Effects of low-frequency tetanic stimulation of the synaptic inputs on different forms of long-term potentiation in the rat hippocampus

In experiments on transversal slices of the dorsal hippocampus of rats, we found that low-frequency stimulation of the mossy fibers (MF) against the background of pre-settled long-term post-tetanic potentiation in the MF-CA3 pyramidal neuron (PN) dendrites synaptic system evoked depotentiation in all studied slices. Depotentiation was considerably decreased by a non-competitive blocker of the NMDA glutamate receptors, ketamine (100 μM), as well as by an inhibitor of calmodulin, trifluoroperazine (10 μM), and an inhibitor of calcineurin, cyclosporin A (250 μM). At the same time, depontentiation was not changed by 50 μM polymixin B, an inhibitor of protein kinase C. Long-term potentiation of synaptic transmission in the Schaffer collaterals (SchC)-CA1 PN dendrites system, which was evoked by 2.5-min-long anoxia/aglycemia episodes, resulted exclusively from enhancement of the NMDA component of population EPSP, while their AMPA component was not modified, i.e., in this case potentiation was of a postsynaptic nature. Under these conditions, low-frequency stimulation of SchC resulted in a further increase in the intensity of synaptic transmission due to increases in both the NMDA and AMPA components of population EPSP. The above form of potentiation could be suppressed by 100 μM ketamine, 10 μM trifuoroperazine, 250 μM cyclosporin A, or 10 μM N-nitro-L-arginine. Weak (near-threshold) high-frequency stimulation of SchC induced long-lasting potentiation of synaptic transmission due to an isolated increase in the AMPA component of population EPSP, i.e., this potentiation was of a postsynaptic nature. In the latter case, low-frequency SchC stimulation resulted in further facilitation of synaptic transmission. Intensive tetanic high-frequency stimulation of the above fibers induced long-term potentiation of a presynaptic nature, while their low-frequency stimulation depotentiated synaptic transmission.     

Neurochemical mechanisms responsible for depotentiation of synaptic transmission

It was established in experiments on murine hippocampal slices that low-frequency (1 sec⁻¹, 15 min) stimulation of the Schaffer collaterals applied 45 to 60 min after their high-frequency repetitive stimulation (60 sec⁻¹, 0.5 sec) results, in 2/3 of the slices, in reduction of the amplitude of population EPSP recorded from pyramidal neurons of theCA1 area, almost to its level before high-frequency stimulation. Depotentiation was practically completely prevented by application of a non-competitive blocker of NMDA glutamate receptors (GR), ketamine (100 μM), was weakened by a blocker of voltage-dependent L-type Ca²⁺ channels, nifedipine (10 μM), and remained significant after a competitive blocker of the AMPA/kainate receptors, CNQX (10 μM), had been applied to the slices. Depotentiation was significantly reduced by 10 μM of a calmodulin inhibitor, trifluoroperazine, by an increase in the intracellular cAMP concentration caused by activation of A2-adenosine receptors and D5-dopamine receptors, but was resistant to the action of 50 μM of a protein kinase C (PKC) inhibitor, polymixin B. Nootropic compounds possessing anti-amnestic activity enhanced the depotentiation. It is suggested that depotentiation is due to an increase in the intracellular Ca²⁺ concentration, activation of protein phosphatases, and dephosphorylation of pre- and post-synaptic substrates involved in the expression of long-term post-tetanic potentiation of synaptic transmission, which result from cooperative activation of NMDA GR and metabotropic GR.     

Neurochemical mechanisms underlying NMDA-independent long-term post-tetanic potentiation of synaptic transmission

