Ameliorative effect of vasopressin-(4-9) through vasopressin V(1A) receptor on scopolamine-induced impairments of rat spatial memory in the eight-arm radial maze

Sex differences of oxytocin and vasopressin in social behaviors

The neuropeptides oxytocin (OT) and vasopressin (VP) are known to mediate social cognition and behaviors in a sex-dependent manner. This chapter reviews the sex-dependent influence of OT and VP on social behaviors, focusing on (1) partner preference and sexual orientation, (2) memory modulation, (3) emotion regulation, and (4) trust-related behaviors. Most studies suggest that OT promotes familiar (opposite-sex) partner preference, strengthens memory, relieves anxiety, and increases trust. However, VP-regulated social cognition has been studied less than OT. VP facilitates familiar (opposite-sex) partner preference, enhances memory, induces anxiety, and influences happiness/anger perception. Detailed sex differences of these effects are reviewed. There is a male preponderance in the use of animal models and many study results are too complex to draw firm conclusions. Clarifying the complex interplay between the OT/VP system and sex hormones in the regulation of social behaviors is needed.

Sexual dimorphism of oxytocin and vasopressin in social cognition and behavior

The neuropeptides oxytocin (OT) and vasopressin (VP) are hormones that are known to mediate social behavior and cognition, but their influence may be sex-dependent. This paper aims to provide a comprehensive review of the sex-related influence of OT and VP on social cognition, focusing on partner preference and sexual orientation, trust and relevant behaviors, memory modulation, and emotion regulation. Most studies have suggested that OT facilitates familiar-partner preference in both sexes, with females being more significant, increased trust in others, especially for male, enhanced memory in either sex, and reduced anxious emotion in males. However, VP-regulated social cognition has been less studied. Other relevant studies have indicated that VP facilitated familiar-partner preference, improved memory, induced empathy formation, increased positive-emotion recognition, and induced anxiety without any sex difference. However, there was a male preponderance among studies, and results were often too complex to draw firm conclusions. Clarifying the interplay between OT/VP and sex hormones in the regulation of social cognition is necessary for further applications.

Basal Forebrain Cholinergic System and Memory

Basal forebrain cholinergic neurons constitute a way station for many ascending and descending pathways. These cholinergic neurons have a role in eliciting cortical activation and arousal. It is well established that they are mainly involved in cognitive processes requiring increased levels of arousal, attentive states and/or cortical activation with desynchronized activity in the EEG. These cholinergic neurons are modulated by several afferents of different neurotransmitter systems. Of particular importance within the cortical targets of basal forebrain neurons is the hippocampal cortex. The septohippocampal pathway is a bidirectional pathway constituting the main septal efferent system, which is widely known to be implicated in every memory process investigated. The present work aims to review the main neurotransmitter systems involved in modulating cognitive processes related to learning and memory through modulation of basal forebrain neurons.

Proinflammatory role of vasopressin through V1b receptors in hapten-induced experimental colitis in rodents: implication in IBD

Vasopressin and its receptors modulate several gut functions, but their role in intestinal inflammation is unknown. Our aims were to determine 1) the localization of V1b receptors in human and rodent colon, 2) the role of vasopressin and V1b receptors in experimental colitis using two approaches: V1b⁻(/)⁻ mice and a selective V1b receptor antagonist, SSR149415, and 3) the mechanisms involved. V1b receptors were localized in normal and inflamed colon from humans and rats. Experimental colitis was induced in rats and mice and some groups were treated before or after colitis induction with oral SSR149415 (3-30 mg/kg). Other groups of mice were submitted to dehydration to increase vasopressin plasma levels, prior to colitis induction. Body weight, damage scores, MPO, and TNF-α tissue levels were determined. Finally, colonic segments of wild-type (WT) and V1b⁻(/)⁻ mice were mounted in Ussing chambers and paracellular permeability in response to vasopressin was studied. V1b receptors were expressed in enterocytes and ganglia cells of the enteric nervous system of human and rat intestine. Expression levels were independent from inflammatory status. Colitis was less severe in rodents treated by either preventive or curative SSR149415 and in V1b⁻(/)⁻ mice. 2,4,6-Trinitrobenzene sulfonic acid induced a strong mortality in dehydrated animals that was reversed by preventive SSR149415 or mast cell stabilizer. Vasopressin significantly increased paracellular permeability in WT, but not in V1b⁻(/)⁻ mice. Preincubation of colon tissues with SSR149415 abolished the vasopressin effect. Similarly, vasopressin had no effect in colonic preparations from WT mice pretreated with mast cell stabilizers. Vasopressin, through V1b receptor interaction, has proinflammatory properties linked to mast cell activation and downstream alterations of the colonic epithelial barrier. These findings underline the potential interest of V1b receptor blockers in gut inflammatory diseases.

