Seasonally Dependent Effect of Ectopic Pituitary Grafts on 24-Hour Rhythms in Serum Prolactin and Gonadotropins in Rats

To assess to what extent the presence of an ectopic pituitary differentially affected circulating prolactin (PRL) and gonadotropin levels at different times of the year, rats kept under 12h light, 12h dark (12:12 LD) photoperi-ods and receiving a pituitary graft or a sham operation in summer or winter were examined 3 months later. In both male and female sham-operated rats, a circadian variation in serum PRL levels was found, with an acrophase varying from 21:53h to 00:54h and the mesor and amplitude higher in spring than autumn in males and higher in autumn than in spring in females. After grafting a pituitary, changes in serum PRL related to time of day were no longer observed. In pituitary-grafted male rats killed during spring, serum PRL levels were higher than controls at only a few time points throughout the 24h cycle, whereas in rats killed during autumn, there were no significant differences in PRL levels between grafted and control rats. Pituitary-grafted female rats killed during spring showed serum PRL levels significantly higher than those of sham-operated rats, while in female rats killed in autumn, PRL levels of pituitary-grafted and sham-operated rats did not differ. Significant variations of luteinizing hormone (LH) related to time of day were found in sham-operated male rats only, with acrophases at 23:52h and 00:24h for spring and autumn, respectively, and the mesor and amplitude of the rhythm significantly higher in autumn. Pituitary transplants suppressed 24h variations in circulating LH and depressed its levels during the two seasons examined. As far as follicle-stimulating hormone (FSH), pituitary grafts decreased circulating levels, with the extent of decrease higher during autumn than in spring. The results indicate that some endocrine consequences of the grafting of an ectopic pituitary are dependent on time of year.     

Seasonally Dependent Effect of Ectopic Pituitary Grafts on 24-Hour Rhythms in Serum Prolactin and Gonadotropins in Rats

To assess to what extent the presence of an ectopic pituitary differentially affected circulating prolactin (PRL) and gonadotropin levels at different times of the year, rats kept under 12h light, 12h dark (12:12 LD) photoperi-ods and receiving a pituitary graft or a sham operation in summer or winter were examined 3 months later. In both male and female sham-operated rats, a circadian variation in serum PRL levels was found, with an acrophase varying from 21:53h to 00:54h and the mesor and amplitude higher in spring than autumn in males and higher in autumn than in spring in females. After grafting a pituitary, changes in serum PRL related to time of day were no longer observed. In pituitary-grafted male rats killed during spring, serum PRL levels were higher than controls at only a few time points throughout the 24h cycle, whereas in rats killed during autumn, there were no significant differences in PRL levels between grafted and control rats. Pituitary-grafted female rats killed during spring showed serum PRL levels significantly higher than those of sham-operated rats, while in female rats killed in autumn, PRL levels of pituitary-grafted and sham-operated rats did not differ. Significant variations of luteinizing hormone (LH) related to time of day were found in sham-operated male rats only, with acrophases at 23:52h and 00:24h for spring and autumn, respectively, and the mesor and amplitude of the rhythm significantly higher in autumn. Pituitary transplants suppressed 24h variations in circulating LH and depressed its levels during the two seasons examined. As far as follicle-stimulating hormone (FSH), pituitary grafts decreased circulating levels, with the extent of decrease higher during autumn than in spring. The results indicate that some endocrine consequences of the grafting of an ectopic pituitary are dependent on time of year.     

Hepatic and pulmonary cytosolic metabolism of epoxides effects of aging on conjugation with glutathione

In liver cytosol from male Fischer 344 rats, glutathione S-transferase specific activities with six epoxide substrates were lower in the 24-month-old (senescent) group than in the 3-month-old (young) group. With lung cytosol from males and liver and lung cytosol from females, specific activities declined with only some of the substrates. Age-related increases in protein content in male and female rat liver occurred by 12 months of age (middle-age) and remained elevated through senescence. In addition, increases in liver weights in males similarly occurred so that total metabolic rates tended to be highest in middle-aged males and similar in young and senescent groups. Few changes similar to these were found in liver cytosol from females or lung cytosol from males or females. Thus, tissue-, sex-, and substrate-specific alterations in epoxide metabolism occurred during aging.     

