Psychoactive Drugs - Brookes Flashcards
What are some methods for studying CNS disorders?
- Imaging techniques
- Studying electrical signals from the brain
- Indirect ‘markers’ for changes in neurotransmitter function
- Post-mortem studies
- Human genetics
Usable models:
- Cellular
- Animal
What are the benefits of using imaging techniques to study CNS disorders?
You can study humans
They are non-invasive
They are longitudinal (can be repeated over time)
Can visualise affect of drugs
What is a negative of using imaging techniques to study CNS disorders?
No intervention studies possible
- You can take a patient and knock out a gene
They also only give out certain types of information
What are some imaging techniques that are used?
Computed Tomography (CT scan)
Magnetic Resonance Imaging (MRI)
Functional Magnetic Resonance Imaging (fMRI)
Diffusion Tensor Imaging (DTI)
Positron Emission Spectroscopy (PET)
Single Photo Positron Emission Spectroscopy (SPEC)
Electroencephalography (EEG)
Magnetoencephalography (MEG)
What is a CT scan?
Computed tomography
Spatial resolution is not great, it is of several mm
Used for things such as viewing ventricular size
Not generally used in research
What is MRI and fMRI?
Magnetic Resonance Imaging and Functional Magnetic Resonance Imaging
Has a resolution of less than 1mm
They are able to reveal patterns of activity in the intact brain
Activity utilises oxygen - microvasculature increases blood flow to active regions
Widely used in research
What is DTI?
Diffusion Tensor Imaging
Uses the properties of water which is constrained within neural cells
Good for mapping white matter tracts
Allows the mapping of pathways and investigation of aberrant connectivity
What is PET and SPEC?
PET is Positron Emission Tomography
SPEC is Single Photon Positron Emission Tomography (main advantage does not require on site cyclotron)
To work, an unstable positron emission isotope is created in a cyclotron (e.g. O^15)
The isotope is then injected and distributed according to the relative activity of the brain regions
Very good spatial resolution
What is EEG and MEG?
Electroencephalography and Magnetoencephalography
Measures electrical signals from brain
Good for looking at surface activities
Not good for deeper brain signals
Spatial resolution is not good
Very good temporal resolution
What is the ENIGMA?
International consortia that are specifically looking at different types of modalities within CNS disorders
Able to look at much larger cohorts
What are some indirect markers for changes in neurotransmitter function?
Levels of neurotransmitters/metabolites:
- In cerebrospinal fluid
- In plasma
- In urine (e.g. decreased dopamine (and metabolites) in Parkinson’s disease)
- Binding to platelets
What are the advantages and limitations of studying a post mortem brain?
Ability to study via imaging AND biochemistry
- Includes analysis of protein (e.g. receptor) levels, RNA levels, localisation of proteins/RNA
Culture of human tissue and electrophysiological analysis
It is now possible to culture human brain and keep it alive in a dish for up to 6 weeks
Limitations:
- Endpoint analysis only
- Time post-death to freeze/fix/analyse
- Variability between samples
- Cause of death
- Patient may have been on medication
What are some model organisms used in modelling CNS disorders?
Model organisms:
- Drosophila
- C.Elegans
- Mice
- Rats
- Primates
What are some advantages and disadvantages of using model organisms to model CNS disorders?
Advantages
- Malleable
- Drug screening
- Can do studies with multiple time points, interventions
- Often used after genetic studies have been carried out so specific genes can be knocked in or out
- Able to mimic environmental/neurochemical changes seen in disease
Disadvantages
- Mimic some but not all human
What is the definition of:
Psychotomimetic
Psychotropic
Psychoactive
Psychotomimetic:
- This term refers to drugs that mimic the symptoms of psychosis, such as hallucinations, delusions, or paranoia.
Psychotropic:
- This is a broader term that refers to drugs that affect mood, perception, or behavior by acting on the central nervous system.
Psychoactive:
- This is the broadest term, referring to any substance that alters brain function, resulting in changes in mood, consciousness, perception, or behavior.
What are some hallucinogens and their active ingredients?
Naturally Occuring:
- Ayahuasca (Harmaline)
- Peyote (Mescaline)
- Magic Mushrooms (Psilocybin)
Synthesised:
- Lysergic Acid Diethylamide (LSD)
What is the difference between the potency of LSD and other naturally occurring hallucinogens?
