Effects of central administration of highly selective opioid mu-, delta- and kappa-receptor agonists on plasma luteinizing hormone (LH), prolactin, and the estrogen-induced LH surge in ovariectomized ewes

A reduction in endogenous opioid inhibition (disinhibition) of GnRH secretion is thought to be permissive for the preovulatory GnRH/LH surge. There are no published studies of the effects of highly specific opioid receptor agonists on the LH surge in any species, and the relative importance of the opioid receptor subtypes mu, delta and kappa in the mechanism of disinhibition is unknown. In sheep, attempts to block the LH surge with opiates have been largely unsuccessful, and there is little evidence for reduced opioid inhibition during the GnRH/LH surge. The opioid receptor subtypes regulating PRL secretion in sheep are also unknown. Conscious, ovariectomized ewes with permanent third ventricular cannulae were injected with estradiol benzoate (EB) 50 micrograms or oil im (t = 0 h). In this model, EB elicits a time-delayed surge in LH secretion after 13-18 h. Jugular venous blood was sampled at half hourly intervals between-2 and 0 h and 10 and 26 h. From 12-20 h, infusions were made into the third ventricle of either the highly specific mu-agonist DAGO (10, 20 or 40 nmol/h), the delta-agonist DPDPE (40 nmol/h), the kappa-agonist U50488 (40 nmol/h) or saline (vehicle). In oil-treated animals (n = 4-6), DAGO infusion at 20 and 40 nmol/h reduced plasma LH whereas DPDPE or U50488 had no effect. In EB-treated animals (n = 6), DAGO (40 nmol/h) delayed the LH surge (mean +/- SEM time to surge onset 21.4 +/- 0.3 h vs. 14.0 +/- 0.4 h in controls, P < 0.0001). DAGO at 10 nmol/h did not alter surge onset and at 20 nmol/h had variable effects. DPDPE or U50488 did not affect LH surge timing or amplitude. All doses of DAGO increased plasma PRL, whereas DPDPE and U50488 had no effect. We conclude that, in ovariectomized ewes, activation of opioid mu-receptors, but not delta- or kappa-receptors, inhibits GnRH secretion, can block the estrogen-induced GnRH/LH surge and increases PRL secretion. The results are consistent with the disinhibition hypothesis.

Effects of exposure regimen on changes in sensitivity to the effects of cocaine on schedule-controlled behavior in rhesus monkeys

It has been reported that conditions of drug exposure can influence changes in sensitivity to cocaine upon repeated administration. In the present experiment, the behavioral effects of prolonged exposure to continuous or intermittent infusion of cocaine were compared in rhesus monkeys responding under a multiple component fixed-ratio (FR) schedule of food presentation. In order to quantify changes in sensitivity to cocaine, cumulative dose-response functions for acute cocaine were determined using a multiple schedule comprised of six 10-minute components separated by time-out periods of 3 minutes. Initially, cocaine decreased responding in a dose-related manner. Continuous infusion of cocaine (4 mg/kg per day) for a period of 4 weeks resulted in a 2- to 4-fold shift to the right in the cocaine dose-response function, i.e., tolerance developed. In contrast, when the same daily dose of cocaine was injected intermittently (1.0 mg/kg per injection) four times/day to different monkeys, there was no change in the effects of cocaine on responding. The present results support the notion that the dosing regimen is an important determinant of changes in sensitivity to the behavioral effects of cocaine. Additionally, since sensitization failed to develop upon intermittent administration of cocaine, behavioral baseline may play a role in changes in sensitivity to cocaine.

Role of central mu, delta-1, and kappa-1 opioid receptors in opioid-induced muscle rigidity in the rat

BACKGROUND

Opioids appear to produce their physiologic effects by binding to at least three types of opioid receptors, the mu (mu), delta (delta), and kappa (kappa) receptors. Muscle rigidity occurs after administration of supra-analgesic doses of potent mu-preferring agonists like alfentanil. The role of different supraspinal opioid receptors in this rigidity has been addressed only recently. To elucidate the contribution of central mu, delta, and kappa receptors to muscle rigidity, the effects of intracerebroventricularly administered opioid receptor-selective agonists and antagonists on alfentanil-induced muscle rigidity were examined in rats.

