Neuro 15 - Ach Cont. And Catecholamines

Front 1

what’s the effect of Ach esterase inhibitors?

Back 1

inhibitors of acetylcholine esterase will potentiate the effects of ACh

Front 2

what are some other, more dangerous inhibitors of ACh esterase? what’s their common chemical property?

Back 2

other inhibitors of ACh esterase include nerve gases like tabun and sarin, and an insecticide parathion. the common chemical property is that they are all phosphate esters with an “active ester bond,” P-CN, P-F, etc. that are highly polar and very susceptible to hydrolysis

Front 3

what’s the mechanism for parathion, tabun and sarin? what are the effects on ACh?

Back 3

when the insecticide or nerve gases gain access to the neuromuscular junction, the ACh esterase will hydrolyze it and you get a phospho-serine intermediate, which is irreversible, so it’s like a suicide reagent for the enzyme. it potentiates the effects of ACh in an uncontrolled way

Front 4

what is PAM? how does it work?

Back 4

pyridine aldoximine methiodide (PAM), an older drug, an antidote for nerve gas poisoning. the pyridinium group orients in the anonic site and the monoxine moiety hydrolyzes off the phosphate ester to regenerate active enzyme

Front 5

soldiers are given muscarinic antagonists. what do they protect against?

Back 5

muscarinic antagonists to protect the SNS from overstimulation

Front 6

what are the 4 steps of ACh synthesis up to release?

Back 6

1) single condensation reaction generates ACh in the cytoplasm, 2) the enzyme choline acetyltransferase catalyzes the rxn (CAT - located uniquely in cholinergic neurons), 3) the ACh is taken up into the vesicles, concentrated there, 4) released upon depolarization

Front 7

what are the 3 steps following ACh release?

Back 7

1) where at the post-synaptic site, it interacts with the cognate receptors: nicotinic and muscarinic, which have different functions and localizations in the CNS and periphery. 2) ACh is rapidly hydrolyzed by choline esterase, generating acetate and choline, 3) the latter of which is taken up into the nerve terminal through a specific transporter to be used in de novo synthesis.

Front 8

what’s the key step for regulation of ACh levels?

Back 8

Reuptake of choline is key to regulation of ACh levels –Choline is rate limiting in ACh biosynthetic pathway; it’s a well-regulated system even at the level of the individual neuron

Front 9

where are cholinergic neurons found?

Back 9

you should know all motor neurons are cholinergic, cell bodies in the ventral horn, projections etc.

Front 10

what degenerates in Huntington disease?

Back 10

striatal interneurons degenerate in Huntington disease

Front 11

what’s one thing that degenerates in the brains of people with Alzheimer’s disease? and what has been an attempted therapeutic strategy?

Back 11

cell bodies in the nucleus basalis* with rather diffuse projections are important because they seem to degenerate early in the brains of people with Alzheimer’s disease, so some of the therapeutic strategies had been to increase cholinergic transmission in early-onset patients to help with cognitive function

Front 12

what do congenital myasthenic syndromes affect? what happens with neurotransmission?

Back 12

Presynaptic Function. would all reduce neurotransmission at the neuromuscular junction, translating into muscle weakness but very rare (e.g. Choline acetyltransferase (CAT) deficiencies reduce the amount of ACh available for release (6 cases); Paucity of presynaptic vesicles (1 case), may be embryonically lethal

Front 13

what’s Myasthenia gravis? what’s a hallmark feature?

Back 13

Myasthenia gravis is a neuromuscular disorder characterized by weakness of skeletal muscle. hallmark feature: Ptosis with eyebrows up

Front 14

what are 7 common features of myasthenia gravis?

Back 14

Common features of MG: 1) Only the motor system is impaired, 2) Likely to affect cranial nerves and limb muscles, 3) Fatigability of muscle upon use*, 4) Severity of disease can vary dramatically during course of a single day. 5) No clinical signs of denervation which would lead to muscle atrophy and loss of reflexes but 6) reflexes intact and 7) weakness can be reversed by acetylcholinesterase inhibitors

Front 15

what do you see on electromyleogram (EMG) with MG patients?

Back 15

Characteristic abnormal electromyleogram (EMG) seen with MG patients, the fast-stem muscle stimulation decline/decrement

Front 16

what is the hypothesis explaining MG? what type of disease is it?

