INFLUENCE OF TESTOSTERONE ON LHRH RELEASE, LHRH MESSENGER-RNA AND PROOPIOMELANOCORTIN MESSENGER-RNA IN MALE SHEEP

The mechanism whereby testosterone (T) reduces pulsatile LHRH and LH r elease is unknown. We tested the hypothesis that hypothalamic levels o f LHRH mRNA decrease and proopiomelanocortin (POMC) mRNA increase coin cident with reduced LHRH release induced by either long-term or short- term T treatment in male sheep. Experiment 1 examined the effect of lo ng-term T exposure on LHRH and LH release and LHRH and POMC mRNA level s. Yearling Suffolk rams were castrated and assigned to one of four tr eatments: 1) castrated (n = 4); 2) castrated, portal cannula (n = 5); 3) castrated +T (n = 4) and 4) castrated +T, portal cannula (n = 4), T -treated males received ten 10-cm silastic T-implants immediately afte r castration. Surgical placement of devices for collecting hypophyseal -portal blood occurred 2 to 3 months after castration. Seven to 10 day s after surgery, blood samples were collected at 10-min intervals for 8 h from portal cannulated males or for 5 h from non-cannulated males to assess pulsatile LHRH and/or LH release. Immediately after blood sa mple collection, hypothalamic tissue was collected for in situ measure ment of LHRH or POMC mRNA. T-treatment decreased (P<0.01) mean LHRH an d LH and decreased (P<0.01) LHRH and LH pulse frequency. T did not sig nificantly affect (P>0.10) silver grain area per LHRH neuron, but decr eased (P<0.01) silver grain area per POMC neuron, Portal cannulation t ended to decrease (P=0.057) silver grain area per LHRH neuron without significantly affecting (P>0.10) LHRH cell numbers while reducing (P<0 .01) silver grain area per POMC neuron and POMC cell numbers. A second experiment examined the effect of 72 h of T-infusion on LHRH and POMC mRNA levels. Castrated yearling males were assigned to receive either vehicle (n = 4) or T (768 ug/kg/day;n = 4). Blood samples were collec ted at 10 min intervals for 4 h prior to and during the final 4 h of i nfusion. Infusion of T decreased (P<0.01) mean LH and LH pulse frequen cy. T did not significantly affect (P>0.10) silver grain area per LHRH neuron or LHRH cell numbers. T reduced (P<0.01) silver grain area per POMC neuron without affecting (P>0.10) POMC cell number. We reject ou r hypothesis and conclude that reduced LHRH or heightened POMC gene ex pression are not mechanisms whereby T reduces pulsatile LHRH release i n male sheep.