Intrahypothalamic pituitary grafts elevate prolactin in the cerebrospinal fluid and attenuate prolactin release following ether stress

Anterior pituitary (AP) tissue grafted into the hypothalamus of female rats inhibits the luteotrophic prolactin (PRL) secretion which normally follows mating. Dopamine blockade has been shown to overcome this inhibition, suggesting that the grafts suppress PRL release from the in situ pituitary by the action of graft PRL increasing dopamine activity in the hypothalamus. To examine whether PRL levels in the cerebrospinal fluid (CSF) were elevated by the AP grafts, CSF samples were taken from 5 control rats and 10 rats bearing intrahypothalamic AP grafts. Mean PRL concentrations in the CSF of the control rats were 3.0 +/- 0.8 ng/ml. The grafted rats had significantly higher concentrations of PRL in their CSF, averaging 23.2 +/- 4.2 ng/ml (P less than 0.005). Plasma PRL concentrations were similar in the control and grafted rats. PRL release in response to 5 min of ether stress was examined in 8 control and 11 grafted rats. In control animals, PRL rose from 4.2 +/- 1.5 to 44.7 +/- 9.0 ng/ml following exposure to ether, but the response was significantly attenuated in the grafted rats, peaking at 9.3 +/- 1.4 ng/ml (P less than 0.001). This inhibition of response due to the grafts was evident within 1 week of graft placement. The results confirm that the presence of intrahypothalamic AP grafts led to the accumulation of supranormal PRL concentrations in the CSF. This elevated PRL suppressed pituitary PRL release in response to ether stress, probably by an autoregulatory feedback activation of the inhibitory tuberoinfundibular dopaminergic neurons in the hypothalamus.     

Robust growth hormone (GH) secretion in aged female rats co-administered GH-releasing hexapeptide (GHRP-6) and GH-releasing hormone (GHRH)

Aging is associated with a blunted growth hormone (GH) secretory response to GH-releasing hormone (GHRH), in vivo. The objective of the present study was to assess the effects of aging on the GH secretory response to GH-releasing hexapeptide (GHRP-6), a synthetic GH secretagogue. GHRP-6 (30 micrograms/kg) was administered alone or in combination with GHRH (2 micrograms/kg) to anesthetized female Fischer 344 rats, 3 or 19 months of age. The peptides were co-administered to determine the effect of aging upon the potentiating effect of GHRP-6 on GHRH activity. The increase in plasma GH as a function of time following administration of GHRP-6 was lower (p less than 0.001) in old rats than in young rats; whereas the increase in plasma GH secretion as a function of time following co-administration of GHRP-6 and GHRH was higher (p less than 0.001) in old rats than in young rats (mean Cmax = 8539 +/- 790.6 micrograms/l vs. 2970 +/- 866 micrograms/l, respectively; p less than 0.01). Since pituitary GH concentrations in old rats were lower than in young rats (257.0 +/- 59.8 micrograms/mg wet wt. vs. 639.7 +/- 149.2 micrograms/mg wet wt., respectively; p less than 0.03), the results suggested that GH functional reserve in old female rats was not linked to pituitary GH concentration. The differential responses of old rats to individually administered and co-administered GHRP-6 are important because they demonstrate that robust and immediate GH secretion can occur in old rats that are appropriately stimulated. The data further suggest that the cellular processes subserving GH secretion are intact in old rats, and that age-related decrements in GH secretion result from inadequate stimulation, rather than to maladaptive changes in the mechanism of GH release.     

Relationship between bioassay and radioimmunoassay measurement of prolactin in the IPL rat, a hypoprolactinemic rat strain

