Plasticity in neurotransmitters of the central auditory system

Auditory Research Group
Southern Illinois University
School of Medicine
Excitatory and Inhibitory Neurotransmission in
Central Auditory Structures
•  Tinnitus and Aging: Loss of Acoustic Input and
Maladaptive Plasticity
•  Signal Detection and Binaural Localization
•  The Impact of Aging and Deafferentation on
the Pharmacology and Physiology of
CANSInferior Colliculus
• 
• 
Top-down bottom-up Processing
Central Auditory Pathways
(CANS)
Adapted from Netter atlas
What are the criteria for identifying a substance as a
neurotransmitter?
Why is this an important question for these lectures?
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The substance must be present within the presynaptic neuron.
Enzymes and precursors required to synthesize the substance are
present in presynaptic neurons. Transmitters: glutamate, glycine and GABA are
also used for protein synthesis and other metabolic reactions in all neurons, their
presence is not sufficient evidence to establish them as neurotransmitters.
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The substance must be released in response to presynaptic
depolarization, and the release must be Ca2+-dependent.
Specific receptors for the substance must be present on the
postsynaptic cell. A neurotransmitter cannot act on its target unless
specific receptors for the transmitter are present in the postsynaptic
membrane. Demonstrate receptors by application of exogenous
transmitter mimics the postsynaptic effect of presynaptic stimulation.
Pharmacologic identity agonists and antagonists that alter the normal
postsynaptic response have the same effect when the substance in
question is applied exogenously.
PLASTICITY
Major Neurotransmitters of the CANS:
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Glutamate (GLU) Receptors-Excitatory
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GABA Receptors-Inhibitory (Mostly)
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AMPA- Fast
NMDA-Slower, rectifying, development, plasticity
mGLURs-Metabotropic/G-protein coupled
GABAA-Fast, synaptic, extrasynaptic
GABAB-slower metabotropic G-protein coupled
Glycine Receptors-Inhibitory (also NMDA excitatory cofactor)
Nicotinic Cholinergic Receptors-postsynaptic excitatory,
presynaptic modulators of excitation and inhibition
Glutamate Excitatory Neurotransmission
• 
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Neuronal glutamate (Glu)
Glutamine (Gln)
Vesicular glutamate transporters (vGluTs)
SNARE proteins mediate the interaction and fusion
of vesicles with the presynaptic membrane.
•  Ionotropic glutamate receptors (NMDA receptors
(NMDARs) and AMPA receptors (AMPARs)
•  Metabotropic glutamate receptors (mGluR1 to mGluR8)
on membranes of post- & presynaptic neurons and glial cells.
•  Glutamate is cleared from the synapse by excitatory
AA transporters (EAATs) on glial cells
(EAAT1 and EAAT2) and on neurons (EAAT3 and EAAT4).
Projections to CANS
Central Auditory Pathway
Transmitter of Acoustic Nerve Synapses?
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Mimicry and Blockade
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AMPA receptors
NMDA receptors
Acoustic Nerve to Cochlear Nucleus
AP5
Quis
DNQX
Unit Recordings in Real Time
What is the Excitatory Neurotransmitter at Acoustic Nerve Synapses
in the Cochlear Nucleus?
Homeostatic Plasticity in Sensory
Systems
What Happens When Sensory Input to
the Brain or Spinal Cord is Degraded?
All central auditory structures can
adjust.
All central auditory structures balance
excitation and inhibition as well as
bottom-up and top-down influences.
Inhibition, Auditory Coding and Maladaptive Plasticity
Age or Noise-Dependent Loss
of Peripheral Sensitivity/ Partial
Deafferentation
Age or noiseDependent
Deficiencies in
Auditory Processing
Maladaptive
Plastic
Normal
Changes
Central Auditory System
in Central Auditory
InhibitionSystem
Inhibition
(GABAergic, Glycinergic
(GABAergic,
Glycinergic
Neurotransmission)
Neurotransmission)
Inaccurate Neural
Coding of :
Accurate
Temporally Complex
Temporal/Speech
Processing,
Spatial Localization,
Stimuli,
Localization,
Auditory
Attention,
Auditory
Attention,
Gating
Gating
Plastic Changes Related to GABA in
Somatosensory Systems
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Whisker trimming in adult rats reduced GAD levels
25%. Muscimol binding was reduced 10-25%
resulting in signs of cortical disinhibition (Fuchs and Salazar,
1998; Skangiel-Kramska, 1994).
