Ovariectomized Rhesus Monkeys Research Articles

Gonadotropin responsiveness to estrogens and low-level progesterone in the ovariectomized rhesus monkey

When estrogen is administered to gonadectomized rhesus monkeys in sufficient quantity, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels are initially suppressed and gonadotropin secretion is subsequently augmented. This study was designed to examine the ability of various orally administered estrogens to elicit biphasic pituitary responses in adult castrate rhesus monkeys and to investigate the role of low-level progesterone in altering these responses. In an incomplete block design, 13 treatments were constructed: (E 2): 0.5, 5.0, 50 μg/kg; (DES): 0.05, 0.5, 5.0 μg/kg; zearalenone (Z): 10, 100, 1000 μg/kg; or oil vehicle, each combined with progesterone. The remaining treatments (5.0, 0.5, and 100 μg/kg of E 2, DES, and Z, respectively) did not include progesterone. During six intervals, E 2, DES, or Z was administered orally on Days 1, 2, and 3 and blood was sampled on Days 0, 3, and 8 of each period. A main effect of estrogen treatment was observed for LH and FSH secretion on Day 3 but not Day 8. There was no main effect of progesterone alone on release of either LH or FSH on days 3 or 8, nor were significant carry-over effects of progesterone, E 2, DES, or Z on LH or FSH concentration apparent by Day 8 of any treatment. However, progesterone synergized with E 2 to suppress Day 3 LH levels. Conversely, DES and Z interacted with progesterone to facilitate LH secretion at this time. FSH was not altered by the combination of progesteron and E 2, DES, or Z but was dose-dependently influenced by estrogen alone, with maximally suppressive effects on Day 3 occurring at the high dose level of E 2, DES, and Z. We have demonstrated a fundamental animal bioassay model that, in addition to existing models, might be used to monitor and predict the physiologic consequences of human exposure to estrogenic compounds from the food chain. These data also indicate that low blood levels of progesterone may synergize with estrogens to selectively stabilize estrogen-induced pituitary responses.

Dexamethasone Treatment Prevents the Inhibitory Effect of Corticotropin-Releasing Hormone on Gonadotropin Release in the Primate

Corticotropin-releasing hormone (CRH) has been shown to inhibit gonadotropin secretion and this effect is mediated by endogenous opioid peptides, presumably stimulated by CRH. Since glucocorticoids are known to block the CRH-induced ACTH response, it can be hypothesized that by concurrently preventing endogenous opioid peptide release, they would also prevent the inhibitory action of CRH on gonadotropin secretion. We tested this hypothesis in 4 ovariectomized rhesus monkeys, pretreated with dexamethasone (DEX; 1.5 mg b.i.d. for 5 days). In experiment 1, the effects of a 5 h i.v. hCRH infusion with or without DEX pretreatment and of physiological saline were compared. Blood samples were taken at 15-min intervals during a 3 hour preinfusion control and throughout the infusion. Sera were assayed for luteinizing hormone (LH), follicle-stimulating hormone (FSH) and cortisol by RIA. In the absence of DEX pretreatment, LH and FSH levels were progressively decreased during the CRH infusion: by hour 5, LH and FSH areas under the curve were 34.1 ( +/- 7.6) and 65.3% ( +/- 2.5) (mean % of preinfusion control values; + SE), respectively (p less than 0.01 vs. saline). In contrast, DEX pretreatment prevented the CRH-induced gonadotropin decrease: by hour 5, LH and FSH areas under the curve were 91.9 ( +/- 9.0) and 99.0% ( +/- 5.7) (n.s. vs. saline). In experiment 2, we tested whether DEX-treated monkeys would remain responsive to the gonadotropin inhibitory action of an opiate agonist. After a 3 hour preinfusion control baseline, morphine (9 mg i.v.) was given as a bolus injection to the same 4 animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Endometrial responses to a deciduogenic stimulus in ovariectomized rhesus monkeys treated with oestrogen and progesterone.

