Promazine and chlorpromazine for prolonged spinal anesthesia in
rats
Yu-Wen Chen1,2,*, PhD, Chin-Chen Chu3,4, MD, PhD, Yu-Chung Chen5, MS, Chung-Dann Kan6, MD, Jhi-Joung Wang2, MD, PhD.
1 Department of Physical Therapy, China Medical University, Taichung, Taiwan 2 Department of Medical Research, Chi-Mei Medical Center, Tainan, Taiwan 3 Department of Anesthesiology, Chi Mei Medical Center, Tainan, Taiwan
4 Department of Recreation and Health-Care Management, Chia Nan University of Pharmacy & Science, Tainan, Taiwan
5 Division of Physical Therapy, Department of Physical Medicine and Rehabilitation, Cheng Hsin General Hospital, Taipei, Taiwan
6 Department of Surgery, National Cheng Kung University Medical Center, Tainan, Taiwan
*Corresponding Author: Yu-Wen Chen, PhD.
Department of Physical Therapy China Medical University
No.91 Hsueh-Shih Road, Taichung 40402, Taiwan Phone: 886-4-22053366 ext 7327
FAX: 886-4-22065051
Abstract
Thought promazine and chlorpromazine elicited cutaneous anesthesia, no study of spinal anesthesia with chlorpromazine and promazine has been reported. This study was to examine whether chlorpromazine and promazine produce spinal anesthesia. Using a rat model via intrathecal injection, we tested spinal blockades of motor function and nociception by promazine, chlorpromazine or bupivacaine, and so were dose–response studies and durations. We demonstrated that chlorpromazine and promazine elicited dose-dependent spinal blockades in motor function and nociception. On the 50% effective dose (ED50) basis, the rank of potency of these drugs was bupivacaine > promazine > chlorpromazine (P < 0.05 for the differences). On an equipotent basis (25% effective dose [ED25], ED50, and ED75), the block duration caused by chlorpromazine or promazine was longer than that caused by the long-lasting local anesthetic bupivacaine (P < 0.01 for the differences). Chlorpromazine and promazine, as well as bupivacaine, showed longer duration of sensory block than that of motor block. Our data reported that intrathecal promazine and chlorpromazine with a more sensory-selective action over motor blockade had less potent and longer-lasting spinal blockades when compared with bupivacaine.
Key Words: Chlorpromazine; Promazine; Bupivacaine; Spinal blockade; Motor function; Nociception
The typical antipsychotics began with the serendipitous discovery of the antipsychotic activity of chlorpromazine, one of phenothiazine-type antipsychotics including a phenothiazine ring with different substituents attached at the 2 and 10 positions [12]. Besides, there is a growing body of evidence that chlorpromazine blocks the voltage-gated Na+ currents [17, 18]. One of the most extensive pharmacological studies of chlorpromazine was demonstrated that chlorpromazine (1%) produced anesthesia of the sciatic nerve of guinea pigs [5], and it had been recently known that chlorpromazine and promazine elicited infiltrative anesthesia of skin in rats [8].
Long-lasting local anesthetics are frequently administered intrathecally for various procedures or pathologies [3], and spinal anesthesia is a relatively simple technique, which gives adequate surgical conditions via injecting a small amount of local anesthetic
[11]. To the best of our knowledge, no study of spinal anesthesia with chlorpromazine and promazine has been reported to date. The aim of this study was to examine, using a rat model of spinal punctures, whether chlorpromazine and promazine produced long-acting spinal anesthesia and then compared with bupivacaine, a long-lasting local anesthetic.
The experimental protocols were approved by the Institutional Animal Care and Use Committee of China Medical University (Taiwan), and conformed to the recommendations and policies of the International Association for the Study of Pain (IASP). Male Sprague-Dawley rats (295-345g) were purchased from the National Laboratory Animal Centre, Taipei, Taiwan. They were housed in groups of three, with food and water freely available until the time of experiments. The climate controlled room maintained at 24 degree C with approximately 50% relative humidity on a 12-h
light/dark cycle (6:00 AM–6:00 PM).
Promazine HCl, chlorpromazine HCl, and bupivacaine HCl were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA). All drugs were freshly prepared in 5% dextrose as solution before intrathecal injections. After intrathecal injections, the low pH of these plain solutions (ranging from 5.3 to 7.1) is likely to be buffered quickly by the cerebral spinal fluid (pH 7.4).
Two studies were carried out. In study 1, in a dose-dependent manner, the potencies of promazine (0.38, 0.54, 0.60, 0.75, 1.00, 1.35, 1.75 μmol), chlorpromazine (0.50, 0.63, 0.75, 1.00, 1.50, 1.75, 2.50 μmol), and bupivacaine (0.18, 0.23, 0.32, 0.43, 0.75, 0.90 μmol) on spinal anesthesia were performed (n=8 rats for each dose of each drug). Then, the spinal anesthetic effects of promazine at 1.75 μmol and chlorpromazine at 2.50 μmol were compared with those of bupivacaine at 0.90 μmol (n=8 rats for each dose of each drug). In study 2, on an equipotent basis (25% effective dose [ED25], ED50, ED75), the spinal block duration (full recovery time) caused by promazine or chlorpromazine was compared with that caused by bupivacaine (n=8 rats for each dose of each drug).
