BDNF has a unique role in the hippocampus, where it is involved in learning and LTP [66]. It is widely recognised that expression of the BDNF is reduced in the hippocampus of depressed patients and in the hippocampus of AnMD [33, 67], although the cause-effect relationship of this observation is not clear. Furthermore, chronic stress is known to cause a down-regulation of BDNF protein and mRNA in the hippocampus [68], an effect that is closely linked to depression [53]. Conversely, while treatment with StAD increased the expression of BDNF mRNA in the hippocampus [69, 70, 71], repeated electroconvulsive seizure treatment can also increase expression of the BDNF protein in the hippocampus [72].

The effect of ketamine on the hippocampus is also closely linked to that of the BDNF; indeed, the presynaptic modulation of vesicular glutamate release [73] and the postsynaptic modulation of glutamate receptor trafficking can be attributed to BDNF, as demonstrated by the increase of AMPA receptors in hippocampal slices and neuronal culture [74]. Bearing in mind that in the hippocampus, as well as in the mPFC, ketamine activates the cellular and molecular mechanisms that underlie synaptic strengthening and dendritic spine formation, it is quite likely that BDNF mediates these effects, and that ketamine can generate them through the release of BDNF [49, 51]. Direct evidence of this effect is provided by Autry et al. [75], who observed that ketamine (3 mg/kg i.p.) increased BDNF protein expression in the hippocampus after only 30 minutes’ treatment. Recently, it has been suggested that ketamine can increase BDNF release in the hippocampus through the blockade of postsynaptic NMDA receptors [37] followed by synthesis of eukaryotic Elongation Factor2 (eEF2), similar to that proposed by Autry et al. [75]. Moreover, these authors provided evidence for an increase of TrkB phosphorylation, an indicator of TrkB activation in the hippocampus, suggesting that BDNF-TrkB signalling in the hippocampus is involved in the antidepressant response to ketamine [74]. Complementary to this observation is the finding that a loss of hippocampal BDNF per se is not sufficient to mediate depression-like behaviour, but BDNF in the dentate girus might be essential in mediating the therapeutic effects of antidepressants [76].

Together with BDNF, hippocampal volume and hippocampal neurogenesis are parameters that can be assessed to test the response of ketamine or StAD in AnMD or in patients affected by MDD. Indeed, the volume of the hippocampus is reduced in MDD patients [77, 78] and although a few studies have reported contrasting results [79], most have confirmed this observation. Interestingly, the antidepressant effects of ketamine were observed in patients who have a relatively smaller hippocampus and were refractory to StAD [80]. Another crucial issue in the aetiology of depression is the hippocampal neurogenesis, a process by which 700 new-born hippocampal granule cells are normally added daily in adult humans [81]. This process should allow the distinguishing and storing of similar experiences as distinct memories; the weakening of this process is of relevance as a biomarker of MDD, although some limits should be considered [82]. Neuroimaging and post-mortem studies in MDD patients have shown that neuronal atrophy is likely to be associated with a reduction of neurogenesis [83, 84, 85, 86]. Similar to the effects on hippocampal BDNF, chronic stress decreases hippocampal neurogenesis, while StAD can reverse this decrease [87, 88, 89]. It is known that neurogenesis can be stimulated either by therapy with StAD, or by physical exercise by activating not only progenitors but also stem cells, which are normally not stimulated [90]. Interestingly, the delay in the onset of the StAD effect after chronic administration has often been attributed to the time needed for new neurons to be recruited into memory circuits.

Given this, it is even more challenging to try and understand how ketamine produces its rapid and long-lasting antidepressant effects. Deyama and Duman [51], who recently reviewed this issue, have suggested that neurogenesis is involved in the sustained but not in the fast actions of ketamine. Although it reconciles the effects of ketamine with those of StAD, this hypothesis raises a doubt about the indispensability of the reactivation of hippocampal neurogenesis for the therapeutic manifestation of the rapid effects of antidepressants. On the other hand, a further testimony to the crucial role of hippocampal neurogenesis has been provided by Yamada and Jinno [91], who reported that, just two days after the administration of a single dose (10 mg/kg i.p.), ketamine elevated the density of neuronal progenitors and new-born granule cells in the ventral but not in the dorsal hippocampus in adult mice. Notably, the densities of neuronal stem cells were not affected by ketamine in either area. The preferential action of ketamine on ventral hippocampus (vHipp) neurogenesis is consistent with the role of this area in emotional processes and memories, while the dorsal part is related to spatial memory [92]. These results not only confirm neurogenesis to be a complex phenomenon but also suggest that new therapeutic tools, to be used as a complementary therapy of StAD, could generate a partial and temporary reactivation of neurogenesis.

