Postnatal development of corticofugal modulation in the auditory thalamus in rats

Abstract

To examine how acoustically evoked neuronal responses are modulated by corticofugal innervation, functionally activated neurons along the auditory pathway were examined in conscious and anesthetized adult Sprague Dawley rats that were subjected to 70dB noise in a sound-proof room. Neuronal activation was defined by the expression of Fos protein. In conscious rats, Fos expression was observed in all auditory relays spanning from medulla to cortex. In the medial geniculate body (MGB), Fos labeling was observed in its medial and ventral subdivisions (MGv) but not in the dorsal subdivision. In ketamine/xyaline anesthetized rats in which cortical activities are known to be suppressed, Fos-positive neurons were found only in auditory relays in the brainstem but not in the MGB and the cortex. That Fos-positive neurons in the MGv were observed only after both activation of the auditory cortex (by microinjecting bicucullin, a GABAergic receptor antagonist) and presentation of acoustic stimulus suggests that corticofugal modulation is a prerequisite for Fos expression within MGB. This also implies that in conscious rats, corticofugal modulation plays an important role in the activation of thalamic neurons during acoustic stimulation. Experiments were also conducted to examine the developmental profile of corticothalamic modulation in MGB of conscious postnatal rats (P7 to adult). Fos-positive neurons were first observed within the brainstem at postnatal day 11 (P11). Fos-positive MGB neurons were not observed until P13; numbers peaked at P15, and thereafter dropped to adult level at P21. These suggest that maturation of corticothalamic modulation of acoustic signals occurs during the second and third postnatal weeks.