Vasopressin maintains long-term potentiation in rat lateral septum slices

Top-down regulation of motivated behaviors via lateral septum sub-circuits

How does cognition regulate innate behaviors? While the cognitive functions of the cortex have been extensively studied, we know much less about how cognition can regulate innate motivated behaviors to fulfill physiological, safety and social needs. Selection of appropriate motivated behaviors depends on external stimuli and past experiences that helps to scale priorities. With its abundant inputs from neocortical and allocortical regions, the lateral septum (LS) is ideally positioned to integrate perception and experience signals in order to regulate the activity of hypothalamic and midbrain nuclei that control motivated behaviors. In addition, LS receives numerous subcortical modulatory inputs, which represent the animal internal states and also participate in this regulation. In this perspective, we argue that LS sub-circuits regulate distinct motivated behaviors by integrating neural activity from neocortical, allocortical and neuromodulatory inputs. In addition, we propose that lateral inhibition between LS sub-circuits may allow the emergence of functional units that orchestrates competing motivated behaviors.

Neurobiology of the lateral septum: regulation of social behavior

Social interactions are essential for mammalian life and are regulated by evolutionary conserved neuronal mechanisms. An individual's internal state, experiences, and the nature of the social stimulus are critical for determining apt responses to social situations. The lateral septum (LS) - a structure of the basal forebrain - integrates abundant cortical and subcortical inputs, and projects to multiple downstream regions to generate appropriate behavioral responses. Although incoming cognitive information is indispensable for contextualizing a social stimulus, neuromodulatory information related to the internal state of the organism significantly influences the behavioral outcome as well. This review article provides an overview of the neuroanatomical properties of the LS, and examines its neurochemical (neuropeptidergic and hormonal) signaling, which provide the neuromodulatory information essential for fine-tuning social behavior across the lifespan.

Nicotine-Mediated ADP to Spike Transition: Double Spiking in Septal Neurons

The majority of neurons in lateral septum (LS) are electrically silent at resting membrane potential. Nicotine transiently excites a subset of neurons and occasionally leads to long lasting bursting activity upon longer applications. We have observed simultaneous changes in frequencies and amplitudes of spontaneous action potentials (AP) in the presence of nicotine. During the prolonged exposure, nicotine increased numbers of spikes within a burst. One of the hallmarks of nicotine effects was the occurrences of double spikes (known also as bursting). Alignment of 51 spontaneous spikes, triggered upon continuous application of nicotine, revealed that the slope of after-depolarizing potential gradually increased (1.4 vs. 3 mV/ms) and neuron fired the second AP, termed as double spiking. A transition from a single AP to double spikes increased the amplitude of after-hyperpolarizing potential. The amplitude of the second (premature) AP was smaller compared to the first one, and this correlation persisted in regard to their duration (half-width). A similar bursting activity in the presence of nicotine, to our knowledge, has not been reported previously in the septal structure in general and in LS in particular.

The regulation of social recognition, social communication and aggression: vasopressin in the social behavior neural network

Neuropeptides in the arginine vasotocin/arginine vasopressin (AVT/AVP) family play a major role in the regulation of social behavior by their actions in the brain. In mammals, AVP is found within a circuit of recriprocally connected limbic structures that form the social behavior neural network. This review examines the role played by AVP within this network in controlling social processes that are critical for the formation and maintenance of social relationships: social recognition, social communication and aggression. Studies in a number of mammalian species indicate that AVP and AVP V1a receptors are ideally suited to regulate the expression of social processes because of their plasticity in response to factors that influence social behavior. The pattern of AVP innervation and V1a receptors across the social behavior neural network may determine the potential range and intensity of social responses that individuals display in different social situations. Although fundamental information on how social behavior is wired in the brain is still lacking, it is clear that different social behaviors can be influenced by the actions of AVP in the same region of the network and that AVP can act within multiple regions of this network to regulate the expression of individual social behaviors. The existing data suggest that AVP can influence social behavior by modulating the interpretation of sensory information, by influencing decision making and by triggering complex motor outputs. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Association study between single nucleotide polymorphisms in promoter region of AVPR1A and Korean autism spectrum disorders

To determine the association between arginine vasopressin receptor 1A gene (AVPR1A) and autism spectrum disorders (ASDs), we examined 3 single nucleotide polymorphisms (SNPs), namely, rs7294536, rs3759292, and rs10877969, in the promoter region of AVPR1A by using a family-based association test (FBAT) in 151 Korean trios. Our results demonstrated a statistically significant association between autism and SNPs (additive model: rs7294536, chi(2)=9.328, df=2, P=0.002; rs10877969, chi(2)=11.529, df=2, P<0.001) as well as between autism and haplotype analysis (additive model: chi(2)=14.122, df=3, P=0.003). In addition, we found that ADI-R scores calculated by using a diagnostic algorithm for failure to develop peer relationships (A2) were higher in subjects having the AA genotype than in subjects having the AG and GG genotypes of rs7294536. Thus, our study provides evidence for a possible association between these SNPs and the phenotype of ASDs.