METHODS

Rats in which chronic intracerebroventricular cannulae had been implanted received an intracerebroventricular infusion of either saline or a mu (D-Ala2,N-Me-Phe4-Gly5-olenkephalin; DAMGO), delta(1) (D-Pen2,D-Pen5-enkephalin; DPDPE), or kappa(1) (trans-(+/-)-3,4-dichloro-N-methyl-N-(2-(1-pyrrolidinyl)- cyclohexyl)-benzene-acetamide methane sulfonate; U50,488H) opioid agonist. Ten minutes later, they received either saline or the mu-agonist alfentanil subcutaneously. Muscle rigidity was assessed using hindlimb electromyographic activity. Different groups of animals were pretreated with an intracerebroventricular infusion of either saline or a mu (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2; CTAP), delta (naltrindole), or kappa(1) (norbinaltorphimine) opioid antagonist before administration of either saline or a selective intracerebroventricular agonist.

RESULTS

The mu agonist DAMGO alone dose-dependently induced muscle rigidity. This effect was antagonized by pretreatment with the mu-selective antagonist CTAP. Neither DPDPE nor U50,488H, when administered alone, affected muscle tone. However, both the delta(1) and kappa(1) agonists dose-dependently attenuated alfentanil-induced rigidity. This antagonism of alfentanil rigidity was abolished after pretreatment with the delta (naltrindole) and kappa(1) (nor-binaltorphimine) antagonists, respectively.

CONCLUSIONS

The present data demonstrate that whereas systemic opiate-induced muscle rigidity is primarily due to the activation of central mu receptors, supraspinal delta(1) and kappa(1) receptors may attenuate this effect. This finding is consistent with previous demonstrations of functional interactions between different central opioid receptor populations in other opiate effects, and could have important pharmacotherapeutic implications.

Continuous co-administration of dextromethorphan or MK-801 with morphine: attenuation of morphine dependence and naloxone-reversible attenuation of morphine tolerance

N-Methyl-D-aspartate (NMDA) receptor antagonists have been repeatedly shown to attenuate the development of opiate tolerance and dependence in rodents. In the present experiments, continuous subcutaneous infusion of either MK-801 (0.01 mg/kg/h but not 0.005 mg/kg/h) or DM (0.133, 0.67 and 1.33 mg/kg/h) reliably prolonged the antinociceptive effect of continuous subcutaneous infusion of morphine sulfate (2.0 mg/kg/h), indicating attenuation of the development of morphine tolerance. Furthermore, this prolonged antinociception was completely reversible by naloxone (10 mg/kg, i.p.). Doses of MK-801 and DM that were equipotent in attenuating morphine tolerance (0.01 mg/kg/h and 1.33 mg/kg/h, respectively) revealed different profiles of effects, however, on locomotor activity and naloxone-precipitated abstinence/withdrawal symptoms. With regard to locomotor activity, rats having received continuous (48 h) subcutaneous infusion of morphine sulfate and MK-801, but not rats having received morphine sulfate and DM, displayed a reliable and striking increase in locomotor activity as compared with rats having received morphine alone. With regard to naloxone-precipitated withdrawal symptoms, continuous (48 h) subcutaneous co-infusion of either MK-801 (0.01 mg/kg/h) or DM (1.33 mg/kg/h) with morphine attenuated naloxone-precipitated hyperalgesia as compared with rats infused with morphine alone. MK-801 (0.01 mg/kg/h) was more effective than DM (0.133, 0.67, or 1.33 mg/kg/h), however, in reducing other naloxone-precipitated withdrawal symptoms (teeth chattering, jumping and wet dog shakes). The effects of MK-801 on all withdrawal symptoms were confounded, however, by the appearance of flaccidity following naloxone administration to rats having received MK-801 and morphine. These results extend previous observations by showing that the prolonged antinociception observed following co-administration of morphine and an NMDA antagonist is completely naloxone-reversible, supporting the notion that this antinociception reflects prolongation of an opioid receptor-mediated effect. The different profiles of side effects associated with MK-801 and DM, however, suggest that (1) attenuation of naloxone-precipitated withdrawal symptoms by MK-801 may be an artifact of toxicity, and (2) DM may prove clinically useful for the prevention of morphine tolerance, given its lack of observable side effects when administered concurrently with morphine to rodents.

