the schedule of MK-801. Our results demonstrate that the subchronic administration of MK-801 produces behavioural tolerance and down-regulation of N-methyl-D-aspartate binding sites and suggest differential regulation of the domains of the N-methyl-D-aspartate receptor-ionophore complex.

Phencyclidine receptors and N-methyl-D-aspartate antagonism: electrophysiologic data correlates with known behaviours

Using cortical wedges and isolated frog spinal cords, the potency of a series of psychoactive phencyclidine (PCP) and sigma receptor ligands as antagonists of N-methyl-D-aspartate (NMDA) has been compared with their potency in neurochemical and behavioural studies. Phencyclidine receptor, but not sigma or kappa, ligands were selective antagonists of NMDA on both preparations. Combination studies suggested that dissociative anaesthetics and sigma benzomorphans act at the same site. The relative potencies of the drugs as NMDA antagonists correlated well with their potency in PCP receptor binding studies in vitro and in PCP discrimination studies in vivo.

High-dose naloxone in older normal subjects: implications for Alzheimer's disease

The behavioral and cognitive effects of naloxone HCl, in doses of 5 micrograms/kg, 0.1 mg/kg, and 2.0 mg/kg administered as an IV bolus, were assessed in a double-blind, placebo-controlled, randomized study of eight normal subjects ranging in age from 44 to 74 years (mean 63). Naloxone produced mild behavioral effects with slight cognitive impairment after the 2.0 mg/kg dose only. The threshold, dose dependency, characteristics, and magnitude of these behavioral effects were similar to what has previously been reported in young normal subjects, but markedly different from those observed in patients with dementia of the Alzheimer type (DAT) matched in age to the current study sample. These data suggest that the metabolic fate of naloxone is not substantially affected by age within the range studied. The findings of this study provide further support for a role for endogenous opiate systems in the modulation of behavior and cognition, and suggest that the unusual behavioral sensitivity of patients with DAT to naloxone cannot be accounted for by the effect of age.

Opposite effects of restraint on morphine analgesia and naloxone-induced jumping

It has been demonstrated that the effects of exogenous opiates like morphine could be modified by exposure of an organism to stress, but it is uncertain whether this modification is due to the action of endogenous opioid peptides released by stressful stimuli. The stress of restraint produced an antinociceptive response in mice measured by a latency to escape from a hot plate and, in addition, markedly potentiated analgesia induced by low doses of morphine. Both effects were antagonized by naloxone in a dose-dependent manner. On the other hand, restraint reduced the naloxone-precipitated jumping after single morphine injection. Morphine analgesia and a jumping response were not correlated when tested in two different strains of mice. It is suggested that the enhancement of morphine analgesia by restraint and the reduction in naloxone-induced jumping are mediated via independent mechanisms.

Opioid kappa receptors correlate with the development of conditioned suppression of motility in mice

Mice exhibited a marked suppression of motility (conditioned suppression) when placed in the same environment in which they had previously received an electric footshock. The conditioned suppression of motility was potentiated by U-69,593, a selective kappa (non-mu) agonist. This effect of U-69,593 was antagonized by MR-2266, a kappa antagonist. Additionally, in contrast to U-69,593, MR-2266 alone attenuated the conditioned suppression of motility. These results suggest that the activation of kappa receptors may be responsible for the development of conditioned suppression of motility.

The effect of opiates upon prostatic carcinoma cell growth

The effect of opiate receptor agonists upon cell growth of the prostatic carcinoma cell line DU145 were studied. Dynorphin-A increased growth significantly with a peak response at 10(-13) M, of 21 +/- 4% (mean +/- SEM). The dose response curve had a typical inverted-U shape. Dynorphin fragments 1-13 and 1-7 also increased growth at 10(-13) M, while the 2-13 fragment failed to increase growth. Naloxone increased growth at high concentration (10(-7) M) suggesting a stimulatory effect, while at the same time blocking the effect of dynorphin-A. This data demonstrates that agents which stimulate opiate receptors, especially the kappa receptor agonist dynorphin, increase the growth of prostatic carcinoma, and that this effect is controlled by changes at the N-terminal end of the peptide. This effect is blocked by Naloxone.

