Bioassay of ACTH using isolated cortex cells. Applications: structure activity relationship for ACTH and analogues, assay of corticotropin-releasing factor, and assay of plasma ACTH

Invited review: Adrenocortical function in avian and non-avian reptiles: Insights from dispersed adrenocortical cells

Herein we review our work involving dispersed adrenocortical cells from several lizard species: the Eastern Fence Lizard (Sceloporus undulatus), Yarrow's Spiny Lizard (Sceloporus jarrovii), Striped Plateau Lizard (Sceloporus virgatus) and the Yucatán Banded Gecko (Coleonyx elegans). Early work demonstrated changes in steroidogenic function of adrenocortical cells derived from adult S. undulatus associated with seasonal interactions with sex. However, new information suggests that both sexes operate within the same steroidogenic budget over season. The observed sex effect was further explored in orchiectomized and ovariectomized lizards, some supported with exogenous testosterone. Overall, a suppressive effect of testosterone was evident, especially in cells from C. elegans. Life stage added to this complex picture of adrenal steroidogenic function. This was evident when sexually mature and immature Sceloporus lizards were subjected to a nutritional stressor, cricket restriction/deprivation. There were divergent patterns of corticosterone, aldosterone, and progesterone responses and associated sensitivities of each to corticotropin (ACTH). Finally, we provide strong evidence that there are multiple, labile subpopulations of adrenocortical cells. We conclude that the rapid (days) remodeling of adrenocortical steroidogenic function through fluctuating cell subpopulations drives the circulating corticosteroid profile of Sceloporus lizard species. Interestingly, progesterone and aldosterone may be more important with corticosterone serving as essential supportive background. In the wild, the flux in adrenocortical cell subpopulations may be adversely susceptible to climate-change related disruptions in food sources and to xenobiotic/endocrine-disrupting chemicals. We urge further studies using native lizard species as bioindicators of local pollutants and as models to examine the broader eco-exposome.

Effects of food restriction on steroidogenesis in dispersed adrenocortical cells from Yarrow's Spiny Lizard (Sceloporus jarrovii)

Changes in energy balance can lead to functional alterations at all levels of the hypothalamic-pituitary-adrenal (HPA) axis. However, relatively little is known about how energy balance affects functional properties of adrenocortical cells themselves. We investigated effects of restricted food intake on sensitivity to ACTH and rates of steroidogenesis in adrenocortical cells isolated from growing female and male Yarrow's Spiny Lizards (Sceloporus jarrovii). At the end of the feeding regimen, we assayed acute (3h) progesterone (P(4)), corticosterone (B), and aldosterone (ALDO) production in response to ACTH in dispersed adrenocortical cells. Food restriction depressed growth rate by about 50% in both males and females but did not alter baseline plasma B measured at 10 weeks in either sex. At the cellular level, food restriction had the following effects: (1) increased basal B production in both sexes and basal ALDO production in males, (2) increased net maximal rates of production of P(4), B, and ALDO in response to ACTH, and (3) no overall effect on adrenocortical cellular sensitivity to ACTH. There were modest sex differences: overall rates of P(4) production were 46% greater in cells from females than from males, and in response to food restriction, the net maximal rate of ALDO production was 50% greater in cells from males than from females. Our results demonstrate that food restriction in S. jarrovii increases adrenocortical cellular rates of steroid production without affecting overall cellular sensitivity to ACTH.

Seasonal alterations in adrenocortical cell function associated with stress-responsiveness and sex in the eastern fence lizard (Sceloporus undulatus)

We characterized steroidogenic properties of dispersed adrenocortical cells from field-active male and female eastern fence lizards (Sceloporus undulatus) to investigate whether alterations in cell function could, in part, explain seasonal variation in baseline and stress-induced plasma corticosterone (B). Lizards were collected during the breeding and postbreeding seasons and shortly prior to hibernation. Dispersed cells in vitro produced B, aldosterone (ALDO), and progesterone in response to 8-Br-cAMP, 25-(OH)cholesterol, adrenocorticotropin (ACTH; as little as 100 fM), and angiotensin II. Maximal progesterone, B, and ALDO responses to ACTH were roughly 1000%, 500%, and 100% greater than corresponding basal values. Angiotensin II was an effective steroidogenic stimulant but much less so than ACTH. Corticosteroid production exhibited considerable steroid-specific variation among seasons. Maximal ACTH-induced B production was lower in the postbreeding season than at either of the other two measurement points, essentially opposite to the pattern for ALDO. Males and females generally produced B at similar rates, but ALDO and progesterone showed numerous sex differences that usually covaried between the two steroids. Cellular sensitivity to 25-(OH)cholesterol and angiotensin II showed few sex differences or seasonal changes. In contrast, sensitivity to ACTH decreased markedly from the breeding to the postbreeding season in males, corresponding to the decrease in stress-responsiveness, and in both sexes was considerably lower prior to hibernation than during the breeding season. Under some conditions, plasma B may be limited by the production capacity of adrenocortical cells. In summary, seasonal variations in body condition, reproductive activity, and baseline and stress-induced plasma B may be attributed at least in part to alterations in adrenocortical cell steroidogenic function.

