Reproductive endocrinology of free-living nestling and juvenile starlings, Sturnus vulgaris; an altricial species

Plasma concentrations of luteinizing hormone (LH), prolactin and testosterone, and pituitary content of LH and prolactin, were measured in free-living starlings, Sturnus vulgaris , from hatching until 12 weeks of age.
Plasma LH concentrations were elevated in both sexes until four days after hatching, then they decreased. Throughout the period, plasma LH levels were low compared to those in breeding adults but were comparable to levels in post-breeding photorefractory adults. Pituitary LH content increased until 12 days after hatching, but this increase was due to physical growth during this period. Plasma prolactin concentration and pituitary prolactin content increased dramatically during the nestling period. The increase in pituitary prolactin content was in excess of that accounted for by increasing size. Plasma prolactin remained high during the immediate post-fledging period, but had started to decrease by 12 weeks after hatching. Plasma testosterone concentrations were lower than those in breeding adults, but generally higher than in post-breeding photorefractory adults. The gonads of both sexes remained regressed.
These results suggest that the reproductive system of nestling and juvenile starlings is in a similar state to that of post-breeding photorefractory adult starlings. The comparatively high levels of testosterone may reflect involvement in sexual differentiation.     

Thyroidectomy results in termination of photorefractoriness in starlings (Sturnus vulgaris) kept in long daylengths

Four groups of 10 male starlings were transferred from short daylengths (8 h light/day) to long daylengths (18 h light/day), which caused the tests to develop rapidly to maximum size and then to decrease to minimal size as birds became photorefractory. Birds were surgically thyroidectomized at 8, 16 or 28 weeks. A fourth group was left intact. Testicular volume and plasma FSH and prolactin concentrations were measured. After 42 weeks all birds were castrated and plasma FSH was measured during the next 6 weeks. Testicular growth began in all thyroidectomized birds between 4 and 8 weeks after thyroidectomy. By 42 weeks, the testes of all thyroidectomized birds were large, whereas those of intact birds were still of minimal size. Plasma FSH concentrations remained low in all birds and plasma prolactin values, originally elevated by long daylengths, decreased at a similar rate in thyroidectomized and intact birds. After castration at 42 weeks, plasma FSH values increased rapidly in all thyroidectomized birds but remained low in non-thyroidectomized birds. The results demonstrate that thyroidectomy of photorefractory starlings does not induce immediate testicular growth but may initiate a process which eventually terminates photorefractoriness in a way similar to that caused by return to short daylengths.     

Thyroxine can mimic photoperiodically induced gonadal growth in Japanese quail

Summary Pharmacological doses of thyroxine are able to mimic the effects of long photoperiods in Japanese quail. In birds maintained on short daylengths thrice-weekly injections of 100 g thyroxine cause full testicular maturation at rates not greatly different from those seen if quail are exposed to long days. Thyroxine stimulates increases in the secretion of FSH and LH, in pituitary prolactin content and in the hypothalamic content of Gn-RH. The effects are dose-dependent. If female quail kept on short daylengths are given thyroxine their ovaries develop and they lay eggs. In castrated male quail on short days thyroxine causes a ten-fold increase in circulating LH within a week. Thyroxine injections are also capable of maintaining quail in a photorefractory state even when they are transferred to short daylengths. The results suggest that thyroxine mimics long days by acting high in the photoneuroendocrine system and does not simply act to facilitate hormone secretion per se. This is in line with growing evidence in mammals and birds that parts of the photoperiodic machinery are sensitive to thyroid hormones.Abbreviations Gn-RH gonadotropin releasing hormone - FSH follicle stimulating hormone - LH luteinizing hormone - T ₄ thyroxine     

Testosterone induction of song in photosensitive and photorefractory male sparrows

