Chemical messengers - Dan Brierley Flashcards
describe the synthesis of neurotransmitter in an axon
synthesis and formation of vesicles in main body
transport down acon terminal
ap down axon via salt conduction
influx of ca2+ due to AP - voltage mediated ion channels
ca2+ cause presynaptic vesicles to fuse with the presynaptic membrane and release neurotrans into the synaptic cleft
neurotransmitter diffuses across synaptic cleft and binds to receptors on post
initiate excitatory or inhibitory response
methods of neurotransmitter removal
diffusion
reuptake
degradation by enzymes ie acetylcholinesterase
over what timescale do neurotransmitters act
depends on function
from ms to months/years
ms - impulse, neurotransmitter release and rapid transmission ie ca2+ and ligand gated channels
minutes to days - synaptic plasticity, pharmacalogical tolerance ie down reg of neuroactive drugs
fast neurotransmission uses
ligand gated channels
slow neurotransmission uses
g protein coupled receptors
ligand gated ion channels are called
ionotropic receptors - intrinsic ion channels that change shape following the brinding of an extracellular ligand to the binding site
how to ligand gated ion channels work
allow diff but specific ions to pass thought - bind to specific protein receptor
short latency period - disocciation closes channel and vise versa
what do modulatory sites on ligand gated ion channels do
allow for the modification of the channels activity ie other chemicals bind and determine opening closing
g protein couped receptors are called
metabotropic receptors - not mediated by channels as link with other g protein channel - neurotransmitter bind to g protein coupled receptor activatin g g protein which activates second messenger (neurotrans is first messenger)
what does a second messenger do in g protein coupled receptor
can bind to channels elsewhere on membrane or activate intermediate molecules inside the cell
slower response - must go through more stages to influence
BUT have wider response as second messenger activate range of proteins/genes
what are neuromodulators
released by neurones and astrocytes for a slow pre or post synaptic response - operate g protein couples and can have short/long term influence
neurotransmitter criteria
synthesised and storable
released via exocytosis
interact with specific receptors
must have a mechanism for inactivation
what occurs in nitric oxide neuromodulation
(NO) present in 2% of neurons accross the brain - predominantly in the cerebellum/hippocampus
NO increase due to ca2+ increase and immediate release as cant be stored
short term nitric oxide neuromodulation
activates guanylate cyclase enzyme which produces the 2nd messenger cyclicGMP which leads to phosphorylative reactions (inhibitory and excitatory responses)
long term nitric oxide neuromodulation
NO reacts with free redicals that produces toxic peroxynitrile leading to neuronal death
what occurs in endocannabinois neuromodulation
there are endogenous ligands for cannabinoid receptors in the brain - CB1 is a GPCR receptor that mediates the effect of delta9-tetrahydrohydrocannabinol (THC)
Activation of the CB1 causes suppression of synaptic transmission in various regions of the CNS
What are the two major endocannabinoids (eCBs)
anandamide and 2-arachidonylglycerol (2-AG) are endogenous ligands for CB1 which are produced and released from central neurons in a manner dependent on neural activity and intracellular Ca2+.
Where is the CB1 receptor distributed
cotrical regions of the brain, basal ganglia, cerebellar cortex, thall and hypothallamus
how to endocannabinoids function as retrograde messengers
endocannabinoids released from postsynaptic neurons in a calcium-dependent manner and act retrogradely onto presynaptic cannabinoid receptors to suppress neurotransmitter release.
1 - presynaptic release and bind to receptor
2- increase intracellular ca2+ causing demand to eCBs from membran bound precursors
3- release of eCBs bind to presynaptic CB1 receptors and decrease intracellular ca2+ preventing neurotransmitter release
4- eCBs reuptake into post - degredation by FAAH enzyme into ethanolamide and arachidonic acid
5- eCBs via CB1 inhibits neurotransmitter release of gutamate/dopamine/GABA
what effect do eCBs have on behaviour
- modulatory effect on learning, movement, appetite and memory
Why is electrical brain stimulation used
allows examination of the neural regulation of behavior - stereotaxic methods where electrode placed in brain and brief electric current passed through activating the cell membrane and producing an excitatory response
describe olds and milner (1954) study
concerned that the electrical stimulation they were
routinely giving might be aversive
found that particular brain regions that, when stimulated,
produce behaviour that implied the electrical
brain stimulation was rewarding
rat with an electrode implanted
into particular areas of the brain would repeatedly
return to the place in the cage where the electrical
stimulation had been applied - provide the positive emotional
consequences associated with natural rewards
what is intracranial self stimulation
developed to show that animals
will learn operant contingencies i.e. they will use
energy and effort to obtain high frequency, pulsed
electrical stimulation of specific brain regions (pleasure centres)
stimulator is
attached to an electrode that is stereotaxically
implanted into the rats brain. When the rat presses a
lever, the stimulator sends a pulse to the rats brain.
