Arginine vasopressin induces the expression of c-fos in the mouse septum and hippocampus

The role of serotonin, vasopressin, and serotonin/vasopressin interactions in aggressive behavior

Aggression control has been investigated across species and is centrally mediated within various brain regions by several neural systems that interact at different levels. The debate over the degree to which any one system or region affects aggressive responding, or any behavior for that matter, in some senses is arbitrary considering the plastic and adaptive properties of the central nervous system. Nevertheless, from the reductionist point of view, the compartmentalization of evolutionarily maladaptive behaviors to specific regions and systems of the brain is necessary for the advancement of clinical treatments (e.g., pharmaceutical) and novel therapeutic methods (e.g., deep brain stimulation). The general purpose of this chapter is to examine the confluence of two such systems, and how their functional interaction affects aggressive behavior. Specifically, the influence of the serotonin (5HT) and arginine vasopressin (AVP) neural systems on the control of aggressive behavior will be examined individually and together to provide a context by which the understanding of aggression modulation can be expanded from seemingly parallel neuromodulatory mechanisms, to a single and highly interactive system of aggression control.

Intranasal application of vasopressin fails to elicit changes in brain immediate early gene expression, neural activity and behavioural performance of rats

Intranasal administration has been widely used to investigate the effects of the neuropeptides vasopressin and oxytocin on human behaviour and neurological disorders, although exactly what happens when these neuropeptides are administered intranasally is far from clear. In particular, it is not clear whether a physiological significant amount of peptide enters the brain to account for the observed effects. In the present study, we investigated whether the intranasal administration of vasopressin and oxytocin to rats induces the expression of the immediate-early gene product Fos in brain areas that are sensitive to centrally-administered peptide, whether it alters neuronal activity in the way that centrally-administered peptide does, and whether it affects behaviour in the ways that are expected from studies of centrally-administered peptide. We found that, whereas i.c.v. injection of very low doses of vasopressin or oxytocin increased Fos expression in several distinct brain regions, intranasal administration of large doses of the peptides had no significant effect. By contrast to the effects of vasopressin applied topically to the main olfactory bulb, we saw no changes in the electrical activity of olfactory bulb mitral cells after intranasal vasopressin administration. In addition, vasopressin given intranasally had no significant effects on social recognition or short-term recognition memory. Finally, intranasal infusions of vasopressin had no significant effects on the parameters monitored on the elevated plus maze, a rodent model of anxiety. Our data obtained in rats suggest that, after intranasal administration, significant amounts of vasopressin and oxytocin do not reach areas in the brain at levels sufficient to change immediate early gene expression, neural activity or behaviour in the ways described for central administration of the peptides.

Effects of gonadotropin releasing hormone and estradiol on c-fos expression in the rat hippocampus

The effects of ovariectomy, estradiol, and gonadotropin releasing hormone (GnRH) on the protein synthetic activity of hippocampal neurons were studied with immunohistochemistry for the proto-oncogene c-fos. Ovariectomy caused a reduction in the number of c-fos-positive neurons to 16% in area CA(1) and to 25% in area CA(3) when compared to that in the intact control animal. The dentate gyrus was only slightly affected. The decline in the number of c-fos-immunoreactive neurons in the Ammon's horn was partially reversed by a single intravenous injection of estradiol which resulted in the expression of c-fos in 71% of the neurons in area CA(1) and 74% in CA(3) when compared to the numbers of positive cells in the control animals. Similarly, intracerebroventricular injections of 10 muM GnRH caused an increase in the number of c-fos-positive cells to 68% in area CA(1) and to 52% in area CA(3) compared to that in the control animals. The induction of c-fos synthesis after estradiol and GnRH was transient and reached a maximum after 1 to 2 h before it declined to pretreatment levels after 8 h. The results suggest that both estradiol and GnRH exert specific effects on protein synthesis in certain neurons of the hippocampus and that these effects of the hormones are, at least in part, mediated by c-fos.

Induction of the arginine vasopressin-enhanced green fluorescent protein fusion transgene in the rat locus coeruleus

We examined the effects of intracerebroventricular (i.c.v.) administration of colchicine on the expression of the arginine vasopressin (AVP)-enhanced green fluorescent protein (eGFP) fusion gene in rats. In rats administered i.c.v. vehicle (control), eGFP fluorescence was observed in the supraoptic nucleus (SON), the magnocellular division of the paraventricular nucleus (PVN), the suprachiasmatic nucleus (SCN), the median eminence (ME) and the posterior pituitary. Two days after i.c.v. administration of colchicine, eGFP fluorescence was markedly increased in the SON, the magnocellular and parvocellular divisions of the PVN, the SCN, the ME and the locus coeruleus (LC). Immunohistochemical staining for eGFP confirmed the distribution of fluorescence in both groups. In the colchicines-administered groups, immunohistochemistry for tyrosine hydroxylase (TH) revealed that the eGFP fluorescence was co-localised with TH-immunoreactivity in the LC. Similarly, in situ hybridization histochemistry for eGFP mRNA revealed a significant increase in gene expression in the LC, the SON and the PVN 12-48 h after administration of colchicine. Our results indicate that the synthesis of AVP-eGFP is upregulated in noradrenergic neurones in the LC after colchicine administration. This implies that AVP and noradrenaline, originating from LC neurones, might play a role in response to chronic stress.

