Seung P. Kwak

Corticosteroids regulate brain hippocampal 5-HT1A receptor mRNA expression

Using in situ hybridization techniques, the expression of 5-HT1A receptor mRNA was measured within the hippocampal formation after bilateral adrenalectomy (ADX). After 24 hr ADX, 5-HT1A receptor mRNA expression was significantly increased in all hippocampal subfields in ADX animals relative to sham-operated controls (SHAM). The magnitude of the increase was most pronounced within CA2 (127%) and CA3/4 (94%)- subfields of dorsal hippocampus, intermediate in the dentate gyrus (73%), and least within CA1 (60%). Administration of exogenous corticosterone (CORT) at the time of ADX maintained the level of 5-HT1A receptor mRNA expression within the range of SHAM animals. In vitro receptor autoradiographic analysis of 5-HT1A receptors in adjacent sections from the same animals indicated a simultaneous increase in 5- HT1A binding throughout the hippocampus in response to ADX. 5-HT1A binding increased to a similar extent (approximately 30%) in CA subfields and dentate gyrus but remained within SHAM levels in CORT- replaced animals. 5-HT1A receptor mRNA levels were also increased in hippocampal subregions of 1 week ADX animals relative to SHAM animals. Within both CA1 and CA2 subfields, the increments were approximately double those observed after 1 d ADX. 5-HT1A receptor binding was increased in every hippocampal subfield to a similar extent as that observed after 1 d ADX. Increases in both 5-HT1A receptor mRNA expression and 5-HT1A receptor binding were preventable by administration of exogenous CORT at the time of ADX. Hippocampal 5-HT1C receptor mRNA and D1 receptor mRNA expression were not significantly altered by either acute or chronic ADX treatment. These data indicate that adrenal steroids may selectively regulate hippocampal 5-HT1A receptors at the level of 5-HT1A receptor mRNA expression.

hVH‐5: A Protein Tyrosine Phosphatase Abundant in Brain that Inactivates Mitogen‐Activated Protein Kinase

Abstract: A novel protein tyrosine phosphatase [homologue of vaccinia virus H1 phosphatase gene clone 5 (hVH‐5)] was cloned; it shared sequence similarity with a subset of protein tyrosine phosphatases that regulate mitogen‐activated protein kinase. The catalytic region of hVH‐5 was expressed as a fusion protein and was shown to hydrolyze p‐nitrophenylphosphate and inactivate mitogen‐activated protein kinase, thus proving that hVH‐5 possessed phosphatase activity. A unique proline‐rich region distinguished hVH‐5 from other closely related protein tyrosine phosphatases. Another feature that distinguished hVH‐5 from related phosphatases was that hVH‐5 was expressed predominantly in the adult brain, heart, and skeletal muscle. In addition, in situ hybridization histochemistry of mouse embryo revealed high levels of expression and a wide distribution in the central and peripheral nervous system. Some specific areas of abundant hVH‐5 expression included the olfactory bulb, retina, layers of the cerebral cortex, and cranial and spinal ganglia. hVH‐5 was induced in PC12 cells upon nerve growth factor and insulin treatment in a manner characteristic of an immediate‐early gene, suggesting a possible role in the signal transduction cascade.

A novel receptor-type protein tyrosine phosphatase is expressed during neurogenesis in the olfactory neuroepithelium

Tyrosine phosphorylation plays a central role in the control of neuronal cell development and function. Yet, few neuronal protein tyrosine phosphatases (PTPs) have been identified. We examined rat olfactory neuroepithelium for expression of novel PTPs potentially important in neuronal development and regeneration. Using the polymerase chain reaction with degenerate DNA oligomers directed to the conserved tyrosine phosphatase domain, we identified 6 novel tyrosine phosphatases. One of these, PTP NE-3, is a receptor-type PTP expressed selectively in both rat brain and olfactory neuroepithelium. In the olfactory neuroepithelium, PTP NE-3 expression is restricted to neurons and describes a novel pattern of expression with a high level in the immature neurons and a lower level in mature olfactory sensory neurons.

