Induction of Fos-like immunoreactivity by opioids in guinea-pig brain

In the present study the effects of intracerebroventricular (i.c.v.) administration of 100 nmol of morphine, the selective mu-receptor agonist DAMGO, the delta-receptor agonist DPDPE and the kappa-receptor agonist U50,488H, on the induction of Fos-like immunoreactivity (Fos-LI) in the guinea-pig brain were investigated using immunohistochemical techniques. Guinea-pigs given i.c.v. injection of opioids showed marked increases in the number of Fos-LI nuclei within a large number of brain regions, several of which, including hypothalamic nuclei, paraventricular thalamic nucleus, the amygdala, periaqueductal gray, superior and inferior colliculi, the piriform and entorhinal cortices, have been shown to be activated under stressful or aversive conditions. Pretreatment with the opioid antagonist, naltrexone, before administration of morphine or U50,488H, inhibited Fos-LI induction indicating that the effects of the opioids were mediated by opioid receptors. U50,488H administration resulted in higher numbers of Fos-LI stained neurons compared to morphine in most regions other than the nucleus accumbens and interpeduncular nucleus. Morphine and DAMGO produced significantly higher numbers of Fos-LI neurons in the nucleus accumbens shell region than U50,488H, which may reflect the more powerful reinforcing/rewarding effects of mu-receptor agonists. Thus the present study supports a critical role for the nucleus accumbens shell region in the reinforcing/rewarding effects of opioids.     

Antinociception produced by an ascending spino-supraspinal pathway

Studies in mice and rats have shown that antinociception produced by intrathecal (i.t.) administration of opioids can be partially inhibited by intracerebroventricular (i.c.v.) administration of naloxone. In this study we tested the hypothesis that this inhibition by i.c.v. naloxone results from antagonism of supraspinal endogenous opioid-mediated antinociception produced by the action of i.t. opioids on an ascending antinociceptive pathway. In rats lightly anesthetized with urethane/alpha-chloralose, i.t. DAMGO, i.t. lidocaine, or spinal transection at T5-T6 all attenuated the trigeminal jaw opening reflex (JOR) (i.e., were antinociceptive), an effect that was antagonized in each case by i.c.v. naloxone. These findings support the suggestion that there exists a pathway that ascends from the spinal cord to a supraspinal site that tonically inhibits antinociception mediated by supraspinal opioids. When activity in this ascending pathway is suppressed (e.g., by i.t. opioids or local anesthetics or by spinal cord transection), antinociception mediated by supraspinal opioids is disinhibited. To determine the supraspinal site(s) at which endogenous opioid-dependent antinociception is evoked by i.t. opioids, we microinjected naloxone methiodide into several supraspinal sites. Microinjection of naloxone methiodide into nucleus accumbens but not into the rostral ventral medulla (RVM) or the periaqueductal gray matter (PAG) antagonized the suppression of the JOR produced by i.t. DAMGO or lidocaine. The possibility that this ascending pathway may represent a source of spinal input to mesolimbic circuitry involved in setting the state of sensorimotor reactivity to noxious stimuli is discussed.     

Pretreatment with protein kinase C activator phorbol 12,13-dibutyrate attenuates the antinociception induced by mu- but not epsilon-opioid receptor agonist in the mouse

The effects of pretreatment with a protein kinase C activator, phorbol 12,13-dibutyrate, on antinociception induced by i.c.v.-administered mu-opioid receptor agonist (D-Ala2, NMePhe4, Gly(ol)5) enkephalin (DAMGO) or morphine and epsilon-opioid receptor agonist beta-endorphin were studied in male ICR mice. The tail-flick responses were used for antinociceptive tests. I.c.v. pretreatment with phorbol 12,13-dibutyrate (50 pmol) for 30 or 60 but not 10 min attenuated antinociception induced by i.c.v.-administered DAMGO. I.c.v. pretreatment with phorbol 12,13-dibutyrate (10 and 50 pmol) for 60 min caused a dose-dependent attenuation of DAMGO (19.5 pmol)- or morphine (6.0 nmol)-induced antinociception. The dose-response curve for DAMGO-induced antinociception was shifted to the right by 7.3-fold by i.c.v. pretreatment with phorbol 12,13-dibutyrate (50 pmol) for 60 min. However, the i.c.v.-administered beta-endorphin-induced antinociception was not affected by the same pretreatment with phorbol 12,13-dibutyrate. The attenuation of i.c.v.-administered DAMGO- and morphine-induced antinociception by phorbol 12,13-dibutyrate was reversed by concomitant i.c.v. pretreatment with a selective protein kinase C inhibitor calphostin C. These results suggest that activation of protein kinase C by phorbol 12,13-dibutyrate leads to the desensitization of mu-, but not epsilon-opioid receptor-mediated antinociception. These findings also provide additional evidence for differential intracellular modulation on antinociceptive action of mu- and epsilon-opioid receptor agonists.     

