In vitro and in vivo temporal aspects of ACTH secretion: stimulatory actions of corticotropin-releasing hormone and vasopressin in cattle.

The objective of the present study was to evaluate the temporal aspects associated with corticotropin-releasing hormone (CRH) and vasopressin (VP) stimulated bovine adrenocorticotropic hormone (ACTH) secretion in vitro and in vivo. For the in vitro studies, bovine anterior pituitary glands were enzymatically dispersed to establish primary cultures. On day 5 of culture, cells were challenged for 3 h with medium alone (Control) or various combinations and concentrations of bovine CRH (bCRH) and VP. Both CRH and VP each increased (P < 0.05) ACTH secretion. Maximal increases in ACTH secretion occurred in response to 0.1 microM CRH (5.5-fold) and 1 microM VP (3.7-fold), relative to Control cells. The in vivo portion of the study examined possible temporal differences in the activation of the pituitary-adrenal axis by CRH and VP. Jersey cows were randomly assigned to one of four groups (n = 8 cows/group): (i) Control (saline); (ii) bCRH (0.3 microg/kg BW); (iii) VP (1 microg/kg BW) and (iv) bCRH (0.3 microg/kg BW) + VP (1 microg/kg BW). Jugular blood samples were collected at 15-min intervals for 4 h pre- and for 6 h post-treatment; samples were also taken at 1, 5 and 10 min post-treatment. Plasma concentration of ACTH did not differ among treatment groups for the 4-h pre-treatment period. At 1 min post-treatment, bCRH + VP, VP and bCRH increased ACTH secretion by 22.4-, 9.6- and 2.2-fold, respectively, relative to Control (32.7 +/- 7.2 pg/ml). Maximal plasma concentration of ACTH occurred at 5, 10 and 15 min post-treatment for the VP (1017.7 +/- 219.9 pg/ml), bCRH + VP (1399.8 +/- 260.1 pg/ml) and bCRH (324.8 +/- 126.2 pg/ml) treatment groups respectively. Both the in vitro and in vivo data demonstrated that while VP acutely activates the bovine pituitary-adrenal axis, CRH-induced ACTH secretion is slower in onset but of longer duration. The present study also provides insight into the dynamics of ACTH and cortisol (CS) responsiveness to CRH and VP in cattle.

Visual documentation of ovine pituitary gland development with magnetic resonance imaging following zeranol treatment.

The objective of the current study was to determine whether magnetic resonance imaging (MRI) could be successfully utilized to document the effect of an oestrogenic anabolic agent on pituitary gland growth. The experimental animals consisted of two 1/2 sibling Suffolk wethers (castrated rams), which received either no implant (control, n = 1) or a 24 mg zeranol implant at day 0 and day 42 (zeranol; n = 1). Animals were anaesthetized with propofol and supported with oxygen during the MRI procedure. A mobile MRI unit with a 0.5 tesla (T), superconducting magnet was used to obtain 3 mm thick, non-contrast enhanced, T1-weighted (TR 500-600, TE25) sagittal, transverse and dorsal images of the pituitary gland. Sagittal images were recorded only when the mesencephalic aqueduct and infundibulum were distinctly visible in the same image. Pituitary glands were imaged at 14-day intervals for 70 days to determine if and when the anabolic effects of zeranol on pituitary gland growth could be visualized using MRI techniques. Three separate measurements of the pituitary gland dimensions made with the on-screen cursor were averaged to calculate pituitary gland dimensions and volume. A computer-assisted image analysis system and laser film images were used to determine pituitary gland area. Increases in pituitary gland volume for control and zeranol-treated animals were evident within 14 days, and by the end of the 70-day study, the increase in pituitary volume for the zeranol-treated animal was three times greater than that of the control animal. Overall, our results indicate that MRI technology can be successfully used to document the development of the pituitary gland in vivo. Application of knowledge gained from this novel approach to study the growth, development and function of endocrine glands over time, and within the same animal, will enhance human and animal endocrine diagnostic procedures.

Origin and ontogeny of mammalian ovarian neurons.

