Intranasally Applied Neuropeptide S Shifts a High-Anxiety Electrophysiological Endophenotype in the Ventral Hippocampus towards a "Normal"-Anxiety One

The neurobiological basis of pathological anxiety and the improvement of its pharmacological treatment are a matter of intensive investigation. Here, using electrophysiological techniques in brain slices from animals of the high anxiety-related behavior (HAB) and normal anxiety-related behavior (NAB) mouse model, we show that basal neurotransmission at ventral hippocampal CA3-CA1 synapses is weaker in HAB compared to NAB mice. We further demonstrate that paired-pulse facilitation (PPF) and long-term potentiation (LTP) at these synapses are more pronounced in slices from HAB animals. Based on previous findings, we also examined whether intranasal delivery of neuropeptide S (NPS), which increasingly emerges as a potential novel treatment option for anxiety symptoms occurring in a variety of diseases like anxiety disorders, posttraumatic stress disorder, and major depression, impacts on the high-anxiety electrophysiological endophenotype in HAB mice. Strikingly, we detected enhanced basal neurotransmission and reduced PPF and LTP in slices from NPS-treated HAB animals. Collectively, our study uncovers a multifaceted high-anxiety neurophysiological endophenotype in the murine ventral hippocampus and provides the first evidence that an intranasally applied neuropeptide can shift such an endophenotype in an anxiety-regulating brain structure towards a “normal”-anxiety one.     

Presynaptic GABAB Receptors Regulate Hippocampal Synapses during Associative Learning in Behaving Mice

GABA_B receptors are the G-protein-coupled receptors for GABA, the main inhibitory neurotransmitter in the central nervous system. Pharmacological activation of GABA_B receptors regulates neurotransmission and neuronal excitability at pre- and postsynaptic sites. Electrophysiological activation of GABA_B receptors in brain slices generally requires strong stimulus intensities. This raises the question as to whether behavioral stimuli are strong enough to activate GABA_B receptors. Here we show that GABA_B1a^-/- mice, which constitutively lack presynaptic GABA_B receptors at glutamatergic synapses, are impaired in their ability to acquire an operant learning task. In vivo recordings during the operant conditioning reveal a deficit in learning-dependent increases in synaptic strength at CA3-CA1 synapses. Moreover, GABA_B1a^-/- mice fail to synchronize neuronal activity in the CA1 area during the acquisition process. Our results support that activation of presynaptic hippocampal GABA_B receptors is important for acquisition of a learning task and for learning-associated synaptic changes and network dynamics.     

Gating of action potential propagation by an axonal A-like potassium conductance in the hippocampus: a new type of non-synaptic plasticity.

Synaptic plasticity is usually considered as the main form of activity-dependent functional plasticity in the mammalian brain. Short- and long-term regulation of synaptic transmission have been shown in various types of excitatory synapses including cortical and hippocampal synapses. In this review, we discuss a novel form of non-synaptic plasticity that involves voltage-gated K+ conductances in CA3 pyramidal cell axons. With experimental and theoretical arguments, we show that axons cannot only be considered as a simple structure that transmit reliably the action potential (AP) from the cell body to the nerve terminals. The axon is also able to express conduction failures in specific axonal pathways. We discuss possible physiological conditions in which these axonal plasticity may occur and its incidence on hippocampal network properties.     

Impaired long-term depression in P2X3 deficient mice is not associated with a spatial learning deficit

The hippocampus is a brain region critical for learning and memory processes believed to result from long-lasting changes in the function and structure of synapses. Recent findings suggest that ATP functions as a neurotransmitter or neuromodulator in the mammalian brain, where it activates several different types of ionotropic and G protein-coupled ATP receptors that transduce calcium signals. However, the roles of specific ATP receptors in synaptic plasticity have not been established. Here we show that mice lacking the P2X3 ATP receptor (P2X3KO mice) exhibit abnormalities in hippocampal synaptic plasticity that can be restored by pharmacological modification of calcium-sensitive kinase and phosphatase activities. Calcium imaging studies revealed an attenuated calcium response to ATP in hippocampal neurons from P2X3KO mice. Basal synaptic transmission, paired-pulse facilitation and long-term potentiation are normal at synapses in hippocampal slices from P2X3KO. However, long-term depression is severely impaired at CA1, CA3 and dentate gyrus synapses. Long-term depression can be partially rescued in slices treated with a protein phosphatase 1-2 A activator or by postsynaptic inhibition of calcium/calmodulin-dependent protein kinase II. Despite the deficit in hippocampal long-term depression, P2X3KO mice performed normally in water maze tests of spatial learning, suggesting that long-term depression is not critical for this type of hippocampus-dependent learning and memory.     

