Defective place cell activity in nociceptin receptor knockout mice with elevated NMDA receptor-dependent long-term potentiation

Abstract

There is growing evidence that NMDA receptor-dependent long-term potentiation (LTP) in the hippocampus mediates the synaptic plasticity that underlies spatial learning and memory. LTP deficiencies correlate well with spatial memory deficits and LTP enhancements may improve spatial memory. In addition, LTP deficiencies are associated with abnormal place cells as expected from the spatial mapping hypothesis of hippocampal function. In contrast, nothing is known on how enhanced NMDA receptor-dependent LTP affects place cells. To address this question we recorded place cells from mice lacking the nociceptin receptor (NOP1/ORL1/OP4) that have enhanced hippocampal LTP. We found that the enhanced LTP was mediated by NMDA receptors, did not require L-type calcium channels, and occurred only when high frequency tetanizing stimulus trains were used. Place cells in nociceptin receptor knockout mice were abnormal in several ways: they were less stable, had noisier positional firing patterns, larger firing fields and higher discharge rates inside and outside the firing fields. Our results suggest that excessive LTP can cause subnormal hippocampal place cell function. The effects of LTP enhancement on place cell function may therefore also depend on molecular details of synaptic plasticity, including the relationship between stimulus frequency and synaptic strength, and not merely on the magnitude of synaptic strength increases. The data have important clinical implications on development of strategies to improve cognitive function.

Figure 1. Altered place cell firing fields in NocRKO mice

A, examples of hippocampal CA1 place cell firing fields from WT and NocRKO mice. The circular outline represents the grey cylindrical recording chamber with the white cue card at 3:00 o'clock. Increasing place cell firing rates are sorted in the colour order: yellow, orange, red, green, blue, purple. The firing rate is exactly 0 Hz for yellow pixels. Most NocRKO rate maps were noisier than those of WT mice (as seen by more of the darker pixels and fewer yellow pixels). The median rate in the purple (highest rate) category is given at the bottom left of each map; the corresponding coherence values are listed at the bottom right of each map. B, histogram of coherence values of firing fields for 19 WT and 21 NocRKO complex spike pyramidal cells. The mean values for each genotype are shown on the coherence axis.

Figure 2. Poor stability of place cell firing fields in NocRKO mice

Representative series of 5 sessions from 3 WT (top) and 3 NocRKO (bottom) place cells. Note that in session B (1 h interval) the cue card was rotated 90 deg clockwise; however, the map shown has been rotated counter-clockwise 90 deg for simplicity of visual comparison. Each row shows repeated recordings from the same place cell with the time intervals indicated above. Tetrode waveforms for each session are displayed below the firing rate maps. The consistency of waveforms demonstrates the temporal stability of the recordings.

Figure 3. Inaccurate positional pattern of place cell firing fields in NocRKO mice

A, similarity scores for pairs of recording sessions separated by different intervals. Similarity was calculated only at 0 deg (i.e. no optimization) with the cue card rotation for session B subtracted. The similarity for WT firing fields was higher than for NocRKO cells at all intervals indicating greater stability (statistical differences by t test are shown). B, maximum similarity scores for pairs of recording sessions separated by different intervals. Maximum similarity was found by rotating the second session of a pair against the first in 1 deg steps; the cue card rotation for session B was subtracted. The WT firing fields were more stable than the NocRKO fields according to this measure. C, the angle at which maximum similarity occurred was also compared. The smaller rotation required to maximize similarity once more indicates the greater stability of WT firing fields. D, polar plot of the rotations required to maximize similarity after cue card rotation. For both genotypes, the rotations systematically follow the card rotation. Nevertheless, the instances are much more concentrated near 90 deg for WT place cells than for NocRKO place cells.

Figure 4. Synaptic transmission in hippocampal slices

A, representative field excitatory postsynaptic potentials (fEPSPs, shown superimposed) were recorded from CA1 stratum radiatum region in response to increasing strength of Schaffer collateral stimulation. WT traces are shown in black (left) and NocRKO traces appear in grey (right). B, the slope of the evoked fEPSP was plotted as a function of the presynaptic volley amplitude for both WT (black diamonds) and NocRKO (grey circles) littermates (n = 8 and 10 slices, respectively, each with 8 fixed stimulation intensity levels per slice).

Figure 5. Paired-pulse facilitation is unchanged in NocRKO mice

A, representative responses for WT (black traces, left) and NocRKO (grey traces, right) are shown. Paired stimuli delivered at intervals of 60, 150, 250 and 400 ms are shown in superimposed format. B, the facilitation ratio (slope of second fEPSP divided by slope of first fEPSP) was plotted for WT (n = 8) and NocRKO (n = 10) slices. Paired-pulse facilitation was measured with the stimulus intensity set to make the first fEPSP slope 40% of maximum spike-free value. Note that the two curves are virtually the same.

Figure 6. Enhanced NMDA receptor-dependent LTP in slices from NocRKO mice

A, sample fEPSPs recorded before and 90 min after (superimposed) 100 Hz tetanic stimulation to induce LTP at Schaffer collateral–CA1 synapses. From left to right: NocRKO, WT and NocRKO in the presence of NMDA receptor blocker d-AP5. B, LTP time course showing ensemble averages of the normalized fEPSP slope (per cent of average pre-tetanus fEPSP slope) for each experimental group (NocRKO, n = 6; WT, n = 6; NocRKO in d-AP5, n = 4). Each data point represents 6 responses averaged over 1 min, and for clarity only every third time point is shown. Time of tetanus delivery is shown, as well as time of drug addition for NocRKO slice group receiving d-AP5. C, LTP induced by 50 Hz stimulation (n = 6). D, induction of LTP by 20 Hz stimulation (n = 5). E, magnitude of LTP at 30 min induced by different tetanus frequencies. The LTP in NocRKO slices is significantly greater than that of WT slices at 100 Hz.

Figure 7. Superior LTP in NocRKO slices independent of L-type calcium channels

A, example fEPSPs recorded in the presence of verapamil (Ver), before and 90 min after 100 Hz tetanic stimulation (traces shown superimposed) to induce LTP at Schaffer collateral–CA1 stratum radiatum synapses. B, LTP time course showing group averages of the normalized fEPSP. Time of drug addition and time of tetanus delivery are shown (NocRKO, n = 8; WT, n = 6). C, sample fEPSPs recorded from CA1 basal dendrites (stratum oriens), before and 60 min after 200 Hz tetanic stimulation (traces shown superimposed) to induce L-type calcium channel-dependent LTP. Traces on the left are from WT slices and traces on the right are from WT slices recorded in the presence of verapamil. Scale as per A. D, basal dendrite LTP time course in WT slices induced by 200 Hz stimulation either in the presence of verapamil (n = 4) or vehicle (n = 4).