IUPHAR review- Preclinical models of neuropathic pain: Evaluating multifunctional properties of natural cannabinoid receptors ligands

Abstract

Neuropathic pain remains prevalent and challenging to manage and is often comorbid with depression and anxiety. The new approach that simultaneously targets neuropathic pain and the associated comorbidities, such as depression and anxiety, is timely and critical, given the high prevalence and severity of neuropathic pain and the lack of effective analgesics. In this review, we focus on the animal models of neuropathic pain that researchers have used to investigate the analgesic effects of cannabidiol (CBD) and Beta-Caryophyllene (BCP) individually and in combination while addressing the impact of these compounds on the major comorbidity (e.g., depression, anxiety) associated with neuropathic pain. We also addressed the potential targets/mechanisms by which CBD and BCP produce analgesic effects in neuropathic pain models. The preclinical studies examined in this review support CBD and BCP individually and combined as potential alternative analgesics for neuropathic pain while showing beneficial effects on depression and anxiety.

1. Introduction

Pain is defined as an unpleasant sensory and emotional experience associated with or resembling that associated with actual or potential tissue damage [1]. Pain, one of the most common reasons adults seek medical care, [2] has been linked to restrictions in mobility and daily activities,[3,4] anxiety and depression, [3] sleep deprivation, and reduced quality of life. [3,4] Chronic pain (Pain that persists for ≥ 3 months is defined as chronic[5]) contributes to an estimated $560 billion each year in direct medical costs, lost productivity, and disability programs. [6,7] Chronic pain, especially neuropathic pain, is the most severe form of chronic pain. People with chronic neuropathic pain often have hyperalgesia (decreased pain threshold), allodynia (extreme sensitivity to touch), and ongoing pain (sensations of pins and needles, shooting, burning, stabbing, and electric shocks). A lesion or disease of the somatosensory nervous system causes neuropathic pain. [8] Examples of the causes of neuropathic pain include diabetic neuropathy, Human immunodeficiency virus −1 (HIV-1) infection or Acquired immunodeficiency syndrome (AIDS), central nervous system disorders (e. g., stroke), shingles, chemotherapy drugs (Cisplatin, Paclitaxel, etc.), amputation, spinal nerve compression or inflammation, or trauma. [5].

Multiple mechanisms underlie neuropathic pain have been reported. [9] Potential peripheral mechanisms include ectopic activity in lesioned or adjacent nerves, neuromas, or dorsal root ganglia, whereas central mechanisms involve sensitization of the spinal cord, brain areas involved in pain, [10] and/or dysfunction of the descending pain modulatory system.[11–13].

Current drugs used for neuropathic pain management include gabapentinoids (e.g., Gabapentin and Pregabalin), tricyclic antidepressants (e.g., Amitriptyline), and selective serotonin–norepinephrine reuptake inhibitors (e.g., Duloxetine) as the first-line drugs for neuropathic pain. Lidocaine and Capsaicin are recommended as second-line drugs. However, these options for neuropathic pain management are often ineffective and have associated side effects. [14–17] Better outcomes can be achieved by developing new and improved therapeutics or more immediately by identifying favorable drugs that are already available or emerging as potential new analgesics.

There is a growing interest in the possible medicinal use of cannabis. There are more than 560 constituents have been identified in cannabis. [18] Besides the major psychoactive component delta-9-tetrahydrocannabinol (Δ⁹-THC), there are also non-psychoactive compounds such as cannabidiol (CBD) and Beta-Caryophyllene (BCP).

CBD is a minor cannabinoid found in the cannabis plant that has gained attention as a therapeutic agent over the past several years due to its lack of central nervous system (CNS) side effects. [19].

CBD in humans exhibits no evidence of abuse or dependence potential, and there is no evidence of public health-related problems associated with pure CBD.[20] The FDA-approved Epidiolex (CBD) oral (p.o.) solution for treating seizures associated with two rare and severe forms of epilepsy, Lennox-Gastaut syndrome, and Dravet syndrome, in patients two years of age and old.

Terpenes, the largest phytochemicals found in cannabis and other plants, are the source of various aromas and flavors. They are lipophilic compounds that easily cross membranes, including the blood-brain barrier. [21] BCP is a natural bicyclic sesquiterpene that is a common constituent in many essential oils, including those derived from Cannabis sativa. BCP is a natural selective agonist for the cannabinoid type 2 receptor (CB₂). [22] BCP is an FDA-approved food additive [23] that has several beneficial effects, such as analgesia, antioxidant, and anti-inflammation. [22,24–32].

In this review, we focus on the animal models of neuropathic pain that researchers have used to investigate the analgesic effects of CBD and BCP individually and in combination. We also address the beneficial effect of these compounds on the major comorbidities (e.g., depression, anxiety) associated with neuropathic pain. Finally, we addressed the potential targets/mechanisms by which CBD and BCP produce analgesic effects in neuropathic pain models.

2. Neuropathic pain animal models

First, we provided a brief description of the pain-like behavior tests used to examine the effect of CBD and BCP (Table 1).

Table 1.

Behavioral tests used to examine the effect of CBD and/or BCP in neuropathic pain models.

