Catecholamine and NPY efferents from the ventrolateral medulla to the amygdala in the rat.

Anatomical tracing studies have demonstrated an efferent pathway to central nucleus of the amygdala (CeA) from the ventrolateral medulla. The combined retrograde tracing/immunohistochemical method was used to test for the presence of catecholamines and neuropeptide Y (NPY) in ventrolateral medulla neurons that innervate the CeA. Numerous retrogradely labeled neurons were observed in the ventrolateral medulla caudal to the area postrema. Fewer retrogradely labeled neurons were also observed in the rostral ventrolateral medulla. Retrogradely labeled neurons immunoreactive for both tyrosine hydroxylase and NPY were found in the caudal ventrolateral medulla. Double-labeled phenylethanolamine-N-methyltransferase neurons were present at levels rostral to the area postrema. A substantial portion of the ventrolateral medulla projection to the CeA arises from adrenergic cells of the C1 group, because nearly 40% of the retrogradely labeled cells were also immunoreactive for phenylethanolamine-N-methyl-transferase. The high percentage of double-labeled NPY-immunoreactive neurons suggests NPY is colocalized in CeA-projecting catecholamine neurons, indicating that input from the ventrolateral medulla to the CeA primarily arises from C1 adrenergic neurons that also express NPY. This contrasts with previous data suggesting the 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 is chemically coded in terms of a dorsal noradrenergic and a ventrolateral adrenergic pathway.