GABA interneurons are the cellular trigger for ketamine's rapid antidepressant actions

A single subanesthetic dose of ketamine, an NMDA receptor (NMDAR) antagonist, produces rapid and sustained antidepressant actions in depressed patients, addressing a major unmet need for the treatment of mood disorders. Ketamine produces a rapid increase in extracellular glutamate and synaptic formation in the prefrontal cortex, but the initial cellular trigger that initiates this increase and ketamine's behavioral actions has not been identified. To address this question, we used a combination of viral shRNA and conditional mutation to produce cell-specific knockdown or deletion of a key NMDAR subunit, GluN2B, implicated in the actions of ketamine. The results demonstrated that the antidepressant actions of ketamine were blocked by GluN2B-NMDAR knockdown on GABA (Gad1) interneurons, as well as subtypes expressing somatostatin (Sst) or parvalbumin (Pvalb), but not glutamate principle neurons in the medial prefrontal cortex (mPFC). Further analysis of GABA subtypes showed that cell-specific knockdown or deletion of GluN2B in Sst interneurons blocked or occluded the antidepressant actions of ketamine and revealed sex-specific differences that are associated with excitatory postsynaptic currents on mPFC principle neurons. These findings demonstrate that GluN2B-NMDARs on GABA interneurons are the initial cellular trigger for the rapid antidepressant actions of ketamine and show sex-specific adaptive mechanisms to GluN2B modulation.

Medial PFC AMPA receptor and BDNF signaling are required for the rapid and sustained antidepressant-like effects of 5-HT1A receptor stimulation

We previously reported that the serotonergic system is important for the antidepressant-like effects of ketamine, a non-competitive N-methyl-D-aspartate receptor antagonist, which produces rapid and long-lasting antidepressant effects in patients with major depressive disorder (MDD). In particular, selective stimulation of the 5-HT_1A receptor in the medial prefrontal cortex (mPFC), as opposed to the somatic 5-HT_1A autoreceptor, has been shown to play a critical role in the antidepressant-like actions of ketamine. However, the detailed mechanisms underlying mPFC 5-HT_1A receptor-mediated antidepressant-like effects are not fully understood. Here we examined the involvement of the glutamate AMPA receptor and brain-derived neurotrophic factor (BDNF) in the antidepressant-like effects of 5-HT_1A receptor activation in the mPFC. The results show that intra-mPFC infusion of the 5-HT_1A receptor agonist 8-OH-DPAT induces rapid and long-lasting antidepressant-like effects in the forced swim, novelty-suppressed feeding, female urine sniffing, and chronic unpredictable stress tests. In addition, the results demonstrate that the antidepressant-like effects of intra-mPFC infusion of 8-OH-DPAT are blocked by co-infusion of an AMPA receptor antagonist or an anti-BDNF neutralizing antibody. In addition, mPFC infusion of 8-OH-DPAT increased the phosphorylation of signaling proteins downstream of BDNF, including mTOR, ERK, 4EBP1, and p70S6K. Finally, selective stimulation of the 5-HT_1A receptor increased levels of synaptic proteins and synaptic function in the mPFC. Collectively, these results indicate that selective stimulation of 5-HT_1A receptor in the mPFC exerts rapid and sustained antidepressant-like effects via activation of AMPA receptor/BDNF/mTOR signaling in mice, which subsequently increase synaptic function in the mPFC, and provide evidence for the 5-HT_1A receptor as a target for the treatment of MDD.

N-Methyl-D-aspartate receptor antagonist d-methadone produces rapid, mTORC1-dependent antidepressant effects