Test	Stimulus	Pain-like behavior
Von Frey	The filament is applied with increasing force (grams) until a paw withdrawal response is elicited. The force at which this response occurs is designated as the paw withdrawal threshold.	Mechanical allodynia [41]
Hargreaves	Infrared heat stimulus. A focused infrared beam is delivered to the plantar surface of the hind paw. The paw withdrawal latency (seconds) to this stimulus is recorded.	Heat hypersensitivity [42]
Acetone	The cooling stimulus is the acetone applied to the mid-plantar surface of each hind paw. Time spent elevating, licking, biting, and shaking the stimulated paw is recorded.	Cold allodynia [43]
Hot/cold-plate	Animals are placed on a hot plate (55 °C) or a cold plate (10 °C). The time latency for the animal to lick its right/left foot is measured.	Thermal hypersensitivity [44]
Vocalization	vocalizations were evoked by brief (10 s) innocuous (100–300 g/6 mm2) and noxious (1000–1500 g/6 mm2) stimuli applied to the left hind paw using a calibrated forceps connected to a force transducer whose output.	Vocalization [45]
CPP	Following the 3-day pre-conditioning period, the animals received the appropriate control paired with a randomly chosen chamber in the morning and the appropriate drug treatment paired with the other chamber 4 h later for the single trial conditioning. On test day, 20 h, animals were placed in the CPP box with access to all chambers, and their behavior was recorded for 15 min for analysis for chamber preference. For multi-trial conditioning, the animals underwent conditioning across six days with alternating treatment-chamber pairings.	Ongoing pain [46]

Second, we provided a brief description of the neuropathic pain models used to test the effect of CBD and BCP.

2.1. Spinal and sciatic nerve injury-related neuropathic pain models

The chronic constriction injury (CCI) is induced by four loosely applied ligatures proximal to the trifurcation of the sciatic nerve. [33] The spared nerve injury (SNI) model is produced by tibial and common peroneal nerve lesions; the sural nerve was kept intact. [34] The Partial nerve ligation (PNS) is induced by a tight ligation of one-third to half of the sciatic nerve. [35] The spinal cord injury (SCI) model consists of a laminectomy at the thoracic vertebral level followed by spinal cord compression. [36].

2.2. Chemotherapy-induced neuropathic pain models

Several chemotherapeutic agents have been found to induce neuropathic pain as a side effect. [61,62]. Among these chemotherapeutic agents, paclitaxel is routinely used for breast, ovarian, head, neck, and lung cancer patients. [62] Cisplatin (cis-diammine-dichloro-platinum II) is another chemotherapy for leukemia, lymphomas, breast, testicular, ovarian, head and neck, cervical cancers, and sarcomas. [63] The Paclitaxel neuropathic pain model consists of intraperitoneal (i.p.) administration of Taxol. [37] The Cisplatin neuropathic pain model consists of injection of Cisplatin (i.p.) one (3 mg/ kg), two (2 mg/kg), or three (1 mg/kg) times a week up to a cumulative dose of 15 or 20 mg/kg. [38].

2.3. Diabetes neuropathic pain model

The Streptozotocin (STZ) neuropathic pain model consists of subcutaneous (s.c.) injection of STZ). [39].

2.4. Human immunodeficiency virus −1 (HIV)- therapy-induced neuropathic pain

Anti-retroviral therapy (HAART), the most effective treatment for acquired immunodeficiency syndrome (AIDS), which contains nucleoside reverse transcriptase inhibitors (NRTIs) as an active component, such as 2′,3′-dideoxycytidine (ddc) can produce neuropathic pain. [40] The ddc neuropathic pain model consists of systemic administration of 2′,3′-dideoxycytidine (ddC) in animals. [40].

3. CBD, BCP, and neuropathic pain

3.1. CBD and neuropathic pain

The analgesic effect of CBD has been tested in several neuropathic pain models, summarized in Fig. 1.

Fig. 1.

Schematic representative of neuropathic pain models tested for the effect of CBD.

3.1.1. CBD and sciatic nerve injury-related neuropathic pain

Several groups have studied the effect of CBD on neuropathic pain resulting from sciatic nerve injury using different routes of administration. CBD (5 mg/kg) injected subcutaneously (s.c.) daily for seven days in rats, starting from day 15 after SNI induction produced an antiallodynic effect. [47] In the CCI model, intraperitoneal (i.p.) injection of CBD (5 mg/kg) produced an antiallodynic effect. [48] CBD administered orally also produced a dose-response (D-R) partial reversal of the CCI-induced reduction in mechanical allodynia and thermal hypersensitivity with a median effective dose (ED₅₀) of 51 and 38 mg/kg, respectively. [49] Repeated administration of CBD (2.5–20 mg/kg. p.o.) daily for seven days attenuated both thermal and mechanical hyperalgesia in a D-R manner in rats with CCI. [50] In mice with SNI, a single application of CBD (20 mg/kg, p.o.) alleviated mechanical hypersensitivity 4 h after administration. [51] CBD (10 mg/ml, p.o.) gelatin formulation self-administration over three weeks produced an antiallodynic effect in mice with PNS. [52] Direct administration of CBD (5, 10, and 30 μg) in the hind paw reduced mechanical allodynia in mice with CCI, with a peak of antinociceptive action 15 min after the administration of CBD. [53].

