Vasopressin facilitates excitatory transmission in slices of the rat dorso-lateral septum

Amniotic fluid or its fatty acids produce actions similar to diazepam on lateral septal neurons firing rate

Human amniotic fluid (AF) contains eight fatty acids (FATs), and both produce anxiolytic-like effects in adult rats and appetitive responses in human newborns. The medial amygdala and lateral septal nucleus function are related to social behavior, but the action of AF or its FATs in this circuit is known. We obtained 267 single-unit extracellular recordings in Wistar rats treated with vehicle (1 mL, s.c.; n = 12), human AF (1 mL, s.c.; n = 12), a FAT mixture (1 mL, s.c.; n = 13), diazepam (1 mg/kg, i.p.; n = 11), and fluoxetine (1 mg/kg, p.o.; n = 12). Compared with the vehicle group, the spontaneous septal firing rate in the AF, FAT mixture, and diazepam groups was the lowest and in the fluoxetine group the highest. Cumulative peristimulus histograms indicated that the significant change in septal firing occurred only in the AF and FAT mixture groups and exclusively in those neurons that increased their firing rate during amygdala stimulation. We conclude that human AF and its FATs produce actions comparable to anxiolytic drugs and are able to modify the responsivity of a circuit involved in social behavior, suggesting facilitation of social recognition processes by maternal-fetal fluids.

Neuromodulation by oxytocin and vasopressin

Oxytocin (OT) and vasopressin (VP) are two closely related neuropeptides, widely known for their peripheral hormonal effects. Specific receptors have also been found in the brain, where their neuromodulatory actions have meanwhile been described in a large number of regions. Recently, it has become possible to study their endogenous neuropeptide release with the help of OT/VP promoter-driven expression of fluorescent proteins and light-activated ion channels. In this review, I summarize the neuromodulatory effects of OT and VP in different brain regions by grouping these into different behavioral systems, highlighting their concerted, and at times opposite, effects on different aspects of behavior.

GABAA receptor activation in the lateral septum reduces the expression of conditioned defeat and increases aggression in Syrian hamsters

Exposure to social stressors can cause profound changes in an individual's physiology and behavior. In Syrian hamsters, even a single social defeat results in conditioned defeat, which includes an abolishment of territorial aggression and the emergence of high levels of submissive behavior. The purpose of the current study was to determine whether the lateral septum (LS) is a component of the putative neural circuit underlying conditioned defeat. Experiment 1 explored the possibility that plasticity in the LS is necessary for the induction of conditioned defeat. Infusions of the protein synthesis inhibitor, anisomycin, prior to defeat training, however, failed to alter conditioned defeat during testing on the following day, suggesting that synaptic plasticity in the LS is not critical for defeat-induced suppression of aggression. Experiment 2 tested whether the LS is necessary for the expression of conditioned defeat. Infusions of the GABA(A) agonist muscimol into the LS prior to testing significantly increased aggression and decreased submission in previously defeated animals suggesting that the LS is an important component of the neural circuit mediating the expression of both aggression and submission in conditioned defeat. Experiment 3 examined whether the effects of muscimol on aggression were dependent on prior social defeat. Non-defeated animals receiving muscimol infusions prior to testing with a non-aggressive intruder displayed significantly more aggression than did hamsters receiving control injections. Thus, these data suggest that the activation of GABA(A) receptors in the LS increases aggression regardless of whether or not a hamster has previously experienced social defeat.

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.

The vasopressin system--from antidiuresis to psychopathology

Vasopressin is a neuropeptide with multiple functions. In addition to its predominantly antidiuretic action after peripheral secretion from the posterior pituitary, it seems to fulfill--together with its receptor subtype--all requirements for a neuropeptide system critically involved in higher brain functions, including cognitive abilities and emotionality. Following somatodendritic and axonal release in distinct brain areas, vasopressin acts as a neuromodulator and neurotransmitter in multiple and varying modes of interneuronal communication. Accordingly, changes in vasopressin expression and release patterns may have wide-spread consequences. As shown in mice, rats, voles, and humans, central vasopressin release along a continuum may be beneficial to the individual, serving to adjust physiology and behavior in stressful scenarios, possibly at the potential expense of increasing susceptibility to disease. Indeed, if over-expressed and over-released, it may contribute to hyper-anxiety and depression-like behaviors. A vasopressin deficit, in turn, may cause signs of both diabetes insipidus and total hypo-anxiety. The identification of genetic polymorphisms underlying these phenomena does not only explain individual variation in social memory and emotionality, but also help to characterize potential targets for therapeutic interventions. The capability of both responding to stressful stimuli and mediating genetic polymorphisms makes the vasopressin system a key mediator for converging (i.e., environmentally and genetically driven) behavioral regulation.

