Hormonal and neurotransmitter roles for angiotensin in the regulation of central autonomic function

In this review we present the case for both hormonal and neurotransmitter actions of angiotensin II (ANG) in the control of neuronal excitability in a simple neural pathway involved in central autonomic regulation. We will present both single-cell and whole-animal data highlighting hormonal roles for ANG in controlling the excitability of subfornical organ (SFO) neurons. More controversially we will also present the case for a neurotransmitter role for ANG in SFO neurons in controlling the excitability of identified neurons in the paraventricular nucleus (PVN) of the hypothalamus. In this review we highlight the similarities between the actions of ANG on these two populations of neurons in an attempt to emphasize that whether we call such actions "hormonal" or "neurotransmitter" is largely semantic. In fact such definitions only refer to the method of delivery of the chemical messenger, in this case ANG, to its cellular site of action, in this case the AT1 receptor. We also described in this review some novel concepts that may underlie synthesis, metabolic processing, and co-transmitter actions of ANG in this pathway. We hope that such suggestions may lead ultimately to the development of broader guiding principles to enhance our understanding of the multiplicity of physiological uses for single chemical messengers.