1. In the rat variation of metabolic heat production is the principal effector of thermoregulation. There is a continuous relationship between ambient temperature and metabolic rat over the whole range of tolerable environmental temperature. The mechanism that controls metabolic rate is unknown; this paper reports an attempt to test whether thyroid hormones provide the controlling pathway. 2. First, the changes in metabolic rate and in the plasma concentrations of thyroid stimulating hormone (TSH), triiodothyronine (T3) and thyroxine (T4) were measured in rats living in a controlled environment, first at 23 degrees C and then at 6 degrees C. Metabolic rate increased from approximately 290 to 470 kJ day-1 when the temperature was lowered, a factor of ca 1.6, and the diurnal rhythm disappeared. The concentration of TSH increased from approximately 320 to 450 ng ml-1 (with loss of diurnal rhythm) and of T3 from ca 0.7 to 1.0 nmol l-1, a factor of ca 1.4 in each case. T4 concentration did not change. 3. Next, a dose schedule of T3 was found that, when injected I.V. via indwelling jugular cannulae in the same rats in an environment at 23 degrees C, maintained an increase in T3 concentration rather greater than had been found at 6 degrees C. 4. This dose of T3, given to the same rats at 23 degrees C, did not affect metabolic rate (or its diurnal pattern). 5. It is therefore unlikely that the increase in T3 concentration evoked the increase in metabolic rate when ambient temperature was changed from 23 to 6 degrees C; and therefore that the thyroid controls variation of metabolic rate in 'everyday' thermoregulation in the rat.

SUMMARYThe afferent innervation of the uterus might be expected to grow during pregnancy as the size of the uterus increases. Substance P-like immunoreactivity (SPLI) has been measured as a means of monitoring the changes in the afferent innervation of the urogenital tract of rats during pregnancy and following parturition. The great growth of uterine tissue during pregnancy causes an overall decrease in SPLI concentrations during pregnancy, but it has been found that the amount of SPLI present per uterine horn increases nearly 3-fold by the end of pregnancy. This increase is greater in uterine horns that contain more fetuses, suggesting that the SPLI innervation expands to a greater extent in uterine horns that undergo greater degrees of hypertrophy. There is a significant correlation between SPLI content and the number or total weight of fetuses throughout the latter two-thirds of pregnancy. There is a fall in SPLI content of uterine horns following parturition, but not to a statistically significant degree, and this may be related to the release of the peptide during parturition.

SUMMARYNon-obese and obese Zucker rats were exposed to 6 OC for 20 days. At the end of the experiment thyroid stimulating hormone (TSH), circulating thyroid hormones, and the weights of the thyroid gland and a brown adipose tissue pad were measured. TSH and 3,5,3'-triiodothyronine concentrations and brown adipose tissue weights increased in response to cold in both phenotypes but by larger amounts in obese rats. Free thyroxine was lower in obese rats. There appears to be no defect in thyroid function in congenitally obese rats exposed to cold, or in their ability to develop brown adipose tissue.

The concentrations of substance P-like immunoreactivity (SPLI) were measured in mesenteric nerves and vessels of control rats and rats treated with capsaicin during the second day of life. The results showed that there was a significantly higher concentration of SPLI in the jejunum than in the terminal ileum, and that there was a loss of SPLI from these tissues following neonatal treatment with capsaicin. The results suggest that the substance P-containing afferent innervation of the upper small intestine is greater than in the terminal ileum. The significance of these results to the Intensity or Summation hypothesis of visceral pain, and to the thresholds of visceral reflexes from different areas of the small intestine is discussed.

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