Dietary protein restriction stress in the domestic chicken (Gallus gallus domesticus) induces remodeling of adrenal steroidogenic tissue that supports hyperfunction

The stress of dietary protein restriction in the immature domestic turkey (Meleagris gallopavo) induces adrenal steroidogenic hypofunction that is associated with an alteration in the proportion of density-dependent subpopulations of steroidogenic cells within the adrenal gland. In contrast, when imposed on immature chickens, this nutritional stressor induces long-term enhancement of adrenal steroidogenic function. However, whether this alteration in function is accompanied by a remodeling of chicken adrenal steroidogenic tissue as in the turkey is not known. To address this question, immature cockerels (2 weeks old) were fed established isocaloric synthetic diets containing either 20% (control) or 8% (restriction) soy protein for 4 weeks. Adrenal glands were processed for the isolation of defined, density-separable, adrenal steroidogenic cell subpopulations: three low-density adrenal steroidogenic cell subpopulations [LDAC-1 (rho = 1.0285-1.0430 g/ml), LDAC-2 (rho = 1. 0430-1.0485 g/ml), and LDAC-3 (rho = 1.0485-1.0500 g/ml)] and one high-density subpopulation [HDAC (rho = 1.0510-1.0840 g/ml)]. The steroidogenic function of these cell subpopulations was assessed. Protein restriction consistently, but differentially, enhanced maximal ACTH-induced corticosterone production by the subpopulations: values of LDAC-1, -2, and -3 and HDAC from protein restricted birds were, respectively, 116, 43, 33, and 20% greater than those of corresponding cell subpopulations from control birds. However, it had contrasting influences on maximal ACTH-induced aldosterone production by the cell subpopulations. Whereas the value of LDAC-1 from protein-restricted birds was 70% greater than that from control birds, the values for LDAC-2 and -3 were not different from those of the control, and the value for HDAC was 22% less than that of the control. Protein restriction also altered the cell subpopulation composition of the adrenal gland: compared to control, it increased the proportion of LDAC-1 by 46% and decreased the proportion of LDAC-3 and HDAC by 34 and 20%, respectively. Thus, dietary protein restriction increased the proportion of cells (i.e., LDAC-1) having the greatest enhancement in corticosteroid production. This pattern of remodeling of chicken adrenal steroidogenic tissue in response to dietary protein restriction contrasts sharply with the pattern that occurs in another galliform species, the domestic turkey.

Dietary protein restriction stress in the domestic fowl (Gallus gallus domesticus) alters adrenocorticotropin-transmembranous signaling and corticosterone negative feedback in adrenal steroidogenic cells