Furthermore, it has been suggested that ketamine activation of hippocampal neural activity may bring about protective action against stress-induced depressive-like behaviour, as suggested by the results of Mastrodonato et al. [93]. They showed that a single injection of ketamine (30 mg/kg, one week before), selectively increased DeltaFosB expression in the vHipp though only in mice exposed to social defeat stress [93]. Interestingly, the inactivation of the vHipp with lidocaine was able to prevent the sustained but not the acute antidepressant effect of ketamine, evaluated by forced swim test in Sprague-Dawley rats [94]. The authors therefore suggested that the activity in the vHipp-mPFC pathway is both necessary and sufficient for the antidepressant-like effect of ketamine to take place [94]. In short, we are suggesting that the antidepressant action of ketamine may occur as a result of a direct effect on the hippocampus, as supported by its effect on BDNF release and neurogenesis, although the connections that the hippocampus establishes with the mPFC and the NAc/Ventral Tegmental Area (VTA) suggest that it would be more appropriate to consider the global interaction of ketamine with a hypothetical primary circuit of depression (Fig. 3). This is also in consideration of the role of the mPFC in the cognitive aspects of depression, that of NAc/VTA in motivational behaviour and that of the hippocampus in its ability to control emotional memories.

Fig. 3.

Schematic representation of interactions among the brain areas that are involved in the “primary circuit of depression”, according with our hypothesis. Abbreviations: PFC, prefrontal cortex; Hipp, hippocampus; NAc, nucleus accumbens; VTA, ventral tegmental area.

Among the brain areas affected by depression, the mPFC has been widely investigated and many stimulating hypotheses about its role in depression have been advanced on the basis of several objective observations [95]. Among these, it is of some relevance that post-mortem studies have reported a reduced volume of several mPFC subregions in patients suffering from MDD and/or exhibiting suicidal thoughts and behaviours [96]. In particular, a reduction in size and density of neurons of dorsolateral mPFC has been observed [97, 98]. In addition, a decrease in synapse-related genes has been observed in AnMD [99]. An interesting report by Deyama and Kaneda [100] showed that a single injection of BDNF into the mPFC of mice produced long-lasting (8 days) antidepressant effects, in some way similar to the rapid and sustained antidepressant effect produced by ketamine [69]. Moreover, in the mPFC of subjects with MDD, the expression level of BDNF transcript variants and synaptic markers are selectively reduced [101].

Several studies have shown that the increased release of BDNF in mPFC may be the means by which ketamine produces its antidepressant effects. Indeed, preclinical studies have shown that both (S)-ketamine (10 or 50 mg/kg i.p.) and the ketamine metabolite (2S,6S)-HNK (10 mg/kg i.p.) stimulate BDNF release in the mPFC of mice [102, 103]. Furthermore, it was observed that the antidepressant effects of ketamine (10 mg/kg i.p.) can be blocked by the administration of an antibody against BDNF in the mPFC of rats [104]. Additionally, a direct effect of ketamine (0.5 $\mu$M) on BDNF release was observed in a primary culture of cortical neurons, and, interestingly, this release can be blocked by NBQX, an AMPA receptor antagonist, or by Verapamil, an antagonist of L-type calcium channels [104]. It has been suggested that ketamine increases BDNF release through inhibition of the eEF2, resulting in reduced phosphorylation of eEF2 [104].

