Effects of kappa opioids in an assay of pain-depressed intracranial self-stimulation in rats

Abstract

Rationale: Selective, centrally acting kappa opioid agonists produce antinociception in a wide range of preclinical assays, but these compounds perform poorly as analgesics in humans. This discrepancy may be related to the behavioral depressant effects of kappa agonists. Kappa antagonists do not typically produce antinociception, but they produce antidepressant-like effects in some preclinical assays.

Objective: The objective of this study was to test the hypothesis that the kappa agonist U69,593 and the kappa antagonist norbinaltorphimine would produce pronociceptive and antinociceptive effects, respectively, in an assay of pain-depressed behavior.

Methods: Effects of U69,593 (0.056-0.56 mg/kg), norbinaltorphimine (10-32 mg/kg), and morphine (3.2 mg/kg) were evaluated on the stimulation of a stretching response and the depression of intracranial self-stimulation (ICSS) of the medial forebrain bundle produced in rats by a common noxious stimulus (intraperitoneal administration of dilute lactic acid).

Results: U69,593 produced a dose-dependent blockade of acid-stimulated stretching but only exacerbated acid-induced depression of ICSS. Thus, U69,593 produced antinociception in the assay of pain-stimulated behavior but pronociceptive effects in the assay of pain-depressed behavior. Norbinaltorphimine did not alter acid-stimulated stretching or acid-induced depression of ICSS. The mu opioid agonist morphine blocked both acid-stimulated stretching and acid-induced depression of ICSS.

Conclusions: These results support the hypothesis that prodepressant effects of kappa agonists may limit their clinical utility as analgesics. These results do not support the use of kappa antagonists to treat depressant effects of pain. These findings illustrate the potential value of using complementary assays of pain-stimulated and pain-depressed behaviors for preclinical evaluation of candidate analgesics.

Figure 1. U69,593 and morphine block acid-stimulated stretching

Abscissa: dose U69,593 in mg/kg, IP. Ordinate: Number of stretches observed during a 30-min observation period. Each bar shows mean ± SEM for 6 rats. For U69,593, one-way ANOVA indicated a significant main effect of U69,593 dose [F(3,15)=, p=0.0156]. *Asterisk indicates a significant difference from “Veh” as determined by the Dunnett’s post hoc test, p<0.05. For morphine, a paired t-test indicated a significant effect of morphine (p<0.05).

Figure 2. Lactic acid treatment depresses ICSS

Abscissa: Frequency of electrical brain stimulation in Hz (log scale). Ordinate: Percent maximum control response rate. The average baseline ICSS frequency-rate curve for the entire study in this group of rats is shown by the gray line for comparison, but these data were not included in statistical analysis. Effects of treatment with drug vehicle + lactic acid vehicle and drug vehicle + 1.5% lactic acid are shown by diamonds and circles, respectively. All points show mean ± SEM for 6 rats. In a comparison of ICSS frequency-rate curves after treatment with lactic acid vehicle and 1.5% lactic acid, there was a significant main effect of frequency [F(9,100)=38.5, p<0.001], a significant main effect of lactic acid treatment [F(1,100)=16.1, p<0.001], but not a significant frequency × treatment interaction [F(9,100)=1.8, p=0.088]. Filled circles indicate frequencies at which reinforcement rates after lactic acid were significantly lower than rates after vehicle treatment as determined by the Holman-Sidak post hoc test, p<0.05.

Figure 3. U69,593 fails to block acid-induced depression of ICSS

(a) The left and center panels show U69,593 effects on full frequency-rate curves. Abscissae: Frequency of electrical brain stimulation in Hz (log scale). Ordinates: Percent maximum control response rate. The left panel shows data for U69,593 administered as a pretreatment to lactic acid vehicle. There was a significant main effect of frequency [F(9,200)=68.9; p<0.001], a significant main effect of U69,593 dose [F(3,200)=28.6, p<0.001], and a significant frequency × dose interaction [F(27,200)=4.1; p<0.001]. The center panel shows data for U69,593 administered as a pretreatment to 1.5% lactic acid. There was a significant main effect of frequency [F(9,200)=30.4; p<0.001], a significant main effect of U69,593 dose [F(3,200)=5.7, p<0.001], but not a significant frequency × dose interaction [F(27,200)=1.1; p=0.264]. Filled symbols indicate frequencies at which reinforcement rates after U69,593 treatment were significantly lower than rates after vehicle alone (left panel) or acid alone (center panel) as determined by the Holman-Sidak post hoc test, p<0.05. (b) The right panel shows U69,593 effects on the total number of stimulations per component. Abscissa: Dose U69,593 in mg/kg. Ordinate: Total Stimulations per component. The asterisk indicates a significant lactic acid-induced depression of ICSS as determined by paired t-test (p<0.05). Dollar signs indicate a significant depression of ICSS by U69,593 as determined by significant one-way ANOVA in the respective lactic acid vehicle or 1.5% lactic acid treatment conditions, followed by the Dunnett post hoc test (p<0.05). All data show results from 6 rats.