Mammalian ovaries contain sympathetic neurons expressing the low affinity neurotropin receptor (p75NTR). To date neither the role these neurons might play in ovarian physiology nor their embryological origin is known. Immunohistochemistry was used to detect postnatal changes in distribution and number of both p75NTR-positive and tyrosine hydroxylase-positive neurons in rhesus monkey ovaries. Pig fetuses were used to map the pathway of ovarian neuronal migration during embryonic development. Antiserum to p75NTR revealed the presence of isolated neurons and neurons clustered into ganglia in 2-month-old monkey ovaries. After 8 months, the neurons exhibited well-developed processes, and other than being more extensively interlaced, the localization and morphology did not change after 2 yr of age. Total number of p75NTR-positive neurons present decreased gradually between 2 months and 12 yr of age and declined markedly with reproductive aging. Conversely, the subpopulation of neurons immunoreactive to anti-tyrosine hydroxylase increased significantly at puberty and then declined with the loss of reproductive capacity. By d 21 of fetal life in the pig, p75NTR neurons had migrated medially from the neural crest to form the paraaortic autonomic ganglia. Some neurons migrated ventrally from the ganglia and then continued ventrolaterally to enter the genital ridge. By d 27, neurons had entered the developing ovary, and by d 35, the migration was complete with neurons demonstrating immunoreactivity to NeuN, a neuron-specific marker. Results demonstrate that p75NTR-expressing ovarian neurons originate from the neural crest and that a catecholaminergic subset is associated with pubertal maturation of the ovary and subsequent reproductive function.

Calcium/calmodulin-dependent protein kinase II involvement in release of gonadotropin-releasing hormone.

Involvement of calcium/calmodulin-dependent protein kinase II (CaM kinase II) in regulation of GnRH release was tested by determining the effect of CaM kinase II antagonists (KN-62 or KN-93) on GnRH release from rat or cattle infundibular (stalk median eminence) explants. Preincubation of male rat infundibular explants for 30 min with KN-62 (0.5, 1, 5 or 10 microM) 1.5 h prior to the addition of 59.3 mM (high) K+ resulted in a dose-dependent suppression of GnRH release. A longer pretreatment period (2 h) of rat infundibular explants with KN-62 (1 or 10 microM) appeared to enhance the suppressive effect of the CaM kinase II antagonist. Exposure (2 h) of rat infundibular explants to 10 microM, but not 0.1 microM KN-93, resulted in a complete inhibition of high K+-induced GnRH release. Exposure of steer infundibular explant halves to KN-62 (50 or 100 microM) or KN-93 (50 microM) inhibited high K+-induced GnRH release. Likewise, treatment of heifer infundibular explant halves with KN-93 (50 microM) abolished high K+-induced GnRH release. The period of exposure required for KN-62 to elicit its effect was relatively short since exposure of KN-62 (100 microM) for only 91-150 min of incubation was sufficient to block high K+-induced GnRH release from steer infundibular explant halves. In conclusion, these results: (1) support the hypothesis that CaM kinase II is involved in GnRH release from the rat and cattle infundibulum, (2) demonstrate that the effect of CaM kinase II on GnRH release from cattle infundibula is independent of reproductive state, (3) confirm previous reports supporting Ca2+ and CaM involvement in GnRH release from rat and cattle infundibula and (4) establish that infundibular explants incubated in vitro are useful for studying selected mechanisms regulating hypothalamic neurohormone release from neuron terminals.

Magnetic resonance imaging of the brain of normal neonatal foals.

Magnetic resonance imaging (MRI) was performed on the brain of 5 normal, anesthetized, neonatal (age 3-to-6 days) Quarter Horse foals. The objectives of the study were to develop a technique for imaging the brain of neonatal foals, and to ascertain their normal brain anatomy. Intravenous propofol was administered for induction and maintenance of general anesthesia. Using spin echo MR techniques, T1 weighted sagittal and transverse views, and spin density and T2 weighted transverse views were successfully made of each foal. MR images provided excellent visualization of many anatomic structures of the brain and head. MRI of the brain is feasible for selected neonatal equine patients.

Mifepristone modulation of ACTH and CRH regulation of bovine adrenocorticosteroidogenesis in vitro.

