The cholinergic influences on the oxytocin activity of the hypothalamus and neurohypophysis in long-term dehydrated male rats

The cholinergic influences on the oxytocin activity of the hypothalamus and neurohypophysis in long-term dehydrated male rats. Acta Physiol. Pol., 1978, 29 (1): 17-25. Rats dehydrated up to 12 days were injected intraperitoneally with carbachol or atropine sulfate in daily doses of 20 microgram/100 g and 1.0 mg/100 g, respectively. In not dehydrated rats atropine increased the oxytocin activity of the hypothalamus and neurohypophysis; carbachol did not influence the oxytocin potency of the hypothalamus but augmented it in the neurohypophysis. During long-term dehydration both carbachol and atropine intensified the depletion of oxytocin in the hypothalamus as well as in the neurohypophysis.     

Studies on the vasopressin and oxytocin storage in the hypothalamus and neurohypophysis

The effects of modified adrenergic transmission on the bioassayed storage of vasopressin and oxytocin in the hypothalamus and neurohypophysis under conditions of stress (cold or immobilization), disturbed water balance and pinealectomy are reviewed. Alpha-adrenergic mechanisms seem to be included in the response of vasopressinergic and oxytocinergic neurones to stress; on the other hand, impulses of osmoreceptor origin are of importance in regulatory processes affecting the functional response of these neurones to altered alpha-adrenergic transmission and also to melatonin. The beta-adrenergic (and, to some extent, also the alpha-adrenergic) transmission is probably involved in the neural mechanisms of the pineal-neurohypophysial relationship. Furthermore, a possible regulatory role of cholecystokinin in water metabolism and release of neurohypophysial hormones is suggested.     

The vasopressin and oxytocin content in the hypothalamus and neurohypophysis as influenced by reserpine treatment during long-term dehydration in the white rat.

In dehydrated rats both neurohypophysial hormones diminished in hypothalamus as well as in the neurohypophysis. Oxytocin disappearef from the hypothalamus and neurohypophysis at a more rapid rate than vasopressin did. The minimal content of vasopressin and oxytocin in the hypothalamus was observed during 3rd--4th day, but even in extreme dehydration it was found to be relatively high: 65 per cent of vasopressin and 27 per cent of oxytocin as compared with intact animals. At that time the neurohypophysial vasopressin and oxytocin content were almost fully exhausted. In dehydrated and additionally reserpinized animals (10 mg/kg intraperitoneally, then each 48 hr 5 mg/kg of initial body weight) the vasopressin and and oxytocin hypothalamus and neurohypophysis changed in a similar manner. In some experimental groups the decrease of neurohormones in both sites was more marked under reserpine treatment. The drug seems therefore rather to potentiate the effects of physiological stimulation of osmodetectors. So the existence of monoaminergic stimulatory synapses, directly involved in the neural pathway between the osmodetector and the neurosecretory cell, appears to be hardly probable.     

Melatonin and the synthesis of vasopressin in pinealectomized male rats

The pineal hormone, melatonin, is known to modify, under different experimental conditions, neurohypophysial hormone secretion in the rat. The aim of this study was to investigate the effect of melatonin on the vasopressin biosynthesis rate in the hypothalamus of either pinealectomized or sham-operated rats, using the colchicine method. To estimate whether colchicine affects the function of the neurohypophysis in these animals, the neurohypophysial and plasma vasopressin levels were also measured. The vasopressin synthesis rate was increased after pineal removal, when compared with sham-operated animals, and melatonin strongly inhibited the rise in the hormone synthesis due to pinealectomy. After pineal removal plasma vasopressin concentration was significantly elevated, and melatonin attenuated this effect. On the contrary, the neurohypophysial vasopressin content was significantly decreased after pinealectomy, but it was not further modified by melatonin.Thus, melatonin suppresses the synthesis and secretion of vasopressin in pinealectomized rats. The present results confirm our previous reports as to the inhibitory impact of the pineal on both vasopressin synthesis and release and suggest that melatonin may mediate the effect of the pineal gland on vasopressinergic neuron activity.     

