Dr Margaret Piggott

1
Neurochemistry of the Dementiasand
transmitter-based therapies

• Dr Margaret Piggott
• margaret.piggott_at_ncl.ac.uk
• margaret.piggott_at_ntw.nhs.uk

2

• Examining neurotransmitter mechanisms is
  important because

• Different dementias have
  different neurochemical profiles
  with implications for treatment
• Neurochemical changes underlie symptoms
• Antipsychotic, anxiolytic, pro-cognitive and
  antidepressant drugs all
• Modulate Transmitter Systems

3
You will have varying familiarity with
neuroscience Apologies for fact-laden stuff How
much you know already? Covering things that may
be in MCQ
4
NEUROCHEMISTRY OF THE DEMENTIAStransmitter
therapies
THE OXFORD TEXTBOOK OF OLD AGE PSYCHIATRY (Psychia
try in the Elderly 4th edition) Chapter
6 Neurochemical pathology of neurodegenerative
disorders of old age Piggott MA and Court JA
(2008) (in revision)
Parkinsons Disease Dementia, edited by Professor
Murat Emre Chapter 13 - Neurochemistry of
Parkinsons disease dementia Piggott MA and Perry
EK (2010)
Early-Onset Dementia, edited by Professor John R
Hodges Chapter 9 Neurochemical pathology
in degenerative dementias Elaine Perry, Rose
Goodchild and Margaret Piggott (2001)
5
Neurotransmitter types Amino acids glutamate,
aspartate,
D-serine, glycine, ?
amino butyric acid (GABA),
Biogenic amines dopamine, serotonin,
norepinephrine, epinephrine,
histamine Others acetylcholine, adenosine,
anandamide, nitric oxide Peptides over 50
peptide neurotransmitters, somatostatin,
substance P, ? endorphin
Neurotransmitters activate one or more types of
receptors. The effect on the postsynaptic cell
depends on the properties of those receptors
6
Cholinergic system

• Cholinergic cell nuclei
• The nucleus basalis of Meynert projects to
  neocortex
• Cholinergic cells in the medial septum/diagonal
  band project to hippocampus and entorhinal cortex
• Cholinergic interneurons intrinsic to the
  striatum
• Brainstem pedunculopontine (PPN) neurons project
  to thalamus
  

Cholinergic nuclei numbers - http//www.acnp.org/g
4/gn401000012/ch012.html
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Cholinergic terminal
Synthesising enzyme choline acetyltransferase
(ChAT) Acetylcholine released from synaptic
vesicles in response to depolarisation
Acetylcholine interacts with receptors
(muscarinic and nicotinic) on the pre and
postsynaptic membrane Acetylcholine in the
synaptic cleft is removed by degrading enzyme
acetylcholinesterase (AChE)
8
Muscarinic receptors
M1
M1

• Five subtypes M1 - M5
• All metabotropic (G-protein coupled receptors)
• M1 postsynaptic
• cortex, hippocampus, striatum,
• low in thalamus, none in cerebellum
• M2 - cortex, hippocampus, thalamus, striatum,
  cerebellum and brainstem,
• M4 - mainly in striatum, also in cortex
• M3 M5 substantia nigra, thalamus and
  hippocampus
• M1, M3, M5 stimulate, M2 M4 inhibit
• - overlapping distribution

M4/M2
M2
M1
M2
Autoradiographs from frozen post mortem tissue
9
?
10
Neuronal nicotinic receptor (?4?2) distribution
temporal cortex
striatum
cerebellum
thalamus
occipital cortex
midbrain
11
DOPAMINERGIC SYSTEM
nigrostriatal mesolimbic mesocortical dopamine
pathways
Thalamus
12
Dopamine receptors (all GPCR)

• D2, D3, D4 inhibitory, D1 D5 stimulatory
• D2 and D1 in striatum gt thalamus gt cortex
• D3 is limbic, in nucleus accumbens, ventral
  pallidum, limbic thalamus (not cortex)
• D4 - despite high affinity for clozapine, links
  to ADHD, receptor protein has very low density in
  human
• - many polymorphisms,
• and 48bp repeat (2x 4x or 7x) in third
  intracytoplasmic loop
• - D4 variants not linked to disease (except
  ADHD, 7x repeats)
• - D4 variants not associated with clinical
  response
• -defective gene 2 population ?
  low sensitivity to dopamine and clozapine
• D5 low density cholinergic neurons,
  sub-thalamic nucleus
• ? antipsychotic drug potencies correlate with
  their ability to block D2 ?

