Sergio R. Ojeda

Activation of luteinizing hormone-releasing hormone release advances the onset of female puberty.

The juvenile-peripubertal transition period in the female rat is associated with an ovarian-independent afternoon increase in the amplitude of plasma luteinizing hormone (LH) pulses. To determine if the immature pituitary could be activated to cause precocious puberty juvenile female rats were subjected for 4 days to a microprocessor-driven pulsatile intravenous administration of LH-releasing hormone (LHRH) at a dose that produced a peripubertal pattern of LH release. To determine if the LHRH neurons themselves could be prematurely activated to induce such a pattern of plasma LH, and hence lead to precocious puberty, the neuroexcitatory amino acid analog N-methyl-DL-aspartic acid (NMA) was similarly administered. The time of puberty (vaginal opening and first ovulation) was advanced by both the LHRH and NMA treatments, by 5 and 7 days, respectively. Ovarian weight and incidence of corpora lutea at first diestrus were similar in all animals regardless of treatment, but a juvenile body weight was retained by the animals that underwent precocious puberty. Therefore, just as the adenohypophysis can be driven by exogenous LHRH to initiate puberty, the LHRH neuronal system can be precociously activated by the episodic administration of an excitatory amino acid analog that is known to interact with specific brain receptors. It is likely, therefore, that sexual maturation is limited by factors that lie further upstream in the hypothalamo-pituitary axis (e.g., the neuronal circuits that impinge upon LHRH-producing neurons).

Prostaglandin E2-Induced Release of Luteinizing Hormone-Releasing Factor (LRF)

Prostaglandin E2 (PGE2) injected into the third ventricle (3rd V) of conscious ovariectomized rats bearing a permanent jugular cannula increased the percentage of plasma samples showing detectable immunoassayable LRF levels at 1, 3 and 5 min after injection. This percentage was small at 2 and 4 min. When plasma LRF and LH titers were measured in animals injected intraventicularly with PGE2 and decapitated 5 min later, both LRF and LH were significantly higher than control values of diluent-injected animals. These results indicate that PGE2 is acting on the hypothalamus to induce release of LRF sufficient in mangitude to be detected by measuring this neurohormone directly in peripheral plasma.

Activation of estradiol-positive feedback at puberty: Estradiol sensitizes the LHRH-releasing system at two different biochemical steps

Experiments were performed to examine whether estradiol (E2) can influence some of the intraneuronal mechanisms involved in luteinizing hormone-releasing hormone (LHRH) release during the onset of puberty in the female rat. The capacity of median eminence (ME) nerve terminals to secrete LHRH, as determined by both their basal release of LHRH and by their response to prostaglandin E2 (PGE2) in vitro, increased significantly during the juvenile-early peripubertal periods of development (postnatal days 22-34). Ovariectomy (OVX) on day 22 led to a striking reduction in LHRH response to PGE2 on day 34. E2 administered via s.c. Silastic capsules, at a dose that reproduces juvenile serum E2 levels, restored the response. Simulation of first proestrous serum E2 levels in late juvenile (28-day-old) female rats enhanced both the sensitivity and the responsiveness of LHRH-containing terminals to PGE2. Furthermore, E2 enhanced the sensitivity and the responsiveness of LHRH terminals to norepinephrine (NE). This effect appeared to be related to both the increased LHRH response to PGE2 and an enhanced sensitivity of the PGE2-synthesizing pathway to NE. This is because MEs from E2-treated rats showed a marked increase in PGE2 release in response to a NE concentration which was barely effective in untreated controls. It is suggested that one of the mechanisms by which E2 activates the first preovulatory discharge of LHRH release in the female rat is by facilitating the occurrence of two different but sequentially related biochemical events: the stimulation of PGE2 formation by NE and the enhancement of LHRH release by PGE2.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for participation of a catecholaminergic mechanism in the post-castration rise in plasma gonadotropins.

Orchidectomy produced a rise in plasma gonadotropin levels after 16 h, which was completely prevented by an intraperitoneal injection of DL- α -methyltyrosine ( α -MT

Further Studies on the Maturation of the Estrogen Negative Feedback on Gonadotropin Release in the Female Rat

To evaluate estrogen negative feedback in infantile female rats, 9-day-old rats were ovariectomized (OVX) and treated with different doses of estradiol benzoate (Eb, s.c. once daily for 2 days); plasma LH, FSH and estradiol (E2) levels were then determined by RIA. The responses of these rats were compared with those of 25-day-old rats subjected to the same experimental procedures. In intact sham-operated controls, plasma FSH was elevated on days 9--13 and low on days 25--29, whereas LH did not change significantly throughout the period studied. OVX at day 9 or 25 increased the levels of both hormones, with the response faster and larger in the older rats. Treatment with Eb was more effective in suppressing the post-castration rise in gonadotropin levels in 27-day-old than in 11-day-old rats. At day 27 plasma E2 was elevated to pre-castration values by as little as 10 ng of Eb, whereas at day 11 this dose produced plasma E2 titers even higher than pre-castration values. Plasma E2 titers were high at 9--13 days of age and low at days 25--29. OVX on day 9 decreased E2 2--4 days later to about 1/2 the initial concentration, whereas adrenalectomy (ADRX) or ADRX-OVX was followed by an almost complete disappearance of the steroid. When E2 was injected at day 10 in intact rats to elevate plasma E2, E2 remained elevated when measured 3 to 120 min after its injection, but on day 25, 50% of the injected E2 had disappeared from plasma in 90 min. These results provide additional support for the view that estrogen negative feedback, even though present in infantile rats, is less effective than later in life and hence indicate that high gonadotropin titers observed at days 9--13 in the presence of high E2 may be caused by the relative ineffectiveness of the feedback at these early ages. The high E2 titers appear to be caused by an enhanced rate of production of E2 by the adrenals and ovaries and by a reduced metabolic clearance of the steroid.

