Diet can modify the response of rodents to acute and chronic nociceptive stimuli. Most research has been done on the role of diet in acute pain tests (1–3), while there is only sparse evidence on the effect of diet in attenuating chronic pain in experimental animals (4,5). Most data on the effect of diet on chronic pain come from our work on the partial sciatic nerve ligation (PSL) model (6). Using this model we have shown that levels of neuropathic pain after PSL depended significantly on the type of dietary protein rats consumed. For example, consumption of soy-rich diets significantly suppressed the development of tactile and heat allodynia and hyperalgesia after a PSL injury (7–9), whereas albumin-rich diets enabled the development of significant sensory disorders (10).
Recently, we (10) found that dietary fat could also suppress chronic neuropathic pain in PSL-injured rats and that this effect is augmented by an interaction between dietary fat and protein. Dietary fatty acids, especially polyunsaturated fatty acids (PUFA), have been shown to significantly affect multiple physiological and pathological processes in experimental animals (11) and humans (12). Moreover, dietary fat possessed analgesic properties in acute experimental heat pain tests in rats (3). No previous studies, however, considered a possible role of fat in attenuating experimental chronic pain states that could be more relevant in clinical situations. Indirect support for a possible role for fatty acids in attenuating chronic neuropathic pain derives from data showing that dietary PUFAs suppress the production and function of inflammatory cytokines (13), whose role in the development of neuropathic pain in PSL rats is well demonstrated.
Our aims in this study were to verify that dietary fat has antinociceptive properties in PSL-injured rats independently of other dietary components and to learn whether this effect could be attributed to levels of ω-3 or ω-6 PUFA in the diet. For that purpose we tested six PSL rat groups fed identical diets, differing solely in their fat source.
This study was approved by the McGill University Institutional Animal Care and Use Committee and conducted according to its regulations. Experiments adhered to the guidelines for animal experimentation of the International Association for the Study of Pain.
Six groups of male Wistar rats (Charles River Laboratories, Montreal, Canada; 10–11 rats/group), weighing 225–250 g at the beginning of the experiment were used. Rats were housed in standard colony conditions (three rats per cage; ambient temperature of 22°C ± 0.5°C; water and food supplied ad libitum; day-night cycle with lights switched on at 7 am and off at 7 pm). Surgery and behavioral tests were done between 10 am and 5 pm, by a single investigator blinded as to the rats’ group allocation. Rats underwent 2 15-min acclimatization periods in the testing chamber before the first sensory test.
From the time they were weaned until the beginning of experiment rats were fed a regular commercial rodent chow (as per Charles River routine; Purina 5075®). Starting 1 wk before PSL injury and for 1 wk thereafter, rats were fed the tested diet. A presurgical feeding period of 1 wk was considered to suffice for the purpose of this study based on our previous results (9). All rats received a commercially available fat-free diet (AIN 93G; Bio-Serv CO, Frenchtown, NJ). This diet comprised 52.7% cornstarch, 17% sucrose, 20% protein (>85% casein), 5% cellulose, and 3.5% minerals, vitamins and trace elements, and it contained <0.15% fat. Over the same time period, five groups of rats were supplemented daily at noon with 1 mL of commercially available oil via gavage. The oils were corn, soy, canola, sunflower, or hemp bought in local stores. The amount of fat was calculated based on 5% fat content in most commercial rodent chows and an average daily food consumption of 20 g per adult rat. These oils were chosen for their availability for daily use, their high levels of PUFAs, and the significant differences in their ω-3 and ω-6 content (Table 1). The sixth group of rats served as a control and received 1 mL of plain water via gavage. An investigator blinded as to their identity supplemented oil and water.
Daily consumption of food was calculated per cage by subtracting the food remaining in the cage from the amount given the day before (no allowance was made for spillage). Daily intake per rat was calculated by dividing by the number of rats per cage. Rats’ weights were recorded before PSL and at the end of the experiment 1 wk later.
PSL was done under inhaled anesthesia with 2%–3% isoflurane. The right sciatic nerve was exposed high on the thigh, near the trochanter. Using a mini-needle, an 8-0 silk suture was inserted in the middle of the nerve, trapping in a tight ligation the dorsal one-third to one-half of the nerve thickness. The wound was closed layer by layer with absorbable sutures (6).
The responses of rats to tactile and thermal stimuli were determined before surgery and on days 3 and 7 thereafter. The withdrawal threshold to repetitive touch was measured with von Frey hairs (1, 2, 4, 6, 8, 10, 15, and 26 g; North Coast Medical, San Jose, CA). The tested hair was indented 5 times, at a rate of 2 per second, in the midplantar skin of the hindpaw until the hair bowed. If subthreshold, the stimulus intensity was increased by using the next hair. At threshold, rats responded by hindpaw elevation. Groups were considered to develop tactile allodynia if their mean post-PSL thresholds were significantly lower compared with their baseline means.
