The central nucleus of amygdala (CeA), a limbic forebrain structure, has an important role in the integration of emotional and cardiovascular responses (Davis, 1992). Recently, we have provided anatomical evidence for an inhibitory neural pathway by which cardiovascular responses to stress may be, in part, mediated by attenuation of baroreceptor reflexes at the level of the NTS (Saha et al. 2000, 2002). The present study was designed to investigate the organisation of projections from the CeA to neurones in the dorsal vagal motor nucleus (DVN), the nucleus ambiguus (NA) and the rostral ventrolateral medulla (RVLM). The first two nuclei contain cardioinhibitory vagal preganglionic neurones and the RVLM contains tonically active neurons that project to preganglionic sympathetic neurons in the spinal cord.
All recovery experiments were performed on 280-300 g rats (n = 12) anaesthetised with halothane (95 % in O2) with buprenorphine analgesia as required. The CeA terminals in the medulla were labelled by microinjecting an anterograde tracer, biotin dextran amine (BDA, 0.2 µl of 10 % in saline) stereotaxically into the CeA. Neurones in the DVN and NA were identified by applying retrograde tracer, cholera toxin B subunit (CTb, 0.5-1 µl of 1 % in saline) into the nodose ganglion, and neurones in the RVLM were identified by phenylethanolamine N-methyl-transferase (PNMT) immunocytochemistry or by expression of the immediate early gene c-fos following infusion of sodium nitroprusside (1 mg ml-1 in saline for 1 h under pentobarbitone anaesthesia), at a rate sufficient to maintain mean arterial pressure at least 25 % below the resting level. At the end of the experimental protocol, the rats were humanely killed by perfusion with aldehyde fixative, and vibratome sections of the brain stem were processed for light and electron microscopy.
BDA labelled CeA terminals were found to make synaptic contact with neurones retrogradely labelled with CTb in the DVN and NA, and also with PNMT-positive neurones in the RVLM. NP-induced hypotension produced c-fos expression in the RVLM. Many anterogradely labelled varicose fibres were observed in close apposition with neurones with Fos-immunoreactive nuclei in the RVLM. The results suggest that neural projections from the CeA may directly influence the activity of medullary neurones, including sympathetic premotor neurones and parasympathetic preganglionic neurones that are involved in regulation of blood pressure and other autonomic functions in response to stressful conditions.
This work was supported by the Wellcome Trust.