Distribution of Phosphorylated TrkB Receptor in the Mouse Hippocampal Formation Depends on Sex and Estrous Cycle Stage

Abstract

TrkB is a neurotrophin receptor important for the synaptic plasticity underlying hippocampal-dependent learning and memory. Because this receptor is widely expressed in hippocampal neurons, the precise location of TrkB activation is likely important for its specific actions. The goal of this study was to identify the precise sites of TrkB activation in the mouse hippocampal formation, and to determine any changes in the distribution of activated TrkB under conditions of enhanced BDNF expression and hippocampal excitability. Using electron microscopy, we localized TrkB phosphorylated at tyrosine 816 in the hippocampal formation of male mice and female mice under conditions of naturally low circulating estradiol and naturally high circulating estradiol, when BDNF expression, TrkB signaling, and synaptic plasticity are enhanced. To compare relative amounts of pTrkB in each group, we counted profiles containing pTrkB-immunoreactivity (pTrkB-ir) in all hippocampal subregions. pTrkB-ir was in axons and axon terminals, dendrites and dendritic spines of neurons in the hippocampal formation, but the majority of pTrkB-ir localized to presynaptic profiles. pTrkB-ir also was abundant in glial profiles, which were further identified as microglia using immunofluorescence and confocal microscopy. Axonal and glial pTrkB-ir and pTrkB-ir in the CA1 stratum radiatum were more abundant in high-estradiol states (proestrus females) than low-estradiol states (estrus and diestrus females and males). These findings suggest that presynaptic TrkB is positioned to modulate estradiol-mediated and BDNF-dependent synaptic plasticity. Furthermore, they suggest a novel role for TrkB in microglial function in the neuroimmune system.

The cellular and synaptic location of activated TrkB in mouse hippocampus during limbic epileptogenesis

“…Indeed, quantification at low power (100×) in stratum radiatum of CA1 in the present study revealed no significant differences between NS and KA treated animals, despite robust increases following KA when pY816+ dendritic shafts and spines were quantified at high power (630×). These results are consistent with ultrastructural evidence localizing pY816 immunoreactivity to dendritic shafts of CA1 pyramids and a minority (5%) of spines within stratum radiatum of CA1 under basal conditions (Spencer-Segal et al, 2011); notably, activated TrkB was clustered next to the postsynaptic density. Light microscopic studies have also colocalized pTrkB immunoreactivity with a subset of PSD-95 (5%) in stratum radiatum of CA1, and the fraction of pTrkB colocalizing with PSD-95 increased approximately 2-fold following an unsupervised learning paradigm (Chen et al, 2010a;Chen et al, 2010b).…”

“…These results confirm earlier work that reported increased pTrkB immunoreactivity in stratum lucidum during epileptogenesis, yet was unable to identify the cellular structures containing this immunoreactivity (Binder et al, 1999a;He et al, 2004;He et al, 2002;He et al, 2010). The present results are also consistent with recent ultrastructural evidence localizing pY816 immunoreactivity to mossy fiber axons and a minority (5%) of boutons within SL under basal conditions (Spencer-Segal et al, 2011); pY816 immunoreactivity within boutons was most often affiliated with small synaptic vesicles opposite the synaptic contact.…”

Lower Serum Tropomyosin Receptor Kinase B Levels in Patients with Schizophrenia

“…[25,26] The distribution of phosphorylated TrkBs was found to be affected by estradiol levels. [27] However, the real mechanism of estrogen that influences TrkB protein is still unknown.…”

Estrogen Effects on Cognitive and Synaptic Health Over the Lifecourse

“…An electron microscopic study revealed that phosphorylated-TrkB (pTrkB), the activated form of TrkB, is localized primarily to axons and axon terminals, but is also present in dendritic shafts and spines of hippocampal neurons (202). There was also abundant pTrkB in microglial profiles identified using immunofluorescence and confocal microscopy.…”