The Role of CB2 Receptor in the Recovery of Mice after Traumatic Brain Injury
Cannabis is one of the most widely used plant drugs in the world today. In spite of the large number of scientific reports on medical marijuana, there still exists much controversy surrounding its use and the potential for abuse due to the undesirable psychotropic effects. However, recent developments in medicinal chemistry of novel non-psychoactive synthetic cannabinoids have indicated that it is possible to separate some of the therapeutic effects from the psychoactivity.
We have previously shown that treatment with the endocannabinoid 2-AG, which binds to both CB1 and CB2 receptors 1 h after traumatic brain injury in mice, attenuates neurological deficits, edema formation, infarct volume, blood–brain barrier permeability, neuronal cell loss at the CA3 hippocampal region, and neuroinflammation. Recently, we synthesized a set of camphor-resorcinol derivatives, which represent a novel series of CB2 receptor selective ligands.
Most of the novel compounds exhibited potent binding and agonistic properties at the CB2 receptors with very low affinity for the CB1 receptor, and some were highly anti-inflammatory. This selective binding correlated with their intrinsic activities. HU-910 and HU-914 were selected in the present study to evaluate their potential effect in the pathophysiology of traumatic brain injury (TBI). In mice and rats subjected to closed-head injury and treated with these novel compounds, we showed enhanced neurobehavioral recovery, inhibition of tumor necrosis factor a production, increased synaptogenesis, and partial recovery of the cortical spinal tract. We propose these CB2 agonists as potential drugs for development of novel therapeutic modality to TBI.
Keywords : cannabinoids; CB2 receptor; HU-910; HU-914; traumatic brain injury
Today, cannabis is one of the most widely used plant drugs in the world. In line with therapeutic benefits, smoking cannabis produces a number of acute, dose-dependent psychotropic effects, such as ‘‘feeling high,’’ relaxation, and euphoria. In spite of more than 3500 citations in the PubMed on medical marijuana, there still exists much controversy surrounding its medical use and the potential for abuse due to the unwanted psychotropic effects. However, recent developments in medicinal chemistry of novel nonpsychoactive synthetic cannabinoids have indicated that it is possible to separate some of the therapeutic effects from the undesirable psychoactivity.
At least two types of cannabinoid receptors, CB1 and CB2, have been identified in mammals.1,2 Both receptors belong to the class of G-protein–coupled receptors and have a seven-transmembrane domain structure. Activation of either receptor blocks forskolininduced accumulation of intracellular cyclic adenosine 3¢,5¢- monophosphate and involves linkage to Gi and/or Go-proteins.3 CB2 receptor shares 44% protein identity with CB1,4 but they display a distinct tissue distribution, pharmacological profile, and physiological roles pattern.
In the central nervous system (CNS), the CB1 receptor is located at the terminals of central and peripheral neurons, where it usually mediates inhibition of several neurotransmissions.5 CB2 receptor is
widely distributed in cells of the immune system and was for many years referred to as ‘‘peripheral receptor’’ since it was initially cloned from a leukocyte cell line.2 Upon activation, CB2 receptor has been reported to alter immune functions like cell migration and cytokines release6 and the mechanisms underlying the CB1- induced psychotropic effects have not been attributed to the CB2 receptor. CB1 receptor is also shown to be associated with ondemand protection against acute excitotoxicity in central nervous system neurons and after brain injury.7,8 However a number of more recent studies have shown that in contrast to the previous claims, CB2 is also expressed in low levels in the CNS, where it appears more in microglia rather than in neurons.9 Thus, activation of CB2 with full agonists reduced, while antagonists worsened, post-traumatic inflammation.10 Surprisingly, modulating microglia activation with a CB2 inverse agonist was recently shown to rescue neuronal loss after mild TBI.11 These findings may have a significant impact on drug discovery and on our understanding of the biology of the endocannabinoid (eCB) system.
The cannabinoid receptors can be activated not only by plantderived and synthetic ligands, but also by endogenous derivatives of fatty acids, particularly arachidonic acid, produced in mammalian tissues and often referred to as ‘‘endocannabinoids.’’ The first endocannabinoids to be discovered were N-arachidonoyl ethanolamine (anandamide) and 2-arachidonoyl glycerol (2-AG),12–14 which activate,
with different affinities, both types of receptors. Later, a novel family of fatty-acids, amino acids (e.g., arachidonoyl-serine), also was identified as endocannabinoid-like compounds,15 which exhibit no
direct binding to these receptors, but probably have an indirect or allosteric effect,16,17 probably mediated via the CB receptors.18–21
Although considered to be located mostly in the immune system, CB2 receptors are now well recognized on resident inflammatory cells within the CNS, on microglial and dendritic cells,22,23 and on brain endothelial cells.24 Activation of these receptors attenuates the inflammatory response by inhibiting the release of proinflammatory mediators and by diminishing leukocyte chemotaxis and extravasation into the brain parenchyma.25 CB2 agonists also were found to decrease cytochrome-C release, inhibit apoptosis, and exert anti-inflammatory effects in a diverse range of animal models.26,27
Traumatic brain injury (TBI) is a major cause of mortality and disability worldwide, followed by long-term physical and cognitive consequences. TBI-induced deleterious cerebral effects involve a complex of primary and secondary damages due to glutamate toxicity, oxidative stress, ionic and metabolic imbalance, inflammation, and ischemia. The observation of the TBI-induced multifactorial pathology, along with the pharmacological profile of the eCBs, prompted us to investigate the effects of 2-AG after TBI. Our studies revealed that TBI-induces a 10-fold increase in 2-AG in the injured hemisphere 4 h after TBI, and that treatment with synthetic 2-AG attenuated edema formation, infarct volume, blood-brain
barrier permeability, neuronal cell loss at the CA3 hippocampal region, and neuroinflammation.8,21,28,29
Recently, we synthesized a series of camphor-resorcinol derivatives, which represent a novel series of CB2 receptor selective ligands. The synthesis of the novel ligands, interaction with CB receptors, and anti-inflammatory properties are described in details in the PhD thesis by Magid,30 as well as in patents,31–32 and its protective effects in hepatic ischemia/reperfusion injury were reported
by Horvath and colleagues.33
Most of the novel compounds exhibited potent binding affinities at the CB2 receptors in the low micromolar range, with up to 200- fold selectivity over the CB1 receptor. This binding discrimination correlated with their intrinsic activities in activating the CB2 receptors. We report now an evaluation of the effects of two compounds in this series, HU-910 and HU-914 (Fig. 1), in the pathophysiology of TBI.
Unlike the endogenous 2-AG, mentioned above, HU-910 is highly selective towards CB2 and was shown to attenuate oxidative stress, inflammation, and cell death associated with hepatic ischemia/ reperfusion injury. HU-914 is a low-affinity partial selective CB2 agonist, which demonstrated high anti-inflammatory properties in lipopolysaccharide (LPS)-stimulated primary macrophages. 30,33 Hence, we chose to evaluate these two compounds, which possess the target engagement and pharmacological requirements for the potential neuroprotective effect following TBI.