INNERVATION OF THE AMYGDALOID COMPLEX BY CATECHOLAMINERGIC CELL GROUPS OF THE VENTROLATERAL MEDULLA

The projections to the amygdaloid complex (AMG), originating in the ca techolaminergic cell groups of the ventrolateral medulla (VLM), were s tudied in the rat by using either the retrograde tracer fluorogold (FG ) or the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in combination with tyrosine hydroxylase (TH) and/or phenylethanolami ne-N-methyltransferase (PNMT) immunohistochemistry. In the first serie s of experiments, injections of FG were made into regions of the centr al nucleus of the amygdala (ACe) where dense TH and PNMT immunoreactiv ity was previously observed, and then sections of the brainstem were p rocessed for TH and PNMT immunoreactivity. FG retrogradely labelled ne uronal cell bodies were observed throughout the rostrocaudal extent of VLM, bilaterally, with a contralateral predominance. Approximately 44 % of the FG labelled cell bodies in VLM were also immunoreactive to th e catecholamine biosynthetic enzymes TH and/or PNMT. Most of these cat echolaminergic neurons were part of the Al noradrenergic cell group in the caudal VLM and to a lesser extent part of the Cl adrenergic cell group in the rostral VLM. In the second series of experiments, PHA-L w as iontophoresed into VLM at different rostrocaudal levels where in th e previous series of experiments FG retrogradely labelled cell bodies were observed. Transverse sections of the forebrain and brainstem were then processed for the demonstration of PHA-L and either TH or PNMT i mmunoreactivity in cell bodies, axons, and presumptive axon terminals. PHA-L injection sites within either the caudal or rostral VLM resulte d in labelled axons and terminal bouton-like swellings primarily in th e contralateral AMG and to a lesser extent in the ipsilateral AMG. The ACe was observed to receive the greatest innervation from either VLM site. Additionally, PHA-L labelled fibers and presumptive terminal bou tons were observed within the intercalated, medial, basomedial, and ba solateral nuclei of the AMG. Most of the PHA-L labelled fibers and pre sumptive terminal boutons in the AMG after a caudal VLM (A1 region) in jection also displayed TH immunoreactivity, whereas after a PHA-L inje ction into the rostral VLM (Cl region) all of the labelled axons and a xon terminals in the AMG also were immunoreactive to PNMT. These data demonstrate that catecholaminergic neurons in A1 and C1 regions of VLM innervate the AMG and suggest that these VLM neurons may be involved in relaying afferent information directly to the AMG which influences the activity of AMG neurons controlling autonomic, endocrine, and beha vioural functions.