The nitric oxide/cyclic GMP system at the supraspinal site is involved in the development of acute morphine antinociceptive tolerance

The role of the supraspinal nitric oxide (NO)/cyclic GMP system in the development of acute morphine antinociceptive tolerance was investigated by use of the mouse 55 degrees C warm-water tail-flick test. A single intracerebroventricular (i.c.v.) pretreatment of mice with morphine (3 nmol, 140 min before testing) produced an acute antinociceptive tolerance to subsequent i.c.v. doses of morphine, as demonstrated by a 120-fold rightward shift of the morphine dose-response curve. When co-administered with morphine (140 min before testing), the NO synthase inhibitors: N-nitro-L-arginine methyl ester (L-NAME), 3-bromo-7-nitroindazole, 7-nitroindazole and NG-monomethyl-L-arginine, attenuated the development of morphine tolerance. All four NO synthase inhibitors completely blocked the rightward shift of the morphine dose-response curve caused by i.c.v. morphine pretreatment (3 nmol, 140 min before testing). This effect was partially antagonized by L-arginine, but not D-arginine, in a dose-dependent manner. Also, D-NAME did not block the development of tolerance. Like the NO synthase inhibitors, LY-83,583, a guanylyl cyclase inhibitor, blocked the development of tolerance, which suggests that NO acting through the cyclic GMP pathway is involved in the development of acute antinociceptive tolerance. The effects of increased NO production on acute morphine antinociceptive tolerance were also studied. When co-administered with morphine (140 min before testing), neither L-arginine (100 nmol) nor the NO donors, sodium nitroprusside (5 nmol) and isosorbide dinitrate (10 nmol), had any effect on the magnitude of morphine antinociceptive tolerance. These results suggest that NO, acting through the cyclic GMP pathway, mediates the development of acute antinociceptive tolerance, but that NO production does not alter the magnitude of antinociceptive tolerance.     

Characterization of the membrane-associating domain of the sperm adhesive protein, bindin

Male Swiss-Webster mice were rendered tolerant to morphine by subcutaneous implantation of a morphine pellet, each containing 75 mg morphine base, for 3 days. Mice implanted with placebo pellets served as controls. A high degree of tolerance to the analgesic effect of morphine developed as evidenced by decreased analgesic response to various doses of morphine. A selective kappa-opiate agonist, U-50,488H (8, 16 and 32 mg/kg, i.p.) produced dose-dependent analgesic and hypothermic effects in mice implanted with placebo pellets. A significant decrease in the analgesic and hypothermic effects of U-50,488H was observed in morphine tolerant mice as compared to placebo-treated mice. Mice were rendered tolerant to U-50,488H by injecting the drug (25 mg/kg, i.p.) twice daily for 4 days. Vehicle injected mice served as controls. Tolerance to the analgesic and hypothermic effects of U-50,488H in mice injected chronically with the drug was evidenced by the decreases in the intensity of these responses when compared to those observed in vehicle injected controls. Morphine produced a dose-dependent analgesic and hypothermic effects in mice injected chronically with vehicle but the intensity of these effects was significantly lower in mice injected chronically with U-50,488H. These results indicate that a substantial tolerance to analgesic and hypothermic effects of U-50,488H develops in morphine tolerant mice. The effect of chronic injections of U-50,488H on the binding of [3H]ethylketocyclazocine (EKC) and [3H]D-Ala2,MePhe4,Gly-ol5-enkephalin (DAMGO) to whole brain and spinal cord kappa- and mu-opiate receptors was determined.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo injection of antisense oligodeoxynucleotides to G alpha subunits and supraspinal analgesia evoked by mu and delta opioid agonists

