Synergistic interactions of endogenous opioids and cannabinoid systems

Abstract

Cannabinoids and opioids are distinct drug classes historically used in combination to treat pain. Δ⁹-THC, an active constituent in marijuana, releases endogenous dynorphin A and leucine enkephalin in the production of analgesia. The endocannabinoid, anandamide (AEA), fails to release dynorphin A. The synthetic cannabinoid, CP55,940, releases dynorphin B. Neither AEA nor CP55,940 enhances morphine analgesia. The CB1 antagonist, SR141716A, differentially blocks Δ⁹-THC versus AEA. Tolerance to Δ⁹-THC, but not AEA, involves a decrease in the release of dynorphin A. Our preclinical studies indicate that Δ⁹-THC and morphine can be useful in low dose combination as an analgesic. Such is not observed with AEA or CP55,940. We hypothesize the existence of a new CB receptor differentially linked to endogenous opioid systems based upon data showing the stereoselectivity of endogenous opioid release. Such a receptor, due to the release of endogenous opioids, may have significant impact upon the clinical development of cannabinoid/opioid combinations for the treatment of a variety of types of pain in humans.

Introduction

The cannabinoid/opioid interaction differs in that cannabinoids generally fall into two categories — those that enhance the antinociceptive effects of morphine in the spinal cord (Δ⁹-THC {THC} for example) and those that do not enhance spinally administered morphine (CP55,940 {CP55} for example) [48]. The mechanisms by which the cannabinoids produce antinociception are as yet unclear. We believe that our data indicate that the mechanism by which the cannabinoids produce antinociception involves dynorphin release spinally and that the `greater than additive effects' of the cannabinoids with morphine 1, 36 and the delta opioid, DPDPE, are due to the initial release of dynorphin A peptides and the subsequent breakdown of the dynorphin A to leucine enkephalin [28]. We hypothesize that the functional coupling of the mu/delta and mu/kappa receptors leads to enhanced antinociceptive effects of morphine and DPDPE by the cannabinoids. We envision cannabinoid-induced release of dynorphins as an indirect process due to the disinhibition of yet unknown neuronal processes. The localization of the cannabinoid receptors involved in dynorphin release are not known. We hypothesize that in the spinal cord, cannabinoids produce antinociceptive effects via the direct interaction of the cannabinoid receptor with Gi/o proteins resulting in a decreased c-AMP production [47], as well as hyperpolarization via interaction with specific potassium channels [3]. Thus, the cannabinoids may produce disinhibition by decreasing the release of an inhibitory neurotransmitter in dynorphinergic pathways. The net result of such an effect may be an increase in dynorphin release. The events which precede and follow the release of dynorphin remain unclear. The dynorphin most likely is a modulator of other `down-stream' systems (possible substance P release or interaction with NMDA-mediated events) which culminate in antinociception upon administration of cannabinoids. What has proved intriguing is the observation that cannabinoids differ in their interactions with dynorphins (and subsequently with mu and delta opioids) 16, 17.

THC appears to interact with the dynorphin A system 28, 44, while CP55 appears to interact with and release dynorphin B [27], although CP55 is clearly cross-tolerant to THC [6]. THC is not cross-tolerant to dynorphin B, but is cross-tolerant to the dynorphins of the `A' type [44]. In addition, as animals are rendered tolerant to THC, the levels of dynorphin A released are decreased. Thus, tolerance to THC involves a decrease in the release of dynorphin A [16].

The kappa antagonist, nor-binaltorphimine (nor-BNI), and dynorphin antisera block THC-induced antinociception, but do not block THC-induced catalepsy, hypothermia, or hypoactivity 28, 38, 43. In addition, the discovery of the bi-directional cross tolerance of THC and CP55 to kappa agonists using the tail-flick test [38] and to dynorphin A [44], indicates that cannabinoids interact in a yet-to-be-determined manner with kappa opioids. The attenuation of the antinociceptive effects of THC by antisense oligonucleotides to the cloned kappa-1 opioid receptor further implicates the release of endogenous kappa opioids in the mechanism of action of the cannabinoids [26]. Dynorphin antibodies block THC-induced antinociception, and prevention of the metabolism of dynorphin A (1–17) to dynorphin (1–8) or to leucine enkephalin prevents the enhancement of morphine-induced antinociception by the THC [28].