Previous work with growing chickens (Gallus gallus domesticus) indicates that transient dietary protein restriction induces long-term enhancement of adrenal steroidogenic function in response to adrenocorticotropin (ACTH). The present study investigated two possible cellular functions mediating this enhanced response: (a) ACTH signal transduction and dissemination and (b) short-loop feedback inhibition of ACTH-induced corticosterone production by exogenous corticosterone. Cockerels (2 weeks old) were fed isocaloric synthetic diets containing either 20% (control) or 8% (restriction) soy protein for 4 weeks. Adrenal glands were processed for the isolation of adrenal steroidogenic cells nearly devoid of chromaffin cells ( approximately 90% adrenal steroidogenic cells). Results of experiments to assess signal transduction and dissemination indicated that protein restriction selectively enhanced ACTH-induced corticosterone production mediated by the cyclic AMP (cAMP)-dependent pathway. In addition, protein restriction substantially counteracted exogenous corticosterone-dependent inhibition of acute ACTH-induced corticosterone production (by 40.7% vs control). The proximal portion of the cAMP pathway seemed most affected by this stressor. Protein-restricted cells exhibited enhanced homologous sensitization to ACTH (136% greater than that of control cells) which appeared to be localized at a step(s) prior to or at the formation to cAMP. Also, maximal ACTH-induced cAMP production and sensitivity to ACTH in terms of cAMP production by protein-restricted cells were, respectively, 2.2 and 15.8 times those of control cells. However, variable results were obtained from other experiments designed to pinpoint the altered early steps in ACTH-transmembranous signaling. For example, with intact cells, cAMP responses to cholera toxin (CT) and forskolin (FSK) did not corroborate the results suggesting an augmentation of ACTH-signal transduction induced by protein restriction. Furthermore, basal and stimulatable (by ACTH, CT, FSK, and NaF) adenylyl cyclase activities from membranes from protein-restricted cells were, respectively, 47.2 and 40.2% less than those from control cells (normalized to 10(7) cell equivalents of crude membranes). Collectively, these findings suggest that protein restriction stress potentiates ACTH-induced corticosterone secretion by chicken adrenal steroidogenic cells in at least two ways: (1) on the proximal end, by modulating unknown factors which enhance cellular sensitivity to ACTH, ACTH receptor-adenylyl cyclase coupling, and adenylyl cyclase activity, and (2) on the distal end, by suppressing end-product corticosterone negative feedback, thus facilitating an increase in net corticosterone secretion.

Corticosterone secretion in response to serotonin and ACTH by perfused adrenal of normal and athymic nude mice

Adrenocortical secretory responses to chemical mediators and ACTH in CD1 ICR nu/nu (athymic) mice were compared with those in CD1 ICR (normal) mice. The bilateral adrenals of normal or athymic mice were perfused in situ with artificial medium equilibrated by 95% O2 + 5% CO2. Infusion of serotonin induced the secretory response of corticosterone significantly at 10 nM and markedly at 100 nM and the response at 1000 or 10000 nM declined as compared with that at 100nM in normal mice. Total corticosterone secretion in response to 100 or 1000 nM serotonin in athymic mice was about one fourth that in normal mice, respectively. Corticosterone responses to ACTH at the range of 10 to 300 pg/ml in athymic mice were comparable to those in normal mice. Infusion of histamine, platelet activating factor(PAF), or compound 48/80 did not induce significant corticosterone response in both normal and athymic mice. The data suggest that the congenital defect of the thymus and/or hair causes the hyporesponsiveness of adrenocortical cells to serotonin although the adrenal cortex of athymic mice is able to perform its function in response to ACTH.

The use of immunoradiometric assay for the measurement of ACTH in human plasma

Bioassay and immunoassay techniques for the measurement of ACTH in human plasma have provided sensitive and specific results but have also met with some skepticism as to their reliability in some clinical circumstances. The recent development of a supersensitive two-site immunoradiometric assay for ACTH may resolve sole of the limitations of previous assays and greatly facilitate the evaluation of pituitary-adrenal disorders.

The melanotropin potentiating factor and beta-endorphin do not modulate the alpha-melanotropin-or adrenocorticotropin-induced corticosteroidogenesis in purified isolated rat adrenal cells

The ability of two pro-opiomelanocortin-derived peptides, the melanotropin potentiating factor (MPF) and beta-endorphin (beta EP), to affect corticosteroid production was studied in purified isolated rat adrenal cells. Addition of MPF or beta EP, in doses from 5 pg to 5 micrograms, alone did not result in a corticosterone production. Furthermore, no effect of MPF or beta EP in doses from 5 pg to as high as 5 micrograms for both peptides upon the ACTH or alpha-MSH-induced corticosteroidogenesis was observed (p greater than 0.1). It is concluded that both MPF and beta EP do not influence the steroidogenic activity in the adrenal gland. Use of these peptides for discrimination of the ACTH/alpha-MSH receptor interactions is suggested.