Naloxone administration does not relieve the inhibition of gonadotropin release in food-restricted, lactating rats

Lactational anovulation is an important factor in determining birth spacing in women living in developing countries. Therefore, a more comprehensive understanding of the mechanisms involved in the relationships among lactation, nutrition and ovulation is important. This study was designed using the food-restricted, lactating rat to examine whether endogenous opioids might be involved in depressing gonadotropin release. Females were mated after 65 d of age and, beginning on d 42 of life, offered food in unrestricted amounts (control) or were food restricted to 50% of what the controls consumed. On d 15 of lactation, dams were injected with either naloxone hydrochloride (3 mg/kg body weight) or saline and killed 0, 15, 30 or 60 min later. Plasma was analyzed for luteinizing hormone, follicle-stimulating hormone and prolactin. Food restriction decreased plasma concentrations of luteinizing hormone and follicle-stimulating hormone (P < 0.005). Naloxone administration marginally influenced follicle stimulating hormone (P < 0.1), but not luteinizing hormone concentration regardless of diet group. The interaction among diet group, drug group and time of killing was significant for plasma prolactin concentration (P < 0.05). Food restriction lowered prolactin concentrations, but this effect was diminished with increasing time after injection of naloxone. Furthermore, the magnitude of the effect of food restriction was lessened and even reversed with treatment of naloxone. These results indicate that endogenous opioids are not the primary mechanism suppressing luteinizing hormone release in food-restricted lactating rats.

Opioid growth factor tonically inhibits human colon cancer cell proliferation in tissue culture

Native opioid peptides serve as growth factors in a number of normal and neoplastic cells and tissues, including the prevention and delayed growth of human colon cancer xenografts in nude mice. This study examined the hypothesis that opioids exert a direct inhibitory influence on tumor cell growth by the use of a tissue culture model. The naturally occurring pentapeptide [Met5]enkephalin depressed growth of HT-29 human colon cancer cells from 17 to 41% at 12-72 h after administration of 10(-6)M concentration; consistent with previously defined nomenclature, this peptide was termed opioid growth factor (OGF). OGF action exhibited a dose-response relationship, was reversible and not cytotoxic, and was opioid receptor mediated. Growth inhibition by OGF was not dependent on serum, and was noted in the two other human colon cancer cell lines examined WiDr and COLO 205. This peptide continually repressed growth because an increase in cell number was noted when cells were exposed to the potent opioid antagonist naltrexone or an antibody to OGF. Both OGF and its receptor, zeta (zeta), were found in colon cancer cells by immunocytochemistry, and receptor binding assays revealed a nuclear-associated receptor with a dissociation constant of 8.9 nM and a maximum binding capacity of 43 fmol/mg of protein. OGF was produced and secreted by the tumor cells. These results lead to the suggestion that OGF has a direct, tonic, inhibitory action on the growth of human colon cancer cells and contribute to our understanding of the mechanisms underlying the marked antitumor effect of this peptide in nude mice inoculated with human colon cancer cells.

The stimulatory effect of opioids on mitogen-activated protein kinase in Chinese hamster ovary cells transfected to express mu-opioid receptors

The mitogen-activated protein (MAP) kinase of Chinese hamster ovary cells (CHO mu66 cell line) transfected to express mu-opioid receptors was markedly activated by mu agonists. The rank order of effectiveness of agonists was approximately the same as the rank order of their binding affinities to the mu receptor. The delta and kappa receptor-specific agonists cyclic[D-Pen2, D-Pen5]enkephalin and U69,593 showed a very weak stimulatory effect. The mu agonist-stimulated MAP kinase activity peaked at approximately 4-8 min and lasted almost 1 hr. The stimulatory effect of mu agonists was antagonized by the opioid receptor antagonist naltrexone and inhibited by pretreatment of cells with pertussis toxin. This opioid-induced activation of MAP kinase activity may have a role in the long term effects of opioids.

The mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) [but not D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP)] produces a nonopioid receptor-mediated increase in K+ conductance of rat locus ceruleus neurons

