Attenuation of IL‐6 Signalling Enhances Hippocampal Long‐Term Potentiation in Dystrophin‐Deficient mdx Mice

Abstract

In addition to progressive immobility, boys with Duchenne muscular dystrophy commonly exhibit cognitive impairment1, a trait also exhibited by mdx mice, which similarly lack hippocampal dystrophin. Interleukin (IL)-6 is also linked to impaired cognitive function2 and is elevated in dystrophinopathies. Hippocampal long term potentiation (LTP) is the proposed molecular correlate of memory formation, but descriptions of this phenomenon in mdx mice are conflicting3. The aim of the present study was to assess hippocampal LTP in wild type (WT) and mdx mice and to determine whether blocking IL-6 signalling modified LTP in mdx mice. In accordance with the European Directive 2010/63/EU, mdx and WT mice (8-12 weeks old) were anaesthetised with isoflurane. An intrathecal injection of monoclonal anti-IL-6 receptor antibodies (xIL-6R, 0.2mg/kg) or saline was administered and recognition memory was assessed using the novel object recognition test. After euthanasia, LTP, induced by high frequency stimulation of Schaffer collateral-CA1 fibres, was compared between mdx and WT mice, in both untreated and treated mice. All fEPSP slope speeds (mV/ms) are provided as a normalised percentage of averaged 20-minute fEPSP slope pre-LTP baselines. Statistical analyses were performed using 2-way ANOVA and unpaired t tests with data expressed as mean normalised post-LTP slope speed ± SEM. Contrary to previous studies3, the magnitude of LTP was reduced in untreated mdx slices relative to WT (2-way ANOVA, Interaction – F (179, 2327) = 1.370, p=0.0012, Genotype – F (1, 13) = 8.517, p=0.0120, n=6-9). Importantly, following in vivo administration of xIL-6R, the slope of the post-LTP excitatory post synaptic potential (EPSP) was enhanced in mdx hippocampal slices in comparison to in vivo administration of saline in mdx mice (mdx saline 199.5 ± 15.90, mdx xIL-6R 282.3 ± 3.528, n=3-4, p= 0.0074). No change was detected in WT mice. With regards to behaviour, novel object recognition in mdx mice was not different to WT mice, and this was not further modified by in vivo blockade of IL-6 signalling. Consistent with reports of cognitive deficits associated with loss of neuronal dystrophin, the molecular correlate of memory formation, LTP, is decreased in mdx hippocampal slices relative to WT comparators. Local changes in pro-inflammatory mediators, linked to loss of dystrophin could underlie changes in hippocampal function, and indeed, blocking IL-6 receptors normalised LTP evoked in mdx mice. Although recognition memory was not changed by this intervention, our findings support a role for IL-6 in deficiencies in hippocampal function in dystrophin-deficient mdx mice at a cellular level. 1. Snow WM, Anderson JE, Jakobson LS. Neuropsychological and neurobehavioral functioning in Duchenne muscular dystrophy: a review. Neurosci Biobehav Rev.2013;37(5):743-52. 2. Gruol DL. IL-6 regulation of synaptic function in the CNS. Neuropharmacology. 2015;96(Pt A):42-54. 3. Vaillend C, Billard JM, Laroche S. Impaired long-term spatial and recognition memory and enhanced CA1 hippocampal LTP in the dystrophin-deficient Dmd(mdx) mouse. Neurobiol Dis. 2004;17(1):10-20.