Stress at the synapse: the synaptic action of acute behavioural stress and the protective effect of psychiatric drugs

Purpose of the study: Recent neuroimaging and histopathological studies on psychiatric patients have shown morphometrical and functional modifications in brain areas with glutamate predominance. There are many evidences that repeated exposure to different stressful events represents a risk factor for neuropsychiatric diseases. Preclinical studies showed that the exposure of rodents to stress produces many concomitans observed in human pathology. Indeed, in rats stress induces morphometrical alterations in brain areas probably due to neuronal atrophy and associated with hyperactivation of excitatory amino acid transmission [1]. Aim of this work was to study the effect of acute stress on glutamate release, to analyze the mechanisms whereby stress modifies glutamate release and to understand if these changes are dampened by chronic antidepressants. Methods: Rats were chronically (2 weeks) treated with vehicle or drugs employed for therapy of mood/anxiety disorders and then subjected to a standard Footshock (FS)-stress protocol [2]. Immediately after FS-stress, prefrontal/frontal cortex (P/FC) was dissected and synaptosomes were purified on Percoll gradients; glutamate release was measured [3]. SNARE complexes were measured in un-boiled samples of synaptic membranes by SDS-PAGE and Western blot. Electrophysiological experiments were performed on acute P/FC slices [3]. Changes in vesicles mobilization were measured by Total Internal Reflection Fluorescence Microscopy (TIRFM). Results: Acute FS-stress induced a marked increase of circulating corticosterone (CORT) in all stressed rats (vehicle- and antidepressant (AD)-treated) and a rapid (non genomic) increase of glutamate release from synaptosomes of P/FC via selective activation of glucocorticoid receptor. The increase of glutamate release was prevented by chronic AD treatments [3]. On the molecular level, FS-stress induced a rapid accumulation of SNARE complexes in presynaptic membranes of rats pre-treated or not with ADs. Patch-clamp recordings on P/FC pyramidal neurons revealed that FS-stress induced changes in paired-pulse facilitation (PPF) and its Ca2+-dependence, consistent with an increase in glutamate release. Chronic desipramine (DMI) completely prevented this effect [3]. Because the number of SNARE complexes per vesicle is fixed, the accumulation induced by stress suggests that FS-stress may increase the size of the readily releasable pool (RRP) of vesicles (docked vesicles). Therefore we measured the release of glutamate from P/FC synaptosomes of control and FS-stressed rats evoked by sucrose (250–500 mM), which mobilizes the RRP. In line with SNARE complex accumulation, RRP size was markedly increased in P/FC synaptosomes from stressed rats (vehicle and ADs-treated). Furthermore, we investigated the effects of stress on synaptic vesicle kinetics with TIRFM. Since the effects of CORT on glutamate release and SNARE complex accumulation seem to be non genomic, we incubated synaptosomes in vitro with CORT and we visualized with TIRFM the changes induced by CORT and in vitro depolarization on vesicle mobilization. Conclusions: Overall, the combined results of this study give more insight into the basic mechanisms whereby behavioural stress affects excitatory transmission in the forebrain and show a novel effect of ADs that could be related to their therapeutic action.