The somatostatin analogues D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) and D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP) have been used widely as selective antagonists of mu-opioid receptors. Actions of CTOP and CTAP on the membrane properties of rat locus ceruleus neurons were studied using intracellular recordings of membrane currents in superfused brain slices. CTOP increased a K+ conductance with an EC50 of 560 nM. The maximal conductance increase produced by CTOP (10 microM) was similar to that produced by high concentrations of the mu-opioid agonists D-Ala-Met-enkephalinglyol (1 microM) and Met-enkephalin (10 microM), as well as an alpha 2-adrenoceptor agonist (UK14304, 3 microM) and somatostatin (1 microM). The K+ current produced by CTOP was not antagonized by naloxone (1 microM), suggesting it was not mediated by mu-opioid receptors. The K+ currents induced by high concentrations of CTOP desensitized to 42% of the initial maximum after prolonged superfusion (t1/2 = 247 sec). In the presence of fully desensitized CTOP responses, somatostatin (1 microM) still produced near-maximal K+ currents; i.e., there was no cross-desensitization, which suggests that CTOP might act on a receptor distinct from somatostatin receptors. However, the converse did not apply; high concentrations of CTOP (30 microM) did not produce any additional current in the presence of desensitized somatostatin responses. No cross-desensitization was observed between CTOP (10-30 microM) and Met-enkephalin (30 microM) or nociceptin (3 microM) regardless of the order of drug application. Cyclo-(7-aminoheptanoyl-Phe-D-Trp-Lys-Thr[Bzl], antagonized both somatostatin-(KD = 10 microM) and CTOP-(KD = 8 microM) induced K+ currents with similar potency. Concentrations of CTOP (100 nM) that produced a small K+ current partially antagonized the actions of Met-enkephalin (10 microM) on mu-opioid receptors. In contrast to CTOP, CTAP produced no K+ current at concentrations of 300 nM and 1 microM and little current at 10 microM. CTAP potently antagonized K+ currents produced by the mu-opioid receptor agonist D-Ala-Met-enkephalin-glyol, with an equilibrium dissociation constant of 4 nM (Schild analysis). CTAP did not antagonize K+ currents produced by CTOP or somatostatin. These results demonstrate that CTOP is a potent and efficacious agonist at nonopioid receptors, whereas CTAP is a potent mu-opioid receptor antagonist with little nonopioid agonist activity in rat locus ceruleus neurons. The receptor activated by CTOP has yet to be fully resolved but seems to be similar to the somatostatin type 2 receptor or perhaps to a receptor closely related to somatostatin or opioid receptors.

Inhibition by MK-801 of morphine-induced conditioned place preference and postsynaptic dopamine receptor supersensitivity in mice

Intraperitoncel injection of morphine (5 mg/kg) in mice every other day for 8 days produced conditioned place preference (CPP). CPP effects were evaluated by assessing the difference in time spent in the drug-paired compartment and the saline-paired compartment of the place conditioning apparatus. The injection of a noncompetitive NMDA antagonist, MK-801 (0.05 and 0.1 mg/kg. IP), prior to and during morphine treatment in mice inhibited morphine-induced CPP. The development of postsynaptic dopamine (DA) receptor supersensitivity in mice displaying a morphine-induced CPP was evidenced by the enhanced response in ambulatory activity to the DA agonist, apomorphine (2 mg/kg). MK-801 inhibited that development of postsynaptic DA receptor supersensitivity. MK-801 also inhibited apomorphine-induced climbing behavior, suggesting that MK-801 inhibits dopaminergic activation mediated via the NMDA receptor. These results suggest that the development of morphine-induced CPP may be associated with the development of postsynaptic DA receptor supersensitivity. The development of morphine-induced CPP and DA receptor supersensitivity may be closely related to NMDA receptor-mediated dopaminergic activity, because morphine-induced changes in sensitivity to apomorphine, as well as apomorphine-induced climbing behavior in morphine treated mice, were both blocked by MK-801.

Low-dose apomorphine attenuates morphine-induced enhancement of brain stimulation reward

Thresholds for brain stimulation reward (BSR) delivered to the medial forebrain bundle-lateral hypothalamus were determined by means of a rate free psychophysical method. Lower doses of apomorphine (0.5 to 0.2 mg/kg) produced modest elevations in BSR thresholds. A 0.4 mg/kg dose of apomorphine resulted in emergence of stereotypic behaviors and the loss of stimulus control. Morphine's BSR threshold lowering effects were significantly blocked by the concurrent administration of a 0.1 mg/kg dose of apomorphine. These results support the hypothesis that presynaptic dopamine neurons are involved in the mediation of morphine's reinforcing effects and that dopamine autoreceptor agonists may be of some use in the pharmacotherapy of opiate abuse.

Differentiation of sigma ligand-activated receptor subtypes that modulate NMDA-evoked [3H]-noradrenaline release in rat hippocampal slices

