COLLATERAL AXONAL PROJECTIONS FROM VENTROLATERAL MEDULLARY NONCATECHOLAMINERGIC NEURONS TO CENTRAL NUCLEUS OF THE AMYGDALA

Retrograde tract-tracing techniques were used to investigate whether c atecholaminergic neurons in the ventrolateral medulla (VLM) send colla teral axonal projections to both central nuclei of the amygdala (ACe) in the rat. Rhodamine-labelled latex microspheres or fluorogold (2%) w ere microinjected into the region of either the right or left ACe. Aft er a survival period of 10-12 days, the rats were sacrificed and trans verse sections of the brainstem were processed immunohistochemically f or the identification of cell bodies containing the catecholamine bios ynthetic enzymes tyrosine hydroxylase (TH) or phenylethanolamine-N-met hyltransferase (PNMT). Neuronal perikarya containing the retrogradely transported tracers were observed throughout the rostrocaudal extent o f VLM, bilaterally. Approximately 10% of the retrogradely labelled neu rons were observed to contain both retrograde tracers. The majority (7 9 +/- 6.8%) of these double labelled neurons were located within the c audal VLM and their number decreased rostrally. In addition, the propo rtion of double labelled neurons to single labelled neurons in VLM dec reased rostrally; approximately 11% in the caudal VLM and 6% in the ro stral VLM. Furthermore, approximately 21% of all VLM neurons that proj ected to ACe were found to be catecholaminergic: 75% of these were imm unoreactive to TH and 25% to PNMT. However, no neurons were found in V LM that contained both retrograde tracers and immunoreactivity to TH o r PNMT. These data demonstrate that axons originating from non-catecho laminergic neurons in VLM bifurcate to innervate ACe bilaterally. Alth ough the function of these VLM neurons that project to both ACe is not known, they may be the anatomical substrate by which VLM neurons rela y simultaneously autonomic and/or visceral sensory information to infl uence the activity of ACe.