Abdominal Vagotomy Delays the Onset of Puberty and Inhibits Ovarian Function in the Female Rat

To examine the influence that the vagus nerve may have on ovarian development and on the timing of puberty, both anterior and posterior vagi were sectioned at the abdominal level in 24-day-old female

On the Hypothalamic Mechanism by Which Prostaglandin E2 Stimulates Growth Hormone Release

It appears that prostaglandin E2 (PGE2) stimulates pituitary GH release by acting both on hypothalamic and pituitary sites. To determine if the hypothalamic components are at lea

Release of Prostaglandin E2 from the Hypothalamus Depends on Extracellular Ca2+ Availability: Relation to LHRH Release

The present experiments were performed to investigate the role of extracellular Ca2+ in the process of prostaglandin E2 (PGE2) release from the median eminence (ME) of the hypothalamus. Changes in the release of LHRH were also evaluated. Incubation of ME fragments with different concentrations of K+ induced a dose-related increase in PGE2 and LHRH release. The effect of K+ depended upon the Ca2+ concentration in the incubation medium. A Ca2+ concentration of 0.1 mM was sufficient to permit a significant response in both PGE2 and LHRH release to K+. The effect of a maximal K+ concentration (56 mM) was almost completely obliterated by omitting Ca2+ from the incubation medium and by chelating the remaining Ca2+ with EDTA or EGTA. The Ca2+ ionophore A23187, tested at different concentrations (1-50 microM) significantly increased the release of both PGE2 and LHRH from the ME in a Ca2+-dependent manner. A Ca2+ concentration of 0.25 mM allowed maximal LHRH response to the ionophore, but permitted only a partial (50%) PGE2 response. Blockade of Ca2+ channels with Verapamil, a Ca2+ entry blocker, prevented the effect of K+ on both PGE2 and LHRH release in a dose-dependent manner. The results demonstrate that release of PGE2 from ME nerve terminals depends upon the concentration of extracellular Ca2+ and suggest that PGE2 release is initiated by an increase in Ca2+ influx to the terminals. In addition, the data provide further evidence that release of LHRH is, to a significant extent, a function of Ca2+ influx through Ca2+ voltage-dependent channels.

An Automated System for the Study of Pulsatile Hormone Secretion in the Immature Rat

A procedure is described for prolonged sampling of blood from unrestrained immature rats which overcomes some of the problems associated with contemporary techniques. 1 or 2 days before sampling, the animal is fitted with two indwelling catheters, one inserted into the right jugular vein and the other into a femoral vein. On the day of the experiment blood is continuously withdrawn from the jugular vein (30 microliter/min) using a peristaltic pump and is dispensed into sample tubes every 5 min by means of an automatic fraction collector. A second channel of the same pump continuously infuses a blood replacement mixture (at 37 degrees C) into the femoral vein thereby keeping the animals blood volume constant. In the present study this technique was used to sample blood from both ovariectomized and immature, intact female rats for as long as 5-7 h. The procedure did not appear to disturb the normal locomotion, feeding, drinking and sleeping behavior of the animals. Plasma luteinizing hormone (LH) profiles obtained from the ovariectomized rats showed a normal pulsatile pattern of LH secretion. The frequency and amplitude of the pulses was not affected, even when the animals were bled on 2 consecutive days. Moreover, the procedure permitted accurate detection and characterization of pulsatile LH release in intact, immature rats although the episodes of LH secretion were more variable and much less pronounced. It is believed that this novel sampling technique could prove to be a valuable tool in the study of pulsatile hormone secretion in small laboratory rodents.

Changes in Pituitary Responsiveness to LH-RH during Puberty in the Female Rat: Initiation of the Priming Effect

Pituitary responsiveness was evaluated after a single or two consecutive i.v. injections of LH-RH at the time of puberty in the female rat. A single injection of LH-RH (20 or 100 ng/100 g b.w.) significantly elevated plasma LH levels in prepubertal rats (anestrous phase of puberty, A); the response became minimal in early proestrus (EP) prior to the preovulatory peak of gonadotropins. In contrast to the adult, this response barely increased on the preovulatory day (late proestrus, LP) when the decapeptide was injected at a time of day prior to the gonadotropin discharge. However, once the preovulatory surge of gonadotropins had started, there was a clear increase in responsiveness to LH-RH. Response to LH-RH was low during the 1st day of vaginal opening (estrus) and during the 2nd and 3rd day after vaginal opening (diestrus). The FSH response to LH-RH was greater during estrus than at EP or LP. At a time prior to the preovulatory surge of LH, the LH response to a 2nd injection of LH-RH administered 60 min after the initial injection was enhanced only during LP. When the 2nd injection of the neurohormone was given after the proestrous gonadotropin surge had started, the response was markedly enhanced. Uterine weight, used as an index of estrogen secretion, was low in A, increased significantly during EP and became maximum at LP, declining thereafter during estrus and diestrus. Ovulation usually occurred in the night prior to the day of vaginal opening. The results suggest that in spite of elevated estrogen levels, the pituitary LH response to LH-RH only increases around the time of puberty when endogenous release of the neurohormone presumably occurs to elevate plasma LH. This increase in LH-RH release,