Astrocytes mediate in vivo cholinergic-induced synaptic plasticity

Long-term potentiation (LTP) of synaptic transmission represents the cellular basis of learning and memory. Astrocytes have been shown to regulate synaptic transmission and plasticity. However, their involvement in specific physiological processes that induce LTP in vivo remains unknown. Here we show that in vivo cholinergic activity evoked by sensory stimulation or electrical stimulation of the septal nucleus increases Ca²⁺ in hippocampal astrocytes and induces LTP of CA3-CA1 synapses, which requires cholinergic muscarinic (mAChR) and metabotropic glutamate receptor (mGluR) activation. Stimulation of cholinergic pathways in hippocampal slices evokes astrocyte Ca²⁺ elevations, postsynaptic depolarizations of CA1 pyramidal neurons, and LTP of transmitter release at single CA3-CA1 synapses. Like in vivo, these effects are mediated by mAChRs, and this cholinergic-induced LTP (c-LTP) also involves mGluR activation. Astrocyte Ca²⁺ elevations and LTP are absent in IP₃R2 knock-out mice. Downregulating astrocyte Ca²⁺ signal by loading astrocytes with BAPTA or GDPβS also prevents LTP, which is restored by simultaneous astrocyte Ca²⁺ uncaging and postsynaptic depolarization. Therefore, cholinergic-induced LTP requires astrocyte Ca²⁺ elevations, which stimulate astrocyte glutamate release that activates mGluRs. The cholinergic-induced LTP results from the temporal coincidence of the postsynaptic activity and the astrocyte Ca²⁺ signal simultaneously evoked by cholinergic activity. Therefore, the astrocyte Ca²⁺ signal is necessary for cholinergic-induced synaptic plasticity, indicating that astrocytes are directly involved in brain storage information.     

The effect of prolonged hypothermia on the structure and function of the neurons of surviving slices of guinea pig and suslik hippocampus]

Hippocampi of the guinea pig and ground squirrel were used to examine the neuronal structure of CA1 and CA3 field in surviving slices incubated in warm chamber to rehabilitate the functional activity; then the slices were subjected to a prolonged cooling under zero temperatures. It was shown that the structure of the guinea pig hippocampal slices, first of all of the neuronal cytoskeleton, was disturbed at 1.5 h warm incubation. A further 1 h cooling in a refrigerator resulted in a significant swelling of synapses, dendrites and their organelles. In 24 h the structure was severely damaged. It was determined for the first time that the ground squirrel hippocampal slices revealed in the same conditions a high tolerance to both warming and long-term cooling. Even after 5 days in a refrigerator the neurons and dendrites could be clearly seen in slices, nearly intact ultrastructure of their cytoskeleton, synapses and organelles was preserved. A correlation was found between the structural integrity and the functional activity of hippocampal slices. It was suggested that the cytoskeleton may play a crucial role in maintaining the neuronal structure under conditions of both heating and cooling of the hippocampal slices. These data are useful to further elaborating appropriate methods for prolonging the term of incubation and to neurobiological investigation of surviving brain slices.     

Transgenic mice expressing dominant-negative activin receptor IB in forebrain neurons reveal novel functions of activin at glutamatergic synapses

