An interaction between PPADS, an ATP antagonist, and a moderately intense sound in the cochlea

Purinergic Signaling and Cochlear Injury-Targeting the Immune System?

Hearing impairment is the most common sensory deficit, affecting more than 400 million people worldwide. Sensorineural hearing losses currently lack any specific or efficient pharmacotherapy largely due to the insufficient knowledge of the pathomechanism. Purinergic signaling plays a substantial role in cochlear (patho)physiology. P2 (ionotropic P2X and the metabotropic P2Y) as well as adenosine receptors expressed on cochlear sensory and non-sensory cells are involved mostly in protective mechanisms of the cochlea. They are implicated in the sensitivity adjustment of the receptor cells by a K⁺ shunt and can attenuate the cochlear amplification by modifying cochlear micromechanics. Cochlear blood flow is also regulated by purines. Here, we propose to comprehend this field with the purine-immune interactions in the cochlea. The role of harmful immune mechanisms in sensorineural hearing losses has been emerging in the horizon of cochlear pathologies. In addition to decreasing hearing sensitivity and increasing cochlear blood supply, influencing the immune system can be the additional avenue for pharmacological targeting of purinergic signaling in the cochlea. Elucidating this complexity of purinergic effects on cochlear functions is necessary and it can result in development of new therapeutic approaches in hearing disabilities, especially in the noise-induced ones.

Purinergic signaling in the organ of Corti: Potential therapeutic targets of sensorineural hearing losses

Purinergic signaling is deeply involved in the development, functions and protective mechanisms of the cochlea. Release of ATP and activation of purinergic receptors on sensory and supporting/epithelial cells play a substantial role in cochlear (patho)physiology. Both the ionotropic P2X and the metabotropic P2Y receptors are widely distributed on the inner and outer hair cells as well as on the different supporting cells in the organ of Corti and on other epithelial cells in the scala media. Among others, they are implicated in the sensitivity adjustment of the receptor cells by a K⁺ shunt and can attenuate the cochlear amplification by modifying cochlear micromechanics acting on outer hair cells and supporting cells. Cochlear blood flow is also regulated by purines. Sensorineural hearing losses currently lack any specific or efficient pharmacotherapy. Decreasing hearing sensitivity and increasing cochlear blood supply by pharmacological targeting of purinergic signaling in the cochlea are potential new therapeutic approaches in these hearing disabilities, especially in the noise-induced ones.

Cochlear administration of adenosine triphosphate facilitates recovery from acoustic trauma (temporary threshold shift)

BACKGROUND

Adenosine triphosphate (ATP) has often been used in the treatment of acoustic trauma although evidence supporting its clinical use was lacking. The aim of this study was to evaluate the chronic effects of ATP on acoustic trauma in guinea pigs.

METHODS

We infused ATP into the perilymph of the guinea pig cochlea concurrently with intense noise exposure to investigate the effect of ATP on the process of recovery after acoustic trauma. We assessed auditory brainstem response (ABR) thresholds to evaluate cochlear function.

RESULTS

After noise exposure (120 dB SPL, 5 h), ABR thresholds showed an increase of approximately 50 dB SPL that returned to normal after 14 days. Cochlear function in ATP-treated ears recovered more quickly than in control ears. The effect of ATP was inhibited by the administration of the ATP receptor antagonist: pyridoxal- phosphate-6-azophenyl-2',4'-disulfonic acid.

CONCLUSION

These results suggest that ATP mitigates the effects of noise trauma through the ATP receptor.

PPADS, an ATP antagonist, attenuates the effects of a moderately intense sound on cochlear mechanics

Increasing attention is being given to the role of neurotransmitters and other signaling substances in the damage induced by intense sound to the cochlea. Adenosine triphosphate (ATP) is one example of a putative neurotransmitter that may alter cochlear mechanics during sound exposure. The purpose of the present study was to test the hypothesis that endogenous extracellular ATP has a role in the generation of the changes in cochlear mechanics induced by moderate intense sound exposure. Guinea pigs were exposed to either: (1) a perilymphatic administration of pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 1 mM), an ATP antagonist; (2) a moderately intense sound (6 kHz tone, 95 dB SPL, 15 min); or (3) a combination of the PPADS and the sound. The effects on the cubic distortion product otoacoustic emissions (DPOAEs; 2f1-f2) were monitored using three sets of equal level primaries (f1=9.25 kHz, f2=10.8 kHz, 2f1-f2=7.7 kHz; f1=7.2 kHz, f2=8.4 kHz, 2f1-f2=6 kHz; f1=5.55 kHz, f2=6.5 kHz, 2f1-f2=4.6 kHz). PPADS alone had no effect on the cubic DPOAEs monitored. The intense sound alone suppressed all three cubic DPOAEs. The combination of PPADS with the intense sound induced a suppression of the cubic DPOAEs that was equal to or greater than induced by the intense sound alone at f2=10.8 kHz but was equal to or less than induced by the intense sound at f2=8.4 and 6.5 kHz. After washing the PPADS out of the cochlea with artificial perilymph, all three cubic DPOAEs were suppressed less in the PPADS with intense sound treatment group than in the intense sound alone group. The PPADS appeared to provide protection from the intense sound. Results are consistent with the hypothesis that extracellular ATP is involved in the changes in cochlear mechanics induced by moderately intense sound exposure.