Responsiveness of isolated adrenocortical cells from lean and obese Zucker rats to ACTH

Morning plasma corticosterone concentrations have been reported to be elevated in obese Zucker rats compared with lean rats. The aim of this study was to determine if differences in adrenal sensitivity or maximal responsiveness to adrenocorticotropic hormone (ACTH)-(1-24) could account for this disparity. Serum and adrenal glands were collected from lean and obese, male and female Zucker rats (10-13 wk old) between 10:00 and 11:00 A.M. Adrenocortical cells were isolated and challenged with ACTH-(1-24). The serum corticosterone and ACTH concentrations were significantly greater in obese males compared with lean males (45.3 +/- 10.3 vs. 23.2 +/- 1.45 ng/ml and 156.6 +/- 15.3 vs. 113.3 +/- 9.4 pg/ml, respectively). Although serum corticosterone concentrations were similar in female rats, serum ACTH concentrations tended (P = 0.07) to be lower in obese female rats than in lean female rats (67.6 +/- 9.3 vs. 103.5 +/- 15.1 pg/ml, respectively). The median effective concentration (EC50) and the maximal corticosterone response per microgram of DNA of dose-response curves derived from lean and obese rats were not significantly different. Additionally, a morphometric evaluation of adrenal tissue from lean and obese rats suggested that cells of the zona glomerulosa were smaller in obese rats than in lean rats. Our data confirm that morning serum corticosterone concentrations are elevated in 10- to 13-wk-old male Zucker rats. This difference does not appear to be due to differences in the sensitivity or maximal secreting capacity of adrenocortical cells to ACTH.

Polyhormonal regulation of avian and mammalian corticosteroidogenesis in vitro

1. The combined actions of ACTH, corticosterone and prolactin (PRL) in the acute regulation of corticosteroidogenesis were investigated using isolated adrenocortical cells from intact and hypophysectomized (hypox) rats (Rattus norvegicus) and from intact male domestic fowl (Gallus gallus domesticus). 2. Exogenous corticosterone suppressed to about 50% ACTH-induced corticosterone production of cells from either species. This suppression, in part, was due to corticosterone degradation. 3. oPRL, in the presence or absence of ACTH, raised corticosterone production of hypox rat cells, but not intact rat and domestic fowl cells. 4. In addition, oPRL counteracted the corticosterone-induced suppression of net ACTH-stimulated corticosterone production of hypox rat and intact domestic fowl cells, but not intact rat cells. 5. The potency of oPRL with domestic fowl cells was 4 times that with hypox rat cells. 6. Furthermore, in domestic fowl cells, the effect of oPRL was Ca2+-dependent.

Existence of different forms of adrenocorticotropin in rat hypothalamus

Adrenocorticotropin (ACTH)-like immunoreactivity and bioactivity were extracted from rat hypothalamus and fractionated by high pressure liquid chromatography. Analysis of the fractions either by radioimmunoassay or bioassay (corticosteroid production from rat adrenal cells) revealed several peaks of immunoreactivity and bioactivity. Only 20-25% of total ACTH-like immunoreactivity and bioactivity eluted with the same retention time as authentic ACTH 1-39. The results suggest that different forms of ACTH exist in rat hypothalamus.

Effect of ACTH, beta-endorphin, morphine and naloxone on the release of cortisol by isolated adrenal glands

The release of cortisol (determined by RIA) from isolated slices of adrenal glands of guinea pigs is stimulated by ACTH, by beta-endorphin, and by morphine in a concentration-dependent way; naloxone gives a small stimulation which is not related to its concentration. Naloxone inhibits the effect of ACTH (1.11 X 10(-11) M) in a competitive manner with an IC50 of about 3.10(-9) M. Also morphine and beta-endorphin inhibit the effect of ACTH, but not in competitive manner. Naloxone (10(-9)-10(-7) M) gives a concentration-related inhibition of the increase of cortisol release produced by morphine (10(-8) M) and by beta-endorphin (1.44 X 10(-10) M). These data suggest a similarity in the conformation of ACTH, beta-endorphin, morphine and naloxone towards the binding sites of ACTH of the guinea pig adrenal glands.

