Presumptive adrenergic neurons containing phenylethanolamine N-methyltransferase immunoreactivity in the medulla oblongata of neonatal swine

Given the importance of the swine (Sus scrofa) as an animal model for human development, physiology and disease, neurons containing the epinephrine-synthesizing enzyme, phenylethanolamine N-methyltransferase (PNMT), were mapped in the medulla oblongata of neonatal swine as a first step in identifying their roles in central autonomic control. Neurons were labeled immunocytochemically by using an antiserum to PNMT raised in rabbits against trypsin-treated enzyme purified from the bovine adrenal gland. The general regional organization of neurons expressing PNMT (-like) immunoreactivity (ir) in the neonatal swine was similar to data obtained in other species and, in some aspects, more closely resembled the pattern observed in the primate brain. Immunolabeled cells appeared to be more abundant and caudally more extensive than observed in other adult animals. PNMT-immunoreactive (ir) neuronal somata, however, were largely confined to the reticular formation in the ventrolateral quadrant and the nucleus tractus solitarii (NTS) and more restricted in distribution than those expressing tyrosine hydroxylase (TH) and dopamine β-hydroxylase (DβH)-ir on serial transverse sections. A close correspondence was observed between the distributions of TH-and PNMT-ir neurons and processes throughout the CI and C2 areas. However, in the CI and C3 regions TH-ir neurons outnumbered those containing DβH and PNMT-ir. In contrast, cell groups enriched in PNMT-ir neurons and processes were characterized by relatively weak DβH-ir. In the ventrolateral medulla (VLM), PNMT-ir cell bodies were concentrated rostrally and extended from the caudal pole of the facial nucleus to a level posterior to the calamus scriptorius. The rostral VLM was characterized by an admixture of bipolar and multipolar primarily medium-diameter immunostained neurons. A prominent cell column (condensation) organized ventromedially to the nucleus ambiguus pars compactus (NAc). A loosely organized cluster bordered the lateral aspect of the special visceral efferent column; another smaller aggregate was located in the ventromedial reticular formation adjacent to the inferior olive. At middle medullary levels, PNMT-ir neurons formed two distinct subgroups (dorsal and ventral) interrupted by a band of precerebellar relay neurons that extended between the medial and lateral limbs of the lateral reticular nucleus of Walberg. At obex, the dorsal cell group formed a diagonal array and assumed a position dorsal and dorsolateral to the medial limb of LRN. This group was distinguished by bipolar neurons with axes of orientation directed perpendicularly to the majority of neurons in the rostral VLM or those lying near the caudal ventromedullary surface. PNMT-ir neurons in the dorsomedial medulla were concentrated in the NTS and extended from the rostral medulla towards the spinal-medullary junction. As observed in the human brainstem, a dorsal group of neurons, smaller in diameter than those in the VLM and intensely immunoreactive, formed a dense circular condensation at the dorsolateral edge of the nucleus subpostrema (NSP) subjacent (ventrolateral) to the area postrema (AP). Another heterogenous cell group was identified in the main body of the NSP; most cells were small in size and concentrated caudally. The NSP, a major target of the subdiaphragmatic vagus, was characterized by a dense matrix of immunoreactive processes and cell bodies with frequently branching dendrites ramifying locally or extending into the AP or subjacent NTS. A third distinct group of small ovoidal-fusiform cells occurred in the medial aspect of the commissural nucleus of Cajal and represented a caudal consolidation of those in the NSP. Smaller numbers of less intensely labeled cells occurred in the AP. Cells were sparsely immunostained for PNMT in the C3 area of the rostral dorsomedial reticular formation and medial longitudinal fasciculus and unlabeled in the periventricular gray. The distributions of PNMT-ir neurons in the VLM and NTS in neonatal swine are consistent with a role for adrenergic pathways in central autonomic control. Subsequent efforts following their maturation may provide a structural basis for cardiorespiratory reflex differentiation as occurs postnatally.