Concentrations of extracellular free zinc (pZn)e in the central nervous system during simple anesthetization, ischemia and reperfusion.

"Free Zn2+" (rapidly exchangeable Zn2+) is stored along with glutamate in the presynaptic terminals of specific specialized (gluzinergic) cerebrocortical neurons. This synaptically releasable Zn2+ has been recognized as a potent modulator of glutamatergic transmission and as a key toxin in excitotoxic neuronal injury. Surprisingly (despite abundant work on bound zinc), neither the baseline concentration of free Zn2+ in the brain nor the presumed co-release of free Zn2+ and glutamate has ever been directly observed in the intact brain in vivo. Here, we show for the first time in dialysates of rat and rabbit brain and human CSF samples from lumbar punctures that: (i) the resting or "tonic" level of free Zn2+ signal in the extracellular fluid of the rat, rabbit and human being is approximately 19 nM (95% range: 5-25 nM). This concentration is 15,000-fold lower than the "300 microM" concentration which is often used as the "physiological" concentration of free zinc for stimulating neural tissue. (ii) During ischemia and reperfusion in the rabbit, free zinc and glutamate are (as has often been presumed) released together into the extracellular fluid. (iii) Unexpectedly, Zn2+ is also released alone (without glutamate) at a variable concentration for several hours during the reperfusion aftermath following ischemia. The source(s) of this latter prolonged release of Zn2+ is/are presumed to be non-synaptic and is/are now under investigation. We conclude that both Zn2+ and glutamate signaling occur in excitotoxicity, perhaps by two (or more) different release mechanisms.

Characterization of extracellular accumulation of Zn2+ during ischemia and reperfusion of hippocampus slices in rat.

The mammalian CNS contains an abundance of chelatable zinc that is sequestered in the vesicles of glutamatergic presynaptic terminals and co-released with glutamate. Considerable Zn(2+) is also released during cerebral ischemia and reperfusion (I/R) although the mechanism of this release has not been elucidated. We report here the real time observation of increase of the concentration of extracellular Zn(2+) ([Zn(2+)](o)), accompanied by a rapid increase of intracellular free Zn(2+)concentration, in the areas of dentate gyrus (DG), CA1 and CA3 in acute rat hippocampus slices during ischemia simulated by deprivation of oxygen and glucose (OGD) followed by reperfusion with normal artificial cerebrospinal fluid. A brief period of OGD caused a sustained increase of [Zn(2+)](o). Subsequent reperfusion with oxygenated medium containing glucose resulted in a further increase of [Zn(2+)](o). Longer periods of OGD caused greater increases of [Zn(2+)](o,) and subsequent reperfusion caused still further increases of [Zn(2+)](o,) regardless of OGD duration. The Zn(2+) chelator CaEDTA (10 mM) significantly reduced the increase of [Zn(2+)] induced by OGD and reperfusion. Significant regional differences of [Zn(2+)](o) over the areas of the DG, CA1 and CA3 were not observed during I/R. Neither sodium channel blockade by tetrodotoxin (2 microM), perfusion with nominally calcium-free medium nor anatomical disassociation of the DG, CA1 and CA3 regions from one another by lesioning affected the increase of [Zn(2+)](o). The non-specific nitric oxide synthase (NOS) inhibitor, Nomega-nitro-l-arginine methyl ester (1 mM), however, blocked the increase of [Zn(2+)](o) during ischemia and reperfusion. The data indicate the important role of NO in causing the release of Zn(2+) during I/R and suggest that NOS inhibitors may be used to reduce Zn(2+)-induced neuronal injury.

Rapid translocation of Zn(2+) from presynaptic terminals into postsynaptic hippocampal neurons after physiological stimulation.

