Localization of neuronal nitric oxide synthase-immunoreactivity within sub-populations of noradrenergic A(1) and A(2) neurons in the rats

The noradrenergic cells of the ventrolateral medulla (VLM) and the nucleus tractus solitarii (NTS) are implicated in the control of a variety of cardiovascular, respiratory and neuroendocrine functions. The present study has used antibodies raised against rat neuronal nitric oxide synthase (nNOS) and tyrosine hydroxylase (TH) to determine whether nNOS is expressed by A(1) and A(2) noradrenergic neurons. Double-labelling immunofluorescence experiments revealed that approximately 10% of TH-immunoreactive cells in the rostral NTS and 6% in the caudal NTS, were immunoreactive for nNOS. In the rostral VLM, only 1% of cells were double-labelled while approximately 9% of the TH-immunoreactive cells in the caudal VLM were immunoreactive for nNOS. These findings indicate that sub-populations of the A(1) and A(2) noradrenergic neurons are capable of generating nitric oxide and suggest a direct role for this neuronal messenger in the regulation of noradrenergic activity within the brain.