Facilitatory effect of hypothalamo-neurohypophysial tract stimulation on milk ejection frequency in the lactating rat

The relative contribution of magnocellular and parvocellular neurones to the patterning of milk ejections was studied by activation of various parts of the hypothalamo-neurohypophysial tract. During suckling of anaesthetised lactating rats, electrical stimulation of the neurohypophysis/neural stalk (15 Hz, 1.5 s on/1.5 s off, 10 min) evoked an increased frequency of milk ejections as detected by intramammary pressure recording. Bilateral stimulation of the paraventricular nuclei produced a similar facilitation in the post-stimulus period, but stimulation of the supraoptic nuclei (with either low current or current sufficient to evoke peripheral oxytocin release) had no effect on the occurrence of milk ejections. These data are consistent with the hypothesis that parvocellular but not magnocellular neurones participate in the regulation of milk ejection frequency.