Jason M. Dwyer

Glutamate N-methyl-D-aspartate Receptor Antagonists Rapidly Reverse Behavioral and Synaptic Deficits Caused by Chronic Stress Exposure

BACKGROUND Despite widely reported clinical and preclinical studies of rapid antidepressant actions of glutamate N-methyl-D-aspartate (NMDA) receptor antagonists, there has been very little work examining the effects of these drugs in stress models of depression that require chronic administration of antidepressants or the molecular mechanisms that could account for the rapid responses. METHODS We used a rat 21-day chronic unpredictable stress (CUS) model to test the rapid actions of NMDA receptor antagonists on depressant-like behavior, neurochemistry, and spine density and synaptic function of prefrontal cortex neurons. RESULTS The results demonstrate that acute treatment with the noncompetitive NMDA channel blocker ketamine or the selective NMDA receptor 2B antagonist Ro 25-6981 rapidly ameliorates CUS-induced anhedonic and anxiogenic behaviors. We also found that CUS exposure decreases the expression levels of synaptic proteins and spine number and the frequency/amplitude of synaptic currents (excitatory postsynaptic currents) in layer V pyramidal neurons in the prefrontal cortex and that these deficits are rapidly reversed by ketamine. Blockade of the mammalian target of rapamycin protein synthesis cascade abolishes both the behavioral and biochemical effects of ketamine. CONCLUSIONS The results indicate that the structural and functional deficits resulting from long-term stress exposure, which could contribute to the pathophysiology of depression, are rapidly reversed by NMDA receptor antagonists in a mammalian target of rapamycin dependent manner.

GSK-3 Inhibition Potentiates the Synaptogenic and Antidepressant-Like Effects of Subthreshold Doses of Ketamine

A single dose of the short-acting NMDA antagonist ketamine produces rapid and prolonged antidepressant effects in treatment-resistant patients with major depressive disorder (MDD), which are thought to occur via restoration of synaptic connectivity. However, acute dissociative side effects and eventual fading of antidepressant effects limit widespread clinical use of ketamine. Recent studies in medial prefrontal cortex (mPFC) show that the synaptogenic and antidepressant-like effects of a single standard dose of ketamine in rodents are dependent upon activation of the mammalian target of rapamycin (mTOR) complex 1 (mTORC1) signaling pathway together with inhibitory phosphorylation of glycogen synthase kinase-3 (GSK-3), which relieves its inhibitory in influence on mTOR. Here, we found that the synaptogenic and antidepressant-like effects of a single otherwise subthreshold dose of ketamine were potentiated when given together with a single dose of lithium chloride (a nonselective GSK-3 inhibitor) or a preferential GSK-3β inhibitor; these effects included rapid activation of the mTORC1 signaling pathway, increased inhibitory phosphorylation of GSK-3β, increased synaptic spine density/diameter, increased excitatory postsynaptic currents in mPFC layer V pyramidal neurons, and antidepressant responses that persist for up to 1 week in the forced-swim test model of depression. The results demonstrate that low, subthreshold doses of ketamine combined with lithium or a selective GSK-3 inhibitor are equivalent to higher doses of ketamine, indicating the pivotal role of the GSK-3 pathway in modulating the synaptogenic and antidepressant responses to ketamine. The possible mitigation by GSK-3 inhibitors of the eventual fading of ketamine’s antidepressant effects remains to be explored.

mTOR activation is required for the antidepressant effects of mGluR2/3 blockade

Recent studies demonstrate that ketamine, a fast-acting antidepressant, rapidly activates the mammalian target of rapamycin (mTOR) and increases synaptogenesis in the prefrontal cortex. Because of the side-effect and abuse potential of ketamine we are investigating alternative agents that produce similar effects. Here, we demonstrate that a single dose of LY 341495, an mGluR₂/₃ antagonist, produces ketamine-like biochemical and behavioural actions. LY 341495 administration rapidly (1 h) activates the mTOR pathway (mTOR, p70S6K, 4E-BP1) and subsequently (24 h later) increases levels of synaptic proteins (PSD-95, GluR1 and Synapsin I), similar to the effects of ketamine. Finally, the antidepressant effects of LY 341495 in the rat forced swim test are completely blocked by the mTOR inhibitor, rapamycin. The results indicate that the antidepressant actions of LY 341495 are mediated by activation of mTOR and suggest that this and other mGluR₂/₃ antagonists could produce rapid antidepressant effects in depressed patients.

Cell atrophy and loss in depression: reversal by antidepressant treatment.

Depression is associated with structural alterations in limbic brain regions that control emotion and mood. Studies of chronic stress in animal models and postmortem tissue from depressed subjects demonstrate that these structural alterations result from atrophy and loss of neurons and glial cells. These findings indicate that depression and stress-related mood disorders can be considered mild neurodegenerative disorders. Importantly, there is evidence that these structural alterations can be blocked or even reversed by elimination of stress and by antidepressant treatments. A major focus of current investigations is to characterize the molecular signaling pathways and factors that underlie these effects of stress, depression, and antidepressant treatment. Recent advances in this research area are discussed and potential novel targets for antidepressant development are highlighted.