Currently available antidepressants have a delayed onset and limited efficacy, highlighting the need for new, rapid and more efficacious agents. Ketamine, an NMDA receptor antagonist, has emerged as a new rapid-acting antidepressant, effective even in treatment resistant patients. However, ketamine induces undesired psychotomimetic and dissociative side effects that limit its clinical use. The d-stereoisomer of methadone (dextromethadone; REL-1017) is a noncompetitive NMDA receptor antagonist with an apparently favorable safety and tolerability profile. The current study examined the rapid and sustained antidepressant actions of d-methadone in several behavioral paradigms, as well as on mTORC1 signaling and synaptic changes in the medial prefrontal cortex (mPFC). A single dose of d-methadone promoted rapid and sustained antidepressant responses in the novelty-suppressed feeding test (NSFT), a measure of anxiety, and in the female urine sniffing test (FUST), a measure of motivation and reward. D-methadone also produced a rapid reversal of the sucrose preference deficit, a measure of anhedonia, in rats exposed to chronic unpredictable stress. D-methadone increased phospho-p70S6 kinase, a downstream target of mTORC1 in the mPFC, and intra-mPFC infusion of the selective mTORC1 inhibitor rapamycin blocked the antidepressant actions of d-methadone in the FUST and NSFT. D-methadone administration also increased levels of the synaptic proteins, PSD95, GluA1, and Synapsin 1 and enhanced synaptic function in the mPFC. Studies in primary cortical cultures show that d-methadone also increases BDNF release, as well as phospho-p70S6 kinase. These findings indicate that d-methadone induces rapid antidepressant actions through mTORC1-mediated synaptic plasticity in the mPFC similar to ketamine.

Ketamine rapidly reverses stress-induced impairments in GABAergic transmission in the prefrontal cortex in male rodents

Dysfunction of medial prefrontal cortex (mPFC) in association with imbalance of inhibitory and excitatory neurotransmission has been implicated in depression. However, the precise cellular mechanisms underlying this imbalance, particularly for GABAergic transmission in the mPFC, and the link with the rapid acting antidepressant ketamine remains poorly understood. Here we determined the influence of chronic unpredictable stress (CUS), an ethologically validated model of depression, on synaptic markers of GABA neurotransmission, and the influence of a single dose of ketamine on CUS-induced synaptic deficits in mPFC of male rodents. The results demonstrate that CUS decreases GABAergic proteins and the frequency of inhibitory post synaptic currents (IPSCs) of layer V mPFC pyramidal neurons, concomitant with depression-like behaviors. In contrast, a single dose of ketamine can reverse CUS-induced deficits of GABA markers, in conjunction with reversal of CUS-induced depressive-like behaviors. These findings provide further evidence of impairments of GABAergic synapses as key determinants of depressive behavior and highlight ketamine-induced synaptic responses that restore GABA inhibitory, as well as glutamate neurotransmission.

Sestrin modulator NV-5138 produces rapid antidepressant effects via direct mTORC1 activation

Preclinical studies demonstrate that rapid acting antidepressants, including ketamine require stimulation of mTORC1 signaling. This pathway is regulated by neuronal activity, endocrine and metabolic signals, notably the amino acid leucine, which activates mTORC1 signaling via binding to the upstream regulator sestrin. Here, we examined the antidepressant actions of NV-5138, a novel highly selective small molecule modulator of sestrin that penetrates the blood brain barrier. The results demonstrate that a single dose of NV-5138 produced rapid and long-lasting antidepressant effects, and rapidly reversed anhedonia caused by chronic stress exposure. The antidepressant actions of NV-5138 required BDNF release as the behavioral responses are blocked by infusion of a BDNF neutralizing antibody into the medial prefrontal cortex (mPFC) or in mice with a knock-in of a BDNF polymorphism that blocks activity dependent BDNF release. NV-5138 administration also rapidly increased synapse number and function in the mPFC, and reversed the synaptic deficits caused by chronic stress. Together, the results demonstrate that NV-5138 produced rapid synaptic and antidepressant behavioral responses via activation of the mTORC1 pathway and BDNF signaling, indicating that pharmacological modulation of sestrin is a novel approach for development of rapid acting antidepressants.

Stress-Induced Neuronal Colony Stimulating Factor 1 Provokes Microglia-Mediated Neuronal Remodeling and Depressive-like Behavior

BACKGROUND

Chronic stress exposure causes neuronal atrophy and synaptic deficits in the medial prefrontal cortex (PFC), contributing to development of anxiety- and depressive-like behaviors. Concomitantly, microglia in the PFC undergo morphological and functional changes following stress exposure, suggesting that microglia contribute to synaptic deficits underlying behavioral consequences.

METHODS

Male and female mice were exposed to chronic unpredictable stress (CUS) to examine the role of neuron-microglia interactions in the medial PFC during development of anxiety- and depressive-like behaviors. Thy1-GFP-M mice were used to assess microglia-mediated neuronal remodeling and dendritic spine density in the medial PFC. Viral-mediated knockdown of neuronal colony stimulating factor 1 (CSF1) was used to modulate microglia function and behavioral consequences after CUS.