The endogenous cannabinoid, AEA, appears to differ from THC and CP55 in its lack of interactions with dynorphinergic systems 38, 44. Despite similarities in the profile of action to classical cannabinoids, distinct differences between AEA and other cannabinoids in terms of behavioral effects have been reported 28, 38, 44, 45. AEA appears to differ from the traditional cannabinoids in that it is not active following icv. administration in several behaviors which are characteristic of cannabinoids. Other differences between anandamide and THC have been observed in tasks involving learning and memory [13], drug discrimination [49] and modulation by agonists and antagonists of classical neurotransmitter systems [45]. AEA which is neither blocked by the kappa antagonist, nor-BNI, nor cross-tolerant to any dynorphins 37, 44, 45 is cross-tolerant to THC and CP55 and displaces binding of the traditional cannabinoids 4, 33, 37, 45. Anandamide fails to enhance the activity of any opioid and does not release dynorphin A 28, 44, 45.

Hence three cannabinoids, representing three different classes, induce antinociceptive activity via the cannabinoid receptor, yet differentially modulate dynorphinergic systems. These differences may reflect differences in the interactions of cannabinoids with the cannabinoid CB₁ receptor or activities of functional subtypes of the cannabinoid CB₁ receptor in the spinal cord. It is difficult to envision such diverse dynorphin release profiles for the drugs through actions at one receptor subtype. The mechanisms underlying the differential release of dynorphins by THC versus CP55 and AEA thus remains unknown.

Two distinct cannabinoid receptors have been cloned, the CB1 receptor which is predominantly located in the central nervous system [18], and the CB2 receptor which is found on immune cells and on peripheral tissues [21]. In addition, a splice variant of the CB1 receptor termed the CB1A receptor has been identified [34]. The discovery of the cannabinoid CB1 receptor antagonist, SR141716A [30] and the discovery of the first endogenous cannabinoid-like ligand, anandamide {AEA}, [4] greatly facilitated work with the cannabinoids and complements the discovery and cloning of the cannabinoid receptors. The newly described cannabinoid CB₂ receptor antagonist, SR144528 [31], will be of great help in elucidating cannabinoid receptor subtypes. Receptor-ligand binding studies have produced evidence suggesting the existence of cannabinoid CB₁ receptor subtypes [41]. We evaluated THC, CP55, and AEA alone and in combination with SR141716A (SR), a CB1 antagonist, in order to better characterize potential diversity in interactions of the cannabinoids with the cannabinoid (CB1) receptor. The effects of SR on AEA-induced antinociception were mixed. The maximum attenuation of AEA-induced antinociception (intrathecally administered, i.t.) by SR (i.t.) was only 38%. SR (administered intraperitoneally, i.p.) blockade of ANA was complete, but the AD₅₀ was nearly 15-fold higher than that required to block THC or CP55. In addition, SR (i.p. or i.t.) failed to block the hypothermic effects of AEA (i.t.), while completely reversing the hypothermic effects of THC (i.t.). Such data are suggestive of either a differential interaction of the cannabinoids at the CB1 receptor or the existence of subtypes of the CB1 receptor [46].

In addition to the use of the CB1 antagonist, SR141716A, in this paper we report the stereoselectivity of opioid peptide release by the administration of levonantradol, a synthetic cannabinoid which we have shown to produce antinociception and enhance the activity of morphine [48], and dextronantradol, its inactive stereoisomer. We chose this pair of stereoisomers since the drugs had been previously evaluated in several test systems in mice and rats. Clearly, many other stereoisomeric pairs of cannabinoids have been tested in other systems (see Ref. [15] for review) but most have not been tested via the i.t. route of administration and have not been evaluated for the ability to enhance the antinociceptive effects of morphine. We hypothesized that levonantradol, which was the most potent cannabinoid that we tested [48] would release more dynorphin and produce antinociceptive effects at lower doses in the rat than THC, while dextronantradol would not release dynorphin. Such work was designed to show the stereoselectivity and thus, receptor mediation, of the dynorphin release.