Song in male songbirds is activated by the sex steroid testosterone (T). Using male song sparrows (Melospiza melodia), we compared effects of T in the normal spring state of photosensitivity (i.e., when the pituitary-gonadal axis is sensitive to stimulation by increasing daylength) and in the late summer-early fall state of photorefractoriness (i.e., when they are insensitive to increasing daylength). Photosensitive males experienced short days for 8 weeks and then long days for another 22 weeks to induce photorefractoriness. T implants were given to the birds twice, first when on short days and photosensitive, and second when on long days and photorefractory. Song rates were compared among 5 conditions: (1) photosensitive, short days, low T titers; (2) photosensitive, short days, high T titers; (3) photosensitive, long days, high T titers; (4) photorefractory, long days, low T titers; and (5) photorefractory, long days, high T titers. Plasma levels of T were monitored throughout the experiment by radioimmunoassay. T was equally effective in inducing song in both the photosensitive and photorefractory conditions. Thus, no seasonal change was found in the sensitivity to hormone action of the neural target sites mediating this behavior in song sparrows. Photosensitive birds sang at a higher rate when on long days than when on short days, however, even though there was no concomitant increase in plasma levels of T. This finding suggests that environmental factors can alter the expression of song activated by similar levels of T.     

Hypothalamic Ventricular Ependymal Thyroid Hormone Deiodinases Are an Important Element of Circannual Timing in the Siberian Hamster (Phodopus sungorus)

Exposure to short days (SD) induces profound changes in the physiology and behaviour of Siberian hamsters, including gonadal regression and up to 30% loss in body weight. In a continuous SD environment after approximately 20 weeks, Siberian hamsters spontaneously revert to a long day (LD) phenotype, a phenomenon referred to as the photorefractory response. Previously we have identified a number of genes that are regulated by short photoperiod in the neuropil and ventricular ependymal (VE) cells of the hypothalamus, although their importance and contribution to photoperiod induced physiology is unclear. In this refractory model we hypothesised that the return to LD physiology involves reversal of SD expression levels of key hypothalamic genes to their LD values and thereby implicate genes required for LD physiology. Male Siberian hamsters were kept in either LD or SD for up to 39 weeks during which time SD hamster body weight decreased before increasing, after more than 20 weeks, back to LD values. Brain tissue was collected between 14 and 39 weeks for in situ hybridization to determine hypothalamic gene expression. In VE cells lining the third ventricle, expression of nestin, vimentin, Crbp1 and Gpr50 were down-regulated at 18 weeks in SD photoperiod, but expression was not restored to the LD level in photorefractory hamsters. Dio2, Mct8 and Tsh-r expression were altered by SD photoperiod and were fully restored, or even exceeded values found in LD hamsters in the refractory state. In hypothalamic nuclei, expression of Srif and Mc3r mRNAs was altered at 18 weeks in SD, but were similar to LD expression values in photorefractory hamsters. We conclude that in refractory hamsters not all VE cell functions are required to establish LD physiology. However, thyroid hormone signalling from ependymal cells and reversal of neuronal gene expression appear to be essential for the SD refractory response.     

Sexual maturation and moult in juvenile Starlings Sturnus vulgaris in response to different daylengths

Starlings, like most other species, show no gonadal development until spring of the year after they hatch, even though they hatch and attain full body size during long days. This could be because they develop in a physiological state analogous to that of photorefractory adults and so need to experience short days in order to activate the reproductive system. To test this possibility, young were hand-reared under different photoperiodic regimes. Young raised under constant long days showed no gonadal development, nor did birds initially raised under short days and then transferred to long days at 3 weeks of age. However, birds transferred from short to long days at 10 weeks of age did show gonadal development, followed by gonadal regression, while birds raised under constant short days showed slow continual gonadal development. This last group, unlike the other three groups, did not moult into adult plumage. Since 4 weeks of long days are required to terminate photorefractoriness in adult Starlings, these results demonstrate that the reproductive system of young birds is in a similar state to that of photorefractory adults, and hence that puberty is analogous to the termination of photofractoriness.     

Effect of histamine and acetylcholine on hypophysial stalk plasma dopamine and peripheral plasma prolactin levels.

To further define the role of dopamine in the regulation of prolactin secretion, we studied the effect on prolactin and hypothalamic dopamine secretion of histamine and acetylcholine (ACh) injected into the lateral ventricle of urethane anesthetized diestrus-1 rats. Histamine (10 μg) caused a 592% increase in plasma prolactin levels and a 26% decrease in stalk plasma dopamine levels. ACh (50 μg) caused a 2090% increase in plasma prolactin levels but no significant change in stalk plasma dopamine concentration.To determine if the 26% fall in stalk plasma dopamine following histamine administration could account for the 6-fold increase in plasma prolactin, we measured the effect on prolactin secretion of a similar decrease in administered dopamine. During an infusion of physiologic levels of dopamine, a 25% decrease in arterial plasma dopamine concentration resulted in only a 2-fold increase in prolactin secretion.The results of these experiments suggest that the effect of histamine on prolactin secretion may be mediated in part by decreased hypothalamic secretion of dopamine but that an additional hypothalamic hormone is probably involved. The stimulatory effect of ACh on prolactin secretion is not mediated by dopamine. These data are consistent with the growing evidence for the participation of multiple hypothalamic factors in the regulation of prolactin secretion.     