Unidirectional analgesic cross-tolerance between morphine and levorphanol in the rat

Clinically, patients often demonstrate incomplete cross-tolerance between opiate analgesics. Although dispositional and pharmacokinetic factors may be a factor, our results suggest that differences in selectivity of various opioids for those opioid receptor subtypes involved in analgesia, mu 1, kappa and delta, also play an important role. In binding studies, levorphanol potently labelled all 3 classes whereas morphine was relatively selective for mu sites. Levorphanol infusions yielded tolerance to both morphine and levorphanol while morphine infusions selectively produced tolerance to morphine. This unidirectional tolerance might be due to the selectivity of morphine for mu receptors compared to levorphanol's ability to interact more potently with other relevant receptor subtypes. These observations raise the possibility that the order in which different opioid analgesics are administered may be of clinical significance.

Gender determinants of opioid mediation of swim analgesia in rats

Continuous cold-water swims (CCWS) and intermittent cold-water swims (ICWS) elicit respective nonopioid and opioid analgesic responses in adult male rats. The present experiment evaluated whether gender differences were observed in naloxone's (14 mg/kg, SC) ability to alter differentially CCWS and ICWS analgesia on the tail-flick and jump tests in age-matched and weight-matched intact rats and in gonadectomized rats. CCWS analgesia was unaffected by naloxone on either test in age-matched males and females. Naloxone significantly reduced ICWS analgesia on the tail-flick (45%) and jump (37%) tests in intact males, but not age-matched females. Naloxone significantly reversed ICWS analgesia in weight-matched males on the tail-flick (1-14 mg/kg, 30-32%) and jump (14 mg/kg, 31%) tests. Naloxone also significantly reduced ICWS analgesia on the tail-flick (32%) and jump (41%) tests in castrated males, but not ovariectomized females. Changes in swim hypothermia could not account for the above effects. These data indicate gender differences in naloxone's differential modulation of swim analgesia, and reflect further differences in pain-inhibitory responses as a function of gender.

At which 'sigma' site are the spinal actions of ketamine mediated?

Electrophysiological experiments have been performed on rat spinal nociceptive reflexes to examine whether the spinal actions of ketamine are mediated at the haloperidol-sensitive sigma-site (which is particularly concentrated in the spinal ventral horn) or at the other site which has been referred to as sigma and which is characterized by phencyclidine (PCP), an antagonist of N-methylaspartate (NMA). The effects of ketamine on nociceptive reflexes were not affected by haloperidol 1-100 micrograms/kg i.v. The anticonvulsive agent MK-801, which is active at the PCP site but relatively inactive at the haloperidol-sensitive sigma-site, behaved like ketamine in reducing nociceptive reflexes at NMA-blocking doses; its actions were not affected by haloperidol at up to 1 mg/kg. Ketamine thus appears to manifest its spinal actions at the PCP site, a finding supportive of these spinal nociceptive reflexes being mediated by NMA receptors.

Differential sensitivity of opioid-induced feeding to naloxone and naloxonazine

The high-affinity mu-1 opioid binding site has been implicated in some opioid responses (e.g., supraspinal analgesia) but not others (e.g., respiratory depression) by comparing the actions of naloxone, a short-acting, non-selective antagonist, and naloxonazine, an irreversible and selective mu-1 antagonist. The mu-1 site has been implicated in the opioid component modulating free feeding and deprivation-induced feeding, but not glucoprivic feeding. The present study compared naloxone and naloxonazine antagonism of hyperphagia induced by morphine, ethylketocyclazocine (EKC), dynorphin and d-ala2,d-leu5-enkephalin (DADL) in rats. Morphine produced a dose-dependent (0.01-5 mg/kg) hyperphagia in mildly food-deprived rats that was blocked by naloxone (0.01-10 mg/kg). Naloxonazine (10 mg/kg) shifted the morphine hyperphagia dose-response curve to the right. These effects could not be fully accounted for by the intrinsic hypophagic properties of these antagonists. EKC produced a dose-dependent (0.5-5 mg/kg) hyperphagia which was blocked by naloxone (10 mg/kg) only at low effective EKC doses. Naloxonazine (10 mg/kg) failed to affect EKC hyperphagia. Naloxone, but not naloxonazine also blocked dynorphin and DADL hyperphagia. These results indicate that feeding induced by opiate and opioid agonists are differentially mediated by the mu-1 and other opioid binding sites; these data contrast with the modulation by the mu-1 site of the supraspinal analgesia induced by each of these agonists.