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.

Vasopressin and oxytocin decrease excitatory amino acid release in adult rat supraoptic nucleus

Oxytocin and vasopressin reduce the amplitude of excitatory postsynaptic responses in magnocellular neuroendocrine cells of the supraoptic nucleus (SON). To test whether synaptic glutamate release is modulated by these neuropeptides, we examined the combined effect of vasopressin and oxytocin on depolarization-induced glutamate and aspartate release from acutely dissected rat SON or fronto-parietal cortex punches. Glutamate release was stimulated with 60 mm K+ for 5-10 min and measured using ion exchange chromatography or high-performance liquid chromatography. During depolarization with high K+, extracellular glutamate levels increased, on average, to 204% of control values. In the presence of vasopressin/oxytocin, K+-stimulated glutamate and aspartate release were significantly reduced by 34% and 62%, respectively, in the SON. Treatment with the aminopeptidase inhibitor amastatin did not mimic the effects of exogenous vasopressin/oxytocin on glutamate or aspartate release, suggesting that, under the conditions tested here, amastatin treatment may produce more complex effects. The effects of exogenous neuropeptides are likely mediated by oxytocin and/or vasopressin receptors, as the oxytocin- and V1a-receptor antagonist, Manning Compound (10-100 micro m), partially reversed the effects of vasopressin/oxytocin on SON glutamate release. In contrast, in cortical punches, glutamate release was enhanced by high K+, but vasopressin/oxytocin did not significantly reduce glutamate/aspartate release, consistent with the relatively sparse distribution of vasopressin/oxytocin receptors in fronto-parietal cortex. These findings suggest that locally released oxytocin and vasopressin may autoregulate SON magnocellular neuroendocrine cell activity in part by modulating the release of excitatory amino acids from afferent terminals targeting these cells and/or from other cellular sources.

Vasopressin- and oxytocin-induced activity in the central nervous system: electrophysiological studies using in-vitro systems

During the last two decades, it has become apparent that vasopressin and oxytocin, in addition to playing a role as peptide hormones, also act as neurotransmitters/neuromodulators. A number of arguments support this notion: (i) vasopressin and oxytocin are synthesized not only in hypothalamo-neurohypophysial cells, but also in other hypothalamic and extrahypothalamic cell bodies, whose axon projects to the limbic system, the brainstem and the spinal cord. (ii) Vasopressin and oxytocin can be shed from central axons as are classical neurotransmitters. (iii) Specific binding sites, i.e. membrane receptors having high affinity for vasopressin and oxytocin are present in the central nervous system. (iv) Vasopressin and oxytocin can alter the firing rate of selected neuronal populations. (v) In-situ injection of vasopressin and oxytocin receptor agonists and antagonists can interfere with behavior or physiological regulations. Morphological studies and electrophysiological recordings have evidenced a close anatomical correlation between the presence of vasopressin and oxytocin receptors in the brain and the neuronal responsiveness to vasopressin or oxytocin. These compounds have been found to affect membrane excitability in neurons located in the limbic system, hypothalamus, circumventricular organs, brainstem, and spinal cord. Sharp electrode intracellular recordings and whole-cell recordings, done in brainstem motoneurons or in spinal cord neurons, have revealed that vasopressin and oxytocin can directly affect neuronal excitability by opening non-specific cationic channels or by closing K(+) channels. These neuropeptides can also influence synaptic transmission, by acting either postsynaptically or upon presynaptic target neurons or axon terminals. Whereas, in cultured neurons, vasopressin and oxytocin appear to mobilize intracellular Ca(++), in brainstem slices, the action of oxytocin is mediated by a second messenger that is distinct from the second messenger activated in peripheral target cells. In this review, we will summarize studies carried out at the cellular level, i.e. we will concentrate on in-vitro approaches. Vasopressin and oxytocin will be treated together. Though acting via distinct receptors in distinct brain areas, these two neuropeptides appear to exert similar effects upon neuronal excitability.

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.

The suprachiasmatic nucleus-paraventricular nucleus interactions: a bridge to the neuroendocrine and autonomic nervous system

Vasopressin (VP) is one of the principal neurotransmitters of the suprachiasmatic nucleus (SCN). By means of anatomical, physiological and electrophysiological techniques we have demonstrated that VP containing pathways from the SCN serve to affect neuroendocrine and 'autonomic' neurons in the paraventricular nucleus. By direct and indirect connections VP serves to inhibit corticosterone secretion, not only by affecting ACTH secretion but also by controlling the adrenal cortex via a neuronal route. Apart from controlling the pineal and adrenal, we also observed that the SCN is able to influence the heart. Subjecting rats or humans to light affects heart rate in a dose-dependent manner. These results suggest an important role for the SCN and VP in the SCN in the regulation of neuroendocrine and autonomic functions.

