Abstract
Background
Local anesthetics exert their anesthetic and analgesic effects by blocking the sodium channels in the nervous system. Phenothiazine-type antipsychotics also block sodium channels, but the local anesthetic characteristics of these drugs have not been evaluated. The aim of this study was to evaluate the cutaneous analgesic effect of phenothiazine-type antipsychotics.
Methods
Using a subcutaneous injection model in rats, we tested the cutaneous analgesic effects of six phenothiazine-type antipsychotics (mesoridazine, promazine, chlorpromazine, fluphenazine, perphenazine and triflupromazine) at a dose of 0.6 μmol, and compared them with those of bupivacaine and lidocaine. A saline injection was used as the control.
Results
All six phenothiazine-type antipsychotics elicited cutaneous analgesia. At the dose of 0.6 μmol, the potencies of mesoridazine and promazine were similar to that of bupivacaine; the other four drugs were less potent (p<0.001 for each comparison). Mesoridazine had a longer duration of action than bupivacaine (p<0.001). In terms of ED50 values, mesoridazine was more potent and longer-acting than bupivacaine and lidocaine (p<0.01 for each comparison).
Conclusion
Of the antipsychotic drugs tested, mesoridazine is potentially the best candidate for development into a potent, long-acting local anesthetic. However, toxicity studies are needed before these agents can be used clinically as analgesics.
Keywords
analgesia: cutaneous; bupivacaine; mesoridazine;
1. Introduction
Phenothiazine-type antipsychotics are a group of drugs comprising the same three-ring structure as phenothiazine but have different side chains.1,2 They have been used in clinical practice as antipsychotic drugs for more than 60 years, and appear to induce their effects by acting as dopamine receptor antagonists.1,2 However, in receptor-binding assays, phenothiazine-type antipsychotics block dopamine receptors as well as histamine, catecholamine, acetylcholine, serotonin and sodium channel receptors.1,2 Although phenothiazine-type antipsychotics have a long history of clinical use, the therapeutic efficacy related to sodium-channel blocking has not been clearly explored.
Sodium-channel blockade is the principal mechanism of action of local anesthetics.3−5 Through this mechanism, local anesthetics elicit cutaneous analgesia, neural blockade, and spinal and epidural anesthesia.4 Because phenothiazine-type antipsychotics block sodium channels, they may also have local anesthetic effects.6−8 Therefore, we evaluated the cutaneous analgesic effects of phenothiazinetype antipsychotics. Using a subcutaneous injection rat model, we tested the cutaneous analgesic effects of six phenothiazine-type antipsychotics (mesoridazine, promazine, chlorpromazine, fluphenazine, perphenazine and triflupromazine) and compared them with those of bupivacaine and lidocaine. A saline injection was used as the control.
2. Methods
All experiments were carried out in male Sprague- Dawley rats weighing 200−250 g, purchased from the Laboratory Animal Center at the National Yang-Ming University, Taipei, Taiwan. The rats were housed in groups of three for at least 1 week in a climatecontrolled room maintained at 21ºC, 50% relative humidity and a 12-hour light/dark cycle (lights on at 6:00 AM). Food and water were available ad libitum until testing. All tests were done in accordance with the recommendations and policies of the International Association for the Study of Pain, and the protocol was approved by the Animal Investigation Committee of the Chi Mei Medical Center.
Two local anesthetics (bupivacaine and lidocaine) and six phenothiazine-type antipsychotics (mesoridazine, promazine, chlorpromazine, fluphenazine, perphenazine and triflupromazine) purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA) were used. All tested agents were freshly prepared and dissolved in saline (0.9% NaCl).
In the first of our three studies, we evaluated the potency and duration of action of the local anesthetics and phenothiazine-type antipsychotics at a dose of 0.6 μmol (0.6 mL) for cutaneous analgesia. In the second study, we evaluated the dose-response relationships of the local anesthetics versus those of the most potent antipsychotics identified in Study 1. In the third study, we evaluated the durationresponse relationships of the drugs analyzed in Study 2.
Before behavioral tests, the rats were familiarized with the specific experimental environment and procedures. One day before the subcutaneousinjections of drugs, the hair on the dorsal surface of the thoracolumbar region (10 × 10 cm) of the rats was shaved.
We used a 0.6-mL 30-gauge needle to subcutaneously inject the test drugs into the dorsal surface of the thoracolumbar region of the rats, which were not anesthetized. The drugs were prepared and injected by one investigator. For consistency, another experienced investigator (YWC), blinded to the treatment, was responsible for evaluating the extent of cutaneous analgesia in all rats.
Cutaneous analgesia was evaluated using the cutaneous trunci muscle reflex.9,10 During the experiment, the cutaneous analgesic effects of the drugs were recorded as a percentage of the possible effect (%PE). The maximal %PE value was presented as a percentage of the maximal PE (%MPE). The duration of action was defined as the time from drug injection (time = 0) to full recovery of the cutaneous trunci muscle reflex (no analgesic effect was found).
In Study 1, the %MPEs and the duration of action of two local anesthetics and six antipsychotics in rats (n = 12 rats per drug) were determined after subcutaneous injection. We then compared the %MPEs and the durations of action of these drugs.
