Cannabinoid receptors

Endocannabinoids bind to cannabinoid receptors as neurotransmitters:¹
Unlike other neurotransmitters, endocannabinoids do not act from the presynapse to the postsynapse, but are released from the postsynapse of the downstream neuron and bind to CB1R on presynaptic upstream GABAergic and glutamatergic² neurons. The presynaptic neuron then reduces or stops glutamate release, which reduces excitation. However, if the activation of the cannabinoid receptors lasts longer and affects a large number of neurons (e.g. during drug intake), this also terminates GABA signaling.² As GABA has a dampening effect, the activation level rises again and the excitatory processes increase again.³⁴

There are two cannabinoid receptors, CB1R and CB2R.⁵ CB3R has occasionally been mentioned as a third cannabinoid receptor, but this is unlikely to be the case.

• 1. CB1 receptor (CB1R)
  • 1.1. CB1R Occurrence
  • 1.2. What is regulated by CB1R
  • 1.3. Paths of action of CB1R
  • 1.4. CB1R for various disorders
  • 1.5. CB1R agonists and antagonists
    • 1.5.1. CB1R agonists
    • 1.5.2. Primarily peripherally active CB1R agonists
    • 1.5.3. CB1R antagonists
    • 1.5.4. Allosteric modulators of CB1R
      • 1.5.4.1. Positive allosteric modulators (CB1R-POM)
      • 1.5.4.2. Negative allosteric modulators (CB1R-NOM)
    • 1.5.5. Binding strength to CB1R
  • 1.6. What influences the CB1R effect
• 2. CB2 receptor (CB2R)
• 3. GPR55: probably not a CB3 receptor
• 4. Downregulation, upregulation, desensitization

1. CB1 receptor (CB1R)¶

The psychoactive effects of cannabis in the brain are mediated by the binding of its main component ∆9-tetrahydrocannabinol (THC) to CB1R.⁶ Selective CB1R antagonists trigger a withdrawal syndrome in cannabinoid-dependent animals.⁶⁷⁸

CBRs are G protein-coupled receptors (GPCRs) and are primarily coupled to Gi/o proteins. They inhibit adenylyl cyclase and promote mitogen-activated protein kinase (MAPK). CB1R is also coupled to specific ion channels via Gi/o proteins and acts directly on Gs proteins, which activate adelylate cyclase.⁹ CB1R also mediates autapic LTD via G(q) proteins.¹⁰

1.1. CB1R Occurrence¶

CB1Rs increase continuously during adolescence, reaching a maximum during puberty. After that, the number of CB1Rs decreases again.¹¹¹²³

Unlike CB2R, which occurs pre- and postsynaptically, CB1R is only found presynaptically.¹³

The CB1R (cannabinoid 1 receptor) is present in many organs, sometimes in low concentrations.
It is one of the most common receptors in the brain and the most abundant G protein-coupled receptor there.¹⁴
The CB1R finds itself:⁶¹⁵

• in the brain in:

  • Basal ganglia
    • Nucleus accumbens¹⁶¹⁷¹⁸
      • at presynaptic glutamatergic terminals¹⁶
        • from the PFC, the basolateral amygdala and the ventral hippocampus¹⁹ but not from the thalamus
      • on inhibitory GABAergic terminals of FSIs, but not on LTSIs, MSNs or CINs¹⁶¹⁹
    • Dorsal striatum¹⁶¹⁸ and ventral²⁰
      • on FSI and MSN collaterals and to a lesser extent on CIN and LTSI¹⁶
      • on GABAergic interneurons²⁰
      • in a large part of the large non-spiny cholinergic interneurons²⁰
      • on striatonigral neurons expressing substance P and dopamine D1 receptors²¹, which also preferentially express M4 muscarinic receptors²⁰
      • on striatopallidal neurons
        • that express enkephalin and dopamine D2 receptors²¹
        • that preferentially express M1 muscarinic receptors²⁰
      • presynaptic at glutamatergic terminals of afferent corticostriatal (VGLUT1-positive) and thalamostriatal (VGLUT2-positive) axons²²
    • in mouse dorsal striatum, TH immunogold particles were found in less than one percent of axonal profiles with CB1R immunoperoxidase labeling, indicating that CB1R expression in axon terminals of nigrostriatal dopaminergic neurons is very low²³
  • Globus pallidus²⁴
  • corticolimbic regions (involved in cognition, memory and motivation)
    • PFC²⁴
      • particularly in layers II, III and V and to a lesser extent in layer VI²⁵²⁰
      • In all cortical layers predominantly in GABAergic interneurons
        • namely in²⁶
          • Cholecystokinin-positive
          • vasoactive intestinal peptide-positive
          • Somatostatin-positive
        • CB1R on GABAergic neurons promote behavioral inhibition²⁷
        • CB1R on GABAergic terminals are more frequent than on glutamatergic terminals²⁸
      • To a lesser extent also in glutamatergic terminals and pyramidal neurons²⁶
        • CB1R expressed in cortical glutamatergic neurons promote novelty seeking²⁷
      • in noradrenergic terminals²⁹
      • CB1R in the vMPFC regulate the baroreflex³⁰
  • Caudatus putamen
    • CB1R in caudate and putamen form heteromers with adenosine A2A receptors. CB1R-mediated motor inhibition in the striatum is completely dependent on the activation of A2AR (in the form of CB1R-ADA2 heteromers) and is abolished by A2AR antagonists. The inhibitory effect of CB1R agonists on hyperlocomotion induced by the D2 agonist quinpirole was blocked by A2AR antagonists MSX-3 as well as by the CB1R antagonist rimonabant³¹
  • Midbrain (mesencephalon)
    • Nucleus entopeduncularis³²
    • Substantia nigra³²
    • VTA¹⁷
    • periaqueductal gray (PAG, involved in pain processing)³³
  • Spinal cord³²
  • limbic system (associated with stress reactions)
    • Hippocampus³²
      • paraventricular nucleus
      • especially on cholecystokinin-positive GABAergic interneurons³⁴
    • Amygdala³³
      • central amygdala³⁵
  • Cerebellum (cerebellum)³²³⁶
  • Diencephalon
    • Thalamus³³
    • Hypothalamus³³
  • Pyriform cortex (primary olfactory cortex)¹⁸
  • Brain stem
    • rostral ventromedial medulla (RVM, involved in pain processing)³³
    • Pons (in small quantities)
  • in oligodendrocytes in the white matter of certain brain regions³⁷³⁸ at least during brain development:³⁹
    • Corpus callosum, commissura anterior (commissura rostralis), stria terminalis, fornix, areas of the white matter of the brain stem

