Effect of alterations in pulsatile luteinizing hormone release on ovarian follicular atresia and steroid secretion on diestrus 1 in the rat estrous cycle

This study examined the importance of pulsatile luteinizing hormone (LH) release on diestrus 1 (D1; metestrus) in the rat estrous cycle to ovarian follicular development and estradiol (E2) secretion. Single injections of a luteinizing hormone-releasing hormone (LHRH) antagonist given at -7.5 h prior to the onset of a 3-h blood sampling period on D1 reduced mean blood LH levels by decreasing LH pulse amplitude, while frequency was not altered. Sequential injections at -7.5 and -3.5 h completely eliminated pulsatile LH secretion. Neither treatment altered the total number of follicles/ovary greater than 150 mu in diameter, the number of follicles in any size group between 150 and 551 mu, or plasma E2, progesterone, or follicle-stimulating hormone (FSH) levels. However, both treatments with LHRH antagonist significantly increased the percentage of atretic follicles in the ovary. These data indicate that: 1) pulsatile LH release is an important factor in determining the rate at which follicles undergo atresia on D1; 2) reductions in LH pulse amplitude alone are sufficient to increase the rate of follicular atresia on D1; 3) an absence of pulsatile LH release for a period of up to 10 h on D1 is not sufficient to produce a decline in ovarian E2 secretion, most likely because the atretic process was in its early stages and had not yet affected a sufficient number of E2-secreting granulosa cells to reduce the follicle's capacity to secrete E2; and 4) suppression or elimination of pulsatile LH release on D1 is not associated with diminished FSH secretion.

Pulsatile luteinizing hormone release during pregnancy in the rat

The present studies were designed to characterize LH release during pregnancy in the rat. Unanesthetized animals with jugular cannulae were bled for 3 h between 1000-1300 h on days 6-8, 14-16 or 22 of gestation (50 microliters whole blood/5 min). Plasma estradiol and progesterone values both increased from days 6-8 to days 14-16. However, while plasma estradiol levels increased further between days 14-16 and day 22, plasma P levels had declined 86%. The percent coefficients of variation obtained for alterations in blood LH levels at each stage of pregnancy were all significantly greater than intraassay variation, indicating that LH release was pulsatile at each stage. Although there were no significant differences in mean blood LH levels, pulse amplitude, or frequency between days 6-8 and 14-16, the individual patterns of LH release clearly varied between these 2 groups, and most notably within the 14-16 day group. Fifty-three percent (9 of 17) of the LH records in rats on days 14-16 were nonpulsatile compared to only 20% (3 of 15) on days 6-8. However, despite a trend toward an absence of pulsatile LH release on days 14-16, mean frequency at this time did not differ from days 6-8, since on days 14-16 the remaining 8 animals demonstrated 3.5 pulses/3 h, while on days 6-8 the other 12 rats averaged only 2.5 pulses/3 h. On day 22, there was a marked increase in mean blood LH levels compared with either days 6-8 or 14-16. This increase was due to an increase in mean LH pulse frequency. All 15 rats demonstrated pulsatile LH secretion, a significantly greater incidence of pulsatile LH release than on days 14-16 (100% vs. 47%). These data demonstrate that LH release is pulsatile during pregnancy in the rat, and changes in the characteristics of this secretion occur at different stages of gestation.

