An AMPA receptor potentiator modulates hippocampal expression of BDNF: an in vivo study

Abstract

AMPA receptor activation has been demonstrated to increase the neuronal expression of brain derived neurotrophic factor (BDNF). In the present study, we investigated the effect of a novel AMPA receptor potentiator (LY404187) and its active isomer (LY451646) on the expression of BDNF protein and mRNA, as well as TrkB mRNA in rat hippocampus. LY404187 administered for 7 days (1 mg/kg) significantly increased the number of BDNF immunopositive cells in the dentate gyrus, but not other hippocampal subfields. Chronic treatment (7 days) with LY451646 (0.5 mg/kg, comparable to 1 mg/kg of LY404187) increased the level of both BDNF and TrkB mRNA expression in the dentate gyrus, CA3 and CA4 of the hippocampus. However, chronic treatment with lower doses of LY451646 (0.125 and 0.25 mg/kg) decreased the level of BDNF and TrkB mRNA in hippocampus, whilst the highest used dose of LY451646 (1 mg/kg) had no effect on BDNF and TrkB mRNA in hippocampus. In contrast, acute treatment with LY451646 produced an increase in BDNF mRNA levels at doses of 0.125 and 0.25 mg/kg in the hippocampus (CA4, CA3 and dentate gyrus, but not in CA1). LY451646 at 0.5 mg/kg had no effect, but at 1.0 mg/kg decreased the level of BDNF mRNA in hippocampus. Acute treatment with LY451646 did not affect the TrkB receptor mRNA levels in hippocampus. Our results demonstrate that biarylpropylsulfonamide AMPA receptor potentiators are capable of modulating the expression of BDNF and TrkB mRNA in a dose- and time-dependent manner. The increase in both BDNF protein and mRNA expression in the dentate gyrus but not in CA1 indicates a specific role of AMPA receptors in the regulation of BDNF expression in this hippocampal subfield. The regulation of BDNF expression by biarylpropylsulfonamids such as LY451646 may have important therapeutical implications for this class of molecule in the treatment of depression and other CNS disorders.

Introduction

Brain–derived neurotrophic factor (BDNF) is a member of the neurotrophin family of nerve growth factors. This neurotrophic factor is most abundantly expressed in the central nervous system. Through its high affinity interactions with a tyrosine kinase receptor [TrkB] (Lindvall et al., 1994), BDNF enhances the growth and maintenance of several neuronal systems, serves as a neurotransmitter modulator, and participates in plasticity phenomena, such as long-term potentation and learning (Schinder and Poo, 2000). Further, exogenous administration of BDNF produces antidepressant-like effects in behavioral despair models (Siuciak et al., 1997), analgesia (Siuciak et al., 1994), and reduces blood glucose in obese diabetic (db/db) mice (Ono et al., 1997). Increases in BDNF expression have been reported in hippocampus following chronic antidepressant administration in both animals (Nibuya et al., 1995, Russo-Neustadt et al., 1999) and humans (Chen et al., 2001). In contrast, decreases in the expression of this neurotrophin have been reported following immobilization stress (Smith et al., 1995) and in Parkinson’s disease (Howells et al., 2000). In toto, these findings indicate that elevating BDNF levels may represent a novel strategy in the treatment of a range of neuropsychiatric disorders.

It has been known for more than decade that AMPA receptor activation increases BDNF gene expression in neurons (Zafra et al., 1990). The role of AMPA receptor activation in the mediation of BDNF expression may involve two independent mechanisms, the first involving AMPA receptor-dependent depolarization opening L-type (voltage sensitive) calcium channels. Increases in [Ca⁺⁺] may activate BDNF expression through multiple mechanisms, including a CRE element in the BDNF promoter region (Shieh et al., 1998, Tao et al., 1998). Hayashi et al. (1999) reported as a second, Ca⁺⁺ and Na⁺ independent mechanism, with AMPA receptor activation directly linked to the activation of Lyn (a member of the Src family of non-receptor protein tyrosine kinases). Lyn, physically associated with AMPA receptors, then activates a mitogen–activated protein kinase (MAPK) signalling pathway (Hayashi et al., 1999).

AMPA receptors are a subfamily of ionotropic glutamate receptors mediating fast excitatory transmission in the central nervous system. High densities of AMPA receptors are present in the hippocampus, septum, cerebral cortical areas, and striatum (Stensbol et al., 1999). These receptors, like other ligand-gated ion channels, possess multiple, allosteric modulatory sites that represent targets for small molecules. One such class of compounds, called AMPA receptor positive modulators or AMPA receptor potentiators, do not themselves activate AMPA receptors, but in the presence of agonist (e.g., glutamate and AMPA) reduce the rate of receptor desensitization and/or deactivation (Bleakman and Lodge, 1998, Borges and Dingledine, 1998). Several chemical classes of AMPA receptor potentiator have been reported, including pyrrolidones (piracetam, aniracetam), benzothiadiazides (cyclothiazide), benzoylpiperidines (CX516, CX546) and more recently, the biarylopropylsulfonamides (LY392098, LY404187, its active isomer LY451646). It has been reported that positive modulators of AMPA receptors can enhance the expression of BDNF protein and mRNA in cerebellar granule cells (Hayashi et al., 1999), in primary cortical neurons (Legutko et al., 2001) and explants of hippocampus and enthorinal cortex (Lauterborn et al., 2000). To date, there has been only one study in vivo of an increase in BDNF mRNA level induced by AMPA positive modulators (Lauterborn et al., 2000).

Both electrophysiological (Baumbarger et al., 2001, Gates et al., 2001, Miu et al., 2001) and neurochemical studies (Linden et al., 2001) have shown that biarylpropylsulfonamides (Ornstein et al., 2000, Zarrinmayeh et al., 2001) are far more potent than previously described AMPA receptor potentiators. As there is no in vivo data on the impact of AMPA receptor potentiators on the expression of BDNF protein, and limited data on the mRNA expression (Lauterborn et al., 2000), we evaluated the effects of a novel AMPA potentiator from the biarylpropylsulfonamide family (LY404187 and its active isomer LY451646) on BDNF protein and mRNA expression in the rat brain. Both LY404187 and LY451646 were active in cloned cells lines expressing human iGluR receptors, potentiated currents in hippocampal neurons and penetrated the brain. The racemic mixture (LY404187) was employed in probe studies, examining the effect of this biarylpropylsulfomamide on BDNF protein levels. In subsequent studies, the active enantiomer (LY451646) was used to examine its effects on the expression of BDNF and TrkB mRNA.