Influence of histamine and pentobarbitone on plasma and CSF vasopressin levels of hypophysectomized rats

Chronic Cardiovascular Effects of Central Vasopressin in Conscious Rats

To clarify the cardiovascular effects of central vasopressin (AVP), a chronic intracerebroventricular (ICV) infusion of AVP was performed in conscious Wistar normotensive rats. Animals were divided into 3 groups: 1) AVP 1 ng/hr (Low), 2) AVP 100 ng/hr (High), and 3) saline (control) ICV infusion. After a 6 day control period, AVP or saline was continuously infused into the lateral cerebroventricle at a rate of 1 microliter/hr using osmotic minipump for 7 days. As a result, a dose-related elevation of AVP concentration in CSF was achieved. Systolic blood pressure in both Low and High AVP infusion was slightly (7-12 mmHg) but significantly higher than that in control. ICV infusion of AVP did not alter urine volume, electrolytes excretion or osmolality, and AVP vascular antagonist injected intravenously failed to affect mean arterial pressure. Furthermore, plasma catecholamines and renin activity did not differ significantly among the groups. Thus, chronic ICV infusion of AVP induced the elevation of blood pressure, which is due to centrally mediated effect of AVP.

Meninges play a neurotrophic role in the regeneration of vasopressin nerves after hypophysectomy

Following hypophysectomy the regenerating fibers of magnocellular neurons are known to establish new neurohemal connections with reorganized vasculatures in the median eminence, which lead to establishment of a posterior pituitary-like structure. In order to examine the role of the meninges (the pia mater and the arachnoid) in this regeneration process, we implanted the meningeal tissues obtained from neonatal rat pups into the third ventricle of the adult rats, and then hypophysectomized the host animals. Ten days after hypophysectomy, the meningeal tissue grafts were found to be densely innervated by regenerating vasopressin-immunoreactive fibers. Such fibers had dots and frequently formed large punctuations. On the contrary, few vasopressin fibers were found within the cortical tissue grafts. Further, the exposure of primary hypothalamic cell cultures to the medium conditioned by meningeal cell cultures promoted not only the survival of vasopressin-immunoreactive neurons but also the outgrowth and aborization of the neurites. The survivals of cortical and cerebellum neurons in culture were also promoted by the conditioned medium. These findings raise the possibility that the meninges play an important role in the axonal regeneration process after hypophysectomy.

Release of oxytocin into blood and into cerebrospinal fluid induced by naloxone in anaesthetized morphine-dependent rats: the role of the paraventricular nucleus

Abstract Opioid actions on oxytocin secretion into blood and cerebrospinal fluid (CSF) were investigated in urethane-anaesthetized female rats after intracerebroventricular (icv) infusion of morphine sulphate or vehicle for 5 days. Serial femoral arterial blood samples and cisterna magna CSF samples were collected for radioimmunoassay. Naloxone was given to assess endogenous opioid tone in icv vehicle-infused rats and to precipitate withdrawal in morphine-dependent animals. Initial plasma oxytocin concentration was not affected by icv morphine infusion. In control rats receiving icv vehicle, naloxone increased plasma oxytocin 11-fold within 5 min, and in icv morphine-infused rats, naloxone increased plasma oxytocin 80-fold within 5 min. In both groups, 90 min after naloxone plasma oxytocin was still 5 and 10 times, respectively, the initial concentration. Without naloxone, neither plasma nor CSF oxytocin concentration changed significantly with time (up to 90 min) in either icv treatment group. In the icv vehicle group, there was a 2-fold increase in CSF oxytocin 90 min after naloxone. In the icv morphine-infused group, CSF oxytocin was increased 5-fold 40 min after naloxone. In another group of icv morphine-infused rats, intravenous infusion of oxytocin to achieve plasma levels similar to those seen after naloxone, did not significantly increase CSF oxytocin. In a further group of icv morphine-infused rats, [(3)H]oxytocin was infused intravenously immediately after naloxone was given; in these rats oxytocin transfer from blood to CSF could account at most for only 20% of the increase in CSF oxytocin after naloxone. A further group of rats underwent bilateral microknife ablation of the paraventricular nuclei (PVN) 9 days before icv vehicle or morphine infusions were started; blood and CSF samples were collected under urethane anaesthesia. Initial concentrations of oxytocin in CSF and in plasma were similar in both groups with PVN ablation. In all PVN-lesioned rats initial plasma concentrations of oxytocin were undetectable (<5 pg/ml) and thus less than in intact rats. In contrast, initial levels of oxytocin in CSF were 8-fold greater in PVN-lesioned rats than in intact animals. Naloxone increased plasma oxytocin concentration in the icv vehicle group at least 10-fold within 30 min and in the icv morphine group at least 100-fold within 5 min. CSF oxytocin in the icv vehicle group was not altered by naloxone, but in the icv morphine group CSF oxytocin was increased 5-fold 40 min after naloxone. There were no consistent differences between the icv vehicle- and icv morphine-treated groups in the initial plasma levels of vasopressin, growth hormone and adrenocorticotrophin; PVN ablation did not affect adrenocorticotrophin levels. After naloxone growth hormone levels did not change, vasopressin concentration rose moderately only after 90 min and only in the icv vehicle-treated group, and adrenocorticotrophin concentrations decreased with time whether or not naloxone was given. The results imply an endogenous opioid tone on neurons releasing oxytocin into CSF, and morphine-dependence of these neurons. Furthermore, in PVN-lesioned rats, magnocellular supraoptic neurons could be a source of oxytocin release into CSF.

