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K Scarbrough

Publications and source records attributed to K Scarbrough.

30 records · Page 2Linked to original sources
PubMed1991

Diurnal rhythmicity of norepinephrine activity associated with the estradiol-stimulated luteinizing hormone surge: effect of age and long-term ovariectomy on hemispheric asymmetry.

The age-related decline in female reproductive capacity in rats is accompanied by an inability to respond positively to estradiol (E2) treatment. This age-related change is associated with a loss in diurnal rhythmicity of norepinephrine (NE) activity in brain areas important in the control of LH. Decreased exposure to ovarian secretions during adulthood delays certain aspects of neuroendocrine aging. We tested the hypothesis that long-term ovariectomy (OVX) would delay the age-related loss of diurnal rhythmicity in NE activity in microdissected hypothalamic nuclei. Intrigued by reports of lateralization of hypothalamic function, we also assayed NE activity in the left and right sides of the hypothalamus separately. Young (2-3 mo) and middle-aged (11-12 mo) rats that exhibited regular estrous cycles were OVX. One week later (Day 0) these short-term OVX animals (Y-ST, MA-ST) plus a group of middle-aged (11-12 mo) rats that were OVX at 3 mo (MA-LT) were treated with E2. On Day 4, the rate constant of NE activity in microdissected hypothalamic nuclei was determined at 0900 h and 1500 h using the alpha-methyl-para-tyrosine method. Rate constants were compared by t-test to determine diurnal rhythmicity. Y-ST rats exhibited a diurnal rhythm in NE activity in the median eminence, which was absent in MA-ST rats. Long-term OVX spared animals this "age-related" loss in rhythmicity since MA-LT rats demonstrated a significant increase in NE activity from morning to afternoon.(ABSTRACT TRUNCATED AT 250 WORDS)

Age Factors↗
PubMed1991

Age-related changes in proopiomelanocortin (POMC) gene expression in the periarcuate region of ovariectomized rats.

Aging of the female reproductive system results in a decline in estrous cyclicity which is due, in part, to alterations in hypothalamic function. Opioid peptides, especially the proopiomelanocortin (POMC)-derived neuropeptide, beta-endorphin, are thought to play a role in maintaining normal patterns of LH secretion. Previous studies have shown that the level of hypothalamic beta-endorphin and POMC messenger RNA (mRNA) decreases in old animals; however, it is unknown whether opioid peptides are involved in age-related reproductive decline. To determine whether POMC gene expression changes with age and is related to reproductive status, we assessed POMC mRNA levels by in situ hybridization histochemistry in the periarcuate region of young (3-4 months), middle-aged (10-12 months), and old (17-19 months) ovariectomized rats. Two methods of quantitation were used: 1) slides were apposed to x-ray film and POMC mRNA levels were quantitated over the entire periarcuate region, and 2) the same slides were dipped in emulsion and the level of POMC was quantitated in individual cells. POMC mRNA levels decreased 20-30% by the time animals were middle-aged, and no further decline was noted in the old animal groups. The decrease in POMC mRNA levels in the middle-aged and old animals occurred regardless of their reproductive status prior to ovariectomy. In addition, there was a 30-40% decline in the number of cells expressing POMC mRNA in middle-aged and old animals, suggesting an overall age-related decline in POMC gene expression in middle-aged and old animals independent of reproductive status.

Aging↗
PubMed1991

Measurement of peptide secretion and gene expression in the same cell.

A combined reverse hemolytic plaque-in situ hybridization assay was developed to allow analysis of the relationship between peptide secretion and gene expression within individual cells. We used the pituitary lactotroph as a model system, but this strategy should be widely applicable. It can be used to test hypotheses regarding if and when peptide secretion and gene expression are coupled in any system in which antibodies to the secreted peptide and probes complementary to the mRNA are available. Using the mRNA hybridization signal to identify certain cell types, this method may also be useful in further studies on the biochemical mechanism of peptide secretion. In addition, questions regarding whether a cell known to secrete a given peptide contains other specific mRNAs and the relationship between these mRNAs and the secretion of the peptide can be studied using this strategy. We found striking heterogeneity among lactotrophs in both gene expression and PRL secretion and a lack of correlation of these parameters within individual lactotrophs under every treatment examined. We also present the first direct visualization and quantitation of the percentage of nonsecreting PRL mRNA-containing cells after estradiol treatment and in the presence or absence of the PRL secretagogue, TRH. Finally, we found that in ovariectomized rats, nonsecreting lactotrophs exhibited significantly higher levels of PRL mRNA than lactotrophs that were actively secreting PRL during the assay.

