Ganong's Review of Medical Physiology, 23rd Edition

278 SECTION III Central & Peripheral Neurophysiology
1
FIGURE 18–7 Structure of bovine prepropressophysin (left) and prepro-oxyphysin (right). Gly in the 10 position of both peptides is
necessary for amidation of the Gly residue in position 9. aa, amino acid residues. (Reproduced with permission from Richter D: Molecular events in expression
of vasopressin and oxytocin and their cognate receptors. Am J Physiol 1988;255:F207.)
arginine vasopressin, neurophysin II, and a glycopeptide (Figure
18–7). Prepro-oxyphysin, the precursor for oxytocin, is a
similar but smaller molecule that lacks the glycopeptide.
The precursor molecules are synthesized in the ribosomes
of the cell bodies of the neurons. They have their leader
sequences removed in the endoplasmic reticulum, are packaged
into secretory granules in the Golgi apparatus, and are
transported down the axons by axoplasmic flow to the endings
in the posterior pituitary. The secretory granules, called
Herring bodies, are easy to stain in tissue sections, and they
have been extensively studied. Cleavage of the precursor molecules
occurs as they are being transported, and the storage
granules in the endings contain free vasopressin or oxytocin
and the corresponding neurophysin. In the case of vasopressin,
the glycopeptide is also present. All these products
are secreted, but the functions of the components other than
the established posterior pituitary hormones are unknown.
ELECTRICAL ACTIVITY OF
MAGNOCELLULAR NEURONS
1
2
3
4
Signal peptide
Vasopressin
Neurophysin II
Glycopeptide
19 aa
9 aa
95 aa
39 aa
2 3 4
-Gly-Lys-Arg- -Arg-
The oxytocin-secreting and vasopressin-secreting neurons
also generate and conduct action potentials, and action potentials
reaching their endings trigger release of hormone from
them by Ca 2+ -dependent exocytosis. At least in anesthetized
rats, these neurons are silent at rest or discharge at low, irregular
rates (0.1–3 spikes/s). However, their response to stimulation
varies (Figure 18–8). Stimulation of the nipples causes a
synchronous, high-frequency discharge of the oxytocin neurons
after an appreciable latency. This discharge causes release
of a pulse of oxytocin and consequent milk ejection in postpartum
females. On the other hand, stimulation of the vasopressin-secreting
neurons by a stimulus such as hemorrhage
causes an initial steady increase in firing rate followed by a
prolonged pattern of phasic discharge in which periods of
high-frequency discharge alternate with periods of electrical
quiescence (phasic bursting). These phasic bursts are gener-
1
1
2
3
Signal peptide
Oxytocin
Neurophysin I
2 3
19 aa
9 aa
93 aa
-Gly-Lys-Arg- -Arg/His
ally not synchronous in different vasopressin-secreting neurons.
They are well suited to maintain a prolonged increase in
the output of vasopressin, as opposed to the synchronous, relatively
short, high-frequency discharge of oxytocin-secreting
neurons in response to stimulation of the nipples.
A
Unit
Rate
B
Control
HFD
Intramammary pressure
5 mL blood removed
5 mL blood removed (+ 20 min)
1 min
10/s
FIGURE 18–8 Responses of magnocellular neurons to
stimulation. The tracings show individual extracellularly recorded action
potentials, discharge rates, and intramammary duct pressure. A)
Response of an oxytocin-secreting neuron. HFD, high-frequency discharge;
ME, milk ejection. Stimulation of nipples started before the onset
of recording. B) Responses of a vasopressin-secreting neuron,
showing no change in the slow firing rate in response to stimulation of
nipples and a prompt increase in the firing rate when 5 mL of blood
was drawn, followed by typical phasic discharge. (Modified from Wakerly
JB: Hypothalamic neurosecretory function: Insights from electrophysiological studies
of the magno-cellular nuclei. IBRO News 1985;4:15.)
ME
ME
50/s