(2R,6R)-hydroxynorketamine rescues chronic stress-induced depression-like behavior through its actions in the midbrain periaqueductal gray

Highlights

• Both intraperitoneal and intra-vlPAG administration of (2R,6R)-HNK rescue chronic stress-induced depression-like behavior.

• (2R,6R)-HNK rapidly rescues chronic stress-induced depression-like behavior.

• Antidepressant effects of (2R,6R)-HNK persist for up to 21 days.

• (2R,6R)-HNK increases glutamate release presynaptically and surface GluR1 expression postsynaptically in the vlPAG.

• Blockage of AMPAR in the vlPAG antagonizes antidepressant effects of (2R,6R)-HNK.

Abstract

It has been widely reported that ketamine rescues chronic stress-induced depression-like behavior, but the underlying cellular mechanisms of the rapid antidepressant actions of ketamine remain largely unclear. Both male and female Sprague-Dawley rats were used and received modified learned helplessness paradigm to induce depression-like behavior. Depression-like behavior was assayed and manipulated using forced swim tests, sucrose preference tests and pharmacological microinjection. We conducted whole-cell patch-clamp electrophysiological recordings in the midbrain ventrolateral periaqueductal gray (vlPAG) neurons. Surface and cytosolic glutamate receptor 1 (GluR1) α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor expression were analyzed using Western blotting. Phosphorylated GluR1 expression was quantified using Western blotting analysis. The results showed that a single systemic administration of a ketamine metabolite (2R,6R)-hydroxynorketamine (2R,6R-HNK) rapidly rescued chronic stress-induced depression-like behavior and persisted for up to 21 days. Consistently, the chronic stress-induced diminished glutamatergic transmission and surface GluR1 expression in the vlPAG were also reversed by a single systemic injection of (2R,6R)-HNK. Furthermore, bath application of (2R,6R)-HNK increased the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs) in the vlPAG. Further evidence for the antidepressant action of (2R,6R)-HNK is provided by the finding that microinjection of (2R,6R)-HNK into the vlPAG exhibited a rapid-acting and long-lasting antidepressant effect. This antidepressant effect of (2R,6R)-HNK was prevented by the intra-vlPAG microinjection of AMPA receptor antagonist CNQX. Together, the current results provide evidence that (2R,6R)-HNK rescues chronic stress-induced depression-like behavior with rapid-acting and long-lasting antidepressant effects through enhancement of AMPA receptor-mediated transmission in the vlPAG.

Introduction

Repeated exposure to psychological or environmental stressors leads to an elevated risk of psychiatric disorders such as depression (Duman et al., 2016; Moylan et al., 2013; Wohleb et al., 2016a; Zhuo, 2017). Major depressive disorder (MDD), which has a lifetime prevalence of 17% in the United States (Kessler et al., 2005), is among the leading contributors to the global burden of disease (Whiteford et al., 2013). The currently available pharmacotherapies for depression are associated with limited therapeutic efficacy and a delayed response onset of weeks to months (Gaynes et al., 2009). Although intense research efforts during the past decades have yielded in-depth insights into the molecular mechanisms of depression (Gerhard et al., 2016; Harmer et al., 2017; Wohleb et al., 2016a), the field of antidepressant research has had little success in the development of next-generation and faster-acting antidepressants. These intractable issues highlight the need for more efficacious and rapid-acting medications to alleviate MDD.

The midbrain periaqueductal gray (PAG) has been considered a critical nucleus of the circuitry that participates in stress-induced responses such as depression-like behavior and antinociception. (Ho et al., 2018; Johnson et al., 2016; Lau and Vaughan, 2014; Vaughan, 2006; Wang et al., 2016). A previous study demonstrated that the PAG controlling the emotional coping responses to different stressors comprises distinct subregions (Paredes et al., 2000). It has been widely accepted that the PAG contributes to processing many aspects of emotional responses to organismal stress with preferential activation of a specific PAG subregion reflecting the type of emotional coping reaction triggered and whether the stressor is physical or psychological (Keay and Bandler, 2001). The dorsolateral PAG (dlPAG) has been shown to participate in both acute and chronic stress-induced maladaptive responses (De Oliveira et al., 2001). However, the role of the ventrolateral periaqueductal gray (vlPAG) contributing to stress-induced maladaptive behavior is largely unclear. Recently, the vlPAG, a novel brain region governing stress-induced depression-like behavior, has been demonstrated to regulate behavioral despair and anhedonia (Ho et al., 2018; Johnson et al., 2016). A previous study reported that depression-like behaviors were neutralized by knockdown of the galanin receptor 1 in the ventral periaqueductal gray, demonstrating the crucial role of the ventral periaqueductal gray in chronic stress-induced depression-like behavior (Wang et al., 2016). Recently, we found that diminished glutamatergic transmission in the vlPAG contributes to the development of chronic stress-elicited depression-like behavior (Ho et al., 2018). The vlPAG is thus an important location for stress-associated depression-like behavior and for studying its underlying cellular and molecular mechanisms.

The currently available pharmacotherapies provide some symptom relief for patients with depression, but these medications have significant limitations, including onset time lag and low efficacy (Duman et al., 2016; Gerhard et al., 2016). Thus, more rapid-acting and efficacious antidepressants are sorely needed (Abdallah, 2017). A previous study reported that ketamine rapidly improves depressive symptoms within 72 h after administration in patients with MDD (Berman et al., 2000). Zarate and colleagues found that the levels of a ketamine metabolite, (2R,6R)-hydroxynorketamine (2R,6R-HNK), were consistently high in patients with MDD after ketamine infusion (Zarate et al., 2012). Interestingly, a recent study demonstrated that systemic application of (2R,6R)-HNK exerted rapid antidepressant behavioral actions within 24 h through enhancement of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor function and expression in hippocampal synapses (Zanos et al., 2016). Previously, we found that chronic stress-induced depression-like behavior was attributed to the hypofunction of glutamatergic transmission in the vlPAG (Ho et al., 2018). Therefore, in the current study, we sought to investigate whether the ketamine metabolite (2R,6R)-HNK rescues chronic stress-induced depression-like behavior via increasing glutamatergic transmission in the vlPAG, and we explored its underlying mechanisms.