Endocrinological and behavioural effects of p-chlorophenylalanine (PCPA) oral administration to broody turkey hens (Meleagris gallopavo)

Changes in plasma levels of prolactin and LH, feed intake, water consumption, behavioural pattern and ovarian activity were recorded after oral administration of PCPA to broody turkey hens. A decrease in prolactin concentration was measured, from day 3, in 3 out of the 5 birds treated with 100 mg PCPA/kg body weight (BW) for 3 consecutive days. In these hens, broodiness was disrupted on day 6 and feeding activity subsequently increased to levels of photorefractory hens. Neither LH concentrations nor ovarian activity were affected after treatment with PCPA. Moreover, PCPA treatment was ineffective at a 50 mg/kg BW dose. These results confirm that a serotoninergic mechanism is probably involved in prolactin release and moreover suggest that prolactin is implicated in maintaining broody behaviour. However, the reductions in the plasma concentration of prolactin induced by PCPA were not sufficient to restore the hypothalamic-hypophyseal-ovarian axis to a physiological status characteristic of the laying hen. Therefore, PCPA does not appear to be a useful method of treating broodiness in commercial turkey hens.     

Growth hormone release inhibiting hormone: actions on thyrotrophin and prolactin secretion after thyrotrophinreleasing hormone.

The hypothalamic tetradecapeptide growth hormone release inhibiting hormone (GH-RIH) blocked the thyrotrophin response to thyrotrophin-releasing hormone (TRH) in normal people and in patients with primary hypothyroidism. This inhibition was dose related. The TRH-induced prolactin release was not affected by GH-RIH. This dissociation of the thyrotrophin and prolactin responses to TRH by GH-RIH suggests that there are different mechanisms for release of thyrotrophin and prolactin and that only the former is affected by GH-RIH.     

Reproductive photorefractoriness in rams and accompanying changes in the patterns of melatonin and prolactin secretion

Exposure of rams to alternating 16-week cycles of long and short days (16L:8D and 8L:16D) results in periods of testicular regression followed by testicular redevelopment, and there is an inverse relationship between the blood levels of prolactin and testis activity. In this study, two groups of rams were held under long or short day lengths for a period of 94 weeks. When held under either long or short days for more than 16 weeks, the animals showed spontaneous changes in gonadal activity and in the secretion of prolactin, both of which were no longer correlated with the prevailing photoperiod, i.e., they became photorefractory. The photorefractoriness was characterized by cyclical changes in testis function which were independent of day length. The period of these spontaneous cycles was similar during both treatment regimens (long days: 40.9 +/- 1.5 weeks; short days: 38.1 +/- 0.33 weeks), suggesting the presence of an endogenous pacemaker for the reproductive system. The changes in blood prolactin levels during photorefractoriness were no longer correlated with testis activity, and though cyclical, the period lengths differed under the two regimens (long days: 31.8 +/- 1.4 weeks; short days: 48.6 +/- 0.3 weeks). The rates of change in testis function and prolactin secretion were slower during the refractory state than during the photosensitive state. Upon switching the rams to a different photoperiod after the 94 weeks of exposure to fixed day lengths, the rams showed relatively rapid testicular and prolactin responses. Photoperiodic information appears to be relayed to the endocrine system through the daily pattern of melatonin secretion by the pineal. We measured the daily blood levels of melatonin on several occasions during phases of photosensitivity and photorefractoriness in the same group of rams. During the first 21 weeks under both lighting treatments, the rams showed synchronized daily patterns in their blood levels of melatonin, with elevated levels occurring mainly during the daily period of darkness. Similar synchronized daily rhythms were also seen when the rams were switched to a different photoperiod following 94 weeks of exposure to either long or short days. Between Weeks 21 and 94, the daily rhythms of melatonin did not occur consistently in all rams; often, the patterns differed markedly between individual rams held under the same day length and peak levels of melatonin were not always confined to periods of darkness.(ABSTRACT TRUNCATED AT 400 WORDS)     