In experiments performed on rat transversial slices of the rat dorsal hippocampus, we found that high-frequency tetanic stimulation of the mossy fibers (MF) and short-term action of 1 μM kainic acid on the slices resulted in long-term potentiation of the population spikes evoked inCA3 pyramidal neurons by single stimuli applied to the MF. The tetanus-and kainate-induced potentiations of synaptic transmission were accompanied by a decrease in the degree of paired facilitation at a 50-msec-long interstimulus interval; they were additive, prevented by 10 μM CNQX, a competitive antagonist of AMPA/kainate receptors, and insensitive to 100 μM ketamine, a noncompetitive antagonist of NMDA-glutamate receptors. Both types of potentiation were enhanced by 10 μM (1S, 3R)-ACPD, an agonist of metabotropic glutamate receptors, as well as by 1 μM pyracetam or 50 μM dichlothiazide, substances weakening AMPA/kainate receptor desensitization. The effects produced by high-frequency tetanic stimulation of the MF and by kainic acid were prevented by 50 μM polymixin B, a protein kinase C blocker, and weakened by 10 μM trifluoroperazine, a calmodulin inhibitor, or 1 μM pirenzepine, an M1 acetylcholine receptor blocking agent. In total, the above data suggest that the tetanus- and kainate-induced potentiations of transmission in the synapses formed by the MF and dendrites ofCA3 pyramidal neurons are due to the combined activation of pre-synaptic high-affinity kainate-preferring receptors, located in the membranes of the MF varicosities, and post-synaptic phosphoinositide metabolism-coupled metabotropic glutamate receptors and 1 and M1 acetylcholine receptors. This activation results in a significant increase in the activity of epsilon-form protein kinase C, phosphorylation of protein substrates involved in vesicular glutamate release from the MF varicosities, and long-term enhancement of presynaptic glutamate release.     

Mechanisms of Long-Lasting Depression of Synaptic Transmission in the CA1 Region of the Rat Hippocampus

Low-frequency tetanic stimulation (2 sec^-1, 5 min) of Schaffer collaterals (SchC) in superfused slices of the dorsal hippocampus of 12- to 15-day-old rats was demonstrated to evoke homosynaptic long-lasting depression (LLD) of synaptic transmission. The same procedure applied to hippocampal slices of mature (8-week-old or older) rats failed to elicit LLD. Low-frequency tetanic stimulation of the alveus in hippocampal slices, applied under conditions of intensified NMDA glutamate receptor functioning, led to the development of heterosynaptic LLD of synaptic transmission in the SchC–dendrites of the CA1 pyramidal neurons system. Both LLD cases were either absent or weakened when hippocampal slices were treated with a competitive blocker of the NMDA glutamate receptors, D-2-amino-5-phosphonovalerate (50 M). Morphine hydrochloride (10 M), as well as inhibitors of calmodulin and calcineurin (trifluoroperasine and cyclosporin A in concentrations of 1 and 50 M, respectively), interfered with induction of LLD or decreased its intensity. A blocker of the L-type voltage-dependent Ca²⁺ channels, nifedipine (10 M), did not influence homosynaptic LLD, but decreased heterosynaptic depression. Both types of depression of synaptic transmission were facilitated upon application of substances possessing a nootropic activity, 1 mM pyracetam or 5 M carbacetam. A blocker of NO synthase, N-nitro-L-arginine (10 M) did not alter either type of LLD. When hippocampal slices were influenced with a blocker of the A1 adenosine receptors, 1,3-dipropyl-8-phenylxanthine (1 M, 15 min), both LLD forms were intensified, and the development of homosynaptic LLD of synaptic transmission became possible in hippocampal slices of mature rats. When hippocampal slices were treated with an inhibitor of protein kinase C, polymyxin B (50 M, 15 min), intensification of LLD and, in particular, the development of homosynaptic LLD of synaptic transmission were observed. When an inhibitor of phospholipase A2, mepacrine (25 M, 15 min), was applied to hippocampal slices, both forms of LLD of synaptic transmission were significantly suppressed.     

Dynamic control of presynaptic Ca(2+) inflow by fast-inactivating K(+) channels in hippocampal mossy fiber boutons

Analysis of presynaptic determinants of synaptic strength has been difficult at cortical synapses, mainly due to the lack of direct access to presynaptic elements. Here we report patch-clamp recordings from mossy fiber boutons (MFBs) in rat hippocampal slices. The presynaptic action potential is very short during low-frequency stimulation but is prolonged up to 3-fold during high-frequency stimulation. Voltage-gated K(+) channels in MFBs inactivate rapidly but recover from inactivation very slowly, suggesting that cumulative K(+) channel inactivation mediates activity-dependent spike broadening. Prolongation of the presynaptic voltage waveform leads to an increase in the number of Ca(2+) ions entering the terminal per action potential and to a consecutive potentiation of evoked excitatory postsynaptic currents at MFB-CA3 pyramidal cell synapses. Thus, inactivation of presynaptic K(+) channels contributes to the control of efficacy of a glutamatergic synapse in the cortex.     

