Effects of μ-opioid receptor agonists in assays of acute pain-stimulated and pain-depressed behavior in male rats: role of μ-agonist efficacy and noxious stimulus intensity

Abstract

Pain is associated with stimulation of some behaviors and depression of others, and μ-opioid receptor agonists are among the most widely used analgesics. This study used parallel assays of pain-stimulated and pain-depressed behavior in male Sprague-Dawley rats to compare antinociception profiles for six μ-agonists that varied in efficacy at μ-opioid receptors (from highest to lowest: methadone, fentanyl, morphine, hydrocodone, buprenorphine, and nalbuphine). Intraperitoneal injection of diluted lactic acid served as an acute noxious stimulus to either stimulate stretching or depress operant responding maintained by electrical stimulation in an intracranial self-stimulation (ICSS). All μ-agonists blocked both stimulation of stretching and depression of ICSS produced by 1.8% lactic acid. The high-efficacy agonists methadone and fentanyl were more potent at blocking acid-induced depression of ICSS than acid-stimulated stretching, whereas lower-efficacy agonists displayed similar potency across assays. All μ-agonists except morphine also facilitated ICSS in the absence of the noxious stimulus at doses similar to those that blocked acid-induced depression of ICSS. The potency of the low-efficacy μ-agonist nalbuphine, but not the high-efficacy μ-agonist methadone, to block acid-induced depression of ICSS was significantly reduced by increasing the intensity of the noxious stimulus to 5.6% acid. These results demonstrate sensitivity of acid-induced depression of ICSS to a range of clinically effective μ-opioid analgesics and reveal distinctions between opioids based on efficacy at the μ-receptor. These results also support the use of parallel assays of pain-stimulated and -depressed behaviors to evaluate analgesic efficacy of candidate drugs.

Fig. 1.

Effects of μ-opioid agonists in the assay of acid-stimulated stretching. Abscissae: dose in milligrams per kilogram (log scale). Ordinates: percent control stretching. All points show mean data ± S.E.M. from 5–8 rats, and ED₅₀ values are reported in Table 1. The mean ± S.E.M. numbers of control stretches for each group were as follows: (A) methadone, 13.3 ± 4.3; fentanyl, 13.9 ± 2.7; morphine, 14.0 ± 7.6; (B) hydrocodone, 12.7 ± 3.1; buprenorphine, 12.3 ± 4.1; and nalbuphine, 12.9 ± 2.8.

Fig. 2.

Depression of ICSS by 1.8% lactic acid. (A) The left panel compares effects of pretreatment with vehicle plus vehicle and vehicle plus 1.8% lactic acid on full frequency-rate curves for all 35 rats used in the first phase of ICSS experiments. Abscissa: frequency of electrical brain stimulation in hertz (log scale). Ordinate: ICSS rate expressed as %MCR. Two-way ANOVA indicated a significant main effect of frequency [F_(9,306) = 295.4, P < 0.001] and acid treatment [F_(1,34) = 86.8, P < 0.001], and the interaction was also significant [F_(9,306) = 10.2, P < 0.001]. The acid noxious stimulus significantly depressed ICSS at all frequencies (Holm–Sidak post hoc test, P < 0.05). (B) The right panel shows summary data for lactic acid effects on the total number of stimulations per component. Abscissa: pretreatment condition. Ordinate: percent baseline number of stimulations per component. The downward arrow indicates that lactic acid produced a significant decrease in ICSS at one or more frequencies in the full frequency-rate curve. LA, lactic acid; Veh, vehicle.

Fig. 3.

