Topic 27: Endocannabinoids Flashcards
What are cannabinoid receptors?
THC was first isolated in 1964
the receptor responsible for THC binding was only identified in 1988: significant THC binding was observed in the cortex, basal ganglia, hippocampus, and cerebellum
in 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
What are CB1 receptors in the CNS?
two receptors for cannabinoids are found in humans:
CB1 is expressed widely in the CNS
CB2 is expressed primarily on immune cells: only in microglia in the CNS
cannabinoid receptors are G-protein coupled receptors: most Gi-coupled, rarely Cs-coupled
putative CB3 receptor for CBD recently proposed: GPR55 antagonist
What are the characteristics of CB1 receptors?
CB1 receptors are G-protein coupled, primarily to Gi
inhibitory through effects on adenylate cyclase (decreased cAMP), GIRK (increased K+ efflux), and Ca2+ channels (decreased Ca2+ influx)
CB1 receptors are exclusively expressed presynaptically: present on glutamatergic, GABAergic, and monoaminergic nerve terminals
What are endocannabinoids?
THC is a specific partial agonist of the CB receptors
endocannabinoids are the endogenous ligands for the CB receptors, 6 putative endocannabinoids have been identified
anandamide was first identified in 1992
2-AG was identified in 1994-5; these two endocannabinoids are the best described in the CNS
like phytocannabinoids (from plants), endocannabinoids are highly lipophilic
What is the retrograde signaling of endocannabinoids?
endocannabinoids do not function as classic neurotransmitters
endocannabinoids are capable of free diffusion through membranes: they cannot eb packaged into secretory vesicles
primary function is retrograde signaling to modulate presynaptic neurotransmitter release
release from postsynaptic compartments is through activity-dependent synthesis mechanisms
What is endocannabinoid synthesis?
endocannabinoids are formed from arachidonoyl-containing phospholipids
2-AG is synthesized in activity dependent process (such as Gq signaling to phospholipase C - PLC) from the hydrolysis of diacylglycerol (DAG) by DAG lipase (DAGL)
anandamide is similarly synthesized by cleavage of phospholipids by phospholipases A2, C and D
What are the steps of the 2-AG synthesis?
PIP2 is cleaved by PLC into IP3 and DAG
DAG-lipase converts DAG to 2-AG
What is the depolarization-induced suppression of inhibition?
a specific form of short-term synaptic plasticity
post-synaptic depolarization can activate endocannabinoid synthesis (e.g., dendritic backpropagation of strong stimuli)
endocannabinoids are released and suppress presynaptic GABA release: net decrease in inhibitory transmission leads to increased output from cell
What is the metabolism of endocannabinoids?
two key enzymes are responsible for the breakdown of endocannabinoids
fatty-acid amide hydrolase (FAAH) metabolizes anandamide
monoacyglycerol lipase (MAGL) metabolizes 2-AG
both are normally expressed to attenuate endocannabinoid signaling
activity dependent expression can regulate cannabinoid signaling
What are the genetic variants of FAAH?
a common genetic variant in FAAH (P129T) decreases the rate of breakdown of anandamide (~20% of North Americans): increased level 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 conditioned fear responses: decreased anxiety and increased 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
How are endocannabinoids involved in 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
excess activity of the HPA axis is implicated in a number of psychiatric disorders including depression, anxiety, bipolar
in many systems anandamide functions as a tonic (steady-state) regulator of activity while 2-AG functions as a phasic (on demand) regulator
How is the HPA axis regulated?
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 signaling: 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
What is the limbic control of the HPA axis?
under steady-state conditions CB1 antagonism results in an increase in glucocorticoid release
suggestive of steady-state endocannabinoid suppression of the HPA axis
microinjection of CB1 antagonists into the hypothalamic paraventricular nucleus (PVN) does not affect glucocorticoid release
direct microinjection into the basolateral amygdala (BLA) increases glucocorticoid release
BLA projections indirectly provide excitatory input to the PVN
How is FAAH impacted by the stress response?
under conditions of stress anandamide levels in the BLA drop as a result of FAAH induction
decreased anandamide levels release inhibitory tone leading to excitatory output to the PVN
CB1 agonists administered to the BLA suppress stress-induced activation of the HPA axis
What is the quick negative feedback pathway of glucocorticoids?
rapid feedback in the PVN can be blocked by microinjection of CB1 antagonists
role of endocannabinoids in controlling negative feedback
