Central nucleus of amygdala projections to rostral ventrolateral medulla neurones activated by decreased blood pressure.

The central nucleus of amygdala (CeA) participates in cardiovascular regulation during emotional behaviour but it has not been established whether any of these effects are mediated through its direct connections to blood pressure-regulating neurones in the rostral ventrolateral medulla (RVLM). The RVLM contains barosensitive neurones that maintain resting blood pressure via their projections to sympathetic preganglionic neurones in the thoracic spinal cord. In this study on rats, we used combined anterograde neuronal tracing of CeA projections with confocal and electron microscopic immunohistochemical detection of phenylethanolamine-N-methyltransferase, the adrenaline-synthesizing enzyme present in C1 catecholamine neurones of the RVLM, and Fos, the protein product of the c-fos proto-oncogene. Fos expression in barosensitive neurones was stimulated by an intravenous infusion of the hypotensive agent sodium nitroprusside. Injection of the tracer biotin dextran amine (10-kDa form) into the CeA resulted in anterograde labelling of axons and varicosities throughout the RVLM without retrograde labelling of somata in any brain area. With confocal microscopy, presumptive CeA terminals were found in close apposition to adrenergic (phenylethanolamine-N-methyltransferase-immunoreactive) and non-adrenergic neurones that displayed Fos-immunoreactive nuclei in response to decreased blood pressure. Electron microscopic analysis confirmed that some labelled terminals of CeA axons made synaptic contact with c-fos-activated adrenergic neurones. The results provide evidence that cardiovascular influences elicited from the CeA during stressful events may be mediated, at least in part, via monosynaptic neural projections to barosensitive sympathetic blood pressure-regulating neurones in the RVLM.