Excitatory effect of dopamine on oxytocin and vasopressin reflex releases in the rat.

The involvement of dopamine in the release of oxytocin and vasopressin was investigated in lactating rats during suckling or after changes in plasma osmolality. The effects of intraventricular injections of dopamine, agonists and antagonists, were tested on electrical unit activity of oxytocinergic or vasopressinergic cells in the paraventricular nucleus, on intramammary pressure (index of oxytocin release) and diuresis (index of vasopressin release). In urethane-anaesthetized lactating suckled rats, dopamine (1 microgram), apomorphine (2.5 and 5 micrograms) facilitated the established milk-ejection reflex, increasing the frequency and the amplitude of neurosecretory bursts of oxytocinergic cells. They also triggered the reflex in lactating rats without milk-ejections during suckling. The small doses injected were in no way such as to induce an acceleration in firing rate of oxytocinergic cells or an increase in mammary pressure. In alcohol-loaded rats, during water diuresis, dopamine (2 micrograms) and apomorphine (5 micrograms) activated the depressed vasopressinergic cells and inhibited diuresis. These facilitatory effects were progressive, reaching a maximum 10-15 min after injection. Haloperidol (5 micrograms) and alpha-flupentixol (10 micrograms) had an inhibitory effect on both types of neurosecretory cells in urethane-anaesthetized rats. They prevented the reflex activation of oxytocinergic cells induced by suckling and of vasopressinergic cells after a hyperosmotic stimulus (1 ml i.p 9% NaCl solution). These inhibitory effects were not of the "all-or-none' type. So, we can postulate that dopamine regulates the reflex release of oxytocin and vasopressin in the hypothalamus. On the one hand, dopamine permits and controls the periodic activation of oxytocinergic cells as long as the mothers are being suckled. On the other hand, it modulates the activity of vasopressinergic cells whenever the plasma osmolality changes.