Ethanol modulates neuropeptide-degrading aminopeptidases at synapse level in calcium-dependent conditions

AIMS

To investigate the role of aminopeptidases in the pathways to peptides neurotransmission/neuromodulation ending in the actions of ethanol (EtOH) on the brain.

METHODS

The effects of EtOH on alanyl-, arginyl-, cystyl-, leucyl- and tyrosyl-aminopeptidase activities were studied under basal/resting and K+-stimulated conditions at the synapse level, using mouse frontal cortex synaptosomes and their incubation supernatant in a Ca2+-containing or Ca2+-free medium.

RESULTS

Under basal conditions, synaptosome aminopeptidase activities showed an inhibitory or biphasic response depending on the concentration of EtOH used and the aminopeptidase assayed, whereas supernatant activities showed a more complex response. Under K+-stimulated conditions, EtOH inhibited all synaptosome aminopeptidases assayed in presence of Ca2+. However, in absence of Ca2+, different responses were obtained depending on the concentration of EtOH used. In the supernatant, the highest concentration of EtOH inhibited the K+-stimulated increase on aminopeptidase activities, although the lowest concentration enhanced the release in presence of Ca2+. In absence of it, EtOH blocked the K+-stimulated decrease or increased the activity depending on the concentration of EtOH used.

CONCLUSIONS

The changes on aminopeptidase activities induced by EtOH may reflect the functional status of their corresponding endogenous substrates. EtOH may influence opioid peptides, oxytocin, vasopressin and the brain renin-angiotensin system through their degrading enzymes.

Current state of knowledge about the mechanisms of alcohol tolerance

Far from being a simple homeostatic response to the presence of ethanol in the brain, tolerance is now recognized as a complex process which can develop within various time frames (acute, rapid, chronic) and in which the drug interacts with various environmental and cognitive factors, including associative and operant learning. A major question is whether the acute form is an innate adaptive response, which is converted into the rapid and chronic forms by the action of these external influences. So far, all behavioral and neuropharmacological manipulations that alter chronic tolerance also modify rapid and acute tolerance in similar ways. These include lesions of serotonergic forebrain projections, blockade of NMDA-type glutamate receptors and calcium "L" channels, central or peripheral injection of vasopressin and blockade of vasopressin V1 receptors. Cytochemical and immunofluorescence studies, combined with use of retrograde tracers, indicate the existence of a septohippocampal circuit which may mediate the interactions of these diverse elements in the production and maintenance of tolerance. There is limited evidence that development of tolerance leads to increased consumption of ethanol in experimental animals, but the clinical significance of these findings remains to be proven.

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.

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.

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

In order to clarify the mechanism by which pGlu-Asn-Cys(Cys)-Pro-Arg-Gly-NH(2) (vasopressin-(4-9)), a major metabolite C-terminal fragment of [Arg(8)]-vasopressin (vasopressin-(1-9)), improves learning and memory, we used several different drugs such as an acetylcholine receptor antagonist, a Ca(2+)/calmodulin-dependent protein kinase II inhibitor, vasopressin receptor antagonists and L-type Ca(2+) channel blocker to disrupt spatial memory in rats. Moreover, we examined the effect of vasopressin-(4-9) on acetylcholine release in the ventral hippocampus using microdialysis. Vasopressin-(4-9) (10 fg/brain, i.c.v.) improved the impairment of spatial memory in the eight-arm radial maze induced by scopolamine, pirenzepine and Ca(2+)/calmodulin -dependent protein kinase II inhibitor. Pirenzepine, a vasopressin V(1A) receptor antagonist, and L-type Ca(2+) channel blocker, but not a vasopressin V(2) receptor antagonist, suppressed the effects of vasopressin-(4-9) on scopolamine-induced impairment of spatial memory. Moreover, vasopressin-(4-9) did not affect acetylcholine release in the ventral hippocampus of intact rats or of scopolamine-treated rats as assessed by microdialysis. These results suggest that vasopressin-(4-9) activates vasopressin V(1A) receptors on the postsynaptic membrane of cholinergic neurons, and induces a transient influx of intracellular Ca(2+) through L-type Ca(2+) channels to interact with muscarinic M(1) receptors. The activation of these processes by vasopressin-(4-9) is critically involved in the positive effect of vasopressin-(4-9) on scopolamine-induced impairment of spatial memory.

Differential increase in Fos immunoreactivity in hypothalamic and septal nuclei by arginine8-vasopressin and desglycinamide9-arginine8-vasopressin

