Tramadol, but not its major metabolite (mono-O-demethyl tramadol) depresses compound action potentials in frog sciatic nerves

Abstract

Background and purpose: Although tramadol is known to exhibit a local anaesthetic effect, how tramadol exerts this effect is not understood fully.

Experimental approach: The effects of tramadol and its metabolite mono-O-demethyl-tramadol (M1) on compound action potentials (CAPs) were examined by applying the air-gap method to frog sciatic nerves, and the results were compared with those of other local anaesthetics, lidocaine and ropivacaine.

Key results: Tramadol reduced the peak amplitude of the CAP in a dose-dependent manner (IC50=2.3 mM). On the other hand, M1 (1-2 mM), which exhibits a higher affinity for mu-opioid receptors than tramadol, did not affect CAPs. These effects of tramadol were resistant to the non-selective opioid receptor antagonist naloxone and the mu-opioid receptor agonist, DAMGO, did not affect CAPs. This tramadol action was not affected by a combination of the noradrenaline uptake inhibitor, desipramine, and the 5-hydroxytryptamine uptake inhibitor, fluoxetine. Lidocaine and ropivacaine also concentration-dependently reduced CAP peak amplitudes with IC50 values of 0.74 and 0.34 mM, respectively.

Conclusions and implications: These results indicate that tramadol reduces the peak amplitude of CAP in peripheral nerve fibres with a potency which is less than those of lidocaine and ropivacaine, whereas M1 has much less effect on CAPs. This action of tramadol was not produced by activation of mu-opioid receptors nor by inhibition of noradrenaline and 5-hydroxytryptamine uptake. It is suggested that the methyl group present in tramadol but not in M1 may play an important role in producing nerve conduction block.

Figure 1

Recordings of CAPs from frog sciatic nerve fibres by using the air-gap method. (a) Representative recording of CAP. This illustrates how the peak amplitude and HPD of the CAP were measured. (b) Recordings of CAPs for a period of 60 min. In this and subsequent figures, dashed line in recordings denotes the peak level of CAP in the control.

Figure 2

Tramadol (1 mM) reduces the peak amplitude of CAP recorded from frog sciatic nerve fibres with a slow time course. (a) The peak amplitudes of CAP before (open circles) and under the action of tramadol for a period of 20 min (closed circles), which are plotted against stimulus strength used to elicit the CAP. Recordings of the CAPs elicited at 0.2, 0.3 and 1.5 V under the two conditions are shown in the lower. (b) Recordings of CAPs in the control, at 6, 12 and 20 min after exposure to tramadol and thereafter 12, 30 and 60 min in the absence of tramadol. (c) Average time course of changes in CAP peak amplitudes following exposure to tramadol for 20 min, relative to that before the soaking, obtained from four sciatic nerves. In this and subsequent figures, each point with vertical bars represents the mean and s.e.m. and dotted line denotes the control value. The s.e.m. of the values without a vertical bar was within the size of symbol. All data points after washout of tramadol differed from that before drug treatment (control).

Figure 3

Tramadol reduces CAP peak amplitude in a dose-dependent manner in the frog sciatic nerve. (a) Recordings of CAPs in the control (left) and 20 min after exposure to tramadol at concentrations of 0.2, 2 and 5 mM (right); these were obtained from different sciatic nerves. (b) The peak amplitude of CAP recorded from fibres of sciatic nerves exposed to tramadol at various concentrations for 20 min, relative to that in the control, plotted against tramadol concentration. Each of the data points was obtained from four sciatic nerves. The s.e.m. of the values without a vertical bar was within the size of symbol. The dose–response curve was drawn according to the Hill equation (IC₅₀: 2.3 mM; n_H: 1.7).

Figure 4

CAP peak amplitude reduction produced by tramadol (1 mM) in the frog sciatic nerve is not due to the activation of μ-opioid receptors. (a) Recordings of CAPs in control conditions or with naloxone (10 μM) with or without tramadol. (b) Average time course of changes in CAP peak amplitudes following treatment with naloxone and with both naloxone and tramadol, relative to that before drug treatment, obtained from four sciatic nerves. The s.e.m. of the values without a vertical bar was within the size of symbol. (c) Recordings of CAPs under control conditions and 20 min after treatment with the μ-opioid receptor agonist DAMGO (1 μM).

Figure 5

The tramadol metabolite, M1, reduces the peak amplitudes of CAPs recorded from frog sciatic nerve fibres less effectively than tramadol. (a) Chemical structures of tramadol and M1. (b) The peak amplitude of CAP recorded from fibres in sciatic nerves treated with M1 at various concentrations for 20 min, relative to that in the control, plotted against M1 concentration. (c) Recordings of CAPs in control conditions and 20 min after treatment with M1 (5 mM). (d) Recordings of CAPs in the control conditions, at 10 and 20 min after exposure to M1 and then at 10 and 20 min exposure to tramadol. (e) Average time course of changes in CAP peak amplitude following treatment with M1 and then tramadol, relative to that before drug treatment, obtained from four sciatic nerves. The s.e.m. of each data point was within the size of symbol.

Figure 6

Effects of lidocaine and ropivacaine on CAPs recorded from frog sciatic nerve fibres. (a, d) Average time course of a change in CAP peak amplitude following treatment with (a) lidocaine (1 mM) or (d) ropivacaine (0.2 mM), relative to that in the control, obtained from four sciatic nerves. Insets in (a) and (d) show CAPs in the control (dotted line) and 20 min under the action of lidocaine (1 mM, a) or ropivacaine (0.2 mM, d; straight line). An asterisk (^*) shown below the data point indicates that there is no difference from the dotted line (value before drug treatment). (b, e) Recordings of CAPs in the control (left) and 20 min after the beginning of soaking the sciatic nerve into lidocaine (0.1, 0.5 and 2 mM; b)- or ropivacaine (0.01, 0.5 and 1 mM; e)-containing solution (right); these were obtained from different sciatic nerves. (c, f) The peak amplitude of CAP recorded from fibres in sciatic nerves treated for 20 min with lidocaine (c) or ropivacaine (f) at various concentrations, relative to that in the control, plotted against the concentration of the local anaesthetic. Each of the data points in (c) and (f) was obtained from 3–4 sciatic nerves. The dose–response curves in (c) and (f) were drawn according to the Hill equation (c: IC₅₀=0.74 mM, n_H=1.7; f: IC₅₀=0.34 mM, n_H=1.7). In (a), (c), (d) and (f), the s.e.m. of the values without a vertical bar was within the size of symbol.