Inhibition of histone deacetylase 2 increases apoptosis and p21Cip1/WAF1 expression, independent of histone deacetylase 1

Histone deacetylases (HDACs) 1 and 2 share a high degree of homology and coexist within the same protein complexes. Despite their close association, each possesses unique functions. We show that the upregulation of HDAC2 in colorectal cancer occurred early at the polyp stage, was more robust and occurred more frequently than HDAC1. Similarly, while the expression of HDACs1 and 2 were increased in cervical dysplasia and invasive carcinoma, HDAC2 expression showed a clear demarcation of high-intensity staining at the transition region of dysplasia compared to HDAC1. Upon HDAC2 knockdown, cells displayed an increased number of cellular extensions reminiscent of cell differentiation. There was also an increase in apoptosis, associated with increased p21Cip1/WAF1 expression that was independent of p53. These results suggest that HDACs, especially HDAC2, are important enzymes involved in the early events of carcinogenesis, making them candidate markers for tumor progression and targets for cancer therapy.

The NUS MBBS-PhD programme: nurturing clinician-scientists for tomorrow

The MBBS-PhD programme is a significant milestone in medical education in Singapore. In July 2000, the Faculty of Medicine, National University of Singapore launched this programme in collaboration with the Institute of Molecular and Cell Biology, with support from the Economic Development Board, and the Agency for Science, Technology and Research, Singapore. The objectives of the programme are to nurture and develop the talents of the brightest medical students by integrating clinical and basic biomedical research training, as well as to stimulate advanced basic and applied research in areas of growing importance to clinical medicine. The programme also aims to train clinician-scientists who will interface basic biology and clinical practice to solve biomedical problems and spearhead biomedical research initiatives in Singapore. Successful MBBS-PhD graduates can pursue career tracks in clinical research, basic biomedical research or in the biotechnology industry.

One hundred years of physiology education in Singapore

Physiology is the study of normal function in the body and how genes, proteins, organ systems interact to maintain health. It provides a foundation for the health sciences profession and life science research. Physiology education in Singapore began soon after the establishment of the Federated States Government Medical School in 1905. The importance of Physiology to medical education was recognised by the appointment of a separate lecturer in Physiology in 1906, followed by the appointment of Professor James Argyll Campbell as the first King Edward VII professor and endowed Chair in Physiology in 1912. The teaching of Physiology in the early days was focused on the basics of normal function with little correlation to clinical problems and application. However, by the 1970s, first-year medical students were given the opportunity to visit hospitals where they were tutored by clinicians to help them apply Basic Physiology to clinical problems. Curriculum changes in the subsequent years emphasised a reduction in content, integration among preclinical subjects, independent learning and clinical relevance. Physiology is taught not only to medical but also dental, pharmacy and life science students. The teaching of Physiology to science students is a collaborative effort between the Department of Physiology, Faculty of Medicine and the Department of Biological Sciences, Faculty of Science. A lot of the teaching of Physiology to life science students occurs not in classrooms but in the laboratories, where students work closely with research supervisors and mentors on research projects. There has been a very significant increase in the number of students doing research projects in Physiology in recent years, especially in the areas of Cell Physiology, Immunology and Neurobiology. The completion of the human genome sequence poses new challenges to understand function, especially how genes, proteins and organ systems interact to sustain function. Physiology education will be increasingly important in the undergraduate and graduate life science and medical curriculum. Further, the country's vision of being the biomedical R&D and healthcare hub for the region means that Physiology education must remain at the forefront to prepare the next generation of doctors, clinician-scientists, researchers and entrepreneurs.

Differential expression of Hox A5 in human colon cancer cell differentiation: a quantitative study using real-time RT-PCR

