EEG effects of subcutaneous and intracerebroventricular injections of arginine vasopressin in the rat

Vasopressin as a Possible Link between Sleep-Disturbances and Memory Problems

Normal biological rhythms, including sleep, are very important for a healthy life and their disturbance may induce-among other issues-memory impairment, which is a key problem of many psychiatric pathologies. The major brain center of circadian regulation is the suprachiasmatic nucleus, and vasopressin (AVP), which is one of its main neurotransmitters, also plays a key role in memory formation. In this review paper, we aimed to summarize our knowledge on the vasopressinergic connection between sleep and memory with the help of the AVP-deficient Brattleboro rat strain. These animals have EEG disturbances with reduced sleep and impaired memory-boosting theta oscillation and show memory impairment in parallel. Based upon human and animal data measuring AVP levels, haplotypes, and the administration of AVP or its agonist or antagonist via different routes (subcutaneous, intraperitoneal, intracerebroventricular, or intranasal), V1a receptors (especially of hippocampal origin) were implicated in the sleep-memory interaction. All in all, the presented data confirm the possible connective role of AVP between biological rhythms and memory formation, thus, supporting the importance of AVP in several psychopathological conditions.

The angiotensin AT1 receptor antagonist, losartan, induces barrel rotation in the rat

Intracerebroventricular injections of [Arg8]vasopressin (500 ng/rat) or endothelin-1 (70 ng/rat) into the right lateral ventricle induced rotation along the long axis of the body (barrel rotation) in rats. Losartan (10-200 microg/rat), an angiotensin AT1 receptor antagonist, also evoked barrel rotation, which was not inhibited by vasopressin and endothelin receptor antagonists. However, barrel rotation was not observed after injections of high doses of another angiotensin II receptor antagonist, [Sar1,Ile8]angiotensin II (100 microg/rat), or after angiotensin II (10 microg/rat). The results indicate that losartan does evoke barrel rotation which may be not mediated via vasopressin and endothelin receptors.

Plasma arginine vasopressin and motor activity in major depression

BACKGROUND

Previously, we found that mean plasma concentrations of arginine vasopressin (AVP), but not of oxytocin (OT), were higher in depressed patients than in healthy controls. Plasma AVP concentrations were positively correlated to clinically rated psychomotor retardation. To further explore this previously reported relation we studied psychomotor retardation by means of an activity monitor, which is a more fine-focused and more objective instrument to analyze motor retardation than a clinical rating scale.

METHODS

Plasma AVP and OT concentrations, and day- and nighttime wrist activity were measured in 48 in- and outpatients with major depression and 30 healthy controls during a period of 5 consecutive days and nights.

RESULTS

Principal components analysis revealed three components of motor activity: motor activity during wakefulness, motor activity during sleep, and the awake/sleep time ratio. In patients and controls an inverse relationship between plasma AVP concentrations and motor activity during wakefulness was found. Patients with elevated AVP plasma levels showed increased motor activity during sleep.

CONCLUSIONS

These results suggest that high plasma AVP levels are related to the clinical picture of daytime psychomotor retardation and nighttime motor activity in major depression. Mean plasma OT concentrations were not related to measures of motor activity.

Desmopressin and vasopressin increase locomotor activity in the rat via a central mechanism: implications for nocturnal enuresis

PURPOSE

Nocturnal enuresis is characterized by nocturnal urine volumes exceeding bladder capacity and by inability to wake up to the stimulus of a full bladder. Desmopressin (DDAVP) is believed to be efficient in treating nocturnal enuresis by reducing nocturnal urine production. However, clinical observations indicate an additional mode of action since the drug appears to modify sleep architecture, apparently improving the patient's ability to awaken to the stimulus of a full bladder. Because of this, a possible arousing effect of DDAVP was studied.

MATERIALS AND METHODS

The tentative ability of DDAVP and the endogenous hormone vasopressin (AVP) to produce locomotor stimulation in resting rats after both intracerebroventricular and subcutaneous administration was used as an animal model of arousal. In addition brain monoamine biochemistry was analyzed.

