Pathology of hair cells and neurones
Overview / Aminoglycosides / Noise Trauma / Excitotoxicity / Presbycusis / Tinnitus
Pictures : R. Pujol

Excitotoxic injury at endings of auditory nerve




Immediately after perilymphatic perfusion with glutamate or its agonists such as AMPA or kainate (ref.c1, c3), all afferent dendrites below the IHC are swollen, their membranes are ruptured and their cytoplasmic content is lost (left). At higher magnification (right), a remnant of the postsynaptic membrane (red arrow) faces a presynaptic body. The IHC is disconnected from the auditory nerve and cochlear potentials are lost. The same type of damage occurs immediately after acoustic trauma (ref. c5) or ischaemia (ref.c1, c3) but it can be blocked by an AMPA antagonist such as DNQX. A protective effect of dopamine, a lateral efferent neurotransmitter, is also observed.
scale bar: 10 µm (Left); 0,3 µm (right)

Synaptic repair after an excitotoxic injury (ref. c4)

JL. Puel et al.
See also animated drawing

1) Normal synaptic complex below an IHC: a presynaptic body in the IHC faces an afferent dendrite (blue), which itself is contacted by a lateral efferent vesiculated ending (pink). The cochlear action potential is normal.
Acute excitotoxicity: the bouton of the afferent dendrite has ruptured and only remnants of its membrane are seen; a piece of the postsynaptic membrane is still attached to the presynapse. Cochlear action potential is absent.
3) 1 day post injury: the afferent dendrite has begun to grow upwards and some filopodia already reach the IHC. In the meantime the widow efferents are directly contacting the IHC. Cochlear action potential begins to recover.
5 days post injury. The normal synaptic pattern is found again, resulting in full functional recovery, demonstrated by the compound action potential.

In man, this mechanism is probably the basis for recovery after sudden deafness caused by ischaemia or noise trauma.
At stage 2 and 3, the nerve ending undergoes drastic changes, some of them because an overexpression of NMDA glutamate receptors may trigger abnormal spontaneaous firing, which is probably the origin of tinnitus.
After repetitive excitotoxic injuries, the repair mechanism will stop when the neurone itself dies.

Protocol for testing acute and chronic local pharmacology in the guinea pig cochlea (ref. c4)

JL. Puel
1- osmotic minipump (Azlet Corp) which delivers a drug (eg agonist, antagonist, antisense molecule) over 3,5,7, or 15 days. After placing the catheter in a tiny hole in the scala tympani (3), the pump is inserted under the dorsal skin;
2- recording plug to monitor cochlear potentials: the plug is connected to the round window electrode (4);
5- eardrum;
6- micropipette which delivers a single shot of drug (for instance, AMPA to induce excitotoxicity) or chronic dose if connected to minipump;
7- hole (in scala vestibuli) for perfusion outlet (for acute pharmacology).
This protocol of intracochlear perfusion is used in animals (guinea pigs or rats) to explore the molecular pharmacology of the first auditory synapse in the cochlea.
Currently this protocol is being used to check the effect of molecules in speeding up the repair process, protecting the neurone, or stopping an increased firing of the fibre (tinnitus).
In humans, whilst intracochlear pharmacology cannot be envisaged, a substitutive trans-tympanic protocol is going to be tested. In this way in the near future a specific and local pharmacology for the cochlea may be successfully developed.


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