RESULTS

CUS promoted anxiety- and depressive-like behaviors that were associated with increased messenger RNA levels of CSF1 in the PFC. Increased CSF1 messenger RNA levels were also detected in the postmortem dorsolateral PFC of individuals with depression. Moreover, microglia isolated from the frontal cortex of mice exposed to CUS show elevated CSF1 receptor expression and increased phagocytosis of neuronal elements. Notably, functional alterations in microglia were more pronounced in male mice compared with female mice. These functional changes in microglia corresponded with reduced dendritic spine density on pyramidal neurons in layer 1 of the medial PFC. Viral-mediated knockdown of neuronal CSF1 in the medial PFC attenuated microglia-mediated neuronal remodeling and prevented behavioral deficits caused by CUS.

CONCLUSIONS

These findings revealed that stress-induced elevations in neuronal CSF1 provokes microglia-mediated neuronal remodeling in the medial PFC, contributing to synaptic deficits and development of anxiety- and depressive-like behavior.

Spine synapse remodeling in the pathophysiology and treatment of depression

Clinical brain imaging and postmortem studies provide evidence of structural and functional abnormalities of key limbic and cortical structures in depressed patients, suggesting that spine synapse connectivity is altered in depression. Characterization of the cellular determinants underlying these changes in patients are limited, but studies in rodent models demonstrate alterations of dendrite complexity and spine density and function that could contribute to the morphological and functional alterations observed in humans. Rodent studies demonstrate region specific effects in chronic stress models of depression, including reductions in dendrite complexity and spine density in the hippocampus and prefrontal cortex (PFC) but increases in the basolateral amygdala and nucleus accumbens. Alterations of spine synapse connectivity in these regions are thought to contribute to the behavioral symptoms of depression, including disruption of cognition, mood, emotion, motivation, and reward. Studies of the mechanisms underlying these effects demonstrate a role for altered brain derived neurotrophic factor (BDNF) signaling that regulates synaptic protein synthesis. In contrast, there is evidence that chronic antidepressant treatment can block or reverse the spine synapse alterations caused by stress. Notably, the new fast acting antidepressant ketamine, which produces rapid therapeutic actions in treatment resistant MDD patients, rapidly increases spine synapse number in the PFC of rodents and reverses the effects of chronic stress. The rapid synaptic and behavioral actions of ketamine occur via increased BDNF regulation of synaptic protein synthesis. Together these studies provide evidence for a neurotophic and synaptogenic hypothesis of depression and treatment response and indicate that spine synapse connectivity in key cortical and limbic brain regions is critical for control of mood and emotion.

Evaluating effects of EPO in rodent behavioral assays related to depression

The cytokine erythropoietin (EPO) is an important regulator of hematopoesis and has well-known tissue protective properties. Neurotrophic action is implicated as mechanistically important in the treatment of depression, and neurotrophic actions of EPO suggest potential therapeutic utility of an EPO-like mechanism in depressive disorder. Rodent behavioral models that are responsive to clinically used antidepressants as well as to neurotrophic compounds can be used to assess potential antidepressant properties of EPO and EPO-like compounds. Rodent models described here are the forced-swim test (FST), a hyponeophagia test and the novel object recognition test. Each of these models provides different information and relevance to depression and each can be tested with EPO and similar compounds.

Models of depression

The incidence of depressive illness is high in the United States and worldwide, and the inadequacy of currently available drug treatments contributes to the significant health burden associated with depression. A basic understanding of the underlying disease processes in depression is lacking, and therefore, recreating the disease in animal models is not possible. Currently used models of depression attempt to produce quantifiable correlates of human symptoms in experimental animals. The models differ in the degree to which they produce features that resemble a depressive-like state, and models that include stress exposure are widely used. Paradigms that employ acute or subchronic stress exposure include learned helplessness, forced swim test, and tail suspension test, which employ relatively short-term exposure to inescapable or uncontrollable stress and can reliably detect antidepressant drug response. Longer-term models include chronic mild stress models, early-life stress models, and social conflict models, which may more accurately simulate processes that lead to depression. These models each have varying degrees of face, construct, and predictive validity for depression and contribute differently to our understanding of antidepressant processes.

