EEG correlates of osmotic activation of the neurohypophyseal milk-ejection mechanism

¹ Department of Anatomy, School of Medicine, University of California at Los Angeles, Los Angeles, and Veterans Administration Hospital, Long Beach, California

A study was made in unanesthetized rabbits immobilized with curare of electroencephalographic (EEG) changes accompanying activation of the neurohypophysis by intracarotid injections of hypertonic saline. The release of neurohypophyseal hormone was monitored by recording the milk-ejection response (intramammary pressure). EEG recordings were obtained from concentric bipolar electrodes placed stereotaxically in various cortical and deep regions of the brain. Every milk-ejection response induced by injection of hypertonic saline was preceded by a brief burst of fast EEG activity in certain areas of the brain. This distinctive EEG activity was recorded most consistently from the deep portion of the olfactory tubercle, and it preceded the calculated time of release of neurohypophyseal hormone by about 10–15 sec. Injections of isotonic saline failed to elicit either the EEG change or the milk-ejection response. It is proposed that osmosensitive elements may lie in or closely related to the olfactory tubercle.