The distribution of vasopressin and oxytocin in the hypothalamoneurohypophysial system of the guinea-pig

The effect of an osmotic stimulus on the release of neurohypophysial hormones in the cat: preferential release of vasopressin with a possible involvement of the area postrema

Abstract In cats anaesthetized with intravenous chloralose, the injection of 0.05 to 0.4 ml 1.54 M NaCl solution (hypertonic saline, HS) into a lateral cerebral ventricle caused a large release of vasopressin. The concentration of vasopressin greatly exceeded that of oxytocin in the same samples of plasma. Vasopressin was also released when HS was injected into the fourth ventricle and into the cisterna magna from which there is no access in the cat to the ventricles, but it was less effective by these routes than when injected into a lateral ventricle in the same cat. This suggests a possible action of HS on circumventricular organs related to the third ventricle but also indicates an additional site of action reached from the subarachnoid space which would give access to the ventral and dorsal surfaces of the brainstem. Vasopressin was not released on topical application of HS to the 'nicotine sensitive area' on the ventral surface of the brainstem where nicotine acts to release vasopressin without oxytocin. Vasopressin, however, was released without detectable oxytocin on topical appliction of HS to the dorsal surface of the brainstem either outside the fourth ventricle or to the floor of the ventricle at its distal extremity, in the region of the obex. A possible site where HS acts to cause a preferential release of vasopressin on injection into a lateral ventricle is the area postrema, a circumventricular organ which impinges on the walls of the fourth ventricle at the obex. Preferential release of vasopressin might then be mediated by a selective neural input, possibly through the nucleus of the tractus solitarius, from osmoreceptors in the area postrema to the vasopressin-secreting cells in the supraoptic and paraventricular nuclei.

Central inhibition by gamma-aminobutyric acid and muscimol of the release of vasopressin and oxytocin by an osmotic stimulus in the rat

1. In water-loaded rats under ethanol anaesthesia, the injection of 2-4 microliters 1.54M NaCl solution (hypertonic saline:HS) into a lateral cerebral ventricle (i.c.v.) produced an antidiuretic and a pressor response, together with increased urinary excretion of vasopressin and 'oxytocin-like radioimmunoreactivity' (OLRI). In lactating rats HS also produced a milk-ejection response which was shown to be due to the release of oxytocin. 2. The injection of 20-40 micrograms gamma-aminobutyric acid (GABA) or 40-80 ng muscimol i.c.v. 2 min before HS inhibited the antidiuretic, pressor and milk-ejection responses and reduced the urinary excretion of vasopressin and OLRI. 3. The pressor response to HS was abolished by a ganglion blocking agent but it was not reduced by a vasopressin antagonist. After the antagonist, the antidiuretic response to HS was abolished and the pressor response was accompanied by a diuresis both of which were blocked by muscimol. 4. The threshold dose of HS for an antidiuretic response was 4-8 times higher on injection into the cisterna magna (i.cist.) than when injected i.c.v. GABA, i.v. or i.cist, did not inhibit the response to HS i.c.v. 5. The results confirm other evidence that, in the rat, in contrast some other species, an osmotic stimulus causes release of both vasopressin and oxytocin. This release is blocked by GABA and muscimol. These drugs and HS act at a site reached not from the subarachnoid space but from the cerebral ventricles, probably the hypothalamus. The pressor response to HS under the experimental conditions used is due entirely to central sympathetic stimulation and this effect, as well as the release of vasopressin and oxytocin, is blocked by muscimol.

Thirst and vasopressin secretion following central administration of angiotensin II in rats with lesions of the septal area and subfornical organ

Various dipsogenic stimuli, including peripheral and central administration of angiotensin II, have been shown to be capable of releasing vasopressin from the neurohypophyseal system. Studies were carried out in the rat to investigate whether the septal area, which contains a high concentration of angiotensin-sensitive cells and has neural connections with hypothalamic vasopressin-secreting neurons, mediated the stimulatory effect produced by angiotensin II on vasopressin release. Rats with electrolytic lesions in the region of the septal area had increased daily water consumption and urine output when these lesions included the medioventral or lateral nuclei of the septal forebrain, but not when the lesion involved the subfornical organ. No difference was observed in drinking responses following water deprivation or intracerebroventricular injection of angiotensin II in all experimental groups. In addition, the impaired ability to maintain water homeostasis (polyuro-polydipsic syndrome) of septal-lesioned rats was associated septal-lesioned rats was associated with decreased levels of circulating radioimmunoassayable vasopressin. Furthermore, the vasopressin release which occurred in response to intracerebroventricular angiotensin II in normal controls, sham-lesioned and subfornical organ-lesioned rats was significantly attenuated in rats with electrolytic lesion of the medioventral or lateral septal area. Since cells in the lateral septal area are excited by iontophoretic application of angiotensin II, the present data might be consistent with the hypothesis that the stimulatory effect produced by central administration of angiotensin II on vasopressin release rests upon the integrity of the lateral septal area.