For 5 consecutive days repeated intracerebroventricular (i.c.v.) administration of antisense oligodeoxynucleotides (ODNs) to G alpha subunit mRNAs was used to impair the function of mouse Gi1, Gi2, Gi3 and Gx/z regulatory proteins. Decreases of 20 to 60% on the G alpha-like immunoreactivity could be observed in neural structures of mouse brain, an effect that was not produced by a random-sequence ODN used as a control. The ODN to Gi1 alpha subunits lacked effect on opioid-evoked analgesia. In mice injected with the ODN to Gi2 alpha subunits the antinociceptive activity of all the opioids studied appeared greatly impaired. The ODN to Gi3 alpha subunits reduced the effects of the selective agonists of delta opioid receptors, [D-Pen2,5]-enkephalin and [D-Ala2]deltorphin II. Conversely, the analgesia evoked by opioids binding mu opioid receptors, [D-Ala2, N-MePhe4,Gly-ol5]enkephalin and morphine, appeared consistently and significantly attenuated in mice injected with the ODN to Gx/z alpha. The effect of the neuropeptide beta-endorphine-(1-31) agonist at mu and delta receptors was also reduced by ODNs to Gi3 alpha or Gx/z alpha subunits. l.c.v. injection of antibodies directed to these G alpha subunits antagonized opioid-induced analgesia with a pattern similar to that observed for the ODNs. Thus, the mu and delta opiod receptors regulate different classes of G transducer proteins to mediate the analgesic effect of agonists. The in vivo antisense strategy and the use of specific antibodies to G alpha subunits gave comparable results, indicating that in the neural tissue the mRNAs and the G alpha subunits can be accessed by the corresponding ODNs and IgGs.     

Morphine application to peripheral tissues modulates nociceptive jaw reflex

This study assessed the effect of peripherally applied opioids on the electromyographic activity reflexly evoked in digastric and masseter muscles by injection of the small-fiber excitant and inflammatory irritant mustard oil (MO) into the temporomandibular joint. In 39 anaesthetized rats, local pretreatment of joint tissues with morphine (15 nmol) significantly depressed the jaw muscle responses compared with saline, and the depression was antagonized by simultaneous local injection of the opiate antagonist naloxone (2.7 nmol); systemic morphine pretreatment (15 nmol, i.v.) did not influence the muscle responses. The naloxone-reversible depression of the MO-evoked muscle responses by local, but not systemic morphine, supports the presence of peripheral opioid receptors that may have a role in modulating nociceptive responses.     

Regulation of morphine antiallodynic efficacy by cholecystokinin in a model of neuropathic pain in rats

Neuropathic pains have often been classified as opioid-resistant. Here, spinal (intrathecal) actions of morphine and nonmorphine opioids have been studied in a nerve ligation model of neuropathic pain in rats. Mechanical allodynia was evaluated using von Frey filaments. Nerve-injured animals exhibited allodynia that was stable for up to 6 weeks after the surgery. Morphine did not alter allodynia at doses up to 300 nmol (100 micrograms). In contrast, [D-Ala2, NMPhe4, Gly-ol]enkephalin (DAMGO), a high-efficacy mu opioid agonist, produced a significant, dose-related antiallodynic action. [D-Ala2, Glu4]deltorphin (delta agonist) produced a significant antiallodynic effect only at 300 nmol, reaching approximately 70% of the maximum. Coadministration of morphine with a dose of [D-Ala2, Glu4]deltorphin, which was inactive alone, produced a significant and long-lasting antiallodynic action that was antagonized by NTI (delta receptor antagonist); NTI alone had no effect. Although blockade of cholecystokinin-B (CCKB) receptors with L365,260 did not produce effects alone, a significant antiallodynic action was observed when coadministered with morphine; this elevation of nociceptive threshold was abolished by NTI. The finding that DAMGO, but not very large doses of morphine, produced antiallodynic actions suggests that the ability of mu opioids to alleviate the allodynia is related, in part, to efficacy at postsynaptic mu receptors. At an inactive dose, a delta agonist or a CCKB antagonist enhanced morphine antiallodynic efficacy in an NTI-sensitive fashion. CCKB receptor blockade may enhance endogenous enkephalin actions, resulting in enhancement of morphine efficacy through a mu-delta receptor interaction.     

