Selective inhibition of [D-Ala2, Glu4]deltorphin antinociception by supraspinal, but not spinal, administration of an antisense oligodeoxynucleotide to an opioid delta receptor

Evidence in vivo has suggested the existence of subtypes of the delta opioid receptor (DOR), which have been termed delta 1 and delta 2. These proposed DOR subtypes are thought to be activated by [D-Pen2, D-Pen5]enkephalin (DPDPE, delta 1) and [D-Ala2, Glu4]deltorphin (delta 2). Recent work in which an antisense oligodeoxynucleotide (oligo) to a cloned DOR was administered by the intrathecal (i.th.) route has demonstrated a reduction in the antinociceptive actions of both i.th. DPDPE and [D-Ala2, Glu4]deltorphin, but not of [D-Ala2, NMPhe4, Gly-ol]enkephalin (DAMGO, mu agonist) in mice. The present investigation has extended these observations by administering the same DOR antisense oligo sequence by the intracerebroventricular (i.c.v.) route and evaluating the antinociceptive actions of i.c.v. agonists selective for delta, mu and kappa receptors. I.th. treatment with DOR antisense oligo, but not mismatch oligo, significantly inhibited the antinociceptive actions of both i.th. DPDPE and [D-Ala2, Glu4]deltorphin but not of i.th. DAMGO or U69,593 (kappa agonist), confirming previous data. In contrast, i.c.v. DOR antisense oligo, but not mismatch oligo, selectively inhibited the antinociceptive response to i.c.v. [D-Ala2, Glu4]deltorphin without altering the antinociceptive actions of i.c.v. DPDPE, DAMGO or U69,593. The data suggest that the cloned DOR corresponds to that pharmacologically classified as delta 2 and further, suggest that this delta receptor subtype may play a major role in eliciting spinal delta-mediated antinociception.     

Antinociceptive evaluation of 14 beta-(bromoacetamido)-7,8-dihydro- N(cyclopropylmethyl)-normorphinone in mice

This study evaluated the supraspinal opioid effects of 14 beta-(bromoacetamido)-7,8-dihydro-N(cyclopropylmethyl)-normorphinone+ ++ (N-CPM-H2BAMO) in the mouse acetic acid-induced writhing and tail-flick assays. In the writhing test, N-CPM-H2BAMO produced a time- and dose-dependent antinociception after i.c.v. administration, with a 50% antinociceptive response being obtained with 0.28 (0.19-0.39) nmol when given 10 min before testing. The antinociceptive effect of N-CPM-H2BAMO was antagonized in a dose-dependent manner by the kappa-selective opioid receptor antagonist, nor-binaltorphimine. In the mouse tail-flick assay, N-CPM-H2BAMO failed to produce any antinociception after i.c.v. administration. N-CPM-H2BAMO produced a dose-dependent antagonism of morphine-induced antinociception but not antinociception induced by the delta-opioid receptor agonist [D-Pen2,D-Pen5]enkephalin. Nor-binaltorphimine (0.3 nmol) at dose that completely antagonized N-CPM-H2BAMO-induced antinociception in the writhing assay did not prevent the antagonistic effect of N-CPM-H2BAMO on morphine-induced antinociception. Therefore, these data indicate that N-CPM-H2BAMO produces antinociception by acting at supraspinal kappa-opioid receptors in the writhing assay, and also acts as a mu-opioid receptor antagonist.     

Kappa1- and kappa2-opioid receptors mediating presynaptic inhibition of dopamine and acetylcholine release in rat neostriatum

