How do you identify if a neurotransmitter is fast or slow acting?
By the type of channel it activates; (ionotropic = fast, metabotrophic = slow)
Ionotropic receptors are what?
Gated ion channels that are opened by binding of ligand = ions flow; results in a quick effect
Metabotropic receptors are what?
receptors that work through a 2nd messenger system when activated = slow effect; i.e GPCRs
What are the 4 major types of receptor classes?
GPCRs, RTKs, Ligand-gated ICs, NTFRs
How is the signaling of neurotransmitters (NTs) terminated?
- enzymatic degradation
- diffusion away from the synapse
- accumulation in glial cells
What are the possible effects that an NT can have on its target receptor?
stimulator, inhibitor, or modulator
What ion is required for the exocytosis (release) of an NT from a presynaptic neuron?
What are the 5 major biogenic amines?
- norepi (NE)
- Dopa (DA)
- Serotonin (5-HT)
What amino acid are catecholamines (Epi, NE, and Dopa) derived from?
What is the rate-limiting step in catecholamine synthesis?
tyrosine –> L-dopa
Which catecholamine is L-dopa converted into?
Tyrosine –> L-dopa–> Dopa– >NE
What enzyme is needed to convert NE –> Epi?
phenylethanolamine N-methyltransferase (PNMT)
What are the 2 major enzymes in catecholamine production?
tyrosine hydroxylase and PNMT
That is the sequence of catecholamine synthesis
Where does the conversion of dopamine –> NE occur?
in vesicles called chromaffin granules
Where is NE –> Epi?
What moves Epi from the cytosol into vesicles?
What are COMTs and MAOs?
enzymes that inactive catecholamines convert them into VMA
What products of catecholamine metabolism is used to measure catecholamine production?
Where are NE producing neurons found?
Locus coeruleus = crucial to waking up/awareness; projects to almost all areas of CNS
Where is Epi primarily released?
Where is dopamine made in the CNS?
substantia nigra (black substance) and ventral tegmental area of brain stem
Where does the substantia nigra project to?
striatum (lentiform nuclei & caudate nuclei)
Where does the ventral tegmental area project is DA fibers to?
prefrontal cortex and parts of the limbic system
What are the 4 major systems that use DA?
- substantia nigra = motor
- mesolimbic = VTA –> Nucleus accumbens
- mesocortical = VTA–> frontal cortex
- tuberinfundibular = hypothalamus –> anterior pituitary (prolatin suppression)
where is histamine made in the CNS?
the tuberomammillary nucleus (TMN) of the posterior hypothalamus; project throughout brain
What cognitive state is histamine associated with?
How does histamine down regulate itself?
activation of H3 receptors on presynaptic neuron says there is enough histamine
Diamine oxidase degrades what?
What nuclei make serotonin (5-HT)
raphe nuclei of the brainstem
What amino acid is 5-HT derived from?
What is the synthesis pathway for serotonin?
- tryptophan –> 5-HTP via Tryptophan hydroxylase
- 5-HTP–> serotonin via 5-HT decarboxylase
What is the rate-limiting step in serotonin synthesis?
tryptophan –> 5-HTP via tryptophan hydroxlase
What is the function of serotonin?
control attention and mood; involved in depression
How is serotonin metabolized?
MAO upon reuptake; so cytosol and vesicles
What type of receptors does serotonin activate?
- 6 isoforms of metabotropic receptors
- 1 isoform of ionotropic receptors (type 3 receptor)
Where are Cholinergic NTs made in the CNS?
- Pons/midbrain (pontomesencephalotegmental complex)
- nuclesu basalis (frontal cortex)
- septal nuclei (limbic system)
What cognitive tasks are cholinergic/ACh secreting neurons important for?
Learning and memory
What are the CNS functions of Ach?
- consciousness (arousal/wakefulness)
- voluntary motion
- initiation of REM
Is Ach an inhibitory or excitatory NT
What moves Ach from the cytosol into vesicles?
VAchT (sometimes seen as VAT)
What enzyme breaksdown Ach?
What type of receptors are activated by Ach?
- Nicotinic (ionotropic)
- muscarinic (metabotropic)
There are 5 isoforms of Muscarinic (Ach) receptors. Which isoforms use Gq and Gi GPCRs?
- Gq = M1, M3, M5
- Gi = M2, M4
Which isoform of muscarinic receptors are found in the CNS?