Radial nerve section reduced GABA levels in
primate cortex (Kaas,1991).
l  Partial peripheral nerve injury promotes a selective
loss of GABAergic inhibition in the superficial
dorsal horn of the spinal cord
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(Moore et al., 2002).
Plastic Changes Related to GABA in
the Visual System
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Retinal lesions reduced GABA levels in cortical
regions receiving projections from the damaged
area (Rosier et al., 1995).
Blockade of peripheral visual input activity
resulted in a reversible 50% reduction in the
number of GAD immunoreactive neurons in
visual cortex (Jones, 1990).
Monocular deprivation in adult primates produced
a rapid down-regulation in GABA and certain
GABAA receptor subunits in deprived-eye
columns of primary visual cortex (Hendry and Miller,
1996).
Inhibitory Neurotransmission at CANS
Synapses?
Mimicry (agonist)
Blockade (antagonist)
Glycine receptor
glycine
strychnine
GABAA receptor
GABA
Muscimol
gaboxidol (THIP)
GABAB
gabazine
bicuculine
What Roles Do Inhibitory Circuits Play in
Acoustic Signal Processing at Different Levels
of the Auditory Neuroaxis?
Dynamic range adjustment
l  Improved signal in noise
l  Frequency Receptive Fields
l  Temporal response properties
l  Binaural response properties
l  Complex signal processing
l  Auditory Learning
l  Or CN and SOC
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What we know about one
inhibitory circuit
Dorsal Cochlear Nucleus (DCN):
a. Glycine Neurotransmission
b. Role of Inhibitory Neurotransmission
c. Impact of Aging on inhibition
d. Impact of Sound Exposure on inhibition
Glycine and DCN
10µm
Red= GlyR α1 ; Green= Gephyrin
Glycine and DCN
Godfrey, et al., J. Histochem. Cytochem., 25:417-431, 1977
ME Rubio – J. Comp. Neurol, 2004
DCN Fusiform Cell Response Properties
40
30
20
10
0
IN 0
(mV)
-10
-20
Intensity (dB SPL)
-30
-40
-50
-60
-70
-80
0.4
0.6
0.8
1
1.2
Spikes/sec
Time (s)
Spikes/Sec
Frequency (kHz)
Time (ms)
1
Glycine and Intensity Coding in DCN
175
Rate (Spikes/sec)
140
105
70
35
Control
Strychnine
0
0
20
40
dB SPL (dB)
Caspary et al., 1987-chinchilla, Davis & Young, 2000-cat;, Wang et al., 2011.
60
80
Reducing Glycine Inhibition Predicts Age-related SAM
Coding Deficits
Backoff et al., 1999, Hearing Res.
Age-Related Plastic Inhibitory Changes in
Central Auditory Pathways
Why Study Age-Related Hearing loss?
30% 65 or older
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l  25% of US
population over 65 by
2020
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Assumptions Regarding Aging and Inhibitory
Neurotransmission
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Age-related hearing loss can be thought of, in part, as the
result of a slow progressive partial peripheral
deafferentation.
Hypothesis: Partial peripheral deafferentation leads to a
selective down-regulation of inhibitory neurotransmission
along the entire auditory neuraxis.
The underlying signal for central neurotransmitter changes
likely involves age-related changes in the magnitude and
pattern of ascending neural activity.
As yet unknown trophic/trafficing/signaling/protein folding/
degradation factors are likely involved.
Approach: Examine functional and molecular
neurochemical markers of glycinergic and GABAergic
neurotransmission in a rat model of aging.
Why
is
loss
of
central
auditory
inhibitory
Assumptions Regarding Aging and Inhibitory
transmitter function
a significant component of
Neurotransmission
age-related hearing loss?
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Seniors have more difficulty with temporal
processing tasks than younger adults, especially
in noisy environments.
Superthreshold psychoacoustic findings from
seniors with hearing-loss matched to young adult
controls show age-related decrements in
temporal tasks.
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gap-detection
duration coding
speech understanding.
Damping provided by inhibitory circuits allows
neural responses to follow the temporal fine
structure of complex acoustic signals.
Reuters; UN Population Aging Development 2009: Olshansky plot
Why
Study
Aging?
