Whole-cell voltage-clamp investigation of the role of PKC in muscarinic inhibition of IAHP in rat CA1 hippocampal neurons

Endocannabinoids Induce Lateral Long-Term Potentiation of Transmitter Release by Stimulation of Gliotransmission

Endocannabinoids (eCBs) play key roles in brain function, acting as modulatory signals in synaptic transmission and plasticity. They are recognized as retrograde messengers that mediate long-term synaptic depression (LTD), but their ability to induce long-term potentiation (LTP) is poorly known. We show that eCBs induce the long-term enhancement of transmitter release at single hippocampal synapses through stimulation of astrocytes when coincident with postsynaptic activity. This LTP requires the coordinated activity of the 3 elements of the tripartite synapse: 1) eCB-evoked astrocyte calcium signal that stimulates glutamate release; 2) postsynaptic nitric oxide production; and 3) activation of protein kinase C and presynaptic group I metabotropic glutamate receptors, whose location at presynaptic sites was confirmed by immunoelectron microscopy. Hence, while eCBs act as retrograde signals to depress homoneuronal synapses, they serve as lateral messengers to induce LTP in distant heteroneuronal synapses through stimulation of astrocytes. Therefore, eCBs can trigger LTP through stimulation of astrocyte-neuron signaling, revealing novel cellular mechanisms of eCB effects on synaptic plasticity.

Metabotropic-mediated kainate receptor regulation of IsAHP and excitability in pyramidal cells

Kainate receptors (KARs) on CA1 pyramidal cells make no detectable contribution to EPSCs. We report that these receptors have a metabotropic function, as shown previously for CA1 interneurons. Brief kainate exposure caused long-lasting inhibition of a postspike potassium current (I(sAHP)) in CA1 pyramidal cells. The pharmacological profile was independent of AMPA receptors or the GluR5 subunit, indicating a possible role for the GluR6 subunit. KAR inhibition of I(sAHP) did not require ionotropic action or network activity, but was blocked by the inhibitor of pertussis toxin-sensitive G proteins, N-ethylmaleimide (NEM), or the PKC inhibitor calphostin C. These data suggest how KARs, putatively containing GluR6, directly increase excitability of CA1 pyramidal cells and help explain the propensity for seizure activity following KAR activation.

Calcium-dependent persistent facilitation of spike backpropagation in the CA1 pyramidal neurons

Sodium-dependent action potentials initiated near the soma are known to backpropagate over the dendrites of CA1 pyramidal neurons in an activity-dependent manner. Consequently, later spikes in a train have smaller amplitude when recorded in the apical dendrites. We found that depolarization and resultant Ca(2+) influx into dendrites caused a persistent facilitation of spike backpropagation. Dendritic patch recordings were made from CA1 pyramidal neurons in mouse hippocampal slices under blockade of fast excitatory and inhibitory synaptic inputs. Trains of 10 backpropagating action potentials induced by antidromic stimulation showed a clear decrement in the amplitude of later spikes when recorded in the middle apical dendrites. After several depolarizing current pulses, the amplitude of later spikes increased persistently, and all spikes in a train became almost equal in size. BAPTA (10 mm) contained in the pipette or low-Ca(2+) perfusing solution abolished this depolarization-induced facilitation, indicating that Ca(2+) influx is required. This facilitation was present in Galpha(q) knock-out mice that lack the previously reported muscarinic receptor-mediated enhancement of spike backpropagation. Therefore, these two forms of facilitation are clearly distinct in their intracellular mechanisms. Intracellular injection of either calmodulin binding domain (100 micrometer) or Ca(2+)/calmodulin-kinase II (CaMKII) inhibitor 281-301 (10 micrometer) blocked the depolarization-induced facilitation. Bath application of a membrane-permeable CaMKII inhibitor KN-93 (10 micrometer) also blocked the facilitation, but KN-92 (10 micrometer), an inactive isomer of KN-93, had no effect. These results suggest that increases in [Ca(2+)](i) cause persistent facilitation of spike backpropagation in the apical dendrite of CA1 pyramidal neuron by CaMKII-dependent mechanisms.

