Inhibition by tramadol of muscarinic receptor-induced responses in cultured adrenal medullary cells and in Xenopus laevis oocytes expressing cloned M1 receptors.

Shigematsu A.

Department of Anesthesiology, University of Occupational and Environmental Health, School of Medicine, Kitakyushu, Japan.

Tramadol is a widely used, centrally acting analgesic, but its mechanisms of action are not completely understood. Muscarinic receptors are known to be involved in neuronal function in the brain and autonomic nervous system, and much attention has been paid to these receptors as targets of analgesic drugs in the central nervous system. This study investigated the effects of tramadol on muscarinic receptors by using two different systems, i.e., a Xenopus laevis oocyte expression system and cultured bovine adrenal medullary cells. Tramadol (10 nM-100 microM) inhibited acetylcholine-induced currents in oocytes expressing the M1 receptor. Although GF109203X, a protein kinase C inhibitor, increased the basal current, it had little effect on the inhibition of acetylcholine-induced currents by tramadol. On the other hand, tramadol did not inhibit the current induced by AlF4-, a direct activator of GTP-binding protein. In cultured bovine adrenal medullary cells, tramadol (100 nM-100 microM) suppressed muscarine-induced cyclic GMP accumulation. Moreover, tramadol inhibited the specific binding of [3H]quinuclidinyl benzilate (QNB). Scatchard analysis showed that tramadol increases the apparent dissociation constant (Kd) value without changing the maximal binding (Bmax), indicating competitive inhibition. These findings suggest that tramadol at clinically relevant concentrations inhibits muscarinic receptor function via QNB-binding sites. This may explain the neuronal function and anticholinergic effect of tramadol.

Online source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11561087&dopt=Abstract tramadol



Effects of chronic tramadol on pre- and post-synaptic measures of monoamine function.

Stamford JA.

Academic Department of Anaesthesia and Intensive Care, St Bartholomew's and The Royal London School of Medicine and Dentistry, Royal London Hospital, Whitechapel, UK.

The atypical analgesic tramadol has strong structural similarities to the antidepressant venlafaxine and is a mixed noradrenaline (NA) and serotonin (5-HT) uptake inhibitor. Because tramadol has been found active in the forced swim test, a common predictor of antidepressant efficacy, we therefore examined the effects of chronic tramadol on various pre- and post-synaptic monoamine measures. Male Wistar rats (150-200 g) received tramadol (20 mg/kg i.p.) or vehicle for 21 days and were sacrificed 24 h after the last dose. Quantitative autoradiography revealed that specific frontocortical [3H]dihydroalprenolol and [3H]ketanserin binding was lower in the chronic tramadol group than controls (beta: 37+/-8 and 217+/-56 fmol/mg; 5-HT2A: 23+/-3 and 44+/-7 fmol/mg, respectively, p < 0.05). Chronic tramadol had no effect on the magnitude of electrically stimulated noradrenaline (NA) efflux or uptake in locus coeruleus (LC) slices. Although dexmedetomidine (10 nM) decreased LC NA efflux equally (by approximately 60%) in chronic tramadol and vehicle groups, desipramine (50 nM) increased LC NA efflux more in vehicle (to 164+/-7%) than tramadol-treated rats (144+/-6%; p < 0.05). Chronic tramadol had no effect on dorsal raphe (DRN) or median raphe (MRN) 5-HT efflux. However, 5-HT uptake in tramadol-treated rats was slower (p < 0.05) in MRN and nearly so (p = 0.055) in DRN. The selective 5-HT1A agonist 8-OH-DPAT reduced 5-HT efflux in both DRN and MRN. Its effect in DRN was greater in rats given chronic tramadol than in vehicle controls (54+/-2 versus 32+/-6% reduction in 5-HT efflux, respectively). In conclusion, we suggest that tramadol has many of the pre- and postsynaptic neurochemical features of a conventional antidepressant, as might be predicted from its pharmacology.

Online source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11565620&dopt=Abstract tramadol



Gas chromatographic method using nitrogen-phosphorus detection for the measurement of tramadol and its O-desmethyl metabolite in plasma and brain tissue of mice and rats.

Raffa RB.

Temple University School of Pharmacy, Philadelphia, PA 19140, USA.

A method that allows the measurement of plasma and brain levels of the centrally-acting analgesic tramadol and its major metabolite (O-desmethyl tramadol) in mice and rats was developed using gas chromatography equipped with nitrogen-phosphorus detection (GC-NPD). Plasma samples were extracted with methyl tert.-butyl ether (MTBE) and were injected directly into the GC system. Brain tissue homogenates were precipitated with methanol, the resulting supernatant was dried then acidified with hydrochloric acid. The aqueous solution was washed with MTBE twice, alkalinized, and extracted with MTBE. The MTBE layer was dried, reconstituted and injected into the GC system. The GC assay used a DB-1 capillary column with an oven temperature ramp (135 to 179 degrees C at 4 degrees C/min). Dextromethorphan was used as the internal standard. The calibration curves for tramadol and O-desmethyl tramadol in plasma and brain tissue were linear in the range of 10 to 10000 ng/ml (plasma) and ng/g (brain). Assay accuracy and precision of back calculated standards were within +/- 15%.

Online source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11710575&dopt=Abstract tramadol



Plasma glucose-lowering effect of tramadol in streptozotocin-induced diabetic rats.

Chang CJ.

Department of Pharmacology, College of Medicine, National Cheng Kung University, Tainan City, Taiwan. jtcheng mail.ncku.edu.tw

The effect of tramadol on the plasma glucose level of streptozotocin (STZ)-induced diabetic rats was investigated. A dose-dependent lowering of plasma glucose was seen in the fasting STZ-induced diabetic rats 30 min after intravenous injection of tramadol. This effect of tramadol was abolished by pretreatment with naloxone or naloxonazine at doses sufficient to block opioid mu-receptors. However, response to tramadol was not changed in STZ-induced diabetic rats receiving p-chlorophenylalanine at a dose sufficient to deplete endogenous 5-hydroxytrptamine (5-HT). Therefore, mediation of 5-HT in this action of tramadol is ruled out. In isolated soleus muscle, tramadol enhanced the uptake of radioactive glucose in a concentration-dependent manner. The stimulatory effects of tramadol on glycogen synthesis were also seen in hepatocytes isolated from STZ-induced diabetic rats. The blockade of these actions by naloxone and naloxonazine indicated the mediation of opioid mu-receptors. The mRNA and protein levels of the subtype 4 form of glucose transporter in soleus muscle were increased after repeated treatments for 4 days with tramadol in STZ-induced diabetic rats. Moreover, similar repeated treatments with tramadol reversed the elevated mRNA and protein levels of phosphoenolpyruvate carboxykinase in the liver of STZ-induced diabetic rats. These results suggest that activation of opioid mu-receptors by tramadol can increase the utilization of glucose and/or decrease hepatic gluconeogenesis to lower plasma glucose in diabetic rats lacking insulin.

Online source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11723065&dopt=Abstract tramadol