Medial preoptic area involvement in norepinephrine-induced suppression of pulsatile luteinizing hormone release in ovariectomized rats

This study examined whether the medial preoptic area (MPOA) is a site which mediates the inhibitory effects of norepinephrine (NE) on pulsatile luteinizing hormone (LH) secretion in ovariectomized rats. Animals were bled continuously at a rate of 50 microliter whole blood/7 min for 2 h prior to push-pull perfusion in the MPOA, and during a 2-3 h period of perfusion of the MPOA (20 microliter/min) with artificial CSF, or 2 or 20 pg NE/min. In another group of rats LH levels were only determined during a 2-3 h period of MPOA perfusion with CSF. Pulsatile LH release was not affected by push-pull perfusion with CSF when a comparison was made to preperfusion LH values in the same rats. Moreover, LH levels obtained from rats only bled during MPOA perfusion with CSF were not different from LH values obtained during the preperfusion periods in the other groups. However, push-pull perfusion of the MPOA with 2 or 20 pg NE/min significantly suppressed mean LH levels by causing a 35-45% reduction in pulse frequency. No decrease occurred in LH pulse amplitude. Therefore, these studies demonstrate that NE acting at the level of the MPOA can suppress pulsatile LH release solely by decreasing LH pulse frequency.

Lack of ovarian steroid negative feedback on pulsatile luteinizing hormone release between estrus and diestrous day 1 in the rat estrous cycle

The object of this study was to determine the roles of ovarian estradiol (E2) and progesterone (P) in regulating pulsatile LH release between estrus and diestrous day 1 (D1) in the rat estrous cycle. Three groups of rats were bled at rates of 75 or 100 microliter whole blood/6 or 8 min, respectively, between 0930-1230 h on estrus or 24 h after either sham ovariectomy (OVX) on estrus (i.e. on D1) or OVX on estrus. There were no differences in plasma E2 and P levels in rats between estrus and early D1. However, after OVX on estrus, plasma levels of both steroids declined and were significantly lower 24 h later than values in D1 controls, indicating an active ovarian secretion of both hormones in this interval of the rat cycle. A significant increase in mean blood LH levels occurred between estrus and D1 due to an increase in LH pulse amplitude and frequency. After OVX on estrus, all parameters of pulsatile LH release also increased within 24 h, but mean blood LH levels as well as LH pulse amplitude and frequency were virtually identical to values in D1 controls, despite the decline in plasma E2 and P levels. Thus, OVX did not augment the increases in LH pulse amplitude and frequency that occur between estrus and D1. This demonstrates that the increase in pulsatile LH release from estrus to early D1 occurs in the absence of ovarian steroid negative feedback; the increases in LH pulse amplitude and frequency are not under negative feedback control by the low plasma levels of E2 and P present at this time. These data stand in direct contrast to the presence of prominent ovarian steroid negative feedback systems operative between D1 and diestrous day 2, and diestrous day 2 and proestrus.

Stimulatory or inhibitory effects of angiotensin II upon LH secretion in ovariectomized rats: a function of gonadal steroids

The effects of intraventricular infusions of artificial cerebrospinal fluid (aCSF) or angiotensin II (AII) on LH secretion were investigated in rats that had been ovariectomized for 8 days. In untreated ovariectomized rats, the mean whole blood concentration of LH as well as the amplitude, frequency, and nadir of the LH pulses were not affected by infusion of aCSF or 15 ng AII/h, but were suppressed in a dose-dependent fashion by infusion of AII at doses of 150 or 600 ng/h. The AII receptor antagonist, saralasin, blocked the inhibitory effect of AII, demonstrating the specificity of the response to AII. In ovariectomized rats pretreated with estradiol, infusion of AII did not modify mean blood LH levels. However, in ovariectomized rats pretreated with both estradiol and progesterone, infusions of AII at 150 or 600 ng/h produced dose-dependent increases in mean LH concentrations. The results demonstrate both inhibitory and stimulatory effects of AII upon LH secretion, the direction of the effect being determined by gonadal steroids.