RESULTS

The intracerebroventricular injection of AVP (0.1 and 1 microgram.) and the intracerebroventricular (0.1, 1, 10 and 100 microgram.) and subcutaneous (90 and 180 microgram.) injections of DDAVP were both associated with a significant increase in the locomotor activity of the animals compared with controls. The biochemical analysis of cerebral monoamines indicated that DDAVP lowers brain dopamine levels after both types of administration.

CONCLUSIONS

These results suggest that DDAVP exerts a stimulatory effect in the CNS, which is also observed after peripheral administration. There are also indications for an increase in central dopamine turnover which could explain the registered increase in locomotor activity.

Effects of neurotensin on EEG and event-related potentials in the rat

Neurotensin has neuromodulatory actions on multiple brain functions including motor, sensory and limbic processes. However, little is known about how neurotensin affects general arousal and/or attention states. The present study evaluated the effects of neurotensin on spontaneous brain activity as well as auditory evoked responses using electrophysiological measures. Electroencephalographic and event-related potential recordings were obtained in awake animals following intracerebroventricular administration of neurotensin (1.0, 10.0 and 30.0 micrograms). Twenty rats were implanted with recording electrodes in the frontal cortex, dorsal hippocampus, amygdala and nucleus accumbens. Neurotensin was found to produce a dose-related effect on behavior and electrophysiological measures. Lower doses (10 micrograms) produced no obvious behavioral changes, but significantly reduced EEG power in the lower frequency ranges (2-6 Hz) in the frontal cortex, the anterior amygdaloid complex and the nucleus accumbens. At higher doses (30 micrograms), rats appeared behaviorally inactivated, and EEG power was reduced in all structures in both the lower frequency ranges (2-6 Hz) and the higher frequency ranges (8-32 Hz). Auditory processing, as assessed by event-related potentials, was affected most significantly in amygdala and dorsal hippocampus. In the amygdala, the amplitude of the P3 component of the auditory event-related potential was increased significantly by doses of 10.0 and 30.0 micrograms. In the dorsal hippocampus, the amplitude and the area of the N1 component was increased dose dependently and significance was reached at the 30 micrograms dose. These electrophysiological findings indicate that neurotensin does not reduce the arousal level of the animals and in fact may enhance neurosensory processing in limbic areas through increased arousal and/or enhanced stimulus evaluation.

Mortyn Jones Memorial Lecture. Limbic regions mediating central actions of oxytocin on the milk-ejection reflex in the rat

Central oxytocin administration has a profound facilitatory effect on the patterning of the milk-ejection reflex in the lactating rat. Lesion and microinjection studies indicate that this action is, in part, mediated via a population of limbic neurones in the bed nuclei of the stria terminalis and ventrolateral septum, which have been shown to possess oxytocin receptors and to be activated by selective oxytocin-receptor agonists in vitro. In vivo electrophysiological recordings reveal that some of these neurones display cyclical activity which is highly correlated to each milk ejection, and are rapidly activated following i.c.v. administration of oxytocin, coincident with the facilitation of milk ejection activity. A hypothetical model is proposed in which this population of limbic neurones serves to gate the activity of a pacemaker which, in turn, coordinates the bursting of hypothalamic magnocellular neurones. The oxytocin innervation of these neurones and their expression of oxytocin receptors increases in the postpartum period, and the resultant enhanced sensitivity leads to a greater facilitatory response during lactation. Inhibitory opioid and noradrenergic inputs which converge on these oxytocin-sensitive neurones may function to switch off the facilitatory circuit during periods of stress. Thus, this population of limbic neurones participates in the regulation of neuroendocrine activity during lactation by providing an appropriate degree of feedback to alter the patterning of the milk-ejection reflex.

Vasopressin prolongs behavioral and cardiac responses to mild stress in young but not in aged rats

In young male Wistar rats sudden silence superimposed on low intensity background noise evokes a relative decrease in heart rate. This bradycardia is accompanied by immobility behavior. In the present study, involving young (3 month), late-adult (14 month), aged (20 month), and senescent (25 month) rats the magnitude of the stress-induced bradycardia shows an age-related reduction while the behavioral immobility response remained unchanged during the process of aging. Arginine-8-vasopressin (AVP, 6 micrograms/kg SC) administered 60 min prior to the experiment led to a prolonged behavioral and cardiac stress response in young and late-adult rats, but not in aged and senescent animals. The peripheral and central mechanisms possibly involved in the failure of systemically applied AVP to improve bradycardiac stress responses in aged rats are discussed.