Psilocybin: 250ug/kg dose lasts for 3 hours
Mescaline: 15mg/kg dose lasts for 12 hours
LSD: 3ug/kg dose lasts for 10 hours
LSD significantly more potent
It is so potent it is most likely to act very specifically at receptor sites within the brain
Why was LSD synthesised?
Ergotism is a disease that presents with gangrene and psychosis
It is caused by eating rye bread contaminated with fungus
The fungus releases ergot alkaloids which cause peripheral vasoconstriction
The aim was to derive a substance with a similar action to treat haemorrhage
In 1943 Albert Hoffmann synthesised LSD for the first time, and accidentally ingested it, resulting in psychotic event
What are the affects of LSD?
Somatic
Perceptual
Psychological
The distortion of sensory perception indicates an effect in pathways that process sensory information
How was the LSD mechanism of action discovered?
There is a cross tolerance between LSD and mescaline (taking one or the other requires larger dose of the latter)
This suggests that both psychotomimetics act at the same class of receptor site
The structures of LSD and mescaline are similar to 5-HT
Early in vitro pharmacological studies showed that LSD interacts with 5-HT receptors in the peripheral vasculature
In the brain: LSD is a 5-HT receptor agonist/partial agonist
Importance of raphe neurones in LSD mechanism
LSD decreases firing rate of Raphe Neurones (5-HT1A receptor)
Raphe Neurones send extensive projection to the forebrain
BUT… mescaline shows cross tolerance with LSD…and does not affect raphe neurone firing, so there is more to it
In addition, rats are able to detect LSD in their system with raphe neurones ablated
Importance of the locus coeruleus in LSD mechanism?
Also acts at the Locus Coeruleus
LSD increases activity in locus coeruleus neurons
LSD increases activity of subsets of neurons in the cortex
5-HT2A receptors in cortex and thalamus modulate signal
Which 5-HT receptors does LSD work through?
Because LSD and mescaline have cross tolerance, it is possible to determine which receptors LSD work through by finding out which receptors BOTH drugs work through
Both LSD and mescaline work through 5-HT2a/2c receptors
It was also found that LSD has a very high affinity for 5-HT2A
Importance of 5-HT2A neurones in the brain?
5-HT2A neurones are highly expressed on pyramidal neurones in the cortex, which is where sensory information is processed
LSD increases the activity of layer V pyramidal neurones
Provide a summary for the potential sites of action for LSD?
Decreases firing rate of raphe neurones
Increases activity in locus coeruleus neurones
Increases activity of layer V pyramidal neurones in the cortex
Expanded primary visual cortex functional connectivity
What is Phencyclidine?
It is a psychotomimetic drug
Phencyclidine (PCP) is a ‘dissociative’ anaesthetic
Same class as Ketamine
Causes a catatonic-like state without muscle relaxation
Withdrawn from clinical use in 1965 due to ‘emergence phenomenon’W
How is PCP and LSD used in disease modelling?
Both LSD and PCP have been used in animal studies as drug-induced models for schizophrenia
Where do psychoactive drugs with abuse potential act on the brain?
Psychoactive drugs with abuse potential have
common actions on the limbic system of the brain
Specifically:
- Amygdala
- Ventral tegmental area
- Nucleus accumbens
How were the reward centres of the brain initially discovered?
James Olds and Peter Milner (1954)
Rat implanted with stimulating electrodes, self-administer (ICSS)
“The results indicate that various places exist in the brain where electrical stimulation is rewarding in the sense that the experimental animal will stimulate itself in these places frequently and regularly for long periods of time if permitted to do so.”
The behaviour is called REINFORCEMENT
The region where the electrodes were implanted when this behaviour was viewed is the septal area (now known as the medial forebrain bundle)
What is the medial forebrain bundle?
Medial forebrain bundle contains a mixture of axonsthat originate in the midbrain/medulla, andproject anteriorly to innervate areas throughout the brain including:
- 5-HT axons from raphe neurons
- Noradrenergic axons from locus coeruleus neurons
- Dopaminergic axons from ventral tegmental area
What is cocaine?
It is a plant compound
Stimulant and also local anesthetic
Anesthetic action through blocking voltage-gated sodium channels
Cocaine mechanism of action
Cocaine affects catecholamine neurotransmission
Cocaine binds with high affinity to monoamine, including dopamine, transporters and blocks them on presynaptic terminal
Prevents recycling of catecholamines
Increased levels of catecholamine in synaptic space
What is the Conditioned Place Preference (CPP) Task?
