Vasopressin-induced calcium signaling in cultured hippocampal neurons

Haploinsufficiency of the arginine-vasopressin gene is associated with poor spatial working memory performance in rats

Behavioral pharmacological studies have implicated a role for the neurophysin arginine-vasopressin in learning and memory. Vasopressin, and its analogues, can produce either improvements or impairments in mnemonic functions, effects that depend upon the agent administered, the memory process measured and the task employed. As recent data have implicated vasopressin in regulating the cognitive functions of the prefrontal cortex, we sought to determine whether changes in vasopressinergic tone would affect a form of memory that is dependent upon this brain region. To that end, we used a genetic approach to examine how haploinsufficiency of the vasopressin gene affects working memory performance. Specifically, we tested a naturally occurring null-mutant rat on an operant delayed-non-match-to-position task. Male and female heterozygous and wild-type rats were trained to perform this working memory task, and the effects of varying the delay across which they had to maintain task information were systematically varied. Although vasopressin-deficient rats omitted fewer trials and completed trials more quickly, they exhibited delay-dependent deficits of choice accuracy. The genotype effects were not modified by sex. Collectively, these data indicate that even partial vasopressin deficiency can trigger deficits of spatial working memory performance and add to the growing body of results supporting a regulatory control of neocortical-dependent cognitive functions by this neurohormone.

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.

Vasopressin-induced cytoplasmic and nuclear calcium signaling in cultured cortical astrocytes

We sought to determine whether vasopressin V(1a) receptor (V(1a)R) mRNA detected in cortical astrocytes [Brain Res. Mol. Brain Res. 45 (1997) 138] was translated into functional receptors by investigating the effector calcium signaling cascade associated with the vasopressin V(1a) receptor subtype. Analysis of intracellular calcium dynamics using the calcium-sensitive dye fura-2 AM indicated that exposure of cortical astrocytes to V(1) vasopressin receptor agonist, [Phe(2),Orn(8)]-oxytocin, induced a marked dose-dependent increase in intracellular calcium which was abolished by depletion of extracellular calcium. V(1) agonist treatment induced a rapid increase in calcium signal in both the cytoplasm and nucleus, which was followed by an accumulation of the calcium signal in the nucleus, suggesting translocation of cytoplasmic calcium into the nucleus. The nuclear calcium signal was sustained for several seconds followed by translocation back to the cytoplasm. Following the nuclear-to-cytoplasmic calcium translocation, total free intracellular calcium concentration decreased. The dynamic calcium cytoplasmic and nuclear localization was confirmed by laser scanning confocal microscopy coupled with the calcium-sensitive dye fluo-3 AM. To determine the source of calcium, V(1) agonist-induced (45)Ca(2+) uptake and [(3)H]IP(1) accumulation were investigated. V(1) agonist induced significant and rapid uptake of (45)Ca(2+) and a significant dose-dependent increase in [(3)H]IP(1) accumulation in cortical astrocytes. To our knowledge, this is the first documentation of a vasopressin receptor-induced calcium signaling cascade in cortical astrocytes and the first documentation of vasopressin receptor induction of nuclear calcium signaling.

Effect of N-glycosylation on ligand binding affinity of rat V1a vasopressin receptor

A rat Vla vasopressin (rVla) receptor has two putative N-glycosylation sites at 14th and 27th amino acid asparagine in the extracellular N-terminus. In the present study, we examined the possible roles of N-glycosylation of the N-terminus in the receptor function. Three point mutants for deglycosylated rVla receptor were generated in which the 14th and/or the 27th asparagine was replaced with glutamine, namely N14Q, N27Q, and N14:27Q, each tagged with an enhanced green fluorescent protein (EGFP) at their C-termini, and transfected to COS-7 or HEK292 cells. The two single mutants and a double mutant have progressively smaller molecular mass compared to the wild-type receptor as determined by immunoblot analysis, indicating that the two sites are effectively glycosylated in vivo. The maximal ligand binding capacities of three mutant receptors were comparable to that of wild-type (17.02 +/- 1.32 pmol/g protein) with modest changes in ligand binding affinities: N27Q and N14:27Q had decreased binding affinities compared to N14Q and wild-type receptors. The reduced binding affinities of the deglycosylated mutants are not likely due to the impaired intracellular transport since their traffickings were indistinguishable from one another. Taken together, these results suggest that the N-glycosylation at the two sites of the N-terminus of rV1a receptor minimally affects the surface expression and trafficking of the receptor.

