Elmar Friderichs

(–)-(1R,2R)-3-(3-Dimethylamino-1-ethyl-2-methyl-propyl)-phenol Hydrochloride (Tapentadol HCl): a Novel μ-Opioid Receptor Agonist/Norepinephrine Reuptake Inhibitor with Broad-Spectrum Analgesic Properties

(–)-(1R,2R)-3-(3-Dimethylamino-1-ethyl-2-methyl-propyl)-phenol hydrochloride (tapentadol HCl) is a novel μ-opioid receptor (MOR) agonist (Ki = 0.1 μM; relative efficacy compared with morphine 88% in a [35S]guanosine 5′-3-O-(thio)triphosphate binding assay) and NE reuptake inhibitor (Ki = 0.5 μM for synaptosomal reuptake inhibition). In vivo intracerebral microdialysis showed that tapentadol, in contrast to morphine, produces large increases in extracellular levels of NE (+450% at 10 mg/kg i.p.). Tapentadol exhibited analgesic effects in a wide range of animal models of acute and chronic pain [hot plate, tail-flick, writhing, Randall-Selitto, mustard oil colitis, chronic constriction injury (CCI), and spinal nerve ligation (SNL)], with ED50 values ranging from 8.2 to 13 mg/kg after i.p. administration in rats. Despite a 50-fold lower binding affinity to MOR, the analgesic potency of tapentadol was only two to three times lower than that of morphine, suggesting that the dual mode of action of tapentadol may result in an opiate-sparing effect. A role of NE in the analgesic efficacy of tapentadol was directly demonstrated in the SNL model, where the analgesic effect of tapentadol was strongly reduced by the α2-adrenoceptor antagonist yohimbine but only moderately attenuated by the MOR antagonist naloxone, whereas the opposite was seen for morphine. Tolerance development to the analgesic effect of tapentadol in the CCI model was twice as slow as that of morphine. It is suggested that the broad analgesic profile of tapentadol and its relative resistance to tolerance development may be due to a dual mode of action consisting of both MOR activation and NE reuptake inhibition.

Interaction of μ-opioid receptor agonists and antagonists with the analgesic effect of buprenorphine in mice

Buprenorphine is a potent opioid analgesic with partial agonistic properties at μ‐opioid receptors. This study investigated the interaction potential with several full μ‐agonists in the tail‐flick test in mice. We further examined the reversibility of buprenorphine antinociception by different μ‐opioid receptor antagonists.

Anticonvulsant and proconvulsant effects of tramadol, its enantiomers and its M1 metabolite in the rat kindling model of epilepsy

The centrally acting analgesic tramadol has recently been reported to cause seizures at re‐commended dosages in patients, whereas animal experiments had indicated that seizures only occur in high, toxic doses. Tramadol has a dual mechanism of action that includes weak agonistic effects at the mu‐opioid receptor as well as inhibition of monoamine (serotonin, norepinephrine) re‐uptake. Its major (M1) metabolite mono‐O‐desmethyltramadol, which is rapidly formed in vivo, has a markedly higher affinity for mu receptors and may thus contribute to the effects of the parent compound. Furthermore, the pharmacological effects of tramadol appear to be related to the different, but complementary and interactive pharmacologies of its enantiomers. In the present study, we evaluated (±)‐tramadol, its enantiomers, and its M1 metabolite ((+)‐enantiomer) in the amygdala kindling model of epilepsy in rats. Adverse effects determined in kindled rats were compared to those in nonkindled rats. At doses within the analgesic range, (±)‐tramadol and its enantiomers induced anticonvulsant effects in kindled rats. However, at only slightly higher doses seizures occurred. With (±)‐tramadol, generalized seizures were observed at 30 mg kg−1 in most kindled but not in nonkindled rats. The (−)‐enantiomer induced myoclonic seizures at 30 mg kg−1 in most kindled but not in nonkindled rats, although myoclonic seizure activity was observed in some nonkindled rats at 10 or 20 mg kg−1. Seizures were also observed after the (+)‐enantiomer and the (+)‐enantiomer of the M1 metabolite, but experiments with higher doses of these compounds were limited by marked respiratory depression. The data demonstrate that kindling enhances the susceptibility of rats to convulsant adverse effects of tramadol and its enantiomers, indicating that a preexisting lowered seizure threshold increases the risk of tramadol‐induced seizures.

Actions of tramadol on micturition in awake, freely moving rats.

