Blockade of NMDA receptors unmasks a long-term depression in synaptic efficacy in rat prefrontal neurons in vitro

Postsynaptic levels of [Ca2+]i needed to trigger LTD and LTP

Long-term potentiation (LTP) and long-term depression (LTD) in CA1 pyramidal neurons are both triggered by a postsynaptic rise in intracellular Ca2+ concentration ([Ca2+]i). We used photolysis of postsynaptic caged Ca2+ compounds to search for differential thresholds for activation of these processes. Long-lasting potentiation (LLP) resembling LTP, and long-lasting depression (LLD) resembling LTD, were evoked by [Ca2+]i elevations of comparable magnitude and duration in different cells. No distinctions in threshold for these processes were detectable. LLP was occluded by tetanically induced LTP and blocked by calmodulin inhibition, and LLD was occluded by electrically induced LTD and blocked by phosphatase inhibition.

NMDA receptor-dependent LTD in different subfields of hippocampus in vivo and in vitro

In simulations with artificial neural networks, efficient information processing and storage has been shown to require that the strength of connections between network elements has the capacity to both increase and decrease in a use-dependent manner. In contrast to long-term potentiation (LTP) of excitatory synaptic transmission, activity-dependent long-term depression (LTD) has been difficult to demonstrate in forebrain in vivo. Theoretical arguments indicate that coincidence of presynaptic excitation and low-magnitude postsynaptic activation are the necessary prerequisites for LTD induction. Here we report that stimulation paradigms which cause 1) sufficient excitation to result in NMDA receptor activation and simultaneously 2) attenuate the level of postsynaptic activation by recruitment of GABAA receptor-mediated inhibition consistently produce LTD of commissural input to area CA1 in the hippocampus of anesthetized adult rats, and of the perforant path input to the dentate gyrus in the hippocampus of anesthetized and unanesthetized adult rabbits. A functionally similar pre- and postsynaptic activation pattern applied to the hippocampal slice preparation by injecting hyperpolarizing current into the postsynaptic cell during NMDA receptor-mediated excitation also was effective in consistently inducing LTD. Results of studies in vitro show that Ca2+ influx through the NMDA channel is necessary for the induction of LTD, and moreover, that NMDA receptors also participate in the expression of LTD. Our findings demonstrate a general mechanism for the implementation of a theoretically derived learning rule in adult forebrain in vivo and in vitro and provide justification for the inclusion of use-dependent decreases of connection weights in formal models of cognitive processing.

Dopamine favours the emergence of long-term depression versus long-term potentiation in slices of rat prefrontal cortex

In the present study, we have investigated possible interactions between dopamine and long-term changes in synaptic efficacy induced in layer V pyramidal cells by tetanization of afferents from layer I-II. In the absence of dopamine, we confirmed that high frequency stimulation of excitatory afferents induced long-term potentiation, long-term depression or no change. Inversely, in the presence of dopamine, we have found that the same tetanus led to long-term depression in synaptic transmission in a majority of cells, but no more long-term potentiation. These results suggest that in rat prefrontal cortex, dopamine may determine the direction of activity dependent changes in synaptic efficacy and therefore, plays a functional role in the physiology of this structure.

NMDA receptor-dependent long-term potentiation in the hippocampal afferent fibre system to the prefrontal cortex in the rat

This study investigated the role of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor in the induction of long-term potentiation (LTP) in the hippocampal-prefrontal cortex pathway in vivo. Field potentials evoked by electrical stimulation of the CA1/subicular region were recorded in the prelimbic area of the prefrontal cortex under continuous perfusion of artificial cerebrospinal fluid in anaesthetized rats. High-frequency stimulation of the CA1/subicular region induced LTP of the evoked response in the prelimbic area of the prefrontal cortex. LTP was completely blocked when the selective NMDA receptor antagonist D-(-)2-amino-5-phosphonopentanoic acid (D-AP5; 200 microM), was perfused during the tetanus. Perfusion of D-AP5 did not affect normal transmission or pre-established LTP. These results demonstrate that induction of LTP in the hippocampal-prefrontal cortex pathway is an NMDA receptor-dependent process.

Developmental shift from long-term depression to long-term potentiation at the mossy fibre synapses in the rat hippocampus

During development, in the CA1 hippocampal region, long-term potentiation (LTP) starts appearing at postnatal (P) day 7 and reaches its maximal expression towards the end of the second postnatal week. However, LTP is often preceded by long-term depression (LTD), an activity-dependent and long-lasting reduction of synaptic strength. LTD can be induced by sustained, low-frequency stimulation of the afferent pathway and is dependent on activation of N-methyl-D-aspartate (NMDA) receptors. We report here that, in the CA3 hippocampal region, during a critical period of postnatal development, between P6 and P14, a high-frequency stimulation train (100 Hz, 1 s) to the mossy fibres in the presence of the NMDA receptor antagonist (+)-3-(2-carboxy-piperazin-4-yl)-propyl-1-phosphonic acid (CPP; 20 microM) induced LTD. The depression of the amplitude of the field excitatory postsynaptic potential (EPSP) was 28 +/- 7% (n = 21). This form of LTD was NMDA-independent and synapse-specific. When a tetanus was applied in the presence of CPP and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 50 microM), which blocked the field EPSP, it failed to induce LTD upon washout of CNQX. LTD was probably postsynaptic in origin since it did not affect paired-pulse facilitation. A rise in extracellular calcium concentration (from 2 to 4 mM) produced LTP instead of LTD. At the end of the second postnatal week, the same high-frequency stimulation train to the mossy fibres induced LTP as in adult neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyrotropin-releasing hormone enhances excitatory postsynaptic potentials in neocortical neurons of the rat in vitro