Further studies on norepinephrine-induced suppression of pulsatile luteinizing hormone release in ovariectomized rats

The present study examined three aspects of the inhibitory effects of continuous intraventricular infusion of norepinephrine (NE) on pulsatile luteinizing hormone (LH) release in ovariectomized, nonsteroid-primed rats: whether the inhibitory effects of NE infusion were exerted on LH pulse frequency and/or amplitude; whether central nervous system desensitization occurred in response to the inhibitory effects of continuous NE infusion on pulsatile LH secretion, and whether dopamine of serotonin were involved as possible interneuronal transmitter mediators of NE-induced suppression of pulsatile LH release. Unanesthetized rats with external jugular cannulae were bled continuously at a rate of 50 microliters whole blood/7 min for 2 h prior to infusion and for 2-3 h during continuous intraventricular infusion of artificial cerebrospinal fluid or NE. Infusion of cerebrospinal fluid had no effect on pulsatile LH release, while continuous infusion of 0.3 or 1.8 micrograms NE/h for 2-3 h produced suppression of pulsatile LH secretion. Although desensitization to the stimulatory effects of NE on LH release in ovariectomized, steroid-primed rats had been observed to occur rapidly within 90 min after the onset of infusion, desensitization to the inhibitory effect of NE on pulsatile LH release did not occur even after continuous infusion of NE for periods up to 20 h. Mean blood LH levels were as low in rats bled 17-20 h after the onset of NE infusion as in those bled at 0-3 h. The suppressive effect of NE on pulsatile LH release was not prevented by prior blockade of dopamine or serotonin receptors with pimozide or metergoline, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of dopamine receptor blockade or norepinephrine synthesis inhibition on acute, ovariectomy-induced increases in pulsatile luteinizing hormone release in the rat

The initial aim of the present studies was to examine the influence of blockade of dopamine (DA) receptors with pimozide or inhibition of norepinephrine (NE) synthesis with U-14,624 on acute, ovariectomy (OVX)-induced changes in pulsatile LH release. Either treatment instituted at the time of OVX suppressed or inhibited the rapid increase in LH pulse amplitude and frequency normally occurring within 24 hr following ovarian removal on diestrus 1. While administration of pimozide at either 24 hr or 48 hr following OVX suppressed pulsatile LH release by selectively reducing LH pulse frequency, by 8 days following OVX pimozide failed to exert any effect on LH pulse frequency and therefore on pulsatile LH secretion. To determine if there was a transient critical period following OVX of at least 2 days but less than 8 when endogenous DA was excitatory to pulsatile LH release, piribedil (a DA receptor agonist) was given 24 hr following OVX. Rather than increase LH secretion, piribedil markedly suppressed pulsatile LH release indicating that DA does not stimulate LH secretion in acutely ovariectomized rats. These experiments indicate that (1) NE is involved in stimulating the acute, OVX-induced increase that occurs in pulsatile LH release; (2) DA receptor blockade by pimozide has a differential effect on pulsatile LH secretion which depends on the time following OVX when the compound is administered; (3) this differential effect cannot be explained by a transient critical period of a few days duration following OVX during which DA is excitatory to pulsatile LH release.

The influence of progesterone and estradiol on the acute changes in pulsatile luteinizing hormone release induced by ovariectomy on diestrus day 1 in the rat