Rats will press a lever many hundreds of times to
obtain ICSS
similarities between ICSS and natural rewards
ICSS elicits a natural motivated behaviour ie eating, drinking, maternal behaviour or copulation in the presence of the appropriate goal object.
Increases in natural motivation ie food or water
deprivation, increases self-stimulation
rates.
Study of ICSS and natural reward
In a study that compared lever pressing for brain stimulation and lever pressing for a natural reinforcer, when rats pressed a lever to inject a small quantity of chocolate milk into their mouths through an intraoral tube, they behave remarkably like self-stimulating rats.
Reward processes all work via the same processes
brain areas critical in reward
1- Anterior Bed Nucleus project to Ventral Tegmental Area
2- vent teg to Nucleaus Accumbens (dopamine primary transmittor)
3- nuc acc to Ventral Pallidum and anterior bed
nucleus of the MFB using enkephalin, an endogenous
opioid neurotransmitter
also involved - Amygdala and Raphe Nuclei
amphetamine, cocaine, opiates,
cannabis, ketamine all act on the NAc
opiates, alcohol, barbiturates and nicotine
all act on the ventral tegmental area.
define wanting
motivation for reward, which includes both (1) incentive salience ‘wanting’ processes that are not necessarily conscious and (2) conscious desires for incentives or cognitive goals
define liking
ctual pleasure component or hedonic impact of a reward. Pleasure comprises two levels- (1) core ‘liking’ reactions that need not necessarily be conscious; (2) conscious experiences of pleasure
define learning
associations, representation, and predictions
about future rewards based on past experiences.
include (1) explicit and cognitive predictions and (2) implicit knowledge as well as well as associative conditioning
dopamine and wanting
2 nd-stage dopaminergic fibres, and release of dopamine
within the nucleus accumben were considered to directly
mediate reward, or the experience of pleasure
now consider that dopamine fibres are activated by
wanted incentive stimuli and have an essential role in the
acquisition and direction of natural reward- and drug-seeking behaviour
Electrophysiological and microdialysis studies show that DA system activity is increased by the presence of natural rewards (food, water, sexual mates, etc), or stimuli that predict their availability – rather than by consummatory behaviour
describe Pecina et al 2003
genetic knockdown approach where a mutation in the dopamine transporter (DAT) in mice meant that levels of the transporter were significantly reduced, resulting in
mutant mice having 70% elevated levels of synaptic dopamine. (As reuptake prevented)
examined the consequences of the elevated synaptic dopamine on:
(1) spontaneous food intake
(2) incentive motivation and learning to obtain a palatable sweet reward in a runway task
(3) affective “liking” reactions elicited by the taste of sucrose
Findings Pecina et al 2003 spontaneous food intake
Food intake of DAT slightly higher than wild-type mice over 4 week period.
Mutant mice ate 21% more chow pellets average
Their higher food consumption meant mutant mice had
heavier body weights
pecina et al 2003 runway paradigm
Runway learning and performance for a food
reward generates acquisition learning curves, as
well as running speed and other measures of
incentive motivation i.e measures of wanting.
three compartments: a start box, a central runway and
a goal box – which mice want to get to as
contains sweet treat.
Protocol starts by placing the mice in the goal
box, then move the start box further and further
away from the goal box so learn to run down to
goal
Findings pecina et al 2003 incentive motivation and learning
1-3 habituation to apparatus
4-6 training
7-11 goal
DAT knockdowns were faster, i.e.
They reached the reward more quickly than wild
types each day during the training phase (sessions
8–11).
DAT knockdown mutant mice showed enhanced
acquisition and incentive performance for a
sweet reward in the runway task
Findings Pecina et al liking reactions
The different taste-elicited affective reactions
of mice and rats are the same as the affective
facial reactions of human infants, great apes,
and monkeys.
