Tamás Friedmann

Peripheral versus central antinociceptive actions of 6-amino acid-substituted derivatives of 14-O-methyloxymorphone in acute and inflammatory pain in the rat.

Opioid analgesics with restricted access to the central nervous system represent a new approach to the treatment of severe pain with an improved safety profile. The objective of this study was to investigate the peripheral and central components of the antinociceptive actions of the 6-amino acid conjugates (glycine, alanine, and phenylalanine) of 14-O-methyloxymorphone. Their antinociceptive activities were compared with those of the centrally penetrating μ-opioid agonists morphine, fentanyl, and 14-O-methyloxymorphone. In the tail-flick test in rats, the 6-amino acid conjugates were 45- to 1170-fold more potent than morphine after i.c.v. administration and 19- to 209-fold after s.c. administration. They showed potencies similar to fentanyl after s.c. administration and were more potent after i.c.v. application. The time course of action was different between s.c. and i.c.v. administration, with significant long-lasting effects after i.c.v. administration. Systemic administration of the peripherally selective opioid antagonist naloxone methiodide antagonized the effects after s.c. but not after i.c.v. administration in the tail-flick test. Subcutaneous 6-amino acid derivatives also elicited antihyperalgesic effects in the formalin test in rats, which were reversed by systemically administered naloxone methiodide. Although morphine exerts its analgesic effects by central and peripheral mechanisms, the investigated new opioids interact primarily with peripheral opioid receptors after s.c. administration. The present data indicate that the 6-amino acid conjugates of 14-O-methyloxymorphone have limited access to the central nervous system and can mediate antinociception at peripheral sites. Also, they might find clinical application when the central actions of opioids are unwanted.

Serotonin reuptake inhibitors fluoxetine and citalopram relax intestinal smooth muscle.

Selective serotonin reuptake inhibitor antidepressants (SSRIs) exert depressant effects on cardiac myocytes and vascular smooth muscle cells by inhibiting Ca2+ channels. We hypothesized that the SSRIs fluoxetine and citalopram affect the contractile activity of intestinal smooth muscle by interfering with Ca2+ entry and (or) signaling pathways. The effects of fluoxetine and citalopram on contractions of guinea-pig ileum longitudinal muscle-myenteric plexus preparations (LMMP) were compared with the effects of the voltage-operated Ca2+ channel inhibitors nifedipine and diltiazem. In a concentration-dependent manner, nifedipine, diltiazem, fluoxetine, and citalopram elicited relaxation of LMMPs contracted by electrical field stimulation (EC50 values of 4 x 10(-7) M, 1.4 x 10(-6) M, 1.4 x 10(-5), and 6.8 x 10(-6) M, respectively). Nifedipine, diltiazem, fluoxetine, and citalopram also relaxed LMMPs contracted with a depolarizing concentration of KCl (48 mM; EC50 values of 1.8 x 10(-8) M, 1.4 x 10(-7) M, 3.7 x 10(-6) M, and 6.3 x 10(-6), respectively), a response that could be reversed by increasing the extracellular Ca2+ concentration (2.5-30 mM). These data suggest that fluoxetine and citalopram elicit relaxation of intestinal smooth muscle, likely by inhibiting Ca2+ channel(s). This effect may be of clinical importance.

Spinal interaction between the highly selective μ agonist DAMGO and several δ opioid receptor ligands in naive and morphine-tolerant mice.

Since the discovery of opioid receptor dimers their possible roles in opioid actions were intensively investigated. Here we suggest a mechanism that may involve the μ-δ opioid heterodimers. The exact role of δ opioid receptors in antinociception and in the development of opioid tolerance is still unclear. While receptor up-regulation can be observed during the development of opioid tolerance no μ receptor down-regulation could be detected within five days. In our present work we investigated how the selective δ opioid receptor agonists and antagonists influence the antinociceptive effect of the selective μ receptor agonist DAMGO in naïve and morphine-tolerant mice. We treated male NMRI mice with 200 μmol/kg subcutaneous (s.c.) morphine twice daily for three days. On the fourth day we measured the antinociceptive effect of DAMGO alone and combined with delta ligands: DPDPE, deltorphin II (agonists), TIPP and TICPψ (antagonists), respectively, administered intrathecally (i.t.) in mouse tail-flick test. In naive control mice none of the δ ligands caused significant changes in the antinociceptive action of DAMGO. The treatment with s.c. morphine resulted in approximately four-fold tolerance to i.t. DAMGO, i.e. the ED₅₀ value of DAMGO was four times as high as in naive mice. 500 and 1000 pmol/mouse of the δ₁ selective agonist DPDPE enhanced the tolerance to DAMGO while 1000 pmol/mouse of the δ₂ selective agonist deltorphin II did not influence the degree of tolerance. However, both δ antagonists TIPP and TICPψ potentiated the antinociceptive effect of i.t. DAMGO, thus they restored the potency of DAMGO to the control level. The inhibitory action of DPDPE against the antinociceptive effect of DAMGO could be antagonized by TIPP and TICPψ. We hypothesize that during the development of morphine tolerance the formation of μδ heterodimers may contribute to the spinal opioid tolerance. δ ligands may affect the dimer formation differently. Those, like DPDPE may facilitate the dimer formation hence inhibit the antinociceptive effect of DAMGO by causing virtual μ receptor down-regulation. Ligands that do not affect the dimer formation do not influence antinociception either but ligands with the presumed capability of disconnecting the dimers may decrease the spinal tolerance to DAMGO.

