in the body fluids (interoceptive stimuli). External ph ysico-chemical changes in
the environment may also influence the release of hormones: these include temperature, light, noxious chemicals, injury, anoxia and the proximity of other animals
with all their associated implications, friendly or otherwise. These stimuli may be
translated by the nervous system into fear, frustration , anxiety , rage and pain (the
more extreme being termed 'stress'), or in certain instances, as when the opposite
sex is involved, (usually) desire and pleasure. Such responses ma y involve the release of endocrine secretions.
Ph ysiological levels of hormones appear in the blood for varying periods of
time, dependent on their rate of release and their metabolic fate . The destruction
and removal (clearance) from the circulation, depends to some extent on all tissues
of the body, but in many instances the blood, liver and kidney are especially concerned. The survival of hormones in the circulation may vary from a few minutes,
as with the neurohypophysial hormones, to several hours for thyroxine. The balance between release and destruction determines the circulating concentration, and
could influence the physiological role of the hormone. Thus, the neurohypophysial
hormone oxytocin is destroyed by an enzyme that appears in the blood of women
during pregnancy, a time when the uterine-contracting ability of the peptide is undesirable.
(Ix) Adenohypophysis. Corticotrophic and growth hormones may be released in response to 'stress', while thyrotrophic hormone secretion is inhibited by such stimuli . The norma] release of gonadotrophic hormones ma y also be upset by unusual
exteroceptive stimuli, and this may alter the normal processes of the reproductive
cycle. The implantation of fertilized eggs in mice can be prevented by exposure
of the female to the odour of a strange male mouse (PARKES and BRUCE, 1961),
while the emotionally caused vagaries of the human menstrual cycle have plagued
most of us.
The normal ph ysiological stimuli that control the secretion of corticotrophin,
are the circulating levels of the steroid hormones secreted by its target organ, the
adrenocortical tissue. High circulating levels of the adrenocortical steroids inhibit,
or reduce, release of corticotrophin, while low concentrations facilitate its discharge. Such a control mechanism is an example of the 'negativ efeed-back' system .
The pituitary gonadotrophic-gonad and thyrotrophic-thyroid axes also exert
mutual control in this manner. The receptor site for the feed-back inhibition by
the steroid hormones is in the median eminence region of the hypothalamus,
though it is still considered possible that a direct effect may also occur in the adenohypophysis itself. Thyroxine influences TSH discharge by a direct action on the
anterior pituitary. Stimuli (apart from stress) may also impinge on these functional
axes. In the instance of the gonadotrophic hormones, this may rest in the obscurity
of a hypothalamic 'biological clock,' which can be influenced by such factors as
the external temperature, light (day length), rainfall and the nutritional state of the
animal.
The mechanisms that control the release of corticotrophin are of a dual nature;
the effects of 'stress' take place rapidly within minutes and are little, if at all, affected
by the steroid feed-back system, while the changes initiated by steroid levels only
take place after man y days. Prolactin is released in response to 'suckling,' a mammalian prerogative, but in teleost fishes it apparently takes place when the animals
57
the environment may also influence the release of hormones: these include temperature, light, noxious chemicals, injury, anoxia and the proximity of other animals
with all their associated implications, friendly or otherwise. These stimuli may be
translated by the nervous system into fear, frustration , anxiety , rage and pain (the
more extreme being termed 'stress'), or in certain instances, as when the opposite
sex is involved, (usually) desire and pleasure. Such responses ma y involve the release of endocrine secretions.
Ph ysiological levels of hormones appear in the blood for varying periods of
time, dependent on their rate of release and their metabolic fate . The destruction
and removal (clearance) from the circulation, depends to some extent on all tissues
of the body, but in many instances the blood, liver and kidney are especially concerned. The survival of hormones in the circulation may vary from a few minutes,
as with the neurohypophysial hormones, to several hours for thyroxine. The balance between release and destruction determines the circulating concentration, and
could influence the physiological role of the hormone. Thus, the neurohypophysial
hormone oxytocin is destroyed by an enzyme that appears in the blood of women
during pregnancy, a time when the uterine-contracting ability of the peptide is undesirable.
(Ix) Adenohypophysis. Corticotrophic and growth hormones may be released in response to 'stress', while thyrotrophic hormone secretion is inhibited by such stimuli . The norma] release of gonadotrophic hormones ma y also be upset by unusual
exteroceptive stimuli, and this may alter the normal processes of the reproductive
cycle. The implantation of fertilized eggs in mice can be prevented by exposure
of the female to the odour of a strange male mouse (PARKES and BRUCE, 1961),
while the emotionally caused vagaries of the human menstrual cycle have plagued
most of us.
The normal ph ysiological stimuli that control the secretion of corticotrophin,
are the circulating levels of the steroid hormones secreted by its target organ, the
adrenocortical tissue. High circulating levels of the adrenocortical steroids inhibit,
or reduce, release of corticotrophin, while low concentrations facilitate its discharge. Such a control mechanism is an example of the 'negativ efeed-back' system .
The pituitary gonadotrophic-gonad and thyrotrophic-thyroid axes also exert
mutual control in this manner. The receptor site for the feed-back inhibition by
the steroid hormones is in the median eminence region of the hypothalamus,
though it is still considered possible that a direct effect may also occur in the adenohypophysis itself. Thyroxine influences TSH discharge by a direct action on the
anterior pituitary. Stimuli (apart from stress) may also impinge on these functional
axes. In the instance of the gonadotrophic hormones, this may rest in the obscurity
of a hypothalamic 'biological clock,' which can be influenced by such factors as
the external temperature, light (day length), rainfall and the nutritional state of the
animal.
The mechanisms that control the release of corticotrophin are of a dual nature;
the effects of 'stress' take place rapidly within minutes and are little, if at all, affected
by the steroid feed-back system, while the changes initiated by steroid levels only
take place after man y days. Prolactin is released in response to 'suckling,' a mammalian prerogative, but in teleost fishes it apparently takes place when the animals
57