Effect of changes in the intrahippocampal vasopressin on memory retrieval and relearning

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.

Coprophagy prevention alters microbiome, metabolism, neurochemistry, and cognitive behavior in a small mammal

Many small mammals engage in coprophagy, or the behavior of consuming feces, as a means to meet nutritional requirements when feeding on low-quality foods. In addition to nutritional benefits, coprophagy may also help herbivores retain necessary gut microbial diversity and function, which may have downstream physiological effects, such as maintaining energy balance and cognitive function. Here, we used collars to prevent Brandt's vole (Lasiopodomys brandtii) from engaging in coprophagy and monitored changes in microbial community structure, energy metabolism, and cognitive performance. In this research, we found that coprophagy prevention decreased alpha diversity of the gut microbiota, and altered proportions of microbial taxa such as Bacteroidetes, Firmicutes, and Oscillospira. Preventing coprophagy resulted in a reduced body mass, and increased food intake. Importantly, coprophagy prevention decreased vole cognitive behavior and altered levels of neurotransmitters in brain. Daily acetate administration was able to reverse some of the coprophagy prevention-induced changes in microbiota composition, metabolism, neurochemistry, and cognitive behavior. These findings identify the functional importance of coprophagy behavior and interactions between the gut microbiota, energy metabolism, and neurological function. Our results suggest that coprophagy contributes to stabilizing the gut microbiota, promoting microbial metabolism, maintaining host energy balance and, consequently, altering cognitive performance.

Arginine vasopressin prevents against Abeta(25-35)-induced impairment of spatial learning and memory in rats

Amyloid beta protein (Abeta) is thought to be responsible for loss of memory in Alzheimer's disease (AD). A significant decrease in [Arg(8)]-vasopressin (AVP) has been found in the AD brain and in plasma; however, it is unclear whether this decrease in AVP is involved in Abeta-induced impairment of spatial cognition and whether AVP can protect against Abeta-induced deficits in cognitive function. The present study examined the effects of intracerebroventricular (i.c.v.) injection of AVP on spatial learning and memory in the Morris water maze test and investigated the potential protective function of AVP against Abeta-induced impairment in spatial cognition. The results were as follows: (1) i.c.v. injection of 25 nmol Abeta(25-35) resulted in a significant decline in spatial learning and memory; (2) 1 nmol and 10 nmol, but not 0.1 nmol, AVP injections markedly improved learning and memory; (3) pretreatment with 1 nmol or 10 nmol, but not 0.1 nmol, AVP effectively reversed the impairment in spatial learning and memory induced by Abeta(25-35); and (4) none of the drugs, including Abeta(25-35) and different concentrations of AVP, affected the vision or swimming speed of the rats. These results indicate that Abeta(25-35) could significantly impair spatial learning and memory in rats, and pretreatment with AVP centrally can enhance spatial learning and effectively prevent the behavioral impairment induced by neurotoxic Abeta(25-35). Thus, the present study provides further insight into the mechanisms by which Abeta impairs spatial learning and memory, suggesting that up-regulation of central AVP might be beneficial in the prevention and treatment of AD.

Arginine vasopressin prevents amyloid beta protein-induced impairment of long-term potentiation in rat hippocampus in vivo

Amyloid beta protein (Abeta) is thought to be responsible for the loss of memory in Alzheimer's disease (AD). A significant decrease in [Arg(8)]-vasopressin (AVP) in the AD brain has been found. However, it is unclear whether the decrease in AVP is involved in Abeta-induced impairment of memory and whether AVP can protect against Abeta-induced neurotoxicity. The present study examines the effects of intracerebroventricular (i.c.v.) injection of AVP on hippocampal long-term potentiation (LTP), a synaptic model of memory, and investigates the potential protective function of AVP in Abeta-induced LTP impairment. The results showed that (1) i.c.v. injection of different concentrations of AVP or Abeta(25-35) did not affect the baseline field excitatory postsynaptic potentials (fEPSPs); (2) AVP administration alone induced a significant increase in HFS-induced LTP, while Abeta(25-35) significantly suppressed HFS-induced LTP; (3) Abeta(25-35)-induced LTP suppression was significantly prevented by the pretreatment with AVP; (4) paired-pulse facilitation did not change after separate application or co-application of AVP and Abeta(25-35). These results indicate that AVP can potentiate hippocampal synaptic plasticity and dose-dependently prevent Abeta(25-35)-induced LTP impairment. Thus, the present study provides further insight into the mechanisms by which Abeta impairs synaptic plasticity and suggests an important approach in the treatment of AD.

