Single high strength afferent volleys can produce long-term potentiation in the hippocampus in vitro

Although hippocampal long-term potentiation (LTP) is normally elicited by tetanization of an afferent input, it may also be induced by pairing afferent volleys with strong depolarizing conditioning stimuli. In extracellular recordings made from area CA1 of hippocampal slices bathed with a picrotoxin-containing solution, long-lasting potentiation was produced by high strength single volleys alone. Potentiation occurred with intervals between high strength stimuli as great as one per minute. Tetanization-induced LTP was no greater than tetanization-induced LTP plus high strength single volley potentiation, given in either order. These data suggest that single afferent volleys can induce LTP under conditions of reduced inhibition.     

Mu opiates inhibit long-term potentiation induction in the spinal cord slice

Long-term potentiation (LTP) involves a prolonged increase in neuronal excitability following repeated afferent input. This phenomenon has been extensively studied in the hippocampus as a model of learning and memory. Similar long-term increases in neuronal responses have been reported in the dorsal horn of the spinal cord following intense primary afferent stimulation. In these studies, we utilized the spinal cord slice preparation to examine effects of the potently antinociceptive mu opioids in modulating primary afferent/dorsal horn neurotransmission as well as LTP of such transmission. Transverse slices were made from the lumbar spinal cord of 10- to 17-day-old rats, placed in a recording chamber, and perfused with artificial cerebrospinal fluid also containing bicuculline (10 microM) and strychnine (1 microM). Primary afferent activation was achieved in the spinal slice by electrical stimulation of the dorsal root (DR) or the tract of Lissauer (LT) which is known to contain a high percentage of small diameter fibers likely to transmit nociception. Consistent with this anatomy, response latencies of LT-evoked field potentials in the dorsal horn were considerably slower than the response latencies of DR-evoked potentials. Only LT-evoked field potentials were found to be reliably inhibited by the mu opioid receptor agonist [D-Ala(2), N-Me-Phe(4), Gly(5)] enkephalin-ol (DAMGO, 1 microM), although evoked potentials from both DR and LT were blocked by the AMPA/kainate glutamate receptor antagonist 6-cyano-7-nitroquinoxalene-2,3-dione. Moreover repeated stimulation of LT produced LTP of LT- but not DR-evoked potentials. In contrast, repeated stimulation of DR showed no reliable LTP. LTP of LT-evoked potentials depended on N-methyl-D-aspartate (NMDA) receptor activity, in that it was attenuated by the NMDA antagonist APV. Moreover, such LTP was inhibited by DAMGO interfering with LTP induction mechanisms. Finally, in whole cell voltage-clamp studies of Lamina I neurons, DAMGO inhibited excitatory postsynaptic current (EPSC) response amplitudes from LT stimulation-evoked excitatory amino acid release but not from glutamate puffed onto the cell and increased paired-pulse facilitation of EPSCs evoked by LT stimulation. These studies suggest that mu opioids exert their inhibitory effects presynaptically, likely through the inhibition of glutamate release from primary afferent terminals, and thereby inhibit the induction of LTP in the spinal dorsal horn.     

Phorbol ester-induced hippocampal long-term potentiation is counteracted by inhibitors of protein kinase C

Summary As was shown previously (Reymann et al. 1988), the protein kinase C (PKC)-inhibitor polymyxin B prevents the maintenance of electrically induced long-term potentiation (LTP) of synaptic transmission to CA1 neurons, indicating that post-translational phosphorylation processes mediated by PKC are involved in mechanisms underlying this form of synaptic plasticity. To make sure that 1.) the polymyxin B actually acts against PKC activation and 2.) the long-lasting potentiation elicited by phorbol esters (Malenka et al. 1986) is mediated by PKC-activation, we have tested polymyxin B as well as the potent PKC-inhibitor K-252b during phorbol ester-induced LTP. 4-beta-phorbol-12,13-dibutyrate (PDBu) — a known activator of protein kinase C, induces a remarkable potentiation at concentrations as low as 0.5 M. When 20 M polymyxin B or 40 nM K-252b was administered to rat hippocampal slices prior to such a weak phorbol ester treatment, this potentiation did not develop with the exception of a small increase in the population spike in spite of polymyxin B-treatment (42% instead of 120% increase at 2 h after PDBu). In contrast, spike potentiation induced by high concentrations of PDBu (10 M) could not be counteracted by 100 M polymyxin B. It is concluded that at low concentrations the phorbol ester-induced potentiation is mainly mediated by a selective activation of protein kinase C and that the prevented maintenance of electrically induced LTP by polymyxin B is in fact due to inhibition of this kinase. The spike potentiation developed faster than that of the EPSP raising the possibility that PDBu activates two separate PKC-dependent processes.     