Subcutaneous or intracerebroventricular injection of either arginine8-vasopressin or desglycinamide9-arginine8-vasopressin has been shown to facilitate memory, reduce or reverse the effects of amnesic drugs, and maintain tolerance to some effects of ethanol. These actions of vasopressin (and, by inference, of desglycinamide9-arginine8-vasopressin) are mediated by vasopressin V1 receptors in brain, via a c-fos-dependent mechanism, but the receptors at which the desglycinamide analog acts have not been identified. The precise central sites are also not known, but evidence of several types suggested the anterior hypothalamus and septum as probable loci of vasopressin action. In the present work, this question was studied by immunocytochemistry, using antibodies against Fos and Fos-like proteins. The numbers of Fos-immunoreactive nuclei were counted in several related brain regions and structures, after administration of arginine8-vasopressin, des-Gly9-[Arg8]-vasopressin or saline. A subcutaneous injection of vasopressin, but not of saline, enhanced Fos expression in the paraventricular, supraoptic and suprachiasmatic nuclei of the hypothalamus, but the desglycinamide analog stimulated Fos expression only in the suprachiasmatic nucleus. Vasopressin injection significantly increased the number of Fos-immunoreactive cells in the intermediate lateral septum, medial septum, and dorsal and ventral divisions of the lateral septum. In contrast, the desglycinamide analog increased the numbers of Fos-immunoreactive cells in the dorsal and intermediate portions of the lateral septum, but caused no change in the medial septum, and a decrease in the ventral portion of the lateral septum. Increased Fos expression was also found in the subfornical organ after subcutaneous injection of either vasopressin or the desglycinamide analog. Double labeling with antibodies against Fos protein and against vasopressin revealed that most of the vasopressin-induced Fos-immunoreactive cells in the supraoptic, paraventricular and suprachiasmatic hypothalamic nuclei are also vasopressin immunoreactive, i.e. they are vasopressin-producing neurons. These findings suggest that a circuit involving V1 receptors in the subfornical organ, connecting fibres to the suprachiasmatic nucleus, and vasopressinergic projections from the suprachiasmatic nucleus to the lateral septum, may play a central role in mediating the actions of both vasopressin and its desglycinamide analog in the maintenance of ethanol tolerance.

Fos protein expression induced by intracerebroventricular injection of vasopressin in unconditioned and conditioned mice

Arginine8-vasopressin (AVP) has been shown to improve memory consolidation in various mnemonic tasks. Our previous studies have pointed out the involvement of the hippocampus in memory consolidation and retrieval processes during discriminative learning by mice. The present study attempts to determine what other brain areas besides the hippocampus might be involved in the enhancing effect of intracerebroventricularly (i.c.v.) injected AVP on memory consolidation in a visual discrimination task using a polyclonal antibody that acts against Fos and Fos-like proteins. For behavioral testing, AVP was i.c.v. injected at the behaviorally active dose of 2 ng after the last learning session and improvement in consolidation processes was assessed in a retention session. Changes in Fos and Fos-like protein expression were determined in non-conditioned and conditioned mice. In non-conditioned mice, AVP i. c.v. injected at a dose of 2 ng evoked a time-dependent increase in Fos and Fos-like protein expression in the dentate gyrus (DG), CA1 and CA3 hippocampal fields, lateral septum (LS), bed nucleus of the stria terminalis, and basolateral and central amygdaloid nuclei, with a peak 120 min after the injection in most of the these brain areas. In contrast, in conditioned mice, an increase in the level of Fos expression, assessed 120 min after the end of learning and the injection of AVP, was detected only in the DG, ventral CA3 hippocampal field, and LS. Thus, the pattern observed after post-training injection of AVP was not the same as that evoked by AVP alone, since among the limbic structures activated following AVP alone, only the DG, the CA3 hippocampal field, and the LS seem to be involved in the enhancing effect of AVP on memory consolidation in discriminative learning.

Biochemistry of vasopressin fragments

Vasopressin (VP) undergoes a step-wise aminopeptidase conversion process in the brain, leading to accumulation of several metabolites. Some of these metabolites, in particular [pGlu4,Cyt6]VP 4-9 and 4-8, show behavioral effects comparable to VP, but are more potent and selective than VP. Most data favor the existence of a separate receptor for the VP metabolites distinct of the classical VP and oxytocin receptors, although its identity has remained obscure thus far. The characterization of this receptor is a major challenge to understand how the brain VP system generates and regulates divers central functions.

Vasopressin binding sites in the central nervous system: distribution and regulation

High affinity binding sites for vasopressin (VP) are widely distributed within the rat brain and spinal cord. Since their presence is associated with neuronal sensitivity to VP application, their anatomical distribution maps structures which could be activated by endogenous VP. Interestingly, marked species-related differences of the VP receptor distribution have been revealed. Some evidence has also been provided that mechanisms of receptor regulation may vary among species. In the rat, the expression of VP binding sites in some motor nuclei shows remarkable plasticity, in particular up-regulation after axotomy. These data suggest that VP may, in addition to affecting motoneuronal excitability, act as a trophic factor onto motoneurones.

Vasopressin released within the septal brain area during swim stress modulates the behavioural stress response in rats

The aim of the present study was to investigate the physiological significance of the neuropeptide arginine vasopressin (AVP) released within the septum, in the behavioural response of rats to stress. In the first experiment, rats were chronically implanted with a microdialysis probe aimed at the mediolateral or ventral septum to monitor the local release of AVP in response to 10 min of forced swimming in 20 degrees C warm water. Exposure to this stressor caused a significant increase in AVP release in both the mediolateral (174 +/- 21%, P < 0.01) and ventral septum (220 +/- 33%, P < 0.01). In contrast, microdialysates collected outside the mediolateral septum or in the lateral ventricle remained at prestress levels throughout the dialysis period. Furthermore, unstressed control animals failed to show significant alterations in vasopressin release in the mediolateral septum. In a second experiment, the introduction of the V1 receptor antagonist d(CH2)5Tyr(Me)AVP into the mediolateral septum via inverse microdialysis concomitant with stressor exposure caused the rats to spend an increased time floating and a reduced time swimming compared to vehicle-treated rats. This effect was acute and also detected 24 h after antagonist administration. Taken together, these findings demonstrate a significant activation of the septal vasopressinergic system in response to swim stress. Furthermore, our data support the view that AVP released within this brain area is involved in the generation of active behavioural strategies aimed at coping with new and challenging situations.