The transforming growth factor beta family member activin is an important regulator of development and tissue repair. It is strongly up-regulated after acute injury to the adult brain, and application of exogenous activin protects neurons in several lesion models. To explore the role of endogenous activin in the normal and acutely damaged brain, we generated transgenic mice expressing a dominant-negative activin receptor IB (dnActRIB) mutant in forebrain neurons. The functionality of the transgene was verified in vivo. Hippocampal neurons from dnActRIB mice were significantly more vulnerable to intracerebroventricular injection of the excitotoxin kainic acid than those from control littermates, indicating a crucial role of endogenous activin in the rescue of neurons from excitotoxic insult. Because dnActRIB is only expressed in neurons, but not in glial cells, activin affords protection at least in part through a direct action on endangered neurons. Unexpectedly, the transgenic mice also revealed a prominent novel role of activin in glutamatergic neurotransmission in the intact adult brain. Electrophysiologic examination of excitatory synapses onto CA1 pyramidal cells in hippocampal slices of dnActRIB mice showed a reduced NMDA current response, which was associated with impaired long term potentiation. This is the first demonstration that activin receptor signaling is essential to optimize the performance of neuronal circuits in the mature brain under physiological conditions.     

Altered presynaptic vesicle release and cycling during mGluR-dependent LTD

The site of modification of synaptic transmission during long-term plasticity in the mammalian hippocampus remains controversial. Here we used a fluorescent marker of presynaptic activity, FM 1-43, to directly image presynaptic function during metabotropic glutamate receptor-dependent long-term depression (mGluR-LTD) at CA3-CA1 excitatory synapses in acute hippocampal slices. We found a significant decrease in the rate of FM 1-43 release in response to synaptic stimulation following induction of mGluR-LTD, providing direct evidence for altered presynaptic function. Moreover, we found that mGluR-LTD causes several changes in FM dye release properties that are consistent with a change in the mode of vesicle cycling, possibly involving a switch from a full fusion mode of release to a "kiss-and-run" mode of release through the transient opening of a fusion pore.     

Leptin Regulation of Synaptic Function at Hippocampal TA-CA1 and SC-CA1 Synapses: Implications for Health and Disease

Growing evidence indicates that the endocrine hormone leptin regulates hippocampal synaptic function in addition to its established role as a hypothalamic satiety signal. Indeed, numerous studies show that leptin facilitates the cellular events that underlie hippocampal learning and memory including activity-dependent synaptic plasticity and glutamate receptor trafficking, indicating that leptin may be a potential cognitive enhancer. Although there has been extensive investigation into the modulatory role of leptin at hippocampal Schaffer collateral (SC)-CA1 synapses, recent evidence indicates that leptin also potently regulates excitatory synaptic transmission at the anatomically distinct temporoammonic (TA) input to hippocampal CA1 neurons. The cellular mechanisms underlying activity-dependent synaptic plasticity at TA-CA1 synapses differ from those at SC-CA1 synapses and the TA input is implicated in spatial and episodic memory formation. Furthermore, the TA input is an early target for neurodegeneration in Alzheimer’s disease (AD) and aberrant leptin function is linked to AD. Here, we review the evidence that leptin regulates hippocampal synaptic function at both SC- and TA-CA1 synapses and discuss the consequences for neurodegenerative disorders like AD.

     

Wnt-7a modulates the synaptic vesicle cycle and synaptic transmission in hippocampal neurons

Wnt signaling is essential for neuronal development and the maintenance of the developing nervous system. Recent studies indicated that Wnt signaling modulates long term potentiation in adult hippocampal slices. We report here that different Wnt ligands are present in hippocampal neurons of rat embryo and adult rat, including Wnt-4, -5a, -7a, and -11. Wnt-7a acts as a canonical Wnt ligand in rat hippocampal neurons, stimulates clustering of presynaptic proteins, and induces recycling and exocytosis of synaptic vesicles as studied by FM dyes. Wnt-3a has a moderate effect on recycling of synaptic vesicles, and no effect of Wnt-1 and Wnt-5a was detected. Electrophysiological analysis on adult rat hippocampal slices indicates that Wnt-7a, but not Wnt-5a, increases neurotransmitter release in CA3-CA1 synapses by decreasing paired pulse facilitation and increasing the miniature excitatory post-synaptic currents frequency. These results indicate that the presynaptic function of rat hippocampal neurons is modulated by the canonical Wnt signaling.     