Novel approach to demonstrate high efficacy of μ opioids in the rat vas deferens: A simple model of predictive value

14-O-Methyloxymorphone and 14-methoxymetopon were reported as highly selective and potent micro opioid receptor agonists. The aim of this study was to demonstrate the opioid activity of these compounds in vitro and in vivo in comparison to oxymorphone, morphine and DAMGO. The micro opioid receptor efficacy, full or partial agonist nature of opioids was analyzed in the rat vas deferens (RVD) bioassay. Compared to oxymorphone, 14-O-methyloxymorphone and 14-methoxymetopon showed greater affinities to the rodent brain micro opioid receptors in receptor binding assays. In isolated organs 14-O-methyloxymorphone and 14-methoxymetopon were 3-10-fold more potent than the micro agonist opioid peptide, DAMGO. All tested compounds reached at least 70% maximum inhibition in mouse vas deferens (MVD) except morphine and oxymorphone. In the RVD, morphine could not exceed 50% inhibition of the twitches while 14-O-methyloxymorphone and 14-methoxymetopon showed inhibitory effects more than 70%. Oxymorphone reached only 4% maximal agonist effect and antagonized the inhibitory effect of DAMGO. The investigated morphinans produced dose-dependent antinociceptive activities in mice and rats. Both, 14-O-methyloxymorphone and 14-methoxymetopon are highly efficacious micro opioid receptor agonists in the RVD exhibiting full micro agonist properties. The RVD tissue contains mu receptors indicated by the comparable K(e) values of the micro antagonist naltrexone against DAMGO in the MVD. RVD may be a good alternative to assess the mu receptor efficacy of opioid agonists providing a more physiological environment for the ligand-receptor interaction than other efficacy measuring methods such as the [(35)S]GTPgammaS binding assay.

Different μ opioid receptor subtypes may mediate C-6 substituted oxycodone derivatives induced antinociceptive gastrointestinal and respiratory actions

To examine the relative roles of μ-opioid receptor subtypes in spinal and supraspinal antinociception, in gastrointestinal transit (GIT) and respiratory activity we assessed the effects of μ-1 selective antagonist, naloxonazine (NXZ) and the non-selective antagonist naloxone (NX) on actions produced by subcutaneous (s.c), intrathecal (i.t.) or intracerebroventricular (i.c.v.) injections of the C-6 -oxime,-hydrazone, -phenylhydrazone, -semicarbazone, -p-NO 2 -phenylhydrazone and -thiosemicarbazone derivatives of oxycodone. The compounds tested were observed to produce marked antinociceptive effects, in all routes of administration, sensitive towards NXZ antagonism

Spinal interaction between μ and δ opioid receptors in naive and morphine-tolerant rats

Background The role of δ opioid receptors in opioid antinociception and tolerance development is still unclear. In the spinal cord of morphine-tolerant mice δ receptor ligands given intrathecally (i.t.) differently influenced the antinociceptive effect of the μ agonist D-Ala2-methyl-glycinol (DAMGO). The δ1 agonist D-Pen2,5-enkephalin (DPDPE) inhibited, the δ2 agonist deltorphin II did not alter, and the δ antagonist cha-TIPPψ potentiated the effect of DAMGO. We hypothesized that during the development of morphine tolerance the formation of μ-δ heterodimers may contribute to the spinal μ opioid tolerance. Delta ligands may affect the dimer formation differently. Those, like DPDPE may facilitate the dimer formation, hence inhibit the antinociceptive effect of DAMGO by causing virtual μ receptor down-regulation. Ligands that do not affect the dimer formation do not influence antinociception but ligands with the presumed capability of disconnecting the dimers may decrease the spinal tolerance to DAMGO. The δ ligand profile in morphine-tolerant rats, were also studied.

The peripheral antinociceptive effect of DAMGO and 6β-glycine-substituted 14-O-methyloxymorphone (HS-731) after systemic administration in a mouse visceral pain model

Inhibition of nociception through activation of peripheral μ opioid receptors (MOR) avoiding the central adverse effects of opioids has added a new possibility to manage pain control. Here we report the antinociceptive effects of the peripherally restricted MOR agonist HS-731, the peptide DAMGO, and morphine in the writhing test in mice. S.c. and i.c.v. HS-731 dose-dependently and completely inhibited writhing, being 24–598 times more potent than the two MOR-selective agonists DAMGO and morphine. However, extremely high s.c./i.c.v. potency ratios were calculated for HS-731 and DAMGO and much lower for morphine. Remarkably, a long duration of action was induced by HS-731 and much shorter by morphine and DAMGO. The antinociceptive effects of systemic opioids were reversed by s.c. naloxone while i.c.v. administration of the MOR selective antagonist CTAP significantly abolished the antinociceptive effect of s.c. morphine but completely failed to antagonize the effects of systemic HS-731 or DAMGO. In addition, in the rat vas deferens HS-731 and DAMGO, but not morphine, showed high intrinsic efficacy and naltrexone-sensitive agonist response at MOR by depressing electrically-evoked contractions of this organ. These data demonstrate that the selectivity and high efficacy of HS-731 and DAMGO at peripheral MOR as well as their inability to cross the blood brain barrier are a cornerstone for producing peripheral antinociception after systemic administration.