Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC13
Neurons from the cardiovascular region of the nucleus tractus solitarius target GABAergic and presympathetic neurons of the paraventricular nucleus of the hypothalamus
V. S. Affleck1, S. Pyner1
1. School Biological & Biomedical Sciences, University of Durham, Durham, United Kingdom.
The paraventricular nucleus of the hypothalamus (PVN) and nucleus tractus solitarius (NTS) are critical components of the neural circuit that controls the cardiovascular system1. Presympathetic neurons of the PVN regulate sympathetic activity, thus contributing to cardiovascular control. These neurons receive information about cardiovascular status from neurons in the NTS2. Furthermore, transynaptic labelling has shown the presence of GABAergic interneurons that modulate presympathetic PVN neuronal activity3. The question therefore is; do the GABAergic interneurons also receive an NTS input providing a further pathway to control sympathetic activity? This is important because elevated sympathetic nerve activity, strongly associated with cardiovascular disease, is partly generated from the presympathetic PVN neurons1. This study used anterograde (Biotin dextran amine-BDA) and retrograde (Fluorogold-FG or cholera toxin subunit B-CTB) tracing with immunohistochemistry for GABAergic neurons to identify the terminal neuronal targets of the ascending projection from the NTS. Experiments were performed on male Wistar rats (n= 6) in accordance with the Animals (Scientific Procedures) Act, 1986. For labelling of NTS projecting axons, animals were anaesthetised i.p. with medetomidine (0.25 ml/100g) and ketamine (0.06 ml/100g). Animals received an injection of 10% BDA into the NTS (R/C 13.68 mm, L 0.5 mm, D/V 8.00 mm)4,5. After a 14 day recovery period, animals were re-anaesthetised (as previously described) to inject 2 % FG or 0.5 % CTB into the spinal cord2. Post-operatively, the animals were administered buprenophine (0.1 ml/kg). Following a further recovery period (7-14 days) animals were humanely killed (pentobarbital 60 mg/kg i.p.) perfused fixed (4 % paraformaldehyde-PFA or 4 % PFA and 0.5 % glutaraldehyde) with removal of brain and spinal cord. Frozen sections were processed to reveal BDA, CTB and GABAergic interneurons in the PVN2,3. Tissues were examined under bright field or epifluorescence for the existence of putative connections between the NTS and GABAergic interneurons and presympathetic neurons of the PVN. Ascending axons from the NTS coursed through and around the PVN nucleus. The NTS terminal axons showed numerous varicosities, some of which appeared to closely appose the somata and dendrites of presympathetic neurons and the GABAergic neurons adjacent to the PVN. We have confirmed and extended our earlier finding such that axonal fibres from the NTS target presympathetic neurons of the PVN and GABAergic interneurons surrounding the PVN. Functionally, this neural circuit underpins the regulation of sympathetic outflow and therefore has the potential to contribute to the generation of abnormal sympathetic activity by the PVN.
Where applicable, experiments conform with Society ethical requirements