Episodic patterns of luteinizing hormone and follicle-stimulating hormone release: differential secretory dynamics and adrenergic control in ovariectomized rats.

Long term (4 weeks) ovariectomized rats were bled sequentially at 5-min intervals for 5 h via indwelling intraatrial cannulas. Plasma LH and FSH secretory patterns were determined from the same plasma samples by RIA. Hormonal profiles were subjected to power spectral analysis to determine periodicities of plasma LH and FSH. Distinct and regular release patterns were observed for LH, with significant periodicities between 20-40 min. In contrast to LH, FSH oscillations were neither as distinct nor as regular. However, significant periodicities in FSH (50-60 min) were often detected. At times, plasma LH and FSH appeared to be synchronized, but there were numerous instances of differential secretion. The effects of intracerebroventricular infusion of norepinephine (NE) and clonidine (an alpha 2-agonist) were tested in another group of animals. After a 2- to 3-h control bleeding period each animal bearing a chronic third ventricle cannula received an intracerebroventricular infusion of 0.3 mumol NE, clonidine, or vehicle. Blood sampling was continued for 2-3 h after infusion. Intracerebroventricular infusion of NE caused rapid and potent inhibition of LH secretion with FSH affected to a lesser extent. NE infusion decreased mean plasma LH levels and LH pulse amplitude while causing a marked increase in pulse period. Although mean FSH levels declined after NE infusion, secretory episodes of FSH were detectable even in the absence of pulsatile LH secretion. Infusion of an equimolar dose of clonidine produced a biphasic response in LH, a transient elevation followed by a decrease in overall plasma levels. In contrast to LH, plasma FSH levels showed only a delayed decrease after clonidine infusion. No significant changes in pulse amplitude or pulse period for either gonadotropin were observed. These data show that plasma FSH, like LH, oscillates in a periodic manner, but when compared with episodic LH secretion there are both quantitative and qualitative differences. Although the neural mechanisms involved in periodic LH release are also involved to a lesser extent in FSH secretion, it appears that independent regulatory mechanisms exist for LH and FSH as well.

Electrical stimulation of mesencephalic noradrenergic pathway: effects on luteinizing hormone levels in blood of ovariectomized and ovariectomized, steroid-primed rats.

This study examined the effect of electrical stimulation of the dorsal mesencephalic tegmentum (DMT) region on blood LH levels in long term ovariectomized (OVX), pentobarbital-anesthetized rats, with the aim of activating the principal ascending noradrenergic (NE) bundle. In OVX unprimed rats, electrical stimulation (with parameters of 100 Hz, 0.5-msec effective biphasic pulses, 150-200 microA, and 15 sec on/off for up to 1.5 h) of the DMT region inside the ascending NE bundle either completely or partially inhibited the pulsatile pattern of blood LH levels characteristic of OVX animals; stimulation outside the NE bundle was ineffective. Pretreatment of OVX rats with a serotonin synthesis inhibitor, p-chlorophenylalanine (320 mg/kg, ip) 71 h before electrical stimulation of the DMT did not affect the stimulation-induced inhibition of pulsatile LH release. On the other hand, pretreatment of OVX rats with a tyrosine hydroxylase inhibitor, alpha-methyl-p-tyrosine (250 mg/kg, ip) 4.5 h before electrical stimulation of the DMT was effective in preventing the stimulation-induced inhibition of pulsatile LH release, thereby supporting a NE-mediated mechanism. In OVX rats primed with estradiol benzoate alone (5 micrograms/100 g BW for 2 days) or with 50 micrograms estradiol benzoate and 25 mg progesterone, electrical stimulation in the DMT was ineffective in altering the low, nonpulsatile blood levels of LH. The results in OVX unprimed rats suggest that activation of the ascending NE system can inhibit pulsatile LH release, thus indicating a possible functional importance of inhibitory (in addition to well-documented stimulatory) NE synapses in the modulation of LH release.

Effects of intraventricular norepinephrine on preoptic-anterior hypothalamic electrical activity in the freely-moving rat: modulation by ovarian steroid hormones.

In freely-moving female rats the effects of intraventricular infusion of norepinephrine (NE) on multiunit activity (MUA) were examined in the Diagonal Band of Broca (DBB), medial preoptic area (MPOA) and anterior hypothalamic area (AHA), regions containing neurons producing luteinizing hormone-releasing hormone (LHRH). NE was infused at a dosage known to depress plasma LH levels in ovariectomized (OVX) rats and elevate plasma LH concentrations in OVX-estrogen-progesterone-primed (EBP) animals. It was found that in adult OVX rats MUA in the brain areas listed above was almost invariably inhibited by NE (20 of 23 cases or 87%; the other three cases showed no change in electrical activity). However, after priming with estrogen and progesterone only about one-third of the OVX-EBP rats gave an inhibitory response, with another one-third showing no change in MUA and the final one-third of the cases actually giving an excitatory MUA response to NE--the DBB neurons being the most positive in the regard. Thus it appears that responsiveness of LHRH and/or adjacent neurons to the modulatory action of NE may itself be modulated by the influence of gonadal steroids.

