a) The Neurohypophysis
The neurohypophysis of fish is not as well developed as that of the tetrapods, it
lacks a distinct neural lobe while the portal circulation between the median eminence and the adenohypophysis is not usually well defined. This tissue in the fishes
nevertheless contains peptides that have a similar structure to those in the tetrapods.
Vasotocin has a widespread distribution in the fishes (Table 7.3 and 7.4) ranging
phyletically from the agnathans to the lungfishes and is subsequently perpetuated
in the amphibians, reptiles and birds (Fig. 7.1) (SAWYER, 1969). It appears to be
present, usually in small amounts, in some, but not all of the Chondrichthyes. Vasotocin may have been the original parent neurohypophysial hormone in vertebrates from which others have been derived as a result of successive mutations (see
SAWYER, 1967a). Such mutations presumably gave rise to isotocin, mesotocin, oxytocin and glumitocin. Speculation regarding the genetic changes which may have
occurred is very tempting, but has become successively more complicated as further
such peptides have been found and chemically characterized (for a discussion see
GESCHWIND , 1967; SAWYER, 1968). Vasotocin in the neurohypophysis is usually
associated with the presence of another peptide with a similar amino acid makeup. This is not always apparent as vasotocin seems to be present by itself in the
agnathan and possibly in the chondrostean fishes (paddlefish). Mesotocin and
sometimes oxytocin, is present, along with vasotocin, in reptiles and the Amphibia
and it is interesting that both these pep tides have also been identified among their
closest living piscine relatives, the Choanichthyes (lungfishes). The rest of the bony
fishes lack meso toe in and oxytocin but possess instead, isotocin, a peptide that differs from mesotocin by the presence of serine at position 4 in the molecule (see
Holocephali
AVT and oxytocin
Skates
Sharks
AVTand
AVT and
glumitocin
EOPI
~/
Elasmobranchs
<.
Amphibians
AVT, mesotocin
and ? oxytocin
Teleostei
AVT and isotocin
I
Holostei
. :
Chondrostei AVT
/
CHONDRICHTHYES OSTEICHTHYES
"'./
Cyclostomes
AVT
Fig. 7.1 Phyletic distribution of the neurohypophysial peptides in the fishes and the Amphibia. AVT = argininevasotocin,EOP I = an unidentified oxytocin-like principle found
in some elasmobranchs. (From SAWYER, 1969).
203
The neurohypophysis of fish is not as well developed as that of the tetrapods, it
lacks a distinct neural lobe while the portal circulation between the median eminence and the adenohypophysis is not usually well defined. This tissue in the fishes
nevertheless contains peptides that have a similar structure to those in the tetrapods.
Vasotocin has a widespread distribution in the fishes (Table 7.3 and 7.4) ranging
phyletically from the agnathans to the lungfishes and is subsequently perpetuated
in the amphibians, reptiles and birds (Fig. 7.1) (SAWYER, 1969). It appears to be
present, usually in small amounts, in some, but not all of the Chondrichthyes. Vasotocin may have been the original parent neurohypophysial hormone in vertebrates from which others have been derived as a result of successive mutations (see
SAWYER, 1967a). Such mutations presumably gave rise to isotocin, mesotocin, oxytocin and glumitocin. Speculation regarding the genetic changes which may have
occurred is very tempting, but has become successively more complicated as further
such peptides have been found and chemically characterized (for a discussion see
GESCHWIND , 1967; SAWYER, 1968). Vasotocin in the neurohypophysis is usually
associated with the presence of another peptide with a similar amino acid makeup. This is not always apparent as vasotocin seems to be present by itself in the
agnathan and possibly in the chondrostean fishes (paddlefish). Mesotocin and
sometimes oxytocin, is present, along with vasotocin, in reptiles and the Amphibia
and it is interesting that both these pep tides have also been identified among their
closest living piscine relatives, the Choanichthyes (lungfishes). The rest of the bony
fishes lack meso toe in and oxytocin but possess instead, isotocin, a peptide that differs from mesotocin by the presence of serine at position 4 in the molecule (see
Holocephali
AVT and oxytocin
Skates
Sharks
AVTand
AVT and
glumitocin
EOPI
~/
Elasmobranchs
<.
Amphibians
AVT, mesotocin
and ? oxytocin
Teleostei
AVT and isotocin
I
Holostei
. :
Chondrostei AVT
/
CHONDRICHTHYES OSTEICHTHYES
"'./
Cyclostomes
AVT
Fig. 7.1 Phyletic distribution of the neurohypophysial peptides in the fishes and the Amphibia. AVT = argininevasotocin,EOP I = an unidentified oxytocin-like principle found
in some elasmobranchs. (From SAWYER, 1969).
203