Effects of oxytocin and vasopressin on retrieval of passive avoidance response in melatonin-treated and/or pinealectomized male rats

The role of the pineal gland and its hormone-melatonin-as to the impact of vasopressin (VP) and/or oxytocin (OT) on the regulation of behavior was studied, the passive avoidance task being chosen as an experimental model. The results showed that VP facilitated the avoidance latency during the first retention trial; after pinealectomy, however, VP was ineffective in this regard. Intraperitoneal application of OT was ineffective in modifying the passive avoidance latency when compared with respective saline-treated animals. Melatonin alone, when injected to shamoperated animals 30 min before behavioral experiment, did not affect the passive avoidance response in SA- or OT-treated rats, but blocked the VP-induced lengthening of the passive avoidance latency in the first retention trial. In pinealectomized and OT-treated animals the passive avoidance latency during the second retention trial was severely diminished by melatonin when compared to respective control. We conclude that: a) VP needs a regulated pineal function for developing short-term effects on the passive avoidance response and b) the effect of OT on the avoidance latency in pinealectomized rats develops after melatonin treatment as a long-term effect.     

Changes in oxytocin and vasopressin content in posterior pituitary and hypothalamus following pantethine treatment

Pantethine, a cysteamine precursor, depletes somatostatin in the cerebral cortex and hypothalamus and prolactin in the anterior pituitary and hypothalamus. This study investigated the effect of pantethine on oxytocin and arginine vasopressin content in the posterior pituitary and hypothalamus. Male Long-Evans rats were injected intraperitoneally with escalating doses of pantethine (i.e., 146.7 mg, 293.4 mg and 586.6 mg/100 gm body weight). Hormone content was determined by radioimmunoassay. Three hours after pantethine treatment, the oxytocin content in the posterior pituitary and the hypothalamus was markedly reduced with all doses of the drug. Vasopressin content in the posterior pituitary and hypothalamus was decreased but to a lesser extent than oxytocin and only with the highest dose of pantethine. Pantethine may act to reduce oxytocin and vasopressin content through intracellular conversion to cysteamine. The exact mechanism of action of pantethine on oxytocin and vasopressin remains to be elucidated.     

Electron microscopy of the neurohypophysis in normal and histamine-treated rats

Summary The fine structure of the neurohypophysis has been studied in normal and histamine-treated rats with particular reference to capillary relationships and to the neurosecretory vesicles. Certain new information on the pericapillary space has been developed and is discussed with reference to the existing literature on the posterior hypophysis and other endocrine organs. The membrane-bounded pericapillary space penetrates deeply between surrounding nerve terminals and pituicyte processes, seemingly forming a pervasive metabolic lake which undoubtedly is of physiological significance for metabolic and secretory exchange.Following histamine treatment, the neurosecretory vesicles lose their electrondense centers, the mitochondria in the nerve terminals become swollen, and the capillary endothelium shows evidence of increased pinocytosis. In one rat subjected to painful stimuli, only the first and last of the above alterations are prominent. The experimental results are interpreted in the light of previous work done by other authors, as additional evidence for the identity of the stainable neurosecretion of light microscopy and the neurosecretory vesicles of electron microscopy.The author is greatly indebted to Prof. Dr. med. W. Bargmann for the use of electron microscope facilities and for other invaluable help during the course of the work. To Frau Dr. A. Knoop of the Anatomical Institute in Kiel, and to Frl. Dr. Weichan of Siemens & Halske AG in Berlin, for aid in obtaining the micrographs, a grateful acknowledgement is extended.Presented in part at the 55th meeting of the Anatomische Gesellschaft in Frankfurt/Main, April 9–12, 1958.United States Public Health Service Special Research Fellow, on leave from Department of Anatomy, University of Minnesota Medical School, Minneapolis, Minnesota, USA.     

Prolactin increases the density of striatal dopamine receptors in normal and hypophysectomized male rats

Administration of prolactin to adult male rats, by s.c. injection, significantly increases the density of the striatal dopamine (DA) receptors, without altering the apparent affinity of the receptors for [³H]spiroperidol. Larger doses of prolactin are required to increase the density of the striatal DA receptors in hypophysectomized rates compared to normal rats. These results suggest that prolactin might be the common mediator of the increase in striatal DA receptor density produced by either estrogen or haloperidol administration. Monitoring and/or altering prolactin levels might be informative in neurologic or psychiatric disorders involving striatal DA neurotransmission.     