1. The effects of selective opioid receptor agonists and antagonists on N-methyl-D-aspartate (NMDA, 10 microM)-induced release of [3H]-dopamine and [14C]-acetylcholine (ACh) from superfused neostriatal slices were studied to investigate the possible occurrence of functional kappa-opioid receptor subtypes in rat brain. 2. The kappa receptor agonists (-)-ethylketocyclazocine ((-)-EKC), U69593 and the endogenous opioid peptide dynorphin A1-13 caused a naloxone-reversible inhibition of NMDA-induced [3H]-dopamine release, with pD2 values of about 9, 8.5 and 8.2, respectively, whereas both the mu agonist Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAMGO) and the delta agonist D-Pen2-D-Pen5-enkephalin (DPDPE) were ineffective in this respect. The inhibitory effect of submaximally effective concentrations of dynorphin A1-13, U69593 and (-)-EKC on NMDA-induced [3H]-dopamine release were not changed by the delta1/delta2-opioid receptor antagonist naltrindole (up to a concentration of 1 microM, but reversed by the kappa receptor antagonist nor-binaltorphimine (nor-BNI), with an IC50) as low as 0.02 nM, indicating the involvement of U69593-sensitive kappa1-opioid receptors. 3. NMDA-induced [14C]-ACh release was reduced in a naloxone-reversible manner by DPDPE (pD2 about 7.2), dynorphin A1-13 (pD2 6.7) and EKC (pD2 6.2), but not by U69593 and DAMGO. The inhibitory effect of a submaximally effective concentration of DPDPE, unlike those of dynorphin A1-13 and (-)-EKC, on NMDA-induced [14C]-ACh release was antagonized by naltrindole with an IC50 of 1 nM, indicating the involvement of delta-opioid receptors in the inhibitory effect of DPDPE. On the other hand, the inhibitory effects of dynorphin A1-13 and (-)-EKC on [14C]-ACh release were readily antagonized by nor-BNI with an IC50 of about 3 nM. A 100 fold higher concentration of nor-BNI also antagonized the inhibitory effect of DPDPE, indicating the involvement of U69593-insensitive kappa2-opioid receptors in the inhibitory effects of dynorphin A1-13 and (-)-EKC. 4. Although naloxone benzoylhydrazone (NalBzoH), displaying high affinity towards the putative kappa3-opioid receptor, antagonized the inhibitory effects of dynorphin A1-13 and (-)-EKC on [3H]-dopamine and [14C]-ACh release as well as that of U69593 on [3H]-dopamine release, it displayed a low apparent affinity (IC50 about 100 nM) in each case. 5. In conclusion, whereas activation of kappa1-opioid receptors causes presynaptic inhibition of NMDA-induced dopamine release, kappa2 receptor activation results in inhibition of ACh release in rat neostriatum. As such, this study is the first to provide unequivocal in vitro evidence for the existence of functionally distinct kappa-opioid receptor subtypes in the brain.     

Effects of naturally acquired decompensated mitral valve regurgitation on the renin-angiotensin-aldosterone system and atrial natriuretic peptide concentration in dogs

We examined whether opioids, especially morphine, would centrally elicit scratching in mice and determined some characteristics of the scratch-inducing action of opioids. When intracisternally (i.c.) injected, morphine (0.1-3 nmol/mouse) produced a dose-dependent increase in scratching of the face, but not of the ears, head and body trunk. When injected intradermally into the rostral part of the back, morphine (at most potent i.c. dose of 3 nmol/mouse or higher) did not increase the scratching of the injected site. Facial scratching of the mouse induced by i.c. injection of morphine (0.3 nmol/mouse) was almost abolished by distraction and by naloxone (1 mg/kg, s.c.). [D-Ala2, N-Me-Phe4, Gly5-ol]Enkephalin (DAMGO) (0.03-2 nmol), but not [D-Pen2,5]enkephalin (DPDPE) and U-50,488, dose-dependently elicited facial scratching by i.c. injection. These results suggest that morphine and DAMGO increased facial scratching, probably mediated by central opioid mu-receptors in mice, and such scratching was due to a sensation, probably itching. The present animal model may be useful for analyzing opioid-mediated central itching.     

