Dorothy T. Krieger

ACTH, beta-lipotropin, and related peptides in brain, pituitary, and blood.

Publisher Summary This chapter presents the demonstration of a precursor molecule containing adrenocorticotropic hormone (ACTH) and β -lipotropin in multiple tissues, which has the potential to be processed into products both similar to and different from those elaborated by the anterior and intermediate pituitary lobes. This has raised a number of fundamental questions with regard to its biosynthetic pathways and regulation and to the functions of the peptide products. The chapter discusses the nature and regulation of extrapituitary form of ACTH family of peptides and the forms secreted by human pituitary, both in normal subjects and in diseases characterized by disorders of ACTH regulation. Both ACTH and β -lipotropin can serve as precursors for other biologically active peptides. In humans, ACTH and β -melanotropin ( β -MSH) are secreted concomitantly and occur within the same cell of the anterior pituitary. The distribution of ACTH and β -lipotropin cells is identical. Within the arcuate nucleus, these cells in colchicine-treated rats are distributed throughout its rostrocaudal extent as for α -MSH in the vinblastine-treated rat. The concept of a hypothalamic center distributing ACTH, α -MSH, or β -endorphin via axons to regulate many other brain regions is comparable to the oxytocin and vasopressin fibers in many extra hypothalamic sites that originate in the magnocellular paraventricular nucleus of the hypothalamus. The ACTH- β -lipotropin and vasopressin-oxytocin systems are unique among known brain peptides as their perikarya is restricted to the hypothalamus. The hypothalamus is the center of the ACTH- β -lipotropin brain.

Presence of immunoassayable β-lipotropin in bovine brain and spinal cord: Lack of concordance with ACTH concentrations

Abstract Discrete areas of freshly obtained adult bovine brain were assayed for their content of immunoreactive β-lipotropin (β-LPH), ACTH and β-endorphin. Highest concentrations (pg/100ug protein) of β-LPH were present in hypothalamus (517 ± 81), hippocampus (218 ± 60), central grey rostral mesencephalic level, pons, striatum, and spinal cord (163–258). Lesser concentrations (49–138) were present in other parts of the limbic system, brain stem, cortex and thalamus. Immunoreactive ACTH concentrations were highest in hypothalamus (1702 ± 487) and hippocampus (210 ± 40), with markedly lesser concentrations (5–24) being present in all the other aforementioned areas. Immunoreactive β-endorphin concentrations in hypothalamus were 1990 ± 510, in hippocampus 280 ± 50.

Rhythms of acth and corticosteroid secretion in health and disease, and their experimental modification

Detailed (q 5–30 min) sampling studies of both plasma ACTH and cortisol concentrations in normal human subjects demonstrates an early morning major circadian rise and a decline over the subsequent 24 h period, on which are superimposed lesser, episodic fluctuations (both synchronous and asynchronous) of plasma ACTH and cortisol concentrations. Marked concordance of bioassayable and immunoassayable plasma ACTH concentrations is present. The results of studies of ACTH-cortisol periodicity in patients with localized CNS disease or ocular blindness support the thesis of a neural origin of such periodicity. It is suggested that such a periodicity is endogenous, with the observed time of peaking related to some aspect(s) of the sleep-wake cycle, but synchronized by the dark-light cycle. Animal studies indicate that altered feeding schedules (without alteration of light-dark cycles) can alter the phase of corticosteroid periodicity, perhaps by alteration of the sleep-wake cycle. The presence of abnormal pituitary-adrenal periodicity in patients with Cushings disease in remission lends support to the thesis of a neural etiology of this disease. Alteration of neonatal corticosteroid levels in the rat is associated with only temporary disruption of the normal periodicity of plasma cortocosteroid levels, and alterations of neonatal CNS amine content is not associated with any disruption of such periodicity.

