Stress and corticosterone increase the readily releasable pool of glutamate vesicles in synaptic terminals of prefrontal and frontal cortex.

Stress and glucocorticoids alter glutamatergic transmission, and the outcome of stress may range from plasticity enhancing effects to noxious, maladaptive changes. We have previously demonstrated that acute stress rapidly increases glutamate release in prefrontal and frontal cortex via glucocorticoid receptor and accumulation of presynaptic SNARE complex. Here we compared the ex vivo effects of acute stress on glutamate release with those of in vitro application of corticosterone, to analyze whether acute effect of stress on glutamatergic transmission is mediated by local synaptic action of corticosterone. We found that acute stress increases both the readily releasable pool (RRP) of vesicles and depolarization-evoked glutamate release, while application in vitro of corticosterone rapidly increases the RRP, an effect dependent on synaptic receptors for the hormone, but does not induce glutamate release for up to 20 min. These findings indicate that corticosterone mediates the enhancement of glutamate release induced by acute stress, and the rapid non-genomic action of the hormone is necessary but not sufficient for this effect.

Pharmacological characterization of BDNF promoters I, II and IV reveals that serotonin and norepinephrine input is sufficient for transcription activation.

Compelling evidence has shown that the effects of antidepressants, increasing extracellular serotonin and noradrenaline as a primary mechanism of action, involve neuroplastic and neurotrophic mechanisms. Brain-derived neurotrophic factor (BDNF) has been shown to play a key role in neuroplasticity and synaptic function, as well as in the pathophysiology of neuropsychiatric disorders and the mechanism of action of antidepressants. The expression of BDNF is mediated by the transcription of different mRNAs derived by the splicing of one of the eight 5' non-coding exons with the 3' coding exon (in rats). The transcription of each non-coding exon is driven by unique and different promoters. We generated a gene reporter system based on hippocampal and cortical neuronal cultures, in which the transcription of luciferase is regulated by BDNF promoters I, II, IV or by cAMP response element (CRE), to investigate the activation of selected promoters induced by monoaminergic antidepressants and by serotonin or noradrenaline agonists. We found that incubation with fluoxetine or reboxetine failed to induce any activation of BDNF promoters or CRE. On the other hand, the incubation of cultures with selective agonists of serotonin or noradrenaline receptors induced a specific and distinct profile of activation of BDNF promoters I, II, IV and CRE, suggesting that the monoaminergic input, absent in dissociated cultures, is essential for the modulation of BDNF expression. In summary, we applied a rapidly detectable and highly sensitive reporter gene assay to characterize the selective activation profile of BDNF and CRE promoters, through specific and different pharmacological stimuli.