Cannabinoid CB1 receptor

Role in extinction and sensitization

Say it with me

• Kah nab a noid

• Ka banna boy

outline

• Receptor and receptor subtypes– Location, function and distribution

• Endogenous ligand

• Kamprath paper

• Pamplona et al paper– Results– Clinical applications

CB receptor subtypes

• CB1-found in brain

• CB1A-very little info, but does exist

• CB2-found in immune cells (not in brain)– CB1 is of most interest to us today– acts via G-protein which, when activated,

inhibits adenylate cyclase as well as voltage gated CA++ channels, stimulates K+ channels

Distribution of CB1

• Cerebellum-– motor function

• Cortex-

– association

• Basal Ganglia• Limbic system

– Hippocampus-learning– Amygdala-fear

• NO CB1 in thalamus,medulla or brainstem

Limbic CB1 receptors

• Exclusively localized to GABA and Cholecystokinin (CCK) containing presynaptic terminals– Receptor activates G-protein that suppresses

GABA release• Less GABA=more anxiety?

– In hippocampus- release of ACh is inhibited

CB1 in a RAT BRAIN

Relative density of CB1R in select brain regions

Anandamide

• Endogenous cannabinoid– Synthesis not known

• When bound in high doses, behaviors include: hypothermia, analgesia, hypomobility, catalepsy

• LIPID NT, can pass membrane, thought to be synthesized on demand

• Acts on membrane bound and intracellular locations

Anandamide

• Brain levels rival that of DA (felder et al 96)

• Binds to both CB1 and CB2

– Acts as agonist• Highest conc in hippo, striatum, ctx and

cerebellum• THC acts as CB1 agonist• Some diet drugs act as CB1 antagonist

– Rimonabant (aka SR141716)

Kamprath et al Neuroscience 06

• Sensitization: nonassociative learning– General increase in reponse to potentially

harmful stimuli after aversive experience– E.g. inescapable footshock=alter behav& cort

• Fear conditioning: tone paired with shock– Re-exposure to tone activates memory of the

tone-punishment association and induces response

– E.g. rats freeze upon re-exposure to tone

Extinction vs Habituation

• Extinction: new association btwn tone and the nonappearance of predicted punisment (safety learning)– Suppresses the expression of the memory of

the tone-shock pairing

• Habituation: repeated non-reinforced tone presentation will lead to a decrease in response to the tone

Role of CB1

• CB1 role in extinction is limited to aversive testing conditions – Genetic ablation or pharmacological blockade

of CB1 impairs the extinction of fear memories

• CB1 plays no role in conditioning to tasks involving positive reinforcement

Goals of study

• Examine the role of CB1 in extinction and habituation of acquired fear responses – Utilize genetic mutant CB -/- and ‘wild type’

CB+/+– Utilize SR selective CB1 antagonist

CB-/-

CB+/+

sensitized

sensitizedconditioned

conditioned

No difference in acquisition

Conditioning: 80 dB tone with .7 mA footshock next day tone only in novel envir

CB-/- show prolonged and stronger freezing to tone

Sensitization: .7 mA footshock only NO TONE next day tone only in novel envir

CB-/- show prolonged and longer freezing to tone

Cond Sens

CB-/- showed same freezingBehavior with or without shock

Can CB-/- associate memories?

• Because no difference in freezing was seen between tone-shock pair and tone alone in CB-/- animals, authors wanted to make sure that CB-/- animals could in fact form an associative memory i.e. associate the tone with the impending shock

• Utilize electrophysiology to record auditory evoked potentials in CA1 region of hippo

• No significant differences between CB-/- and CB+/+ in the potentiation of auditory-evoked potentials

• Both groups have similar activation in CA1 region when exposed to loud tone

Within session vs long term

• Next, authors wanted to see what changes in extinction are seen over time between the two groups

• Testing occurred as before, with the addition of another testing round 5 days after the first test

CB-/-sensitized

conditioned

CB+/+

sensitizedconditioned

CB-/- again froze longerAnd stronger both 1 day and 6 days after conditioning

CB+/+ animals treated withSR show same trend as theMutant strain, even 5 dayslater