Competitive and non-competitive NMDA antagonists block the development of antinociceptive tolerance to morphine, but not to selective mu or delta opioid agonists in mice

N-Methyl-D-aspartate (NMDA) receptor antagonists have been shown to block the development of antinociceptive tolerance to morphine. Assessment of the effects of NMDA antagonists on development of antinociceptive tolerance to selective opioid mu (mu) and delta (delta) agonists, however, has not been reported. In these experiments, selective mu and delta receptor agonists, and morphine, were repeatedly administered to mice either supraspinally (i.c.v.) or systemically (s.c.), alone or after pretreatment with systemic NMDA antagonists. Antinociception was evaluated using a warm-water tail-flick test. Repeated i.c.v. injections of mu agonists including morphine, fentanyl, [D-Ala2, NMePhe4, Gly-ol]enkephalin (DAMGO) and Tyr-Pro-NMePhe-D-Pro-NH2 (PL017) or [D-Ala2, Glu4]deltorphin, a delta agonist, or s.c. injections of morphine or fentanyl, produced antinociceptive tolerance as shown by a significant rightward displacement of the agonist dose-response curves compared to controls. Single injections or repeated administration of MK801 (a non-competitive NMDA antagonist) or LY235959 (a competitive NMDA antagonist) at the doses employed in this study did not produce behavioral toxicity, antinociception or alter the acute antinociceptive effects of the tested opioid agonists. Consistent with previous reports, pretreatment with MK801 or LY235959 (30 min prior to agonist administration throughout the tolerance regimen) prevented the development of antinociceptive tolerance to i.c.v. or s.c. morphine. Neither NMDA antagonist, however, affected the development of antinociceptive tolerance to i.c.v. fentanyl, DAMGO, or [D-Ala2, Glu4]deltorphin. Additionally, MK801 pretreatment did not affect the development of antinociceptive tolerance to i.c.v. PL017 or to s.c. fentanyl. Further, MK801 pretreatment also did not affect the development of tolerance to the antinociception resulting from a cold-water swim-stress episode, previously shown to be a delta-opioid mediated effect. These data lead to the suggestion that the mechanisms of tolerance to receptor selective mu and delta opioids may be regulated differently from those associated with morphine. Additionally, these findings emphasize that conclusions reached with studies employing morphine cannot always be extended to 'opiates' in general.     

Cytokines serum levels as the markers of thyroid activation in Graves'' disease

We examined whether central somatostatin prevents an inhibitory effect of central calcitonin-gene related peptide (CGRP) on pancreatic secretion in conscious male Wistar rats (330-330 g). Rats were prepared with separate cannulas for draining bile and pancreatic juice and with a duodenal cannula and an extrajugular vein cannula. In addition, another cannula was stereotactically implanted into the left lateral cerebral ventricle. Rats were placed in restraint cages and experiments were conducted 4 days after the operation without anesthesia. An injection of CGRP (0.1, 1.0 nmol/10 microl) into the left lateral cerebral ventricle (i.c.v.) inhibited pancreatic secretion dose-dependently. To confirm the inhibitory effect of CGRP (i.c.v.) was mediated via sympathetic nerves, phentolamine was injected intravenously (i.v.) bolus (0.5 mg kg(-1)) 0.5-h before CGRP (i.c.v.), followed by continuous infusion of 0.2 mg kg(-1) h(-1). Phentolamine (i.v.) reversed the inhibition produced by CGRP (i.c.v.). An injection of 4 nmol/10 microl somatostatin (i.c.v.) 5 min prior to CGRP injection diminished the inhibitory effect of CGRP (i.c.v.). It is concluded that centrally administered somatostatin diminished the inhibitory action of CGRP (i.c.v.) on pancreatic secretion, probably via inhibiting autonomic (sympathetic) nerve excitation at the central site.     