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.

New topics in vasopressin receptors and approach to novel drugs: role of the vasopressin receptor in psychological and cognitive functions

Arginine vasopressin (AVP) is a neurohypophyseal peptide best known as an antidiuretic hormone. AVP receptors have been classified into three subtypes: V1a, V1b, and V2 receptors. V1a receptor (V1aR) and V1b receptor (V1bR) are widely distributed in the central nervous system, including the septum, cortex, hippocampus, and hypothalamus. Clinical studies have demonstrated an involvement of AVP in psychiatric disorders. In the present study, we examined the performance of V1aR or V1bR knockout (KO) mice compared to wild-type (WT) mice in behavioral tests. V1aR and V1bR KO mice exhibited deficits of social behavior and prepulse inhibition in comparison to WT mice. Moreover, V1aR KO mice exhibited reduced anxiety-like behavior and impairment of spatial learning. These results suggest that V1aR and V1bR play an important role in psychological and cognitive functions.

Alcohol preference in mice lacking the Avpr1a vasopressin receptor

[Arg(8)]-vasopressin (Avp), a nonapeptide hormone, is known to regulate blood pressure, water balance, and a variety of behaviors such as anxiety, aggression, and bonding. Although some evidence that Avp modifies ethanol consumption and some of the effects of ethanol on behavior have been reported, the role of Avp in alcohol consumption and preference is poorly understood. The Avp1a receptor (Avpr1a) is ubiquitously expressed in the central nervous system. To determine the role of Avp signaling on the behavioral effects of alcohol, we examined voluntary ethanol consumption in mice with targeted disruptions of the Avpr1a knockout (Avpr1a KO) gene. Avpr1a KO mice displayed both increased ethanol consumption and preference compared with wild-type (WT) mice. Enhanced ethanol consumption was dramatically and reversibly reduced by treatment with N-methyl-D-aspartic acid antagonists. Basal glutamate release was elevated around the striatum in Avpr1a KO mice. Elevation of extracellular glutamate was also produced in WT mice by local application of an Avpr1a antagonist though a dialysis probe, and this elevation was quickly reversed by stopping the perfusion. These results suggest that Avp can inhibit the release of glutamate from the presynaptic terminal via the Avp1a receptor and that elevation of glutamate levels owing to loss of the inhibitory effect via Avp-Avpr1a signaling may play an important role in the preference for ethanol.

