Pharmacologically diverse antidepressants facilitate TRKB receptor activation by disrupting its interaction with the endocytic adaptor complex AP-2.

Several antidepressant drugs activate tropomyosin-related kinase B (TRKB) receptor, but it remains unclear whether these compounds employ a common mechanism for TRKB activation. Here, using MS, we found that a single intraperitoneal injection of fluoxetine disrupts the interaction of several proteins with TRKB in the hippocampus of mice. These proteins included members of adaptor protein complex-2 (AP-2) involved in vesicular endocytosis. The interaction of TRKB with the cargo-docking µ subunit of the AP-2 complex (AP2M) was confirmed to be disrupted by both acute and repeated fluoxetine treatments. Of note, fluoxetine disrupted the coupling between full-length TRKB and AP2M, but not the interaction between AP2M and the TRKB C-terminal region, indicating that the fluoxetine-binding site in TRKB lies outside the TRKB:AP2M interface. ELISA experiments revealed that in addition to fluoxetine, other chemically diverse antidepressants, such as imipramine, rolipram, phenelzine, ketamine, and its metabolite 2R,6R-hydroxynorketamine, also decreased the interaction between TRKB and AP2M in vitro Silencing the expression of AP2M in a TRKB-expressing mouse fibroblast cell line (MG87.TRKB) increased cell-surface expression of TRKB and facilitated its activation by brain-derived neurotrophic factor (BDNF), observed as levels of phosphorylated TRKB. Moreover, animals haploinsufficient for the Ap2m1 gene displayed increased levels of active TRKB, along with enhanced cell-surface expression of the receptor in cultured hippocampal neurons. Taken together, our results suggest that disruption of the TRKB:AP2M interaction is a common mechanism underlying TRKB activation by several chemically diverse antidepressants.

Isoflurane produces antidepressant effects and induces TrkB signaling in rodents.

A brief burst-suppressing isoflurane anesthesia has been shown to rapidly alleviate symptoms of depression in a subset of patients, but the neurobiological basis of these observations remains obscure. We show that a single isoflurane anesthesia produces antidepressant-like behavioural effects in the learned helplessness paradigm and regulates molecular events implicated in the mechanism of action of rapid-acting antidepressant ketamine: activation of brain-derived neurotrophic factor (BDNF) receptor TrkB, facilitation of mammalian target of rapamycin (mTOR) signaling pathway and inhibition of glycogen synthase kinase 3ß (GSK3ß). Moreover, isoflurane affected neuronal plasticity by facilitating long-term potentiation in the hippocampus. We also found that isoflurane increased activity of the parvalbumin interneurons, and facilitated GABAergic transmission in wild type mice but not in transgenic mice with reduced TrkB expression in parvalbumin interneurons. Our findings strengthen the role of TrkB signaling in the antidepressant responses and encourage further evaluation of isoflurane as a rapid-acting antidepressant devoid of the psychotomimetic effects and abuse potential of ketamine.

The impact of Bdnf gene deficiency to the memory impairment and brain pathology of APPswe/PS1dE9 mouse model of Alzheimer's disease.

Brain-derived neurotrophic factor (BDNF) importantly regulates learning and memory and supports the survival of injured neurons. Reduced BDNF levels have been detected in the brains of Alzheimer's disease (AD) patients but the exact role of BDNF in the pathophysiology of the disorder remains obscure. We have recently shown that reduced signaling of BDNF receptor TrkB aggravates memory impairment in APPswe/PS1dE9 (APdE9) mice, a model of AD. The present study examined the influence of Bdnf gene deficiency (heterozygous knockout) on spatial learning, spontaneous exploratory activity and motor coordination/balance in middle-aged male and female APdE9 mice. We also studied brain BDNF protein levels in APdE9 mice in different ages showing progressive amyloid pathology. Both APdE9 and Bdnf mutations impaired spatial learning in males and showed a similar trend in females. Importantly, the effect was additive, so that double mutant mice performed the worst. However, APdE9 and Bdnf mutations influenced spontaneous locomotion in contrasting ways, such that locomotor hyperactivity observed in APdE9 mice was normalized by Bdnf deficiency. Obesity associated with Bdnf deficiency did not account for the reduced hyperactivity in double mutant mice. Bdnf deficiency did not alter amyloid plaque formation in APdE9 mice. Before plaque formation (3 months), BDNF protein levels where either reduced (female) or unaltered (male) in the APdE9 mouse cortex. Unexpectedly, this was followed by an age-dependent increase in mature BDNF protein. Bdnf mRNA and phospho-TrkB levels remained unaltered in the cortical tissue samples of middle-aged APdE9 mice. Immunohistological studies revealed increased BDNF immunoreactivity around amyloid plaques indicating that the plaques may sequester BDNF protein and prevent it from activating TrkB. If similar BDNF accumulation happens in human AD brains, it would suggest that functional BDNF levels in the AD brains are even lower than reported, which could partially contribute to learning and memory problems of AD patients.

The responsiveness of TrkB to BDNF and antidepressant drugs is differentially regulated during mouse development.