Isolated adrenocortical cells of the domestic fowl (Gallus domesticus): steroidogenic and ultrastructural properties

Isolated adrenocortical cells from White Leghorn chickens (Gallus domesticus) were compared to those from rats (Rattus norvegicus). Cells were prepared from collagenase-dispersed adrenal glands of sexually mature male animals. Corticosterone was measured by radioimmunoassay after incubation for 2 h with steroidogenic agents. Of the four ACTH analogues used, three were 6-17 times more potent with rat cells than with fowl cells (potencies were indicated by half-maximal steroidogenic concentrations). However, 9-tryptophan (O-nitrophenylsulfenyl) ACTH was 8 times more potent with fowl cells than with rat cells, thus suggesting that ACTH receptor differences exist between the two cell types. In addition, cAMP analogues were 10 times more potent with rat cells than with fowl cells suggesting that fowl corticosteroidogenesis is less dependent on cAMP than is rat corticosteroidogenesis. At equal cell concentrations, rat cells secreted 20-40 times more corticosterone than did chicken cells when they were maximally stimulated. Although rat cells converted 8 times more pregnenolone to corticosterone than did fowl cells, the half-maximal steroidogenic concentration for pregnenolone-supported corticosterone synthesis was the same for both cell types (about 5 microM). This suggests that fowl cells have lower steroidogenic enzyme content rather than lower steroidogenic enzyme activity. An unusual feature seen in the isolated fowl adrenocortical cells was an abundance of intracellular filaments.

The effects of gonadotropins on metabolism of isolated rat granulosa cells

FSH in vitro, but not LH, increased the O2 uptake of isolated granulosa cells from 23 day old rats previously treated with DES or with DES and FSH. Dose response studies showed that the cells were most sensitive to FSH when the cellular binding of FSH was highest. LH increased the O2 uptake of granulosa cells of untreated 30 day old rats. DES treatment inhibited the LH induced rise in O2 uptake when the rats were implanted with DES capsules unless FSH was injected to induce LH receptors. Addition of dbcAMP in vitro increased O2 uptake of granulosa cells from 30 day old rats at concentrations 10X lower than those required to stimulate O2 uptake in cells from 23 day old rats treated with DES alone.

Cortisol production by dispersed guinea-pig adrenal cells; a specific, sensitive and reproducible response to ACTH....and its fragments

Naturally occurring steroids and peptide hormones, tested at supraphysiological concentrations, were without effect on basal and human (h) 1-39 ACTH (NIBSC code 74/555, 25 ng/l (5.5 X 10(-12) mol/l] stimulated cortisol production. Further, low concentrations of angiotensin II, N-pro-opiocortin (N terminal fragment 1-76) and gamma-MSH all of which have been reported to synergise with ACTH with regard to cortisol production, were without significant effect alone or in combination with ACTH over the range 2.2 X 10(-13) to 5.5 X 10(-12) mol/l. The activity of h 1-39 was compared with that of the ACTH related peptides 1-24, 1-18, 1-17, 1-16, 1-13-NH2 (alpha MSH), 1-10 and 4-10. The dose responses were parallel and the same maximal cortisol output was observed with all the peptides except the 1-10 fragment. Half maximal stimulation occurred at 3.1 X 10(-12) (1-24), 4.4 X 10(-12) (h 1-39), 1.5 X 10(-11) (1-39), 3.3 X 10(-10) (1-18), 5 X 10(-9) (1-13-NH2), 8 X 10(-9) (1-17), 2 X 10(-7) (1-16) and 1 X 10(-5) (4-10) mol/l respectively. Interference by the above ACTH-derived peptides in cortisol secretion by the cells in response to 5.5 X 10(-12) mol/l h 1-39 ACTH was minimal over the range 5.2 X 10(-12)-2.2 X 10(-6) mol/l. The sensitivity of the adrenal cells to h 1-39 ACTH was such that 2 ng/l (4.4 X 10(-13) mol/l) provoked cortisol secretion over the control (P less than 0.05, n = 17). The coefficient of variation within assay for each dose on the full standard curve (2.2 X 10(-13)-1.1 X 10(-10) mol/l) was less than 10% (n = 6). Half maximal stimulation was given by 14.5 ng/l (3.2 X 10(-12) mol/l). Between control and 1.1 X 10(-10) mol/l ACTH there was a 32 +/- 8 (mean +/- SD, n = 9) fold change in cortisol production.

