Facilitation of milk ejection-related activation of oxytocin-secreting neurones by osmotic stimulation in the rat

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Abstract
Extracellular recordings were made from antidromically identified neurosecretory cells in the paraventricular nucleus of the hypothalamus in urethane-anesthetized lactating rats. Sixty-six percent of the rats tested (n=80) showed reflex milk ejections during suckling. Oxytocin neurones could be distinguished from other neurosecretory neurones by a characteristic high-frequency burst of spikes displayed before a reflex milk ejection. Twenty-three of the oxytocin neurones recorded from twenty-three individual animals were tested for the effect of intraperitoneal injection of 1.5 M NaCl (1 ml), which raised the plasma osmotic pressure from 299.3±2.4 (SE) mosmole/kg to 313.0±2.4 mosmole/kg. The injection significantly increased not only the firing rate of the oxytocin neurones (from 1.7±0.3 spikes/s to 4.9±0.6 spikes/s) but also the number of spikes per burst (from 53.6±7.4 to 75.5±7.5) and burst duration (from 3.38±0.22 s to 3.80±0.18 s). The amplitude of reflex milk ejection was also increased to 1.9 times. However, the injection did not change the interval between bursts. On the other hand, intraperitoneal injection of 0.15 M NaCl affected neither these parameters of oxytocin neurones nor the amplitude of milk ejection. Some of the antidromically identified neurones recorded in the rats which showed no detectable reflex milk ejection during suckling displayed intermitent bursts of action potentials. Ten of these neurones in ten individual rats were tested for the effect of 1.5 M NaCl. The number of spikes per burst was significiantly increased and reflex milk ejection was induced by the injection in eight of these rats. Then the bursts became milk ejection-related. These results indicate that increased plasma osmotic pressure influences the milk ejection reflex by enhancing the burst activity of oxytocin neurones which precedes reflex milk ejection.