Mechanisms involved in tetanus-induced potentiation of fast IPSCs in rat hippocampal CA1 neurons

In the present study, possible mechanisms involved in the tetanus-induced potentiation of gamma-aminobutyric acid-A (GABA-A) receptor-mediated inhibitory postsynaptic currents (IPSCs) were investigated using the whole cell voltage-clamp technique on CA1 neurons in rat hippocampal slices. Stimulations (100 Hz) of the stratum radiatum, while voltage-clamping the membrane potential of neurons, induces a long-term potentiation (LTP) of evoked fast IPSCs while increasing the number but not the amplitude of spontaneous IPSCs (sIPSCs). The potentiation of fast IPSCs was input specific. During the period of IPSC potentiation, postsynaptic responses produced by 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride and baclofen, GABA-A and GABA-B agonists respectively, were not significantly different from control. CGP 36742, a GABA-B antagonist, blocked the induction of tetanus-induced potentiation of evoked and spontaneous IPSCs, while GTPgammaS, an activator of G proteins, substitution for GTP in the postsynaptic recording electrode did not occlude potentiation. Since GABA-B receptors work through G proteins, our results suggest that pre- but not postsynaptic GABA-B receptors are involved in the potentiation of fast IPSCs. A tetanus delivered when GABA-A responses were completely blocked by bicuculline suggests that GABA-A receptor activation during tetanus is not essential for the induction of potentiation. Rp-cAMPs, an antagonist of protein kinase A (PKA) activation, blocks the induction of potentiation of fast IPSCs. Forskolin, an activator of PKA, increases baseline evoked IPSCs as well as the number of sIPSCs, and a tetanic stimulation during this enhancement uncovers a long-term depression of the evoked IPSC. Sulfhydryl alkylating agents, N-ethylmaleimide and p-chloromercuribenzoic acid, which have been found to presynaptically increase GABA release and have been suggested to have effects on proteins involved in transmitter release processes occurring in nerve terminals, occlude tetanus-induced potentiation of evoked and spontaneous IPSCs. Taken together our results suggest that LTP of IPSCs originates from a presynaptic site and that GABA-B receptor activation, cyclic AMP/PKA activation and sulfhydryl-alkylation are involved. Plasticity of IPSCs as observed in this study would have significant implications for network behavior in the hippocampus.     

2-Amino-4-phosphonobutyrate selectively eliminates late phases of long-term potentiation in rat hippocampus

The possible involvement of 2-amino-4-phosphonobutyrate (APB) recognition sites in mechanisms enabling the maintenance of long-term potentiation (LTP) was investigated in rat hippocampal slices. The action of D(-)- and L(+)-isomers of APB was tested on orthodromic EPSP and spike responses recorded extracellularly from CA1 pyramidal cells. If a moderate concentration (50 microM) of one or the other APB isomer was present during tetanization, posttetanic and early long-term potentiation developed nearly normally. However, from 2h onward LTP of both EPSP and spike potentiation was eliminated in an irreversible manner (8 h experiment). D-APB (L-isomer not tested) applied shortly after tetanization caused nearly the same delayed decline of LTP. No consistent effects of APB were seen in non-tetanized slices. Considering previous findings these data suggest that besides the obligatory NMDA receptor activation an APB-sensitive component expressed during and after tetanization is a necessary step for subsequent mechanisms enabling the late maintenance of LTP.     