Vasopressin: behavioral roles of an "original" neuropeptide

Vasopressin (Avp) is mainly synthesized in the magnocellular cells of the hypothalamic supraoptic (SON) and paraventricular nuclei (PVN) whose axons project to the posterior pituitary. Avp is then released into the blood stream upon appropriate stimulation (e.g., hemorrhage or dehydration) to act at the kidneys and blood vessels. The brain also contains several populations of smaller, parvocellular neurons whose projections remain within the brain. These populations are located within the PVN, bed nucleus of the stria terminalis (BNST), medial amygdala (MeA) and suprachiasmatic nucleus (SCN). Since the 1950s, research examining the roles of Avp in the brain and periphery has intensified. The development of specific agonists and antagonists for Avp receptors has allowed for a better elucidation of its contributions to physiology and behavior. Anatomical, pharmacological and transgenic, including "knockout," animal studies have implicated Avp in the regulation of various social behaviors across species. Avp plays a prominent role in the regulation of aggression, generally of facilitating or promoting it. Affiliation and certain aspects of pair-bonding are also influenced by Avp. Memory, one of the first brain functions of Avp that was investigated, has been implicated especially strongly in social recognition. The roles of Avp in stress, anxiety, and depressive states are areas of active exploration. In this review, we concentrate on the scientific progress that has been made in understanding the role of Avp in regulating these and other behaviors across species. We also discuss the implications for human behavior.

The acute intoxicating effects of ethanol are not dependent on the vasopressin 1a or 1b receptors

Studies of the role of vasopressin (Avp) in mediating the effects of ethanol have focused on Avp's role in altering kidney function via its action through the vasopressin 2 receptor. However, alcohol consumption also has central effects that are poorly understood. There is evidence that Avp may mediate ethanol consumption as well as some of ethanol's behavioral effects. Centrally only two Avp receptor subtypes are expressed: the 1a receptor (Avpr1a) and the 1b receptor (Avpr1b). To determine the extent to which these receptors mediate the behavioral effects of alcohol, we used mice with targeted disruptions of either their Avpr1a or Avpr1b gene. We examined the effects of genotype on the acute intoxicating effects of ethanol as well as on voluntary ethanol consumption. Surprisingly, our findings indicate that there is no interaction between either the Avpr1a or Avpr1b and ethanol on motor coordination, hypothermia, mood, or voluntary ethanol consumption.

Association of vasopressin 1a receptor levels with a regulatory microsatellite and behavior

Vasopressin regulates complex behaviors such as anxiety, parenting, social engagement and attachment and aggression in a species-specific manner. The capacity of vasopressin to modulate these behaviors is thought to depend on the species-specific distribution patterns of vasopressin 1a receptors (V1aRs) in the brain. There is considerable individual variation in the pattern of V1aR binding in the brains of the prairie vole species, Microtus ochrogaster. We hypothesize that this individual variability in V1aR expression levels is associated with individual variation in a polymorphic microsatellite in the 5' regulatory region of the prairie vole v1ar gene. Additionally, we hypothesize that individual variation in V1aR expression contributes to individual variation in vasopressin-dependent behaviors. To test these hypotheses, we first screened 20 adult male prairie voles for behavioral variation using tests that measure anxiety-related and social behaviors. We then assessed the brains of those animals for V1aR variability with receptor autoradiography and used polymerase chain reaction to genotype the same animals for the length of their 5' microsatellite polymorphism in the v1ar gene. In this report, we describe the results of this discovery-based experimental approach to identify potential gene, brain and behavior interrelationships. The analysis reveals that V1aR levels, in some but not all brain regions, are associated with microsatellite length and that V1aR levels in those and other brain regions correlate with anxiety-related and social behaviors. These results generate novel hypotheses regarding neural control of anxiety-related and social behaviors and yield insight into potential mechanisms by which non-coding gene polymorphisms may influence behavioral traits.

Dopamine D2 receptor plays a role in memory function: implications of dopamine-acetylcholine interaction in the ventral hippocampus

RATIONALE

The role of the hippocampal dopaminergic system in mnemonic function has not been clarified yet.

OBJECTIVE

We previously reported that the dopamine D2 receptor (D2R) is involved in the regulation of acethylcholin (ACh) release in the hippocampus. In this study, we further investigated ACh-dopamine (DA) interaction in the hippocampus and its involvement in mnemonic function.

METHODS

For experiment 1, rats fed with Cholin (Ch)-deficient chow were used. We examined the effects of D2R antagonist, raclopride, on cognitive performance using a passive avoidance task. We further carried out in vivo microdialysis to assess the effect of infusion of D2R agonist, quinpirole, into the ventral hippocampus on its capacity to release ACh. For experiment 2, rats fed with normal chow were used. The performance of a radial arm maze task was assessed to examine the effects of hippocampal injection of D2R agonist, quinpirole, on memory impairment induced by scopolamine, a muscarinic ACh antagonist.

