Secretion of Anterior Pituitary Hormones in Rats with Ventromedial Hypothalamic Lesion

Abstract

The aspects of the secretion of the anterior pituitary hormones were studied using male rats with electrolytic lesions of the bilateral ventromedial hypothalamic area (HTL) in various experimental conditions.The results obtained were as follows : 1. There was no significant difference in base-line level and in response of plasma growth hormone (GH) to the intravenous administration of prostaglandin E2 between the HTL and control groups, although plasma GH response to the intravenous administration of pentobarbital (PB) was significantly decreased in HTL rats. The decrease in GH secretion was observed even in the early stage after HTL. These results suggested that the impaired GH response to PB in HTL rats was induced by HTL itself rather than obesity due to HTL.2. There was no significant difference in base-line level and in response of plasma prolactin (PRL) to the intravenous administration of sulpiride between the two groups at 1 week after the operation. Plasma PRL response to the intravenous administration of sulpiride in HTL rats at 10 weeks after the operation was significantly decreased, but showed a normal response after the consecutive loads of sulpiride given subcutaneously for 6 days. On the other hand, plasma PRL response to the intravenous administration of chlorpromazine in HTL rats was significantly decreased both at 1 and at 10 weeks after the operation. These results suggested that the decreased PRL response in HTL rats might have resulted from the reduced secretion of the PRL releasing factor, although the possibility of the increased secretion of the PRL-inhibiting factor cannot be excluded.3. The plasma base-line level of luteinizing hormone (LH) was significantly decreased, but there was no change in the plasma follicle-stimulating hormone (FSH) level in HTL rats. There was no significant difference in plasma LH and FSH responses to the intravenous administration of LH-RH between the two groups. There was no significant change in plasma LH response to the subcutaneous administration of clomiphene in low and high doses for 5 days in HTL rats, but plasma FSH response was increased in both groups to the same degree. These results suggested that the LH-RH secretion was impaired in HTL rats and that the control mechanism of the secretion of LH and FSH by clomiphene might be different.4. The plasma base-line level of thyrotropin (TSH) and plasma TSH response to methyl-mercapto-imidazole given subcutaneously for 5 days or to TRH given intravenously was decreased in HTL rats, although not significantly.5. The Plasma base-line level of corticosterone showed no significant difference between the two groups.These results indicate that the plasma base-line levels and/or pituitary reserves of anterior pituitary hormones are significantly decreased or tend to be reduced in HTL rats. The impaired secretion of these hormones, particularly of GH, probably contributes to the development of hypothalamic obesity.