Figure 1.
Timeline of the general experimental design.
Figure 2.
Fentanyl self-administration in CFA-induced inflammation.
Complete Freund’s Adjuvant was injected intraplantarly (i.pl.) the day before the first self-administration session. (A) Mechanical hyperalgesia was assessed through day 19 of the study. CFA-treated hindpaws (inverted closed triangles) demonstrated significantly reduced (p < 0.05) thresholds to paw withdrawal relative to contralateral hindpaws (open triangles) or saline-treated controls (circles). Saline (B) or CFA (C) was injected i.pl. in the left hindpaw and establishment of fentanyl self-administration assessed by bar pressing on the active lever compared to the control lever. Responses represent lever presses on one of two bars. The first bar (active lever) delivers 70 µL of fentanyl (10 µg/ml) (triangles). Pressing the control lever results in no reward and is indicative of non-specific activity (circles). (D) The magnitude of discrimination between the active and control levers are displayed for the duration of the testing period for CFA (closed circles) and saline-treated (open triangles) mice. (E) Analysis of the AUC for the groups in B and C show that mice that had CFA-induced hyperalgesia did not lever press for fentanyl. (F) Analysis of the AUC for the groups in E show that while saline-treated mice discriminated between the active and control levers, mice with CFA-induced hyperalgesia did not. (*significance was determined by ANOVA; p < 0.05; n=8 per group).
Figure 3.
Fentanyl self-administration in peripheral nerve injury: (A-F).
(A) Spinal nerve ligation was performed the day before the first self-administration session. Mechanical hyperalgesia was assessed through day 42 of the study. Hindpaws ispilateral to the injury (closed circles) demonstrated significantly (p < 0.05) reduced thresholds to paw withdrawal relative to contralateral hindpaws (open circles), sham-operated (triangles) or naive controls (squares). Naive (B) and Nerve-injured (C) mice were assessed for establishment of fentanyl self-administration by comparing bar pressing on the active lever with that on the control lever. (D) The magnitude of discrimination between the active and control levers are displayed for the duration of the testing period for naive (closed circles) and nerve-injured (open triangles) mice. (E) Analysis of the AUC for the groups in B and C indicate that mice with SNL-induced hyperalgesia did not lever press for fentanyl. (F) Analysis of the AUC for the groups in D show that naive mice discriminated between the active and control levers, while mice with SNL-induced hyperalgesia did not. (*significance for AUC was determined by Student’s t-test; p < 0.05 n=7 per group). (G-K) In a separate experiment nerve-injured mice were compared to sham-operated control mice. Sham (G) and nerve-injured (H) mice were assessed for establishment of fentanyl self-administration by comparing bar pressing on the active lever with that on the control lever. (I) The magnitude of discrimination between the active and control levers are displayed for the duration of the testing period for sham-operated (closed diamonds) and nerve-injured (open triangles) mice. (J) Analysis of the AUC for the groups in G and H show that mice with SNL-induced hyperalgesia did not lever press for fentanyl. (K) Analysis of the AUC for the groups in panel I show that the discrimination scores of sham-operated mice differ from that of the nerve-injured mice. # indicates a statistical trend for AUC was determined by Student’s t-test; p < 0.1, n=8 per group.
Figure 4.
Chemotherapeutic-induced neuropathic pain.
(A) Vincristine or saline was injected daily for 9 consecutive days. Mechanical hyperalgesia was assessed through day 14 of the study. Hindpaws of vincristine-treated subjects (triangles) demonstrated significantly (p < 0.05) reduced thresholds to paw withdrawal relative to hindpaws of saline-treated mice (circles) in a dose-related manner. N=8 per group. Saline-treated (B) or vincristine-treated (C) mice were assessed for establishment of fentanyl self-administration assessed by comparing bar pressing on the active lever with that on the control lever. (D) The magnitude of discrimination between the active and control levers are displayed for the duration of the testing period for vincristine (open triangles) and saline-treated (closed circles) mice. (E) Analysis of the AUC for the groups in B and C show that mice with vincristine-induced hyperalgesia did not lever press for fentanyl. (F) Analysis of the AUC for the groups in D show that the discrimination scores of saline-treated mice significantly differ between that of the vincristine-treated mice. *indicates significance for AUC was determined by ANOVA; p < 0.05, n=6 per group.
Figure 5.
Food-maintained responding in CFA-, SNL- and vincristine-induced hyperalgesia.
When compared against their respective control groups, food-maintained responding was not different between CFA-treated (A) versus saline-treated subjects (B), nerve-injured (C) versus sham-operated (D) mice, or vincristine-treated (E) versus saline-treated (F) mice. Food-maintained responding and control lever presses were recorded in daily 2 h sessions and the final area under the curve is represented in panel G for all groups. *indicates significant difference in responding between the respective control and active lever within experimental group was determined by ANOVA, p < 0.05., n=6 per group.