Comparison of the effect of hypothalamic neuropeptides upon luteinizing hormone secretion by cultured rat anterior pituitary cells.

Bovine median eminence contains a factor difference from gonadotropin-releasing hormone (GnRH) than increases basal luteinizing hormone (LH) secretion and potentiates GnRH-stimulated LH release. We compared the effect of hypothalamic neuropeptides on basal and GnRH-stimulated LH secretion using rat pituitary cells under static incubation conditions to determine if any of them mimics the LH-releasing activity no attributable to GnRH present in bovine median eminence extracts. Both, galanin and neurotensin (10(-9)-10(-5)) stimulated basal LH secretion in a dose-response manner. Galanin increased 3-4 fold and neurotensin doubled the basal LH secretion. The GnRH antagonist Nal-Glu 10(-6) M abolished the effect of 10(-7) M GnRH and 10(-5)M neurotensin, but did not block the LH-releasing activity of galanin. Leucin-enkephalin, beta-endorphin, substance P and neuropeptide Y (NPY) did not alter basal LH secretion. Neuropeptides produced three types of response on GnRH-stimulated LH release. First, leucine-enkephalin and beta-endorphin (10(-9)-10(-5) M) showed a dose-dependent inhibition of GnRH-stimulated LH release. At 10(-5) M the inhibition was complete with leucine-enkephalin and only 30% with beta-endorphin. Both were blocked by naloxone. Second, substance P showed an inverted U type response on GnRH-stimulated LH secretion. At 10(-9) M this peptide potentiated the action of GnRH. This effect decreased when the dose of substance P was increased to 10(-7) M and turned inhibitory at 10(-5) M when 10(-7) M GnRH was used. Third, galanin and NPY potentiated the effect of GnRH on LH secretion. Neurotensin had no effect on GnRH-stimulated LH release. In conclusion, rat gonadotrophs present diverse responses to neuropeptides at physiological concentrations, and -apart from GnRH-galanin is most likely the other factor present in bovine median eminence extracts that stimulates LH secretion. The data lend further support to a role of galanin in the control of LH secretion.