Identification of C1 presympathetic neurons in rat rostral ventrolateral medulla by juxtacellular labeling in vivo.

Schreihofer AM, Guyenet PG.
J Comp Neurol. 1997 Nov 3;387(4):524-36.

   Before this paper, it was known that some RVLM neurons were barosensitive, but showing the phenotypes of which ones were linked to SNA had been difficult. There were multiple lines of evidence stating that C1 neurons were involved, but the presympathetic non-C1 cells complicated the issue. In fact, experiments that measured conduction velocities of spinally projecting neurons suggested that C1 neurons might not be a major player, despite making up 50-70% of the cells projecting to the IML.

   In this study, they recorded 96 cells in 41 rats, and 87% of the cells were spinally projecting (tested via antidromic activation). Many times the blood pressure needed to be increase to stop spontaneous action potentials in order to induce an antidromic one. The distance between recording and stimulating electrodes was estimated at 35mm, which allowed them use constant antidromic latencies to estimate conduction velocity. They attemped to label 67 of the 87 cells and recovered 49 of the ones they attempted. They found that faster conducting (and more spontaneously active) cells were harder to label than slower ones (presumed to be C1). When they looked for TH immunoreactivity, they were able to recover 39 labeled, processed, spinally projecting cells.

   The cells broke roughly into thirds as non-TH-ir cells, slow TH-ir cells, and fast TH-ir cells. They applied a metric of TH immunoreactivity densitometry to find that slow TH-ir cells had much greater signal than fast TH-ir cells. All slow firing cells were TH-ir. They confirmed this by labeling analyzing another set of 5 labeled cells (3 slow, 2 fast) and staining for PNMT and got the same results - slow cells showed stain while fast cells didn't. They also reconstructed the neurons similar to how our lab has, but they didn't find any differences between the 3 types of neurons in terms of size or shape, which is something I would have been interested in seeing.

   The main findings here were that ~70% of the spinally projecting cells they found were indeed C1 cells, and that ALL of the slow firing spinally projecting cells were C1. This was also evidence that C1 are inhibited by increases in blood pressure, and their function was most likely sympathoexcitatory. It also helps explain why previous papers did not report C1 spinally projecting neurons - because they only recorded fast neurons, which selected against ~50% of the C1 neurons. -DH