The activity of identified supraoptic neurones and their response to acetylcholine applied by iontophoresis.

1. In urethane/pentobarbitone anaesthetized male rats, the hypothalamus and pituitary stalk were exposed by a transpharyngeal approach. Cells recorded with glass micro-electrodes from the ventral hypothalamus near the bifurcation of the internal carotid artery were identified as supraoptic neurones only when their antidromic action potential evoked at constant latency by stimulation of the pituitary stalk was cancelled by collision with a spontaneously occurring action potential.2. More than 80% of the identified cells were spontaneously active. The frequency of firing was often slow (under 1 Hz) and rather irregular. More than half the cells, however, had a more distinctive firing pattern which recurred every 1-3 min and consisted of a period of low or absent activity alternating with periods of quite marked discharge which occasionally exceeded 20 Hz. This periodicity was retained when the over-all activity was enhanced by an intracarotid injection of 0.2 ml. 5% (w/v) NaCl.3. Of fifty-two cells tested, thirty-six were excited by acetylcholine applied by iontophoresis from a multibarrelled micropipette with average ejection currents of 35 +/- 5 nA (S.D. of thirty-six observations). Ten of these cells were also excited by nicotine applied with similar currents.