• peripherally in lower density in nerve and other tissues involved in the regulation of emotions, stress and responsiveness:⁴⁰

  • Axon terminals
  • Glial cells
  • Reproductive system (uterus, ovary, testicles, prostate)
  • some glandular systems (e.g. adrenal gland)
  • Adipose tissue
  • Hearts
  • Liver
  • Lung
  • Bone marrow
  • Thymus
  • Microcirculation

• especially on axon terminals.⁴¹ CB1Rs are subject to a permanent cycle of endocytosis and recycling (only) in the somatodendritic compartment. As a result, CB1Rs are continuously removed from the plasma membrane by endocytosis in the somatodendritic compartment, but not in the axons. As a result, this leads to a disproportionate occurrence of CB1Rs on axons.⁴²⁴³

  • CB1R is found more frequently on GABAergic than on glutamatergic presynaptic terminals^{4435 28} and on neurons expressing DRD1⁴⁵
  • Activation by endocannabinoids, which are synthesized and released in postsynaptic terminals in response to postsynaptic depolarization⁴⁶⁴⁷
  • this underlines their importance for inhibitory and excitatory regulation of the brain⁴⁸

• on membranes and mitochondria of astrocytes, where it is involved in the regulation of neuronal/synaptic function via crosstalk between astrocytes and neurons⁴⁹

CB1R form heteromers with:⁵⁰

• Dopamine receptors
  • postsynaptic heteromers.⁵¹
  • DRD2⁵²
• Adenosine A2A receptors
  • in caudate and putamen form heteromers with . CB1R-mediated motor inhibition in the striatum is completely dependent on the activation of A2AR (in the form of CB1R-ADA2 heteromers) and is abolished by A2AR antagonists. The inhibitory effect of CB1R agonists on hyperlocomotion induced by the D2 agonist quinpirole was blocked by A2AR antagonists MSX-3 as well as by the CB1R antagonist rimonabant.³¹
  • 5HT1A⁵²
  • 5HT2A⁵²
  • AT1⁵²
  • GPR55⁵²
  • SST5⁵²
  • OX1⁵²
  • OX2⁵²
  • μOR⁵²
  • δOR⁵²
  • CB2R⁵²
  • GPR55⁵²
  • CXCR4⁵²

1.2. What is regulated by CB1R¶

CB1R is involved in the regulation of:

