Fos expression in the auditory thalamus is triggered by corticofugal projections

Abstract

In the present study, we investigated relationships between Fos expression in the auditory thalamus and 1) corticofugal activation, 2) neuronal firing patterns, and 3) neurotransmitters and receptors. The ventral (MGv) and dorsal divisions of the medial geniculate body (MGB) had no Fos expression with only acoustic stimulus under aneathesia, but did when the ipsilateral auditory cortex was deinactivated with injection of bicuculline (BIM), a GABAA receptor antagonist. Meanwhile, a strong Fos expression was also found in the BIM-injected cotex as well as the ipsilateral thalamic reticular nucleus. Simutaneously monitoring neural activities in the thalamus and cortex through multi-channel extracellular recording showed that an oscillatory activity (1-3 Hz), characterized by highly synchronized episodes of tetanus burst firing, appeared in the corticothalamic network after BIM injection in the cortex. The spontaneous neuronal activities in the MGB and the auditory cortex before the BIM injection were not or weakly synchronized, and the time lag between them was up to hundreds of milliseconds if synchronized. After the BIM injection the neuronal activities became highly synchronized with a narrow time lag of less than 50 milliseconds in the thalamus to the cortex. We would conclude that cortically derived highly synchronized oscillation (1-3 Hz) induced Fos expression in the corticothalamic network, including MGv, in which the primary ascending excitation could not induce Fos expression. Injection of glutamate neurotransmitter directly to the thalamus caused Fos expression when the animal was allowed to wake up, indicting that glutamate was involved in the corticofugal activation as well as Fos expression in MGv. Injections of glutamate receptor antagonists, AP-5 (an NMDA receptor antagonist) and /or CNQX (an AMPA receptor antagonist) into MGB showed a suppression effect on the Fos expression elicited by the corticofugal activation, indicating that both NMDA and AMPA receptors played an important role herein.