1. It is now widely accepted that there are two classes of sigma (sigma) binding sites, denoted sigma(1) and sigma(2), and recently sigma(3) subtype has been proposed. Selective sigma(1) and sigma(2) receptor agonists are known to modulate the neuronal response to N-methyl-D-aspartate (NMDA) in vivo and in vitro. To identify the site of action of a series of recently synthesised high affinity sigma ligands, the present in vitro series of experiments was carried out on NMDA-evoked [3H]-noradrenaline ([3H]-NA) overflow from preloaded hippocampal slices of the rat. 2. The ligands (+)-cis-N-methyl-N-[2,(3,4-dichlorophenyl) ethyl]-2-(1-pyrrolidinyl) cyclohexylamine (BD-737) and (+)-pentazocine, considered as the prototypic sigma(1) agonists, potentiated the NMDA response from 10 nM to 100 nM. This potentiation faded between 100 nM and 1 microM ligand concentrations. On the other hand, 1,3-di(2-tolyl)guanidine (DTG), a mixed sigma(1)/sigma(2) agonist, at concentrations greater than 100 nM inhibited the NMDA-evoked [3H]-NA release. Spiperone, considered as active on putative sigma(3) receptors, was without effect on the NMDA response, or on the potentiating effect of BD-737. 3. The high affinity sigma antagonists haloperidol and 1[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine (BD-1063), inactive by themselves on the NMDA-induced response, at concentrations above 30 nM totally prevented the potentiating effect of (+)-pentazocine (100 nM) as well as the inhibitory effect of DTG (300 nM) on NMDA-evoked [3H]-NA release. Whereas haloperidol and BD-1063, at concentrations < 1 microM, were inactive on the potentiating effect of BD-737 (100 nM). 4. 4-(4-Chlorophenyl)-alpha-4-fluorophenyl-4-hydroxy-1-piperidinebutanol (reduced haloperidol), N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine (BD-1008), inactive by themselves on the NMDA-evoked [3H]-NA release, failed to reverse the effects of (+)-pentazocine and DTG, but at concentrations of 30 nM to 1 microM antagonised the BD-737-induced potentiation of the NMDA response. Conversely, N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE-100) blocked the effects of (+)-pentazocine as well as those of BD-737, but not those of DTG. 5. The present results provide in vitro functional evidence for a sigma receptor type preferentially sensitive to BD-737, reduced haloperidol, BD-1008 and also to NE-100, that differs from the already identified sigma(1), sigma(2) and sigma(3) sites.

Effects of meperidine on the pancreatic and biliary sphincter

BACKGROUND

Opioids are traditionally avoided during sphincter of Oddi manometry because of indirect evidence suggesting that these agents cause sphincter of Oddi spasm. This study was undertaken to determine the direct effects of meperidine on the biliary and pancreatic sphincter.

METHODS

Forty-seven patients were prospectively evaluated by sphincter of Oddi manometry in the conventional retrograde fashion. Manometry was initially performed with intravenous diazepam sedation alone. The manometry was repeated 3 to 5 minutes after meperidine was administered.

RESULTS

The basal sphincter pressure of the biliary sphincter, pancreatic sphincter, and the combined sphincter group were not significantly altered by meperidine. Concordance (normal versus abnormal) between the basal sphincter pressure before and after meperidine was seen in 44 of 47 patients (94%). Meperidine produced a significant increase in the pancreatic, biliary, and combined sphincter phasic frequency and a significant decrease in the phasic duration. The pancreatic and combined sphincter phasic pressures were significantly reduced following meperidine administration. Seventeen manometry tracings (36%) were believed to be qualitatively better after meperidine, while only four (8.5%; p < .001) were qualitatively better with diazepam alone.

CONCLUSION

Meperidine can be used for additional analgesia during sphincter of Oddi manometry if the basal sphincter pressure is the parameter used to determine therapy.

Inhibition of morphine tolerance by NMDA receptor antagonists in the formalin test

5-Nitro-6,7-dimethyl-1,4-dihydro-2,3-quinoxalinedione (ACEA-1328) was characterized in vitro for antagonism of excitatory amino acid receptors, and subsequently tested in vivo and compared with MK-801 for phencyclidine (PCP)-like motor stimulation, antinociception, and effects on morphine tolerance in mice. Assayed on rat cerebral cortical glutamate receptors expressed in Xenopus oocytes ACEA-1328 showed potent (Kb approximately 40 nM) antagonism at NMDA receptor/glycine sites and moderate (Kb approximately 3 microM) antagonism at non-NMDA receptors. In both cases inhibition was predominantly competitive. ACEA-1328 was weak, or inactive, at NMDA receptor glutamate recognition sites, metabotropic receptors and opioid binding sites. In the formalin and rotarod tests ACEA-1328 and MK-801 produced both antinociception and disturbances of motor coordination. MK-801 caused a PCP-like motor stimulatory effect, whereas ACEA-1328 was devoid of such an effect. In tolerance studies, ACEA-1328 and MK-801 each blocked morphine tolerance in the formalin test, the effect of ACEA-1328 was dose-dependent. Our data contribute to a growing body of evidence which suggests that activation of NMDA receptors is critical for the development of opioid tolerance, and that antagonism at NMDA receptor/glycine sites may have potential as a means of diminishing tolerance with no PCP-like motor stimulatory side effects.

