Intensive venous sampling of adrenocorticotropic hormone in rats with sham or paraventricular nucleus lesions.

Adrenocorticotropic hormone (ACTH) secretion from the anterior pituitary is predominantly regulated by corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) synthesized in neurons of the paraventricular nucleus (PVN) of the hypothalamus. Secretion of ACTH occurs in pulsatile bursts. To explore the relationship between hypothalamic control and the pulsatile pattern of ACTH secretion, we measured ACTH in 2 min blood samples over 4 h in rats with intact and lesioned PVN during hypovolemic-stress or control conditions and also measured median eminence (ME) levels of CRH, AVP, and oxytocin (OT). Mean plasma ACTH was highest in the sham lesioned-hypovolemic group, lowest in the sham lesioned-control group and intermediate in the two PVN-lesioned groups. CRH in the ME was negligible in the lesioned animals and correlated with OT and AVP. Pulsatile secretion was observed despite PVN ablation. Visual inspection of composite time series suggested different temporal patterns of ACTH secretion. Principal components analysis of the individual ACTH time series revealed three significant eigenvectors which correlated differentially with the three treatment groups. Neither lesioned group had the steep rise over 10 min seen in plasma ACTH in the non-lesioned groups. Delayed ACTH rise after 30-60 min occurred in all but the sham control group. Our data suggest that CRH is responsible for immediate secretion of ACTH in response to hypovolemic stress and that regulators from non-PVN sites may be responsible for more delayed secretion of ACTH in this setting. The persistence of ME AVP and OT levels in the face of > 90% reduction in ME CRH levels leaves open the question of a role for one or both of these peptides in the delayed ACTH response following stress onset and in the generation of pulsatile ACTH secretory bursts.

Coincident plasma ACTH and corticosterone time series: comparisons between young and old rats.

Senescence is accompanied by a reduced ability to respond to a variety of physical and behavioral stressors. A sizable literature has been devoted to the interplay between hypothalamic-pituitary-adrenocortical axis dysfunction and senescence; yet, the precise interactions remain an enigma. Adrenocorticotropic hormone (ACTH) is secreted in pulsatile bursts generating complex signals in the plasma compartment that must be "read" by adrenocortical cells in order to initiate appropriate secretory responses. We have previously demonstrated subtle differences between young and old rats in the pattern of fluctuations in plasma ACTH concentrations over time, despite no difference in mean levels. The present work addressed the physiological significance of these differences in the plasma ACTH signal by analyzing the corresponding plasma corticosterone concentration time series and the relationship between these two hormones over time. Time series of integrated 10-min ACTH and corticosterone concentrations were collected over 4 h at the time of diurnal activation and analyzed in the time and frequency domains. The time of onset of the diurnal surge occurred 20 min later in old rats, and the ratio of corticosterone to ACTH was less at the time of onset and peak of the diurnal surge. Corticosterone levels were lower in old rats and mean ACTH and corticosterone levels were correlated in young but not old rats, as were maximum levels of the two hormones. Cross-correlation of ACTH and corticosterone time series and comparison of spectra were consistent with smoother fluctuations in plasma corticosterone in old animals with less variability at time scales less than 55 min. We conclude that age may be associated with a delay in diurnal activation of the HPA axis, a loss of sensitivity of adrenal corticosterone secretion to plasma ACTH levels, and a relative loss of high frequency variability in the corticosterone signal, as seen in many physiological systems with age.

Model simulations of ACTH pulsatility.

Using high intensity venous sampling (1-2 min integrated intervals) we have observed rapid (< 10 min) large amplitude (up to 80 pg/ml) fluctuations in plasma ACTH concentrations in addition to variations at longer time scales. We developed a mathematical model to assess whether plausible physiological explanations could account for our observations and compared model simulations with time series from two human subjects. Three key features enabled the model to accurately simulate the observed time series. 1) The pattern of instantaneous secretory events comprising a pulse followed a Poisson process during baseline activity and rapidly shifted to a step function pattern during a pulsatile episode. 2) The fraction of secreted ACTH shunted between a fast and slow clearance mechanism varied biphasically between baseline and pulsatile states. 3) A brief rate-sensitive suppression of secretion was invoked when secretory rates increased above a threshold amount.

High intensity venous sampling reveals subtle alterations in plasma adrenocorticotropin patterns in old rats.