Vasopressin regulates human sleep by reducing rapid-eye-movement sleep

In two double-blind experiments, effects of intravenous infusion of arginine vasopressin (AVP) on sleep were evaluated in 2 groups of 10 men (20-35 yr). In experiment I, subjects were tested on two occasions, during which they received either placebo or 0.33 IU/h AVP. In experiment II, on three different occasions, subjects received either placebo or 0.66 or 0.99 IU/h AVP. Infusions were administered between 2200 and 0700 h. Nocturnal plasma AVP concentrations were close to the upper limit of the normal physiological range during 0.66 IU/h AVP (16.6 +/- 2.2 pg/ml) but markedly exceeded this range during 0.99 IU/h AVP (25.0 +/- 1.6 pg/ml). Results indicate primary effects of AVP on rapid-eye-movement (REM) sleep, with moderate reductions in REM sleep during 0.33 IU/h AVP (averaging -10.5%) and with substantial reductions in REM sleep (-24.0%) during 0.66 IU/h AVP. During 0.99 IU/h AVP the effect did not further increase (-24.4%). Less consistent effects of AVP were an increase in stage 2 sleep and in time awake. Effects of AVP were not mediated by changes in cortisol or blood pressure. Results suggest AVP to participate in REM sleep regulation under normal physiological conditions.

Cardiovascular instability induced by intracerebroventricular administration of vasopressin or corticotrophin-releasing factor in conscious long evans and brattleboro rats

Abstract Conscious Long Evans and Brattleboro rats, chronically instrumented with pulsed Doppler probes around left renal and superior mesenteric arteries and the distal abdominal aorta, had vasopressin or corticotrophin-releasing factor administered intracerebroventricularly. In Long Evans rats, vasopressin caused no cardiovascular instability, but in Brattleboro rats, vasopressin caused the development of marked oscillations in blood pressure and heart rate. Under the latter conditions, falls in blood pressure were accompanied by increases in hindquarter blood flow. Similar oscillations in blood pressure, heart rate and regional haemodynamics were seen in Long Evans and Brattleboro rats given corticotrophin-releasing factor. These results indicate that centrally-administered vasopressin and corticotrophin-releasing factor can have an important influence on the control of regional haemodynamics. In some respects the effects of the peptides resemble the changes seen during the defence reaction.

Effect of centrally administered oxytocin on the association between cortical electroencephalogram and milk ejection in the rat

Abstract The milk-ejection reflex of the rat is closely associated with synchronized activity of the cortical electroencephalogram (EEG), and the frequency of milk ejections has been shown to be greatly facilitated by central oxytocin. The following experiments were undertaken to examine the changes in the EEG during facilitation of the reflex by central oxytocin. Intracerebroventricular injection of 1 mU (2.2 ng) oxytocin during suckling caused a rapid increase in the frequency of milk ejections but no change in the predominantly synchronized pattern of the EEG. However, after a delay (11.3 +/- 0.8 min, mean +/- SE) there appeared to be an increasing proportion of desynchronization, which correlated with cessation of the facilitated milk-ejection responses. Hence, the observed EEG desynchronization may signal activation of mechanisms inhibitory to the milk-ejection reflex. In the absence of the suckling stimulus oxytocin also caused a change to desynchronization. However, this effect was more pronounced and commenced after a much shorter latency (1.7 +/- 0.4 min, mean +/- SE; P < 1.001), suggesting that the desynchronizing effect of oxytocin on the EEG can be attenuated by the suckling stimulus. These results demonstrate two phases in the action of central oxytocin in the suckled rat. During the initial phase, the milk-ejection reflex is facilitated and although there may be a concomitant desynchronizing influence on the EEG this is prevented by the influence of the suckling stimulus. In the later phase, this desynchronizing influence predominates and is accompanied by cessation of milk-ejection responses. Although milk ejections were generally restricted to periods of a synchronized EEG as previously reported, during the oxytocin-induced change to desynchronization a number of milk ejections were observed to occur in the absence of a synchronized EEG. These results provide further evidence that the association between milk ejection and the EEG state is not an absolute causal relationship.