Vasopressin-induced neurotrophism in cultured neurons of the cerebral cortex: dependency on calcium signaling and protein kinase C activity

Neuronal process outgrowth has been postulated to be one of the fundamental steps involved in neuronal development. To test whether vasopressin can influence neuronal development by acting on the outgrowth of neuronal processes, we determined the neurotrophic action of the memory-enhancing peptide, vasopressin, in neurons derived from the cerebral cortex, a site of integrative cognitive function and long-term memory. Exposure to V(1) receptor agonist significantly increased multiple features of nerve cell morphology, including neurite length, number of branches, branch length, number of branch bifurcation points and number of microspikes. The dose-response profile of V(1) receptor agonist-induced neurotrophism exhibited a biphasic function, with lower concentrations inducing a significant increase while higher concentrations generally induced no significant effect. The neurotrophic effect of V(1) receptor activation did not require growth factors present in serum. Analysis of the regional selectivity of the vasopressin-induced neurotrophic effect revealed significant V(1) receptor agonist-induced neurotrophism in occipital and parietal neurons, whereas frontal and temporal neurons were unresponsive. Results of experiments to determine the mechanism of vasopressin-induced neurotrophism demonstrated that vasopressin-induced neurotrophism is dependent on V(1)a receptor activation, requires L-type calcium channel activation and activation of both pathways of the phosphatidylinositol signaling cascade, inositol trisphosphate and protein kinase C. These studies are the first to describe a functional cellular response for vasopressin in the cerebral cortex. The findings are discussed with respect to their implications for understanding the role of vasopressin-induced neurotrophism, the associated signaling pathways required for this response, and the ability of vasopressin to enhance memory function.

Spatial, cellular and temporal basis of vasopressin potentiation of norepinephrine-induced cAMP formation

This study investigated the spatial distribution of vasopressin V(1) and beta(1)-adrenoceptors within hippocampal subfields and lamina in an attempt to localize the site(s) of interaction between these two receptor systems. In addition, the cell types, neuronal and glial, in which the vasopressin-induced neuromodulation occurs, were identified. Lastly, the temporal constraints of the potentiation induced by vasopressin were investigated. Results of these analyses demonstrated multiple sites within the hippocampus where the interaction between vasopressin and norephinephrine could occur. Moreover, vasopressin-induced potentiation of adrenergic stimulated cyclase occurred in both hippocampal neurons and glia whereas it did not occur in undifferentiated neurons. Analysis of the temporal constraints of vasopressin-induced potentiation revealed that pre-activation of the vasopressin V(1) receptor for 1 min yielded greater potentiation than simultaneous exposure to vasopressin and norepinephrine. These data provide insights into the spatial and temporal characteristics for the interaction between the vasopressin receptor and adrenoceptor systems and provide a cellular and biochemical rationale for the behavioral findings of Kovács and De Wied.

Vasopressin increases [Ca(2+)](i) in differentiated astrocytes by activation of V1b/V3 receptors but has no effect in mature cortical neurons

Vasopressin (AVP) plays an important role in regulation of astrocytic, but not neuronal, water content and cell volume during hydro-osmotic challenge. To investigate the intracellular mechanism(s) signaling this response, [Ca(2+)](i) was measured fluorometrically in cultured cerebrocortical astrocytes and neurons, obtained from neonatal and fetal mouse brains, and matured during the culturing period. In astrocytes, [Ca(2+)](i) increased with an EC(50) of between 10(-10) and 10(-9) M AVP, the maximum increase was approximately 100 nM, and the response was independent of extracellular Ca(2+), identifying the receptor as being of the V1b/V3 subtype. In contrast, AVP had no effect on [Ca(2+)](i) in cortical neurons. This cellular difference is consistent with the ability of AVP to increase water permeability in astrocytes but not in neurons.

