Vestibular critical period, maturation of central vestibular neurons, and locomotor control.

Mutant mice are a good model to study to what extent the postnatal activity of sensory afferents is necessary for the maturation of central neurons. In particular, the question arises whether the signals carried by the first-order vestibular neurons, which encode information on the head movement of pups, are necessary for the maturation of second-order vestibular neurons. To address that question, juvenile and adult transgenic, vestibular-deficient mutants where a null mutation of the KCNE1 potassium-channel gene leads to degeneration of all hair cells of the inner ear just after birth were studied. These KCNE1(-/-) mutants are deaf and show quasi-constant head bobbing and a permanent shaker/waltzer phenotype. This behavior is not due to persistent abnormalities of the membrane properties of central vestibular neurons, because their maturation is delayed but not impaired by the absence of sensory vestibular information. On the other hand, the data shed light on how the membrane properties of vestibular neurons might be modified according to functional requirements or following lesions. The expression levels of the protein calretinin that regulates the intracellular free-calcium concentration in central vestibular neurons could play a major role both in intact animals and following labyrinthectomy. By comparing the KCNE1(-/-) mutant mice to other vestibular-deficient animals, it was concluded that the suppression of vestibular inputs during a "critical period" of postnatal development can induce a permanent circling behavior, but that this phenotype is not always due to congenital vestibular deficiency.