In Study 2, we established dose-response curves for two local anesthetics and the most potent antipsychotic agent identified in Study 1 using the %MPE for each drug (n = 8 rats for each dose of each drug). For nonlinear regression analysis, we fitted these curves using the computer-derived NLIN (nonlinear) procedure (SAS 9.1; SAS Institute, Cary, NC, USA).9,11 After curve-fitting, the 25%, 50%, and 75% effective doses (ED25, ED50, and ED75) were calculated for each drug and the potencies (ED50) of the two local anesthetics and the antipsychotic drug were compared. To exclude the possibility of vehicle or systemic effects of drugs on cutaneous analgesia, two control groups were also established. One control group (n = 8 rats) received a subcutaneous injection of saline; another control group (n = 8 rats for each drug) received a subcutaneous injection of saline in conjunction with an intraperitoneal injection of the antipsychotic or local anesthetic drug at twice the ED75 dose.
In Study 3, the durations of cutaneous analgesic action of the two local anesthetics and the most potent antipsychotic agent were evaluated. The rats were given subcutaneous injections of drugs at the ED25, ED50 or ED75 (n = 8 rats for each dose of each drug) and the duration of action of each drug was measured.
Values are presented as mean ± standard error of the mean. Differences in %MPE, duration of action, and ED50 between drugs were evaluated by one-way analysis of variance (ANOVA) followed by the pairwise Tukey’s honestly significant difference (HSD) test. The differences in durations between drugs at the ED25, ED50, and ED75 were evaluated using two-way ANOVA followed by the pairwise Tukey’s HSD test. In the control groups, one-way ANOVA and Dunnett’s test were used to evaluate the effects of medications. Bonferroni correction was used when appropriate. SPSS version 10.0.7 (SPSS Inc., Chicago, IL, USA) was used for all analyses. Statistical significance was set at p < 0.05.
3. Results
The time-course of cutaneous analgesia was assessed after the rats had been treated with a single dose (0.6 μmol) of the local anesthetics or the antipsychotics (Figure 1A). At this dose, bupivacaine elicited an 83 ± 6% blockade (%MPE) with a duration of action of about 53 ± 4 minutes, whereas mesoridazine showed an 85 ± 5% blockade (%MPE) with a duration of action of about 265 ± 15 minutes (Figure 1B).
The %MPEs on cutaneous analgesia of the local anesthetics and antipsychotics at the dose of 0.6 μmol are shown in Figure 2. Lidocaine was significantly less potent than bupivacaine for cutaneous analgesia. Mesoridazine, promazine and bupivacaine were similarly potent, whereas chlorpromazine, fluphenazine, perphenazine and triflupromazine were less potent than bupivacaine. All rats recovered completely after the study. The time to full recovery for each treatment is shown in Figure 3. At the dose of 0.6 μmol, the duration of cutaneous analgesia was greater with mesoridazine than with bupivacaine.
Dose-response curves were constructed for mesoridazine, bupivacaine and lidocaine and the ED50 values were calculated (Figure 4 and Table 1). On an ED50 basis, the order of potency was mesoridazine > bupivacaine > lidocaine (Table 1). In the control groups, no sedation or loss of motor control was encountered after the intraperitoneal injection of a larger dose (2 × ED75) of bupivacaine, lidocaine or mesoridazine. Moreover, the rats subcutaneously injected with saline or intraperitoneally injected with the test drugs did not exhibit cutaneous analgesia. Compared with bupivacaine and lidocaine at the ED25, ED50 and ED75 doses, mesoridazine had a longer duration of action (Figure 5). All of the rats recovered completely after the study.
Download full-size image
Download full-size image
Download full-size image
Download full-size image
Download full-size image
4. Discussion
In this study, we evaluated the effects of six phenothiazine-type antipsychotic agents on cutaneous analgesia. As expected, all six drugs elicited cutaneous analgesia. We also found that mesoridazine, a phenothiazine-type antipsychotic, was more potent and had a longer duration of action than bupivacaine, a known potent and long-acting local anesthetic.
In vitro sodium channel blocking activities of phenothiazine-type antipsychotics (i.e. fluphenazine, trifluoperazine and chlorpromazine) have previously been reported.6−8 Fluphenazine was shown to block tetrodotoxin-sensitive and tetrodotoxinresistant sodium channels in a dorsal root ganglionderived clonal cell line and in primary cerebrocortical neurons.7 Trifluoperazine was found to block the sodium channel subtypes Nav1.7 and Nav1.4 in cell lines transfected with sodium channels.8 Meanwhile, chlorpromazine was reported to block sodium channels in a model of single myelinated nerve fibers.6 Because sodium channel blockade is the principal mechanism of action of local anesthetics,3−5 we hypothesized
that phenothiazine-type antipsychotics have a local anesthetic effect. In this study, we found that these drugs did elicit cutaneous analgesia.
We also found that mesoridazine, one of the phenothiazine-type antipsychotics, was a more potent and longer-acting cutaneous analgesic than bupivacaine. This implies that mesoridazine dissociates more slowly from its binding site than either lidocaine or bupivacaine. However, we did not design further tests to evaluate this hypothesis. Nevertheless, this effect may be beneficial for patients who require long-acting cutaneous analgesia. Therefore, the clinical use of mesoridazine warrants further investigation. We did not examine whether these drugs caused local toxicity at the injection site, which should be investigated before these agents can enter clinical use.
To exclude possible effects of the vehicle or systemic effects of the tested drugs, we included two control groups in this study. Neither of the control groups experienced cutaneous analgesia, confirming that these drugs were directly responsible for the cutaneous analgesia.
In conclusion, all six phenothiazine-type antipsychotics produced cutaneous anesthesia in rats after subcutaneous injections. Mesoridazine was more potent and had a longer duration of cutaneous analgesia than bupivacaine, a potent, longacting local anesthetic.