Introduction

Many factors, such as ionic composition of medium, type of drugs and conduction frequency change or prevent the nerve action potential conduction on whole nerve.

Tramadol, a synthetic opioid in the aminocyclohexanol group, is a centrally acting analgesic. In a clinical study, it is postulated that, it has a local anesthetic effect similar to lidocaine following intradermal injections ₁,₂. But, effects on nerve conduction are not show at in vitro nerve preparations. However, in a patch clamp study demonstrated that meperidine, an opioid, has local anesthetic-like activity because of its Na⁺ channel blocking potency₃. But its affinity to peripheral nerve Na⁺ channels is lower when comparing to lidocaine. In addition meperidine inhibit different K⁺ channels of the peripheral nerve3, ₄.

In this study, in order to explain tramadol effects on nerve compound resting potential (CRP) and compound action potential (CAP), we benefit from known effects of local anesthetics. Local anesthetics block nerve conduction without affecting resting membrane potential by inhibiting Na⁺ channels in a concentration dependent manner₅. Additionally, they can bind more effectively to the active and in active states than to the resting states of the Na⁺ channels. Therefore, in addition to tonic block observe low conduction frequency, they produce additional block at high conduction frequency called phasic block₆, ₇,₈.

In previous researches demonstrated that, if pHs of local anesthetics in the test solution change, conduction blocks produced by them alter. Raising the pH of the external medium enhance the non-ionic form of the local anesthetics. Thus, more local anesthetic cross the membrane and accumulate with in the axoplasm, and nerve conduction blocks increase7, ₉,₁₀.

The purpose of this research was to investigate nerve conduction blocks of tramadol and effects of chancing pH on these blocks, and explain these effects comparing to be known effects of local anesthetics.

Materials and Methods

Preparation

Solutions

Stimulation and Recording instruments

Grass S48 stimulator and stimulus isolation unit (SIU5), Grass P16 microelectrode AC/DC amplifier, Hitachi VC-6523 digital storage oscilloscope, Cole Parmer pen recorder with 2 channels, Master flex perfusion pump with 8 channels and A/D card + PC computer were used in the experiments.

Before the experiments, the sciatic nerves were desheathed and then placed in sucrose-gap apparatus₁₁, ₁₂ (Fig.1) for stimulation and recording. At the experiments, nerves were stimulated supramaximally (1.5 to 2 times of maximal) square wave voltage pulses with 0.05 ms duration. When the control values were recorded (Fig.2), nerve was stimulated as tonic by single stimulus or as phasic by repetitive stimulus at 10 Hz train, lasting 1000 ms and 40 Hz train, lasting 500 ms respectively.

Figure 1

Figure 1: Diagram of Sucrose-gap recording technique: Sucrose-gap apparatus having 4 pools made by using Plexiglas. Pool A containing a pair of platinum stimulating electrodes is filled with mineral oil in order to preserve nerves against getting dry; Pool B contains Ringer's or test solution; Pool C contains non ionic isotonic sucrose and Pool D contains isotonic KCl solution. Pools are isolated each other with vaseline-silicon oil mixture. The potential difference between pool B and D was recorded by using Agar Bridge Ag-AgCl electrodes13. All solutions were perfused in the rate of 2-3 ml per minute. All of the experiments were carried out in room temperature (21-23 C).

Figure 2

Figure 2: Amplitude of CAP and CRP and time parameters of CAP were measured in this study. A: Amplitude (V) and time parameters (TDE and RT) of CAP and effects of tramadol on CAP. B: Effects of tramadol on CRP.

After recording the CRP and CAP control values, test solutions were added into the test pool, nerve was stimulated once per 5 min (tonic stimulation); the responses were recorded for 35 min. At the end of this period, recorded CAP was accepted as a non-frequency dependent response (tonic response). Frequency dependent responses (phasic response) were recorded, according the protocol mentioned above (Fig.3). All records were transferred to the computer in order to measure and to evaluate them afterwards. Applications of test solutions were done only if nerves had a control CAP amplitude of at least 35 mV.

Figure 3

Figure 3: An example record of production of tonic and phasic blocks on nerve conduction by 6.6 mM tramadol and the formula used in calculation. Tonic block which is defined as the decrease in amplitude of the response to single stimuli, was calculated as percentage of control CAP amplitude. Phasic block is the decrease in amplitude of the last pulse of train (figured as star *) compared with the same pulse under control conditions.

Statistical analysis

Results

Effects of tramadol on nerve conduction

Tonic blocks (it was shown as 0 Hz in Fig.4) were increased as concentration dependent. These blocks were at 1.6 mM (chosen minimum concentration) 14.3% ±0.8 and at 9.9 mM (chosen maximum concentration) 67.7% ±1.5. When conduction frequency was increased, tramadol produced additional block to tonic blocks. Phasic blocks (it was shown as 40 Hz in Fig.4) were at 1.6 mM 27.1% ±1.2 and at 9.9 mM 87.4% ±1.8.

Figure 4

Figure 4: Blocks of tramadol concentrations at pH 7.4. Numbers in parentheses indicate the number of experiment at each concentration.