Preliminary experiences with a bioassay for adrenocorticotrophin (ACTH) in unextracted human plasma using dispersed guinea-pig adrenal cells

A simple, accurate and precise dispersed cell bioassay suitable for the assay of ACTH levels greater than 50 ng/1 in unextracted human plasma is described. Isolated adrenal cells were prepared by tryptic digestion of the guinea-pig adrenal gland and cortisol production by these cells was controlled specifically by ACTH. The cortisol response to 2 ng/1 of human pituitary ACTH (1-39) was always significantly (P less than 0.05, n = 17) different from the control. ACTH concentrations between 1 and 20 ng/1 gave a 14 +/- 3 fold (n = 17) increment in cortisol response. Dilutions of test plasmas were parallel with standard ACTH (1-39) when the plasma concentrations were maintained throughout at 4% (1 in 25 dilution) by the addition of dexamethasone-suppressed plasma ('ACTH free'). Using a 25 fold plasma dilution, the limit of detection of ACTH in unextracted plasma was 50 ng/1. Recovery of ACTH (1-39) spiked into dexamethasone suppressed plasma at concentrations of 250 and 100 ng/1 was 97 +/- 10% (n = 17) and 114 +/- 15% (n = 17) respectively. The within assay coefficient of variation (CV) of both quality control and patients' samples never exceeded 9% while between assay variation was less than or equal to 13%. The sample throughput was 30 plasma samples/week/technician.

Novel variants of adrenocorticotrophic hormone in porcine anterior pituitary

Extracts of porcine anterior pituitary contain several corticotrophic variants of ACTH 1-39. They were isolated by adsorption chromatography, ion-exchange chromatography and reverse-phase high-performance liquid chromatography. Four variants were then identified as starting and ending at positions corresponding to ACTH 1-38, 1-37, 7-39 and 7-38. Several of these fragments were recovered in chromatographically multiple forms. Although all fragments isolated had corticotrophic activity (measured on isolated rat adrenal cells) those with a shortened N-terminal region had a lower potency than those with an intact N-terminal region. Corticotrophic activity of porcine ACTH 7-38 was detected in a preparation with a beta-aspartyl shift at position 25. However, in agreement with previous studies, synthetic human ACTH 7-38, in which an intact Asn-Gly bond was structurally proven, possessed no such activity. The results indicate that position 25 as well as positions 1-6 are important for corticotrophic activity, and that a deamidative beta-aspartyl shift at position 25 can influence the activity.

Glucocorticoid control of steroidogenesis in isolated rat adrenocortical cells

The role of end-product glucocorticoids in the regulation of corticosteroidogenesis in isolated adrenocortical cells was investigated. Trypsin-isolated cells from male rat adrenal glands were incubated with or without corticotropin (ACTH) and with or without corticosterone. Endogenous corticosterone production was determined by radioimmunoassay at the end of incubation. Cessation of ACTH-induced corticosterone production was apparent after 2-4 h of incubation. The suppression occurred later with lower cell concentrations. Corticosterone production was partially restored after washing the suppressed cells. Supernatant fluid from suppressed cell suspensions also suppressed steroidogenesis of a fresh population of cells. However, the suppressing property of the supernatant fluid was abolished after the removal of corticosterone by charcoal-dextran treatment, suggesting that corticosterone or other steroids caused the suppression. Exogenous corticosterone induced suppression over a wide range of ACTH concentrations, but did not change the half-maximal steroidogenic concentration of ACTH, indicating that the suppression does not change the sensitivity of the cells to ACTH. Suppression occurred within 30-60 min after corticosterone had been added to the incubation medium either at the start of incubation or while steroidogenesis was in progress. Suppression varied directly with the concentration of exogenous corticosterone. These data indicate that glucocorticoids can directly and acutely suppress corticosteroidogenesis and thus control adrenocortical function in concert with other regulators such as ACTH and Ca2+.

Hypothalamic and pituitary periodicity demonstrated by isolated rat adrenal cells

Rats were maintained under standardized conditions of food and water ad libitum and a lighting schedule of 12 h light/we h dark for seven days. Animals were sacrificed at 0500 and 1700 h and tissues were removed and washed. Isolated intact adrenal and pituitary cells were prepared by collagenase treatment. Using a sequential incubation procedure, the release of pituitary ACTH by hypothalamic acid extracts (CRF) was assayed by stimulation of corticosteroid secretion from isolated adrenal cells. Maximum stimulation of adrenal steroids was achieved with hypothalamic extracts and pituitary cells from rats sacrificed at 1700 h, and minimum values were obtained at 0500 h. Intermediate levels were obtained when hypothalamic extracts at one time point were incubated with pituitary cells at the other. The results substantiate the late afternoon peak level of hypothalamic and pituitary secretion occurring prior to the peak activity pattern of the rat.