It is apparently incongruent that ketamine activates BDNF release because it reduces rather than stimulates glutamate transmission. One explanation for this apparent incongruity holds that NMDA receptor blocking activity occurs pre-synaptically on the terminals of GABA interneurons in the medial mPFC and thereby reduces the inhibitory activity of these neurons. Thus, the reduction of GABA inhibitory activity would allow an increased release of glutamate, which through NMDA receptors would in turn activate BDNF release; this is followed by rapid exposure of AMPA receptors located on pyramidal neurons projecting towards subcortical areas, or on other pyramidal cells [19, 37, 51, 105, 106]. Activation of AMPA receptors allows calcium entry via voltage-dependent calcium channels and the subsequent release of both BDNF and the vascular endothelial growth factor. These two factors induce transduction and synthesis of further factors that mediate activation of synaptogenesis and dendritic spine maturation, including GluA1 and postsynaptic density protein 95 through the TrkB/Flk-1—mammalian Target of Rapamycin (mTOR) complex1 signalling pathway [107]. Likewise, these investigators suggested that direct activation of postsynaptic AMPA receptors, resulting in increased extracellular glutamate and GABA levels in the mPFC, is involved in the antidepressant effects of the ketamine metabolite HNK [37].

The rapid effects of ketamine on neuronal plasticity in the mPFC has been observed in rodents [108]; these authors have shown that a single administration of a low dose of ketamine promptly activated the mTOR signalling, resulting in a long-lasting increase of spine and synapse-associated proteins in the rat mPFC. Notably, repeated stress induces an opposite effect, with dendritic spine loss in the rat mPFC [109]. Furthermore, Moda-Sava et al. [110], demonstrated that ketamine reversed the loss of postsynaptic dendritic spines induced by chronic stress or chronic corticosterone in a mouse model of depression. These authors also demonstrated that specific elimination of postsynaptic dendritic spines on mPFC projection neurons was temporarily associated with the manifestation of depressive behaviour and that antidepressant doses of ketamine, in addition to counteracting behavioural manifestations, resulted in regeneration of the spines; they further suggested that synaptogenesis might be a means by which the long-lasting antidepressant effects (but not the rapid ones) of ketamine are manifested [110]. This last hypothesis is not in agreement with the increased level of synaptic proteins (i.e., PSD-95, synapsin-1, and GluA1) that Li et al. [108] observed as early as 2 h after ketamine administration. Therefore, it can be attested that a local activity-dependent synaptogenesis in the mPFC pyramidal neurons is an essential feature of the antidepressant effects of ketamine [51].

The NAc receives excitatory glutamatergic innervation from different brain areas involved in depression such as the hippocampus, mPFC, basolateral amygdala and thalamus [111, 112, 113, 114] (Fig. 1), supporting the hypothesis of its key role in modulating individual susceptibility to depression and thus its potential to be a target area for the antidepressant actions of ketamine. The VTA provides the dopaminergic input to the NAc [115] but also to many other brain areas involved in depression such as the mPFC, hippocampus, BLA and BNST [116]. VTA neurons are in turn innervated by GABA projection from NAc and BNST, whereas glutamate neurons originating in the mPFC, LHb and amygdala also innervate VTA [19] (Figs. 1,4).

Fig. 4.

Schematic representation of glutamate (Glu), GABA and dopamine (DA) inputs and outputs among the brain areas involved in the “primary circuit of depression”. Abbreviations: PFC, prefrontal cortex; Hipp, hippocampus; NAc, nucleus accumbens; VTA, ventral tegmental area.

Interestingly, inputs from the hippocampus and mPFC converge on the same NAc neuron, a feature that would suggest the existence of a hippocampal gating of other limbic inputs [117] and in particular that vHipp afferents to NAc regulate susceptibility to depression [118]. The NAc is a central component of the ventral striatum and integrates limbic and cortical signals to modulate goal-directed behaviour through the modulatory action of dopamine [119]. For its role in the reward circuit, in motivated behaviour, in hedonic appraisal, mood regulation and in anhedonia in general (decreased drive for activities because of the reduced capacity to feel pleasure and satisfaction for the associated outcomes), NAc has been implicated in the aetiology of depression [28, 120].