IPL (Institut Pasteur, Lyon) nude, hypoprolactinemic rats exhibit delayed puberty and a complete lack of lactation. To characterize the secretion of circulating forms of prolactin (PRL) of these rats, PRL concentrations were measured in serum and pituitaries of males and females under various physiological conditions. Two assay methods, a radioimmunoassay (RIA) and a sensitive bioassay (NB2BA) were employed. Normal rats of the Sprague-Dawley strain were tested simultaneously, as controls. The pituitary content of PRL, estimated either by RIA or by NB2BA, in IPL nude males and females was similar to that of normal male and female rats. On the contrary, serum PRL levels of IPL male rats, measured by RIA or NB2BA, were significantly reduced when compared to normal rats. In both groups, there was a close correlation between the results obtained by the two methods, the NB2BA estimates being higher. However, the NB2BA/RIA ratio was significantly decreased in serum from IPL nude rats compared to controls, indicating that the circulating form of PRL was less bioactive in this group. Castrated male rats injected with estradiol showed sharply increased PRL values as estimated by RIA or NB2BA. The increase was greater (35-fold) in IPL nude rats then in normal rats (9-fold), but these increases resulted in serum PRL levels being similar in the two groups. However, the NB2BA/RIA ratio remained significantly reduced in IPL nude rats. In female rats, PRL was measured during different physiological states: estrus, diestrus, proestrus at 1000, 1200, and 1600 h and Days 1 and 21 of gestation and 2 days postpartum.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential changes in the mechanisms controlling luteinizing hormone and prolactin secretion in middle-aged female rats

Serum luteinizing hormone (LH) and prolactin (PRL) concentrations were measured in young (3-4 month old) and middle-aged (10-12 month old) intact female rats on proestrus, in ovariectomized rats after two estrogen injections (estradiol benzoate; EB, 10 micrograms/100 g body weight, s.c.) or after preoptic stimulation in EB-primed ovariectomized rats. Only animals showing regular 4-day estrous cycles were selected for the experiment. The magnitude of proestrous LH surge was significantly smaller in middle-aged than in young rats. Two BE injections, at noon on Days 0 and 3, in ovariectomized middle-aged rats failed to induce surges in LH secretion on Day 4 whereas the same treatment produced LH surges in ovariectomized young rats. The preoptic electrochemical stimulation (50 microA for 60 sec) produced a prompt rise in serum LH levels in ovariectomized EB-primed young but not in middle aged rats. The preoptic stimulation with a larger current (200 microA) induced LH secretin in middle-aged rats. In none of these situations serum PRL concentrations were different between young and middle-age rats. These results suggest differential aging rates in the preoptic mechanisms governing LH and PRL secretion in the rat. The function of the preoptic ovulatory center in responding to the estrogen positive feedback action and inducing LH secretion may become impaired and independent of the PRL control mechanism, even before the regular estrous cycle terminates.     

Sexual dimorphism in the renin-angiotensin system in aging spontaneously hypertensive rats

In young adult spontaneously hypertensive rats (SHR), mean arterial pressure (MAP) is higher in males than in females and inhibition of the renin-angiotensin system (RAS) eliminates this sex difference. After cessation of estrous cycling in female SHR, MAP is similar to that in male SHR. The purpose of this study was to determine the role of the RAS in maintenance of hypertension in aging male and female SHR. At 16 mo of age, MAP was similar in male and female SHR (183+/-5 vs. 193+/-8 mmHg), and chronic losartan (40 mg.kg-1.day-1 po for 3 wk) reduced MAP by 52% (to 90+/-8 mmHg, P<0.05 vs. control) in males and 37% (to 123+/-11 mmHg, P<0.05 vs. control) in females (P<0.05, females vs. males). The effect of losartan on angiotensin type 1 (AT1) receptor blockade was similar: MAP responses to acute doses of ANG II (62.5-250 ng/kg) were blocked to a similar extent in losartan-treated males and females. F2-isoprostane excretion was reduced with losartan more in males than in females. There were no sex differences in plasma renin activity, plasma angiotensinogen or ANG II, or renal expression of AT1 receptors, angiotensin-converting enzyme, or renin. However, renal angiotensinogen mRNA and protein expression was higher in old males than females, whereas renal ANG II was higher in old females than males. The data show that, in aging SHR, when blood pressures are similar, there remains a sexual dimorphism in the response to AT1 receptor antagonism, and the differences may involve sex differences in mechanisms responsible for oxidative stress with aging.     