Erythropoietin induction by electroconvulsive seizure, gene regulation, and antidepressant-like behavioral effects

BACKGROUND

The neuroprotective and trophic actions of erythropoietin (EPO) have been tested in several animal models of insult, injury, and neurodegeneration. Recent studies in human volunteers demonstrated that EPO improves cognition and also elicits antidepressant effects. It is believed that the behavioral effects are mediated by EPO's trophic effect on neuronal systems. We therefore tested whether EPO is able to alter behavior and brain gene expression in rats.

METHODS

The expression of EPO and EPO receptor (EPOR) in multiple brain regions was examined by quantitative polymerase chain reaction, in situ hybridization, and immunohistochemistry. The regulation of EPO and the transcription factor hypoxia-induced factor-alpha (HIF1alpha) after electroconvulsive seizure (ECS) was investigated. Behavioral effects of EPO were tested in the rodent forced swimming and novelty-induced hypophagia (NIH) models. EPO gene profiles were obtained by microarray analysis of the hippocampus after intracerebroventricular infusion.

RESULTS

EPO and EPOR were widely expressed in the brain albeit at low levels. Highest level of EPO and EPOR were in the choroid plexus and striatum, respectively. Peripheral administration of EPO was sufficient to produce a robust antidepressant-like effect in the forced swim and NIH tests. Gene expression profiles revealed that EPO induces the expression of neurotrophic genes such as brain-derived neurotrophic factor, VGF (nonacronymic), and neuritin.

CONCLUSIONS

EPO is induced by ECS and independently exhibits antidepressant-like efficacy in the forced swim and NIH tests. EPO regulates the expression of genes implicated in antidepressant action and appears to be a candidate molecule for further testing in neuropsychiatry.

Peripheral insulin-like growth factor-I produces antidepressant-like behavior and contributes to the effect of exercise

Growth factors in the brain are important to depression and it's treatment and we assessed the ability of peripherally administered insulin-like growth factor-I (IGF-I) to influence behavior related to depression. We found that mice that received chronic IGF-I treatment showed antidepressant-like behavior in forced-swim and novelty-induced hypophagia (NIH) tests and increased sucrose consumption after chronic mild unpredictable stress exposure. Additionally, peripheral anti-IGF-I administration blocked exercise-induced antidepressant effects in the forced-swim test (FST). These results support the functional relevance of neurotrophic mechanisms to depression and extend this idea to include neurotrophic factors in the periphery.

Voluntary exercise produces antidepressant and anxiolytic behavioral effects in mice

Reports of beneficial effects of exercise on psychological health in humans are increasingly supported by basic research studies. Exercise is hypothesized to regulate antidepressant-related mechanisms and we therefore characterized the effects of chronic exercise in mouse behavioral paradigms relevant to antidepressant actions. Mice given free access to running wheels showed antidepressant-like behavior in learned helplessness, forced-swim (FST) and tail suspension paradigms. These responses were similar to responses of antidepressant drug-treated animals. When tested under conditions where locomotor activity was not altered, exercising mice also showed reduced anxiety compared to sedentary control mice. In situ hybridization analysis showed that BDNF mRNA was increased in specific subfields of hippocampus after wheel running. We chose one paradigm, the FST, in which to investigate a functional role for brain-derived neurotrophic factor (BDNF) in the behavioral response to exercise. We tested mice heterozygous for a deletion of the BDNF gene in the FST after wheel-running. Exercising wild-type mice showed the expected antidepressant-like behavioral response in the FST but exercise was ineffective in improving FST performance in heterozygous BDNF knockout mice. A possible functional contribution of a BDNF signaling pathway to FST performance in exercising mice was investigated using the specific MEK inhibitor PD184161 to block the MAPK signaling pathway. Subchronic administration of PD184161 to exercising mice blocked the antidepressant-like behavioral response seen in vehicle-treated exercising mice in the FST. In summary, chronic wheel-running exercise in mice results in antidepressant-like behavioral changes that may involve a BDNF related mechanism similar to that hypothesized for antidepressant drug treatment.

A role for MAP kinase signaling in behavioral models of depression and antidepressant treatment

BACKGROUND

Brain-derived neurotrophic factor (BDNF) is upregulated in the hippocampus by antidepressant treatments, and centrally administered BDNF can produce antidepressant-like effects in rodent behavioral models of depression. BDNF-regulated signaling pathways are thus potential targets for investigation of antidepressant mechanisms.