Hebbian synapses in cortical and hippocampal pathways

Use-dependent alterations in synaptic efficacy are believed to form the basis for such complex brain functions as learning and memory and significantly contribute to the development of neuronal networks. The algorithm of synapse modification proposed by Hebb as early as 1949 is the coincident activation of pre- and postsynaptic neurons. The present review considers the evolution of experimental protocols in which postsynaptic cell depolarization through the recording microelectrode was used to reveal the manifestation of Hebb-type plasticity in the synaptic inputs of the neocortex and hippocampus. Special attention is focused on the inhibitory control of the Hebb-type plasticity. Disinhibition within the local neuronal circuits is considered to be an important factor in Hebbian plasticity, contributing to such phenomena as priming, primed burst potentiation, hippocampal theta-rhythm and cortical arousal. The role of various transmitters (acetylcholine, norepinephrine, gamma-amino-butyric acid) in disinhibition is discussed with a special emphasis on the brain noradrenergic system. Possible mechanisms of Hebbian synapse modification and their modulation by memory enhancing substances are considered. It is suggested that along with their involvement in disinhibition processes these substances may control Hebb-type plasticity through intracellular second messenger systems.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Research on tolerance: what can we learn from history?

The concept of tolerance to ethanol has evolved gradually over the past two centuries, and all of the basic clinical features, as they are now understood, have been clearly recognized for nearly 100 years. The basic mechanisms involved in central nervous system tolerance, however, have been elucidated only in the past 20 to 30 years. Little progress was made as long as tolerance was viewed as a purely cellular or physiological adaptation to alcohol, and researchers used overly simple paradigms based on mere exposure to the drug. With the recognition that learning, both operant and associative, can play a major role in the development of tolerance to alcohol and cross-tolerance to other drugs, a radical change in research approaches became possible. Most of the neural mechanisms related to learning and memory are now known to be involved in the development and retention of tolerance, and the simplistic models used in earlier research must now be abandoned. Nevertheless, a review of the history of past research points to a number of important lessons for future work, including the following: (1) many of the present concepts were enunciated by astute observers many decades ago, and research was hindered because this older literature was forgotten; (2) for many decades progress was slow because of a narrow focus on specific techniques, questions, and hypotheses that overlooked important research in related disciplines; (3) the course of research is often irregular, and past questions may have to be revisited with new approaches--but these are more likely to be fruitful if based on knowledge of past history; and (4) excellent researchers often obtain apparently contradictory findings, but the disagreements may hold the key to deeper understanding of the phenomena, and should not be brushed over by ignoring the minority findings and interpretations. As in all scientific research, the most important requirement for major progress is the formulation of good questions or hypotheses: the results yielded by the best available techniques can be only as good as the questions they are meant to answer.

Antisense oligonucleotide to c-fos blocks the ability of arginine vasopressin to maintain ethanol tolerance

Administration of the neuropeptide, arginine vasopressin, can reduce the rate of dissipation of functional ethanol tolerance in mice that have acquired that tolerance. We previously showed that intracerebroventricular vasopressin administration can also produce an increase in septal c-fos mRNA levels. To evaluate the role of the increased expression of c-fos in the ability of vasopressin to maintain tolerance, ethanol-tolerant mice were given intracerebroventricular injections of vasopressin in the presence or absence of an antisense oligonucleotide to c-fos. The antisense oligonucleotide completely blocked the ability of vasopressin to maintain ethanol tolerance, while a missense oligonucleotide was without effect. The antisense oligonucleotide also attenuated the increase in septal c-fos mRNA levels caused by vasopressin. The results provide evidence for a role of c-fos expression in the maintenance of ethanol tolerance by vasopressin.

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.

Brain-derived neurotrophic factor, neurotrophin-3 and neurotrophin-4/5 maintain functional tolerance to ethanol

Neurotrophins and growth factors not only affect neuronal development, but also maintain neuronal survival and influence neuronal function in the adult brain, and affect various cognitive processes related to learning and memory. Functional tolerance to ethanol represents an adaptive change in the central nervous system that has been hypothesized to have mechanisms in common with those underlying learning or memory. In the present work, the effects of neurotrophins on ethanol tolerance were compared to the effect of the neuropeptide, arginine vasopressin, which maintains (reduces the rate of dissipation of) both ethanol tolerance and memory. Functional tolerance to ethanol was induced in C57BL/6J mice by feeding them an ethanol-containing liquid diet, and the effect of neurotrophins on the rate of dissipation of tolerance to the hypnotic effect of ethanol was assessed. Human recombinant brain-derived neutrophic factor, neurotrophin-3 and neurotrophin-4/5, injected intracerebroventricularly once daily following ethanol withdrawal, maintained ethanol tolerance, while tolerance dissipated in ethanol-fed mice injected with vehicle (artificial cerebrospinal fluid) or with basic fibroblast growth factor. The results demonstrate that some neurotrophins can modulate neuroadaptation to ethanol, supporting the hypothesis that these factors can influence the function of postmitotic neurons in the adult brain.