Effect of I.C.V. injection of AT4 receptor ligands, NLE1-angiotensin IV and LVV-hemorphin 7, on spatial learning in rats

Central administration of angiotensin IV (Ang IV) or its analogues enhance performance of rats in passive avoidance and spatial memory paradigms. The purpose of this study was to examine the effect of a single bolus injection of two distinct AT4 ligands, Nle1-Ang IV or LVV-haemorphin-7, on spatial learning in the Barnes circular maze. Mean number of days for rats treated with either Nle1-Ang IV or LVV-haemorphin-7 to achieve learner criterion is significantly reduced compared with controls (P < 0.001 and P < 0.05 respectively). This is due to enhanced ability of the peptide-treated rats to adopt a spatial strategy for finding the escape hatch. In all three measures of learning performance, (1) the number of errors made, (2) the distance travelled and (3) the latency in finding the escape hatch, rats treated with either 100 pmol or 1 nmol of Nle1-Ang IV or 100 pmol LVV-haemorphin-7 performed significantly better than the control groups. As early as the first day of testing, the rats treated with the lower dose of Nle1-Ang IV or LVV-haemorphin-7 made fewer errors (P < 0.01 and P < 0.05 respectively) and travelled shorter distances (P < 0.05 for both groups) than the control animals. The enhanced spatial learning induced by Nle1-Ang IV (100 pmol) was attenuated by the co-administration of the AT4 receptor antagonist, divalinal-Ang IV (10 nmol). Thus, administration of AT4 ligands results in an immediate potentiation of learning, which may be associated with facilitation of synaptic transmission and/or enhancement of acetylcholine release.

Management of urinary incontinence and nocturnal enuresis in attention-deficit hyperactivity disorder

PURPOSE

We sought to determine whether attention-deficit hyperactivity disorder (ADHD) influences the resolution of urinary incontinence (UI, or diurnal and nocturnal wetness) and monosymptomatic nocturnal enuresis (NE).

MATERIALS AND METHODS

We performed a retrospective review of patients with ADHD, UI and NE. Individuals with UI were treated with timed voiding, and anticholinergics were added only after timed voiding failed. Patients with NE were treated with either an enuretic alarm, desmopressin or imipramine. Statistical comparisons used a control population matched for age, sex, IQ, and urinary and gastrointestinal symptoms.

RESULTS

The presence of ADHD had a negative effect on the resolution of incontinence, with 68% of the patients with ADHD becoming continent compared to 91% of controls (p <0.01). Two factors impact the resolution of wetness in patients with ADHD-treatment noncompliance and IQ. Treatment noncompliance was found in 48% of the patients with ADHD compared to 14% of controls (p <0.01). The IQ of patients with ADHD affected success, with 32% of children with an IQ of less than 84 achieving continence compared to 80% of those with an IQ of 84 or greater (p <0.01). Patients with ADHD and NE responded similarly to controls when using desmopressin and imipramine. However, they were less likely to exhibit a durable response following management with an enuretic alarm (19% vs 66%, p <0.01).

CONCLUSIONS

Treatment of urinary incontinence in children with ADHD is impaired compared to those without ADHD, and is directly affected by compliance and IQ.

AVP (4-8) improves concept learning in PFC-damaged but not hippocampal-damaged rats

A position reversal task was used to test the memory-enhancing effects of the arginine vasopressin analog [pGlu4, Cyt6] AVP (4-8) at a dose of 1.5 microg/kg. Rats received either sham operations (SHM), medial prefrontal cortex lesions (PFC), or hippocampal lesions (HIP). The peptide was administered daily, via s.c. injection, 30 min prior to training to half of the animals in each group. As expected, all animals performed equally well on the initial position habit and the first reversal. Overall, it was found that AVP (4-8)-treated animals performed significantly better across trials than saline (SAL)-treated animals. Further analysis showed that PFC animals that received AVP (4-8) (PFC+AVP) performed significantly better (at the level of controls) across trials than saline-treated PFC (PFC+SAL) animals. Sham animals that received the AVP (4-8) analog (SHM+AVP) only showed significant improvement on the last two reversals when compared to the sham saline-treated animals (SHM+SAL), which was likely due to a ceiling effect as performance reached high levels early in the reversal task. Trial 2 analysis across reversals revealed enhanced cognitive abilities in both sham groups (SHM+SAL, SHM+AVP) and the PFC+AVP group, but not in the PFC+SAL, HIP+AVP or the HIP+SAL groups. Hippocampal lesioned animals performed poorly on the task and injections of AVP (4-8) did not improve their performance. It is thus concluded that AVP (4-8) enhanced the acquisition of concept learning (win-stay/loose-shift) in this paradigm in PFC-damaged animals and ameliorated the perseverance habit that is often seen in PFC animals on this task. It is suggested that AVP (4-8) might have an enhancing effect on general cognitive abilities that is not limited to memory processes.