• Coordination of movements⁵³
  • Motor function is co-regulated by endocannabinoids that act on the subthalamic nucleus. This effect is mediated by CB1R, which are located outside the subthalamic nucleus, presumably in the globus pallidus and the PFC, among others.²⁴
• Reward/motivation⁵⁴⁵⁵
• Spatial orientation⁵³
• Sensory perception (taste, smell, touch, hearing)
• Appetite⁵⁶⁵⁷⁵⁸
  • The CB1R antagonist rimonabant inhibited the effects of appetite stimulants (cocaine, morphine and food) and reduced interest in sucrose, beer and alcohol⁵⁹
  • Endocannabinoids modulate appetite and energy expenditure via CB1R. AEA has different effects on feeding behavior depending on the state of satiety and the CB1R location (peripheral or central):⁶⁰
    • Peripheral: The AEA reuptake inhibitor AM404 reduced food intake in partially satiated animals, not in unsatiated animals
    • Central: AM404 induced unrestrained eating (hyperphagia) in unsatiated animals. The inverse CBR agonist SR141716A suppressed this.
  • THC use is often reported to have an appetizing effect
  • Insulin mediates endocannabinoid–mediated LTD in dopamine neurons in the VTA⁶¹
    • Insulin suppresses glutamatergic input
    • The reduced arousal drive reduces the interest in food
  • Milk intake of the infant
    • CB1R antagonists prevent the milk intake of infants on the first to fourth day and can therefore lead to their death⁶²
      • The puppies find the nipples but cannot suckle⁶³
      • Licking openly offered milk works⁶⁴
      • Effect of rimonabant and VCHSR1 is equally strong, although rimonabant binds 14 times more strongly to CB1R than VCHSR1.⁶⁵ It is therefore questionable whether the effect is mediated via CB1R alone.
    • CB1R-KO mice only stop taking milk on the first day of life⁶², after which this normalizes⁶⁶
• Diuresis (different from CB2R)⁶⁷
• Mental performance
• Learning and memory⁵⁴⁶⁸⁶⁹
• Short-term memory
  • CB1R agonists impair short-term memory⁷⁰
• Motivation⁵³
• Fear^71727374
  • CB1R on cortical glutamatergic neurons reduce anxiety when activated⁷⁵
  • CB1R on GABAergic neurons slightly increases anxiety⁷⁵
    • The GABAergic pathway of endocannabinoids is involved in mediating the anxiolytic effect of benzodiazepines. Endocannabinoids can improve the anxiolytic effect of benzodiazepines.⁷⁶
  • Blockade of CB1R prevents the glucocorticoid-induced decrease in conditioned freezing behavior⁷⁷
    • The CB1R antagonist AM281 given i.p. as well as intra-hippocampally promoted contextual fear memory⁷⁸
  • Endogenous CB1R agonists have an anxiolytic effect and prevent the occurrence of pathological anxiety states⁷⁹⁹
    • CB1R activation (by AEA or 2-AG) leads to deletion of the fear memory⁸⁰
      • AEA had a primary effect on signal transmission in the amygdala
      • 2-AG induced a stronger DA release from the mesolimbic signaling pathway⁸¹
    • Endocannabinoids positively modulate avoidance behavior through increased dopamine release
      • The inverse CB1R agonist rimonabant dramatically reduced dopamine release in animal models of warning signal (light cue) and resulting active avoidance of electric foot shocks; similarly, inhibition of 2-AG⁸²
    • The high-dose AEA reuptake inhibitor AM404 inhibited contextual fear memory⁷⁸
    • In contrast, 2-AG promoted the expression of conditioned fear via CB1R binding on GABAergic neurons⁷⁵
    • The CB1R antagonist SR141716 increased conditioned freezing⁷⁵
    • The CBR2 antagonist AM630 reduced conditioned freezing⁷⁵
    • The TRPV1 antagonist SB366791 did not affect freezing⁷⁵
    • The CBR2 agonist JWH133 did not affect freezing⁷⁵
    • The CBR1 and CBR2 agonist CP55.940⁸³ increased conditioned freezing⁷⁵
    • The endocannabinoid reuptake inhibitor AM404 reduced conditioned freezing⁷⁵
    • The endocannabinoid reuptake inhibitor VDM11 did not affect freezing⁷⁵
    • Co-administration with SR141716 or SB366791 confirmed involvement of CB1 and TRPV1⁷⁵
    • The AEA degradation inhibitor URB597 reduced the conditioned freezing⁷⁵
    • The 2-AG degradation inhibitor JZL184 enhanced conditioned freezing⁷⁵
    • The anxiety-promoting effects of 2-AG are mediated via CB1R in GABAergic neurons⁷⁵
• Defense behavior⁸⁴
  • AM-404 increased the defensive behavior to the smell of an attacker in rats
  • However, the CB1R antagonist AM251 did not influence the defense behavior
• Risk behavior / learning to avoid risks⁸⁵
• Amphetamine-mediated LTD in the amygdala⁸⁶
  • Amphetamine-LTD is dose-dependently blocked by the cannabinoid CB1 receptor antagonist AM251. Neither dopamine, serotonin 1A, nor noradrenaline α2 nor GABA-B receptor antagonists affected amphetamine LTD.
