Peripheral neurotransmission 1

What does the somatic nervous system typically innervate?

Skeletal muscle

What does the autonomic nervous system typically innervate?

Cardiac, smooth muscle, secretory epithelia and glands

What is the main type of transmission in PNS?

Chemical synaptic transmission

What is the main neurotransmitter for the somatic NS?

ACh

In the ANS what is unique about the sympathetic post-ganglionic nerves at the effector tissues?

Forms bulbous expansions/varicosities that are distributed along their axons within their target organ. At each varicosity autonomic axons form an “en passant” synapse with the target tissue.

Where is ACh released?

-All autonomic ganglia

-All postganglionic parasympathetic nerve endings

-Adrenal medulla and sweat glands 

Where is NA released from?

All sympathetic postganglionic nerves except sweat glands

What is predominantly released from adrenal medulla?

Mainly adrenaline and some NA into the blood

NANC (non-adrenergic non-cholinergic) transmitters

-ATP,

-Nitric oxide (NO)

-Some neuropeptides such as vasoactive intestinal peptide (VIP) (parasympathetic) and neuropeptide Y (NPY) (sympathetic)

-Serotonin (5-HT)

-GABA

-Dopamine

Where are small molecule neurotransmitter synthesised?

Within nerve termini and accumulate into clear core vesicles

Where are neuropeptides synthesised?

Cell body then packaged into large dense core vesicles that reach nerve termini through axonal transport.

What is synaptotagmin?

Calcium sensor

What is synaptobrevin?

v-SNARE

Where are VGCa2+C clustered?

At the active zone

What are 3 major sites for pharmacological intervention at the periphery?

Site A: presynaptic nerve terminal

Site B: synaptic/junctional cleft

Site C: post-synaptic nerve membrane

Two ways pharmacological agents can modulate peripheral neurotransmission?

a) Direct action

b) Indirect action

Direct action

Agent directly binds to the postsynaptic target receptors (ionotropic/LGICs and/or metabotropic/GPCRs) as an agonist, antagonist, allosteric modulators or as a pore blocker (for LGICs), resulting in stimulation/enhancement/reduction/ blockade of neurotransmission

Indirect action

Agent does not directly bind to the postsynaptic targets of the release neurotransmitters, instead it modulates the amount of neurotransmitter released in various ways.

ACh synthesis

-availability of choline within the pre-synaptic nerve terminal is critical

-either be secured from the diet or hydrolysis of the released ACh.

-Often this reuptake of choline within the pre-synaptic nerve terminal can be rate limiting

How does choline uptake work?

mediated by a high affinity choline transporter that cotransports Na+ and can be inhibited pharmacologically

hemicholinium

Experimental compound that inhibits choline transporter

What acetylates choline?

ACh by Choline AcetylTransferase (CAT). The acetyl group comes from Acetyl CoA originating from the mitochondria.

triethylcholine

Dummy acetylcholine (false neurotransmitter (e.g. acetyltriethylcholine)) too fool choline transporter and CAT

ACh Storage

-ACh is loaded into cholinergic vesicles by a Vesicular ACh Transporter (VAChT) that at the same time extrudes H+.

-H+ accumulates into the vesicles by a V-type ATPase

-ACh containing vesicles are often clear core (also contains ATP) and in some cases, dense core (also contains a neuropeptide such as VIP). Both ATP and VIP function as a co-transmitters at some synapses.

ACh is released

-via exocytosis

Fate of the released ACh - binding to receptors at postsynaptic/junctional membrane

2 major classes - muscarinic (mAChRs and nicotinic (nAChRs).

These receptors differ in their structures, how they work and in their different affinities for agents that mimic the action of ACh (= cholinomimetic agents).

Nicotinic ACh receptors (nAChRs)

-ligand-gated ion channels (LGICs)

-belonging to the 'Cys-Loop' family

-present at the NMJs, autonomic ganglia, adrenal medulla, the CNS and some other non-neuronal cells

-a pentamer consisting of alpha,beta,gamma,delta and epsilon subunits with at least 2 alpha subunits present

-Each subunit consists of four transmembrane (M1-M4) helices of which the M2 helix from all five subunits together form the pore

-Minimally 2 molecules of ACh are required for activation

-Most nAChR isoforms are permeable to Na+ and K+

-one of the CNS isoforms namely the (alpha7)5 channel, is highly Ca2+ permeable

-nAChRs can be divided into the Nmuscle (Nm) and Nneuronal (Nn) subtypes

Muscarinic ACh receptors (mAChRs)