CBD was also tested for its effect on ongoing pain using conditioned place preference (CPP). [54] Rats treated with CBD (0.3, 3, and 10 mg/kg, i.p.) produced significant place preference for the CBD-paired chamber in CCI neuropathic pain model. This suggests that CBD relieves the ongoing pain. [54] This is important because it shows that CBD is effective against ongoing (CPP) and sensory (Von Frey test), at least in the CCI model.

In addition to systemic administration, the effect of central administration of CBD was also studied by several groups. The microinjection of CBD (5, 30, or 60 nmol) into the prelimbic cortex 21 days after CCI in rats showed an antiallodynic effect. [55] Using the same neuropathic pain model, intrathecal (i.t.) administration of CBD (100 nmol) significantly increased the mechanical paw withdrawal threshold. [56] In the PSN model, CBD (10 μg, i.t.) alleviated thermal and mechanical hypersensitivity in both male and female mice. [57].

3.1.2. CBD and spinal cord injury-related neuropathic pain

In the SCI neuropathic pain model, CBD (0.1–5.0 mg/kg i.p.) reduced mechanical hypersensitivity in a dose- and time-dependent manner with maximal reduction at 60 min post-injection for 3 mg/kg and 5 mg/kg doses in rats. [58] A dose 5 mg/kg was significantly more potent than other doses at 60–120 mins post-injection. [58] Similarly, another D-R study showed an antiallodynic effect of CBD (2.5–20 mg/kg, i.p.) in rats.[59].

The effect of CBD was also reproduced in a longitudinal study in the SCI model. [60] CBD (1.5 mg/kg, i.p.) injected for ten weeks reduced thermal sensitivity in mice. [60].

3.1.3. CBD and chemotherapy-induced neuropathic pain

The potential analgesic effect of CBD in chemotherapy induces neuropathic pain has been studied by several groups. [64] The chemotherapies used to test for the analgesic effect of CBD are Paclitaxel and Cisplatin.

Several groups using a wide range of doses reported the improvement of mechanical allodynia by CBD. For example, D-R studies of CBD (0.625–20.0 mg/kg, i.p.) showed attenuation of mechanical allodynia in Paclitaxel-induced neuropathic pain in mice. [65] Paclitaxel-induced mechanical allodynia was prevented by the administration of CBD (2.5–10 mg/kg, i.p.) in mice. [66,67] CBD (10 mg/kg, i.p.) treatment produced an antinociceptive effect on Paclitaxel-induced neuropathic pain using the Von Frey test in mice. [68] CBD (56, 100 mg/kg, i.p.) attenuated paclitaxel-induced allodynia in mice. [69].

The analgesic effect of CBD is maintained after chronic administration. CBD (10 mg/kg, i.p.) treatment twice a week for six weeks reduced paclitaxel-induced thermal hypersensitivity as determined by the hot and cold plate method in mice. [64].

In cisplatin-induced neuropathic pain, CBD (2 mg/kg, i.p.) reduced mechanical allodynia in mice. [70]. However, CBD had no significant impact on mechanical allodynia in cisplatin-induced neuropathic pain in mice across a dose range of 5–20 mg/kg, i.p. [71].

3.1.4. CBD and diabetes-induced neuropathic pain

In the STZ model, a single injection of CBD (3 mg/kg, i.p.) showed an antiallodynic effect for two hours after treatment in rats. The dose of 0.3 mg/kg of CBD showed effect only one hour after treatment, and no effect was found after the injection of the CBD at a dose of 0.1 mg/kg. [72] The antiallodynic effect of CBD is also reproduced after daily injection for 14 days (0.1, 0.3, and 3 mg/kg, i.p.).[72].

Using different routes of injection, mice receiving intranasal CBD produced thermal hypersensitivity and tactile allodynia in STZ mice. [73] This effect was maintained during CBD delivery and for the additional four assessments over two months following the discontinuation of CBD. [73].

In Table 2, we summarized the literature that studied the analgesic effect of CBD in animal models for neuropathic pain while addressing the dose, route, tests used, species, weight, age, and key outcomes.

Table 2.

Summary of the effects of CBD treatment in neuropathic pain animal models. The rows highlighted in green represent the chronic administration of CBD studies [47–60,64–77].