Streptozotocin-induced diabetes selectively enhances antinociception mediated by delta 1- but not delta 2-opioid receptors

We assessed the effect of diabetes on antinociception produced by intracerebroventricular injection of delta-opioid receptor agonists [D-Pen2,5]enkephalin (DPDPE) and [D-Ala2]deltorphin II. The antinociceptive effect of DPDPE (10 nmol), administered i.c.v., was significantly greater in diabetic mice than in non-diabetic mice. The antinociceptive effect of i.c.v. DPDPE was significantly reduced in both diabetic and non-diabetic mice following pretreatment with 7-benzylidenenaltrexone (BNTX), a selective delta 1-opioid receptor antagonist, but not with naltriben (NTB), a selective delta 2-opioid receptor antagonist. There were no significant differences in the antinociceptive effect of [D-Ala2]deltorphin II (3 nmol, i.c.v.) in diabetic and non-diabetic mice. Furthermore, the antinociceptive effect of i.c.v. [D-Ala2]deltorphin II was significantly reduced in both diabetic and non-diabetic mice following pretreatment with NTB, but not with BNTX. In conclusion, mice with diabetes are selectively hyper-responsive to supraspinal delta 1-opioid receptor-mediated antinociception, but are normally responsive to activation of delta 2-opioid receptors.     

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.     

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.     

Differential antagonism by MK-801 against antinociception induced by opioid receptor agonists administered supraspinally in mice

Various doses of MK-801 ((+/-)-5-methyl-10,11-dihydro-5H-dibenzo(a,d) cyclohepten-5, 10-imine maleate), a non-competitive N-methyl-D-aspartic acid (NMDA) receptor antagonist (0.001-1 microgram) injected intracerebroventricularly (i.c.v.) alone did not show any antinociceptive effect. MK-801 (0.001-1 microgram i.c.v.) dose dependently attenuated the inhibition of the tail-flick and hot plate responses induced by i.c.v. administered morphine (1 microgram), [D-Pen2, D-Pen5]enkephalin (DPDPE; 10 micrograms), and U50,488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeoce tamide ) 60 micrograms). However, the inhibition of the tail-flick and hot plate responses induced by i.c.v. administered beta-endorphin (1 microgram) was not changed by i.c.v. administered MK-801. Our results indicate that, at the supraspinal level, NMDA receptors are involved in the production of antinociception induced by supraspinally administered morphine, DPDPE, and U50,488H but not beta-endorphin.     

The competence-related abilities of women criminal defendants

Delta9-tetrahydrocannabinol (delta9-THC) elicits antinociception in rodents through the central CB1 cannabinoid receptor subtype. In addition. Delta9-THC stimulates the release of dynorphin-related peptides leading to kappa-opioid spinal antinociception. In this work we describe the effect of a mixture of thiorphan (a neutral endopeptidase EC3.4.24.11 inhibitor) and bestatin (an aminopeptidase inhibitor), administered i.c.v., on the antinociceptive effect of peripherally administered delta9-THC in mice. As in the case of morphine or DAMGO ([D-Ala2.N-Me-Phe4,Gly-ol]enkephalin), a mu-selective opioid receptor agonist, the mixture of enkephalin-degrading enzyme inhibitors also enhanced the antinociceptive effect of delta9-THC. This effect was blocked by the CB1 cannabinoid receptor antagonist, SR-141,716-A, as well as by naloxone. The kappa-opioid receptor antagonist nor-binaltorphimine, administered i.t., also antagonized the effect of this combination. Similar results were obtained with the mu-opioid receptor antagonist beta-funaltrexamine after i.c.v. administration. These results demonstrate the involvement of both mu-opioid supraspinal and kappa-opioid spinal receptors in the interaction of both opioid and cannabinoid systems regulating nociception in mice.     

