Endocannabinoids Flashcards
What are endocannabinioids?
- By 1990 researchers had cloned the gene for the
brain cannabinoid receptor (termed CB1 receptor) - Endogenous ligands for the cannabinoid
receptors (endocannabinoids) were first
identified in 1992
Describe CB1 and CB2
CB1 is expressed widely in the CNS
» CB2 is expressed primarily on immune cells
‒ Only microglia in the CNS
Putative CB3 receptor for CBD recently
Describe CB1 receptors
- Inhibitory through effects on adenylate
cyclase (↓ cAMP), GIRK (↑ K+ efflux),
and Ca2+-channels (↓ Ca2+ influx) - CB1 receptors are exclusively
expressed presynaptically
Deescribe endocannabinoids
THC is a specific partial agonist of the CB receptors.
Anandamide was first identified in 1992 and named
from the Sanskrit word ananda, meaning ‘bliss’.
2-AG was identified in 1994-5. These two
endocannabinoids are the best described in the
CNS.
Describe endocannbinoid synthesis
Endocannabinoids are formed from
arachidonoyl-containing phospholipids
(arachidonic acid is an essential ω6 fatty acid
found in peanut oil).
2-AG is synthesized in activity dependent
processes (such as Gq signalling to
phospholipase C - PLC) from the hydrolysis of
diacylglycerol (DAG) by DAG lipase (DAGL).
Describe retrograde signalling
- Endocannabinoids do not function as classic
neurotransmitters - Endocannabinoids are capable of free diffusion
through membranes - They cannot be packaged into secretory vesicles
- Primary function is retrograde signalling to
modulate presynaptic neurotransmitter release - Release from postsynaptic compartments is
through activity-dependent synthesis
mechanisms
Describe depolarization induced suppression of inhibition
- Post-synaptic depolarization can
activate endocannabinoid synthesis - Endocannabinoids are released and
supress presynaptic GABA release
Describe metabolism of endocannabinioids
- Two key enzymes are responsible for
the breakdown of endocannabinoids - Fatty-acid amide hydrolase (FAAH)
metabolizes anandamide - Monoacylglycerol lipase (MAGL)
metabolizes 2-AG
Describe endocannabinoid breakdown
- FAAH
- A common genetic variant in FAAH (P129T) decreases the rate of breakdown of anandamide (~20% of
North Americans) - Increased levels of anandamide throughout the brain
- FAAH P129T variant is associated with a greatly reduced risk of anxiety disorder and PTSD
- FAAH P129T mice show decreased anxiety in tests such as the elevated plus maze and more rapid
extinction of conditioned fear responses - Anxiety ↓ and fear extinction ↑ in humans carrying the mutation
- FAAH P129T is also overrepresented in problem users of street drugs, suggesting increased risk of
drug and alcohol abuse - BUT associates with decreased craving and withdrawal in cannabis users
Describe stress and anxiety
- Endocannabinoids are highly involved in regulation of the physiological response to
psychological stressors - Acute stress activates two distinct pathways:
- Neuronal response – activation of the sympathetic nervous system via descending pathways from
hypothalamus - Neuroendocrine response – activation of the hypothalamic-pituitary-adrenal axis (HPA axis) leading
to release of glucocorticoid hormones - In many systems anandamide functions as a tonic (steady-state) regulator of activity
while 2-AG functions as a phasic (on demand) regulator
Describe HPA axis regulation
Hypothalamic release of CRH leads to pituitary
release of ACTH into circulation, and stimulates
adrenal release of cortisol.
Negative feedback is provided by cortisol to
the hypothalamus to attenuate stress signalling
– this feedback process may be impaired in
depression and anxiety disorders.
The limbic system provides a number of
modulatory inputs to the hypothalamus that are
critical in HPA response to psychological
stressors.
describe limbic control of HPA axis
Under steady-state conditions
CB1 antagonism results in an
increase in glucocorticoid release
Direct microinjection into the
basolateral amygdala (BLA)
increases glucocorticoid release
Describe anandamine’s role in stress response
- Under conditions of stress
anandamide levels in the BLA drop as
a result of FAAH induction. - Decreased anandamide levels release
the inhibitory tone leading to
excitatory output to the PVN. - CB1 agonists administered to the BLA
supress stress-induced activation of
the HPA axis.
