Innervation of the amygdaloid complex by catecholaminergic cell groups of the ventrolateral medulla

The projections to the amygdaloid complex (AMG), originating in the catecholaminergic cell groups of the ventrolateral medulla (VLM), were studied in the rat by using either the retrograde tracer fluoro-gold (FG) or the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in combination with tyrosine hydroxylase (TH) and/or phenylethanolamine-N-methyltransferase (PNMT) immunohistochemistry. In the first series of experiments, injections of FG were made into regions of the central nucleus of the amygdala (ACe) where dense TH and PNMT immunoreactivity was previously observed, and then sections of the brainstem were processed for TH and PNMT immunoreactivity. FG retrogradely labelled neuronal 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 the catecholamine biosynthetic enzymes TH and/or PNMT. Most of these catecholaminergic neurons were part of the A1 noradrenergic cell group in the caudal VLM and to a lesser extent part of the C1 adrenergic cell group in the rostral VLM. In the second series of experiments, PHA-L was iontophoresed into VLM at different rostrocaudal levels where in the 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 immunoreactivity in cell bodies, axons, and presumptive axon terminals. PHA-L injection sites within either the caudal or rostral VLM resulted in labelled axons and terminal bouton-like swellings primarily in the 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 boutons were observed within the intercalated, medial, basomedial, and basolateral nuclei of the AMG. Most of the PHA-L labelled fibers and presumptive terminal boutons in the AMG after a caudal VLM (A1 region) injection also displayed TH immunoreactivity, whereas after a PHA-L injection into the rostral VLM (C1 region) all of the labelled axons and axon 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 behavioural functions.