Stress-induced changes in hippocampal function

Exposure of an organism to stress leads to activation of the sympatho-adrenomedullary system and the hypothalamo-pituitary-adrenal axis. Consequently, levels of noradrenaline, peptides like vasopressin and CRH, and corticosteroid hormones in the brain rise. These hormones affect brain function at those sites where receptors are enriched, like the hippocampus, lateral septum, amygdala nuclei, and prefrontal cortex. During the initial phase of the stress response, when hormone levels are high, these compounds mostly enhance excitability and promote long-term potentiation. Later on, when hormone levels have subsided but gene-mediated effects of corticosteroids start to appear, the excitability is normalized to the pre-stress level, in the CA1 hippocampal area, but possibly less so in the dentate gyrus and amygdala. A disturbed balance between these early and late phases of the stress response as well as a shift toward the relative contribution of the dentate/amygdala pathways may explain why the normal restorative capacity fails in vulnerable people experiencing a life-threatening situation, which could contribute to the development of PTSD.

Glutamate antagonists attenuate the action of NC-1900, a vasopressin fragment analog, on passive avoidance task performance in mice

To examine the relationship between glutamate receptors and the action of NC-1900 on a step-through passive avoidance (PA) task in mice, MK-801, an NMDA receptor blocker, and (S)-4-carboxyphenylglycine (4CPG), a group I metabotropic receptor antagonist, were administered intraventricularly (i.c.v.) singly or as co-injections. The i.c.v. injection of MK-801 (0.8 microg) or 4CPG (2 microg) decreased the latency on the PA task. NC-1900 (1 ng/kg, subcutaneously (s.c.)) alone prolonged the latency on the retention trial in the PA task. MK-801 (0.2 and 0.8 microg) or 4CPG (0.5 and 2 microg) significantly inhibited the action of NC-1900, while the s.c. injection of NC-1900 did not affect latency in mice that received i.c.v. co-injection of MK-801 and 4CPG at any of the doses tested. These results suggest that glutamate receptors participate in the action of NC-1900 on learning and memory in PA task performance.

Regulation of affect by the lateral septum: implications for neuropsychiatry

Substantial evidence indicates that the lateral septum (LS) plays a critical role in regulating processes related to mood and motivation. This review presents findings from the basic neuroscience literature and from some clinically oriented research, drawing from behavioral, neuroanatomical, electrophysiological, and molecular studies in support of such a role, and articulates models and hypotheses intended to advance our understanding of these functions. Neuroanatomically, the LS is connected with numerous regions known to regulate affect, such as the hippocampus, amygdala, and hypothalamus. Through its connections with the mesocorticolimbic dopamine system, the LS regulates motivation, both by stimulating the activity of midbrain dopamine neurons and regulating the consequences of this activity on the ventral striatum. Evidence that LS function could impact processes related to schizophrenia and other psychotic spectrum disorders, such as alterations in LS function following administration of antipsychotics and psychotomimetics in animals, will also be presented. The LS can also diminish or enable fear responding when its neural activity is stimulated or inhibited, respectively, perhaps through its projections to the hypothalamus. It also regulates behavioral manifestations of depression, with antidepressants stimulating the activity of LS neurons, and depression-like phenotypes corresponding to blunted activity of LS neurons; serotonin likely plays a key role in modulating these functions by influencing the responsiveness of the LS to hippocampal input. In conclusion, a better understanding of the LS may provide important and useful information in the pursuit of better treatments for a wide range of psychiatric conditions typified by disregulation of affective functions.

An 8-day extensive elemental, but not contextual, fear conditioning potentiates hippocampal-lateral septal synaptic efficacy in mice

Previous findings have suggested a critical role for hippocampal-lateral septal (HPC-LS) synaptic transmission in the modulation of elemental vs. contextual fear conditioning. Pharmacologically- or electrophysiologically-induced increases in HPC-LS neurotransmission were shown to be associated with both an increase in elemental and a decrease in contextual fear conditioning. However, elemental conditioning, induced by an unconditional stimulus (US) that was explicitly paired with a simple conditional stimulus (CS), did not result in any change in this neurotransmission when two tone CS-footshock US pairings were provided. The present experiment was thus designed to investigate directly, in mice, whether extensive elemental conditioning (repeated CS-US pairings) could induce an increase in HPC-LS neurotransmission. For that purpose, over 8 days, an elemental conditioning group was repeatedly submitted to CS-US pairings in either one context (A) or another (B) depending on the training day. Hence, whichever the context, the tone CS was the relevant predictive stimulus for the occurrence of the footshock US. In contrast, a contextual conditioning group was submitted to the same regimen except that the US was delivered only in context A and was never paired with the CS, making, thereby, the context A the relevant predictor for the US regardless of the occurrence of the tone CS. Results show that during re-exposure of the animals to either context A or B, a significant increase in HPC-LS neurotransmission was selectively associated with the repeated elemental conditioning. This study supports the idea that changes in HPC-LS neurotransmission may modulate the strength of simple CS-US associations, and suggests that alterations of hippocampal functioning might be involved.