Fifteen different homeobox genes were identified from normal colon mucosa, untreated COLO 205 and herbimycin A treated COLO 205 cells in a degenerate primer RT-PCR screen. Several of the homeobox genes, including Cdx-1, Cdx-2, Pdx-1 and Hox A5, showed a trend toward differential expression in normal colon mucosa, and undifferentiated COLO 205 cells. Hox A5 was recently shown to suppress growth and induce p53-dependent apoptosis. To determine if Hox A5 was differentially expressed in differentiation of colon epithelial cells, we quantified Hox A5 expression by real-time quantitative RT-PCR. Expression of Hox A5 was 5.3- and 4.8-fold higher in normal colon mucosa compared to COLO 205 and HT-29 cells, respectively, suggesting that Hox A5 expression was higher in differentiated compared to undifferentiated colon epithelial cells. To avoid the complexity of tissue specimens and the influence of individual variation in Hox A5 expression, the effect of differentiation on Hox A5 expression was studied in COLO 205 cells treated with herbimycin A. The quantitative study showed that Hox A5 expression was increased when COLO 205 cells were induced to differentiate. The expression of Hox A5 was about 2-fold higher in the cells treated for 48 h compared to the untreated poorly-differentiated cells. The present study shows that Hox A5 may be involved in intestinal cell differentiation, in addition to its role in apoptosis.

Characterization and expression of the mouse pregnant specific uterus protein gene and its rat homologue in the intestine and uterus

The pregnant specific uterus protein gene (Psup) is a novel mouse gene expressed in pregnant uterus. This paper describes the identification and expression of the rat homologue of Psup. The gene is highly expressed in the duodenum. Expression decreases in a proximal-distal gradient in the small intestine and was not detected in the cecum and colon. The pattern of expression in the mouse was similar. Expression of Psup in the mouse was localized to the epithelial cells in the intestine and pregnant uterus by in situ hybridization. The data show tissue-specific expression of Psup.

Ribosomal protein L7a gene is up-regulated but not fused to the tyrosine kinase receptor as chimeric trk oncogene in human colorectal carcinoma

Ribosomal protein L7a (rp L7a) was identified in a subtractive hybridization screen as a gene up-regulated in human colorectal cancer. Expression of rp L7a was greater than 2-fold higher in tumors compared to adjacent normal mucosa in 72% of the patients studied (n=36). rp L7a was also up-regulated in concomitant polyps. The number of patients with rp L7a T/N ratio of >2 was significantly higher in the female (16/18) than in the male (10/18). rp L7a expression was also significantly higher in females with lymph node involvement compared to males. These results indicate that rp L7a expression is related to tumor growth in colorectal cancer especially in females, where it may also be related to tumor spread. There was no correlation of rp L7a expression with tumor cell differentiation. We also show that rp L7a does not exist as a fusion oncogene (trk-2h) in colorectal cancer.

Syncollin is differentially expressed in rat proximal small intestine and regulated by feeding behavior

Gradients of gene expression are maintained along the proximal-distal axis of the mammalian small intestine despite a continuously regenerating epithelium. To study the molecular mechanisms responsible for this phenomenon, we utilized a subtractive hybridization strategy to isolate genes differentially expressed in the duodenum but not ileum. We isolated and sequenced 15 clones. The clones were fragments of genes encoding lipases, proteases, and an esterase. A novel clone was characterized and subsequently shown to encode syncollin, a secretory granule protein that binds to syntaxin in a calcium-sensitive manner. RT-PCR and S1 nuclease protection assay were used to clarify the 5'-end of syncollin. Syncollin was expressed in the rat pancreas, spleen, duodenum, and colon. In situ hybridization localized syncollin expression in the pancreas to acinar cells and in the duodenum to villus epithelial cells.

Stimulation of anterior pituitary galanin and prolactin gene expression in suckling rats

Recent evidence suggests that galanin may regulate prolactin (PRL) secretion during lactation. In this article, we describe the regulation of anterior pituitary galanin and PRL gene expression during pregnancy and after parturition in the rat. Expression of galanin and PRL in the anterior pituitary were significantly higher at d 20 of pregnancy compared to diestrus. One day after parturition, galanin mRNA levels increased a further 4.5-fold. This post partum increase in gene expression was not observed for PRL. The increase in galanin gene expression was maintained above the diestrous level for at least 10 d after parturition. PRL mRNA expression, on the other hand, was largely unchanged after parturition. Although the increase in galanin gene expression 1 d after parturition was independent of suckling, subsequently, galanin gene expression was significantly higher in nursing mothers. Anterior pituitary galanin gene expression was 12-fold higher in nursing mothers compared with those that were not, 3 d after parturition. Similarly, PRL gene expression was significantly lower in mothers who were not suckling their pups 3 d after parturition. Initiation of suckling alone was insufficient to stimulate galanin and PRL expression. Despite suckling for 2 d, removal of the suckling stimulus subsequently resulted in a rapid decrease in galanin gene expression. Hence, the stimulatory effect of suckling on galanin expression requires a sustained suckling stimulus. In conclusion, the data support the hypothesis that anterior pituitary galanin plays an important role during lactation, likely acting to amplify lactotroph stimulation through paracrine and autocrine mechanisms.