Neuropathic pain model	Dose and route	Specie/sex/weight/age	Test used	Key outcome	Ref
CCI	5–30 μg, paw	Male mice Swiss (30–40 g)	Von Frey	Attenuated mechanical allodynia	[53]
Paclitaxel	85.14 mg/kg, i.p.	Male and female Mice C57BL/6J (21,7–42.2 g)	Von Frey	Attenuated mechanical allodynia	[69]
Paclitaxel	10 mg/kg, i.p.	Male mice C57BL/6J (8–10 weeks)	Von Frey	Attenuated mechanical allodynia	[68]
Paclitaxel	10 mg/kg, i.p.	Male mice C57BL/6J (4–5 weeks)	Von Frey, Hargraves, hot and cold plate	Attenuated mechanical allodynia and thermal hypersensitivity	[64]
pSNL	Gelatin formulation (1 mg/15 ml/g) p.o.	Male mice C57BL/6J (4 months)	Ultrasound vocalization, Von Frey, hot-plate	Attenuated mechanical allodynia and thermal hypersensitivity	[52]
pSNL	10 μg, i.t.	Male and female mice C57BL/6J (8–12 weeks)	Hargreaves	Attenuated mechanical allodynia and thermal hypersensitivity	[57]
CCI	0.3, 3, 10, or 30 mg/kg, i.p.	Male rats Wistar (250g)	Von Frey, acetone, CPP	Reverse CPP	[54]
CCI	15, 30, and 60 nmol, prelimbic cortex	Male rats Wistar (100 g)	Von Frey	Attenuated mechanical allodynia	[55]
SCI	5–20 mg/kg, i.p	Female rats Wistar (200–220 g)	Von Frey	Attenuated mechanical allodynia	[59]
CCI	2.5–20 mg/kg, p.o.	Male rats Wistar (200–220 g)	Von Frey Hargreaves	Attenuated mechanical allodynia and thermal hypersensitivity	[50]
CCI	0.01–56, s.c.	Male mice C57BL/6J (8–9 weeks)	Von Frey and acetone	Attenuated mechanical allodynia and thermal hypersensitivity	[74]
SCI	2.5 mg/kg, i.p.	Female mice C57BL/6J (7–8 weeks)	Von Frey and acetone	Attenuated mechanical allodynia and thermal hypersensitivity	[60]
SNI	5 mg/kg, s.c.	Male rats Wistar (6 weeks)	Von Frey	Attenuated mechanical allodynia	[47]
Paclitaxel	5 and 10 mg/kg, i.p.	Male and female mice C57BL/6J	Vonfery and acetone	Attenuated mechanical allodynia	[67]
Paclitaxel	2.5 and 5 mg/kg, i.p.	Female mice C57BL/6J (19–20 g)	Von Frey	Attenuated mechanical allodynia	[66]
Paclitaxel	1 and 10 mg mg/kg i.p.	Male mice C57BL/6J (18–20 g)	Von Frey	Attenuated mechanical allodynia	[65]
Cisplatin	2 mg/kg, i.p.	Male mice C57BL/6J (25–30 g)	Von Frey	Attenuated mechanical allodynia	[70]
STZ	2 mg/kg, intranasal	Male mice CD1 (25–30 g)	Von Frey and Hargreaves	Attenuated mechanical allodynia and thermal hypersensitivity	[73]
STZ	0.1 and 3 mg/kg, i.p.	Male rats Wistar (180–220 g)	Von Frey	Attenuated mechanical allodynia	[72]
CCI	5 mg/kg. i.p.	Male Mice Swiss (25.35 g)	Von Frey	Attenuated mechanical allodynia	[48]
Cisplatin	5, 10, or 20 mg/kg), i.p.	Male and female mice C57BL/6J (10–12 weeks)	Von Frey	No effect	[71]
CCI	100 nmol, i.t.	Male mice C57BL/6J (8–12 weeks)	Von Frey and acetone	Attenuated mechanical allodynia	[56]
CCI	100 mg/kg, p.o.	Male mice C57BL/6J (8–10 weeks)	Von Frey and acetone	Attenuated mechanical allodynia and thermal hypersensitivity	[49]
SNL	100 ug, i.t. DH-CBD	Male mice C57BL/6J (8 weeks)	Hargraves	Attenuated thermal hypersensitivity	[75]
CCI	10–100 mg/kg, p.o.	Male rats Sprague Dawley (10 weeks)	Von Frey	Attenuated mechanical allodynia	[76]
SNI	20 mg/kg, p.o.	Male mice C57BL/6J (8–10 weeks)	Von Frey	Attenuated mechanical allodynia	[51]
SCI	0.1–5 mg/kg, i.p.	Male and female rats Sprague Dawley (140–200 g)	Von Frey and acetone	Attenuated mechanical allodynia and thermal hypersensitivity	[58]
CCI	25 mg/kg, p.o	Male and female Sprague Dawley (70–200 g)	Von Frey	Attenuated mechanical allodynia	[77]

3.2. BCP and neuropathic pain

The analgesic effect of BCP has been tested in several neuropathic pain, summarized in Fig. 2.

Fig. 2.

Schematic representative of neuropathic pain models tested for the effect of BCP.

3.2.1. BCP and sciatic nerve injury-related neuropathic pain

BCP administration (1, 5, and 10 mg/kg, p.o.) was tested in the PNL neuropathic pain model. [26] BCP produced an antiallodynic effect at lower doses (1 mg/kg and 5 mg/kg). At high doses, BCP showed a more enhanced antiallodynic effect. [26] However, only 1 mg/kg BCP reduced thermal hyperalgesia. [26] An analgesic effect of BCP was also reproduced in the CCI neuropathic pain model. [78,79].

3.2.2. BCP and spinal cord injury-related neuropathic pain

In the SNI neuropathic pain model, CBD was tested at 0.1–5.0 mg/kg i.p., in male and female rats. [58] CBD reduced mechanical hypersensitivity in males and females in a dose- and time-dependent manner [58]. However, CBD effects appeared to be shorter lasting in females, returning towards pre-drug baseline by at approximately 90–120 min after administration. [58].

