Relationship of calbindin D-28k with afferent neurons to the rostral ventrolateral medulla in the rat.

The phenylethanolamine-N-methyltransferase (PNMT)-containing neurons in the rostral ventrolateral medulla (RVLM) (the C1 adrenergic group) have been implicated in the generation of the tonic sympathetic nerve activity. Using a double-labeling immunohistofluorescence technique, we found that 34.6 +/- 11.4% (mean +/- S.D.) of PNMT immunoreactive neurons in the RVLM were immunoreactive for Calbindin D-28k (CaBP), a Vitamin D-dependent calcium binding protein. Since CaBP is probably involved in regulating intracellular calcium concentrations in cells that are metabolically or electrically very active, our results suggest that at least some C1 adrenergic neurons (those containing calbindin) may have calcium mediated high metabolic or electrophysiologic activity that is associated with generating tonic nerve function. The RVLM has wide connections with many different nuclei in the brain which are known to contain clusters of neurons that express immunoreactivity to CaBP. In order to determine whether CaBP could be used as a molecular marker for projection neurons to the RVLM or to identify a subpopulation of projection neurons containing CaBP, we sought to determine the relationships between CaBP and the neurons that project to RVLM. Following injections of the retrograde tracer FluoroGold (FG) into the rat RVLM, sections containing retrogradely labeled neurons in (1) the nucleus tractus solitarii (NTS), (2) the contralateral RVLM, (3) the area postrema, (4) the mesencephalic central gray (mCG), (5) the lateral hypothalamus (LH), (6) the substantia innominata (SI), and (7) the paraventricular hypothalamic nucleus (PV) were tested for CaBP immunoreactivity. Although many retrogradely labeled neurons were found amidst many CaBP immunoreactive neurons in each of these nuclei, only a subpopulation of the retrogradely labeled neurons expressed CaBP immunoreactivity. The NTS demonstrated the higher proportion of double-labeled cells (mean 31.5 +/- 4.3%), whereas the lower proportion corresponded to the contralateral RVLM (mean 9.6 +/- 3.2%). On the other hand, both the retrogradely labeled neurons and the CaBP immunoreactive neurons in each of these nuclei were often found in regions containing a great number of adrenergic axons (i.e. immunoreactive for PNMT). Our results suggest that: (1) Two types of adrenergic RVLM neurons could be found, those containing CaBP and those lacking this calcium binding protein. (2) CaBP is not a common marker for the afferent neurons to the RVLM, but rather is found in selective subsets of them. (3) Both the non-CaBP projection neurons and the CaBP immunoreactive neurons in these nuclei may be innervated by adrenergic fibers.