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Presbycusis: Age-Related loss of sensitivity to acoustic stimulation
Age and reported hearing loss:
l  18% of Americans 45-64 years old :30% of Americans 65-74 years old
l  47%-66% of Americans 75 years old or older
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Resulting/Related Central Auditory Processing Deficits:
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Temporal Processing/Speech understanding
Extracting meaningful sounds within background noise
Attention-demanding processing tasks
FBN Rat Model of Aging
Median Life Span: 36mos. Maximal Life Span: 44mos.
Young (4-7 mos.) Middle (20-22 mos.) Aged (28-32 mos.)
100
F344 OHC
FBN OHC
F344 IHC
FBN IHC
Percent Missing
75
50
25
0
0
50
25
75
100
Percent Distance from Apex
FBN rats: Young 4-6 months old (n=21); Aged 32-38 months old (n=17)
Turner et Al., 2005, Comp Med.
Wang et al.,2009a, Neuroscience
Gap Detection: Aging
Control
no
Gap
Variable
Gap
Background BBN
Startle
Variable
GAP
Widths
GAPs randomly varied between 1-50msec
Startle
Response
Aging: Behavioral GAP Detection
Young
Aged
Response re: no gap
1.2
1
0.8
0.6
0.4
0.2
s
50
m
s
15
m
s
10
m
s
5m
s
4m
s
3m
s
2m
s
1m
ba
se
St
l
ar ine
tle
O
nl
y
0
Gap Duration
Animal Model for these Studies
FBN rats: Young 4-6 months old (n=8); Aged 32-38 months old (n=8).
Wang, Turner et al., 2009a, Neuroscience
Central Auditory Pathway
Age-Related Inhibitory
Changes in DCN
Adapted from Netter atlas
Single Unit Characteristic Frequency and Threshold
Single Unit Thresholds from Young and Aged Fusiform Cells
Young (n=93)
Aged (n=88)
Threshold (dB SPL)
55
35
15
-5
1
10
Frequency (kHz)
100
Mean Discharge Rate: Young vs. Aged
Caspary et al., 2005, J Neurosci.
Fusiform Cell CF
Rate-Level
Functions
Reducing Glycine Inhibition Predicts Age-related SAM
Coding Deficits
Backoff et al., 1999, Hearing Res.
Age-Related Changes in Temporal Modulation Transfer Function
0.6
0.6
0.5
0.5
0.5
0.4
0.4
0.4
Aged 100%
Young 50%
Young 50%
Aged 50%
Aged 50%
Young 20%
Young
Young20%
20%
Aged 20%
Aged
Aged20%
20%
0.3
0.3
0.3
0.2
0.2
0.2
BMF
BMF
BMF
0.1
0.1
0.1
181
1801
.02.2
101
7067
.64.4
646
040
383
08.0
5.5
222
62.6
3.3
1313
4.4
5.5
8080
4747
.6.6
2828
.3.3
00
1616
.8.8
Vector
Vector
strength
(R)
Vectorstrength
strength(R)
(R)
Young 100%
Modulation
frequency
(Hz)
Modulationfrequency
frequency(Hz)
(Hz)
Modulation
Schatteman et a., 2008, Neuroscience
Glycine Neurotransmission in DCN
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Glycine: primary inhibitory neurotransmitter of DCN
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Post-synaptic receptors: pentameric
heteromeric: 3α2β or 2α3β
homomeric: 5α
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Gephyrin at glycinergic synapses
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Anchoring protein
Receptor transporter
Moss & Smart, 2001
[3H] Strychnine Binding in DCN
YoungControl
AgedControl
YoungExposed
Specific Binding(fmol/mg)
DCN Glycine Receptors: Young & Aged
Bound/Free
100
75
50
Saturation Binding Isotherm
2000
1500
Young
Aged
1000
500
0
0
10
20
30
40
[3H]Strychnine (nM)
25
0
0
1000
2000
3000
4000
Bound (fmol/mg protein)
(n=4young&4old)
Wang et al., 2009a, neuroscience
Glycine Subunits & Gephyrin
Young
HF
MF
DCN
GlyR α1 Protein
LF
Scale bar = 10 µm
ROD
Aged
Wang et al., 2009a, neuroscience
Red=GlyR α1; Green=gephyrin
BDNF Protein
*
BDNF
*
35
Young
Aged
Relative Optical Density
30
25
20
15
10
5
0
LF
MF
HF
(n = 4 young 4 old)
LF,MF,HF= low, middle and high frequency thirds of DCN
Wang et al.,20011, Neuroscience
Young-Adult
Glycinergic synapses
Aged
Heteromeric glycine receptor: 3α2β
Homomeric glycine receptor: 5α
WHAT ABOUT THE SAME
DCN SYSTEM FOLLOWING A
SOUND EXPOSURE
CONSISTENT WITH
BEHAVIORAL DEVELOPMENT
OF TINNITUS?