A protein phosphatase is involved in the cholinergic suppression of the Ca(2+)-activated K(+) current sI(AHP) in hippocampal pyramidal neurons

The slow calcium-activated potassium current sI(AHP) underlies spike-frequency adaptation and has a substantial impact on the excitability of hippocampal CA1 pyramidal neurons. Among other neuromodulatory substances, sI(AHP) is modulated by acetylcholine acting via muscarinic receptors. The second-messenger systems mediating the suppression of sI(AHP) by muscarinic agonists are largely unknown. Both protein kinase C and A do not seem to be involved, whereas calcium calmodulin kinase II has been shown to take part in the muscarinic action on sI(AHP). We re-examined the mechanism of action of muscarinic agonists on sI(AHP) combining whole-cell recordings with the use of specific inhibitors or activators of putative constituents of the muscarinic pathway. Our results suggest that activation of muscarinic receptors reduces sI(AHP) in a G-protein-mediated and phospholipase C-independent manner. Furthermore, we obtained evidence for the involvement of the cGMP-cGK pathway and of a protein phosphatase in the cholinergic suppression of sI(AHP), whereas release of Ca(2+) from IP(3)-sensitive stores seems to be relevant neither for maintenance nor for modulation of sI(AHP).

Muscarinic receptor activation induces depolarizing plateau potentials in bursting neurons of the rat subiculum

Acetylcholine functions as a neuromodulator in the mammalian brain by binding to specific receptors and thus bringing about profound changes in neuronal excitability. Activation of muscarinic receptors often results in an increased excitability of cortical cells. It is, however, unknown whether such an action is present in the subiculum, a limbic structure that may be involved in cognitive processes as well as in seizure propagation. Most rat subicular neurons are endowed of intrinsic membrane properties that make them fire action potential bursts. Using intracellular recordings from these bursting cells in a slice preparation, we report here that application of the cholinergic agonist carbachol (CCh, 30-100 microM) to medium containing ionotropic excitatory amino acid receptor antagonists reduces burst-afterhyperpolarizations (burst-AHPs) and discloses depolarizing plateau potentials that outlast the triggering current pulses by 140-2,800 ms. These plateau potentials appear with CCh concentrations >50 microM and are dependent on the resting membrane potential and on the intensity/duration of the triggering pulse; are recorded during application of tetrodotoxin (1 microM, n = 5 neurons); but are markedly reduced by replacing 82% of extracellular Na(+) with equimolar choline (n = 6). Plateau potentials also are abolished by Co(2+) (2 mM; n = 5) or Cd(2+) (1 mM; n = 2) application and by recording with electrodes containing the Ca(2+) chelator bis(2-aminophenoxy)ethane-N, N,N',N'-tetraacetic acid (0.2 M; n = 6). CCh-induced burst-AHP reduction and plateau potentials are reversed by the muscarinic antagonist atropine (0.5 microM, n = 7). In conclusion, our findings demonstrate a powerful muscarinic modulation of the intrinsic excitability of subicular bursting cells that is predominated by the appearance of plateau potentials. These changes in excitability may contribute to physiological processes such as learning or memory and play a role in the generation of epileptiform depolarizations. We propose that, as in other limbic structures, muscarinic plateau potentials in the subiculum are mainly due to a Ca(2+)-dependent nonselective cationic conductance.

Muscarinic acetylcholine receptors in the hippocampus, neocortex and amygdala: a review of immunocytochemical localization in relation to learning and memory

Immunocytochemical mapping studies employing the extensively used monoclonal anti-muscarinic acetylcholine receptor (mAChR) antibody M35 are reviewed. We focus on three neuronal muscarinic cholinoceptive substrates, which are target regions of the cholinergic basal forebrain system intimately involved in cognitive functions: the hippocampus; neocortex; and amygdala. The distribution and neurochemistry of mAChR-immunoreactive cells as well as behaviorally induced alterations in mAChR-immunoreactivity (ir) are described in detail. M35+ neurons are viewed as cells actively engaged in neuronal functions in which the cholinergic system is typically involved. Phosphorylation and subsequent internalization of muscarinic receptors determine the immunocytochemical outcome, and hence M35 as a tool to visualize muscarinic receptors is less suitable for detection of the entire pool of mAChRs in the central nervous system (CNS). Instead, M35 is sensitive to and capable of detecting alterations in the physiological condition of muscarinic receptors. Therefore, M35 is an excellent tool to localize alterations in cellular cholinoceptivity in the CNS. M35-ir is not only determined by acetylcholine (ACh), but by any substance that changes the phosphorylation/internalization state of the mAChR. An important consequence of this proposition is that other neurotransmitters than ACh (especially glutamate) can regulate M35-ir and the cholinoceptive state of a neuron, and hence the functional properties of a neuron. One of the primary objectives of this review is to provide a synthesis of our data and literature data on mAChR-ir. We propose a hypothesis for the role of muscarinic receptors in learning and memory in terms of modulation between learning and recall states of brain areas at the postsynaptic level as studied by way of immunocytochemistry employing the monoclonal antibody M35.