Activation of inhibitory pathways suppresses the induction of long-term potentiation in neurons of the rat lateral septal nucleus

Long-term potentiation of the hippocampal-septal pathway was examined by intracellular recording techniques. High frequency stimulation (two 100-Hz 1-s trains with a 20-s interval between them) of the hippocampal CA3 area resulted in a transient depolarization in rat lateral septal nucleus neurons. High frequency stimulation was followed by a facilitation of fast and slow inhibitory postsynaptic potentials, lasting for more than 2 h, but not by a long-lasting increase in the excitatory postsynaptic potential in the normal solution. Long-term potentiation (>2 h) of the excitatory postsynaptic potential did not appear in 74% of neurons tested, even when the fast inhibitory postsynaptic potential was blocked by bicuculline (30 microM), a GABA(A) receptor antagonist. High frequency stimulation produced long-term potentiation of the excitatory postsynaptic potential in the Mg(2+)-free solution containing bicuculline. When the fast and slow inhibitory postsynaptic potentials were blocked by GABA(A) and GABA(B) receptor antagonists (bicuculline and CGP 55845A respectively), high frequency stimulation produced a large and sustained depolarization followed by long-term potentiation of the excitatory postsynaptic potential. However, the excitatory postsynaptic potential was not enhanced by administration of these drugs after termination of high frequency stimulation. Pretreatment with 2-amino-5-phosphonopentanoate, a NMDA receptor antagonist, resulted in loss of long-term potentiation in both sets of experiments. Paired-pulse stimulation of the hippocampal CA3 region with interstimulus intervals between 200 and 800 ms depressed the second excitatory postsynaptic potential in the presence of bicuculline. CGP 35348, a GABA(B) receptor antagonist, reversed the depression of excitatory postsynaptic potentials to facilitation. These data suggest that high frequency stimulation of hippocampal CA3 neurons enhances the efficacy of GABAergic inhibitory circuits which, in turn, depress the ability of lateral septal nucleus neurons to express long-term potentiation.

5-Hydroxytryptamine facilitates spatiotemporal propagation of optical signals in the hippocampal-septal pathway

The role of 5-hydroxytryptamine (5-HT) on the propagation of neuronal excitation in the hippocampal-septal pathway was examined in a brain slice by optical and electrophysiological recording techniques. After electrical stimulation of the fimbrial pathway, optical signals first occurred at the caudal region of lateral septal nucleus (LSN), then propagated toward the rostral region of LSN. All of the evoked optical signals were blocked by tetrodotoxin (TTX). The optical signal that propagated to the LSN was blocked by either the removal of external Ca(2+) or bath-application of 6-cyano-7-nitroquinoxaline-2,3-(1H,4H)-dione (CNQX). Bath-application of 5-HT (1-50 microM) to the LSN for 10 min produced an increase in the propagation area of the optical signal and prolonged the falling phase of the optical signal. Bicuculline blocked the 5-HT-induced facilitation of the optical signal. 8-Hydroxy-di-n-propylamino tetralin (8-OH-DPAT), a selective 5-HT(1A) agonist, mimicked the facilitation of 5-HT. 1-(2-Methoxyphenyl)-4-(4-phthalimidobutyl)piperazine (NAN-190), a 5-HT(1A) antagonist, blocked the facilitation induced by 5-HT. 5-HT enhanced the amplitude of the field potential in septal slices, where the optical signals had been enhanced. These results indicate that 5-HT increases the efficacy of excitatory synaptic transmission in the hippocampal-septal circuit via 5-HT(1A) receptors of LSN neurons.

Modulatory actions of steroid hormones and neuropeptides on electrical activity in brain

Electrophysiological studies over the past decades have shown that many compounds in addition to 'classical' neurotransmitters affect electrical activity in the brain. These compounds include neuropeptides synthesized in brain as well as compounds which are released from peripheral sources and subsequently enter the brain compartment, such as corticosteroid hormones from the adrenal gland. In the present review, this principle is illustrated by describing the effects of two substances, i.e. vasopressin and corticosterone. Neuropeptides and corticosteroid hormones add at least two essential aspects to information processing in the brain. First, they both act conditional, i.e. they modulate the actions of 'classical' neurotransmitters, rather than changing basal neuronal activity by themselves. Second, the time-frame in which modulation of electrical properties takes place differs from that generally seen with 'classical' neurotransmitters. Neuropeptides modulate electrical activity over a period of minutes, while effects of corticosteroid hormones usually become apparent after at least an hour but then last for hours. In this way, neuropeptides and steroid hormones expand the repertoire of responses through which the brain reacts to environmental challenges.

