The Ghrelin/Growth Hormone Secretagogue Receptor System Is Involved in the Rapid and Sustained Antidepressant-Like Effect of Paeoniflorin

Abstract

Major depressive disorder (MDD) is a debilitating mental illness affecting people worldwide. Although significant progress has been made in the development of therapeutic agents to treat this condition, fewer than half of all patients respond to currently available antidepressants, highlighting the urgent need for the development of new classes of antidepressant drugs. Here, we found that paeoniflorin (PF) produced rapid and sustained antidepressant-like effects in multiple mouse models of depression, including the forced swimming test and exposure to chronic mild stress (CMS). Moreover, PF decreased the bodyweight of mice without affecting food intake and glucose homeostasis, and also reduced the plasma levels of total ghrelin and the expression of ghrelin O-acyltransferase in the stomach; however, the plasma levels of ghrelin and the ghrelin/total ghrelin ratio were unaffected. Furthermore, PF significantly increased the expression of growth hormone secretagogue receptor 1 alpha (GHSR1α, encoded by the Ghsr gene) in the intestine, whereas the levels of GHSR1α in the brain were only marginally downregulated following subchronic PF treatment. Finally, the genetic deletion of Ghsr attenuated the antidepressant-like effects of PF in mice exposed to CMS. These results suggested that increased GHSR1α expression in the intestine mediates the antidepressant-like effects of PF. Understanding peripheral ghrelin/GHSR signaling may provide new insights for the screening of antidepressant drugs that produce fast-acting and sustained effects.

Keywords: antidepressant; growth hormone secretagogue receptor 1α; intestine; major depressive disorder; paeoniflorin.

FIGURE 1

Paeoniflorin (PF) produced antidepressant-like effects in the forced swimming test (FST). (A) Schematic diagram of the experimental design. (B) Duration of immobility in the FST (n = 8–10). (C) Total distance traveled during the open field test (OFT) (n = 8–10). (D) Duration of immobility in the FST after treatment with PF at the dosage of 2.5, 5, or 10 mg/kg (n = 10). SAL, saline; FLX, fluoxetine; IMI, imipramine; HAL, haloperidol. The data are expressed as means ± SEM. **P < 0.01.

FIGURE 2

Paeoniflorin (PF) produced sustained antidepressant-like effects in the forced swimming test (FST). (A) Schedule for detecting the long-lasting antidepressant-like effect of PF. (B,C) Statistical data for the FST (B) and the open field test (OFT) (C) conducted 28 days after subchronic PF treatment (n = 10). (D,E) The latency (D) and the food intake (E) in the novelty suppressed feeding test (n = 7–8). SAL, saline. The data are expressed as means ± SEM. *P < 0.05, ***P < 0.001.

FIGURE 3

The rapid and sustained antidepressant-like effect of paeoniflorin (PF) during chronic mild stress (CMS). (A) Schematic diagram of the experimental design for the treatments with PF, imipramine (IMI), and saline (SAL) during exposure to CMS. (B,C) The results of the sucrose preference test (B) and the coat score assay (C) during exposure to CMS (n = 10–11). The data are expressed as means ± SEM. *P < 0.05, **P < 0.01, ***P < 0.001; n.s., not significant.

FIGURE 4

The effects of paeoniflorin (PF) on metabolic status. (A) Schematic representation of the experimental design (n = 8). (B) Bodyweight gain following PF/saline (SAL) administration. (C) Cumulative food intake. (D) Plasma glucose levels during the glucose tolerance test. The data are expressed as means ± SEM. *P < 0.05.

FIGURE 5

The effects of paeoniflorin (PF) on the mRNA levels of factors in the peripheral ghrelin/GHSR system. (A) qPCR analysis of Ghrl, Mboat4, and Ghsr mRNA in the stomach, intestine, and liver in adult C57BL/6 mice (n = 8). (B) The effects of the 7-day treatment with PF or saline (SAL) on the mRNA levels of Ghrl and Mboat4 in the stomach (n = 8). (C) The effects of PF or SAL on the mRNA levels of Ghrl, Mboat4, and Ghsr in the intestine (n = 8). The data are expressed as means ± SEM. **P < 0.01, ***P < 0.001.

FIGURE 6

The effects of paeoniflorin (PF) on the levels of proteins in the peripheral ghrelin/GHSR system. (A) Compared with saline (SAL) treatment, 7-day treatment with PF decreased the plasma levels of total ghrelin (n = 8). (B) The plasma levels of ghrelin in C57BL/6 mice treated with PF or SAL for 7 consecutive days (n = 8). (C) The ghrelin/total ghrelin ratio in the plasma following subchronic treatment with PF or SAL (n = 8). (D) The plasma levels of total ghrelin in adult C57BL/6 mice after 24 h of fasting (n = 10). (E) The protein levels of GOAT in the stomach after 7 days of treatment with PF or SAL (n = 8). (F) The protein levels of GHSR1α in the intestine were higher with PF administration than with SAL administration (n = 8). The data are expressed as means ± SEM. *P < 0.05, ***P < 0.001.

FIGURE 7

The effects of paeoniflorin (PF) on the levels of GHSR1α in the brain. (A–E) Seven days of PF treatment did not affect the protein levels of GHSR1α in the medial prefrontal cortex (mPFC) (A), nucleus accumbens (NAc) (B), hippocampus (C), ventral tegmental area (VTA) (D), or Edinger–Westphal nucleus (EW) (E) (n = 8) when compared with saline (SAL) treatment. (F) Subchronic PF treatment decreased the protein levels of GHSR1α in the nucleus tractus solitarii (NTS) (n = 8). The data are expressed as means ± SEM. *P < 0.05.

FIGURE 8

The deletion of Ghsr attenuated the antidepressant-like effect of paeoniflorin (PF). (A) Genotyping shows the downregulation of Ghsr mRNA in Ghsr^–/– mice and their heterozygous littermates (Ghsr^+/–) compared with the wild-type controls (Ghsr^+/+). (B) Schematic diagram of the experimental design. (C,D) The sucrose preference test and coat score assay following exposure to chronic mild stress (CMS) (n = 6, 7, 9, and 10 for Ghsr^+/+-CON (control; not exposed to CMS), Ghsr^–/–-CON, Ghsr^+/+-CMS, and Ghsr^–/–-CMS, respectively). The data are expressed as means ± SEM. *P < 0.05, **P < 0.01, ***P < 0.001; n.s., not significant.