NAc mesolimbic pathway and the BDNF expression, has a fundamental role in mediating the effects of social defeat stress and those of antidepressants; in particular, after local deletion of BDNF in the VTA, the effects of social defeat stress on gene activation in the NAc was lost or reversed, as was the effect of chronic fluoxetine [121]. BDNF signalling is highly represented in the VTA-NAc circuit; in particular, one major source of BDNF in both regions is provided by a glutamatergic afferent from mPFC, hippocampus and amygdala, while release of BDNF is recognised by TrkB receptors expressed on VTA dopaminergic and NAc GABAergic neurons [28]. In general, the changes in BDNF release induced by chronic stress in the NAc further support its pivotal role in the generation of depression [28]. Additionally, the phasic stimulation of VTA neurons that project to NAc, but not those that project to the mPFC, induced susceptibility to social defeat stress in mice [122]. In particular, stress increases the release of BDNF in the NAc, an effect that was mediated by Corticotrophin Releasing Factor (CRF) [123]. It was subsequently suggested that BDNF–TrkB signalling but not dopamine signalling in the VTA-NAc circuit, was an essential component in the development of depressive-like behaviours induced by chronic stress [124]. Interestingly, it has also been shown that a BDNF increase (namely 90% elevation) in the NAc occurs only in susceptible mice [125]. Direct evidence of the NAc-BDNF role in promoting depression-like behaviour has been provided by the observation that direct infusion of BDNF into the NAc increases the immobility in forced swim tests (FST), while inhibition of TrkB signalling in the NAc decreases immobility [126].

Looking at the role of NAc in mediating the antidepressant effect of ketamine, it is of interest that ketamine and LY341495 (a potent and selective antagonist of group II metabotropic glutamate receptor [127]), but not the selective serotonin reuptake inhibitor (SSRI) citalopram, increased the number of spontaneously active dopamine neurons in the VTA, and the extracellular dopamine levels in the NAc as well as in the mPFC [128], an effect that was shown to be AMPA-dependent. Although the abuse potential of ketamine [129] would suggest there may be direct ketamine action on dopamine transmission through an interaction with dopamine reuptake, or with dopamine receptors, it has been demonstrated that such an occurrence is unlikely [130].

Stress is known to affect synaptic plasticity by inhibiting LTP and/or facilitating long term depression (LTD) in the hippocampus of rodents [131] and we also know that ketamine and its metabolite (2R,6R)-HNK restore LTP and long-term memory in the Wistar-Kioto rat model of depression that likely own an alteration of dopamine stores [132, 133]. As regards NAc, ketamine restores normal neuron population activity and restores LTP in the ventral subiculum-NAc pathway, thus suggesting that normal LTP in the NAc shell prevents the generation of helplessness [134]. On the other hand, a single low dose injection of ketamine (3–5 mg/kg) dose-dependently induced the impairing of LTP in the NAc at 24 h after administration; this effect was maintained for 7 days and was not associated with any alteration of basal synaptic transmission mediated by AMPA and NMDA receptors [135]. Similar results were previously shown by Réus et al. [136], who reported that both ketamine and imipramine impaired histone deacetylation in the NAc; this function was found to increase in the NAc but not in the mPFC, hippocampus, and amygdala in maternally deprived adult Wistar rats. This may well be relevant with regard to long-term changes related to stress and antidepressant treatment.

Further evidence of NAc involvement in ketamine’s effects was obtained by Abdallah et al. [137] in a MDD subgroup of patients displaying an enlarged bilateral NAc volume; these authors observed that ketamine treatment normalized NAc volume in patients who achieved remission. On the whole, the role of NAc in the appearance of peculiar symptoms of depression and in the antidepressant effects of ketamine supports its pivotal role in a primary circuit of depression; interestingly, the effects of stress, as well as the response to antidepressants (i.e., StAD and ketamine) on BDNF related actions in the VTA-NAc circuit appear to be opposite to those observed in the mPFC and hippocampus (Fig. 2).