Prolactin receptors in the brain during pregnancy and lactation: implications for behavior

Numerous studies have documented prolactin regulation of a variety of brain functions, including maternal behavior, regulation of oxytocin neurons, regulation of feeding and appetite, suppression of ACTH secretion in response to stress, and suppression of fertility. We have observed marked changes in expression of prolactin receptors in specific hypothalamic nuclei during pregnancy and lactation. This has important implications for neuronal functions regulated by prolactin. In light of the high circulating levels of prolactin during pregnancy and lactation and the increased expression of prolactin receptors in the hypothalamus, many of these functions may be enhanced or exaggerated in the maternal brain. The adaptations of the maternal brain allow the female to exhibit the appropriate behavior to feed and nurture her offspring, to adjust to the nutritional and metabolic demands of milk production, and to maintain appropriate hormone secretion to allow milk synthesis, secretion, and ejection. This review aims to summarize the evidence that prolactin plays a key role in regulating hypothalamic function during lactation and to discuss the hypothesis that the overall role of prolactin is to organize and coordinate this wide range of behavioral and neuroendocrine adaptations during pregnancy and lactation.     

Thyroxine treatment prevents the development of photosensitivity in european starlings (Sturnus vulgaris)

Summary In starlings, the breeding season is terminated by a state of photorefractoriness. Birds remain completely reproductively inactive as long as long days are maintained, and only exposure to short days restores photosensitivity. Two experiments investigated the role of different doses of thyroxine in the development of photosensitivity in castrated starlings. First, photorefractory castrated male starlings were moved from long (18L:6D) to short (8L:16D) days, and received in the drinking water either 1 or 10 mg · 1^-1 thyroxine for the first 7 weeks of a 14-week observation period. Control birds regained photosensitivity after 5 weeks of short days, as signaled by a spontaneous increase in plasma LH, whereas the return to photosensitivity was delayed until weeks 7 and 9 in the 1- and 10-mg · 1^-1 thyroxine-treated birds, respectively. In the second experiment, the effect of different doses of thyroxine was explored at the level of the hypothalamic Gn-RH neurosecretory neurones. The acquisition of photosensitivity in control birds transferred from long to short days was characterized by a marked increase in hypothalamic Gn-RH content (while long-day controls maintained low Gn-RH content). Doses of 10 and 20 mg · 1^-1 of thyroxine completely prevented the return to photosensitivity, as seen through changes in either plasma LH concentrations or hypothalamic Gn-RH content, while a dose of 1 mg · 1^-1 allowed a partial recovery of photosensitivity, as hypothalamic Gn-RH content increased to an intermediate level and the spontaneous rise in plasma LH occurred slowly but steadily.Abbreviations Gn-RH gonadotrophin-releasing hormone - LH luteinizing hormone - LHRH-I luteinizing hormone releasing hormone     

Tachykinins and the control of prolactin secretion

Tachykinins are present in the pituitary gland and in brain areas involved in the control of the secretion of pituitary hormones. Tachykinins have been demonstrated to stimulate prolactin release acting directly on the anterior pituitary gland. These peptides have also been revealed to be able to act at the hypothalamic level, interacting with neurotransmitters and neuropeptides that have the potential to affect prolactin secretion. Tachykinins seem to act by stimulating or inhibiting the release of the factors that affect prolactin secretion. Among them, tachykinins have been demonstrated to stimulate oxytocin and vasopressin release, which in turn results in prolactin release. Tachykinins also potentiated the response to vasoactive intestinal peptide (VIP) and reinforced the action of glutamate, which in turn result in prolactin release. They have also been shown to interact with serotonin, a neurotransmitter involved in the control of prolactin secretion. In addition, tachykinins have been shown to inhibit GABA release, a neurotransmitter with prolactin-release inhibiting effect. This inhibition may result in an increased prolactin secretion by removal of the GABA inhibition. On the other hand, tachykinins have also been shown to stimulate dopamine release by the hypothalamus, an action that results in an inhibition of prolactin release. Dopamine is a well known inhibitor of prolactin secretion. In conclusion, although tachykinins have been shown to have a predominantly stimulatory effect on prolactin secretion, especially at the pituitary level, under some circumstances they may also exert an inhibitory influence on prolactin release, by stimulating dopamine release at the hypothalamic level.     

