Ganong's Review of Medical Physiology, 23rd Edition

278
SECTION III
Central & Peripheral Neurophysiology

arginine vasopressin, neurophysin II, and a glycopeptide (Fig-
ure 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 pack-
aged into secretory granules in the Golgi apparatus, and are
transported down the axons by axoplasmic flow to the end-
ings 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 mol-
ecules 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 vaso-
pressin, 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

The oxytocin-secreting and vasopressin-secreting neurons
also generate and conduct action potentials, and action poten-
tials 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, irreg-
ular rates (0.1–3 spikes/s). However, their response to stimu-
lation varies (Figure 18–8). Stimulation of the nipples causes a
synchronous, high-frequency discharge of the oxytocin neu-
rons after an appreciable latency. This discharge causes release
of a pulse of oxytocin and consequent milk ejection in post-
partum females. On the other hand, stimulation of the vaso-
pressin-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-

ally not synchronous in different vasopressin-secreting neu-

rons. They are well suited to maintain a prolonged increase in

the output of vasopressin, as opposed to the synchronous, rel-

atively short, high-frequency discharge of oxytocin-secreting

neurons in response to stimulation of the nipples.

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.)

1

1

2

3

4

2 3 4

Signal peptide

Vasopressin

Neurophysin II

Glycopeptide

19 aa

9 aa

95 aa

39 aa

-Gly-Lys-Arg- -Arg-

1

1

2

3

2 3

Signal peptide

Oxytocin

Neurophysin I

19 aa

 9 aa

93 aa

-Gly-Lys-Arg- -Arg/His

FIGURE 18–8

Responses of magnocellular neurons to

stimulation.

The tracings show individual extracellularly recorded ac-

tion potentials, discharge rates, and intramammary duct pressure.

A)

Response of an oxytocin-secreting neuron. HFD, high-frequency dis-

charge; ME, milk ejection. Stimulation of nipples started before the on-

set 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.)

Unit

Rate

Control

5 mL blood removed

5 mL blood removed (+ 20 min)

1 min 10/s

ME

ME

50/s

A HFD

B

Intramammary pressure