Heat pain was tested with the Hargreaves device (Ugo Basile, Comerio, VA, Italy). The time elapsed from stimulus onset until hindpaw lifting (withdrawal threshold) and the duration of hindpaw lifting until it was placed again on the floor (response duration) were recorded. Each hind paw was tested 3 times, 2 min apart, and the response of individual rats was calculated as the average of 3 trials. Based on previous experiments with the PSL and other partial nerve injury models (e.g., the chronic constriction injury [CCI] model), groups were considered to develop heat hyperalgesia if their post-PSL thresholds were significantly lower or if their post-PSL response durations were significantly longer compared with their baseline means.
Only post-PSL injury data from the ipsilateral, operated, side were used in the analysis. Levels of postoperative sensory abnormalities were calculated by averaging the responses at days 3 and 7 after PSL, creating a single post-PSL score/rat. An average post-PSL score was calculated for each dietary group. To demonstrate the development of tactile allodynia and heat hyperalgesia after PSL, the postoperative sensory levels were compared with preoperative levels within each dietary group using paired Student’s t-tests. A difference score was computed for every rat by subtracting the preoperative value from the average post-PSL value, and results were summarized across dietary groups. To investigate the differences among groups one-way analysis of variance was performed with post hoc Student’s t-tests. Mean daily food consumption and weight gain were also compared among groups using analysis of variance.
The association between experimental pain behavior and dietary ω-3 and ω-6 (including ω-6/ω-3 ratio) intake was explored using linear and nonlinear regression analyses. Significance was set at the 5% level, and all tests were two-sided.
The experimental dietary groups did not differ in their daily solid food consumption (average, 17.6 ± 0.42 g/rat) and weekly weight gain (average, 34.4 ± 1.74 g/rat). There was a 200-fold difference in ω-3 intake between groups consuming hemp and sunflower oils, containing the highest and lowest levels of ω-3, respectively (Table 2). Similarly, there was a 3.2 fold difference in ω-6 consumption between rats fed sunflower and canola oils, containing the highest and lowest levels of ω-6, respectively. A 295-fold difference in the ω-6/ω-3 ratio has been calculated for the sunflower versus the canola oils (Table 2).
All six groups of rats developed a significant tactile allodynia after PSL (P ≤ 0.001). There was no significant difference in threshold response, however, among groups (P = 0.41, analysis of variance) (Fig. 1). Dietary content of ω-3 or ω-6 had no effect on tactile allodynia, and no correlation was observed between the dietary ω-3/ω-6 ratio and tactile allodynia.
There was no significant difference in the threshold response to heat among groups (mean ± sem difference: canola −0.8 ± 1.2 s; water 0.4 ± 1.3 s; corn 0.7 ± 1.4 s; hemp 1.0 ± 1.1 s; soy 1.2 ± 1.5 s; sunflower 3.0 ± 1.6 s; P = 0.44). The dietary content of ω-3 or ω-6 had no effect on the threshold response to heat after PSL, and there was no relationship between threshold response and ω-3/ω-6 ratio.
Significant heat hyperalgesia developed in the 5 experimental groups receiving the soy (P < 0.05), sunflower (P < 0.008), canola (P < 0.001), and hemp (P < 0.001) oils and water (P < 0.004) (Fig. 1). Rats fed the corn oil did not develop heat hyperalgesia after PSL (P < 0.09). The response duration to heat was significantly different among groups (P = 0.005; analysis of variance). Heat hyperalgesia of rats fed hemp oil, developing the most robust response, was significantly larger compared with rats fed corn oil, developing the least pain model (difference score: 24.3 ± 4.1 s versus 6.1 ± 3.1 s, respectively; P < 0.001). Although reaching only marginally significant differences, rats supplemented with water only developed suppressed heat hyperalgesia compared to hemp-fed rats (P < 0.07) and increased heat response compared to corn-fed rats (P < 0.06). No correlation was found between dietary ω-6 consumption or the ω-3/ω-6 ratio and levels of heat hyperalgesia. However, a highly significant correlation between the ω-3 dietary intake and duration of withdrawal response was found (r2 = −0.35, P = 0.006) (Fig. 2).
This study shows that levels of neuropathic pain in rats undergoing PSL could depend on dietary oil modification. The consumption of corn and soy oil was associated with decreased neuropathic pain levels, whereas canola and hemp oils enabled the development of a significant heat hyperalgesia. In addition, we unexpectedly found that the tendency to develop neuropathic pain may correlate with dietary levels of ω-3, but not ω-6, PUFA. The effect of oil was independent of other dietary constituents because except for the supplemented oil, all six tested groups consumed the same amounts of identical diet during the entire study period. Together with our preliminary findings (10) these data show that common dietary oils could be associated with reduced pain levels after PSL in rats.