All animals were handled to familiarize them with the experiments and to minimize stress-induced analgesia before intrathecal injections. The drugs were injected intrathecally on unanaesthetized rats as previously described [7, 13]. In brief, a 27-gauge needle attached to a 50-μl syringe (Hamilton, Reno, Nevada) was inserted into the midline of the lumbar 4-5 (L4-L5) intervertebral space and 50-μl of drugs was injected. Rats were then checked for paralysis of two hind limbs, indicative of a spinal blockade. Rats that displayed unilateral blockades were excluded from the study and killed by using an overdose of sevoflurane.
For consistency, one experienced investigator, who was blinded to the identity of the injected drugs, was responsible for handling all the animals and behavioral evaluation. Rats were evaluated before medication and at 1, 3, 5, 7, 10, 15, and 20 min afterwards, then again at 10-min interval until 1 h, at 15 min interval until 2 h, and at 30 min interval until 6 h. The magnitude of spinal blockades was described as the percent of possible effect (% PE). The maximum blockade in a time course of spinal anesthesia with drugs was described as the percent of maximal possible effect (% MPE).
After intrathecal injections, nociception and motor function were evaluated as previously described [6, 10]. In brief, nociception was evaluated according to the withdrawal reflex or vocalization elicited by pinching a skin fold on each rat's back at 1 cm from the proximal part of the tail, the lateral metatarsus of the two hind limbs, and the dorsal part of the mid-tail. Nociceptive blockade was graded as 0 (absent or 100% MPE), 1 (75% MPE), 2 (50% MPE), 3 (25% MPE), and 4 (normal nociception or 0% MPE) [9]. At each testing time, only one pinch was given to each of the four testing areas, and the time interval between stimulations at different areas was around 2 s.
Motor function was assessed by measuring 'the extensor postural thrust' of the right hind limb of each rat and was measured as the gram force, which resisted contacting the platform via the heel applied to the digital platform balance (Mettler Toledo, PB 1502-S, Switzerland). The decrease in force, resulting from extensor muscle tone, was considered motor deficit (block). The pre-injection control value was considered a 0% motor blockade or 0% MPE, and a force less than 20 g was interpreted as a 100% motor blockade or 100% MPE [6].
each dose of each drug), dose-response curves were constructed. The curves were then fitted by using SAS NLIN procedures (SAS Institute Inc., Carey, NC), and the value of ED50, defined as the doses that caused 50% spinal blockades of motor function and nociception, were obtained [10, 15]. The ED25 and ED75 of drugs were obtained via using the same SAS NLIN procedures that were used to derive the ED50. On an equipotent basis (ED25, ED50, ED75), the full recovery time (duration) of each blockade, defined as the interval from drug injection to full recovery (0% MPE), was measured and compared. Furthermore, that area under curves (AUCs) of spinal blockades of drugs was estimated
via using Kinetica version 2.0.1 (InnaPhase Corporation, Philadelphia, PA).
Values are presented as mean SEM or ED50 values with 95% confidence interval (95% CI). The differences in the ED50s (Table 1) among drugs or the differences in the %MPE, duration, and AUCs of drugs (Table 2) were evaluated by one-way analysis of variance (ANOVA), followed by the pairwise Tukey’s honest significance difference (HSD) test. The differences in durations (Fig. 3) among drugs were evaluated by two-way ANOVA followed by pairwise Tukey's HSD test. SPSS for Windows (version 17.0) was used for all statistical analyses. Statistical significance was set at P < 0.05.
Chlorpromazine and promazine, as well as bupivacaine, showed a dose-dependent effect on spinal anesthesia in rats (Fig. 1). The ED50s of chlorpromazine, promazine, and bupivacaine are shown in Table 1. On the ED50 basis, the ranks of potencies in motor function and nociception were bupivacaine > promazine > chlorpromazine (P < 0.05;
Table 1). Furthermore, the sensory/nociceptive blockade (ED50) was more potent than the motor blockade for chlorpromazine, promazine, and bupivacaine (P < 0.05; Table 1).
(% MPE) in motor function and nociception with duration of action of about 88 and 238 min, respectively (Fig. 2 and Table 2). Chlorpromazine at 2.50 μmol showed 100 and 100% of blockades in motor function and nociception with duration of action of about 106 and 263 min, respectively (Table 2). At a dose of 0.90 μmol, bupivacaine demonstrated 100 and 100% of blockades in motor function and nociception with duration of action of about 35 and 88 min, respectively (Table 2). Intrathecal injection of 5% dextrose (vehicle) produced no spinal anesthesia.