M1 (Gq –>IP3 & DAG –> increased Ca2+)
Where are Ach nicotinic receptors found?
NMJ, autonomic ganglia, and CNS
What are the 2 main inhibitory amino acids?
GABA & glycine
Where is GABA main found? what type of neurons?
cortical & cerebellar inhibitory interneurons
Which amino acid is GABA derived from?
glutamate (which is excitatory; oh the irony)
What are the 2 major GABA receptor isoforms?
- GABAa = opens Cl- ionotropic channels
- GABAB = metabotropic –> increased K+ efflux
Which GABA receptor is the most common, is ionotropic and conducts chloride?
Which GABA receptor is metabotropic and results in intracellular increased K+
What drugs target and augment GABA receptors?
Benzodiazepines, barbiturates & ethanol = sedative action
What is the major inhibitory NT in the spinal cord?
glycine = action on glycine receptor
What enzyme converts Glutamate into GABA?
What is the major categorical difference between monoamines, cholinergic, & amino acid NTs vs opioids?
- monoamines, cholinergic, and amino acids NTs are small molecules
- Opioids are neuropeptides
What are the 3 opioid peptides that we care about?
endorphins, enkephalins, and dynorphins
Which opioid peptide is derived from POMC and targets mu receptors?
What are the effects of Mu receptors?
- analgesia & sedation
- respirator depression
- increased GH & prolaction
What is the mechanism behind Mu receptors?
Increases K+ efflux –> hyperpolarization
What are the effects of Kappa opioid receptor activation?
- analgesia & sedation
What is the mechanism that Kappa & Delta opioid receptors use to elicit their effect?
decrease Ca2+ influx
What are the effects of delta opioid receptors?
Where are nociceptin receptors found and what is their effect?
found throughout brain and produces anxiety, depression & tolerance to Mu-opioid agonists
What is excitotoxicity?
- Proposed mechanism to explain continual neuron death after an ischemic event.
- Based on the idea that overstimulation of EAA system can cause cell death even in neurons not directly involved in the ischemic event.
Where do we see evidence of excitotoxicity existing?
- Cerebral ischemia or stroke
- Mechanical trauma to CNS
What are the areas most affected by the ischemic event (anoxic core)?
- Oxygen deprivation
- Cells unable to meet metabolic needs
- Depolarization of membrane
Within minutes of an ischemic event (anoxic core) of a neuron, what is seen?
- ATP levels fall to 0 inside the neurons
- Na+-K+-ATPase function ceases
- Cell depolarizes
- Leads to action potential and release of neurotransmitters including EAAs
After minutes of an ischemic event occurring, high amounts of EAAs accumulate in the synapse. Why?
- Excessive EAA released
- EAA re-uptake is Na+ dependent is decreased d/t Less Na+ around in synaptic cleft
- NMDAR activation is Ca2+ dependent
- Less Ca2+ around in synaptic cleft
When CNS ischemia has occurred, increased intracellular [Ca2+] initiates excitotoxicity. How does it do this? (4)
- Activation of phospholipase A2
- Activation of calcineurin (phosphatase)
- Activation of Mu-calpain (protease)
- Activation of apoptotic pathway
Excess activation of these enzymes disrupts normal cellular function
Regarding CNS ischemia, describe the activation of phospholipase A2.
- Release of arachidonate from membrane and causes physical damage to the membrane
- Arachidonate acts at ryanodine receptor on ER
- Release of Ca2+ from intracellular stores
- ER: “unfolded protein response” – stops making protein
- Activation of eIF2α-kinase
- Mitochondria: impaired function
Regarding CNS ischemia, describe the activation of μ-calpain (protease).
• Spectrin (more structural damage to cell)
• eIF4G (eukaryotic induction factor 4G – protein synthesis)
• Others – metabolic impairment
Regarding CNS ischemia, describe the activation of calcineurin.
•Among other things, activates NOS
•Increases NO synthesis
The activation of the three (3) enzymes (phospholipase A2, Mu-calpain, calcineurin). during CNS ischemia, causes a disruption of mitochondrial and ER function, which further increases what?
free cytosolic calcium
During CNS ischemia, what does an increase in free cytosolic Ca2+ lead to?
Increase in mitochondrial membrane disruption, and apoptotic pathways are activated.
Describe the basic idea regarding reperfusion injury. (9)
- CNS ischemia lead to an apoptoic neuron.
- Blood is reperfused to the area (no longer ischemic), but neuron is no longer “normal” since apoptosis started.