To establish an artificial model for the study of decidual cell reaction in the rhesus monkey, long-term ovariectomized animals were treated with oestrogen followed by progesterone and an artificial deciduogenic stimulus was applied on day 16 of the treatment cycle. Histological examinations of the endometrial samples on different days following stimulation showed prominent epithelial plaque reaction by day 20. Marked subepithelial oedema surrounding plaque cells was also noted. On day 32, degenerating plaque cells with leucocytic infiltration were found. Beginning on day 24 of hormone treatment, stromal fibroblasts adjacent to plaque and glands showed swelling with rounding-up and enlargement of nuclei; these stromal cells appeared more decidual-like with moderate amounts of hypertrophy and few were binucleate by day 28. Endometrial granular cells became more conspicuous and appeared to increase in numbers adjacent to glands and blood vessels. Endometrial glands showed no notable changes, and were mostly tortuous with columnar epithelium and apocrine-type secretions. Around days 40 and 48 of the treatment cycle, the luminal margins of the glands became bosselated, and significant amounts of secretion were present in the glandular lumen. Measurement of immunoreactive concentrations of oestradiol-17 beta and progesterone in endometria from traumatized, hormone-treated monkeys revealed significant (P less than 0.01) decreases in levels of oestradiol with time while the level of progesterone remained unchanged (P greater than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Prolactin and gonadotrophin dynamics in response to antagonists of LHRH and dopamine in ovariectomized rhesus monkeys: a dissection of their common secretion.

This experiment was designed to study the prolactin (PRL) and gonadotrophin response to an inhibitory analogue of luteinizing hormone releasing hormone (LHRH-A; ORG 30276) and a dopamine antagonist, metoclopramide (MCP), in castrated female rhesus monkeys. The LHRH-A given as an i.v. bolus followed by an infusion 2 h later induced a significant decline in circulating PRL levels reaching a maximum suppression 1 h after initiation of the LHRH-A infusion. Intravenous administration of MCP, performed 1 h after beginning the LHRH-A infusion, resulted in a prompt and abrupt rise in PRL levels in the LHRH-A group as well as in the controls. Luteinizing hormone (LH) and follicle stimulating hormone (FSH) levels showed a progressive decline during LHRH-A treatment. Injection of MCP did not affect the levels of LH and FSH in either of the groups. The data obtained demonstrate in primates that the prolactin inhibitory effect of LHRH-A is easily overcome by the dopamine antagonist, MCP, whereas the LHRH-A-induced gonadotrophin suppression remains unaffected by MCP.

Luteinizing hormone releasing hormone (LHRH) neuroterminals mapped using the push-pull perfusion method in the rhesus monkey

In vivo luteinizing hormone releasing hormone (LHRH) release was measured in conscious, ovariectomized rhesus monkeys using a push-pull cannula inserted into the stalk-median eminence, and the relationship between sampling location and LHRH release was examined. Within an individual animal in which multiple experiments were conducted with different cannula placements, LHRH pulse frequency was consistent. In contrast, LHRH pulse amplitude and mean LHRH release varied with cannula tip location in a pattern which reflected the anatomical distribution of LHRH-immunoreactive fibers described for the rhesus monkey. These results suggest that our push-pull perfusion method is reliable for the in vivo measurement of LHRH and perhaps other neuropeptides and/or neurotransmitters, as well.

The inhibitory action of corticotropin-releasing hormone on gonadotropin secretion in the ovariectomized rhesus monkey is not mediated by adrenocorticotropic hormone.

Earlier observations in our laboratory indicated that i.v. infusion of human/rat corticotropin-releasing hormone (hCRH) suppresses pulsatile luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release in ovariectomized rhesus monkeys. Since cortisol secretion increased significantly as well, it was not possible to exclude the possibility that this inhibitory effect of hCRH on gonadotropins was related to the activation of the pituitary/adrenal axis. The purpose of the present study was to determine the role of pituitary/adrenal activation in the effect of hCRH on LH and FSH secretion. We compared the effects of 5-h i.v. infusions of hCRH (100 micrograms/h, n = 7) and of human adrenocorticotropic hormone (ACTH) (1-24) (5 micrograms/h, n = 3; 10 micrograms/h, n = 3, 20 micrograms/h, n = 3) to ovariectomized monkeys on LH, FSH, and cortisol secretion. As expected, during the 5-h ACTH infusions, cortisol levels increased by 176-215% of baseline control, an increase similar to that observed after CRH infusion (184%). However, in contrast to the inhibitory effect observed during the CRH infusion, LH and FSH continued to be released in a pulsatile fashion during the ACTH infusions, and no decreases in gonadotropin secretion were observed. The results indicated that increases in ACTH and cortisol did not affect LH and FSH secretion and allowed us to conclude that the rapid inhibitory effect of CRH on LH and FSH pulsatile release was not mediated by activation of the pituitary/adrenal axis.

