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

The pre-Botzinger complex (pre-BotC) 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-BotC area was defined as a 500-mum-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 mum(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-BotC 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-BotC of adult rats.