Differential control of three after-hyperpolarizations in rat hippocampal neurones by intracellular calcium buffering

1. The whole-cell recording technique, combined with internal perfusion, was used to study the effects of intracellular Ca2+ buffering on fast, medium and slow after-hyperpolarizations (fAHP, mAHP and sAHP) in hippocampal CA1 pyramidal neurones in rat brain slices at room temperature. 2. The action potentials and the fAHP were unaffected by 100 microM to 3 mM concentrations of the internally applied fast Ca2+ chelator BAPTA. At higher (10-15 mM) concentrations, BAPTA inhibited the fAHP and prolonged the decay of the action potential, suggesting that the corresponding large-conductance Ca2+-activated K+ channels are located close to the sites of Ca2+ entry during an action potential. Addition of Ca2+ to the BAPTA-containing solution (at a ratio of 4.5 [Ca2+] : 10 [BAPTA]) to maintain the control level of [Ca2+]i did not prevent the effects of high concentrations of BAPTA. 3. The mAHP, activated by a train of action potentials, was inhibited by internally applied BAPTA within the range of concentrations used (100 microM to 15 mM), and this effect could not be reversed or prevented by addition of Ca2+ to the BAPTA-containing solution. The inhibition of the mAHP by BAPTA could also be observed after blockade of the hyperpolarization-activated IQ type mixed Na+-K+ current (also known as Ih) component of the mAHP by bath-applied 3-5 mM Cs+, suggesting that the inhibition of the mAHP by BAPTA is due to inhibition of the depolarization-activated IM (muscarinic) type K+ current. 4. The sAHP, activated by a train of action potentials, was potentiated by 100-300 microM internally applied BAPTA, both with and without added Ca2+. At 1-2 mM or higher concentrations, the potentiation of the sAHP by BAPTA without added Ca2+ was transient and was followed by a fast decrease. With added Ca2+, however, BAPTA caused a persistent potentiation of the sAHP with more than a 10-fold increase in duration for periods exceeding 1 h even at concentrations of the buffer as high as 10-15 mM. Earlier reports showing a blockade of the sAHP by BAPTA, based on experiments without added Ca2+, were apparently due to a sharp reduction in intracellular free [Ca2+] and to a high intracellular concentration of the free buffer. 5. Internally applied BAPTA caused a prolongation of the spike discharge during an 800 ms-long depolarizing current step. At 100-300 microM BAPTA, but not at 1-2 mM or higher concentrations, this effect could be reversed by addition of Ca2+. The effects of BAPTA on the spike discharge occurred in parallel with the changes in the sAHP time course, which was more prolonged at higher concentrations of the buffer. 6. The concentration-dependent differential control of the three types of AHP in hippocampal neurones by BAPTA is related to modulation of intracellular Ca2+ diffusion by a fast acting mobile Ca2+ buffer.

RGS4 inhibits signaling by group I metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) couple to heterotrimeric G-proteins and regulate cell excitability and synaptic transmission in the CNS. Considerable effort has been focused on understanding the cellular and biochemical mechanisms that underlie regulation of signaling by G-proteins and their linked receptors, including the mGluRs. Recent findings demonstrate that regulators of G-protein signaling (RGS) proteins act as effector antagonists and GTPase-activating proteins for Galpha subunits to inhibit cellular responses by G-protein-coupled receptors. RGS4 blocks Gq activation of phospholipase Cbeta and is expressed broadly in rat brain. The group I mGluRs (mGluRs 1 and 5) couple to Gq pathways to regulate several effectors in the CNS. We examined the capacity of RGS4 to regulate group I mGluR responses. In Xenopus oocytes, purified RGS4 virtually abolishes the mGluR1a- and mGluR5a-mediated but not the inositol trisphospate-mediated activation of a calcium-dependent chloride current. Additionally, RGS4 markedly attenuates the mGluR5-mediated inhibition of potassium currents in hippocampal CA1 neurons. This inhibition is dose-dependent and occurs at concentrations that are virtually identical to those required for inhibition of phospholipase C activity in NG108-15 membranes and reconstituted systems using purified proteins. These findings demonstrate that RGS4 can modulate mGluR responses in neurons, and they highlight a previously unknown mechanism for regulation of G-protein-coupled receptor signaling in the CNS.