METHODS

We examined the effects of inhibition of MAPK kinase (MEK) in mouse behavioral models for depression including interactions with effects of antidepressant drugs. We also assessed the behavioral consequences of a heterozygous gene deletion for BDNF combined with MEK inhibition or stress.

RESULTS

Acute administration of the MEK inhibitor PD184161 produced depressive-like behavior. PD184161 blocked the antidepressant-like effects of desipramine and sertraline in the forced swim test and blocked the effects of desipramine in the tail suspension test. Heterozygous deletion of BDNF alone did not influence behavior in the forced swim test but resulted in a depressive phenotype when combined with a low-dose MEK inhibitor or stress exposure.

CONCLUSIONS

We demonstrate that acute blockade of MAPK signaling produces a depressive-like phenotype and blocks behavioral actions of antidepressants. We also demonstrate in BDNF heterozygous knockout mice an example of a how a defined genetic alteration can confer vulnerability to a pharmacologic or environmental challenge resulting in a depressive behavioral phenotype.

TNFalpha signaling in depression and anxiety: behavioral consequences of individual receptor targeting

BACKGROUND

Increased serum levels of TNFalpha and other pro-inflammatory cytokines have been found in patients with major depression and several other psychiatric conditions. In rodents, these cytokines produce symptoms commonly referred to as "sickness behavior." Some of these, including reduced feeding and decreased social and exploratory behavior, are reminiscent of those seen in depressed patients. Interpretation of these effects is complicated by the malaise caused by acute injections of pro-inflammatory cytokines, however. Thus, it is unclear whether cytokines are involved in the etiology of depressive symptoms.

METHODS

We used a panel of behavioral assays to assess TNFR1(-/-) and TNFR2(-/-) mice for anxiety and depression-like behaviors.

RESULTS

We show that deletion of either TNFR1 or TNFR2 leads to an antidepressant-like response in the forced swim test and that mice lacking TNFR2 demonstrate a hedonic response in a sucrose drinking test compared with wildtype littermates. In addition, deletion of TNFR1 leads to decreased fear conditioning. There were no differences in behavior in anxiety tests for either null mutant.

CONCLUSIONS

These results are consistent with the hypothesis that TNFalpha can induce depression-like symptoms even in the absence of malaise and demonstrate that both receptor subtypes can be involved in this response.

Neurobiology and treatment of anxiety: signal transduction and neural plasticity

The stress-dependence and chronic nature of anxiety disorders along with the anxiolytic effectiveness of antidepressant drugs suggests that neuronal plasticity may play a role in the pathophysiology of anxiety. Intracellular signaling pathways are known in many systems to be critical links in the cascades from surface signals to the molecular alterations that result in functional plasticity. Chronic antidepressant treatments can regulate intracellular signaling pathways and can induce molecular, cellular, and structural changes over time. These changes may be important to the anxiolytic effectiveness of these drugs. In addition, the signaling proteins implicated in the actions of chronic antidepressant action, such as cAMP response element binding protein (CREB), have also been implicated in conditioned fear and in anxiety. The cellular mechanisms underlying conditioned fear indicate roles for additional signaling pathways; however, less is known about such mechanisms in anxiety. The challenge to identify intracellular signaling pathways and related molecular and structural changes that are critical to the etiology and treatment of anxiety will further establish the importance of mechanisms of neuronal plasticity in functional outcome and improve treatment strategies.

Fear conditioning and latent inhibition in mice lacking the high affinity subclass of nicotinic acetylcholine receptors in the brain

Nicotine can enhance performance in several tests of cognition but the specific nicotinic receptor subtypes mediating these effects are largely unknown. Knock out mice lacking the beta2 subunit of the nicotinic receptor were evaluated in fear conditioning and latent inhibition tasks to begin to determine which receptor subtypes mediate the cognitive effects of nicotine. Young (2-4 months) knock out and wild type mice did not differ in either contextual or tone-conditioned fear, but aged (9-20 months) knock out males were impaired in freezing to both context and tone compared to aged wild type males. No differences in fear conditioning were observed between aged knock out and wild type females. Latent inhibition of fear to a pre-exposed tone, as measured by behavioral freezing, was also assessed. Both knock out and wild type mice displayed similar levels of latent inhibition, although overall levels of freezing were lower in knock out mice. These results support a previous study showing spatial learning deficits in aged beta2 subunit knock out mice [EMBO J. 18 (1999) 1235] and suggest that performance of other cognitive tasks may not be influenced by absence of beta2 subunit-containing receptors.