Endogenous Opioid Peptides Modulate the Effect of Corticotropin-Releasing Factor on Gonadotropin Release in the Primate*

Stress can induce endocrine abnormalities and menstrual dysfunction in the primate. Here, we examine the effects that CRF, the principal neurohormone in control of the hypothalamic-pituitary-adrenal axis, exerts on pulsatile gonadotropin secretion and the role that the endogenous opioid peptides may play in this phenomenon. Ovariectomized rhesus monkeys were given a 5-h continuous iv infusion of physiological saline (2 ml/h), human CRF (100 micrograms/2 ml . h), or hCRF plus the opiate receptor antagonist naloxone (2 mg/2 ml/h; 5 mg in two experiments; n = 7 experiments/group). LH and FSH concentrations were measured at 15-min intervals for a 3-h preinfusion baseline control, during the 5-h infusion, and during a 2-h postinfusion observation period, while cortisol concentrations were measured at frequent intervals during the entire experiment. CRF infusion produced a progressive and significant decrease in both LH and FSH. Mean areas (+/- SE) under the LH and FSH curves during the 5-h CRF infusion, expressed as a percentage of preinfusion baseline, were 59.9 +/- 4.6% and 83.0 +/- 3.1% (+/- SE), respectively (P less than 0.001 and P less than 0.01 vs. saline controls). Large amplitude LH pulses were abolished during the CRF infusion. However, after cessation of CRF infusion, there was a rapid resumption of LH pulsatile release in four of the seven experiments. Addition of naloxone to CRF prevented the CRF-mediated suppression of LH and FSH release. Mean areas for LH and FSH during the 5-h combined infusion were 100.3 +/- 6.6% and 99.6 +/- 4.3% of the preinfusion baseline, respectively (P less than 0.001 and P less than 0.05 vs. CRH alone; NS vs. saline), and pulsatile LH secretion was maintained. Regardless of whether naloxone was administered, CRF increased cortisol levels significantly. Mean cortisol levels at the end of the CRF and CRF plus naloxone infusions were 48.2 +/- 10.4 and 52.9 +/- 7.4 micrograms/dl (+/- SE), respectively, compared to 21.0 +/- 3.0 with saline (P less than 0.05). These results demonstrate that in the ovariectomized rhesus monkey, CRF suppresses the secretion of both LH and FSH, and this effect can be sustained. They also indicate that the CRF inhibitory action on gonadotropin is primarily mediated by endogenous opioid peptides, independent of glucocorticoid levels.

Corticotropin-releasing hormone inhibits gonadotropin secretion in the ovariectomized rhesus monkey.

To evaluate whether the compromised gonadotropin secretion frequently occurring during stressful conditions in the primate may be related to an inhibitory action of CRH, the effects of ovine (oCRH) or human (hCRH) CRH on gonadotropin and cortisol secretion were studied in ovariectomized rhesus monkeys. LH secretion (assessed as area under the curve) decreased 35% and 21%, and cortisol increased 37% and 90%, 1-3 h after single iv injections of 200 and 500 micrograms oCRH, respectively (P less than 0.05 vs. pre-CRH control period; n = 4-7/dose). Single injections of 200 and 500 micrograms hCRH, respectively, resulted in 35% and 24% decreases in LH and 40% and 79% increases in cortisol secretion (P less than 0.05). Injections of 100 micrograms oCRH and hCRH elicited significant (P less than 0.05) increases in cortisol release (37% and 31%, respectively), but did not affect LH secretion. A 5-h infusion of hCRH (100 micrograms/h) reduced LH levels (23%, 49%, 59%, 61%, and 62% during the first through the fifth hour, respectively; P less than 0.05 for hours 2-5). FSH secretion also decreased during the hCRH infusion (26%, 33%, 42%, 46%, and 49% during the first through the fifth hour, respectively; P less than 0.05 for hours 3-5), while cortisol increased 76%. These data demonstrate that exogenous CRH administration results in inhibition of LH and FSH secretion in ovariectomized rhesus monkeys. These results are consistent with the hypothesis that elevated CRH levels could contribute to decreased LH and FSH secretion and, thus, disruption of reproduction function under conditions of stress in primates.