13
Major transmitters



glutamate
(excitatory) and GABA (inhibitory)

• Glutamate and GABA (?-amino butyric acid) form
  basis of neurotransmission
• GABA neurons are interneurons in cortex, can be
  interneurons or projection neurons in subcortical
  areas (e.g. striatal projection neurons)
• Glutamate neurons are projection neurons
  
  corticocortical, thalamocortical,
  cortical-subcortical (corticofugal)

14
Glutamate receptors
Multiple glutamate receptor subtypes, subunits
and splice variants
NMDA receptors Mg2 block long
term potentiation (LTP), learning and memory
Na/ Ca2
Asp
Mg2
2
Glu
2
(Ca
)

H
Na

NMDA
PCP
Glu
Mg2
2
Mg
Glu
AMPA
Group II
Glu
G
2
Ca
cAMP
AC
IP
3
ATP
DAG
PI-PLC
G
PIP
2
Group I
Glu
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Glutamate neurotransmission

• Glutamate has role in cognition at normal
  concentrations (LTP)
• Reduced glutamate affects learning and memory
• Excess glutamate leads to excitotoxic cell death
  (Ca)
• Alzheimers disease - both too much and too
  little glutamate at different
  times
• Glutamatergic pyramidal neurones in entorhinal
  cortex and hippocampus are particularly
  vulnerable to tangle formation and cell loss

16
GABA receptors
GABAA chloride ion channel, post-synaptic Diffe
rent combinations of subunits have different
pharmacology and cellular and regional
distributions diverse pharmacological properties
of GABAA drugs
GABAB metabotropic G-protein coupled receptor
(GPCR)
Many drug development programmes target GABA and
glutamate Benzodiazepines positively modulate
GABAA and increase chloride conductance Negative
GABA modulators could enhance cognition
Modafinil decreased GABA transmission and
increased glutamate
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SEROTONERGIC SYSTEM (5-HT)
18
SEROTONIN Receptors

• 7 classes of serotonin receptors, 5HT1 - 7
• All GPCR (except 5HT3 - ligand-gated ion channel)
  
• 5HT4 - presynaptic, stimulate release of
  transmitters
• This array of receptor subtypes provides huge
  signalling possibilities
• alternate splicing increases the number of
  proteins
• oligomerisation increases the number of
  complexes
• multiple G-proteins allow crosstalk between
  receptor families
  

19
NORADRENERGIC SYSTEM
multiple ?- and ?-adrenergic receptors all
metabotropic GPCR
20
HISTAMINE SYSTEM
.
4 Histamine Receptor types all GPCR
21
Any more neurotransmitters? Adenosine,
Cannabinoid Neuropeptide Transmitters (Substance
P, Orexin, Neurotensin, Somatostatin, Substance
Y, Opioids etc) human genome shows more than
300 potential GPCR About half remain orphan
receptors, endogenous ligands unknown
Receptor heteromers and oligomers A2A, D2,
mGluR5 and M1 receptors form raft of receptors
GPCR e.g. histamine H3, can have
constitutive spontaneous activity
where G-protein coupled in absence of agonist
22
Agonist or Antagonist?

• If it causes a response, it's an agonist
• If it causes a response that is relatively
  smaller than the response to another agonist,
  it's a partial agonist
• If it inhibits the response caused by an agonist,
  it's an antagonist
• If there is some baseline level of activity in
  the absence of agonist and the drug inhibits
  that, it's an inverse agonist

23
AD, DLB
Alzheimers Global cognitive impairment Memory
impairment plus
impaired language (aphasia)
impaired movement (apraxia)
impaired recognition
(agnosia) or disturbed
executive functioning Gradual decline No
disturbance of consciousness Additional
features anxiety, wandering,
depression, psychosis

