Regulation of noradrenaline release by presynaptic receptor systems.

Organ cultured hypothalamo-neurohypophyseal (HNS) explants from rats were maintained in vitro for up to 9 days. These explants contain the supraoptic nucleus with its vasopressin (AVP)- producing neurons, their axonal projections through the median eminence and axon terminals in the neurohypophysis. The presence of cells containing AVP was confirmed in 5 day cultured explants irnmunocytochemically. The rate of AVP release by the HNS explants remained constant over 9 days in culture. However, due to simultaneous degradation of AVP by enzymes released into the culture medium by the explant, it was necessary to develop a sampling protocol which allowed simultaneous estimation of AVP release and degradation. Utilizing this protocol, the rate of AVP release by the cultured HNS was 309 ± 32 pg\h\explant on day 3 in culture. This rate remained essentially unchanged during the subsequent 2 days in culture. This rate of hormone release corresponds closely with other in vivo and in vitro measures of AVP secretion. ...

Angiotensin stimulation of vasopressin release from the rat hypothalamo-neurohypophyseal system in organ culture.

Publisher Summary This chapter discusses the regulation of vasopressin (AVP) release from the neurohypophysis and the known effects of afferents to the supraoptic nucleus (SON). The signals regulating AVP release from the neurohypophysis for the maintenance of these functions involve osmoreceptive mechanisms located primarily in the hypothalamus and signals relayed from the cardiovascular volume and baroreceptors. AVP release is also altered by the changes in emotional states, pain, exercise, and the act of drinking. The AVP neurone itself is osmosensitive, in which increases in the osmolarity of the extracellular fluid cause depolarization of the membrane and a resulting sensitization of the neurone to other excitatory inputs. However, this inherent osmosensitivity is inadequate to generate firing of the neurone and subsequent AVP release by itself. The ability of osmotic stimulation to override noradrenergic inhibition emphasizes the prominent role of the osmotic control of AVP release and is consistent with the postulated role for the noradrenergic innervation in conveying signals from the cardiovascular baro- and volume-receptors. The interaction between noradrenaline and osmotic stimulation in determining AVP release would fit with a role for noradrenaline in transmitting cardiovascular information as the osmotic regulatory mechanisms can override the effects of noradrenaline. However, the information conveyed by the other chemically identified afferents to the SON remain to be established, and the mechanisms, which integrate all of these signals to achieve a rate of hormone release appropriate to the physiological circumstances, remain to be elucidated.

Regulation of Vasopressin Release by Neurotransmitters, Neuropeptides and Osmotic Stimuli

Components of the renin-angiotensin system were studied in established cell culture lines of 3T3 and SV3T3 mouse fibroblasts. The renin content in 3T3 cells was significantly higher than in virus-transformed SV3T3 cells. With time after infection, renin decreased in Simian virus 40 transformed cells, while it increased steadily in mock-infected 3T3 cells. In contrast to renin, angiotensinase activity was higher in SV3T3 cells.



Angiotensin II stimulated cell proliferation in 3T3 mouse fibroblasts and decreased their renin content in a dose-related manner. In contrast, saralasin, an angiotensin receptor antagonist, inhibited cell growth in 3T3 and SV3T3 cells and caused an increase of cellular renin concentration. The angiotensin fragments angiotensin (2–8) heptapeptide and angiotensin (4–8) pentapeptide had no effect on cell growth. A significant negative correlation was found between cell proliferation and renin levels in 3T3 and SV3T3 cells irrespective of the treatment.



Our results indicate (1) that angiotensin II may be involved in cell growth regulation, (2) that a negative feedback exists between angiotensin II added and intracellular renin content, and (3) that virus infection causes a decrease in intracellular renin synthesis, while non-specific angiotensinase activity is increased under this condition.

Effects of angiotensin II and angiotensin II antagonist saralasin on cell growth and renin in 3T3 and SV3T3 cells.

The release of vasopressin (VP) from the neural lobe occurs in response to action potentials, which are generated at the perikaryon and propagated down the axon to the nerve terminal located in the posterior pituitary. The generation of electrical signals and subsequent hormonal release occurs in response to synaptic release of neuroactive agents from nerve terminals in the hypothalamus. VP release may also be regulated by neuroactive agents released from nerve terminals in the posterior pituitary. Figure 1 summarizes our current knowledge of the neuronal projections and identified transmitters to the supraoptic nucleus (SON) and neural lobe. This literature is reviewed in detail by Swanson and Sawchenko (1983) (see also Chapter 1, this volume). It is apparent that this anatomical evidence implicates numerous neurotransmitters and neuropeptides as candidates for regulating hormonal release from the neural lobe. An even greater number of neuroactive substances have been localized to the paraventricular nucleus (PVN), but the complexity of that nucleus suggests that these additional agents may subserve functions other than regulation of hormonal release from the neurohypophysis. Thus, this chapter concentrates on neuroactive substances localized in SON and the neural lobe in evaluating the physiological regulation of VP release from the neural lobe by neurotransmitters and neuropeptides.

Effect of Neurotransmitters and Neuropeptides on Vasopressin Release

SUMMARY

1. Calcium did not influence the spontaneous release of vasopressin from rat neurohypophyses in vitro when used in concentrations of 0·05, 0·5 and 2·8 mM in the bathing medium.



2. Stimulation of the basal output of vasopressin by angiotensin II (1 · 10–9 M) required at least 0·5 mM calcium in the medium.



3. Angiotensin II stimulated the release of vasopressin within 2·5 min of incubation, maximal release was observed after 10 min.



4. Angiotensin II rapidly promoted the accumulation of tissue cyclic AMP; maximal accumulation was observed after 5 min of incubation.



5. Theophylline and dibutyryl cyclic AMP produced varying degree of stimulation of the release of vasopressin.



6. Increases in vasopressin secretion and in the accumulation of cyclic AMP were always present when neurohypophyses were exposed to optimal concentrations of angiotensin II. The results presented suggested that cyclic AMP may be an intermediate step for the release of vasopressin by endogenous angiotensin II.

Stimulation by angiotensin II of the release of vasopressin from incubated rat neurohypophyses---possible involvement of cyclic AMP.

(1971) has suggested that the production of a false transmitter metabolite of amphetamine, p-hydroxynorephedrine, which replaced noradrenaline within the sympathetic neuronal vesicles, is liberated on subsequent amphetamine challenge, and is responsible for the attenuation in hyperthermia observed with repeated doses. However, recent studies on sex and species differences in amphetamine metabolism and tolerance militate against the false transmitter hypothesis (Sever & Caldwell, in preparation). P.S.S. is in receipt of an M.R.C. Junior Research Fellowship.

P. J. Boucher

Papers

Stimulation by angiotensin II of the release of vasopressin from incubated rat neurohypophyses---possible involvement of cyclic AMP.

Inactivation of prostaglandin E2 by rabbit lung homogenates