EVIDENCE THAT HYPOTHALAMIC NEUROPEPTIDE-Y GENE-EXPRESSION INCREASES BEFORE THE ONSET OF THE PREOVULATORY LH SURGE

Neuropeptide Y (NPY), a 36 amino acid residue peptide, is involved in stimulation of LHRH and LH surges on proestrus and those induced by ov arian steroids in ovariectomized (ovx) rats. Recently, we observed tha t NPY gene expression in the medial basal hypothalamus (MBH) was incre ased before the onset of the LH surge in the ovarian steroid-primed ov x rats. Since the ovarian steroidal milieu during the estrous cycle is markedly different from that prevailing after ovarian steroid injecti ons in ovx rats, we evaluated in cycling rats the temporal relationshi p between MBH preproNPY mRNA levels and the preovulatory LH surge on t he day of proestrus and compared that with diestrus II, concomitant wi th basal LH levels. PreproNPY mRNA levels in the MBH were measured by solution hybridization/RNAse protection assay, using a cRNA probe, On the day of diestrus II, preproNPY mRNA levels changed little between 1 000 and 1800 h. Quite unexpectedly, preproNPY mRNA levels at 1000 h on proestrus were similar to diestrus II levels, despite additional expo sure to ovarian steroids during this interval. However, from these low levels at 1000 h, the preproNPY mRNA profile displayed a biphasic ris e. During the first phase, preproNPY mRNA rose significantly at 1200 h and remained elevated at 1300 and 1400 h concomitant with basal serum LH levels, Thereafter, a second rise in preproNPY mRNA began at 1500 h, peaked rapidly at 1600 h and declined significantly at 1800 h. This secondary activation of NPY gene expression occurred with a slow, two -fold increase in serum LH at 1500 h, followed by a rapid ascension to peak levels at 1800 h and was associated with an increase at 1400 h o f serum progesterone levels which reached their peak at 1800 h. These results demonstrate that a dynamic, biphasic augmentation in hypothala mic NPY gene expression occurs selectively on proestrus, and that the first incremental response is observed some time before the onset of p reovulatory LH hypersecretion. Because preproNPY mRNA levels at 1000 h on proestrus were similar to the low levels seen on the preceding die strous II phase, a neural timing mechanism, and not changes in ovarian hormone levels during this phase may be responsible for the increase in NPY gene expression after 1000 h of proestrus. Because of our previ ous observations that progesterone can rapidly augment preproNPY mRNA in the MBH and because a rise in serum progesterone occurs hours befor e the onset of the LH surge, we suggest that the secondary rise in pre proNPY mRNA is facilitated by this antecedent increase in serum proges terone. Cumulatively, these results are in accord with the thesis that activation of hypothalamic NPY gene expression is one of the key earl y neural events initiated by the neural clock that times the preovulat ory LHRH and LH surges.