Back 16

rabbit gave rise to the MG hypothesis that the production of auto-antibodies against the ACh nicotinic receptor as cause of the disease, (best evidence is when injecting antibodies from people with MG and inject into animal model causing disease, “passive transfer”)

Front 17

what 2 structural changes do you see at the neuromuscular junctions of patients with myasthenia gravis?

Back 17

Reduced AChR sites at the neuromuscular junctions of myasthenia patients, by 90% in biopsies; 2) can also see damaged synapses (probably due to inflammation), certainly part of the story

Front 18

what happens to antibodies in patients with MG? what are the consequences?

Back 18

1) if normal turnover is 5-7 days, in MG, antibodies cross-link; they are bivalent and this will increase turnover as receptors are aggregated by antibodies and then go through lysosomal destruction --> decline in receptor number (measured clinically in patients);

Front 19

what’s the overall goal of treatment in MG? what agents are used?

Back 19

to enhance transmission across the synapse. cholinesterase inhibitors are effective because whatever receptors are present, the ACh will be increased and its action will be prolonged at the synapse, which seems to help. Need to worry about immunological involvement as well, so immunosuppressants are often prescribed, and ways to reduce inflammation (steroid therapy) (remember case, thymectomy, plasmaphoresesis 2x/wk)

Front 20

what’s Lambert-Eaton syndrome? compare and contrast with MG.

Back 20

Lambert-Eaton syndrome: Rare myasthenic disease also involving proximal muscle weakness – most characteristic but weakness seen in RESTING muscle and improves with use. Different from MG patients who tend to have leg weakness and difficulty walking.

Front 21

what’s the muscle response in Lambert-Eaton syndrome? what ion is involved?

Back 21

Very different muscle response than seen with MG patients (like opposite), called facilitation with fast rep. stimulation. calcium tends to accumulate and you will then get a more normal type of release. Repeated stimulation leads to Ca accumulation in presynaptic terminal, restoring normal release

Front 22

what are some symptoms you see in Lambert-Eaton syndrome? what’s the etiology in many patients?

Back 22

In Lambert-Eaton syndrome, you see autonomic dysfunction e.g. dry mouth, dry eyes, constipation. In a significant fraction of patients, it’s a paraneoplastic disorder, meaning it can be associated with small cell lung cancer tumors. It, too, is an autoimmune disease.

Front 23

how do scientists show that a disease is autoimmune?

Back 23

for autoimmune diseases, you can passively transfer disease to mice via antibodies from people with the disease. unknown why certain people start making these antibodies and others don’t

Front 24

people with Lambert-Eaton syndrome have high titers of antibodies against the ____ channel

Back 24

people with Lambert-Eaton syndrome have high titers of antibodies against the calcium channel

Front 25

what happens with calcium channels in Lambert-Eaton syndrome?

Back 25

by blocking the amount of calcium entering the cell with the autoantibodies, it causes dysfunction. as the neuron depolarizes, normally you get calcium influx and release, but this doesn’t happen properly.

Front 26

Why would tumors lead to anti-Ca channel antibody production?

Back 26

in the small-cell lung carcinomas, some have calcium channels on their surface, so immune system is being presented these calcium channels in an abnormal way, so the immune system in these patients make antibodies in response

Front 27

what’s the overall strategy for treating Lambert-Eaton syndrome?

Back 27

you need to enhance calcium current, can do so with potassium channel blockers, because K+ efflux is a repolarizing event for the membrane so if you block repolarization, you’ll have longer depolarization and thus increased calcium influx (e.g. 3,4-DAP); non-used muscle strength being restored. choline esterase inhibitors don’t work… “that’s what we see”

Front 28

contrast the 2 types of Myasthenic acquired diseases.

Back 28

2 types of Myasthenic acquired diseases: post-synaptic (MG on ACh nicotinic receptors) vs. pre-synaptic effect (Lambert-Eaton syndrome on calcium channels)

Front 29

what are the 2 famous catecholamines? what’s the main structural difference?

Back 29

dopamine -> dopaminergic; norepinephrine/ noradrenaline -> noradrenergic (hydroxyl group at the beta position)

Front 30

what’s the precursor for all catecholamines?

Back 30

Tyrosine serves as the precursor taken up from the circulation for all of the catecholamines.

Front 31

what’s the first step in the dopamine synthesis pathway?

