Involvement of postsynaptic G proteins in hippocampal long-term potentiation

Group I metabotropic glutamate receptor signaling via Galpha q/Galpha 11 secures the induction of long-term potentiation in the hippocampal area CA1

Heterotromeric G-proteins of the Gq family are thought to transduce signals from group I metabotropic glutamate receptors (mGluRs) in central neurons. We investigated roles of this cascade in hippocampal long-term potentiation (LTP) by using null-mutant mice lacking the alpha subunit of Gq (Galphaq) or G11 (Galpha11). We found no obvious abnormalities in the morphology, layer structure, expression of NMDA receptors, and basic parameters of excitatory synaptic transmission in the hippocampus of Galphaq mutant mice. We used theta burst stimulation (TBS) (3-10 burst trains at 5 Hz; each train consisted of five stimuli at 100 Hz) to induce LTP at Schaffer collateral to CA1 pyramidal cell synapses. Conventional TBS with 10 burst trains induced robust LTP in wild-type, Galphaq mutant, and Galpha11 mutant mice. Weak TBS with three burst trains consistently induced LTP in wild-type mice. In contrast, the same weak TBS was insufficient to induce LTP in Galphaq and Galpha11 mutant mice. In wild-type mice, the LTP by weak TBS was abolished by inhibiting group I mGluR or protein kinase C (PKC) but not by blocking muscarinic acetylcholine receptors. Prior activation of group I mGluR by an agonist significantly enhanced the LTP by weak TBS in wild-type mice. However, this priming effect was absent in Galphaq mutant mice. These results indicate that the signaling from group I mGluR to PKC involving Galphaq/Galpha11 does not constitute the main pathway for LTP, but it secures LTP induction by lowering its threshold in the hippocampal area CA1.

Synapse-selective impairment of NMDA receptor functions in mice lacking NMDA receptor epsilon 1 or epsilon 2 subunit

1. We have explored the effects of targeted disruption of the N-methyl-D-aspartate (NMDA) receptor epsilon 1 or epsilon 2 subunit gene on NMDA receptor-mediated excitatory postsynaptic currents (NMDA EPSCs) and long-term potentiations (LTPs) at the two types of synapse in mouse hippocampal CA3 pyramidal neurons: those formed by the commissural/associational (C/A) and fimbrial (Fim) inputs. 2. Electrophysiological experiments were performed in hippocampal slices prepared from both wild-type and epsilon 1- or epsilon 2-disrupted mice using extracellular and whole-cell patch recording techniques. To assess the epsilon 1, epsilon 2 and zeta 1 subunit expression at cellular levels, we performed non-isotopic in situ hybridization with digoxigenin-labelled cRNA probes. 3. We could record EPSCs in response to the stimulations to either of the C/A and Fim afferents from a single CA3 pyramidal neuron. The epsilon 1, epsilon 2 and zeta 1 subunits were expressed together in individual CA3 neurons. 4. The epsilon 1 subunit disruption selectively reduced NMDA EPSCs and LTP in the C/A-CA3 synapse without significantly affecting those in the Fim-CA3 synapse, whereas the epsilon 2 subunit mutation diminished NMDA EPSCs and LTP in the Fim-CA3 synapse with no appreciable functional modifications in the C/A-CA3 synapse. 5. These results suggest that NMDA receptors with different subunit compositions function within a single CA3 pyramidal cell in a synapse-selective manner.

