Eighteen ovariectomized rhesus mon-keys were stereotaxically implanted with multiple intracranial cannulae, four on each side of the midline, spaced 1.5–2.0 mm apart. The pulsatile pattern of LH release, characteristic of the castrated state, was not altered by such implantation or phencyclidine sedation. Systemic injections of 800 ng of estradiol reduced LH levels and attenuated the pulsatile type of LH secretion. Single injections of l/1000th the peripheral dose of estradiol at specific intrahypothalamic sites also depressed LH levels, mimicking the effects of iv administration. Most of the responsive sites were situated in the mediobasal hypothalamus and included the supra-chiasmatic, infundibular, ventromedial and mam-millary complex nuclei. Injections in other hypothalamic sites generally did not produce a response. Saline injections in estrogen responsive areas were ineffective in 10 out of 12 instances. Injections into adjacent extra-hypothalamic sites were usually ineffective, except for sites located within the fields of Forel.
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