Short-latency interneurons in the dorsal nucleus of the little skate, Raja erinacea

The elasmobranch electrosensory system is the most thoroughly understood of the non-teleost electrosensory systems and is useful for studying central nervous system mechanisms for the separation of behaviorally relevant signals from self-generated noise. In the little skate, Raja erinacea, the electrosensory primary afferents are responsive to electrical potentials created during the animal's own ventilation, while second-order neurons in the dorsal nucleus of the medulla suppress responses to ventilatory potentials (self-generated noise) but retain their extreme sensitivity to electric signals in the environment. The selective suppression of ventilatory noise in second-order cells is due in part to the fact that ventilatory potentials stimulate all receptors equally and simultaneously. The neuronal circuitry mediating rejection of such 'common mode' signals in the dorsal nucleus likely includes inhibitory interneurons. This study describes physiological and anatomical characteristics of a group of dorsal nucleus interneurons that are distinguished from previously described interneurons by their shorter orthodromic activation latencies and higher spontaneous firing rates. The interneurons show sustained responses to an external dipole stimulus and respond well during simultaneous activation of all afferents by a 'common mode' stimulus. Intracellular labeling indicates that the short-latency interneurons are located in the central and peripheral zones of the dorsal nucleus and the extent of their labeled processes is limited to the projection area of afferents from a single ampullary cluster. These features are consistent with a hypothesized role for these interneurons in inhibiting second-order cells, including inhibition which contributes to common mode rejection.

Distinct but overlapping populations of commissural and GABAergic neurons in the dorsal nucleus of the little skate, Raja erinacea

In the little skate, Raja erinacea, the electrosensory primary afferents are responsive to electrical potentials created during the animal's own ventilation, while second-order electrosensory cells in the dorsal nucleus of the medulla suppress the responses to ventilatory potentials but retain their extreme sensitivity to important environmental electric signals. Previous electrophysiological studies indicate a role for a commissural pathway between the bilateral dorsal nuclei in ventilatory noise suppression. In the present study, retrograde tracers were used to label dorsal nucleus commissural cells. Large round or triangular and thin elongate commissural cells occur in the central zone of the dorsal nucleus where the primary afferent fibers terminate. Elongate commissural cells also occur in the peripheral zone which is the cell body area of the major efferents of the dorsal nucleus. Immunohistochemical studies indicate that stellate cells of the molecular layer and round or triangular cells of the central zone comprise the GABA-immunoreactive cell groups of the dorsal nucleus. A subpopulation of the round commissural cells in the central zone are GABA-immunoreactive and may be candidates for mediators of common-mode noise rejection in the dorsal nucleus of skates. The non-GABAergic commissural cells may mediate crossed inhibition through an inhibitory transmitter other than GABA or may supply crossed excitation to the dorsal nucleus.

A role for GABAergic inhibition in electrosensory processing and common mode rejection in the dorsal nucleus of the little skate, Raja erinacea

The electrosensory primary afferents in elasmobranchs are responsive to electric potentials created by the animal's own ventilation, while the second-order neurons (AENs) which receive this afferent input in the medulla suppress responses to ventilatory potentials but retain their extreme sensitivity to electric signals in the environment. Ventilatory potentials are common mode signals in elasmobranchs and a common mode rejection mechanism is one way the AENs suppress ventilatory noise. By pressure injecting the GABA-A receptor antagonist SR95531 while extracellularly recording from AENs, we tested the hypothesis that the subtractive circuitry that selectively reduces common mode signals in AENs utilizes GABA, and that a GABAergic component of the dorsal nucleus commissural pathway mediates crossed inhibition of AENs. Local application of SR95531 increased the spontaneous activity and the responsiveness of AENs to electrosensory stimuli. AEN responses to a common mode stimulus were selectively increased compared to responses to a localized stimulus due to SR95531 application. Contralateral inhibition of AENs was blocked by SR95531, indicating that GABAergic commissural cells may inhibit AENs when the contralateral side of the body is stimulated, as with common mode stimulation. We conclude that GABAergic inhibition contributes significantly to the shaping of AEN responses including common mode rejection.