Vasopressin in the lateral septum promotes elemental conditioning to the detriment of contextual fear conditioning in mice

Previous experiments using a classical fear conditioning paradigm have provided evidence that the processing of contextual conditional stimuli (CSs) by the hippocampus would be controlled by the amygdala through a modulation of hippocampal-lateral septal (H-LS) excitability. More specifically, our suggestion was that vasopressin release into the LS would occur in an elemental conditioning case [pairing CS-US (unconditional stimulus) procedure] and would result in less hippocampal-dependent contextual stimuli processing (i.e. overshadowing of CSs by the simple CS). Conversely, when an unpairing CS-US procedure is used, this would result in more contextual stimuli processing through a decrease in vasopressin release into the LS. The aim of the present experiment was to test this hypothesis using intraseptal injection of vasopressin or its V1/V2 antagonist. In agreement with this hypothesis, results suggest that vasopressin release into the LS would constitute a device by which priority is given to the more salient simple stimulus to the detriment of contextual information.

Vasopressin and amastatin induce V(1)-receptor-mediated suppression of excitatory transmission in the rat parabrachial nucleus

We examined actions of arginine vasopressin (AVP) and amastatin (an inhibitor of the aminopeptidase that cleaves AVP) on synaptic currents in slices of rat parabrachial nucleus using the nystatin-perforated patch recording technique. AVP reversibly decreased the amplitude of the evoked, glutamate-mediated, excitatory postsynaptic current (EPSC) with an increase in paired-pulse ratio. No apparent changes in postsynaptic membrane properties were revealed by ramp protocols, and the inward current induced by a brief application of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid was unchanged after AVP. The reduction induced by 1 microM AVP could be blocked by a V(1) AVP receptor antagonist, [d(CH(2))(5)(1)-O-Me-Tyr(2)-Arg(8)]-vasopressin (Manning compound, 10 microM). Bath application of an aminopeptidase inhibitor, amastatin (10 microM), reduced the evoked EPSC, and AVP induced further synaptic depression in the presence of amastatin. Amastatin's effects also could be antagonized by the Manning compound. Corticotropin-releasing hormone slightly increased the EPSC at 1 microM, and coapplication with AVP attenuated the AVP response. Pretreatment of slices with 1 microg/ml cholera toxin or 0.5 microg/ml pertussis toxin for 20 h did not significantly affect AVP's synaptic action. The results suggest that AVP has suppressant effects on glutamatergic transmission by acting at V(1) AVP receptors, possibly through a presynaptic mechanism involving a pertussis-toxin- and cholera-toxin-resistant pathway.

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

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

The vasopressin deficient Brattleboro rats: a natural knockout model used in the search for CNS effects of vasopressin

Behavioral neuroscience is using more and more gene knockout techniques to produce animals with a specific deletion. These studies have their precedent in nature. A mutation may result in a limited genetic defect, as seen in the vasopressin (VP) deficiency in the Brattleboro rat. The mutation is in a single pair of autosomal loci, and the sequences of VP gene from wild-type and homozygous Brattleboro rats are identical except for a single nucleotide deletion in the second exon. The deletion results in the synthesis of an altered VP precursor that is unable to enter the secretory pathway. The genetic disturbance results in a central diabetes insipidus comparable to that found in humans. Starting with our work during the early 1970s we found that the genetic defect in the availability of VP causes deficits in central nervous system (CNS) functions. Behavioral processes from cognition to drug tolerance appeared to be disturbed by the absence of VP, but not all behaviors are affected. The specificity of the absence of VP in causing behavioral deficits is shown in many cases. However, certain deficits are due to genetic factors other than the deletion of the VP gene. The picture is further complicated by differences in testing conditions, the absence of proper controls, i.e. heterozygous and wild-type Brattleboro rats, sex, compensation phenomena, and the absence of neuropeptides co-localized with VP. Interestingly, an age dependent spontaneous shunt to a heterozygous phenotype in vasopressinergic neurons might also compensate for the disturbance. Accordingly, findings in knockout animals should be interpreted with caution. One should realize that brain functions are modulated by multiple neuropeptides and that neuropeptides possess multiple CNS effects.

