Morphology of rostral medullary neurons with intrinsic pacemaker activity in the rat.

Sun MK, Stornetta RL, Guyenet PG.
Brain Res. 1991 Aug 9;556(1):61-70.

   In slices, and in some in vivo preparations, some presympathetic RVLM neurons fire spontaneously, even when glutamate receptors are blocked. They had previously shown that spontaneously active spinally projecting RVLM neurons were non-C1 type cells, so this study followed up on that by looking at these "pacemaker" neurons and get phentotypic information about them, including cell type and morphology. To do this, they used rat medulla slices and intracellular recording with peroxidase- or lucifer yellow-filled electrodes. They located and recorded from spontaneously active cells before injecting the labeling compound.
   They found that the pacemaker cells had fusiform or triangular bodies that were significantly larger compared to 46 PNMT positive cells they measured, though they note that there may have been some bias toward recording larger cells. They found that the cells had fairly simple dendrites which mostly spread toward the ventral surface of the slice and then out to the ventromedial and dorsolateral edges, which didn't seem to be unique to the pacemaker cells (C1 cells may also fit this pattern). This made a lot of sense since previous experiments had been done by some groups simply by dripping drugs onto the ventral surface of the RVLM.
   They identified what they believed to be axons (though this was not confirmed by electron microscopy) through their thin, smoother appearance which was different from the light branching seen on most dendrites. The axons projected dorsally and medially, similar to C1 cells. This was in contrast to the lateral course seen by another group, though they suggest that might have been a different subpopulation of cells. They also did not find axonal branching (ascending and descending) seen by others in C1 neurons and say this may be more evidence of different cell types. I admit to being a little cofused about this interpretation since they say the branching has been shown by others to be ~2.6mm away from the body, and their slices were only 500um thick. They had to use cells towards the middle of their slices to limit cutting off dendrites, so that means they could only really observe ~250um. Maybe I'm just misinterpreting the text, because in the conclusions they state that their prior electrophysiological studies DO support axonal branching. Either way, it is nice to learn more about the types of cells I can expect to see in my reconstruction study. -DH