BACKGROUND: Previous studies suggest that the responsiveness of TrkB receptor to BDNF is developmentally regulated in rats. Antidepressant drugs (AD) have been shown to increase TrkB signalling in the adult rodent brain, and recent findings implicate a BDNF-independent mechanism behind this phenomenon. When administered during early postnatal life, ADs produce long-lasting biochemical and behavioural alterations that are observed in adult animals. METHODOLOGY: We have here examined the responsiveness of brain TrkB receptors to BDNF and ADs during early postnatal life of mouse, measured as autophosphorylation of TrkB (pTrkB). PRINCIPAL FINDINGS: We found that ADs fail to induce TrkB signalling before postnatal day 12 (P12) after which an adult response of TrkB to ADs was observed. Interestingly, there was a temporally inverse correlation between the appearance of the responsiveness of TrkB to systemic ADs and the marked developmental reduction of BDNF-induced TrkB in brain microslices ex vivo. Basal p-TrkB status in the brain of BDNF deficient mice was significantly reduced only during early postnatal period. Enhancing cAMP (cyclic adenosine monophosphate) signalling failed to facilitate TrkB responsiveness to BDNF. Reduced responsiveness of TrkB to BDNF was not produced by the developmental increase in the expression of dominant-negative truncated TrkB.T1 because this reduction was similarly observed in the brain microslices of trkB.T1(-/-) mice. Moreover, postnatal AD administration produced long-lasting behavioural alterations observable in adult mice, but the responses were different when mice were treated during the time when ADs did not (P4-9) or did (P16-21) activate TrkB. CONCLUSIONS: We have found that ADs induce the activation of TrkB only in mice older than 2 weeks and that responsiveness of brain microslices to BDNF is reduced during the same time period. Exposure to ADs before and after the age when ADs activate TrkB produces differential long-term behavioural responses in adult mice.

The antidepressant-like effects of glutamatergic drugs ketamine and AMPA receptor potentiator LY 451646 are preserved in bdnf⁺/⁻ heterozygous null mice.

Accumulating evidence suggests that biogenic amine-based antidepressants act, at least in part, via regulation of brain-derived neurotrophic factor (BDNF) signaling. Biogenic amine-based antidepressants increase BDNF synthesis and activate its signaling pathway through TrkB receptors. Moreover, the antidepressant-like effects of these molecules are abolished in BDNF deficient mice. Glutamate-based drugs, including the NMDA antagonist ketamine, and the AMPA receptor potentiator LY 451646, mimic the effects of antidepressants in preclinical tests with high predictive validity. In humans, a single intravenous dose of ketamine produces an antidepressant effect that is rapid, robust and persistent. In this study, we examined the role of BDNF in expression of the antidepressant-like effects of ketamine and an AMPA receptor potentiator (LY 451646) in the forced swim test (FST). Ketamine and LY 451646 produced antidepressant-like effects in the FST in mice at 45 min after a single injection, but no effects were observed one week after a single ketamine injection. As previously reported, the effects of imipramine in the forced swim test were blunted in heterozygous BDNF knockout (bdnf(+/-)) mice. However ketamine and LY 451646 produced similar antidepressant-like responses in wildtype and bdnf(+/-) mice. Neither ketamine nor LY 451646 significantly influenced the levels BDNF or TrkB phosphorylation in the hippocampus when assessed at 45 min or 7 days after the drug administration. These data demonstrate that under the conditions tested, neither ketamine nor the AMPA-potentiator LY 451656 activate BDNF signaling, but produce a characteristic antidepressant-like response in heterozygous bdnf(+/-) mice. These data indicate that unlike biogenic amine-based agents, BDNF signaling does not play a pivotal role in the antidepressant effects of glutamate-based compounds. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Antidepressant drugs transactivate TrkB neurotrophin receptors in the adult rodent brain independently of BDNF and monoamine transporter blockade.

BACKGROUND: Antidepressant drugs (ADs) have been shown to activate BDNF (brain-derived neurotrophic factor) receptor TrkB in the rodent brain but the mechanism underlying this phenomenon remains unclear. ADs act as monoamine reuptake inhibitors and after prolonged treatments regulate brain bdnf mRNA levels indicating that monoamine-BDNF signaling regulate AD-induced TrkB activation in vivo. However, recent findings demonstrate that Trk receptors can be transactivated independently of their neurotrophin ligands. METHODOLOGY: In this study we examined the role of BDNF, TrkB kinase activity and monoamine reuptake in the AD-induced TrkB activation in vivo and in vitro by employing several transgenic mouse models, cultured neurons and TrkB-expressing cell lines. PRINCIPAL FINDINGS: Using a chemical-genetic TrkB(F616A) mutant and TrkB overexpressing mice, we demonstrate that ADs specifically activate both the maturely and immaturely glycosylated forms of TrkB receptors in the brain in a TrkB kinase dependent manner. However, the tricyclic AD imipramine readily induced the phosphorylation of TrkB receptors in conditional bdnf⁻/⁻ knock-out mice (132.4±8.5% of control; P = 0.01), indicating that BDNF is not required for the TrkB activation. Moreover, using serotonin transporter (SERT) deficient mice and chemical lesions of monoaminergic neurons we show that neither a functional SERT nor monoamines are required for the TrkB phosphorylation response induced by the serotonin selective reuptake inhibitors fluoxetine or citalopram, or norepinephrine selective reuptake inhibitor reboxetine. However, neither ADs nor monoamine transmitters activated TrkB in cultured neurons or cell lines expressing TrkB receptors, arguing that ADs do not directly bind to TrkB. CONCLUSIONS: The present findings suggest that ADs transactivate brain TrkB receptors independently of BDNF and monoamine reuptake blockade and emphasize the need of an intact tissue context for the ability of ADs to induce TrkB activity in brain.