are transferred from salt water to fresh water (ENSOR and BALL, 1968). These latter
observations remind us that mechanisms for the ph ysiological release of hormones
may differ among the vertebrates, in a manner commensurate with their differing
roles.
(fJ) Neurohypophysis. The neurohypophysis has direct neural connections with the
brain, and release of its hormones may be initiated through these by a variety of
stimuli, man y of which are not specifically related to the ph ysiological responses.
Thus, neurohypophysial secretion may be initiated by a variety of noxious physical
and chemical stimuli, pain and even emotion. Stimuli for release in normal physiological circumstances have been studied almost exclusively in mammals, but these
have been extrapolated to the non-mammalian groups. As in the instance of the
adenohypophysis care must be taken to dist inguish between such secretion in response to non-specific stimuli, and those that may be init iated in relation to precise
homeostatic requirements and adjustments.
Osmotic stimuli playa major role in regulating the release of the antidiuretic
hormone vasopressin in mammals, and probably also have this effect in other tetrapods. Unequivocal evidence for this was provided by the classical experiments
of VERNEY (1947) with dogs . VERNEY showed that injections of hypertonic solutions of sodium chloride into the common carotid artery, resulted in a reduction
of the urine flow of hydrated bitches. An increase of onl y 2 % in the osmotic concentration of the blood produced a 90% reduction in urine flow . Injections of glucose also resulted in such responses, but urea was ineffective, presumably reflecting
its perrneant nature which fails to initiate an osmotic change. JEWELL and VERNEY
(1957) repeated these experiments and tied off various branches of the common
carotid artery in an attempt to localize the region of the brain that responds to these
osmotic changes. It was found that the blood supply to the anterior hypothalamus
was essential; this region includes the supraoptic nucleus, and it has been suggested
that small vesicular cells near this may function as the osmoreceptors. Osmotic
modulation of such receptors then may be transmitted cholinergically to the neurons of the supraoptico-hypophysial tract, and result in increased, or decreased,
release of vasopressin from the nerve terminals. Osmotic stimuli such as dehydration and immersion in hypertonic saline solutions have been shown to deplete
the neurohypophysis of its stored peptides in some birds, amphibians, fishes and
mammals, and this probably reflects release of the hormones. Vasotocin hasalso
been measured in the circulation of frogs and toads, following such osmotic excitation (BENTLEY, 1969a).
In mammals, vasopressin is released in response to a variety of non-osmotic
stimuli such as haemorrhage, exposure to an increased environmental temperature,
suckling, administration of anaesthetics, and following a large variety of major and
minor noxious stimuli. A depression of hormone release follows exposure to cold ,
a rising blood alcohol level, and some tranquilizing drugs like chlorpromazine.
Some of these stimuli are also effective in frogs and toads. The second
neurohypophysial peptide present in mammals is oxytocin; commensurate with
its probable physiological roles in initiating 'milk let down' from the mammary
gland and assisting uterine contractions during parturition, it is secreted in
response to suckling and stimulation of the female genitalia. Vasopressin is simultaneously released . Most non-mammalian species also possess two such neuro58
observations remind us that mechanisms for the ph ysiological release of hormones
may differ among the vertebrates, in a manner commensurate with their differing
roles.
(fJ) Neurohypophysis. The neurohypophysis has direct neural connections with the
brain, and release of its hormones may be initiated through these by a variety of
stimuli, man y of which are not specifically related to the ph ysiological responses.
Thus, neurohypophysial secretion may be initiated by a variety of noxious physical
and chemical stimuli, pain and even emotion. Stimuli for release in normal physiological circumstances have been studied almost exclusively in mammals, but these
have been extrapolated to the non-mammalian groups. As in the instance of the
adenohypophysis care must be taken to dist inguish between such secretion in response to non-specific stimuli, and those that may be init iated in relation to precise
homeostatic requirements and adjustments.
Osmotic stimuli playa major role in regulating the release of the antidiuretic
hormone vasopressin in mammals, and probably also have this effect in other tetrapods. Unequivocal evidence for this was provided by the classical experiments
of VERNEY (1947) with dogs . VERNEY showed that injections of hypertonic solutions of sodium chloride into the common carotid artery, resulted in a reduction
of the urine flow of hydrated bitches. An increase of onl y 2 % in the osmotic concentration of the blood produced a 90% reduction in urine flow . Injections of glucose also resulted in such responses, but urea was ineffective, presumably reflecting
its perrneant nature which fails to initiate an osmotic change. JEWELL and VERNEY
(1957) repeated these experiments and tied off various branches of the common
carotid artery in an attempt to localize the region of the brain that responds to these
osmotic changes. It was found that the blood supply to the anterior hypothalamus
was essential; this region includes the supraoptic nucleus, and it has been suggested
that small vesicular cells near this may function as the osmoreceptors. Osmotic
modulation of such receptors then may be transmitted cholinergically to the neurons of the supraoptico-hypophysial tract, and result in increased, or decreased,
release of vasopressin from the nerve terminals. Osmotic stimuli such as dehydration and immersion in hypertonic saline solutions have been shown to deplete
the neurohypophysis of its stored peptides in some birds, amphibians, fishes and
mammals, and this probably reflects release of the hormones. Vasotocin hasalso
been measured in the circulation of frogs and toads, following such osmotic excitation (BENTLEY, 1969a).
In mammals, vasopressin is released in response to a variety of non-osmotic
stimuli such as haemorrhage, exposure to an increased environmental temperature,
suckling, administration of anaesthetics, and following a large variety of major and
minor noxious stimuli. A depression of hormone release follows exposure to cold ,
a rising blood alcohol level, and some tranquilizing drugs like chlorpromazine.
Some of these stimuli are also effective in frogs and toads. The second
neurohypophysial peptide present in mammals is oxytocin; commensurate with
its probable physiological roles in initiating 'milk let down' from the mammary
gland and assisting uterine contractions during parturition, it is secreted in
response to suckling and stimulation of the female genitalia. Vasopressin is simultaneously released . Most non-mammalian species also possess two such neuro58