Zn(2+) is found in glutamatergic nerve terminals throughout the mammalian forebrain and has diverse extracellular and intracellular actions. The anatomical location and possible synaptic signaling role for this cation have led to the hypothesis that Zn(2+) is released from presynaptic boutons, traverses the synaptic cleft, and enters postsynaptic neurons. However, these events have not been directly observed or characterized. Here we show, using microfluorescence imaging in rat hippocampal slices, that brief trains of electrical stimulation of mossy fibers caused immediate release of Zn(2+) from synaptic terminals into the extracellular microenvironment. Release was induced across a broad range of stimulus intensities and frequencies, including those likely to induce long-term potentiation. The amount of Zn(2+) release was dependent on stimulation frequency (1-200 Hz) and intensity. Release of Zn(2+) required sodium-dependent action potentials and was dependent on extracellular Ca(2+). Once released, Zn(2+) crosses the synaptic cleft and enters postsynaptic neurons, producing increases in intracellular Zn(2+) concentration. These results indicate that, like a neurotransmitter, Zn(2+) is stored in synaptic vesicles and is released into the synaptic cleft. However, unlike conventional transmitters, it also enters postsynaptic neurons, where it may have manifold physiological functions as an intracellular second messenger.

Induction of mossy fiber --> Ca3 long-term potentiation requires translocation of synaptically released Zn2+.

The mammalian CNS contains an abundance of chelatable Zn(2+) sequestered in the vesicles of glutamatergic terminals. These vesicles are particularly numerous in hippocampal mossy fiber synapses of the hilar and CA3 regions. Our recent observation of frequency-dependent Zn(2+) release from mossy fiber synaptic terminals and subsequent entry into postsynaptic neurons has prompted us to investigate the role of synaptically released Zn(2+) in the induction of long-term potentiation (LTP) in field CA3 of the hippocampus. The rapid removal of synaptically released Zn(2+) with the membrane-impermeable Zn(2+) chelator CaEDTA (10 mm) blocked induction of NMDA receptor-independent mossy fiber LTP by high-frequency electrical stimulation (HFS) in rat hippocampal slices. Mimicking Zn(2+) release by bath application of Zn(2+) (50-100 microm) without HFS induced a long-lasting potentiation of synaptic transmission that lasted more than 3 hr. Moreover, our experiments indicate the effects of Zn(2+) were not attributable to its interaction with extracellular membrane proteins but required its entry into presynaptic or postsynaptic neurons. Co-released glutamate is also essential for induction of LTP under physiological conditions, in part because it allows Zn(2+) entry into postsynaptic neurons. These results indicate that synaptically released Zn(2+), acting as a second messenger, is necessary for the induction of LTP at mossy fiber-->CA3 synapses of hippocampus.

beta-NAAG rescues LTP from blockade by NAAG in rat dentate gyrus via the type 3 metabotropic glutamate receptor.

N-Acetylaspartylglutamate (NAAG) is an agonist at the type 3 metabotropic glutamate receptor (mGluR3), which is coupled to a Gi/o protein. When activated, the mGluR3 receptor inhibits adenylyl cyclase and reduces the cAMP-mediated second-messenger cascade. Long-term potentiation (LTP) in the medial perforant path (MPP) of the hippocampal dentate gyrus requires increases in cAMP. The presence of mGluR3 receptors and NAAG in neurons of the dentate gyrus suggests that this peptide transmitter may inhibit LTP in the dentate gyrus. High-frequency stimulation (100 Hz; 2 s) of the MPP resulted in LTP of extracellularly recorded excitatory postsynaptic potentials at the MPP-granule cell synapse of rat hippocampal slices. Perfusion of the slice with NAAG (50 and 200 microM) blocked LTP. Neither 50 nor 200 microM NAAG produced N-methyl-D-aspartate receptor currents in the granule cells of the acute hippocampal slice. The group II mGluR antagonist ethyl glutamate (100 microM) and a structural analogue of NAAG, beta-NAAG (100 microM), prevented the blockade of LTP by NAAG. Paired-pulse depression of the excitatory postsynaptic potential at 20- and 80-ms interpulse intervals (IPI) was not affected by NAAG or beta-NAAG. beta-NAAG did not affect inositol trisphosphate production stimulated by the agonist glutamate in cells expressing the group I mGluR1alpha or mGluR5. beta-NAAG blocked the decrease in forskolin-stimulated cAMP by the group II mGluR agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) but not the group III mGluR agonist L(+)-2-amino-4-phosphonobutyric acid in cerebellar granule cells. In cells transfected with mGluR3, but not mGluR2, beta-NAAG blocked forskolin-stimulated cAMP responses to glutamate, NAAG, the nonspecific group I, II agonist trans-ACPD, and the group II agonist DCG-IV. We conclude that beta-NAAG is a selective mGluR antagonist capable of differentiating between mGluR2 and mGluR3 subtypes and that the mGluR3 receptor functions to regulate activity-dependent synaptic potentiation in the hippocampus.

