Maintenance of hypothalamic GnRH release during lactation in the rat: a push-pull perfusion study

The activity of the gonadotropin-releasing hormone (GnRH) pulse generator during lactation was assessed by direct determination of GnRH levels impinging upon the pituitary gland. Sprague-Dawley rats were implanted on day 15 of pregnancy with a push-pull perfusion cannula directed to the anterior pituitary. All implanted animals showed normal parturition, maternal behavior and lactation. Push-pull perfusions were performed in 15 rats suckling 11.0 +/- 0.8 pups (range 4-15) on day 7-20 of lactation and repeated on diestrous 1 after weaning in some of the same animals. GnRH content of the samples was assayed by RIA. GnRH pulses were clearly detected during lactation. Mean GnRH secretion rate was 1.9 +/- 0.3 pg/10 min (chi +/- SE, range between 0.5 and 3 pg/10 min) and interpulse interval was 37.5 +/- 1.7 min (range between 27 and 50 min). There was a significant decrease of about 19% in the interpulse interval after weaning. There was no significant difference in GnRH pulse amplitude nor in GnRH secretion rate between lactation and diestrous. These results demonstrate that nursing does not suppress the GnRH pulse generator in the rat.