Differential involvement of protein kinase C in basal versus acetylcholine-regulated prolactin secretion in rat anterior pituitary cells during aging

Although it is well known that plasma concentration of prolactin (PRL) increases during aging in rats, how the anterior pituitary (AP) aging per se affects PRL secretion remains obscure. The objectives of this study were to determine if changes in the pituitary PRL responsiveness to acetylcholine (ACh; a paracrine factor in the AP), as compared with that to other PRL stimulators or inhibitors, contribute to the known age-related increase in PRL secretion, and if protein kinase C (PKC) is involved. We also determined if replenishment with aging-declined hormones such as estrogen/thyroid hormone influences the aging-caused effects on pituitary PRL responses. AP cells were prepared from old (23-24-month-old) as well as young (2-3-month-old) ovariectomized rats. Cells were pretreated for 5 days with diluent or 17beta-estradiol (E(2); 0.6 nM) in combination with or without triiodothyronine (T(3); 10 nM). Then, cells were incubated for 20 min with thyrotropin-releasing hormone (TRH; 100 nM), angiotensin II (AII; 0.2-20 nM), vasoactive intestinal peptide (VIP; 10(-9)-10(-5) M), dopamine (DA; 10(-9)-10(-5) M), or ACh (10(-7)-10(-3) M). Cells were also challenged with ACh, TRH, or phorbol 12-myristate 13-acetate (PMA; 10(-6) M) following PKC depletion by prolonged PMA (10(-6) M for 24 h) pretreatment. We found that estrogen priming of AP cells could reverse the aging-caused effects on pituitary PRL responses to AII and DA. In hormone-replenished cells aging enhanced the stimulation of PRL secretion by TRH and PMA, but not by AII and VIP. Aging also reduced the responsiveness of cells to ACh and DA in suppressing basal PRL secretion, and attenuated ACh inhibition of TRH-induced PRL secretion. Furthermore, ACh suppressed TRH-induced PRL secretion mainly via the PMA-sensitive PKC in the old AP cells, but via additional mechanisms in young AP cells. On the contrary, basal PRL secretion was PKC (PMA-sensitive)-independent in the old AP cells, but dependent in the young AP cells. Taken together, these results suggest differential roles of PMA-sensitive PKC in regulating basal and ACh-regulated PRL responses in old versus young AP cells. The persistent aging-induced differences in AP cell responsiveness to ACh, DA, TRH, and PMA following hormone (E(2)/T(3)) replenishment suggest an intrinsic pituitary change that may contribute, in part, to the elevated in vivo PRL secretion observed in aged rats.     

Hypothalamic luteinizing hormone-releasing hormone (LHRH) and LH secretion in aging female rats

Age-related changes in hypothalamic luteinizing hormone-releasing hormone (LHRH) and luteinizing hormone (LH) secretion were studied in young (6 months), middle-aged (12 months) and old (18 months) female rats. The LHRH levels in the mid-hypothalamic area were higher in intact middle-aged and old females than in young ones. Additionally, there was no age difference in the hypothalamic LHRH levels in male rats. In order to clarify the significance of this age-related increase in female rats, we examined the effects of progesterone treatment in estrogen-primed ovariectomized young and old rats on the LHRH levels in the median eminence (ME) and on plasma LH levels. We found phasic changes in ME-LHRH and plasma LH levels in estrogen-primed rats following progesterone treatment in rats of both ages, but the progesterone-induced change in ME-LHRH levels tended to be delayed in old rats compared with young females. This delay may correspond to the delayed onset, slow and low magnitude of plasma LH increase in old females. The ME-LHRH levels were generally higher in old rats than in young rats. Nevertheless, we found that the increase in plasma LH in response to progesterone treatment in estrogen-primed ovariectomized females was smaller in old rats than young rats. These results suggest that the LHRH secretory mechanism changes with age in female rats. Such alterations may result in the accumulation of LHRH in the mid-hypothalamic area and an increase in ME-LHRH.     