• DLB
• Progressive cognitive decline
• plus two out of three Core Features
• Cognitive fluctuation of with variation in
  attention and alertness
• Recurrent visual hallucinations
• Spontaneous features of parkinsonism
• REM sleep behaviour disorder, neuroleptic
  sensitivity, low DaTSCAN, falls and syncope,
  transient loss of consciousness, severe autonomic
  dysfunction, hallucinations in other modalities,
  delusions, depression

24
Dementia with Lewy bodies and Parkinsons disease
dementia

• spectrum
• very similar clinically
• pathologically probably indistinguishable
• movement disorder before dementia by gtone year ?
  PDD
• movement disorder within one year of dementia, or
  later, or not at all ? DLB
• 20 of DLB no EPS, while PDD begins with levodopa
  responsive Parkinsonism
• Some dopaminergic and cholinergic receptor
  differences (compensatory changes in PD esp. D2
  up-regulation in PD)

25
Cortical cholinergic markers in AD

• Post-mortem loss
• ChAT activity 35-50
  Choline uptake 60
  AChE activity 40-60
  Nicotinic binding 30-70

Muscarinic M1 receptor reduced efficiency of
coupling to G-protein as disease progresses,
reduced receptor density late in disease
In vivo imaging loss of AChE, vesicular ACh
transporter, M1 and
nicotinic receptor
Biopsy 3.5 yrs disease, ACh markers reduced up
to 50
26
Cholinergic Changes in DLBpost-mortem
neurochemistry

• More extensive cholinergic loss than AD
  (cortex and brainstem
  rather than hippocampus)
• In vivo PET loss of cortical
  acetylcholinesterase (AChE) in DLB exceeds
  AD
• Cortical ChAT loss greater than in AD
• Striatal ChAT loss
• Retained cortical M1 receptors and G-protein
  coupling
• Reduced striatal M1 receptors
• Cortical ?4?2 nicotinic receptors reduced as in
  AD, but much more reduced in striatum

27
Clinical consequences of cholinergic losses
Memory hippocampus Learning hippocampus,
cortex Attention cortex, thalamus Consciousnes
s, sleep, and dreaming - brainstem,
thalamus, cortex Movement, balance and motor
regulation striatum, brainstem,
thalamus Visual function cortex, thalamus
28
Cholinergic loss correlates with Cognitive Decline
Reduced choline acetyltransferase (ChAT) in
temporal and frontal cortex correlates with
cognitive impairment
8
7
6
5
4
3
2
1
0
0
1
2
3
4
5
6
7
8
9
Dementia rating
and in DLB and PDD
29
Visual Hallucinations
Prevalence of recurrent complex VH in different
disorders relates to the extent of cortical ChAT
loss
Level of cholinergic activity
picture of hallucination by artist with PD
Rate of hallucinations
Inferior temporal cortex
30
In DLB, more reduced ChAT is associated with
visual hallucinations
ChAT activity in temporal cortex
Presence of VH is good predictor of response to
ChEI
DLB with and without visual hallucinations
31
Hallucinations related to nicotinic receptors in
DLB

• Imaging
• reduced 5IA85480 binding to ?4?2 nicotinic
  receptors in DLB in striatum and frontal,
  temporal and cingulate cortex
• Increased ?4?2 in occipital cortex associated
  with hallucinations

32
Fluctuations related to nicotinic receptors in DLB

• Temporal cortex nicotinic receptor ?4?2 reduced
  in DLB/PDD
• Greater reduction in cortex and thalamus in cases
  without fluctuations
  

