Metabotropic glutamate receptor 1 activity generates persistent,N-methyl-d-aspartate receptor-dependent depression of hippocampal pyramidal cell excitability

mGlu1 receptor as a drug target for treatment of substance use disorders: time to gather stones together?

Modulation of the mGlu1 receptor was repeatedly shown to inhibit various phenomena associated with exposure to abused drugs. Efficacy in preclinical models was observed with both positive and negative allosteric modulators (PAMs and NAMs, respectively) using essentially non-overlapping sets of experimental methods. Taken together, these data indicate that the mGlu1 receptor certainly plays a significant role in the plasticity triggered by the exposure to abused drugs and is involved in the maintenance of drug-seeking and drug-taking behaviors. Understanding whether modulation of the mGlu1 receptor activity can also affect drug-seeking and drug-taking in humans could have a significant impact on the future development of medications in this field. We argue that the mGlu1 receptor NAMs have a significant value as potential tools for human experimental pharmacology that could help to validate methods used in preclinical research. Compared with the PAMs, the mGlu1 receptor NAMs appear to be better candidates for this role due to the following: (1) a number of highly potent, selective, and chemically diverse mGlu1 receptor NAMs to choose from; (2) availability of high-quality PET ligands to monitor target exposure; and (3) a rich pharmacological profile with a number of effects that can complement anti-addictive action (e.g., anxiolytic/antidepressant) and may also serve as additional pharmacodynamic readouts during the preclinical-to-clinical translation. We believe that the mGlu1 receptor NAMs have a significant value as potential tools for human experimental pharmacology that could help to validate methods used in preclinical research.

Differential ability of the dorsal and ventral rat hippocampus to exhibit group I metabotropic glutamate receptor-dependent synaptic and intrinsic plasticity

Background:

The hippocampus is critically involved in learning and memory processes. Although once considered a relatively homogenous structure, it is now clear that the hippocampus can be divided along its longitudinal axis into functionally distinct domains, responsible for the encoding of different types of memory or behaviour. Although differences in extrinsic connectivity are likely to contribute to this functional differentiation, emerging evidence now suggests that cellular and molecular differences at the level of local hippocampal circuits may also play a role.

Methods:

In this study, we have used extracellular field potential recordings to compare basal input/output function and group I metabotropic glutamate receptor-dependent forms of synaptic and intrinsic plasticity in area CA1 of slices taken from the dorsal and ventral sectors of the adult rat hippocampus.

Results:

Using two extracellular electrodes to simultaneously record field EPSPs and population spikes, we show that dorsal and ventral hippocampal slices differ in their basal levels of excitatory synaptic transmission, paired-pulse facilitation, and EPSP-to-Spike coupling. Furthermore, we show that slices taken from the ventral hippocampus have a greater ability than their dorsal counterparts to exhibit long-term depression of synaptic transmission and EPSP-to-Spike potentiation induced by transient application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine.

Conclusions:

Together, our results provide further evidence that the information processing properties of local hippocampal circuits differ in the dorsal and ventral hippocampal sectors, and that these differences may in turn contribute to the functional differentiation that exists along the hippocampal longitudinal axis.

Neurophysiological modification of CA1 pyramidal neurons in a transgenic mouse expressing a truncated form of disrupted-in-schizophrenia 1

A t(1;11) balanced chromosomal translocation transects the Disc1 gene in a large Scottish family and produces genome-wide linkage to schizophrenia and recurrent major depressive disorder. This study describes our in vitro investigations into neurophysiological function in hippocampal area CA1 of a transgenic mouse (DISC1_tr) that expresses a truncated version of DISC1 designed to reproduce aspects of the genetic situation in the Scottish t(1;11) pedigree. We employed both patch-clamp and extracellular recording methods in vitro to compare intrinsic properties and synaptic function and plasticity between DISC1_tr animals and wild-type littermates. Patch-clamp analysis of CA1 pyramidal neurons (CA1-PNs) revealed no genotype dependence in multiple subthreshold parameters, including resting potential, input resistance, hyperpolarization-activated ‘sag’ and resonance properties. Suprathreshold stimuli revealed no alteration to action potential (AP) waveform, although the initial rate of AP production was higher in DISC1_tr mice. No difference was observed in afterhyperpolarizing potentials following trains of 5–25 APs at 50 Hz. Patch-clamp analysis of synaptic responses in the Schaffer collateral commissural (SC) pathway indicated no genotype-dependence of paired pulse facilitation, excitatory postsynaptic potential summation or AMPA/NMDA ratio. Extracellular recordings also revealed an absence of changes to SC synaptic responses and indicated input–output and short-term plasticity were also unaltered in the temporoammonic (TA) input. However, in DISC1_tr mice theta burst-induced long-term potentiation was enhanced in the SC pathway but completely lost in the TA pathway. These data demonstrate that expressing a truncated form of DISC1 affects intrinsic properties of CA1-PNs and produces pathway-specific effects on long-term synaptic plasticity.

