Glutamatergic neurons in the lateral periaqueductal gray innervate neurokinin-1 receptor-expressing neurons in the ventrolateral medulla of the rat.

The neural pathways underlying the respiratory responses elicited by electrical or chemical stimulation of the lateral part of the periaqueductal gray (lPAG) remain unsettled. In the present study, we examined the lPAG projection to neurokinin-1 receptor (NK1R)-immunoreactive (ir) neurons in the ventrolateral medulla (VLM) which have been implicated in the control of respiration. After biotinylated dextranamine (BDA) injection into the lPAG, NK1R-ir neurons in the rostral VLM were embedded in the plexus of BDA-labeled fibers. At the electron microscopic level, the BDA-labeled terminals made asymmetrical synaptic contacts predominantly with dendrites and additionally with somata of the NK1R-ir neurons. Using retrograde tracing combined with in situ hybridization, we demonstrated that the vast majority of the lPAG neurons projecting to the rostral VLM were positive for vesicular glutamate transporter 2 (VGLUT2) mRNA, but not for glutamic acid decarboxylase 67 mRNA. Using a combination of anterograde tracing and immunohistochemistry, we further demonstrated that the lPAG axon terminals with VGLUT2 immunoreactivity made close apposition with the NK1R-ir neuronal profiles in the rostral VLM. These data suggest that lPAG neurons exert an excitatory influence on NK1R-expressing neurons in the rostral VLM for the control of respiration.