Distribution and developmental changes in vasopressin V2 receptor mRNA in rat brain

The extrarenal presence of vasopressin V2 receptors (V2Rs) has been suggested but never been demonstrated. We investigated the distribution of V2R mRNA expression in the brain of newborn and adult rats using the reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization (ISH). RT-PCR assay revealed V2R mRNA in the cerebellum of both newborn and adult rats, and in the cerebrum of newborn but not adult rats. Further examination of the newborn rat brain by ISH showed dense localization of that V2R mRNA in the choroid plexus, and the neurons of the hippocampus and granular layer of the cerebellum. Epithelial cells and vascular endothelial cells were strongly stained in the choroid plexus. Moreover, expression of V2R mRNA in the hippocampus of the newborn rat decreased with age and could not be detected in rats more than 2 weeks old, while in the cerebellum V2R mRNA expression was stable and did not change with development. The results of our study have demonstrated the presence and distribution of V2R in the brain for the first time, and also show that expression of V2R mRNA in the hippocampus changes dynamically during the process of development. V2R may play an important role in the development of the central nervous system.

Peripheral injection of arginine8-vasopressin increases Fos in specific brain areas

Learned behaviors and tolerance to ethanol can be maintained by peripheral injection of arginine8-vasopressin (vasopressin) under conditions in which they would otherwise be lost. However, the sites of this action in the brain have not been clearly identified. Using a polyclonal antibody raised against Fos and Fos-like proteins, we have demonstrated increases in immunoreactive Fos and Fos-like proteins in the suprachiasmatic, supraoptic and paraventricular nuclei of the hypothalamus, and lesser increases in piriform cortex and amygdala, of the rat 2 h after a s.c. injection of vasopressin. Our results suggest that the exogenous vasopressin may exert its central action by activating a cellular immediate early gene in specific brain regions.

Angiotensin II induces the expression of c-fos mRNA in the central nervous system of the rat

We have investigated the effects of intracerebroventricular injections of angiotensin II in conscious rats on the expression of c-fos messenger RNA (mRNA) in the caudate nucleus, hypothalamus, midbrain and brainstem using semi-quantitative polymerase chain reaction and Northern Blots. RNA analysis revealed the presence of c-fos transcripts in the midbrain and brainstem following icv injections of ANG II. ANG II (1, 10, 100 ng) induced a substantial increase in c-fos mRNA in the brainstem which was significant after 10 ng ANG II, and less after 100 ng. This effect was time-dependent being detectable within 15 minutes and maximal after 60 minutes. This ANG II-induced c-fos mRNA expression was totally inhibited by icv pretreatment with the ANG II-AT1 receptor antagonist, losartan. Our data show for the first time that stimulation of central periventricular AT1 receptors induces the expression of c-fos mRNA in the brain. Thus, ANG II, in addition to its short-term regulatory actions, can participate through transcription factors in neuroplastic processes.

Vasopressin V1 receptor in rat hippocampus is regulated by adrenocortical functions

Two subtypes of arginine vasopressin (AVP) receptors (V1 and V2) have been distinguished. In this study, we examined the characteristics of AVP binding in rat hippocampus and the effects of bilateral adrenalectomy and adrenal steroids on its [3H]AVP binding. [3H]AVP binding to rat liver and the hippocampal membranes was strongly inhibited by the V1 antagonist, OPC-21268. ADX resulted in a significant decrease in the Bmax of AVP binding in the hippocampus. Chronic treatment with aldosterone and corticosterone restored the ADX-induced reduction, but treatment with dexamethasone did not. These results suggest that the AVP V1 receptor in the hippocampus is regulated by adrenocortical neuroregulatory functions.

Decreased tolerance to ethanol-induced hypothermia in long-term castrate male rats

A potential role for central stores of vasopressin in the development of tolerance was studied in the long-term castrate rat. Vasopressin stores in the septal region are known to be dramatically depressed following long-term castration. Sprague-Dawley male rat littermates were castrated at 26 days of age or given a sham surgery. Experiments began when animals reached 130 days of age. Tolerance to the hypothermic effects of ethanol occurred in intact but not castrate animals over the course of six daily IP injections of 3.0 g/kg ethanol. Both groups exhibited tolerance to the length of time needed to return to baseline temperature over the 6 days of ethanol injections. Tolerance to this effect of ethanol was still evident in intact animals but not castrates following another injection of ethanol 1 week later. No tolerance developed to the rebound hyperthermia that occurred in both groups. Blood ethanol levels did not differ significantly between castrate and intact littermates administered a single dose of ethanol. Overall, these results support the hypothesis that endogenous vasopressin is involved in the development of some aspects of tolerance to ethanol.

Vasopressin induction of long-lasting potentiation of synaptic transmission in the dentate gyrus

Vasopressin receptors are present in both the developing and mature dentate gyrus of the rat brain and are of the V1 vasopressor type. Because vasopressin has been shown to influence memory function when injected into the dentate gyrus, the influence of this peptide on an electrophysiological model of learning and memory using the field excitatory postsynaptic potential (EPSP) of the dentate gyrus was investigated. Results of these studies showed that nanomolar concentrations of [Arg8]-vasopressin induced a prolonged increase in the amplitude and slope of the evoked population response in the presence of 1.5 mM calcium. Moreover, the expression of the vasopressin-induced potentiation of the EPSP persisted following removal of vasopressin from the perfusion medium. The vasopressin-induced sustained increase has been termed long-term vasopressin potentiation (LTVP). The closely related neuropeptide oxytocin had no effect upon the EPSP of the dentate gyrus. Preincubation of hippocampal slices in a selective V1 antagonist blocked the expression of LTVP. The ability of the V1 antagonist to block LTVP demonstrates that the potentiation induced by vasopressin is receptor-specific. In the presence of 2.5 mM calcium, the effect of vasopressin was opposite to that observed in 1.5 mM calcium. Under the conditions of 2.5 calcium, vasopressin induced a prolonged depression in the amplitude and slope of the EPSP. Expression of both potentiation and depression appeared within 5 minutes of application and persisted for the length of the observation, 60 minutes. These experiments demonstrate that vasopressin can induce long-lasting changes in the excitability of dentate gyrus neurons that are both calcium-dependent and receptor-specific.