The Effect of Season on the Induction of Sexual Behavior by Estradiol in Female Rhesus Monkeys1

Rhesus monkeys housed in an outdoor environment are seasonal breeders, with ovulations and concomitant sexual behavior limited to the fall and winter months. To determine if there is a seasonal difference in the capacity of physiological levels of estradiol (E2) to induce sexual behavior in ovariectomized rhesus monkeys housed outdoors, subjects living in a social group were treated with subcutaneous E2 implants in a counter-balanced design during the nonbreeding season (May-July) and during the breeding season (September-November). Serum E2 levels were monitored by obtaining blood samples twice a week. Three levels of E2 were studied: baseline (less than 30 pg/ml), follicular (100 pg/ml), and periovulatory (200 pg/ml). Two of five adult males in the group were injected with human chorionic gonadotropin (hCG) twice a week to insure that males with high testosterone levels were present during each season. Focal observations of behavior of 15 minutes' duration on each subject were conducted 5 days per week, with frequencies and durations of social, sexual, and solitary behaviors recorded. Concomitant 2-h group scans were made to record all occurrences of mounting behavior. Neither heterosexual serial mounting nor seminal plugs were ever observed in E2-treated females during the summer months. In contrast, copulation and seminal plugs were observed in subjects at both treatment levels during the fall. While E2-treated females engaged in homosexual mounting behavior during both summer and fall, E2 treatment resulted in heterosexual copulation only during the fall. Changes in patterns of social behavior paralleled changes in sexual behavior and were significantly affected by treatment and season, with more male-female interactions during the fall months.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular and tissue mechanisms of primate uterine endometrial renewal: an hypothesis.

Recent advances in hormonal regulation of cyclic uterine differentiation and preimplantation biology are evident in the remarkable announcements that normal live primate newborns can be produced through artificial menstrual cycles in ovariectomized rhesus monkeys and women (1–3). Such fertile menstrual cycles are produced in rhesus monkeys by a program of insertion and removal of subcutaneous silastic implants of estradiol and progesterone that serve as effective “artificial ovaries” until the placenta takes over the maintenance of pregnancy. These tangible demonstrations provide new stimulus for experimental analysis of endometrial cell and tissue compartments in the primate (rhesus) uterus with regard to the hormonal mechanisms that control cyclic renewal.

Release rates of catecholamines, GABA and beta-endorphin in the preoptic area and the mediobasal hypothalamus of the rhesus monkey in push-pull perfusates: correlation with blood hormone levels.

Push-pull cannulae were implanted into the preoptic area and into the mediobasal hypothalamic median eminence complex of ovariectomized rhesus monkeys. After recovery, perfusion of the implanted areas was performed over a period of 56 h before and following estradiol benzoate treatment. This treatment results in a drop of LH levels followed by an increase. Catecholamine (norepinephrine, epinephrine and dopamine) concentrations in perfusates collected at 15 min intervals fluctuated tremendously prior to treatment with the estrogen. These fluctuations were largely reduced in perfusates of both structures following the estrogen treatment. They reoccurred at the time of increasing LH levels. beta-endorphin and GABA concentrations were also measured in the perfusates of both structures. Occasional secretory bursts were observed without any obvious relation to the estrogen treatment. It is concluded that catecholamine release in the preoptic area and in the mediobasal hypothalamic median eminence complex is of a pulsatile nature in ovariectomized rhesus monkeys. This pulsatility is largely reduced or abolished following estrogen treatment. The reduced pulsatility may bear a signal character for the release of LH.

Sites of action of testosterone in the brain of the female primate.

