CATECHOLAMINE AND NPY EFFERENTS FROM THE VENTROLATERAL MEDULLA TO THEAMYGDALA IN THE RAT

Anatomical tracing studies have demonstrated an efferent pathway to ce ntral nucleus of the amygdala (CeA) from the ventrolateral medulla. Th e combined retrograde tracing/immunohistochemical method was used to t est for the presence of catecholamines and neuropeptide Y (NPY) in ven trolateral medulla neurons that innervate the CeA. Numerous retrograde ly labeled neurons were observed in the ventrolateral medulla caudal t o the area postrema. Fewer retrogradely labeled neurons were also obse rved in the rostral ventrolateral medulla. Retrogradely labeled neuron s immunoreactive for both tyrosine hydroxylase and NPY were found in t he caudal ventrolateral medulla. Double-labeled phenylethanolamine-N-m ethyltransferase neurons were present at levels rostral to the area po strema. A substantial portion of the ventrolateral medulla projection to the CeA arises from adrenergic cells of the C1 group, because nearl y 40% of the retrogradely labeled cells were also immunoreactive for p henylethanolamine-N-methyltransferase. The high percentage of double-l abeled NPY-immunoreactive neurons suggests NPY is colocalized in CeA-p rojecting catecholamine neurons, indicating that input from the ventro lateral medulla to the CeA primarily arises from C1 adrenergic neurons that also express NPY. This contrasts with previous data suggesting t he catecholaminergic projection from the nucleus of the solitary tract to the CeA originates from the A2 noradrenergic cell group. Thus, the input to the CeA from catecholaminegic groups in the caudal medulla i s chemically coded in terms of a dorsal noradrenergic and a ventrolate ral adrenergic pathway.