Dysregulation of the hypothalamic-pituitary-adrenocortical axis has been theoretically linked to the processes of aging for decades. To investigate the effects of age on high frequency rhythms of plasma ACTH at the time of circadian activation, integrated 2-min blood samples were collected over 4 h in 10 young and 14 old rats with simultaneous plasma volume replacement. Plasma ACTH time series were analyzed in the time and frequency domains. Relative to young rats, old rats had a significantly later onset of the diurnal surge, more spectral power (R2) at lower frequencies, a lack of correlation between the slope of the spectral background continuum and the R2 at periods less than approximately 11 min, a stretching of the time scale in the composite spectra by 18.5%, and an amplitude reduction of the major composite spectral peak by 31%. These findings support the existence of subtle, but significant, alterations in the pattern of plasma ACTH with age and a delayed response of the hypothalamic-pituitary-adrenocortical axis to circadian activation. The differences in spectra suggest a weaker coupling with age between the high frequency signal input (that may reflect depolarization of groups of corticotrophs) and the system response, which could account for the delay in onset of the diurnal surge seen in the time domain.

Effect of cocaine injections on the neuroendocrine response to the serotonin agonist MK-212.

This study was undertaken to examine whether several of the hormones that can be released by activation of serotonin receptors will be affected by long-term cocaine administration. Male rats received cocaine injections (15 mg/kg, IP) twice daily for 7 days. Forty-two hr after the last cocaine injection, the rats were challenged with increasing doses (0, 1, 5, 10 mg/kg, IP) of the 5-HT1/5-HT2 agonist MK-212 (6-chloro-2-[1-piper-azinyl]-pyrazine). The following observations were made: (1) cocaine reduced the rate of body weight gain; (2) cocaine inhibited the stimulatory effect of MK-212 on plasma vasopressin, oxytocin, and prolactin concentrations and on plasma renin activity and concentration; (3) cocaine did not inhibit the stimulatory effect of MK-212 on plasma ACTH or corticosterone concentrations. The data indicate that a wide-spectrum 5-HT (serotonin) agonist such as MK-212 can reveal differential neuroendocrine responses. This effect could be related to cocaine-induced changes in the different 5-HT receptor subtypes that regulate the secretion of these hormones.

Prior chronic exposure to cocaine inhibits the serotonergic stimulation of ACTH and secretion of corticosterone.

The effect of long-term pretreatment with cocaine on serotonergic regulation of ACTH (adrenocorticotropic hormone; corticotropin) and secretion of corticosterone in rats was investigated. The following observations were made: (1) Pretreatment with cocaine had no significant effect on basal levels of ACTH and corticosterone in plasma. However, cocaine caused a reduction in the ability of the 5-HT (5-hydroxytryptamine, serotonin) releaser p-chloroamphetamine (PCA) to increase corticosterone in plasma, 42 hr after the last injection of cocaine. (2) Exposure to cocaine for 7 days was sufficient to produce a maximal inhibition of the PCA-induced increase in ACTH in plasma. (3) The inhibitory effect of cocaine on PCA-induced release of ACTH was more marked than on corticosterone. (4) Conversely, the dose-dependent stimulatory effect of two 5-HT1 agonists, RU 24969 (5-methoxy-3-(1,2,3,4-tetrahydro-4-pyridinyl)-1H-indole) and m-CPP (m-chlorophenylpiperazine), on ACTH and corticosterone was not reduced by 7 days of exposure to cocaine. Taken together, these findings indicate that pretreatment with cocaine reduced the function of serotonergic nerve-terminals but not postsynaptic receptors, that stimulate ACTH and secretion of corticosterone.

Pulsatile ACTH and cortisol in goats: effects of insulin-induced hypoglycemia and dexamethasone.

Insulin-induced hypoglycemia is a metabolic stress that stimulates secretion of adrenocorticotropic hormone (ACTH) and cortisol in a number of animal species. Dexamethasone is a potent synthetic glucocorticoid that suppresses the secretion of ACTH and cortisol. Both ACTH and cortisol exhibit complex secretory patterns demonstrating ultradian and circadian rhythms. This work investigated the pattern of ACTH and cortisol response to hypoglycemia in goats and the effect of dexamethasone on this response. Five goats were pretreated with dexamethasone (0.1 mg/kg) and 5 with saline. Blood samples were taken every 2 min for 60 min before and 60 min after administration of insulin (2.5 IU/kg, i.v.). Immunoreactive ACTH and cortisol were measured in all samples and glucose in selected samples. Data sets were analyzed for significant pulses with the Cluster Analysis program. Complete data sets were compared as well as those for each 30-min interval. Plasma glucose was lower than preinsulin levels at 10 min, declined rapidly between 10 and 30 min, and remained low 30-60 min after insulin injection in both treatment groups. Controls showed a rapid rise in ACTH and cortisol beginning 30 +/- 10 min postinsulin. The increase in mean plasma hormone levels during hypoglycemia was predominantly due to an increase in amplitude of secretory pulses for ACTH and cortisol compared with the 30 min before insulin. Dexamethasone significantly lowered mean ACTH and cortisol levels and prevented alteration in plasma ACTH and cortisol secretion during hypoglycemia but did not totally ablate pulsatile activity of either hormone. The amplitude of ACTH and cortisol pulses was significantly decreased by dexamethasone treatment. The frequency of cortisol but not ACTH pulses was also significantly decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

High resolution spectral analysis of plasma adrenocorticotropin reveals a multifactorial frequency structure.

