Unitary inhibition recorded from two synaptically-connected CR cells
CR interneurons during the late stages of AD were readily identifiable under ID-DIC during experiments, (in striking contrast to CCK or SST cells that were not easily visualised), allowing us to perform paired recording between two CR cells. We performed paired recording in theAppNL-F/NL-F animals only due to the very technically challenging nature of these experiments, hampered by the age of the mice. However, supplementary Figure 1(E-F) shows examples of paired recordings performed in young healthy control rodents.
Consistent with the finding that the sIPSPs recorded in CR cells were sensitive to α5-SOP002, unitary IPSPs recorded between two CR cells in SR were also reduced in peak amplitude and width at half amplitude following bath-application of α5-SOP002 at -55mV (Figure 4E). The decrease in amplitude and width was: 51.20 ± 7.36 % (P <0.05, n =3, paired, two-tailed Student’s t-test) and 28.25 ± 1.02 % (P <0.01, n =3, paired, two-tailed Student’s t-test) of control IPSPs recorded inAppNL-F/NL-F , respectively. Bath-application of the α1 subunit-selective agonist, zolpidem did not change the IPSP properties at these synapses, which was consistent with previous studies that reported insensitivity to zolpidem at synapses involving presynaptic dendrite-preferring cells (Ali et al., 2008). Subsequent addition of the broad spectrum benzodiazepine site agonist, diazepam (after α5-SOP002) enhanced IPSP amplitude by 186.59 ± 41.45 % (P <0.05, n =3, one-way ANOVA) and width at half amplitude by, 37.31 ± 6.71 % (P >0.05, n =3, one-way ANOVA with post-hoc Bonferroni’s test) of control IPSPs recorded in AppNL-F/NL-F mice (Figure. 4 (E-F)).
The recorded (putative) CR –expressing interneurons, recovered post-hoc were usually oval with 2-3 vertically orientated primary beaded dendrites, usually from opposite poles, with fine axons containing small/medium sized boutons originated from the soma or a primary dendrite and ramified quite sparsely in mid-SR, as described previously (Shi et al., 2019) These cells resembled previously published CR cells (Gulyas et al., 1996).