RESULTS

In experiment 1, rats fed with Ch-deficient chow showed impaired performances indicated by prolonged latency on retention trials of a passive avoidance task following the hippocampal injection of D2R antagonist, and showed reduced capacity to release ACh following the injection of D2R agonist compared with rats fed with normal chow. In experiment 2, memory impairment induced by the intraperitoneal injection of scopolamine was ameliorated by the injection of D2R agonist into the ventral hippocampus.

CONCLUSION

These results indicate the possible involvement of hippocampal ACh-DA interaction in mnemonic processing.

Effect of pretraining administration of NC-1900, a vasopressin fragment analog, on memory performance in non- or CO2-amnesic mice

In the present study, we investigated the facilitative effect of NC-1900, a new arginine vasopressin (AVP(1-9)) fragment analog, on memory performance in eight-arm radial maze or passive avoidance (PA) tasks in nonamnesic and amnesic (PA tasks only) mice. In the radial maze, all injections (subcutaneous) were given daily 60 min before each trail. NC-1900 (1 ng/kg)-treated animals showed enhancement of performance, and AVP(4-9) (1 microg/kg), an AVP(1-9) fragment, had similar effects, although the effective dose was 1000-fold higher. In the PA task, all drugs were administrated subcutaneously 60 min before the acquisition trial (Acq.), and the amnesic mice were exposed to CO(2) just after the Acq. NC-1900 (1 ng/kg) enhanced the memory performance of nonamnesic mice and ameliorated CO(2)-induced amnesia. AVP(4-9) (1 microg/kg) had a similar effect, although only at higher doses, while AVP(1-9) (0.1-1 microg/kg) had no effect. The facilitating effect of NC-1900 on nonamnesic mice was inhibited by coinjection [Pmp(1)-Tyr(Me)(2)]-AVP (Pmp,Tyr-AVP; 1 microg/kg), a V(1A) antagonist, but not by OPC-31260, a vasopressin(2) (V(2)) antagonist. The effect of NC-1900 on CO(2)-induced amnesia was also decreased by coinjection of Pmp,Tyr-AVP or [deamino-Pen(1), Me-Tyr(2)]-AVP (10 microg/kg), both of which are V(1) antagonists. These results suggested that NC-1900 has a more potent effect on facilitation of memory via the V(1A) receptor than AVP(4-9) in non- and CO(2)-amnesic conditions.

Ameliorative and Exacerbating Effects of [pGlu4,Cyt6]AVP(4–9) on Impairment of Step-Through Passive Avoidance Task Performance by Group II Metabotropic Glutamate Receptor-Related Drugs in Mice

To examine the effect of the arginine-vasopressin fragment, [pGlu(4),Cyt(6)]AVP((4-9)) (AVP4-9), on group II metabotropic glutamate receptor (mGluR2/3) agonist and antagonist induced impairment of passive avoidance (PA) task performance, AVP4-9 or phorbol 12-myristate 13-acetate (PMA) was administered in the presence of mGluR2/3-related drugs that induced the impairment of the step-through-type PA task performance. The PA task performance was evaluated in terms of the latency (the time that elapsed prior to entry into the dark compartment) at 24 h after the electrical stimulation. The subcutaneous injection of AVP4-9 at 1 mug/kg had the greatest facilitative effect on the performance, and the facilitative effect of AVP4-9 was inhibited by NPC-15437, a specific protein kinase C (PKC) inhibitor. The injection of AVP4-9 ameliorated PA task performance impairment induced by DCG-IV, an mGluR2/3 agonist. Intracisternal injection of PMA, a PKC activator, also ameliorated the DCG-IV-induced impairment. High doses of AVP4-9 exacerbated the PA task performance impairment induced by LY341495 (an mGluR2/3 antagonist), and PMA injection (1 mug) also exacerbated the impairment induced by the antagonist. These results suggest that an increase in the activity of the PKC-signaling pathway may not always facilitate PA task performance; therefore, AVP4-9 can either enhance or inhibit memory performance in mice.