Influence of concentration and application time of maleic acid on dentin permeability

We reported previously that electroacupuncture (Acu) applied to the acu-point equivalent to GV4 in the mouse just before the 2,4,6-trinitrochlorobenzene (TNCB) challenge suppressed the delayed type hypersensitivity (DTH) through endogenous opioidergic systems in the brain, and the pituitary was pivotal in this immunosuppression. The purpose of the present study was to compare the suppressive effects of Acu with those of single, acute doses of morphine on TNCB-DTH in intact and hypophysectomized (HPX) mice. Subcutaneous morphine 10 mg/kg in ddY mice, 30 mg/kg in BALB/c mice or intracisternal morphine 40 micrograms/mouse in BALB/c mice given just before TNCB challenge suppressed (40-53%) the maximal extent of ear swelling at 24 hrs after challenge in intact mice. In HPX mice, the suppressive effects of intracisternal morphine 10 and 100 micrograms/mouse were less pronounced than those observed in intact mice and there was no significant difference between intact and HPX groups. In addition, suppressive effects observed with Acu or subcutaneous morphine (30 mg/kg) were effectively antagonized by pretreatment with intracisternal naloxone at a dose of as low as 2 micrograms/mouse. Naloxone alone had no effect of its own. These results suggest that 1) the activation of opioid receptor-mediated pathways in the brain, which occurs when opioids are endogenously released (Acu) or exogenously given (morphine), is important in the suppression of TNCB-induced DTH, a cell-mediated immune response, and 2) the pituitary is less pivotal in the suppressive effects of acute morphine than in those of Acu.     

Anti asialoglycoprotein receptor antibodies and soluble interleukin-2 receptor levels as marker for inflammation in autoimmune hepatitis

Previous results using an amphibian model showed that systemic and spinal administration of opioids selective for mu, delta and kappa-opioid receptors produce analgesia. It is not known whether non-mammalian vertebrates also contain supraspinal sites mediating opioid analgesia. Thus, opioid agonists selective for mu (morphine; fentanyl), delta (DADLE, [D-Ala2, D-Leu5]-enkephalin; DPDPE, [D-Pen2, D-Pen5]-enkephalin) and kappa (U50488, trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl] benzeneacetamide methanesulfonate; CI977, (5R)-(544alpha,744alpha,845beta)-N-methyl-N-[7-(1-p yrr olidinyl)-1-oxaspiro[4,5]dec-8yl]-4-benzofuranaceta mide++ + monohydrochloride) opioid receptors were tested for analgesia following i.c.v. administration in the Northern grass frog, Rana pipiens. Morphine, administered at 0.3, 1, 3 and 10 nmol/frog, produced a dose-dependent and long-lasting analgesic effect. Concurrent naltrexone (10 nmol) significantly blocked analgesia produced by i.c.v. morphine (10 nmol). ED50 values for the six opioids ranged from 2.0 for morphine to 63.9 nmol for U50488. The rank order of analgesic potency was morphine > DADLE > DPDPE > CI977 > fentanyl > U50488. These results show that supraspinal sites mediate opioid analgesia in amphibians and suggest that mechanisms of supraspinal opioid analgesia may be common to all vertebrates.     

The analgesic activity of crotamine, a neurotoxin from Crotalus durissus terrificus (South American rattlesnake) venom: a biochemical and pharmacological study

Crotamine, a 4.88 kDa neurotoxic protein, has been purified to apparent homogeneity from Crotalus durissus venom by gel filtration on Sephadex G-75. When injected (i.p. or s.c.) in adult male Swiss mice (20-25 g), it induced a time-dose dependent analgesic effect which was inhibited by naloxone, thus suggesting an opioid action mechanism. When compared with morphine (4 mg/kg), crotamine, even in extremely low doses (133.4 microg/kg, i.p., about 0.4% of a LD50 is approximately 30-fold more potent than morphine (w/w) as an analgesic. On a molar basis it is more than 500-fold more potent than morphine. It is also much more potent than the lower molecular weight crude fractions of the same venom. The antinociceptive effects of crotamine and morphine were assayed by the hot plate test and by the acetic acid-induced writhing method. Therefore, both central and peripheral mechanisms should be involved. Histopathological analysis of the brain, liver, skeletal muscles, stomach, lungs, spleen, heart, kidneys and small intestine of the crotamine injected mice did not show any visible lesion in any of these organs by light microscopy. Since crotamine accounted for 22% (w/w) of the desiccated venom, it was identified as its major antinociceptive low molecular weight peptide component.     