Vasopressin and oxytocin action in the brain: cellular neurophysiological studies

During the last two decades it has become apparent that vasopressin (VP) and oxytocin (OT), in addition to playing a role as peptide hormones, also act as neurotransmitters. Morphological studies and electrophysiological recordings have shown a close anatomical correlation between the presence of these receptors and the neuronal responsiveness to VP or OT. These compounds have been found to affect membrane excitability in neurons located in the hippocampus, hypothalamus, lateral septum, brainstem, spinal cord and superior cervical ganglion. Sharp electrode intracellular and whole-cell recordings, done in brainstem motoneurons, have revealed that VP and OT can directly affect neuronal excitability by opening non-specific cationic channels. These neuropeptides can also influence synaptic transmission, by acting either postsynaptically or upon presynaptic target neurons or axon terminals. Whereas in some hypothalamic neurons OT appears to mobilize intracellular calcium, as revealed by calcium imaging techniques, in the brainstem the action of this neuropeptide is mediated by a second messenger which is distinct from the second messenger activated in peripheral target cells. Future studies should be aimed at elucidating the properties of the cationic channels responsible for the neuronal action of VP and OT, at identifying the brain-specific second messengers activated by these neuropeptides and at determining whether endogenous VP and OT can exert neuronal effects similar to those elicited by exogenous neuropeptides.

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.

Electrophysiological studies of neurohypophysial neurons and peptides

We have used hypothalamic slices of the supraoptic nucleus (SON) to investigate synaptic control of magnocellular vasopressinergic and oxytocinergic neurons. With the use of perforated patch recording techniques we identified and isolated excitatory or inhibitory postsynaptic currents elicited by electrical stimulation of afferent fibers. Both inhibitory and excitatory afferent fibers displayed presynaptic GABAB receptors; the GABAB agonist, baclofen caused a dose-dependent suppression of the evoked potentials in the absence of any effects on postsynaptic input resistance. Further evidence for a presynaptic locus included an increase in paired pulse ratio and a lack of effect on currents elicited by exogenously applied muscimol (a GABAA receptor agonist) or AMPA (a glutamate agonist). With the use of an GABAB receptor antagonist we demonstrated an action of endogenously released GABA, acting at GABAB receptors on excitatory terminals, to reduce excitatory transmission. In addition to presynaptic modulation by GABA of afferent inputs, we also observed actions of vasopressin and oxytocin, released from dendrites of magnocellular SON neurons, to gate afferent, excitatory transmission in the SON. Exogenously applied vasopressin and oxytocin, or these peptides when released by depolarizing stimuli of magnocellular neurons, reduced the size of evoked excitatory postsynaptic potentials at a presynaptic locus. We have also observed actions of arginine vasopressin to modulate the action of glutamate in slices of the ventral septal area and to attenuate a glutamate-mediated excitatory postsynaptic current in slices of the parabrachial nucleus.

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.

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.

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.

Colocalization of gamma-aminobutyric acid with vasopressin, vasoactive intestinal peptide, and somatostatin in the rat suprachiasmatic nucleus

The seemingly contradictory observations in previous publications that gamma-aminobutyric acid (GABA) is detected in all cell bodies of the suprachiasmatic nucleus (SCN) and that terminals originating from the SCN are only 20-30% GABA positive prompted us to investigate whether this might be explained by a preference of colocalization in terminals of certain peptidergic neurons in the SCN or by a day/night rhythm in GABA synthesis. At three different circadian times, animals were perfusion fixed, and their SCNs were stained for vasopressin (VP), somatostatin (SOM), or vasoactive intestinal polypeptide (VIP). Subsequently, the number of GABA peptide-positive terminals was determined using GABA postembedding staining in ultrathin sections. It appeared that the highest percentage of colocalization with GABA was detected in VIP terminals (38%) and the lowest in VP terminals (15%). No differences in colocalization percentages could be observed in any parameter at any circadian time. In the dorsomedial hypothalamus, one of the target areas of the VP and VIP fibers from the SCN, a colocalization of GABA within VP and VIP terminals was found similar to that in the SCN. In the region of the somatostatin-containing neurons in the SCN, a number of axoaxonal contacts could be observed that sometimes exhibited synaptic specializations. In nearly all cases, the axoaxonic terminals contained GABA and/or SOM. The conclusion is that the high level of intrinsic GABAergic connections in the SCN represents a putatively powerful mechanism to synchronize or shut down the activity of the SCN. We discuss the possibility that, depending on the firing frequency of the neurons, the colocalization of GABA with all peptides under investigation allows for the selection of which transmitter is released, the peptidergic one or the amino acid.

