Spexin-Based Galanin Receptor Type 2 Agonist for Comorbid Mood Disorders and Abnormal Body Weight

Abstract

Despite the established comorbidity between mood disorders and abnormal eating behaviors, the underlying molecular mechanism and therapeutics remain to be resolved. Here, we show that a spexin-based galanin receptor type 2 agonist (SG2A) simultaneously normalized mood behaviors and body weight in corticosterone pellet-implanted (CORTI) mice, which are underweight and exhibit signs of anhedonia, increased anxiety, and depression. Administration of SG2A into the lateral ventricle produced antidepressive and anxiolytic effects in CORTI mice. Additionally, SG2A led to a recovery of body weight in CORTI mice while it induced significant weight loss in normal mice. In Pavlovian fear-conditioned mice, SG2A decreased contextual and auditory fear memory consolidation but accelerated the extinction of acquired fear memory without altering innate fear and recognition memory. The main action sites of SG2A in the brain may include serotonergic neurons in the dorsal raphe nucleus for mood control, and proopiomelanocortin/corticotropin-releasing hormone neurons in the hypothalamus for appetite and body weight control. Furthermore, intranasal administration of SG2A exerted the same anxiolytic and antidepressant-like effects and decreased food intake and body weight in a dose-dependent manner. Altogether, these results indicate that SG2A holds promise as a clinical treatment for patients with comorbid mood disorders and abnormal appetite/body weight.

Keywords: appetite; body weight; depression; galanin receptor 2 agonist; intranasal administration; post-traumatic stress disorder.

Figure 1

Complexity of GAL/SPX and receptor systems and pharmacological properties of SG2A. The natural ligand GAL binds to GAL₁ receptor and GAL₂ receptor with high affinity but shows relatively low affinity to GAL₃ receptor. SPX has high potency to activate GAL₂ receptor and GAL₃ receptor but does not activate GAL₁ receptor (Kim et al., 2014). The synthetic ligand SPX-based GAL₂ receptor agonist (SG2A) exhibits exclusive selectivity toward GAL₂ receptor but not GAL1 receptor and GAL₃ receptor (Reyes-Alcaraz et al.,2016). GAL receptors have a wide range of physiological functions through subtype-specific signaling pathways. Upon stimulation by a ligand, GAL₁ receptor and GAL₃ receptor induce inhibitory G_i signaling, whereas GAL₂ receptor triggers stimulatory G_q signaling. The GAL-induced GAL₂ receptor activation induces G_q-coupled signaling followed by the β-arrestin (βA)-mediated pathway, including receptor desensitization/internalization and initiation of an alternative signaling. Thus, GAL behaves as a classical agonist equally inducing G_q-coupled and β-arrestin-mediated pathways. Unlike GAL, SPX and SG2A exhibit a biased GAL₂ receptor agonism toward the G_q-coupled pathway over β-arrestin-mediated pathway such that they induce receptor internalization much less than GAL (Reyes-Alcaraz et al., 2018).

Figure 2

Effects of SG2A on body weight change and anxiety/depression-like behaviors in mice with corticosterone pellet implantation (CORTI) and in sham-operated mice. (A) Experimental scheme for SG2A administrations and behavioral tests after CORTI. SG2A or vehicle (Veh) was administered to CORTI or sham-operated mice for 8 consecutive days, during which body weight measurement, the sucrose preference test (SPT), elevated plus maze test (EPMT), open field test (OFT), and tail suspension test (TST) were conducted. After all behavioral tests, blood and brain samples were collected for determination of CORT levels, neurogenesis in the dentate gyrus of the hippocampus, and tryptophan hydroxylase (TPH) protein levels in the dorsal raphe nucleus. (B,C) Body weight changes during 3 weeks of CORT implantation and after SG2A administration [arrow in panel (B)] for 7 days. Effects of SG2A on sucrose preference (D), total fluid consumption (E), times in the open-arm in the EPMT (F) and in the center in the OFT (G), and immobility of mice in the TST (H). (I) Immunohistochemistry of BrdU (red), doublecortin [(DCX) green] and DAPI (blue) (left) and mean number of BrdU+DCX+ cells (right) in the DG of the hippocampus (5–6 slices were counted per mouse). (J) Western blot for tryptophan hydroxylase [(TPH) top] and TPH levels in the DRN (bottom). Data are presented as means ± SEMs; ^∗P < 0.05 and ^∗∗P < 0.01 vs. Sham-Veh, ^#P < 0.05 and ^##P < 0.01 vs. CORTI-Veh. Numbers in parentheses indicated the numbers of animals used for each group. Scale bar, 50 μm.

Figure 3

Effect of SG2A on fear memory consolidation and extinction. (A) Decreased fear memory consolidation by SG2A. Fear memory acquisition curves during seven repeated tones and foot shock pairing (left). SG2A was administered immediately after acquisition, and contextual fear memory (middle) and auditory fear memory (right) were determined 24 h later. (B) Facilitated extinction of consolidated fear memory by SG2A. 24 h after memory acquisition, freezing behaviors were scored during 20 trials of a 30-s tone without foot shock in the distinct context in the presence or absence of SG2A. (C) Innate fear response is unaltered by SG2A. Freezing behaviors of mice exposed to TMT were scored. (D) Recognition memory by SG2A in the Y maze. No significant difference (n.s.) was found between Veh- and SG2A-treated groups. (E) Novel object recognition test. Vehicle- and SG2A-treated mice similarly showed a preference for the novel object. Data are presented as means ± SEMs; ^∗P < 0.05 and ^∗∗P < 0.01 vs. Veh, #P < 0.05 between training and test group. Numbers in parentheses indicated the numbers of animals used for each group.

Figure 4

Neurons responding to SG2A. Immunohistochemistry for c-fos⁺ (red) neurons in mice administered SG2A or vehicle (Veh) for 1–2 h. The Percentage of cells double immunopositive for c-fos and TPH (serotonergic neurons) in the DRN (A), TH (dopaminergic neurons) in the VTA (B) and SN (C), NeuN in PFC (D) and NAc (E), CamKIIα in BLA (F), GAD67 in CeA (G), NeuN in the DG of the hippocampus (H), CRH in PVN (I), and POMC in ARC (J) were counted (3–4 slices were counted per mouse). (K) Depolarization of POMC neuron membrane potential in response to applications of SG2A (left); summary of acute effects of SG2A on the membrane potentials of responsive POMC neurons (right). (L) α-MSH secretion in cultured POMC neurons in response to SG2A. Data are presented as means ± SEMs; ^∗P < 0.05 and ^∗∗P < 0.01 vs. Veh. Numbers in parentheses indicate the numbers of animals used for each group. Scale bar, 50 μm.

Figure 5

Effects of intranasally administered SG2A on anxiety, depression-like, and feeding behaviors. Normal mice received various concentrations of SG2A (1, 3, and 10 μg/mouse). Anxiolytic effect of SG2A in the EPMT (A) and OFT (B). Antidepressant-like effect of SG2A in the TST (C) and FST (D). Cumulative changes in food intake (E) and body weight (F) during 7 consecutive administrations of SG2A. Data are presented as means ± SEMs; ^∗P < 0.05 and ^∗∗P < 0.01 vs. Veh. Numbers in parentheses indicate the numbers of animals used for each group.