Modos de operación

As expected in this model, T4 was more severely depressed than T3, indicating that the dams were hypothyroxinémie rather than hypothyroid. The consistently high TSH levels in Tx dams indicated that the pituitary feedback response was normal, and also that this response was not able to compensate for the decreased synthetic capacity of the thyroid. Partial compensation was seen however, in that T4 levels in Tx dams did not fall so steeply during pregnancy as in N dams. Nevertheless, as serum T3 levels were reduced in the Tx dams, some degree of tissue hypothyroidism was probably present.

Table 3.9 ProteiniDNA ratios in postnatal normal (N) and partially thyroidectomised (Tx) dam progeny brain regions

A)

Region Age Dam status

(pnd) ProteiniDNA r a t i o Cerebral cortex 10 N 24.08 ± 2.00 Tx 23.33 ± 1.18 20 N 38.22 ± 1.65 Tx 42.48 ± 6.60 30 N 47.28 ±4.81 Tx 49.0815.71 Cerebellum 10 N 5.5910.28 Tx 5.7910.43 20 N 6 .0 110.39 Tx 8.491 1.25* 30 N 10.0011.56 Tx 9.56 1 0 .4 7 Brain stem 10 N 19.521 1.43 Tx 21.201 1.38 20 N 30.581 1.67 Tx 32.1517.74 30 N 40.25 1 8.99 Tx 43 .4817.46 Subcortex 10 N 23.631 1.50 Tx 26.1913.02 20 N 38.2612.43 Tx 38.5616.99 30 N 39.08 15 .5 4 Tx 41.3113.73

Values are mean ± SEM; n = 5

Statistical analysis, NS - No significant difference

2-wav ANOVA Cerebral cortex Cerebellum Brain stem Subcortex Age P < 0.0005 P < 0.0005 P < 0.0005 P < 0.005 Treatment NS NS NS NS Age-treatment interaction NS NS NS NS Fisher’s PLSD /. Treatment-related *P < 0.05 Tx vs. N dam progeny

II. Age-related Cerebral cortex Cerebellum Brain stem Subcortex N dam progeny 20 vs. 10 pnd P < 0.005 NS P < 0.05 P < 0.05 30 vs. 20 pnd NS P < 0.005 NS NS Tx dam progeny 20 vs. 10 pnd P < 0.0005 P < 0.05 NS NS 30 vs. 20 pnd NS NS NS NS

Body weight was reduced in Tx dam fetuses from 16 to 21 dg. This parameter had largely normalised by the postnatal stages of development, although a trend for lower body weight could still be seen at birth in Tx dam progeny. Brain weight in contrast, was not affected pre- or postnatally in Tx dam progeny, neither were brain protein or DNA levels. These results indicate the presence of a mechanism sparing the brain from the effects of maternal hypothyroidism such as that proposed by Morreale de Escobar et.al. (96), involving increased 5 D-II activity (section 1.5.4). Fetal litter size was significantly reduced in Tx dams at 16 and 19 dg, and there was a tendency for lower litter sizes at 14 dg (the lack of significance at this age was probably due to the smaller number of litters studied). This decrease in Tx dam litter size is expected as maternal hypothyroidism inhibits ovulation (460). No such effect is seen at 21 dg, however, due to the decrease in control litter size between 19 and 21 dg, for which no simple explanation exists. A further decrease is seen in viable pups from Tx dams on the day of birth. This could be due to a number of reasons, including difficulties during parturition, late-stage metabolic compromise and decreased maternal inclination. In addition, Tx dams may cannibalise pups they see as unfit—which may also explain the normalisation of body weight at this age (i.e. the runts of the litter are removed). Decreased litter size in Tx dam progeny may also provide another explanation for why serum T4 levels do not fall as rapidly as controls. The decrease in serum total T4 levels in normal dams as gestation progresses has been previously reported (94,408), and may occur due to increased placental demand for TH as a source of iodine for the fetal thyroid. Thus the fall in circulating maternal T4 levels is steepest between 16 and 19 dg, the period during which the fetal thyroid becomes active. The reduced litter size in Tx dams equates to reduced placental T4 demand, hence the maternal T4 level drops less rapidly in Tx dams, conserving the limited maternal T4 pool.

Certain aspects of placental development were disturbed in Tx dams; protein content and proteiniDNA ratio (cell size) were reduced at 19 dg, although protein concentration, DNA content (cell number) and DNA concentration (cell density) appeared normal. The placental compromise was largely limited to 19 dg, and did not seem to affect fetal development (body weight being compromised from 16 dg). Late stage placental dysfunction in Tx dams may partly explain the decrease in the number of viable progeny per litter that occurs between 21 dg and birth. Placental dysfunction may have arisen as a result of maternal metabolic compromise, alternatively, the observed changes may be due to a direct effect of TH deficiency on placental development.

In summary, the use of a partially thyroidectomised rat dam model results in reduced Tx dam serum T4 levels, but does not appear to induce severe metabolic compromise in the

dams despite serum T3 levels being depressed. Hence all parameters of brain growth were normal in pre- and postnatal Tx dam progeny, and body weight normalised after birth. Of particular significance is the normal development of the cerebellum, a region which develops largely postnatally and is therefore particularly sensitive to maternal nutritional deficiencies during the suckling period. Similarly, placental abnormalities were only seen at 19 dg and did not appear to impinge upon fetal development, as judged by the absence of late fetal abnormalities.

CHAPTER 4.