A group of glutamatergic interneurons expressing high levels of both neurokinin‐1 receptors and somatostatin identifies the region of the pre‐Bötzinger complex

Abstract The pre‐Bötzinger complex (pre‐BötC) is a physiologically defined group of ventrolateral medullary neurons that plays a central role in respiratory rhythm generation. These cells are located in a portion of the rostral ventrolateral medulla (RVLM) that is difficult to identify precisely for lack of a specific marker. We sought to determine whether somatostatin (SST) might be a marker for this region. The rat pre‐BötC area was defined as a 500‐μm‐long segment of ventrolateral medulla coextensive with the ventral respiratory group. This region was identified by juxtacellular labeling of neurons with respiratory‐related activity and by its location rostral to the phrenic premotor neurons. It contained most of the SST‐ir neuronal somata of the RVLM. These cells were small (107 μm 2 ) and expressed high levels of preprosomatostatin mRNA. They were strongly neurokinin 1 receptor (NK1R)‐ir and were selectively destroyed by saporin conjugated with an NK1R agonist (SSP‐SAP). Most SST‐ir neurons (>90%) contained vesicular glutamate transporter 2 (VGLUT2) mRNA, and terminals immunoreactive for SST and VGLUT2 protein were found in their midst. Few SST‐ir neurons contained GAD‐67 mRNA (<1%) or preproenkephalin mRNA (6%). Retrograde labeling experiments demonstrated that over 75% of the SST‐ir neurons project to the contralateral pre‐BötC area, but none projects to the spinal cord. In conclusion, the RVLM contains many neurons that express preprosomatostatin mRNA. A subgroup of these cells contains high levels of SST and NK1R immunoreactivity in their somata. These glutamatergic interneurons identify a narrow region of the RVLM that appears to be coextensive with the pre‐BötC of adult rats. J. Comp. Neurol. 455:499–512, 2003. © 2002 Wiley‐Liss, Inc.