Lecture 5 - PNS/Cholinergic Transmission Flashcards
What are muscarine and nicotine?
Alkaloids from fly agaric mushroom (amantia muscaria) and tobacco plant (nicotaine tabacum) - mimick Ach (some but not all therefore, >1 class of Ach receptor)
Systemic effects of Ach
Causes drop in BP due to vasodilation (M3 receptors). Larger dose = bradycardia as well (M2 receptors). If atropine added, Ach has no effect - much larger dose causes increase in BP and tachycardia (stimulation of sympathetic ganglia) followed by 2nd rise in BP (release of adrenaline from adrenal gland).
Nature of cholinergic receptors?
mAch or nAch - 3D structure = rigid and differ significantly. Ach can assume 3D structure similar to both, therefore one neuroT molecule can interact with different receptor proteins based on molecular flexibility.
Nicotinic receptors?
Located postsynaptically in all autonomic ganglia and at NMJ - function as excitatory receptor for post-synpatic cell - release of sufficient quantitiy of Ach from PreSynaptic cell causes excitatory response in autonomic ganglion cells and somatic muscle fibres.
nAchR = 5 subunits - all composedof pentamers arranged around central pore, always 2 alpha-subunits (essential for Ach binding).
Ach receptor = ion channel –> Na+/K+ flux –> excitation.
Nicotinic receptor blocking drugs?
Physiological Ach antagonists = HEMICHOLINIUM = competitive inhibitor or choline uptake. VESAMICOL = blocks Ach transport into synapatic vessels.
Inhibit Ach release = BOTULINUM TOXIN + B-BUNGAROTOXIN = specifically inhibit Ach release.
Non-depolarising blocking agents (post-synaptically) = TUBOCURARINE and structurally similar drugs such as PANCURONIUM GALLAMINE and VECURONIUM are competitive antagonists of Ach @ NMJ endplate.
Suxamethonium?
Depolarising blocking agent - acts as antagonist of Ach receptors at endplate. Unlike Ach it diffuses slowly to the endplate and conc is maintained long enough to cause loss of electrical activity.
Mainly used to faciliate tracheal intubation but can be used in surgery for muscle relaxation and flaccid paralysis.
Would think it causes tetany. nAchR binding causes depolarisation and Ca2+ release from SR, but not broken down by acetylcholinesterase + so motor and end=plate remains depolarised, preventing Ach from causing depolarisation. Ca2+ pumped back into the SR, taken up by mitochondria and pumped out of cell causing cytoplasmic [Ca2+] to decrease –> relaxation.
Side effects = malignant hyperthermia, hyperkalaemia, bradycardia, cardiac arrest.
Acetylcholinesterases?
Break down Ach at NMJ. Anti ACE drugs such as NEOSTIGMINE enhance cholinergic transmission.
Myasthenia gravis - 1/3 the number of receptors due to antibody/
Nicotinic Ach receptors?
Fast acting molecular switch, composed of: extracelluar ligand binding domain and gated, membrane spanning pore. Opened by NT binding and resultant transient conformational change –> ion flow –> change in membrane potential.
Cys-loop family - share common features - pentameric with entral pore, cation selective, Ach binding sites (2) on alpha-subunits, four membrane spanning regions in each subunit.
Constricted central region due to interactios of amino acids –> tight hydrophobic girdle –> barrier for ion flux. Binding –> conformational change that makes it permeable to ions.
Describe conformational change in nAchRs?
Ach induced rotations in subunits transmitted to gate through M2 helices. Rotations destabilise the gate causing the helices to adopt alternative configuration that is permeable.
Helices move freely because they are separated from outer protein wall + connected by flexible loops containing glysine residues. S-S disulphide bridge pvot in ligand binding domain, anchored to the fixed outer shell of the pore.
Location of muscarinic Ach receptors?
Located post synaptically at parasympathetic neuro-effector junction - act to increase/decrease activity of effector cells. Also located post-synaptically neuroeffector junction of sympathetic fibres in sweat glands; function to increase sweating, antimuscarinics can interfere with sweating.
Also located on endothelial cells of blood vessels - not innervated by cholinergic nerve fibres but are sensitive to circulating molecules.
Types of muscarinic receptors?
5 discovered (M1 - M5 = GPCRs)
M1, M3 + M5 = coupled to PLC beta by Gq –> generate IP3 and DAG.
M2 + M4 negatively coupled to andeylyl cyclase by Gi –> decrease cAMP.
Gby subunit can also modulate ion channel acitivty, including Na+, K+, Ca2+ and cyclase nucleotide-gated channels.
Logothetic et al, 1987?
First direct effector identified for Gby was activation of cardiac muscarinic-gated inwardly rectifying K+ channel responsible for M-current in atrial myocytes.
Similar direct currents have been described for inhibition of neuronal M-current (mediated by M4 mAchR); inhibition of VG Ca2+ channels (Cav2), but also L-type and T-type and the hyperpolarisation-activated cyclic nucleotide-gated HCN family of channels (IF or funny current) - KEY ROLE IN CARDIAC AND NEURONAL PACEMAKING.
Model of mAchR?
Helical wheel model - Asp147 = involved in ion-ion interaction with ammonium group of Ach. Series of Thr and Tyr resiudes critical for high affinity Ach binding.
M1?
NEURAL - CNS and peripheral neurones.
Mainly excitatory. Interactions with PLC (cleaves PIP2) –> IP3 and DAG. Excitation caused by decrease n K+ conductance –> activates PLC –> depolarisation.
Limited supply of PIP2 so activation of mAchR –> depletion. M-channels (VG K+ channels) are gated by voltage but require a certain density of PIP2 in membrane for open state to be stabilised, therefore close, therefore decrease K+ conductance.
M2?
CARDIAC - heart and presynaptic terminals of PNS/CNS
Inhibitory - increase K+ channel conductance and inhibit Ca2+ channels. Responsible for vagal inhibition of heart. Increase in K+ conductance due to activation of GP gated inward rectifier K+ channels (GIRK) by beta/gamma subunits (BROWN, 2010).
Gai - ADP-ribosylated pertussis toxin (PTX) produced by bordatella pertussis and thus INACTIVATED. Unable to inhibit adenylyl cyclase production of cAMP. Currently being investigated for treatment of AI conditions (MS) - (weber, 2010).