Age-related alterations in prolactin secretion by individual cells as assessed by the reverse hemolytic plaque assay

The objectives of this study were to determine: 1) if lactotropes from old rats, compared to those from young rats, secrete a greater amount of prolactin (PRL) per cell, 2) if the percentage of pituitary cells secreting PRL changes with age; and 3) how estradiol (E2), dopamine (DA), or thyrotropin-releasing hormone (TRH), or the combination of these factors influences both of these parameters in old rats. To meet these objectives we used the reverse hemolytic plaque assay (RHPA), because this method allows us to determine both the percentage of pituitary cells secreting prolactin during the experimental period and the amount of hormone released by each secreting pituitary cell. These parameters were measured in young (2-3 mo old) or old (17-19 mo old) female Sprague-Dawley rats. Animals were ovariectomized (OVX) for 10 days or OVX for 1 wk and then treated with E2 for 3 days. Rats were killed, anterior pituitaries were removed, and cells were enzymatically dispersed and prepared for use in the RHPA. Pituitary cells were treated in vitro with vehicle, DA, or PRL, old OVX and E2-treated rats exhibited a greater percentage of secretory cells than young at both 1 and 2 h of incubation. Administration of E2 increased the percentage of cells secreting PRL in both young and old rats. DA reduced the percentage of cells secreting PRL at the highest dose tested (10(-5) M) regardless of age or E2 status following incubation for 1 h.(ABSTRACT TRUNCATED AT 250 WORDS)     

Gender differences on the effects of aging on cardiac and peripheral adrenergic stimulation in old conscious monkeys

We examined the effects of gender and aging on cardiac and peripheral hemodynamic responses to beta-adrenergic receptor (beta-AR) stimulation in young (male = 5.9 +/- 0.4 yr old and female = 6.5 +/- 0.7 yr old) and old (male = 19.8 +/- 0.7 yr old and female = 21.2 +/- 0.2 yr old) conscious monkeys (Macaca fascicularis), chronically instrumented for measurements of left ventricular (LV) and arterial pressures as well as cardiac output. Baseline LV pressure, the first derivative of LV pressure (LV dP/dt), cardiac index, mean arterial pressure, total peripheral resistance (TPR), and heart rate in conscious monkeys were not different among the four groups. Increases in LV dP/dt in response to 0.1 microg/kg isoproterenol (Iso) were diminished (P < 0.05) in old males (+99 +/- 11%) compared with young males (+194 +/- 18%). In addition, the inotropic responses to norepinephrine (NE) and forskolin (FSK) were significantly depressed (P < 0.05) in old males. Iso-induced reductions of TPR were less (P < 0.05) in old males (-28 +/- 2%) than in young males (-49 +/- 2%). The changes of TPR in response to NE and FSK were also significantly attenuated (P < 0.05) in old males. However, the LV dP/dt responses to BAY y 5959 (15 microg. kg-1. min-1), a Ca2+ channel promotor independent of beta-AR signaling, were not significantly different between old and young males. In contrast to results in male monkeys, LV dP/dt and TPR responses to Iso, NE, and FSK in old females were similar to those observed in young females. Thus both cardiac contractile and peripheral vascular dynamic responses to beta-AR stimulation are preserved in old female but not old male monkeys. This may explain, in part, the reduced cardiovascular risk in the older female population.     

Immuno-electron-microscopic study of the prolactin cells in the pituitary gland of male Wistar rats during aging

Summary Prolactin cells were identified by means of immunocytochemistry with protein-A gold as a marker on ultrathin sections of the pituitary gland of young (3–4 months), middle-aged (16–19 months), and aged (26–30 months) male Wistar rats. Point-counting volumetry revealed that the prolactin (PRL) cell-volume density in middle-aged rats was significantly increased in comparison to the volume densities in young and aged rats. Within the PRL-cell population, four types of PRL cells were distinguished on the basis of the shape and size of their secretory granules. During aging, dramatic changes occurred in the relative volumes of the four cell types. The volume percentage of cells with round granules (type I, granule diameter 150–250 nm, and type IIA, granule diameter 250–350 nm) increased from ±30% in young rats to ±90% in old rats. The volume percentage of cells with round and polymorphic granules (type IIB; granule diameter 350–400 nm and type III; granule diameter 500–600 nm) decreased from ±70% in young rats to ±7% in old rats. Age-related changes in serum PRL levels were not found. It is concluded that although during the life span of the male Wistar rat considerable changes in PRL-cell volume densities and in the ratios of PRL-cell types occur serum, PRL levels remain more or less constant.     

