LONG-TERM CONTINUOUS-INFUSION OF GNRH DOES NOT INHIBIT RESPONSES TO CONCURRENT PULSATILE GNRH INJECTIONS IN THE EWE

Pituitary responses to pulsatile injections of GnRH administered durin g continuous infusion of GnRH were studied in progesterone-dominated s easonally anoestrous ewes. A total of 20 ewes were randomly divided in to 4 equal-sized groups (5 ewes per group) and continuously infused wi th 250 ng/h of GnRH for up to 21 days. All ewes received 5 i.v. inject ions of 500 ng GnRH each at 2-h intervals 4 days before the start and again on days 2, 9 and 21 (Group 1), day 9 only (Group 2) and day 21 o nly (Group 3) of continuous infusion of GnRH. Ewes in Group 4 were giv en only continuous infusion of GnRH for a period of 9 days and served as controls. In response to continuous infusion of GnRH, control ewes (Group 4) showed a significant increase in LH concentrations until day 2, after which, the mean LH concentrations were not different from th e pre-treatment values. During different stages of continuous infusion of GnRH (day 2, 9 and 21) pulsatile injections of GnRH resulted in a pulsatile pattern of LH secretion. However, the mean LH pulse amplitud e during GnRH infusion was significantly reduced compared to that obse rved before the start of infusion. These results show that the pituita ry gland retains its ability to respond to pulsatile GnRH administrati on after it ceases to respond to concurrent continuous infusion.