Collateral axonal projections from ventrolateral medullary non-catecholaminergic neurons to central nucleus of the amygdala

Abstract

Retrograde tract-tracing techniques were used to investigate whether catecholaminergic neurons in the ventrolateral medulla (VLM) send collateral axonal projections to both central nuclei of the amygdala (ACe) in the rat. Rhodamine-labelled latex microspheres or fluorogold (2%) were microinjected into the region of either the right or left ACe. After a survival period of 10–12 days, the rats were sacrificed and transverse sections of the brainstem were processed immunohistochemically for the identification of cell bodies containing the catecholamine biosynthetic enzymes tyrosine hydroxylase (TH) or phenylethanolamine- N-methyltransferase (PNMT). Neuronal perikarya containing the retrogradely transported tracers were observed throughout the rostrocaudal extent of VLM, bilaterally. Approximately 10% of the retrogradely labelled neurons were observed to contain both retrograde tracers. The majority (79 ± 6.8%) of these double labelled neurons were located within the caudal VLM and their number decreased rostrally. In addition, the proportion of double labelled neurons to single labelled neurons in VLM decreased rostrally; approximately 11% in the caudal VLM and 6% in the rostral VLM. Furthermore, approximately 21% of all VLM neurons that projected to ACe were found to be catecholaminergic: 75% of these were immunoreactive to TH and 25% to PNMT. However, no neurons were found in VLM that contained both retrograde tracers and immunoreactivity to TH or PNMT. These data demonstrate that axons originating from non-catecholaminergic neurons in VLM bifurcate to innervate ACe bilaterally. Although the function of these VLM neurons that project to both ACe is not known, they may be the anatomical substrate by which VLM neurons relay simultaneously autonomic and/or visceral sensory information to influence the activity of ACe.