A role for the ovaries in maturational processes of hypothalamic neurons containing luteinizing hormone- releasing hormone

The role of the ovaries in the maturation of the hypothalamic LHRH neuron has been investigated. Female rats were castrated at 4 weeks of age, and the subcellular distribution as weLl as the content of LHRH in the hypothalamic homogenates was analyzed 2.5, 12, and 20 weeks after ovariectomy. Hypothalami were homogenized in an isoosmotic sucrose solution, and a 900 × g supernatant fluid was prepared. Free granules and synaptosomes containing LHRH were separated by means of continuous sucrose density gradient centrifugation. In hypothalamic homogenates of intact immature female rats, 35% of the LHRH was associated with free granules and 65% was associated with synaptosomes; in homogenates of adult rats, 57% of the LHRH was associated with free granules and 43% was associated with synaptosomes. Ovariectomy did not alter the postnatal changes in the fractional distribution of LHRH between these two subcellular compartments. The LHRH content of the hypothalamus as well as the amount of LHRH sequestered in free granules and in synaptosomes prepared from intact female rats increased as a function of age of the donor animals, but this increase was abolished by ovariectomy. In addition, ovariectomy resulted in a significant (P < 0.02) decrease in the LHRH content of the hypothalamus, which was evident 2.5 weeks postoperatively. This reduction appears to be a consequence of a preferential decrease in the amount of LHRH present in structures which give rise to synaptosomes, i.e. presynaptic terminals. It is concluded that 1) the increased accumulation of LHRH which occurs during maturation of the hypothalamic LHRH neuron is dependent on ovarian function, 2) the subcellular distribution of LHRH is independent of ovarian function, and 3) the changes in the subcellular distribution of LHRH which occur during postnatal development are independent of ovarian function but may be dependent on age.