Corticosterone and stress reduce synaptic potentiation in mouse hippocampal slices with mild stimulation

Elevation of circulating corticosterone levels, either through exogenous administration of the hormone or following stress exposure, is known to reduce hippocampal synaptic potentiation in rodents. It is presently debated whether this reduction is due to activation of hippocampal glucocorticoid receptors or is primarily caused in other brain structures projecting to the hippocampus. To address this issue, we examined whether synaptic potentiation in hippocampal slices from mice with low basal corticosterone levels was altered 1-4 h after a brief in vitro administration of 100 nM corticosterone. Population spike and field excitatory postsynaptic potential (fEPSP) were recorded in the cell and dendritic layers, respectively, of the CA1 area, in response to Schaffer collateral/commissural fiber stimulation. Basal characteristics of the stimulus-response relationship were not affected by corticosterone treatment, except that after corticosterone treatment the maximal fEPSP slope was reduced while the excitability ratio was increased. For studies on potentiation of the fEPSP and population spike, stimulus intensities were chosen to evoke half maximal responses before potentiation; this intensity was significantly lower for the fEPSP than for the population spike. Primed burst potentiation of the fEPSP but not population spike was significantly attenuated after corticosterone treatment. When using a more rigorous stimulation paradigm, i.e. theta burst potentiation, synaptic potentiation was not affected by corticosterone. Raising corticosterone levels in mice by exposure to a psychosocial stressor led to comparable results in subsequent in vitro experiments; stress reduced primed burst potentiation only of the fEPSP. These data support that corticosterone affects synaptic potentiation in the mouse via direct activation of hippocampal glucocorticoid receptors but only when using mild stimulation conditions.

Dependence on morphine impairs the induction of long-term potentiation in the CA1 region of rat hippocampal slices

The effect of chronic morphine treatment on hippocampal CA1-long-term potentiation (LTP) was examined in vitro. The field excitatory postsynaptic potential (fEPSP) was recorded from stratum radiatum of area CA1 following stimulation of Schaffer collaterals in slices taken from control and morphine-dependent rats. To induce LTP, a 100-Hz primed burst stimulation (PBs) was used. Slices from rats exposed to chronic morphine showed no effect on baseline synaptic responses. Slices from control rats or rats exposed to chronic morphine maintained in ACSF with either morphine or naloxone also had no effect on baseline synaptic responses. Control slices perfused with medium containing either morphine or naloxone as well as both drugs exhibited hippocampal CA1 LTP. Similarly, slices from morphine-dependent rats maintained in ACSF with either naloxone or just morphine free ACSF also exhibited hippocampal CA1 LTP. However, slices from morphine-dependent rats maintained in ACSF with morphine significantly attenuated hippocampal CA1 LTP. These findings suggest that hippocampal CA1-LTP can still be achieved in slices from morphine-dependent rats exhibiting morphine withdrawal through mechanisms that may be inhibited by opiate exposure. Such studies can be helpful in understanding the neurophysiological substrate of memory deficits seen in opiate addicts.

Redistribution of syntaxin mRNA in neuronal cell bodies regulates protein expression and transport during synapse formation and long-term synaptic plasticity

Syntaxin has an important role in regulating vesicle docking and fusion essential for neurotransmitter release. Here, we demonstrate that the distribution of syntaxin mRNA in cell bodies of sensory neurons (SNs) of Aplysia maintained in cell culture is affected by synapse formation, synapse stabilization, and long-term facilitation (LTF) produced by 5-HT. The distribution of the mRNA in turn regulates expression and axonal transport of the protein. Syntaxin mRNA and protein accumulated at the axon hillock of SNs during the initial phase of synapse formation. Significant numbers of granules containing syntaxin were detected in the SN axon. When synaptic strength was stable, both mRNA and protein were targeted away from the axon hillock, and the number of syntaxin granules in the SN axon was reduced. Dramatic increases in mRNA and protein accumulation at the axon hillock and number of syntaxin granules in the SN axon were produced when cultures with stable connections were treated with 5-HT that evoked LTF. Anisomycin (protein synthesis inhibitor) or KT5720 (protein kinase A inhibitor) blocked LTF, accumulation of syntaxin mRNA and protein at the axon hillock, and the increase in syntaxin granules in SN axons. The results indicate that without significant effects on overall mRNA expression, both target interaction and 5-HT via activation of protein kinase A pathway regulate expression of syntaxin and its packaging for transport into axons by influencing the distribution of its mRNA in the SN cell body.