Facilitative effect of a novel AVP fragment analog, NC-1900, on memory retention and recall in mice

In order to determine the mechanism of action of a new AVP(4-9) analog, NC-1900, on memory processes, memory retention and retrieval tests were conducted in a step-through passive avoidance (PA) task in mice. The administration of NC-1900 facilitated memory retention and retrieval in the PA task through vasopressin1A (V1A) receptors but not V2 receptors. The effect of NC-1900 on memory retention test performance appeared to be due to activation of the protein kinase C (PKC) signaling pathway via V1A receptors; however, the modulation of PKC was not essential for the facilitative effect of the new peptide in the retrieval test. The facilitation of memory retrieval by NC-1900 may also be mediated by other non-PKC-dependent signaling pathways, such as the phospholipase C-inositol trisphosphate pathway.

V1a receptor knockout mice exhibit impairment of spatial memory in an eight-arm radial maze

In this study, we examined the performance of vasopressin V1a receptor (V1aR) and vasopressin V1b receptor (V1bR) knockout (KO) mice compared to wild-type (WT) mice in an eight-arm radial maze. V1aR KO mice exhibited an impairment of spatial memory in comparison to WT mice. By contrast, we did not observe any significant differences between the V1bR KO mice and the WT mice in the eight-arm radial maze. Moreover, OPC-21268, a selective V1aR antagonist, impaired spatial memory in the eight-arm radial maze in WT mice characterized by an increased number of errors. These results suggest that the V1aR controls spatial memory in mice.

Memory Impairment Means Less Pain for Mice

BACKGROUND

Clinical observations have reported that individuals with memory deterioration, like in Alzheimer's disease, display a lesser pain sensibility than patients with no cognitive impairment.

OBJECTIVE

To clarify the link between pain and loss of memory, we studied how memory-impaired mice behave when submitted to hotplate nociceptive tests.

METHODS

For 5 days (D1-D5), male CD1 mice were injected daily intraperitonealy with saline or scopolamine (s, an anticholinergic drug, 0.2 mg/kg) or ketamine (k, an N-methyl-D-aspartate receptor antagonist (NMDAr), 2.5 mg/kg), at doses leading to memory impairment with no analgesic effect. From D6 to D9, all received saline only. They were placed on the hotplate and removed at the first sign of discomfort, response time being recorded.

RESULTS

From D1 to D5, reaction time decreased significantly in controls only and did not change in mice with scopolamine or ketamine. From D6 to D9, response times decreased (p < 0.05 (s) and p < 0.0001 (k)) to reach the steady state of control animals. At D5, response time was significantly prolonged for scopolamine (p < 0.01) and ketamine (p < 0.05), compared to controls.

CONCLUSION

These results show that pain sensibility needs the integrity of the central cholinergic and of the NMDA systems, and that mice with memory impairment display a lesser pain sensibility than normal mice. Further research on the complex interactions of receptors and neurotransmitters involved in pain and cognition could assist in gaining a better understanding of pain and analgesia in patients with memory impairment and in demented individuals.

Norleucine1-Angiotensin IV alleviates mecamylamine-induced spatial memory deficits

The brain angiotensin AT4 receptor subtype has been implicated in cognitive processing. We initially established that intracerebroventricular administration of the nAChR-antagonist mecamylamine (mec) interfered with spatial memory performance in male Sprague-Dawley rats. Next we demonstrated that mec-induced deficits in spatial memory were overcome by the AT4 receptor-agonist Norleucine1-Angiotensin IV (Nle1-Ang IV). Nle1-Ang IV could not, however, compensate for spatial learning impairments precipitated by both mec and the mAChR-antagonist scopolamine. These findings support the importance of the AT4 receptor in cognitive processing and suggest that the ability of Nle1-Ang IV to improve spatial memory deficiencies may be dependant upon the brain cholinergic system.

