PH3113 Unit 1

what controls excitablility of a cell

the presence of voltage gated na+ channels

role of nav channel

initiating/rising phase of the action potential, voltage dependent gating leading to depolarisation

distribution of nav channels

plasma membrane, concentrated at nodes of ranvier in myelinated neurones

subtypes of nav channels

9 alpha subunit isoforms

structure of nav channels

multimeric transmembrane complexes

• large pore forming alpha subunit

• one or two smaller beta subunits (regulate alpha)

voltage sensing

S4 moves in response to membrane voltage changes

this causes a conformational change in the channel which allows the gate to open, allowing current to flow

fast innactivation

channels can be open, closed or innactivates

domains III and IV linker acts as a physical barrier to ion permeation. IFM amino acids bind to hydrophobic residues in the cytoplasmic vestibule

local anaesthetic mechanism

action by reversible nav channel blockade

lipophilicity important for cell permeation and charge for block

mode of entry dependent on drug

sodium voltage channels are targets in pain, block ap in all sensory, motor and autonomic fibres

use dependent block

use dependent block

la inhibition of na current increases with repetitive depolarisations

results from open and innactivated channels having greater affinity than closed/resting channels

repetitive depolarisations = increased chance that la encounters open or innactivated channels

antiepileptics and vgc

lamotrigine, phenytoin and carbamazepine bind preferentially to open sodium channels during inactivation

little effect on normal ap

voltage and use dependent block only when periods of sustained high frequency discharges

ca2+

flexible, precisely regulated, achieved by antiport and uniport mechanisms

kept low in cytosol, high in extracellular environment and intracellular stores

temporal and spatial control of cytostolic signals

cav channel theory of excitation

hallmark of excitable cells

e is even more positive than ena

translate electrical signals into chemical signals

regulation of intracellular events possible

cav channel structure

alpha 1 subunit forms ion conducting pore

alpha 2 delta subunit = trafficking and drug binding

beta and gamma subunit = trafficking and biophysical properties

pregabalin mechanism

binds to alpha 2 delta

prevents ca2+ influx

prevents neurotransmitter release blocking signal

for neuropathic pain and epilepsy

nach-r

found highly in electric organ of electric fish

fluorescent a-bungarotoxin binds with high selectivity for protein

nicotinoid receptor structure

made up of 5 subunits

each subunit has 2 TM spanning helices

M2 TM domain lines the pore

charged amino acids at the top and bottom create selectivity

gate = ring of leucines, closed when no ligand, determines pore size

5HT-3 receptors in PNS

involvement in

• triggering exocytosis

• reflex bradycardia

• hypotension

• pain sensation

5HT-3 receptors in CNS

involvement in

• fast epsps

• neurotransmitter release

5HT-3 agonist use

may alter mood

ie varenicline

5HT-3 antagonist use

nausea and vomiting

• ondansetron

post operative neuropathic pain

headache and migraine

GABAc receptors

10 fold more sensitive to GABA than A

holomeric and heteromeric forms in humans exist

AMPA glutamate receptors

widespread throughout CNS, regional specificity for subtypes

responsible for fast synaptic excitation

kainate glutamate receptors

slight permeability to ca

not highly expressed, not involved at synapses as much

more subtle role in ensuring neurons will fire in response to another signal

NMDA receptors

pore is permeable to ca2+ but not mg2+

expressed throughout CNS/PNS

excitatory

structurally complex

P2X receptors

fast synaptic transmission

neuron smooth muscle communication

ATP mediated lysis of antigen presenting cells

GCPR

mediate cellular responses to neurotransmitters, hormones and other ligands

responsible for sight, smell and taste

7 TM proteins associated with a heterotrimeric G protein

they bind GTP and GDP

classes of human GPCRs

A. rhodopsin like

B1. secretin like

B2. adhesion

C. metabotropic glutamate receptors

D. frizzled

rhodopsin like GPCR

largest group

short extracellular N terminus

receptors for most transmitters

secretin like GPCR

larger extracellular ligand binding domain on the n terminus

receptors for peptide hormones

4 targets = calcitonin, GLP-1 agonists, glucagon, parathyroid hormone

central homeostatic function

metabotropic glutamate receptors GPCR

large extracellular n terminal domain responsible for ligand recognition

all function as dimers = extra regulation

GABAb, taste receptors, mGluRs, Ca2+ sensing receptor

mGluRs

group 1 = post synaptic

group 2/3 = presynaptic

modulate other receptors

modify excitotoxic activity

involved in synaptic plasticity

differential distribution in the brain

agonist GPCR regulation

stimulate the receptor by stabilising an active conformation

anatgonist GPCR regulation

prevent the receptor going into the active signal conformation by getting in the way of the agonist

inverse agonist GPCR regulation

will stabilise one or more of the inactive conformations of the receptor

inverse agonists and constitutive activity

regulators

decrease g protein receptor binding and reduce basal activity of receptors OR reduction in receptor binding affinity

induces conformational change in receptor, produces variable affinity for g proteins, so they have different actions.