An electronic open field.

An electronic open field made with inexpensive, off-the-shelf materials is described. Its automatic operation frees the researcher from tedious and time consuming direct observations in experiments measuring exploratory behavior, goal preference, etc. Its numeric output can be read from the display or interfaced to printers, recorders, computers, etc. As an example of its operation, experimental results measuring sexual motivation (proceptivity) of female rats are reported.

Electrophysiologic correlates of steroid modulation of luteinizing hormone release.

In both ovariectomized (OVX) and steroid-primed OVX freely moving rats, attempts were made to correlate the effects of intraventricular norepinephrine (NE) on multiunit activity (MUA) of different brain regions with NE-induced alterations in blood LH levels. MUA-recording electrodes were implanted in the diagonal band of Broca (DBB), medial preoptic area (MPOA), arcuate nucleus (ARC) and/or ventromedial hypothalamic nucleus (VMH). Steroid priming included 50 micrograms estradiol benzoate (EB) and 25 mg progesterone (P) 3 days prior to experiment. The unanesthetized animals were bled via indwelling atrial cannulas before and after intraventricular infusion of NE (10 micrograms in 2 microliters over 2 min). In OVX-primed rats NE lengthened the interval between episodic LH peaks and decreased mean blood LH levels. In contrast, in OVX-EBP-primed rats, NE stimulated an LH surge. Concurrent recording of MUA revealed that, in OVX-unprimed rats, NE dramatically depressed MUA in both DBB-MPO and ARC-VMH neurons. However, in OVX-EBP-primed rats, while still markedly inhibiting ARC-VMH units, NE failed to depress MUA recorded in DBB-MPO sites (some units were actually excited by NE), perhaps reflecting the higher ratio of LHRH neurons/inhibitory neurons in DBB-MPO.

Gonadotropin secretion following intraventricular norepinephrine infusion into neonatally androgenized female rats.

To determine whether norepinephrine (NE) will stimulate gonadotropin secretion in androgenized female rats, we administered NE (0.3 microM in 2 microliter saline; pH 6.0-6.5) into the third ventricle of females which had been treated neonatally with two different doses of testosterone propionate (TP; 10 or 100 micrograms) on day 5 of life. This treatment rendered these animals anovulatory as evidenced by polyfollicular ovaries and a persistent vaginal estrus condition which occurred by 100 days of age. Those females which were determined to be anovulatory were ovariectomized 4 weeks prior to the stereotaxic implantation of an intracerebroventricular (ICV) cannula. Ten to twelve days following cannula placement, animals were primed with either estradiol benzoate (EB; 30 micrograms) or EB and progesterone (P; 5 mg). Two days following steroid priming, NE was infused into the third ventricle at 15.00 h and blood samples were taken via an intra-atrial catheter at 15-min intervals for 2 h before and 2 h after administration. In both TP-treated groups, NE caused rapid and significant (p less than 0.01) elevations in plasma luteinizing hormone (LH) concentrations when animals were primed with EB as compared with control females given acid saline infusions (pH 6.0-6.5). Similar responses to NE were seen after additional priming with P at the same time as EB. Follicle-stimulating hormone (FSH) values did not increase following NE or saline infusion to any of the groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Postcastration rise in plasma gonadotropins is blocked by a luteinizing hormone-releasing hormone antagonist.

The dependence of the acute increases in plasma gonadotropins following castration on luteinizing hormone-releasing hormone (LHRH) was assessed with the use of a potent LHRH antagonist [ALHRH; (Nac-L-Ala1,p-Cl-D-Phe2,D-Trp3,6) LHRH]. Blood samples were collected from male and female rats at the time of castration and 2, 4, 8, 12, 24 and 48 h following and plasma gonadotropin levels were determined. Immediately following castration (diestrus I for females) animals received one of the following treatments: females-vehicle, 100 micrograms ALHRH, 50 micrograms estrogen benzoate (EB), or 100 micrograms ALHRH + 50 micrograms EB; males-vehicle, 100 micrograms ALHRH, 500 micrograms testosterone propionate (TP), or 100 micrograms ALHRH + 500 micrograms TP. ALHRH blocked the selective increase in plasma follicle-stimulating hormone (FSH) observed in female rats as well as the parallel increases in both gonadotropins seen in male rats following castration. Administration of EB or ALHRH + EB to females significantly suppressed both gonadotropins compared with control levels. However, EB alone did not completely block the rise in plasma FSH in females. In males, all three treatments significantly suppressed the increases in both gonadotropins when compared with control levels. These data demonstrate that hypothalamic LHRH plays an essential role in the acute elevations of plasma gonadotropins following castration in rats. In addition, these data suggest that the selective rise of FSH in females is dependent on LHRH stimulation of pituitary gonadotropes.