Changes in oxytocin content in the magnocellular neurons of the rat hypothalamus following water deprivation or estrogen treatment

Summary The effect of water deprivation or estrogen treatment on the oxytocin content of rat hypothalamic cells was examined using a quantitative immunohistological technique. Oxytocin-containing cells were visualized using the immunoperoxidase technique of Sternberger and a primary antiserum directed against oxytocin. The optical density of the darkest 3.2 m diameter spot in the cytoplasm of a cell was used as a measure of the oxytocin content of that cell. Water deprivation produced a significant decrease in anti-oxytocin staining in the anterior commissural nucleus of males and females. There was a similar decrease in the paraventricular nucleus of males, but not in the paraventricular nucleus of females or the supraoptic nucleus of either males or females. Estrogen treatment of ovariectomized female rats produced a fall in anti-oxytocin staining in the anterior commissural, but not paraventricular or supraoptic nuclei.     

Presence of microglia-like cells in the neurohypophysis of normal and dehydrated rats

By use of electron microscopy, in addition to conventional pituicytes small irregular cells were found in the hypophysial neural lobe of both normal and dehydrated adult rats. These cells were characterized by their irregular profiles, showing many multidirectional cytoplasmic processes, and by their dense nuclei and cytoplasm. They were located in both perivascular and interstitial positions. The cells in the first site were more numerous and showed frequent contacts with the parenchymal or endothelial basal laminae. Both showed basically the same submicroscopic features, displaying a well-developed Golgi apparatus and several dense bodies, presumably primary and secondary lysosomes. Cells were related to axons and nerve endings lying in the intercellular and in the perivascular spaces. Their ultrastructural features closely resembled those of microglial cells. The participation of these cells in the activities of neural lobe is discussed.     

Effect of alcohol on the content of sex steroid receptors in the hypothalamus and hypophysis of male rats

In experimental dipsomania model (formation of physical dependence by method of intensive alcoholization) we have studied receptor binding of testosterone (T) and estradiol (E2) in the hypothalamus and pituitary body of mature male rats. Administration (at 10 and 16 h) of 25% ethanol-saline solution at a dose of 7.5 g/kg of body weight in the course of 5 days significantly decreased serum T level but did not change serum LH and FSH levels. Essential reduction of the nuclear androgen receptors in the preoptic-anterior hypothalamic area (POA), mediobasal hypothalamus (MBH) and adenohypophysis was noted in alcohol-treated rats. Unlike androgen receptors the number of the nuclear E2-binding sites in PaO was significantly increased in these males. Thus the results of the present paper demonstrate that multiple administration of ethanol stipulates deficit of serum T, androgen receptors in MBH and pituitary body that possibly results in separation of negative feedback mechanism between the gonads and pituitary body. Increase of specific binding of E2 to nuclear receptors in PoA might appear to explain feminization of alcohol-treated rats.     

The effect of intraventricular 6-hydroxydopamine on the content of oxytocin and vasopressin in the hypothalamus and pituitary gland of water-deprived rats

The effect of intraventricular 6-hydroxydopamine on the content of oxytocin and vasopressin in the hypothalamus and pituitary gland of water deprived rats. Acta Physiol. Pol., 1977, 28 (6): 497-504. Rats received one infusion of 200 microgram 6-hydroxydopamine with 25 microgram of ascorbic acid into the lateral cerebral ventricle. After 57 days some rats were deprived of water for 4, 8 or 12 days. Then, the animals were sacrificed by decapitation. Oxytocin was determined in extracts from the posterior pituitary lobe and hypothalamus by the method of Van Dongen and Hays, while the vasopressin content was determined by the method of Dekanski. It was found that 6-hydroxydopamine injection into the cerebral ventricles causes a rise in oxytocin content in the hypothalamus and prevents its fall during--4--12 days of dehydration.     

