Spinal cord injury after thoracic aortic surgery is a disastrous and unpredictable complication, with a reported prevalence ranging from 0.9% to 40% (1,2). The mechanism of spinal cord injury during operations on the thoracic aorta is thought to be related primarily to direct tissue ischemia.
Of particular importance is a recent clinical and experimental observation that neuraxial administration of morphine after transient aortic occlusion (as for repair of aortic aneurysm) triggers the development of spasticity and that this is reversed by subsequent naloxone treatment (3). Furthermore, we also demonstrated that repetitive intrathecal (IT) morphine induced irreversible paraplegia after spinal cord ischemia in rats (4). These findings indicated that neuraxial morphine during and after thoracic aortic occlusion could be harmful to spinal neurological function.
Because postoperative analgesia, especially by a neuraxial opioid, is an important factor in reducing the incidence of postoperative complications (5), an opioid that provides safety with regard to neurological function should be considered after thoracic or thoracoabdominal aneurysm repair surgery. Although buprenorphine and pentazocine are often used as analgesics during the postoperative period, it is not known whether these opioids are safe with respect to preservation of neurological function after spinal cord ischemia. The aim of this study was to characterize the effect of IT buprenorphine and pentazocine after a noninjurious interval of spinal ischemia in rats.
All experimental procedures were performed according to a protocol approved by the Animal Care Committee at the University of the Ryukyus, Okinawa, Japan.
Male Sprague-Dawley rats (300–400 g), implanted with IT catheters by the method described by Yaksh and Rudy (6), were used. Details of the aortic occlusion model have been reported (7). Briefly, rats were anesthetized and maintained with 1.0%–2.5% isoflurane delivered by a mask. A thermocouple to measure paravertebral muscle temperature was placed at 1 cm in depth of the lumbar paravertebral muscle. A 24-guage Teflon catheter was inserted into the tail artery for monitoring distal arterial blood pressure. For the induction of spinal ischemia, a left femoral artery was isolated, and a 2F Fogarty catheter was placed into the descending thoracic aorta so that the tip of the catheter reached the level of the left subclavian artery. A 20-gauge Teflon catheter connected to an external blood reservoir (38.0°C) was inserted into the left carotid artery. To induce spinal ischemia, the balloon catheter was inflated with 0.05 mL of saline, and blood was allowed to flow into the external reservoir to control the proximal arterial blood pressure (i.e., above the level of aortic occlusion) at 40 mm Hg during the period of aortic occlusion. In this study, 6 min of spinal ischemia was chosen on the basis of our previous experiments that showed only a minimal or no deleterious effect on spinal neuronal function, with nearly complete recovery observed after 24–48 h of reperfusion (7). After ischemia, the balloon was deflated, and blood was reinfused over a period of 60 s. Protamine sulfate (4 mg) was subcutaneously administered. All arterial catheters were then removed, incisions were closed, and rats were allowed to recover for 3 days.
The rats were assigned to one of the following four groups according to the kind of injection administered 30 min after recovery from anesthesia (n = 6 per group): group 1 (control): IT saline injection (10 μL); group 2: IT injection of morphine (30 μg); group 3: IT injection of large-dose buprenorphine (4 μg); and group 4: IT injection of large-dose pentazocine (150 μg).
During reperfusion, recovery of motor and sensory function was assessed by the following grading system (7): motor function was quantified by assessment of ambulation and placing and stepping responses. For statistical purposes, ambulation (walking with lower extremities) was graded as follows: 0 = normal, 1 = toes flat under the body when walking, but ataxia present, 2 = knuckle walking, 3 = movement in lower extremities but unable to knuckle walk, or 4 = no movement; drags lower extremities. The placing and stepping reflex was assessed by dragging the dorsum of the hindpaw over the edge of a surface. This normally evokes a coordinating lifting and placing response (e.g., stepping), which was graded as follows: 0 = normal, 1 = weak, or 2 = no stepping. The motor deficit index (MDI) was the sum of the scores (walking with lower extremities plus placing and stepping reflex). The MDI was calculated for each rat at each time interval after ischemia (30 min, 1, 2, 4, 8, 24, and 72 h). The presence of spasticity or flaccidity was determined when an exaggerated flexion response to a pinch of the hindpaw was elicited. Flaccidity was defined as no tone in response to limb extension or pinching. Neurological function was assessed by two observers without knowledge of the treatment group (SN and MK).
Drugs used were morphine sulfate and buprenorphine hydrochloride, which were purchased from Otsuka Pharmaceutical Co, Ltd (Tokyo, Japan). Pentazocine was purchased from Sankyou Pharmaceutical Co, Ltd. Drugs for IT administration were mixed such that all doses were delivered in a total volume of 10 μL, followed by 10 μL of saline to flush the catheter.
Data for MDI were expressed as median, and other data were expressed as mean ± sd. Statistical analysis of physiological data was performed by one-way analysis of variance followed by Fisher’s protected least-squares difference test. For analysis of neurological outcome in individual groups, significant overall values were obtained by the Friedman test followed by the Wilcoxon signed rank test. Specific comparisons between experimental groups at individual time points after reflow were made with the Kruskal-Wallis test followed by the Tukey-Kramer test. A P value of <0.05 was considered significant. Statistical analyses were performed using SPSS software 8.0.1 for Windows from the SPSS Institute (Chicago, IL).