The aim of this study was to determine whether the rapid increases in LH pulse amplitude and frequency that occur within 24 h after ovariectomy (ovx) on diestrus day 1 (D1) were due to the removal of progesterone (P) and/or estradiol (E). Initial studies demonstrated that plasma levels of E and P were 18.2 +/- 1.2 pg/ml and 34.1 +/- 3.2 ng/ml, respectively, between the evening of D1 and the morning of D2 in our colony of intact rats. Immediately after ovx and jugular venous cannulation on the morning of D1, rats were implanted either with empty Silastic capsules or capsules capable of restoring physiological levels of E and P to the control values reported above. These rats were continuously bled (75 microliter/6 min) for 3 h 1 day after ovx for analysis of pulsatile LH release, and then additional plasma samples were gathered for determination of E and P levels. Rats with empty capsules had decreased levels of E and P and increases in mean blood LH levels, LH pulse amplitude, and pulse frequency. Animals with E capsules had physiological levels of E and decreased levels of P, but no suppression of the acute post-ovx increase in pulsatile LH release. In contrast, animals with P capsules had physiological plasma levels of P, decreased levels of E, and a marked reduction in the acute LH response to ovx. This suppression was due entirely to a decrease in LH pulse amplitude, as pulse frequency was not altered. Rats with E and P capsules had physiological levels of these hormones, which resulted in an even greater reduction in the acute LH response to ovx. This suppression was due to decreases in both LH pulse amplitude and pulse frequency. The effect of P on LH pulse amplitude was centrally mediated, since the in vitro response to LHRH of anterior pituitary fragments from P-implanted rats was the same as that of anterior pituitary fragments taken from rats with empty capsules. These studies demonstrate that the acute increase in LH pulse amplitude that occurs within 24 h after ovx on D1 is due to the absence of a central inhibitory effect of ovarian P, while the rapid increase in LH pulse frequency is due to the loss of both ovarian E and P.(ABSTRACT TRUNCATED AT 400 WORDS)

Acute and long-term changes in central and pituitary mechanisms regulating pulsatile luteinizing hormone secretion after ovariectomy in the rat

These experiments examined the time course of changes in the characteristics of pulsatile luteinizing hormone (LH) secretion that occur after ovariectomy (OVX) in the rat, and compared the response of the brain and pituitary to the absence of ovarian steroid negative feedback. The literature indicates that the brain could respond to OVX by altering the frequency and/or amplitude of the pulsatile luteinizing hormone releasing hormone (LH-RH) release which triggers pulsatile LH secretion, while the pituitary could respond by altering basal LH output and/or its response to LH-RH. In vivo experiments examined changes in mean blood LH levels, LH pulse amplitudes and pulse frequencies in control rats on diestrus 1 (D1) and at 7 h, 14 h, 1, 2 or 8 days, or 3 weeks following OVX. In addition, anterior pituitaries from rats on D1 or rats ovariectomized for 1, 2 or 8 days, or 3 weeks, were incubated in vitro with or without LH-RH to examine changes in both basal and LH-RH-induced LH release. Mean blood LH levels increased within 17-24 h following OVX. This acute, rapid elevation was due to increases in both LH pulse frequency and pulse amplitude. Blood LH levels continued to increase over a 3-week period. However, the long-term increase seen at 8 days or 3 weeks was not only due to increases in LH pulse frequency and amplitude, but also to dramatic increases in basal LH secretion. The frequency of pulsatile LH release was maximal within 8 days, with no further increases occurring over the next 2 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

A possible role for the brain renin-angiotensin system in the regulation of LH secretion

To investigate the possibility that angiotensin II (ANG II) is involved in the regulation of luteinizing hormone (LH) secretion, ANG II was injected intraventricular ANG II caused an increase in plasma LH concentrations in 10 min, whereas intravenous ANG II in the same dose had no significant effect. Intraventricular administration of the ANG II antagonist, saralasin, during the afternoon of proestrus induced a significant decrease in the number of rats ovulating and abolished the ovulatory surge in LH secretion seen in saline-injected control animals. Intravenous saralasin in two different doses failed to inhibit ovulation and produced only a small decrease in the LH surge. In animals treated with intraventricular saralasin, the increase in plasma LH concentration produced by intravenous injection of 50 ng of LH-releasing hormone (LHRH) was normal, indicating no alteration in the sensitivity of the gonadotrops to LHRH. Intraventricular administration of the converting-enzyme inhibitor, enalapril diacid, inhibited ovulation and the ovulatory surge, whereas systemically administered enalapril diacid had no effect on LH secretion. The data suggest that ANG II generated in the brain may play a significant role in the regulation of LH secretion on the day of proestrus.