[3H] Strychnine Binding (GlyR) in DCN:
Control & Exposed
Saturation Binding Isotherm
Specific Binding
(fmol/mg protein)
2000
Bound/Free
150
100
1500
Control
Exposed
1000
500
0
0
10
20
30
40
[ 3H] Strychnine (nM)
50
0
∆ Slope (kd): Affinity
∆ X-axis (Bmax): Binding
Density
0
1000
2000
3000
4000
Bound (fmol/mg protein)
FBN rats (n=4 controls; 4 exposed); Unilateral, 17kHz, 1hr, 116dB
(Wang et al.,
GlyR Subunit & Gephyrin Protein Changes in the
DCN of Rats with Behavioral Evidence of Tinnitus
50
GlyRα1
GlyRα2
GlyRα3
LF
MF
*
40
Protein (% from Control)
Gephyrin
*
30
HF
*
20
10
0
HF
-10
-20
-30
DCN
MF
LF
*
* *
-40
FBNrats(n=15controls;15exposed);Unilateral,17kHz,1hr,116dB
(Wangetal.,2009)
Break time
Neurotransmitters
Binaural Localization
& Professor Oliver’s
Favorite Structure the
Inferior Colliculus
Assignment: Listen to the
Virtual barber shop must wear
headphones
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https://www.youtube.com/watch?
v=IUDTlvagjJA
Quick Review of Localization
of Sound in space
Why is this an important task?
l  Know where any threats are coming from
l  Know where your prey is
l  Optimization of coding of communication
sounds?
l  Signal in noise detection?
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Localization of Sound in Space
What are the cues needed to do this task?
l  Interaural Intensity Difference (IID)-LSO,MNTB
l  Interaural Time Difference (ITD)-MSO
l  Spectral Cues (asymmetric temporal notches)
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DCN
Superior Olivary Complex
IID Coding
Henrique von Gersdorff & J. Gerard G. Borst
Superior Olivary Complex
LSO
&
MSO
Superior Olivery Complex
GABA: GABAA & GABAB
Age-Related Inhibitory
Changes in IC
Adapted from Netter atlas
Homeostatic Plasticity and Inhibitory Amino Acid Neurotransmitters
Lüscher & Keller, 2001
Subunit
composition
and
associated
proteins
determine:
These
dynamic
processes
profoundly
affect
the strength
of GABAand glycinergic
signaling:
Receptors
undergo
either
recycling
and/or
degradation
Receptor
Trafficking
Inhibition
maintains
the
dynamic
balance
between
excitation
and inhibition
in the auditory
neuraxis
Location
synaptic
or
extrasynaptic
Ubiquitination
Oligomerization
Critically
Involved
in
Altered
Receptor
Function
with
age
but
not
well
studied
posttranslational
modifications
Peak
current
Virtually
all ofwith
these
processes
been &
shown
to be activity
dependant
Duration/desensitization
of the ipsc have
Interaction
associated
transport
anchoring
proteins
Termination characteristics including: phasic or tonic inhibition
IC: Age-Related Presynaptic GABA changes
Inferior Colliculus-F344 Rat
Gutierrez et al., 1994
130
Young
Old
120
% of Young Adult
110
100
90
80
Young
70
60
50
40
30
20
10
0
Old
Total
Cells
GABA
Cells
GABA
Content
GABA
Release
GAD
Activity
GABA
GABAA
GABAB
Terminals Receptors Receptors
Caspary et. al., 1999; 2008;Gutierrez et al., 1994
IC: Age-related Postsynaptic GABAA Receptor Changes
Caspary et al.,2008
Old GABAAR has less a1 and g1 which young receptor does not have
FBN Flunitrazepam
Young
Old
TBOB Binding in the IC and Aging
100
(5 w/o GABA)
[3H]TBOB Bound
120
80
*
60
40
20
*
3 Months (n=8)
26 Months (n=8)
-9
-8
-7
Log [GABA]
-6
-5
P1 Synaptosome Preparation
Inferior Cerebellum
Colliculus
Sample Removal
(<3 min)
Homogenize
(Glass/Glass)
Centrifuge
(1000xg-15min)
Homogenize
(Glass/Teflon)
Resuspend P1
(Synaptosomes)
Centrifuge
(1000xg-15min)
Chloride Flux and Aging
Mean change from basal GABA-evoked 36Cl uptake into microsacs prepared from the IC of young-adult, middle-aged, and aged
F344 rats at 30 µM and 100 µM GABA. Each age group's mean flux (nM 36Cl uptake/100 µl sample) at 0 GABA (basal level) was
assigned a value of 0.