Effect of aging on corticosterone secretion in diestrous rats

The roles of age and prolactin (PRL) in regulating glucocorticoid secretion in diestrous rats were investigated. Adrenal zona fasciculata-reticularis (ZFR) cells from young, adult, middle (mid)-aged, and old female rats were isolated. Estrous cycle stage was determined by light microscopy after vaginal smears. Blood samples were collected from right jugular vein at 0, 30, 60, and 120 min after challenge with adrenocorticotropin (ACTH). During the diestrous phase, plasma levels of estradiol and progesterone were lower in mid-aged and old rats than in either young or adult rats. Age-dependent increases of the basal levels of plasma PRL and corticosterone were observed. No difference of ACTH-increased plasma concentrations of corticosterone was observed among young, adult, mid-aged, and old rats. Aging increased the basal, ACTH-, PRL-, forskolin (an adenylate cyclase activator)-, and 3-isobutyl-l-methylxanthine (IBMX, a non-selective phosphodiesterase inhibitor)-stimulated release of corticosterone and production of adenosine 3', 5'-cyclic monophosphate (cAMP) in ZFR cells. However, the 8-Br-cAMP (a membrane-permeable cAMP)-stimulated release of corticosterone was not affected by age. Taken together, these data indicated that aging increased corticosterone secretion in female rats during diestrous phase, which is in part due to an increase in cAMP accumulation. In conclusion, aging and PRL play a stimulatory role in the co-regulation of corticosterone secretion.     

Sexual Dimorphism in Development of Kidney Damage in Aging Fischer-344 Rats

BackgroundAging kidneys exhibit slowly developing injury and women are usually protected compared with men, in association with maintained renal nitric oxide.ObjectivesOur purpose was to test 2 hypotheses: (1) that aging intact Fischer-344 (F344) female rats exhibit less glomerular damage than similarly aged males, and (2) that loss of female ovarian hormones would lead to greater structural injury and dysregulation of the nitric oxide synthase (NOS) system in aging F344 rat kidneys.MethodsWe compared renal injury in F344 rats in intact, ovariectomized, and ovariectomized with estrogen replaced young (6 month) and old (24 month) female rats with young and old intact male rats and measured renal protein abundance of NOS isoforms and oxidative stress.ResultsThere was no difference in age-dependent glomerular damage between young or old intact male and female F344 rats, and neither ovariectomy nor estrogen replacement affected renal injury; however, tubulointerstitial injury was greater in old males than in old females. These data suggest that ovarian hormones do not influence these aspects of kidney aging in F344 rats and that the greater tubulointerstitial injury is caused by male sex. Old males had greater kidney cortex NOS3 abundance than females, and NOS1 abundance (alpha and beta isoforms) was increased in old males compared with both young males and old females. NOS abundance was preserved with age in intact females, ovariectomy did not reduce NOS1 or NOS3 protein abundance, and estrogen replacement did not uniformly elevate NOS proteins, suggesting that estrogens are not primary regulators of renal NOS abundance in this strain. Nicotinamide adenine dinucleotide phosphate oxidase-dependent superoxide production and nitrotyrosine immunoreactivity were increased in aging male rat kidneys compared with females, which could compromise renal nitric oxide production and/or bioavailability.ConclusionsThe kidney damage expressed in aging F344 rats is fairly mild and is not related to loss of renal cortex NOS3 or NOS1 alpha.     

Sex differences in circulating and renal angiotensins of hypertensive mRen(2). Lewis but not normotensive Lewis rats