Thaw-mount autoradiography was used to map target neurons for tritiated steroids in the brains of ovariectomized rhesus monkeys. In five females, studies were performed 60 min after the administration of 3H-estradiol. Of these, two females were pretreated for five days with daily injections of 2 mg testosterone propionate, one female was pretreated with daily injections of 2 mg dihydrotestosterone propionate, and two females were pretreated with oil vehicle. For comparison, two additional females were administered 3H-testosterone and were killed 30 min later. Following 3H-estradiol administration in females pretreated with oil, there were numerous labeled neurons in the medial preoptic nucleus (n.), anterior hypothalamic area, ventromedial hypothalamic n., arcuate n., bed n. of stria terminalis, lateral septal n., and regions of the amygdala. In females pretreated with testosterone propionate, labeling was virtually abolished in all these areas with the exceptions of the lateral septal n. and arcuate n., presumably because estradiol formed from testosterone competed with 3H-estradiol for estrogen-binding sites. Pretreatment with dihydrotestosterone propionate had no effect on the labeling following the administration of 3H-estradiol. Areas where labeling was blocked, therefore, represent regions where testosterone acted as an estrogen, and it would appear that regional differences in the activity of aromatizing enzymes determine which estrogen target sites in the brain can interact with estrogenic metabolites of testosterone in female primates.

The effect of morphine on the electrophysiological activity of the hypothalamic luteinizing hormone-releasing hormone pulse generator in the rhesus monkey.

In ovariectomized rhesus monkeys, the electrophysiological manifestation of luteinizing hormone-releasing hormone (LHRH) pulse generator activity was arrested by morphine and reinitiated by naloxone. These responses were noted within 1 and 2 min, respectively, after the intravenous injection of the drugs. Naloxone given alone had no effect. These results support the view that opioids modulate pulsatile gonadotropin release by an action on the hypothalamic LHRH pulse generator.

Gonadotropin-Releasing Hormone Pulses in Third Ventricular Cerebrospinal Fluid of Ovariectomized Rhesus Monkeys: Correlation with Luteinizing Hormone Pulses*

Morphological evidence suggests that GnRH may be released into cerebrospinal fluid (CSF) of the third ventricle. Therefore, a method of cannulating the third ventricle of monkey brains was developed for the purpose of examining GnRH secretion in primates. A stainless steel guide cannula was stereotaxically implanted into the third ventricle of 14 ovariectomized rhesus monkeys. A Silastic cannula for collecting CSF was inserted via the guide cannula into the ventral portion of the ventricle, permitting repeated CSF sampling for long time periods from the same animal. One week to 6 months after cannulation, CSF was collected continuously for periods of 5-10 h at 2 different rates (480 and 120 microliter/h) from conscious monkeys seated in chairs. Samples were divided into 15-min fractions, and the GnRH concentration in each was determined by RIA. In contrast to most previous studies, third ventricular CSF was found to contain significant concentrations of GnRH. GnRH was detected in 40 of 50 collections. Concentrations ranged from less than 8 to greater than 800 pg/ml, a range similar to that observed in hypophyseal portal blood. Furthermore, fluctuations within individual collections indicated that GnRH was released in pulses. The mean GnRH pulse frequency during the higher rate of CSF withdrawal was 0.43 +/- 0.06 pulses/h (n = 31), while the mean pulse amplitude was 91 +/- 7 pg/ml (n = 64). Neither parameter was influenced by the rate of CSF removal, as frequency was 0.52 +/- 0.08 pulses/h (n = 19) and amplitude was 94 +/- 11 pg/ml (n = 82) during the lower collection rate. However, the CSF withdrawal rate had a profound influence on LH secretion. In 12 of 17 collections at the higher rate, LH levels plummeted to undetectable concentrations during the first 2 h of CSF exfusion and remained low throughout the collection period. Pituitary responsiveness was not reduced, as a GnRH bolus (0.25 or 2.5 micrograms) after 6 h of CSF removal elicited a dose-dependent stimulation of LH secretion. In contrast, a higher incidence of normal pulsatile LH secretion (12 of 19 collections) was observed when the CSF withdrawal rate was reduced. During these 12 collections, LH and GnRH pulses occurred at regular intervals and exhibited similar pulse frequencies (mean +/- SE, 0.76 +/- 0.07 and 0.67 +/- 0.09 pulses/h for LH and GnRH, respectively). Most GnRH and LH pulses were synchronized, as 86% of all GnRH pulses (43 of 50) were accompanied by a LH pulse.(ABSTRACT TRUNCATED AT 400 WORDS)

Testosterone administration to ovariectomized female rhesus monkeys (Macaca mulatta) reduces the frequency of pulsatile luteinizing hormone secretion.