3.2.3. BCP and chemotherapy induced neuropathic pain

CBD’s acute and chronic effect was tested in paclitaxel-induced neuropathic pain in mice. [28] Paclitaxel-treated animals with BCP (12.5, 25, or 50 mg/kg, p.o.) reduced the mechanical allodynia but did not affect thermal hypersensitivity. [28] A high dose of BCP (200 mg/kg, i.p.) produced a time-dependent antiallodynic effect. [80]. Daily administration of CBD (25 mg/kg, p.o.) for six days also reduced the mechanical allodynia. [28].

3.2.4. BCP and diabetes-associated neuropathic pain

BCP (10 mg/kg/60 μL, p.o.) mitigated pain-like behaviors (mechanical allodynia and thermal hypersensitivity) in the STZ neuropathic pain model in mice. [81].

3.2.5. BCP and HIV-1 therapy-induced neuropathic pain related

Some HIV-1 medications can induce pain independently of HIV-1. For example, zidovudine, zalcitabine, didanosine, and stavudine can induce neuropathic pain as a side effect. [82].

The effect of BCP in a model of HIV-1-related neuropathic pain (2′–3′-dideoxycytidine, ddc) has been studied. [83] Administration of ddC (25 mg/kg i.p.) produced a significant reduction in withdrawal threshold to mechanical stimuli (mechanical allodynia) in mice. [83] The oral administration of BCP (25 mg/kg) also prevented the development of ddC-induced mechanical allodynia. [83].

In Table 3, we summarize the literature that studied the analgesic effect of BCP in neuropathic pain models while addressing the dose, route, species, tests used, weight, age, and key outcomes.

Table 3.

Summary of the effects of BCP treatment in neuropathic pain animal models. The rows highlighted in blue represent the chronic treatment of CBD studies [26,28,58,78,80,81,83].

Neuropathic pain model	Dose and route	Specie/sex/weight/age	Test used	Key outcome	Ref
CCI	178 mg/kg, i.p.	Male mice C57BL/6J (18–35 g)	Von Frey and hot plate	Attenuated mechanical allodynia and thermal hypersensitivity	[78]
Paclitaxel	25 mg/kg, p.o.	Male mice Swiss (12.5–50g)	Von Frey and Hargreaves	Attenuated mechanical allodynia and thermal hypersensitivity	[28]
STZ	10 mg/kg/60 μL, p.o.	Male mice BALB/C	Von Frey and hot plate	Attenuated mechanical allodynia and thermal hypersensitivity	[81]
Paclitaxel	200 mg/kg, i.p.	Male and female mice CD-1 (5–8 weeks)	Von Frey	Attenuated mechanical allodynia	[80]
ddc	25 mg/kg, p.o.	Female mice BALB/c (8–12 weeks)	Von Frey, cold and hot plate	Attenuated mechanical allodynia and thermal hypersensitivity	[83]
SCI	10–50 mg/kg, i.p.	Male and female rats Sprague Dawley (140–200 g)	Von Frey and acetone	Attenuated mechanical allodynia and thermal hypersensitivity	[58]
PNL	1, 5 and 10 m/kg, p.o.	Male mice C57BL/6J (3–4 months)	Von Frey and Hargreaves	Attenuated mechanical allodynia and thermal hypersensitivity	[26]

Using different neuropathic pain models, injection routes, doses, and species, these preclinical data support an analgesic effect of CBD in neuropathic pain. Interestingly, several groups did not observe analgesic tolerance after chronic administration of CBD. This is important because neuropathic pain is a chronic condition that requires repeated treatment for an extended period.

4. Potential targets of the analgesic effect of CBD and BCP in neuropathic pain

4.1. Potential targets of the analgesic effect of CBD

In terms of pain, the best-characterized targets of CBD are the transient receptor potential vanilloid 1 receptor (TRPV1), the serotonin 1 A receptor (5-HT_1A receptor), and inflammatory mediators.

4.1.1. Role of 5-HT_1A

The role of 5-HT_1A has been studied in three neuropathic pain models.

In the STZ neuropathic pain, the pretreatment with the selective 5-HT1A receptor antagonist WAY 100135 (3 μg/rat, i.t.) attenuated the antiallodynic effect of CBD (3 mg/kg, i.p.) in rats.[72].

In the SNI model of neuropathic pain, WAY 100635 (2 mg/kg/day, s. c., for 7 days) attenuated the antiallodynic effect of CBD (5 mg/kg, s.c.) in mice. [47] Using a different selective 5-HT_1A receptor antagonist, NAN-190 (10 μg/paw), and a different neuropathic pain model, CCI, the antiallodynic effect of the CBD (30 μg/paw) was partially reversed in mice with CCI. [53] Using the same neuropathic pain model, the analgesic effect of central administration of CBD (30 nmol microinjected directly into the prelimbic cortex) was attenuated by the pretreatment with WAY-100635 in the prelimbic cortex in rats. [55].

In the chemotherapy neuropathic pain model, CBD (2.5–10 mg/kg, i. p.) prevented paclitaxel-induced mechanical allodynia. This effect was reversed by co-administration of WAY 100635.[66] The role of 5-HT_1A in the analgesic effect of CBD has been reproduced using the same neuropathic pain model and the same antagonist in mice. [69].