During all of the experiments, body temperature (paravertebral muscle temperature) ranged between 37.4°C and 38.0°C. Baseline distal arterial blood pressure was 83 ± 6 mm Hg and decreased to 6 ± 1 mm Hg at the end of 6 min of aortic occlusion in all of the experiments. No significant differences among experimental groups were detected in the physiological data (Table 1).
In group 1 (control group), all rats had modest and transient motor weakness (MDI, 2–4) at 1 h after reperfusion, followed by gradual recovery over 8–24 h and walked almost normally at 72 h. All group 2 rats (IT morphine) gradually developed spasticity of their hindlimbs after the IT morphine injection, with the peak of motor deficit observed between 2 and 4 h after spinal cord ischemia. Although there were significant differences in the MDI at 2–8 h after spinal cord ischemia compared with the values before IT injection (0.5 h after spinal cord ischemia), morphine-induced spasticity diminished over the 24 h after injection. Neither IT buprenorphine nor pentazocine after a noninjurious interval of aortic occlusion increased spasticity of the hindlimbs throughout the experiment. At 72 h after spinal cord ischemia, rats in all groups had almost normal motor function, and there were no significant differences in MDI among groups. With the exception of group 2, there were no significant differences in neurological function among groups throughout this experiment (Fig. 1).
The present study demonstrated that IT morphine, but not pentazocine or buprenorphine, induces spastic paraparesis after a noninjurious interval of spinal cord ischemia. It is suggested that the effect of various opioids on motor function after a noninjurious interval of spinal cord ischemia is opioid-specific.
A limitation of the present study is the lack of histopathological analysis. It was not determined whether neuraxial morphine, but not pentazocine or buprenorphine, caused injury in spinal motor neurons. However, in our previous study (3), neuraxial morphine after a short period of aortic occlusion induced spastic paraparesis via the spinal opioid receptor, and this spasticity may be mediated in part by inhibition of γ-amino butyric acid (GABA)ergic system (8). Furthermore, histopathological analysis revealed that large doses (9) or repetitive IT morphine injections (4) were likely to induce neuronal degeneration in the ventral horn, suggesting that neuraxial morphine might be harmful to neurological function after spinal cord ischemia. Although neither pentazocine nor buprenorphine induced spasticity after spinal cord ischemia, histopathological assessment should be made.
The potency of spinal pentazocine and buprenorphine for antinociception is about one fifth and 10 times of that of morphine, respectively (10,11). According to those data, 150 μg of pentazocine and 4 μg of buprenorphine was approximately equipotent to 30 μg of morphine after IT delivery. Although neither 150 μg of pentazocine nor 4 μg of buprenorphine caused neurological dysfunction, whether larger doses of these could induce spasticity after a noninjurious interval of spinal cord ischemia is unknown. However, doses larger than those used in the current study are unlikely to be clinically used.
Although our previous studies (3,4,9) did not focus on the mechanism by which IT morphine induces spasticity after a noninjurious interval of spinal ischemia, in those reports, we speculated on several mechanisms. Morphine-induced neurological dysfunction after a noninjurious interval of spinal cord ischemia is characterized by spastic paraplegia (4,8). With interneuronal networks in the spinal cord, previous studies indicated that in spastic paraplegia or paraparesis excitatory tonic activity in the spinal motor neurons may predominate as the result of a decrease in inhibition mediated by inhibitory (GABAergic or glycinergic) interneurons (12). Therefore, it can be postulated that IT morphine after spinal ischemia might block the inhibitory systems’ input to motoneurons, leading to an increase in hindlimb spasticity.
The differences in the pharmacological mechanism between morphine and pentazocine depend on the occupancy of opioid receptors. Activation of the μ opioid receptor by morphine can induce membrane hyperpolarization through the activation of an inwardly rectifying K+ channel (13). In contrast, the κ opioid receptor occupied by pentazocine seems to be distinctly coupled and was shown to depress Ca2+ conductance in neurons (14). Considering that there is inhibition of GABAergic interneurons by hyperpolarization through an increase of an inwardly K+ conductance (15), activation of μ opioid receptors by morphine rather than κ opioid receptors by pentazocine is likely to block spinal inhibitory interneurons, leading to an increase in spasticity. However, buprenorphine, despite being a partial agonist of μ opioid receptor, did not produce spastic paraplegia. Considerable evidence supports the existence of at least two μ opioid receptor subtypes in the central nervous system, namely, μ1- and μ2-opioid receptors (16,17). Buprenorphine is reported to act selectively at μ1-opioid receptors to produce an antinociceptive effect in mice (18). Supposing that morphine-inducing spastic paraplegia is mediated by activation of the μ2-opioid receptor in the spinal cord, it is reasonable to expect that IT buprenorphine failed to induce spastic paraparesis after a short period of aortic occlusion.
In conclusion, the present study shows that neither IT pentazocine nor buprenorphine can increase spasticity of hindlimbs after a noninjurious interval of spinal cord ischemia in rats. These results suggest that the effect of opioids on motor function after a noninjurious interval of spinal cord ischemia is opioid-specific.
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