Mifepristone (RU486), bovine corticotropin-releasing hormone (CRH), arginine vasopressin (VP), adrenocorticotropin (ACTH1-24), and protein kinase activators (forskolin, [FSK]; phorbol 12-myristate 13-acetate [PMA]) were used in vitro to investigate their direct effect on adrenocorticosteroidogenesis. Bovine adrenocortical fasciculata/reticularis cells (2 x 10(5) viable cells/well) were cultured for 3 d in medium supplemented with 10% fetal calf serum. After incubation for an additional 24 hr in serum-free medium, cells were treated with serum-free medium alone (Control) or various concentrations of ACTH, CRH, VP, FSK, PMA, RU486, and/or various concentrations for 1, 2, 4, or 24 hr. Medium content of cortisol and progesterone were determined by radioimmunoassays. ACTH, CRH, FSK, and PMA each stimulated (P < 0.05) secretion of cortisol in time- and dose-related manners. Although these agents stimulated (P < 0.05) secretion of progesterone in a dose-related manner, medium content of progesterone declined (P < 0.05) over time. The minimal effective doses of ACTH and CRH required to stimulate (P < 0.05) secretion of cortisol relative to the Control over a 4-hr culture period were 0.01 nM and 3 nM, respectively. Relative to observations at 1 hr posttreatment, 24-hr treatment with ACTH or CRH increased the medium content of cortisol by an additional 19.8- and 48-fold, respectively (whereas content of progesterone declined over that time period). VP-stimulated secretion of cortisol was time- (P < 0.05) but not dose-related. Specifically, by 24-hr posttreatment, the medium content of cortisol was increased (P < 0.05) 4.6-fold relative to the quantity of cortisol secreted by 1-hr postaddition of VP (0.01 to 1 microM). Co-treatment with RU486 (1 microM) decreased (p < 0.05) FSK-, ACTH- and CRH-stimulated secretion of cortisol by 77, 27, and 56%, respectively. Similarly, the stimulatory effects of ACTH and CRH on progesterone secretion were reduced (P < 0.05) by 40 and 22%, respectively, by co-addition of RU486. The inhibitory action of RU486 on production of cortisol was no longer apparent by 24 hr after treatment. These observations indicate that RU486 can act as a steroid agonist and as well as an antagonist. These data characterize time- and dose-related direct actions of ACTH, CRH, and RU486 on adrenocorticosteroidogenesis. This information will assist efforts to clarify complex intra-adrenal interactions of neurohormones, growth factors, and endogenous steroids.

Calcium involvement in luteinizing hormone-releasing hormone release from the bovine infundibulum.

Bovine infundibular (stalk median eminence) explants were incubated in vitro to test the hypothesis that calcium (Ca) is involved in the release of luteinizing hormone-releasing hormone (LHRH) from LHRH neuron terminals in cattle. Right and left infundibular halves from individual heifers and/or steers were randomly assigned to either control or treated (EGTA [a Ca chelator] or verapamil [an L-type Ca channel antagonist]) groups. Each half was incubated in 600 microliters of Krebs-Ringer bicarbonate medium (KRB) in the presence or absence of a treatment agent for 180 min. At 30-min intervals, 500-microliters samples were removed from each incubated and replaced with fresh media. Spontaneous (basal) and depolarization-induced (60 mM potassium) LHRH release was evaluated by radioimmunoassay of the LHRH content in the media incubated from 91 to 120 and 121 to 150 min of culture, respectively. The effect of treatment on depolarization-induced LHRH release was analyzed by comparing the differences between spontaneous and depolarization-induced LHRH release in control and treated groups. Spontaneous LHRH release was not different between control and 1.25 mM EGTA- or 100 microM verapamil-treated halves from steers. In contrast, steer infundibular halves incubated with EGTA (replacing Ca in KRB and chelating any Ca in the media) released less LHRH during depolarization than did control halves. In addition, verapamil-treated (to block Ca uptake by the terminal) infundibular halves from steers or heifers released less LHRH in response to depolarization than did control halves.(ABSTRACT TRUNCATED AT 250 WORDS)

Luteinizing hormone-releasing hormone gene expression in the bovine brain: anatomical localization and regulation by ovarian state.

