Figure 4. Electrical stimulation of the GHT evokes inhibitory and excitatory responses in the SCN.

A, schematic of the slice preparation showing stimulation electrode and pMEA placement used. B, electrical stimulation (concentric electrode; 1.2 mA, 200 μs) of the IGL region evoked inhibitory responses at a subset of SCN recording sites (n = 21 sites from 12 slices; representative recording site shown). C, electrical stimulation of the GHT also evoked excitatory responses in a second group of SCN recording sites from these 12 slices (n = 27 sites; left: representative recording site shown), which could be abolished by bath application of ionotropic glutamate receptor blockers (CNQX, 20 μm and d‐AP5, 50 μm; right: representative recording site shown of four SCN recording sites tested). D, GHT‐evoked inhibitory responses (time to peak, 10 ms moving window) were typically slower and exhibited more variable timing than excitatory responses. E, where tested (n = 12 sites), short trains of pulses (five stimuli at 50 ms, i.e. 20 Hz, or five stimuli at 25 ms, 40 Hz) did not evoke significantly larger inhibitions than single pulse stimulation (one‐way ANOVA; P = 0.98). F, GHT‐evoked (1.2 mA, 200 μs; 5 × 40 Hz train) inhibitory responses (middle and bottom) but not excitatory responses (top) were blocked by bath application of the GABA_AR antagonist, BIC (20 μm; representative SCN sites shown from a total of 23 excitatory and five inhibitory responses tested). G and H, quantification of the effect of BIC and CNQX/D‐AP5 relative to predrug values (black bars) for excitatory (G), and inhibitory responses (H). Data were analysed by a paired t test (except CNQX in G, where only four cells were tested). ^** P < 0.01; n.s., P > 0.05.