Human studies on the mu opiate receptor agonist fentanyl: neuroendocrine and behavioral responses

The neuroendocrine and behavioral responses to the potent mu opiate receptor agonist Fentanyl (FE) have been systematically investigated in healthy male volunteers. These volunteers received, according to a randomized block design, different doses of FE: 0.1 mg/70 kg (n = 11), 0.2 mg/70 kg (n = 11), 0.25 mg/70 kg (n = 8), and saline (n = 11). FE induced a pronounced dose-dependent increase of plasma prolactin concentrations, which was significant at the lowest dose. In contrast, growth hormone was significantly stimulated by the highest FE dose only. Moreover, FE induced a maximum reduction of plasma cortisol concentrations at the lowest dose (0.1 mg/70 kg). In parallel, marked euphoric responses were also observed at this lowest FE dose. These results suggest a mu specific influence on all neuroendocrine and behavioral parameters investigated. Different responses of these parameters to different doses of FE, however, suggest a differential modulation of these parameters by the mu receptor agonist FE.

A role played by sigma receptors in the conditioned suppression of motility in mice

Mice exhibited marked suppression of motility (conditioned suppression) when placed in the same environment in which they had previously received an electric footshock. The conditioned suppression was attenuated by cyclazocine and N-allylnormetazocine (SKF-10047), sigma agonists. The effect of these drugs was reduced by chronic pretreatment with cyclazocine. This behavioral change was related to the change in binding activity of [3H]-phencyclidine to sigma receptors (defined using non-radioactive SKF-10047). In the chronic vehicle-pretreated conditioned suppression group, the Kd and Bmax values of [3H]-phencyclidine binding at the high affinity site were increased when compared to those in the chronic vehicle-pretreated control group. The increased values were restored to the control levels by acute treatment with cyclazocine and SKF-10047. On the other hand, in the chronic cyclazocine-pretreated conditioned suppression group, acute cyclazocine and SKF-10047 treatment failed to change the increased values of Kd and Bmax at the high affinity site. The present behavioral and receptor-binding experiments suggest that the activation of nervous system mediated by sigma receptors may be responsible for the attenuation of conditioned suppression of motility.

The significance of multiple CNS opioid receptor types: a review of critical considerations relating to technical details and anatomy in the study of central opioid actions

The study of the CNS actions of opioids is complicated by the presence of both multiple opioid receptors and endogenous ligands in the brain. The recent descriptions of opioid isoreceptors, of tonic opioid systems, and of multiple opioid receptors on a single neuron are further technical details which must be considered. In the use of various opiates and opioid peptides to study physiological systems, the multiple opioid affinities of these compounds, as well as potential non-opioid actions, must be controlled for in the experimental design. In conjunction with the multiple receptor affinities of various opiates is the problem of receptor dualism with some drugs; particularly with the agonist/antagonist analgesics. Species differences in the relative proportions of different opioid receptor populations also limit any generalizations of a finding in one species. These limitations in the study of opioid receptors will be discussed with reference to previous neurochemical, neuroendocrine, electrophysiological and behavioral reports of multiple opioid receptors.

Different effects of ethylketocyclazocine on phencyclidine- and N-allylnormetazocine-induced stereotyped behaviors in rats

The effects of ethylketocyclazocine (EKC) on the stereotyped behaviors induced by intraperitoneal injection of phencyclidine (PCP) or N-allylnormetazocine (SKF 10,047) were examined. EKC markedly antagonized PCP-induced stereotyped behaviors such as sniffing, head-weaving, turning and backpedalling. On the other hand, EKC failed to antagonize SKF 10,047-induced stereotyped behaviors, which are PCP-like stereotyped behaviors, except sniffing and head-weaving at 0-15 min after the SKF 10,047 injection. PCP-induced turning and backpedalling were potentiated by pretreatment with SKF 10,047, while PCP-induced sniffing and head-weaving were not. EKC failed to affect the enhancing effect of SKF 10,047 on PCP-induced turning and backpedalling. These results suggest that part of the PCP- and SKF 10,047-induced stereotypy may be mediated by different neuronal mechanisms.

