The phosphoinositide 3-kinase and p70 S6 kinase regulate long-term potentiation in hippocampal neurons.

The mechanisms by which long-term changes in synaptic efficacy (e.g., long-term potentiation) are maintained are not well understood. There is evidence that reorganization of the neuronal actin cytoskeleton is important for consolidation of long-term potentiation. In non-neuronal cells, phosphoinositide 3-kinase and p70 S6 kinase have been shown to regulate actin polymerization. We have investigated the subcellular localization of these enzymes in cultured hippocampal pyramidal neurons and their possible role in hippocampal long-term potentiation. Immunohistochemical analysis revealed enrichment of both enzymes in the growth cones and filopodia of extending neurites, whereas p70 S6 kinase was also present at the soma. Antibodies to the phosphorylated form of p70 S6 kinase confirmed its activity in these locations. Interestingly, both enzymes displayed strong colocalization with F-actin in discrete regions of developing neurites. In hippocampal slices, the maintenance of long-term potentiation was attenuated by either rapamycin or 2-(4-morpholinyl)-8-phenyl-1(4H)-1-benzopyran-4-one, inhibitors of p70 S6 kinase and phosphoinositide 3-kinase, respectively. Our findings provide evidence for a novel biochemical pathway involving phosphoinositide 3-kinase and p70 S6 kinase that is important for the maintenance of hippocampal long-term potentiation, possibly via regulation of actin dynamics.

Metabotropic glutamate receptors trigger homosynaptic protein synthesis to prolong long-term potentiation.

We investigated the mechanisms by which previous "priming" activation of group I metabotropic glutamate receptors (mGluRs) facilitates the persistence of long-term potentiation (LTP) in area CA1 of rat hippocampal slices. Priming of LTP was elicited by either pharmacological or synaptic activation of mGluRs before a weak tetanic stimulus that normally produced only a rapidly decaying phase of LTP that did not involve protein synthesis or mGluRs. Pharmacological priming of LTP persistence by a selective group I mGluR agonist was blocked by an inhibitor of group I mGluRs and by inhibitors of translation, but not by a transcriptional inhibitor. The same mGluR agonist increased (35)S-methionine incorporation into slice proteins. LTP could also be facilitated using a synaptic stimulation priming protocol, and this effect was similarly blocked by group I mGluR and protein synthesis inhibitors. Furthermore, using a two-pathway protocol, the synaptic priming of LTP was found to be input-specific. To test for the contribution of group I mGluRs and protein synthesis to LTP in nonprimed slices, a longer duration control tetanization protocol was used to elicit a more slowly decaying form of LTP than did the weak tetanus used in the previous experiments. The persistence of the LTP induced by this stronger tetanus was dependent on mGluR activation and protein synthesis but not on transcription. Together, these results suggest that mGluRs couple to nearby protein synthesis machinery to homosynaptically regulate an intermediate phase of LTP dependent on new proteins made from pre-existing mRNA.

Long-lasting increase in cellular excitability associated with the priming of LTP induction in rat hippocampus.

The mechanisms underlying the facilitation (priming) of long-term potentiation (LTP) by prior activation of metabotropic glutamate receptors (mGluRs) were investigated in area CA1 of rat hippocampal slices. In particular, we focused on whether a long-lasting increase in postsynaptic excitability could account for the facilitated LTP. Administration of the mGluR agonist 1S, 3R-aminocyclopentanedicarboxylic acid (ACPD) produced rapid decreases in the amplitude of both the slow spike afterhyperpolarization (AHP(slow)) and spike frequency adaptation recorded intracellularly from CA1 pyramidal cells. These changes persisted after drug washout, showing only a slow decay over 20 min. ACPD also caused a leftward shift of the field EPSP-population spike relation and an overall increase in population spike amplitude, but this effect was not as persistent as the intracellularly measured alterations in cell excitability. ACPD-treated cells showed increased spike discharges during LTP-inducing tetanic stimulation, and the amplitude of the AHP(slow) was negatively correlated with the degree of initial LTP induction. The beta-adrenergic agonist isoproterenol also caused excitability changes as recorded intracellularly, whereas in extracellular experiments it weakly primed the induction but not the persistence of LTP. ACPD primed both LTP measures. Isoproterenol administration during the tetanus occluded the priming effect of ACPD on initial LTP induction but not its effect on LTP persistence. We conclude that the persistent excitability changes elicited by ACPD contributes to the priming of LTP induction but that other ACPD-triggered mechanisms must account for the facilitated persistence of LTP in the priming paradigm.

