Applied Pharmacology of the Nervous and Endocrine System Modulators of Neuronal Excitability

UNIVERSITYOF PORTSMOUTH

M33319 - Applied Pharmacology of the Nervous and Endocrine System
Modulators of Neuronal Excitability
Ngan Pan Bennett Au, PhD
Lecturer in Pharmacology (Neuroscience)
School of Pharmacy and Biomedical Science,
University of Portsmouth

Learning Objectives

• Identify the various classes of drugs that target axonal voltage-gated ion
  channels, and their mechanisms of action in altering neuronal activity.
• Identify the various classes of drugs that target monoaminergic
  neurotransmitters/receptors/transporters, and their mechanisms of action
  in altering neuronal activity.
• Identify the appropriate clinical indications for these different drugs.

Drugs Covered in This Lecture

a
Somatodendritic
Axonal
Intracellular recording
Action potentials
Excitatory inputs
Kv4.2
Kv3 throughout dendrite
Proximal
dendrites
Kv2.1
EPSPs
0
-
8
88
Distal
dendrites
PP
888
0
-
8
P
Nodes of Ranvier
Nav, KCNQ. Kv3.1b
HCN
Cav
Inhibitory
inputs
IPSPs
AIS
Nav, KCNQ
Dendritic action potentials
4
Neurotransmitter
release
Dopamine neuron
Noradrenaline neuron
5-Hydroxytryptamine neuron
Tyrosine
Amph.
MPP+
Cocaine
GBR 12935
WIN 35,428
RTI-121
Amph.
L-DOPA
L-DOPA
Cocaine
Nisoxetine
Reboxetine
5-hydroxytryptophan
DA
DA
5-HT
*
d
DA
DA
5-HT
NA
DAT
1
NET
SERT
DA
Adrenergic .
autoreceptors
5-HT
autoreceptors
DA receptors
Adrenergic receptors
5-HT receptors
Drugs targeting axonal
voltage-gated ion channels
Tyrosine
Tryptophan
Amph.
MDMA
Cocaine
Fluoxetine
Paroxetine
Sertraline
Back propagation
Nav, Kv1, Cav
Presynaptic
nerve
terminals
P
Soma
Kv1
IXPs
8
O
8
autoreceptors
Drugs targeting monoaminergic
neurotransmitters

Types of Voltage-Gated Ion Channels

a
Somatodendritic
Axonal
Intracellular recording
Action potentials
Excitatory inputs
Kv4.2
Kv3 throughout dendrite
Proximal
dendrites
Kv2.1
EPSPs
Soma
Kv1
IXPs
0
o
A
Distal
dendrites
8
88
O
=
Nodes of Ranvier
Nav. KCNQ. Kv3.1b
0
HCN
Cav
Inhibitory
inputs
IPSPs
AIS
Nav, KCNQ
O
o
Dendritic action potentials
Back propagation
Neurotransmitter
release
Membrane potential (mv
3 voltage-gated Na+
channel inactivated
(no ion flow)
voltage-gated K+
4 channel opens,
K+ leaves cell
2 voltage-gated Na+
channel opens;
time
Na+ rushes in
resting potential re-established,
Na+ channel closed (not
inactive)
-50
1 voltage-gated Na+
channel
closed
time of stimulation

• Voltage-gated sodium (Nav) ion channels
• Voltage-gated potassium (Kv) ion channels
• Voltage-gated calcium (Cav) ion channels
• Abundantly expressed in axon initial segment (AIS)
  . Determine the threshold for firing an action potential
• Action potential propagates along the axon and reach the axonal terminal
• Activate Cav
• Calcium influx and neurotransmitter release
  4
  +50
  Nav, Kv1, Cav
  Presynaptic
  nerve
  terminals
  0
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Voltage-Gated Sodium Channels

• 10 different Nav identified in mammals
• Nav1.1 - Nav1.9 and atypical NavX
• Based on the difference in a-subunit
• All are expressed in nervous system, except Nav1.4
• CNS: Nav1.1 - Nav1.3, Nav1.5, Nav1.6
• PNS: Nav1.7 - Nav1.9
• a-subunit (260 kDa): pore-forming
• 4 homologous transmembrane domain I-IV
• Each domain has six hydrophobic a-helical transmembrane segments (S1-S6)
• S4: voltage sensor (change in membrane potential) - related to activation of
  Nav
• TTX (tetrodotoxin)-sensitive: Nav1.1, Nav1.2, Nav1.3, Nav1.4, Nav1.6, Nav1.7
• TTX-resistant: Nav1.5, Nav1.8, Nav1.9
• 3 intracellular loops
  A
  ₿1/3-subunit
  c-subunit
  ฿ 2/4-subunit
  =
  =
  IV
  NH
  NH
  Voltage
  sensor
  Outside
  C
  C
  wow !!
  1 2345 6
  12 345 6 12345 6
  12345 6
  Inside
  COOH
  COOH
  PY
  motif
  ER-retention
  signals (Na,1.5)
  Endocytic
  motif
  AnkyrinG
  -binding site
  Di-leucine
  motif
  B
  Na
  -
  Pore loops
  1
  5
  4
  2
  13
  IV
  =
  IFM
  + Inactivation gate
  5
  COOH
  NH