Diurnal CRH mRNA Rhythm in the Hypothalamus: Decreased Expression in the Evening Is Not Dependent on Endogenous Glucocorticoids

Levels of hypothalamic corticotropin-releasing hormone (CRH) mRNA and plasma glucocorticoids vary diurnally as a result of circadian influences on the hypothalamopituitary-adrenal axis. CRH mRNA expression increases from morning to afternoon in rats but decreases rapidly near the onset of dark as glucocorticoids reach peak concentrations in plasma. Since glucocorticoids are normally inhibitory on hypothalamic CRN mRNA expression, we determined whether the glucocorticoid secretion at the diurnal peak reduced CRH mRNA concentration in the evening. We found that adrenalectomy did not prevent the decrease in CRH mRNA levels near the onset of dark. It appears that the drop in CRH mRNA expression occurs via a steroid-independent mechanism. While the mean CRH mRNA level increased after adrenalectomy, the shape of the CRH mRNA rhythm remained unchanged except in the morning. Interestingly, adrenalectomy increased CRH mRNA levels disproportionately in the morning, producing a sharp rise followed by a plateau during the light phase instead of the gradual rise observed in intact animals. We subsequently treated adrenalectomized animals with corticosterone pellets to determine whether a constant steroid signal was sufficient in restoring the normal shape of the mRNA rhythm during the light phase. Results indicate that the endogenous steroid rhythm is not necessary for generating the normal CRH mRNA rhythm during the light phase. Instead, a constant exposure to corticosterone at approximately 50% of the daily mean (2.4-3 micrograms/dl) appears to be sufficient for regulation of the mRNA rhythm.(ABSTRACT TRUNCATED AT 250 WORDS)

Negative feedback regulation following administration of chronic exogenous corticosterone.

Administration of exogenous glucocorticoids is known to suppress the HPA axis and has been reported to occupy brain glucocorticoid receptors, eventually leading to down‐regulation. To determine the effects of chronic corticosterone administration on HPA axis function, corticosterone was administered as both 25% and 50% corticosteronekholesterol pellets. Rats were sacrificed 6 days after corticosterone pellet implantation. The 25% corticosterone pellets produced a small increase in morning corticosterone concentrations but no change in evening ACTH or corticosterone secretion. The 50% corticosterone pellets produced constant corticosterone concentrations of 5–6 pg/dl, with no circadian variation in corticosterone, indicating inhibition of evening ACTH and corticosterone secretion. The 25% corticosterone pellets produced no significant decrease in thymus weight or in adrenal weight; 50% corticosterone pellets produced significant decreases in thymus weight and adrenal weight. Neither 25% nor 50% corticosterone pellets produced significant decreases in GR in hippocampus and cortex. The 50% corticosterone pellets treatment resulted in a decrease in anterior pituitary POMC mRNA levels, a decrease in baseline and oCRH stimulated ACTH release from the anterior pituitary, and a near complete inhibition of the AM and PM response to restraint stress. These results suggest that: 1) the HPA axis was able to adjust to the small increase in glucocorticoids produced by the 25% cort pellets with minimal disturbances in function and 2) 50% corticosterone pellets exert a significant inhibitory effect on stress and diurnal ACTH secretion which appears to be exerted at the pituitary as well as possible inhibitory effects on brain.

α, β, and γ Mineralocorticoid Receptor Messenger Ribonucleic Acid Splice Variants: Differential Expression and Rapid Regulation in the Developing Hippocampus1

Two different types of corticoid receptor molecules bind circulating corticosterone in brain: mineralocorticoid receptors (MR) and glucocorticoid receptors. MR exhibit the highest affinity for the endogenous glucocorticoid in the rat, corticosterone. During development, low corticosterone levels influence neurogenesis, and these effects are probably MR mediated. Three MR complementary DNA clones, α, β, and γ, have been identified in the rodent. All of these MR complementary DNA clones have identical coding regions, but differ significantly at the 5′-untranslated end. Although the functional significance of these three messenger RNA (mRNA) species remains unknown, one hypothesis is that they reflect the ability of the brain to regulate the expression of MR, allowing multiple factors to differentially control transcription in a tissue- and time-specific manner. To investigate this possibility, we examined the presence of these distinct mRNA forms in the developing rat hippocampus (HC). In situ hybridization...

Changes in Proopiomelanocortin Primary Transcript Levels in the Anterior Pituitary Accompany Increased Adrenocorticotropin Secretion During the Diurnal Surge

Proopiomelanocortin (POMC) gene transcription in the anterior pituitary varies during stress and glucocorticoid feedback. These changes appear to parallel alterations in peptide release. The diurnal rhythm of the hypothalamo‐pituitary‐adrenal axis also involves the periodic excursion of adrenocorticotropin (ACTH) levels in plasma, but it is not clear whether the diurnal release is accompanied by changes at the transcriptional level. In the present study, we have initially characterized the heteronuclear species of POMC (hnPOMC) RNA found in the anterior pituitary by a Northern blot analysis and subsequently used this method to quantitate relative changes in the levels of heteronuclear transcript during diurnal stimulation. Two species of RNA migrating at 6.0 kb and 4.1 kb were found in the nuclear fraction of the anterior pituitary. Successive probing by various POMC cRNAs indicated that the 6.0 kb fragment was the primary transcript and the 4.1 kb fragment corresponded to the intron A‐containing processing intermediate of POMC.