Neuromodulation of memory in the hippocampus by vasopressin

The involvement of [Arg(8)]vasopressin in memory processes was analyzed in the hippocampal structure, since we have reported that this is one of the main central target structures of the vasopressin-enhancing effect on memory. This structure is functionally differentiated along its dorsoventral axis, and the expression of the vasopressinergic system is dependent upon whether the dorsal or ventral part of the hippocampus is involved. For this reason, the effect of vasopressin injected into hippocampus was evaluated on the basis of the site of injection. We have shown, using a Go-No Go visual discrimination task with mice that both parts of the hippocampus are involved in the effect of endogenous or exogenous vasopressin, but with higher sensitivity for the ventral part. Based on the expression of Fos protein following intracerebroventricular injection of vasopressin in unconditioned or conditioned mice, we confirmed the greater involvement of the ventral hippocampus in the enhancing effect of vasopressin on memory processes. The effect of the peptide seems specific, since only a few of the hippocampal cells that expressed Fos protein in the unconditioned mice did so in the conditioned mice (cells in the dentate gyrus and the CA3 hippocampal field). Moreover, we have shown that in the ventral hippocampus, vasopressin generates different behavioral effects whether treatment is performed at the beginning or in the middle of the learning process, suggesting that the mnemonic context is an important factor for understanding the effect of vasopressin on memory in the ventral hippocampus.

Sexual dimorphism in numbers of vasotocin-immunoreactive neurons in brain areas associated with reproductive behaviors in the roughskin newt

Vasotocin (VT) and vasopressin control many endocrine and neuroendocrine functions, including the regulation of reproductive behaviors. In the roughskin newt (Taricha granulosa), VT administration can enhance courtship behaviors in males and egg-laying behaviors in females. This study used immunohistochemistry to investigate whether there are sex differences in VT in specific brain areas, and whether these differences persist in nonbreeding animals. Numbers of VT immunoreactive (ir) cell bodies were counted in males and females collected in February, April, June, and August. Radioimmunoassay of plasma samples confirmed that testosterone and 5alpha-dihydrotestosterone concentrations were higher in males than females, and that 17beta-estradiol concentrations were higher in females than males. In 11 brain areas, no sexual or seasonal differences in the number of VTir cells were found. But in 3 brain regions-the bed nucleus of the stria terminalis (BNST), the nucleus amygdalae dorsolateralis (AMYG), and the anterior preoptic area (aPOA)-there were significantly greater numbers of VTir cells in males than in females, and these differences did not change seasonally. In the aPOA, an area important to male sex behaviors, the sexual dimorphism in VTir was particularly pronounced. In four brain regions, there were significantly greater numbers of VTir cells in females than males, but only in specific seasons. In April-collected (breeding) animals, more VTir cells were found in females than in males in the populations of VT cells within the pars dorsalis hypothalami and ventromedial hypothalamus, brain regions frequently associated with stress responses and female mating behaviors. In August-collected (nonbreeding) animals, more VTir cells were found in females than in males, in the region of the bed nucleus of the decussation of the fasciculus lateralis telencephali and in the nucleus visceralis superior, nucleus isthmi region. Significantly greater numbers of VTir cells were observed in the magnocellular preoptic area of males and females collected in February. These results indicate that the functional interactions between gonadal steroid hormones and VT are complex and appear to involve site-, sex-, and season-specific regulatory mechanisms. Furthermore, it seems likely that populations of VT neurons in the BNST, AMYG, and aPOA are involved in regulating male-specific behaviors, and that the VT neurons in the pars dorsalis hypothalami/ventromedial hypothalamus may be involved in female-specific behaviors.

