The action of the A1 noradrenergic region on the activity of subfornical organ neurons in the rat

Electrical stimulation of the A1 noradrenergic region of the ventrolateral medulla produced synatic excitation ( n=19, 9%), inhibition ( n=5, 2%) or no effect ( n=184, 89%) in the activity of a total of 208 neurons in the subfornical organ (SFO) in male rats under urethane anesthesia. Almost all ( n=18) of the excitatory responses ( n=19) were blocked by microiontophoretically-applied phentolamine, an α-adrenergic antagonist, but not by timolol, a β-adrenergic antagonist. The inhibitory response of all the neurons ( n=5) tested was prevented by iontophoretically applied timolol, but not by phentolamine. Approximately a half ( n=9) of SFO neurons that demonstrated the excitatory response to A1 region stimulation exhibited an increase in neuronal activity in response to hemorrhage (10 ml/kg b.w.t.), while remaining neurons ( n=10) were unresponsive. Hemorrhage did not cause any change in the activity of all the neurons ( n=5) that demonstrated the inhibitory response to A1 region stimulation. These results suggest that the medullary inputs to approximately 10% of SFO neurons tested are mediated by α-adrenergic excitatory and β-adrenergic inhibitory pathways, and imply that a part of the excitatory pathways may transmit the peripheral baroreceptor information.