Maternal thyroid status influences early brain development and, consequently, cognitive and motor function in humans and rats. The biochemical targets of maternal thyroid hormone (TH) action in fetal brain remain poorly defined. A partially thyroidectomized rat dam model was therefore used to investigate the influence of maternal hypothyroxinemia on the specific activities of cholinergic and monoaminergic neurotransmitter metabolic enzymes in the developing brain. Maternal hypothyroxinemia was associated with reduced monoamine oxidase (MAO) activity in fetal whole brain at 16 and 19 days gestation (dg). A similar trend was observed for choline acetyltransferase (ChAT) activity. In contrast, DOPA decarboxylase (DDC) activity was markedly elevated at 21 dg. Further study of these enzymes at 14 dg showed no differences between normal and experimental progeny - suggesting they become TH sensitive after this age. Tyrosine hydroxylase (TyrH) and acetylcholinesterase (AChE) activities were unaffected prenatally. During postnatal development, the activities of TyrH, MAO, DDC and, to a lesser extent, AChE were increased in a brain region- and age-specific manner in experimental progeny. The prenatal disturbances noted in this study may have wide-ranging consequences since they occur when neurotransmitters have putative neurotropic roles in brain development. Furthermore, the chronic disturbances in enzyme activity observed during postnatal life may affect neurotransmission, thereby contributing to the behavioural dysfunction seen in adult progeny of hypothyroxinemic dams.
IM Evans, AK Sinha, MR Pickard, PR Edwards, AJ Leonard and RP Ekins
M. R. Pickard, A. K. Sinha, L. Ogilvie and R. P. Ekins
The influence of maternal hypothyroxinaemia on early brain and placental development was examined in a partially thyroidectomized (parathyroid-spared; TX) rat dam model. Ornithine decarboxylase (ODC) specific activity, along with more general indices of cell growth, were determined in prenatal whole brain (at 15, 19 and 22 days of gestation), postnatal brain regions (at 5, 10 and 14 days) and placenta.
Maternal hypothyroxinaemia resulted in reductions in fetal body weight, brain weight, brain DNA content and brain total protein content at 15 days of gestation; the latter effect persisting until 19 days of gestation. Further changes in brain cell growth were observed near term, when an increase in the DNA concentration was accompanied by a decrease in the total protein: DNA ratio. Growth of the postnatal brain regions appeared normal, with the exception of an isolated increase in the protein content of the cerebellum at postnatal day 5. Determination of the specific activity of brain ODC revealed a complex pattern of change in the progeny of TX dams, super-imposed upon the normal ontogenetic decline. In the fetal brain, activity was initially deficient at 15 days of gestation but was increased at 22 days of gestation relative to controls. The compromise extended into the postnatal period; ODC specific activity being transiently reduced in the brainstem, the subcortex and the cerebral cortex. Placental development was less consistently affected; wet weight, gross indices of cell growth (DNA content, DNA concentration, total protein: DNA ratio) and ODC specific activity were all normal in the TX dam. However, cytosolic and total protein concentrations were reduced at 15 and 19 days of gestation respectively.
These results demonstrate abnormal fetal brain cell development as a consequence of maternal hypothyroxinaemia. The damage extended into the neonatal period, well after the onset of fetal thyroid hormone synthesis. Although the reduced supply of maternal thyroxine to the fetal brain may play a major role in this dysgenesis, factors such as the impairment of placental function must be taken into consideration.
Journal of Endocrinology (1993) 139, 205–212
MR Pickard, AK Sinha, LM Ogilvie, AJ Leonard, PR Edwards and RP Ekins
The influence of maternal hypothyroxinemia on the expression of the glucose transporters, GLUT1 and GLUT3, in rat fetal brain and placenta was investigated. Fetal growth was retarded in hypothyroxinemic pregnancies, but only before the onset of fetal thyroid hormone synthesis. Placental weights were normal, but placental total protein concentration was reduced at 19 days gestation (dg). Immunoblotting revealed a decreased abundance of GLUT1 in placental microsomes at 16 dg, whereas GLUT3 was increased. Fetal serum glucose levels were reduced at 16 dg. In fetal brain, the concentration of microsomal protein was deficient at 16 dg and the abundance of parenchymal forms of GLUT1 was further depressed, whereas GLUT3 was unaffected. Northern hybridization analysis demonstrated normal GLUT1 mRNA levels in placenta and fetal brain. In conclusion, maternal hypothyroxinemia results in fetal growth retardation and impaired brain development before the onset of fetal thyroid function. Glucose uptake in fetal brain parenchyma may be compromised directly, due to deficient GLUT1 expression in this tissue, and indirectly, as a result of reduced placental GLUT1 expression. Though corrected by the onset of fetal thyroid hormone synthesis, these deficits are present during the critical period of neuroblast proliferation and may contribute to long term changes in brain development and function seen in this model and in the progeny of hypothyroxinemic women.