-All mAChRs are metabotropic

-Besides ACh, they also recognize muscarine, an alkaloid present in certain poisonous mushrooms

-show only a weak affinity for nicotine

-present on the membranes of effector tissues including heart, smooth muscles,glands

-5 identified subtypes of mAChRs, of which M1, M2 and M3 are the most important isoforms in the periphery

-M2 subtype is found pre-synaptically on the endings of noradrenergic nerves and cholinergic nerves where its activation causes a decrease in transmitter release

Fate of the released ACh - degradation at the synaptic/junctional cleft

-most of the ACh is rapidly hydrolyzed by acetylcholine esterase (AChE) which is tethered to the post-synaptic membrane

-A 'soluble' form of AChE is present in the cytoplasm of the presynaptic nerve but understandably it doesn't affect the life span of the released AChE.

Presynaptic modulation of ACh release.

-mediators, including ACh itself, acting on presynaptic receptors

-muscarinic M2 receptors participate in inhibition of ACh release in autocrine manner

-NA via its cognate receptors can also inhibit the release of ACh in paracrine manner

-At the NMJs presynaptic nicotinic receptors facilitate ACh release, a mechanism that may allow the NMJs to function reliably during prolonged high-frequency activity

-agent interfering with the generation of action potential and/or Ca2+ signals in the presynaptic nerve terminal will also affect ACh release

Where does muscle-type nAChRs normally present?

the muscle-type nAChRs (Nm) are present at the upper part of the junctional fold

Where do VGNa+C normally present at NMJ junctions?

the voltage-gated sodium channels (Navs) are present deep in these folds as well as at the non-junctional parts of the sarcolemma

Where is AChE present in NMJ?

At the NMJ, ACh is hydrolysed by AChE present in the basal lamina within the junctional cleft

Agents that directly target Nm - neuromuscular blocking agents

-directly bind to the Nm receptors and inhibit them

-main clinical use is as adjuvant to general anaesthesia to relax skeletal muscles for surgery and endotracheal intubation

-significantly increase the safety of anaesthesia thereby minimizing respiratory and cardiovascular depression and allowing rapid recovery

-These drugs do not cross the blood-brain barrier (BBB) and their action remains restricted to the periphery

Depolarizing neuromuscular blocking agents

-agonists of the Nm receptors

-not readily hydrolyzed by the AChE within the junctional cleft

-depolarization they cause last longer

-a brief period of repetitive excitation eliciting transient and repetitive muscle excitation (fasciculations)

-followed by blockade of neuromuscular transmission and flaccid paralysis (phase I block)

-paralysis occurs because sustained depolarized state of the sarcolemma leaves all the Nav channels in inactivated state

Suxamethonium

succinylcholine- Depolarizing neuromuscular blocking agents

What is phase 2 block?

Wwith increasing concentrations of suxamethonium over time, the depolarizing (phase I) block slowly turns into a non-depolarizing (phase II) block. During the latter, the membrane gradually repolarizes but now the Nm channels become desensitized.

Suxamethonium has several unwanted effects…

-hyperkalemia (+ relevant cardiovascular risk) in patients with burns, trauma or muscle injury.

-fast onset and brief duration of action are required, e.g. with tracheal intubation.

-The very brief duration of action is due to rapid hydrolysis in the plasma by butyrylcholinesterase (BuChE, also known as pseudocholinesterase).

-This enzyme is synthesized in the liver and patients with liver disease or genetic deficiency of this enzyme therefore run the risk of prolonged action of suxamethonium.

Non-depolarizing neuromuscular blocking agents

-competitive antagonists of the Nm channels

-compete with the endogenous ACh without stimulating it

-prevent depolarization of the sarcolemma and inhibit muscular contraction, leading to a flaccid paralysis.

curare

-non-depolarising type blocker

-cocktail of natural alkaloids

-D-tubocurarine is not used clinically due to its poor selectivity between the ganglionic and NMJ-specific nAChRs

-its early analogs are by now replaced by safer, synthetic agents

What are the 2 groups chemical agents divide into?

-the aminosteroid group (the ones often with curonium suffix) such as rocuronium

-the isoquinolinium group (the ones often with the curium suffix) such as atracurium

What is the duration of action of non-depolarising NMJ blocking drugs?

-slower onset of action than suxamethonium.

-They can be short-acting (<30 minutes)

-intermediate-acting (30–40 minutes)

-Long-acting (60–120 minutes)

-Drugs with a shorter or intermediate duration of action, such as atracurium and rocuronium, are more widely used than those with a longer duration of action.