Heparin/heparan sulfate interacting protein gene expression is up-regulated in human colorectal carcinoma and correlated with differentiation status and metastasis

We applied a subtractive hybridization strategy to obtain genes that are differentially expressed in colorectal carcinoma. Heparin/heparan sulfate interacting protein (HIP) was shown to be up-regulated in colorectal carcinoma. A study of 53 patients with documented colorectal carcinoma showed that 70% of the tumors had HIP tumor-to-normal ratios (expression in tumor tissue compared to expression in normal mucosa) of >2. In six patients with concomitant polyps, HIP expression in the polyps was similar to the carcinoma, showing that up-regulation of HIP may be an early event in tumorigenesis. A significant inverse correlation between HIP levels and the presence of distant metastasis (Duke's stage D) was noted. Similarly, HIP expression was also related to differentiation status in human colorectal carcinoma cell lines. HIP expression was lower in the poorly differentiated COLO 205 cell line compared to the well-differentiated HT-29 cell line. The correlation was further strengthened by studies in COLO 205 cells that were induced to differentiate with herbimycin A treatment. HIP expression was significantly higher when the cells were induced to differentiate. Withdrawal of herbimycin A resulted in a reversal of morphological changes associated with differentiation and an associated decrease in HIP expression. These studies indicate that HIP is an important molecule for cell-cell and cell-extracellular matrix adhesion. The up-regulation of HIP may be an early event in tumorigenesis, and its increased expression may facilitate growth and local invasion. A lower expression of HIP in tumors results in decreased cell adhesion, favoring metastasis. HIP is a candidate marker of abnormal cell growth in the colon and a prognostic marker for colorectal carcinoma.

Identification and characterization of a novel rat triosephosphate isomerase gene in remnant ileum after massive small bowel resection

This paper describes the identification and characterization of a novel cDNA encoding a putative protein of 254 amino acids that is highly homologous to triosephosphate isomerase. The cDNA was isolated by subtractive hybridization and was differentially expressed in the remnant rat ileum after massive small bowel resection. The novel triosephosphate isomerase was named rsTPI (resection-induced TPI) and the putative protein encoded RSTPI. The nucleotide and amino acid sequences of rsTPI and RSTPI were about 60% and 62% homologous to Giardia lamblia TPI and TPI, respectively. Active catalytic sites (Lys 13, His 95, and Glu 167) and the peptide motifs, AYEPVWSIGT and GGASLKPEF found in other triosephosphate isomerases were conserved in RSTPI. rsTPI expression was detected in normal ileum and pancreas by reverse transcription-polymerase chain reaction. Expression of rsTPI in remnant rat ileum was detectable by northern blot analysis one week after massive small bowel resection. Expression increased significantly by 2.8-fold between one and two weeks after surgery. High levels were maintained for at least one month after surgery. The up-regulation of triosephosphate isomerase expression in the remnant small intestine after massive resection indicates that it may play an important role in the adaptive process.

Increase in plasma leptin and Lep mRNA concentrations by food intake is dependent on insulin

Obese (Lep) gene expression and leptin secretion are regulated by changes in food intake. However, the mechanism by which leptin concentrations are altered by fasting and feeding is unclear. Since these changes occur in parallel with changes in plasma insulin, it is possible that the changes observed are mediated by insulin. To test this hypothesis, we studied the role of insulin in the regulation of Lep gene expression in epididymal fat and leptin secretion during feeding. As shown previously, fasted animals showed significant reductions in Lep mRNA, plasma leptin, and plasma insulin concentrations. Conversely, feeding increased plasma insulin, Lep mRNA, and plasma leptin. In streptozotocin (STZ)-treated animals, plasma insulin concentrations were low. This was associated with low Lep mRNA and plasma leptin concentrations. Changes in food intake, whether fasting or feeding, did not significantly alter plasma insulin levels in STZ-treated animals. Under these circumstances, Lep mRNA and plasma leptin concentrations also remained low. Our results demonstrate that the decrease in Lep mRNA and plasma leptin during fasting and the increase with feeding are dependent on changes in the plasma insulin concentration.