Plasma gonadotropin,prolactin levels and hypothalamic tyrosine hydroxylase activity in rats during estrous cycle,after overiectomy and after blockade of catecholamine biosynthesis

Plasma gonadotropin, prolactin levels and hypothalamic tyrosine hydroxylase activity were evaluated at 0900, 1200 and 1700 h during diestrus, proestrus and estrus, ovariectomized and after systemic administration of reserpine or α-methyl p-tyrosine, which interfere with catecholamine biosynthesis, in rats. Gonadotropin and prolactin levels showed peak values during the afternoon of proestrus, while hypothalamic tyrosine hydroxylase activity was markedly lowered at 1200 on proestrus. Gonadotropin levels were slightly lowered whereas prolactin concentrations and hypothalamic tyrosine hydroxylase activity were significantly increased by reserpine. Depletion of hypothalamic dopamine by reserpine apparently resulted in significant elevation of prolactin levels which inturn induce tyrosine hydroxylase. Gonadotropin levels and hypothalamic tyrosine hydroxylase activity were significantly suppressed after the administration of α-methyl p-tyrosine. Prolactin levels, however, were elevated significantly. These results indicate that catecholamines are involved in the control of gonadotropin and prolactin release during estrous cycle and inhibition of catecholamines biosynthesis by α-methyl p-tyrosine could result in suppression of gonadotropin levels, whereas removal of tonic inhibition of hypothalamic dopamine by α-methyl-p-tyrosine elevate prolactin levels.     

The role of domperidone in the regulation of prolactin release in rats

Effects of domperidone, a dopamine antagonist, on prolactin release in female rats were studied. Oral administration of domperidone for 14 days caused a significant increase in serum prolactin levels in mature female rats. The routes by which domperidone exerted its effects on prolactin release were studied by a in vitro incubation system using rat pituitary tissues. Pituitary halves were incubated with (1) domperidone, (2) dopamine, (3) dopamine plus domperidone, (4) hypothalamic extracts from rats which had been treated with control meal (control hypothalamic extract), (5) control hypothalamic extract plus domperidone, and with (6) hypothalamic extract from rats which had been treated with domperidone for 14 days (domperidone-treated hypothalamic extract). Pituitary halves, when incubated alone, released a significant amount of prolactin into the incubation medium after 24 hours incubation, which was completely inhibited by dopamine or control hypothalamic extract. The addition of domperidone could not reverse the inhibitory effect of dopamine or control hypothalamic extract. On the other hand, domperidone-treated hypothalamic extract showed no inhibitory effects on prolactin release. These results indicated that domperidone could increase serum prolactin levels in female rats by acting primarily at the hypothalamus.     

Short-day experience is not a prerequisite for the termination of photorefractoriness in the reproductive cycle of baya weaver,Ploceus philippinus

In most photoperiodic avian forms (irrespective of temperate or tropical distribution) including the baya weaver,Ploceus philippinus, seasonal reproduction comes to an end due to the development of a photoperiodically controlled photorefractory phase when birds cease to respond to the stimulatory effect of long days. In the present paper photoperiodic control of the termination of photorefractory phase has been examined by studying the effect of short-day exposure lasting 4–6 months on long-day response of birds. Results indicate that unlike in other photoperiodic birds short-day exposure of winter is not a prerequisite for the termination of photorefractory phase in the reproductive cycle of baya weaver. Artificial long days on the other hand hasten the termination of this phase. Refractory phase in baya weaver, therefore, unlike that in temperate forms, is a temporary state resulting most likely from a sequel of physiological events triggered by long days of spring/summer which temporarily mask the photostimulatory response. Spontaneous termination of photorefractoriness in birds of tropical habitats may have a selective value imparting to the reproductive cycle the necessary elasticity for adaptation to diverse ecological conditions.     