Back 31

the first step is hydroxylation of tyrosine in meta position, catalyzed by Tyrosine hydroxylase, and the product is L-dopa

Front 32

what’s a marker protein for dopaminergic neurons?

Back 32

The enzyme (tyrosine hydroxylase) is uniquely localized in catecholaminergic neurons, located in the cytoplasm and highly localized in the nerve terminals of neurons (just like cholinergic), the major site of biosynthesis is at the nerve terminals itself

Front 33

what’s involved in the rate-limiting step in dopamine synthesis?

Back 33

Tyrosine hydroxylase is involved in the rate-limiting step, the regulatory step here; it’s activated by calcium and cAMP and other second messengers

Front 34

what controls the local rate of dopamine synthesis?

Back 34

end-product inhibition; negative feedback. there’s actually allosteric binding site on the enzyme which is capable of recognizing the product, dopamine or norepinephrine. if there’s too much product being made, then you get this feedback effect. this is important for the local rate of synthesis

Front 35

what inactivates tyrosine hydroxylase? reversible or irreversible? why important?

Back 35

Norepinephrine binds tyrosine and forms an inactive complex, which is reversible. this regulatory feedback is key to keeping everything in balance, to keeping a steady pool of neurotransmitters, enough there to be released but not too much!

Front 36

in what scenario would tyrosine hydroxylase be inactivated?

Back 36

during periods of relatively low activity, NE will tend to accumulate in the nerve terminal, tyrosine hydroxylase will be converted into an inactive form

Front 37

when would tyrosine hydroxylase levels become active? slow or fast?

Back 37

if you go to a high-impulse flow condition and NE levels decline as it’s being released in the nerve terminal. tyrosine hydroxylase, by mass action, will tend to be liberated and you go from an inactive to an active form; this can happen very quickly!

Front 38

if you had to make more tyrosine hydroxylase rather than activate it, would the process be slow or fast?

Back 38

if you had to make more enzyme (which occurs up in the cell body and is transported down to the nerve terminal, etc.), there’s no way to have a regulatory process that will quickly respond to changes in impulse flow like this.

Front 39

what are autoreceptors?

Back 39

autoreceptors are presynaptic receptors activated by the transmitter released from the same synapse (found in D2 receptors).

Front 40

what’s the role of autoreceptors in the dopaminergic biosynthetic pathway?

Back 40

they regulate tyrosine hydroxylase. autoreceptors are involved in negative-feedback loop mediated through activation of presynaptic receptors.

Front 41

what happens if dopamine levels rise to too high a level?

Back 41

normally, dopamine will act at its post-synaptic site, but if dopamine levels rise to a level that’s too high, there’s negative feedback. you begin to activate autoreceptors which (through signaling) dampen tyrosine hydroxylase activity at the local synapse.*

Front 42

what happens to L-dopa?

Back 42

L-dopa is not around very long. it’s rapidly converted into dopamine by an enzyme called DOPA decarboxylase (a very active though not very specific enzyme)

Front 43

what is L-dopa used for therapeutically? why?

Back 43

in Parkinson’s patients where losing dopaminergic activity, L-dopa is used as a drug in early stages to compensate partially for that loss. by providing more of this direct precursor of dopamine you can improve neurological function.

Front 44

where is dopamine made? where is it stored?

Back 44

after dopamine is made in the cytosol, it is taken up into vesicles at the nerve terminals (like ACh)

Front 45

what’s the final step of NE synthesis in noradrenergic nerve terminals? what enzyme is involved?

Back 45

a final step in the noradrenergic nerve terminals is the hydroxylation at the beta position of DA by the enzyme DBH, dopamine-beta-hydroxylase.

Front 46

where is DBH?

Back 46

DBH is located in the synaptic vesicles themselves. the last step in the noradrenergic neurons does take place in those vesicles. dopamine is taken up and converted into norepinephrine there.

Front 47

what’s a marker protein for noradrenergic neurons in the brain?

Back 47

DBH can be used as a marker for noradrenergic innervation because it’s specific

Front 48

Back 48

(norepi only in noradrenergic neurons)

Front 49

what two enzymes are involved in the degradation of catecholamines?

Back 49

there are 2 *important* catabolic processes for degradation of the catecholamines: Monamine oxidase (MAO) and Catecholamine-O-methyl transferase (COMT). Neither one of these enzymes is responsible for inactivation of synaptic activity** they are more catabolic, part of the steady-state turnover.