On the role of nitric oxide in hippocampal long-term potentiation

Nitric oxide (NO) functions in several types of synaptic plasticity, including hippocampal long-term potentiation (LTP), in which it may serve as a retrograde messenger after postsynaptic NMDA receptor activation. In accordance with a prediction of this hypothesis, and with previous findings using guinea pig tissue, exogenous NO, when paired with a short tetanus (ST) to afferent fibers, generated a stable NMDA receptor-independent potentiation of rat CA1 hippocampal synaptic transmission that occluded LTP. Contrary to predictions, however, the pairing-induced potentiation was abolished in the presence of NO synthase inhibitors, indicating that endogenous NO is required for exogenous NO to facilitate LTP. Periodic application of NO while endogenous NO synthesis was blocked indicated that a tonic low level is necessary on both sides of the NO-ST pairing for the plasticity to occur. A similar dependence on tonic NO seems to extend to LTP, because application of an NO synthase inhibitor 5 min after tetanic stimulation blocked LTP as effectively as adding it beforehand. The posttetanus time window during which NO operated was restricted to <15 min. Inhibition of the guanylyl cyclase-coupled NO receptor indicated that the potentiation resulting from NO-ST pairing and the NO signal transduction pathway during early LTP are both through cGMP. We conclude that NO does not function simply as an acute signaling molecule in LTP induction but has an equally important role outside this phase. The results resonate with observations concerning the role of the hippocampal NO-cGMP pathway in certain types of learning behavior.

The role of cortical cholinergic pre- and post-synaptic receptors in taste memory formation

A number of studies have implicated cholinergic activity in the mediation of learning and memory processes. However, the specific role of muscarinic receptors in memory formation mechanisms is less known. The aim of the present study is to evaluate the effects of muscarinic antagonist M2 presynaptic receptor, AFDX-116 (0.5mM) and M1 and M3 post-synaptic receptor pirenzepine (100mM), as well as a non-selective muscarinic antagonist, scopolamine (136mM), in the insular cortex (IC) during acquisition and retrieval of conditioned taste aversion (CTA). In addition, we evaluate the effects of those antagonists in cortical ACh release by in vivo microdialysis and the effects on the induction of in vivo LTP in the BLA-IC projection. The results showed that the cortical microinjections of scopolamine and pirenzepine, but not AFDX-116, produced significant disruption in the acquisition of CTA, without effects during retrieval. Microinjections of scopolamine and AFDX-116 produced significant cortical ACh release, while infusions of pirenzepine did not produce any release. Application of scopolamine and pirenzepine diminished induction of LTP in the BLA-IC projection, but not AFDX-116, as compared with vehicle. The induction of BLA-CI LTP seems to be modulated by post-synaptic muscarinic acetylcholine receptors and not by pre-synaptic muscarinic receptors. These results suggest a differential involvement of cholinergic receptors during acquisition and retrieval of aversive memory formation, as well as a differential role of muscarinic receptors in the biochemical and electrophysiological processes that may underlay aversive memory.

Cortico-striatal synaptic plasticity in endothelial nitric oxide synthase deficient mice

Nitric oxide (NO) is a retrograde messenger involved in the processes of learning and memory. The role of the endothelial isoform of nitric oxide synthase (eNOS) in striatal synaptic plasticity was investigated in eNOS-deficient (eNOS(-/-)) and wild type (WT) mice. Tetanic stimulation of cortical afferents in WT mice evoked either long-term potentiation (LTP), or long-term depression (LTD) of cortico-striatal transmission. Both these plasticity related phenomena were NMDA-receptor-dependent; LTD was blocked by sulpiride, a dopamine D2-receptor antagonist. LTP occurrence in slices from eNOS(-/-) mice was significantly reduced when compared with WT mice. The NOS inhibitor NL-ARG reduced the occurrence of LTP and increased the occurrence of LTD in WT mice, resembling the balance of LTP/LTD in eNOS(-/-) mice. Impairment of NO-synthesis thus shifts striatal plasticity towards LTD. This indicates a possible involvement of eNOS from endothelia in neuronal modulation.

