Exam 2: Serotonin and Ach

what kind of molecule is 5 HT?

indolamine (core structure is indole with amine sidechain)

T or F: 5 HT only serves as a neurotransmitter

F: serves as an NT, neuromodulator, and hormone

synthesis of 5HT

1. Tryptophan is converted to 5-HTP by tryptophan hydroxylase (TPH)

2. 5-HTP is converted to 5-HT by AADC (AKA tryptophan decarboxylase) enzyme

difference in location of TPH enzyme subtypes

TPH1: converts tryptophan to 5-HTP (later converted to 5-HT) in non-neuronal cells located in gut and pineal gland

TPH 2: does same function in neuronal cells

which enzyme is a better indicator of 5-HT presence: TPH or AADC?

TPH

what is the rate-limiting enzyme in 5-HT synthesis?

TPH; conversion of tryptophan to 5-HTP occurs at slower rate than the conversion of 5-HTP to 5-HT

how can TPH enzyme activity be sped up?

via phosphorylation by protein kinases

where is the majority of 5-HT found in the body?

in the gut (not the brain!)

rat study of food intake and 5-HT synthesis: high protein low carbs

found that feeding rats more protein-rich foods (high in tryptophan) leads to increases in blood tryptophan but not necessarily brain tryptophan (to be converted to 5-HT)

suggests that tryptophan has to compete with other amino acids to get across the blood brain barrier

rat study of food intake and 5-HT synthesis: high carbs low protein

surprisingly found an increase in brain tryptophan

carbs cause a release. of insulin from the pancreas; insulin stimulates uptake. of glucose and many amino acids (not tryptophan) which reduces competition at the BBB allowing more tryptophan to cross and make 5-HT

indolamine storage and release

stored in synaptic vesicles using vesicular monoamine transporters (VMAT); same as storage and release of catecholamines

where is VMAT1 located?

adrenal medulla

where is VMAT2 located?

CNS

neurons release a _____ amount of NT

predetermined

purpose of VMATs

protects NT from degradation by enzymes

how is 5-HT released?

via exocytosis when the nerve terminal is stimulated

T or F: 5-HT can be released independently from nerve cell firing

true; MDMA/molly/ecstasy can cause this

how is release of 5-HT inhibited?

via autoreceptors

terminal receptors: reduce amount of Ca2+ that enters the terminal and increases opening of K+ channels causing hyperpolarization

somatodendritic autoreceptors: reduce cell firing rate

5-HT somatodendritic autoreceptor

5-HT1A receptor subtype

5-HT terminal autoreceptor

5-HT1B or 5-HT1D receptor subtype

how are indolamines inactivated?

reuptake into nerve terminal through SERT; then NT is either repackaged or metabolized to prevent excessive NT accumulation in the synapse

what enzyme metabolizes 5-HT

monoamine oxidase enzyme (MAO-A)

what metabolite is measured for 5-HT presence? concentration 5-HT

5-HIAA

concentration 5-HT in body (increasing to decreasing)

CSF→bloodstream→urine

where are cell bodies of 5-HT neurons in the brain?

raphe nuclei

where do 5-HT neurons project?

• all forebrain regions

  • dysfunction: mood, social behavior, appetite/digestion, sleep, memory, sexual behaviors

• cerebellum

  • dysfunction: motor learning

• spinal cord

  • sensory, motor, autonomic functions

how do 5-HT neurons act during wakefulness?

each cell produces tonic firing (slow but regular)

seratonin important for wakefulness!

how do 5-HT neurons act during slow-wave sleep?

cell firing slows down from tonic firing (triggered by GABA) and becomes more irregular

how do 5-HT neurons act during REM sleep?

firing almost completely stops

what is a phasic burst of activity?

• burst of firing triggered by glutamate or ACh

• facilitates motor output in cats and suppresses sensory processing

• in rodents, there are phasic bursts in relation to rewards and punishments

what nuclei are important in the sleep-wake cycle and release 5-HT?

raphe nuclei

total number of 5-HT receptor types?

14

metabotropic receptors for 5-HT to know

5-HT 1A, 5-HT 2A, 5-HT 5B

what happens when 5-HT binds to 5-HT1A receptor? where are these receptors found?

adenylyl cyclase is inhibited, decreases cAMP (PKA); these receptors can be autoreceptors or postsynaptic receptors

what happens when 5-HT binds to 5-HT2A receptor? where are these receptors found?

activation of phospholipase C which increases PKC

where are 5-HT2A receptors found?

cerebral cortex, striatum, nucleus accumbens

which receptor subtype do hallucinogenic drugs bind to?

