Antinociceptive and Immune Effects of Delta-9-Tetrahydrocannabinol or Cannabidiol in Male Versus Female Rats with Persistent Inflammatory Pain

Abstract

Chronic pain is the most common reason reported for using medical cannabis. The goal of this research was to determine whether the two primary phytocannabinoids, delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD), are effective treatments for persistent inflammatory pain. In experiment 1, inflammation was induced by intraplantar injection of Complete Freund's adjuvant (CFA). Then THC (0.0-4.0 mg/kg, i.p.) or CBD (0.0-10 mg/kg, i.p.) was administered twice daily for 3 days. On day 4, THC, CBD, or vehicle was administered, and allodynia, hyperalgesia, weight-bearing, locomotor activity, and hindpaw edema were assessed 0.5-4 hours postinjection. In experiment 2, CFA or mineral oil (no-pain control)-treated rats were given THC (2.0 mg/kg, i.p.), CBD (10 mg/kg, i.p.), or vehicle in the same manner as in experiment 1. Four hours postinjection on day 4, serum samples were taken for analysis of cytokines known to influence inflammatory pain: interleukin (IL)-1β, IL-6, IL-10, interferon (IFN)-γ, and tumor necrosis factor (TNF)-α THC dose-dependently reduced pain-related behaviors but did not reduce hindpaw edema, and little tolerance developed to THC's effects. In contrast, CBD effects on inflammatory pain were minimal. THC produced little to no change in serum cytokines, whereas CBD decreased IL-1β, IL-10, and IFN-γ and increased IL-6. Few sex differences in antinociception or immune modulation were observed with either drug, but CFA-induced immune activation was significantly greater in males than females. These results suggest that THC may be more beneficial than CBD for reducing inflammatory pain in that THC maintains its efficacy with short-term treatment in both sexes and does not induce immune activation. SIGNIFICANCE STATEMENT: The pain-relieving effects of cannabidiol (CBD) and delta-9-tetrahydrocannabinol (THC) are examined in male and female rats with persistent inflammatory pain to determine whether individual phytocannabinoids could be a viable treatment for men and women with chronic inflammatory pain. Additionally, sex differences in the immune response to an adjuvant and to THC and CBD are characterized to provide preliminary insight into immune-related effects of cannabinoid-based therapy for pain.

Fig. 1.

Timeline for behavioral experiment (Experiment 1). On day 1 (D1), rats were baselined and then injected with CFA in the right hindpaw. One hour post-CFA, rats received CBD, THC, or vehicle. Rats received the same injection again at approximately 1700 hours on day 1 and twice daily on days 2 and 3 (D2, D3). On day 4 (D4), rats that had previously received THC or CBD received another injection of the same dose, whereas rats that had previously received vehicle received either another injection of vehicle or an acute injection of THC or CBD. Rats were tested on behavioral assays from 30 to 240 minutes postinjection on day 4. Rats were tested again on day 8 (D8) but did not receive injections on days 5–8.

Fig. 2.

THC and CBD effects in male and female rats on the von Frey test on day 4 (means of day 4 time course data are shown). In the THC experiment (A and B), the mechanical threshold in rats that received vehicle treatment was ∼40% BL, indicative of allodynia. Acute and repeated THC were antiallodynic in both males (A) and females (B) (all P ≤ 0.010). In the CBD experiment (C and D), the mechanical threshold in rats that received vehicle treatment was ∼55% BL, which was also indicative of allodynia. CBD 2.5 mg/kg was antiallodynic (significant effect not shown; see Supplemental Fig. 2 for data pooled across sexes). Each bar is the mean ± 1 S.E.M. of 8–12 male or female rats. *Significantly different from same-sex, vehicle-treated controls (P < 0.05).

Fig. 3.

CBD effects on the von Frey test on day 4, with data pooled across sexes (mean of day 4 time course data are shown). CBD 2.5 mg/kg was antiallodynic; this effect was not different when CBD was given acutely vs. repeatedly. Each bar is the mean ± 1 S.E.M. of 16–24 rats. *Significantly different from vehicle-treated controls (P < 0.05).

Fig. 4.

THC and CBD effects in male and female rats on the Hargreaves test on day 4 (means of day 4 time course data are shown). In the THC experiment (A and B), the thermal response latency in rats that received vehicle was ∼75% BL, indicative of hyperalgesia. THC produced dose-dependent antihyperalgesia that was greater in females than males. In males, all doses of THC were antihyperalgesic, and no tolerance developed when THC was given repeatedly. In females, THC 1–4 mg/kg was antihyperalgesic, and tolerance developed to this effect when THC was given repeatedly. In the CBD experiment (C and D), the thermal response latency in vehicle-treated rats was ∼100% BL, indicating no hyperalgesia. CBD did not alter thermal response latencies in either sex. Each bar is the mean ± 1 S.E.M. of 8–12 male or female rats. *Significantly different from same-sex, vehicle-treated controls (P < 0.05); #Significantly different from same dose administered acutely (P < 0.05).

