Hypothalamic Somatostatin Levels in the Rat : With Special References to Development, Nyctohemeral Cycle, Sex Difference and Food Restriction

Abstract

Using a sensitive and specific radioimmunoassay for somatostatin (somatotropin release-inhibiting factor : SRIF), hypothalamic SRIF levels were investigated in Sprague Dawley rats.1) The developmental patterns of hypothalamic immunoreactive SRIF (IR-SRIF) content and concentration were examined in normal rats (including females) until weaning, from the 4th day through the 70th day after birth. Animals were sacrificed by decapitation without anesthesia between 12 : 00 h and 14 : 00 h. The brains were immediately removed and frozen on dry-ice. Hypothalamic tissues were dissected out and extracted with 2.5 ml of 2 N acetic acid. Hypothalamic IR-SRIF contents were increased progressively from 1.87 ± 0.02 ng (Mean ± SE) on the 4th day to 3.01 ± 0.19 ng on the 7th day and 4.86 ± 0.34 ng on the 14th day of age, respectively. Thereafter the contents were elevated markedly to 10.5 ± 1.0 ng on the 22nd day of weaning and followed by a slow increment until the 36th day of age Coincidental with the pubertal period, IR-SRIF contents were again increased markedly tc 22.9 ± 0.3 ng on the 42nd day and reached the adult level of 27.4 ± 0.8 ng on the 46th day and of 27.3 ± 0.5 ng on the 70th day of age. As expressed in terms of pg IR-SRIF per mi wet weight, the developmental patterns of hypothalamic IR-SRIF concentrations appeared to be more distinctly biphasic than those of the contents.2) The diurnal fluctuations of hypothalamic IR-SRIF levels were observed every 4 h over a 24 h period in adult intact male rats housed for two weeks in a controlled light (14 h of light starting at 05 : 30 h) and temperature (22 ± 2°C, 60 ± 5% of humidity) room and given free access to food and water. Hypothalamic IR-SRIF levels were increased from 22.6 ± 1.3 ng in content, 1016 ± 43 pg/mg wet weight in concentration at 20 : 00 h to 27.5 ± 0.4 ng, 1147 ± 41 pg/mg wet weight at 04 : 00 h and to a maximum of 28.5 ± 0.9 ng, 1217 ± 33 pg/ mg at 08 : 00 h, respectively, and followed by a gradual decline toward 12 : 00 h and 16 : 00. Furthermore, it is of interest that the hypothalamic levels of immunoreactive TRH fluctuated reciprocally in these rats.3) A sex difference on hypothalamic IR-SRIF levels was examined. Hypothalamic IR-SRIF levels of male rats were not different from those of females of the same age (46 days) but slightly lower than those of females (53 days) of the same body weight.4) The effect of food restriction for 24 h on hypothalamic IR-SRIF levels was studied in adult female rats. IR-SRIF levels in fasted rats were significantly reduced as compared with fed controls. (29.2 ± 0.5 ng vs 33.3 ± 0.9 ng in content, 1215 ± 47 pg/mg wet weight vs 1426 ± 53 pg/mg wet weight in concentration) In conclusion, these observations indicate that hypothalamic IR-SRIF levels show apparent physiological fluctuations, especially on development, nyctohemeral cycle and food restriction in the rat. The possible roles of hypothalamic SRIF in this species are discussed.