Hanna Antila

Fear Erasure in Mice Requires Synergy Between Antidepressant Drugs and Extinction Training

Long-term loss of fearful memories can be achieved through a combination of antidepressant drugs and exposure therapy. Antidepressant drugs and psychotherapy combined are more effective in treating mood disorders than either treatment alone, but the neurobiological basis of this interaction is unknown. To investigate how antidepressants influence the response of mood-related systems to behavioral experience, we used a fear-conditioning and extinction paradigm in mice. Combining extinction training with chronic fluoxetine, but neither treatment alone, induced an enduring loss of conditioned fear memory in adult animals. Fluoxetine treatment increased synaptic plasticity, converted the fear memory circuitry to a more immature state, and acted through local brain-derived neurotrophic factor. Fluoxetine-induced plasticity may allow fear erasure by extinction-guided remodeling of the memory circuitry. Thus, the pharmacological effects of antidepressants need to be combined with psychological rehabilitation to reorganize networks rendered more plastic by the drug treatment.

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 TrkBF616A 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.

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.

VGF (TLQP-62)-induced neurogenesis targets early phase neural progenitor cells in the adult hippocampus and requires glutamate and BDNF signaling

The neuropeptide VGF (non-acronymic), which has antidepressant-like effects, enhances adult hippocampal neurogenesis as well as synaptic activity and plasticity in the hippocampus, however the interaction between these processes and the mechanism underlying this regulation remain unclear. In this study, we demonstrate that VGF-derived peptide TLQP-62 specifically enhances the generation of early progenitor cells in nestin-GFP mice. Specifically, TLQP-62 significantly increases the number of Type 2a neural progenitor cells (NPCs) while reducing the number of more differentiated Type 3 cells. The effect of TLQP-62 on proliferation rather than differentiation was confirmed using NPCs in vitro; TLQP-62 but not scrambled peptide PEHN-62 increases proliferation in a cell line as well as in primary progenitors from adult hippocampus. Moreover, TLQP-62 but not scrambled peptide increases Cyclin D mRNA expression. The proliferation of NPCs induced by TLQP-62 requires synaptic activity, in particular through NMDA and metabotropic glutamate receptors. The activation of glutamate receptors by TLQP-62 activation induces phosphorylation of CaMKII through NMDA receptors and protein kinase D through metabotropic glutamate receptor 5 (mGluR5). Furthermore, pharmacological antagonists to CaMKII and PKD inhibit TLQP-62-induced proliferation of NPCs indicating that these signaling molecules downstream of glutamate receptors are essential for the actions of TLQP-62 on neurogenesis. We also show that TLQP-62 gradually activates Brain-Derived Neurotrophic Factor (BDNF)-receptor TrkB in vitro and that Trk signaling is required for TLQP-62-induced proliferation of NPCs. Understanding the precise molecular mechanism of how TLQP-62 influences neurogenesis may reveal mechanisms by which VGF-derived peptides act as antidepressant-like agents.

Neuronal plasticity and neurotrophic factors in drug responses

Neurotrophic factors, particularly brain-derived neurotrophic factor (BDNF) and other members of the neurotrophin family, are central mediators of the activity-dependent plasticity through which environmental experiences, such as sensory information are translated into the structure and function of neuronal networks. Synthesis, release and action of BDNF is regulated by neuronal activity and BDNF in turn leads to trophic effects such as formation, stabilization and potentiation of synapses through its high-affinity TrkB receptors. Several clinically available drugs activate neurotrophin signaling and neuronal plasticity. In particular, antidepressant drugs rapidly activate TrkB signaling and gradually increase BDNF expression, and the behavioral effects of antidepressants are mediated by and dependent on BDNF signaling through TrkB at least in rodents. These findings indicate that antidepressants, widely used drugs, effectively act as TrkB activators. They further imply that neuronal plasticity is a central mechanism in the action of antidepressant drugs. Indeed, it was recently discovered that antidepressants reactivate a state of plasticity in the adult cerebral cortex that closely resembles the enhanced plasticity normally observed during postnatal critical periods. This state of induced plasticity, known as iPlasticity, allows environmental stimuli to beneficially reorganize networks abnormally wired during early life. iPlasticity has been observed in cortical as well as subcortical networks and is induced by several pharmacological and non-pharmacological treatments. iPlasticity is a new pharmacological principle where drug treatment and rehabilitation cooperate; the drug acts permissively to enhance plasticity and rehabilitation provides activity to guide the appropriate wiring of the plastic network. Optimization of iPlastic drug treatment with novel means of rehabilitation may help improve the efficacy of available drug treatments and expand the use of currently existing drugs into new indications.

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 3β (GSK3β). 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.

Repeated brief isoflurane anesthesia during early postnatal development produces negligible changes on adult behavior in male mice

Brain development is a complex process regulated by genetic programs and activity-dependent neuronal connectivity. Anesthetics profoundly alter neuronal excitability, and anesthesia during early brain development has been consistently associated with neuroapoptosis, altered synaptogenesis, and persistent behavioral abnormalities in experimental animals. However, the depth, and even more the duration and developmental time point(s) of exposure to anesthesia determine the neuropathological and long-term behavioral consequences of anesthetics. Here, we have investigated adulthood phenotypic changes induced by repeated but brief (30 min) isoflurane anesthesia delivered during two distinct developmental periods in male mice. A set of animals were subjected to anesthesia treatments at postnatal days 7, 8 and 9 (P7-9) when the animals are susceptible to anesthesia-induced neuroapoptosis and reduced synaptogenesis. To control the potential influence of (handling) stress, a separate group of animals underwent repeated maternal separations of similar durations. Another set of animals were exposed to the same treatments at postnatal days 15, 16 and 17 (P15-17), a developmental time period when anesthetics have been shown to increase synaptogenesis. Starting from postnatal week 9 the mouse phenotype was evaluated using a battery of behavioral tests that assess general locomotor activity (home cage activity, open field), learning and memory (water maze) and depression- (saccharin preference, forced swim test), anxiety- (light-dark box, stress-induced hyperthermia) and schizophrenia- (nesting, prepulse inhibition) related endophenotypes. Apart from mild impairment in spatial navigation memory, exposure to anesthesia treatments during P7-9 did not bring obvious behavioral alterations in adult animals. Importantly, maternal separation during the same developmental period produced a very similar phenotype during the water maze. Mice exposed to anesthesia during P15-17 showed mild hyperactivity and risk-taking behavior in adulthood, but were otherwise normal. We conclude that significantly longer administration periods are needed in order for early-life repeated exposures to anesthetics to produce behavioral alterations in adult mice.

TrkB signaling underlies the rapid antidepressant effects of isoflurane

Neuronal plasticity induced by signaling through BDNF receptor TrkB has been implicated in the actions of antidepressants, including the rapid-acting antidepressant ketamine. We show that isoflurane induced transphosphorylation of TrkB by Src family kinases, stimulates the mTor signaling pathway and promotes neuronal plasticity and antidepressant-like behavior in rodents. Our findings provide a neurobiological basis for the clinically observed antidepressant effects of isoflurane and encourage its further evaluation as a rapid-acting antidepressant treatment devoid of the psychotomimetic side effects of ketamine.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 3β (GSK3β). 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.