Organization of presumptive catecholamine-synthesizing neurons in the canine medulla oblongata

Immunocytochemical methods were used to identify cells and processes containing two major catecholamine (CA)-biosynthetic enzymes in areas of the canine medulla implicated in autonomic control. Antisera were employed against tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT). These enzymes respectively catalyze the conversions of tyrosine to l -DOPA and noradrenaline to adrenaline. Immunocytochemical studies laid the groundwork for subsequent investigations in conscious dog in which we characterized an area of cardiovascular control in the rostral ventrolateral medulla (RVLM). In the anatomical studies, previously unidentified neuronal somata and processes were demonstrated in the canine medulla. Presumptive adrenergic (C1) neurons in the canine RVLM were subjacent to the nucleus ambiguus (NA) and most numerous at a level where the compact and semicompact divisions of NA merged. In contrast to their distribution in rodents, C1 neurons were skewed caudally and did not extend rostrally to the caudal pole of the facial nucleus. C1 neurons were also relatively less concentrated in the RVLM. A large number of C1 neurons extended dorsally into the lateral tegmental field (LTF). Most C1 neurons in the LTF (like those in the A1 area) were aligned with catecholaminergic (TH- and PNMT-ir) processes traversing the intermediate reticular zone. Since the numbers and locations of TH- and PNMT-ir neurons in the C1 area of the RVLM and rostral LTF were virtually identical on adjacent sections, it can be implicity inferred that the enzymes are co-localized to the same somata and that these neurons are capable of biosynthesizing adrenaline. The C1 and A5 areas were clearly separated by a transitional zone, sparsely populated by TH-ir somata (1–2 cells per section), where the facial nucleus and rostral pole of the NA pars compacta (NAc) occupied the same level. A5 neurons were more abundant and complexly organized than suggested by previous CA-histofluorescence data. In addition, a new parvicellular subgroup was identified and composed of neurons containing TH but not PNMT. In contrast to other species, the A1 cell group was not confined to the VLM. A large number of A1 neurons extended into the caudal LTF and were situated between the nucleus tractus solitarii-motor vagal complex (NTS-X) and caudal VLM (CVLM). In contrast to previous reports, presumptive adrenergic (TH- and PNMT-ir) cell groups were more densely represented in the C2-3 areas of the canine NTS and dorsomedial reticular formation. C3 neurons were more numerous than those in the C2 area and appeared to represent a dorsomedial extension of cells in the C1 area of the LTF. In contrast to CA-histofluorescence data, the NTS also was enriched in cell groups staining for TH in the A2 area. A more complex and heterogeneous distribution of neuronal somata and punctate varicosities were immunolabeled for PNMT and/or TH than observed previously. Suggestive evidence was also obtained that putative adrenergic (TH- and PNMT-ir) terminals may be spatially segregated from those containing other catecholamines (i.e. only TH). Cranial nerve afferents traversed the spinal trigeminal complex and coursed caudally through the solitary tract. These fibers were immunostained for TH but not PNMT. Primary visceral afferents thus might employ dopamine and/or norepinephrine as a neurotransmitter or neuromodulator. Finally, structures were found to express intense TH- or PNMT-ir in areas previously characterized by weak or imperceptible CA-histofluorescence. These cell groups in the canine medulla may, therefore, either store low levels of catecholamines or may synthesize their presumptive neurotransmitter on demand.