Regulation of hippocampal synaptic function by the metabolic hormone leptin: Implications for health and disease

Significant advances have been made in our understanding of the hormone, leptin and its CNS actions in recent years. It is now evident that leptin has a multitude of brain functions, that extend beyond its established role in the hypothalamic control of energy balance. Additional brain regions including the hippocampus are important targets for leptin, with a high density of leptin receptors (LepRs) expressed in specific hippocampal regions and localised to CA1 synapses. Extensive evidence indicates that leptin has pro-cognitive actions, as it rapidly modifies synaptic efficacy at excitatory Schaffer collateral (SC)-CA1 and temporoammonic (TA)-CA1 synapses and enhances performance in hippocampal-dependent memory tasks. There is a functional decline in hippocampal responsiveness to leptin with age, with significant reductions in the modulatory effects of leptin at SC-CA1 and TA-CA1 synapses in aged, compared to adult hippocampus. As leptin has pro-cognitive effects, this decline in leptin sensitivity is likely to have negative consequences for cognitive function during the aging process. Here we review how evaluation of the hippocampal actions of leptin has improved our knowledge of the regulatory brain functions of leptin in health and provided significant insight into the impact of leptin in age-related neurodegenerative disorders linked to cognitive decline.     

Role of the adenylate cyclase system in cholinergic modulation of synaptic transmission in the hippocampus

Involvement of the adenylate cyclase system in cholinergic modulation of synaptic transmission was investigated in area CA1 in rat hippocampal slices. Microiontophoretic application of acetylcholine as well as addition of carbachol to the superfusate or of tolbutamide (a cAMP-dependent protein kinase inhibitor) depressed transmission at synapses formed by Schaffer collaterals and commissural fibers with dendrites of pyramidal cells belonging to hippocampal area CA1. Both numbers of free quanta of neurotransmitter and the likelihood of transmitter release decreased following carbachol action. Atropine suppressed the inhibitory action of carbachol on synaptic transmission. Dibutyryl cAMP and forskolin increased the amplitude of synaptic potentials and suppressed, either partially or in full, the inhibitory effects of cholinomimetics on synaptic potentials. It was concluded that cholinomimetics and activators of the adenylate cyclase system exert opposing effects on neurotransmission at synapses formed between Schaffer collaterals/commissural fibers and dendrites of pyramidal neurons belonging to hippocampal area CA1.Institute of Biophysics, Academy of Sciences of the USSR, Pushchino. Translated from Neirofiziologiya, Vol. 21, No. 4, pp. 435–442, July–August, 1989.     

Role of different voltage-gated Ca2+ channels in cortical spreading depression: specific requirement of P/Q-type Ca2+ channels

Gain-of-function mutations in CaV 2.1 (P/Q-type) Ca2+ channels cause familial hemiplegic migraine type 1 (FHM1), a subtype of migraine with aura. Knockin (KI) mice carrying FHM1 mutations show increased neuronal P/Q-type current and facilitation of induction and propagation of cortical spreading depression (CSD), the phenomenon that underlies migraine aura and may activate migraine headache mechanisms. We recently studied cortical neurotransmission in neuronal microcultures and brain slices of FHM1 KI mice, and showed (1) gain-of-function of excitatory neurotransmission, due to increased action potential-evoked Ca2+ influx and increased probability of glutamate release at pyramidal cell synapses, but unaltered inhibitory neurotransmission at fast-spiking interneuron synapses, and (2) a causative link between enhanced glutamate release and facilitation of CSD induced by brief pulses of high K+ in cortical slices. Here, we show that after blockade of either the P/Q-type Ca2+ channels or the NMDA receptors, CSD cannot be induced in wild-type mouse cortical slices. In contrast, blockade of N- or R-type Ca2+ channels has only a small inhibitory effect on CSD threshold and velocity of propagation. Our findings support a model in which Ca2+ influx through presynaptic P/Q-type Ca2+ channels with consequent release of glutamate from recurrent cortical pyramidal cell synapses and activation of NMDA receptors are required for initiation and propagation of the CSD involved in migraine.     