The Lateral Habenula (LHb) has recently been implicated in the coding of negative emotions [138]. In particular, the functions of the LHb are associated with anxiety, stress, pain, avoidance learning, attention, human reward processing, psychosis and depression [139, 140, 141]. In consequence of the strong inhibition of dopamine neurons after a weak electrical stimulation of the LHb, it has been proposed that LHb provides a measure of control on VTA dopamine neurons, negatively influencing reward-related signals [138]. The capacity to reduce dopamine-mediated reward responsiveness validates the involvement of LHb in the occurrence of anhedonia, a typical feature in MDD patients [142] and possibly, in stress induced depression as reported by Shabel et al. [143]; these authors observed that acute stress can transform LHb reward responses into punishment-like neuronal signs. LHb is primarily a glutamatergic nucleus, and its output reaches the VTA and the dorsal raphe nuclei (DRN), where an inhibition of monoamine neurons can occur through the activation of GABAergic inhibitory neurons [144]. Glutamatergic LHb neurons are transiently activated by acute stress or aversive stimuli and likely mediate a wide range of responses, ranging from stress evasion to value-based decision-making [145]. The LHb receives projections from the mPFC, providing a link for conveying cognitive and affective information from prefrontal and insular regions to the midbrain monoaminergic centres [146] (Fig. 5).

Fig. 5.

Schematic representation of interactions among the lateral habenula (LHb) and the brain areas that are involved in the “primary circuit of depression”, according with our hypothesis. Some interactions of dorsal and median raphe with these brain areas are represented schematically. Other abbreviations: PFC, prefrontal cortex; Hipp, hippocampus; NAc, nucleus accumbens; VTA, ventral tegmental area; RN, raphe nuclei.

Based on this assumption, and considering the role of ketamine in the mPFC, it should not be surprising that ketamine may well produce its rapid antidepressant action through modulating the LHb function either directly or through the modulation of mPFC activity. Indeed, in a rat and mouse model of depression, in which burst-evoking photo-stimulation of the LHb can result in despair and anhedonia, it has been reported that because it blocks NMDA receptor-dependent burst firing in the LHb, ketamine can reduce stimulation of inhibitory neurons in the VTA, disinhibiting the NAc reward centre [142]. Similarly, ketamine can persistently (up to 72 h post injection) reverse maternal deprivation-induced intrinsic neuronal excitability in the LHb and decrease immobility in the forced swimming test in late adolescent male rats [147].

As far as the LHb BDNF role in AnMD is concerned, the few reports available have not enabled us to gain any clearer understanding of the mechanism involved. In addition, BDNF immunoreactivity in the habenula is mostly distributed in the medial part and is concentrated in fibers [148]. BDNF level expressions in the LHb were found to be higher than in controls of Chronic Restraint Stress (CRS) exposed mice, together with higher levels of GluN2B and c-Fos. Interestingly, the systemic administration of the specific GluN2B antagonist rislenemdaz, a drug used in phase II clinical trials as an antidepressant, reversed CRS effects while its infusion in the LHb decreased immobility time in CRS-exposed mice [149]. These authors have also showed that knocking down BDNF in the LHb prevented CRS-induced despair-like behaviour and inhibited neuronal activity in the LHb. Curiously, 6-hydroxydopamine lesions in the ventral region of the caudate putamen determined a reduction of BDNF and TRkB mRNA levels in the hippocampus, as well as in the BDNF mRNA in the habenula. By contrast, the lesions did not influence BDNF and TRkB expression in the NAc and in the VTA, whereas chronic imipramine reversed BDNF mRNA reduction in the dentate girus of the hippocampus [150].

A recent interesting study investigated the role of the LHb in depression-associated inflammation by using the Unpredictable Chronic Stress (UCS) animal model of depression [151]. These researchers observed an increase of inflammatory cytokines in the CNS, especially in the LHb; conversely, local administration of tumor necrosis factor-alpha in the LHb, can induce depressive-like behaviour in control rats. Lastly, they showed that lesioning LHb prevented the impairment of hippocampal neurogenesis and a reduction of the hippocampal function in UCS exposed rats [151].

Summing up, it appears that an inappropriate LHb firing activity could play a decisive role in the aetiology of depression, while its peculiar intrinsic activity and connections with the other brain areas involved in depression means that we can consider the LHb to be a further site through which the rapid action of ketamine occurs [152] (Figs. 5,6).