Implantation of normal fetal preoptic area into hypogonadal mutant mice: Temporal relationships of the growth of gonadotropin-releasing hormone neurons and the development of the pituitary/testicular axis

Central nervous system tissue which included the preoptic area (an area rich in gonadotropin-releasing hormone neurons) was taken from normal 17-day fetal mice and transplanted into the infundibular recess of the third ventricle of the hypothalamus of 90-day male mutant hypogonadal mouse hosts that are unable to synthesize the neurohormone, gonadotropin-releasing hormone. The growth and development of gonadotropin-releasing hormone neurons and fibers in the donor and host tissue as well as recovery of the pituitary-testicular axis were followed from 10 to 120 days post-implantation. Testicular growth was evident in 94% of the hypogonadal animals within 30 days post-implantation, continued for 90 days but showed no further increase during the remainder of the experiment. Increases in seminal vesicle weight, an index of testosterone secretion, were measurable at 30 days and continued through to the end of the experiment. Pituitary concentrations of gonadotropins were doubled at 30 days over that seen in the control mutant mouse and were maintained thereafter at normal or supranormal concentrations. In contrast plasma levels of gonadotropins, although above baseline at 30 days, never reached normal circulating levels. Nevertheless, it appeared that the concentration of luteinizing hormone achieved was sufficient to initiate and maintain testicular growth and testosterone secretion for the entire duration of the experiment. Immunocytochemical analysis of brain tissue was used to determine the presence and numbers of gonadotropin-releasing hormone neurons in the transplant and the distribution of their fibers in the donor and host tissue. The numbers of immunoreactive gonadotropin-releasing hormone neurons present at the time of sacrifice ranged from 3 to 140. Fiber outgrowth from the donor cells into the host was noted as early as 10 days post-implantation and the density of outgrowth continued to increase over the course of the experiment. Positive fibers tended to accumulate over the tuberoinfundibular sulci as they do in normal animals. In those instances where the transplant was placed a long distance from the median eminence, the gonadotropin-releasing hormone axons grew on the internal surface of the third ventricle until they reached these specific exit zones. These studies indicate that in the mutant hypogonadal mouse, central nervous system transplants from normal fetal mice can maintain the function of the pituitary-gonadal axis for periods of up to 120 days post-implantation. Outgrowth of the neurosecretory fibers begins very soon after implantation and the axons tend to follow pathways seen in normal tissue.(ABSTRACT TRUNCATED AT 400 WORDS)

β-lipotropin in brain: Localization in hypothalamic neurons by immunoperoxidase technique

Summaryβ-Lipotropin (β-LPH) has been localized in hypothalamus and pituitary of sheep and ox by the immunoperoxidase technique. In both species β-LPH was found in perikarya of arcuate neurons as well as in cells of the anterior and intermediate lobes of the pituitary. A large number of immunoreactive axons were found in the arcuate region; some appeared to innervate other neurons and others projected to portal capillaries. Stained fiber segments were also scattered throughout the hypothalamus. The presence of β-LPH in hypothalamic neurons supports the possibility that brain β-LPH may be a precursor for opiate-like or other peptides which may be involved in neuromodulation or neurohormonal activities.

Circadian Pattern of Plasma 17-Hydroxycorticosteroid: Alteration by Anticholinergic Agents

Atropine, administered to cats just prior to the time of the expected circadian rise in levels of 17-hydroxycorticosteroid in plasma, blocks this rise. Atropine does not alter this circadian pattern when administered at other times in the circadian cycle. Results similar to those obtained with atropine have been observed with short-acting barbiturates. Dibenzyline administered just prior to the time of the expected circadian rise is ineffective in blocking this rise. These findings support the hypothesis that the circadian pattern of plasma 17-hydroxycorticosteroid levels reflects activation, by the central nervous system, of the hypothalamicpituitary-adrenal axis during a critical time period in the circadian cycle.

Circadian corticosteroid periodicity: critical period for abolition by neonatal injection of corticosteroid.