Alterations in regional concentrations of endogenous opioids following traumatic brain injury in the cat

Delayed injury following trauma to the central nervous system (CNS) may be due to the release or activation of endogenous factors. Endogenous opioid peptides have been proposed as one such class of injury factors, based on pharmacological studies demonstrating a therapeutic effect of naloxone and other opiate receptor antagonists following CNS injury. However, changes in brain opioid concentrations following injury have not been evaluated. In the present study, we measured regional alterations in dynorphin (ir-Dyn), leucine-enkephalin (ir-Enk) and beta-endorphin immunoreactivity (ir-End) following low- (1.0-2.0 atmospheres (atm)) or high- (3.0-4.0 atm) level fluid-percussion brain injury in the cat. A significant decrease in ir-End was observed in the hypothalamus at 2 h following high- but not low-level injury. No changes were observed in tissue ir-Enk following either level of injury. Severe brain trauma but not low-level injury caused a significant increase in ir-Dyn in the striatum, frontal cortex, parietal cortex, pons and medulla. In the anterior pituitary, a significant increase in ir-End and a significant decrease in ir-Dyn was observed at 2 h following both levels of injury. Pathological damage to brain tissue after injury was most pronounced in those regions showing significant increases in ir-Dyn but not other opioids. In the medulla, the increase in ir-Dyn but not ir-End or ir-Enk was also significantly correlated with a fall in systemic mean arterial pressure (MAP) at 2 h following high- but not low-level injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Immediate and long-term effects of opiate antagonists on postictal behaviour following amygdala kindling in the rat

Male Wistar rats implanted with bipolar electrodes in the amygdaloid complex were kindled. Subcutaneous injection of naloxone or naltrexone in low doses--0.12 and 0.24 mg/kg, respectively--dramatically reduced the postictal behavioural depression (BD) at 10 or 60 min. Remarkably, the BD was still reduced one day later. It would appear that the brain mechanisms involved in postictal BD use mu-receptors since BD is quite sensitive to low doses of the preferential antagonists naloxone and naltrexone. The long-term effects, the most novel aspect of these studies, are probably related to immediate effects but could be produced through slow genomic processes or alteration of the response to endogenously released enkephalins.

Beta endorphin modulation of the glucoregulatory effects of repeated epinephrine infusion in alloxan-diabetic and normal dogs

When repeated epinephrine infusions are given to normal dogs as a partial stress model, there is exaggerated hyperglycaemia, associated with reduced plasma insulin levels and markedly decreased glucose clearance. In the present study, we have examined the hormonal and metabolic responses to two successive 60-min epinephrine (0.1 microgram . kg-1 . min-1) infusions with or without concomitant infusion of beta endorphin (0.3 microgram . kg-1 . min-1) in 6 alloxan-diabetic dogs. These studies have been compared to similar studies in 5 normal dogs. In the diabetic dogs, plasma glucose rose from 12.3 +/- 2.2 to 16.2 +/- 2.4 mmol/l (p less than 0.001) in response to the first epinephrine infusion and rose further to 18.1 +/- 2.5 mmol/l (p less than 0.001) during the second epinephrine infusion. The increases in plasma glucagon and glucose production were comparable with both infusions, but considerably greater than previously observed in normal dogs. In normal dogs, beta endorphin diminished the insulin response to the first epinephrine infusion (p less than 0.02), and abolished this response to the second (p less than 0.05). In addition beta endorphin also diminished the glucagon response to the second epinephrine infusion (p less than 0.01) and greatly potentiated epinephrine-induced suppression of glucose metabolic clearance during both infusions (p less than 0.001). However, beta endorphin did not appreciably alter the hyperglycaemic response to epinephrine due to a concomitant attenuation of the epinephrine-induced increase in hepatic glucose production. In contrast to normal dogs, beta endorphin did not modulate the effects of either the first or second epinephrine infusion on glucose kinetics in diabetic dogs. Also, beta endorphin failed to inhibit glucagon or insulin secretion in response to epinephrine in the diabetic animals. Since the alloxan-diabetic and normal dogs respond differently to the combined infusion of beta endorphin and epinephrine we conclude that the effects of beta endorphin observed in the normal dogs are dependent upon intact pancreatic endocrine function.