Post-tetanic potentiation of GABAergic IPSCs in cultured hippocampal neurons is exclusively time-dependent

Incremental induction of NMDAR-STP and NMDAR-LTP in the CA1 area of ventral hippocampal slices relies on graded activation of discrete NMDA receptors

N-methyl-d-aspartate receptor (NMDAR)-dependent short- and long-term types of potentiation (STP and LTP, respectively) are frequently studied in the CA1 area of dorsal hippocampal slices (DHS). Far less is known about the NMDAR dependence of STP and LTP in ventral hippocampal slices (VHS), where both types of potentiation are smaller in magnitude than in the DHS. Here, we first briefly review our knowledge about the NMDAR dependence of STP and LTP and some other forms of synaptic plasticity. We then show in new experiments that the decay of NMDAR-STP in VHS, similar to dorsal hippocampal NMDAR-STP, is not time- but activity-dependent. We also demonstrate that the induction of submaximal levels of NMDAR-STP and NMDAR-LTP in VHS differs from the induction of saturated levels of plasticity in terms of their sensitivity to subunit-preferring NMDAR antagonists. These data suggest that activation of distinct NMDAR subtypes in a population of neurons results in an incremental increase in the induction of different phases of potentiation with changing sensitivity to pharmacological agents. Differences in pharmacological sensitivity, which arise due to differences in the levels of agonist-evoked biological response, might explain the disparity of the results concerning NMDAR subunit involvement in the induction of NMDAR-dependent plasticity.This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Differential regulation of STP, LTP and LTD by structurally diverse NMDA receptor subunit-specific positive allosteric modulators

Different types of memory are thought to rely on different types of synaptic plasticity, many of which depend on the activation of the N-Methyl-D Aspartate (NMDA) subtype of glutamate receptors. Accordingly, there is considerable interest in the possibility of using positive allosteric modulators (PAMs) of NMDA receptors (NMDARs) as cognitive enhancers. Here we firstly review the evidence that NMDA receptor-dependent forms of synaptic plasticity: short-term potentiation (STP), long-term potentiation (LTP) and long-term depression (LTD) can be pharmacologically differentiated by using NMDAR ligands. These observations suggest that PAMs of NMDAR function, depending on their subtype selectivity, might differentially regulate STP, LTP and LTD. To test this hypothesis, we secondly performed experiments in rodent hippocampal slices with UBP714 (a GluN2A/2B preferring PAM), CIQ (a GluN2C/D selective PAM) and UBP709 (a pan-PAM that potentiates all GluN2 subunits). We report here, for the first time, that: (i) UBP714 potentiates sub-maximal LTP and reduces LTD; (ii) CIQ potentiates STP without affecting LTP; (iii) UBP709 enhances LTD and decreases LTP. We conclude that PAMs can differentially regulate distinct forms of NMDAR-dependent synaptic plasticity due to their subtype selectivity.

This article is part of the Neuropharmacology Special Issue on ‘Glutamate Receptors – NMDA receptors’.

•

NMDAR-dependent STP, LTP and LTD can be dissociated pharmacologically

•

GluN2A/2B PAM UBP714 potentiates LTP and reduces LTD

•

GluN2C/D PAM CIQ potentiates STP without affecting LTP

•

NMDAR pan-PAM UBP709 potentiates LTD and reduces LTP

Kinetic analysis of evoked IPSCs discloses mechanism of antagonism of synaptic GABAA receptors by picrotoxin

BACKGROUND AND PURPOSE

Although picrotoxin is a well-established antagonist of GABA(A) receptors, detailed studies of its action on inhibitory synaptic transmission have not previously been made.

EXPERIMENTAL APPROACH

Electrophysiological techniques were used to study the action of picrotoxin on inhibitory postsynaptic currents (IPSCs) evoked in hippocampal neurones, in culture and slice preparations prepared from Wistar rat embryos and juveniles, respectively.

KEY RESULTS

Picrotoxin gradually reduced the amplitude of GABA(A) receptor-mediated eIPSCs in a concentration-dependent manner. This was accompanied by a marked acceleration of the eIPSC decay kinetics, which, in contrast to the effect on amplitude, developed immediately and was completely reversed on washing. The decaying phase of the IPSC could be resolved into two components; 30 microM picrotoxin reduced tau(fast) by 34% and increased its relative amplitude, while tau(slow) was reduced by 38%, and its relative amplitude decreased. The area under the decaying phase of the normalized eIPSC showed an immediate reduction by 36% in 30 microM picrotoxin. With increasing concentrations of picrotoxin, this normalized area converged towards 55% of the control, indicating that the rate of relaxation and block has a finite maximum. This implies that picrotoxin does not act by a pore-occluding mechanism (open-channel blocking), and suggests allosteric stabilization of desensitized receptor states as a more likely alternative. This was corroborated by modelling, based on two established microscopic GABA(A) receptor transition schemes.

CONCLUSIONS AND IMPLICATIONS

Although the identity of the stabilized state has not been determined unequivocally, picrotoxin effectively traps synaptic GABA(A) receptors in a desensitized state.

Imaging of Ca2+ responses mediated by presynaptic L-type channels on GABAergic boutons of cultured hippocampal neurons

We have previously demonstrated that L-type Ca(2+) channels are involved in post-tetanic potentiation (PTP) of GABAergic IPSCs in cultured hippocampal neurons. Here we have used intracellular Fluo-3 to detect [Ca(2+)](i) in single GABAergic boutons in response to stimulation that evokes PTP. During control stimulation of the presynaptic GABAergic neuron at 40 Hz for 1-2 s, DeltaF/F(0) increased rapidly to a peak value and started to decline shortly after the train ended, returning to baseline within 10-20 s. The L-type channel blocker, isradipine (5 microM), had no significant effect on the amplitude or kinetics of the Ca(2+) signal. Following blockade of N- and P/Q-type Ca(2+)-channels, the amplitude was reduced by 52.9+/-3%. Isradipine caused a reduction of the remaining response (by 26.6+/-5%, P<0.01), that was fully reversible on washing. The L-type channel "agonist", BayK 8644 (8 microM), caused a significant enhancement of the peak (by 18.7%+/-7%, P<0.05). The rising phase of the Ca(2+) signal, which is related to the rate of entry of Ca(2+) into the bouton, was decreased by isradipine (by 25.5+/-6%, P<0.05) and enhanced by BayK 8644 (by 45.2%+/-16%, P<0.05). These Ca(2+) imaging experiments support the putative role of L-type channels in PTP of GABAergic synapses on cultured hippocampal neurons. We expect L-channels to be few in number, although they may couple strongly to intracellular signalling cascades that could amplify a signal that regulates synaptic vesicle turnover in the GABAergic boutons.