Effects of Tramadol on time parameters of CAP

Tramadol extended the time parameters of CAP according to control CAP (Fig.2.B and Fig.5). T_DE (depolarization time) was extended 40% ±9 and RT (half-repolarization time) was extended 73% ± 8 at 3.3 mM tramadol concentration (Fig.5). When conduction frequency increased, extension of T_DE increased, but extension of RT unchanged. When time parameters of CAP measured, measurements error occurred through large blocks of CAP at high concentrations. Thus, Changes shown in graphics for only low tramadol concentrations.

Figure 5

Figure 5: 3.3 mM tramadol was extended time of CAP (A). Effects of tramadol on time parameters of CAP at pH 7.4 (B).

Effects of pH on conduction blocks of tramadol

In the absence of tramadol, pH of Ringer (in the test pool) changed from 7.4 to 9.2 and 5.5. Ringer with pH 7.4 values accepted as control. CAP amplitude blocked 2.2% ±0.5 (tonic block) and 3.4% ±0.6 (phasic block). When pH of Ringer was elevated to 9.2, tonic block was 5.7% ±0.5 and phasic block was 6% ±0.7.Thesevalues were not shown in graphics. In the presence of tramadol (Fig.6), when pHs of test solutions elevated from 5.5 to 9.2 tonic conduction blocks enhanced from 4.5% ±0.5 to 39.5% ±1 at 1.6 mM, and from 14.5% ±1,1 to58.3% ±1,3 at 3.3 mM. Increasing the nerve conduction frequency enhanced conduction blocks. Phasic blocks increased from 7.1% ±0.7 to 49.1% ±1.2 at 1.6 mM, and from 20.2% ±1.0 to 77.9% ±0.9 at 3.3 mM.

Figure 6

Figure 6: Relationship between pH and blocks of tramadol at different frequencies. Numbers in parentheses indicate the number of experiment at each concentration.

Effects of tramadol on CRP

Discussion

Our results demonstrated that, increasing the tramadol concentrations of the test solution and nerve conduction frequency resulted with an increase in blocks of tramadol without affecting CRP. In order to explain tramadol effects on nerve conduction, we used known effects of local anesthetics. When conduction blocks produced by tramadol compared with lidocaine6, at the same proportion of tonic and phasic conduction blocks, tramadol concentration was approximately 3 and 6 times more than lidocaine's, respectively. This comparison demonstrated that tramadol has a local anesthetic effect, but this effect weaker than lidocaine.

If nerve conduction frequency increases, local anesthetics occur additional blocks to tonic blocks₁₄, ₁₅. These blocks could be explained by using modulated receptor hypothesis₁₆, ₁₇. This hypothesis includes two pathways; hydrophobic drugs (such as benzocaine) are used hydrophobic pathway. They have capability of passing nerve membrane and reach to the binding site of the Na+ channels even if the channels are closed. However, hydrophilic drugs (such as lidocaine) are used hydrophilic pathway. When they access to axoplasm, they are reach to the binding site and are able to move through the open gates in the Na+ channels. In the presence of lidocaine, high conduction frequency increase Na+ channel inactivation, and more lidocaine can reach to the binding site of the Na+ channel. As a result of decreased number of Na+ channels that contribute to the occurrence of the CAP, CAP amplitude gets smaller with increased conduction frequency. Phasic blocks of tramadol much weaker than lidocaine's as if compare with tonic blocks of them. Therefore, tramadol might be used hydrophobic pathway instead of hydrophilic pathway to produce conduction blocks. Because, hydrophobic drugs such as benzocaine are competent of passing through the nerve membrane and thus can easily reach to the binding site in the Na+ channels even if channels are closed₁₈. In addition, tramadol broaden the CAP. It is known that, Na+ channels responsible from depolarization and K+ channels responsible from repolarization at CAP. Tramadol more extended half-repolarization time than depolarization time. In a study, it is postulated that, meperidine inhibited K+ channels more than Na+ channels3. Considering these, it is said that tramadol has probably similar effect to meperidine.

It is known that, if pHs of test solutions increase, conduction blocks produced by local anesthetics enhance. Formation of these blocks could be explained that way; only non-ionic forms of local anesthetics can cross the membrane through passive diffusion and accumulate within the axoplasm. When the pH of external medium increase, non-ionic forms of local anesthetics increase and more local anesthetic penetrate to axoplasm and reach to binding site of Na+ channels9, 16. Thus, their nerve conduction blocks increase. However, if the pHs of external medium decrease, their blocks decrease because of the non-ionic forms of local anesthetics. In present study, when the pH of the test solution increased, conduction blocking potency of tramadol enhanced. These results also demonstrated that tramadol has an action mechanism similar to local anesthetics.

Conclusion

In conclusion, when tramadol block the nerve conduction, it could act as weaker local anesthetics. Probably, action mechanism of tramadol could be similar to hydrophobic local anesthetics. Blocking potency of tramadol was changed as pH dependent at direction with local anesthetics.

Correspondence to

Tufan Mert Department of Biophysics, School of Medicine, University of Cukurova, 01330 Balcali, Adana, Turkey Tel.: +90-322-3386060-3472 Fax: +90-322-3386847 E-mail : tufanmert@yahoo.com