Chronic enhancement of brain oxytocin levels causes enduring anti-aggressive and pro-social explorative behavioral effects in male rats

Oxytocin (OXT) has been implicated in the regulation of social behaviors, including intermale offensive aggression. Recently, we showed that acute enhancement of brain OXT levels markedly suppressed offensive aggression and increased social exploration in resident rats confronted with an intruder in their home territory. Moreover, a different responsivity to the exogenous OXTergic manipulation was observed among individuals based on their baseline aggression. In this study we aimed at evaluating the behavioral response to chronically enhancing or attenuating central OXT levels, and at scrutinizing whether the trait-aggression moderates the treatment-induced behavioral changes. To this end, resident male wild-type Groningen rats were continuously (via osmotic minipumps) intracerebroventricularly infused with synthetic OXT or a selective OXT receptor (OXTR) antagonist for 7 days. Changes in behavior were assessed performing a resident–intruder test before and at the end of the treatment period, as well as after 7 days of withdrawal. Chronic infusion of OXT was found to selectively suppress aggression and enhance social exploration. Chronic blockage of OXTRs instead increased introductory aggressive behavior (i.e. lateral threat), yet without affecting the total duration of the aggression. The magnitude of the anti-aggressive changes correlated positively with the level of baseline aggression. Interestingly, OXT-induced behavioral changes persisted 7 days after cessation of the treatment. In conclusion, these findings provide further evidence that enhanced functional activity of the central OXTergic system decreases social offensive aggression while it increases social explorative behavior. The data also indicate that chronically enhancing brain OXT levels may cause enduring anti-aggressive and pro-social explorative behavioral effects.     

Acute nicotine administration increases somatostatin content and binding in the rat hypothalamus.

Within 4 minutes a single, intravenous injection of nicotine (0.3 mg/Kg) induced increases in somatostatin-like immunoreactivity concentrations in the rat hypothalamus but not in the striatum. These changes were associated with a significant increase in the specific binding of somatostatin to putative receptor sites in hypothalamic membranes, while no significant changes were found in striatum. The enhancement of somatostatin binding resulted from a rapid increase in the number of available receptors rather than a change in receptor affinity. This effect appears to be mediated by nicotinic cholinergic receptors, because pretreatment with a centrally active nicotinic receptor antagonist, mecamylamine (5.0 mg/Kg i.v.), prevented the nicotine-induced changes in somatostatin content and binding in the hypothalamus. Mecamylamine alone had no observable effect on the hypothalamic somatostatinergic system. These results suggest that the rat hypothalamic somatostatinergic system can be regulated by nicotine-like acetylcholine receptors.     

Specific immunoelectronmicroscopic localization of vasopressin and oxytocin in the neurohypophysis of the rat

Summary An immunoelectronmicroscopic method for the specific localization of neurohypophyseal hormones was developed in neurohypophyses of Wistar and Brattleboro rats, the latter strain being homozygous for diabetes insipidus. If the proper precautions were omitted, a marked cross reactivity between antivasopressin and antioxytocin preparations was found. Cross reaction of an antivasopressin plasma with oxytocin, at a dilution of less than 11600, resulted in electron density of all granules within neurosecretory fibres of the Brattleboro and Wistar neurohypophyses. However, this cross reactivity could be eliminated either by sufficient dilution of the antiplasma, or by its purification. Purification of the antibodies was performed by absorption to agarose beads coated with the cross reacting component. Upon incubation with antivasopressin (diluted unpurified 11600 or purified 180) and unpurified antioxytocin (1400) plasma, sections of a Wistar neurohypophysis revealed two types of neurosecretory fibres, containing either electron dense or lucent granules. Oxytocin and vasopressin containing neurosecretory fibres were found as clusters in the neurohypophysis. The specificity of both unpurified antivasopressin (11600) and antioxytocin (1400) plasma was confirmed on serial sections of a Wistar neurohypophysis, alternately incubated with the solutions of the two antibodies.These data prove that the one-cell-one-hormone hypothesis holds true for the hypothalamic-neurohypophyseal system.The authors wish to thank Dr. L.A. Sternberger (Edgewood Arsenal, Md., U.S.A.) for the peroxidase-anti-peroxidase complex, Dr. J.G. Streefkerk (Free University, Amsterdam) and the members of our project group on neuroendocrinology for their suggestions and critical remarks, and Mrs. M. Mud, Mr. P. Wolters and Mrs. A. van der Velden for their skilful assistance