Gating of NMDA receptor-mediated hippocampal spike timing-dependent potentiation by mGluR5

Hippocampal long-term potentiation (LTP) is believed to be important for learning and memory. Experimentally, the pairing of precisely timed pre- and postsynaptic spikes within a time window of ∼10 ms can induce timing-dependent LTP (tLTP), but the requirements for induction of tLTP change with development: in young rodents single postsynaptic spikes are sufficient to induce tLTP, whereas postsynaptic burst firing appears to be required in the adult. However, hippocampal neurons in vivo show theta-modulated single spike activities also in older hippocampus. Here we investigated the conditions for single spike pairing to induce tLTP at older CA3–CA1 synapses. We found that the pairing of single pre- and postsynaptic spikes could induce tLTP in older hippocampus when the postsynaptic neuronal membrane was depolarized and the pairing frequency exceeded ∼4 Hz. The spike frequency requirement is postsynaptic, as tLTP could still be induced with presynaptic stimulation at 1 Hz as long as the postsynaptic spike frequency exceeded ∼4 Hz, suggesting that postsynaptic theta-frequency activity is required for the successful induction of tLTP at older CA3–CA1 synapses. The induction of tLTP was blocked by an NMDA receptor antagonist and by the selective mGluR5 blockers, MPEP and MTEP, whereas activation of mGluR1 and mGluR5 by DHPG relieved the postsynaptic spike frequency requirement for tLTP induction. These results suggest that activation of mGluR5 during single-spike pairing at older CA3–CA1 synapses gates NMDA receptor-dependent tLTP.

Dendritic EGFP-STIM1 activation after type I metabotropic glutamate and muscarinic acetylcholine receptor stimulation in hippocampal neuron

Several signaling pathways in neurons engage the endoplasmic reticulum (ER) calcium store by triggering calcium release. After release, ER calcium levels must be restored. In many non-neuronal cell types, this is mediated by store-operated calcium entry (SOCE), a cellular homeostatic mechanism that activates specialized store-operated calcium channels (SOC). Although much evidence supports the existence of SOCE in neurons, its importance has been difficult to determine because of the abundance of calcium channels expressed and the lack of SOC-specific pharmacological agents. We have explored the function of the SOCE-inducing protein STIM1 in neurons. In EGFP-STIM1-expressing hippocampal neurons, the sarco- and endoplasmic reticulum calcium ATPase (SERCA) inhibitor thapsigargin caused rapid aggregation (i.e., activation) of STIM1 in soma and dendrites. Upon STIM1 activation by thapsigargin, a dramatic reduction in STIM1 mobility was detected by fluorescence recovery after photobleaching (FRAP). By triggering release of ER calcium with 3,5-dihydroxyphenylglycine (DHPG) or carbachol (Cch), agonists of type I metabotropic glutamate receptors (mGluR) and muscarinic acetylcholine receptors (mAChR), respectively, STIM1 was activated, and calcium entry (likely to represent SOCE) occurred in dendrites. It is therefore possible that neuronal SOCE is activated by physiological stimuli, some of which may alter the postsynaptic calcium signaling properties.

