Anesth Analg 2001;93:1293-1296
© 2001 International Anesthesia Research Society
OBSTETRIC ANESTHESIA
An Analysis of Excitatory Amino Acids, Nitric Oxide, and Prostaglandin E2 in the Cerebrospinal Fluid of Pregnant Women: The Effect on Labor Pain
Ming-Man Hsu*
,
Yen-Yen Chou*,
Yi-Chen Chang*,
Tz-Chong Chou, and
Chih-Shung Wong*
*Department of Anesthesiology, National Defense Medical Center and Tri-Service General Hospital; Graduate Institute of Medical Sciences, National Defense Medical Center, Taipei, Taiwan, Republic of China
Address correspondence and reprint requests to Chih-Shung Wong, Department of Anesthesiology, National Defense Medical Center and Tri-Service General Hospital, 325, Chenggung Rd., Section 2, Nei-Hu, 114, Taipei, Taiwan, ROC. Address e-mail to w82556{at}ndmctsgh.edu.tw
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Abstract
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It is still unclear which neurotransmitters are involved in labor pain. We measured the concentrations of excitatory amino acids, nitric oxide, and prostaglandin E2 in the cerebrospinal fluid (CSF) of pregnant women, particularly in those with labor pain. The patients included in the study consisted of women who underwent cesarean delivery either with labor pain (Labor Pain group, n = 40) or without labor pain (Nonlabor Pain group, n = 58). All patients received spinal anesthesia (intrathecal injection of 10–12 mg of bupivacaine) for the operation, and 2 mL of CSF was collected before bupivacaine injection. Concentrations of aspartate and glutamate (0.50 ± 0.06 µM and 0.79 ± 0.10 µM, respectively) were significantly larger in the Labor Pain group than in the Nonlabor Pain group (0.35 ± 0.03 µM and 0.54 ± 0.04 µM, P < 0.05). There were no significant differences in the concentrations of nitric oxide and prostaglandin E2 between the groups. A positive correlation was found between CSF concentrations of excitatory amino acids and labor pain.
IMPLICATIONS: The excitatory amino acids, aspartate and glutamate, play a role in labor pain. N-methyl-D-aspartate receptor antagonists may be useful for labor pain and postlabor uterine contraction pain relief.
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Introduction
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Several studies, both in animals and humans, have demonstrated that many neurotransmitters and neuromodulators, such as excitatory amino acids (EAAs), nitric oxide (NO), and prostaglandin E2 (PGE2), are involved in nociception. Increased release of EAAs, acting as nociceptive neurotransmitters, occurs in the spinal cord after activating of N-methyl-D-aspartate receptors by noxious stimuli (1). In the formalin pain animal model, glutamate (Glu) and substance P are involved in the development of persistent pain and hyperalgesia (2). In a clinical study, Castillo et al. (3) observed that the concentration of Glu in the cerebrospinal fluid (CSF) was larger in stroke patients with headache than in those without headache. In our own clinical study, we found an increase in the CSF levels of EAAs, aspartate (Asp), and Glu, in terminal cancer patients who had severe pain that could not be relieved by morphine. Olofsson et al. (4) found that only Asp levels were increased in pregnant women with labor pain compared with pregnant women without labor pain.
In addition to EAAs, NO and PGE2 also play important roles in the central nervous system in spinal nociceptive processing, hyperalgesia, and neuropathic pain development (5–9). In an electrophysiologic study, NO prolonged chemically induced nociception in rats (10). Moreover, NO levels are increased in certain painful neurologic diseases. Asahara et al. (11) demonstrated that CSF levels of NO and its metabolites are significantly higher in patients with lumbar pain than in control patients. However, another report demonstrated a negative correlation between CSF levels of EAAs or NO and labor pain (4). Several lines of evidence show that PGE2 plays a critical role in the augmentation of nociception processing in the spinal cord (8,9). Friese et al. (12) showed that PGE2 is involved in visceral pain. Moreover, inflammatory cytokines, such as interleukin-1, increase PGE2 production, resulting in sciatic pain (13).
The roles of EAAs, NO, and PGE2 in the mechanism of visceral pain are still not clear. To study visceral pain, it is important to work with a type of pain that is well defined and easily observed. Labor pain, a homogenous and well defined visceral pain, can be used for studying acute visceral pain (14). In the present study, we examined CSF concentrations of EAAs, NO, and PGE2 in pregnant women, particularly in pregnant women with labor pain.
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Methods
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This study was approved by the Protection of Human Subjects IRB of the Tri-Service General Hospital. Subjects were allocated to two groups. Pregnant women scheduled for elective cesarean delivery without labor pain formed the Nonlabor Pain group (n = 58), whereas the Labor Pain group consisted of pregnant women who failed to deliver the fetus via the vagina during active labor with cervix dilation between 4 to 6 cm without any progress in labor stage and suffered severe pain for several hours (between 2 to 4 h), and finally underwent emergency Cesarean delivery (n = 40). All subjects received spinal anesthesia (intrathecal injection of 0.5% bupivacaine; 10–12 mg) for their operations and 2 mL of CSF was collected from the spinal needle (#25) before injection. Samples were centrifuged through micro-spin filters (retention size 5000 d) and the filtrate was stored at -70°C until analysis for EAAs, NO, and PGE2 levels.