Sex differences in blood pressure are evident in experimental models and human subjects, yet the mechanisms underlying this disparity remain equivocal. The current study sought to define the extent of male-female differences in the circulating and tissue renin-angiotensin aldosterone systems (RAASs) of congenic mRen(2). Lewis and control Lewis rats. Male congenics exhibited higher systolic blood pressure than females [200 +/- 4 vs. 146 +/- 7 mmHg, P < 0.01] or Lewis males and females [113 +/- 2 vs. 112 +/- 2 mmHg, P > 0.05]. Plasma ANG II levels were twofold higher in male congenics [47 +/- 3 vs. 19 +/- 3 pM, P < 0.01] and fivefold higher than in male or female Lewis rats [6 +/- 1 vs. 6 +/- 1 pM]. ANG I levels were also highest in the males; however, plasma ANG-(1-7) was higher in female congenics. Male congenics exhibited greater circulating renin and angiotensin-converting enzyme (ACE) activities, as well as angiotensinogen, than female littermates. Renal cortical and medullary ANG II levels were also higher in the male congenics versus all the other groups; ANG I was lower in the males. Cortical ACE2 activity was higher in male congenics, yet neprilysin activity and protein were greater in the females, which may contribute to reduced renal levels of ANG II. These data reveal that sex differences in both the circulating and renal RAAS are apparent primarily in the hypertensive group. The enhanced activity of the RAAS in male congenics may contribute to the higher pressure and tissue injury evident in the strain.     

Differences in the stress response of prolactin in young and aged female rats

The drawing of blood by orbital sinus puncture (OSP) under ether anesthesia is known to produce a marked increase in serum prolactin (PRL levels in young cycling female rats. The effect of this stressful procedure on PRL release was compared in young and aged female rats. Nonstressed PRL levels were obtained from blood drawn by decapitation. Whereas OSP with a one-minute ether exposure induced a marked increase in PRL levels in young rats on all days of the estrus cycle, older cycling female rats on the day of diestrus -1 and aged rats exhibiting prolonged diestrus (PD) showed virtually no increase above nonstressed levels. However, increasing the ether exposure time to five minutes did produce a rise in PRL levels. Old cycling female rats on the day of estrus and aged rats exhibiting constant estrus (CE) did show a PRL increase comparable to that seen in young animals. Ovariectomy (OVX) completely abolished the stress response seen in aged CE rats. The response, though markedly decreased, was still present in young ovariectomized rats. These experiments show that the stress-induced rise in PRL promoted by OSP under either anesthesia is markedly diminished in aged rats exhibiting a diestrus state. The attenuated response seen in these rats is believed due to factors characteristic of the diestrous state of aging.     

Reduced gonadotropin release in response to progesterone or gonadotropin releasing hormone (GnRH) in old female rats

Ovariectomized rats that were 3–4, 12 or 22 months old were injected s.c. with 4 mg, of testosterone propionate and 3 days later were injected s.c. with 2.8 mg. progesterone or the oil vehicle. Blood samples were collected by heart puncture 5 hrs. later. Serum levels of LH and FSH decreased significantly as age increased. Progesterone significantly increased serum LH and FSH levels regardless of age. The increase in serum LH concentration attributed to progesterone was greatest in the young and least in the old rats. To determine if age effects were due to differences in pituitary response to GnRH, ovariectomized rats that were 2.5 to 23 months old were injected i.v. with GnRH at doses of 100 ng or 40 ng/100 g body weight or were primed with 25 mg progesterone and 50 μg estradiol-benzoate 3 days before an injection of 2 ng GnRH/100 g body weight. Blood was obtained by heart puncture before and 20 min. after GnRH. In each experiment serum LH levels significantly decreased with increasing age but were significantly elevated by GnRH. This increase in serum LH level in response to GnRH declined with increasing age. The data suggest that the elevation in serum LH level in response to GnRH declines as a result of aging in female rats and that this effect is independent of circulating ovarian steroid levels.     

Presence and possible site of action of secretin in the rat pituitary and hypothalamus

Secretin-like immunoreactivity was detected in extracts of several rat brain structures by radioimmunoassay, most notably in the pituitary, hypothalamus, pineal and septum. Its localization to these structures suggested that it might play a role in neuroendocrine events similar to its structural homolog vasoactive intestinal peptide. Dose-related stimulations (MED, 10(-7) M) of prolactin (PRL) release were observed after incubation of synthetic secretin with dispersed, cultured pituitary cells from male and ovariectomized (OVX) female rats. In OVX females, i.v. infusion of a high dose of secretin (10 micrograms) resulted in a significant elevation of PRL levels. Doses of secretin as low as 0.1 micrograms when administered into the third cerebroventricle were capable of significantly inhibiting PRL release in both males and OVX females, suggesting an ultrashort-loop, negative feedback of secretin. Secretin can now be added to the growing list of putative PRL-releasing agents.