Expression of c-fos in restricted areas of the basal forebrain and brainstem following single or combined intraventricular infusions of vasopressin and corticotropin-releasing factor

Vasopressin has been shown to be localized in specific central nervous system (CNS) sites. There is considerable evidence that it can act as a central neurotransmitter and it has been ascribed a variety of putative roles in the CNS. To identify those regions of the brain capable of responding to this peptide, 250 pmol vasopressin were infused into the lateral ventricle intracerebroventricular of conscious, handled male rats, and their brains processed for fos-immunohistochemistry 60 min later. Increases in fos-immunoreactivity, compared with cerebrospinal fluid-infused controls, were found in specific regions of the basal forebrain and brainstem: the central nucleus of the amygdala, ventrolateral septum, parvocellular divisions of the paraventricular nucleus of the hypothalamus, dorsal tuberal nucleus and locus coeruleus. Pre-infusion of 2500 pmol of a V1a antagonist prevented or reduced the expression of c-fos by intracerebroventricular vasopressin in all areas except the dorsal parvocellular paraventricular nucleus, implying that in most (but not all) areas the actions of vasopressin are mediated by the V1a receptor. Central administration of vasopressin had no effect on plasma corticosterone levels. Vasopressin and corticotropin-releasing factor act synergistically on the anterior pituitary to cause release of adrenocorticotropic releasing hormone and have corresponding synergistic interactions on behaviour. Infusion of 250 pmol corticotropin releasing factor produced a similar but not identical pattern of fos-like immunoreactivity to that of vasopressin. Activation of the parabrachial nucleus was observed, but there was no significant effect on the lateral septum and apparent increases in the medial parvocellular division of the paraventricular nucleus and locus coeruleus were not significant. Corticotropin releasing factor also caused a marked rise in plasma corticosterone. When the two peptides were infused together (125 pmol each) no evidence for synergy was found, in terms of the number of neurons activated to express c-fos. The induction of differential patterns of fos-like immunoreactivity by vasopressin and corticotropin-releasing factor in specific regions of the limbic forebrain and brainstem has implications for the individual roles they play in the CNS.

cFos labeling in rat superior colliculus: activation by normal retinal pathways and pathways from intracranial retinal transplants

Previous studies in this laboratory have shown that intracranial retinal transplants can establish both anatomical and functional connections with the host brain. Embryonic rat retinae transplanted intracranially into neonatal host brains are capable of evoking appropriate physiological and behavioral responses when illuminated. The present study employs the specific expression of the cFos protein to identify brain regions in which immediate-early gene activation can be recognized in response to flash stimuli delivered to retinal transplants and to normal intact retinae. Stimulation of the intact eye induced significant cFos expression in various visual centers, including the stratum griseum superficiale of the superior colliculus and the pretectal area, but not in the dLGN, suprachiasmatic nucleus, or retina. Animals with functional transplants expressed cFos throughout the depth of the stratum griseum superficiale and stratum opticum of the superior colliculus, thus apparently activating an additional population of superior collicular cells. Like the eye-stimulated animals, animals with functional transplants failed to elicit significant cFos expression in the dLGN or transplanted retina. This study indicates that intracranial retinal transplants are capable of forming functional connections with the host superior colliculus which not only mediate transient changes in electrical activity but also may affect gene expression through the induction of the c-fos gene.

Vasopressin administration via microdialysis into the septum interferes with the acquisition of spatial memory in rats

The role of vasopressin (AVP) in the septohippocampal system in spatial memory was studied in 27 male hooded rats of the Long-Evans strain. The rats were implanted with a septal microdialysis probe and assigned to 3 groups. Two days later, they were trained on 3 consecutive days (12 daily trials) to locate the hidden underwater platform in the Morris water maze (MWM) while the probes were perfused with either artificial cerebrospinal fluid (aCSF) or aCSF containing vasopressin or the V1 antagonist d(CH2)5Tyr(Me)AVP (AAVP). Another group of rats (n = 8) remained untreated. Groups receiving microdialysis of aCSF or AAVP acquired the MWM task at the same rate as untreated animals. On the other hand, place navigation learning was significantly impaired by microdialysis of AVP during all sessions. The results indicate that endogenous AVP (at least that affecting the V1 receptor subtype) is not indispensable for the acquisition of spatial memories in the MWM, whereas excessive presence of synthetic AVP interferes with spatial learning.