Experiments were conducted to identify neurons in the bovine brain that express the LHRH gene and to determine whether LHRH mRNA levels are influenced by the ovaries. Two groups of postpubertal heifers were utilized: heifers killed during the mid-luteal phase of the estrous cycle (LUTEAL, n = 5) and heifers killed 14-16 wk following ovariectomy (OVX, n = 5). In situ hybridization was performed through use of a 32P-end-labeled deoxyoligonucleotide (59 mer) complementary to the human LHRH mRNA sequence. LHRH-expressing neurons were identified in the diagonal band of Broca, the preoptic area, and the anterior hypothalamus in a manner consistent with immunocytochemical localization. Reduced silver grains, proportional to LHRH mRNA content, were quantified (in pixels, 45x objective) with an image analysis system. Expected serum hormone concentration differences between endocrine states were confirmed by radioimmunoassay for progesterone (LUTEAL > OVX, p < 0.01) and for LH (OVX > LUTEAL, p < 0.01). Compared to the OVX group, LUTEAL heifers had 34% fewer LHRH-expressing neurons (p < 0.05); on the average, these neurons possessed 28% fewer pixels/cell (p < 0.01), indicating fewer copies of LHRH mRNA per cell. When the numbers of pixels in all labeled cells were totalled, LUTEAL animals had 57% fewer pixels (p < 0.05) than did the OVX females--probably reflecting a decrease in LHRH synthetic capacity in the LUTEAL animals. Therefore, during the mid-luteal phase of the bovine estrous cycle, ovarian steroid (i.e., luteal progesterone) suppression of LHRH release (as reflected by serum LH) is coincident with decreased LHRH mRNA in the brain.(ABSTRACT TRUNCATED AT 250 WORDS)

A possible dual mechanism involved in the inhibitory influence of suckling on luteinizing hormone release in the ovariectomized rat.

A study was conducted to further understand involvement of the endogenous opioid peptides in suckling-induced inhibition of LH release in ovariectomized rats. The first experiment was designed to determine the effect of an opioid antagonist, naloxone (NAL, 1.0 mg. kg-1h-1), on the increase in peripheral LH concentration 18 h after pup removal and on the decrease in LH concentration 18 h after pup return. Infusion of NAL during the 18 h after pup removal or during the 18 h after pup return neither accentuated nor attenuated serum LH concentrations. The second experiment was designed to determine the effect of NAL on peripheral LH concentrations in continuously suckled rats. Serum LH increased (p less than 0.10 and p less than 0.005, respectively) in response to 18 and 36 h of NAL infusion. The third experiment was designed to determine the effect of pup removal during NAL infusion on serum LH. Peripheral LH concentrations were not different in the rats treated with 36 h of NAL infusion whether they were suckled for the duration of the infusion or nonsuckled for the last 18 h of infusion. These results suggest that suckling may inhibit LH release through two mechanisms. The first may be an opioid-independent or enhanced opioid tone mechanism important for the initiation of the inhibitory effect of suckling on LH release, while the second may be an opioid-dependent mechanism important for the sustained inhibitory effect of suckling on LH release.

A priming effect of naloxone on in vitro LHRH release from the hypothalamus of mid-luteal ewes.

The possible involvement of endogenous opioid peptides (EOPs) in LHRH release from hypothalami of ewes during the breeding season was investigated using an in vitro perifusion system. Hypothalami were procured in December from ovariectomized (OVX; 62-65 days before the experiment; n = 6) and mid-luteal (ML; n = 7) Western White-Face ewes. Hypothalami were mid-sagitally sectioned into halves containing the preoptic area, mediobasal hypothalamus, and infundibulum (median eminence). The left half (treated) received two 30-min challenges (beginning at 130 and 250 min, respectively, after onset of perifusion) of 500 microM naloxone (NAL) followed by a 30-min 60-mM potassium (K) challenge (at 370 min after onset of perifusion). The right half served as the control, receiving only K at the same time as the treated tissue. Both NAL challenges elicited (p less than 0.05) LHRH release from tissues of both ML and OVX ewes. Release of LHRH by hypothalami from ML, but not from OVX, ewes was greater (p less than 0.01) after the second than after the first NAL challenge. These results are consistent with the view that an inhibitory opioid influence exists on LHRH release from ovine hypothalami. The release of LHRH in response to NAL was dependent on the ovarian status in vivo since the priming effect of NAL on subsequent NAL-induced LHRH release occurred only from the hypothalami of ML ewes. We suggest from these results that EOPs may modulate LHRH release from ovine hypothalami in an ovarian steroid-dependent and independent manner.