EAAs were assayed by high-pressure liquid chromatography with fluorescence detection (Gilson Model 121, set at 428 nm; Gilson, Middleton, WI) as previously reported (15). In brief, Asp and Glu were determined by precolumn derivatization with an o-phthalaldehyde/t-butylthiol reagent and iodoacetamide/methanol scavenger. The derivatized sample was injected onto a C18 reversed phase column and a linear gradient of 2 eluents used to separate the amino acids; eluent A was 0.1 M sodium acetate buffer, pH 6.8/acetonitrile (80:20) and eluent B acetonitrile/double-distilled water (80:20). The mobile phase flow rate was 0.4 mL/min.
Total NOx (NO + NO2- + NO3-) was determined as previously described (16) by using a chemiluminescence detector (model 270B; Sievers Research Inc., Boulder, CO). The NOx signals from the detector were displayed and analyzed by a PC-based data recording and analyzing system. Background concentrations of NOx were measured and subtracted. All test samples were analyzed in duplicate.
PGE2 levels were quantified by using a commercially available enzyme immunoassay (EIA) kit (Assay Designs, Inc., Ann Arbor, MI) in accordance with the manufacturer’s protocol. Samples containing high PGE2 levels were diluted in the assay buffer provided with the kit. The optical density at 405 nm was read on an EIA Reader (MRX; Dynex Technology Inc., Glen Allen, VA) and the PGE2 concentration determined by using the Endpoint Program (MRX; Dynex Technology Inc.).
All data were presented as the mean ± SEM. Statistical analysis was performed by analysis of variance with post hoc test and Pearson correlation. A P value < 0.05 was considered statistically significant.
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Results
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There were no significant differences in the characteristics of the Labor Pain and Nonlabor Pain groups (Table 1). The CSF concentrations of Asp were significantly larger in the Labor Pain group (0.50 ± 0.06 µM) than in the Nonlabor Pain group (0.35 ± 0.03 µM, P < 0.05) (Fig. 1). The CSF concentration of Glu was also significantly higher in the Labor Pain group (0.79 ± 0.10 µM) than in the Nonlabor Pain group (0.54 ± 0.04 µM, P < 0.05). There was no significant difference in CSF concentrations of NO and PGE2 between the two groups (Table 2).
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Discussion
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In the present study, we demonstrated differences in CSF concentrations of EAAs between the two groups, EAAs levels being higher in pregnant women with labor pain than in those without labor pain. However, CSF concentrations of NO and PGE2 did not differ significantly between the groups.
In animal studies, a positive correlation has been seen between levels of EAAs (Asp and Glu) and NO and nociception (17,18). In clinical studies, Samuelsson et al. (19) reported a larger concentration of NO in the CSF of patients with cancer-related pain than in control patients, and Larson et al. (20) demonstrated that pain intensity in fibromyalgia patients is related to NO production. It is therefore reasonable to propose that there may be a positive correlation between CSF concentrations of EAAs and NO and labor pain. A larger CSF Asp concentration was observed in pregnant women who had labor pain than that of nonlabor pain patients in the present study, which is in agreement with previous results (4). Moreover, an increase of the Glu level in the CSF was also observed in the Labor Pain group in the present study, and this supports the involvement of EAAs in labor pain. However, in contrast to the report of Olofsson et al. (4) of lower NO levels in an active labor pain group than in a pain-free group, in the present study, no difference in NO concentration was seen between the two groups. The NO levels were approximately 8 µM in both groups. They were higher than the Labor Pain patients who delivered the fetus via vagina, and lower than the Nonlabor Pain patients who had cesarean delivery. This discrepancy may be attributed to CSF sampling at different labor stages.
PGE2 is involved in visceral pain (12). Minami et al. (21) reported that intrathecal PGE2 administration can induce both allodynia and an increase in the CSF NO concentration, possibly via the glutamatergic system. However, in the present study, we did not find any significant difference in either PGE2 or NO levels between the Labor Pain and Nonlabor Pain groups. It is not clear whether NO or PGE2 could influence the intensity of labor pain. In our present study, we did not measure labor pain intensity, which might explain why we did not find a correlation between NO and PGE2 levels and labor pain.
The contradictions and discrepancies seen in these animal and clinical studies suggest that many factors may influence pain behavior, such as experience of labor pain. Physical and psychological factors may influence pain intensity and its expression during labor (22–24). In the present study, patients in the Labor Pain group had severe pain for several hours and the psychological and physical conditions may therefore have influenced the intensity and expression of labor pain. The involvement of all of these factors means that labor pain is a multidimensional experience. According to the mechanisms of supraspinal pain proposed by Melzack and Casey (25), three major psychological dimensions of pain, the sensory-discriminative, motivational-affective, and cognitive-evaluative dimensions, may interact with physiologic specialized systems, and thus influence pain expression. Considering all these findings and conceptual models, the mechanisms of visceral pain, particularly during the labor state, are complicated, and psychological and physical factors should also be considered in this kind of study.
In summary, our present results show that EAAs may be involved in labor pain development. There was no significant difference in NO and PGE2 levels in patients with or without labor pain undergoing cesarean delivery. In labor pain studies, the pain intensity and the psychological and physical condition of the patients should also be considered.
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Acknowledgments
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This study was supported by the Foundation of the Tri-Service General Hospital of the Republic of China (TSGH-C90-60).
The authors thank Mr. Tai-Chung Law for assistance with EIA program management and Mr. Fu-Gong Lin for statistical assistance.
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Accepted for publication July 25, 2001.
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Abstract
Introduction
, Nonlabor Pain group; 
, Labor Pain group.