Vasopressin in the mammalian brain: the neurobiology of a mnemonic peptide

We have sought to understand the mechanisms by which VP can enhance memory function and in the process determine whether VP fulfills the requirements for neurotransmitter status. The latter goal of proving the neurotransmitter status of VP has been achieved through our findings and the results of many of the scientists contributing to this volume. With respect to elucidating the mechanisms by which VP can enhance memory function, results of our work have shown that VP and its receptors are present in brain regions known to be involved in memory function, that release of VP is inhibited by a factor that inhibits memory function, that VP can significantly enhance the morphological complexity and outgrowth of neurons involved in memory function, that second messenger systems held to be involved in learning and memory, cyclic AMP and calcium signaling pathways, are potentiated and activated by VP, that electrophysiological models of memory function are induced by VP, and that when animals remember a learned association VP content in brain increases over time during the active phase of remembering. Collectively, these studies have taught us a great deal about the sites and mechanisms of VP action and have led us to pursue avenues of investigation that we would not have imagined 15 years ago when we began this work. We stand on the threshold of a new era in our research as we begin our studies of the role VP and its receptors play in the cerebral cortex. Thus far, results of these studies are quite exciting and promise to yield fascinating insights into the complexities of VP action in the most highly developed region of the mammalian brain, the cerebral cortex, the site of abstract reasoning, judgment, complex analysis and the repository of those memories that last a life-time.

Effects of vasopressin and related peptides on neurons of the rat lateral septum and ventral hippocampus

The effects of vasopressin (VP), VP fragments and propressophysin glycopeptide on neuronal activities in the septum-hippocampus complex of rats were studied in vitro and in vivo. The frequency of the hippocampus theta rhythm in Brattleboro rats homozygous for diabetes insipidus was significantly slower than that of heterozygous litter mates and normal rats. Intracerebroventricular micro-injection of des-glycine-amide vasopressin corrected for several hours the frequency deficit of the theta rhythm in the homozygous Brattleboro rats and the centrally administered VP slowed down theta rhythm in normal rats. Microinotophoretically administered VP excited single neurons in the lateral septum of ventral hippocampus, and/or facilitated the responses of these neurons to glutamate and to stimulation of the glutamatergic afferent fibers in the fimbria bundle. The excitatory effects of VP vanished within seconds after termination of the peptide administration, however, the peptide-induced enhancement of glutamate and syntatically induced excitations were sustained for up to 60 min after the peptide administration. In vitro, pM concentrations of VP, VP 4-8 and C-terminus glycopeptide of propresophysin facilitated for 30-60 min the glutamate-mediated EPSPs in neurons of the lateral septum or the ventral hippocampus. The EPSPs increase in the lateral septum neurons was not prevented by pretreatment with antagonist of the V1a type of the vasopressin receptor. The resting membrane potential and input resistance were not affected by the peptides. A low-frequency electrical stimulation in the diagonal Band of Broca or in the Bed nucleus of the stria terminals, sources of the vasopressinergic innervation of the septum, facilitated the negative wave of the filed potentials responses evoked in the lateral septum by stimulating the fimbria bundle fibers in control Long-Evans and Brattleboro rats heterozygous for diabetes insipidus. The field potential increase was sustained for several hours after the stimulation, and it was not occluded by long-term potentiation elicited by high frequency stimulation of the fimbria bundle afferent fibers. Brattleboro rats homozygous for diabetes insipidus failed to show the filed potential increase after the diagonal band stimulation. It is suggested that the long-lasting facilitation of glutamate-mediated excitations might be a physiological action of the propressophysin-derived peptides in the septum-hippocampus complex which, in concert with other forms of synaptic plasticity like the long-term potentiation, facilitates the hippocampus-mediated forms of learning and memory. This action is presumably related to the memory enhancing effect of the propressophysin-derived peptides.

