Efficacy of Intrathecal Morphine in a Model of Surgical Pain in Rats.

Concerns over interactions between analgesics and experimental outcomes are a major reason for withholding opioids from rats undergoing surgical procedures. Only a fraction of morphine injected intravenously reaches receptors responsible for analgesia in the central nervous system. Intrathecal administration of morphine may represent a way to provide rats with analgesia while minimizing the amount of morphine injected. This study aimed to assess whether morphine injected intrathecally via direct lumbar puncture provides sufficient analgesia to rats exposed to acute surgical pain (caudal laparotomy).In an initial blinded, randomised study, pain-free rats received morphine subcutaneously (MSC, 3mg.kg-1, N = 6), intrathecally (MIT, 0.2mg.kg-1, N = 6); NaCl subcutaneously (NSC, N = 6) or intrathecally (NIT, N = 6). Previously validated pain behaviours, activity and Rat Grimace Scale (RGS) scores were recorded at baseline, 1, 2, 4 and 8h post-injection. Morphine-treated rats had similar behaviours to NaCl rats, but their RGS scores were significantly different over time and between treatments. In a second blinded study, rats (N = 28) were randomly allocated to one of the following four treatments (N = 7): MSC, 3mg.kg-1, surgery; MIT, 0.2mg.kg-1, surgery; NIT, surgery; NSC, sham surgery. Composite Pain Behaviours (CPB) and RGS were recorded as previously. CPB in MIT and MSC groups were not significantly different to NSC group. MSC and MIT rats displayed significantly lower RGS scores than NIT rats at 1 and 8h postoperatively. RGS scores for MIT and MSC rats were not significantly different at 1, 2, and 8h postoperatively. Intraclass correlation value amongst operators involved in RGS scoring (N = 9) was 0.913 for total RGS score. Intrathecal morphine was mostly indistinguishable from its subcutaneous counterpart, providing pain relief lasting up to 8 hours in a rat model of surgical pain. Further studies are warranted to clarify the relevance of the rat grimace scale for assessing pain in rats that have received opioid analgesics

Ethogram used for behavioural observations in phase 1.

<p>Adapted from references [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0163909#pone.0163909.ref036" target="_blank">36</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0163909#pone.0163909.ref049" target="_blank">49</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0163909#pone.0163909.ref051" target="_blank">51</a>].</p

Treatment groups in phase 2 of the study.

<p>Treatment groups in phase 2 of the study.</p

Average Action Unit RGS scores for treatments 2.1 (NaCl SC + sham surgery); 2.2 (NaCl IT + surgery); 2.3 (morphine SC + surgery) and 2.4 (morphine IT + surgery) over time (Solid line = median, box = 1^st and 3^rd quartiles, whiskers = minimum and maximum, ★,○ = Outliers).

<p>N = 28. <i>P</i>-values are indicated where differences are significant (<0.05).</p

Within-subjects comparison between post treatment time-points with baseline with associated <i>p</i>-values for differences where significance was found (phase 1 of the study).

<p>Differences are significant if p<0.05. AU: Action Unit; SC: Subcutaneous; IT: Intrathecal; ND: No significant difference.</p

Representation of mean duration or frequency of the most commonly observed behaviours in pain-free rats (1^st phase study) ± 2 standard error to the mean.

<p>N = 24, pooled data for treatment groups 1.1 to 1.4. There was no significant treatment effect between groups. <b>A</b>: significant decrease of rearing frequency over time (p<0.001); <b>B</b>: significant decrease of time spent walking over time (p<0.001); <b>C</b>: Time spent climbing is significantly higher at baseline than at any further time point (P = 0.036); <b>D:</b> significant increase of the time spent inactive (p = 0.001).</p

Composite Pain Behaviour scores and associated <i>p</i>-values for phase 2 of the study.

<p>Composite Pain Behaviour scores and associated <i>p</i>-values for phase 2 of the study.</p

Treatment groups in phase 1 of the study.

<p>Treatment groups in phase 1 of the study.</p

Mean frequencies for Composite Pain Behaviour (CPB) scores across treatments and time points.

<p>Data presented ± 2 standard error of the mean. N = 28. Significant differences (*) are explained in the text and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0163909#pone.0163909.t005" target="_blank">Table 5</a>.</p