BDNF Release Is Required for the Behavioral Actions of Ketamine

Background: Recent studies demonstrate that the rapid antidepressant ketamine increases spine number and function in the medial prefrontal cortex (mPFC), and that these effects are dependent on activation of glutamate α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors and brain-derived neurotrophic factor (BDNF). In vitro studies also show that activation of AMPA receptors stimulates BNDF release via activation of L-type voltage-dependent calcium channels (VDCC). Methods: Based on this evidence, we examined the role of BDNF release and the impact of L-type VDCCs on the behavioral actions of ketamine. Results: The results demonstrate that infusion of a neutralizing BDNF antibody into the mPFC blocks the behavioral effects of ketamine in the forced swim test (FST). In addition, we show that pretreatment with nifedipine or verapamil, two structurally-different L-type calcium channel antagonists, blocks the behavioral effects of ketamine in the FST. Finally, we show that ketamine treatment stimulates BDNF release in primary cortical neurons and that this effect is blocked by inhibition of AMPA receptors or L-type VDCCs. Conclusions: Taken together, these results indicate that the antidepressant effects of ketamine are mediated by activation of L-type VDCCs and the release of BDNF. They further elucidate the cellular mechanisms underlying this novel rapid-acting antidepressant.

Activation of Mammalian Target of Rapamycin and Synaptogenesis: Role in the Actions of Rapid-Acting Antidepressants

Antidepressants that produce rapid and robust effects, particularly for severely ill patients, represent one of the largest unmet medical needs for the treatment of depression. Currently available drugs that modulate monoamine neurotransmission provide relief for only a subset of patients, and this minimal efficacy requires several weeks of chronic treatment. The recent discovery that the glutamatergic agent ketamine produces rapid antidepressant responses within hours has opened a new area of research to explore the molecular mechanisms through which ketamine produces these surprising responses. Clinical and preclinical findings have exposed some of the unique actions of ketamine and identified a cell-signaling pathway known as the mammalian target of rapamycin. Activation of mammalian target of rapamycin and increased synaptogenesis in the prefrontal cortex are crucial in mediating the antidepressant effects of ketamine. Importantly, the synaptic actions of ketamine allow rapid recovery from the insults produced by exposure to repeated stress that cause neuronal atrophy and loss of synaptic connections. In the following review, we explore some of the clinical and preclinical findings that have thrust ketamine to the forefront of rapid antidepressant research and unveiled some of its unique molecular and cellular actions.

Rapid Antidepressant Actions of Scopolamine: Role of Medial Prefrontal Cortex and M1-subtype Muscarinic Acetylcholine Receptors

Clinical studies demonstrate that scopolamine, a non-selective muscarinic acetylcholine receptor (mAchR) antagonist, produces rapid therapeutic effects in depressed patients, and preclinical studies report that the actions of scopolamine require glutamate receptor activation and the mechanistic target of rapamycin complex 1 (mTORC1). The present study extends these findings to determine the role of the medial prefrontal cortex (mPFC) and specific muscarinic acetylcholine receptor (M-AchR) subtypes in the actions of scopolamine. The administration of scopolamine increases the activity marker Fos in the mPFC, including the infralimbic (IL) and prelimbic (PrL) subregions. Microinfusions of scopolamine into either the IL or the PrL produced significant antidepressant responses in the forced swim test, and neuronal silencing of IL or PrL blocked the antidepressant effects of systemic scopolamine. The results also demonstrate that the systemic administration of a selective M1-AChR antagonist, VU0255035, produced an antidepressant response and stimulated mTORC1 signaling in the PFC, similar to the actions of scopolamine. Finally, we used a chronic unpredictable stress model as a more rigorous test of rapid antidepressant actions and found that a single dose of scopolamine or VU0255035 blocked the anhedonic response caused by CUS, an effect that requires the chronic administration of typical antidepressants. Taken together, these findings indicate that mPFC is a critical mediator of the behavioral actions of scopolamine and identify the M1-AChR as a therapeutic target for the development of novel and selective rapid-acting antidepressants.

Ribosomal protein S6 kinase 1 signaling in prefrontal cortex controls depressive behavior

Significance The molecular pathophysiology associated with depression remains largely unknown. Recent evidence suggests that signaling pathways downstream of mechanistic target of rapamycin complex 1, such as p70 S6 kinase 1 (S6K1), can lead to structural changes in the medial prefrontal cortex (mPFC) of rats, which are associated with antidepressant responses. We used a viral-mediated approach to control S6K1 activity in the mPFC. Enhanced S6K1 activity produced antidepressant-like effects and resilience to chronic stress, whereas decreased S6K1 activity produced prodepressive behavior. Together, these studies demonstrate that aberrant activity of protein synthesis pathways may underlie the pathology of depression, and demonstrate that direct modulation of this pathway can control depressive behavior in a bidirectional manner. Further understanding of these signaling pathways may contribute to improved treatments for major depressive disorder. Current treatments for major depressive disorder (MDD) have a time lag and are ineffective for a large number of patients. Development of novel pharmacological therapies requires a comprehensive understanding of the molecular events that contribute to MDD pathophysiology. Recent evidence points toward aberrant activity of synaptic proteins as a critical contributing factor. In the present studies, we used viral-mediated gene transfer to target a key mediator of activity-dependent synaptic protein synthesis downstream of mechanistic target of rapamycin complex 1 (mTORC1) known as p70 S6 kinase 1 (S6K1). Targeted delivery of two mutants of S6K1, constitutively active or dominant-negative, to the medial prefrontal cortex (mPFC) of rats allowed control of the mTORC1/S6K1 translational pathway. Our results demonstrate that increased expression of S6K1 in the mPFC produces antidepressant effects in the forced swim test without altering locomotor activity. Moreover, expression of active S6K1 in the mPFC blocked the anhedonia caused by chronic stress, resulting in a state of stress resilience. This antidepressant response was associated with increased neuronal complexity caused by enhanced S6K1 activity. Conversely, expression of dominant-negative S6K1 in the mPFC resulted in prodepressive behavior in the forced swim test and was sufficient to cause anhedonia in the absence of chronic stress exposure. Together, these data demonstrate a critical role for S6K1 activity in depressive behaviors, and suggest that pathways downstream of mTORC1 may underlie the pathophysiology and treatment of MDD.