Coping with stress in rats and mice: differential peptidergic modulation of the amygdala-lateral septum complex

This chapter focuses on the parvicellular vasopressin (VP) system originating from the medial nucleus of the amygdala (MeA) and bed nucleus of the stria terminalis (BNST). The vasopressinergic fibers of these nuclei innervate a number of limbic brain areas including the septum-hippocampal complex. Interestingly, this VP system is sexually dimorphic and the VP synthesis in this system depends on circulating gonadal steroids. Studies in rats and mice show that the variation in the lateral septal VP network within the male gender is as large as the variation between the sexes as reported in the literature. Non-aggressive males are characterized by a far more extensive VP network and a higher VP content in the lateral septal area than aggressive males. A review of the literature on the function of lateral septal VP in the organization of behavior reveals not only a modulatory role of behavior in a social context, but also of fear- and anxiety-related behaviors. It is argued that these seemingly diverse functions might be explained by the concept of coping style. Extensive behavioral and physiological analyses in a variety of animal species show that males may be characterized by the way in which they cope with environmental challenges in general. Aggressive males tend to cope actively with their environment whereas non-aggressive males seem to accept the situation as it is more easily. In several tests, we determined the effects of chronic infusion of the V1 receptor antagonist locally into the lateral septal area in male rats. The main conclusion from these experiments is that LS VP does not modulate coping style in general. However, the experiments confirm the idea that LS VP has a certain degree of functional specificity in social behavior and social learning tasks. Together with the observation that the size and distribution of the vasopressinergic system may be highly variable between individual males in relation to their coping style, this suggests that the lateral septal vasopressinergic system is involved in the differential capacity of individuals to cope behaviorally with challenges of a social nature.

Role of vasopressin in learning and memory in the hippocampus

The involvement of arginine8-vasopressin (VP) in learning and memory in the hippocampus is examined in mice using a discriminative learning task. Bilateral dorsal hippocampal lesion blocks the enhancing effect of intracerebroventricular (i.c.v.) injection of VP on retrieval and relearning processes. An additional study showed that immunoneutralization of dorsal hippocampal endogenous VP inhibited the facilitating effect of i.c.v. injection of VP, suggesting that hippocampus is essential for the expression of VP's behavioral effects. Using in situ microinjection, a greater sensitivity of the ventral part of the hippocampus to the memory enhancing effects of VP has been reported. This effect is mediated by vasopressin V1 type receptors and oxytocin receptors. Then, we examined the effects on behavior of VP applied to the ventral hippocampus, in relation to the time of treatment during learning. When the animals have no previous information about the task to learn, a deleterious effect of VP appears (pre-first session treatment). Regarding memory consolidation, the effects of VP may depend upon the previous level of performance acquired by the animals since, when injected after the first learning session, the peptide slightly delayed performance, whereas when the injection took place after the second learning session, it enhanced learning. Concerning memory retrieval, the effects of VP depend on the quality of the previously stored information. The fact that VP did not generate the same behavioral effects when the treatment was performed at the beginning or in the middle of the learning processes, suggests that mnemonic context is an important factor in understanding the effect of VP on memory in the ventral hippocampus. Finally, the role of hippocampal adrenergic receptors in the enhancing VP effects on memory retrieval has been examined. The facilitatory effects of VP seem to depend upon the functional state of both alpha- and beta-adrenergic receptors, but further studies will be necessary to clarify the role played by each receptor type in retrieval processes, and to determine the relationships that might exist between them.

The behavioral effect of vasopressin in the ventral hippocampus is antagonized by an oxytocin receptor antagonist

[Arg8]vasopressin improved long-term retrieval processes and relearning in a go-no go visual discrimination task when bilaterally microinjected at a dose of 25 pg/animal into the ventral hippocampus of mice, 10 min prior to the retention session. We had shown that this enhancing effect is antagonized by pretreatment with equal or lower doses (25 pg or 1 ng) of the vasopressin V1 receptor antagonist, (d(CH2)5Tyr(Me)-vasopressin). The present study was an attempt to determine whether the vasopressin V2 receptor antagonist or oxytocin receptor antagonist is as effective as the vasopressin V1 receptor antagonist to block the behavioral effect of vasopressin in the ventral hippocampus. We tested the effect of 25 pg of [d(CH2)5-D-Ile2,Ile4,Arg8]vasopressin, a vasopressin V2 receptor antagonist, and [d(CH2)5,Tyr(Me)2,Thr4,Tyr-NH9(2)]ornithine vasotocin, an oxytocin receptor antagonist, under the same experimental conditions as those used to test the effect of the vasopressin V1 receptor antagonist. The results showed that the vasopressin V2 receptor antagonist microinjected into the ventral hippocampus did not alter the enhancing effect of vasopressin on retrieval and relearning. In contrast, the oxytocin receptor antagonist blocked the vasopressin-enhancing effect on retention processes. We can conclude from the data that both vasopressin V1 receptors and oxytocin receptors seem to be involved in the enhancing effect of vasopressin on memory retention. In contrast, the vasopressin V2 receptors do not seem to be involved in the effect of the peptide.