N-methyl-D-aspartate receptor activation is required for the induction of both early and late phases of long-term potentiation in rat hippocampal slices

The possible involvement of N-methyl-D-aspartate (NMDA) receptors in mechanisms enabling the maintenance of long-term potentiation (LTP) was investigated in rat hippocampal slices. The action of the specific NMDA receptor antagonists (-)-2-amino-7-phosphonoheptanoic acid (D-APH) and 2-amino-5-phosphonovaleric acid (DL-APV) as well as of the inactive isomer L-APH was tested on orthodromic population excitatory postsynaptic potential (EPSP) and population spike (PS) responses recorded extracellularly from CA1 pyramidal cells. If the active D-isomer of APH (10 microM) or DL-APV (50 microM), but not if L-APH was present during tetanization, both EPSP and spike potentiation were markedly reduced or even blocked for the whole recording period (8 h after tetanization). It is concluded that the NMDA receptor component expressed during tetanization is a necessary step not only for initiation but also for subsequent mechanisms enabling late phases of synaptic LTP. Some remaining potentiation of the population spike may be related to a second, NMDA-independent mechanism.     

Adenosine acting on A1 receptors protects NO-triggered rebound potentiation and LTP in rat hippocampal slices

Exposure of hippocampal slices to nitric oxide (NO) results in a depression of CA1 synaptic transmission. Under 0.2-Hz stimulation, washout of NO leads to a persistent potentiation that depends on N-methyl-D-aspartate (NMDA) receptors and endogenous NO formation and that occludes tetanus-induced long-term potentiation (LTP). The experiments were initially aimed at determining the relationship between the NO-induced synaptic depression and rebound potentiation. The adenosine A1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) partially inhibited the depression produced by the NO donor diethylamine NONOate (300 microM). It also led to a complete block of both the rebound potentiation and the subsequent tetanus-induced LTP. LTP was preserved in the presence of DPCPX if the stimulation frequency was reduced to 0.033 Hz or if the NO application was omitted. The NO-triggered rebound potentiation was restored if the experiment (DPCPX followed by exogenous NO) was conducted in the presence of an NMDA antagonist. The restored potentiation was completely blocked by the NO synthase inhibitor, L-nitroarginine. It is concluded that the NO-induced depression is partially mediated by increased release of endogenous adenosine acting on A1 receptors. Moreover, tonic A1 receptor activation by adenosine protects LTP and the rebound potentiation from being disabled by untimely NMDA receptor activity. Hence, the NO-induced depression and rebound potentiation are linked in the sense that the depression helps to preserve the capacity of the synapses to undergo potentiation. Finally, the results give the first example of exogenous NO eliciting an enduring potentiation of hippocampal synaptic transmission that is dependent on endogenous NO formation, but not on NMDA receptors.     

A decrease in firing threshold observed after induction of the EPSP-spike (E-S) component of long-term potentiation in rat hippocampal slices

Summary Two components of long-term potentiation (LTP) are distinguished with extracellular recording electrodes: a synaptic and an EPSP-Spike (E-S) component. The latter consists of the enhancement produced in the population spike amplitude in excess of that predicted by EPSP potentiation alone. The experiments carried out in this study were designed to investigate intracellular correlates of E-S potentiation and to examine the hypothesis that an increased postsynaptic excitability underlies E-S potentiation. CA1 pyramidal neurons were synaptically activated from stratum radiatum. LTP, defined as a stable increase in the probability of firing to afferent stimulation, was found to be related to a decrease in the intracellular PSP peak amplitude and slope required to fire the cells at a probability of 0.5. These changes were accompanied by a decrease in threshold to direct activation. No significant changes in input resistance or resting potential were recorded. These excitability changes were only observed in cells displaying LTP; they were not related to the potentiation of the synaptic component (PSP amplitude). Our results support the hypothesis that different mechanisms underlie the two components of LTP, and that a reduction in threshold for neuronal discharge accompanies tetanus-induced E-S potentiation. It is suggested that an increase in the ratio of synaptically evoked excitation/inhibition and a reduction in tonic synaptic inhibition through GA-BA_A channels contribute to E-S potentiation.     