Vasopressin and oxytocin receptor mRNA expression during rat telencephalon development

We investigated the developmental expression of vasopressin and oxytocin receptor and peptide mRNA using semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) and Southern blot hybridization. Messenger RNAs for both vasopressin receptor subtypes V(1)a and V(2)were present in the telencephalon from embryonic day 12 to day 20. Both V(1)a and V(2)receptor mRNA increased on day 13 and then remained stable from embryonic day 13 to day 20. Messenger RNA for the vasopressin peptide was also detected in the telencephalon from day 12 to day 20, indicating that vasopressin could be synthesized within the rat cerebral cortex during rat embryonic development. Oxytocin receptor mRNA expression was also present in the telencephalon, but expression levels varied considerably from day 12 to day 20. No oxytocin mRNA expression was detected during rat telencephalon development. Temporal patterns of vasopressin receptor and vasopressin peptide mRNA expression along with oxytocin receptor mRNA suggest a temporal role for vasopressin- and oxytocin-mediated actions during rat telencephalon development.

Behavioral pharmacology of neuropeptides related to melanocortins and the neurohypophyseal hormones

Neuropeptides are peptides which affect the nervous system. They are derived from large precursor molecules. These are converted to neurohormones, neuropeptides of the "first generation", which can be further converted to neuropeptides of the "second generation". This review is a brief survey of the nervous system effects of neuropeptides derived from pro-opiomelanocortin (POMC) and the neurohypophyseal hormones. Processing of these molecules results in neuropeptides of the first and second generation which have similar, different, more selective or even opposite effects. Among those are effects on learning and memory processes, grooming, stretching and yawning, social, sexual and rewarded behavior, aging and nerve regeneration, thermoregulation, pain, sensitivity to seizures, and cardiovascular control. Results of animal studies as well as those of clinical studies suggest that these neuropeptides may be beneficial in aging, neuropathy, memory disturbances and schizophrenia. Most of these nervous system effects in animal studies were found before receptors in the nervous system for the various neuropeptides were detected. G-protein-coupled receptors for the neuropeptides of the "first generation", i.e., melanocortin receptors, opioid receptors, and neurohypophyseal hormone receptors have been found, in contrast to the receptors for neuropeptides of the "second generation", although there are indications that G-protein coupled receptors for these may be present in the brain.

The effects of vasopressin on hippocampal slices from trained rats

The effects of vasopressin on hippocampal slices from control and pretrained rats were compared. Prolonged presentation of both combined and noncombined conditioned stimuli (CS) and unconditioned stimuli (US) increases the sensitivity of hippocampal neurons to vasopressin. The effect of this was that responses to application of DG-AVP after training and active control treatment was biphasic, consisting of initial excitation and subsequent inhibition; similar effects were seen in control slices when the peptide concentration was increased. Increases in the excitatory phase occurred with presentation of the CS and US and were independent of the training factor. The inhibitory phase increased to a greater extent on presentation of combinations of the CS and US. It is suggested that increases in depression in response to application of vasopressin in trained animals, as compared with active controls, are due to a higher level of free intracellular calcium occurring as a result of the training procedure.

Glucocorticoid regulation of vasopressin V1a receptors in rat forebrain

Vasopressin V1a receptors (V1aRs) are expressed in the septum of the rat brain where they are thought to mediate several of the physiologic and behavioral effects of this neuropeptide. We have investigated the effects of adrenal steroids on forebrain V1aRs. Rats were bilaterally adrenalectomized (ADX) and hormone replaced with either corticosterone (CORT), dexamethasone (DEX) or aldosterone (ALDO) at different concentrations. V1aR mRNA was evaluated using in situ hybridization, and V1aR binding site density was quantified using a specific iodinated V1aR antagonist [125I]d(CH2)5Sar7-AVP (125I-SAVP). V1aR density in the dorsolateral septum and the bed nucleus of the stria terminalis (BNST) decreased significantly with adrenalectomy, and 5 micrograms/100 g b.wt. of DEX was able to restore V1aR binding to levels comparable to those of sham operated controls in both regions. ALDO replacement also elevated V1aR binding in the BNST but not in the septum. In ADX animals given corticosterone in their drinking water, V1aR mRNA levels detected by in situ hybridization increased significantly over the ADX rats given saline. In order to understand the molecular basis of this effect, a putative genomic clone encoding the rat V1aR was isolated, and sequence analysis of the 5' flanking region has revealed the presence of several putative glucocorticoid response elements (GREs). Gel retardation assays were performed using these putative GREs, and two appear to be active in protein binding in glucocorticoid receptor containing nuclear extracts. The glucocorticoid effects on V1aR mRNA and binding, and the presence of putative active GREs in the promoter of the V1aR gene strongly implicate a role for adrenal steroids in the regulation of V1a receptor gene expression in glucocorticoid receptor and/or mineralocorticoid receptor expressing tissues.