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thyrotropin-releasing hormone neurons that indirectly regulate the thyroid gland;
corticotrophin-releasing hormone neurons that indirectly control the adrenal gland,
magnocellular vasopressin neurons that control the kidney and magnocellular oxy-
tocin neurons that are responsible for controlling parturition and lactation; in ad-
dition, smaller, centrally projecting oxytocin neurons regulate gastric function, and
centrally projecting vasopressin neurons that regulate body temperature and blood
pressure, some of which project into the spinal cord (as do some oxytocin neu-
rons, a subpopulation that seems to be involved in penile erection). Below this, the
suprachiasmatic nucleus is the body's principal circadian pacemaker; one popula-
tion of neurons here makes vasoactive intestinal peptide, another makes vasopressin;
these cells are governed by clock genes that confer 24-h cyclicity on their behav-
ior. Behind the suprachiasmatic nucleus is the arcuate nucleus, that in addition to
growth-hormone releasing-hormone neurons contains leptin-sensitive neuropeptide
Y neurons that regulate feeding, dopamine neurons that regulate the secretion of pro-
lactin, opioid (b-endorphin) neurons that impact on many neuronal systems through
extensive central projections, and a large population of centrally projecting somato-
statin neurons of unknown function. Above this, the ventromedial nucleus contains
specialised glucoreceptive neurons, and alongside it the lateral hypothalamus con-
tains orexin neurons; orexin is linked to sleep and wakefulness, and orexin knock-out
results in narcolepsy.
We have not gone far in the hypothalamus yet, and we have described only some
of the best-known populations, and neglected subpopulations of interneurons and
many distinctive subnuclei. In addition, the individual cells vary even within a given
population: these
homogeneous populations
are far from clones. Moreover, individ-
uals in one population interact to differing extents with individuals of many other
populations, and these interactions differ from cell to cell even within a population.
The populations are not fully interconnected but neither are they as separable
as we would like. Take for instance the magnocellular oxytocin neurons of the
hypothalamus-and we probably know as much or more about these as about any
cells in the brain (see [3]). These are simple neurons in many respects; there are
about 3,000 of them in the rat brain, and each has a single axon that projects to the
neural lobe of the pituitary gland. Oxytocin, released from the nerve endings in the
pituitary, controls milk let-down in response to suckling, and it controls the progress
of parturition by its actions on the uterus. But in the rat, oxytocin also controls the
excretion of sodium at the kidney. Moreover, centrally released oxytocin is involved
in maternal behavior, sexual behavior, and affiliative behaviors generally, and stress
responsiveness, and the magnocellular oxytocin system is involved in these behav-
iors through secretion of oxytocin from its dendrites rather than from classical nerve
endings. Dendritic secretion unfortunately does not parallel secretion from axonal
endings - the mechanisms underlying dendritic secretion differ in important ways
from those that govern axonal secretion. The diversity of roles played by oxytocin
shows both that the oxytocin neurons receive very functionally diverse inputs, and
also that they influence many other neuronal populations, including some to which
they are not synaptically connected, even indirectly. It would be dangerous to think
that oxytocin neurons are exceptionally complicated, just because we know more
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