Ear Manipulations Reveal Importance of Gravity Input for Orientation Development

Karen Elliott Thompson

University of Iowa

On Earth

• Gravity provides linear but not angular acceleration

• In Space, astronauts experience microgravity

GRAVITY

Image from http://upload.wikimedia.org/wikipedia/commons/9/98/Aldrin_Apollo_11_original.jpg

Image from http://t.wallpaperweb.org/wallpaper/space/1280x1024/shuttle_99_2_1280.jpg

Microgravity

• ALL linear acceleration is associated with angular acceleration due to centrifugal forces acting on the inner ear

• Perceived as self-movement

Return to Gravity on Earth

• Gravity again provides linear but not angular acceleration

• Brain adds the perception of angular acceleration

• Body attempts to counter with motor and eye movements

• Results in difficulty walking, standing, and stabilizing gaze

GRAVITY

Goal

To understand how gravity shapes the nervous system to guide behavior

• Want to look at areas where synaptic plasticity in gravity perception occur

Gravity and Movement Perception • Linear Acceleration

– Utricle

– Saccule

• Angular Acceleration – Semicircular canals

Image from Kopecky et al., 2012

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hc

http://www.bthchiroonline.com/wordpress/wp-content/uploads/2012/11/inner_ear.jpg

Neuronal Network

• The plasticity between gravity input and motor output

occurs in the vestibular nuclei – Eron et al.(2008) Adaptationof orientation vectors of otolith-related central vestibular neurons to

gravity. Journal of Neurophysiology 75: 1928-1932

Brain

Spinal Cord

Experiment:

• Manipulate the gravity sensor: the ear – Remove one ear

– Add one ear

• Determine how asymmetrical and mismatched gravity detection shapes behavior and nervous system – Swimming

– Sensory projections

– Target neuron

Image from: http://petra-aqua.com/gfx/foto/1954.jpg

http://www.shechterlab.org/wp-content/uploads/2009/08/frog-eggs.jpg

Transplantation Method

• Donor ear from a Xenopus embryo (stage 25-27) is transplanted to host, rostral to native ear

– Ear in native orientation

– Ear rotated by 90 degrees

1-eared animal 3-eared animal

Transplantation Method

• Donor ear from a Xenopus embryo (stage 25-27) is transplanted to host, rostral to native ear

– Ear in native orientation

– Ear rotated by 90 degrees

• Embryos allowed to grow until early tadpole (stage 46)

• Swimming monitored

Rationale

• Native orientation – Sensory epithelia

aligned – Respond the same to

a given stimulus

• Rotated by 90°

– Sensory epithelia not aligned

– Respond differently to a given stimulus

Transplantation Results

Ear Removed 3 Ears Normal 3 Ears Rotated Normal Rotated

Swimming was most affected when ear was removed or when 3rd ear was rotated

3 ears, all normal orientation 3 ears, one rotated 90°

1 ear 2 ears, Normal orientation

Afferent innervation depends upon orientation

Normal Orientation Rotated by 90°

Single z-series Images Single z-series Images

Animal A Animal B Animal C Animal D

• Lipophilic dye-soaked filter paper was injected into native (red) and transplanted ears (blue, false colored green)

• Brain removed and sensory

neurons were imaged with confocal microscope

• Single optical sections were

observed

Loss of input or gain of input affected dendritic branching of target neurons

• Dendritic development of brain neurons depends upon neuronal input – Loss of input (no ear) results in reduced branching – Gain of input (extra ear) results in increased branching

• Blocking afferent activity does not affect dendritic branching

(Goodman and Model, 1990), therefore branching is independent of the ear’s orientation

Proposed Mechanism

Normal Rotated

Testing the mechanism: Determine whether ear manipulations affect initial direction of movement

Conclusions

• Sensory neurons of transplanted ears rotated 90° interfere with vestibular processing as indicated by the aberrant swimming, similar to that of one-eared frogs.

• Partial overlap and segregation of sensory neurons implies that differential gravity sensation affects guidance of afferent axons in addition to molecular mechanisms.

• Altered sensory neuron input affects the dendritic branching of target neurons, such as the Mauthner cell.

Future Directions

• Alter gravity using a ‘frog elevator’

– Simulated parabolic flight

http://umps.med.univ-tours.fr/images/eng_parabole1.jpg

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Acknowledgements

• Bernd Fritzsch

• Israt Jahan

• Ning Pan

• Tian Yang

• Jennifer Kersigo

• Ryan Ries

• Jeremy Duncan

• Ben Kopecky

• Hannah Maher

• Alex Perk

• Dr. Douglas Houston

• Dr. James Buchholz

• Dr. Dan Weeks

• Funding – NASA base grant

– NIH

– Iowa Center for Molecular Auditory Neuroscience