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.

Post-ischemic administration [correction of administeration] but not pre-ischemic administration [correction of administeration] of NG-nitro-L-arginine prevents spatial memory impairments and apoptosis by an inhibition of a delayed increase in NOx- in the hippocampus following repeated cerebral ischemia

In the present study, we investigated the effects of N(G)-nitro-L-arginine (L-NAME), an inhibitor of nitric oxide synthase, on repeated cerebral ischemia-induced impairment of spatial memory of the 8-arm radial maze in rats. Repeated ischemia (10 min ischemia x 2 times with 1 h interval) impaired the spatial memory in the 8-arm radial maze test and produced apoptosis in the hippocampus 7 days after final occlusion, and gradually increased the NO(x)(-) levels approximately 30-180 min after the second reperfusion. Post-ischemic administration of L-NAME at a dose of 50 mg/kg, i.p. 30 min following the second occlusion, significantly attenuated the repeated ischemia-induced impairment of spatial memory in the 8-arm radial maze test and suppressed apoptosis in the hippocampus, and also significantly suppressed a delayed increase in the NO(x)(-) levels induced by repeated ischemia. However, pre-ischemic administration of L-NAME at a dose of 50 mg/kg, i.p. 30 min before the first occlusion, caused about 90% mortality (the mortality rate of vehicle-treated group was 10%). These results suggest that the delayed generation of NO(x)(-) may cause spatial memory impairment and induction of apoptosis in the hippocampus in rats subjected to repeated ischemia.

Ameliorative effect of NC-1900, a new AVP4-9 analog, through vasopressin V1A receptor on scopolamine-induced impairments of spatial memory in the eight-arm radial maze

The mechanism by which NC-1900, a new pGlu-Asn-Cys(Cys)-Pro-Arg-Gly-NH(2) (AVP(4-9)) analog, improves spatial memory in rats using an eight-arm radial maze was examined. Even at very low doses (0.2 ng/kg for s.c., 1 microg/kg for p.o., 1 fg for i.c.v.) NC-1900 improved scopolamine-induced impairment of spatial memory. NC-1900 (1 ng/kg, s.c.) also improved impairment of spatial memory induced by pirenzepine, a muscarinic(1) (M(1)) receptor antagonist, and by KN-62, a Ca2+/calmodulin (CaM)-dependent protein kinase II inhibitor. [Pmp(1), Tyr(Me)(2)]-Arg(8)-vasopressin, a vasopressin(1A) (V(1A)) receptor antagonist, and nicardipine, L-type Ca2+ blocker, but not OPC-31260, a V(2) antagonist, suppressed the effect of NC-1900 on scopolamine-induced impairment of spatial memory. A microdialysis study showed that NC-1900 did not affect acetylcholine release in the ventral hippocampus (VH) of intact rats or of scopolamine-treated rats. NC-1900 (1 microM) increased [Ca2+](i) in the VH than in the dorsal hippocampus (DH). Pretreatment with nicardipine (1 microM) and Ca2+ -free conditions inhibited the NC-1900-induced [Ca2+](i) response in the VH. Whereas co-administration of NC-1900 (1 microM) and carbachol (500 microM) increased [Ca2+](i) in the VH. Moreover, nicardipine concentration-dependently inhibited the increase in [Ca2+](i) induced by the co-administration of NC-1900 and carbachol in the VH. These results suggest that NC-1900 activates the V(1A) receptor at the postsynaptic cholinergic nerve, and causes a transient influx of intracellular Ca2+ through L-type Ca2+ channels, to interact with the M(1) receptor. The activation of these Ca2+ -dependent processes induced by NC-1900 may be involved in the positive effect of NC-1900 on scopolamine-induced impairment of spatial memory.