Temporal cortex
4
?
3
3H epibatidine fmol/mg
2
1
In an environment of reduced cholinergic
activity, a higher density of nicotinic receptors
could amplify small transmitter changes leading
to variations in consciousness and attention
16
6
0
FC
-FC
Fluctuations impair ADL and are over seconds,
minutes, hours, and days
33
Dopamine in DLB
Dopamine concentration and dopamine transporters
are reduced in DLB, almost to the same extent as
in Parkinsons disease
Control
Alzheimer
DLB no EPS
DLB EPS
Autoradiographs of dopamine transporter
34
Dopamine transporters
in PD, PDD, DLBEPS, and AD
posterior caudate
posterior putamen
1.0
1.0
0.8
0.8
0.6
0.6
125I PE2I binding fmol/mg
0.4
0.4
0.2
0.2
0.0
0.0
Control
Significant loss even in
DLB with no EPS support for FP-CIT SPECT
(DaTSCAN) in AD/DLB discrimination
PD no dementia
PDD
DLBEPS
DLB no EPS
AD
35
Striatal D2 receptors in PD, DLB and AD
Control
PD
DLB
caudate
putamen
???
15
12
27
8
controls
PD
DLB
AD
36
Cortical D2 receptors reduced in DLB and PDD
20/21
22
Ent cx
normal
20
36
36
36
22
21
20
nsb
20
21

DLB/PDD
22
22
Ent cx
22
Ent cx
Ent cx
21
21
36
36
21
36
20
20
20
40 reduction in DLB (30 in PDD) in D2 receptors
in temporal cortex no change in AD
37
Temporal cortex D2 decline with MMSE
0.7
DLB and PDD, Ba 20 N20, r0.58, p0.008
0.6
0.5

• Consistent with
• Neuroleptics impair cognition
• D2 PET in hippocampus correlates with memory

0.4
125I epidepride binding fmol/mg
0.3
0.2
0.1
DLB
PDD
0.0
0
5
10
15
20
25
30
MMSE
38
Thalamic D2 receptors elevated in PD (50)
compared to controls and other disease groups
parafascicular
ventral area centromedian


8
8
centromedian
7

6
6
6
reticular nucleus
5
u
4
2.0
4
4

1.5
3
2
1.0
2
2
0.5
1
0
0
0.0
0
10
3
9
9
5
7
3
8
8
4
12
6
11
11
5
7
3
8
8
4
ventroposterior
8

laterodorsal nucleus
6
5

paraventricular nucleus
4
4
12

u
3
u
control
2
2
PD no dementia
10
1
PDD
0
0
7
5
6
7
3
8
12
5
12
10
6
MD
DLB EPS
7

6
6
DLB - EPS
5
4
4
3
2
2
1
0
0
12
5
9
10
5
11
6
8
9
4
39
Raised D2 in DLB/PDD with fluctuations in cortex
and in thalamic nuclei with a role in maintenance
of consciousness
with DOC
parafascicular
8
without DOC
centromedian
6
6
reticular nucleus
5

1.5
4
4

1.0
125I epidepride fmol/mg

3
0.5
2
2
0.0
12
5
1
0
0
9
6
10
6
with DOC
without DOC
D2
cingulate cortex
1.00
mediodorsal
MD
4
0.75
3


2
0.50
1
0.25
0
10
5
reticular
CM/pf
0.00
6
5
40
Dopamine mechanisms

• Elevated D2 receptors in PD - compensates for
  low dopamine
• Reduced D2 receptors in DLB and PDD may
  correlate with
• poor levodopa response and neuroleptic
  sensitivity
• D2 receptors decline as PD progresses
  faster
  in cortex than striatum and thalamus

D2 receptors are on GABA interneurons i.e.
inhibiting inhibitory neurons - a higher density
of D2 receptors will amplify small transmitter
changes
41
Glutamate markers in AD inconsistent reports
Reduced NMDA binding and NMDAR1 mRNA expression
in AD
Cortical pyramidal neurone loss leads to reduced
glutamate activity and cognitive impairment in AD

42

• With reduced NMDA receptors in AD, odd that NMDA
  antagonist memantine effective
• - it blocks NMDA receptor better than Mg2
• But reduced membrane potential
• (due to pathology, reduced energy metabolism)
  
  leads to release voltage dependent Mg2 block of
  NMDA
• ? and excessive, neurotoxic entry of Ca2
• So Memantine efficacy in moderate-severe AD with
  heavier pathology
• acting as uncompetitive, low-affinity,
  open-channel blocker
• limiting excessive glutamate
• reducing signal to noise
• Memantine is also a D2 agonist, 5HT3 antagonist

43
Serotonergic abnormalities

• neurone loss tangles in raphe, reduced 5HT
• relatively retained 5-HT function linked to
  more psychosis (AD and DLB)
• 5-HT2A receptors more reduced with severe
  dementia
• 5HT receptor polymorphisms linked to
• Aggression, Psychosis, Depression, Anxiety

