Neurotransmitters and neuroplasticity during cochlear development: in vivo and in vitro studies

The adult auditory receptor and pathway are organized by highly complex circuits that appear during embryonic development. Several different neuroplasticity processes during development and maturation have clearly contributed to its final structural and functional organization. It seems especially interesting that an important over-sprouting of nerve fibres is first observed and later re-accommodates until reaching the final structure. During earlier development stages afferent nerve fibres from type I spiral ganglion cells reach all of the immature hair cells, before their differentiation into inner (IHCs) and outer (OHCs) hair cells. They were all recognized by the precocious ability to respond to glutamate agonists, which means an early high degree of maturation. Later, only type I afferent nerve fibres (kainic acid sensitive), connecting OHCs, will degenerate and disappear. The recent finding of some molecular differences between both types could justify this dissimilar maturation. In addition, an over-sprouting affects the olivary efferent fibres. In the same way as the afferents, these efferent nerve fibres promptly begin to express neurotransmitters even though a relevant number will degenerate during maturation and after the onset of hearing. Two types of nerve fibres must be considered: stable fibres, which remain into adult life, and transitory ones that reach a high degree of maturation but then disappear. All these neurotransmitters, and transitory nerve fibres, could be involved in neuroplasticity.