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.

A facilitatory role of vasopressin in hypoxia/hypoglycemia-induced impairment of dopamine release from rat striatal slices

The excitatory amino acid, glutamate plays a crucial role in the pathogenesis of brain damage caused by anoxia and/or hypoglycemia. Although vasopressin (VP) also acts as an excitatory transmitter in the CNS, little is known about its effect on hypoxic and/or ischemic brain damage. In this study, we investigated the effect of arginine vasopressin (AVP) on hypoxia/hypoglycemia-induced impairment of dopamine release from striatal slices. Striatal slices were incubated in hypoxia-/hypoglycemia-inducing medium with or without AVP (0.01-1.0 microM) for 20 min. After 1-3 h of washout in normal medium, high K(+)-evoked dopamine release from the slices were examined. Hypoxia/hypoglycemia-induced decrease of striatal dopamine release was reversed by the removal of Ca2+ in the medium, but not by VP1- or VP2-receptor antagonist. In contrast, AVP potentiated the hypoxia/hypoglycemia-induced decrease of dopamine release in the striatum. This AVP-induced deterioration of the striatal response was antagonized by VP2 receptor antagonist, but not by VP1 receptor antagonist. The present results suggest that AVP may play a facilitatory role in hypoxia/hypoglycemia-induced dopamine release deficit mediated through the activation of VP2 receptor.

The impaired long-term potentiation in the CA1 field of the hippocampus of cognitive deficient microencephalic rats is restored by D-serine

Rat embryos exposed on gestational day 15 to methyl-azoxymethanol acetate develop a microencephaly characterized primarily by a hypoplasia of the neocortex and CA fields of the hippocampus that in adulthood is associated with disturbances in learning. In brain slices prepared from microencephalic rats, we have examined the field excitatory postsynaptic potentials and population spike in the CA1 field of the hippocampus evoked by stimulation of the stratum radiatum. These parameters did not differ from those obtained in slices from control rats. High frequency stimulation of the stratum radiatum afferent fibres, which readily induced long-term potentiation of the field excitatory postsynaptic potentials and population spike in the CA1 field of the hippocampus of control rats, failed to induce long-term potentiation in that of microencephalic rats. High frequency stimulation of the perforant path readily elicited long-term potentiation in the dentate gyrus of both control and microencephalic rats. Picrotoxin had no apparent effect on field excitatory postsynaptic potentials and population spike in the CA1 field of the microencephalic rats, indicating that little GABAergic inhibition was present in slices from these rats. D-2-Amino-phosphonovalerate suppressed the field potentials in slices from microencephalic rats by more than 50%, suggesting that N-methyl-D-aspartate receptors contributed markedly to the synaptic responses evoked by single stimuli. D-Serine, but not picrotoxin, restored long-term potentiation in the CA1 field of the microencephalic rats. The D-serine effect was prevented by pretreating the slices with either 7-chloro-kynurenate or D-2-amino-phosphonovalerate. The failure to induce long-term potentiation, if also found in vivo, may be among the factors related to the learning deficits displayed by these rats.

The C-terminal glycopeptide of propressophysin potentiates excitatory transmission in the rat lateral septum

Effects of peptides synthesized from the same precursor as vasopressin, i.e. the C-terminal 39-amino acid long glycopeptide and neurophysin II, were investigated for biological activities in electrophysiological experiments in brain slices of the rat lateral septum. These slices contained the glycopeptide as the predominant form and a fragment of it, amino acid sequence 22-39, as a minor form (8% of the glycopeptide 1-39), as shown by high performance liquid chromatography of extracts and by radioimmunoassay. None of the peptides, neurophysin II, the glycopeptide 1-39 and the fragment 22-39, tested in a concentration of 10(-12) M, had measurable effects on the resting membrane potential of the neurons. The glycopeptide and the fragment 22-39, however, increased, in some cells, for tens of minutes the excitatory postsynaptic potentials evoked in these neurons by stimulation of the fimbria fibers. The increase in input resistance, seen in many septal neurons treated with either of the peptides was not correlated with the excitatory postsynaptic potential increase. Neurophysin II affected neither the excitatory postsynaptic potentials nor the input resistance of the neurons. It is concluded that the glycopeptide 1-39 and the fragment 22-39 possess biological activities amongst which the facilitation of excitatory amino acid transmission on lateral septum neurons. Therefore, these peptides derived from the vasopressin precursor may act in concert with vasopressin to establish facilitation of excitatory transmission in the brain.