Binding of [3H][D-Ala2, MePhe4, Gly-ol5] enkephalin, [3H][D-Pen2, D-Pen5]enkephalin, and [3H]U-69,593 to airway and pulmonary tissues of normal and sensitized rats

The role of endogenous opioid peptides in the regulation of bronchomotor tone, as well as in the pathophysiology of asthma is uncertain. We have studied the binding of highly selective [3H]labeled ligands of mu-([D-Ala2, MePhe4, Gly-ol5]enkephalin; DAMGO), delta ([D-Pen2, D-Pen5]enkephalin; DPDPE), and kappa-(U-69,593) opioid receptors to membranes of trachea, main bronchus, lung parenchyma and pulmonary artery obtained from normal (unsensitized) and actively IgE-sensitized rats acutely challenged with the specific antigen. [3H]DAMGO, [3H]DPDPE and [3H]U-69,593 bound to membranes of normal and sensitized tissues at a saturable, single high-affinity site. The rank order of receptor densities in normal tissues was delta- > or = kappa- > or = mu-, with lung parenchyma exhibiting the greatest binding capacity for delta- and mu- receptors compared to the other regions examined. The Kd values showed small differences between ligands and regions tested. The mu- and delta-opioid receptor densities were decreased in sensitized main bronchus and lung parenchyma, respectively, compared to normal tissues. By contrast, kappa-opioid receptor density was augmented in sensitized lung parenchyma but an increase in Kd values was also observed. These differential changes in the density and affinity of opioid receptor types may be related to alterations in endogenous opioid peptides during the process of sensitization.     

Effects of mu and delta opioid agonists and antagonists on affective vocal and reflexive pain responses during social stress in rats

The present experiments evaluated the influence of intraventricular mu and delta opioid receptors on affective vocal and reflexive responses to aversive stimuli in socially inexperienced, as well as defensive and submissive responses in defeated, adult male Long-Evans rats. Defeat stress consisted of: (1) an aggressive confrontation in which the experimental intruder rat exhibited escape, defensive and submissive behaviors [i.e., upright, supine postures and ultrasonic vocalizations (USV)], and subsequently, (2) protection from the resident stimulus rat with a wire mesh screen for 10-20 min. Defeat stress was immediately followed by an experimental session with tactile startle (20 psi). The mu opioid receptor agonists morphine (0.1-0.6 microg i.c.v.) and [D-Ala2-N-Me-Phe4-Gly5-ol]-enkephalin (DAMGO; 0.01-0.3 microg i.c.v.), and the delta opioid receptor agonist [D-Pen2,5]-enkephalin (DPDPE; 10-100 microg i.c.v.) dose-dependently decreased startle-induced USV and increased tail-flick latencies in socially inexperienced and defeated rats. Of greater interest, morphine, DAMGO and DPDPE increased the occurrence of the submissive crouch posture, and defeated rats were more sensitive than socially inexperienced rats to the startle-induced USV-suppressive and antinociceptive effects of morphine and DPDPE. The antinociceptive effects of DAMGO were likewise obtained at lower doses in defeated rats. Finally, the USV-suppressive effects of morphine and DAMGO were reversed with the mu receptor antagonist naltrexone (0.1 mg/kg i.p.), but the USV-suppressive effects produced by DPDPE were not reversed with the delta receptor antagonist naltrindole (1 mg/kg i.p.). These results confirm mu, but not delta opioid receptor activation as significant in affective vocal, passive-submissive behavior, as well as reflexive antinociception. Furthermore, similar to previous studies with restraint and electric shock stress, the facilitation of mu opioid effects on vocal responses and antinociception is consistent with the proposal that defeat stress activated endogenous opioid mechanisms.     

Mechanism of the dynorphin-induced dualistic effect on free intracellular Ca2+ concentration in cultured rat spinal neurons