Induction of long-term potentiation leads to increased reliability of evoked neocortical spindles in vivo

Large amplitude electroencephalographic spindle waves (7-14 Hz) occur spontaneously in the neocortex during both sleep and awake immobility, and it has been proposed that synchronous neuronal activation during spindles may contribute to learning-related synaptic plasticity. Spindles can also be evoked in the sensorimotor cortex by electrical stimulation of cortical or thalamic inputs in the rat. To determine if strengthening cortical synapses can affect the initiation and maintenance of electrically evoked spindles, stimulation pulses were delivered at a range of intensities to the corpus callosum or ventrolateral thalamus in the awake rat before and after the induction of long-term potentiation (LTP) by tetanization of the corpus callosum. The morphology of evoked spindles was similar to that of naturally occurring spindles. Spindles were evoked less reliably during slow-wave sleep than during waking, and this was correlated with smaller synaptic responses during slow-wave sleep. Similar to previous findings, daily tetanization of the corpus callosum for 15 days decreased the early component and increased the late component of synaptic responses evoked by corpus callosum stimulation, but did not significantly affect synaptic responses evoked by thalamic stimulation. Similarly, LTP induction increased the reliability with which low-intensity corpus callosum stimulation evoked spindles, but increases in spindles evoked by thalamic stimulation were not significant. Synaptic potentiation and the increased reliability of spindles developed with a similar time-course over the 15-day LTP induction period. These results reflect strong correlations between the strength of cortical layer V activation and the initiation of spindles in the sensorimotor cortex, and support the idea that monosynaptic and polysynaptic horizontal collaterals of layer V neurons can play a significant role in the initiation of spindles.     

The EPSP-spike (E-S) component of long-term potentiation in the rat hippocampal slice is modulated by GABAergic but not cholinergic mechanisms

Long-term potentiation of synaptic efficacy (LTP) can be shown to consist of two components: a synaptic and an excitatory postsynaptic potential (EPSP)-spike (E-S) component. The E-S component is expressed as a leftward shift in the curve relating population spike amplitude as a function of EPSP slope. The participation of cholinergic and GABAergic processes in E-S potentiation was studied in field CA1 of rat hippocampal slices. Atropine, a muscarinic antagonist, did not prevent tetanus-induced E-S potentiation. The cholinergic agonist carbachol and the GABAA antagonist picrotoxin produced a leftward shift in the E-S relation; picrotoxin, but not carbachol, prevented the expression of tetanus-induced E-S potentiation. These observations indicate that an increase in the ratio of evoked excitation to inhibition and/or a reduction in tonic inhibition mediated by the activation of GABAA receptors contribute to E-S potentiation produced by high-frequency stimulation.     

Nitric oxide is required for the maintenance but not initiation of ganglionic long-term potentiation

The role of nitric oxide in long-term potentiation of the nicotinic pathway of synaptic transmission in the isolated superior cervical ganglia of rat was studied. Long-term potentiation was induced by a brief tetanizing pulse (tetanus, 20 Hz/20 s) to the preganglionic nerve. The amplitude of the extracellularly recorded postganglionic compound action potential was used as an index of synaptic transmission. Pretreatment with the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (10 microM) or L-N(G)-nitro-arginine (10 microM) 30 min before tetanus, inhibited long-term potentiation. The inactive enantiomer of the nitric oxide synthase inhibitor, N(G)-nitro-D-arginine methyl ester (10 microM), failed to inhibit the long-term potentiation when given 30 min before the tetanus. Washout of L-N(G)-nitro-arginine, but not N(G)-nitro-L-arginine methyl ester, resulted in complete recovery of long-term potentiation. The nitric oxide synthase inhibitor had no significant effect on the basal ganglionic neurotransmission or post-tetanic potentiation. Furthermore, established long-term potentiation was blocked by superfusion of ganglia with N(G)-nitro-L-arginine methyl ester 1 h after a tetanus. Pretreatment of ganglia with the nitric oxide donor, sodium nitroprusside (100 microM), or the nitric oxide synthase substrate, L-arginine (1 mM), completely prevented the inhibitory effects of N(G)-nitro-L-arginine methyl ester on the tetanus-induced long-term potentiation. These findings present evidence for a requirement of nitric oxide for the maintenance but not induction of long-term potentiation in rat isolated superior cervical ganglia.     