Fragile X (fmr1) mRNA expression is differentially regulated in two adult models of activity-dependent gene expression.

We sought to determine whether the fragile X mental retardation gene fmr1 is regulated in long-term potentiation (LTP) and electroconvulsive shock (ECS). In situ hybridization of fmr1 mRNA in hippocampus of rats given LTP in vivo showed no change in fmr1 mRNA levels relative to control. However, ECS induced a selective increase in fmr1 mRNA expression in the dentate gyrus (DG) granule cell layer at 6 h post-ECS. The ECS paradigm may unmask relevant activity-dependent regulatory mechanisms that modulate fmr1 gene transcription in vivo.

Acute cold stress leading to elevated corticosterone neither enhances synaptic efficacy nor impairs LTP in the dentate gyrus of freely moving rats.

Exposure to stress has previously been found to impair long-term potentiation (LTP) in the hippocampus. Exposure to stress has also been proposed to induce an LTP-like effect. We examined the effect of acute cold stress on synaptic transmission, neuronal excitability, and LTP induction in the medial perforant path-granule cell synapse of freely moving rats. After obtaining baseline recordings of evoked field potentials at room temperature (23 degrees C), rats were transferred to an environmental cage maintained at 4 degrees C (cold group) or 23 degrees C (control group) and, 90 min later, high-frequency stimulation (HFS) was applied to the medial perforant path. Serum corticosterone measured in trunk blood from rats without implanted electrodes was significantly elevated in cold exposed (28. 7 microg/dl) rats relative to control (6.6 microg/dl). Despite increased corticosterone levels indicative of stress activation, cold exposed rats exhibited LTP of the fEPSP slope and population spike of similar magnitude and time course as controls. In addition, there was no stress-specific effect on the fEPSP slope or population spike and no effect on paired-pulse plasticity. Surprisingly, despite extensive cage acclimation, transferring rats to the environmental cage was associated with a reduction in population spike amplitude and an enhancement in paired-pulse facilitation. The results show that acute cold stress leading to elevated serum corticosterone levels neither induces LTP-like increases in synaptic efficacy nor impairs tetanus-evoked LTP in the dentate gyrus of freely moving rats. Thus, impaired working memory during cold stress is not due to an inability of perforant path synapses to express LTP.

LTP in the lateral perforant path is beta-adrenergic receptor-dependent.

Norepinephrine induces an activity-independent long-lasting depression of synaptic transmission in the lateral perforant path input to dentate granule cells, whereas high frequency stimulation induces activity-dependent long-term potentiation (LTP). We investigated the role of endogenous activation of beta-adrenergic receptors in LTP of the lateral and medial perforant paths under conditions affording selective stimulation of these pathways in the rat hippo-campal slice. Propranolol (1 microM), a beta-receptor antagonist, blocked LTP induction of both lateral and medial perforant path-evoked field excitatory postsynaptic potentials. The results indicate a broad requirement for norepinephrine in different types of synaptic plasticity, including activity-independent depression and activity-dependent LTP in the lateral perforant path.

Endogenous activation of mu and delta-1 opioid receptors is required for long-term potentiation induction in the lateral perforant path: dependence on GABAergic inhibition.

