Age-related hearing loss (presbycusis) is a complex state which may reflect pathological influences along the entire auditory system. For certain kinds of age-related hearing loss, there is a dramatic loss of speech understanding without a parallel loss in pure-tone thresholds. This loss is especially severe under noisy conditions. Central auditory dysfunction in aging maybe in response to partial peripheral deaffentation and maybe similar to changes observed with noise exposure or ototoxic drugs. Changes in the central auditory processor may involve changes in the synthesis, degradation, uptake, release, and receptor sensitivity of neurotransmitters. Studies in our laboratory are focused on the understanding of two closely related questions in auditory research:

1. How do inhibitory neurotransmitters function within known auditory circuits to encode
    acoustic information?
2. What is the impact of aging on inhibitory neurotransmission in the auditory system?

   Studies in most central auditory structures have found that inhibitory neurotransmitters are critically involved in coding acoustic information. Coding of these signals appears vulnerable to the aging process. Studies of circuits witin the central auditory system undergo age-related changes related to pre and postsynaptic GABAergic and glycinergic neurotransmission. Ongoing projects in the laboratory are focused on how the brain changes its ability to process auditory signals as we age. Neurochemical and molecular studies are examining how receptors in auditory structures change their structural makeup with aging, and the mechanisms which trigger these age-related receptor changes. A clear understanding of age-related changes within central auditory circuits could lead to the development of pharmacotherapy for age-related hearing loss.

Techniques:
• Auditory system single unit electrophysiology:

Coding studies:
• Iontophoretic studies of auditory system
• Aging electrophysiology studies
• Aging neurochemical studies of the auditory neuraxis:
• Western blotting of receptor subunit proteins and glutamic acid decarboxylase
• In situ hybridization of receptor subunit proteins
• Receptor binding/pharmacology of the GABAA receptor
• Chloride flux/age-related changes in GABAA receptor function

RECENT PUBLICATIONS:


Caspary DM, Ling L, Turner JG, Hughes LF.
Inhibitory neurotransmission, plasticity and aging in the mammalian central auditory system. J Exp Biol. 2008 Jun;211(Pt 11):1781-91. Pub Med Link >

Bauer CA, Turner JG, Caspary DM, Myers KS, Brozoski TJ.
Tinnitus and inferior colliculus activity in chinchillas related to three distinct patterns of cochlear trauma.J Neurosci Res. 2008 Apr 25. [Epub ahead of print] Pub Med LInk >

Schatteman TA, Hughes LF, Caspary DM. Aged-related loss of temporal processing: Altered responses to amplitude modulated tones in rat dorsal cochlear nucleus.
Neuroscience. 2008 Feb 29. [Epub ahead of print] Pub Med Link >

Brozoski, T.J., C.A. Bauer, D.M. Caspary: Elevated Fusiform Cell Activity in the Dorsal Cochlear Nucleus of Chinchillas with Psychophysical Evidence of Tinnitus. J. Neurosci. 22(6):2383-90, 2002.

Wang, J., S.L. McFadden, D.M. Caspary, R.J. Salvi: Gamma-Aminobutyric Acid Circuits Shape Response Properties of Auditory Cortex Neurons. Brain Res. 944:219-231, 2002.

Caspary, D.M., P.S. Palombi, L.F. Hughes: GABAergic Inputs Shape Responses to Sinusoidally Amplitude Modulated Stimuli in the Chinchilla Inferior Colliculus. Hearing Res. 168:163-173, 2002.

Turner, J.G., L.F. Hughes, D.M. Caspary: Divergent Response Properties of Layer V Neurons in Rat Primary Auditory Cortex. Hearing Res. 202:129-140, 2005.

Turner, J.G., J. Parrish, L.F. Hughes, L.A. Toth, D.M. Caspary: Hearing in Laboratory Animals: Strain Differences and Non-Auditory Effects of Noise. Comparative Medicine 55:12-23, 2005.

Ling, L.L., L.F. Hughes, D.M. Caspary: Age-Related Loss of the GABA Synthetic Enzyme in Rat Primary Auditory Cortex. Neurosci. 132:1103-1113, 2005.

Turner, J.G., L.F. Hughes, D.M. Caspary: Effects of Aging on Receptive Fields in Rat Primary Auditory Cortex Layer V Neurons. J. Neurophysiol.94:2738-2747, 2005.

Caspary, D.M., T.A. Schatteman, L.F. Hughes: Age-Related Loss of Response Inhibition in Rat Dorsal Cochlear Nucleus. J Neurosci. 23;25(47):10952-10959, 2005.

Brozoski, T.J., D.M. Caspary, C.A. Bauer: Marking multi-channel silicon-substrate electrode recording sites using radiofrequency lesions. J. Neurosci. Methods 150:185-191, 2006.

Turner, J.G., T.J. Brozoski, C.A. Bauer, J.L. Parrish, K. Myers, L.F. Hughes, D.M. Caspary: Rapid Tinnitus Screening in Rats Behavioral Neurosci. 120:188-195. 2006.

Caspary, D.M., L.F. Hughes, T.A. Schatteman, J.G. Turner: Age-Related Changes in the Response Properties of Cartwheel Cells in Rat Dorsal Cochlear Nucleus. Hearing Res. 214:on-line, 2006

RECENT CHAPTERS:

Caspary, D.M., R.J. Salvi, R.H. Helfert, T.J. Brozoski, and C.A. Bauer: Neuropharmacology of Noise Induced Hearing Loss in Brainstem Auditory Structures. In: Noise Induced Hearing Loss: Mechanisms of Damage and Means of Prevention. (D. Henderson, D. Prasher, R. Kopke, R.J. Salvi and R. Hamernik, eds.), NRN Publications, London. 2002. pp: 169-186.

Kelly, J.B. and Caspary, D.M.: Pharmacology of the Inferior Colliculus. In: The Inferior Colliculus, J.A. and C.E. Schreiner (Eds.), Springer-Verlag: New York. 2005. pp. 248-281.

Turner, J.G. and Caspary, D.M.: Comparison of Two Rat Strains of Aging: Peripheral Pathology and GA Changes in the Inferior Colliculus. In: Auditory Plasticity, Prague Symposium Proceedings, Syka J. and Merzanich M. (Eds.), 2005. pp. 217-225.