Differential effects of intraventricular luteinizing hormone releasing hormone (LH-RH) and norepinephrine on electrical activity of the arcuate nucleus in the proestrous rat.

Effects of intraventricular infusions of LH-RH and norepinephrine (NE) on the electrical activity of the arcuate nucleus were investigated in normally cycling proestrous rats. Under urethane anesthesia, recordings were made of amplitude-discriminated multiple unit spike activity and integated multiunit activity (MUA) in parallel with cortical EEG. Control infusions of saline (2 microliter, isotonic, pH 5.5) were ineffectual, but LH-RH (0.5 microgram) induced a significant increase in both multiunit spike activity and integrated MUA. While the response appeared to be continuous, statistical analysis revealed 2 phases: a quick rise which persisted for approximately 5 min, followed 15 min later by a longer-lasting elevation in activity. The onset of the 2nd increase corresponded with the attainment of peak values of pituitary LH output. Subsequent treatment with 20 microgram NE, on the other hand, resulted in a marked depression of activity. The fact that NE depresses arcuate neuronal activity at dose levels which cause the release of LH and that LH-RH increases activity within the same population of neurons, while possibly mediating an 'ultrashort-loop' negative feedback effect, suggest that this responsive component of the arcuate nucleus, perphaps the tuberoinfundibular dopaminergic system of neurons, is inhibitory to LH release.

Differential gonadotropin secretion: blockade of periovulatory LH but not FSH secretion by a potent LHRH antagonist.

The dependence of periovulatory gonadotropin secretion on LHRH was assessed with the use of a potent LHRH antagonist [ ALHRH ; (Nac-L- Ala1 ,p-Cl-D-Phe2,D-Trp3,6)LHRH]. Blood samples were collected hourly from 14.00 h proestrus (P) through 09.00 h estrus (E) from intact cycling female rats. ALHRH was administered at 09.00 or 13.00 h P before the proestrous increases in gonadotropins had commenced or at 23.00 h P after the LH and primary FSH surges had occurred but preceding the secondary FSH surge. Antagonist given at 09.00 or 13.00 h P completely blocked the LH release with levels remaining undetectable in most animals (less than 30 ng/ml) throughout the sampling period. However, administration of antagonist at these times failed to block completely the primary FSH surge although peak values were reduced when compared with controls, which displayed normal gonadotropin surges. In addition, ALHRH administered at 23.00 h failed to alter the magnitude or other characteristics of the secondary FSH surge when compared with controls. The present study demonstrates that the estrous surge of FSH in the rat is independent of acute hypothalamic release of LHRH. Furthermore, although the proestrous release of FSH is to a large extent LHRH dependent, our data suggest that some other mechanism may also contribute to this primary FSH surge.

Ovarian steroid modulation of norepinephrine action on luteinizing hormone release. Analogous effects in male and female rats.

The modulatory actions of ovarian steroids on the norepinephrine (NE) induced alterations in luteinizing hormone (LH) secretion were examined in male and female rats. Long-term (4 weeks) castrated male and female rats bearing chronic third-ventricle cannulae were implanted with intra-atrial catheters. Animals were bled sequentially at 5- or 15-min intervals for 4-6 h, and plasma LH secretory patterns were determined by radioimmunoassay. After a 2- to 3-hour control bleeding period, castrated unprimed rats received an intracerebroventricular (ICV) infusion of 0.3 mumol of arterenol bitartrate or vehicle. Blood sampling was continued for an additional 2-3 h after infusion. In female rats NE caused a rapid and potent inhibition of episodic LH secretion which was characterized by decreases in mean plasma LH, mean pulse amplitude, and pulse frequency. Similarly, ICV infusion of NE into male rats resulted in decreased mean plasma LH and pulse frequency. In a second series of experiments, castrated male and female rats were primed with estrogen and progesterone 2 days prior to the bleeding/infusion session. Animals were bled sequentially and infused with 0.3 mumol arterenol bitartrate or vehicle during a morning or afternoon session. ICV infusion of vehicle had no effect on plasma LH in either sex regardless of the time of day. NE infusion into estrogen and progesterone primed female rats resulted in significant elevations of plasma LH during both morning and afternoon periods. In estrogen and progesterone primed male rats, NE infusion resulted in a marked facilitation of LH release, similar to that observed in female rats.(ABSTRACT TRUNCATED AT 250 WORDS)