Regulation of leptin expression and secretion by corticosteroids and insulin. Implications for body weight

Leptin is an important hormone that has potent effects on appetite and body weight. The regulation of leptin gene expression and secretion by corticosteroids and insulin was studied in the rat. Adrenalectomy resulted in a significant reduction in leptin gene expression and secretion. The reduction was corrected by hormonal replacement with corticosterone pellets, showing that normal levels of circulating corticosteroids are required to maintain leptin expression and secretion in the body. Chronic treatment with dexamethasone (DEX) over 3 wk did not significantly increase leptin gene expression and secretion, contrary to earlier reports using shorter treatment paradigms. The profound weight loss associated with chronic DEX treatment may have abrogated the direct stimulatory effect of DEX on leptin gene expression and secretion, indicating a possible crosstalk between corticosteroids and leptin in the regulation of body weight. Shorter-term treatment of animals with DEX (3.7 micrograms/g body wt; 24 h) increased leptin gene expression and secretion about 2-fold and 1.4-fold, respectively. The increase was independent of circulating insulin concentrations. In streptozotocin-treated rats, short-term DEX treatment increased leptin gene expression and secretion about 3.5-fold and 2-fold, respectively. The data indicate that circulating leptin concentrations and adipose tissue leptin expression are dependent on corticosteroids and insulin. Although acute DEX treatment resulted in a stimulatory effect on leptin secretion and expression, chronic DEX treatment did not. The stimulatory effect of DEX on leptin is independent of circulating insulin concentrations.

Regulation of anterior pituitary galanin gene expression by thyroid hormone

Thyroid hormone is required for basal and estrogen-induced expression of anterior pituitary galanin. Steady-state anterior pituitary galanin mRNA levels decreased 6-fold in hypothyroid rats after 3 weeks of treatment. Similarly, hypothyroidism resulted in a 2.6-fold decrease in estrogen induction of galanin gene expression. The effect of thyroid hormone on anterior pituitary galanin gene expression appears to be exerted, at least in part, at the pituitary itself. Transient expression assays in GH3 cells suggest the involvement of transcriptional mechanisms in the regulation of galanin gene expression by thyroid hormone. A region between -41 and -132 bp upstream of the transcriptional start site confers thyroid hormone responsiveness to the galanin gene. Gel-mobility shift assays show specific binding of 'SPI-like' proteins in GH3 nuclear extracts to this region of the galanin gene. This binding was greatly enhanced by thyroid hormone.

Coexpression of galanin and adrenocorticotropic hormone in human pituitary and pituitary adenomas

Galanin is a neuropeptide that regulates the secretion of several pituitary hormones, including prolactin (PRL) and growth hormone (GH). Galaninlike immunoreactivity (Gal-IR) and galanin mRNA in the rat anterior pituitary is cell lineage specific, with predominant expression in lactotrophs and somatotrophs. The authors examined the cellular distribution of human Gal-IR in seven normal postmortem pituitaries and 62 pituitary tumors by immunoperoxidase staining. In contrast to the rat, Gal-IR in human anterior pituitaries was present in corticotrophs scattered throughout the gland, but not in lactotrophs, somatotrophs, thyrotrophs, or gonadotrophs. Distinct Gal-IR also was present in hyperplastic and neoplastic corticotrophs in 19 of 22 patients with Cushing's disease. In noncorticotroph cell tumors, unequivocal Gal-IR was present in 5 of 11 GH-secreting tumors associated with clinical acromegaly, 9 of 18 nonfunctioning pituitary adenomas, and 2 of 14 prolactinomas. Of these galanin-positive tumors, four of the five GH-secreting adenomas, six of the nine nonfunctioning adenomas, and both of the prolactinomas also contained adrenocorticotropic hormone immunoreactivity (ACTH-IR). Immunostaining and in situ hybridization on adjacent sections using an 35S-labeled probe complementary to human galanin mRNA demonstrated predominant galanin expression in normal corticotrophs. Immunoelectron microscopy confirmed the presence of Gal-IR in pituitary cells characteristic of corticotrophs in both normal and neoplastic pituitaries. Thus, as in the rat, galanin gene expression in the human pituitary is cell-type specific. Unlike the rat, however, human galanin gene expression is restricted to the corticotroph lineage. Studies of tumors confirmed the observed coexpression of galanin and adrenocorticotropic hormone. The divergent cell type specificity of galanin production in human and rat pituitaries reflects different patterns of gene activation in these two species. In addition, these results suggest that galanin in the human pituitary may participate locally in the regulation of the hypothalamic-pituitary-adrenal axis.