Synaptic plasticity at archicortical and neocortical levels

Research carried out by the author and his collaborators, devoted to analysis of the properties and neurophysiological mechanisms of long-term (for several hours) potentiation, is surveyed. Long-term potentiation of focal potentials and unitary responses of strictly hippocampal structures (areas CA₁ and CA₃) in the unanesthetized rabbit is described. Enhancement of excitatory (EPSPs) and inhibitory (IPSPs) postsynaptic potentials was found after tetanization. No corresponding changes of sensitivity to acetylcholine or acetylcholinesterase activity were found by microiontophoretic and histochemical methods during long-term potentiation. Statistical analysis of EPSPs evoked by microstimulation, based on the quantal hypothesis of synaptic transmission, showed an increase in the number of quanta of transmitter release during potentiation. Long-term potentiation of focal potentials during stimulation of the subcortical white matter in surviving neocortical slices and also long-term potentiation of focal and unitary responses of the sensomotor cortex of the unanesthetized rabbit are described. Potentiation of the "indirect" component of the global response of the pyramidal tract was found. The data suggest the presence of long-term potentiation of monosynaptic neocortical responses. It is concluded that the main mechanism of both hippocampal and neocortical long-term potentiation is increased efficiency of excitatory synapses. It is postulated that synapses modified in this way are used in the formation of memory traces.Brain Institute, All-Union Mental Health Research Center, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 16, No. 5, pp. 651–665, September–October, 1984.     

Effects of blockers of voltage-operated potassium channels on an NMDA component of excitatory synaptic transmission in theCA1 subfield of the rat hippocampus

The effects of voltage-operated potassium channel blockers on evoked excitatory synaptic transmission were studied in theCA1 subfield of rat hippocampal slices. Incubation with 50 μM 4-aminopyridine (n=27), 300 nM α-dendrotoxin (n=3), or 5 to 25 mM tetraethylammonium (n=7) resulted in an enhancement of the peak amplitude of excitatory postsynaptic currents (EPSC) and significant prolongation of their decay at strong stimuli, due to an increased contribution of NMDA receptors into EPSC. In five experiments, the presence of an AMPA receptor antagonist, 4-aminopyridine, led to the appearance of NMDA receptor-mediated field excitatory postsynaptic potentials (fEPSP). It is suggested that various modulations increasing presynaptic Ca²⁺ entry and, consequently, glutamate release may increase an NMDA component of synaptic transmission via excitation of polysynaptic excitatory pathways and/or due to glutamate spillover to distant extrasynaptic NMDA receptors.     

Arachidonic Acid, but Not Sodium Nitroprusside, Stimulates Presynaptic Protein Kinase C and Phosphorylation of GAP-43 in Rat Hippocampal Slices and Synaptosomes

Abstract: Activation of protein kinase C (PKC) and phosphorylation of its presynaptic substrate, the 43-kDa growth-associated protein GAP-43, may contribute to the maintenance of hippocampal long-term potentiation (LTP) by enhancing the probability of neurotransmitter release and/or modifying synaptic morphology. Induction of LTP in rat hippocampal slices by high-frequency stimulation of Schaffer collateral-CA1 synapses significantly increased the PKC-dependent phosphorylation of GAP-43, as assessed by quantitative immunoblotting with a monoclonal antibody that recognizes an epitope that is specifically phosphorylated by PKC. The stimulatory effect of high-frequency stimulation on levels of immunoreactive phosphorylated GAP-43 was not observed when 4-amino-5-phosphonovalerate (50 µM), an N-methyl-d -aspartate (NMDA) receptor antagonist, was bath-applied during the high-frequency stimulus. This observation supports the hypothesis that a retrograde messenger is produced postsynaptically following NMDA receptor activation and diffuses to the presynaptic terminal to activate PKC. Two retrograde messenger candidates—arachidonic acid and nitric oxide (sodium nitroprusside was used to generate nitric oxide)—were examined for their effects in hippocampal slices on PKC redistribution from cytosol to membrane as an indirect measure of enzyme activation and PKC-specific GAP-43 phosphorylation. Bath application of arachidonic acid, but not sodium nitroprusside, at concentrations that produce synaptic potentiation (100 µM and 1 mM, respectively) significantly increased translocation of PKC immunoreactivity from cytosol to membrane as well as levels of immunoreactive, phosphorylated GAP-43. The stimulatory effect of arachidonic acid on GAP-43 phosphorylation was also observed in hippocampal synaptosomes. These results indicate that arachidonic acid may contribute to LTP maintenance by activation of presynaptic PKC and phosphorylation of GAP-43 substrate. The data also suggest that nitric oxide does not activate this signal transduction system and, by inference, activates a distinct biochemical pathway.     

