Receptors, Neurotransmitters, and Signal Transduction Flashcards
What is the action of an inotropic receptor?
Excitability
NT binding –> Na+ influx (excitation) or Cl- influx (inhibition)
What is the composition of an inotropic receptor?
Composed of multiple transmembrane receptors that form an ion channel
What are some examples of inotropic receptors?
NMDA, nicotinic ACh, 5HT-3, GABA-A
What is the composition of a metabotropic receptor?
- GPCRs
- Each receptor generally has 7 transmembrane regions
What is the action of a metabotropic receptor?
Genetic transcription via a second messenger system (cAMP or IP3)
The second messengers can affect both gene transcription and ion channel permeability
What are some examples of metabotropic receptors?
ACh, catecholamines (NE, epi, DA), peptides, and serotonin
What is the action of receptor tyrosine kinases?
Synaptic plasticity via gene transcription
Activation of these receptors by growth factors and neurotrophic factors can influence axon generation, migration, apoptosis, and plasticity
What is the action of nuclear receptors?
Lipophilic ligands (hormones) pass through the membrane and bind cytoplasmic receptors, these then enter the nucleus and influence gene transcription
What is the action of somatodendritic autoreceptors?
Regulation of the presynaptic neuron’s firing rate; generally these are inhibitory
K+ channels open –> decreased cAMP
What is the action of nerve terminal autoreceptors?
To decrease the amount of NT released from the presynaptaic neuron by closing Ca2+ channels
What are heteroreceptors?
Pre-synaptic GPCRs with non-specific ligands that are always inhibitory
Explain the mechanism of receptor desensitization.
Over phosphorylation of GPRC –> binding of arrestin protein –> blockage of G-protein cascade –> receptor reset
Explain the mechanism of receptor down-regulation.
Prolonged desensitization –> internalization and degradation of the GPCR
Explain the mechanism of receptor up-regulation.
Chronic antagonism of the GPCR –> increased receptors on the membrane –> increased sensitivity to the NT
Explain the mechanism of tardive dyskinesia.
Chronic D2 blockade –> increased D2 receptor expression –> increased sensitivity to DA
Therefore, treatment with medications that increase DA will make TD worse.
TD movements end up looking similar to that of patients with excess DA
Describe the pathophysiologic differences in TD and EPS
EPS is caused by too little DA (hypokinetic), and adding DA improves the condition (anticholinergics decrease ACh but increase DA)
TD is caused by hypersensitivity to DA due to the increased number of receptors
What are the 5 criteria for a NT?
- NTs are synthesized and released from neurons
- NTs are released from nerve terminals in chemically or pharmacologically identifiable form
- NTs interact with post-synaptic receptors and have the same effect on both the pre- and post-synaptic neurons
- Interaction with postsynaptic receptor displays a specific pharmacology
- Actions are terminated by active processes
Describe the action of partial agonist?
A ligand that has a weaker effect on ion channels or second messenger systems.
Alone, it acts as a weak agonist. In the presence of a full agonist, it will compete with/act as an antagonist of the full agonist.
Describe the action of an antagonist.
A ligand with no intrinsic activity of its own. When bound to a receptor alone, the receptor stays in its resting state.
In the presence of an agonist, an antagonist works to block the agonist and to return the receptor to its resting state.
Overall, an antagonist will return a receptor to its basal activity.
Describe the action of an inverse agonist.
A ligand that will depress receptor activity.
**An antagonist in the presence of an inverse agonist would return the receptor to its basal activity.
What are the projections of the serotonergic system?
From the Raphe nucleus to:
- Frontal cortex –> mood
- Basal ganglia (5HT-2A/C) –> repetitive movement and OCD
- Limbic area –> anxiety and panic
- Hypothalamus (5HT-3) –> regulation of appetite and eating behavior
Describe the production of serotonin.
L-tryptophan in presynaptic neuron –> 5-hydroxytryptophan (5HTP) via tryptophan hydroxylase
5HTP is converted to serotonin (5HT) and packaged into vesicles by VMAT
Describe the destruction of serotonin.