- Much of the new O2 will end up as free radicals somewhere (peroxides)
- Increase in O2 leads to production of ATP
- But enzymes currently activated in necrotic neuron aren’t “normal”
- Kinases take ATP –> ADP + PO4
- Phosphorylation, further modifying enzyme action
- Phosphorylation of eIF2α kinase leads to a decrease in protein synthesis & activates caspase 3, which increases apoptotic signaling
- Nitric oxide adds to the cascade by contributing to edema via damage to capillary endothelial cells and vasodilation
Is there any way to stop reperfusion injury from happening?
Preventing this cycle is difficult at best
To date, most experimentally successful treatments are pre-treatments that focus on the NMDA receptors.
What is the 5-HT2a receptor associated with?
smooth muscle contraction
What is the 5-HT3 receptor associated with?
What is the 5-HT6 receptor associated with?
high-affinity for several antidepressants (SSRIs/SNRIs)
What is the 5-HT2c receptor associated with?
role in controlling normal body weight and preventing seizures
Which serotonin receptor uses a Gi receptor mechanism?
Which serotonin receptor uses a Gq receptor mechanism?
From what molecule are enkephalins derived from?
What receptor is the cause of respiration depression during opioid o/d?
Mu opioid receptor
people on chronic opioid treatment are more likely to have frequent constipation, why?
stimulation of Mu-opioid receptors
What are the 2 main endocannabinoids?
Anandamide & 2-AG
What is unique/atypically about endocannabinoids?
not stored in vesicles, released from pre-synaptic terminal, have retrograde actions
What is the most abundant cannabinoid receptor in the brain?
What kind of GPCR are CB1 receptors
G i = reduce Ca2+ influx or enhance K+ efflux
Wher are CB 1 receptors found on neurons?
presynaptic terminal = allow for retrograde action
What are CB1 receptors associated with in the spinal cord?
modulation of nociception
What are CB1 receptors associated with in the neocortex?
protects against excitotoxicity
What is important about CB2 receptors?
- anti-inflammatory properties systemically
- found on microglia in brain (uncertain of role)
What are the 2 primary excitatory amino acids?
- glutamate (Know this one)
From which kerbs cycle intermediate is glutamate derived from?
What receptors types are associated with glutamate receptors?
- AMPAs & KAs
What are the two major glutamate ionotropic receptors we care about?
NMDA & AMPA
What is the functional difference between AMPA and NMDA receptors
- AMPA = fast (Na/K+ flux)
- NMDA = slow (voltage gated and Ca2+ flux)
Why do NMDA receptors have a slow response to activation?
Are voltage-gated (Mg2+ blocking channel)
What modulators can bind to NDMA receptors?
- glycine (required for opening)
- Mg2+ (blocker/plug)
- PCP (blocker)
What amino acid is a co-agonist and is required for the activation of NMDA receptors?
What is the role of Mg2+ in regards to NMDA receptors?
sits inside channel to block it; removed when RMP is above ~60mV
What are the roles of AMPA and NMDA receptors in regards to EPSPs?
- AMPA = fast depolarization
- NMDAs = determine duration of depolarization
Where are AMPA and NMDA receptors typically found?
on post-synaptic neurons typically
Which post-synaptic receptor associated w/ EAAs is associated w/ Na+ influx
Which post-synaptic receptor associated w/ EAAs is associated with Ca2+ influx
What class of drugs bind to and inhibit AMPA receptors? What other class of receptors are affected by the same class of drugs?
Benzodiazepines; GABAa receptors
What type of neurons are non-NMDA receptors (AMPAs) associated with?
Primary afferents & UMNs
What type of neurons are NMDARs associated with?
- Long-term synaptic changes (LTP)
What are neurons/cognitive tasks are Metabotropic receptors associated with?
- motor systems
How are EAAs cleared from the synaptic cleft?
- reuptake by neurons
- glial cells
Why is nitric oxide (NO) unique as an NT?
- not stored in synaptic vesicles
- release is Ca2+ independent
- diffuses through membranes
- retrograde and anterograde signaling
How do NMDARs regulate NO?
influx of Ca2+ binds to calcineurin which activates NOS –> NO production
What are the neural effects of NO?
- smooth muscle relaxation
- role in memory
- enhances NT release in CNS
- can lead to excitotoxicity!!!
Here is another image regarding CNS ischemia and what an increase in cytosolic Ca2+ can lead to.