GALR1 galanin receptor mRNA is co-expressed by galanin neurons but not cholinergic neurons in the rat basal forebrain

The neuropeptide galanin (GAL) has been proposed to be an inhibitory modulator of cholinergic transmission in the hippocampus and may impair memory by directly affecting the activity of basal forebrain (BF) cholinergic neurons. Alternatively, GAL may act indirectly and modulate the activity of other neurotransmitter systems which, in turn, influence cholinergic transmission. We have used double in situ hybridization histochemistry to evaluate the co-expression of the GAL receptor subtype, GALR1, within cholinergic neurons in the medial septum/diagonal band of adult male rats. In alternate brain sections, we assessed the co-expression of GALR1 mRNA within another forebrain cell group implicated in memory functions, the neurons of the bed nucleus of the stria terminalis (BNST) and medial amygdala (AMe) which co-express vasopressin (VP) and GAL and project to septo-hippocampus. Despite the abundance of GALR1 mRNA-expressing neurons in the cholinergic BF, we found no evidence for the co-expression of this receptor subtype within cholinergic neurons in the medial septum/diagonal band. In contrast, we detected an extensive co-expression (95%) of GALR1 mRNA within extrahypothalamic VP/GAL neurons. These results do not support the idea that GAL, acting via the GALR1 receptor, directly impairs BF cholinergic neurons but suggest, instead, that non-cholinergic neurons in the BF may play a role in mediating the inhibitory actions of GAL on cholinergic function. However, our findings provide anatomical evidence that GAL could directly modulate the activity and/or secretion pattern of extrahypothalmic VP/GAL neurons into septo-hippocampal regions.

Effects of arginine8-vasopressin administered at different times in the learning of an appetitive visual discriminative task in mice

A visual discrimination task was used to investigate the effect of the intra-hippocampal injection of arginine8-vasopressin (AVP) in male Balb/c mice at different stages of the learning processes. The peptide was bilaterally microinjected at a dose of 25 pg per animal, i.e. 833 pg/kg, into the ventral hippocampus, in a volume of 0.3 microliter 10 min before either the first or the second learning session, or immediately after the first or second learning session. Following pre-session administration of AVP, no effect of the peptide was observed on the session prior to which it was administered. On the other hand, 48 h after the pre-first session treatment, it seems that AVP animals had trouble learning the task. Following post-session injection of AVP, no effect was observed when the treatment was given after the first learning session and a tendency to improve performance was noted when the treatment was given after the second learning session. Thus, whatever time AVP was injected during learning, little or no effect was observed. These results and previous work on the same behavioral task showing a clear enhancing effect of the peptide on retrieval processes, suggest that prior experience or mnemonic context before AVP treatment is as important a factor in understanding the effects of AVP on memory processes as the administration route or the doses used.

Vasopressin and memory. I. The vasopressin analogue AVP4-9 enhances working memory as well as reference memory in the radial arm maze

The present study examined the effects of vasopressin on memory. Healthy rats were injected with the arginine vasopressin fragment AVP4-9 and the AVP antagonist [beta-mercapto-beta,beta-cyclopentamethylenepropionyl1,O-Et-Pyr 2,Val4,Arg8] and were tested in an eight-arm radial maze for 60 sessions. All injections were given s.c. 30 min prior to testing. AVP4-9 enhanced radial arm maze performance. AVP4-9 treated animals showed enhancement in performance as well as increases in the rate of learning, indicating that they learned the task faster. Furthermore, the overall memory enhancement was due to improved working memory as well as to improved reference memory. These results cannot be explained in terms of changes in locomotor activity because an open field test revealed no differences between groups for both of these compounds. The AVP antagonist did not impair performance in the radial maze. It is concluded that AVP4-9 has a more general effect on memory, one that is not limited to a specific type of memory.