• Social behavior⁸⁷
• Mood⁷¹
• Antidepressant effect⁸⁸
• Cramps
  • The ω-3 endocannabinoid DHEA (docosahexaenoyl ethanolamide) reduces susceptibility to seizures in mice by activating CB1R⁸⁹
  • Status epilepticus was reduced by AM404 and URB597, but not by AM251⁹⁰
• Compulsive behavior⁹¹
  • CB1R inhibits compulsive behavior, TRPV1 promotes compulsive behavior. AEA acts on both receptors.⁹²
  • The compulsive inhibitory effect of AEA diminished after 14 days, while that of the TRPV1 antagonist capsazepine was maintained.⁹²
• Addiction⁹³⁹⁴⁹⁵
  • Nicotine-dependent mice show reduced 5-HT(1A) receptors in the diencephalon:⁹⁶
    • AM251 (CB1R antagonist) reduced abstinence signs
    • AM-404 (which increases AEA) reduced immobility time in a dose-dependent manner
    • The 5-HT(1A) receptor antagonist WAY 100635 abolished the immobility-inhibiting effect of AM404
  • MA-404 alleviates withdrawal symptoms⁹⁷
• Pain⁹⁸⁹⁹
  • Diclofenac inhibits pain via CB1R, GPR55 receptors and mu-opioid receptors of mPFC and ventrolateral periaqueductal gray in the brainstem¹⁰⁰
  • Systemically administered cannabinoids have an analgesic effect via CB1R (but not via CB2R) via spinal 5-HT7 and 5-HT(2A) receptors¹⁰¹
  • CB1R influences migraine, cluster headaches¹⁰²
  • Headaches are associated with increased FAAH and AMT levels (in women) and decreased AEA levels¹⁰³
  • FAAH-KO mice show greatly increased anandamide levels and greatly reduced pain perception¹⁰⁴
  • AM404 reduced mechanical and cold hyperalgesia with minimal effects on motor coordination. AM251 significantly inhibited the analgesic effect of AM404. Capsazepine enhanced the analgesic effect. AM404 has an analgesic effect in the spinal cord, primarily via CB1R, not CB2R.¹⁰⁵
  • TRPV1 receptors appear to play a pain-increasing role in CCI rats¹⁰⁵
• Itching
  • URB597 and JZL184, but not AM404, attenuated serotonin-induced scratching. The CB2R antagonist SR144528 neutralized the inhibitory effect of URB597.¹⁰⁶
• Inhibition of excessive signal transmission by neurotransmitters
• Sexual behavior
  • Inhibition of ejaculation¹⁰⁷
  • CB1R antagonists (in the VTA) during copulation inhibit sexual satiety in male rats¹⁰⁸
• HPA axis / stress
  • Disorder of the CB1R gene (CNR1) caused hyperactivity of the HPA axis¹⁰⁹
  • Stress reduced CB1R¹¹⁰
  • Stress reduces anandamide (AEA) and increases AG-2¹¹⁰
  • Endogenous cannabinoids appear to inhibit the HPA axis via CB1R in the brain
  • Anandamide (AEA)
    • Lowers the corticosterone level in stressed animals⁷⁹
    • Reduces the endocrine stress response of the HPA axis¹¹¹
    • Reduces the release of noradrenaline caused by the stress hormone CRF¹¹² here: via AM-404, which increases AEA, and through the selective CB1R agonist ACEA. In contrast, the CB1R antagonist AM 241 increased CRF-induced noradrenaline release
    • The CBR1 and CBR2 agonist WIN55212-2 decreased corticosterone, while the endocannabinoid reuptake inhibitor AM404 as well as the endocannabinoid degradation enzyme inhibitor URB597 increased corticosterone¹¹³
  • Endogenous cannabinoids inhibit the basal and suppress the stress-induced activity of the HPA axis via the hypothalamus, pituitary gland and adrenal cortex. Stimulation of the central cannabinoid receptors leads to activation of the sympathetic nervous system⁷⁹
  • Activation of the peripheral CB1R inhibits noradrenaline release from the sympathetic terminals and adrenaline release from the adrenal glands⁷⁹
  • CB1Rs may not affect adaptation to daily chronic stress¹¹⁴
  • CB1R mediates the rapid downregulation of the HPA axis by GR agonists (endocrine feedback loop), here by dexamethasone¹¹⁵¹¹⁶ CB1R antagonists prevent downregulation.
  • Administration of the CB1R agonist WIN55,212-2 during singular stress in a PSTD mouse model:¹¹⁷
    • Administered intraperitoneally, prevented the trauma-related changes:
      • In the conditioning of inhibitory avoidance (IA)
      • In the conditioning of extinction
      • Acoustic startle response potentiation (ASR)
      • Inhibition of the HPA axis
    • Administered into the basolateral amygdala, prevented
      • The SPS-induced changes in IA and ASR.
    • The CB1R antagonist AM251 applied to the basolateral amygdala blocked the described effects of WIN55,212-2
• Sleep / Circadian rhythm¹¹⁸
  • CB1R antagonists or FAAH inhibition promote wakefulness
  • Elevated AEA levels, due to AEA administration¹¹⁹ or AMT reuptake inhibition¹²⁰, promote sleep
  • Lack of sleep strengthened the 2-AG rhythm¹²¹