Voltage-Gated Sodium Channels: Beta-Subunit

• ß-subunit (33-36 kDa): auxiliary subunits
• Contains three major parts:
• Independent transmembrane domain
• Small intracellular C-terminal
• Large intracellular N-terminal
• 4 subtypes: 31-34
• Regulate a-subunit (gated kinetics, voltage-dependence,
  localisation)
  Navß isoforms Distribution in neurons
  ß1
  mainly in large or medium (>30 pm) DRG neurons, less in smaller neurons (<25 um)
  ฿ 2
  widely distributed in CNS and PNS neurons, including cerebral and DRG neurons
  ฿ 3
  mainly in small (<25 um) and medium (25-45 um) neurons, less in large (>45 um) neurons
  ฿ 4
  mainly in large neurons, less in medium and small neurons
  A
  ₿1/3-subunit
  c-subunit
  ฿ 2/4-subunit
  =
  =
  IV
  NH
  NH
  Voltage
  sensor
  Outside
  C
  C
  wow !!
  1 2345 6
  12 345 6 12345 6
  12345 6
  Inside
  COOH
  Inactivation
  gate
  COOH
  PY
  motif
  ER-retention
  signals (Na_1.5)
  Endocytic
  motif
  AnkyrinG
  -binding site
  Di-leucine
  motif
  B
  Na
  -
  Pore loops
  1
  5
  4
  2
  13
  IV
  =
  IFM
  + Inactivation gate
  6
  6
  COOH
  NH

Modulators of Voltage-Gated Sodium Channels: Phenytoin

• Act as a non-specific sodium channel blocker
  . Stabilise the Nay channels that are inactivated
• Prolong the refractory period of a neuron
• Blockade of sodium-dependent action potentials depends on:
• Voltage: Nav inactivated at positive (+30mV) membrane potential
• Use: ultimately reduce sustained high-frequency firing of action
  potential
• Time: time required to dissociate from binding of the drug to Nav
  channels
• Reduce synaptic release of glutamate
  Medical indication
• Epilepsy/seizures following neurosurgery or severe head injury
  Open
  Inactivated
  Na+
  Na+
  A
  A
  1
  carbamazepine
  phenytoin
  topiramate
  Na+
  lamotrigine
  valproate
  zonisamide
  Control
  Phenytoin
  1
  On
  Off On
  Off
  peak AP
  +35
  -----
  D
  depolarization (Na' influx)
  repolarization (K+ efflux)
  relative
  refractory
  period
  55
  threshold
  ------
  resting
  - 70
  hyperpolar-
  ization
  absolute
  refractory
  period
  Na+
  transmembrane potential (mV)
  time (ms)
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Modulators of Voltage-Gated Sodium Channels: Carbamazepine

• Act as a non-specific sodium channel blocker
• Also bind to other voltage-gated ion channels (e.g. Cav channels)
• Stabilise the Nay channels that are inactivated
• Prolong the refractory period of a neuron
• Fewer Nay channels to open and prevent the generation of action
  potential
• Reduce neuronal excitability
  Medical indication
• Epilepsy
• Trigeminal Neuralgia (sudden and severe facial pain caused by
  compression of trigeminal nerve)
• Mania (abnormally elevated arousal and energy level)
  Open
  Inactivated
  Na+
  Na+
  A
  A
  1
  carbamazepine
  phenytoin
  topiramate
  Na+
  Na+
  Carbamazepine
  +35
  ----
  D
  depolarization (Na' influx)
  repolarization (K+ efflux)
  K
  relative
  refractory
  period
  . 55
  threshold
  -------
  resting
  . 70
  --
  hyperpolar-
  ization
  absolute
  refractory
  period
  On
  Off
  lamotrigine
  valproate
  zonisamide
  peak AP
  transmembrane potential (mV)
  time (ms)
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Voltage-Gated Calcium Channels

