Periventricular preoptic area neurons coactivated with luteinizing hormone(LH)-releasing hormone (LHRH) neurons at the time of the LH surge are LHRHafferents

Earlier studies demonstrated coactivation of the periventricular preoptic a rea (pePOA) with LHRH neurons at the time of an induced or spontaneous LH s urge, suggesting that the pePOA might regulate LHRH neurons. To investigate this hypothesis, studies were conducted to determine the temporal pattern of pePOA Fos activation during the rat estrous cycle and establish the conn ections of the pePOA neurons with LHRH neurons. Fos activation within LHRH and pePOA neurons showed the same temporal pattern. Both were absent during diestrous I, diestrous II, and the morning of proestrus. Fos was induced i n the pePOA and LHRH neurons beginning on the afternoon of proestrus (4 h b efore lights off), with a decline 8 h later on proestrous evening. Tract-tr acing studies then established the relationship between LHRH and pePOA neur ons. Retrograde labeling with fluorogold determined that a portion of the F os-positive pePOA neurons present at the time of the LW surge sent a projec tion to regions that contain LHRH cells, Anterograde tracer (neurobiotin) i njections established that the pePOA neurons sent axone to the LHRH cells. Taken together, these data indicate that the pePOA provides direct input to LHRH neurons that is likely to stimulate LHRH neurons at the time of the L H surge.