Fig. 5.

THC and CBD effects in male and female rats on the incapacitance (weight-bearing) test on day 4 (means of day 4 time course data are shown). In the THC experiment (A and B), weight-bearing on the inflamed paw was ∼60% BL in rats treated with vehicle, indicating pain-induced suppression of weight-bearing. THC produced small increases in weight-bearing on the inflamed paw in females (B) but not males (A); increases in weight-bearing in females were larger when THC was given repeatedly compared with acutely (B). In the CBD experiment (C and D), weight-bearing on the inflamed paw was ∼65% BL in rats treated with vehicle, indicating pain-induced suppression of weight-bearing. CBD produced a small increase in weight-bearing on the inflamed paw that was similar in both sexes (C and D). Each bar is the mean ± 1 S.E.M. of 8–12 male or female rats. *Significantly different from same-sex, vehicle-treated controls (P < 0.05); #significantly different from same dose administered acutely (P < 0.05).

Fig. 6.

THC and CBD effects in male and female rats on paw thickness (edema) on day 4 (measured 240 minutes postinjection). In the THC experiment (A and B), paw thickness was ∼205% BL in rats treated with vehicle, indicating edema. THC did not alter paw thickness in either sex. In the CBD experiment (C and D), paw thickness was ∼220% BL in rats treated with vehicle, indicating edema. CBD decreased paw thickness at the highest dose tested, 10 mg/kg, similarly in both sexes (significant effect not shown; see Supplemental Fig. 6 for data pooled across sexes). Each bar is the mean ± 1 S.E.M. of 8–12 male or female rats.

Fig. 7.

THC and CBD effects in male and female rats on the locomotor activity test on day 4 (means of day 4 time course data are shown). In the THC experiment (A and B), locomotor activity in rats that received vehicle was ∼45% BL, indicating pain-suppressed behavior. THC decreased locomotor activity similarly in both sexes (significance not shown; see Supplemental Fig. 7 for data pooled across sexes). In the CBD experiment (C and D), locomotor activity in rats that received vehicle was ∼40% BL, indicating pain-suppressed behavior. CBD 2.5 mg/kg increased locomotor activity similarly in both sexes (significance not shown; see Supplemental Fig. 7 for data pooled across sexes). Each bar is the mean ± 1 S.E.M. of 8–12 male or female rats.

Fig. 8.

Body weight during the course of Experiment 1 in male (A and C) and female (B and D) rats treated repeatedly with THC (A and B) or CBD (C and D). CFA decreased body weight in vehicle-treated rats of both sexes to ∼95% BL. Male and female rats treated with THC repeatedly weighed significantly less than rats treated with vehicle, whereas CBD treatment restored day 1 body weight in both sexes. Each point is the mean ± 1 S.E.M. of 8–12 male or female rats.

Fig. 9.

Experiment 2: serum cytokine concentrations in male (left panels: A, C, D, F, G, and I) and female (right panels: B, D, F, H, and J) rats, at 240 minutes postinjection on day 4. (A and B) CFA increased TNF-α in males (A) but not females (B); THC did not alter TNF-α, but CBD decreased TNF-α in only healthy males. (C and D) CFA increased IL-1β in males (C) but not females (D); repeated THC increased IL-1β in males, but subsequent analysis showed this effect was not significant in either mineral oil– or CFA-treated rats. THC did not alter IL-1β in females. CBD decreased IL-1β similarly in healthy males and females. (E and F) CFA increased IL-6 in males (E) but not females (F); THC did not alter IL-6, and CBD increased IL-6 only in healthy males and CFA-treated females. (G and H) CFA did not affect serum IL-10; THC effects on IL-10 were sex- and pain state–dependent, and CBD decreased IL-10 similarly in both sexes, and post hoc analysis showed this was significant only in mineral oil–treated rats. (I and F) CFA did not affect serum IFN-γ; THC did not alter serum IFN-γ, whereas CBD reduced IFN-γ in both sexes. Each bar is the mean ± 1 S.E.M. of four to eight male or female rats. *Significantly different than same-sex, mineral oil + vehicle–treated control group (P < 0.05); ^#significantly different than same-sex, CFA + vehicle–treated group (P < 0.05).