CB1R show significant constitutive activity in vitro (i.e. spontaneous activation in the absence of ligands).⁴²

1.3. Paths of action of CB1R¶

Agonist-activated CB1Rs transmit in three different spatiotemporal waves:¹²²

• the first wave is transient (<10 minutes) and is triggered by heterotrimeric G proteins
• the second wave (>5 minutes) is mediated by β-arrestins
• the third and last wave occurs in intracellular compartments and could be triggered by G-proteins or β-arrestins

Activated CB1 receptors:

• inhibit GABA neurons (significant inhibition of evoked and spontaneous GABA-mediated synaptic events¹⁰³.)
  • Consequences: less inhibition of dopamine neurons. As a result, cannabinoids indirectly increase dopamine levels.
  • The altered sensitivity of the GABA synapses caused by stimulation of the CB1R resulted in increased GABA release. This indicates that the CB1R contributes significantly to the regulation of inhibitory transmission in the striatum.¹⁰³
  • However, the synthetic CB1R agonist HU210 did not cause a reduction in GABA transmission in the striatum, but led to a significant increase in GABA release¹²³
    • Either CB1Rs can promote the release of transmitters under certain circumstances, or HU210 mediates its effects via other pathways than CB1Rs.
  • While inhibitory synapses were predominantly homeostatically reduced after chronic inactivity, decreased endocannabinoid tone strengthened a subset of GABAergic synapses in the hippocampus in vitro, increasing the likelihood of GABA release¹²⁴
• inhibit the overactivity of pain regulation and thus have a pain-relieving effect
• THC as a CB1R agonist has a dopamine-increasing effect in the nucleus accumbens. Adenosine A2A receptor antagonists (e.g. caffeine) counteract this.¹²⁵
  • Since the CB1R is not present in the brainstem, which regulates the respiratory and cardiovascular systems, an overdose of THC should not lead to related deaths³
• CB1R (not TRPV1) mediates the amnestic effect caused by elevated AEA. AM-404 interfered with both memory consolidation and memory retrieval during contextual fear conditioning and abolished LTP induction.¹²⁶
• CB1R increase histamine¹²⁷
  • The CB1R agonists ACEA and mAEA increased histamine release from the posterior hypothalamus, where the histaminergic tuberomammillary nuclei (TMN) are located, as well as from the histaminergic projection areas nucleus basalis magnocellularis and dorsal striatum
  • AM404 in the TMN limited the increased histamine release on the TMN
  • The CB1R antagonist AM251 blocked the release of histamine
• A CB1R agonist reduced and a CB1R antagonist increased AEA in the hypothalamus¹²⁸

THC causes overstimulation of the CB1R, which impairs its ability to regulate food intake, metabolism, cognitive processes and pleasure. This leads to a reduction in memory and motivation and, in the long term, to addiction. Consequences, heavy long-term cannabis use is associated with an increased risk of mental disorders, including psychosis, addiction, depression, suicidality and impaired cognition and motivation.⁶

Long-term agonist administration initially leads to desensitization and later (with the involvement of clathrin-coated pits) to internalization of CB1R. In the absence of an agonist, the receptor is brought back to the cell surface.¹²⁹

CB1R show constitutive receptor activity: Exogenous expression of CB1R alone in neurons of the superior cervical ganglion suppressed Ca2+ current, even in the absence of CB1R agonists.¹³⁰

CB1R effect over time:

• tens of seconds¹⁶
  A brief (5 to 10 seconds) postsynaptic depolarization causes short-term depression (STD - depolarization-induced suppression of inhibition (DSI) or excitation (DSE))
  • of the glutamate input in MSN or FSI or
  • of the GABAergic input of FSI in MSN.
  • is produced by retrograde endocannabinoid mobilization at presynaptic CB1R receptors and enables fine-tuned regulation of ongoing presynaptic input
• long-term effect: long-term depression (LTD)¹⁶
  • of CB1R-expressing
    • glutamatergic terminals from the PFC, the basolateral amygdala and the ventral hippocampus
    • GABAergic terminals from CB1-expressing FSIs

CB1R act in the opposite direction to the D2R.²³

1.4. CB1R for various disorders¶

In epilepsy, a lack of cannabinoid receptors appears to lead to overexcitation.³ Paracetamol was able to terminate the status epilepticus in nerve cells in vitro, which was mediated via the CB1R.¹³¹
Huntington’s disease is associated with a severe loss of CB1R.¹⁰³
Parkinson’s disease (destroyed dopamine neurons) shows increased CB1R expression in the basal ganglia^{132133 134} and increased anandamide levels¹³⁵. Anandamide levels are also elevated in ADHD, but this is thought to result from altered anandamide degradation.¹³⁶ DAT-KO mice, on the other hand, show massively reduced anandamide levels, which indicates that anandamide is influenced by dopamine levels.¹³⁷
In our view, increased CB1R expression could theoretically be a neurophysiological explanation for the reduced risk of addiction from stimulants in ADHD, as these increase dopamine. However, this is contradicted by the fact that there is no known increased risk of THC addiction in Parkinson’s disease, which is also characterized by dopamine deficiency. It is conceivable that Parkinson’s is not characterized by a CB1R deficiency.

Omega 3 is said to increase CB1R.¹³⁸

1.5. CB1R agonists and antagonists¶

1.5.1. CB1R agonists¶

CB1R agonists are among others:

