Glutamatergic neurons in the Kölliker-Fuse nucleus project to the rostral ventral respiratory group and phrenic nucleus: a combined retrograde tracing and in situ hybridization study in the rat.

Kölliker-Fuse nucleus (KF) neurons are considered to excite motoneurons in the phrenic nucleus (PhN) during inspiration through its projection to the PhN and/or to the rostral ventral respiratory group (rVRG), which in turn projects to the PhN, probably by releasing glutamate from their axon terminals. Using a combined retrograde tracing and in situ hybridization technique, here we demonstrate that most of the KF neurons projecting to the PhN and rVRG contain vesicular glutamate transporter 2 (VGLUT2) mRNA but not glutamic acid decarboxylase 67 (GAD67) mRNA, providing definitive evidence that these neurons are glutamatergic. Together with previous data by Stornetta et al. [Stornetta, R.L., Sevigny, C.P., Guyenet, P.G., 2003b. Inspiratory argumenting bulbospinal neurons express both glutamatergic and enkephalinergic phenotypes. J. Comp. Neurol. 455, 113-124], indicating that PhN-projecting rVRG neurons are VGLUT2 mRNA-positive, the present results suggest that the glutamatergic KF-PhN pathway and/or the glutamatergic KF-rVRG-PhN pathway transmit excitatory outputs of KF neurons to the PhN neurons during inspiration.