Galaninergic mechanisms are involved in the regulation of corticotropin and thyrotropin secretion in the rat

Galanin (GAL), a 29-amino acid peptide, affects the secretion of several anterior pituitary hormones, including PRL and GH. Since GAL coexists with vasopressin and CRH in the hypothalamic paraventricular nucleus (PVN), we have studied the pharmacological and physiological actions of GAL on ACTH and TSH secretion in freely moving male rats. Cannulae were surgically implanted in the right atria and brain, intraventricular or adjacent to the PVN, of adult Sprague-Dawley rats. Seven days later, GAL (500 or 1000 ng) or saline was infused into the PVN, and serial blood samples were obtained 5, 10, 20, and 40 min after the infusion. Some animals were also stressed by the inhalation of ether vapors for 2 min after the PVN infusion. Basal ACTH concentrations were increased 2-fold in saline-treated rats; however, plasma ACTH levels were unchanged after GAL infusion. The exposure of rats to ether vapors for 2 min after the infusion of saline into the PVN increased plasma ACTH concentrations from 22.8 +/- 6.0 to 596.6 +/- 59.9 pg/ml 10 min later. However, the infusion of GAL into the PVN attenuated stress-induced ACTH secretion. After GAL infusion, peak ACTH levels (332.7 +/- 84.0 pg/ml) were attained 5 min after ether exposure, followed by a rapid decline at 10 min (P less than 0.001) and 20 min (P less than 0.05). Plasma TSH concentrations were unchanged by GAL or saline infusion and were not affected by ether vapor inhalation. To determine the physiological significance of GAL in the control of ACTH and TSH secretion, endogenous GAL was immunoneutralized by the infusion of 3 microliters GAL antiserum (GAL-AS) into the third cerebral ventricle 25 and 1 h before withdrawing blood samples every 15 min for 6 h. Animals treated with normal rabbit serum (NRS) served as controls. Plasma ACTH concentrations were unchanged by NRS during the 6-h period. However, infusion of GAL-AS raised plasma ACTH concentrations to over 400 pg/ml 75 min after infusion in some animals. In general, plasma ACTH concentrations were increased 4 h of the 6-h sampling period compared to levels in NRS-treated controls. In contrast, GAL-AS reduced TSH concentrations by 50% compared to control values. In contrast to these marked actions of centrally administered GAL, ACTH secretion from dispersed anterior pituitary cells in vitro was unaffected by GAL in concentrations up to 10(-6) M. Furthermore, GAL did not alter CRH (1 nM)-induced ACTH secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of thyroid hormone on the concentration of galanin in the rat brain and pituitary

Galanin (GAL) is a 29-amino acid peptide implicated in neuroendocrine regulation of prolactin, growth hormone and thyrotropin in the rat. GAL-like immunoreactivity and GAL messenger RNA (mRNA) are present in the anterior pituitary (AP) and hypothalamus and the expression of GAL mRNA has been shown to be modulated by peripheral gonadal steroid hormones. In view of possible interactions between members of the steroid/thyroid hormone receptor family and recent data suggesting an effect of GAL on thyrotropin secretion, we investigated the possible influence of thyroid status on GAL concentrations in the hypothalamus and AP of the male rat. Three weeks after the surgical removal of the thyroid gland from male rats, the concentrations of GAL in the median eminence (ME) and AP were reduced 54 and 65%, respectively. Similarly, GAL concentrations were decreased 39% in the ME and 69% in the AP of animals rendered hypothyroid by treatment with propylthiouracil (PTU). The effects of PTU treatment in both regions were reversed by daily T4 injections (50 micrograms/kg). The effects of PTU in the ME were reversed after 2 weeks of T4 treatment, whereas 3 weeks of replacement therapy were required to restore GAL concentrations in the AP. However, T4 treatment of intact control animals did not influence GAL concentrations. This study demonstrates that the presence of thyroid hormones is required for the maintenance of physiological concentrations of GAL in the hypothalamus and AP of the rat. These data also suggest that GAL may be involved in the negative feedback regulation of the hypothalamohypophysial-thyroid axis.