JH Mitchell, F Nicol, GJ Beckett and Arthur JR
Adequate dietary iodine supplies and thyroid hormones are needed for the development of the central nervous system (CNS) and brown adipose tissue (BAT) function. Decreases in plasma thyroxine (T4) concentrations may increase the requirement for the selenoenzymes types I and II iodothyronine deiodinase (ID-I and ID-II) in the brain and ID-II in BAT to protect against any fall in intracellular 3,3',5 tri-iodothyronine (T3) concentrations in these organs. We have therefore investigated selenoenzyme activity and expression and some developmental markers in brain and BAT of second generation selenium- and iodine-deficient rats. Despite substantial alterations in plasma thyroid hormone concentrations and thyroidal and hepatic selenoprotein expression in selenium and iodine deficiencies, ID-I, cytosolic glutathione peroxidase (cGSHPx) and phospholipid hydroperoxide glutathione peroxidase (phGSHPx) activities and expression remained relatively constant in most brain regions studied. Additionally, brain and pituitary ID-II activities were increased in iodine deficiency regardless of selenium status. This can help maintain tissue T3 concentrations in hypothyroidism. Consistent with this, no significant effects of iodine or selenium deficiency on the development of the brain were observed, as assessed by the activities of marker enzymes. In contrast, BAT from selenium- and iodine deficient rats had impaired thyroid hormone metabolism and less uncoupling protein than in tissue from selenium- and iodine-supplemented animals. Thus, the effects of selenium and iodine deficiency on the brain are limited due to the activation of the compensatory mechanisms but these mechanisms are less effective in BAT.
R. E. Hutchison, A. W. Wozniak and J. B. Hutchison
Oestrogen is formed in the female dove brain. The aim of this study was to determine whether (a) the catalytic properties of the brain aromatase are similar to the ovarian enzyme and (b) aromatase activity in the female brain changes during the reproductive cycle and is influenced by steroids and environmental stimuli.
The results show that female preoptic aromatase has a higher substrate affinity than the enzyme in ovarian follicles (apparent K m: preoptic area, 7 nmol/l; ovarian follicles, 29 nmol/l), but a lower activity in the preoptic area (Vmax: preoptic area, 290 fmol/mg tissue per h; ovarian follicles, 843 fmol/mg tissue per h). In intact females with developing follicles, oestradiol-17β formation was higher in the posterior hypothalamus than the preoptic area. Females in a later stage of reproductive development (yolked follicles) had a different distribution of oestrogen formation with increased aromatase activity in the preoptic area. Preoptic and posterior hypothalamic aromatase activity of females paired with males for 10 days was positively correlated (r = 0·84, P = 0·0001; r = 0·75, P = 0·001 respectively) with ovarian development. Females with undeveloped ovaries which interacted with males had higher preoptic aromatase activity than visually isolated females with similar ovarian development, suggesting that behavioural stimuli have direct effects on brain aromatase activity which are independent of the ovary. Oestradiol benzoate treatment increased preoptic and posterior hypothalamic aromatase activity in intact and ovariectomized females, and testosterone propionate treatment increased anterior hypothalamic aromatase activity, but did not affect other areas, indicating that the distribution of induced aromatase activity is steroid-specific. Oestrogen treatment in ovariectomized or intact females did not replicate the maximal hypothalamic aromatase activity seen when the ovary contained yolked follicles.
We conclude that brain aromatase activity is related directly to ovarian condition during the reproductive cycle of the female dove. As in the male, steroids have a role in the regulation of oestrogen formation in the female hypothalamus; behavioural stimuli are also likely to be involved in the control of the brain enzyme.
Journal of Endocrinology (1992) 134, 385–396
C. OLIVER, C. R. PARKER JR and J. C. PORTER
*Laboratoire de Médecine Expérimentale, UER Médecine Nord, Boulevard Pierre Dramard, 13326 Marseille Cedex 3, France and †Department of Obstetrics and Gynecology, Southwestern Medical School, 5323 Harry Hines Boulevard, Dallas, Texas 75235, U.S.A.
(Received 15 March 1977)
Thyrotrophin releasing hormone (TRH) is rapidly degraded when incubated at 37 °C with plasma (Redding & Schally, 1969) or brain homogenates (Bassiri & Utiger, 1974) from adult rats. However, immunoreactive TRH is stable in serum obtained from rats less than 2 weeks old (Oliver, Taurog & Porter, 1974). No loss of biological or immunological TRH activity occurs during incubation with serum from 4- or 16-day-old rats (Neary, Kieffer, Federico, Mover, Maloof & Soodak, 1976). In this report, we have determined the TRH degrading activity of brain homogenates and serum obtained from male rats at various stages of development after birth.
