Selective labeling of visual corpus callosum connections with aspartate in cat and rat

Tritiated neurotransmitter candidates were unilaterally injected in visual cortical regions with abundant corpus callosum connections. D-aspartate (Asp) or gamma-aminobutyric acid (GABA) was injected along the area 17/18 border in cat, and the area 17/18a and 17/18b borders in rat. Retrograde Asp label was found contralaterally in supragranular and infragranular laminae in areas 17, 18, 19, PMLS, and PLLS in cat, and in areas 29, 18b, 17, and 18a in rat. No contralateral GABA label was found in cat or rat. Thus, the cat and rat corpus callosum may use Asp or a related substance as a neurotransmitter.