Microdialysis administration of vasopressin and vasopressin antagonists into the septum during pole-jumping behavior in rats

Wistar rats (n = 95) were trained in a pole-jumping apparatus (10 trials/session/day) to investigate the involvement of centrally and peripherally released endogenous AVP in their acquisition rate and to examine the feasibility of the microdialysis technique for the administration of peptides during a behavioral test. After session 1, a microdialysis probe was implanted into the septum; during sessions 2 and 3 the probe was perfused with artificial cerebrospinal fluid (aCSF) alone or containing either AVP (delivered amount via the probe: 0.2 ng) or the V1 (d(CH2)5Tyr(Me)AVP, 5.0 ng) or the V2/V1 (d(CH2)5-D-Tyr(Et)VAVP, 5.0 ng) antagonist. Administration of AVP via microdialysis into the septum failed to alter the acquisition rate of pole jumping. Also, ip application of both hypertonic saline and the AVP V1 antagonist (10 micrograms) in another experiment failed to show a significant effect upon behavior. Septal administration of the V1 or the V2/V1 antagonist via microdialysis, however, produced a significantly impaired performance. The results indicate that AVP release within the septum is involved in the acquisition of pole-jumping behavior probably mediated by the V1 receptor subtype. An additional involvement of the V2 receptor subtype, however, cannot be entirely excluded. The microdialysis technique proved to be a potent tool to administer substances concomitantly with behavioral tests.

Fos-like proteins in the lumbosacral spinal cord following noxious and non-noxious colorectal distention in the rat

The rat lumbosacral spinal cord was immunocytochemically stained for Fos-like immunoreactivity following repetitive colorectal distention (CRD) to 20, 40 or 80 mm Hg. Following all 3 distention pressures, Fos-like immunoreactive (Fos-ir) nuclei were observed primarily in laminae I-II, V-VII and X, although some labeled nuclei were observed in laminae III-IV. Eighty mm Hg CRD resulted in significantly more Fos-ir nuclei than 20 or 40 mm Hg CRD. Morphological examination of the colon revealed clear signs of inflammation following 80 but not 20 mm Hg CRD. Acute 20 mm Hg CRD is a non-noxious stimulus, suggesting that both noxious and non-noxious visceral stimuli can induce Fos-like immunoreactivity in the spinal cord. It is suggested that activation of neuropeptide-containing small-diameter primary afferents is necessary, although not necessarily sufficient, to induce c-fos in the rat spinal cord.

Genetic approaches to drug dependence

Pharmacogenetic studies with drugs of abuse are proliferating. Many genetic animal models are now available for studies of the mechanisms of action of a variety of drugs. These models provide unique, genetically defined populations of extremely sensitive and insensitive animals for neuropharmacological analyses. John Crabbe and John Belknap describe how molecular biological methods are being applied to these models in combination with more traditional genetic mapping strategies to identify single genes of importance to drug effects. Pharmacogenetic approaches offer the hope of establishing commonalities of mechanisms among abused drugs.

Vasopressin and oxytocin in brain areas of rats selectively bred for differences in behavioral performance

Wistar rats were selectively bred over 10 generations for differences in performance in a footshock-motivated brightness discrimination (BD) test in a Y-maze. High behavioral performance (Wis/HBP) and low behavioral performance (Wis/LBP) rat lines were obtained which differ significantly in all behavioral components tested: frequency of correct responses, number of trials to criterion, response latency (HBP less than LBP), and frequency of freezing behavior (HBP less than LBP), the latter suggesting differences in emotionality. In Wis/LBP rats, furthermore, the normal increase in behavioral performance between the training and the relearning session, which indicates the formation of a memory trace, disappeared during selection. In male breeders sampled during selection of the two lines (Wis/HBP: n = 17; Wis/LBP: n = 21), both arginine vasopressin (AVP) and oxytocin (OXT) contents were measured by radioimmunoassay in the motor cortex, septum/striatum, hippocampus, hypothalamus, medulla oblongata and posterior pituitary. Compared with the Wis/HBP rats, the Wis/LBP rats contained less AVP in the hippocampus (3.1 +/- 0.58 vs 8.3 +/- 1.4 pg/mg wet wt., mean +/- S.E.M., P less than 0.001), but more AVP in the medulla (1.7 +/- 0.20 vs 1.1 +/- 0.18 pg/mg, P less than 0.05). In contrast, no significant differences between the lines were detected with respect to OXT concentrations. In the Wis/LBP rats, moreover, the hippocampal AVP content decreased during selection (r = -0.645, P less than 0.01), while the acquisition response latency increased (r = 0.549, P less than 0.01). As a consequence, a significant, albeit weak, negative correlation (r = -0.483, P less than 0.05) was observed between the individual hippocampal AVP content and the response latency during acquisition. Thus, the results confirm the view that genetically determined differences in the hippocampal content of endogenous AVP may contribute to an individual's level of emotionality and behavioral performance.

c-fos protein-like immunoreactivity: distribution in the human brain and over-expression in the hippocampus of patients with Alzheimer's disease

c-fos protein-like immunoreactivity was investigated in the human brain post mortem, using a polyclonal antiserum raised against the N-terminal conserved peptide of c-fos protein. Immunostaining was found in the cerebral cortex, hippocampus, striatum, thalamus and cerebellum but not in the upper brainstem and the adrenal gland. c-fos-like immunoreactivity predominated in neuronal elements, but was also observed in neuropil and glial cells. In addition to a nuclear localization, the staining could be seen in neuronal dendrites (i.e. in the pyramidal cells of hippocampus or in some cortical areas). In order to analyse the effect of brain injury on c-fos expression, the characteristics of the immunostaining were analysed in the hippocampus of patients deceased with Alzheimer's disease known to be associated with a preferential vulnerability of the pyramidal neurons. No staining was observed in the senile plaques or in neurofibrillary tangles, the histopathological stigmata of the disease. Densitometric measurement of the intensity of c-fos-like staining revealed a significant increase in the hilus, the fimbria and the CA1 field of the pyramidal layer in brains of patients with Alzheimer's disease compared to controls. These modifications may result from a suffering stage of hippocampal cells or from a compensatory mechanism in the still surviving neurons not yet affected by the pathological process.

