8. UROPHYSIS AND CAUDAL NEUROSECRETORY SYSTEM
415
function. Second, the prominent nervous input into these cells should
allow effective transduction of nervous information into hormonal messages important in regulation. Third, the renal portal drainage of the
urophysis suggests obvious target areas still to be investigated as sites of
action. The kidney itself has been implicated only relatively minimally to
date as a target for urophysial action (Maetz et al., 1964). Thus, the descriptions of the sensitivity of teleost blood vessels (Chan and Chester
Jones; see Bern et al., 1967) and urinary bladder (Lederis, 1969) to
urophysial extracts are promising in the development of needed assay
methods, but they must not be allowed to delude one into thinking that
the functions of the urophysis have necessarily been delineated. In view
of the variety of activities indicated (albeit not established), the possibility of more than one hormone-a common occurrence in neurosecretory systems, both vertebrate and invertebrate-cannot be disregarded.
In view of the abundance of nervous connections upon the whole
length of the caudal neurosecretory neuron, its precise regulation would
seem to be assured. If the function of the system be in part ionoregulatory, the data of Yagi and Bern (1965) would indicate the existence of
Na+ receptors somewhere anterior to the caudal system-in the central
nervous system (Fig. 8) or external to it. The caudal neurons themselves
do not seem to be responsive to changes in blood Na+ levels. Again, the
notion of a function for urophysial hormone( s) in the “fine adjustment”
to internal ionic changes cannot yet be rejected.
The ubiquitousness of the system points to some rather vital-if not
indeed essential-contribution. Militating against this notion, however, is
its late appearance in development (Fridberg, 1962a; Sano et al., 1962;
Holmgren, 1964; Imai, 1965). Furthermore, the limited indication of results of urophysectomy formerly provided further evidence of “nonessentiality.” However, we now know that the system has a remarkable ability
to regenerate after total removal in the adult (Fridberg et al., 1966c).
Apparently adult neogenesis of the system is from the same elementsependymal-that give rise to the system in normal ontogeny (Sano et al.,
1962; Fridberg, 1962a).
What is most needed at present for an understanding of the caudal
neurosecretory system is the interest of more comparative physiologists.
It seems very important that the isolated claims of functional significance
made by one laboratory be subjected to confirmatory testing by another
laboratory. The limited history of the field makes it all too evident that
most of the earlier claims as to function have not been substantiated. A
reference standard for comparison of “urotensins” has recently been proposed (Bern and Lederis, 1969), which should allow comparison of urophysial preparations from different laboratories upon a variety of target
systems.
415
function. Second, the prominent nervous input into these cells should
allow effective transduction of nervous information into hormonal messages important in regulation. Third, the renal portal drainage of the
urophysis suggests obvious target areas still to be investigated as sites of
action. The kidney itself has been implicated only relatively minimally to
date as a target for urophysial action (Maetz et al., 1964). Thus, the descriptions of the sensitivity of teleost blood vessels (Chan and Chester
Jones; see Bern et al., 1967) and urinary bladder (Lederis, 1969) to
urophysial extracts are promising in the development of needed assay
methods, but they must not be allowed to delude one into thinking that
the functions of the urophysis have necessarily been delineated. In view
of the variety of activities indicated (albeit not established), the possibility of more than one hormone-a common occurrence in neurosecretory systems, both vertebrate and invertebrate-cannot be disregarded.
In view of the abundance of nervous connections upon the whole
length of the caudal neurosecretory neuron, its precise regulation would
seem to be assured. If the function of the system be in part ionoregulatory, the data of Yagi and Bern (1965) would indicate the existence of
Na+ receptors somewhere anterior to the caudal system-in the central
nervous system (Fig. 8) or external to it. The caudal neurons themselves
do not seem to be responsive to changes in blood Na+ levels. Again, the
notion of a function for urophysial hormone( s) in the “fine adjustment”
to internal ionic changes cannot yet be rejected.
The ubiquitousness of the system points to some rather vital-if not
indeed essential-contribution. Militating against this notion, however, is
its late appearance in development (Fridberg, 1962a; Sano et al., 1962;
Holmgren, 1964; Imai, 1965). Furthermore, the limited indication of results of urophysectomy formerly provided further evidence of “nonessentiality.” However, we now know that the system has a remarkable ability
to regenerate after total removal in the adult (Fridberg et al., 1966c).
Apparently adult neogenesis of the system is from the same elementsependymal-that give rise to the system in normal ontogeny (Sano et al.,
1962; Fridberg, 1962a).
What is most needed at present for an understanding of the caudal
neurosecretory system is the interest of more comparative physiologists.
It seems very important that the isolated claims of functional significance
made by one laboratory be subjected to confirmatory testing by another
laboratory. The limited history of the field makes it all too evident that
most of the earlier claims as to function have not been substantiated. A
reference standard for comparison of “urotensins” has recently been proposed (Bern and Lederis, 1969), which should allow comparison of urophysial preparations from different laboratories upon a variety of target
systems.