tebrates. Growth and lipotrophic hormones help regulate the overall general
protein, carbohydrate and fat metabolism of the tissues but evidence for a direct
role in osmoregulation is lacking. Prolactin promotes the secretion of progesterone
by the ovaries of rats and mice, but in other mammals has a more general role in
instituting the secretion of milk by the mammary glands . In non-mammalian vertebrates the physiological role of prolactin is not clear , but when injected it may
influence a variety of processes including, growth and water and salt metabolism
of some urodele Amphibia and euryhaline fishes.
{I) Pars Intermedia. The intermediate lobe of the pituitary gland secretes a hormone
called melanocyte stimulating hormone (MSH) or intermedin which in the amphibians and some fishes may influence melanophores that are concerned with colour
change (WARING, 1963). In higher vertebrates its role is unknown.
y) Neurohypophysis. Arginine vasotocin is present in representatives of all vertebrate groups from the Agnathan fishes to the birds (see SAWYER, 1968), and is usually accompanied by a second hormone whose structure varies characteristically
in the different taxonomic classes. In the Agnatha, and probably in the
chondrostean fishes, like the sturgeons, vasotocin alone is present. The mammals
secrete vasopressin (antidiuretic hormone, ADH) and oxytocin. In tetrapods vasotocin and vasopressin influence water metabolism by acting on the kidneys and certain other tissues like amphibian skin and urinary bladder. The role of such hormones in the fishes is far from clear . The second hormone in mammals, oxytocin,
assists in the initiation of 'milk let down' from the mammary glands and may aid
the contraction of uterine smooth muscle during parturition. However, in nonmammalian species the role of the analogous hormones are undetermined. Neurohypophysial hormones are stored in highest concentrations in the pars nervosa
(neural lobe), but can also be identified in neurohypophysial tissue extending to
the region of the praeoptic or supraoptic and pa raventricular nuclei in the hypothalamus.
In mammals the median eminence is the site of storage for at least seven further
molecular species that can initiate or inhibit the release of six different hormones
from the pars distalis (see Table 2.2). These substances may be released in response
to neural stimuli from the brain, pass through a portal circulation from the median
eminence to the pars distalis and bring about the specific discharge of a particular
hormone into the systemic circulation (see MCCANN, 1968). The non-mammalian
vertebrates have been somewhat neglected with respect to these transmitters but
there is evidence that at least one, the corticotrophic releasing factor (CRF), is
present in the Amphibia (JORGENSEN and LARSEN, 1967). However JORGENSEN
and LARSEN consider that it is doubtful that such a control system exists generally
in the fishes, as the required anatomical portal circulation onl y occurs sporadically
in diverse species, where it may be preadaptive, rather than of contemporary physiological significance.
51
protein, carbohydrate and fat metabolism of the tissues but evidence for a direct
role in osmoregulation is lacking. Prolactin promotes the secretion of progesterone
by the ovaries of rats and mice, but in other mammals has a more general role in
instituting the secretion of milk by the mammary glands . In non-mammalian vertebrates the physiological role of prolactin is not clear , but when injected it may
influence a variety of processes including, growth and water and salt metabolism
of some urodele Amphibia and euryhaline fishes.
{I) Pars Intermedia. The intermediate lobe of the pituitary gland secretes a hormone
called melanocyte stimulating hormone (MSH) or intermedin which in the amphibians and some fishes may influence melanophores that are concerned with colour
change (WARING, 1963). In higher vertebrates its role is unknown.
y) Neurohypophysis. Arginine vasotocin is present in representatives of all vertebrate groups from the Agnathan fishes to the birds (see SAWYER, 1968), and is usually accompanied by a second hormone whose structure varies characteristically
in the different taxonomic classes. In the Agnatha, and probably in the
chondrostean fishes, like the sturgeons, vasotocin alone is present. The mammals
secrete vasopressin (antidiuretic hormone, ADH) and oxytocin. In tetrapods vasotocin and vasopressin influence water metabolism by acting on the kidneys and certain other tissues like amphibian skin and urinary bladder. The role of such hormones in the fishes is far from clear . The second hormone in mammals, oxytocin,
assists in the initiation of 'milk let down' from the mammary glands and may aid
the contraction of uterine smooth muscle during parturition. However, in nonmammalian species the role of the analogous hormones are undetermined. Neurohypophysial hormones are stored in highest concentrations in the pars nervosa
(neural lobe), but can also be identified in neurohypophysial tissue extending to
the region of the praeoptic or supraoptic and pa raventricular nuclei in the hypothalamus.
In mammals the median eminence is the site of storage for at least seven further
molecular species that can initiate or inhibit the release of six different hormones
from the pars distalis (see Table 2.2). These substances may be released in response
to neural stimuli from the brain, pass through a portal circulation from the median
eminence to the pars distalis and bring about the specific discharge of a particular
hormone into the systemic circulation (see MCCANN, 1968). The non-mammalian
vertebrates have been somewhat neglected with respect to these transmitters but
there is evidence that at least one, the corticotrophic releasing factor (CRF), is
present in the Amphibia (JORGENSEN and LARSEN, 1967). However JORGENSEN
and LARSEN consider that it is doubtful that such a control system exists generally
in the fishes, as the required anatomical portal circulation onl y occurs sporadically
in diverse species, where it may be preadaptive, rather than of contemporary physiological significance.
51