*3.5. Inhibition of the Dopamine D1 Receptor Attenuated the WY-Induced Memory Improvement*

To investigate the link between dopamine and the memory-improving effect of the WY dipeptide, we examined the effect of SCH-23390, a dopamine D1-like receptor antagonist on the WY-dipeptide-induced memory improvement. Whereas WY-dipeptide increased spontaneous alteration in scopolamine-induced amnestic mice with prior treatment with saline (control group), SCH-23390 treatment abolished the memory improvement induced by the WY dipeptide (Figure 5A). These results indicated that the dopamine D1-like receptor was involved in the improvement of spatial memory induced by the WY dipeptide.

To further determine the involvement of the dopamine D1 receptor in the memory-improving effect of the WY dipeptide, the mRNA expression of the dopamine D1 receptor was knocked down in hippocampal neurons using the AAV system expressing artificial miRNA targeting this receptor according to our previous study [10]. While the WY dipeptide administration increased the spontaneous alternation in amnestic mice expressing control miRNA, this memory improvement was not significant with the dopamine D1 receptor knockdown in hippocampal neurons (Figure 5B). These results suggested that the dopamine D1 receptor in the hippocampus is involved in the memory improvement caused by the WY dipeptide at least in part.

**Figure 5.** The dopamine receptor is involved in memory improvement linked to the WY dipeptide. (**A**) Six-week-old Crl:CD1 male mice were orally administered 0 or 1 mg/kg of WY dipeptide and, 40 min later, intraperitoneally injected with 0.85 mg/kg of scopolamine alone or scopolamine plus 0.05 mg/kg of SCH-23390. At 1 h after oral administration of the peptide, mice were allowed to explore the Y-maze for 8 min. Data represent the mean ± SEM of 10 mice per group. (**B**) Eight-week-old Crl:CD1 male mice were administered either a control microRNA or dopamine D1 receptor microRNA containing AAV, which suppresses the dopamine D1 receptor, to the hippocampus. Mice were orally administered 0 or 1 mg/kg of WY dipeptide and, 40 min later, intraperitoneally injected with 0.85 mg/kg of scopolamine. At 1 h after oral administration of the peptide, mice were allowed to explore the Y-maze for 8 min. Data represent the mean ± SEM of 7–8 mice per group. The *p* values were calculated using one-way ANOVA followed by the Tukey–Kramer test. \*\* *p* < 0.01 and \* *p* < 0.05.
