Reduced Gamma Oscillations in a Mouse Model of Intellectual Disability: A Role for Impaired Repetitive Neurotransmission?

Intellectual disability affects 2-3% of the population; mutations of the X-chromosome are a major cause of moderate to severe cases. The link between the molecular consequences of the mutation and impaired cognitive function remains unclear. Loss of function mutations of oligophrenin-1 (OPHN1) disrupt Rho-GTPase signalling. Here we demonstrate abnormal neurotransmission at CA3 synapses in hippocampal slices from Ophn1-/y mice, resulting from a substantial decrease in the readily releasable pool of vesicles. As a result, synaptic transmission fails at high frequencies required for oscillations associated with cognitive functions. Both spontaneous and KA-induced gamma oscillations were reduced in Ophn1-/y hippocampal slices. Spontaneous oscillations were rapidly rescued by inhibition of the downstream signalling pathway of oligophrenin-1. These findings suggest that the intellectual disability due to mutations of oligophrenin-1 results from a synaptopathy and consequent network malfunction, providing a plausible mechanism for the learning disabilities. Furthermore, they raise the prospect of drug treatments for affected individuals

Reduced facilitation in response to high frequency stimulation in <i>Ophn1</i>^−/y neurons.

<p>(<b>a</b>) Representative traces illustrating IPSC summation in response to 10 stimuli delivered at 33 Hz. The responses to first stimuli were normalised. In contrast to <i>Ophn1</i>^+/y neurons, <i>Ophn1</i>^−/y IPSCs (<i>grey trace</i>) showed no summation. (<b>b</b>) IPSC amplitude plotted against stimulus number for 33 Hz trains in <i>Ophn1</i>^+/y (•, n = 20) and <i>Ophn1</i>^−/y neurons (○, n = 14). (<b>c</b>) Maximal IPSC amplitude (mean of the last 5 stimuli) plotted against stimulus frequency. (<i>p</i>: *<0.05, **<0.01, ***<0.001).</p

<i>Ophn1</i>^−/y slices show reduced postsynaptic potentials.

<p>(<b>a</b>) Representative traces of postsynaptic potentials from <i>Ophn1</i>^+/y (black traces) and <i>Ophn1</i>^−/y (grey traces) slices. (<b>b</b>) Stimulus response curve of postsynaptic potentials recorded from the s. radiatum of CA3. PSP slopes were significantly smaller in <i>Ophn1</i>^−/y (n = 5) than in <i>Ophn1</i>^+/y slices (n = 12; p = 0.011, ANOVA). Representative traces of spontaneous EPSCs in <i>Ophn1</i>^+/y (<b>c</b>) and <i>Ophn1</i>^−/y (<b>d</b>). Representative individual spontaneous EPSCs are shown in the right panel. Cumulative frequency plots show that the interevent intervals (<b>e</b>), but not amplitude (<b>f</b>) of EPSCs are altered in <i>Ophn1⁻</i>^/y neurons compared to <i>Ophn1</i>^+/y neurons. (<i>inset</i>) Mean frequency and amplitude of spontaneous EPSCs.</p

Gamma oscillation synchrony and coherence unchanged in <i>Ophn1</i>^−/y slices.

<p>(<b>a</b>) Average gamma waveforms for <i>Ophn1</i>^+/y (<i>black trace</i>) and <i>Ophn1</i>^−/y (<i>grey trace</i>) slices revealed a reduced amplitude without alteration in gamma waveform kinetics (<b>b</b>; <i>grey trace</i>, normalised <i>Ophn1</i>^−/y). The reduced gamma power in <i>Ophn1</i>^−/y slices was not associated with an altered spatial distribution; (<b>c</b>) waveform averages phase-zeroed at the peak of the oscillation recorded from CA3c (<i>black trace</i>), CA3b (<i>dotted trace</i>), CA3a (<i>short dashed trace</i>) and CA1 (<i>long dashed trace</i>) in an <i>Ophn1</i>^+/y slice. (<b>d</b>) Cross-correlation (<i>left panel</i>) and phase lead (<i>right panel</i>) for CA regions with CA3c as the reference, data are expressed as mean±s.e.m. No differences were observed between <i>Ophn1</i>^+/y (<i>filled symbols</i>) and <i>Ophn1</i>^−/y (<i>open symbols</i>) slices.</p

RRP is reduced in CA3 synapses.

<p>Evoked IPSCs recorded during a 20<i>Ophn1</i>^+/y (<b>a</b>, <i>black trace</i>) and <i>Ophn1</i>^−/y neuron (<i>grey trace</i>). The traces are averages of 5 sweeps. (<b>b</b>) The corresponding cumulative evoked IPSC amplitude plot (<i>Ophn1^+/y</i>,•; <i>Ophn1</i>^−/y,○). Data between 1–2 s were fitted by linear regression and back-extrapolated to time 0 to estimate the RRP size (<b>b, c</b>). (<b>c</b>) The mean amplitude of IPSC₁ was unaltered in <i>Ophn1</i>^−/y neurons. (<b>d</b>) Mean P_ves was increased in <i>Ophn1</i>^−/y neurons, whilst the mean number of vesicles (N_syn) forming the RRP was reduced (<b>e</b>) (<i>p</i>: **<0.01, ***<0.005).</p

KA-induced and spontaneous gamma oscillations are differentially affected by ROCK inhibition.

<p>Y–27632 (10 µM) was applied for 20 minutes prior to application of KA (50 nM). (<b>a</b>) Oscillations recorded at 60 minutes post KA application were reduced by Y-27632 application. In contrast, spontaneous oscillations recorded prior to KA application were enhanced in <i>Ophn1</i>^−/y slices (<b>b</b>) (<i>p</i>: *<0.05, ***<0.005).</p

Smaller gamma oscillations in <i>Ophn1</i>^−/y slices.

<p>(<b>a</b>) Application of 50 nM KA induced neuronal synchrony in the gamma frequency range; the power of these oscillations increased over time (<b>e</b>). (<b>b</b>) Spectrogram illustrating the development of the dominant frequency of gamma oscillations in <i>Ophn1</i>^+/y (<i>left panel</i>) and <i>Ophn1</i>^−/y (<i>right panel</i>) slices. (<b>c</b>) Power spectra for <i>Ophn1</i>^+/y (<i>left panel</i>) and <i>Ophn1</i>^−/y (<i>right panel</i>) slices at t = 60 minutes. (<b>d</b>) The peak frequency did not differ significantly between <i>Ophn1</i>^+/y (•) and <i>Ophn1</i>^−/y (○) slices. (<b>e</b>) Summated power of gamma oscillations was reduced in <i>Ophn1</i>^−/y slices.</p