not-yet-known not-yet-known not-yet-known unknown MRS5474 decreased excitability under hyperexcitable conditions We then hypothesized that the effect of MRS5474 upon excitatory transmission could predominantly occur in hyperexcitable neurons. To test this hypothesis, we started by assessing the action of MRS5474 (250 nM) in organotypic rhinal-hippocampal slices, which develop spontaneous activity (Valente et al., 2021). To further enhance spontaneous activity, the extracellular concentration of potassium chloride in the extracellular medium was increased to 8.5 mM during field potential recordings. The frequency of spikes within a burst did not change appreciably throughout the time of recording in control slices (no drug) (Figure 3A and C ). However, the frequency of spikes within a burst was significantly decreased (P<0.05) in slices perfused with MRS5474 (Table 1 and Figure 3B and D ). In 2 out of 7 slices perfused with MRS5474, the bursts were abolished within minutes after adding the drug to the perfusing medium (Figure 3B and D ). In those slices where bursts remained, the frequency of spikes within the burst, at 80–90 minutes, was reduced to 0.65 ± 0.04 (P<0.01 as compared with baseline, Table 1,). When comparing values in control and MRS5474-perfused slices, using time and drug condition as independent variables, the number of bursts at 80–90 minutes in slices perfused with MRS5474 was significantly lower than in control slices (P<0.05, two-way ANOVA, Table 1). The amplitude of spikes within the bursts was not significantly affected by MRS5474 (Table 1). Under non-depolarizing conditions (extracellular perfusing medium composed of Neurobasal A), MRS5474 was virtually devoid of effect in all parameters analysed (Table 1). In clear contrast, CPA (30 nM), an A1R agonist, fully abolished spontaneous activity within 30 minutes after starting its perfusion, which was zero at all parameters evaluated (n=4, data not shown). To further address the effect of MRS5474 on hippocampal synaptic transmission in ‘epileptic-like’ tissue, we tested its effect in hippocampal slices taken from animals with established epilepsy (EE). In hippocampal slices of animals with EE, MRS5474 (250 nM) significantly decreased fEPSP slope (Figure 4A ) by 31 ± 6.0% (n=7 slices from 6 rats, P<0.05), while in slices from control animals, MRS5474 was virtually devoid of effect (Figure 4B) on fEPSPs (% change in slope at the end of application: -1.43±8.6%, n=6, P>0.05).