Glutamate stimulation of arcuate nucleus inhibits responses of subfornical organ neurons to plasma hypernatremia and angiotensin II.

Experiments were done in urethane anesthetized rats to investigate the effect of glutamate (Glu) stimulation of arcuate nucleus of the hypothalamus (Arc) on the discharge rate of subfornical organ (SFO) neurons during changes in plasma sodium concentration and angiotensin II (ANG II) levels. Extracellular recordings were made from 67 histologically verified single neurons within SFO. Of these, 17 (25.4%) were excited by intracarotid infusion of hypertonic NaCl and 21 (31.3%) by intracarotid ANG II infusion. Five (29.4%) of the units excited by hypertonic NaCl were also excited by Glu stimulation of Arc. Similarly, seven (33.3%) of the units excited by ANG II were also excited by Arc stimulation. Additionally, four (19.0%) of the units excited by ANG II were inhibited by Glu stimulation of Arc. The remaining SFO units did not alter their discharge rate to activation of Arc neurons. The response of units to hypertonic NaCl or to ANG II was attenuated during simultaneous activation of Arc. These data suggest that Arc may be involved in modulating the activity of SFO neurons that function in the detection of blood-borne signals from the depletion of intra- and extracellular fluid volumes.