Front 50

what’s MAO? what does it do?

Back 50

Monamine oxidase (MAO), mitochondrial enzyme, takes any amine and converts into its corresponding aldehyde. kind of functions as a scavenger, will chew up a variety of amines, certainly catecholamines in the nerve terminal will be degraded into deaminated metabolites

Front 51

what’s COMT? what does it do?

Back 51

Catecholamine-O-methyl (COMT), the other enzyme, which adds a methyl group to catecholamines, taking it down a metabolic pathway toward degradation and ultimate excretion

Front 52

what’s the effect on catecholamines if you inhibit MAO or COMT?

Back 52

you can inhibit these enzymes, e.g. COMT, and you will NOT potentiate the effects of catecholamines at the nerve terminals (very different from ACh and acetylcholinesterase inhibitors)

Front 53

what inactivates dopamine and what potentiates DA?

Back 53

dopamine (DA) is inactivated by a specific transporter expressed on the pre-synaptic terminal that takes DA back up into the nerve terminal itself. this process is critical because inhibitors for the transporter will indeed potentiate the effects of DA;

Front 54

what can potentiate the effects of NE?

Back 54

similarly, inhibitors of the norepinephrine (NE) transporter will potentiate the effects of NE

Front 55

what are 2 roles of transporter proteins?

Back 55

1) the transporters are responsible for high-affinity reuptake and 2) they have the main role in inactivation / clearance.

Front 56

what’s the gene family for transporters? what’s their structure? what’s a common feature?

Back 56

transporter proteins are part of the SLC6 gene family. are large and complex. their structure: 12-transmembrane segments, assemble into dimers. all share common features e.g. use Na as a co-transporter to bring neurotransmitter back into the nerve terminal

Front 57

what does the DAT transporter transport? what inhibits it(3)? useful in what diseases(2)?

Back 57

the DAT transporter has endogenous substrates DA, NE, Epi. its inhibitors are benztropine, cocaine, amphetamines. drugs inhibiting the DAT transporter have been used in ADHD and Parkinson’s

Front 58

what does the NET transporter transport? what inhibits it(2)? useful in what disease?

Back 58

the NET transporter also has endogenous substrates DA, NE, Epi. its inhibitors are nortriptyline and desipramine, which have been used to treat depression

Front 59

what does the 5-HT transporter transport? what inhibits it (3)? useful in what diseases (3)?

Back 59

the 5-HT transporter transports serotonin. its inhibitors include fluoxetine, MDMA (ecstasy), and escitalopram. therapeutic uses for inhibitors include treatment of OCD, anxiety, and depression

Front 60

there’s a long-term link between catecholamines and what disease?

Back 60

there’s a long-term link between catecholamines and depression

Front 61

if inhibiting SLC6 gene family transporters is useful for treating disease, what does this tell us about the molecular basis for pathophysiology?

Back 61

that some disorders may be due to mutations of various SLC6 gene family transporters or other parts of the pathway. understanding their pharmacology has considerable clinical utility

Front 62

what's the overall strategy in the metabolism of dopamine?

Back 62

catechols can be methylated and can be oxidized, giving rise to various adducts. can occur in either order and in either way.

Front 63

what can happen to aldehydes produced by MAO?

Back 63

the aldehydes produced by MAO can either be reduced to an alcohol or oxidized to an acid

Front 64

what generates HVA? why important?

Back 64

in the case of dopamine, oxidation to DOPAC and ultimately methylation generates homovanillic acid (HVA). so measuring HVA levels in CSF is a diagnostic signal for dopaminergic activity**

Front 65

when are HVA levels used clinically? what’s an example?

Back 65

particularly important in “disorders of basal ganglia. for example, for Parkinsonian disorders, where your patient is losing dopaminergic activity, HVA levels are depressed in the CSF. with treatment, HVA levels will change. (no good signal in the periphery)

Front 66

what’s a useful clinical assay for NE? what’s a disease it’s used for?

Back 66

in the case of norepinephrine, the most useful assay has come from the periphery where oxidation of the aldehyde occurs to the corresponding acid, VMA (vanillylmandelic acid), which is a very useful diagnostic signal for norepinephrine in the periphery. e.g. in pheochromocytoma (NE-producing tumors in the adrenal gland), VMA levels in the urine are highly elevated. (there’s no good signal in the brain, so no useful diagnostic assay in that case)

Front 67

what’s the structure of the adrenergic receptors?