Time series of plasma ACTH concentrations were analyzed with a high resolution spectral analysis program based on digital Fourier transforms. Both coherent signal and stochastic aspects of the time series were analyzed. Samples were collected at 2- and 15-min intervals in control rats and rats immunoneutralized against CRH. The individual and composite spectral distributions revealed significant structure at both the higher and lower ranges of frequencies studied, with multiple periodicities between 4-220 min in both groups. CRH immunoneutralization consistently reduced the amplitude by 82% and compressed the frequency distribution for waveforms with periods longer than 15 min by 23%. A systematic break in the slopes of the background continua occurred between 10 and 15 min in the 2-min time series. This break was unaffected by CRH immunoneutralization. Digital Fourier transform analysis of our ACTH time series suggests a system with a more complex high frequency structure than has previously been appreciated. Our analyses suggest a biological system with the following characteristics: 1) both a fast and a slow response to a fairly constant unspecified fast forcing; 2) the slow response is initiated by the fast response and represents an imperfect integration due to feedback processes; 3) CRH alters the ability of the fast forcing to elicit a slow response without altering the fast response or ACTH clearance; and 4) this alteration consists of both amplitude and frequency modulation in the signal output. This view of ACTH secretion suggests an adaptive and energy-efficient system.

Effects of immunoneutralization of corticotropin-releasing hormone on ultradian rhythms of plasma adrenocorticotropin.

ACTH, like other anterior pituitary peptide hormones, is secreted episodically and demonstrates both circadian and ultradian rhythms. CRH is the major regulator of ACTH release from the pituitary corticotroph. To determine the dependence of ACTH ultradian rhythms on CRH, passive immunoneutralization was used to block the activity of endogenous CRH in rats with indwelling venous catheters. Blood was sampled at 2- and 15-min intervals while blood volume was replaced. Plasma ACTH was measured by RIA. Time-series analysis of plasma ACTH concentrations was performed with PULSAR and Cluster Analysis. The 2 min data demonstrated secretory bursts approximately every 20 min. CRH immunoneutralization had no effect on the frequency of these pulses, but significantly reduced their amplitude. This was the case for raw data as well as data in which lower frequency variation had been filtered out. The 15 min data demonstrated pulsatile secretion, with a secretory episode approximately every 100 min. This lower frequency rhythm was also observed when high frequency components were filtered out of the 2 min data series. Analysis of the 15 min and the filtered 2 min time series showed this rhythm to be almost totally ablated by CRH immunoneutralization. These results suggest that CRH is responsible for amplitude modulation of an underlying CRH-independent rhythm and that through intermittent amplitude modulation of this rhythm a lower frequency rhythm is generated. Comparison between treatment groups of pulses identified by PULSAR or Cluster Analysis yielded similar results, but the programs were discordant with each other. This is the first in vivo evidence of pulsatile ACTH secretion independent of CRH, the first report demonstrating that different ultradian rhythms of ACTH may be regulated by different mechanisms, and the first comparison of PULSAR and Cluster Analysis on plasma ACTH time series.

The effects of stress on plasma ACTH and corticosterone in young and aging pregnant rats and their fetuses.

Compared to younger rats, old rats exhibit prolonged elevations of plasma ACTH and corticosterone (CORT) in response to stress. In addition, CORT crosses the placenta. To investigate whether fetuses of older rats may be exposed to higher concentrations of CORT during development than fetuses of young rats, we compared the effects of stress on hypothalamic-pituitary-adrenal (HPA) axis function in young and aging pregnant rats and their 19-day-old fetuses. The plasma of the mothers and fetuses was assayed for ACTH and CORT by radioimmunoassay. Both young and aging pregnant rats showed a significant increase in plasma ACTH and CORT immediately after exposure to stress. However, aging rats had more prolonged elevations of ACTH and CORT than young rats. This suggests that, like old male rats, aging pregnant rats have an alteration in feedback inhibition of the HPA axis. Prolonged elevation of CORT was also seen in fetuses of aging mothers. These results have important implications concerning the effects of stress during pregnancy at different maternal ages, and for the potential deleterious consequences of prolonged prenatal elevation in stress hormones on the offspring of aging females.

