Comparative Analysis of FOXP2 Expression in the Thalamus of Mice, Rats, and Macaques: Implications for the Evolution of Language Circuits

FOXP2 is a transcription factor essential for the development and function of neural circuits involved in language. Although its expression has been extensively characterized in the cortex and basal ganglia, its organization within the adult thalamus remains poorly understood. In this study, we present a comparative analysis of FoxP2 protein expression across thalamic nuclei in mice, rats, and macaques, with a focus on nuclei associated with higher-order cognitive functions and language-related circuits in humans. We found that FoxP2 is expressed in most thalamic nuclei across species, with a consistent absence in the reticular nucleus and zona incerta. Expression was highest in midline and intralaminar nuclei, whereas the anterior group showed low and variable expression among species. Macaques exhibited broader and, in some nuclei, more intense FoxP2 expression, particularly in associative regions such as the pulvinar, lateral geniculate, and parts of the ventral group, indicating increased specialization of thalamocortical pathways. This distribution suggests a conserved role for FoxP2 in shaping thalamic circuits supporting sensorimotor integration, attention, memory, and linguistic processing. Phylogenetic comparisons further indicate that enhanced FoxP2 expression in associative thalamic territories in primates, likely intensified in humans, may have contributed to the evolution of neural circuits required for speech and language. These findings provide molecular and anatomical insights into how FoxP2 helps organize thalamocortical networks relevant both to language function and to neuropsychiatric disorders involving thalamocortical dysconnectivity.