(±)‐Tramadol, a widely used analgesic, is a racemate stimulating opioid receptors and inhibiting reuptake of noradrenaline and serotonin, that is, pharmacological principles previously shown to influence rat micturition. We studied both (±)‐tramadol and its enantiomers in conscious Sprague–Dawley rats undergoing continuous cystometry. The effects of these agents were compared to those of morphine (μ‐opioid receptor agonist) and tested after pretreatment with naloxone (μ‐opioid receptor antagonist). Cystometries were evaluated before and after intravenous (i.v.), intraperitoneal (i.p.) and intrathecal (i.t.) drug administrations. The most conspicuous effects of i.v. (±)‐tramadol (0.1–10 mg kg−1) was an increase in threshold pressure and an increase in micturition volume. These effects were mimicked by (+)‐tramadol (0.1–5 mg kg−1 i.v.), whereas (−)‐tramadol (5 mg kg−1 i.v.) did not influence threshold pressure and micturition volume. The effects of (±)‐tramadol 5 mg kg−1 on micturition volume were blocked by pretreatment with naloxone 0.3 mg kg−1. Morphine (0.3–10 mg kg−1 i.p.) increased threshold pressure but did not significantly increase micturition volume in doses not resulting in overflow incontinence. (±)‐Tramadol 10 mg kg−1 increased urine production, an effect blocked by desmopressin 25 ng kg−1. (±)‐Tramadol effectively inhibits micturition in conscious rats by stimulating μ‐opioid receptors. A synergy between opioid receptor stimulation and monoamine reuptake inhibition may contribute to the micturition effects.

Spinal antinociception by morphine in rats is antagonised by galanin receptor antagonists

Galanin, a 29 amino acid peptide, has been reported to possess antinociceptive properties at the spinal site and to potentiate opioid-induced antinociception. Our aim was to investigate whether also endogenous galanin interacts with an exogenously administered opioid, morphine, in the rat spinal cord. This question was investigated by use of the recently developed galanin receptor antagonists galantide [M-15, galanin(1–13)-substance P-(5 -11) amide] and M-35 [galanin-(1–13)-bradykinin-(2–9) amide].Nociception was assessed in the rat tail-flick test using radiant heat and the rat Randall-Selitto model of inflammatory pain using vocalization as the nociceptive criterion. Intrathecal (i.t.) injections were performed in rats under ether anaesthesia. Morphine was administered either i.t. or intraperitoneally (i.p.), and the antagonists were injected i.t. [125I]Galanin binding experiments were performed on crude synaptosomal membranes of the rat spinal cord.In the rat tail-flick test, i.t. injection of 3 μg morphine evoked antinociception of about 75% of the maximal possible effect (% MPE). Co-injection of either 2 μg galantide or 2 μg M-35 with morphine almost completely abolished the antinociceptive effect of morphine. I.p. injection of 2.15 mg/kg morphine elicited about 80% MPE when given 10 min prior to i.t. saline injection. Injection of the antagonists instead of saline antagonised the antinociceptive effect of morphine partially thus showing the spinal proportion of the overall antinociceptive effect. In the rat Randall-Selitto test, 3 μg morphine, injected i.t., produced antinociception of almost 100% MPE. Coinjection of the antagonists reduced the maximum effect partially by about 25–35%. I.p. injection of 7.5 mg/kg morphine 10 min prior to Lt. injection of saline elicited an antinociceptive effect of 90–100% MPE; injection of the antagonists instead of saline reduced the peak effect to a similar degree as after i.t. injection of 3 μg morphine. To exclude a direct interference by morphine with the galanin receptor, in vitro binding of [125I]galanin to a spinal synaptosomal fraction was assessed. Morphine, 10 μM, did not interfere with the specific [125I]galanin binding. These results provide further evidence that galanin is involved in spinal nociceptive processing. It seems to be involved in the mediation of the effects of morphine at this site, either as a co-transmitter, or subsequent to µ-receptor activation on nerve terminals or on interneurones.

Galanin receptor antagonists attenuate spinal antinociceptive effects of DAMGO, tramadol and non-opioid drugs in rats

The involvement of endogenous galanin to antinociception elicited by intrathecally (i.t.) or systemically administered drugs from different chemical and therapeutic classes was investigated using the rat Randall-Selitto or the rat tail-flick test, in the absence or presence of the i.t. administered galanin receptor antagonists galantide and M-35. Antinociception elicited by i.t. tramadol (24 micrograms), DAMGO (1 microgram), clonidine (48 micrograms), desipramine (6 micrograms) or fenfluramine (60 micrograms) was attenuated by i.t. galantide (2 micrograms); the attenuation reached significance at least at one time point. A partial antagonism by i.t. galantide was also observed against the antinociception of i.p. tramadol (10 mg/kg), i.v. clonidine (1 mg/kg), i.p. desipramine (1 mg/kg), or i.p. dipyrone (1000 mg/kg), but antinociception by i.p. fenfluramine (30 mg/kg) was not affected. Using M-35 (2 micrograms i.t.), the antinociception of i.t. tramadol or DAMGO was attenuated, but no inhibition was observed when clonidine, desipramine or fenfluramine were used i.t. If drugs were administered systemically, only antinociception of i.p. fenfluramine but not that of i.p. tramadol, or i.v. clonidine, or i.p. desipramine or i.p dipyrone was attenuated. In the rat tail flick test, co-injection of either 2 micrograms i.t. galantide or M-35 with i.t. tramadol (12 micrograms) almost abolished the antinociceptive effect, whereas the antinociception of systemically administered tramadol (4.6 mg/kg i.p.) was only partially attenuated by i.t. galantide and not affected by i.t. M-35. Binding studies in dorsal spinal cord tissue showed no affinity of galantide or M-35 to spinal mu-, or delta-, or kappa-opioid receptors and none of the other drugs interfered with the spinal galanin binding site. These data give further support of at least a partial galanin link in spinal processes of antinociception.