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

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

Differential modulation of lateral septal vasopressin receptor blockade in spatial learning, social recognition, and anxiety-related behaviors in rats

The role of lateral septal vasopressin (VP) in the modulation of spatial memory, social memory, and anxiety-related behavior was studied in adult, male Wistar rats. Animals were equipped with osmotic minipumps delivering the VP-antagonist d(CH2)5-D-Tyr(Et)VAVP (1 ng/0.5 microl per h) bilaterally into the lateral septum (LS). Subsequently, all rats were subjected to four behavioral tests. First, animals were tested in a spatial learning paradigm (Morris water maze; 12 trials), followed by the social recognition test. A possible role for VP in anxiety-related behavior was then studied in the shock-probe burying test and the elevated plus-maze, respectively. The results showed that VP receptor antagonism impaired social recognition and reduced open-arm activity in the plus-maze, while it had no effect on spatial learning (Morris maze) and shock-probe burying behavior. The results indicate a strong task-dependent specificity of lateral septal VP functioning.

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.

Characterization of carbachol-induced rhythmic bursting discharges in neurons from guinea pig lateral septum slices

A brain slice preparation from guinea pigs and intracellular recording techniques were used to examine the effects of carbachol application on the three classes (A, B, and C) of neurons (n = 68, 40 of class A, 12 of class B, 16 of class C) within the mediolateral part of the lateral septum (LSml). Bath application of carbachol elicited a sustained depolarization associated with an increase in membrane input resistance, action-potential firing and triggered rhythmic bursting discharges in 59% of recorded neurons. According to the configuration of these bursts, LSml neurons were classified into type I, II, and III neurons with reference to their response to carbachol. The frequency of spontaneous bursts was increased by depolarization caused by applied DC current in the three types of neurons. Bursts in type II and III neurons were voltage and dose dependent. These dependences were responsible for a continuum of variation in carbachol responses in these two types of neurons. As the neuron depolarized in the presence of carbachol, spontaneous action potentials increased in frequency and slow afterdepolarizing potentials (sADPs) appeared and preceded the occurrence of the first burst. These sADPs from adjacent action potentials appeared to progressively increase to initiate a burst. In the presence of carbachol, sADPs and bursts were also observable after action potentials evoked by depolarizing current pulses at the resting membrane potential (RMP) in LSml neurons. Evoked sADPs and bursts were associated with an apparent increase in input conductance. Application of low Na+ medium blocked both the sADP and bursts. Application of zero Ca2+ medium either 1) blocked completely the generation of sADPs and bursts (n = 16), or 2) did not block bursts (n = 14). Evoked sADPs and bursts were blocked by tetraethylammonium but were resistant to external Cs+. The results indicate that the activation of cholinergic receptors does not differentially affect the three classes of LSml neurons. The responses to carbachol in type II and III neurons form a continuum of variation, whereas these of type I neurons constitute a discrete entity. The selective cholinergic induction of a sADP, and the progressive activation of these sADPs in LSml neurons are thought to be responsible for the onset of the three types of rhythmic bursting discharges. We propose that sADPs and bursts induced by carbachol are generated by a cationic conductance largely permeable to Na+. In a subpopulation of LSml neurons (n = 16), the bursts are dependent on the presence of Ca2+ in the medium.

Differential modulation of changes in hippocampal-septal synaptic excitability by the amygdala as a function of either elemental or contextual fear conditioning in mice

Recent data obtained using a classic fear conditioning paradigm showed a dissociation between the retention of associations relative to contextual information (dependent on the hippocampal formation) and the retention of elemental associations (dependent on the amygdala). Furthermore, it was reported that conditioned emotional responses (CERs) could be dissociated from the recollection of the learning experience (declarative memory) in humans and from modifications of the hippocampal-septal excitability in animals. Our aim was to determine whether these two systems ("behavioral expression" system and "factual memory" system) interact by examining the consequences of amygdalar lesions (1) on the modifications of hippocampal-septal excitability and (2) on the behavioral expression of fear (freezing) resulting from an aversive conditioning during reexposure to conditional stimuli (CSs). During conditioning, to modulate the predictive nature of the context and of a discrete stimulus (tone) on the unconditional stimulus (US) occurrence, the phasic discrete CS was paired with the US or randomly distributed with regard to the US. After the lesion, the CER was dramatically reduced during reexposure to the CSs, whatever the type of acquisition. However, the changes in hippocampal-septal excitability persisted but were altered. For controls, a decrease in septal excitability was observed during reexposure to the conditioning context only for the "unpaired group" (predictive context case). Conversely, among lesioned subjects this decrease was observed in the "paired group" (predictive discrete CS case), whereas this decrease was significantly reduced in the unpaired group with respect to the matched control group. The amplitude and the direction of these modifications suggest a differential modulation of hippocampal-septal excitability by the amygdala to amplify the contribution of the more predictive association signaling the occurrence of the aversive event.