CB1 controls both within session and long term fear adaptation

SR

Pamplona et al Psychopharmacology 2006The cannabinoid receptor agonist

WIN 55,212-2 facilitates the extinction of contextual fear memory and spatial

memory in RATS

Main Objectives

• Examine whether admin of CB1 agonist WIN could faciliate the extinction of recent and/or remote contextual fear memory in rats

• Investigate the role of CB1 antagonist SR in the extinction process

• Determine if not only fear memory but also spatial memory was affected by these drugs

Methods

• Animals– Male Wistar rats, group housed

• Behavorial testing occurred during light phase of the day/night cycle….. Why?

– N of 7-10 animals per group

• Drugs– Win and SR dissolved in 0.9% saline with

10% DMSO and 0.1% Tween 80– Controls received vehicle only

• Animals injected IP 0.2mL per 100 g BW– Win given 30 min before testing– SR given 20 minutes before testing

Behavioral Procedures

• Fear Conditioning – Contextual fear: 3 min in cond chamber and

then received 1 sec 1.5 mA shock, 60 seconds after shock they were removed

– Tone fear: 3 min in cond chamber and then 10 sec of 80 dB tone that co-terminated with 1 sec 1.5 mA shock, 60 seconds after shock they were removed

• Freezing was the behavior measured during subsequent testing

Effects of CB1R on extinction of recent contextual fear memory

• Successive exposures to conditioning chamber were used to assess short term (within exposure) and long term (between exposure) extinction of cond fear

• 24, 48 and 72 hours after contextual conditioning animals were placed back in chamber and freezing recorded– WIN or SR treatment before each session

Control animals froze less on each successive re-exposure1.0 mg/Kg dose of SR disrupted this extinction (they froze more than control group)

Low dose of WIN facilitated extinction (they froze less thancontrols); high dose of WIN disrupted extinction Mid dose of WIN mimicked controlgroup

Dose Dependant Effects!

In the first 3 minutes (retrieval) of the9 minute test block SR had no effectBut WIN at the mid dose showed Decreased freezing time, this meansThat there is no effect of WIN on Memory retrieval

• Effects of WIN on extinction of remote contextual fear

memory

Drug free rats 48 hours after the 5 day extinction protocolWIN treated, contextual condrats froze less than control andSR treated rats

WIN facilitates extinction of remote fear memory

5 days of exposure to contextshows that WIN treated ratsfreeze less, extinction is facilitated in short term fearmemory by WIN

Unconditioned freezing behavior

• Day 1 rats placed in cond chamber for 3 minutes, shocked (1 sec 1.5mA), 60 seconds later they were removed

• Day 2 rats treated with WIN or SR, placed in novel environment and freezing behavior was measured

• Also, separate group of rats treated with WIN and SR and placed in open field to measure effects on locomotion

No effect of WIN or SR on unconditioned freezing or on locomotion in open field test

Day 1 and 2 were training, 6 trials each dayDrug admin on day 3, platformmoved to opposite side, and 6 trials followed

WIN treated animals had decreased latency to find platform on first trial, no diff from controls after that

SR treated animals showedincreased latency on trial 2Compared to control, no difffrom controls after that

Discussion

• Disruption of CB1 cannabinoid signaling decreases the ability of rodents to extinguish fear memories

• WIN acting as a CB1 agonist facilitates fear extinction– WIN also facilitates spatial memory– Not by disrupting acquisition, memory retrieval

or affecting sensory-motor ability

CB1 antagonists

• Administration of selective CB1 antagonists (SR or rimonabant) or genetic ablation of CB1 gene leads to a pronounced deficit in the extinction of conditioned fear– Also leads to deficits in previously learned

spatial information

• In Rats!

• Administration of CB1 agonist WIN facilitates the extinction of contextual fear and may have long term implications

• In humans, pharmacotherapies directed at the endocannabinoid system may help treat psychiatric disorders such as retrival of fear memories, panic, phobias (like being afraid of russians) and PTSD

You may cease learning…….

………………….now!

The End

Pat Ronan

Rules!