• Ca2+: second messenger within the cells for regulation of
  many signalling pathways
• Cav: activate upon membrane depolarisation and
  mediate Ca2+ influx in response to action potentials and
  sub-threshold depolarisation signals.
• Five subunits:
• a1: pore-forming subunit; 4 homologous I-IV domain
  (each contains 6 transmembrane helices S1-S6);
  voltage-sensing
• a28: disulfide-linked glycoprotein dimer; drug target for
  gabapentinoids
• B: intracellular subunit; stabilise the conformation of a1
• y: transmembrane glycoprotein subunit
  Ca2+
  a2
  Y
  α1
  Contraction
  Secretion
  Synaptic transmission
  Protein
  phosphorylation
  Enzyme
  regulation
  Nucleus
  1
  XXX
  Gene transcriptionUNIVERSITYOF
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Types of Voltage-Gated Calcium Channels

• Five types of Cav:
• L-type (long-lasting via DHP receptors)
• N-type (neural; found in brain and PNS)
• P-type (Purkinje; cerebellum)
• R-type (residual; cerebellar granule cells and
  neurons)
• T-type (transient; many neurons, cells with
  pacemaker activity; thalamus)
  Ca2+
  a2
  8
  Y
  α1
  Contraction
  Secretion
  Synaptic transmission
  Protein
  phosphorylation
  Enzyme
  regulation
  Nucleus
  1
  Gene transcriptionTable 1. Subunit composition and function of Ca2+ channel types
  Ca2+ current
  &1
  Subunits
  Specific
  blocker
  Principal physiological functions
  Inherited diseases
  L
  Cav1.1
  DHPs
  Excitation-contraction coupling in
  skeletal muscle, regulation of
  transcription
  Hypokalemic periodic
  paralysis
  Cay1.2
  DHPs
  Excitation-contraction coupling in
  cardiac and smooth muscle,
  endocrine secretion, neuronal
  Ca2+ transients in cell bodies and
  dendrites, regulation of enzyme
  activity, regulation of transcription
  Timothy syndrome: cardiac
  arrhythmia with
  developmental
  abnormalites and autism
  spectrum disorders
  Cav1.3
  DHPs
  Endocrine secretion, cardiac
  pacemaking, neuronal Ca2+
  transients in cell bodies and
  dendrites, auditory transduction
  Cay1.4
  DHPs
  Visual transduction
  Stationary night blindness
  N
  Cav2.1
  @-CTx-GVIA Neurotransmitter release,
  Dendritic Ca2+ transients
  P/Q
  Cav2.2
  @-Agatoxin
  Neurotransmitter release,
  Dendritic Ca2+ transients
  Familial hemiplegic migraine,
  cerebellar ataxia
  R
  Cay2.3
  SNX-482
  Neurotransmitter release,
  Dendritic Ca2+ transients
  T
  Cay3.1
  None
  Cay3.2
  Pacemaking and repetitive firing
  Pacemaking and repetitive firing
  Absence seizures
  Cay3.3
  Abbreviations: DHP, dihydropyridine; @-CTx-GVIA, @-conotoxin GVIA from the cone snail Conus geographus; SNX-482, a
  synthetic version of a peptide toxin from the tarantula Hysterocrates gigas.
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Modulators of Voltage-Gated Calcium Channels: Ethosuximide

• Block T-type calcium channels
• T-type calcium channels are:
• Low-threshold calcium spikes: appear when neuronal
  membrane potentials are below -69mV
• Amplitude of depolarisation is ~25mV: raise the membrane
  potential to -40mV
• Opening of Nav: burst of action potential
• Burst-firing is observed in rodent models of absence epilepsy
  Medical indications
• treatment of absence seizures
  A
  Regular firing
  B
  Burst firing
  10 mV
  40 ms
  -55 mV
  -75 mV
  C
  Action potential
  (high-threshold
  Na+ spike)
  D
  Low-threshold
  Ca2+ spike
  10 mV
  40 ms
  10 mV
  T channels
  opening and closing
  -46 mV
  4 ms
  -63 mV
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Modulators of Voltage-Gated Calcium Channels: Gabapentin

• Ligand of a28 calcium channel subunit (modulate a1 functions and stabilize cellular
  localisation of Cav)
• Bind to a28 and impair its regulatory functions and protein-protein interactions
  with other proteins
• Block the translocation of Cay towards the cell membrane
• Lower the amount of functional Cav at presynaptic terminals
• Reduce neurotransmitter release & neuronal excitability
  Medical indications
• Epilepsy
• Neuropathic pain
• Spasticity (rigid muscles) in multiple sclerosis
  Hyperexcited Neuron
  Presynaptic
  Subunit
  Channel
  Neurotransmitters
  Postsynaptic
  Modulation of Hyperexcited Neuron
  With Pregabalin
  Presynaptic
  Pregabalin
  C2-8
  Subunit
  Ca2+
  Channel
  - --- --
  Neurotransmitters
  Postsynaptic