Modified approach for the selective treatment of temporal lobe epilepsy: transsylvian-transcisternal mesial en bloc resection

1. Intracerebronventricular (i.c.v.) injection of gamma-mangostin (10-40 nmol/mouse), a major compound of the fruit hull of Garcinia mangostana Lin., like ketanserin (10, 20 nmol/mouse, i.c.v.) inhibited 5-fluoro-alpha-methyltryptamine (5-FMT) (45 mg kg(-1), i.p.)-induced head-twitch response in mice in the presence or absence of citalopram (a 5-hydroxytryptamine (5-HT)-uptake inhibitor). 2. Neither the 5-FMT- nor the 8-hydroxy-2-(di-n-propylamino)tetralin (5-HT1A-agonist)-induced 5-HT syndrome (head weaving and hindlimb abduction) was affected by gamma-mangostin or ketanserin. 3. The locomotor activity stimulated by 5-FMT through the activation of alpha1-adrenoceptors did not alter in the presence of gamma-mangostin. 4. 5-HT-induced inositol phosphates accumulation in mouse brain slices was abolished by ketanserin. Gamma-mangostin caused a concentration-dependent inhibition of the inositol phosphates accumulation. 5. Gamma-mangostin caused a concentration-dependent inhibition of the binding of [3H]-spiperone, a specific 5-HT2A receptor antagonist, to mouse brain membranes. 6. Kinetic analysis of the [3H]-spiperone binding revealed that gamma-mangostin increased the Kd value without affecting the Bmax value, indicating the mode of the competitive nature of the inhibition by gamma-mangostin. 7. These results suggest that gamma-mangostin inhibits 5-FMT-induced head-twitch response in mice by blocking 5-HT2A receptors not by blocking the release of 5-HT from the central neurone. Gamma-mangostin is a promising 5-HT2A receptor antagonist in the central nervous system.     

Antagonism of clonidine injected intracerebroventricularly in different models of salt intake

Clonidine, an alpha 2-adrenergic agonist, injected into the brain inhibits salt intake of animals treated by the diuretic model of sodium depletion. In th present study, we address the question of whether central injection of clonidine also inhibits salt intake in animals deprived of water or in the need-free state. Saline or clonidine (30 nmol) was injected into the anterior third ventricle of 24-h sodium-depleted (furosemide + removal of ambient sodium), of 24-h water-deprived and of normovolemic (need-free state) adult male rats. Clonidine injected intracerebroventricularly (i.c.v.) inhibited the 1.5% NaCl intake for 1209 min by 50 to 90% in every model tested. Therefore, different models of salt intake are inhibited by i.c.v. injection of clonidine. Idazoxan, an alpha 2-adrenergic antagonist, injected i.c.v. at a dose of 160 nmol, inhibited the effect of clonidine only in the furosemide + removal of ambient sodium model of salt intake. This indicates that the antagonism of this effect by idazoxan is dependent on the body fluid/sodium status of the animal.     

Results of systematic speech sound monitoring in children with cleft palate

Two potent inhibitors of nitric oxide synthase (NOS), namely, NG-nitro-L-arginine (NNA) and NG-monomethyl-L-arginine (NMMA) were administered intracerebroventricularly (i.c.v.) in morphine-dependent mice to investigate their effects on abrupt withdrawal and naltrexone-precipitated abstinence signs. Male Swiss-Webster mice were rendered dependent on morphine by subcutaneous implantation of a morphine pellet containing 75 mg of morphine base. Mice implanted with placebo pellets served as controls. NMMA or NNA administered i.c.v. had minimal effects on body weight loss and hypothermia that occur during abrupt withdrawal of morphine. When administered i.c.v., both NNA or NMMA (0.1, 1 and 10 micrograms/mouse) dose-dependently inhibited naltrexone-induced stereotyped jumping behavior in mice. I.c.v. administration of NMMA also attenuated withdrawal induced fecal pellet formation. This effect, however, was not dose-dependent. In conclusion, these results suggest that brain NO plays an important role in the expression of behavioral signs of morphine withdrawal syndrome. In addition, these results support the idea that NOS inhibitors may be potentially useful in the treatment of opioid withdrawal syndrome.     