• 2-Arachidonylglycerol (2-AG)
  • complete⁴⁰ CB1R and CB2R agonist⁴¹³⁹
  • low bias for the inhibition of cAMP and the phosphorylation of extracellular signal-regulated kinase 1/2 (pERK1/2)¹²²
• ACEA (arachidonyl-2’chloroethylamide/N-(2chloroethyl)-5Z,8Z,11Z,14Z-eicosatetraenamide)¹⁴⁰
  • selective¹⁴¹²² / highly selective CB1R agonist⁵⁴
• ACPA (arachidonoylcyclopropylamide), selective CB1R agonist⁵⁴
• Anandamide (AEA, N-arachidonoylethanolamine)
  • Partial agonist of CB1R and CB2R¹⁰³, lower affinity for CB2R than for CB1R⁴⁰
  • It is possible that AEA only acts as a partial CB1R agonist without 2-AG and as a competitive antagonist in the presence of 2-AG ¹⁴²
    • With low receptor density or a limited number of post-receptor effectors, AEA (like THC) can antagonize the CB1R signals triggered by 2-AG.^14310144145
  • Bias at CB1R for the inhibition of cAMP¹²²
• AM2389 [9β-hydroxy-3-(1-hexyl-cyclobut-1-yl)-hexahydrocannabinol]⁶⁷
• AM-356 ((R)-methanandamide), a synthetic anandamide analog
• AM4054 [9β-(hydroxymethyl)-3-(1-adamantyl)-hexahydrocannabinol]⁶⁷
• BAY 59-3074 (3-[2-cyano-3-(trifluoromethyl)phenoxy]phenyl-4,4,4-trifluoro-1-butanesulfonate), partial agonist¹⁴⁶
• Catechins from the tea plant (Camellia sinensis)¹⁴⁷
• CRA13 (SAB-378, naphthalen-1-yl-(4-pentyloxynaphthalen-1-yl)methanone)
  • also CB2R agonist, limited CNS penetration
  • caused up to 90 percent reversal of hyperalgesia in neuropathic rat model of mechanical hyperalgesia, via CB1R¹²²
• CP 55940 / CP 55,940⁸³ ((-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxy-propyl)cyclohexanol)¹⁴⁸ Full agonist¹⁴⁹
  • Bias for the inhibition of cAMP¹²²
• HU210 (strong)¹²³ (HU = Hebrew University)
  • Bias for the inhibition of cAMP¹²²
  • also a strong CB2R agonist¹⁵⁰
• HU-320
  • anti-inflammatory; shows no psychotropic effect¹⁵¹
• mAEA (R(+)-methanandamide)¹²⁷
  • Bias for the inhibition of cAMP¹²²
• THC (Δ9-tetrahydrocannabinol), partial agonist¹⁵²
  • Δ9-Tetrahydrocannabinol has psychoactive metabolites, such as 11-hydroxy-THC¹⁵³
• WIN 55,212 [R-(1)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone mesylate]⁶⁷
  • low bias for the inhibition of cAMP and the phosphorylation of extracellular signal-regulated kinase 1/2 (pERK1/2)¹²²
• Yangonin from the kava plant (Piper methysticum)¹⁵⁴

1.5.2. Primarily peripherally active CB1R agonists¶

In practice, the use of CB1R agonists usually fails due to the psychotropic effect on CB1R in the brain. However, there are also CB1R agonists that do not have a psychotropic effect, mostly because they can only cross the blood-brain barrier to a limited extent:

• 10919 is a moderate affinity CB1R agonist (Ki = 143 nM, EC50 = 1440 nM) and CB2R agonist (Ki = 14 nM) with a moderate brain/plasma ratio of 0.74.¹²²
• AZ11713908 is a high affinity CB1R agonist (pIC50 8.4, pEC50 8.0) and CB2R agonist (pIC50 9.0) with 7% and 5% uptake in brain compared to plasma, respectively. AZ11713908 reduced mechanical allodynia (pressure and touch pain sensitivity) in the rat spinal nerve ligation model at 2.5 μmol/kg with 100% efficacy. Of WIN 55,212, this required 8 μmol/kg.¹²²
• AZD1940 is a high affinity CB1R agonist (pKi 7.93) and CB2R agonist (pKi 9.06). Brain-plasma partition coefficient in rats: 0.04. Unfortunately, AZD1940 showed only limited analgesic efficacy against capsaicin-induced pain and hyperalgesia and also mild to moderate adverse effects in the CNS (central cannabinoid side effects) and gastrointestinal tract. AZD1940 was not pursued further after Phase II studies.¹²²
• CRA13 is a CB1R and CB2R agonist with very limited CNS penetration.¹²²
• HU-320 is an anti-inflammatory CB1R agonist without psychotropic effects¹⁵¹

In addition, some endocannabinoids act as biased ligands, i.e. they only trigger certain G-protein signaling pathways at the receptor.¹²²

1.5.3. CB1R antagonists¶

CB1R antagonists are among others:

• Anandamide (AEA, N-arachidonoylethanolamine)
  • It is possible that AEA acts as a competitive antagonist in the presence of 2-AG and as a partial CB1R agonist only in the absence of 2-AG¹⁴²
    • With low receptor density or a limited number of post-receptor effectors, AEA (like THC) can antagonize the CB1R signals triggered by 2-AG.^14310144145
• AM251 (N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1-H-pyrazole-3-carboxamide)⁹⁹¹⁵⁵
  • reduces alcohol self-administration in mice¹⁵⁵
• AM 281²⁴
• AM4113 (neutral antagonist)¹⁵⁶
• AM6527¹⁵⁷
• LH-21 (almost exclusively peripherally active CBR antagonist)¹⁵⁸
• LY320135: Antagonist/inverse agonist¹⁵⁹
• O-2050¹⁵⁹
• O-Arachidonylethanolamide (Virodhamin)
  • partial CB1R antagonist¹⁶⁰
• Rimonabant (SR 141716A, N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride)
  • selective¹⁶¹²² CB1R receptor antagonist/partial agonist^{148131 162} inverse agonist¹⁶³
• VCHSR1 (5-(4-chlorophenyl)-3-[(E)-2-cyclohexylethenyl]-1-(2, 4-dichlorophenyl)-4-methyl-1H-pyrazole)
  • neutral CB1R antagonist, 1/14 of the binding of rimonabant¹⁶⁴

