Seasonal variation in prolactin and TSH releasing activity in the hypothalamus of Rana temporaria

Abstract

Radioimmunoassay of plasma prolactin (PRL) and thyroid stimulating hormone (TSH) has been performed after injection into rats of hypothalamic extracts obtained from R. temporaria. An increase in plasma prolactin followed the injection of an extract of the pars ventralis tuberis cinerei collected from frogs during April and May. A decrease was seen after the injection of hypothalami obtained during November and December. A decrease in plasma TSH was found with tuberal extracts collected during January, February, March and April, whereas no change was observed with extracts of tuberal tissue collected in other months. Comparisons of the TSH/PRL ratios after injections of the various extracts revealed ratios in favour of TSH release during fall caused by a decrease in prolactin; while prolactin was favoured in spring by both an increase in prolactin release and a decrease in TSH release. In a separate series of experiments, the releasing activities for prolactin also could be detected in extracts made from the preoptic area, but not from the median eminence. In these experiments also an increase in prolactin was seen following the injection of extracts collected during April and May, but not during June. It is concluded that the hypothalamus of R. temporaria contains factors which are able to influence both TSH and PRL secretion in the rat, and that these activities are not restricted to the pars ventralis tuberis cinerei. During the period of reproduction a PRF activity may facilitate life in water. Outside this period, PRF activity may disappear and be replaced by PIF activity. TSH secretion does not seem to be stimulated by a releasing factor, but an inhibitory activity is found during the period when PRF occurs.