Contextual conditioned fear blocks the induction but not the maintenance of lateral septal LTP in behaving mice

High-frequency stimulation (HFS) of the fimbria induces long-term potentiation (LTP) in the lateral septum. This study was aimed at investigating the effect of contextual fear conditioning on septal LTP with the use of behaving C57 BL/6 mice as subjects. For the acquisition of contextual fear conditioning, animals were placed in a conditioning chamber, where they were subjected to footshocks (FSs, 0.6 mA); the following day (retention), animals were reexposed to the chamber. Animals from the first group received HFS in their home cages before being submitted to conditioning; animals from the second group were first submitted to conditioning before receiving HFS during reexposure to the conditioning chamber; animals from the third group were submitted to the same regimen as those from the second group, except that no FS was delivered in the conditioning chamber; and animals from the fourth group received FS in the conditioning chamber but were maintained in their home cages the day after for LTP induction. Before conditioning, animals from the first group, placed in a familiar context (home cage), displayed an LTP of the N3 wave of septal field potential. After conditioning, reexposure of these animals to the conditioning chamber produced a transient decrease in the amplitude of N3 but did not interfere with the duration of maintenance of LTP. Conversely, in animals from the second group, when HFS was applied during reexposure to the conditioning chamber the induction of LTP was totally blocked. However, mice from the two other groups (3rd and 4th) displayed normal levels of LTP. Taken together with previous findings, these data suggest that contextual conditioned fear may interfere with certain forms of learning via blockade of hippocampal-septal LTP.

Z-321, a prolyl endopeptidase inhibitor, augments the potentiation of synaptic transmission in rat hippocampal slices

The present study investigated the effects of arginine-vasopressin (AVP) and (1-[3-(2-indanylacetyl)-L-thioprolyl] pyrrolidine (Z-321), an inhibitor of prolyl endopeptidase (PEP; (EC 3.4.21.26)) which degrades AVP in vitro, on the short-lasting potentiation of the field excitatory postsynaptic potentials (EPSP) coupled with a weak tetanus. The EPSP, after the electrical stimulation of the Schaffer collateral/commissural pathway, were recorded in the CA1 region of rat hippocampal slices. AVP at 10(-8) M and Z-321 at 10(-4) M augmented the potentiation induced by the weak tetanus; the magnitude of the post-tetanic potentiation of the EPSP was enhanced and the potentiation lasted for 60 min. In contrast, the racemic D-thioprolyl compound of Z-321, which virtually lacks any inhibitory effects on PEP, failed to affect the potentiation at 10(-4) M. The facilitatory effect of Z-321 was reversed by the application of [d(CH2)5,Tyr(Me)2]AVP (10(-8) M), an antagonist of the AVP V1 receptors, indicating that the effect of Z-321 was mediated through the V1 receptors. These findings suggest that Z-321 augmented the potentiation due to its inhibitory influence on the AVP degradation by PEP.

Polysynaptic potentiation in the lateral septum following stimulation of the fimbria in anesthetized rats

In anesthetized rats, electrical stimulation of fimbria fibers evoked, in the ipsilateral lateral septum (LS), a field potential consisting of two negative components: an initial negativity (N2-3 complex wave) of high amplitude at 6.7 ms (+/- 0.8 ms; peak latency) and a slow negative wave (N4 wave) of small amplitude at 14.4 ms (+/- 2.4 ms). The N2-3 complex wave represents the monosynaptic activation of LS neurons while the N4 wave corresponds to polysynaptic activation of neurons in the mediolateral part of the LS. In this study, we investigated the effects of high-frequency stimulation of fimbria fibers on LS field potentials and compared them with those observed in the CA3 area. Tetanic stimulation of the fimbria did not change the characteristics of the N2-3 wave but induced a long-lasting increase in amplitude and slope of the N4 wave. A positive correlation was found between the magnitude of CA3 LTP and lateral septal polysynaptic potentiation of the N4 component. These results indicate that patterns of stimulation delivered to the same input fibers (fimbria fibers) produce similar changes in a polysynaptic input to the LS and in a monosynaptic input to the CA3 and emphasize the complexity of signal processing in serial networks.

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.

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

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

A long-lasting increase and decrease in synaptic excitability in the rat lateral septum are associated with high and low shuttle box performance, respectively

In a series of experiments with rats, using evoked field potentials, the influence of massed trial training in 2-way shuttle box avoidance and step-through passive avoidance tasks was studied on the synaptic excitability of the lateral septum (LS) neurons and on the induction of long-term potentiation in the lateral septum in vivo. The majority of rats that attained a high performance level in the shuttle box task exhibited, after the shuttle box training, a long-lasting enhancement of synaptic excitability of lateral septum neurons, whereas most of the rats with low performance in the shuttle box showed a long-lasting depression in the LS synaptic excitability. Both types of excitability changes disappeared within 24 h. Neither the first habituation session in the passive avoidance apparatus nor the subsequent one-trial learning in passive avoidance task had a marked influence on lateral septum synaptic excitability. Both high-performance and low-performance rats exhibited a long-term potentiation (LTP)-like potentiation of synaptic excitability of the lateral septum neurons after high frequency stimulation of the fimbria fibers although the amount of LTP in high performance rats was slightly higher than that in low performance animals.