Addictive drugs and stress trigger a common change at VTA synapses

In this issue of Neuron, Saal et al. find that exposure to any of five addictive drugs or exposure to a brief stressor produces a shared cellular modification of excitatory synapses in the ventral tegmental area (VTA). This common response may represent a starting point for dissecting early changes that underlie addiction.

Inhibition of long-term potentiation by interleukin-8: implications for human immunodeficiency virus-1-associated dementia

Human immunodeficiency virus type 1 (HIV-1)-infected mononuclear phagocytes (MP; brain macrophages and microglia) secrete a number of toxic factors that affect the pathogenesis of HIV-1-associated dementia (HAD). The identification and relative role of each MP toxin for neuronal dysfunction during HAD are not well understood. Interleukin-8 (IL-8), a CXC chemokine involved in leukocyte activation and chemotaxis, is constitutively produced by MP, and elevated levels of IL-8 mRNA were detected in the brains of patients with HIV-1 encephalitis (HIVE) by both ribonuclease protection assays and real-time PCR. To determine the role that IL-8 might play in the neuronal dysfunction in HAD, we studied its effect on synaptic transmission and plasticity in the CA1 region of hippocampus, the seat of learning and memory. Bath application of IL-8 (50 ng/ml) to rat hippocampal slices had no effect on basal synaptic transmission. However, IL-8 was shown to inhibit long-term potentiation (LTP) in a concentration-dependent manner. In control and IL-8-treated slices, the LTP magnitudes were 167.8% +/- 11.9% (mean +/- SE; n = 17) and 122.2% +/- 16.2% of basal levels (n = 13), respectively. These differences were statistically significant (P < 0.05). Preincubation of hippocampal slices with a monoclonal CXCR2 antibody (2 microg/ml) but not control IgG (2 microg/ml) blocked IL-8-induced inhibition of LTP. The expression of CXCR2 receptors in the CA1 region was shown by Western blot assays. The induction of IL-8 in HAD, its inhibition of LTP, and the expression of its receptor, CXCR2, in the hippocampus all suggest that it plays a role in the cognitive dysfunction associated with HAD.

Effects of arginine-vasopressin (AVP) on long-term potentiation in intact anesthetized rats

We studied the effects of the neuropeptide arginine-vasopressin (AVP) on the long-term potentiation (LTP) paradigm in the dentate gyrus (DG) of urethane intact anesthetized rats. Intracerebroventricular injection of 1 microg of the hormone in 1 microl of physiological solution 3 min before tetanization, produced a significant increase in both components of the perforant path-evoked potentials (EP) in the DG. The effects were already evident 1 min after tetanization. Amplitude of the EPs increased continuously for the 2h of recording time, reaching values 100% above baseline, reference levels. In contrast, in previous in vitro studies, enhancement of LTP with AVP appeared only after 15 min of exposure of the hippocampal slice to the hormone, increased EPSPs were no higher than 50% from baseline, reached a plateau at 40 min decreasing slowly thereafter. Not only quantitative but also qualitative differences can be observed between in vitro and in vivo intact preparations in response to identical hormones. This study emphasizes the importance of hormone neurotransmitter interactions in determining electrophysiological characteristics of response to AVP.

Dependence on morphine leads to a prominent sharing among the different mechanisms of long-term potentiation in the CA1 region of rat hippocampus

Here, we examined chronic exposure to morphine to determine if this treatment shifted LTP mechanism in the CA1 field in vitro. Long-term potentiation (LTP) of population spikes induced by a 200 Hz theta pattern primed bursts (PBs) stimulation. Verapamil was used to isolate NMDA-dependent LTP. In control slices, a 200 Hz tetanus induced a compound potentiation, consisted of two pharmacologically separable components: nmdaLTP and vdccLTP. LTP in slices taken from morphine dependent rats was completely abolished by either APV or verapamil. These data suggest that morphine dependence in rats does not interfere with the induction and maintenance of hippocampal CA1 LTP. While in control rats both NMDA and voltage-dependent Ca(2+) channel (VDCC) antagonists must have been used concurrently to prevent the induction of LTP, in morphine-dependent rats, each of the antagonist could prevent the LTP induction suggesting a tighter coupling between these two calcium influx regulating processes.

