Andrew N. Margioris

The corticotropin releasing hormone gene is expressed in human placenta.

Maternal plasma immunoreactive corticotropin-releasing hormone (IR-CRH) increases progressively with pregnancy. This elevated plasma IR-CRH is presumably secreted by the placenta. To investigate further this hypothesis, we searched for the CRH mRNA and its peptide product in full term human placentae. Using a radiolabelled 48-mer oligonucleotide probe complementary to a portion of human CRH mRNA, we identified a 1300 nucleotide RNA from human placenta and rat hypothalami. We next examined the chromatographic characteristics of the placental IR-CRH. The bulk of the IR-CRH extracted from placenta and the IR-CRH secreted in vitro by placental fragments had the same chromatographic profiles as synthetic CRH. These findings indicate that the CRH gene is expressed in human placenta and imply that this organ is a site of CRH biosynthesis during pregnancy.

Stress and Reproduction: Physiologic and Pathophysiologic Interactions between the Stress and Reproductive Axes

Stress is a ubiquitous feature of life. Although its prevalence as a factor in health and disease is difficult to ascertain, it is arguably a significant risk to the well being of the overall population. Stress and the response to stressful events, on the other hand, have played an important part in human survival and evolution. From earliest times, people have dealt with harsh elements and adverse situations requiring both behavioral and physiological responses to protect themselves and their societies.

Human Placenta and the Hypothalamic-Pituitary-Adrenal Axis

Maternal plasma bound and free cortisol rises during pregnancy, while maternal plasma IR-ACTH is initially low but soon rises inspite of the further increase of plasma free cortisol. This rise of plasma ACTH during pregnancy, can not be attributed to changes of plasma levels of estrogens or progesterone. It is possible that the human placenta is responsible for the rise in maternal plasma ACTH during pregnancy. There are two possible mechanisms by which this effect could occur: 1) by the placental secretion of CRH into the maternal circulation, which stimulates the maternal pituitary to secrete ACTH, and 2) the secretion of placental POMC-derived peptides. Recent data indicate that the human placenta is capable of both of these actions: A) The POMC and CRH genes are expressed in human placenta; B) the human term placenta is able to secrete both CRH and POMC-derived peptides in vitro; C) the CRH present in the plasma of pregnant women is bioactive and in sufficient levels to be effective on maternal pituitary; D) synthetic hCRH can stimulate the release of placental POMC peptides in vitro. We conclude that the human placenta may be a modulator of the HPA axis during pregnancy in a number of possible ways. Additional experimental work should clarify the intriguing interaction between the HPA axis and the human placenta during pregnancy, labor and delivery.

The Clinical Implications of Corticotropin-Releasing Hormone

We now appreciate that the brain is the most prolific of all endocrine organs producing scores of neurohormones within and beyond the boundaries of the endocrine hypothalamus. The idea that the brain functions as a gland, however, is not new. Indeed, the evolution of thought leading to the identification of corticotropin releasing hormone (CRH) began around 400 B.C. (1,2). At this time, Hippocrates, in his work entitled De Glandulis, states explicitly, “The flesh of the glands is different from the rest of the body, being spongy and full of veins; they are found in the moist part of the body where they receive humidity... and the brain is a gland as well as the mammae.”

The PC12 rat pheochromocytoma cell line expresses the prodynorphin gene and secretes the 8 kDa dynorphin product

Most adrenal chromaffin cells synthesize opioids derived from proenkephalin but not from prodynorphin. However, human pheochromocytomas and the PC12 rat pheochromocytoma cell line synthesize dynorphins. The aim of this study was to confirm the presence of the authentic prodynorphin transcript and its dynorphin product in PC12 cells. We have found that the sequence of a 458 bp cDNA fragment derived from RT-PCR amplification of total PC12 RNA was in complete accordance with the published sequence of the equivalent region of the prodynorphin gene. It encodes the potent endogenous kappa opioid agonists alpha-neo-endorphin, dynorphin A and dynorphin B. Furthermore, immunoaffinity-purified PC12 cell extracts were subjected to RP-HPLC. Most of its IR-dynorphin eluted on a peak exhibiting the retention time of similarly treated rat anterior pituitary. The expression of the prodynorphin gene in pheochromocytomas can be explained as either the result of (a) the process of dedifferentiation of chromaffin cells to pheochromocytoma which may thus cause the expression of a previously unexpressed prodynorphin or that (b) those pheochromocytomas expressing the prodynorphin gene derive from the few, centrally located chromaffin cells, which express this gene even under normal conditions.

Clinical Presentation and Diagnosis of Cushing’s Syndrome

Prolonged exposure to high levels of glucocorticoids results in a syndrome first described by Cushing. Harvey Williams Cushing (1869–1939) reported the clinical syndrome of amenorrhea, bruising, cutaneous striae, facial plethora, high blood pressure, hirsutism, and myopathy and attributed it to “pituitary basophilism” (1). Bishop and Close were the first to name the new syndrome after Cushing in a case recognized as such the year Cushing published his paper (2). Albright was the first to recognize the role of glucocorticoids in this syndrome (3). A few years later, excessive secretion of corticotropin (ACTH) from pituitary was shown to be the cause of Cushing’s disease (4). The ectopic ACTH syndrome was described by the same person who later established the dexamethasone suppression test, the long-standing golden standard for the differential diagnosis of Cushing’s syndrome (5).

Molecular Development of the Hypothalamic-Pituitary-Adrenal (HPA) Axis

The human hypothalamus lies ventrally to the thalamus and extends from the rostral part of the optic chiasm to the caudal edge of the mamillary bodies. It originates in the alar plate of diencephalon, its sulcus becoming identifiable by the 5th wk of gestation. The hypothalamus is a highly complex organ coordinating central nervous system (CNS) centers and neuroendocrine, metabolic, autonomic nervous and behavioral functions. A system of nerve fibers and portal vessels connects the hypothalamus to the pituitary. The axon terminals of neurons originating in several hypothalamic nuclei, including the supraoptic, the paraventricular, the arcuate, and the ventromedial nucleus, form the median eminence located rostrally to the mamillary bodies. The median eminence communicates with the anterior pituitary via a local portai venous System, which permits the direct hematogenous communication between neurons and endocrine cells. The hypothalamic neurons regulating the function of anterior pituitary corticotrophs are located in the parvocellular part of the paraventricular hypothalamic nucleus. They synthesize the neuropeptides corticotropin-releasing hormone (CRH) and arginine-vasopressin (AVP), which, packaged in secretory vesicles, travel the neural axons and reach the median eminence, where they are placed in specifie secretory areas beneath the subplasmalemal actin network ready to be secreted into the portai System.

Corticotropin Releasing Factor (Hormone): Physiological and Clinical Implications

In 1948, Harris suggested the possibility of humoral control of the pituitary gland by the hypothalamus (1–3). Saffran and Schally (4), and Guillemin and Rosenberg (5) demonstrated the presence of a hypothalamic corticotropin releasing (CRF) factor in 1955. Vale and coworkers isolated ovine CRF (oCRF) in 1981 (6). Shortly thereafter, Schally et al. described the composition of porcine CRF (pCRF) (7), and Rivier et al. that of rat CRF (rCRF) (8). Finally, the genes of both ovine and human CRF (hCRF) were sequenced and the amino acid composition of the corresponding peptides deduced (9,10). Rat and human CRF appear to be chemically identical. The structures of oCRF and hCRF (or rCRF) are shown in Figure 1. Human CRF differs from the oCRF molecule by 7 amino acids, giving the two peptides 83% homology.