Calbindin-immunoreactive neurons in the reticular formation of the rat brainstem: Catecholamine content and spinal projections

Calbindin-DB8k (calbindin) is a calcium-binding protein that is distributed widely in the rat brain. The localisation of calbindin immunoreactivity in the medulla oblongata and its colocalisation with adrenaline-synthesising neurons [phenylethanolamine-N-methyltransferase-immunoreactive (PNMT-IR)] w as examined (Granata and Chang [1994] Brain Res. 645:265-277). However, det ailed information about the distribution of calbindin-IR neurons in the ret icular formation of the medulla oblongata in particular is lacking. In this report, the authors address this issue with an emphasis on the quantitatio n of calbindin-IR neurons, catecholamine neurons [tyrosine hydroxylase (TH) -IR, or PNMT-IR], and spinally projecting neurons in the ventral brainstem. Rats received injections of the retrograde tracing agent cholera toxin B ( CTB) into the thoracic spinal cord or into the superior cervical ganglion. Immunocytochemistry was used to reveal calbindin, TH, PNMT, and CTB immunor eactivity. Ten calbindin-IR cell groups were identified within the pontomed ullary reticular formation. Seven previously undescribed but distinct clust ers of calbindin-LR neurons were found. Within the ventral pens, a populati on of calbindin-IR neurons occurred dorsal but adjacent to the A5 cell grou p. These calbindin-IR neurons did not contain either TH or PNMT immunoreact ivity, and few if any of these neurons projected to the spinal cord. A dist inct group of calbindin-IR neurons was present in the ventral medulla. Seve nty-five percent of these calbindin-IR neurons contained TH immunoreactivit y, 45% contained PNMT immunoreactivity, and 21% were spinally projecting ne urons. Spinally projecting, calbindin-IR neurons were a subpopulation of PN MT-IR cells. In the caudal ventral medulla, no TH-IR or PNMT-IR cells were calbindin-IR. In the intermediolateral cell column, close appositions of ca lbindin-IR terminals on identified sympathetic preganglionic neurons as wel l as calbindin-IR synapses indicated that these neurons may affect directly the sympathetic outflow. The results demonstrate for the first time the ex istence of a new subpopulation of spinally projecting, PNMT-IR neurons in t he rostral ventrolateral medulla. (C) 2000 Wiley-Liss, Inc.