Subfornical organ--supraoptic nucleus connections: an electrophysiologic study in the rat.

Extracellular recordings from antidromically-identified neurosecretory cells in the rat supraoptic nucleus (SON) indicate that electrical stimulation (1 Hz, 50 microseconds, 200 microA) in the subfornical organ (SFO) alters the excitability of 89% (n = 31) of phasically-active (putative vasopressin-secreting) and 94% (n = 16) of continuously-active (putative oxytocin-secreting) neurons; 45% of cells display a long latency (mean 80.2 +/- 20.5 ms, S.D.) prolonged (150-350 ms) increase in excitability; 26% of cells demonstrate a similar excitation, preceded by a brief decrease in firing at a latency of 30.5 +/- 13.1 ms; 15% of cells display only a depression in their activity, lasting up to 150 ms. Ninety percent of non-neurosecretory (i.e. non-antidromic) neurons (n = 19) within or above th SON also display orthodromic excitatory or inhibitory responses to SFO stimulation; however, these cells usually respond with shorter latencies, and none demonstrate the prolonged excitation seen among neurosecretory cells. With SON stimulation, antidromic activation observed from 6 of 18 SFO neurons (latency range of 12-27 ms) confirms a projection from SFO to the SON area. These data suggest a predominantly facilitatory influence of SFO neurons on the excitability of both vasopressinergic and oxytocinergic neurosecretory cells in the rat, thereby supporting a role for the SFO in body water balance.