Cooling of the auditory cortex modifies neuronal activity in the inferior colliculus in rats

There are powerful pathways descending from the auditory cortex (AC) to the inferior colliculus (IC), yet their function is not fully understood. The aim of this study is to examine the effects of a reversible cortical inactivation, achieved by cooling of the AC, on the responses of neurons in the rat IC. Extracellular single-unit or multi-unit activity was recorded in the IC of anaesthetized rats with a 16-channel multielectrode probe introduced along the IC dorso-ventral axis through the dorsal cortex (DCIC) to the central nucleus of the IC (CIC). Cooling of the AC produced an increase in spontaneous activity and magnitude of the sound-evoked response in 47% of the IC neurons. Maximal changes in the neuronal activity were observed in the DCIC and the central part of the CIC. The final segments of the sustained responses to 60 ms stimuli and the off responses were more affected than the onset segments. Inactivation of the AC resulted in a suppression of the post-excitatory inhibition and neuronal adaptation, which was reflected in a pronounced enhancement of synchronized responses to a series of fast repeated clicks. The response parameters recovered, at least partly, to the pre-cooling levels 1 h after the cooling cessation. The frequency tuning properties of the IC neurons did not show any significant changes during the cooling period. The results demonstrate that AC cooling inactivates excitatory corticofugal pathways and results in a less activated intrinsic inhibitory network in the IC.