Donepezil in a Narrow Concentration Range Augments Control and Impaired by Beta-Amyloid Peptide Hippocampal LTP in NMDAR-Independent Manner

Acetylcholinesterase (AChE) inhibitor donepezil is widely used for the treatment of Alzheimer’s disease (AD). The mechanisms of therapeutic effects of the drug are not well understood. The ability of donepezil to reverse a known pathogenic effect of β-amyloid peptide (Abeta), namely, the impairment of hippocampal long-term potentiation (LTP), was not studied yet. The goal of the present study was to study the influence of donepezil in 0.1–10 μM concentrations on control and Abeta-impaired hippocampal LTP. Possible involvement of N-methyl-d-aspartate receptors (NMDARs) into mechanisms of donepezil action was also studied. LTP of population spike (PS) was studied in the CA1 region of rat hippocampal slices. Change of LTP by donepezil treatment had a bell-shaped dose–response curve. The drug in concentrations of 0.1 and 1 μM did not change LTP while in concentration of 0.5 μM significantly increased it, and in concentration of 5 and 10 μM suppressed LTP partially or completely. Abeta (200 nM) markedly suppressed LTP. Addition of 0.1, 0.5 or 1 μM donepezil to Abeta solution caused a restoration of LTP. N-methyl-d-aspartate (NMDA) currents were studied in acutely isolated pyramidal neurons from CA1 region of rat hippocampus. Neither Abeta, nor 0.5 μM donepezil were found to change NMDA currents, while 10 μM donepezil rapidly and reversibly depressed it. Results suggest that donepezil augments control and impaired by Abeta hippocampal LTP in NMDAR-independent manner. In general, our findings extend the understanding of mechanisms of therapeutic action of donepezil, especially at an early stage of AD, and maybe taken into account while considering the possibility of donepezil overdose.     

Long-term potentiation at hippocampal perforant path-dentate astrocyte synapses

Accumulated evidence indicates that astroglial cells actively participate in neuronal synaptic transmission and plasticity. However, it is still not clear whether astrocytes are able to undergo plasticity in response to synaptic inputs. Here we demonstrate that a long-term potentiation (LTP)-like response could be detected at perforant path-dentate astrocyte synapses following high-frequency stimulation (HFS) in hippocampal slices of GFAP-GFP transgenic mice. The potentiation was not dependent on the glutamate transporters nor the group I metabotropic glutamate receptors. However, the induction of LTP requires activation of the NMDA receptor (NMDAR). The presence of functional NMDAR was supported by isolating the NMDAR-gated current and by identifying mRNAs of NMDAR subunits in astrocytes. Our results suggest that astrocytes in the hippocampal dentate gyrus are able to undergo plasticity in response to presynaptic inputs.     

<Emphasis Type="Italic">In vivo</Emphasis> injected mitochondria-targeted plastoquinone antioxidant SkQR1 prevents β-amyloid-induced decay of long-term potentiation in rat hippocampal slices

Addition of 200 nM β-amyloid 1–42 (Abeta) to a rat hippocampal slice impairs the induction of a long-term post-tetanic potentiation (LTP) of population spike (PS) in pyramidal neurons of the CA1 field of hippocampus. Intraperitoneal injection into the rat of the mitochondria-targeted plastoquinone derivative SkQR1 (1 μmol/kg of weight given 24 h before the slices were made) abolishes the deleterious effect of Abeta on LTP. These data demonstrate that SkQR1 therapy is able to compensate the Abeta-induced impairments of long-term synaptic plasticity in the hippocampus, which are the main cause of loss of memory and other cognitive functions associated with Alzheimer’s disease.     