Endogenous adenosine regulates the effects of low-frequency stimulation on the induction of long-term potentiation in CA1 neurons of guinea pig hippocampal slices

A train of low-frequency afferent stimuli (LFS, 1 Hz, 1000 pulses), given 60 min prior to a tetanus (100 Hz, 100 pulses), suppresses the induction of long-term potentiation (LTP) in which a short-term potentiation decreases gradually back to the pre-tetanic level within 40-50 min (LTP suppression). We investigated the effects of adenosine A1 or A2 receptor antagonists (8-cyclopentyltheophylline (8-CPT) and CP-66713, respectively) on LTP suppression in CA1 neurons of guinea pig hippocampal slices. When the LFS was delivered in the presence of 8-CPT (1 microM), LTP suppression was not significantly affected. However, when LFS was delivered in the presence of CP-66713 (10 microM), LTP suppression was inhibited, leading to successful LTP induction. These results indicate that endogenous adenosine, acting via A2 receptors, is involved in the mechanism of LTP suppression.     

Inhibition of protein synthesis in the dentate gyrus, but not the entorhinal cortex, blocks maintenance of long-term potentiation in rats

We examined whether the critical protein synthesis for maintenance of perforant path long-term potentiation (LTP) takes place in the dentate gyrus or the entorhinal cortex. Field potential recordings were made of responses in the dentate gyrus to stimulation of the perforant path in urethane-anaesthetized rats. Anisomycin (10 micrograms) injected into the dentate gyrus, but not the entorhinal cortex, 1 h prior to tetanization led to nearly complete decay of perforant path LTP of the excitatory postsynaptic potential (EPSP) within 3 h. Intra-dentate injection of neither actinomycin D (a mRNA synthesis inhibitor) nor boiled anisomycin affected LTP maintenance over 6 h. These results suggest that the proteins necessary for the maintenance of LTP over 6 h are synthesized in the dentate gyrus from already existing mRNA without involving protein synthesis in the cell bodies of the afferent fibres.     

Differential effects of protein kinase inhibitors on pre-established long-term potentiation in rat hippocampal neurons in vitro

The possibility that a permanent protein kinase C (PKC) activity is necessary for the maintenance of long-term potentiation (LTP) was investigated in rat hippocampal slices. The action of the potent kinase inhibitors K-252a, K-252b and staurosporine on LTP of orthodromic population excitatory postsynaptic potentials (EPSPs) recorded from CA1 pyramidal cells was tested both during tetanization and after establishment of LTP. Confirming earlier studies, all inhibitors applied during tetanization at a concentration of 50 nM eliminate late LTP. Only staurosporine, but not K-252a or K-252b, blocked already established late LTP (i.e. late application). Normal synaptic transmission was influenced only weakly by staurosporine. Considering that all inhibitors have similar potencies against PKC and were all effective if applied during tetanization these data suggest that the late maintenance of LTP depends on a staurosporine/H7-sensitive process (or kinase) rather than permanent activation of PKC.     

2-Chloroadenosine decreases long-term potentiation in the hippocampal CA1 area of the rat

The effect of the adenosine (ADO) analogue 2-chloroadenosine (CADO) on frequency-induced long-term potentiation (LTP) of the responses evoked by stimulation of the Schaffer fibres and recorded in CA1 area was studied in hippocampal slices of the rat. CADP significantly decreased LTP of the population spikes (PS) (EC50 = 0.28 microM), and LTP of the field excitatory postsynaptic potentials (f.e.p.s.p) (EC50 = 0.33 microM). These effects were reversed by the ADO receptor antagonist 8-phenyltheophylline (8-PT) (2.5 microM). It is concluded that CADO decreases LTP through activation of a xanthine-sensitive ADO receptor.     