Noradrenergic Abnormalities

• Extensive neuron loss locus coeruleus, reductions
  in noradrenaline, increased turnover in surviving
  neurons linked to upregulation of the
  noradrenaline transporter
• In PD noradrenaline loss linked to ? PDD
• Noradrenaline changes may be related to
• Aggression, Psychosis, Depression

44
Fronto-Temporal Dementia
Younger onset (45 60 years) Pathology most
apparent in the II and deep cortical layers,
coinciding with location of D2 and 5HT1 receptors
Neurotransmitter losses Neurotransmitter losses
Serotonin concentration and transporters reduced, 5HT1A and 5HT2A receptors reduced Compulsive behaviours, sweet and carbohydrate consumption
Dopamine concentration and transporters reduced, D2 receptors elevated in striatum Rigidity, flat facies, depression
Norepinephrine and some neuropeptide transmitters slight reduction Anxiety, suspiciousness, restlessness
Acetylcholine little or no reduction greater imbalance DA/ACh in striatum may exacerbate EPS
GABA, glutamate - unchanged
45
Cholinergic Therapy - Residual receptor
availability
Cholinesterase inhibitors delusions,
hallucinations, agitation, aggression, anxiety,
apathy, as well as cognition (implying
cholinergic mechanisms) Galantamine (Reminyl, or
Razadyne) AChEI and nicotinic receptor
allosteric modulator
Donepezil (Aricept)
AChEI Rivastigmine (Exelon) AChEI and
BuCHEI
46
Why might DLB Patients respond to Cholinergic
Treatment?

• Cortical muscarinic receptors up-regulated
• M1 receptors remain coupled to G-proteins
  (unlike AD)
• ACh very reduced
• Less neuron loss or cortical atrophy
• Little or no tangle burden
• Symptoms fluctuate
• potential for higher function
• to be restored
• Low M1 receptors in striatum
• avoids worsening parkinsonism
• AChEI only inhibit 30 AChE activity

DLB
AD
Control
PDD
PD
47
Cholinergic and dopaminergic influence and
consequences
Neuronal survival Alzheimer pathology Cognitive
impairment
See table of anticholinergic medications many
regularly used by the elderly. Implications
Anticholinergic Medication Use and Cognitive
Impairment in the Older Population The MRC
Cognitive Function in Ageing Study.
Fox et al JAGS 2011
Smoking (and coffee drinking) inversely
associated with PD, not with AD (most studies)
48
Normal elderly (female) smokers and non-smokers
Nicotine use (tobacco) associated with lower
plaque densities in normal elderly
49
CHOLINERGIC TRANSMISSION Reduces Alzheimer-type
pathology

• Muscarinic M1 Agonists reduce A? levels in CSF in
  AD
• In triple-Tg-AD mouse, M1 agonist AF267B rescued
  cognitive deficits
  
  and reduced A? and tau pathology
• 
  (dicyclomine M1
  antagonist)
• Cholinesterase inhibitors may reduce amyloid

Reviews Fisher A., Neurotherapeutics 5 2008,
433-442 Caccamo A., Current Alzheimer Research. 6
2009112-7
50

Alzheimer pathology increased in PD in relation
to antimuscarinic drugs
? ?
acute lt2y, chronic 2-18y Anticholinergics
benztropine, orphenadrine, trihexyphenidyl,
oxybutynin Groups matched for age and PD duration
51
NEUROLEPTIC MEDICATION IS ASSOCIATED
WITH INCREASED TANGLE DENSITY IN DLB/PDD
anterior cingulate cortex
frontal cortex
p0.04
Tangle density

• NL
• (23)

NL (17)

• NL
• (23)

NL (17)
DLB/PDD matched for age, duration of PD, duration
of dementia, MMSE, prevalence of delusions and
visual hallucinations
52
Cognitive and Neuropsychiatric Symptoms in
dementia Can Cholinergic and Dopaminergic
Mechanisms Explain All?
Not quite glutamate, serotonin and
noradrenaline also important
other influences need elucidation