Projections from the dorsal peduncular cortex to the trigeminal subnucleus caudalis (medullary dorsal horn) and other lower brainstem areas in rats.

This study has revealed direct projections from the dorsal peduncular cortex (DP) in the medial prefrontal cortex (mPfC) to the trigeminal brainstem sensory nuclear complex and other lower brainstem areas in rats. We first examined the distribution of mPfC neurons projecting directly to the medullary dorsal horn (trigeminal subnucleus caudalis [Vc]) and trigeminal subnucleus oralis (Vo) which are known to receive direct projections from the lateral prefrontal cortex (insular cortex). After injections of the retrograde tracer Fluorogold (FG) into the rostro-dorsomedial part of laminae I/II of Vc (rdm-I/II-Vc), many neurons were labeled bilaterally (with an ipsilateral predominance) in the rostrocaudal middle level of DP (mid-DP) and not in other mPfC areas. After FG injections into the lateral and caudal parts of laminae I/II of Vc, or the Vo, no neurons were labeled in the mPfC. We then examined projections from the mid-DP by using the anterograde tracer biotinylated dextranamine (BDA). After BDA injections into the mid-DP, many axons and terminals were labeled bilaterally (with an ipsilateral predominance) in the rdm-I/II-Vc, periaqueductal gray and solitary tract nucleus, and ipsilaterally in the parabrachial nucleus and trigeminal mesencephalic nucleus. In addition, the connections of the mid-DP with the insular cortex were examined. Many BDA-labeled axons and terminals from the mid-DP were also found ipsilaterally in the caudalmost level of the granular and dysgranular insular cortex (GI/DI). After BDA injections into the caudalmost GI/DI, many axons and terminals were labeled ipsilaterally in the mid-DP. The projections from the mid-DP to the rdm-I/II-Vc and other brainstem nuclei suggest that mid-DP neurons may regulate intraoral and perioral sensory processing (including nociceptive processing) of rdm-I/II-Vc neurons directly or indirectly through the brainstem nuclei. The reciprocal connections between the mid-DP and caudalmost GI/DI suggest that this regulation may involve mid-DP interactions with the caudalmost GI/DI neurons.