Leptin reverses long-term potentiation at hippocampal CA1 synapses

The hormone leptin crosses the blood brain barrier and regulates numerous neuronal functions, including hippocampal synaptic plasticity. Here we show that application of leptin resulted in the reversal of long-term potentiation (LTP) at hippocampal CA1 synapses. The ability of leptin to depotentiate CA1 synapses was concentration-dependent and it displayed a distinct temporal profile. Leptin-induced depotentiation was not associated with any change in the paired pulse facilitation ratio or the coefficient of variance, indicating a post-synaptic locus of expression. Moreover, the synaptic activation of NMDA receptors was required for leptin-induced depotentiation as the effects of leptin were blocked by the competitive NMDA receptor antagonist, D-aminophosphovaleric acid (D-AP5). The signaling mechanisms underlying leptin-induced depotentiation involved activation of the calcium/calmodulin-dependent protein phosphatase, calcineurin, but were independent of c- jun NH₂ terminal kinase. Furthermore, leptin-induced depotentiation was accompanied by a reduction in α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor rectification indicating that loss of glutamate receptor 2 (GluR2)-lacking AMPA receptors underlies this process. These data indicate that leptin reverses hippocampal LTP via a process involving calcineurin-dependent internalization of GluR2-lacking AMPA receptors which further highlights the key role for this hormone in regulating hippocampal synaptic plasticity and neuronal development.     

Comparing fluctuations of synaptic responses mediated via AMPA and NMDA receptor channels--implications for synaptic plasticity

Glutamate-releasing synapses are essential in fast neuronal signalling. Plasticity at these synapses is important for learning and memory as well as for the activity-dependent control of neuronal development. We have evaluated the trial-to-trial fluctuations of excitatory postsynaptic currents mediated by glutamate receptors of the AMPA and NMDA types in CA1 pyramidal cells. By using the whole cell patch clamp technique in brain slices from young rats, we have demonstrated that the relative variability of AMPA and NMDA receptor mediated responses, expressed as the coefficient of variation, is similar for these two types of responses [Brain Res. 800 (1998) 253-259]. The present paper summarizes and discusses these results in relation to current theories on hippocampal synaptic plasticity, especially with regard to the ideas of glutamate spillover and silent synapses. Our finding of a correspondence between AMPA and NMDA responses with respect to fluctuations is compatible with our previous finding of equal relative changes of the two during activity induced synaptic plasticity. However, the results argue against the glutamate spillover model according to which the effect of glutamate--and hence the induction of plasticity--may spread unspecifically between synapses. But how can silent synapses become functional if no spread of glutamate occurs and no initial signal is present to trigger the functionalization? Is it necessary that NMDA responses are present at these synapses, which are then silent merely with respect to AMPA receptors, or do other alternatives exist? Our discussion aims to elucidate these questions.     

TRPV1 channels mediate long-term depression at synapses on hippocampal interneurons

TRPV1 receptors have classically been defined as heat-sensitive, ligand-gated, nonselective cation channels that integrate nociceptive stimuli in sensory neurons. TRPV1 receptors have also been identified in the brain, but their physiological role is poorly understood. Here we report that TRPV1 channel activation is necessary and sufficient to trigger long-term synaptic depression (LTD). Excitatory synapses onto hippocampal interneurons were depressed by either capsaicin, a potent TRPV1 channel activator, or the endogenously released eicosanoid, 12-(S)-HPETE, whereas neighboring excitatory synapses onto CA1 pyramidal cells were unaffected. TRPV1 receptor antagonists also prevented interneuron LTD. In brain slices from TRPV1-/- mice, LTD was absent, and neither capsaicin nor 12-(S)-HPETE elicited synaptic depression. Our results suggest that, in the hippocampus, TRPV1 receptor activation selectively modifies synapses onto interneurons. Like other forms of hippocampal synaptic plasticity, TRPV1-mediated LTD may have a role in long-term changes in physiological and pathological circuit behavior during learning and epileptic activity.     

Extracellular Ca2+ depletion contributes to fast activity-dependent modulation of synaptic transmission in the brain

Synaptic activation is associated with rapid changes in intracellular Ca(2+), while the extracellular Ca(2+) level is generally assumed to be constant. Here, using a novel optical method to measure changes in extracellular Ca(2+) at high spatial and temporal resolution, we find that brief trains of synaptic transmission in hippocampal area CA1 induce transient depletion of extracellular Ca(2+). We show that this depletion, which depends on postsynaptic NMDA receptor activation, decreases the Ca(2+) available to enter individual presynaptic boutons of CA3 pyramidal cells. This in turn reduces the probability of consecutive synaptic releases at CA3-CA1 synapses and therefore contributes to short-term paired-pulse depression of minimal responses. This activity-dependent depletion of extracellular Ca(2+) represents a novel form of fast retrograde synaptic signaling that can modulate glutamatergic information transfer in the brain.     