Loss of hypothalamic dopaminergic control of prolactin secretion in the hyperprolactinaemic rat

The possibility that chronic hyperprolactinaemia results in loss of the ability of hypothalamic dopamine activity to inhibit prolactin secretion was studied in rats. Two degrees of hyperprolactinaemia (moderate and gross) were induced in the animals following the chronic administration of two different doses of oestradiol valerate. In rats with high chronic serum prolactin concentrations (approximately 20 times normal) there was a profound increase in prolactin secretion following inhibition of brain dopamine (DA) synthesis by 3-iodo-L-tyrosine, indicating intact and highly active hypothalamic DA-inhibitory control of prolactin release. However, the degree of hypothalamic inhibition of prolactin release relative to normal controls was significantly reduced. In animals with grossly elevated chronic serum prolactin concentrations (approximately 100 times normal) a prolactin response to DA synthesis inhibition was absent despite a highly significant reduction in hypothalamic DA concentrations induced by 3-iodo-L-tyrosine. These observations show that chronic and gross hyperprolactinaemia in the rat results in loss of hypothalamic DA inhibitory control of prolactin secretion. The use of 3-iodo-L-tyrosine to block brain DA synthesis in these studies has provided significant new data relating to prolactin control in hyperprolactinaemic states and indicates that this compound could be a useful clinical tool in the study of human hyperprolactinaemia.     

Expression of vasoactive intestinal peptide receptor messenger RNA in the hypothalamus and pituitary throughout the turkey reproductive cycle

Vasoactive intestinal peptide (VIP) has been implicated in the regulation of avian reproductive activity and appears to act at the level of the hypothalamus and pituitary. This in situ hybridization histochemistry study describes the distribution of VIP receptor mRNA expression in the hypothalamus and the pituitary of reproductively active (laying) and quiescent (nonphotostimulated, incubating, and photorefractory) female turkeys and characterizes the differences observed in VIP receptor gene expression. VIP receptor mRNA, while expressed throughout the hypothalamus, was specifically expressed in areas known to contain GnRH-I neurons in the chicken, i.e., the lateral septum, medial preoptic area, anterior hypothalamus, and paraventricular nucleus. Significant differences in VIP receptor mRNA expression between different reproductive states was observed only within the infundibular nuclear complex. VIP receptor mRNA was markedly less in nonphotostimulated and photorefractory hens as compared with laying and incubating hens. The most dense VIP receptor mRNA was found in the anterior pituitary, where it was 2.4- and 3.0-fold greater in laying and incubating hens, respectively, as compared with that in nonphotostimulated ones. Hens that stopped incubating and became photorefractory displayed pituitary VIP receptor mRNA levels similar to those of nonphotostimulated birds. The changes in pituitary VIP receptor mRNA expression were positively correlated with known changes in pituitary prolactin (PRL) mRNA expression and PRL content and release. These findings indicate that the variations in PRL secretion observed across the turkey reproductive cycle are, in part, regulated by changes in VIP receptors at the pituitary level.     

Enhanced luteinizing hormone release by luteinizing hormone-releasing hormone in incubating female turkeys (Meleagris gallopavo)

To determine what role pituitary responsiveness plays in the suppression of gonadotropin level during incubation in the turkey, the ability of the pituitary to release luteinizing hormone (LH) in response to luteinizing hormone-releasing hormone (LHRH) was compared in incubating, laying, and photorefractory birds. In all three groups, the i.m. injection of LHRH (4 micrograms/kg) increased serum LH levels; however, the LH response was markedly enhanced in the incubating turkeys as compared with the laying (6.6-fold increase over preinjection levels vs. 1.9-fold; p less than 0.05) or the photorefractory birds (9.7-fold vs. 3.1-fold; p less than 0.05). The LHRH-induced LH release was also determined in turkeys as they shifted from the laying to the incubating phase of the reproductive cycle. This response increased (p less than 0.05) in magnitude as the birds started to incubate. The high prolactin level of incubating turkeys does not have a depressing effect on LHRH-stimulated LH release; thus, impaired LH response to LHRH is not a mechanism involved in the diminished gonadotropin secretion of incubating turkeys.     

The role of monoamines in female puberty

The estradiol positive feedback mechanism appears to become mature between days 10 and 20 after birth. Rising serum prolactin levels between day 20 after birth and puberty are correlated with high hypothalamic norepinephrine turnover. High prolactin levels stimulate hypothalamic dopamine (DA) turnover, which may actively inhibit hypothalamic luteinizing hormone-releasing hormone (LHRH) release. Hypothalamic DNA receptor sensitivity is high in 10- to 20-day-old rats and gradually decreases between day 20 after birth and puberty. The reason for this desensitization may be the high hypothalamic DA turnover. This may result in a less strong inhibition of LHRH release allowing the positive feedback action of estradiol to elicit the first preovulatory luteinizing hormone (LH) surge initiating puberty.