In the male rhesus monkey testosterone (T) retards the frequency of intermittent LH secretion. The purpose of the present study was to determine whether this action of T is demonstrable in the female. Five ovariectomized rhesus monkeys, bearing indwelling cardiac catheters, were implanted s.c. on one or more occasions with T-containing Silastic capsules. Sequential blood samples were collected for 8 h every 10 min before T treatment and usually at 1, 2, 4, and 8 days thereafter. Plasma LH concentrations were measured in duplicate by radioimmunoassay and subsequently analyzed with a computerized algorithm. Sustained increments in circulating T (5-13 ng/ml) in ovariectomized monkeys resulted in a progressive reduction in LH pulse frequencies from approximately 1 pulse every 60 min before initiation of T treatment to 1 pulse every 100-150 min at 48 h thereafter. In most cases the deceleration in pulsatile gonadotropin secretion continued, and by 4-8 days of T treatment LH pulse frequencies as low as 1 pulse every 5 h were observed. The onset of the T-induced deceleration in LH pulse frequency was generally associated with an increase in LH pulse amplitude and with a decline in mean LH levels. This LH response in the female to T treatment was similar to that previously reported for male castrates.

Electrophysiological manifestation of luteinizing hormone-releasing hormone pulse generator activity in the rhesus monkey: influence of alpha-adrenergic and dopaminergic blocking agents.

Characteristic increases in neuronal electrical activity associated with the initiation of each LH pulse were recorded from ovariectomized rhesus monkeys bearing multiple chronic electrodes in the medial basal hypothalamus. These electrophysiological manifestations of hypothalamic LHRH pulse generator activity were inhibited by the alpha-adrenergic blocker phentolamine or the alpha 1-adrenoceptor blockers phenoxybenzamine and prazosin. At the dosages used, the effects of single injections of these drugs ranged from a reduction in the frequency of LHRH pulse generator activity to its complete arrest. This was faithfully reflected in the pattern of pulsatile LH discharges. The dopaminergic blocking agent metaclopramide similarly reduced the frequency of the pulse generator or arrested its activity altogether. The alpha 2-adrenoceptor blocker yohimbine had no demonstrable effect on hypothalamic electrical activity at the doses studied. These findings support the view of a central action of alpha 1-adrenergic and dopaminergic blockade on LHRH pulse generator activity and the concept that central adrenergic and dopaminergic inputs can modulate the frequency of the LHRH pulse generator.

Separate negative feedback effects of estrogen on the pituitary and the central nervous system in the ovariectomized rhesus monkey.

The site(s) of the negative feedback action of estrogen on gonadotropin secretion were studied in the ovariectomized rhesus monkey by observing the serum LH and FSH responses to intravenous GnRH injections at various times after implantation of Silastic capsules filled with estrogen. Circulating estrogen concentrations produced by the capsules were within the normal midcycle range for this species. Four h after estrogen implantation, no LH or FSH response was seen to the GnRH injection, indicating a suppressive effect of the steroid directly on the pituitary. Twelve and 22 h after estrogen implantation, however, the LH and FSH responses were equal to or larger than control responses. Since preinjection LH and FSH levels were below control values at these times and the pituitary responded to exogenous GnRH, it appears that endogenous GnRH secretion was affected, indicating an inhibitory action of estrogen on the central nervous system. Thus these experiments suggest 2 separate negative feedback actions of estrogen: a transient one directly on the pituitary and a longer lasting effect on the central nervous system.

Different prolactin, thyrotropin, and thyroxine responses after prolonged intermittent or continuous infusions of thyrotropin-releasing hormone in rhesus monkeys.