A cholinergic link in the reflex release of vasopressin by hypotension in the rat

Inhalation of amyl nitrite in the water-loaded rat under ethanol anaesthesia produced a brief fall of blood pressure followed by a prolonged antidiuretic response. The antidiuretic response to amyl nitrite was accompanied by increased urinary excretion of vasopressin, it was blocked by a specific vasopressin antagonist and by a barbiturate and it was absent in the Brattleboro rat with congenital diabetes insipidus. These results show that the antidiuretic response to the hypotension induced by amyl nitrite is due to the release of vasopressin and that this release is mediated by a neuroendocrine reflex acting through the brain stem. Carbachol and nicotine produced an antidiuretic response on injection into a lateral cerebral ventricle (i. vent.). Carbachol was almost ineffective, but nicotine much more effective, when injected into the cisterna magna (i.cist.) from which in the rat there is no access to the ventricles. Carbachol therefore acts at a site reached from the ventricles, possibly the paraventricular nucleus. Nicotine acts at a more distal site reached from the subarachnoid space. This site may correspond with the nicotine-sensitive area on the ventral surface of the brain stem which has been described in the cat. Atropine blocked the antidiuretic response to carbachol but not that to amyl nitrite. Hexamethonium blocked the antidiuretic response to amyl nitrite as well as that to nicotine and was more effective on i.cist. than i.vent. injection. These results reveal a cholinergic link with a nicotinic but not a muscarinic receptor in the neural pathways controlling the release of vasopressin in response to hypotension. A hypothetical model is presented in which the release of vasopressin is stimulated by a pathway arising from chemoreceptors and inhibited by a second pathway arising from stretch- and baroreceptors. Hypotension acts by suppressing the normally predominant inhibitory pathway and stimulating the excitatory pathway. Hexamethonium is presumed to block transmission at a synapse in the excitatory pathway at the ventral surface or, less probably, at the paraventricular and supraoptic nuclei.

Characterisation of an intermediate in neurophysin biosynthesis in the guinea pig

In addition to oxytocin (OT), vasopressin (AVP) and their respective neurophysins (NPs), another [35S]cysteine incorporating component is present in the guinea pig neurohypophysis. Gel filtration and Con A affinity chromatography revealed that this component was larger than NP and was glycosylated. NP-immunoreactivity was assessed using antisera which distinguish the OT- and AVP-related NPs. Whilst the anti-NP antiserum detected only one component (guinea pig NP), the anti-NP antiserum detected both NP and the glycosylated 35S-labelled component. These results suggest that a significant amount of NP in guinea pig neural lobes bears a glycopeptide extension and represents a partially processed form of the AVP precursor in this species.

Afferent projections to the dorsal motor nucleus of the vagus

The afferent projections to the dorsal motor nucleus of the vagus (DMN) were investigated by iontophoretically infusing horseradish peroxidase (HRP) into that neural region of the rat. After the tetramethylbenzidine histochemical procedure was performed on the HRP-injected brains, projections to the DMN from several areas were observed including the nucleus reticularis parvocellularis and gigantocellularis, the nucleus of the solitary tract, the nucleus raphe obscuris, the principal sensory trigeminal nucleus, and the paraventricular nucleus of the hypothalamus. The pathway originating in the paraventricular nucleus was the only forebrain projection to the DMN, a finding compatible with observation of other investigators.

Anatomical evidence of direct projections from the nucleus of the solitary tract to the hypothalamus, amygdala, and other forebrain structures in the rat

Ascending projections from the caudal (general-visceroceptive) part of the nucleus of the solitary tract (NTS) were studied experimentally in the rat by the aid of the anterograde autoradiographic and the retrograde horseradish peroxidase (HRP) tracer techniques. Microelectrophoretic deposits of tritiated proline and leucine which involved the caudal part of the NTS, the dorsal motor nucleus of the vagus (dmX), and portions of the hypoglossal nucleus, nucleus intercalatus and/or nucleus gracilis were found to label ascending fibers that, besides going to numerous brain stem territories that included prominently the parabrachial area, could also be traced to serveral forebrain structures, namely, the bed nucleus of the stria terminalis (BST), the paraventricular (PA), dorsomedial (HDM) and arcuate (ARC) nuclei of the hypothalamus, the central nucleus of the amygdaloid complex (AC), the medial preoptic area (PM) and the periventricular nucleus of the thalamus (TPV). Smaller isotope injections almost completely confined to the NTS and dmX resulted in lighter labeling of a similar set of parabrachial and forebrain projections, whereas in another case, in which the deposit was almost exclusively limited to the nucleus gracilis, no label was seen in the aforementioned structures. In another series of experiments, aimed at further localizing the neurons of origin of the prosencephalic projections under consideration, small microelectrophoretic HRP injections confined almost totally to BST, PA, HDM, AC, PM or TPV, as well as both small and large injections involving ARC, resulted in labeled neurons situated in the dorsal medullary region, mainly in the medial portion of the NTS at the level of and caudal to the area postrema. Taken together, these observations indicate for the first time the existence of relatively direct conduction lines by which interoceptive information might be conveyed to limbic forebrain structures; some of the possible physiological correlates of these anatomical findings are discussed.

