4. THYROID FUNCTION AND ITS CONTROL IN FISHES
245
Chaney, 1953). Olivereau et al. (1964) have described an increase in
pituitary TSH cells in trout kept for months on a low-iodine diet or radiothyroidectomized. Similarly, goitrogenic drugs given to a variety of teleosts, unlike the reported unresponsiveness of elasmobranchs, activate the
pituitary-thyroid axis, evoke cytological changes in the TSH cells of the
pars distalis, hyperplasia of pharyngeal thyroid tissue, and at the same
time at least partially block thyroxine synthesis (Leloup and Olivereau,
1950; Olivereau, 1954; Barrington and Matty, 1955; Sokol, 1955; Cuckrowski and Chavin, 1964).
Conversely, administration of thyroxine to several species of teleosts
has been shown to be inhibitory for the pituitary-thyroid system, both
morphologically and physiologically ( thyroid uptake and organification of
radioiodine) (M. Fontaine and Wurtz-Arlet, 1952; La Roche, 1952; Berg
et al., 1954; Honma and Murakawa, 1955; B. I. Baker, 1965).
Several workers have shown parallel variations in stainable hypothalamic neurosecretion in teleosts that accompany cyclic changes in
thyroid function, or follow treatment with goitrogens or with thyroxine
(Amy et al., 1956, 1957; Olivereau, 1961; Barranikova, 1964). However,
at this time it is difficult to interpret changes in stainable hypothalamic
neurosecretion in terms of thyrotropic activation by thyrotropin releasing
factors (TRF).
In summary it may be said that reasonably good evidence of diencephalic control over TSH secretion exists only for teleosts, and even this
is not well characterized. Because of the different and characteristic
anatomical relations between the brain and pars distalis in different
groups of fishes, it would seem that this would be a rewarding and significant area for further research.
111. THYROTROPIC FUNCTION OF THE FISH PITUITARY GLAND
The evidence for secretion of a thyrotropin by the cyclostome pituitary
is very limited and for the reasons given below must still be considered
equivocal. In the only extraction experiments reported, Dodd et al., in a
review ( 1963), tabulate data not otherwise published showing that hagfish pituitary contains a very low concentration of TSH activity and
lamprey pituitary a slightly higher level. In the same table (Table 4) it is
reported by Dodd et al. (1963) that frog neurohypophysis contains as
much TSH activity as the lamprey whole pituitary, fish neurohypophysis
three times as much, bird neurohypophysis four times, and mouse neurohypophysis 60 times as much. In these tests, using the McKenzie pro-
245
Chaney, 1953). Olivereau et al. (1964) have described an increase in
pituitary TSH cells in trout kept for months on a low-iodine diet or radiothyroidectomized. Similarly, goitrogenic drugs given to a variety of teleosts, unlike the reported unresponsiveness of elasmobranchs, activate the
pituitary-thyroid axis, evoke cytological changes in the TSH cells of the
pars distalis, hyperplasia of pharyngeal thyroid tissue, and at the same
time at least partially block thyroxine synthesis (Leloup and Olivereau,
1950; Olivereau, 1954; Barrington and Matty, 1955; Sokol, 1955; Cuckrowski and Chavin, 1964).
Conversely, administration of thyroxine to several species of teleosts
has been shown to be inhibitory for the pituitary-thyroid system, both
morphologically and physiologically ( thyroid uptake and organification of
radioiodine) (M. Fontaine and Wurtz-Arlet, 1952; La Roche, 1952; Berg
et al., 1954; Honma and Murakawa, 1955; B. I. Baker, 1965).
Several workers have shown parallel variations in stainable hypothalamic neurosecretion in teleosts that accompany cyclic changes in
thyroid function, or follow treatment with goitrogens or with thyroxine
(Amy et al., 1956, 1957; Olivereau, 1961; Barranikova, 1964). However,
at this time it is difficult to interpret changes in stainable hypothalamic
neurosecretion in terms of thyrotropic activation by thyrotropin releasing
factors (TRF).
In summary it may be said that reasonably good evidence of diencephalic control over TSH secretion exists only for teleosts, and even this
is not well characterized. Because of the different and characteristic
anatomical relations between the brain and pars distalis in different
groups of fishes, it would seem that this would be a rewarding and significant area for further research.
111. THYROTROPIC FUNCTION OF THE FISH PITUITARY GLAND
The evidence for secretion of a thyrotropin by the cyclostome pituitary
is very limited and for the reasons given below must still be considered
equivocal. In the only extraction experiments reported, Dodd et al., in a
review ( 1963), tabulate data not otherwise published showing that hagfish pituitary contains a very low concentration of TSH activity and
lamprey pituitary a slightly higher level. In the same table (Table 4) it is
reported by Dodd et al. (1963) that frog neurohypophysis contains as
much TSH activity as the lamprey whole pituitary, fish neurohypophysis
three times as much, bird neurohypophysis four times, and mouse neurohypophysis 60 times as much. In these tests, using the McKenzie pro-