Effects of electroconvulsive seizures and antidepressant drugs on brain-derived neurotrophic factor protein in rat brain

Abstract

Background

The antidepressant-like effects of brain-derived neurotrophic factor (BDNF) infusions in brain, and the upregulation of BDNF mRNA and its receptor in rats exposed to electroconvulsive seizure (ECS) and antidepressants, suggested a role for increased BDNF protein.

Methods

We measured BDNF protein levels with a two-site enzyme-linked immunosorbent assay (ELISA) in six brain regions of adult male rats that received daily ECS or daily injections of antidepressant drugs.

Results

The BDNF ELISA method was validated by the 50% loss of BDNF protein in the brains of +/− BDNF knockout mice, the 60%–100% recovery of spiked recombinant BDNF, and by the amounts and regional variations of BDNF measured in the six brain regions. Ten consecutive daily exposures to ECS increased BDNF protein in the parietal cortex (219%), entorhinal cortex (153%), hippocampus (132%), frontal cortex (94%), neostriatum (67%), and septum (29%). BDNF increased gradually in the hippocampus and frontal cortex, with a peak response by the fourth day of ECS. Increases peaked at 15 hours after the last ECS and lasted at least 3 days thereafter. Two weeks of daily injections with the monoamine (MAO)-A and -B inhibitor tranylcypromine (8–10 mg/kg, IP) increased BDNF by 15% in the frontal cortex, and 3 weeks treatment increased it by 18% in the frontal cortex and by 29% in the neostriatum. Tranylcypromine, fluoxetine, and desmethylimipramine did not elevate BDNF in the hippocampus.

Conclusions

Elevations in BDNF protein in brain are consistent with the greater treatment efficacy of ECS and MAO inhibitors in drug-resistant major depressive disorder and may be predictive for the antidepressant action of the more highly efficacious interventions.

Introduction

Current treatments for major depressive disorder comprise several classes of antidepressant drugs; for treatment-resistant depression, electroconvulsive therapy remains a treatment of choice. Although the acute pharmacologic actions of the chemical antidepressants are well characterized, the mechanism for their long-term antidepressant efficacy or for the efficacy of electroconvulsive therapy remains less well understood. The molecular effects of electroconvulsive seizure (ECS), an animal model of electroconvulsive therapy, are diverse and comprise increases in levels of neurotransmitters, neuropeptides, and structural changes (Fochtmann 1994). Electroconvulsive seizure affects several brain regions, of which the hippocampus, frontal cortex, neostriatum, entorhinal cortex, temporoparietal cortex, and several monoaminergic nuclei that project to these areas have most commonly been reported. One strategy to identify a mechanism by which electroconvulsive therapy treats depression is to identify mediator(s) that may be shared by ECS and conventional antidepressant drugs but for which the response is greater in ECS. For example, like some antidepressants, ECS decreases β-adrenergic receptors, typically in the dentate gyrus of the hippocampus and the frontal cortex (Biegon and Israeli 1986). It also produces a larger increase in the hippocampal field excitatory postsynaptic potential than does fluoxetine (Stewart and Reid 2000).

Neurotrophic factors are prime candidates for mediating the antidepressant effects of ECS and drugs. As a protein class comprising nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), NT-4/5, and NT-6, the neurotrophins positively modulate monoaminergic neurotransmission Altar et al 1994, Mamounas et al 1995, Martin-Iverson et al 1994, Siuciak et al 1996 and central nervous sytem neuron survival, outgrowth, and neuroprotection (Lindsay et al 1994). Brain-derived neurotrophic factor binds to and phosphorylates its high-affinity tyrosine kinase receptor (TrkB) and activates intracellular signaling pathways including microtubule-associated protein (MAP) kinase and cyclic adenosine monophosphate (cAMP) (Skolnick et al 2001). It has been shown that BDNF promotes serotonin neuron development (Eaton et al 1995), augments serotonin synthesis and turnover Martin-Iverson et al 1994, Siuciak et al 1996) and dramatically increases serotonergic axon fiber densities in intact and fiber-lesioned rats Mamounas et al 1995, Mamounas et al 2000. Serotonergic axon densities are decreased in the prefrontal cortex of suicide patients with major depressive disorder (Austin et al 2002). In the neocortex and hippocampus, BDNF mRNA and protein are highly expressed (Altar et al 1997), and these regions are widely implicated in the pathophysiology of depressive disorders. Intraventricular, dorsal raphe, or hippocampal infusions of BDNF in rats reduce learned helplessness behavior and reduce immobility in the forced swim and inescapable shock models, indicating an antidepressantlike effect Shirayama et al 2002, Siuciak et al 1997. An increase of BDNF mRNA has been documented in animals treated with several antidepressant drugs (Nibuya et al 1995) and seizures Isackson et al 1991, Nibuya et al 1995, Rocamora et al 1992. Manipulations such as stress that produce learned helplessness in animals and precipitate depression in humans decrease BDNF mRNA (Smith et al 1995). In the hippocampus of depressed patients treated with unspecified antidepressants, BDNF protein immunoreactivity was elevated compared with subjects who were not so treated at the time of death (Chen et al 2001). These and related findings suggest a role for BDNF in the etiology and treatment of depression Altar 1999, Duman et al 1997. Clinical trials are currently evaluating the antidepressant activity of small molecules that increase BDNF mRNA and protein release from cultured cells (Skolnick et al 2001).

Virtually all of the studies of BDNF regulation or its role in depression have been through an analysis of the levels of BDNF mRNA and not its protein product. This has in part been due to the lack of available and sensitive tissue assays for BDNF protein. Yet measures of BDNF protein are likely to yield information not available with measurements of BDNF mRNA, because the levels of BDNF mRNA in different brain regions do not necessarily reflect the levels of BDNF protein (Altar et al 1997). This is in large part due to the considerable role of anterograde transport in the trafficking of BDNF throughout the brain Altar et al 1997, Altar and DiStefano 1998, Smith et al 1997. For example, the neostriatum contains no BDNF mRNA yet responds dramatically to exogenous BDNF (Volpe et al 1998) and endogenous BDNF, which is provided to the striatum by striatal and nigral afferents (Altar et al 1997).

This study validated a commercially available ELISA assay for measuring BDNF protein in brain and then quantified the amounts of BDNF protein in different brain regions after a chronic administration of antidepressant drugs and single or repeated ECS treatments. Clinically, each of these treatments is used to treat mood disorders, but they vary with respect to efficacy. Electroconvulsive shocks and tranylcypromine are used in patients for which safer, “first-line” pharmacologic treatments have failed. To assess whether this difference in treatment efficacy is reflected in the alterations of BDNF protein in the brain, we evaluated the degree and time course by which BDNF protein in six rat brain regions is augmented after administration of ECS or different classes of antidepressant drugs.