In order to study the different mechanisms of dynorphin spinal analgesia and neurotoxicity at low and high doses, the effects of various concentrations of dynorphin A-(1-17) on the free intracellular Ca2+ concentration ([Ca2+]i) in the cultured rat spinal neurons were studied using single cell microspectrofluorimetry. While dynorphin A-(1-17) 0.1-100 microM had no significant effect on basal [Ca2+]i, dynorphin A-(1-17) 0.1 and 1 microM significantly decreased the high KCl-evoked peak [Ca2+]i by 94% and 83% respectively. Dynorphin A-(1-17) 10 and 100 microM did not affect the peak [Ca2+]i following K+ depolarization, but in all these neurons there was a sustained and irreversible rise in [Ca2+]i following high-K+ challenge. Pretreatment with the specific kappa-opioid receptor antagonist nor-binaltorphimine 10 microM, but not the competitive NMDA receptor antagonist, DL-2-amino-5-phosphonovalerate (APV) 10 microM, significantly blocked the inhibitory effect of dynorphin A-(1-17) 0.1 microM on peak [Ca2+]i. However, APV 10 microM and nor-binaltorphimine 10 microM significantly antagonized the sustained rise in [Ca2+]i induced by a high concentration of dynorphin A-(1-17) 10 microM. Furthermore, in the presence, and following the addition, of increasing concentrations of dynorphin A-(1-17) (0.1, 1, 10 and 100 microM), the high concentrations of dynorphin A-(1-17) failed to produce a sustained rise in peak [Ca2+]i. These results suggested that dynorphin exerted a dualistic modulatory effect on [Ca2+]i in cultured rat spinal neurons, inducing a sustained and irreversible intracellular Ca2+ overload via activation of both NMDA and kappa-opioid receptors at higher concentrations, but inhibiting depolarization-evoked Ca2+ influx via kappa-opioid but not NMDA receptors at lower concentrations. Serial addition of graded concentrations of dynorphin A-(1-17) prevented the effect of high concentrations of dynorphin A-(1-17) on [Ca2+]i.     

Differences in the influence of AIN-purified and non-purified diets on the development of hypertension in SHR and DOCA-salt hypertensive rats

The effect of nicotine administered supraspinally on antinociception induced by supraspinally administered opioids was examined in ICR mice. The intracerebroventricular (i.c.v.) injection of nicotine alone at doses from 1 to 12 micrograms produced only a minimal inhibition of the tail-flick response. Morphine (0.2 micrograms), beta-endorphin (0.1 microgram), D-Pen2.5-enkephalin (DPDPE; 0.5 microgram), trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl] benzeocetamide (U50, 488H; 6 micrograms) caused only slight inhibition of the tail-flick response. Nicotine dose dependently enhanced inhibition of the tail-flick response induced by i.c.v. administered morphine (0.2 microgram) or beta-endorphin (0.1 microgram). The degree of enhancing effect of nicotine toward beta-endorphin-induced inhibition of the tail-flick response was greater than toward morphine-induced inhibition of the tail-flick response. However, i.c.v. administered nicotine at the same doses was not effective in enhancing the inhibition of the tail-flick response induced by DPDPE (0.5 microgram) or U50, 488H (6 micrograms) administered i.c.v. Our results suggest that stimulation of supraspinal nicotinic receptors may enhance antinociception induced by morphine (a mu-opioid receptor agonist) and beta-endorphin (an epsilon-opioid receptor agonist) administered supraspinally. However, the activation of nicotinic receptors at supraspinal sites may not be involved in enhancing the antinociception induced by DPDPE (a delta-opioid receptor agonist) or U50, 488H (a kappa-opioid receptor agonist) administered supraspinally.     

Effect of 7-nitroindazole on tolerance to morphine, U-50,488H and [D-Pen2, D-Pen5] enkephalin in mice

The effects of 7-nitroindazole (7-NI), an inhibitor of the neuronal nitric oxide synthase (nNOS) which does not increase blood pressure, on tolerance to the antinociceptive activity of mu-(morphine), kappa-(U-50,488H) and delta-([D-Pen2, D-Pen5]enkephalin, DPDPE) opioid receptor agonists were determined in mice. Male Swiss-Webster mice were made tolerant by twice daily injections of morphine (20 mg/kg, s.c.), U-50,488H (25 mg/kg, i.p.) or DPDPE (20 micrograms/mouse, i.c.v.) for 4 days. When tested on day 5, tolerance to their antinociceptive activity was evidenced by decreased response in chronic drug treated mice in comparison to vehicle-injected mice. Concurrent administration of 7-NI (20, 40 or 80 mg/kg, i.p.) with DPDPE did not modify the development of tolerance to the antinociceptive action of DPDPE. However, 7-NI (40 or 80 mg/kg, i.p.) inhibited the development of tolerance to the antinociceptive activity of morphine and U-50,488H but the lower dose of 7-NI (20 mg/kg, i.p.) was not effective. Chronic administration of 7-NI by itself did not modify the acute response to morphine, U-50,488H or DPDPE. It is concluded that a specific inhibitor of nNOS can inhibit tolerance to the antinociceptive activity of mu- and kappa- but not of delta-opioid receptor agonists in mice.     

