Comparison of electroacupuncture and morphine-mediated analgesic patterns in a plantar incision-induced pain model

Abstract

Electroacupuncture (EA) is a complementary therapy to improve morphine analgesia for postoperative pain, but underlying mechanism is not well-known. Herein, we investigated EA-induced analgesic effect in a plantar incision (PI) model in male Sprague-Dawley rats. PI was performed at the left hind paw. EA of 4 Hz and high intensity or sham needling was conducted at right ST36 prior to PI and repeated for another 2 days. Behavioral responses to mechanical and thermal stimuli, spinal phospho-ERK, and Fos expression were all analyzed. In additional groups, naloxone and morphine were administered to elucidate involvement of opioid receptors and for comparison with EA. EA pretreatment significantly reduced post-PI tactile allodynia for over 1 day; repeated treatments maintained analgesic effect. Intraperitoneal naloxone could reverse EA analgesia. Low-dose subcutaneous morphine (1 mg/kg) had stronger inhibitory effect on PI-induced allodynia than EA for 1 h. However, analgesic tolerance appeared after repeated morphine injections. Both EA and morphine could equally inhibit PI-induced p-ERK and Fos inductions. We conclude that though EA and morphine attenuate postincision pain through opioid receptor activations, daily EA treatments result in analgesic accumulation whereas daily morphine injections develop analgesic tolerance. Discrepant pathways and mechanisms underlying two analgesic means may account for the results.

Figure 1

Effect of repeated daily EA treatments on PI-induced pain. (a) Summary of the protocols used in this experiment. BL: baseline on one day before PI; D: postplantar incision day; EA: electroacupuncture; h: hour; PI: plantar incision, marked by a solid triangle; pre: before daily EA treatment. (b) Mechanical allodynia, (c) heat hyperalgesia. Naïve: group without PI surgery or EA treatment, EA: group with repeated EA treatments, EA+PI: group for PI with repeated EA treatments, and Sham+PI: group for PI with repeated sham needle insertions; # < 0.05, ## < 0.01, ### < 0.001 groups versus Naïve; ∗ < 0.05, ∗∗ < 0.01, ∗∗∗ < 0.001 EA+PI versus Sham+PI; one-way ANOVA with Tukey's multiple comparison test; N = 4 (Naïve), 4 (EA), 7 (Sham+PI), and 9 (EA+PI).

Figure 2

Intraperitoneal injections of naloxone (Nal) reversed EA analgesic effect on postincisional pain. (a) Summary of the protocols used in this experiment. Naloxone was injected at doses of 2 mg/kg immediately before anesthesia and 1 mg/kg immediately after PI and repeated the administration mode on D1 and D2. BL: baseline on one day before PI; D: postplantar incision day; EA: electroacupuncture; h: hour; i.p.: intraperitoneal injection; Nal: naloxone, marked with an arrow line; PI: plantar incision, marked with a solid triangle; pre: before daily Nal i.p. and EA treatment. (b) Intraperitoneal injections of naloxone almost completely antagonized EA analgesia to basal post-PI pain levels. Nal: group with i.p. naloxone, Saline+EA+PI: group for PI with repeated saline injections and EA treatments, Nal+Sham+PI: group for PI with repeated naloxone injections and sham needle insertion, and Nal+EA+PI: group for PI with repeated naloxone injections and EA treatments; ### < 0.001 groups versus Nal; & < 0.05 Nal+EA+PI versus Nal+Sham+PI; ∗ < 0.05, ∗∗ < 0.01, ∗∗∗ < 0.001 Saline+EA+PI versus Nal+EA+PI; one-way ANOVA with Tukey's multiple comparison test; N = 4 (Nal), 7 (Nal+Sham+PI), 7 (Nal+EA+PI), and 6 (Saline+EA+PI).

Figure 3

Comparison of analgesic patterns between EA and subcutaneous injection of morphine in incisional pain. (a) Summary of the protocols used in this experiment. BL: baseline on one day before PI; D: postplantar incision day; EA: electroacupuncture; h: hour; s.c.: subcutaneous injection; Mor: morphine, marked with a blank triangle; PI: plantar incision, marked with a solid triangle; pre: before daily Mor i.p. and EA treatment. (b) Mechanical allodynia, PI: group for PI surgery, EA+PI: group for PI with repeated EA treatments, and Mor+PI: group for PI with repeated morphine s.c. injections; # < 0.05, ## < 0.01, ### < 0.001 groups versus PI; ∗∗ < 0.01, ∗∗∗ < 0.001 Mor+PI versus EA+PI; one-way ANOVA with Tukey's multiple comparison test. N = 6 (PI), 9 (EA+PI), and 6 (Mor+PI).

Figure 4

Effect of EA and morphine on ERK activation 30 minutes after PI in the lumbar (L4 or L5) spinal dorsal horns. (a)–(d) are section slices showing spinal dorsal horn at the PI-ipsilateral side in the Naïve group ((a) rats without any operation or treatment), Sham+PI group ((b) rats with PI and sham needling and saline injection), EA+PI group ((c) rats with PI and pretreated EA), and Mor+PI group ((d) rats with PI and preinjected morphine). (e) Numbers of pERK-immunoreactive cells by lamina in the spinal dorsal horn among groups. Note no differences between the EA+PI and Mor+PI groups. Laminas I-II: superficial dorsal horn, laminas III-IV: middle dorsal horn, and lamina V: deep dorsal horn. DH: dorsal horn; ∗∗∗ < 0.001 groups versus Sham+PI, one-way ANOVA with Tukey's multiple comparison test; N = 4 for each group, respectively. Scale bar = 100 μm.

Figure 5

Effect of EA and morphine on Fos expression 3 hr after PI in the lumbar (L4 or L5) spinal dorsal horns. (a)–(d) are section slices showing spinal dorsal horn at the PI-ipsilateral side in the Naïve group ((a) rats without any operation or treatment), Sham+PI group ((b) rats with PI and sham needling and saline injection), EA+PI group ((c) rats with PI and pretreated EA), and Mor+PI group ((d) rats with PI and preinjected morphine). (e) Numbers of Fos-immunoreactive cells by lamina in the spinal dorsal horn among groups. Note no differences between the EA+PI and Mor+PI groups. Laminas I-II: superficial dorsal horn, laminas III-IV: middle dorsal horn, and lamina V: deep dorsal horn. DH: dorsal horn; ∗ < 0.05, ∗∗∗ < 0.001 groups versus Sham+PI, one-way ANOVA with Tukey's multiple comparison test; N = 4 for each group. Scale bar = 100 μm.