Chemically defined feedback connections from infragranular layers of sensory association cortices in the rat.

The primary visual (V1), auditory (AI), and somatosensory (SI) cortices are reciprocally connected with their respective sensory association cortices. In the rat, we have previously demonstrated that some of the connections arising from the secondary somatosensory (SII) and parietal insular (PA) cortices and terminating in the SI, are characterized by the expression of latexin, a candidate protein of carboxypeptidase A inhibitor. Here, by using retrograde tracing and latexin-immunohistochemistry, we show that latexin-expressing neurons in other association cortices of different sensory modalities also contribute to the feedback projections to the corresponding primary sensory cortices. These are the lateral part of the secondary visual cortex (V2L), temporal association cortex, and the dorsal and ventral (AIIv) parts of the secondary auditory belt cortex. Within sublayer VIa of the V2L, AIIv and SII, the majority of the V1-, AI- and SI-projecting neurons respectively, are latexin-immunopositive. In contrast to feedback connections, far fewer latexin-expressing neurons participate in callosal or intrahemispheric feedforward connections. The latexin-expressing neurons constitute a virtually completely different population from corticothalamic neurons within the infragranular layers. Given that latexin might participate in the modulation of neuronal activity by controlling the protease activity, latexin-expressing feedback pathways would play a unique role in the modulation of sensory perception.