To investigate the effects and mechanism of nobiletin (NOB) on slow transit constipation (STC) in rats through dopamine receptor D2 (DRD2).

A total of 70 SD rats were randomly divided into blank group, model group, experimental-L, M, H group, AAV9-NC group and AAV9-DRD2 group, with 10 rats in each group. STC model was established with 15 mg·kg^-1 compound diphenoxylate by intragastric administration except in blank group. In experimental-L, M, H group, 0.1, 0.2, 0.4 g·kg^-1 NOB was administered intragastricly. On the basis of experimental-M group, AAV9-NC group and AAV9-DRD2 group were given AAV9-NC and AAV9-DRD2 100 μL, respectively. Weight and fecal indexes of rats were detected. The expression levels of DRD2 was detected by quantitative real time polymerase chain reaction. Vasoactive intestinal peptide (VIP) and nitric oxide (NO) levels were detected by enzyme-linked immunosorbent assay. Levels of AMP-activated protein kinase (AMPK)/endothelial nitric oxide synthase (eNOS) signaling pathway related proteins were detected by Western blot.

The fecal water content in blank, model, and experimental-L, M, H groups were (35.24±5.25)%, (24.76±3.47)%, (28.38±2.77)%, (32.76±3.49)% and (33.32±4.34)% , respectively; the fecal quantity were (53.20±5.25), (42.40±4.55), (47.10±5.36), (49.50±5.10) and (50.10±5.04) grains, respectively. Compared model group with blank group, and compared experimental-L, -M, -H groups with model group, the above indicators showed statistically significant differences (P<0.05, P<0.01, P<0.001). The relative expression levels of DRD2 mRNA in blank, model and experimental-L, M, H groups were 1.00±0.12, 2.95±0.47, 2.73±0.49, 2.17±0.35 and 2.08±0.32, respectively. The relative expression levels of DRD2 mRNA in the experimental-M and experimental-H groups were significantly lower than those in the model group (all P<0.001), with no significant difference between the two groups (P>0.05); therefore, experimental-M was selected for subsequent experiments. The VIP levels of blank group, model group, experimental-M group, AAV9-NC group and AAV9-DRD2 group were (43.15±4.52), (64.42±7.64), (51.68±5.37), (49.25±4.44) and (56.84±6.41) ng·L^-1, respectively; NO levels were (6.84±0.71), (11.68±1.37), (9.28±0.93), (9.35±0.97) and (10.45±1.03) μmol·L^-1, respectively; the relative expression levels of AMPK protein were 1.00±0.15, 2.42±0.34, 1.68±0.17, 1.55±0.18 and 1.94±0.21, respectively; the relative expression levels of eNOS protein were 1.00±0.11, 3.18±0.35, 2.28±0.23, 2.35±0.27 and 2.95±0.33, respectively. Compared model group with blank group, compared experimental-M group with model group, and compared AAV9-DRD2 group with AAV9-NC group, the above indicators all showed statistically significant differences (all P<0.05).

NOB can improve STC in rats, possibly by down-regulating DRD2 expression, regulating VIP and NO levels, and inhibiting the AMPK/eNOS signaling pathway.