224
J. N. BALL
However, in the urodele Triturus, the prolactin thyroidal effect is apparently one of true thyroidal activation (i.e., elevated secretion of TSH),
( Vellano et aZ., 1967). More recently, Olivereau ( 1968) has found that in
the eel, as in Triturus, prolactin stimulates lS1I uptake by the thyroid, as
well as producing morphological activation. Thus prolactin does not act
as a goitrogen in Anguilh but presumably activates the TSH cells directly
or via the hypothalamus, perhaps by resetting the thyroid hormonsTSH
homeostat to a higher level.
A metabolic action of mammalian prolactin in an intact teleost was
reported by Lee and Meier (1967), who found that daily injections of
prolactin (or LH) induced fattening (increase in the lipid fraction of the
total body) in sexually mature golden topminnows, Fundulus chrysotw.
Prolactin had little or no such effect in hypophysectomized fish, and there
is a marked diurnal variation in the responsiveness of intact fish to the
hormones since injections of prolactin or LH early in the photoperiod
actually caused a loss in lipid in contrast to the gain produced by injections in the middle of the photoperiod. It would appear that the fattening
effect of prolactin requires the presence of some other pituitary factor( s )
which probably undergo a diurnal fluctuation in secretion rate. In addition, differences in the responses of hypophysectomized F. chrysotus depending on the time of day they were injected suggest the presence of
some nonpituitary factor which conditions the tissue response to prolactin
and which varies during the photoperiod. It is interesting to recall that
prolactin and gonadotropins probably synergize in inducing fat deposition
resembling premigratory fattening in the bird, Zonotrichia (Meier and
Farner, 1984).
Another possible metabolic role for fish prolactin has been suggested
by Johansen (1967). He found that the resistance of goldfish to heat
stress was promoted by long photoperiod and was mediated by some
hypophysial mechanism that was still active in the ectopically transplanted gland. Although exogenous prolactin failed to increase the impaired heat resistance of hypophysectomized goldfish, Johansen compared
the behavior of the transplanted goldfish pituitary with that in Poecilia
(Ball et al., 1965) and suggested that endogenous fish prolactin is essential for normal resistance to thermal stress, possibly by alleviating the presumed osmoregulatory disturbances induced by thermal stress. This is an
interesting possibility but requires more direct experimental backing. Another miscellaneous effect of the hormone is seen in the maintenance of
normal levels of circulating erythrocytes and thrombocytes by chronic
prolactin injections in hypophysectomized P. latipinnu (Ball et al.,
1966a); this is not a specific action of prolactin since erythrocytes were
also maintained by ACTH and thrombocytes partially by thyroxine and
J. N. BALL
However, in the urodele Triturus, the prolactin thyroidal effect is apparently one of true thyroidal activation (i.e., elevated secretion of TSH),
( Vellano et aZ., 1967). More recently, Olivereau ( 1968) has found that in
the eel, as in Triturus, prolactin stimulates lS1I uptake by the thyroid, as
well as producing morphological activation. Thus prolactin does not act
as a goitrogen in Anguilh but presumably activates the TSH cells directly
or via the hypothalamus, perhaps by resetting the thyroid hormonsTSH
homeostat to a higher level.
A metabolic action of mammalian prolactin in an intact teleost was
reported by Lee and Meier (1967), who found that daily injections of
prolactin (or LH) induced fattening (increase in the lipid fraction of the
total body) in sexually mature golden topminnows, Fundulus chrysotw.
Prolactin had little or no such effect in hypophysectomized fish, and there
is a marked diurnal variation in the responsiveness of intact fish to the
hormones since injections of prolactin or LH early in the photoperiod
actually caused a loss in lipid in contrast to the gain produced by injections in the middle of the photoperiod. It would appear that the fattening
effect of prolactin requires the presence of some other pituitary factor( s )
which probably undergo a diurnal fluctuation in secretion rate. In addition, differences in the responses of hypophysectomized F. chrysotus depending on the time of day they were injected suggest the presence of
some nonpituitary factor which conditions the tissue response to prolactin
and which varies during the photoperiod. It is interesting to recall that
prolactin and gonadotropins probably synergize in inducing fat deposition
resembling premigratory fattening in the bird, Zonotrichia (Meier and
Farner, 1984).
Another possible metabolic role for fish prolactin has been suggested
by Johansen (1967). He found that the resistance of goldfish to heat
stress was promoted by long photoperiod and was mediated by some
hypophysial mechanism that was still active in the ectopically transplanted gland. Although exogenous prolactin failed to increase the impaired heat resistance of hypophysectomized goldfish, Johansen compared
the behavior of the transplanted goldfish pituitary with that in Poecilia
(Ball et al., 1965) and suggested that endogenous fish prolactin is essential for normal resistance to thermal stress, possibly by alleviating the presumed osmoregulatory disturbances induced by thermal stress. This is an
interesting possibility but requires more direct experimental backing. Another miscellaneous effect of the hormone is seen in the maintenance of
normal levels of circulating erythrocytes and thrombocytes by chronic
prolactin injections in hypophysectomized P. latipinnu (Ball et al.,
1966a); this is not a specific action of prolactin since erythrocytes were
also maintained by ACTH and thrombocytes partially by thyroxine and