1.5.4. Allosteric modulators of CB1R¶

A brief explanation of how allosteric moderation works can be found in the article Neurological basics

• Org27569 (unknown whether POM or NOM)¹²²
  • Bias for the inhibition of cAMP by cannabinoid ligands¹²²
  • no influence on ERK1/2 phosphorylation of cannabinoid ligands¹²²

1.5.4.1. Positive allosteric modulators (CB1R-POM)¶

POM enhance the effect of agonists or inverse agonists.
CB1R-POM are among others:¹²²

• GAT-211
• GAT-228, (+)-enantiomer of GAT-211
• GAT-229, (-)-enantiomer of GAT-211
• ZCZ-011
• RTI-371¹⁶⁵
• Pregnenolone (non-selective CB1R-PAM)
• Lipoxin A4 (non-selective CB1R-PAM)
• Pepcan-12 (non-selective CB1R-PAM)

1.5.4.2. Negative allosteric modulators (CB1R-NOM)¶

NOM inhibit the effect of agonists or inverse agonists.
CB1R-NOM are among others:

• Org29647¹⁶⁶
• Org27759¹⁶⁶

1.5.5. Binding strength to CB1R¶

According to binding affinity in Ki (nM; lower is stronger):¹⁵⁰

HU210: 0.2
JWH-210: 0.5
CP47,497: 0.8
AB-FUBINACA: 0.9
AM2201: 1.0
JWH-073: 8.9
JWH-018: 9.0
XLR-144: 29.0
AEA: 32.0
THC: 41.0
2-AG: 472.0
CBD: 842.0

1.6. What influences the CB1R effect¶

In addition to activation by agonists and inhibition by antagonists, CB1R is also regulated by protein-protein interactions:¹³⁰

• G proteins
  • send CB1R signals to effector molecules (adenylyl cyclase, ion channels)
  • influence the affinity of the CB1R for agonists through negative heterotropic interaction
• CRIP1a (cannabinoid receptor-interacting protein 1a)¹⁶⁷
  • changes the preference for Gi/o subtypes from signaling via Gi3 and Go to signaling via Gi1 and Gi2
  • attenuates the agonist-mediated internalization of CB1Rs
  • inhibits the constitutive receptor action of CB1R, via which it suppresses the Ca2+ current in the absence of agonists in neurons of the superior cervical ganglion¹³⁰
• β-Arrestin 1 and β-Arrestin 2
  • change the cellular signaling patterns
  • desensitize the receptor-G-protein interaction
• GASP (GPCR-associated sorting protein) controls the transport of internalized CB1R to the lysosome, where they are degraded

2. CB2 receptor (CB2R)¶

The CB2R (cannabinoid 2 receptor) is, like the CB1RG protein-coupled, but is less frequently found in the brain⁹⁸ and is primarily associated with the immune system.
There are various isoforms of the CB2R:¹⁶⁸

• CB2RA
• CB2RB
• CB2RC
• CB2RD
  The CB2R finds itself:¹⁵⁴⁴
• in the brain in:
  • PFC
  • Striatum
  • Basal ganglia
  • Amygdala
  • Hippocampus¹⁶⁹
  • VTA
    • CB2Rs are more common in the VTA than in the nucleus accumbens or dorsal striatum in rodents¹⁶⁸
    • CB2Rs are also found on dopamine neurons in the VTA¹⁶⁸
  • Brain stem
  • Glial cells associated with neuritic plaques in Alzheimer’s brains
    • In the healthy brain, only a few CB2Rs are found in the microglial cells and astrocytes. Pathological conditions such as brain injuries, strokes or neurodegenerative diseases greatly increase CB2R expression.⁵⁴
  • Brain capillaries and microvessels
• peripheral
  • Immune cells¹²⁹
    • Mast cells
      • Palmitoylethanolamide, but not anandamide, inhibits the activity of mast cells via CB2R.¹⁷⁰
    • B cells
    • T-cells
    • Macrophages
    • Neutrophils

CB2R is involved in¹²⁹

• Pain regulation
  • acute pain and neuropathic pain
  • the CB2R count increases in neuropathic and inflammatory pain.⁹⁸
• Symptoms of anxiety and depression are mediated by CB1R and CB2R signaling.
  • Chronic administration of CB2R antagonists has an anxiolytic effect¹⁷¹
• neuroprotective effect
• Neuromodulation
• Osteoporosis
• Allergic dermatitis
• Liver problems
• Arteriosclerosis
• Multiple sclerosis
• no psychoactive effect (unlike CB1R)