Agonist-selective endocytosis of mu opioid receptor by neurons in vivo

Opiate alkaloids are potent analgesics that exert multiple pharmacological effects in the nervous system by activating G protein-coupled receptors. Receptor internalization upon stimulation may be important for desensitization and resensitization, which affect cellular responsiveness to ligands. Here, we investigated the agonist-induced internalization of the mu opioid receptor (MOR) in vivo by using the guinea pig ileum as a model system and immunohistochemistry with an affinity-purified antibody to the C terminus of rat MOR. Antibody specificity was confirmed by the positive staining of human embryonic kidney 293 cells transfected with epitope-tagged MOR cDNA, by the lack of staining of cells transfected with the delta or kappa receptor cDNA, and by the abolition of staining when the MOR antibody was preadsorbed with the MOR peptide fragment. Abundant MOR immunoreactivity (MOR-IR) was localized to the cell body, dendrites, and axonal processes of myenteric neurons. Immunostaining was primarily confined to the plasma membrane of cell bodies and processes. Within 15 min of an intraperitoneal injection of the opiate agonist etorphine, intense MOR-IR was present in vesicle-like structures, which were identified as endosomes by confocal microscopy. At 30 min, MOR-IR was throughout the cytoplasm and in perinuclear vesicles. MOR-IR was still internalized at 120 min. Agonist-induced endocytosis was completely inhibited by the opiate antagonist naloxone. Interestingly, morphine, a high-affinity MOR agonist, did not cause detectable internalization, but it partially inhibited the etorphine-induced MOR endocytosis. These results demonstrate the occurrence of agonist-selective MOR endocytosis in neurons naturally expressing this receptor in vivo and suggest the existence of different mechanisms regulating cellular responsiveness to ligands.

Postnatal methadone exposure doe not prevent prenatal methadone-induced changes in striatal cholinergic neurons

On postnatal day 4, rats exposed to methadone prenatally but fostered to control dams, as well as those fostered to dams treated with methadone, exhibited significant reductions in striatal acetylcholine (ACh) content. This suggests that neonatal withdrawal from methadone is not responsible for the effects of prenatal exposure on cholinergic development in the early perinatal period. The effects of perinatal exposure to methadone on serotonin (5HT) and dopamine (DA) metabolism do not appear to be strictly related to changes in ACh content. Although prenatal exposure reduces 5-hydroxyindole acetic acid (5HIAA) content, changes in 5HT content prevent significant changes in the ratio 5HIAA/5HT. Pups exposed to methadone only prenatally (withdrawal group) exhibited a decreased DOPAC/DA ratio, whereas pups in the treatment group exposed to methadone both pre- and postnatally exhibited an increased DOPAC/DA ratio.

The NMDA receptor antagonist MK-801 differentially modulates mu and kappa opioid actions in spinal cord in vitro

We have examined the interactions between NMDA receptors and opioid effects in isolated neonatal rat spinal cord. Electrical stimulation of a lumbar dorsal root evoked a nociceptive-related slow ventral root potential (sVRP) recorded at the corresponding ipsilateral ventral root. The kappa opiate receptor agonist U69,593 (2.5 nM-1 microM) depressed sVRP area by a maximum of 80%, EC50 was approximately 33 nM. Both the non-specific antagonist naloxone and the kappa-specific antagonist nor-binaltorphimine (nor-BNI) antagonized the effects of U69,593. Morphine, a mu agonist, (1 nM-1 microM) depressed sVRP area with an approximate EC50 of 90 nM. The effects of both mu and kappa opioid agonists were selective for the very slow metabotropically mediated components of the sVRP, compared to the relatively fast NMDA receptor-mediated components. The non-competitive N-methyl-D-aspartate (NMDA) antagonist MK-801 (20 nM) had no effect on sVRP area when applied alone but co-applied with morphine significantly potentiated the depressant effects of morphine. In contrast, MK-801 either had no effect on or slightly antagonized the depressant effects of U69,593. Naloxone following morphine produced a significant increase in sVRP area above pre-morphine control values; the increase lasted 30 min or more. Neither naloxone nor nor-BNI was associated with an increase in sVRP area when given alone or following U69,593. MK-801 co-applied with morphine blocked the rebound increase in sVRP area following naloxone. These results suggest that (1) both mu and kappa receptor agonists exert similar selective depressant effects on spinal nociceptive neurotransmission; (2) mu but not kappa agonists exert prolonged excitatory effects that oppose the depression; and (3) NMDA receptors play a role in determining opioid analgesic potency and naloxone-precipitated hyperresponsiveness. The results may be related to initial steps in the development of acute tolerance to mu opioids, and suggest that tolerance to kappa opioids may have a different mechanism.