Evidence for corticotropin releasing factor (CRF) synthesis in the preoptic nucleus of Xenopus laevis tadpoles: a preliminary report based on lesion experiments

For the study of the hypothalamo-hypophysial system of Xenopus laevis tadpoles, hypothalamic lesions were made by means of the electrocoagulation technique. Lesioning of the ventral region of the preoptic nucleus resulted in a decrease of the number of ACTH cells in the pars distalis of the pituitary gland and in a diminution of the PAS-positive reaction of these cells. In addition, regeneration of the neurosecretory cells of the ventral region of the preoptic nucleus observed 6 weeks after lesioning was accompanied by the reappearance of normal PAS-positive ACTH cells in the pars distalis. It is suggested that the neurosecretory cells of the ventral region of the preoptic nucleus of Xenopus laevis tadpoles are related to the ACTH synthesizing cells, probably by producing CRF.

Vasopressin release by nicotine: the site of action

1. In cats anaesthetized with chloralose the release of neurohypophysial hormones was examined after injection of nicotine into the cerebral ventricles or cisterna magna or its topical application through perspex rings to the ventral surface of the brain stem. The release was measured by assaying the hormones in samples of venous blood. 2. Injected into a lateral or the third cerebral ventricle, nicotine (0.5 to 1 mg) produced release of vasopressin without oxytocin. When the aqueduct was cannulated, preventing access to the fourth ventricle and to the subarachnoid space, this release did not occur. 3. Vasopressin was also released without oxytocin when nicotine (0.25 to 2 mg) was injected into the subarachnoid space through the cisterna magna. With this route of administration the nicotine did not enter any part of the ventricular system. 4. Applied through paired perspex rings placed across the ventral surface of the brain stem, nicotine again produced release of vasopressin without ocytocin. The amount of nicotine placed in each ring was usually 80 mug, but a release was obtained with 10 mug and in one experiment with as little as 5 mug. 5. The bilateral region on the ventral surface of the brain stem where nicotine acts when producing release of vasopressin lies lateral to the pyramids and in a longitudinal direction, 6 to 9 mm caudal to the trapezoid bodies. 6. The vasopressin release by nicotine injected intraventricularly or intracisternally, or applied topically to the ventral surface of the brain stem was not due to absorption of nicotine into the blood stream, nor to blood pressure effects. 7. It is concluded that nicotine acts on the ventral surface of the brain stem probably by activating the central projection to the supra-optic and possibly also the paraventricular nuclei of afferent pathways in the sinus and vagus nerves which control the release of vasopressin in response to changes in blood volume or distribution.

Presence of neurophysin and vasopressin in the hypothalamic magnocellular nuclei of rats homozygous and heterozygous for diabetes insipidus (Brattleboro strain) as revealed by immunoperoxidase history

The 3-layer immunoperoxidase-bridge technique was used to study the distribution of neurophysin and vasopressin in the neurosecretory neurons of rats homozygous and heterozygous for diabetes insipidus (Brattleboro strain). In the homozygous rats there was a marked hypertrophy of the hypothalamic magnocellular structures when stained either for neurosecretory material or neurophysin-like antigens. Neurophysin was present in both the paraventricular (PVN) and supraoptic nuclei (SON) of homozygous and heterozygous animals. Less than half of the cells in the PVN and SON were stained for neurophysin. This observation was less apparent when histochemical stains were used to visualize the distribution of neurosecretory material. Although it is generally considered that the homozygous Brattleboro rat does not synthesize vasopressin, a positive reaction was observed in the PVN and SON when anti-[8-lysine]-vasopressin serum was employed in the immunohistochemical procedure.

Bioassay of oxytocin on the superfused mammary gland of the mouse, using an automatic apparatus

1 A sensitive method of assaying oxytocin using the superfused mouse mammary gland is described. The method is shown to be reliable, accurate and precise. Assays are performed automatically and the preparation is stable for at least 18 hours.2 The preparation is relatively insensitive to bradykinin, histamine, 5-hydroxytryptamine, angiotensin, prostaglandin F(2alpha), adrenaline and noradrenaline. The contractions produced by acetylcholine can be abolished by atropine without the sensitivity to oxytocin being affected.3 Vasopressin has a variable activity on the preparation; its potency can be as high as one-fifth that of oxytocin.4 It is concluded that this method compares favourably with other published methods.