Is There Stimulus-Specific Adaptation in the Medial Geniculate Body of the Rat?

Neurons in the auditory cortex (AC) show a reduced response to a repeated stimulus, but briefly resume firing if a novel stimulus is presented (Nat Neurosci 6:391-398, 2003). This phenomenon is called stimulus-specific adaptation (SSA). This same study concluded that SSA is absent in the medial geniculate body (MGB), and is therefore a unique feature of the AC. More recently, however, SSA has been observed in the inferior colliculus (IC) of the rat (Eur J Neurosci 22:2879-2885, 2005; J Neuroci 29:5483-5493, 2009). Since the MGB receives its main inputs from the AC and IC, both of which show SSA, we reexamined the issue of whether neurons in the MGB also show SSA. We used a protocol similar to that of Ulanovsky et al. (Nat Neurosci 6:391-398, 2003) to record extracellular single unit responses in the MGB of the urethane-anesthetized rat. Our data demonstrate that SSA is indeed present throughout the MGB, being more prominent in the dorsal and medial subdivisions. Our results taken together with those from the IC indicate that SSA and enhanced responses to novel stimuli are present at every level from the IC on, and we hypothesize that SSA may be shaped, at least in part, through a bottom-up process.