PB-2, a polysaccharide fraction from lichen Flavoparmelia baltimorensis, peripherally promotes the induction of long-term potentiation in the rat dentate gyrus in vivo

We have previously found that oral or intravenous administration of PC-2, a polysaccharide fraction purified from extracts of lichen Flavoparmelia caperata, facilitates the induction of long-term potentiation (LTP) in the dentate gyrus of anesthetized rats. PC-2 could be useful in the development of therapeutic drugs for senile dementia. However, it has been very difficult to obtain the material Flavoparmelia caperata because of its scarcity. In the present study, we therefore investigated whether PB-2, a polysaccharide fraction from another lichen Flavoparmelia baltimorensis, has similar biological effects. Oral administration of PB-2 (100-200 mg/kg) did not affect basal evoked potentials, but significantly promoted the induction of LTP following tetanic stimulation (30 pulses at 60 Hz) in the dentate gyrus of anesthetized rats. Intravenous injection of PB-2 (1-5 mg/kg) was also effective in promoting the induction of LTP, but intracerebroventricular injection of PB-2 (1-2 mg/brain) was ineffective. PB-2, as well as PC-2, should be valuable in identifying factors that promote synaptic plasticity in the hippocampus.

Kindling suppresses primed-burst-induced long-term potentiation in hippocampal CA1

In a previous study, we showed that partial hippocampal kindling in rats, a model of temporal lobe epilepsy, reduced the efficacy of presynaptic GABA(B) receptors in the CA1 area of hippocampal slices. In this study, we show that long-term potentiation (LTP) induced by theta-frequency primed bursts was suppressed in kindled as compared to control rats, but not in the presence of the GABA(B) receptor antagonists CGP35348 or CGP55845A. This is original evidence that LTP is suppressed by pathophysiological downregulation of GABA(B) autoreceptors. Control of postsynaptic inhibition by presynaptic GABA(B) receptors may provide a compensatory mechanism for controlling paroxysmal activity, with a side effect of blocking synaptic plasticity.

Effects of lidocaine reversible inactivation of the median raphe nucleus on long-term potentiation and recurrent inhibition in the dentate gyrus of rat hippocampus

Considering the fact that median raphe nucleus (MRN) constitutes one of the inputs of the hippocampus, the effects of reversible inactivation of MRN on long-term potentiation (LTP) and recurrent inhibition in the dentate gyrus (DG) of rat hippocampus, in vivo, were examined. Rats were anesthetized with urethane (1.5 g/kg, i.p.). MRN was temporarily suppressed by intra-MRN injection of lidocaine (0.5 microl, 2%). For LTP induction, eight episodes of high frequency stimuli (100 Hz) were delivered to the perforant path (PP), each consisting of 10 stimuli at 100 Hz. Population spikes (PS) and population excitatory post synaptic potentials ((P)EPSP) in DG were recorded 10 min before, and 5, 10, 20, 40, 60 and 120 min after tetanization. MRN inactivation itself had no effect on the amplitude of baseline responses. The PS amplitude and (P)EPSP slope in rats, injected with intra-MRN lidocaine, 5 min before tetanization, were not different from the control group. However, at 120 min PS amplitude was significantly higher than control. Lidocaine injection 5 min after tetanic stimuli caused a significant decrease in PS amplitude (10, 20 and 60 min) and (P)EPSP slope (20 and 40 min) after tetanization. The data showed that inactivation of MRN has no effect on LTP induction in the DG of hippocampus but it does affect its maintenance, and this effect depends on the pre- or post-tetanic inactivation. In the last part of this study, in order to investigate the effect of MRN on the efficacy of recurrent inhibition in the perforant-dentate synapses, paired pulse was applied to the PP at 10 and 20 ms interpulse intervals. Inactivation of MRN increased the amount of recurrent inhibition in the DG with 20 ms interpulse interval. This observation indicates that MRN inhibits the recurrent inhibition mechanism, which is in accordance with the suggested role of MRN neurons on inhibition of hippocampal GABAergic interneurons.

Activation of a tyrosine kinase-MAPK cascade enhances the induction of long-term synaptic facilitation and long-term memory in Aplysia

Tyrosine kinases have been implicated in cellular processes thought to underlie learning and memory. Here we show that tyrosine kinases play a direct role in long-term synaptic facilitation (LTF) and long-term memory (LTM) for sensitization in Aplysia. Tyrosine kinase activity is required for serotonin-induced LTF of sensorimotor (SN-MN) synapses, and enhancement of endogenous tyrosine kinase activity facilitates the induction of LTF. These effects are mediated, at least in part, through mitogen-activated protein kinase (MAPK) activation and are blocked by transcriptional and translational inhibitors. Moreover, brain-derived neurotrophic factor (BDNF) also enhances the induction of LTF in a MAPK-dependent fashion. Finally, activation of endogenous tyrosine kinases enhances the induction of long-term memory for sensitization, and this enhancement also requires MAPK activation. Thus, tyrosine kinases, acting through MAPK, play a pivotal role in LTF and LTM formation.