Arginine-vasopressin fragment 4-9 stimulates the acetylcholine release in hippocampus of freely-moving rats

We examined the effects of arginine-vasopressin (AVP) C-terminal fragment 4-9, which facilitates learning and memory, on the extracellular acetylcholine (ACh) release in hippocampus of freely-moving rats using the microdialysis technique. Following administration of AVP4-9, p-Glu-Asn-Cys[Cys]-Pro-Arg-Gly-NH2, through the dialysis probe into the hippocampus, ACh levels in dialysates from the hippocampus increased markedly in dose and time dependent manner at 2-2.5 and 2.5-3 hr. AVP1-9, the parent peptide, has a similar enhancing effect on ACh release as AVP4-9. Stimulated ACh release by AVP4-9 was significantly inhibited by V1-selective receptor antagonist ([1-(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid), 2-(O-methyl)-tyrosine]AVP), but not by V2-selective antagonist ([1-(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid), 2-D-Ile, 4-Ile]AVP). From these observations, it is demonstrated that AVP4-9 stimulates the ACh release in rat hippocampus via mediating V1-like vasopressin receptors.

Comparative effects of arginine vasopressin, [pGlu4,Cyt6]arginine vasopressin-(4-9) and nerve growth factor on maintenance of functional tolerance to ethanol in mice

Previous work has demonstrated that arginine vasopressin (AVP), acting in the CNS, can maintain functional tolerance to ethanol. We now show that the AVP metabolite peptide, [pGlu4,Cyt6]arginine vasopressin-(4-9), also maintains ethanol tolerance, with a potency similar to that of arginine vasopressin, while nerve growth factor has only a marginal effect. The effects of these peptides on ethanol tolerance correlate with their previously described ability to induce expression of the proto-oncogene, c-fos, in the septum, a putative mechanism underlying peptide effects on neuroadaptive processes.

Age-dependent changes in the activity and isoenzymatic pattern of the phosphofructokinase in different areas of the central nervous systems

Regional activity of phosphofructokinase enzyme and the amount of the isoenzyme C in 7 areas of young, adult and aged rats brain have been studied. The phosphofructokinase activity in substantia nigra decreased in adult and aged rats, but the maximum decrease was found in the septum of aged rats. There is a regional distribution of the isoenzyme C in the different areas studied. There was a general decrease in the amount of this isoenzyme in the areas studied with the exception of the hippocampus and the amygdala. The highest decrease was found in the septal area and in the diagonal band of Broca. These results are discussed in relation to the aging in different regions.

Appearance and transient expression of vasopressin and oxytocin receptors in the rat brain

Binding sites for AVP and for OT were studied by in vitro autoradiography in sections from the brain of rat fetuses, neonates and infants; their distribution was compared to that of the brain of adults. Specific binding sites were first detected in the vagal complex for OT and in the reticular formation for AVP at E14 and E16 respectively. In the perinatal period, other areas become labeled. Approximately one week after birth, a "stable" pattern of distribution is established for AVP binding sites, and a different "stable" pattern obtained for OT binding sites. For both types of sites and in many areas, the density of labeling increases during the next two weeks to reach adult levels, whereas labeling decreases concomitantly in other areas of the brain. The distribution of AVP binding sites is of the adult pattern by the time of weaning. In contrast, the adult pattern of distribution of OT binding sites is only established after puberty, when new OT receptors appear in some regions of the hypothalamus and basal forebrain. "Transient" binding sites for AVP and OT, i.e. sites located in areas which were labeled in neonates but not in weanlings, were shown to have the same ligand affinity than the binding sites present in the adult. Electrophysiological studies suggest that at least some of these "transient" binding sites represent authentic receptors and may be involved in neuronal signaling.

The role of arginine vasopressin in alcohol tolerance

Administration of the neuropeptide, arginine vasopressin, to animals that have acquired functional tolerance to ethanol will maintain such tolerance, even in the absence of further ethanol ingestion by the animals. In mice, this action of the peptide is mediated by central nervous system V1 receptors and requires intact brain noradrenergic systems. Autoradiographic studies have shown that some V1 receptors are localized presynaptically on catecholaminergic neuronal terminals in the mouse lateral septum, suggesting that vasopressin may act via modulation of catecholamine release. In addition, vasopressin has been found to increase mRNA levels for the proto-oncogene, c-fos, in septum and hippocampus, possibly by an action at postsynaptic receptors. Expression of c-fos, which has been hypothesized to play a role in central nervous system neuroadaptation, could transform short-term actions of vasopressin into long-term effects on ethanol tolerance. Studies with vasopressin antagonists indicate that the endogenous peptide influences tolerance, and therefore the effect of chronic ethanol ingestion on vasopressin synthesis and release was studied. In mice and rats, hypothalamic vasopressin mRNA is decreased by chronic ethanol exposure, although effects on plasma vasopressin levels differ in the two species. The effect of ethanol on extrahypothalamic vasopressin synthesis in brain is under investigation. The results suggest mechanisms by which vasopressin can produce long-term changes in central nervous system function, and provide evidence for a disturbance of vasopressin regulation during chronic ethanol ingestion.