Download 3 - Neurobiology of Hearing

M. R. Paper 3
MEDS5377
Effects on Sound Localization Sensitivity Due to Cholinergic Enhancement
Introduction: Acetylcholine (ACh) is a neurotransmitter that has the ability to modify
sensory cortical activity, including the processing of auditory information.1 Although the
effects of modulation of ACh activity in the auditory cortex on experience-dependent,
conditioned responses has been studied,1 there has not been a study that measures the effects
of cholinergic enhancement in the auditory cortex on a sound localization task. It is known
that intra-arterial administration of physostigmine, a cholinesterase inhibitor that can cross the
blood-brain barrier, can cause an increase in the amplitudes in the midlatency cortical-evoked
response (MLR) in humans and rats.2,3 In many behavioural studies, cortical ACh release
from afferent inputs is correlated to a response to a salient stimulus, suggesting that ACh
facilitates neuronal response to relevant stimuli; however, during an auditory task,
physostigmine addition (and thus a prolonged increase in ACh) did not lead to enhanced
stimulus response as measured behaviourally or in an fMRI of the auditory cortex.1 This
result could mean that the cholinergic system relies on precise timing in order to enhance
signal detection, so keeping ACh levels saturated at the synaptic cleft over prolonged periods
actually has an attenuating effect on auditory stimulus conditioning in the cortex—similar to
the effect of an cholinergic antagonist such as scopalamine. Given this prior research,
behavioural measurement of success of a sound localization task under the effects of
physostigmine would give information about the role of ACh in the regulation of auditory
cortical processing. Additionally, taking cortical evoked response (CER) measurements via
electroencephalography (EEG) during this recognition task would provide information about
the physiological basis of cholinergic effects in the auditory cortex. What these two results
combined would mean is that the modulation provided by the cholinergic system directly
plays a role in localization of sound, but must be activated in a time-dependent manner in
order to maximize cortical sensitivity to an auditory stimulus (the final processing center of
the ‘where’ pathway that begins in the inferior colliculus). This research may eventually have
consequences in identification and treatment of diseases like schizophrenia and Alzheimer’s
where the MLR peak is aberrant from the normal timing and amplitude in healthy
individuals.2
Hypothesis: Enhancement of cholinergic activity in the auditory cortex will lead to
decreased overall success rate in a behavioural sound localization task due to oversaturation
of the cholinergic system and increased MLR amplitude compared to a control group’s MLR
and performance data on the same task.
Methods: A normal hearing cat model will be used (n=15), where an experimental group of
cats will be administered physostigmine and a control group (n=5) will not be exposed to the
drug. Cats will be trained without influence of the drug on a behavioural protocol that
involves placing the cat in a semicircular chamber with their head pointed straight ahead
(“zero degrees”), the chamber will have angular markings ranging from -9090 at each end
of the circle. To begin, the cat will approach a visual cue at a given azimuthal position and be
given a food reward upon recognizing and approaching the stimulus. Once this conditioning
has been performed, the stimulus will switch to a simple auditory click stimulus that is well
above the cat hearing threshold dB SPL, which will be at various azimuthal positions and
followed by an identical food reward. Once the conditioning has been established, baseline
MLR will be taken in experimental animals prior to the behavioural protocol to serve as MLR
controls. This setup is similar to a visual-tone paired task that has been performed previously
to measure hearing threshold effects due to physostigmine.4 The experimental animals will
undergo the behavioural protocol 20 minutes after the injection of physostigmine as
previously described4, and an EEG electrode will record from the auditory cortex (placement
of the EEG electrodes has been previously described2). Cannular physostigmine
administration and dosage in cats for behavioural tests has been previously described.5 CER
M. R. Paper 3
MEDS5377
measurements will be taken following the tone and recorded through the long latency
response to a tone. Nine azimuthal positions at 20 intervals (i.e. 90, 70, 50...-90) will be
tested randomly five times each during the protocol, which will last no longer than 2 hours so
as to minimize the time course degradation of the physostigmine.4 The overall success rate
will be the average total percent success of the five tests at each position. MLR time will be
measured beginning around the 50 msec range following the signal and end at the 65 msec
range for all animals.