Analgesic potency of mu and kappa opioids after systemic administration in amphibians

The relative analgesic potency of 11 opioid agents was assessed by using the acetic acid test in amphibians. Systemic administration of the mu agonists, fentanyl, levorphanol, methadone, morphine, meperidine and codeine; the partial mu agonist, buprenorphine; and the kappa agonists nalorphine, bremazocine, U50488 and CI-977 was made by s.c. injection into the dorsal lymph sac of the Northern grass frog, Rana pipiens. All agents produced a dose-dependent and long-lasting analgesia which persisted for at least 4 hr. The analgesic effects of single doses of each agent were significantly blocked or reduced by pretreatment with naltrexone. Systemic opioids produced log dose-response curves which yielded ED50 values ranging from 1.4 nmol/g for fentanyl to 320.9 nmol/g for nalorphine. Comparison of ED50 values gave a rank order of analgesic potency = fentanyl > CI-977 > levorphanol > U50488 > methadone > bremazocine > morphine > buprenorphine > meperidine > codeine > nalorphine. The relative analgesic potency of mu opioids in amphibians was significantly correlated with relative analgesic potency of these same agents obtained on the mouse writhing and hot plate tests. These data suggest that the amphibian model may serve as an adjunct or alternative model for the testing of opioid agents. Furthermore, given the inactivity of kappa opioids on rodent hot plate and tail-flick tests, the acetic acid test in amphibians may be especially well-suited for the assessment of opioid analgesia after administration of kappa-selective opioids.     

Proconvulsant and anticonvulsant effects of Evans blue dye in rodents

The effect of i.c.v. administration of Evans blue to sound sensitive DBA/2 mice and to genetically epilepsy-prone rats was studied. In mice, Evans blue (3.3-52 nmol) induced: hyperlocomotion, wild running, scratching, clonic muscle spasms, tonic seizure (latency 10-45 min), followed by death or recovery. The CD50 value for clonic seizures for Evans blue was 35(23-53) nmol. Pretreatment (45 min) with Evans blue (13-52 nmol, i.c.v.) dose-dependently reduced the incidence of sound-induced seizures in DBA/2 mice (ED50 value against clonic seizures = 30 [15-58] nmol, i.c.v). In rats, Evans blue (104 nmol, i.c.v.) induced electroencephalographic seizures in the hippocampus and cortex and behavioural limbic seizures with a latency of 15-20 min. A reduction in the mean score (from 5 to 2-3) for behavioural seizures was observed which lasted for 4-5 days in rats electrically-kindled daily in the hippocampal CA3 subsector. Sound-induced clonic seizures in kindled and non-kindled rats were reduced for 3-4 days after administration of Evans blue (104 nmol, i.c.v.).     

Association between the amygdala and nucleus of the solitary tract in mu-opioid induced feeding in the rat

The central nucleus of the amygdala (CNA) and the nucleus of the solitary tract (NTS) are important in the regulation of ingestive behavior. We evaluated whether opioid-opioid signaling between the CNA and rostral NTS (rNTS) affect feeding behavior. To test this, rats were doubly cannulated with one cannula placed in the rNTS and one cannula in the CNA, allowing for co-administration of an opioid agonist into one site and an opioid antagonist into the other. Tyr-D-Ala-Gly-(me) Phe-Gly-ol (DAMGO) (2 nmol) injected into the CNA (CNA DAMGO) increased feeding more than two-fold compared to the vehicle-injected rats. This increase in food intake was blocked when doses of 26.5 and 79 nmol of naltrexone (NTX) were injected into the rNTs. In the reverse situation, rNTS DAMGO increased food intake above control levels, and CNA NTX blocked DAMGO-induced feeding when administrated in doses of 26.5 and 79 nmol. This suggests that a bi-directional opioid-opioid signaling pathway exists between the CNA and the rNTS which influences feeding via mu-opioid receptors.