A double dissociation within the hippocampus of dopamine D1/D5 receptor and beta-adrenergic receptor contributions to the persistence of long-term potentiation.

We compared the effects of the D1/D5 receptor antagonist SCH-23390 with the beta-adrenergic receptor antagonist propranolol on the persistence of long-term potentiation in the CA1 and dentate gyrus subregions of the hippocampus. In slices, SCH-23390 but not propranolol reduced the persistence of long-term potentiation in area CA1 without affecting its induction. The drugs exerted reverse effects in the dentate gyrus, although in this case the induction of long-term potentiation was also affected by propranolol. The lack of effect of SCH-23390 on the induction and maintenance of long-term potentiation in the dentate gyrus was confirmed in awake animals. The drug also had little or no effect on the expression of inducible transcription factors. In area CA1 of awake animals, SCH-23390 blocked persistence of long-term potentiation beyond 3 h, confirming the results in slices. To rule out a differential release of catecholamines induced by our stimulation protocols between brain areas, we compared the effects of the D1/D5 agonist SKF-38393 with the beta-adrenergic agonist isoproterenol on the persistence of a weakly induced, decremental long-term potentiation in CA1 slices. SKF-38393 but not isoproterenol promoted greater persistence of long-term potentiation over a 2-h period. In contrast, isoproterenol but not SKF-38392 facilitated the induction of long-term potentiation. These data demonstrate that there is a double dissociation of the catecholamine modulation of long-term potentiation between CA1 and the dentate gyrus, suggesting that long-term potentiation in these brain areas may be differentially consolidated according to the animal's behavioural state.

Priming of long-term potentiation induced by activation of metabotropic glutamate receptors coupled to phospholipase C.

Activation of metabotropic glutamate receptors (mGluRs) with 1-aminocyclopentane-1S,3R-dicarboxylic acid 20 min prior to tetanus facilitates, or "primes," subsequent induction of long-term potentiation (LTP; Cohen and Abraham, J Neurophysiol 1996;76:953-962). In the present study, we investigated the receptor specificity and associated second messenger pathways involved in the mGluR priming effect by using field potentials recorded from area CA1 of rat hippocampal slices. In controls, mild theta-burst or high-frequency (100 Hz) stimulation induced 16% and 21% LTP, respectively. A 10-min application of the group I mGluR agonist 3,5-dihydroxyphenylglycine (DHPG) caused a transient depression of synaptic responses but a significant enhancement of subsequent LTP for both tetanus protocols (45% and 41% LTP, respectively). Maximal LTP, induced by stronger tetanization protocols, was not enhanced by DHPG, nor was mild LTP facilitated by post-tetanic application of DHPG. Priming with agonists selective for group II or III mGluRs had no effect on LTP. The mGluR antagonists L-2-amino-3-phosphonopropionic acid and 1-aminoindan-1,5-dicarboxylic acid inhibited the LTP facilitatory effect of DHPG but not the transient response depression, whereas alpha-methyl-4-carboxyphenylglycine produced the opposite effects. Priming with N-methyl-D-aspartate or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid did not facilitate LTP induction. Prior activation of muscarinic acetylcholine receptors produced at best a weak priming effect. Inhibition of phospholipase C by U-73122 completely abolished the priming of LTP by DHPG. We conclude that mGluR priming of LTP results from biochemical cascades triggered by activation of phospholipase C coupled to group I mGluRs.