Microelectrode mapping of tonotopic, laminar, and field-specific organization of thalamo-cortical pathway in rat

- A combined use of surface and depth microelectrode arrays characterized the auditory thalamo-cortical system in rats.
- Tonotopic and latency maps were found in the ventral division of the auditory thalamus.
- Microstimulation in the thalamus elicited layer-specific, region-specific and tonotopic activation in the auditory cortex.
- Thus, the thalamo-cortical anatomical projection in rats was electrophysiologically confirmed for the first time.

The rat has long been considered an important model system for studying neural mechanisms of auditory perception and learning, and particularly mechanisms involving auditory thalamo-cortical processing. However, the functional topography of the auditory thalamus, or medial geniculate body (MGB) has not yet been fully characterized in the rat, and the anatomically-defined features of field-specific, layer-specific and tonotopic thalamo-cortical projections have never been confirmed electrophysiologically. In the present study, we have established a novel technique for recording simultaneously from a surface microelectrode array on the auditory cortex, and a depth electrode array across auditory cortical layers and within the MGB, and characterized the rat MGB and thalamo-cortical projections under isoflurane anesthesia. We revealed that the ventral division of the MGB (MGv) exhibited a low-high-low CF gradient and long-short-long latency gradient along the dorsolateral-to-ventromedial axis, suggesting that the rat MGv is divided into two subdivisions. We also demonstrated that microstimulation in the MGv elicited cortical activation in layer-specific, region-specific and tonotopically organized manners. To our knowledge, the present study has provided the first and most compelling electrophysiological confirmation of the anatomical organization of the primary thalamo-cortical pathway in the rat, setting the groundwork for further investigation.