4.1.2. The role of TRPV_1.

The potential role of TRPV₁ in the analgesic effect of CBD has been studied in two neuropathic pain models, SZT and CCI.

The role of TRPV₁ in diabetic-related neuropathic pain (STZ) has been reported in rats. [47] The antiallodynic effect of CBD (5 mg/kg, s. c.) was prevented by daily injection of capsazepine (10 mg/kg/day, s.c.) for seven days. [47] The simultaneous administration of CBD (20 mg/kg, p.o.) and the TRPV1 receptor antagonist capsazepine (10 mg/kg, i.p.) reversed the antiallodynic effect of CBD in rats with CCI. [50] This effect was reproduced using another selective TRPV1 antagonist SB-366791 (16 μg/paw). [53] This antagonist partially reversed the CBD (30 μg/paw)-induced antiallodynic effect in the CCI neuropathic pain model in mice. [53].

4.1.3. Role of cytokines/glial cells

Inflammation has an important role in the induction and maintenance of chronic pain. [84–86] Increased levels of cytokines such as interleukin – 1 beta (IL-1β), interleukin-6 (IL-6), and Tumor necrosis factor α (TNFα) [87–89] and glial cells activation [90,91] are involved in chronic pain. Several reviews have addressed the effect of CBD on the immune system and inflammation. [92–94] In this review, we will focus on literature that supports the effect of CBD on proinflammatory cytokines in neuropathic pain models.

The effect of CBD on cytokines and glial cells has been studied in four neuropathic pain models.

In the SCI neuropathic pain model, the treatment of CBD (1.5 mg/kg, i.p.) significantly reduced interleukin-23 (IL-23), CXCL-9, CXCL-11, and interferon-γ (IFN-γ) gene expression in the spinal cord of mice. [60].

In the Paclitaxel-induced neuropathic pain model, CBD (10 mg/kg, i. p.) modulates spinal proinflammatory cytokines. [68] Quantification of IL-1β and TNF-α levels in the spinal cord (L4–L6 segments) of the mice using enzyme linked-immunosorbent assay (ELISA) showed that TNF-α and IL-1β levels were prevented by pretreatment with CBD. [68]. However, other groups have reported no effect of CBD in TNFα and IL6. [50,60] CBD (20 mg/kg, p.o.) did not significantly affect the TNFα levels in the lumbar region and dorsal root ganglia of rats with CCI using ELISA.[50]. Similarly, CBD did not affect the spinal cord TNF-α and IL6 gene expression in neuropathic pain associated with SCI. [60].

In addition to the cytokines, the effect of CBD on glial cells in neuropathic pain models has been reported. The pharmacological intervention showed that administration of the microglia activation inhibitor, minocycline (1000 ng dose, i.t.), and the astrocyte inhibitor, FC (1000 pmol dose, i.t), attenuated CBD (10 mg/kg, i.p) the antiallodynic effect in Paclitaxel-induced neuropathic pain in mice. [68].

The role of the glial cells in the analgesic effect of CBD in neuropathic pain is supported by other groups. Immunoreactivity assessment of Glial fibrillary acidic protein (GFAP) and (Ionized calcium binding adaptor molecule 1 (Iba-1) in the anterior cingulate cortex (ACC), complex basolateral amygdala (BLA), granular layer of the dentate gyrus (GrDG), and CA1 region of the hippocampus in CCI neuropathic pain model showed a reduced GFAP and Iba-1 expression in rats with CCI treated with CBD (3 mg/kg, i.p.). [54] Complementary results showed that CBD (100 mg/kg, p.o.) inhibits CCI-induced microglia activation at the spinal level in rats. [76].

4.1.4. Other targets

Mice treated with CBD (10 mg/kg, i.p) inhibited the spinal expression of Toll-like receptors-4 (TLR4) in Paclitaxel-induced neuropathic pain. [68] In the SCI neuropathic pain model, CBD (1.5 mg/kg, i.p.) also reduced TLR-4 spinal cord gene expression in mice. [60].

Recently, it has been reported that FKBP5 (also known as FKBP51), an FK506-binding protein (FKBP) belonging to the family of immunophilins, is involved in the CBD (100 mg/kg, p.o.) analgesic effect in the CCI neuropathic pain. [76].

The role of the endocannabinoid system in the analgesic effect is still controversial. In the chemotherapy neuropathic pain model, CBD (2.5–10 mg/kg, i.p.) prevented paclitaxel-induced mechanical allodynia, and cannabinoid type 1 receptor (CB₁) antagonist SR141716 (3 mg/kg, i.p.) or the CB₂ antagonist SR144528 (3 mg/kg, i.p.) did not reverse this effect in mice. [66] Similar results were observed in STZ-related neuropathic pain. [72] The antinociceptive effect of CBD (3 mg/kg, i. p.) in rats with STZ-related neuropathic pain was not altered by the pretreatment with CB₁ or CB₂ receptor antagonists (AM251 and AM630, respectively; both at a dose of 1 mg/kg, i.p.). [72].

However, the cannabinoid CB₂ receptor antagonist AM630 at the dose of 4 μg (i.t.) reversed the CBD-induced antinociception in mice in the Paclitaxel-induced neuropathic pain model. [68] Additionally, a significant increase in spinal levels of anandamide (AEA) and 2-Arachidonoylglycerol (2-AG) was observed after CBD treatment. [68].