Fig. 6.

Schematic representation of glutamate (Glu) GABA, dopamine (DA) and serotonin inputs and outputs among the lateral habenula (LHb), and the brain areas involved in the “primary circuit of depression”. Some inputs and outputs of dorsal and median raphe with these brain areas are represented schematically. Other abbreviations: PFC, prefrontal cortex; Hipp, hippocampus; NAc, nucleus accumbens; VTA, ventral tegmental area; RN, raphe nuclei.

The amygdala is composed of a series of heterogeneous subnuclei that differ both morphologically and functionally. Notably, the basolateral amygdala (BLA) is primarily glutamatergic [153], whereas the central nucleus of amygdala (CeA), which includes the centrolateral and the centromedial nuclei, consists of 95% GABAergic medium spiny neurons [154]. The BLA projects to the mPFC, hippocampus and NAc [155] thus interacting directly with the primary circuit of depression. CRS triggers dendritic growth and spinogenesis in the neurons of the BLA, regardless of whether or not they project to the mPFC [150]. CRS leads to increases of BDNF mRNA and BDNF protein and dendritic spine density in the BLA, which are strikingly different from those seen in the hippocampus and in the mPFC [156, 157, 158, 159].

Structural alteration and decreased function in the mPFC and hippocampus, but an increased function and an altered morphology in the amygdala were observed in MDD patients during clinical and post-mortem studies [160, 161]. The decreased connectivity and function in the mPFC may well contribute to the appearance of cognitive symptoms of depression while any augmented function of the amygdala may alter emotions and mood, increase fear and anxiety and hyper-activate the hypothalamic-pituitary-adrenal axis [159]. In this regard, it is of interest to note the suggestion that prelimbic mPFC excites amygdala output, while infralimbic mPFC inhibits it, providing a mechanism of bidirectional modulation of fear expression [162]. In a fear-generalized mouse model, a single administration of ketamine (30 mg/kg i.p.) reversed the reduction of BDNF levels induced by fear generalization in the BLA and in the infralimbic mPFC [163].

As for the evaluation of intracellular proteins (i.e., PSD-95) and NMDA glutamate receptor subunits (i.e., NR2A), it is worth noting that they are both increased in the amygdala of depressed humans, whereas they are decreased in the mPFC [164]. In addition, chronic variable stress reduces histone deacetylases-5 in the CeA of male rats [165], similar to that caused by chronic social defeat stress in mice [166]; by contrast, histone deacetylase does not seem to change in the mPFC after acute or chronic stress [167, 168]. In brief, it seems that the amygdala is definitely involved in the generation of depression, because of the biochemical and morphological changes induced by chronic stress, although there is scarce evidence of its involvement in the antidepressant effects of ketamine. Interestingly, many of the BDNF related or unrelated changes induced by chronic stress in the amygdala are opposite to those seen the hippocampus and in the mPFC.

The BNST is a complex limbic structure that plays a crucial role in the regulation of fear, anxiety and aversion [169, 170, 171], in the integration of stress response [172, 173, 174, 175] as well as in the acquisition of emotions and motivated behaviour [176] (Fig. 7). Consequently, since diminished motivation is an essential feature of depression [28], it is likely that it is extensively involved both in developing and sustaining depression as well as in the mechanism of action of StAD and ketamine [19, 24, 177]. The BNST’s role in anxiety and depression is also evidenced by a serotonergic innervation of the BNST [178, 179]; interestingly, the availability of serotonin reuptake sites in the BNST is positively correlated with individual differences in anxiety behaviour [180]. The BNST receives glutamatergic projections from the mPFC, from the hippocampus and the basomedial amygdale, whereas GABAergic inputs arise from central and medial amygdaloid nuclei [181] (Fig. 8). The noradrenergic innervation of the BNST arises from the A2 region of the nucleus of the solitary tract and the A1 region of the caudal brainstem, with a minor projection from the locus coeruleus [182, 183]. These inputs are involved in stress response [184] and in arousal [185]. In particular, the BNST may act as a node for signal integration and coordination for appropriate homeostatic physiological and behavioural responses, a function which is likely reinforced by more than a dozen neuropeptides co-expressed in BNST neurons [175]. Hence, the insufficient resilience capacity of the BNST in handling long-duration threats associated with chronic stress can be a crucial occurrence in the development of depression.