Several lines of evidence suggest a modulatory effect of thyrotropin-releasing hormone (TRH) on synaptic transmission in the mammalian neocortex. In the present study, the effects of this tripeptide on intracellularly recorded neocortical pyramidal neurons were investigated using rat in vitro brain slice preparations. TRH (5 microM and 50 microM) added to the perfusion medium concentration-dependently increased the excitability of pyramidal neurons, reflected by the number of spikes evoked by a depolarizing current pulse and by the augmentation of the time integral of glutamatergic excitatory postsynaptic potentials (EPSPs). TRH increased preferentially the time integrals of the late components of EPSPs (1-EPSPs) and increased their voltage-dependence. The early components of the EPSPs (e-EPSPs) were changed to much lesser extent. Iontophoretically applied D-2-amino-5-phosphonovalerate (D-APV) antagonized the TRH-induced increase of the 1-EPSPs. TRH also markedly enhanced the depolarizing responses evoked by iontophoretically applied N-methyl-D-aspartate (NMDA), while the depolarizing responses evoked by (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) and L-glutamate were not significantly affected. The depolarizing inward rectification present in all neurons studied was augmented by the higher concentration of TRH. The effects of TRH were incited after about 5 min and were long-lasting. In most neurons the effects of TRH on neuronal excitability did not completely recover during the 45 min washout period. The present data suggest that some of the non-hormonal actions of TRH in the neocortex may be due to an enhancement of glutamatergic synaptic transmission.(ABSTRACT TRUNCATED AT 250 WORDS)

Epileptic activity can induce both long-lasting potentiation and long-lasting depression

Epileptic seizures are associated with massive changes of intracellular ion concentrations which could cause persistent changes in efficacy of afferent excitation. Such alterations were investigated in the CA1 area of hippocampal slices using the acute high potassium model of epilepsy which allows rapid wash of the epileptogenic solution. Field potentials elicited by stimulation of Schaffer collaterals were recorded in stratum pyramidale and stratum radiatum. The experiments revealed that long-lasting potentiation as well as long-lasting depression of field potentials could result from superfusion with high potassium. These changes were NMDA-dependent, but did not depend on the discharge type (interictal vs. ictal) of the slice. It is concluded that epileptic activity cannot only lead to an enhancement of responses to afferent excitation, but also to a long-lasting depression. This depression could represent a self-protective mechanism of the brain which may also be involved in post-ictal depression of cerebral activity.

An investigation of depotentiation of long-term potentiation in the CA1 region of the hippocampus

We have investigated long-term synaptic depression in the CA1 region of rat hippocampal slices. Prolonged low-frequency stimulation (LFS; 900 stimuli delivered at 2 Hz) of the Schaffer collateral-commissural pathway in naïve slices did not induce long-term depression (LTD) of synaptic transmission. However, if long-term potentiation (LTP) was firstly induced in the pathway then LFS generated an LTD-like effect (i.e. depotentiation of LTP). Depotentiation could be induced 2 h (the longest time studied) after the induction of LTP and was stable for the duration of the experiment (followed for up to 40 min). The induction of depotentiation was not blocked by the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonopentanoate, the L-type voltage-gated Ca2+ channel blocker nimodipine or the nitric oxide synthase inhibitor N omega-nitro-L-arginine. However, the magnitude of depotentiation was reversibly reduced, in a stereoselective manner, by the specific metabotropic glutamate receptor (mGluR) antagonist (+)-alpha-methyl-4-carboxyphenylglycine. These results show that prolonged low frequency stimulation can result in an mGluR-dependent depotentiation of LTP.

Noradrenaline decreases transmission of NMDA- and non-NMDA-receptor mediated monosynaptic EPSPs in rat prefrontal neurons in vitro

The effects of noradrenaline on pyramidal cells of layer V of the prefrontal cortex were examined in rat brain slices in vitro. Bath administration of noradrenaline (10 microM) reduced synaptic transmission of afferent inputs from layer 1. The decrease affected all the components of the evoked response and particularly the monosynaptic excitatory postsynaptic potential (EPSP) as evidenced by a reduction of its initial rising slope (mean slope: 71 +/- 11% of its control). Pharmacological dissociation of the NMDA- and non-NMDA-receptor components of the EPSP showed that noradrenaline reduced both (mean EPSP slopes were 71 +/- 8% and 73 +/- 10% of their control, respectively). Alpha 1-, but not alpha-2- or beta-adrenoceptor antagonists prevented the noradrenaline-induced decrease in synaptic efficacy. However, the effect of noradrenaline was not reproduced by alpha 1-adrenoceptor agonists. Lastly, noradrenaline acting through beta-adrenoceptors reduced the slow hyperpolarization that follows a train of action potentials.