Cognitive enhancers and hippocampal long-term potentiation in vitro

We review our works on the pharmacological modulation of long-term potentiation (LTP) at guinea pig hippocampal mossy fiber-CA3 synapses in vitro. The magnitude of tetanus-induced LTP at the mossy fiber synapse was augmented by perfusion of slices with several cognitive enhancers, such as bifemelane (1 microM). The mossy fiber LTP was enhanced by somatostatin (0.32 microM) and inhibited in somatostatin-depleted slices from cysteamine-treated guinea pigs. An involvement of the 5-HT3 receptor also showed that granisetron (0.1 microM) enhanced the mossy fiber LTP. The above-mentioned enhancements by perfused agents were commonly reversed, at least in part, by muscarinic antagonists. However, the magnitude of mossy fiber LTP was bidirectionally modulated by muscarinic stimulations of slices with physostigmine or carbachol at different concentrations. The enhancing effects of high-concentration carbachol was antagonized by pirenzepine, and in contrast, the inhibition by low-concentration carbachol was antagonized in the presence of AF-DX116. When guinea pigs were preinjected with the cholinotoxin AF64A, the magnitude of LTP was decreased in the slices prepared from AF64A-treated animals. These results suggest that endogenous acetylcholine dominantly plays facilitatory roles through muscarinic M1 receptors in the induction of mossy fiber LTP. The pharmacological characterization of mossy fiber LTP may be of help to the evaluation of cognitive enhancers at a neuronal circuit level.

Effect of nitric oxide synthase inhibition on long-term potentiation at associational-commissural and mossy fibre synapses on CA3 pyramidal neurones

1. The sensitivity of long-term potentiation (LTP) to nitric oxide synthase (NOS) inhibition was determined for two synaptic input systems onto CA3 pyramidal neurones the LTP of which display differential sensitivity to N-methyl-D-aspartate (NMDA) receptor antagonists: the fimbrial input which activates the associational-commissural synapses on the distal apical dendrites and the mossy fibre input which synapses on the proximal apical dendrites of CA3 pyramidal neurones. 2. Following high-frequency stimulation (HFS) of the fimbrial input, average e.p.s.p. amplitude increased by 92.4 +/- 22.0% (mean +/- s.e.mean; n = 6 cells) when compared to the pre-HFS average. In the presence of 100 microM N omega-nitro-L-arginine methyl ester (L-NAME), the enhancement was reduced significantly to 32.2 +/- 11.6% (n = 5 cells; P < 0.05). In the presence of 300 microM L-NAME, the inhibition was more complete, with post-HFS e.p.s.p. amplitude increasing an average 6.2 +/- 9.3% (n = 7 cells, P < 0.05). 3. Following high frequency stimulation of the mossy fibre input, average e.p.s.p. amplitude increased by 57.9 +/- 13.0% (n = 6 cells) when compared to the pre-HFS average. The presence of 100 microM L-NAME had no significant effect on the enhancement, averaging 63.6 +/- 5.9% (n = 4 cells; P > 0.05). Similarly, increasing the concentration of L-NAME to 300 microM had no significant effect on the potentiation, with the post-HFS amplitude increasing by an average 55.6 +/- 9.5% (n = 5 cells, P > 0.05). 4. These results suggest that LTP at associational-commissural synapses (fimbrial input) is significantly depressed in the presence of the NOS inhibitor L-NAME, while mossy fibre LTP is unchanged.

Involvement of postsynaptic G-proteins in hippocampal long-term potentiation

The guanine nucleotide binding proteins (G-proteins) are a family of proteins that couple membrane receptors to intracellular second messenger enzymes. Most subtypes of G-protein are highly expressed in hippocampal neurons, implying important roles of G-proteins in the modulation of synaptic transmission. In this article, we review recent studies on the involvement of G-proteins in tetanus-induced long-term potentiation (LTP) of synaptic efficacy in the hippocampus. Such an LTP has been demonstrated to be produced by several mechanisms. In Schaffer collateral-CA1 synapses where induction of LTP requires post- and presynaptic events, G-proteins may regulate the development of LTP presynaptically. In mossy fiber-CA3 synapses where only presynaptic elements are needed to initiate LTP, the contribution of G-proteins may also be localized at presynaptic sites. However, in fimbria-CA3 synapses where LTP is thought to be initiated by NMDA receptor activation at CA1 pyramidal cells, postsynaptic G-proteins are involved in the induction of LTP by affecting membrane depolarization and/or Ca2+ mobilization. The different contribution of G-proteins in the various forms of LTP at anatomically distinct synapses implies localized modification of synaptic transmission by endogenous neurotransmitters via G-protein-coupled receptors.