CB2R agonists inhibit the firing of dopamine neurons and terminal dopamine release.^172173174

CP55,940 causes internalization of CB2R¹⁷⁵
WIN55, 212-2 and other aminoalkylindoles do not cause internalization of CB2R¹⁷⁵
WIN55,212-2 competitively antagonized the CB2R internalization induced by CP55,940¹⁷⁵

Echinacin contains N-isobutylamides, which are potent cannabinoid mimetics that bind to peripheral CB2R on immune cells, but not to CB1R in the CNS.¹⁷⁶
Various spice plants contain beta-caryophyllene, which binds to CB2R.¹⁷⁷
Immune reactions are not mediated by CB2R alone, but also by CB1R and TRPV1.¹⁷⁸

CB2R agonists are among others:

• 2-AG (very weak)¹⁵⁰
• A-836339 (2,2,3,3-tetramethyl-cyclopropanecarboxylic acid [3-(2-methoxy-ethyl)-4,5-dimethyl-3H-thiazol-(2Z)-ylidene]-amide), selective CB2R agonist¹⁷⁹
• ARA (very weak)¹⁵⁰
• AM1241 [1-(methylpiperidin-2-ylmethyl)-3-(2-iodo-5-nitrobenzoyl)indole]⁶⁷
• AM2201¹⁵⁰
• β-Caryophyllene (beta-caryophyllene)¹⁸⁰
  • also binds to PPARα and PPARγ, but not to CB1R¹⁸¹
• BAY 59-3074 (3-[2-cyano-3-(trifluoromethyl)phenoxy]phenyl-4,4,4-trifluoro-1-butanesulfonate), partial agonist¹⁴⁶
• CRA13 (SAB-378, naphthalen-1-yl-(4-pentyloxynaphthalen-1-yl)methanone)
  • also CB1R agonist, limited CNS penetration¹²²
• CBD (weak)¹⁵⁰
• CT-3 (ajulemic acid, lenabasum, AJA, IP-751, JBT-101, anabasum), selective CB2R agonist¹⁸² is a synthetic THC analogue¹⁸³
• CP 55940 / CP 55,940⁸³ ((-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxy-propyl)cyclohexanol)
  • also CB1 agonist¹⁴⁸
• GW405833, selective CB2R agonist¹⁸⁴
• HU210¹⁵⁰
• JWH018¹⁵⁰
• JWH073¹⁵⁰
• JWH133, selective CB2R agonist¹⁸⁵
• JWH210¹⁵⁰
• THC (partial agonist)¹⁵²
  • probably not psychoactive on CB2R
• Vicasinabine (RG7774)
  • selective CB2R agonist¹⁸⁶¹⁸⁷
• XLR-144¹⁵⁰

CB2 antagonists are among others:

• AM630 (6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl](4-methoxyphenyl) methanone)⁶⁷
• SR144528 (N-[(1S)-endo-1,3,3-trimethylbicyclo-[2.2.1]heptan-2-yl]5-(4-chloro-3-methyl-phenyl)-1-(4-methybenzyl)pyrazole-3-carboxamide)¹⁴⁸

According to binding affinity in Ki (nM; lower is stronger):¹⁵⁰

HU210: 0.4
JWH-210: 0.7
XLR-144: 2.1
AM2201: 2.6
JWH-018: 2.9
THC: 36.0
JWH-073: 38.0
CBD: 203.0
2-AG: 1,400.0
AEA: 1,932.0

3. GPR55: probably not a CB3 receptor¶

In addition to CB1R and CB2R, GPR55 was sometimes referred to as CB3R.¹⁰³¹⁸⁸ However, GPR55 has too little correspondence with CB1R and CB2R to be considered a CB receptor⁵⁹

More about medicinal cannabis as a medicine for ADHD at Medicinal cannabis for ADHD

4. Downregulation, upregulation, desensitization¶

Cannabinoid receptors are members of the GPCR family. GPCRs are often subject to dynamic changes in their activity.
A lack of agonists can lead to upregulation.
A prolonged excess of agonists can cause desensitization and downregulation¹⁸⁹
Agonists can trigger time- and temperature-dependent endocytosis (internalization). During endocytosis, some receptor proteins are reintegrated into the cell membrane (recycling), while others are sorted out and degraded by lysosomes, which reduces the number of receptors.¹⁹⁰¹⁹¹

Homologous desensitization
The receptor that has bound a ligand is phosphorylated by a kinase (G-protein-coupled receptor kinase, GRK). This phosphorylation stabilizes the binding between the receptor and arrestins. This prevents interaction with the G protein and interrupts signal transduction.

Heterologous desensitization
The signaling chain of a receptor is interrupted or reduced (independent of ligand binding) due to activation of other receptors on the cell surface. The activated receptors activate via second messenger kinases (e.g. protein kinase A, protein kinase C), which do not phosphorylate the activated receptor itself, but other receptors.

With long-term exposure (about 18 hours), delta9-THC also desensitizes the CB1R.¹⁴⁵