Long-term depression of excitatory synaptic transmission and its relationship to long-term potentiation

In many brain areas, including the cerebellar cortex, neocortex, hippocampus, striatum and nucleus accumbens, brief activation of an excitatory pathway can produce long-term depression (LTD) of synaptic transmission. In most preparations, induction of LTD has been shown to require a minimum level of postsynaptic depolarization and a rise in the intracellular Ca2+ concentration [Ca2+]i in the postsynaptic neurone. Thus, induction conditions resemble those described for the initiation of associative long-term potentiation (LTP). However, data from structures susceptible to both LTD and LTP suggest that a stronger depolarization and a greater increase in [Ca2+]i are required to induce LTP than to initiate LTD. The source of Ca2+ appears to be less critical for the differential induction of LTP and LTD than the amplitude of the Ca2+ surge, since the activation of voltage- and ligand-gated Ca2+ conductances as well as the release from intracellular stores have all been shown to contribute to both LTD and LTP induction. LTD is induceable even at inactive synapses if [Ca2+]i is raised to the appropriate level by antidromic or heterosynaptic activation, or by raising the extracellular Ca2+ concentration [Ca2+]o. These conditions suggest a rule (called here the ABS rule) for activity-dependent synaptic modifications that differs from the classical Hebb rule and that can account for both homosynaptic LTD and LTP as well as for heterosynaptic competition and associativity.

Neocortical long-term potentiation

LTP is a form of activity-dependent synaptic plasticity that has been investigated mainly in the hippocampus. It is considered likely that similar mechanisms may also account for aspects of naturally occurring plasticity in the neocortex. Consequently, an increasing number of studies have been devoted to the investigation of neocortical LTP. Recent results suggest that at least two forms of LTP coexist in layer III of the neocortex. One depends on NMDA-receptor activation and resembles the LTP observed in hippocampal field CA1. A second form is independent of NMDA receptors and requires activation of voltage-sensitive Ca2+ channels.

Synaptic plasticity in an in vitro slice preparation of the rat nucleus accumbens

Extra- and intracellular recordings in slices were used to examine what types of synaptic plasticity can be found in the core of the nucleus accumbens, and how these forms of plasticity may be modulated by dopamine. Stimulus electrodes were placed at the rostral border of the nucleus accumbens in order to excite primarily infralimbic and prelimbic afferents, as was confirmed by injections of the retrograde tracer fluoro-gold. In extracellular recordings, tetanization induced long-term potentiation (LTP) of the population spike in 20 out of 53 slices. The presynaptic compound action potential did not change following LTP induction. For the intracellularly recorded excitatory postsynaptic potentiation, three types of synaptic plasticity were noted: long-term potentiation (16 out of 54 cells), decremental potentiation (eight cells) and long-term depression (LTD; six cells). No correlation was found between the occurrence of potentiation or depression and various parameters of the tetanic depolarization (e.g. peak voltage, integral under the curve). The N-methyl-D-aspartate receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (50 microM; D-AP5) reduced, but did not completely prevent, the induction of LTP. The incidence of LTD was not markedly affected by D-AP5. No difference in LTP was found when comparing slices bathed in dopamine (10 microM) and controls. Likewise, slices treated with a mixture of the D1 receptor antagonist Sch 23390 (1 microM) and the D2 antagonist S(-)-sulpiride (1 microM) generated a similar amount of LTP as controls. In conclusion, both LTP and LTD can be induced in a key structure of the limbic-innervated basal ganglia. LTP in the nucleus accumbens strongly depends on N-methyl-D-aspartate receptor activity, but is not significantly affected by dopamine.

Mechanisms underlying induction of homosynaptic long-term depression in area CA1 of the hippocampus

The mechanisms responsible for long-lasting, activity-dependent decreases in synaptic efficacy are not well understood. We have examined the initial steps required for the induction of long-term depression (LTD) in CA1 pyramidal cells by repetitive low frequency (1 Hz) synaptic stimulation. This form of LTD was synapse specific, was saturable, and required activation of post-synaptic NMDA receptors. Loading CA1 cells with the Ca2+ chelator BAPTA prevented LTD, whereas lowering extracellular Ca2+ resulted in the induction of LTD by stimulation that previously elicited long-term potentiation. Following LTD, synaptic strength could be increased to its original maximal level, indicating that LTD is reversible and not due to deterioration of individual synapses. Induction of homosynaptic LTD therefore requires an NMDA receptor-dependent change in postsynaptic Ca2+ which may be distinct from that required for long-term potentiation.