Bidirectional modulation of long-term potentiation by carbachol via M1 and M2 muscarinic receptors in guinea pig hippocampal mossy fiber-CA3 synapses

Participation of muscarinic M1 and M2 receptors in the modulation of long-term potentiation (LTP) was studied in the mossy fiber-CA3 synapse of guinea pig hippocampal slices. The magnitude of tetanus-induced LTP was attenuated in the presence of 0.01-0.1 microM carbachol, at which concentration the pre-tetanus amplitude of field excitatory postsynaptic potential (fEPSP) was not affected. The attenuation of LTP by the low concentration of carbachol was reversed by an M2 muscarinic antagonist, AF-DX 116, but not by an M1 antagonist, pirenzepine. On the contrary, a high concentration (10 microM) of carbachol decreased the pre-tetanic amplitude of fEPSP, however, the magnitude of LTP was significantly larger than that in control slices in which pre-tetanic amplitude of fEPSP was reduced to the level of carbachol-treated slices by reducing the intensity of stimulation or extracellular Ca2+ concentration. The augmentation of LTP by 10 microM carbachol was blocked by pirenzepine but not by AF-DX 116. These results suggest that the synaptic plasticity in the guinea pig hippocampal mossy fiber-CA3 synapse is inhibited and facilitated by muscarinic agonist through muscarinic M2 and M1 receptors to inhibit and facilitate the LTP, respectively.

A postsynaptic G-protein in hippocampal long-term potentiation

The effects of guanosine triphosphate (GTP)-binding protein (G-protein) blockade on hippocampal LTP at stratum radiatum-CA1 synapses was studied. Bath application of 20 mM lithium chloride (LiCl) inhibited long-term potentiation (LTP) of extracellularly-recorded excitatory postsynaptic potentials (EPSPs). Inclusion of 100 mM LiCl in intracellular recording electrodes was shown to block postsynaptic G-proteins by bath-application of baclofen, an agonist at the G-protein linked gamma-aminobutyric acid (GABAB) receptor. Under normal conditions, GABAB receptor activation causes a hyperpolarization postsynaptically, and a decrease in neurotransmitter release presynaptically. With LiCl in the recording electrodes, the postsynaptically-mediated hyperpolarization was blocked, while the presynaptically-mediated depression of EPSPs was unaffected. With postsynaptic G-proteins blocked in this manner, LTP at these synapses was inhibited. These studies provide evidence for the involvement of a postsynaptic G-protein in LTP of stratum radiatum-CA1 synapses.

The involvement of muscarinic, β-adrenergic and metabotropic glutamate receptors in long-term potentiation in the fimbria-CA3 pathway of the hippocampus

Possible modulatory actions of endogenous neurotransmitters on long-term potentiation (LTP) were investigated in the fimbria-CA3 pathway of rat hippocampal slices. Bath application of atropine (10 microM), but neither timolol (10 microM) nor D,L-2-amino-3-phosphonopropionate (AP3, 100 microM), significantly attenuated LTP induced by 20 pulses of 50 Hz stimulation. When stronger stimulation (3 trains of 100 Hz, 100 pulses) was used for the induction of LTP, timolol significantly attenuated LTP, but atropine and AP3 did not. These results suggest that, under specified conditions, endogenous acetylcholine through muscarinic receptors, and noradrenaline through beta-adrenergic receptors may modulate the generation of LTP in the fimbria-CA3 pathway. Metabotropic glutamate receptors may be involved not in the generation of LTP but in low-frequency synaptic transmission, since 300-1,000 microM AP3 greatly reduced, or abolished synaptic transmission in this pathway.