Synthetic TRH (1 ng, Beckman Instruments, Inc.) diluted in 50 μl phosphate-buffered saline (0·01
J K McQueen
When glial cells were first described over a century ago, in the classic studies by Spanish neuroanatomists Cajal and Hortega, they were envisaged as equivalent to connective tissue, providing only support for the 'real' brain cells – the neurons. However, within the last decade it has become increasingly evident that glial cells play an active and crucial role in two fundamental areas: the development of the mammalian nervous system and the maintenance of normal brain function.
The vast majority of glial cells in the nervous system are classed as macroglia, and, based on structural characteristics, can be further subdivided into oligodendrocytes (oligodendroglia), radial glia and astrocytes (or astroglia). A much smaller group, termed microglia, develop from non-neural tissue and act as macrophages (for review see Thomas 1992). Oligodendrocytes are found in white matter and are the source of the myelin of the axon sheath; their origin and functional properties
J F F Powell, S L Krueckl, P M Collins and N M Sherwood
Three species of fish have become important in the study of reproduction and development. Rockfish are a model for developmental studies of live-bearing perch-like fish, whereas medaka and zebrafish are models for developmental and genetic studies. The forms of GnRH are identified in the brains of each of these fish and in the pituitary of the rockfish to investigate the role of GnRH in reproduction. Here, we report that grass rockfish (Sebastes rastrelliger) have three forms of GnRH in brain extracts as determined by HPLC elution position and RIA. These forms are identified as sea bream GnRH, chicken GnRH-II and salmon GnRH. In contrast, only two forms of GnRH were detected in brain extracts of medaka (Oryzias latipes) and zebrafish (Brachydanio rerio): salmon GnRH and chicken GnRH-II. Rockfish is distinct from medaka and zebrafish in that the most abundant form of GnRH in the rockfish pituitary is sea bream GnRH, whereas this form is absent in the other two fishes. The identification of sea bream GnRH in the rockfish brain and pituitary extracts indicates that the phylogenetic emergence of sea bream GnRH is earlier than the order Perciformes.
Journal of Endocrinology (1996) 150, 17–23
G. J. Boer and M. Rieutort
Levels of GH in serum were assayed during the development of heterozygous (HET) control and vasopressin-deficient homozygous (HOM) Brattleboro rats. In early postnatal growth no differences in GH concentrations were present between HET and HOM rats for the rapid decline in serum levels of GH in the first week and the constant period up to day 24 of age thereafter. However, higher values were found in 55-day-old HOM rats and lower values at the age of 9 months. It is concluded that the stunted development of the body and brain of HOM rats is not GH-related, and that changes or anomalies in GH secretion appear only after neurogenesis has been completed.
N Zmora, D Gonzalez-Martinez, JA Munoz-Cueto, T Madigou, E Mananos-Sanchez, SZ Doste, Y Zohar, O Kah and A Elizur
The cDNA sequences encoding three GnRH forms, sea bream GnRH (sbGnRH), salmon GnRH (sGnRH) and chicken GnRH II (cGnRH II), were cloned from the brain of European sea bass, Dicentrarchus labrax. Comparison of their deduced amino acid sequences to the same forms in the gilthead sea bream, Sparus aurata, and striped bass, Morone saxatilis, revealed high homology of the prepro-cGnRH II (94% and 98% respectively), and prepro-sGnRH (92% to both species). The sbGnRH exhibited dissimilar identities, with high homology to the striped bass (93%), and lower homology (59%) to the gilthead sea bream. Two transcript types were identified for the GnRH-associated peptide (GAP)-sGnRH as well as for the GAP-cGnRH II, which suggests a possible alternative splicing followed by the addition of an early stop codon. In order to obtain antibodies specific for the three GnRH precursors, recombinant GAP proteins were produced. The differential expression of the three GnRHs previously reported in the brain by means of in situ hybridization, using riboprobes corresponding to the GAP-coding regions, was fully confirmed by immunocytochemistry using antibodies raised against the recombinant GAP proteins, indicating that the transcripts are translated into functional proteins. Moreover, this approach allowed us to follow, for the first time, the specific projections of the different cell groups: sGAP fibers are distributed mainly in the forebrain with few projections reaching the pituitary, sbGAP fibers are mainly present in the preoptic area, mediobasal hypothalamus and predominantly project to the pars distalis of the pituitary, whereas cGnRH II fibers have a widespread distribution primarily in the posterior brain, and do not project to the pituitary. These new tools will be extremely useful to study further the development, regulation and functional significance of three independent GnRH systems in the brain of vertebrate species.