Anterior neocortical kindling in vasopressin-deficient rats

Kindling of seizures with stimulation of anterior neocortex was examined in control rats and in Brattleboro rats deficient in arginine-vasopressin (AVP). There were no significant differences between control rats, homozygous Brattleboro rats, and heterozygous Brattleboro rats in the rate and pattern of kindling of generalized seizures. Thus AVP is not critically involved in anterior neocortical kindling.

Barrel rotation in rats induced by intracerebroventricular bradykinin antagonists

Intracerebroventricular (ICV) administration of bradykinin (BK) analogs containing the substitution DPhe7 produced extreme postural distortions within 2-4 min after injection, eventually causing rats to spin repeatedly around their longitudinal axis. This behavior, called barrel rotation, has been previously reported following ICV administration of several other neuropeptides. Episodes lasted 5-20 min; two deaths occurred at high doses, but no other long-term effects were observed. The quantal ED50 of the prototype compound B4162 (DArg0, Thi5,8 DPhe7BK), was 14.9 nmole; all seven other DPhe7 analogs tested elicited a positive response at 20 nmole. Among analogs not containing DPhe7, only BK elicited any activity (20% response rate), and only at 100 nmoles. Structure-activity considerations indicate that this behavior is not mediated by classical kinin receptors. The response rate to 20 nmole B4162 (81%) did not significantly change after pretreatment with ICV BK (100 nmoles), or IP atropine, haloperidol or phenytoin; whereas pretreatment with ICV captopril and muscimol and IP naloxone, diazepam and phenobarbital all significantly inhibited the response. A GABAergic mechanism may be involved in this peptide behavior.

Effects of enhanced cerebrospinal fluid levels of vasopressin, vasopressin antagonist or vasoactive intestinal polypeptide on circadian sleep-wake rhythm in the rat

Several endogenous peptides have been implicated in the regulation of sleep and wakefulness. The present study was carried out in order to determine whether the light-dark rhythm of vasopressin (VP) in the cerebrospinal fluid (CSF) had functional significance in relaying information from the circadian pacemaker, i.e. the suprachiasmatic nuclei (which synthesize VP as well as vasoactive intestinal polypeptide (VIP], to the centra regulating sleep. After constant delivery of VP in the CSF via an Accurel/collodion implant in the lateral ventricle, the VP CSF level was raised from 20-35 pg/ml to ca. 265 pg/ml whereby a VP rhythm in the CSF could no longer be detected. Under these conditions VP was found to increase the arousal state of the rat in the dark period, which resulted in a higher amplitude of the circadian sleep-wake rhythm. Application of the VP antagonist d(CH2)5[Tyr(Me)2]VP partly had opposite effects. A similar approach with central application of VIP resulted in an increase in rapid eye movement and quiet sleep but did not affect the amplitude of the circadian rhythm. It was concluded that although peptide levels in the CSF may show clear temporal variations with the light-dark cycle, this rhythmicity is not causally related to the circadian aspect of sleep-wakefulness. However, both VP and VIP contribute to the regulation of the amount of time spent in sleep and wakefulness and the level of VP in the CSF is correlated with the amplitude of the rhythm.

Centrally injected arginine vasopressin (AVP) facilitates social memory in rats

'Memory' for a juvenile conspecific in male rats can be measured by variation in duration of investigation times when the same juvenile is presented at different intervals. Typically, exposure of an adult male rat to a juvenile results in transient investigatory activity that rapidly declines with repeated exposures at short interexposure intervals (30 min). Longer interexposure intervals (120 min) result in re-investigation with durations similar or greater than the original investigation. Arginine vasopressin (AVP) injected into adult male rats intracerebroventricularly in doses of 0.5-2.0 ng immediately after investigation of the juvenile decreased social investigation of the same juvenile at the long (120 min) interexposure interval. This decrease in investigatory time was similar to that observed after a 30-min interexposure interval in untreated animals. These results support the hypothesis that increasing the availability of AVP in the central nervous system can improve the consolidation of olfactory information and improve conspecific recognition in rats.