The role of neuropeptide Y (NPY) in the control of LH secretion in the ewe with respect to season, NPY receptor subtype and the site of action in the hypothalamus
Neuropeptide Y1–36 (NPY1–36) acts through Y1 and Y2 receptors while the C-terminal NPY fragments NPY18–36 and N-acetyl[Leu28,31]pNPY24–36 act only through the Y2 receptor. We have investigated the effects of intracerebroventricular (i.c.v.) administration of NPY1–36, NPY18–36 and N-acetyl[leu28,31]pNPY24–36 on LH secretion in the ovariectomised (OVX) ewe. These peptides were administered into a lateral ventricle (LV) or the third ventricle (3V) of OVX ewes during the non-breeding and breeding seasons. Microinjections of NPY were also made into the preoptic area (POA) during both seasons to investigate the effects of NPY at the level of the GnRH cell bodies. Tamed sheep were fitted with 19 gauge guide tubes into the LV, 3V or the septo-preoptic area (POA). Jugular venous blood samples were taken every 10 min for 3 h. Sheep were then given NPY1–36 (10 μg), NPY18–36 (100 μg) or saline vehicle into the LV; N-acetyl[Leu28,31]pNPY24–36 (100 μg), NPY1–36 (10 μg or 100 μg), NPY18–36 (10 μg or 100 μg) or saline vehicle into the 3V, or NPY1–36 (1 μg, 5 μg, 10 μg) into the POA. Blood sampling continued for a further 3 h. LH was measured in plasma by radioimmunoassay. LV or 3V injection of 10 μg NPY1–36 caused a small but significant (P18–36 did not produce any significant change in LH pulsatility when injected into the LV, and neither peptide had any effect on plasma prolactin or GH levels. There was a significant (P18–36. Pulse amplitude was reduced by 3V administration of the Y2 agonist, N-acetyl[Leu28–31]pNPY24–36 and 100 μ NPY18–36. When the amplitude of the first post-injection LH pulse was analysed, 10 μg NPY also had a significant (P1–36 (but not 10 μg) decreased (P18–36. Doses of 10 μg NPY1–36 and 100 μg NPY18–36 had greater inhibitory effects on pulse frequency during the breeding season but the suppressive effect of 100 μg NPY was similar between seasons. Microinjections of NPY into the POA decreased (P2 receptor. This inhibition is probably exerted by way of a presynaptic action on GnRH terminals in the median eminence as NPY does not modulate the frequency or amplitude of LH pulses at the level of the GnRH cell bodies in the POA. Journal of Endocrinology (1995) 147, 565–579