Vasopressin and memory. II. Lesions to the hippocampus block the memory enhancing effects of AVP4-9 in the radial maze

Previous work has shown that the arginine vasopressin fragment AVP4-9 enhanced overall performance in the radial arm maze. This enhancement effect was due to improved working memory as well as to improved reference memory. The present study uses a behavioral approach to investigate whether these effects of vasopressin on memory are mediated by the hippocampus. Animals received either NMDA lesions to the hippocampus or sham operations and were then treated with AVP4-9 or saline. The testing was performed in an eight-arm radial maze for 60 sessions, and all injections were given s.c. 30 min prior to testing. The hippocampal lesioned animals showed a marked deficit in working memory that was not ameliorated by AVP4-9; however, the improvement in reference memory produced by the compound was as large as in healthy animals. Since the AVP treatment did not differentiate between the two hippocampal groups, the memory enhancing effects of AVP4-9 were blocked by hippocampal lesions. It is concluded that vasopressin has a more general effect on memory and that its site of action includes but is not limited to the hippocampus.

(+)-Eburnamenine-14-carboxylic acid (2-nitroxyethyl) ester (VA-045), a putative cognitive enhancer, facilitates recovery from concussive brain injury-induced learning and memory impairments in mice

We characterized alterations in the ability of concussive brain injury (CBI) models to perform a water-finding task and examined effects of (+)-eburnamenine-14-carboxylic acid (2-nitroxyethyl) ester (VA-045), a novel apovincaminic acid derivative, on post-traumatic impairments in latent learning and memory processes. Two types of CBI-induced amnesia (retrograde and anterograde) were produced by means of post- or preacquisition head impact using a simple weight-drop device. Profound impairments of latent learning and memory processes related to retention and retrieval were observed in the CBI mice. In the CBI-induced retrograde amnesia model, VA-045 (0.5-4.0 mg/kg) significantly ameliorated impairments of latent learning and retention in both the retention test and the retest. In the CBI-induced anterograde amnesia model, the protective effects of the compound on impairments in latent learning and retention or retrieval were significant in both the retention test and the retest. These results suggested that VA-045 may be a novel cognitive enhancer for attenuating or protecting against the learning and memory dysfunction associated with CBI.

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.

Behavioral consequences of intracerebral vasopressin and oxytocin: focus on learning and memory

Since the pioneering work of David de Wied and his colleagues, the neuropeptides arginine vasopressin and oxytocin have been thought to play a pivotal role in behavioral regulation in general, and in learning and memory in particular. The present review focuses on the behavioral effects of intracerebral arginine vasopressin and oxytocin, with particular emphasis on the role of these neuropeptides as signals in interneuronal communication. We also discuss several methodological approaches that have been used to reveal the importance of these intracerebral neuropeptides as signals within signaling cascades. The literature suggests that arginine vasopressin improves, and oxytocin impairs, learning and memory. However, a critical analysis of the subject indicates the necessity for a revision of this generalized concept. We suggest that, depending on the behavioral test and the brain area under study, these endogenous neuropeptides are differentially involved in behavioral regulation; thus, generalizations derived from a single behavioral task should be avoided. In particular, recent studies on rodents indicate that socially relevant behaviors triggered by olfactory stimuli and paradigms in which the animals have to cope with an intense stressor (e.g., foot-shock motivated active or passive avoidance) are controlled by both arginine vasopressin and oxytocin released intracerebrally.

Inhibition of the vasopressin-enhancing effect on memory retrieval and relearning by a vasopressin V1 receptor antagonist in mice

We have previously shown that [Arg8]vasopressin bilaterally administered into the ventral hippocampus of mice at a dose of 0.025 ng/animal 10 min prior to the retention session, improved long-term retrieval processes and relearning of a Go-No-Go visual discrimination task. The purpose of the present study was to determine whether the vasopressin V1 receptor antagonist, -beta-mercapto-beta,beta-cyclopentamethylenepropionyl1, O-Me-Tyr2,Arg8]vasopressin, d(CH2)5Tyr(Me)vasopressin), is able to block the behavioral effect of arginine-vasopressin in the ventral hippocampus. We first tested the effect of three doses of d(CH2)5Tyr(Me)vasopressin (0.025, 1, and 6.3 ng/animal) in the same experimental conditions as used for arginine-vasopressin. The results showed a dose-dependent deleterious effect of the vasopressin V1 receptor antagonist on retrieval and relearning, suggesting the involvement of endogenous arginine-vasopressin in the ventral hippocampus for these memory processes. Second, we tested the ability of d(CH2)5Tyr(Me)vasopressin to block the enhancing effect of experimentally administered arginine-vasopressin. The antagonist was injected at a dose of 0.025 ng, which had no intrinsic effect on behavior, or at a dose of 1 ng, which had a weak deleterious effect on behavior, followed by administration of 0.025 ng of arginine-vasopressin. The results showed that even at the weakest dose (0.025 ng), d(CH2)5Tyr(Me)vasopressin blocked the enhancing effect of arginine-vasopressin on retrieval and relearning. Thus, as for other behaviors and structures, the antagonist microinjected into the ventral hippocampus prevents the enhancing effect of arginine-vasopressin on long-term retrieval and relearning. However, the exclusive involvement of the vasopressin V1 receptors remain to demonstrate vis-a-vis oxytocin receptors.