Evaluation of the cocaine-like discriminative stimulus effects and reinforcing effects of modafinil

Modafinil [(diphenyl-methyl)sulphinyl-2-acetamide] is a novel psychostimulant drug which is effective in the treatment of narcolepsy and idiopathic hypersomnia. It also has neuroprotective effects in animal models of striatal neuropathology. Although the cellular mechanisms of action of modafinil are poorly understood, it has been shown to have a profile of pharmacological effects that differs considerably from that of amphetamine-like stimulants. There is some evidence that modafinil has central alpha 1-adrenergic agonist effects. In the present study modafinil was evaluated for cocaine-like discriminative stimulus effects in rats and for reinforcing effects in rhesus monkeys maintained on intravenous cocaine self-administration. Modafinil, l-ephedrine and d-amphetamine all produced dose dependent increases in cocaine-lever responding, with maximal levels of 67%, 82% and 100%, respectively. Modafinil produced full substitution in four out of the six rats tested while the highest levels of substitution were associated with substantial response rate decreasing effects. Little evidence was obtained that the discriminative stimulus effects of modafinil were produced by alpha 1-adrenergic activation, based upon results of tests performed in combination with prazosin. In the self-administration procedure, modafinil and l-ephedrine functioned as reinforcers in rhesus monkeys. The reinforcing and discriminative stimulus effects of modafinil-required very high doses: modafinil was over 200 times less potent than d-amphetamine and was also less potent than l-ephedrine. These results show that modafinil has some cocaine-like discriminative stimulus effects and, like other abused stimulants, can serve as a reinforcer at high doses.

Effect of opiates on transmitter release from visualized hypogastric boutons innervating the rat pelvic ganglia

1. The effect of opiates on neurotransmission between visualized hypogastric nerve boutons and postganglionic cell bodies has been examined using extracellular recording of nerve bouton impulses (NBIs) and excitatory postsynaptic currents (e.p.s.cs). 2. Morphine (10 to 40 microM) did not affect neurotransmission in the ganglia. Dynorphin-A (4 microM) and U50488H (1 microM) decreased quantal transmitter release and naloxone (10 microM) reversed these effects. 3. Morphine (10 microM), dynorphin-A (4 microM) and U50488H (1 microM) did not affect either the time course or consistency with which the NBI was recorded. 4. Dynorphin-A (1 to 4 microM) and U50488H (1 microM) decreased the average amplitude of e.p.s.cs by increasing the number of failures to release quanta from single or small groups of 2 to 4 boutons during continuous nerve stimulation at 0.1 Hz. 5. The decrease in quantal release induced by dynorphin-A and U50488H in 0.2 to 0.5 mM [Ca2+]zero was readily reversed by increasing the extracellular calcium ion concentration to 1 mM. 6. It was concluded that kappa-opioid receptors are located on the boutons of the hypogastric nerve and when activated by kappa-opioid receptor agonists reduce quantal release without affecting the NBI.

Effects of intermittent and continuous cocaine administration on dopamine release and uptake regulation in the striatum: in vitro voltammetric assessment

Chronic daily injections of cocaine induce behavioral sensitization to subsequent cocaine challenge, while continuous infusion induces tolerance. Following a 7-day withdrawal period, we examined the effects of these two dosing regimens on: (1) baseline dopamine efflux and uptake following single-pulse electrical stimulation, (2) inhibition of uptake by cocaine; and (3) inhibition of efflux by autoreceptor activation. Cocaine (40 mg/kg per day) was administered to rats for 14 days either continuously by osmotic minipumps or intermittently by once-a-day injections. Minipumps containing saline were implanted in the control group. After 7 days of withdrawal, dopamine kinetics in the caudate was examined using in vitro fast-scan cyclic voltammetry. This technique provides very rapid measurements of dopamine in the extracellular space. Thus, when combined with endogenous dopamine efflux evoked by single-pulse, electrical stimulations, it was possible directly to measure the release and uptake components of the efflux. In the absence of pharmacological agents, no group differences were found in the amount of baseline dopamine released or in the uptake kinetics; the potency of bath-applied cocaine (0.03-60 microM) in inhibiting the uptake was also unaltered in either group. In contrast, the potency of quinpirole (an autoreceptor agonist, 5-250 nM) was significantly decreased and increased in the cocaine injection and pump groups, respectively. Thus, the cocaine administration regimen which produces sensitization results in a functional subsensitivity of release-modulating autoreceptors, while the tolerance-producing regimen results in autoreceptor supersensitivity.