- rhythmic tongue protrusions to sucrose indicating ‘liking’
- gapes to bitter tastes such as quinine, indicating ‘disliking’
DAT knockdown mice did not appear to “like” the taste of sucrose more than wild-type mice in the taste reactivity test, despite their higher incentive motivation or “wanting” for a sweet reward and higher food intake
mutants actually displayed fewer liking reactions at the highest sucrose concentration
what elicits transient fluctuations in dopamine
levels in the shell of the nucleus accumbens
a variety of reinforcers including
food/water, cocaine or intracranial self-stimulation
(ICSS)
Describe Beyene et al 2010 (wanting)
effects of varying reinforcer magnitude on cue-evoked dopamine release in the NAc shell in rats performing ICSS
1- trained to press a lever to receive ICSS
2- audio-visual cues that predict the lever being active, and therefore eliciting ICSS
( 2 second time interval between the cue and the lever becoming active)
3- observe dopamine activity :
• Cue onset = small increase
• Lever presented = bigger increase
amplitude (i.e. how big) of cue-evoked dopamine release in the Nac reflects the magnitude of the predicted reinforcer
how may cocaine have an effect on “wanting” mechanism
Very high magnitude reinforcer (compared to natural
rewards) so elicits huge release of DA
Cocaine molecule blocks DA reuptake by binding to DAT, further increasing DA signalling
pronged mechanism on DA = very potent driver of ‘wanting’ behaviour = high risk of dependence
cheer et al 2007 endocannabinoid influence on CB1 in reward wanting
endocannabinoid system acting via CB1 receptors has modulatory influence over synaptic transmission
blocking CB1 signalling by coadministering the CB1 ANTAGONIST rimonabant with cocaine reduces DA release in NAc
CB1 antagonists reduce ‘wanting’ behaviour for cocaine and similar drugs
Williams and Kirkham - endocannabinoid system in reward processes (wanting)
rats were injected with either the endocannabinoid anandamide, or THC from cannabis, and then feeding behaviour observed increasing doses of AEA reduced time until rats started feeding –
i.e. increased their ‘wanting’ of food.
AEA also increased number of meals and total intake, suggesting ‘liking’ of food may also be increased
what neurotransmitter does the final stage of the reward pathway typically use? (nucleus accumbens to the ventral pallidum)
enkephalin, which is an endogenous opioid.
endogenous opioid release is thought to play a role
in…
consummatory aspects of reward - significant role for opioids to enhance liking reactions: opioid agonists suppress aversive reactions to bitter tastes and potentiate hedonic reactions to sweet taste
evidence supports a crucialmrole for endogenous opioid systems in food reward - role in liking rather than wanting
ie opioid antagonists (ie naloxone) reduce food intake by shortening meals,and human subjects report that previously palatable foods taste less pleasant
describe rideout and parker 1996 (liking w/morphine)
look at ability of morphine (opioid agonist) to modify sucrose palatability - assessed by the taste reactivity test
rats injected with morphine (0.5/2/10mg) before receiving a 10-min intraoral infusion of 2% or 20% sucrose solution
mean duration of ingestive reactions (i.e. tongue protrusion ‘liking’ responses) during each of two 5-min testing blocks
Findings of rideout and parker 1996 (liking w/ morphine)
spent more time displaying ingestive
reactions during an infusion of 20% sucrose than
2% sucrose solution, suggesting that the 20% is
more palatable
2 mg/kg dose of morphine increased duration relative to control for both sucrose concentrations – opioid agonist increased sucrose palatability
describe Tallett et al (2008) (naloxone on liking feeding behaviour)
opioid agonists increase liking - what about antagonists
nalozone = antagonist
f increasing doses
of naloxone on 1-hour free-feeding with a
palatable food
results of tallett et al 2008 nalozone on liking feeding behaviour
naloxone doses of 1 – 5 mg/kg consistently suppresses food consumption
doses below 1mg/kg had no effect.
opioid antagonists decrease ‘liking’ of food
naloxone did not significantly influence other feeding related parameters:
• latency to locate food source,
• latency to feed,
• average length of feeding bouts,
• feeding rate
doesn’t impact on the wanting mechanism – just the liking one.
doesn’t affect the frequency of other behaviours (e.g. walking, grooming etc) - suggesting it’s only to do with food, it doesn’t slow other mechanisms
williams and kirkham liking study
increasing opioid antagonist naloxone decreased the feeding stimulation elicited by THC
increasing doses of naloxone alone slightly reduces food intake, but coadministration of CB1 antagonist rimonabant with naloxone significantly reduces intake.
opioid and eCB systems interact to
mediate ‘liking’ component of feeding