Opioid peptides costored with glutamate have emerged as powerful regulators of long-term potentiation (LTP) induction in several hippocampal pathways. The objectives of the present study were twofold: (1) to identify which opioid receptor types (mu, delta, or kappa) regulate LTP induction at lateral perforant path-granule cell synapses and (2) to test the hypothesis that endogenous opioids regulate LTP induction via modulation of GABAergic inhibition. LTP of lateral perforant path-evoked field EPSPs was induced selectively by high-frequency stimulation applied to the outer third of the molecular layer of the dentate gyrus of rat hippocampal slices. No changes in medial perforant path responses occurred. LTP was blocked when high-frequency stimulation was applied in the presence of the mu receptor antagonist CTAP, the selective delta-1 receptor antagonist BNTX, or the delta-1 and delta-2 receptor antagonist naltrindole. By contrast, the kappa-1 opioid receptor antagonist NBNI had no effect on LTP induction. The role of GABAergic inhibition was investigated by comparing the effect of naloxone on LTP induction in slices maintained in standard buffer and picrotoxin-containing buffer. Naloxone blocked LTP in standard buffer, whereas normal LTP was induced in picrotoxin-treated, disinhibited slices. Finally, NMDA receptor blockade completely inhibited LTP in both standard and disinhibited slices. The results show that mu and delta-1 opioid receptors regulate LTP induction and that this mechanism critically depends on GABAergic inhibition. A key issue then becomes how endogenous opioids fine-tune the activity of intact inhibitory networks in the dentate gyrus, effectively gating synaptic plasticity in specific dendritic strata.

Unilateral LTP triggers bilateral increases in hippocampal neurotrophin and trk receptor mRNA expression in behaving rats: evidence for interhemispheric communication.

Induction of long-term potentiation (LTP) in the dentate gyrus of awake rats triggered a rapid (2 hour) elevation in tyrosine kinase receptor (trkB and trkC) gene expression and a delayed (6-24 hour) increase in brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) gene expression. Depending on the mRNA species, LTP induction led to highly selective unilateral or bilateral increases in gene expression. Specifically, trkB and NT-3 mRNA elevations were restricted to granule cells in the ipsilateral dentate gyrus, whereas bilateral increases in trkC, BDNF, and nerve growth factor (NGF) mRNA levels occurred in granule cells and hippocampal pyramidal cells. Both unilateral and bilateral changes in gene expression were N-methyl-D-aspartate (NMDA) receptor-dependent and LTP-specific. Bilateral electrophysiological recordings demonstrated that LTP was unilaterally induced; this was corroborated by a dramatic unilateral increase in the expression of the immediate early gene zif/268, a marker for LTP, restricted to the ipsilateral granule cells. The results indicate that LTP triggers an interhemispheric communication manifested as selective, bilateral increases in gene expression at multiple sites in the hippocampal network. Furthermore, our findings suggest that physiological plastic changes in the adult brain may involve coordinated, time-dependent regulation of multiple neurotrophin and trk receptor genes.

Muscarinic depression of synaptic transmission and blockade of norepinephrine-induced long-lasting potentiation in the dentate gyrus.