Immunoneutralization of corticotropin-releasing hormone prevents the diurnal surge of ACTH.

To determine whether CRH is required for the evening rise in plasma ACTH, rats were injected at 0800 hr with CRH antiserum (anti-CRH) or normal rabbit serum (NRS). Blood samples were taken through venous catheters at 0800 hr before treatment and at 1300, 1700, and 2100 hr. Plasma was assayed for immunoreactive ACTH and corticosterone. There was no significant difference in pretreatment values between the two groups. Immunoneutralization of CRH abolished the rise in plasma ACTH seen at 1700 hr in the NRS group but had little effect on earlier levels. The diurnal elevation in plasma corticosterone continued after anti-CRH treatment, but peak levels occurred earlier. Plasma ACTH and corticosterone were significantly correlated at the time of the diurnal surge, but not at 0800 hr or 1300 hr in the NRS controls or at any time point in the anti-CRH group. These results suggest that CRH is required for the diurnal surge of plasma ACTH. They also confirm previous observations by others that the adrenal cortex does not require active CRH or a diurnal surge of ACTH in order to exhibit a significant diurnal increase in secretion of corticosterone, and that factors other than CRH may be relatively more active than CRH in regulation of ACTH secretion during the time of circadian inactivity.

Pulsatile ACTH secretion: variation with time of day and relationship to cortisol.

It has long been known that ACTH is secreted in an episodic fashion demonstrating circadian and ultradian rhythms. High intensity venous sampling has recently revealed that in addition to these larger ultradian fluctuations in hormone levels, plasma ACTH in rats demonstrates high frequency, low amplitude oscillations which have been called "micropulses." These micropulses were not detected in previous studies due to sampling intervals of greater than 5 minutes. To investigate the presence of these ACTH micropulses in a primate species, blood samples were drawn from six chair-restrained rhesus monkeys at one-minute intervals for up to 70 minutes and plasma was assayed for immunoreactive ACTH. To assess the variation in ACTH micropulse parameters with time of day and the relationship to cortisol secretion, four of the monkeys were sampled for three 70-minute periods beginning at 0530, 1100, and 1730 hours, and plasma was assayed for immunoreactive ACTH and cortisol. Analysis of the data revealed that ACTH and cortisol are secreted in micropulses in rhesus monkeys with marked individual variation in the pattern of secretion and a concurrence of approximately 75% of ACTH and cortisol micropulses. Difference in pulse amplitude but not frequency appeared to contribute to the circadian variation in mean ACTH levels and a sampling interval of two minutes appeared to be adequate for accurately identifying micropulses of ACTH.

Changes in mean plasma ACTH reflect changes in amplitude and frequency of secretory pulses.

ACTH is secreted in an episodic manner from the anterior pituitary. Unanesthetized rats with indwelling jugular and femoral venous cannulae were continuously bled and simultaneously infused with isotonic fluid by peristaltic pump. Two-minute blood samples were collected for up to five hours in 8 male rats. ACTH was measured by radioimmunoassay. The resulting time series were analyzed for significant secretory pulses with the PULSAR program. Elevations or declines in mean plasma ACTH levels were associated with significant changes in amplitude and frequency of secretory pulses.

Effects of dexamethasone on central and peripheral ACTH systems in the rat.

To investigate the simultaneous effects of dexamethasone on peripheral and central adrenocorticotropic hormone (ACTH) systems, rats were treated with dexamethasone or saline for 4 days. Pituitary, plasma, hypothalamus and cerebrospinal fluid (CSF) were then collected and analyzed for ACTH immunoreactivity. Additionally, hypothalamic tissue extracts were analyzed for corticotropin-releasing hormone (CRH) immunoreactivity. Dexamethasone significantly lowered peripheral levels of ACTH as measured in pituitary and plasma. Hypothalamic ACTH content significantly increased while CSF ACTH significantly decreased with dexamethasone treatment. Hypothalamic CRH concentrations showed a small but statistically insignificant decrease. These results suggest that prolonged exposure to dexamethasone affects central as well as peripheral ACTH activity, corroborate our previous findings in rhesus monkeys of decreased CSF ACTH in response to prolonged dexamethasone treatment, suggest that dexamethasone may inhibit the release of ACTH from hypothalamic neurons into the CSF, and provide evidence that the effect of dexamethasone on pituitary ACTH content is of greater magnitude than its effect on hypothalamic CRH.