Dermorphin analog Tyr-D-Arg2-Phe-sarcosine-induced opioid analgesia and respiratory stimulation: the role of mu 1-receptors?

Tyr-D-Arg2-Phe-sarcosine4 (TAPS), a mu-selective tetrapeptide analog of dermorphin, induced sustained antinociception and stimulated ventilatory minute volume (MV) at the doses of 3 to 100 pmol i.c.v. The doses of 30 and 100 pmol i.c.v. induced catalepsy. The effect of TAPS on MV was in negative correlation with the dose and the maximal response was achieved by the lowest (3 pmol) dose (+63 +/- 23%, P < .05). Morphine, an agonist at both mu 1 and mu 2 sites, at a dose of 150 nmol i.c.v. (equianalgesic to 100 pmol of TAPS decreased the MV by 30%, due to a decrease in ventilatory tidal volume. The antinociceptive effect of TAPS was antagonized by naloxone and the mu 1 receptor antagonist, naloxonazine. Naloxonazine also attenuated the catalepsy produced by 100 pmol of TAPS i.c.v. and the respiratory stimulation produced by 3 pmol of TAPS i.c.v. Pretreatment with 30 pmol of TAPS antagonized the respiratory depression induced by the mu opioid agonist dermorphin (changes in MV after dermorphin alone at 1 or 3 nmol were -22 +/- 10% and -60 +/- 9% and, after pretreatment with TAPS, +44 +/- 11% and -18 +/- 5%, respectively). After combined pretreatment with naloxonazine and TAPS, 1 nmol of dermorphin had no significant effect on ventilation. In contrast, pretreatment with a low respiratory stimulant dose (10 pmol i.c.v.) of dermorphin did not modify the effect of 1 nmol of dermorphin. In conclusion, the antinociceptive, cataleptic and respiratory stimulant effects of TAPS appear to be a related to its agonist action at the mu 1 opioid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Treatment of chronic allodynia in spinally injured rats: effects of intrathecal selective opioid receptor agonists

We examined the effects of intrathecal (i.t.) selective opioid receptor agonists in alleviating mechanical and cold allodynia in spinally injured rats. Both DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin, a mu-opioid receptor agonist) and DPDPE ([D-Phe2,D-Phe5]-enkephalin, a delta-opioid receptor agonist) dose-dependently relieved the chronic allodynia-like behavior at doses selective for their respective receptors. The anti-allodynic effect of DAMGO and DPDPE was reversed by the selective mu- and delta-opioid receptor antagonists CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2) and naltrindole, respectively. In contrast, the selective kappa-opioid receptor agonist U50488H did not alleviate the allodynia-like behavior, but rather enhanced it. The anti-nociceptive and anti-allodynic effect of i.t. DAMGO was blocked by U50488H. Thus, activation of spinal mu- and delta-, but not kappa-opioid receptors produced anti-allodynic effect in this model of central pain. Drugs which act selectively on opioid receptor subtypes may be useful in managing chronic central pain of spinal cord origin.     