Regulation of depotentiation and long-term potentiation in the dentate gyrus of freely moving rats by dopamine D2-like receptors

Dopamine receptors are significantly involved in hippocampus-based cognitive processes. Whereas the involvement of D1-like receptors in hippocampal plasticity has been described, the role of D2-like receptors remains to be clarified. Therefore, we investigated the contribution of D2-like receptors to synaptic transmission, long-term potentiation (LTP) and depotentiation in the dentate gyrus of freely moving rats. Male Wistar rats underwent chronic implantation of a recording electrode in the granule cell layer, a stimulating electrode in the medial perforant path and a cannula in the ipsilateral cerebral ventricle (to enable drug administration). The D2-like receptor agonists quinpirole and noraporphine dose-dependently inhibited basal synaptic transmission. Agonist priming of D2-like receptors with a drug concentration which had no effect on synaptic transmission inhibited depotentiation but did not affect LTP. The agonist effects on depotentiation were prevented by the D2-like antagonist remoxipride. Remoxipride itself did not influence basal synaptic transmission or depotentiation. Interestingly, 'weak' LTP (<4 h) but not 'strong' LTP (>24 h) was inhibited by prior application of remoxipride. These results suggest a specific role for dopamine D2-like receptors in the regulation of both depotentiation and LTP in vivo and offer an important and novel insight as to the involvement of these receptors in processes related to learning and memory.

Early exposure to common anesthetic agents causes widespread neurodegeneration in the developing rat brain and persistent learning deficits

Recently it was demonstrated that exposure of the developing brain during the period of synaptogenesis to drugs that block NMDA glutamate receptors or drugs that potentiate GABA(A) receptors can trigger widespread apoptotic neurodegeneration. All currently used general anesthetic agents have either NMDA receptor-blocking or GABA(A) receptor-enhancing properties. To induce or maintain a surgical plane of anesthesia, it is common practice in pediatric or obstetrical medicine to use agents from these two classes in combination. Therefore, the question arises whether this practice entails significant risk of inducing apoptotic neurodegeneration in the developing human brain. To begin to address this problem, we have administered to 7-d-old infant rats a combination of drugs commonly used in pediatric anesthesia (midazolam, nitrous oxide, and isoflurane) in doses sufficient to maintain a surgical plane of anesthesia for 6 hr, and have observed that this causes widespread apoptotic neurodegeneration in the developing brain, deficits in hippocampal synaptic function, and persistent memory/learning impairments.

Administration of aggregated beta-amyloid peptide (25-35) induces changes in long-term potentiation in the hippocampus in vivo

Intracereroventricular administration of aggregated beta-amyloid protein fragment (25-35) (7.5 nmol/ventricle) was followed one month later by significant changes in the dynamics of long-term potentiation in the hippocampus in vivo, expressed as powerful and stable increases in the amplitude of evoked potentials. This phenomenon may be associated with oxidative stress in the hippocampus, which has previously been demonstrated in this model, and, thus, with disturbances in ion homeostasis.

Overexpression and RNA interference of Ap-cyclic AMP-response element binding protein-2, a repressor of long-term facilitation, in Aplysia kurodai sensory-to-motor synapses

cyclic AMP-response element binding protein-2 (CREB2) is a member of the CREB/transcription factor (CREB/ATF4) family. CREB2 is a transcription factor known to be involved in Aplysia long-term facilitation. To further examine the role of ApCREB2 on long-term synaptic facilitation, we isolated ApCREB2 from Aplysia kurodai in full-length cDNA library, and found that the overexpression of ApCREB2 blocked 5-hydroxytryptamine (5-HT)-induced long-term synaptic facilitation in Aplysia sensory-to-motor synapses. Furthermore, a single pulse of 5-HT, which normally induces only short-term facilitation, in the presence of ApCREB2 inhibition by RNA interference, induced long-term facilitation in Aplysia sensory-to-motor synapses. These results suggest that ApCREB2 is a functional repressor of long-term facilitation in Aplysia sensory-to-motor synapses.

