Galanin is a selective marker of the retrotrapezoid nucleus in rats

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Abstract
The rat retrotrapezoid nucleus (RTN) contains CO2‐activated neurons that contribute to the central chemoreflex and to breathing automaticity. These neurons have two known markers, the transcription factor Phox2b and vesicular glutamate transporter 2 (VGLUT2). Noncatecholaminergic galanin‐immunoreactive (ir) neurons within a region of the lower brainstem that seems identical to what is currently defined as the RTN have been previously described. Here we ask whether these galanin‐expressing neurons are the same cells as the recently characterized CO2‐sensitive neurons of the RTN. By using in situ hybridization, we found that pre‐pro‐galanin (PPGal) mRNA is expressed by an isolated cluster of neurons that is co‐extensive with the RTN as defined by a population of strongly Phox2b‐ir neurons devoid of tyrosine hydroxylase (Phox2b+/TH− neurons). This bilateral structure contains about 1,000 PPGal mRNA‐positive neurons in the rat. The PPGal mRNA‐positive neurons were Phox2b+/TH− and as susceptible to destruction by the toxin [Sar9, Met (O2)11]‐substance P as the rest of the RTN Phox2b+/TH− cells of the RTN. CO2‐activated neurons were recorded in the RTN of anesthetized rats and were labeled with biotinamide. Many of those cells (7/17, 41%, five rats) contained PPGal‐mRNA. In conclusion, galanin mRNA is a very specific marker of the glutamatergic Phox2b+/TH− neurons of the RTN, but galanin mRNA identifies only half of these putative central respiratory chemoreceptors. J. Comp. Neurol. 512:373–383, 2009.

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