Histamine Receptor Expression, Hippocampal Plasticity and Ammonia in Histidine Decarboxylase Knockout Mice

Genetic ablation of the histamine producing enzyme histidine decarboxylase (HDC) leads to alteration in exploratory behaviour and hippocampus-dependent learning. We investigated how brain histamine deficiency in HDC knockout mice (HDC KO) affects hippocampal excitability, synaptic plasticity, and the expression of histamine receptors. No significant alterations in: basal synaptic transmission, long-term potentiation (LTP) in the Schaffer collateral synapses, histamine-induced transient changes in the CA1 pyramidal cell excitability, and the expression of H1 and H2 receptor mRNAs were found in hippocampal slices from HDC KO mice. However, when compared to WT mice, HDC KO mice demonstrated: 1. a stronger enhancement of LTP by histamine, 2. a stronger impairment of LTP by ammonia, 3. no long-lasting potentiation of population spikes by histamine, 4. a decreased expression of H3 receptor mRNA, and 5. less potentiation of population spikes by H3 receptor agonism. Parallel measurements in the hypothalamic tuberomamillary nucleus, the origin of neuronal histamine, demonstrated an increased expression of H3 receptors in HDC KO mice without any changes in the spontaneous firing of “histaminergic” neurons without histamine and their responses to the H3 receptor agonist (R)-α-methylhistamine. We conclude that the absence of neuronal histamine results in subtle changes in hippocampal synaptic transmission and plasticity associated with alteration in the expression of H3 receptors.     

Eugenol depresses synaptic transmission but does not prevent the induction of long-term potentiation in the CA1 region of rat hippocampal slices

Using field potential recording in the CA1 region of the rat hippocampal slices, the effects of eugenol on synaptic transmission and long-term potentiation (LTP) were investigated. Population spikes (PS) were recorded in the stratum pyramidal following stimulation of stratum fibers. To induce LTP, eight episodes of theta pattern primed-bursts (PBs) were delivered. Eugenol decreased the amplitude of PS in a concentration-dependent manner. The effect was fast and completely reversible. Eugenol had no effect on PBs-induced LTP of PS. It is concluded that while eugenol depresses synaptic transmission it does not affect the ability of CA1 synapses for tetanus-induced LTP and plasticity.     

Effects of Chronic Introduction of Antidepressants on NMDA-Induced Damage to Neurons of the Rat Hippocampus and Cerebral Cortex

In in vitro studies on superfused slices obtained from the rat hippocampus and cortex, we found that 50 μM N-methyl-D-aspartate (NMDA) applied to the slices in the presence of 10 μM glycine for 15 min exerts a significant damaging action to neurons of these structures. One hour after termination of the action of NMDA, this was manifested in more than a twofold decrease in the synaptic reactivity of pyramidal neurons of the hippocampal СА1 area and layers II/III of the cerebral cortex. The excitotoxic effect of NMDA was prevented by application of competitive (D-2-amino-5-phosphonovaleric acid, 50 μM) and noncompetitive (ketamine, 100 μM) blockers of NMDA receptors. A blocker of glycine-binding sites of NMDA receptors (compound ТСВ 24.15, 10 μM) weakened NMDA-induced damage to the neurons. A competitive blocker of glutamate АМРА receptors, 6,7-dinitroquinoxaline-2,3-dione (DNQX, 10 μM), and a local anesthetic, lidocaine hydrochloride (50 μM), did not modify the excitotoxic effect of NMDA. A blocker of voltagedependent L-type calcium channels, verapamil (20 μM), demonstrated some trend to intensification of NMDA excitotoxic action. An inhibitor of tyrosine-protein phosphatases, sodium vanadate, when i.p. injected into rats in a dose of 15 mg/kg 6 h prior to the electrophysiological experiment, decreased the damaging action of NMDA. Two-hour-long treatment of cerebral slices with 1 μM genistein, an inhibitor of tyrosine kinases, weakened the neuroprotective effect of sodium vanadate. Chronic injections (14 days in daily doses of 20 mg/kg) of antidepressants belonging to different functional classes (imipramine, fluoxetine, and pyrazidol) into rats decreased (similarly to blockers of NMDA receptors) the excitotoxic action of NMDA receptors. Neuroprotective effects of antidepressants were weakened upon the action of genistein. We conclude that the neuroprotective activity of antidepressants under conditions of excitotoxic action of NMDA is mainly determined by an increase in the activity of tyrosine kinases in the cytoplasm and/or neuronal nucleus.     