Lack of sensitization during place conditioning in rats is consistent with the low abuse potential of tramadol

Based on the recent finding that tramadol (TRAM) produces conditioned place preference (CPP) and dopamine release in the nucleus accumbens, it was suggested that the abuse liability of TRAM may be greater than hitherto assumed. We re-evaluated the effects of TRAM in CPP and behavioral sensitization, in comparison with morphine (MOR) and meptazinol (MEPT), an opioid drug with minimal abuse potential. While MOR produced CPP and very strong locomotor sensitization, TRAM and MEPT produced only CPP. It has been suggested that sensitization plays an important role in the development of addiction, hence our results suggest that the abuse potential of TRAM might resemble more that of MEPT than that of MOR, and they are consistent with the clinical picture, in that although TRAM is not completely devoid of positively reinforcing effects, reports on abuse are rare. The low propensity to induce addiction may be related to the lack of changes in the brain circuitry mediating reward and motivation, as evidenced by the lack of sensitization.

A Quantitative Method for Measuring Antipsoriatic Activity of Drugs by the Mouse Tail Test

Topical treatment of the mouse tail with antipsoriatic drugs enhances orthokeratotic cell differentiation in the epidermal scales. We developed a new evaluation system for this test which allows quantification of drug efficacy. Drugs were applied topically, once daily, 5 times a week, for 2 weeks. Two hours after the last treatment the animals were sacrificed, longitudinal sections of the tail skin were made and prepared for histological examination (hematoxylin-eosin staining). As indicator of orthokeratosis (OK), the length of the granular layer per scale was measured microscopically with a semiautomatic image evaluation unit and related to the total scale length (= % OK per scale). Drug activity was defined by the increase in the total length of orthokeratotic regions, 100% activity corresponds to a granular layer extending over the whole scale length. In this model dithranol and retinoic acid dose-dependently increased orthokeratosis up to 75 and 79%, respectively. Beech tar, in a concentration of 5% induced a 19% increase in orthokeratosis. Morphometric quantification by image analysis of the conversion of parakeratotic into orthokeratotic regions in mouse tail scales induced by topical drug treatment seems to be a suitable and reliable procedure to investigate new drugs from which antipsoriatic activity may be expected.

Absence of emetic effects of morphine and loperamide in Suncus murinus

The house musk shrew Suncus murinus recently has been introduced for the study of emesis. We investigated the emetic effects of the opioids morphine (0.1-21.5 mg/kg i.p.) and loperamide (0.01-10 mg/kg i.p.) and found a complete lack of emetogenic potential. Nicotine, however, dose dependently induced vomiting in the Suncus with an ED50 of 8.8 mg/kg s.c. and a 100% incidence at 20 mg/kg. This drug-induced vomiting was reduced by morphine or loperamide: ED50 values obtained were 1.2 mg/kg i.p. for morphine and 0.7 mg/kg i.p. for loperamide. Naloxone (2 mg/kg s.c.) antagonised the inhibitory effect of morphine (2 mg/kg i.p.) or loperamide (10 mg/kg i.p.). Serotonin (20 mg/kg s.c.) had less reliable emetogenic potency than nicotine in the Suncus with incidences between 50 and 100%. However, the serotonin-induced vomiting was abolished by morphine and loperamide and this inhibition was antagonised by naloxone. These results suggest that systemically administered opioids are pure antiemetics in Suncus murinus in contrast to other animal models and man. Naloxone antagonism indicates that this antiemetic effect is mediated by opioid receptors.

Comparison of the protective effects by human and bovine superoxide dismutase against ischemia- and reperfusion-induced impairment of kidney function in anesthetized rats

In a rat model of ischemia- and reperfusion-induced kidney damage, the protective effects of human superoxide dismutase produced by genetic technology (hum-SOD) were compared to those of bovine superoxide dismutase (bov-SOD). The intravenous infusion of hum-SOD and bov-SOD, started concomitantly with the kidney reperfusion after a 60-min (or 30-min) period of ischemia, significantly improved the renal function (inulin and p-amino-hippuric acid clearance rates) as compared to the vehicle-treated control group. In contrast, inactive apoenzyme of superoxide dismutase (Apo-SOD) did not improve the impaired renal function after the kidney reperfusion. Therefore, the kidney protection by hum-SOD and bov-SOD may reasonably be ascribed to their specific enzymatic function--scavenging of oxygen radicals. In this respect, hum-SOD proved to be as effective as bov-SOD. To our knowledge this is the first report on a direct pharmacologic comparison of superoxide dismutases from natural and recombinant origin.