Effects of methadone (A–C, n = 6) and fentanyl (D–F, n = 5) on control and 1.8% acid-depressed ICSS. Left and center panels show drug effects on full frequency-rate curves when drugs were administered as a pretreatment to vehicle (left panels in A and D) or 1.8% lactic acid (center panels in B and E). Abscissae: frequency of electrical brain stimulation in hertz (log scale). Ordinates: %MCR. Right panels (C and F) show summary data for drug effects on the total number of stimulations per component when drugs were administered as a pretreatment to vehicle (open bars) or acid (filled bars). Abscissae: dose of drug in milligrams per kilogram. Ordinate: percent baseline number of stimulations per component. Statistical results for two-way ANOVA of full frequency-rate curves are as follows. (A) Significant main effects of frequency [F_(9,45) = 41.2, P < 0.001] and dose [F_(4,20) = 5.9, P = 0.003], and a significant interaction [F_(36,180) = 3.0, P < 0.001]. (B) Significant main effects of frequency [F_(9,45) = 41.2, P < 0.001] and dose [F_(4,20) = 3.8, P = 0.018], and a significant interaction [F_(36,180) = 4.2, P < 0.001]. (D) Significant main effect of frequency [F_(9,36) = 72.8, P < 0.001], but not dose [F_(3,12) = 1.1, P = 0.384]; the interaction was significant [F_(27,108) = 4.3, P < 0.001]. (E) Significant main effects of frequency [F_(9,36) = 63.5, P < 0.001] and dose [F_(3,12) = 6.1, P = 0.009], and a significant interaction [F_(27,108) = 1.7, P = 0.030]. Filled symbols indicate a significant difference from vehicle plus vehicle (A and D) or vehicle plus lactic acid (B and E) (Holm–Sidak post hoc test, P < 0.05). Upward/downward arrows indicate that the drug dose produced a significant increase/decrease in ICSS at one or more frequencies when comparing the full frequency-rate curves. LA, lactic acid; Veh, vehicle.

Fig. 4.

Effects of morphine (A–C, n = 6) and hydrocodone (D–F, n = 6) on control and 1.8% acid-depressed ICSS. Details are as in Fig. 3. Statistical results for two-way ANOVA of full frequency-rate curves are as follows. (A) Significant main effect of frequency [F_(9,45) = 91.9, P < 0.001], but not dose [F_(4,20) = 1.2, P = 0.356]; the interaction was significant [F_(36,180) = 2.2, P < 0.001]. (B) Significant main effects of frequency [F_(9,45) = 56.8, P < 0.001] and dose [F_(4,20) = 8.2, P < 0.001], and a significant interaction [F_(36,180) = 2.7, P < 0.001. (D) Significant main effects of frequency [F_(9,45) = 71.0, P < 0.001] and dose [F_(4,20) = 8.0, P < 0.001], and a significant interaction [F_(36,180) = 5.6, P < 0.001]. (E) Significant main effects of frequency [F_(9,45) = 70.0, P < 0.001] and dose [F_(4,20) = 9.4, P < 0.001], and a significant interaction [F_(36,180) = 5.8, P < 0.001]. LA, lactic acid; Veh, vehicle.

Fig. 5.

Effects of buprenorphine (A–C, n = 6) and nalbuphine (D–F, n = 6) on control and 1.8% acid-depressed ICSS. Details are as in Fig. 3. Statistical results for two-way ANOVA of full frequency-rate curves are as follows. (A) Significant main effect of frequency [F_(9,45) = 70.6, P < 0.001] and dose [F_(4,20) = 2.9, P = 0.049], and a significant interaction [F_(36,180) = 3.8, P < 0.001]. (B) Significant main effects of frequency [F_(9,45) = 37.9, P < 0.001] and dose [F_(4,20) = 4.2, P = 0.012], and a significant interaction [F_(36,180) = 2.1, P < 0.001]. (D) Significant main effect of frequency [F_(9,45) = 91.5, P < 0.001], but not dose [F_(3,15) = 1.5, P = 0.261]; the interaction was significant [F_(27,135) = 2.9, P < 0.001]. (E) Significant main effects of frequency [F_(9,45) = 152.2, P < 0.001] and dose [F_(3,15) = 5.1, P = 0.012], and a significant interaction [F_(27,135) = 2.9, P < 0.001]. LA, lactic acid; Veh, vehicle.

Fig. 6.

Dose-effect curves for μ-agonist blockade of acid-induced depression of ICSS. Abscissae: dose in milligrams per kilogram (log scale). Ordinates: percent blockade of acid-induced depression of ICSS. All points show mean data ± S.E.M. from five to six rats, and ED₅₀ values are reported in Table 1.

Fig. 7.

Effect of acid concentration on acid-stimulated stretching (left) and acid-depressed ICSS (right). Abscissae: lactic acid concentration administered intraperitoneally. Left ordinate: number of stretches during the 30-minute observation period. Right ordinate: percent baseline number of stimulations per component. One-way ANOVA (with Dunnett post hoc test; P < 0.05) revealed the following results. (A) Significant effect of acid concentration [F_(6,18) = 16.7, P < 0.001]. (B) Significant effect of acid concentration [F_(12,36) = 22.3, P < 0.001]. The pound sign indicates significantly different from 0% acid as indicated by Dunnett post hoc test (P < 0.05).