4-Aminopyridine-mediated increase in long-term potentiation in CA1 of the rat hippocampus

The effect of 4-aminopyridine (4-AP) on long-term potentiation (LTP) was studied in the hippocampal slice preparation of the rat. Field excitatory postsynaptic potentials (EPSPs) were recorded and evoked in the stratum radiatum of the CA1. Both the low frequency EPSP and LTP of the EPSP were significantly increased by treatment with 4-AP. These effects were inhibited by increasing the magnesium concentration from 1 to 4 mM. Pretreatment with 20 microM DL-2-amino-5-phosphonovalerate antagonized only the increase in LTP produced by 4-AP. It is suggested that 4-AP enhances Ca influx either pre- or postsynaptically and thereby increases LTP.     

PKA参与脊髓背角C-纤维诱发电位LTP的诱导和早期维持

目的: 探讨环一磷酸腺苷依赖的蛋白激酶 (PKA) 在脊髓背角C-纤维诱发电位长时程增强(LTP)的诱导和维持中的作用。方法:细胞外记录技术在脊髓腰膨大部记录背角浅层神经元C-纤维诱发电位。 结果:(1) 8-Br-cAMP诱发脊髓背角C-纤维诱发电位LTP,且8-Br-cAMP-诱导的 LTP 遮蔽强直刺激诱导的LTP。(2) PKA的选择性抑制剂Rp-CPT-cAMPS阻断C-纤维诱发电位LTP的诱导和时间依赖性翻转C-纤维诱发电位LTP。(3)蛋白质合成抑制剂茴香霉素彻底阻断8-Br-cAMP诱发的 LTP。(4)MAPK选择性抑制剂PD98059阻断8-Br-cAMP诱发的LTP。 结论:脊髓背角神经元存在PKA信号途径,并参与脊髓背角C-纤维诱发LTP的诱导和早期维持。     

Blockade of GABAA receptors facilitates induction of NMDA receptor-independent long-term potentiation.

An N-methyl-D-aspartate (NMDA)-independent form of long-term potentiation (LTP), which depends on postsynaptic, voltage-dependent calcium channels (VDCCs), has been demonstrated in area CA1 of hippocampus. GABA acting at GABAA receptors limits postsynaptic depolarization during LTP induction. Blockade of GABAA receptors should therefore enhance activation of postsynaptic VDCCs and facilitate the induction of this NMDA receptor-independent, VDCC-dependent LTP. In agreement with this hypothesis, pharmacological blockade of GABAA receptors in the in vitro rat hippocampal slice increased the magnitude of LTP resulting from a normally effective, high-frequency (200 Hz) tetanic stimulation protocol. In addition, GABAA receptor blockade allowed a lower frequency (25 Hz) and normally ineffective tetanic stimulation protocol to induce this form of LTP. Intracellular recordings from CA1 pyramidal cells revealed that blocking GABAA receptors during tetanic stimulation allowed greater postsynaptic depolarization, increased the number of postsynaptic action potentials fired during the tetanization, and also increased the duration of synaptically evoked action potentials. To mimic the increased action potential firing observed when GABAA receptors were blocked, we paired 25-Hz antidromic stimulation with 25-Hz orthodromic stimulation. Paired antidromic + orthodromic 25-Hz stimulation induced NMDA receptor-independent LTP, whereas neither antidromic nor orthodromic stimulation alone induced LTP. Increased action potential firing can therefore at least partially account for the facilitation of NMDA receptor-independent LTP caused by blockade of GABAA receptors. This conclusion is consistent with prior studies demonstrating that action potentials are particularly effective stimuli for the gating of VDCCs in CA1 pyramidal cell dendrites.