Serotonin in the synaptic cleft undergoes reuptake via transporters (5HTT). From there, serotonin is either:
- repackaged for re-release
- broken down to 5-HIAA via monoamine oxidase (MAO) in the mitochondria
What heteroreceptors modulate serotonin?
- Pre-synaptic alpha1 receptors bind NE –> increased 5HT release
- Pre-synaptic alpha2 receptors bind NE –> decreased 5HT release
Describe the action of serotonin transporters (5HTT).
5HTTs co-transport 5HT and Na+ into the cell while shuttling K+ outside the cell.
5HT-1A
Metabotropic GPCR
- Pre-synaptic receptors are somatodendritic autoreceptors (open K+ channels –> increased firing rate)
- Post-synaptic receptors are associated with expression of trophic factors (promote axon branching) –> SSRIs promote hippocampal neurogenesis
- Antagonists –> synaptic losses and play a role in mood disorders
5HT-1B
Nerve terminal auto-receptor –> decreased NT release
5HT-1D
Nerve terminal auto-receptor –> decreased NT release
5HT-2A
Post-synaptic receptor
- Basal ganglia projections control movement and compulsions
- Mesocortical projections –> decreased DA –> apathy and low libido
- Stimulation of these receptors –> inhibition of orgasm/ejaculation
- Limbic projections –> decreased panic/anxiety
5HT-2C
- Limbic projections –> decreased panic/anxiety
- Causes weight gain
How do SSRIs affect 5HT-2 receptors?
Downregulation of 5HT-2 –> improved 5HT:receptor ratio (serotonin deficit hypothesis)
5HT-3
Inotropic receptors
- Responsible for the GI effects of SSRIs
- Hypothalamic projections regulate appetite and satiety
What are the projections of the dopaminergic system?
- Mesolimbic
- Mesocortical
- Nigrostriatal
- Tuberoinfundibular
Describe the mesolimbic dopaminergic pathway.
VTA –> hypothalamus –> limbic system, frontal lobe, and nucleus accumbens
- Associated with reward behaviors and addiction
- Excess DA in this system is associated with (+) symptoms of schizophrenia and aggression
Describe the mesocortical dopaminergic pathway.
VTA –> cortex and limbic system
- Associated with cognition and motivation
- Deficiency of DA in this system is associated with (-) symptoms and cognitive issues in schizophrenia
Explain the pathophysiology of schizophrenia.
Overactive mesolimbic pathway and underactive mesocortical pathway
Describe the nigrostriatal dopaminergic pathway.
Reticular formation + substantia nigra –> caudate nucleus + putamen
- Deficiency of DA –> Parkinsonian symptoms, akathisia, dystonia
- Excess DA –> chorea, diskinesia, tics
Describe the tuberoinfundibular dopaminergic pathway.
Hypothalamus –> anterior pituitary –> inhibits prolactin release
- DA blockade –> increased prolactin –> galactorrhea, amenorrhea, sexual dysfunction
Explain how DA is produced.
L-tyrosine in pre-synaptic neuron –> L-DOPA via tyrosine hydroxylase –> DA by dexarboxylase –> packaged into vesicles by VMAT
Explain how DA is destroyed.
Reuptake into the pre-synaptic neuron via DAT, then it is either
- Repackaged into vesicles and recycled
- Broken down into dihydroxyphenylalanine or HVA by MAO
How does the DA transporter work?
Na+/K+ pump
How do amphetamines affect the DA transporter?
They reverse the direction of the transporter to release DA
How does cocaine affect DA levels?
It blocks the reuptake of DA
Discuss the D1 receptor.
GPCR that stimulates adenylyl cyclase
- Highest density in the frontal cortex (mesocortical pathway) and is involved in cognitive functioning
Discuss the D2 receptor.
GPCR that inhibits adenylyl cyclase
- On pre-synaptic neurons, these autoreceptors are both somatodendritic and nerve terminal
- Post-synaptically, they have the highest density in the striatum and nucleus accumbens (nigrostriatal, mesolimbic)