Back 67

for all of the adrenergic system, the adrenergic receptors have the classic 7 transmembrane sequence*, a motif that binds and activates G proteins.

Front 68

what are the 2 main families of noradrenergic receptors?

Back 68

Noradrenergic receptors, 2 main families: α receptors and β receptors. various subtypes

Front 69

dopamine receptors are important in what disease?

Back 69

dopamine receptors are important in schizophrenia.

Front 70

compare and contrast DA-1 receptors vs. DA-2 receptors

Back 70

localized in different areas of the striatum. DA-2 receptor found on corticostriatum afferents vs. DA-1 receptor found on interneurons in the striatum. also, DA-1 is linked to adenylate cyclase and regulated by guanine nucleotides.

Front 71

what’s the effector of D1 receptors? what’s the effect of bromocriptine on it?

Back 71

D1 receptors effector is cAMP, bromocriptine (a small-molecule alkaloid) is an antagonist, chromosome 5

Front 72

where are D2 receptors found? what’s the effect of bromocriptine on them? the consequences?

Back 72

D2 receptors are found in the pituitary, and if activated, they inhibit the release of prolactin** bromocriptine is an agonist. (chromosome 11)

Front 73

which dopamine receptors have autoreceptors?

Back 73

D2

Front 74

neuroleptic (anti-psychotic) drugs are potent antagonists of what type of receptors?

Back 74

neuroleptic (anti-psychotic) drugs are potent antagonists D2 receptors.

Front 75

where in the brain is DA? (5)

Back 75

dopamine has rather distinct localizations to the substantia nigra, striatum, olfactory “tubercle,” the frontal cortex and median eminence

Front 76

which catecholamine is more diffusely localized in the brain?

Back 76

NE is more diffusely localized throughout the brain. different from dopamine.

Front 77

what are marker proteins (e.g. DBH) used for?

Back 77

amino fluorescence and marker proteins used to map pathways, creating an “antibody sandwich” e.g. using tyrosine hydroxylase as a marker for dopaminergic pathways, DBH as a marker for noradrenergic receptors

Front 78

what’s a very important pathway for dopamine that you should know?

Back 78

should know the classic nigrostriatal pathway* for dopamine. it has cell bodies in the substantia nigra and ending up in the striatum (And Reha said “caudate putamen”),

Front 79

what pathway degenerates in Parkinson’s disease?

Back 79

the nigrostriatal pathway degenerates in Parkinson’s disease** this pathway connects different parts of the basal ganglia, which are involved in the coordination of motor movement. the degeneration of the pathway accounts for the “interesting spasticity and motor movement you see in patients with Parkinson’s.

Front 80

what’s another important dopaminergic pathway? where are its cell bodies?

Back 80

the other one is the mesolimbic pathway, also with cell bodies in the substantia nigra with projections up through the medial forebrain bundle into the olfactory tubercle, nucleus accumbens and the frontal cortex. this accounts for the localized distribution of dopamine there.

Front 81

NE-producing neurons projection from what region? terminate where?

Back 81

rather diffuse projections from the locus coeruleus throughout the brain terminate in spinal cord, brainstem, basal ganglia and entire isocortex. less active during periods of tonic vegetative behavior

Front 82

why is Parkinson’s disease so important?

Back 82

Parkinson’s Disease, is the classic example of the relationship between catecholamines and disease. you see a loss of critical dopaminergic pathway in the basal ganglia, which is why why L-dopa is a good drug because at least in early stage Parkinson’s, helps compensate for loss of dopamine with neurodegeneration. also why HVA is depressed because losing a major dopaminergic system in the brain.

Front 83

what’s the catecholamine hypothesis of affective disorder?

Back 83

basically posits that low levels of dopamine may explain depression and elevated levels may explain mania in bipolar disorder but turns out the disease are more complicated. e.g. serotonin is also important.

Front 84

what’s the therapeutic strategy based on the catecholamine hypothesis?

Back 84

to give drugs which elevate catecholamine levels, e.g. MOA inhibitors and transport inhibitors, for treatment of depression, which seem to be effective (at least for some)