Bath application of the muscarinic receptor agonist, muscarine, produced a concentration-dependent depression of synaptic activity in the dentate gyrus of hippocampal slices. A concentration of 10 microM muscarine produced a reversible depression that could be competitively antagonized by the muscarinic receptor antagonist pirenzepine. However, other muscarinic receptor subtype (M1-M3) antagonists could also block the effects of muscarine. The rank order of antagonist potency was: 4-diphenylacetoxy-N-methyl-piperidine methiodide (M3/M1 antagonist) > pirenzepine (M1) > AFDX-116 (M2). The depression produced by 10 microM muscarine was not affected by in vivo pretreatment with pertussis toxin, and therefore was not mediated by a pertussis toxin-sensitive G-protein. In addition, high concentrations of muscarine did not affect either basal or isoproterenol-stimulated accumulation of cyclic AMP from slices of dentate gyrus. Muscarine also produced a concentration-dependent blockade of the induction of norepinephrine-induced long-lasting potentiation in the dentate gyrus. Norepinephrine-induced long-lasting potentiation is a form of long-lasting plasticity induced in medial perforant path synapses by beta-adrenergic agonists such as isoproterenol. The muscarinic blockade of norepinephrine-induced long-lasting potentiation was also prevented by pretreatment with pirenzepine. Based on these pharmacological data, we conclude that muscarinic depression of evoked responses, as well as blockade of norepinephrine-induced long-lasting potentiation, involves activation of either M3 or M1, but not M2, muscarinic receptors. These data also demonstrate that in addition to modulating normal synaptic transmission, muscarinic receptors may also play an important role in modulating synaptic plasticity.

Long-lasting potentiation and epileptiform activity produced by GABAB receptor activation in the dentate gyrus of rat hippocampal slice.

Bath application of the GABAB receptor agonist baclofen produced a concentration-dependent long-lasting potentiation (LLP) of the evoked population spike in the dentate gyrus of rat hippocampal slices. A high concentration of baclofen (5 microM) also produced a loss of inhibition that was manifested as the appearance of epileptiform, multiple evoked population spikes and a decrease in paired-pulse inhibition. Both baclofen-induced potentiation and epileptiform activity could be blocked or significantly reduced in slices from pertussis toxin-treated animals (1 microgram, intradentate) or in slices pretreated with the NMDA receptor antagonist D-(-)-2-amino-5-phosphonovaleric acid (10 microM). At a concentration that had no significant effect on individual evoked responses (0.1 microM) but still produced a loss in paired-pulse inhibition, baclofen facilitated the induction of beta-adrenergic receptor-mediated LLP. LLP was induced in the dentate gyrus by bath application of 1 microM, but not 0.1 microM, isoproterenol. Coapplication of baclofen and isoproterenol, both at a concentration (0.1 microM) that individually had no effect on the population spike, produced a synergistic LLP of the population spike. We propose that baclofen produces a selective disinhibitory effect in the granule cell layer of the dentate gyrus by inhibiting the activity of GABAergic interneurons. At a low concentration, the subtle loss of inhibition can facilitate the induction of isoproterenol-induced LLP. At a high concentration, baclofen can produce an LLP that is probably induced by a loss of inhibition.

Free radicals accelerate the decay of long-term potentiation in field CA1 of guinea-pig hippocampus.

Free radicals have been implicated in a number of pathological conditions. To evaluate the neurophysiological consequences of free radical exposure, slices of hippocampus isolated from guinea-pigs were exposed to hydrogen peroxide which reacts with tissue iron to generate hydroxyl free radicals. Long-term potentiation, a sustained increase in synaptic responses, was elicited in field CA1 by high frequency stimulation of an afferent pathway. We found that 0.002% peroxide did not directly affect the responses evoked by stimulation of the afferent pathway but did prevent maintenance of long-term potentiation. Short-term potentiation and paired-pulse facilitation were not affected by peroxide treatment. Peroxide was less effective if removed following high frequency stimulation and was ineffective if applied only after high frequency stimulation. Input/output analysis showed that the increase in synaptic efficacy was reduced with peroxide treatment. Changes in the enhanced ability of the synaptic potential to generate a spike were less apparent. These data show that the interference of free radicals with long-term potentiation may contribute to pathological deficits. It is possible that intracellular calcium regulation is disrupted by peroxide treatment. A number of second messenger systems involved with long-term potentiation are potential targets for free radical attack.

Beta-adrenergic agonist-induced long-lasting synaptic modifications in hippocampal dentate gyrus require activation of NMDA receptors, but not electrical activation of afferents.

