Talk abstract:
Vasopressin and Oxytocin Neurons - Similar
Neurosecretory Cells
with Different Patterns of Activity and Hormone Release
David Brown
Laboratory of Computational Neuroscience
The Babraham Institute
Cambridge, CB2 4AT, UK
db10@cus.cam.ac.uk
Magnocellular vasopressin (VP) and oxytocin (OT) secreting
neurons are co-located in the supraoptic and paraventricular
nuclei of the hypothalamus. They have almost all biophysical
properties in common, yet they fire and secrete hormone in very
different ways.
BIOPHYSICS. They both exhibit action potentials as a result
of influx of Na+ ions, followed by a repolarizing
outward K+ conductance. There are low voltage, slowly
inactivating Ca2+ channels, and also Ca2+
dependent potassium channels, the resulting depolarizing and
hyperpolarizing afterpotentials accounting for some aspects
of the firing patterns. One major biophysical difference is
in intracellular calcium buffering: in most OT cells, intracellular
calcium is effectively buffered involving Calbindin, but most
VP cells do not exhibit this buffering.
FUNCTION. Both hormones are involved in maintenance of plasma
osmotic pressure, VP by involvement in control of water retention,
OT in sodium excretion. OT is additionally involved in events
associated with reproduction: pulses of OT release are temporally
correlated with milk ejections in the lactating female, and
uterine contractions during parturition.
PATTERNS OF ACTIVITY AND HORMONE RELEASE. VP cells, when firing
slowly (i.e. when mean firing rate (mfr) < 3Hz in the rat) fire
in an irregular, near Poisson or slightly over-dispersed, pattern;
as activity increases, they fire phasically with irregular burst
and silence durations, with maximum mfr of about 25Hz. The phases
of activity appear to be uncorrelated between cells, the net
result being a graded output of vasopressin into the peripheral
circulation. OT cells' so-called background firing pattern is
typically underdispersed, i.e. more regular than Poisson. In
the non-pregnant rat, background activity is all that is observed,
with a range of mfr typically between 1 and 10Hz. During lactation
and parturition, OT cells fire briefly - for about 2 secs -
outside these limits, at firing rates up to 50Hz, and at about
5 minute intervals, the bursts being temporally correlated with
milk ejections and uterine contractions. Between bursts, the
activities of OT cells seem to be uncorrelated, but the vast
majority of cells in the four nuclei (one of each type on both
sides of the brain) burst together, burst onset varying by a
maximum of about 0.5 second. Each burst results in a brief,
high concentration, bolus of OT released into the peripheral
circulation - necessarily for its physiological action. Bursting
in OT cells is a network phenomenon, unlike background activity.
The talk will cover the behaviour of these cells, an initial
biophysical model of VP cells, and a skeleton model of OT network
behaviour.
Reference: Leng, G, Brown, D (1997) The origins and significance
of pulsatility in hormone secretion from the pituitary, J. Neuroendocrinology,
9, 493-513. [review]
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1998-1999
Mathematics in Biology