Effects of intraseptally injected glutamatergic drugs on hippocampal sodium-dependent high-affinity choline uptake in "naive" and "trained" mice

We have previously reported that spatial reference memory (RM) training-induced alterations in hippocampal cholinergic activity as measured by sodium-dependent high-affinity choline uptake (SDHACU). Each training session was found to induce an immediate (30 s) increase in SDHACU followed (30 s to 15 min posttest) by a deactivation and long-lasting inhibition (15 min to 24 h) of this cholinergic marker. The present experiments were designed to assess the role of septal glutamatergic receptors in this posttraining cholinergic deactivation. In the first experiment, the effects of intraseptal injections of different doses of glutamic acid and glutamatergic antagonists (kynurenic acid, KYN, and AP5) on hippocampal SDHACU were studied in awake but otherwise resting (i.e., naive) mice. The results showed that glutamic acid at the lowest dose used (5 ng) produced a decrease in SDHACU, whereas both glutamatergic antagonists produced a dose-related increase in this cholinergic marker. It was concluded that septal glutamatergic receptors mediate a tonic inhibitory input on the cholinergic cells. Hence, in a second experiment the effect of intraseptal injections of KYN (5 ng) on the training-induced changes in hippocampal cholinergic activity were assessed following variable amounts of radial maze RM training. Trained mice were injected 20 min before the first or the ninth training session and killed 30 s or 15 min posttraining for determination of SDHACU. KYN slowed the posttesting cholinergic deactivation (disinhibition), this effect being more marked in good learners than in bad learners. The present findings suggest that septal glutamatergic receptors mediate an inhibitory input on the cholinergic cells, and that this input could play a role in memory consolidation.

A comparative study of age-related changes in inhibitory processes and long-term potentiation in the lateral septum of mice

Anaesthetized C57 BL/6 mice of different ages (young: 5 months; middle-aged: 15 months; and old: 21 months) were used to determine whether aging alters the efficiency of synaptic inhibition and long-term potentiation (LTP) in the lateral septum (LS). Electrical stimulation of the fimbria induced field potentials in the ipsilateral LS comprising two initial negative components (N2 and N3) followed by a positive wave of low amplitude. Paired-pulse experiments showed a facilitation of the N2 component and a concomitant depression of the N3 components. Facilitation of the N2 component was stronger in both middle-aged and old mice as compared to young mice, whereas an inverse pattern of changes was observed for inhibition of the N3 component. High-frequency stimulation of the fimbria produced a persistent increase in the N3 amplitude. This LTP was of significantly higher amplitude in both young and middle-aged mice as compared to old mice. These results suggest that aging impairs both inhibitory processes and synaptic plasticity in the mouse LS, but that inhibitory processes appear to be affected earlier.

Metabotropic glutamate receptors are required for the induction of long-term potentiation

Recent observations have led to the suggestion that the metabotropic glutamate receptor may play a role in the induction or maintenance of long-term potentiation (LTP). However, experimental evidence supporting a role for this receptor in the induction of LTP is still inconclusive and controversial. Here we report that, in rat dorsolateral septal nucleus (DLSN) neurons, which have the highest density of metabotropic receptors and show functional responses, the induction of LTP is not blocked by the NMDA receptor antagonist 2-amino-5-phosphonovalerate, but is blocked by two putative metabotropic glutamate receptor antagonists, L-2-amino-3-phosphonopropionic acid and L-2-amino-4-phosphonobutyrate. Furthermore, superfusion of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid, a selective metabotropic glutamate agonist, resulted in a long-lasting potentiation of synaptic transmission similar to that induced by tetanic stimuli. Our results demonstrated that activation of postsynaptic metabotropic receptors is both necessary and sufficient for the induction of LTP in the DLSN, and we suggest that such a mechanism may be important at other CNS synapses.

High-frequency septal stimulation suppresses long-term potentiation (LTP) in the CA1 region of rat hippocampus

The effects of high-frequency stimulation (HFS) of the medial septum/diagonal band (MSDB) on long-term potentiation (LTP) of CA1 extracellular field potentials were assessed in anesthetized rats. Ten rats received HFS of the Schaffer collateral pathway alone, and 10 received MSDB HFS 10 min prior to hippocampal HFS. Septal HFS suppressed LTP development assessed by change in population spike (PS) amplitude 60 min after hippocampal HFS (ANOVA, P less than 0.03). Septal inhibition of LTP development was most prominent when septal HFS had little direct effect on the CA1 PS. These results provide a novel demonstration of the functional heterogeneity of septohippocampal pathways and in vivo modulation of hippocampal LTP by HFS of natural afferent inputs.