Fig. 7.

Schematic representation of interactions among the bed nucleus of stria terminalis (BNST) and the brain areas that are involved in the “primary circuit of depression” according with our hypothesis. Some interactions of dorsal and median raphe with these brain areas are represented schematically. Other abbreviations: PFC, prefrontal cortex; Hipp, hippocampus; NAc, nucleus accumbens; VTA, ventral tegmental area; RN, raphe nuclei.

Fig. 8.

Schematic representation of glutamate (Glu), GABA, dopamine (DA) and serotonin (5-HT) inputs and outputs among the bed nucleus of stria terminalis (BNST) and the brain areas involved in the “primary circuit of depression”. Some inputs and outputs of dorsal and median raphe with these brain areas are represented schematically. Other abbreviations: PFC, prefrontal cortex; Hipp, hippocampus; NAc, nucleus accumbens; VTA, ventral tegmental area; RN, raphe nuclei.

In addition, the BNST receives a dense dopaminergic innervation [186] that clearly involves the circuits that mediate reward appraisal [187, 188] and in stress-induced relapses of drug-seeking [189], thereby suggesting that drug addiction shares common neurocircuitry with depression. Part of this neurocircuitry is the BNST output to the VTA; in fact, it has been shown that GABAergic efferent from the BNST forms symmetrical synapses on VTA neurons, which are mostly GABAergic neurons [190]. The activation of this output, mediated by glutamate innervation in the BNST is likely to disinhibit VTA dopaminergic neurons; in addition, dopamine and norepinephrine projections on the BNST innervate CRF neurons that in turn innervate glutamatergic afferents on BNST projecting neurons to VTA neurons [190, 191]. Although it was previously reported that the electrical stimulation of BNST can produce both excitatory and inhibitory responses in VTA neurons in vivo [192], it was recently reported that inhibitory synaptic transmissions to BNST neurons projecting to VTA are enhanced in rats exposed to chronic stress [193].

Significantly, the 16 unique and complex sub-regions of the BNST [194, 195], and the multiple inputs from many limbic regions, mostly glutamatergic (e.g., basomedial and central amygdala, hippocampus, mPFC) but also GABAergic (from central and medial amygdaloyd nucleus [181]) and its multiple outputs (e.g., VTA, lateral hypothalamus, central nucleus of the amygdala, periacqueductal grey, parabrachial nucleus) [196, 197], does not allow us to predict whether or not ketamine, through its direct interaction with the glutamatergic transmission in the BNST, provides a substantial contribution to the manifestation of its rapid and sustained antidepressant effects. On the other hand, from the direct interactions of BNST with brain areas involved in anhedonia or in depression symptoms in general, it might be presumed that the BNST plays a central role in depression aetiology, as well as in the mechanism of action of antidepressants. This hypothesis is supported by the observation (albeit in a pilot study only) that patients affected by TRD could benefit from deep brain stimulation of the BNST [198]. Unlike other brain areas involved in depression, there have been very few investigations concerning the assessment of the BDNF expression in the BNST. In particular, what has been observed is that early-life stress alters affective behaviours in adult mice through the activation of CRF-BDNF signalling in the oval bed nucleus, which indicates enhanced neuronal plasticity [199].

We have previously observed that StAD [177] and ketamine [24] shared the ability to increase dopamine and noradrenaline extracellular concentration in the BNST; interestingly, the SSRIs citalopram or fluoxetine and the selective blockers of norepinephrine reuptake such as desipramine or reboxetine and ketamine (i.e., 10–40 mg/kg i.p.) similarly increased the output of norepinephrine and dopamine. These data have enabled us to hypothesize that a converging pathway of activation of catecholamine transmission in the BNST could well play a determining role in the effect of antidepressants, although it is hard to predict whether this effect is due to a local action or to the involvement of a complex and integrated circuit of depression.