Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 18 Pituitary Function
The neurosecretory pathway runs from the anterior
hypothalamus, traverses the floor of the ventral hypothalamus,
and terminates in the neural lobe (NL) of the neurohypophysis
on fenestrated blood vessels ( Page, 1986 ). The
system is involved in osmoregulation through the production
and release of vasopressin, and in parturition and nursing
through the secretion of oxytocin.
In the intermediate lobe (IL) the secretory activity is regulated
via direct neuronal inhibitory and stimulatory influences
( Saland, 2001 ). In amphibians ( Vazquez-Martinez et al. ,
2003 ) and reptiles ( Dores et al. , 1987 ), the IL plays an
important role in adaptation to background color. The function
of IL cells in mammals has not been fully established,
but they may play a role in opioid-regulated functions.
A . Anatomical Considerations
The hypothalamus and pituitary control vital functions
such as growth, reproduction, lactation, basal metabolism,
stress response, parameters of immune function, and the
state of hydration. Understanding of the complicated functional
relationship of the hypothalamus to the pituitary
requires an appreciation of the anatomical relationships.
1 . Hypothalamus
Hypophysiotropic neurohormones are produced in several
areas of the hypothalamus. For example, in an immunofluorescence
study of hypothalami of dogs, the majority of
cell bodies with immunoreactivity to corticotropin-releasing
hormone (CRH) was found in the region of the periventricular
and paraventricular nuclei, but they were also
found in the supraoptic and suprachiasmatic area as well as
craniodorsal to the mammillary bodies ( Stolp et al. , 1987 ).
The cell bodies of the neurohormone-producing neurons
that project to the median eminence are in part
intermingled with cell bodies that also synthesize these
neurohormones but project to other brain areas. The majority
of the neurons that project to the median eminence are
found in the preoptic and suprachiasmatic region of the
hypothalamus. Axons containing the same neurohormone
may have synaptic contacts that enable regulation of cellular
function between these neurons.
The neurohormone-producing cells receive a complex
neural input from a variety of chemical messengers, such as
neurotransmitters and other neurohormones. Not only the
neurohormones CRH and arginine vasopressin (AVP), but
in general the combination of a neurohormone and another
chemical messenger, may colocalize within a single neuron.
The neurons of the neurohypophyseal system represent
a more anatomically distinct entity, with cell bodies
located in the paraventricular and supraoptic hypothalamic
nuclei ( Sawchenko and Swanson, 1983 ). However, within
these areas there are also neurons producing a variety of
other neuropeptides.
2 . Neurotransmitter Systems
The major neurotransmitter systems for intercellular communication
within the central nervous system consist of
monoamines and peptides. These chemical messengers
regulate the biosynthesis and release of the hypophysiotropic
neurohormones. Through a network of axodendritic
and axoaxonic contacts, these neurons are connected to the
neurohormone-producing cells. In addition, many monoamines
and peptides are found within the hypophysiotropic
hormone-producing cells, where they are released together
with the neurohormones into the portal system and modify
the effect of hypothalamic hormones on the pituitary.
The biogenic amine neurotransmitters, known to play
a regulating/modulating role in the hypothalamic-pituitary
system, include catecholamines (dopamine, noradrenalin,
and adrenalin), indolamines (serotonin, melatonin), acetylcholine,
γ -aminobutyric acid (GABA), and histamine.
Neuropharmacological agents can be used to alter neurotransmitter
effects and, as a consequence, hypothalamic
and pituitary hormone release.
Many of the peptides with potential effects on hypophysiotropic
hormone release are widely distributed in
hypothalamic and extrahypothalamic areas of the brain.
They include, among many others, peptides common to the
gastrointestinal tract, such as gastrin, cholecystokinin, and
pancreatic polypeptide as well as bombesin, angiotensin II,
galanin, substance P, neurotensin, enkephalins, neuropeptide
Y, natriuretic peptide, the vasoactive intestinal peptide
(VIP), and the peptide histidine isoleucine (PHI). The last
three peptides may, through vasoconstriction and vasodilatation
activities, play an important role in the control of the
portal blood flow.
3 . Vascular System
The releasing and inhibiting hormones are stored in nerve
terminals in the median eminence, where their concentrations
are 10 to 100 times as great as elsewhere in the hypothalamus.
The uniquely organized capillary plexus ( Halasz,
1994 ; Page, 1986 ) of the median eminence is in close proximity
to nerve terminals of the hypophysiotropic neurons.
In contrast to other brain regions, the blood-brain barrier
in the area of the median eminence is incomplete, permitting
protein and peptide hormones as well as other charged
particles to move to the intercapillary spaces and the nerve
terminals contained therein. These terminals respond to
humoral and neuronal stimuli by secreting releasing and
inhibiting factors into the portal system.
The portal capillaries coalesce into a series of vessels
that descend through the pituitary stalk and form a second
capillary plexus that surrounds the AL cells ( Fig. 18-1 ).
Inferior hypophyseal arteries supply the neurohypophysis.
From the primary plexus of the neural lobe (NL), blood
flows not only to the systemic circulation but also to the AL
and the hypothalamus. There is evidence for some degree of