Effects of intracarotid injections of hypertonic solutions on 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 of the effects of elevating the osmotic pressure of the blood flow to the brain by injecting hypertonic solutions into the carotid artery. Stimulation of the release of neurohypophyseal hormone was shown by the occurrence of milk ejection (intramammary pressure rise) corresponding to the effects of intravenous injection of Pitocin (2–20 mu) or Pitressin (10–50 mu). The most favorable conditions for detecting and estimating endogenous hormone release were obtained by transecting the thoracic spinal cord. If the spinal cord was intact the hypertonic injections evoked a sympathetic discharge evidenced by a rise in arterial pressure (eliminated by spinal section), a uterine contraction resembling the response to intravenous injection of 5 µg epinephrine, and an inhibition of the milk ejection response to injected Pitocin. It is concluded that the hypertonic solutions activated nervous mechanisms leading to a release of neurohypophyseal hormone and to a discharge from sympathetic centers in the brain.