Synaptic activation of mGluR1 generates persistent depression of a fast after-depolarizing potential in CA3 pyramidal neurons

Burst firing is an important property of hippocampal pyramidal neurons. Group I metabotropic glutamate receptors (mGluRs) produce a multitude of effects on both the synaptic and intrinsic properties of neurons. We investigated whether brief activation of these receptors results in persistent modifications to the intrinsic excitability of rat hippocampal CA3 pyramidal cells (CA3-PCs). In whole-cell current-clamp recordings, current stimuli consisting of filtered, pseudo-random noise produced action potential firing with a mean frequency of ∼1.5-2 Hz. Analysis of spike intervals revealed that this firing included a substantial component (∼20%) of high-frequency (∼100 Hz) bursting activity. Activation of group I mGluRs with (S)-3,5-dihydroxyphenylglycine [(S)-DHPG] selectively eliminated the high-frequency bursts, an effect that persisted > 30 min after (S)-DHPG washout. The fast after-depolarizing potential (ADP) of CA3-PCs is known to be important for generating high-frequency action potential bursting. This ADP was persistently depressed following a short application of (S)-DHPG. This effect was blocked by the mGluR1 antagonist, (S)-(+)-α-amino-4-carboxy-2-methylbenzeneacetic acid (LY367385). In contrast, the depression was resistant to the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate, N-methyl-D-aspartate (NMDA) and γ-aminobutyric acid (GABA)(A) antagonists. Unlike other manipulations that generate persistent depression of the ADP in CA3-PCs, DHPG-mediated ADP depression was insensitive to the Kv7 channel inhibitor 10,10-bis(4-Pyridinylmethyl)-9(10H)-anthracenone dihydrochloride (XE991) and strong intracellular Ca(2+) buffering by 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). Synaptic activation of mGluRs in the associational-commissural pathway also resulted in persistent depression of the ADP in postsynaptic CA3-PCs, which was blocked by LY367385. These data represent the first evidence that synaptic activation of mGluR1 can modulate the intrinsic excitability properties of hippocampal neurons.

Endoplasmic reticulum dynamics in hippocampal dendritic spines induced by agonists of type I metabotropic glutamate but not by muscarinic acetylcholine receptors

Neurons in the hippocampus exhibit subpopulations of dendritic spines that contain endoplasmic reticulum (ER). ER in spines is important for synaptic activity and its associated Ca(2+) signaling. The dynamic distribution of ER to spines is regulated by diacylglycerol and partly mediated by protein kinase C, metalloproteinases and γ-secretase. In this study, we explored whether pharmacological activation of type I metabotropic glutamate receptors (mGluRs) and muscarinic acetylcholine receptors (mAChRs) known to activate phospholipase C would have any effect on spine ER content. We found that DHPG (100 μM) but not carbachol (10 μM) caused a reduction in the number of spines with ER. We further found that ER Ca(2+) depletion triggered by thapsigargin (200 nM) had no effect on ER localization in spines.

Glutamate-induced depression of EPSP-spike coupling in rat hippocampal CA1 neurons and modulation by adenosine receptors

The presence of high concentrations of glutamate in the extracellular fluid following brain trauma or ischaemia may contribute substantially to subsequent impairments of neuronal function. In this study, glutamate was applied to hippocampal slices for several minutes, producing over-depolarization, which was reflected in an initial loss of evoked population potential size in the CA1 region. Orthodromic population spikes recovered only partially over the following 60 min, whereas antidromic spikes and excitatory postsynaptic potentials (EPSPs) showed greater recovery, implying a change in EPSP-spike coupling (E-S coupling), which was confirmed by intracellular recording from CA1 pyramidal cells. The recovery of EPSPs was enhanced further by dizocilpine, suggesting that the long-lasting glutamate-induced change in E-S coupling involves NMDA receptors. This was supported by experiments showing that when isolated NMDA-receptor-mediated EPSPs were studied in isolation, there was only partial recovery following glutamate, unlike the composite EPSPs. The recovery of orthodromic population spikes and NMDA-receptor-mediated EPSPs following glutamate was enhanced by the adenosine A1 receptor blocker DPCPX, the A2A receptor antagonist SCH58261 or adenosine deaminase, associated with a loss of restoration to normal of the glutamate-induced E-S depression. The results indicate that the long-lasting depression of neuronal excitability following recovery from glutamate is associated with a depression of E-S coupling. This effect is partly dependent on activation of NMDA receptors, which modify adenosine release or the sensitivity of adenosine receptors. The results may have implications for the use of A1 and A2A receptor ligands as cognitive enhancers or neuroprotectants.