allosteric modulation

receptors are regulated by sites distant to orthosteric agonist binding site

can be negative (reduces efficacy), positive (increases efficacy), or silent (no effect but prevents other modulators binding)

g protein signalling

ga, gby are signal transduction molecules that can influence the behaviour of other proteins

type of ga protein determines the major effect of the receptor

receptor sub types have specific alpha subunit to bind

homodimerisation

many GPCRs form dimers to express function

can be via covalent or non covalent interactions and can involve extracellular domains/c terminal tails

complex interactions with multiple g proteins, integration or amplification of signals

heterodimerisation

for GABA receptors, function requires heteromeric expression

opioid delta and kappa receptors also similar

increases functional diversity economically

direct signal amplification

each stage of signalling from activation of receptor to cellular response, the signal is amplified

so few olecules of ligand can have large effect in cell

termination of signal

• stop production of ligand

• modify receptor to prevent ligand binding

• remove receptor and ligand by receptor mediated endocytosis

• remove second messenger

• remove phosphate groups from target proteins

channelopathies def

diseases caused by disturbed function of ion channel subunits or the proteins that regulate them

may be congenital or acquired

mg2+ sites nmda

binding sites important for receptor activation and gating of the ion channel

blocks the channel in a voltage dependent manner

zn2+ sites nmda

sites not needed for receptor activation

affect the efficacy of the channel

blocks the channel in a non competitive and voltage independent manner

polyamine sites nmda

not needed for receptor activation

affects the efficacy of the channel

binds compounds such as spermine or spermidine

potentiating or inhibiting depending on the combination of subunits

glycine sites nmda

important for receptor activation and gating of the ion channel

essential binding component to NR1

benzos and gaba

modulate the action of gaba at receptors containing any2/3 motif

allosteric modulation

benzo full positive modulation

allosteric

can maximise small gaba stimulated cl- currents at many gaba a receptor sub types

benzo select positive modulation

allosteric

can maximise small gaba stimulated cl- currents in selected gaba a receptor sub types

benzo p positive modulation

allosteric

amplify gaba stimulated cl- current to limited extent (partial) in most sub types

barbiturates and gaba a receptors

can positively allosterically modulate

high doses can directly activate the receptor in absence of gaba

site of interaction different than benzos and neurosteroids

neurosteroids and gaba a receptors

allosterically modulate receptors in positive or negative way

high doses may directly activate the receptor

+= allopregnanolone

-= pregnenolone

direct = allopregnanolone +++

oligodendrocytes def

CNS

produce myelin, facilitate transmission

schwann cells def

PNS

produce myelin, facilitate transmission

astrocytes def

enable homeostasis, physical barrier/connector

microglia def

immune cells of the brain

phagocytose dead cells and debris

blood brain barrier

lining = endothelial cells with tight junctions

astrocytes form barrier around blood vessels

prevents certain molecules entering from blood stream

specific transporters allow certain molecules access

pericytes use

stabilisation of capillaries

maintenance of bbb tight junctions

controls blood flow

influences immune cell responses

glycocalyx def

consists of glycoproteins and glycolipids with carbohydrate chains

contributes to selective permeability of bbb

holds a negative change, to repel negatively charged molecules

carrier mediated transcytosis

vesicular transport process where a molecule binds to a specific carrier/receptor on BBB endothelial cells, is endocytosed, transported across the cell in vesicles, and exocytosed into the brain (e.g. transferrin‑ or insulin‑based drug delivery).

transcytosis def

transport of substances across BBB endothelial cells via vesicles, involving endocytosis on the blood side and exocytosis on the brain side, without disrupting tight junctions.

receptor mediate transcytosis

ligand binds to a specific receptor on BBB endothelial cells, triggering endocytosis, vesicular transport across the cell, and exocytosis into the brain. It enables delivery of large or hydrophilic molecules (e.g. via transferrin or insulin receptors).

adsorptive mediated transcytosis

vesicular transport process where positively charged molecules bind non‑specifically to the negatively charged endothelial cell membrane, triggering endocytosis, transport across the cell, and release into the brain. It does not require a specific receptor.

paracellular transport bbb

movement of substances between endothelial cells through tight junctions. At the BBB, this pathway is extremely restricted, allowing only very small, hydrophilic molecules to pass, making it negligible for most drugs.

parkinsons tx aim

to restore a lack of dopamine in the brain

levodopa action

precursor of dopamine, converted by DOPA decarboxylase

increases dopamine availability at synapse

can cross the blood brain barrier unlike dopamine

entacapone and carbidopa action

don’t cross the bbb

stop peripheral break down of levodopa to increase its bioavailability in the brain

local delivery to brain

• intracerebral injection

• intrastriatal injection

• stereotactic injection/surgery

• convection enhanced delivery

convection enhanced delivery

delivery to a local region of the brain

allows a greater diffusion distance of the drug

needs multiple cannulae (more risk of bleeding)

intrathecal injection

injection into CSF

covers a large area of the CNS

can also be intraventricular too

intranasal delivery

uptake by the olfactory neurons and the trigeminal nerve can lead to transfer of drug to the brain

doesn’t work for proteins