Effect of an Agonist of β<Subscript>2</Subscript> Adrenoreceptors on Inhibition in the Rat Hippocampal <Emphasis Type="Italic">CA1</Emphasis> Area Induced by Activation of GABA<Subscript>B</Subscript> Receptors

In experiments on rat brain slices using extracellular recording, we studied the effects of an agonist of β₂ adrenoreceptors, metaproterenol (MPT), on reactions of pyramidal neurons of the hippocampal CA1 area induced by activation of GABA_B receptors. Isolated application of an agonist of GABA_B receptors, baclofen (10 μM), resulted in intense inhibition (by 50% or more during the 1st min of action) of orthodromic field discharges (OFDs) in the pyramidal layer of the above-mentioned area of the hippocampus; the discharges were evoked by electrical stimulation of Schaffer collaterals. On the 3rd to 4th min, OFDs were suppressed nearly completely. After washing out from baclofen, the parameters of the evoked responses never completely restored to the initial level. In all cases, simultaneous application of 150 μM MPT and 10 μM baclofen prevented full manifestation of the inhibitory effect of the latter agent on OFDs of pyramidal neurons. The amplitude of evoked responses decreased, but the relative intensity of inhibition under these conditions during 2-min-long application was significantly lower than that upon isolated action of baclofen. The recovery of the amplitude of evoked responses in the course of washing out under conditions of parallel action of MPT was more rapid and, in some cases, complete. Therefore, our experiments showed that GABA_B-ergic inhibitory transmission in the rat hippocampal CA1 area in vitro can be suppressed significantly by the β₂ adrenoreceptor agonist.     

Effects of mibefradil on synaptic transmission in the hippocampus and on voltage-dependent currents in isolated hippocampal and thalamic neurons of the rat

The effects of a novel anti-hypertensive drug, mibefradil, on voltage-dependent currents in isolated thalamic and hippocampal neurons, as well as on synaptic transmission in the hippocampus have been studied. Mibefradil exerted a potent inhibitory action on low-threshold calcium currents in thalamic neurons (IC₅₀=160 nM). In higher concentrations (1–20 μM), this drug blocked not only low-threshold calcium current but also voltage-dependent sodium and delayed potassium currents in pyramidal hippocampal neurons. The amplitude of population action potentials in hippocampal slices decreased by 55% in the presence of 20μM mibefradil. All of the effects of mibefradil were almost completely reversible. In our experiments, the sensitivity of low-threshold calcium channels in thalamic neurons to mibefradil was higher than that observed on other objects. The ability of mibefradil to block not only calcium currents but also other types of voltage-dependent ion conductances in hippocampal neurons may be considered an essential factor that determines the specificity of the pharmacological profile of this drug.     

Sensitivity of hippocampal interneurons and pyramidal cells to GABA and some of its agonists:in vitro experiments

Effects of GABA and its agonists baclofen and muscimol on the background spike activity of single hippocampal neurons were studied in rat brain slices using an intracellular recording technique. Interneurons localized in thestratum alveus-oriens and pyramidal neurons of thestratum pyramidale showed high sensitivity to GABA (mean ID₅₀=65 µM and 40 µM, ranges 10–140 µM and 3–200 µM), baclofen (ID₅₀=2.6 µM and 3.5 µM, ranges 0.6–20.0 µM and 0.4–30.0 µM), and muscimol (ID₅₀=0.85 µM and 0.21 µM, ranges 0.11–4.0 µM and 0.05–0.45 µM, respectively). Responses of hippocampal neurons to application of GABA or either of its agonists were predominantly inhibitory. A part of interneurons (30%) differed from pyramidal neurons in their irresponsivity or low sensitivity to baclofen applications. GABA- or muscimol-induced inhibition of spike activity in many pyramidal cells was preceded by a short-lasting excitation. Our findings indicate that a part of hippocampal interneurons are very poorly supplied with GABAb receptors. Inhibition of pyramidal cells evoked by activation of GABAa receptors probably develops against the background of accompanying depolarization, which in some cases can result in a provisional excitation of these neurons. The excitatory effects of GABA on the pyramidal cells are mediated by GABAa receptors.Neirofiziologiya/Neurophysiology, Vol. 27, No. 1, pp. 36–44, January–February, 1995.     

Astrocytes mediate in vivo cholinergic-induced synaptic plasticity

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