Animals↗
PubMed1990

Age-related changes in pulsatile luteinizing hormone release precede the transition to estrous acyclicity and depend upon estrous cycle history.

Aging of the female reproductive system in rats is marked by discrete stages in the disappearance of regular estrous cyclicity. We determined at which point in the transition to acyclicity changes in the LH pulse generator could be detected by comparing pulsatile LH release in ovariectomized young rats to three groups of middle-aged rats that were chronologically matched, yet exhibited different stages of reproductive senescence. Since changes in LH pulse amplitudes can result from hypothalamic and/or pituitary factors, we also assessed pituitary responsiveness to exogenous GnRH. Young rats (2-3.5 months) that had shown regular 4- or 5-day estrous cycles and middle-aged rats (9.5-12 months) that exhibited either regular cycles, irregular cycles, or persistent estrus were bilaterally ovariectomized and used 4 weeks later. Rats were implanted with right atrial cannulae and were bled 2 days later at 5-min intervals for 3 h. To test pituitary responsiveness to GnRH, rats were bled at 10-min intervals for 3 h and received GnRH (25 ng/100 g BW, iv) after the first and second hours. The mean inter-peak interval increased in middle-aged irregularly cycling and persistent estrous rats. The frequency distribution of inter-peak intervals was already significantly different in middle-aged regularly cycling compared to that in young regularly cycling rats. Middle-aged rats displayed fewer short inter-peak intervals and a greater frequency of longer intervals between LH pulses. LH pulse duration increased gradually in parallel with increasing reproductive senescence. Pulse amplitude decreased in all groups of middle-aged rats regardless of their prior reproductive status. Mean LH concentrations were significantly lower in middle-aged than in young regularly cycling rats, and a further significant decline was detected in the middle-aged irregularly cycling and persistent estrous groups. Middle-aged regularly and irregularly cycling rats showed decreased pituitary responsiveness to GnRH compared to young regularly cycling rats, whereas middle-aged persistent estrous rats displayed an intermediate level of pituitary responsiveness to GnRH. These data are the first evidence of changing pulse generator function in middle-aged rats tha had previously exhibited no change in the regularity of their estrous cycles. We suggest that such changes may play a role in the age-related transition to acyclicity.

Aging↗
PubMed1990

Diurnal pattern of proopiomelanocortin gene expression in the arcuate nucleus of proestrous, ovariectomized, and steroid-treated rats: a possible role in cyclic luteinizing hormone secretion.

Opiate peptides are thought to modulate the pattern of LH release in female rats. We tested the hypothesis that changes in proopiomelanocortin (POMC) gene expression occur in proestrous (PRO) and ovariectomized (OVX) steroid-treated rats which may explain their unique patterns of LH secretion. Using in situ hybridization, we examined whether diurnal changes in POMC gene expression occur in the arcuate nucleus. Four groups of rats were used in this study. 1) PRO rats were used after exhibiting at least two consecutive 4-day estrous cycles; 2) OVX rats were killed 9 days after ovariectomy; 3) estradiol (E2)-treated rats were OVX for 7 days and then treated for 2 days; and 4) E2-progesterone (P4)-treated rats were treated with E2 as described above, and on day 9 at 1030 h, P4 was administered. Rats were killed at 2300, 0300, 1000, 1300, 1500, 1800, or 2300 h, beginning on the evening of diestrous day 2 or day 8 after ovariectomy. POMC gene expression exhibited a diurnal rhythm on PRO. Levels of mRNA rose during the morning, peaked between 0300-1000 h, and decreased by 2300 h. In E2-treated rats, which exhibited a LH surge similar in timing to the PRO surge, POMC mRNA levels exhibited a diurnal rhythm strikingly similar to that observed in PRO animals. OVX abolished the rhythm; however, average POMC mRNA levels across the 24-h period were not significantly different from those in PRO or E2-treated rats. P4 treatment increased POMC mRNA levels by 2300 h compared to those in all other experimental groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Changing hypothalamopituitary function: its role in aging of the female reproductive system.