Finally, the treatment of CBD (1.5 mg/kg, i.p.) significantly reduced the inducible nitric oxide synthase (iNOS) gene expression in the spinal cord in mice SCI. [60].

A summary of the potential targets by which CBD may induce analgesic effects in neuropathic pain models is shown in Fig. 3.

Fig. 3.

Schematic representative CBD targets for neuropathic pain.

4.2. Potential targets of the analgesic effect of BCP in neuropathic pain

BCP was determined to be a selective CB₂ receptor agonist with moderate affinity (155 nM).[22].

BCP inhibits neuropathic pain through the CB₂ receptor. [26] Using the CB₂ knockout mouse approach, chronic BCP (1 mg/kg, p.o.) treatment did not attenuate PNL-induced mechanical allodynia and thermal hyperalgesia in the CB₂ knockout mice compared to wild-type. [26].

BCP (1 mg/kg, p.o.) treatment prevented astrocytosis and reduced microgliosis.[26] In the chemotherapy neuropathic pain model, paclitaxel, BCP (25 mg/kg, p.o.) was found to prevent paclitaxel-induced microglia upregulation [28] and reduced the spinal cord levels of IL-1β and monocyte chemoattractant protein-1 (MCP-1) levels in mice. [28] Substance P and cytokines such as IL-1β, TNF-α, and IL-6 were also attenuated by BCP (10 mg/kg, p.o.) in the STZ-neuropathic pain model in mice. [81].

In the CCI mice neuropathic pain model, Trans-BCP (10 mg/kg, p.o.) was effective at reducing the IL-1β levels in the sciatic nerve. [95] In the ddc mice neuropathic pain model, BCP (25 mg/kg, p.o.) significantly decreased the proinflammatory cytokine (IL-1β, IFN-γ) mRNA in the paw skin and the brain. [83].

Recently, it has been reported that BCP produced an analgesic effect in Paclitaxel neuropathic pain model in mice via the adenosine A_2A receptor (A_2AR). [80] The pretreatment with adenosine A_2AR selective antagonist istradefylline (3.2 mg/kg, i.p.) decreased the antiallodynic effect of BCP (200 mg/kg, i.p.).[80].

A summary of the potential targets by which BCP may induce analgesic effects in neuropathic pain models is shown in Fig. 4.

Fig. 4.

Schematic representative BCP targets for neuropathic pain.

Neuropathic pain condition is a complex condition that involves several pathways and targets. For this reason, an approach that affects various targets is reasonable for this pain condition. [95] Based on the preclinical data, various targets are involved in the analgesic effect of CBD and BCP in neuropathic pain. This makes these compounds suitable for neuropathic pain.

5. CBD, BCP, and depression and anxiety

Chronic pain is frequently associated with anxiety and depression. The comorbidity of pain and depression has been well-established in the literature. [96–98] On average, between 30% and 60% of pain patients report comorbid depression. [98] It is estimated that around 50% of patients with depression report experiencing some physical pain symptoms. [99]. Anxiety is reported to occur in up to 45% of patients with pain. [100].

The anxiety-like behavior was assessed using the elevated plus maze (EPM), open field (OF), and Marble burying (MBT) test in mice. BCP (50 mg/kg, i.p.) showed improvement in all the parameters observed in the EPM test. [101] BCP increased the time spent in the center of the arena without altering the general motor activity in the OF test. This dose also decreased the number of buried marbles and time spent digging in the MBT. [101] Depression-like behaviors were assessed using novelty-suppressed feeding (NSF), tail suspension test (TST), and forced swim tests (FST). The systemic administration of BCP (50 mg/kg, i.p.) reduced immobility time in the TST and the FST. Finally, BCP treatment decreased feeding latency in the NSF test. [101].

The antidepressant effect of BCP was reproduced by other groups. [102,103] For example, depression-like behaviors were reduced by BCP (25, 50, 100 mg/kg, i.p.) using tail suspension test and FST. [104] oral administration of CBD (10 mg/kg/60 μL) attenuated depression-like behavior assessed with the tail suspension. [81].

The CBD antidepressant and anxiolytic effects using the most common standard animal models for depression and anxiety, various routes of injections, various doses, acute and chronic effects, mice, and rats, and have been previously reviewed in detail. [105] For great interest, two groups were able to show that CBD simultaneously attenuates neuropathic pain-like behaviors and anxiety in neuropathic pain models, SNI and CCI. [47,54] CBD (5 mg/kg, s.c.) treatment reverses SNI–induced anxiety-like behavior in the elevated plus maze test in mice. [47] CBD (0.3–10 mg/kg, i.p.)-induced anxiolytic-like effects in CCI rats in the OF test.[54].

While CBD and BCP showed analgesic effects in several neuropathic pain animal models, these compounds also have antidepressant and anxiolytic effects. This is important because it means that BCD and BCP have the potential to serve as analgesics while addressing the major morbidities associated with chronic pain, depression, and anxiety. The preclinical data provided in this review supports the idea that CBD and BCP can be beneficial for neuropathic pain while simultaneously addressing the associated depression and anxiety.