Pulsatile LH release on diestrus 1 in the rat estrous cycle: relation to brain catecholamines and ovarian steroid secretion

This study examined (1) the possible involvement of catecholamines in the regulation of pulsatile LH release on diestrus 1 (D1) in the rat estrous cycle, and (2) the possible importance of pulsatile LH release on D1 to ovarian estradiol and progesterone secretion on this day of the cycle. Blockade of dopamine receptors on D1 had no effect on LH secretion. However, inhibition of norepinephrine (NE) synthesis or blockade of alpha- but not beta-adrenergic receptors decreased mean blood LH levels, and greatly suppressed pulsatile LH secretion by decreasing both LH pulse frequency and amplitude. In contrast, in the same rats, interference with brain NE function had no effect on plasma FSH levels. These data indicate that NE is an excitatory neurotransmitter in the regulation of pulsatile LH release on D1. Acting through an alpha-adrenergic receptor, this neurotransmitter presumably influences both the frequency of the LHRH pulse generator and the amount of LHRH released per pulse. Moreover, the release of LH and FSH on D1 can be separated, suggesting possible differences in the mechanisms regulating secretion of these two gonadotropins. Regardless of whether pulsatile LH release had been greatly suppressed or not altered, no change occurred in plasma progesterone levels, indicating pulsatile LH release on D1 is not important for the release of progesterone that occurs on this day of the cycle. With respect to estradiol, no consistent pattern emerged between changes in pulsatile LH release induced by interference with brain NE function and plasma estradiol levels. Thus, no definitive conclusion was possible on the potential importance of pulsatile LH release to estradiol secretion on D1.

Effects of intraventricular administration of catecholamines on luteinizing hormone release in morphine-treated rats

Morphine (M) treatment has been shown to suppress LH release in rats. These studies were undertaken to determine whether a decrease in the response of LHRH neurons to excitatory neurotransmitters may be responsible for the depressed LH secretion in M-treated rats. Ovariectomized rats bearing permanent cannulae in the third ventricle of the brain were primed with estradiol benzoate and progesterone; 3 days later, they received M (20 mg/kg, sc) or saline (controls). The effects of two intraventricular (Ivt) 2-min pulses delivered 80 min apart of vehicle (artificial cerebrospinal fluid), dopamine, norepinephrine, or epinephrine (E) on LH release were assessed. Basal blood LH levels were undisturbed by Ivt administration of vehicle in saline-treated rats. Intraventricular infusions of dopamine (5.3 micrograms/pulse) also failed to evoke LH release in saline-treated rats. However, similar pulse norepinephrine or E infusions (5.3 micrograms/pulse) readily elicited well defined episodes of LH hypersecretion. The magnitude and temporal pattern of LH responses in the control and M-treated rats were quite similar. In another experiment, the progesterone-induced afternoon LH surge was blocked by M treatment of estradiol benzoate-primed rats. In these blocked rats, Ivt administration of E evoked rapid and substantial LH secretion. Thus, our results failed to demonstrate any evidence of diminution in the response of LHRH neurons to excitatory neurotransmitters in M-treated rats. On the other hand, they lend credence to the view that a decreased influx of adrenergic signals in the vicinity of the LHRH neurons may result in the suppression of LH release after M administration.

Effect of electrical stimulation of the dorsomedial hypothalamic nucleus on pulsatile LH release in ovariectomized rats

The present experiments were carried out to examine the influence of electrical stimulation of the dorsomedial hypothalamic nucleus (DMH) on pulsatile LH release in ovariectomized rats, and the possible involvement of serotonin (5HT) in mediating any observed effects. Unanesthetized animals were bled continuously through jugular vein cannulae for an initial 1 1/2 h control period. Rats were then stimulated for two 1-hour periods, separated by a 45-min nonstimulation period. DMH stimulation suppressed pulsatile LH release and decreased mean blood LH levels during both stimulation periods in control rats as well as animals in which 5HT synthesis was inhibited by p-chlorophenylalanine or 5HT receptors were blocked by metergoline. The decrease in mean blood LH levels in all groups was due solely to an increase in the LH interpulse interval. LH pulses that did occur during the stimulation-induced suppression did not show a decreased LH pulse amplitude. Rather, in control and metergoline-treated animals this parameter increased during stimulation, suggesting a buildup of readily releasable LHRH and/or pituitary LH. Lastly, estradiol benzoate suppressed pulsatile LH secretion but did not reverse the inhibitory LH response to DMH stimulation. These experiments indicate that the DMH, because of neurons originating within this nucleus and/or fibers passing through, may be a brain region that influences pulsatile LH release in a suppressive manner. This effect is exerted solely on the periodicity of the process and not through a 5HT-mediated mechanism.