A placebo-controlled evaluation of vasopressin for ECT-induced memory impairment

Vasopressin may be involved in normal memory functions and may alleviate certain memory impairments. In this study, the usefulness of vasopressin to relieve electroconvulsive therapy (ECT)-induced memory impairment was evaluated using a placebo-controlled, random assignment, double-blind design. Patients were 33 depressives receiving bilateral ECT. Vasopressin, in a nasal spray, was administered q.i.d. from the first through the fifth ECT. Extensive memory testing evaluated both retrograde and anterograde amnesia; ratings of depression and patient ratings of subjective memory complaints were also obtained. Results did not show statistically significant evidence of benefit from vasopressin, though a number of comparisons were in the predicted direction. The role of vasopressin in reducing memory impairment of various types remains to be elucidated.

Regeneration of neurohypophyseal hormone-producing neurons in hypophysectomized immature rats

Age-related changes in the regeneration of vasopressin (AVP) and oxytocin (OXT) axons after hypophysectomy in rats was immunohistochemically examined. Rats were hypophysectomized at 9, 16, 23, 30 and 90 days of age, and sacrificed 10 days after the operation. AVP or OXT immunoreactivity in the external layer of the median eminence (ME) was generally stronger in hypophysectomized immature rats than in hypophysectomized adult rats, and the age-related difference in immunoreactivity was more conspicuous for AVP axons than OXT ones. The cell body size of AVP or OXT neurons in hypophysectomized adult rats was not significantly different from the value of unoperated or initial control rats. However, the neurons in immature rats became significantly larger after hypophysectomy, compared with those of initial controls. These results indicate that AVP- and OXT-producing neurons in immature rats, as early as at 9 days of age, are endowed with the capacity of axonal rearrangement to the external layer of the ME after hypophysectomy, and that the stronger immunoreactivity in the external layer of the ME in immature rats than in the adult may be due to the differences in the rate of synthesis of neurohypophyseal hormones and the regenerative potency of neurons.

Plasticity of vasopressin- and oxytocin-containing fibers in the median eminence in hypophysectomized young and old mice

Rearrangements of vasopressin- and oxytocin-containing fibers in the external layer of the median eminence after hypophysectomy were compared between young and old mice. In 3-month-old hypophysectomized mice, the increase in the number of fibers containing vasopressin was greater than that observed in 19-month-old hypophysectomized ones, suggesting a decrease in axonal plasticity in old mice. No difference with age was detected for the plasticity of fibers containing oxytocin.

Permeability of the blood-brain barrier to neuropeptides: the case for penetration

Evidence that peptides can cross the blood-brain barrier (BBB) is reviewed. Penetration is suggested by the observations that blood levels correlate with cerebrospinal fluid levels for many peptides and that peripheral administration of peptides results in effects on the CNS. Passage is confirmed by experiments involving administration of a peptide (immunoactive or radioactive) in one compartment and identification of its appearance in the other, supported by such methods as selective labeling, cross-reactivity with highly specific antibodies, and chromatography. The degree of passage varies among peptides and their analogs. The major route of passage is probably by a non-competitive, non-saturable mechanism, wih the physicochemical characteristics of the peptide (e.g. lipophilicity, charge, molecular weight, and protein binding) determining the degree of passage. A competitive transport mechanism also exists for some peptides. Penetration of the BBB via large pores or by pinocytosis does not appear to be of major importance for peptides. Permeability of the BBB to peptides, but not to the larger iodinated albumin, is affected by intraperitoneal administration of aluminum, apparently by an increase in the permeability of the membrane to lipophilic materials.