Changes in female reproductive function occur relatively early during the life span in many mammalian species. Therefore, this physiological system is an excellent model system in which to study the effects of age on specific endocrine relationships since changes occur prior to the occurrence of multiple pathologies associated with later stages of aging. Data from several laboratories suggest that changes in hypothalamic, pituitary and ovarian function may contribute to age-related deterioration of fertility in females. We will focus our attention on the role of hypothalamic changes in the cascade of events that eventually lead to acyclicity and infertility. Data suggest that changes in the diurnal rhythmicity of catecholaminergic neurotransmitters and their receptors occur during middle age. These changes may regulate the pattern of release of GnRH since alterations in the pulsatile pattern of LH secretion also become detectable at this age. Some age-related changes in hypothalamic and pituitary function are not irreversible or absolutely determined. Instead it appears that the ovarian steroidal milieu modulates the rate of aging of several aspects of hypothalamohypophysial function. In summary, changes in hypothalamic and pituitary function appear to contribute to the aging of the female reproductive system.

Aging↗
PubMed1984

Relationship of DNA methylation level to the presence of heterochromatin in mealybugs.

Purified nuclear DNA from two mealybug species was analyzed for its 5-methylcytosine (m5C) content by reversed-phase high-pressure liquid chromatography. We observed that the percent m5C (percentage of cytosines which are methylated) varied between the two species, between males and females of the same species, and between lines with and without supernumerary B chromosomes. This is the first case of a sex-specific difference in overall DNA methylation level. In contrast to a recent report (Deobagkar et al., J. Biosci. [India] 4:513-526, 1982), we found no other modified bases in the DNA. Overall, the percent m5C in Pseudococcus obscurus was two to three times higher than in Pseudococcus calceolariae. In both species, the percent m5C in males was higher than in females, although only in P. calceolariae was the difference statistically significant (0.68 +/- 0.02 versus 0.44 +/- 0.04). The high m5C content in males was correlated with the presence of a paternally derived, genetically inactive set of chromosomes which is facultatively heterochromatic. The presence of constitutive heterochromatin, however, was associated with a lower m5C content. Thus, for example, the percent m5C in females of a P. obscurus line with heterochromatic B chromosomes (1.09 +/- 0.04) was significantly lower than that of a related line lacking such chromosomes (1.26 +/- 0.06). Our findings are discussed with respect to the possible relationship between DNA methylation and heterochromatization.

5-Methylcytosine↗
PubMedDate not supplied

Neuroendocrine concomitants of reproductive aging.

Depletion of ovarian follicles is often thought to be the determining factor in female reproductive aging. However, increasing evidence suggests that neural and neuroendocrine changes play important causative roles in the decline of regular reproductive cycles leading to the menopause. A blunting or suppression in the daily pattern of secretion of several neuroendocrine hormones has been documented in aging laboratory animals and humans. Investigators have designed experiments to test whether these changes reflect multiple unrelated changes in the regulation of each of these hormones, or whether these alterations result from a fundamental change in the time-keeping mechanism that underlie these patterns of hormone secretion. Oscillations that occur approximately every 24 h are a hallmark of most living organisms. These cycles provide the organism with the capability of coordinating events that occur at higher (hourly) and lower (weekly or monthly) frequencies within an individual organism, and with the capability of synchronizing these events with the external environment. In mammals, the hypothalamic suprachiasmatic nucleus is thought to be a master oscillator that regulates most circadian rhythms in mammals. Perturbations in temporal organization occur during aging and influence multiple physiological systems, including reproductive cyclicity in females. Thus, the question for neuroendocrinologists is: Do changes in the cyclic pattern of hormone secretion reflect a change in the master oscillator, and do these changes play a role in female reproductive aging? Data from our laboratory demonstrate that the timing of the preovulatory and steroid-induced luteinizing hormone (LH) surge changes during middle-age in rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