Presence of immunoreactive luteinizing hormone in the rat forebrain

This study demonstrates the presence of immunoreactive LH in rat brain by radioimmunoassay and immunocytochemistry. High levels of radioimmunoassayable LH were identified in the hypothalamus, while significant but lesser quantities were found in the amygdala, septal area, preoptic area, thalamus, caudate nucleus, and hippocampus. Correlative immunocytochemistry localized immunopositive fibers in hypothalamic and several extrahypothalamic brain structures. Immunoreactive cell bodies were seen in colchicine-treated rats in the arcuate nucleus of the hypothalamus. Gel chromatography of hypothalamic extracts revealed that immunoassayable LH coeluted with LH standard and rat pituitary extracts. Possible mechanisms related to the origin and functional neuronal LH are discussed.

Neuroendocrine regulation of pulsatile luteinizing hormone release in the rat

This review considers some of the neuroendocrine factors influencing pulsatile LH secretion. Such release is apparently due to the pulsatile discharge of LHRH from brain peptidergic neurons. This is a physiologically important event since a periodic rather than continuous input signal to the pituitary gland prevents it from becoming refractory to LHRH stimulation. Pulsatile secretion of LH, in the rat at least, does not appear to be regulated sloely by the medial basal hypothalamus. Central noradrenergic, cholinergic, dopaminergic, and serotoninergic systems are involved in influencing episodic LH release, presumably by affecting pulsatile LHRH secretion. Moreover, several hypothalamic as well as extrahypothalamic areas appear to play integral parts in controlling the rhythmic alterations in blood LH levels. These regions include the arcuate and suprachiasmatic nuclei, perisuprachiasmatic area, medial preoptic area, and midbrain dorsal raphe nucleus. Ovarian steroids also exert important influences on pulsatile LH relase, and greatly modify the response of this secretory system to neurotransmitters and stimuli from certain brain regions.

Regional differences in response to electrical stimulation within the medial preoptic-suprachiasmatic region on blood luteinizing hormone levels in ovariectomized and ovariectomized, estrogen-primed rats

The effect of electrical stimulation of the medial preoptic-suprachiasmatic nucleus (MPOA-SCN) region of the forebrain on blood LH levels was studied in ovariectomized, pentobarbital-anesthetized rats. Animals were either not primed with estrogen or previously given 5 micrograms estradiol benzoate (EB)/100 g BW . day for the 2 days before stimulation. Rats were bled continuously (30, 40, or 50 microliter whole blood/5--6 min) through indwelling right atrial cannulae for 1.5 h before stimulation, 1.5 h during stimulation, and up to 1 h afterwards. Whole blood was analyzed for LH by RIA. In ovariectomized, unprimed rats, electrical stimulation of the ventral MPOA consistently increased LH release. In contrast, stimulation of the peri-SCN region (immediately caudal to the MPOA, and lateral and dorsal to, but not within, the SCN), uniformly inhibited pulsatile LH secretion. Activation of the SCN elevated blood LH levels in most unprimed rats tested, but suppression of episodic LH release occasionally occurred. Pretreatment with estrogen resulted in increased LH secretion in response to stimulation of each of these three regions. Estrogen prolonged the LH increase occurring during MPOA stimulation, completely reversed the inhibitory LH response to peri-SCN stimulation, and either reversed any possible inhibitory response to SCN stimulation or greatly increased the magnitude and duration of the increase in blood LH levels produced in the unprimed rat during activation of this nucleus. In summary, the present study indicates that well defined areas in the MPOA-SCN region can have strikingly different effects on LH secretion in the absence of ovarian estrogen and that this steroid is critically important in determining the direction, magnitude, and duration of the LH response to a localized brain stimulus.