Met]enkephalin in the spinal cord is involved in the antinociception induced by intracerebroventricularly-administered etorphine in the mouse

We have recently reported that the antinociception induced by etorphine given i.c.v. is mediated in part by the stimulation of both mu- and epsilon-opioid receptors and the activation of both monoaminergic and opioidergic descending pain control systems. [Xu J. Y. et al. (1992) J. Pharmac. exp. Ther. 263, 246-252]. Since the opioid epsilon-receptor-mediated antinociception induced by beta-endorphin is mediated by the release of [Met]enkephalin and subsequent stimulation of delta-opioid receptors in the spinal cord, the present studies were designed to determine if beta-endorphin-like action is also involved in etorphine-induced antinociception. The tail-flick test was used to assess the antinociceptive response performed in male ICR mice. Etorphine at doses from 5 to 20 ng given i.c.v. produced a dose-dependent inhibition of the tail-flick response. The inhibition of the tail-flick response induced by etorphine given i.c.v. was antagonized by intrathecal pretreatment for 60 min with antiserum against [Met]enkephalin (10 microg), but not with antiserum against [Leu]enkephalin (10 microg) or dynorphin A (1-13) (10 microg). Desensitization of delta-opioid receptors in the spinal cord by intrathecal pretreatment with [Met]enkephalin (5 microg) for 60 min attenuated i.c.v. administered etorphine-induced tail-flick inhibition. However, intrathecal pretreatment with [Leu]enkephalin (5 microg) or dynorphin A (1-17) (0.1 microg) for 60 min did not attenuate i.c.v. administered etorphine-induced tail-flick inhibition. The results indicate that antinociception induced by etorphine given i.c.v. is mediated in part by the stimulation of the epsilon-opioid receptor at the supraspinal sites and by the release of [Met]enkephalin, which subsequently stimulates delta-opioid receptors in the spinal cord.     

Enkephalin analogs containing 4,4-difluoro-2-aminobutyric acid: synthesis and fluorine effect on the biological activity

Analogs of Met-enkephalin and [D-Pen2, D-Pen5]enkephalin (DPDPE) containing the partially fluorinated amino acid 4,4-difluoro-2-aminobutyric acid (DFAB) in the 2- or 3-position of the peptide sequence were synthesized and their opioid activities and receptor selectivities were determined in vitro. The linear fluorinated [D-DFAB2, Met5-NH2]enkephalin showed mu and delta agonist potencies comparable to those of natural [Leu5]enkephalin. The partially fluorinated DPDPE analogs behaved differently as compared with their non-fluorinated correlates. While L-amino acid substitution in position 3 of DPDPE usually resulted in higher delta agonist potency than D-amino acid substitution. [D-DFAB3]DPDPE turned out to be a more potent delta agonist than [L-DFAB3]DPDPE. Furthermore, [D-DFAB3]DPDPE showed over 100-fold higher delta agonist potency than [D-Abu3]DPDPE (Abu = 2-aminobutyric acid), indicating that the fluorine substituents interact favorably with a delta opioid receptor subsite.     

The delta2-opioid receptor subtype stimulates phosphoinositide metabolism in mouse periaqueductal gray matter

The delta(delta)-opioid agonists [D-Pen2,5]enkephalin (DPDPE) and [D-Ala2]deltorphin II increased the formation of inositol phosphates (IPs) in mice periaqueductal gray matter (PAG) slices pre-labeled with myo-[3H]inositol. Both delta-agonists caused an increase in IP accumulation in a dose-dependent manner (1-100 microM) and which was pertussis toxin (0.5 microg/mouse, i.c.v.) sensitive. This effect was blocked by the delta-antagonist ICI-174.864 (10 microM). The presence of subtypes of the delta-opioid receptor (delta1 and delta2) in PAG has been suggested by pharmacological studies. In this brain structure, naltrindrole 5'-isothiocyanate (5'-NTII), but not 7-benzylidenenaltrexone (BNTX), antagonized the effects of DPDPE and [D-Ala2]deltorphin II, suggesting the involvement of a population of delta receptors sensitive to the delta2-antagonist NT II on this effect. To further investigate the participation of delta-receptor subtypes in the stimulation of IPs formation, mice were injected with antisense oligodeoxynucleotides (ODNs) directed to nucleotides 7-26 or 2946 of the cloned delta-receptor mRNA, and PAG slices from these animals were used in in vitro assays. The results demonstrate that the reported increase of phosphoinositide (PI) hydrolysis depends on the agonist activation of the delta2-opioid receptor subtype in the PAG.