Influence of dietary energy intake on prepubertal development of Brahman bulls.

Twelve Brahman bulls (paired by sire, weight and age) were assigned randomly and limit fed to gain either .10 to .25 (moderate gain; MG) or .75 to 1.0 (high gain: HG) kg.hd-1.d-1 to examine the effect of dietary energy on onset of puberty. Hip height (HH), scrotal circumference (SC) and serum samples (20 min for 6 h) were obtained at four times (AGE): 0, 56 and 112 d on feed and after appearance of first motile spermatozoa (FS) in the ejaculate of HG bull of the pair. At FS both bulls of a pair were slaughtered, reproductive tissues were collected and in vitro GnRH release from the median eminence (ME) was measured. Increases in BW, HH and SC were greater (P less than .05) in HG bulls. Basal ME GnRH secretion was greater (P less than .05) in HG bulls. Serum LH concentrations were unchanged by energy level (P greater than .10) but increased (P less than .01) with increasing AGE. AGE and energy level increased (P less than .01) basal, mean and total serum testosterone (T) and these two factors acted synergistically (P less than .01). Height and amplitude of T pulses were increased by energy level (P less than .003) and AGE (P less than .002). Testicular T (P less than .08) and development (P less than .05) were increased in HG bulls. Growth hormone peak height and amplitude concentrations following feeding increased with AGE (P less than .06) but were not altered (P greater than .10) by energy level. Serum triglycerides (P less than .03) and BUN (P less than .003) increased with increasing AGE (P greater than .01). These data indicate that dietary energy level influences onset of puberty most directly at the testicular level.

Modulation of growth hormone (GH) secretion and GH mRNA levels by GH-releasing factor, somatostatin and secretagogues in cultured bovine adenohypophysial cells.

The effects of endogenous hypothalamic neurohormones and activators of second messenger signalling systems on the secretion of GH and on cell content of GH mRNA of cultured bovine adenohypophysial cells were studied. Synthetic bovine GH-releasing factor (bGRF; 100 nmol/l) increased secretion of GH by bovine adenohypophysial cells five-fold relative to control. Forskolin (an adenyl cyclase activator; 10 mumol/l) and the synthetic cyclic AMP analogue dibutyryl cyclic AMP (dbcAMP; 1 mmol/l) increased secretion of GH by 1.9- and 1.7-fold respectively, relative to control. The protein kinase C activator phorbol 12-myristate 13-acetate (PMA), provided at 1 mumol/l or 10 nmol/l, increased GH secretion by 6.6- and four-fold respectively, relative to control. Somatostatin-14 (SRIF-14) attenuated basal, bGRF-, forskolin- and dbcAMP-stimulated secretion of GH by 40, 49, 47 and 67% respectively, but did not, however, diminish PMA-stimulated GH secretion. The content of GH mRNA in cultured bovine adenohypophysial cells increased 2.2-, 1.7- and 3.2-fold by administration of bGRF, forskolin and PMA respectively, relative to control. Although GH mRNA content was unchanged by SRIF-14 treatment relative to control, SRIF-14 did reduce bGRF-stimulated bGH mRNA content by 67%. This study demonstrates that mechanisms subserving GH secretion in bovine adenohypophysial cells (e.g. adenyl cyclase and protein kinase C) may be coupled with mechanisms which regulate expression of the GH gene or with factors affecting message stability.

Suckling inhibits release of luteinizing hormone-releasing hormone from the bovine median eminence following ovariectomy.