5-HT 2A

5-HT1B/1D receptor

agonists; migraine medication

5-HT3 receptor

antagonists; counteract nausea and vomiting associated with chemotherapy treatment

how is aggressive behavior tested in rats?

resident-intruder paradigm; introduce an unfamiliar male into another male’s home and observe the behavior of the resident

role of 5-HT in aggressive behavior

5-HT regulates aggressive behavior, specifically impulsive aggression; when 5-HT decreases mice are more aggressive, impulsive and compulsive

5-HT is important for what behaviors?

• sleep

• aggression

• hunger/eating

• anxiety

• pain signaling

• learning and memory

5-HT and hunger/eating

some 5-HT receptors when activated produce hypophagia (reduced food intake) and weight loss in rodents; led to obesity medications being produced that worked on these receptors; problem was that these medications also bound to 5-HT receptors in the heart and caused heart abnormalities

5-HT and anxiety

• tested using elevated plus maze

• SSRIs (5-HT1A agonists) used to help treat anxiety disorders

5-HT generally _________ pain signaling

inhibits

5-HT1A, 5-HT4, and 5-HT6 have shown promise for improving ______ performance

learning and memory

structure of acetyl choline

organic chemical made up of acetyl CoA and choline

T or F: Ach acts only as an NT

False: acts as a NT, neuromodulator, and possibly an autocrine/paracrine molecule

ACh synthesis

• Ach neurons contain choline acetyltransferase (ChAT) that brings acetyl-CoA and choline together to make Ach

• rate of ACh synthesis is based on 2 things:

  • availability of precursors inside the cell

  • rate of cell firing

ACh storage

stored in synaptic vesicles using vesicular cholinergic transporters (VAChT)

ACh release

released via exocytosis when terminal is stimulated; neurons release a predetermined amount of NT

what enzyme breaks down ACh to inactivate it?

acetylcholinetransferase (AChE); breaks down ACh into choline and acetate (acetic acid)

where is AChE enzyme found?

1. in the presynaptic terminal (metabolizes excess ACh)

2. on. themembrane of the postsynaptic cell (metabolizes ACh in the synapse)

3. At the NMJ

AChE at the neuromuscular junction

ACh reuptake

once ACh is broken down into choline, choline is taken back into the nerve terminal via a choline transporter; blocking the transporter with an antagonist leads to a decrease in ACh production (choline is a necessary precursor that must get recycled back into the cell to make ACh)

How can AChE inhibitors be used to treat diseases like Alz?

these inhibitors decrease the breakdown of ACh to increase the amount of ACh able to bind to the decreasing amount of cholinergic receptors (cholinergic neurons are dying)

_________ produce a nearly irreversible inhibition of AChE activity

organophosphorus (found in insecticides and nerve gases used in biological warfare); causes overexcited ACh receptors and symptoms like runny nose, crying, drooling, blurry vision, and eventually convulsions/death

treatments for organophosphate poisoning

• atropine; competes with ACh for receptor binding (competitive, reversible antagonist); injected IV

• pyridostigmine bromide (PB); competes with organophosphate for binding to AChE; acts as a reversible inhibitor of the enzyme

ACh pathways: parasympathetic

only ACh NT being released in the pathway

ACh pathways: sympathetic

ACh is released brom presynaptic neuron onto sympathetic ganglion neuron; NE is released onto target organ

where are the cell bodies of cholinergic neurons?

• striatum

  • interneurons for DA neurons

• basal forebrain cholinergic system

  • learning, memory, attention, appetite, feeding

• dorsolateral pontine tegmental region

  • exerts excitatory influence on DA firing in VTA, behavioral arousal, sensory processing, initiation of REM sleep

two types of ACh receptors

• nicotinic (ionotropic); respond to nicotine

• muscarinic (metabotropic); respond to muscarine

what type of receptors are nicotinic receptors

ionotropic

where are nicotinic receptors located?

muscle cells (NMJ), ganglionic neurons of both parasympathetic and sympathetic NS, neurons

PNS neurons: mostly postsynaptic (regulate cell firing)

CNS neurons: mostly presynaptic (regulate NT release)

how many subunits make up the nicotinic receptor?

5 (a, B, gamma, delta, epsilon); subunit composition differs between PNS vs. CNS neurons and muscle cell receptors

how many ACh must bind to a nicotinic receptor in a muscle cell?

2

T or F: nicotinic receptors have a desensitized state

true these receptors can enter a desensitized state when there is a lot of ACh in the synapse; this causes the channel to close on the receptor and it is unable to be opened even in presence of agonist

where are muscarinic receptors found?

CNS: neocortex, hippocampus (memory), thalamus, striatum (motor), basal forebrain

PNS: cardiac muscles, smooth muscles, of organs (bronchioles, stomach, intestines, bladder, urogenital organs)

what are the subtypes of the muscarinic receptor? are they excitatory or inhibitory?

M1-M5

M1, M3, M5: excitatory

M2, M4: inhibitory