Isoproterenol induced long-lasting potentiation (LLP) of the medial perforant path-evoked excitatory post-synaptic potential (EPSP) and long-lasting depression (LLD) of the lateral perforant path-evoked EPSP in the absence of perforant path activation. The NMDA receptor antagonist D-(-)-2-amino-5-phosphonovaleric acid [D(-)APV] blocked the induction of LLP and LLD. After wash, a subsequent exposure to isoproterenol induced only LLP of medial perforant path EPSPs; LLD of lateral perforant path-evoked EPSPs did not occur. Our results are consistent with the hypothesis that beta-adrenergic agonist-induced synaptic modifications in the dentate gyrus arise from pre- and postsynaptic events.

Muscarinic receptor activation facilitates the induction of long-term potentiation (LTP) in the rat dentate gyrus.

Bath application of two different concentrations of muscarine produced two different effects on evoked responses in the dentate gyrus of rat hippocampal slices. A concentration of 1 microM muscarine did not affect the evoked population spike or excitatory postsynaptic potential (EPSP), but facilitated the induction of LTP. In contrast, a concentration of 10 microM muscarine depressed both the population spike and EPSP, but had no effect on LTP induction. The M1 muscarinic receptor antagonist pirenzepine (1 microM) blocked the muscarine-induced facilitation of LTP, but had no effect on the depression of evoked responses. These data suggest that activation of M1 receptors can facilitate the induction of LTP.

NMDA receptor antagonists reduce medial, but not lateral, perforant path-evoked EPSPs in dentate gyrus of rat hippocampal slice.

NMDA receptor antagonists produced differential effects on medial and lateral perforant path-evoked excitatory postsynaptic potentials (EPSPs) recorded in the dentate gyrus molecular layer of hippocampal slices. D-(-)-2-amino-5-phosphonovaleric acid (D(-)-APV) and 3[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP) significantly reduced the peak amplitude and total area, but not the initial negative slope, of the medial perforant path-evoked EPSP. Neither antagonist affected any component of the lateral perforant path-evoked EPSP. In contrast, population spikes evoked by stimulation of either pathway were depressed.

Norepinephrine induces pathway-specific long-lasting potentiation and depression in the hippocampal dentate gyrus.

The study presented here indicates that norepinephrine (NE) selectively induces long-lasting modifications of synaptically mediated responses in the dentate gyrus of the rat hippocampal slice. A low concentration of NE (1.0 microM; in the presence of 50 microM phentolamine, an alpha-adrenergic antagonist) or a 1.0 microM concentration of the specific beta-adrenergic agonist isoproterenol induced long-lasting pathway-specific alterations of granule cell electrophysiological responses. Excitatory postsynaptic potentials and population spikes evoked by stimulation of the medial perforant pathway (PP) were potentiated for more than 45 min. In contrast, responses to lateral PP stimulation were depressed for the same period. Both potentiation and depression were blocked by the beta-adrenergic antagonist propranolol (1.0 microM). These results indicate that NE can act differentially on projections to the dentate gyrus arising in the entorhinal cortex. Such selective persistent modifications of cortical circuits may be involved in processes in the mammalian brain underlying attention, learning, and memory.

Long-term potentiation: studies in the hippocampal slice.

