Does gonadotropin-releasing hormone in the cerebrospinal fluid modulate luteinizing hormone release?

The function of gonadotropin-releasing hormone (GnRH) in the cerebrospinal fluid (CSF) is unknown. This study on ovariectomized ewes investigated whether CSF-GnRH has a role in modulating luteinizing hormone (LH) secretion either through an ultrashort-loop feedback system to affect GnRH secretion or to directly act on the pituitary gland after entering the hypothalamohypophysial portal system. In the first experiment, a 3-hour continuous infusion of exogenous GnRH (700 or 7 pg/min; n = 8) was administered into the third ventricle through a permanent indwelling cannula. Jugular LH concentrations were measured as an estimate of the activity of the GnRH 'pulse generator'. To assess the potential for a direct involvement of CSF-GnRH in pituitary stimulation of LH secretion, ewes were also implanted with a cannula to collect hypophysial portal blood. In a first investigation, radioactive (2 x 10(6) cpm I-125-GnRH; n = 3) GnRH was injected into the third ventricle, and the amount of radioactivity present in the portal and jugular blood after the injection measured. In a second investigation, cold GnRH was infused (400 pg/min; n = 3) into the third ventricle for 2 h, and portal and jugular blood collected for the determination of GnRH and LH concentrations, respectively. In the first experiment, neither rate of infusion of GnRH into the third ventricle had any effect on the mean interpulse interval, nadir, pulse amplitude or circulating level of systemic LH, suggesting that CSF-GnRH is not a component of an ultrashort-loop feedback system for GnRH. Furthermore, in the second experiment, despite extremely low levels of radioactivity (maximum: 120 cpm/ml) being detected in hypophysial portal blood (which may not have been intact decapeptide), in the second part of this experiment, no radioimmunoassayable GnRH associated with the period of infusion could be measured. These data demonstrate in ewes that little, if any, CSF-GnRH reaches the hypophysial portal blood, and this compartment of GnRH does not, thus, directly affect the pituitary gland. The present study strongly suggests, therefore, that CSF-GnRH does not modulate LH secretion. Whether this compartment of GnRH is involved in sexual behavior remains to be established.