Describe negative feedback
Glucocorticoids can act directly on the
hypothalamus or pituitary to provide
negative feedback preventing excess
glucocorticoid release
- Rapid feedback in the PVN can be
blocked by microinjection of CB1
antagonists
2-AG levels increase resulting in inhibition
of GABA interneurons
Cortical output to the BNST is disinhibited
Net increase in inhibitory output from the
BNST to the PVN attenuates stress response
Describe endocannabinoids in stress response
- AEA inhibits amygdala activation
- FAAH induction releases AEA
inhibitory tone on stress - 2-AG synthesis in the PVN contributes to
rapid negative feedback - 2-AG synthesis in the PFC and HC
contribute to slow negative feedback
- 2-AG synthesis in the PFC and HC
Describe endocannabinoids in appetite
- CB-1 is proposed to function downstream of
ghrelin to initiate orexigenic effects in the PVN - GHSR signalling (via Gq, PLC, PKC) induces
DAGL activity - 2-AG synthesis increases and signals to
presynaptic CB-1 receptors - CB-1 antagonists can block the orexigenic effects
of ghrelin - CB-1 agonism notably increases appetite,
increases craving for highly rewarding foods, and
increases hedonic value of food - ‘Munchies’
Describe cannabidiol
- CBD has a very low affinity for CB receptor
- CBD is a weak CB1 antagonist and a CB2 inverse
agonist but may act indirectly - CBD paradoxically potentiates the effects of THC
at the CB1 receptor by unknown mechanisms - CBD is also a 5-HT1A receptor agonist
- Contribute to antidepressant and anxiolytic effects
- Possible inhibitor of FAAH
Describe therapeutic effects of CBD
High CBD cannabis is being explored as an anti-convulsant for treatment resistant epilepsy
(such as Dravet syndrome)
- CBD has marked antipsychotic effects in animal models of Schizophrenia
- CBD is a potent antioxidant and has been shown to be neuroprotective in models of ischemic
stroke - Selective breeding of cannabis for recreational use has resulted in current strains being high-
THC, low-CBD– this trend is being reversed by current medical marijuana producers
Describe CB1 psychotherapeutics
- Rimonabant– synthetic CB1 antagonist – orexilytic
- Nabilone– synthetic THC analogue (PO) – antiemetic,
adjunct analgesic for neuropathic pain - Dronabinol (THC, PO) – antiemetic, orexigenic
- Naboximol/Sativex (THC/CBD ~ 1:1) – mouth spray –
MS symptoms incl. spasticity, neuropathic pain
Describe FAAH inhibitors
FAAH inhibitors (experimental)
* Bial– BIA 10-2474 – serious adverse events in a
2016 phase I trial left 1 participant dead and 4 with
permanent neurological impairments –
determined likely due to poor drug and trial
design (low affinity, multiple high doses, and
bioaccumulation of the drug)
- Janssen– JNJ-42165279 – phase II trials for
anxiety/depression suspended after Bial’s adverse
events - Sanofi– SSR-411298 – failed in a trial for
depression, ongoing trials for cancer pain - Vernalis– V-158866 – phase I trials for neuropathic pain
Describe cannabis tolerance and dependence
- In animals, THC administration results in
decreased CB1 receptor expression - Many suggest psychological, not physiological
dependence drives addictions (e.g. behavioural
tolerance rather than pharmacodynamic tolerance)
Describe self-administration in animal models
- THC administration often produces an
aversive stimulus initially in animals - THC self-administration has been
demonstrated in squirrel moneys
first trained on the operant
procedure with cocaine
Describe mesolimbic dopamine
- The mechanisms may be dependent
on opioid receptors (animal models) - Naltrexone abolishes THC self-
administration and blocks NAc
dopamine release - Aversive effects of THC
administration are abolished in κ-
opioid receptor knockout mice - However, in human trials naltrexone
administration increased the
positive subjective effects of
inhaled THC administration in
regular heavy marijuana users
Describe cannabis withdrawal
- THC withdrawal can be precipitated in animals
- Administration of CB1 antagonist after period of prolonged
administration - Wet-dog shakes, excessive self-grooming, hyperactivity
Describe cannabis-related psychosis
- A major concern with the use of cannabis is the
occurrence of psychosis - Cannabis use (particularly during adolescence) is
associated with an increased risk of psychosis - Alcohol, cannabis, amphetamine, or cocaine use
is often a precipitant for the first psychotic
episode in schizophrenia - Cannabis use interacts with genetic risk factors
for SCZ