CPP measures drug reward by associating a substance with a specific environment
Mice are conditioned to a drug-paired chamber, and preference for this chamber indicates the drug’s rewarding effects
How was the conditioned place preference task used to show that cocaine acts through dopamine receptors?
Mice with DA transporter knockout have chronically elevated synaptic dopamine and increased locomotor activity:
- Cocaine administered to these animals produces no change in base-line DA (also no increase in locomotor activity)
- BUT these animals will self-administer cocaine, and display conditioned place preference (CPP) to cocaine
So in instead mice with cocaine-insensitive DA transporter knock-in were used:
- F105important forhigh affinitycocaine binding,but not for DAtransport
- Cocaine in the mutant DAT knock-in micedid not elevate extracellular dopamine orincrease locomotion and did not produce reinforcement as measured by CPP
Proves cocaine acts through dopamine
How can we prove dopamine pathways for cocaine in humans?
Using PET imaging with radioactive 11C-raclopride to label dopamine receptors (D2)
As you can see in the image, dopamine receptors in the basal ganglia are lit up, and are less in cocaine user
There is a dowregulation inD2 receptorsincocaine abusers
This suggest cocaine DOES act through dopamine pathway
Rewardsystem compromised
This is maintained afterdetoxification
What is amphetamine and what are its actions?
It is a stimulant
Actions of amphetamine:
- Appetite suppressant
- Euphoria
- Raises blood pressure
- Can also cause psychosis
Acts on dopamine
What is amphetamines mechanism of action?
Similar to cocaine in such that it blocks reuptake of dopamine into presynaptic terminal
However, they are not blocking the transporter like cocaine, instead, the amphetamine itself is transported into the presynaptic terminal and replaces dopamine
This results in an increased level of catecholamine (dopamine) in the synaptic space
Dopaminergic pathways of addictive drugs?
Dopamine neurone is either acted on directly by drugs or acted upon by other neurones that are acted on by drugs
This dopamine neurone has an inhibitory affect on the nucleus accumbens
The nucleus accumbens ordinarily sends inhibitory neurone projections to the cortex
This means that with an increase of dopamine acting inhibitarily on the nucleus accumbens due to the drugs action, the nucleus accumbens inhibitory neuronal projections into the cortex will be less
Results in increased activity of the cortex
What is the difference between the impulsive stage and the compulsive stage of addiction?
Both feedback loops
Impulsive stage is the craving of pleasurable experience, a craving of a reward
Compulsive stage is in line with withdrawal symptoms
Compulsive stage is a craving for reward AND a craving for relief of withdrawal symptoms
This is due to long term changes within the brain caused by heavy use
Proof that cocaine causes histone acetylation?
Cocaine induces histone acetylation in the nucleus accumbens
Loss of CBP (a histone acetyltransferase) in the NAc prevents cocaine-induced locomotion and CPP
Affect of cocaine on C-Fos
Acute stimulant exposure results in rapid induction of genes including C-Fos
Loss of CBP (a histone acetyltransferase) in nucleus accumbens neurones results in decreased histone acetylation and significantly altered C-Fosexpression in response to stimulants
Change in C-Fos levels are desensitised after chronic stimulant
Why is C-Fos desensitised after chronic stimulant use?
Due to a gene called DeltaFosB
DeltaFosB expression is also upregulated in the nucleus accumbens after stimulant exposure
DeltaFosB is a transcription factors that binds to DNA and binds to C-Fos
Amphetamine exposure causes more DeltaFosB to bind to the C-Fos promoter than normal
This causes a down regulation in C-Fos expression in repeated exposures to stimulants
What happens if you down regulate or up regulate expression of FosB in a cocaine user?
Down regulate:
- Less of an increase in locomotion after stimulant exposure
Up regulate:
- Increased locomotion activity after stimulant exposure
This is because higher levels of FosB, which promotes the expression of genes that enhance the response of neurones to dopamine
What are msP rats?
Genetically bred rats that prefer alcohol over water
Innate or acquired hyperactivity of extrahypothalamic CRF systems is associated with high alcohol preference
What happens to the extrahypothalamic CRF systems with high alcohol preference rats?
There is an innate or acquired hyperactivity
Use of hallucinogens in treating cocaine and methamphetamine abuse disorder?
There is some evidence that hallucinogens such as psilocybin are beneficial in the treatment of substances abuse disorders
The idea is potentially that drugs like psilocybin have neuroplasticity inducing properties that allow the brain to make connections in areas they would not normally make