Pharmacology of long-term potentiation. A model for learning reviewed

Long-term potentiation is widely used as a model for memory formation. Recently, much information concerning this topic like the involvement of protein kinase C, arachidonic acid and N-methyl-D-aspartate receptors has been reported. In this review recent discoveries concerning long-term potentiation and the pharmacological implications for the development of cognition-enhancing drugs are discussed.

Inositol 1,4,5-trisphosphate 3-kinase distribution in the rat brain. High levels in the hippocampal CA1 pyramidal and cerebellar Purkinje cells suggest its involvement in some memory processes

The distribution of inositol 1,4,5-trisphosphate (InsP3) 3-kinase was studied in the adult rat brain, using polyclonal antibodies raised against the purified 50,000-Da rat brain enzyme by immunohistochemistry and Western blot, in addition to enzymatic assay. Immunohistochemically, the enzyme was detected in neurons, where it was localized in the dendrites and at the periphery of the cell bodies. Using selective toxin lesions, the highest enzyme levels were found in the dendrites of hippocampal CA1 pyramidal cells and in neurons in the dorsal portion of the lateral septum, regions both involved in long-term potentiation; and in the dendrites of Purkinje cell subpopulations in the cerebellum, a region involved in long-term depression. High levels were found in neurons in the cortex; in the anterior olfactory nucleus; in the striatum (caudate, putamen, olfactory tubercle, Calleja islets and accumbens); in the central nucleus of the amygdala; in the hippocampal dentate gyrus and in the subiculum. The enzyme was not detected in other brain regions. By Western blot, a 50,000-Da immunoreactive band was present in the cortex, caudate-putamen and cerebellum. This band was most highly stained in the hippocampus. InsP3 3-kinase activity, stimulated by calcium/calmodulin, corresponded to 6172-2638 pmol of InsP4 produced/min/mg protein in the hippocampus followed by frontal and parietotemporal cortex and cerebellum. This activity was below 400 in the brainstem and spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)

Convergent vasopressinergic and hippocampal input onto somatospiny neurons of the rat lateral septal area

Electron microscopic immunocytochemistry, was combined with acute anterograde axon degeneration, following transection of the fimbria-fornix, to describe the innervation of somatospiny neurons by vasopressin-immunoreactive and degenerated hippocamposeptal axon terminals in the rat lateral septal area. Vasopressin-immunopositive boutons characterized by symmetric synaptic membrane specializations, and the degenerated hippocamposeptal axon terminals which form asymmetric synaptic contacts, frequently terminate on the same dendritic and somatic profiles, and particularly on the somata of somatospiny neurons. Although hippocamposeptal fibers predominantly form axospinous synapses in the lateral septal area, they terminate mainly on the dendritic shafts and soma of the vasopressin-receptive neurons. Of 720 vasopressin-immunoreactive terminals in the mediolateral part of the lateral septal area, 80% form synaptic contacts with dendritic shafts; 50% on small (distal) dendritic profiles and 30% on large (proximal) dendrites. Synaptic contacts between vasopressin-immunoreactive terminals and dendritic spines were not observed. The remaining 20% of immunoreactive boutons formed axosomatic synaptic contacts with a total of 58 neurons; 31% of these neurons exhibited somatic spines in the plane of the section analysed. Previous studies have demonstrated that in the lateral septal area vasopressin modulates the action of the excitatory amino acid-containing hypocamposeptal fibers, and also plays a role in the maintenance of long term potentiation evoked by fimbria-fornix stimulation. The convergent vasopressinergic and hippocampal input onto the same somatospiny neurons of the lateral septal area suggests that these neurons are targets of these physiological actions.

Neuromodulation: associative and nonlinear adaptation

Neuromodulation, the interaction between at least two chemical messengers in the nervous system, serves as a mechanism by which biochemical association can occur. A simple, yet compelling, hypothesis is that the criteria for expression of associative learning and memory are subserved by biochemical events which are also associative in nature. A neuromodulatory interaction that has been linked to memory function and which has been the subject of biochemical inquiry is the interaction between the catecholamine, norepinephrine (NE) and the neuropeptide, vasopressin (AVP). Studies described in this report show that vasopressin acts to potentiate norepinephrine (NE)-induced cyclic adenosine monophosphate (cAMP) accumulation in the hippocampus by a calcium-dependent mechanism. Results of these studies are considered in the context of the nonlinear properties of synergism and conditionality and in the context of the associative learning requirements of spatial and temporal coupling. Secondly, the calcium dependency of AVP-induced neuromodulation is considered in relation to the calcium dependency for induction of associative long-term potentiation. Lastly, the potential for changes in neuronal morphology in response to neuromodulatory events is considered. By using vasopressin potentiation of noradrenalin-induced cAMP formation as a model system, I have applied the theoretical framework of associative learning and memory to test the hypothesis that neuromodulation can serve as a biochemical analog of associative cognitive events.