Effects of lipopolysaccharide on 56Fe-particle radiation-induced impairment of synaptic plasticity in the mouse hippocampus

Space radiation, including high-mass, high-Z, high-energy particles (HZE; e.g. (56)Fe), represents a significant health risk for astronauts, and the central nervous system (CNS) may be a vulnerable target. HZE-particle radiation may directly affect neuronal function, or during immunological challenge, it may alter immune system-to-CNS communication. To test these hypotheses, we exposed mice to accelerated iron particles ((56)Fe; 600 MeV/nucleon; 1, 2, 4 Gy; brain only) and 1 month later prepared hippocampal slices to measure the effects of radiation on neurotransmission and synaptic plasticity in CA1 neurons. In a model of immune system-to-CNS communication, these electrophysiological parameters were measured in irradiated mice additionally challenged with the peripheral immunological stressor lipopolysaccharide (LPS) injected intraperitoneally 4 h before the slice preparation. Exposure to (56)Fe particles alone increased dendritic excitability and inhibited plasticity. In control mice (0 Gy), LPS treatment also inhibited synaptic plasticity. Paradoxically, in mice exposed to 2 Gy, the LPS treatment restored synaptic plasticity to levels similar to those found in controls (0 Gy, no LPS). Our results indicate that HZE-particle radiation alters normal electrophysiological properties of the CNS and the hippocampal response to LPS.     

(56)Fe-particle radiation reduces neuronal output and attenuates lipopolysaccharide-induced inhibition of long-term potentiation in the mouse hippocampus

Exposure to space radiation consisting of high-energy charged (56)Fe particles represents a significant health risk for astronauts. (56)Fe-particle radiation affects the synaptic plasticity of the hippocampus and alters its response to the experimental immunological stressor lipopolysaccharide (LPS). We previously showed in mice that 1 month after exposure to (56)Fe-particle radiation, the LPS-induced inhibition of hippocampal long-term potentiation (LTP) was significantly attenuated, resulting in seemingly normal LTP. In the current study, we investigated this phenomenon further at longer times postirradiation. We exposed mice to accelerated iron particles ((56)Fe; 600 MeV/nucleon; 1, 2, 4 Gy; brain only), and 1, 3, 6 or 12 months postirradiation we administered LPS. Four hours after the intraperitoneal LPS injection, we prepared hippocampal slices to measure synaptic excitability and plasticity between CA3-CA1 neurons. In unexposed mice, we confirmed that LPS inhibited LTP at all times. However, in mice exposed to 2 Gy, the LPS-induced LTP inhibition was attenuated and reversed to control values. Such reversal was evident at 1 and 3 months but not 6 and 12 months postirradiation. In addition, at 6 and 12 months postirradiation, we observed inhibition of population spike (PS) amplitudes at 4 Gy that correlated with decrements in dendritic potentials, suggesting synaptic damage. Our data show that (56)Fe-particle radiation affects the response of the hippocampus to an immunological stressor and that the alterations progress over time.     

C-Type natriuretic peptide modulates pre- and postsynaptic properties in hippocampal area CA1 in vitro

The cGMP producing natriuretic peptide receptor B (NPR-B) and its ligand C-type natriuretic peptide (CNP) are widely distributed in the brain and are highly expressed in the hippocampal regions CA1-CA3. To date only limited functional data is available concerning the physiological effects of the peptide hormone in the hippocampus. Therefore, we were interested in how bath application of the peptide hormone might influence synaptic plasticity following high frequency stimulation (HFS). We found that CNP application decreased the population spike (PS) amplitude after HFS, thereby affecting long-term potentiation (LTP) in acute hippocampal slices. To investigate the molecular consequences of CNP application leading to a decrease in PS amplitude, we further analyzed the impact of the hormone on the number of presynaptic synapsin I clusters and number of postsynaptic AMPA receptor subunit GluR1 clusters as well as their co-localization in a primary hippocampal cell culture system. The observed pre-and postsynaptic effects after CNP stimulation of the cGMP pathway in hippocampal cell cultures may underlie the effect of the peptide hormone on LTP.