A large sustained Ca2+ elevation occurs in unstimulated spines during the LTP pairing protocol but does not change synaptic strength

Synapses in the CA1 region of the hippocampus undergo bidirectional synaptic modification in response to different patterns of activity. Postsynaptic Ca2+ elevation can trigger either synaptic strengthening or weakening, depending on the properties of the local Ca2+ signal. During the pairing protocol for long-term potentiation (LTP) induction, the cell is depolarized under voltage-clamp and is given low-frequency synaptic stimulation. As an initial step toward understanding the Ca2+ dynamics during this process, we used confocal microscopy to study the Ca2+ signals in spines evoked by the depolarization itself. This depolarization activates voltage-dependent Ca2+ channels (VDCC), but whether these channels inactivate rapidly or remain functional throughout the long depolarizations used in the pairing protocol remains unknown. Cells were depolarized to 0 mV for 2-3 min. This depolarization led to a large initial elevation of Ca2+ in spines that never decayed back to resting levels. The maintained signal was close to the Kd of the low-affinity (5 microM) Ca2+ dye, Magnesium Green. We attempted to determine the functional role of this elevation, using the Ca2+-channel blocker D-890. The addition of D-890 in the internal solution produced a nearly complete abolition of the Ca2+ elevation during depolarization. Under these conditions, the NMDA conductance was normal, but LTP was almost completely blocked. This might suggest the importance of VDCC in LTP; however, we found that high concentrations of D-890 can directly inhibit calmodulin protein kinase II (CaMKII), an enzyme required for LTP induction. Thus, whereas D-890 is a useful tool for blocking postsynaptic VDCC, it cannot be used to study the contribution of these channels to plasticity. We conclude that the activation of VDCC produces a large and persistent elevation of Ca2+ in all spines, but does not produce either LTP or long-term depression (LTD) in the absence of synaptic stimulation. The possible reasons for this are discussed.

Contribution of NMDA receptor channels to the expression of LTP in the hippocampal dentate gyrus

The role of glutamatergic NMDA receptor channels (NMDARs) in the induction of long-term potentiation (LTP) has been well established. In contrast, whether or not NMDARs contribute to the expression of LTP has been an issue of debate. In this study, we investigated the contribution of NMDARs to LTP expression in the hippocampal dentate gyrus (DG) by stimulating perforant path afferents with short bursts of pulses delivered at a moderate frequency (40 Hz), instead of using the traditional protocol of a single stimulus at a low frequency (<0.1 Hz). The synaptic summation provided by the "burst" protocol enabled us to measure the NMDAR-mediated component of synaptic responses (NMDA component), defined as the NMDAR antagonist D-2-amino-5-phosphonovalerate (APV2+)-sensitive component, in the presence of physiological concentrations of Mg (1 mM). Intracellular recordings were obtained from DG granule cells of rabbit hippocampal slices, and excitatory postsynaptic potentials (EPSPs) were measured in terms of the integrated area of their profiles. At 40 Hz, frequency facilitation of the evoked EPSPs was observed. The NMDA component gradually increased during the five-pulse train and frequency facilitation was significantly reduced after the application of APV. We tested the hypothesis that NMDARs undergo potentiation in LTP by comparing the NMDA/non-NMDA ratio of the synaptic responses in control and LTP groups. An increase in the ratio was observed in the LTP group, strongly suggesting potentiation of NMDARs. To infer changes in conductance at individual synapses based on EPSPs recorded at the soma, we constructed a compartmental model of a morphologically reconstructed DG granule cell. The effect on the NMDA/non-NMDA ratio of changes in AMPA and NMDA component synaptic conductance, and of differences in the distribution of activated synapses, was studied with computer simulations. The results confirmed that NMDARs are potentiated after the induction of LTP and contribute significantly to the expression of potentiation under physiological conditions.

