does not always seem to be so in fishes. In addition, the concentrations of vasotocin
in the neurohypophyses of fish (in relation to their body weight) is usually (though
not invariably) more than 10-fold less than those in terrestrial tetrapods. This probably reflects differences in the development of the neural lobe and possibly also
the physiological significance of the neurohypophysial peptides in the fishes which,
as we shall see, is not at all clear.
b) Corticosteroids
Corticosteroids with the same chemical structures as those found in tetrapods, have
been identified in the blood of a variety of fishes ranging from the agnathans to
the lungfishes (Table 7.5). Cortisol and corticosterone usually predominate but
their presence in the Agnatha is in doubt (WEISBART and IDLER, 1970). Earlier reports that aldosterone is present have not been confirmed, though it can be formed
from steroid substrates during in vitro incubation of the interrenal tissues of a variety of fish. There is some doubt, however, as to whether such products represent
a normal secretion in fish . Other steroids have been found in the blood of fishes ;
in the chondrichthyean, Raja radiata, la-hydroxycorticosterone is the principal
corticosteroid present (IDLER and TRUSCOTT, 1966; 1968). TRUSCOTT and IDLER
(1968b) have shown that the interrenal tissues of eleven species of skates, rays,
dogfish and sharks (Chondrichthyes) produce mainly 1 a-hydroxycorticosterone
during in vitro incubation.
There is abundant evidence to indicate the presence of a corticotrophin-like
hormone in the pituitary gland of fish . RASQUIN in 1951 showed that injection of
mammalian corticotrophin into a teleost fish, Astyanax mexicanus, resulted in a
hypertrophy of its interrenal tissues . When carp pituitaries were implanted into
these fish they produced a similar response. The pituitaries of the salmon, Oncorhynchus keta, and the cod, Gadus morhua, have also been shown to contain
activity which, like mammalian corticotrophin, decreases the ascorbic acid content
of the hypophysectomized rat adrenal cortex (RINFRET and HANE, 1955; W 000HEAD , 1960). Evidence of a more direct nature, indicating that there is a relationship
between the adenohypophysis and the secretion of corticosteroids in fish has been
derived from several types of experiments. Hypophysectomy results in decreased
levels of corticosteroids in eels (LELOUp-HATEY, 1961; BUTLER, DONALDSON, and
CLARK, 1969) and rainbow trout (DONALDSON and McBRIDE, 1967). In addition,
the injection of mammalian corticotrophin increases the circulating corticosteroid
levels in eels (LELOUp-HATEY, 1961; BUTLER et al., 1969) and salmon (HANE et
al., 1966). A negative feed-back control system between the adenohypophysis and
the interrenals also seems to exist, as in mammals. The injection of large doses of
very potent synthetic corticosteroids (for example dexamethasone) suppresses the
activity of the interrenals of salmon (FAGERLUND and McBRIDE, 1969) and eels
(BUTLER et al., 1969). The corticotrophin cells in the adenohypophysis of eels hypertrophy after interrenalectomy (HANKE, BERGERHOFF, and CHAN, 1967; OLIVEREAU and OLIVEREAU, 1968) which is also consistent with the presence of a negativefeed-back system of control. While a corticotrophin-like activity has been found
in the pituitaries of agnathans (STRAHAN, 1959), the evidence for the presence of
206
in the neurohypophyses of fish (in relation to their body weight) is usually (though
not invariably) more than 10-fold less than those in terrestrial tetrapods. This probably reflects differences in the development of the neural lobe and possibly also
the physiological significance of the neurohypophysial peptides in the fishes which,
as we shall see, is not at all clear.
b) Corticosteroids
Corticosteroids with the same chemical structures as those found in tetrapods, have
been identified in the blood of a variety of fishes ranging from the agnathans to
the lungfishes (Table 7.5). Cortisol and corticosterone usually predominate but
their presence in the Agnatha is in doubt (WEISBART and IDLER, 1970). Earlier reports that aldosterone is present have not been confirmed, though it can be formed
from steroid substrates during in vitro incubation of the interrenal tissues of a variety of fish. There is some doubt, however, as to whether such products represent
a normal secretion in fish . Other steroids have been found in the blood of fishes ;
in the chondrichthyean, Raja radiata, la-hydroxycorticosterone is the principal
corticosteroid present (IDLER and TRUSCOTT, 1966; 1968). TRUSCOTT and IDLER
(1968b) have shown that the interrenal tissues of eleven species of skates, rays,
dogfish and sharks (Chondrichthyes) produce mainly 1 a-hydroxycorticosterone
during in vitro incubation.
There is abundant evidence to indicate the presence of a corticotrophin-like
hormone in the pituitary gland of fish . RASQUIN in 1951 showed that injection of
mammalian corticotrophin into a teleost fish, Astyanax mexicanus, resulted in a
hypertrophy of its interrenal tissues . When carp pituitaries were implanted into
these fish they produced a similar response. The pituitaries of the salmon, Oncorhynchus keta, and the cod, Gadus morhua, have also been shown to contain
activity which, like mammalian corticotrophin, decreases the ascorbic acid content
of the hypophysectomized rat adrenal cortex (RINFRET and HANE, 1955; W 000HEAD , 1960). Evidence of a more direct nature, indicating that there is a relationship
between the adenohypophysis and the secretion of corticosteroids in fish has been
derived from several types of experiments. Hypophysectomy results in decreased
levels of corticosteroids in eels (LELOUp-HATEY, 1961; BUTLER, DONALDSON, and
CLARK, 1969) and rainbow trout (DONALDSON and McBRIDE, 1967). In addition,
the injection of mammalian corticotrophin increases the circulating corticosteroid
levels in eels (LELOUp-HATEY, 1961; BUTLER et al., 1969) and salmon (HANE et
al., 1966). A negative feed-back control system between the adenohypophysis and
the interrenals also seems to exist, as in mammals. The injection of large doses of
very potent synthetic corticosteroids (for example dexamethasone) suppresses the
activity of the interrenals of salmon (FAGERLUND and McBRIDE, 1969) and eels
(BUTLER et al., 1969). The corticotrophin cells in the adenohypophysis of eels hypertrophy after interrenalectomy (HANKE, BERGERHOFF, and CHAN, 1967; OLIVEREAU and OLIVEREAU, 1968) which is also consistent with the presence of a negativefeed-back system of control. While a corticotrophin-like activity has been found
in the pituitaries of agnathans (STRAHAN, 1959), the evidence for the presence of
206