Neurokinin-1 receptor-expressing neurons that contain serotonin and gamma-aminobutyric acid in the rat rostroventromedial medulla are involved in pain processing.

UNLABELLED: The rostroventromedial medulla (RVM) is an important area of the endogenous pain-regulating system, in which 5-hydroxytryptamine (5-HT) and gamma-aminobutyric acid (GABA) are 2 main transmitters involved in pain modulation. However, whether 5-HT and GABA are colocalized is poorly understood. By using glutamate decarboxylase 67-green fluorescence protein (GAD67-GFP) knock-in mouse, we confirmed the colocalization of 5-HT and GABA in the RVM, with a main distribution in the raphe magnus nucleus and paragigantocellular reticular nucleus. Interestingly, more than half (51.6%) of the 5-HT/GABA-immunoreactive (ir) neurons expressed neurokinin-1 receptors (NK-1R) and one-third (30.1%) of the 5-HT/GABA/NK-1R-ir neurons projected to the spinal cord, suggesting that substance P (SP) should regulate the activity of 5-HT/GABA-ir spinal cord-projecting neurons. By combining retrograde and anterograde tracing methods, we observed that the cuneiform nucleus, dorsal raphe nucleus, and lateral periaqueductal gray are the main origin nuclei for the SP-ir fibers and terminals in the RVM. Finally, after formalin injection into the mice hind paw, 29.2% SP-ir RVM-projecting neurons from supra-RVM nuclei and 33.1% NK-1R-ir spinal cord-projecting neurons in the RVM were activated. The present study provides potent morphological evidence that 5-HT and GABA are coexistent in RVM-spinal cord-projecting neurons that are also regulated by SPergic projections. PERSPECTIVE: The results will greatly enhance our understanding for the modulation of nociceptive information in the descending pain-regulating system.