The AUCs of spinal anesthesia with chlorpromazine and promazine are significantly greater than that of bupivacaine in Table 2. In addition, chlorpromazine, promazine, and bupivacaine also elicited longer duration of sensory blockade than that of motor blockade (Table 2). On an equianesthetic basis (ED25, ED50, and ED75), the block duration of motor function and nociception of caused by chlorpromazine and promazine (P < 0.01) were longer than those caused by bupivacaine (Fig. 3). All rats recovered completely after intrathecal injections.
Intrathecal chlorpromazine and promazine produced dose-dependent spinal blockades of motor function and nociception in rats. On an equipotent basis, promazine and chlorpromazine showed longer duration of action on spinal anesthesia when compared with bupivacaine.
It has been known that the local anesthetics elicit neural blockades via blocking the Na+ currents in the nervous tissues through the voltage-gated Na+ channels [14]. In addition, there is a growing body of evidence that chlorpromazine is a Na+ channel blocker [2, 17, 18] and thus chlorpromazine holds a local anesthetic effect [4, 5, 8]. Meanwhile, chlorpromazine and promazine can target multiple receptors (i.e. dopamine,
muscarinic acetylcholine, etc.) [1, 16], thus these drugs may modulate the pain pathway in more ways than simply through Na+ blockade. Chlorpromazine and promazine blocked motor and sensory functions, suggesting that these compounds have characteristics of a spinal (local) anesthetic drug. Furthermore, chlorpromazine, promazine, and bupivacaine produced dose-dependent spinal anesthesia. Bupivacaine had almost 3.4- and 2.4-folds higher potency than did chlorpromazine and promazine on spinal anesthesia, respectively. Promazine, a neuroleptic agent, is used mainly as an antipsychotic and antiemetic agent and additionally as an adjunct agent in the management of severe pain [12]. We found that intrathecal chlorpromazine, promazine, and bupivacaine displayed a longer duration of sensory blockade than that of motor blockade (Fig. 2 and Table 2). This is in similarity to clinical impressions that bupivacaine is an option when a more sensory-selective action over motor blockade. Besides, we expected that those pharmacokinetic differences, such as distribution, absorption, and metabolism of chlorpromazine or promazine may account for these differences in the duration of action.
Treatment of local anesthetics is an attractive option for management of postoperative pain and surgical anesthesia [11]. There are few cases where ultrashort spinal anesthesia is needed, and for these they clinically use 2-chloroprocaine and bupivacaine. Our study for the first time reported that chlorpromazine and promazine produced a longer duration of spinal anesthesia when compared with bupivacaine. Furthermore, on an equipotent basis, the duration of spinal blockade in nociception and motor function caused by promazine or chlorpromazine was longer than that caused by bupivacaine (Fig. 3). This effect may be beneficial for patients who require long-acting spinal analgesic.
In this study, we also found that chlorpromazine, promazine, and bupivacaine elicited predominantly nociceptive-specific blockades. The sensory/nociceptive blockade in chlorpromazine, promazine, and bupivacaine had almost 1.1-, 1.2-, and 1.2-folds higher potency (ED50) than the motor blockade, respectively. Clinically, local anesthetics have mostly been used to treat with complete blockage of pain and not to augment potency. Meanwhile, it remains unclear whether chlorpromazine and promazine cause toxicity to the spinal nerves. After intrathecal injections, all rats recovered completely. Further studies on peripheral nerve block and related neural and cardiovascular toxicities will be warranted before the possible use of promazine or chlorpromazine as spinal analgesic in humans.
In conclusion, the preclinical data reported that promazine and chlorpromazine produced a local anesthetic effect on spinal anesthesia in rats. Both promazine and chlorpromazine with a more sensory-selective action over motor blockade is less potent and longer spinal block duration than bupivacaine. The neural block potential by promazine and chlorpromazine is worth studying in the future.
Acknowledgement
The authors gratefully acknowledge the financial support provided for this study by the National Science Council of Taiwan (NSC 100-2314-B-039-017-MY3).
Legends to figures
Fig. 1. The dose-response curves of promazine, chlorpromazine, and bupivacaine on spinal blockades (% MPE) of motor function and nociception in rats (n = 8 at each testing point). Data are mean ± SEM. % MPE = percent of maximal possible effect.
Fig. 2. The time courses of spinal blockades in motor function and nociception with promazine at 1.75 μmol, chlorpromazine at 2.50 μmol, and bupivacaine at 0.90 μmol in rats. Intrathecal 5% dextrose (vehicle) was used as control. Neurological evaluation was obtained before and after drug injection. Values are expressed as mean ± SEM. For each group of the time course study, n = 8 rats.
Fig. 3. Full recovery time (duration) of promazine, chlorpromazine, and bupivacaine on spinal blockades of motor function and nociception at equianesthetic doses (ED25, ED50, ED75) in rats (n = 8 at each testing point). Data are mean ± SEM. The differences in duration were evaluated by using two-way ANOVA and then the pairwise Tukey's HSD test.
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