Effect of lesions in the suprachiasmatic nucleus-retrochiasmatic area on the inhibition of pulsatile LH release induced by electrical stimulation of the midbrain dorsal raphe nucleus

The present studies were designed to determine if the inhibition of pulsatile luteinizing hormone (LH) secretion reported to occur during midbrain dorsal raphe nucleus (DRN) stimulation involved some portion of the suprachiasmatic nucleus(SCN)-retrochiasmatic area (RCA). In animals having lesions that completely destroyed the SCN but did not extend into the RCA, dorsal raphe-induced inhibition of pulsatile LH release was still present. If, however, a significant portion (more than 40%) or all of the RCA was encompassed by the lesion (with variable or no SCN damage), the inhibition induced by DRN stimulation was prevented. This indicates that the SCN is not necessary for the inhibitory effect of DRN stimulation on pulsatile LH release to occur. However, the pathway responsible for this inhibition does appear to project to or through the RCA. Furthermore, pulsatile LH release was reduced, but present, during the control period prior to stimulation in rats with SCN lesions (and no damage to the RCA), and mean blood LH levels for this group were significantly decreased during the control bleeding period. Therefore, the neuronal pathway activating episodic LH release involves, but is not restricted solely to, the SCN.

Effect of intraventricular infusion of catecholamines on luteinizing hormone release in ovariectomized and ovariectomized, steroid-primed rats

This study examined alterations in episodic LH release in response to prolonged, slow infusions of dopamine (DA), norepinephrine (NE), or epinephrine (EPIN) into the thire ventricle in adult, ovariectomized (OVX) rats, and the influence of priming with ovarian steroids on the LH response to the catecholamines. Unanesthetized rats with right atrial cannulae were bled continuously at slow rates for 1--1 1/2 h prior to infusion, 1--1 1/2 h during infusion, and up to 1 h afterwards. The amines were protected from oxidation by ascorbic acid, and infused in an artificial cerebrospinal fluid (CSF) vehicle (pH 7.29--7.33) into the third ventricle at a rate of 25--27 microliter/h. Blood samples were analyzed for LH by radioimmunoassay. In unprimed, OVX rats, infusions of artificial CSF, as well as low doses of DA (2--4 micrograms/h) or NE (0.3--0.6 micrograms/h), had no effect on episodic LH release or mean blood LH levels. However, administration of 8.5 and 17 micrograms DA/h, and 5.5 and 11 micrograms NE/h, resulted in a decrease in blood LH levels and, in most animals, prolonged intervals between peak blood LH levels during infusion or inhibitions which began rapidly and lasted for nearly the entire infusion period or longer. In contrast, infusion of 5.7 and 11.5 micrograms EPIN/h had no effect on blood LH levels in uprimed rats. In OVX rats primed 3 days prior to infusion with 50 micrograms estradiol benzoate and 25 mg progesterone (OEP), administration of CSF or the same doses of DA that previously inhibited episodic LH release had no effect on LH secretion. However, these steroids completely reversed the LH response to 11 micrograms NE/h, with increases in LH relase occurring during infusion. EPIN, in doses ineffective in unprimed rats (5.7 and 11.5 micrograms/h), also caused elevations in blood LH levels in EOP rats. Finally, in rats primed with 5 micrograms estradiol benzoate/100 g b.w./day for the 2 days prior to infusion, none of the three neurotransmitters had any effect on LH release.