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

Bovine median eminence contains a factor different from gonadotropin- releasing hormone (GnRH) that increases basal luteinizing hormone (LH) secretion and potentiates GnRH-stimulated LH release. We compared the effects 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 not attributable to GnRH present in bovine median eminence extracts. Both, galanin and neurotensin (10 ^-9-10 ^{- 5} M) 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. Leucine-enkephalin, β-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 β-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 β-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.