6. CBD and BCP in combination and pain

It has been suggested that compounds in cannabis plants function more efficiently in concert with each other rather than in isolation [24]. A concept is known as the entourage effect. Regarding pain, two publications studied the effect of CBD and BCP in combination in the pain models. [58,106]. Both showed that CBD and BCP in combination, can work in concert to mitigate pain. The combination of CBD and BCP in the SCI neuropathic pain model in rats showed a synergistic effect in mitigating neuropathic pain-like behaviors. [58].

Similarly, using an inflammatory pain model (formalin), the combination of CBD and BCP also synergistically reduced pain-like behaviors. [107] Further, to understand the mechanism by which these two compounds produce synergetic effects, proteomics analysis showed a unique anti-inflammatory pattern. [107] The number of inflammatory mediators affected by the combination was superior in comparison with the individual effect of CDB and BCP. Of great interest is that some inflammatory mediators are uniquely affected by the combination (e.g., EGF, ICAM-1, IGFBP-1, IGFBP-5, IGFBP-6, MMP-2, and MMP-3, VCAM1) compared to the individual effect of CBD and BCP. This combination did not produce any adverse effect on motor and coordination functions and body temperature. [107].

These preclinical data showed that CBD and BCP in combination can work in synergy to manage neuropathic pain.

7. Summary

Strong preclinical evidence of an analgesic effect of CBD and BCP is needed to move forward with the potential use of these compounds in clinics. The preclinical studies examined in this review are in support of CBD and BCP individually and combined as potential alternative analgesics for the treatment of neuropathic pain. The other advantages of these two compounds are:

• The safety profile of CBD and BCP and lack of abuse potential. The World Health Organization (WHO) announced that CBD in humans exhibits no evidence of abuse or dependence potential and that there is no evidence of public health-related problems associated with the use of pure CBD. CBD effects are not associated with central side effects. BCP is approved by the Food and Drug Administration (FDA) as a food additive. BCP is devoid of Δ⁹-THC-like psychotropic effects when administered in doses up to 200 mg/kg [108] and is non-toxic when chronically administered in doses up to 700 mg/kg/day.[109]

• The anxiolytic and antidepressant effects of CBD and BCP are of additional benefit for chronic pain conditions because anxiety and depression are known to be a feature of chronic pain in humans.

• The anti-inflammatory component of BCP and CBD is another strength of this combination in relieving chronic pain because neuroinflammation has an important role in the induction and maintenance of chronic pain.

The preclinical data in this review provide the rationale for continuing to study these two compounds and will set the stage for the drug-development process of CBD and BCP as a potential therapy for neuropathic pain and associated depression and anxiety.

Animal models are key tool for understanding various disease mechanisms and testing novel therapies. Animal models are essential in pain research due to practical and ethical concerns associated with human experimentation. However, the translation aspects of animal models have limitations that need to be taken into consideration. Regarding pain, preclinical research studies of neuropathic pain are mainly performed in young, male, healthy, genetically similar rodents. In contrast, in the clinical setting, neuropathic pain most commonly occurs in middle-aged or elderly patients with heterogeneous genetic backgrounds, and they are often female.

Abbreviation:

2-AG

2-Arachidonoylglycerol

5-HT_1A

Serotonin-1a (5-HT1A) receptors

A_2AR

Adenosine A_2A receptor

AEA

Anandamide

AIDS

Acquired immunodeficiency syndrome

BCP

Beta-Caryophyllene

CB1

Cannabinoid type 1 receptor

CB2

Cannabinoid type 2 receptor

CBD

Cannabidiol

CCI

Chronic constriction injury

CNS

Central nervous system

COX1/2

Cyclooxygenases 1/ 2

CPP

Conditioned place preference

ddc

2′–3′-dideoxycytidine

ED₅₀

Median effective dose

FDA

Food and Drug Administration

FST

Forced swim test

GFAP

Glial fibrillary acidic protein

HIV-1

Human immunodeficiency virus −1

i.p

Intraperitoneal

I.t.

Intrathecal

Iba-1

Ionized calcium binding adaptor molecule 1

ICAM-1

Intercellular Adhesion Molecule 1

IFNγ

Interferon-γ

IGFBP-1

Insulin-like growth factor-binding protein-1

IGFBP-5

Insulin-like growth factor-binding protein-5

IGFBP-6

Insulin-like growth factor-binding protein-6

IL-1β

Interleukin-1β

IL-23

Interleukin 23

IL-6

Interleukin-6

iNOS

Inducible nitric oxide synthase

MBT

Marble burying test

MCP-1

Monocyte chemoattractant protein-1

MMP-2

Matrix metalloproteinase-2

MMP-3

Matrix metalloproteinase-3

NRTI

Nucleoside reverse transcriptase inhibitors

NSF

Novelty-suppressed feeding

OF

Open field

P.o

Oral administration

PSN

Partial sciatic nerve ligation

S.c

Subcutaneous

SCI

Spinal cord injury

SNI

Spared nerve injury

STZ

Streptozotocin

TLR-4

Toll-like receptors-4

TNFα

Tumor necrosis factor α

TRVP1

Transient receptor potential vanilloid 1 receptor

VCAM-1

Vascular Cell Adhesion Molecule 1

Δ⁹-THC

delta-9-tetrahydrocannabinol