Circadian variation of corticosteroid concentrations in rat plasma is suppressed if corticosteroids are administered between days 2 to 4 of neonatal life, but not if they are given between days 12 to 14 of neonatal life. This indicates a critical period for the effect of corticosteroid administration on the central nervous system pathways regulating such periodicity. Circadian periodicity of corticosteroids is not affected by neonatal administration of testosterone or reserpine.

On the origin of the serotonergic input to the intermediate lobe of the rat pituitary

Serotonin-containing nerve fibers have been visualized immunocytochemically in the intermediate lobe of the rat pituitary. A 50% depletion of the serotonin level in the intermediate lobe was obtained in our previous experiment in rats with pituitary stalk transection, which may represent the total neuronally derived serotonin there. In the present studies we have attempted to determine the source of these fibers by examining the effect of hypothalamic and midbrain lesions or fiber transections on serotonin levels and serotonin-containing neuronal elements. Complete hypothalamic deafferentation resulted in a significant reduction of immunostained fibers in the intermediate lobe. A 27% fall in the serotonin content (measured by HPLC and electrochemical detection) and a significant disappearance of immunostained fibers were observed after transecting the ascending fibers from the raphe nuclei towards the hypothalamus. The transection combined with the lesioning of the hypothalamic dorsomedial nuclei resulted in a 50% decrease of serotonin level in the intermediate lobe. The present data therefore suggest that serotonin fibers in the intermediate lobe may originate from cells both in the midbrain raphe and hypothalamic dorsomedial nuclei.

Circadian Periodicity of Plasma 11-Hydroxycorticosteroid Levels in Subjects with Partial and Absent Light Perception

Abnormal circadian patterns of plasma corticosteroid levels were observed in 9 of 12 subjects with total absence of light perception and 5 of 7 subjects with partial absence of light perception secondary to intraocular disease. This abnormality consisted both of elevated levels at given times of day and a lack of reproducibility of the circadian pattern over 2 successive 24-hour periods. In 11 of 13 subjects with abnormal circadian periodicity, an early morning rise in plasma corticosteroid levels still occurred, though in 5 of these 11 subjects this rise occurred somewhat in advance of the usual 8 a.m. peak. This might indicate that the sleep-wake rather than dark-light transition plays a major role in the regulation of this phase of circadian periodicity. There was no correlation between the occurrence of abnormal patterns and either the age of the subject, age at onset or duration of blindness, or the subject’s sleep-activity cycle.

Streptozotocin-Induced Diabetes Is Associated with Reduced Immunoreactive Beta-Endorphin Concentrations in Neurointermediate Pituitary Lobe and with Disrupted Circadian Periodicity of Plasma Corticosterone Levels

Lower concentrations of immunoreactive (IR) beta-endorphin were present in the neurointermediate pituitary lobes of streptozocin-induced diabetic versus control animals at both 2 and 4 weeks after the onset of diabetes. The forms of beta-endorphin-like material present appeared to be similar in both groups when studied with cation-exchange chromatography. Insulin therapy via minipump for 2 weeks did not alter this finding of lowered beta-endorphin concentrations in diabetic animals, despite normalization of blood glucose levels and body weight gain. Lower IR beta-endorphin levels were also found in neurointermediate lobes of weight-restricted rats, but this group had increased plasma IR beta-endorphin concentrations compared to diabetic animals. Concentrations of IR beta-endorphin in microdissected brain regions and in anterior pituitaries of the diabetic animals failed to show consistent changes; in addition, ACTH concentrations in pituitary lobes and plasma did not differ among groups. Circadian rhythmicity of plasma insulin and corticosterone concentrations was absent in the diabetic animals, although food and water intake, while elevated, showed the normal nocturnal pattern of increased ingestion. Furthermore, adrenal hypertrophy was present in the diabetic animals and was accompanied by an elevation of mean plasma corticosterone levels. The present findings indicate that diabetes is associated with a decrease of neurointermediate pituitary lobe synthesis of beta-endorphin, while not affecting the processing of the peptide in this lobe, and confirm previous reports of altered adrenal function in diabetic animals.