Contribution of T-type VDCC to TEA-induced long-term synaptic modification in hippocampal CA1 and dentate gyrus

We have previously reported that exposure to the K+ channel blocker tetraethylammonium (TEA), 25 mM, induces long-term potentiation (LTP) in CA1, but not in the dentate gyrus (DG), of the rat hippocampal slice. During TEA application, stimulation of excitatory afferents results in a strong depolarizing potential after the fast excitatory postsynaptic potential (EPSP) in CA1, but not in DG. We hypothesized that the differential effect of TEA on long-term synaptic modification in CA1 and DG results from different levels of TEA-elicited depolarization in the two cell types. Additional pharmacological studies showed that blockade of T-type voltage-dependent calcium channels (VDCCs) decreased both the magnitude of LTP and the late, depolarizing potential in CA1. Blockade of L-type VDCCs had no such effect. Using computer models of morphologically reconstructed CA1 pyramidal cells and DG granule cells, we tested our hypothesis by simulating the relative intracellular Ca2+ accumulation and membrane potential changes mediated by T-type and L-type VDCCs. Simulation results using pyramidal cell models showed that, with decreased maximum conductance of TEA-sensitive potassium channels, synaptic inputs elicited strong depolarizing potentials similar to those observed with intracellular recording. During this depolarization, VDCCs were opened and resulted in a large intracellular Ca2+ accumulation that presumably caused LTP. When T-type VDCCs were blocked, the magnitudes of both the Ca2+ accumulation and the late depolarizing potential were decreased substantially. Simulated blockade of L-type VDCCs had only a minor effect. Together, our modeling and experimental studies indicate that T-type VDCCs, rather than L-type VDCCs, are primarily responsible for facilitating the depolarizing potential caused by TEA and for the consequent Ca2+ influx. Thus, our findings strongly suggest that the induction of TEA-LTP in CA1 depends primarily on T-type, rather than L-type, VDCCs. Simulation results using modeled granule cells suggests that the failure of TEA to induce LTP in DG is partly due to a low density of T-type VDCCs in granule cell membranes.

The corticotropin-releasing factor receptor 1 antagonist CP-154,526 reverses stress-induced learning deficits in mice

The neuropeptide corticotropin-releasing factor (CRF) coordinates the endocrine responses to stress as a major physiological regulator of the hypothalamic-pituitary-adrenal axis. We assessed the effect of the non-peptidergic CRF receptor 1 antagonist CP-154,526 on stress-induced changes in context-dependent fear conditioning and hippocampal synaptic plasticity. The learning impairment of mice trained immediately after 1 h immobilization could be overcome by preinjection of CP-154,526 before exposure to immobilization. Exposure to acute stress reduced the amount of autophosphorylated Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in the hippocampal CA1 area. When animals were pretreated with CP-154,526 before immobilization, the amount of hippocampal autophosphorylated CaMKII was elevated. Electrophysiological studies in the hippocampal CA1 region of stressed animals revealed no significant effects of the CP-154,526 pretreatment on long-term potentiation but a significant elevation of paired-pulse facilitation (PPF) was observed. The CP-154,526-induced enhancements in fear conditioning and PPF could be prevented by the selective CaMKII inhibitor KN-62. Our results demonstrated that learning impairment after acute stress was antagonized by CP-154,526 pretreatment.

Activity-dependent presynaptic facilitation and hebbian LTP are both required and interact during classical conditioning in Aplysia

Using a simplified preparation of the Aplysia siphon-withdrawal reflex, we previously found that associative plasticity at synapses between sensory neurons and motor neurons contributes importantly to classical conditioning of the reflex. We have now tested the roles in that plasticity of two associative cellular mechanisms: activity-dependent enhancement of presynaptic facilitation and postsynaptically induced long-term potentiation. By perturbing molecular signaling pathways in individual neurons, we have provided the most direct evidence to date that each of these mechanisms contributes to behavioral learning. In addition, our results suggest that the two mechanisms are not independent but rather interact through retrograde signaling.

Bidirectional synaptic plasticity in the cerebellum-like mammalian dorsal cochlear nucleus

The dorsal cochlear nucleus integrates acoustic with multimodal sensory inputs from widespread areas of the brain. Multimodal inputs are brought to spiny dendrites of fusiform and cartwheel cells in the molecular layer by parallel fibers through synapses that are subject to long-term potentiation and long-term depression. Acoustic cues are brought to smooth dendrites of fusiform cells in the deep layer by auditory nerve fibers through synapses that do not show plasticity. Plasticity requires Ca(2+)-induced Ca(2+) release; its sensitivity to antagonists of N-methyl-d-aspartate and metabotropic glutamate receptors differs in fusiform and cartwheel cells.