Caspary et al.,1999
Old
Young
Glutamate
Glutamate
GAD
GAD
GABA
GABA
α2/3, β2, γ1
(A)
GABA
GABA
Nerve
Terminal
α1, β2, γ2
GABA
GABA
(B)
GABA
GABA
GABA
GABA
GABA
GABA
GABA
Synapse
Cl-
GABA
Cl-
Cl-
Cl-
Cl-
(C)
GABA
Cl
-
Cl-
Cl-
Cl-
Cl-
(D)
Post-synaptic neuron
Cl-
Cl-
(E)
CIC
Control
Bicuculline
65
dB
SPL
55
45
35
3
4
I09-05 CIC Pauser
CF = 7.070 kHz @ 18 dB
6
8
10
3
4
6
8
10
GABA Suppresses Responses to
Modulated Stimuli
Nonmonotonic Contralateral
CF Tone RIF
40
30
20
10
C
A24-09
Rostral CIC Pau
CF = 12183@4
A
0
NM - Nonmonotonic
P - Plateauing
M - Monotonic
0
10
20
30
40
50
60
70
Intensity (dB SPL)
Percentage of Units
Discharge Rate (Spikes/s)
B
60
50
40
30
20
10
0
60
50
40
30
20
10
0
Whole IC
Young
Aged
NM
P
M
CIC
NM
P
M
Distribution of AM Gain Function Shape
A
60
Whole IC
40
Young (n=79)
Aged (n=96)
Percentage of Units
20
B
0
60
Bandpass
CIC
Lowpass
Mixed
Flat
Other
Young (n=45)
Aged (n=60)
40
20
C
0
60
Bandpass
Lowpass
Mixed
Flat
Other
ECIC
Young (n=18)
Aged (n=29)
40
20
0
Bandpass
Lowpass
Mixed
Flat
Other
Temporal Coding and Aging Inferior Colliculus
Young adult CBA mouse IC units (open bars) and old units (filled bars). rMTF filter shape
LP,BP,HP. Age-related increases in discharge rates at lower Mfs results in significant
increase in the number of low-pass units.
Walton et al., 2002
Neural Delays in the Aging Population.
N= 17 (18–30 years old) & 17 (60–67 years old)
Age-related shift in neural response timing for onset and transition but not for the steady state portion of da
Anderson S et al. J. Neurosci. 2012;32:14156-14164
Musical Experience Offsets Age-related Delays in Neural Timing
[da]
Parbery-Clark et al.,2012
Augmented Acoustic Environment and
GAD Relative Optical Density
Inhibitory Neurotransmission
AAE
AAE
Female
Male
Aged (22–23 months) male (n = 12) and female (n = 9) CBA mice were housed in either 6 weeks of low-level (70
dB SPL;12hr/night;200msec;2/sec) broadband noise stimulation (AAE) or normal vivarium conditions.
%2Fpets%2Frats-playing-musical-instruments%2F&ei=HBjKUq-tFYrQyAGNkYDICw&bvm=bv.
Turner et al., 2013
What about Sound exposure
in Young Adult IC
Effect of tone exposure on spontaneous
activity in the IC
Manzoor et al/Kaltenbach JNeurophysiol 2011
Physiology: Increased Firing Rate and
Intraburst Rate are Present in the IC
±SEM of
Control
***p<0.001, **p<0.01
Relative to Control
8-9 months post-trauma, Unilateral, 4kHz, 85dB, 1 hr
Bauer et al., 2008
Presynaptic: GABA Synthetic Enzyme, GAD is
Down-Regulated Following Sound Exposure
Rat, Free-field, 12kHz, 10hr, 106dB
Milbrandt et al., Hearing Res, 2000
Santiago Ramón y Cajal
Impact of Aging on Top-down/Bottom-up Processing
Top down modulation
MGB
Young Anes
MGB
Young Awake
Bottom up input
MGB
Aged Awake