Figure 4. Morphine blocks acid-induced depression of ICSS

(a) The left and center panels show morphine effects on full frequency-rate curves. Abscissae: Frequency of electrical brain stimulation in Hz (log scale). Ordinates: Percent maximum control response rate. The left panel shows data for morphine administered as a pretreatment to lactic acid vehicle. There was a significant main effect of frequency [F(9,80)=144.0; p<0.001], but not a significant main effect of morphine dose [F(1,80)=0.01, p=0.912] or a significant frequency × dose interaction [F(9,80)=0.3; p=0.980]. The center panel shows data for morphine administered as a pretreatment to 1.5% lactic acid. There was a significant main effect of frequency [F(9,80)=144.0; p<0.001], a significant main effect of morphine dose [F(1,80)=14.7, p<0.001] and a significant frequency × dose interaction [F(9,80)=6.3; p<0.001]. Filled symbols indicate frequencies at which reinforcement rates after morphine treatment were significantly higher than rates after acid alone (center panel) as determined by the Holman-Sidak post hoc test, p<0.05. (b) The right panel shows morphine effects on the total number of stimulations per component. Abscissa: Dose morphine in mg/kg. Ordinate: Total Stimulations per component. The asterisk indicates a significant lactic acid-induced depression of ICSS as determined by t-test (p<0.05). The dollar sign indicates a significant blockade of lactic acid-induced depression of ICSS as determined by t-test in the 1.5% lactic acid-treated condition (p<0.05). All data show results from 5 rats.

Figure 5. NorBNI does not block acid-stimulated stretching

Abscissa: Treatment condition. Ordinate: Number of stretches observed during a 30-min observation period. Each bar shows mean ± SEM for 6 rats. There was a significant effect of treatment [F(3,18)=12.1, p<0.01]. Asterisk indicates that stretching was significantly lower in the U69,593 + Lactic Acid condition than in all other conditions as determined by the Newman-Keuls post hoc test. In addition, the Lactic Acid alone group was not different from the NorBNI + Lactic Acid group or the NorBNI+U69,593+Lactic Acid group (p>0.05).

Figure 6. NorBNI fails to alter basal ICSS or acid-induced depression of ICSS

(a) The left and center panels show norBNI effects on full frequency-rate curves. Abscissae: Frequency of electrical brain stimulation in Hz (log scale). Ordinates: Percent maximum control response rate. The left panel shows data for norBNI administered as a pretreatment to lactic acid vehicle. There was a significant main effect of frequency [F(9,120)=123.7; p<0.001] and a significant main effect of norBNI dose [F(2,120)=5.3, p=0.006], but not a significant frequency × dose interaction [F(18,120)=0.4; p=0.982]. The center panel shows data for norBNI administered as a pretreatment to 1.5% lactic acid. There was a significant main effect of frequency [F(9,120)=39.0; p<0.001], but not a significant main effect of norBNI dose [F(2,120)=2.1, p=0.12] or a significant frequency × dose interaction [F(18,120)=0.4; p=0.989]. The effects of norBNI were not significantly different from the effects of vehicle alone (left panel) or acid alone (center panel) as determined by the Holman-Sidak post hoc test. (b) The right panel shows norBNI effects on the total number of stimulations per component. Abscissa: Dose norBNI in mg/kg. Ordinate: Total Stimulations per component. The asterisk indicates a significant lactic acid-induced depression of ICSS as determined by paired t-test (p<0.05). However, there was not a significant effect of norBNI on total stimulations during either the lactic acid vehicle or 1.5% lactic acid treatment conditions as determined by one-way ANOVAs. All data show results from 5 rats.