Involvement of arachidonate metabolism in neurotensin-induced prolactin release in vitro

Neurotensin increased in a concentration-dependent manner the level of hypophyseal [³H]arachidonic acid in vitro as well as prolactin release from hemipituitary glands. The effect of 1 μM neurotensin on arachidonate release was already present at 2.5 min, maximal at 5, and disappeared after a 10-min incubation. Neurotensin analogues produced an enhancement of hypophyseal arachidonate similar to their relative potencies in other cellular systems, whereas other peptides (somatostatin and vasoactive intestinal peptide) were devoid of any effect on the concentration of the fatty acid in the pituitary. Seventy micromoles RHC 80267, a rather selective inhibitor of diacylglycerol lipase, completely prevented the neurotensin-stimulated prolactin release and decreased arachidonate release both in basal or in neurotensin-induced conditions. Similar results were obtained with 50 μM quinacrine, a phospholipase A₂ inhibitor. To clarify whether arachidonate released by neurotensin requires a further metabolism through specific pathways to stimulate prolactin release, we used indomethacin and BW 755c, two blockers of cyclooxygenase and lipoxygenase pathways. Thirty micromoles indomethacin, a dose active to inhibit cyclooxygenase, did not affect unesterified arachidonate levels either in basal or in neurotensin-induced conditions; moreover, the drug did not modify basal prolactin release but slightly potentiated the stimulatory effect of neurotensin on the release of the hormone. On the other hand, 250 μM BW 755c, an inhibitor of both cyclooxygenase and lipoxygenase pathways, significantly inhibited both basal and neurotensin-stimulated prolactin release and further potentiated the increase of the fatty acid concentrations produced by 1 μM neurotensin. We suggest that the release of the fatty acid from membrane phospholipids is an essential step in the stimulatory effect of neurotensin on prolactin release. Moreover our results indicate that arachidonate cleavage from membrane phospholipids may occur through the activity of a specific diacylglycerol lipase and/or phospholipase A₂. Finally, the action of the fatty acid on the release of the hormone appears to require further metabolism to lipoxygenase products.