Expected Results: In order to accept the hypothesis, the overall success rate of the
experimental cats—i.e. the success at sound localization averaged over nine positions, should
be significantly lower than in the control animals. Additionally, the total average amplitude
of the MLR primary depolarization (‘P2 wave’)2 in the CER voltage vs. time curve should be
significantly higher in the experimental cats cf. control and baseline MLR measurements. If
these two results are shown, then there are significant physiological and behavioural effects
on sound localization due to physostigmine. In order to reject the hypothesis, either the
success rate or the MLR amplitude must not be significantly different; this would mean that
the physiological-behavioural link could not be made as stated in the hypothesis. One issue
with this research is that the physostigmine as administered acts globally on the brain,
inhibiting other auditory areas along the pathway to the cortex (inferior colliculus, cochlear
nucleus, etc); indeed, physostigmine can have an excitatory effect outside of its enhancing of
ACh metabotropic activity.5 However, it would be difficult due to the relatively sparse
background research into cholinergic activity in higher auditory processing to parcel out what
additional roles ACh is playing in behavioural output of the auditory system at this point.
This is perhaps an avenue for future research, where neurometrics are applied directly to
cholinergic neurons (these have been identified in cats in previous research1) involved in the
auditory pathway in order to determine distinctive roles for subpopulations of these neurons.
References:
1. Thiel CM, et al. “Effects of cholinergic enhancement on conditioning related responses in human
auditory cortex.” European Journal of Neuroscience. (2002) 16:2199-2206.
2. Buchwald JS, et al. “Midlatency auditory evoked responses: differential effects of a cholinergic agonist
and antagonist.” Electroencephalography and clinical Neurophysiology. (1991) 80: 303 – 309.
3. Bhargava VK, Salamy A, McKean CM. “Effects of Cholinergic Drugs on Auditory Evoked Responses
(CER) of Rat Cortex” Neuropharmacology. (1978) 17:1009-1013.
4. Amaro J, Guth PS, Wanderlinder L. “Inhibition of Auditory Nerve Action Potentials by acetylcholine
and physostigmine.” British Journal of Pharmaceuticals and Chemotherapeutics. (1966) 28:207-211.
5. Lowy K, et al. “Antagonism by Cholinergic Drugs of Behavioural Effects in Cats of an Anticholinergic
Psychomimetic Drug and Enhancement by Nicotine.” Neuropharmacology. (1977) 16: 399-403.
M. R. Paper 3
MEDS5377
Grade A
Very good
Doing an EEG during behaviour would be very challenging. It may be easier to do two
separate tests, one behavioural and one using evoked middle latency responses on the same
animals under anaesthesia.
STUDENT CRITIQUE
Of all the papers I have read thus far, this one seems to be the most thought out and
well written. The explanation of the experimental methods and pharmacology was very
thorough. Within the paper there were some suggestions I would make in order to ensure a
reliable result.
The control chosen for this experiment lack the same trauma that is induced though
the arterial insertion of the drug physostigmine. It is important that you make sure that the
difference is sound localization is not due to the fact that the animal is concentrating on the
consequences of the surgery. I would suggest the there be a third control group where the
animal undergoes a surgery and is injected with a saline solution. This would affirm the fact
that the change in sound localization would be due the drug alone.
One other problem I encountered while reading this proposal was that it does not
mention what type of sound is going to be used. Additionally he does not mention whether he
is going to be used ILD or ITD for the sounds localization task. The mention of either would
infer the other. Also the angles that he chose to play the sound from, -90 to +90 at 20 degree
intervals do not include 0 degrees. I feel zero is an important reference point because of the
fact that the ILD and ITD are equal to zero there.
It is also mentioned that the test would last no longer than two hours in order to
minimize the course degradation, but I found that the duration of action for physostigmine is
only 45 minutes(http://www.pcc.vghtpe.gov.tw/old/antidote-information.htm). He may want
to consider administering the drug intravenously and continuously to keep the drug levels
more constant, rather than doing just a single injection.
The last issue that I had with the experiment was that intravenous infusion of
physostigmine in normal subjects leads to a syndrome of psychomotor
inhibition(http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TC2-45RCD5C32&_user=618604&_coverDate=03%2F31%2F1993&_rdoc=1&_fmt=high&_orig=search&_
sort=d&_docanchor=&view=c&_searchStrId=1370673038&_rerunOrigin=google&_acct=C0
00032798&_version=1&_urlVersion=0&_userid=618604&md5=3bb91220accd2cbefa0f3f2a
29e99aa7) . This would lead me to believe that the cats in the experiment may have a difficult
time approaching the sound source as listed in methods section of the proposal. The only way
I may suggest limiting the effect on the motor function would be to administer the drug to the
auditory brain directly and test faster. This could possibly limit the effect of the drug on the
rest of the brain. If a cat is not approaching the target because it cannot walk then you
wouldn’t want to think it was because it couldn’t determine the location of the sound.