Developmental responsiveness of thalamic neurons to vertical sinusoidal linear acceleration

Abstract

Signals arising from the vestibular end-organs are transmitted to the vestibular nucleus and then to higher centers for the perception of spatial orientations. It is evident that gravity-related neurons in different vestibular subnuclei showed distinct profiles of maturation. However, little is known about the maturation profile of gravity-related neurons in the thalamus, an ascending relay station in the central vestibular system. With the use of Fos expression as an activity indicator, we investigated the distribution pattern of gravity-related thalamic neurons in adult and postnatal (P7, 14 and 21) Sprague Dawley rats. Conscious rats were subjected to sinusoidal linear acceleration along the vertical plane, thereby activating saccules and rostral part of the utricles in the inner ear. Labyrinthectomized controls and normal stationary controls showed only a few scattered Fos-immunoreactive (Fos-ir) neurons, confirming the otolithic origin of Fos expression. From P21 onwards, Fos-ir neurons were identified in the thalamic nuclei (namely central medial, paracentral and rhomboid) and one subthalamic nucleus (zona incerta). An age-dependent increase in Fos-ir neurons was observed in these thalamic nuclei. In adult rats, significant number of Fos-ir neurons were also found in other thalamic nuclei (namely centrolateral and mediodorsal). Relatively fewer Fos-ir neurons were however observed in the centrolateral nucleus. These findings support the hypothesis that neurons in thalamic subnuclei of rats differ in maturation profile in the detection of translational head movements.