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]