Long-term potentiation (LTP) is an example of activity-dependent plasticity that was discovered in the hippocampal formation. There is growing evidence that LTP not only is a useful model for mnemonic processes, but also may represent the cellular substrate for at least some kinds of learning and memory. The hippocampal slice preparation has proven exceptionally useful in pharmacological studies of possible mechanisms of LTP. A slice remains viable and stable for several hours, and known concentrations of drugs in the bathing medium can be added and then washed out. Drugs can also be applied under visual guidance from micropipettes to discrete neuronal regions, an accomplishment that is aided by the lamellar organization of the hippocampus. Electrical stimulation of the perforant path (PP) in the molecular layer of the dentate gyrus produces a monosynaptic excitatory postsynaptic potential (EPSP) and action potential, which can be recorded extracellularly as a population EPSP and population spike, respectively. Presentation of a high-frequency train (HFT; 100 Hz X 1 s) to the PP results in a long-lasting (greater than 30 min) potentiation of the maximal EPSP slope and of the population spike amplitude. Similarly, exposure of the slice to norepinephrine (e.g. 20 microM for 30 min) results in a long-lasting potentiation (LLP) of both EPSP and population spike (Stanton and Sarvey (1987) Brain Res. Bull., 18: 115). No such LLP was seen in field CA1 following NE application (Stanton and Sarvey (1985) Brain Res., 361: 276). beta-Adrenergic antagonists, such as propranolol, inhibit both LTP and NE-induced LLP in dentate (Stanton and Sarvey, J. Neurosci., 5: 2169 (1985); Stanton and Sarvey (1985) Brain Res., 361: 276). Cyclic AMP levels are increased by either an HFT or NE (Stanton and Sarvey (1985) Brain Res., 358: 343). Thus, NE, acting through a beta-receptor, appears to be both necessary and sufficient to produce long-lasting enhancement of synaptic responses. Finally, inhibitors of protein synthesis, such as emetine, also block both LTP and NE-induced LLP (Stanton and Sarvey, J. Neurosci., (1984) 4: 3080; Stanton and Sarvey (1985) Brain Res., 361: 276). The N-methyl-D-aspartate (NMDA) excitatory amino acid receptor subtype appears to play a role in a number of forms of neuronal plasticity. Bath-application of a 1 microM concentration of the NMDA antagonists D-2-amino-5-phosphonavaleric acid (AVP) or 3-((+/-)2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) blocked both LTP and NE-induced LLP in the dentate gyrus.(ABSTRACT TRUNCATED AT 400 WORDS)

NMDA receptor antagonists block norepinephrine-induced long-lasting potentiation and long-term potentiation in rat dentate gyrus.

In the in vitro rat dentate gyrus, norepinephrine-induced long-lasting potentiation (NELLP) and long-term potentiation (LTP) of responses to perforant path stimulation were blocked by the N-methyl-D-aspartate (NMDA) receptor antagonists, D(-)-2-amino-5-phosphonovaleric acid (D(-)APV) and 3-[(+/-)-2-carboxypiperazin-4-yl]propyl-1-phosphonic acid (CPP). CPP and D(-)APV, but not L(+)APV, also depressed the orthodromic population spike but not the antidromic spike, which suggests that these receptors may function in low-frequency evoked activity of granule cells. We conclude that NELLP, like LTP in the dentate gyrus, requires NMDA receptor activation.

Slow synaptic inhibition in relation to frequency habituation in dentate granule cells of rat hippocampal slices.

In paired pulse stimulation experiments the mechanism underlying frequency habituation of postsynaptic potentials in dentate granule cells of rat hippocampal slices was studied by measuring extra- and intracellular potentials as well as changes in extracellular calcium [( ([Ca2+]0) and potassium concentrations ([K+]0). Orthodromic stimulation of the perforant path induced in most granule cells a late, slow hyperpolarization (SH), lasting for up to 1.2 s. During the SH the membrane conductance was increased by up to 40%. The reversal potential of the SH was around -90 mV and varied with the [K+]0. Frequency habituation was seen in all cells with the SH, whereas cells which display frequency potentiation had no SH. Lowering of [Ca2+]0 reversed paired pulse induced frequency habituation into frequency potentiation at [Ca2+]0 levels where the SH disappeared. Phaclofen blocked the SH and reversed frequency habituation into frequency potentiation. Elevating [Mg2+]0 also reversed frequency habituation into frequency potentiation and reduced the SH. We conclude that the SH represents a late, slow IPSP which is responsible for frequency habituation in dentate granule cells. We noted that during repetitive stimulation the SH soon started to fade. This effect can in part be attributed to extracellular K+-accumulation as suggested by the K+-dependence of the slow IPSP and the observations of changes in [K+]0 during repetitive stimulation. This could explain why frequency habituation reverses into frequency potentiation during repetitive stimulation.