Long-lasting enhancement of synaptic excitability of CA1/subiculum neurons of the rat ventral hippocampus by vasopressin and vasopressin(4-8)

Vasopressin (VP) is axonally distributed in many brain structures, including the ventral hippocampus. Picogram quantities of VP injected into the hippocampus improve the passive avoidance response of rats, presumably by enhancing memory processes. Vasopressin is metabolized by the brain tissue into shorter peptides, such as [pGlu4,Cyt6]VP(4-9) and [pGlu4,Cyt6]VP(4-8), which preserve the behavioral activity but lose the peripheral activities of the parent hormone. Using brain slices, we investigated whether VP or VP(4-8) affects excitatory postsynaptic potentials (EPSPs) and/or membrane responses to depolarization in neurons of the CA1/subiculum of the ventral hippocampus. The EPSPs were evoked by stimulating the striatum radiatum of the CA1 field; the membrane responses were elicited by current injections. Exposure of slices for 15 min to 0.1 nM solution of these peptides resulted in an increase in the amplitude and slope of the EPSPs in 21 neurons (67%) tested. No consistent change in either the resting membrane potential or the input resistance of the neurons was observed. The peptide-induced increase in EPSPs reached a maximum 30-45 min after peptide application. In 14 of these neurons (66%), the peptide-induced increase in EPSPs remained throughout the entire 60-120 min washout period. In the remaining 7 neurons (33%), the initial increase in EPSPs amplitude was followed by a gradual decline to the pre-administration level. The increase in EPSP amplitude was often, but not always, associated with a decrease in the threshold and increase in the number of action potentials in response to depolarizing current injection. Suppression of GABAA receptor-mediated inhibition and N-methyl-D-aspartate (NMDA) receptor-mediated excitation did not prevent the effects of VP and VP(4-8) on the EPSP amplitude or the threshold for action potentials. The results demonstrate that 0.1 nM concentrations of these neuropeptides can elicit a long-lasting enhancement of the excitability of CA1/subiculum neurons of the ventral hippocampus to excitatory, glutamatergic synaptic input. This novel action of VP and its metabolite in the ventral hippocampus may be the physiological action, mediating the memory-enhancing effect of these peptides.

Involvement of alpha- and beta-noradrenergic receptors in the effects of hippocampal vasopressinergic treatment on retrieval and relearning

Biochemical investigations show that vasopressin interacts with noradrenalin to potentiate noradrenalin-induced accumulation of cyclic-AMP in the hippocampus, via the beta-adrenergic receptors. We previously showed in BALB/c mice that the effects of vasopressin (bilaterally injected at a 25-pg dose) in the ventral hippocampus were more effective than in the dorsal hippocampus on the retrieval and relearning of a Go-No Go visual discrimination task. Considering our results and those reported by biochemical investigations, we evaluated possible noradrenergic-vasopressinergic interaction in the ventral hippocampus under our behavioral conditions. To do so, we first explored the effects of propranolol and phentolamine, antagonists of beta- and alpha-adrenergic receptors, respectively. Second, we assessed the modifications in the vasopressin-induced improvement of retrieval and relearning by pretreating the subjects with either propranolol or phentolamine. Third, we tested the treatments in a locomotor activity task to determine whether the effects demonstrated in the two preceding experiments could be partially due to locomotor alterations by the drug. The results indicated that bilateral injection of propranolol (1 microgram on each side), which did not appear to affect the retention performance itself, completely blocked the enhancement of retrieval and relearning resulting from the vasopressin treatment. In contrast, bilateral injection of phentolamine (1 microgram on each side), which moderately improved retrieval, enhanced the vasopressin effect. The present results lend further support to the view that both noradrenalin and vasopressin play important roles in retrieval and relearning processes. More importantly, they provide additional support for the functional interaction of the noradrenergic and vasopressinergic hippocampal systems.