Serum PRL, TSH, and T4 secretion during prolonged continuous or intermittent iv infusions of TRH were studied in 14 adult ovariectomized rhesus monkeys (Macaca mulatta). For 9 days, TRH was administered intermittently at 0.33 or 3.3 micrograms/min for 6 of every 60 min and continuously at 0.33 micrograms/min. With both modes, the PRL levels and responsiveness to TRH simulation peaked on day 1 and then fell to levels that were still higher than the preinfusion values; levels for the intermittently treated group on days 3-9 were 2- to 4-fold above prestimulation levels and significantly (P less than 0.01) higher than levels for the continuously treated group. Elevated basal levels and PRL responses to TRH pulses were similar during the 0.33 and 3.3 micrograms/min pulses of the 9-day treatment period. For both TRH modes, TSH levels were elevated significantly (P less than 0.001) on day 1 [this increase was higher with continuous infusion (P less than 0.001)] and then fell to preinfusion levels by day 3. Serum T4 also increased during both continuous and intermittent TRH stimulations. However, serum T4 levels were significantly lower (P less than 0.01) after intermittent TRH (both 0.33 and 3.3 micrograms/min) than after continuous (0.33 micrograms) TRH (8 +/- 1.1 and 10 +/- 1.8 micrograms T4/dl vs. 18 +/- 3.1 micrograms, respectively). These PRL and T4 responses were replicated when the mode of administering 0.33 micrograms/min TRH was reversed after 9 days. An iv bolus of TRH (20 micrograms) after 9 days of continuous or intermittent TRH infusion caused significant release of PRL and TSH, an indication that neither mode of administration resulted in pituitary depletion of releasable hormone. We have concluded that intermittent TRH is more effective in elevating serum PRL, and continuous TRH is more effective in raising TSH and T4 levels. Thus, the manner of TRH secretion by the hypothalamus may determine its relative physiological importance in the stimulation of lactotropes and thyrotropes.

Behavior of rhesus monkeys during 28-day cycles of estrogen treatment.

Sixteen ovariectomized rhesus monkeys were paired, each with two males, during thirty-two 28-day periods during which they received one or more of the following schedules of daily injections: (a) different mixtures of estradiol benzoate, progesterone, and testosterone propionate (artificial menstrual cycles), (b) the doses of estradiol benzoate given in artificial cycles but without the other hormones (cyclic estradiol regimens), and (c) appropriate control treatments (625 one-hour behavioral tests). Hormone doses were adjusted to give plasma levels in the physiological range (335 blood samples). Measures of male sexual activity during artificial cycles and during cyclic estradiol regimens showed significant cyclic changes, but these changes were indistinguishable from each other. In four females studied in an operant conditioning paradigm, there was a midcycle dip in the times taken to obtain access to males during the cyclic estradiol regimens. The data indicated that the estrogen state of the female plays a preponderant role in determining the cyclic changes in the behavioral interactions of the pair independently of the effects of other ovarian hormones.

Relationship between prolactin in the serum and cerebrospinal fluid of ovariectomized female rhesus monkeys

The entry of prolactin into the cerebrospinal fluid from the blood, and the relation between levels in the two compartments were studied under a variety of conditions in ovariectomized rhesus monkeys. Prolonged treatment with either domperidone or sulpiride, both dopamine-receptor blockers, elevated prolactin levels in serum and cerebrospinal fluid proportionately equally, so that the cerebrospinal fluid/serum ratio was unchanged from controls (circa 12–20%). Gel filtration showed that only ‘little’ (monomeric) prolactin entered the cerebrospinal fluid in such monkeys. Following acute elevations of blood prolactin after a single injection of either sulpiride or ovine prolactin, cerebrospinal fluid levels increased linearly over a 90 min sampling period, despite falling serum levels. The rate of entry of prolactin into the cerebrospinal fluid was similar after either procedure, and was independent of absolute serum or cerebrospinal fluid levels, suggesting a rate-limiting mechanism. Furthermore, retrograde portal blood flow from the pituitary is not necessary to account for these results. Clearance of prolactin from the third ventricle was studied following intraventricular injection of prolactin. Prolactin was removed from the cerebrospinal fluid by a mechanism whose efficiency compares with that in serum, so that the half-life of prolactin in either compartment is about the same. Measurement of sodium, potassium and calcium in the cerebrospinal fluid during prolonged hyperprolactinaemia showed no change, indicating that the central effects of prolactin are not due to alteration of these electrolytes. These results show that cerebrospinal fluid levels of prolactin, and hence, those surrounding the brain, can be inferred accurately from those in the blood, and suggest that there may be a selective mechanism regulating the entry of prolactin into the cerebral compartment.