Abstract
1. Activity of the paraventricular nucleus neurones was recorded by micro-electrodes during resting conditions and while various osmotic, chemical, direct and indirect neural stimuli were applied. This activity was correlated with evidence of oxytocin release by recording milk ejection responses.2. Seventy-four per cent of all paraventricular nucleus neurones were osmosensitive in that firing was augmented following close arterial injection of hypertonic solutions (1 ml. 1 M-NaCl in 10-15 sec). A very few neurones showed decreased activity and in twenty-two per cent no change at all occurred following injection. Evidence indicated that the observed depressions of cellular firing rate were due to other than osmotic stimulation.3. In post-partum cats an osmotically-induced neurone discharge increase was accompanied by a milk ejection response equivalent to that produced by 2-3 m-u. of oxytocin.4. Paraventricular neurones were also sensitive to acetylcholine. Intracarotid injections of 40-80 mug of acetylcholine greatly increased discharge rates and caused a very definite milk ejection response.5. Stimulation of the nipples by gentle suction, but not by electrical shock, and distension of the uterus in post-partum cats increased unit discharge in the paraventricular nucleus and evoked a milk ejection response.6. Neurones of the paraventricular nucleus, unlike those of the supraoptic nucleus, did not appear to be specifically responsive to electrical stimulation of skin and muscle afferent or of central nervous structures. Such stimuli did cause slight augmentations or depressions in firing rates of cells within and adjacent to the paraventricular nuclei, but many neurones were unaffected. Stimuli such as those applied appeared to have a general central effect involving paraventricular as well as many other neurones.7. Direct electrical excitation of the pituitary stalk produced a milk ejection response in post-partum cats. Electrical pulses applied to the paraventricular nuclei were less effective for reasons discussed.