Galanin is a physiological regulator of spontaneous pulsatile secretion of growth hormone in the male rat

To determine whether galanin (GAL), a 29-amino acid neuropeptide, plays a role in the physiological regulation of the pulsatile secretion of GH and PRL in the male rat, secretory patterns of both hormones were studied in freely moving animals after GAL passive immunoneutralization. Adult male Sprague-Dawley rats were equipped with iv and intracerebroventricular catheters. After 7 days, 3 microliters of a specific GAL antiserum (GAL-AS) or normal rabbit serum (NRS; controls) were infused in the third ventricle of 10 rats, 25 and 1 h before the animals were bled every 15 min for 6 h (1000-1600 h). Plasma GH and PRL concentrations were measured by RIA, and the hormonal secretory patterns were analyzed by the PULSAR program. Control rats, treated with NRS, displayed typical GH secretion, with pulses of high amplitude (167 +/- 27 ng/ml) and low frequency (2.4 +/- 0.2 pulses/6 h), separated by periods of low trough levels (3.8 +/- 0.6 ng/ml). Rats treated with GAL-AS had altered pulsatile GH secretion. Pulse height was markedly reduced (77 +/- 15 ng/ml; P less than 0.01 vs. controls), and peak frequency was higher (3.6 +/- 0.5 pulses/6 h; P less than 0.05), while GH baseline levels and integrated GH secretion over the 6-h sampling period remained unaltered. Injection of rat GH-releasing hormone (1 microgram/rat, iv) caused a similar GH stimulation in both groups of rats, as determined by the peak GH response at 5 min (368 +/- 112 vs. 342 +/- 81 ng/ml) or by the integrated GH response over 1 h (5.13 +/- 1.30 vs. 4.77 +/- 1.15 micrograms.min/ml in NRS- and GAL-AS-treated rats, respectively; P less than 0.05). In contrast to GH, pulsatile secretion of PRL was not affected by the GAL-AS treatment. These results indicate that GAL is a physiological regulator of spontaneous pulsatile secretion of GH, but not PRL, in the male rat. The influence of GAL on GH secretion appears to be exerted within the hypothalamus, mainly by a stimulation of GRF secretion. However, the changes in GH pulse frequency observed after GAL immunoneutralization suggest that GAL might also influence the somatostatin inhibitory tone.

Neuropeptide Y (NPY) and vasopressin (AVP) in the hypothalamo-neurohypophysial axis of salt-loaded or Brattleboro rats

A close anatomical relationship between nerve terminals containing neuropeptide Y (NPY) and vasopressin (AVP) has been demonstrated in the hypothalamic paraventricular (PVN) and supraoptic nuclei (SON). Furthermore, injections of NPY into the SON increased plasma concentrations of AVP in the rat. These data suggest a potential involvement of hypothalamic NPY in fluid homeostasis in the rat. Therefore, we have studied the effect of elevated plasma osmolality on the concentration of NPY and AVP in the hypothalamus and neurointermediate lobe (NIL) of the pituitary gland. Furthermore, we measured the concentration of NPY in the AVP-deficient Brattleboro rat, which suffers from diabetes insipidus and hyperosmolality. Salt-loading increased plasma osmolality and the concentration of AVP from 2.0 +/- 0.5 to 4.1 +/- 0.6 pg/ml after 7 days. The concentration of NPY in the NIL doubled after 7 days of salt-loading, from 7.9 +/- 0.6 ng/mg protein to 15.2 +/- 1.4 ng/mg protein, whereas AVP concentrations fell from 2285.7 +/- 210.9 ng/mg protein to 187.5 +/- 2.5 ng/mg protein. AVP concentrations in the ME increased transiently after 2 days of salt-loading and returned to control levels after 7 days. In contrast, NPY concentrations in the ME were unchanged at 2 days and were increased 61% after 7 days. NPY concentrations also were significantly elevated after 7 days of salt-loading in the preoptic area (POA) and mediobasal hypothalamus (MBH). The concentration of NPY in the NIL of the homozygous Brattleboro rat was 2-fold greater than in the heterozygous Brattleboro rat and 4-fold greater than in Sprague-Dawley rats used as controls.(ABSTRACT TRUNCATED AT 250 WORDS)