Protein kinase C activation decreases activity-dependent attenuation of dendritic Na+ current in hippocampal CA1 pyramidal neurons

Action potentials recorded from the soma of CA1 pyramidal neurons remain relatively uniform in amplitude during repetitive firing. In contrast, the amplitudes of back-propagating action potentials in dendrites decrease progressively during a spike train. This activity-dependent decrease in amplitude is dependent on the frequency of firing during the train and distance from the soma. Previously, we described a property of Na+ channels that provides a plausible mechanism for the activity dependence of the amplitude of the dendritic action potentials: available Na+ current decreases during trains of action potentials through an inactivation, distinct from fast inactivation, that appears rapid in onset, but slow and voltage-dependent in its recovery. In this study we found that activation of protein kinase C by phorbol esters decreased this activity-dependent inactivation of pharmacologically isolated Na+ current in cell-attached dendritic, but not somatic, patches. Similarly in whole cell recordings phorbol esters decreased the attenuation of back-propagating dendritic action potentials during trains. These results indicate a novel effect of protein kinase C on the dendritic Na+ channel and further support the hypothesis that the activity dependence of the dendritic action potentials is derived from the inactivation properties of Na+ channels.

Learning-induced alterations in hippocampal PKC-immunoreactivity: a review and hypothesis of its functional significance

1. To localize protein kinase C (PKC) in the hippocampus, PKC activity measures, mRNA in situ hybridization, and [3H]phorbol ester binding techniques were used until in the 1980s antibodies became available for in situ immunocytochemistry. In the late 1980s, PKC-isoform-specific antibodies were first used to map hippocampal PKC at the cellular and subcellular level. The mammalian hippocampus contains all four Ca(2+)-dependent PKC isoforms, but the (sub)cellular localization is both isoform- and species-specific. 2. Hippocampally-dependent spatial and associative learning in rat, mice and rabbit induce an increase in PKC immunoreactivity (ir) in hippocampal principal cells studied 24 hours after the animals had learned the task. Among the four Ca(2+)-dependent PKC subtypes, this increase is selective for the gamma-isoform. The presence of the gamma-isoform in dendritic spines (the most likely site for synaptic plasticity and information storage), in contrast to PKC alpha, beta 1, and beta 2, may underlie the isoform-selectivity. 3. Compared to fully trained animals, subjects halfway training showed intermediate levels of increased PKC gamma-ir. Poor learners that were not able to learn the task showed considerably less enhanced PKC gamma-ir as compared to good learners. 4. Associative learning induced a decrease in astroglial PKC beta 2 and gamma-ir in those regions where a simultaneous increase in neuronal PKC gamma-ir was observed. This decrease most likely reflects PKC down-regulation, enabling the astrocytes to maintain their K+ buffering capacity necessary to support neuronal activity such as accompanying learning and memory. 5. Western blot analyses revealed that the increase in PKC gamma-ir was not due to an increase in total amount of PKC gamma, translocation, or the proteolytic generation of the fragment PKM. The increase in PKC gamma-ir must therefore reflect a learning-induced conformational change in the PKC gamma molecule that results in the exposure of the antigenic site(s). 6. Although a large number of hippocampal pyramidal cells display learning-induced enhancement of PKC gamma-ir at the 24 hours post-training time point, this does not indicate, however, that all synapses in these neurons are used, or that the maximal PKC signal transduction capacity per call has been reached. 7. The enhanced PKC gamma-ir may reflect a form of activated PKC, since PKC stimulation by phorbol esters (both in hippocampal slices and mildly aldehyde fixed sections) mimicked the increase in PKC gamma-ir similar as seen after learning. 8. The most likely transmitter systems which may have induced the altered PKC gamma-ir are acetylcholine and glutamate. Their contribution and interaction at the cellular level are depicted in a schematic circuit terminating on a CA1 pyramidal cell (Fig. 4). 9. Several functional roles for PKC gamma in learning and memory are discussed, and a hypothetical model is proposed based on an endogeneous PKC inhibitor protein that may explain altered antibody-binding to PKC gamma after learning (Fig. 6). 10. The immunocytochemical approach can contribute significantly to the ongoing attempts to decipher part of the cellular and biochemical mechanism of learning and memory. The development of ever more specific and better characterized antibodies reactive with different sites of proteins like PKC gamma will offer the necessary tools for further immunocytochemical research to help unravel complex brain functions.