Opioid antagonist effects of dezocine in opioid-dependent humans

Dezocine is an opioid mu-partial agonist recently approved for use as an analgesic in the United States. This study characterized the relative agonist versus antagonist effects of dezocine in comparison to naloxone (an opioid antagonist), hydromorphone (an opioid mu-agonist), and placebo (saline solution) in opioid-dependent volunteers. In a residential laboratory, six volunteer male opioid abusers maintained on 30 mg/day oral methadone underwent pharmacologic challenges two to three times per week, 20 hours after the last dose of methadone. Challenges consisted of a double-blind intramuscular injection of dezocine (dose range, 7.5 to 60 mg), hydromorphone (5 and 10 mg), naloxone (0.1 and 0.2 mg), or saline solution. Measures included physiologic indexes, self-reports of drug effects, and observer ratings of drug effects. Naloxone and hydromorphone produced characteristic antagonist-like and agonist-like effects, respectively. Dezocine acted as an opioid antagonist, precipitating a withdrawal syndrome only slightly different from that produced by naloxone. Dezocine's antagonist effects were not directly dose related, but peaked at intermediate doses and declined at higher doses.

Importance of post-drug environmental factors for induction of sensitization to the ambulation-increasing effects of methamphetamine and cocaine in mice

Mice given five repeated administrations of methamphetamine (MAP: 2 mg/kg SC) or cocaine (COC: 20 mg/kg SC) at 3-day intervals in a round tilting-type activity cage (20 cm in diameter) showed sensitization to the ambulation-increasing effect of each drug. The mean 3- or 2-h overall activity count at the fifth administration of MAP or COC, respectively, was 2.3-2.5 times higher than that at the first administration. Mice given MAP or COC 4 times in round spaces (15-30 cm in diameter), where the floor did not tilt, exhibited sensitization as strong as that demonstrated by mice given each drug in the activity cages, when the mice were given the fifth administration in the activity cages. In contrast, mice repeatedly given the drugs in spaces 4-9 cm in diameter never, and those in space 12 cm in diameter only partially, exhibited sensitization to MAP and COC. Furthermore, mice given MAP or COC 4 times in their home cages (25D x 20W x 15H cm, with ten mice in each cage) showed partial sensitization. Repeated administration of saline to mice in activity cages, in the spaces 4-30 cm in diameter, or in the home cages did not cause significant change in the sensitivity to either MAP or COC. These results suggest that repeated experience of the stimulant effect of drug and the resultant ambulation is required for induction of sensitization to MAP and COC in terms of ambulation in mice. It is also suggested that spaces larger than 12 cm in diameter, which correspond to 2-2.5 times as long as the body length without tail, and no interference from other mice are required for induction of strong sensitization to both MAP and COC.

Effects of the delta opioid against BW373U86 in pigeons trained to discriminate fentanyl, bremazocine and water in a three-choice drug discrimination procedure

The delta opioid agonist BW373U86 was examined alone and in combination with mu agonists in pigeons trained to discriminate the mu agonist fentanyl (0.056 mg/kg), the kappa agonist bremazocine (0.017 mg/kg), and distilled water in a three-choice drug discrimination procedure. BW373U86 (0.01-10 mg/kg) produced a dose-dependent increase in fentanyl-appropriate responding and complete generalization to fentanyl in four of five subjects. BW373U86 did not elicit bremazocine-appropriate responding in any of the subjects. Fentanyl-appropriate responding elicited by BW373U86 was antagonized by the delta selective antagonist naltrindole (0.1-10 mg/kg) but not by the mu selective antagonist naloxone (0.1-30.0 mg/kg). When BW373U86 was administered in combination with the mu agonists fentanyl, morphine and nalbuphine, a low dose of BW373U86 (0.01 mg/kg) that elicited primarily water-appropriate responding when administered alone did not produce a significant change in the ED50 values for fentanyl, morphine or nalbuphine. Higher doses of BW373U86 (0.1-1.0 mg/kg) increased levels of fentanyl-appropriate responding elicited by low doses of fentanyl, morphine and nalbuphine to levels similar to those produced by BW373U86 alone. These results indicate that BW373U86 shares discriminative stimulus properties with the mu agonist fentanyl in pigeons, possibly by acting at delta opioid receptors. However, BW373U86 does not potentiate the discriminative stimulus effects of mu agonists or share discriminative stimulus effects with the kappa agonist bremazocine.