Effect of the stress hormone corticosterone on synaptic transmission and astrocytic release

Abstract

Glucocorticoids have been shown to influence neuronal function and metabolism in multiple ways. They regulate a wide array of actions-from maintaining cellular homeostasis to mediating stress responses-via myriad downstream pathways. Stress and stress hormones affect a variety of behavioural and cognitive abilities. The hippocampus is a major site for glucocorticoid action and stress response owing to widespread expression of its two receptor subtypes: mineralocorticoid receptor (MR) and glucocorticoid receptor (GR). In the hippocampus, both neurons and astrocytes express MR and GR abundantly. We examined the specific effects of MR and GR activation at increasing concentrations of corticosterone (the primary stress hormone in rodents) on synaptic transmission. During circadian troughs, when corticosterone concentrations are below 25 nM, hippocampal MRs are ~70% occupied while GRs are ~10% occupied. Gradual increases in corticosterone levels (during stress or circadian variation) therefore produce small changes in MR occupancy but large changes in GR occupancy. We selected concentrations along the rising phase of the circadian peak so that the interplay between MR and GR becomes more pronounced. We studied the effect of 3 hour corticosterone treatment (25, 50, and 100 nM) on:

depolarization mediated calcium influx, vesicular release (live cell fluorescence imaging), and properties of mEPSCs (miniature excitatory postsynaptic currents) using whole cell patch clamp in dissociated neonatal rat hippocampal neurons.

A lower concentration (25 nM), which predominantly activates MR, resulted in enhanced depolarization mediated calcium influx via a transcription dependent process, along with increased mEPSC frequency and larger amplitudes. In contrast, activation of GR with 100 nM corticosterone increased the rate of vesicular release through GR dependent genomic mechanisms. GR activation also significantly increased mEPSC frequency, amplitude, and accelerated decay kinetics. Treatment with 50 nM corticosterone resulted in suppression of neuronal responses, likely reflecting the combined but imbalanced activation of MR and GR. Detailed studies on genomic effects of physiologically relevant corticosterone concentrations on various steps of synaptic transmission are limited. Our results indicate that differential activation of MR and GR targets distinct steps in synaptic transmission. Importantly, corticosterone’s effect on neuronal physiology depends mainly on the extent of GR activation relative to MR. While MR activation with partial GR activation suppresses neuronal responses, progressive GR activation enhances physiological responses.

Neuronal synapses are ensheathed by astrocytes, which regulate synaptic function through gliotransmission. In addition to releasing glutamate, D serine, and ATP, astrocytes also release peptides such as atrial natriuretic peptide (ANP) and brain derived neurotrophic factor (BDNF) via regulated exocytosis. Release of vasoactive peptides like ANP has important implications for neurovascular interactions. We studied the effect of corticosterone on peptidergic vesicle release from astrocytes after brief ionomycin application. Hippocampal astrocytes were transfected with a plasmid encoding pro ANP fused to emerald GFP (ANP.emd). High, stress like corticosterone levels (100 nM and 1 M) for 3 hours:

significantly increased ANP.emd vesicle release via GR dependent genomic pathways, enhanced the mobility of these vesicles, and increased the velocity of inter astrocytic calcium wave propagation.

As both vesicle release and calcium wave propagation were significantly altered, we investigated the role of the cytoskeleton. Increasing corticosterone concentrations resulted in:

enhanced GFAP expression (immunofluorescence), progressive formation of F actin bundles and CLANs (Cross Linked Actin Networks) (FITC–phalloidin staining).

Inhibition of actin and tubulin polymerization prevented corticosterone induced increases in peptidergic vesicle release. Our results suggest that corticosterone modulates the release and dynamics of peptidergic vesicles from astrocytes through extensive microfilament rearrangements. Corticosterone mediated increases in astrocytic release and calcium wave propagation may therefore have widespread consequences on synaptic transmission and neurovascular coupling during stress. Astrocytes could thus play an important physiological role in mediating the stress response through modulation of gliotransmitter release.