Choosing the appropriate basic diet for this experiment was not trivial because both dietary fat and protein could play an analgesic role in PSL rats. Consequently, most purified diets could deviate the results of this study, depending on their specific fat/protein composition. We have chosen to use a casein-based, fat-free diet that eliminated the possibility of fatty components, other than those tested, to affect rats’ pain behavior. A casein-based diet has been chosen because it has been safely used in rodent experiments of long-term feeding (14) and consistently enabled the development of a significant neuropathic pain model after PSL (7–9). Thus, we believe that supplementing rats with this diet optimized our chances to discriminate among oils in accordance with their pain-relieving properties. The oils for this experiment have been chosen considering their availability for daily use and their possible relevance for clinical pain states, their high levels of PUFA, and the significant differences in their ω-3 and ω-6 content. Although supplemented as pure products, a possible effect on pain behavior of other vitamin and trace elements in these oils cannot be excluded.
In accordance with our previous results (7) a casein-based diet enabled the development of significant tactile allodynia and heat hyperalgesia (Fig. 1). Choosing the casein/water diet as a control group enabled the clear differentiation between the pro-analgesic (corn and soy) and the pro-nociceptive (canola and hemp) dietary oils (Fig. 1B). Feeding rats a fat-free diet for a limited period of time has been shown to be safe in experimental rats and was not associated with significant deleterious effects (15).
The oils used in the current study were effective in reducing one type of neuropathic pain behavior, heat hyperalgesia, but not tactile allodynia. Several lines of evidence support the assumption that these two sensory abnormalities could be mediated via different pathophysiological mechanisms. It has been shown that the number of damaged nerve fibers after PSL could predict the development of either tactile or heat hyperalgesia or both (16). A variety of drugs with proven analgesic potency in models of chronic pain have been shown to exert differing analgesic profiles in nerve-injured rats (17). Finally, different receptors trigger thermal hyperalgesia and mechanically evoked pain behavior after nerve injury (18). It is possible, therefore, that the analgesic properties of oil can influence some, but not all, neuropathic pain symptoms in rats.
Fatty acids, specifically PUFAs, are crucial components of neuronal (and other) cell membranes and are substrates of bioactive molecules such as prostaglandins, thromboxanes, and leukotrienes (12,13). PUFA play a significant role in cytokine production and function (13) and act directly upon nuclear receptors (19). Consequently, some fatty acids or their metabolites could act like hormones to control the activity of specific transcription factors (19). While the ω-6 PUFAs have been traditionally regarded as mainly proinflammatory, the ω-3 PUFA have been considered beneficial because of their antiinflammatory properties. For example, replacement of ω-6 PUFA with ω-3 PUFA in cell membranes resulted in a decreased cellular response to inflammatory stimuli in rodents (20). In humans, ω-3 PUFA have been shown to attenuate inflammation in patients with chronic inflammatory diseases (21). The results of the present study, implicating ω-3 PUFA as possible pro-nociceptive compounds, seem to contradict this evidence. It is possible that ω-3 PUFA could exert their pro-nociceptive effect via noninflammatory mechanisms, for example by changing the properties of neuronal cell membrane or by direct effect on the neural cell nucleus, inducing pro-nociceptive neurophysiological changes. Indeed, it has been shown that ω-3 PUFA enhanced conduction through slow tetrodotoxin-resistant sodium channels in rat dorsal root ganglion neurons (22), resulting in hyperpolarization that could augment pain. Alternatively, it has been shown that some partial nerve injuries, like the PSL, create a pain model that is mainly neuropathic and not inflammatory. Indeed, activated macrophages play only a limited role in the onset of mechanical allodynia in the spinal nerve ligation model (23). Similarly, rats undergoing the partial sciatic nerve transaction model, a modification of the PSL model, have been compared with rats undergoing the CCI model, whose nociceptive abnormalities are partially related to inflammation (24). The number of macrophages in the epineurium of the injured nerve was strikingly increased after CCI compared with the transaction model (25). We suggest, therefore, that the antiinflammatory properties of ω-3 PUFA might not be relevant in the PSL injury. It is possible that dietary fat modifies chronic neuropathic pain not by altering the synthesis of proinflammatory prostaglandins and cytokine compounds but by acting on other systems related to the generation and perpetuation of such chronic sensory abnormalities.
In summary, this research shows that commercially available dietary oils could play a role in the development of chronic neuropathic pain after PSL in rats. The effect of oil on pain behavior may be associated with levels of ω-3, but not ω-6, PUFA in the tested oils. This study might open the way for experiments that will further examine the role of PUFAs in nociception in animal models and possibly in humans with neuropathic pain.
The advice of Drs. Gary J. Bennett from the Anesthesia Research Unit and Centre for Research on Pain, and Errol Marliss from the Nutrition and Food Science Centre, McGill University, Montreal, is greatly appreciated. The authors thank Lina Naso for her outstanding technical assistance.
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