The effect of suckling on depletion of hypothalamic LHRH from the median eminence (ME) following ovariectomy (OVX) was determined in cattle. Multiparous, postpartum Holstein cows were assigned randomly to three groups: intact, nonsuckled (INT, n = 4); ovariectomized (3 to 5 d after parturition), nonsuckled (OVX, n = 4); and ovariectomized (3 to 5 d after parturition) and suckled by three calves (OVX-S, n = 5). Blood samples were collected at three periods (1 to 7 d before parturition and 3 to 5 d and 31 to 37 d after parturition) to determine plasma LH concentration. At 31 to 37 d after parturition, all cows were slaughtered and each ME was collected and mid-sagitally sectioned. The left half of each ME was used to determine content and concentration of LHRH. Concentrations of LH and LHRH were determined by RIA. Plasma LH concentration was similar among the three groups at 1 to 7 d before parturition and 3 to 5 d after parturition; however, at 31 to 37 d after parturition, OVX cows had a greater (P less than .05) concentration of LH (2.25 +/- .64 ng/ml) than either INT (.47 +/- .10 ng/ml) or OVX-S (.92 +/- .14 ng/ml) cows. Content of LHRH in the ME of INT (80.12 +/- 15.0 ng) and OVX-S 109.8 +/- 16.4 ng) cows was similar but was greater (P less than .05) than that in OVX cows (48.95 +/- 5.9 ng).(ABSTRACT TRUNCATED AT 250 WORDS)

LHRH and LH in peripubertal female rats following prenatal and/or postnatal ethanol exposure.

The effects of pre- and postnatal exposure to ethanol (ETOH) on LHRH and LH were investigated. Pregnant and/or lactating dams were fed ETOH during: 1) gestation, 2) lactation, or 3) gestation-lactation. Female offspring were decapitated at 30 or 40 days-of-age; trunk blood was collected for plasma LH RIA; and hypothalamic tissues were collected for LHRH RIA. Hypothalamic LHRH content of all ETOH-exposed groups was less than that of non-ETOH-fed controls at 30 and 40 days-of-age (p less than 0.05). Plasma LH concentrations of all ETOH-exposed groups were less than those of non-ETOH-fed controls at 30 and 40 days-of-age (p less than 0.05). Also, at 30 and 40 days-of-age, the plasma LH concentrations of the animals exposed to ETOH during lactation and gestation-lactation were less than those of the animals exposed to ETOH during gestation (p less than 0.05). These data suggest that ETOH exposure during gestation and/or lactation negatively affects hypothalamic LHRH content of female rat offspring. Decreased hypothalamic LHRH content with corresponding lowered plasma LH concentration suggests that ETOH influences development or maturation of hypothalamic LHRH neurons by possibly decreasing their number or synthesizing capability.

Epithalamus of the nine-banded armadillo, Dasypus novemcinctus.

The epithalamus of embryonic, neonatal and adult nine-banded armadillo (Dasypus novemcinctus) was examined for evidence of pineal-like tissue. The evagination of the diencephalic roof (the anlage of the epiphysis) was not found in any embryonic specimens. In the adult brain, the epithalamus is dominated by the sub-commissural organ (SCO) which is surrounded entirely by the posterior commissure. At the most caudal aspect of the SCO, previous investigators have observed pineal-like tissue. Using pineal-specific staining techniques however, we found no evidence of this tissue. Because the armadillo produces melatonin in a rhythmic manner, exhibits exacting circadian rhythms, and shows altered rhythms when exposed to exogenous melatonin, we believe other organs, perhaps the retina or Harderian gland, must be involved in maintaining the coordinated melatonin titer.

Effects of ethanol on hypothalamic luteinizing hormone-releasing hormone (LHRH) in the male rat. An immunocytochemical study.

