n~po~ twenty-s i hth rnnurl conference - the Society for Reproductive ...

NPY EFFECTS ON LH SECRETION IN THE OVARIECTOMISED EWE:Y2 RECEPTOR. INVOLVEMENT, SEASONAL VARIATION AND SITE OF ACTION WITHIN THEHYPOTHALAMUS.Mandi L. Barker-Gibb 1, Jaroslav H. Boublik 2 and lain J. Clarke 11Prince Henry's Institute of Medical Research, P.O. Box 5152, Clayton, Victoria 3168; 2Baker Medical ResearchInstitute, P.O. Box 348 Prahran, Victoria 3181.INTRODUCTIONNeuropeptide Y (NPY) action is mediated throughY1(post-synaptic) and Y2 (pre-synaptic) receptor subtypes.NPY plays a key role in modulating LHsecretion in many species, although the extent towhich this involves action through the differentreceptor sub-types is not clear. In rats, LH secretionappears to be controlled through activation of the Y1receptor, and NPY acts at both the hypothalamic andpituitary level (1). In contrast, NPY does not act onthe pituitary gland in sheep (2). We haveinvestigated possible hypothalamic mechanisms bywhich NPY modulates GnRH/LH release in ewes.Intracerebroventricular (i.c.v.) injections of NPY1-36which acts at the Y1 and Y2 receptors and a specificY2 receptor agonist NPY18-36 were given to identifyreceptor specific actions on LH pulse secretion.Microinjections (1 Ill) of NPY were also given into thepreoptic area (POA) to examine effects at the level ofthe GnRH cell bodies. The ewe is also a highlyseasonal species and the activity of the GnRH 'pulsegenerator' is directly influenced by changes inphotoperiod which occur with season. The effect ofphotoperiod on the GnRH system is mediated via anunidentified neuronal circuit. Thus, we alsoinvestigated seasonal variations in the effects of NPYon LH secretion by giving NPY1-36 into the thirdventricle (3V) during both the breeding (BS) and nonbreedingseason (NBS).METHODSOvariectomised (OVX) ewes were fitted with guidetubes into a lateral ventricle (LV), 3V or POA.Jugular venous blood samples were taken every 10min for 3h. During the SS, sheep were given 10lJgNPY1-36, 100lJg NPY18-36 or saline into theLV;101l9,100lJg NPY1-36, and 100119 NPY 18-36 orsaline into the 3V, and 1.5lJg NPY into the POA.Injections of NPY were also given into the 3V (100lJg)and POA (1,5, 10lJg) during the NBS to examineseasonal effects Sampling continued for 3h postinjection,and samples were assayed for LH. Pre- andpost- injection differences were analysed byStudent's paired t-test.RESULTSInjections of 10 (LV) or 100lJg NPY1-36 (3V) had littleeffect on LH secretion in OVX ewes during the B.S.except to extend (P5 400::r:C!J 200;--0- PACAP 5 min-+- PACAP 30 min-0- VIP5min-tI- VIP 30 min--0- Control____ PACAP 16 7 M__ VIP16 7 MPACAP 16 6 MVIP 16 6 Mo+---r---,--_r_----.---...,.--r-----.a 234 5 6 7Incubation time (h)80 -0--+-~ --­~ 60§8 40~.s::i 20o0-t---,---r---,----.----,------.----,o ~ ~ 00 ~ 100 1~ 1~Incubation time (min)Fig.1 Time-course relationship for PACAP and VIP on intracellular cAMPDose (M)Fig. 2Dose-response curve for PACAP and VIPFig. 3Time-course relationship for the effects of PACAP and VIP on GHsecretion from sheep pituitary cellso+--,---,----,r-~-r-_r_---r-..--~__r_~---.a 2 4 6 8 10 12 14 16 18 20 22 24Incubation time (h)Fig. 4Time-course relationship for the effects of PACAP and VIPon LHsecretion from sheep pituitary cells1 2