These studies were designed to determine if the acute alcohol-induced decreases in luteinizing hormone (LH) seen in previous studies using rats could be due to an inhibitory effect of ethanol (ETOH) on hypothalamic LHRH release. Thus, effects of multiple injections of ETOH on the relative amount of immunoreactive LHRH fibers in the hypothalamus and median eminence (ME) of castrate and intact male rats were determined immunocytochemically. Brains were removed following cardiac perfusion of 10% phosphate-buffered formalin. A block containing the hypothalamus with the ME was isolated from each brain, then postfixed in Bouin's solution. Paraffin sections were rehydrated and stained for LHRH with the peroxidase-antiperoxidase technique using an antiserum to synthetic LHRH conjugated to bovine serum albumen. Differences visualized immunocytochemically between saline-treated intact and castrate rats indicated that the LHRH content of the ME was markedly depleted after castration. Conversely, castrate rats treated with ETOH showed only a slight reduction in immunoreactive LHRH fibers. In ETOH-treated intact animals, the LHRH fiber content of both the hypothalamus and ME appeared to be slightly greater than the saline-treated intact controls. Thus, these data support the hypothesis that ETOH diminishes LHRH release, and hence provides an explanation for the depressed plasma LH levels observed in ETOH-treated intact and castrate rats.

Inhibition of the postovariectomy depletion of hypothalamic luteinizing hormone releasing hormone (LHRH) by suckling.

A study was undertaken to further the understanding of the mechanism by which suckling inhibits the release of pituitary LH and depresses the postovariectomy rise of plasma LH in lactating mammals. To that end, the effect of suckling (10 pups/animal) for 1 or 3 weeks on the LHRH content of the hypothalamus and preoptic area (POA) in ovariectomized and intact rats was examined. Controls consisted of intact and 1 or 3 week ovariectomized, nonlactating animals. Following decapitation, the brains were rapidly removed and blocks containing the POA and the hypothalamus (with median eminence) were isolated. Tissue was extracted with acetic acid and LHRH quantitated via validated RIAs utilizing 2 antisera specific for different portions of the LHRH molecule. Ovariectomy of nonlactating, diestrous animals resulted in a significant decline in hypothalamic LHRH, reaching 50% of control levels by 3 weeks. During the same intervals, plasma LH increased dramatically to 20- and 60-fold over intact controls by 1 and 3 weeks, respectively. In contrast, LHRH levels were not decreased at 1 or 3 weeks in ovariectomized rats which were suckled, at which time plasma LH was greatly depressed. When intact animals were evaluated, the suckling stimulus failed to induce a detectable change in LHRH content of the hypothalamus of LHRH in the POA between any of the treatment or control groups. These data from ovariectomized rats suggest that suckling inhibits LHRH release from the hypothalamus and hence provides an explanation for the depression of plasma LH observed in suckled ovariectomized and intact animals.

Immunocytochemical evidence that suckling inhibits the postovariectomy depletion of median eminence luteinizing hormone releasing hormone.

Suckling has been demonstrated to impair the release of pituitary luteinizing hormone (LH) and to prevent the dramatic increase in plasma LH observed following ovariectomy. In the present study, the effect of suckling (10 pups/animal) for either 1 or 3 weeks on the relative amount of luteinizing hormone releasing hormone (LHRH) present in the hypothalamus and preoptic area of ovariectomized and intact rats was examined using immunocytochemical methodology. Controls consisted of nonlactating animals which were either intact (diestrous) or ovariectomized for 1 or 3 weeks. Brains were removed following transcardial perfusion of phosphate-buffered formalin and Bouin's fixative. After dehydration, clearing and paraffin embedding, the brains were sectioned and LHRH localized by an indirect immunoperoxidase technique. A positive reaction denoting the presence of immunoreactive LHRH was observed over axons and termini throughout the rostral to caudal extent of the median eminence (ME) and surrounding the organum vasculosum of the lamina terminalis (OVLT) in the preoptic area. Ovariectomy resulted in a progressive decline in the concentration of LHRH within the ME as evidenced by a reduction in the intensity of the staining reaction and in the number of axons over which the reaction was observed. In contrast, brains from ovariectomized rats which had been suckled appeared to have concentrations of LHRH in the ME equal to or greater than that of the diestrous controls. Similarly, the concentrations of LHRH In the ME of intact, suckled rats did not differ significantly from that of the diestrous controls. Neither ovariectomy nor suckling produced any observable change in the relative concentration of LHRH located near the OVLT. These data demonstrate that suckling prevents the depletion of LHRH from the ME following ovariectomy and provide evidence for mechanism by which the suckling stimulus may suppress plasma LH.