Els H. Velzing

Glucocorticoids alter calcium conductances and calcium channel subunit expression in basolateral amygdala neurons

Glucocorticoid hormones, which are released in high amounts after stress, enter the brain where they bind to intracellular receptors that are abundant in limbic areas, in particular the hippocampus and amygdala nuclei. Behavioural studies indicate that glucocorticoids modulate learning and memory processes via receptors in the hippocampus and amygdala. So far, the effects of glucocorticoids on amygdala neurons have not been investigated at the cellular and molecular level. We report here that in vitro application of glucocorticoids for 20 min increases 1–4 h later the amplitude of sustained, high‐voltage‐activated calcium currents in principal neurons of the basolateral amygdala. In contrast, the transient, low‐voltage‐activated currents were decreased. We examined whether these functional changes in calcium conductance were accompanied by transcriptional regulation of calcium channel subunits. Analysis of the RNA – collected after recording and then linearly amplified – revealed that glucocorticoid‐mediated increases in sustained calcium currents are associated with a parallel shift in the relative expression of the α1 subunit constituting the pore of the sustained, high‐voltage‐activated (L‐type) calcium channel. These data indicate that glucocorticoids, probably by selectively targeting genes encoding calcium channel subunits, largely alter the calcium influx into basolateral amygdala neurons. These actions could modify amygdala network function and thus contribute to the behavioural effects exerted by the stress hormones via the basolateral amygdala.

Adult hippocampal glucocorticoid receptor expression and dentate synaptic plasticity correlate with maternal care received by individuals early in life

Maternal care in mammals is the prevailing environmental influence during perinatal development. The adult rat offspring of mothers exhibiting increased levels of pup licking/grooming (LG; High LG mothers), compared to those reared by Low LG dams, show increased hippocampal glucocorticoid receptor expression, complex dendritic tree structure, and an enhanced capacity for synaptic potentiation. However, these data were derived from studies using the total amount of maternal care directed toward the entire litter, thus ignoring possible within‐litter variation. We show that the amount of LG received by individual pups within a litter varies considerably. Therefore, we questioned if the amount of LG received by individual pups correlates with and thus putatively predicts later hippocampal structure and function. To this end, LG‐scores were determined during the first postnatal week for all pups in 32 litters and correlated with neuroendocrine and hippocampal parameters in young‐adulthood. Pup LG‐score positively correlated with the glucocorticoid receptor mRNA expression in the adult hippocampus. Moreover, the ability to induce synaptic potentiation in the dentate gyrus in vitro was enhanced in animals with high LG‐scores. Structural plasticity correlated less reliably with LG‐scores early in life and differed between sexes. Male offspring with high LG‐scores displayed fewer newborn neurons, higher brain derived neurotrophic factor expression and tended to have more complex granule cell dendritic trees. We conclude that even moderate variations in early life environment have a major impact on adult hippocampal function. This principle could provide a mechanistic basis for individual differences in susceptibility to psychopathology.

Corticosterone reduces dendritic complexity in developing hippocampal CA1 neurons

Although prolonged stress and corticosteroid exposure induce morphological changes in the hippocampal CA3 area, the adult CA1 area is quite resistant to such changes. Here we addressed the question whether elevated corticosteroid hormone levels change dendritic complexity in young, developing CA1 cells. In organotypic cultures (prepared from P5 rats) that were 14–21 days cultured in vitro, two doses of corticosterone (30 and 100 nM) were tested. Dendritic morphology of CA1 neurons was established by imaging neurons filled with the fluorescent dye Alexa. Application of 100 nM corticosterone for 20 minutes induced atrophy of the apical dendritic tree 1–4 hours later. Fractal analysis showed that total neuronal complexity was reduced twofold when compared with vehicle‐treated neurons. Exposing organotypic slices to 30 nM corticosterone reduced apical length in a more delayed manner: only neurons examined more than 2 hours after exposure to corticosterone showed atrophy of the apical dendritic tree. Neither dose of corticosterone affected the length of basal dendrites or spine density. Corticosterone was ineffective in changing morphology of the apical dendrites when tested in the presence of the glucocorticoid receptor antagonist RU38486. These results suggest that high physiological levels of corticosterone, via activation of the glucocorticoid receptor, can, during the course of only a few hours, reduce the dendritic complexity of CA1 pyramidal neurons in young, developing hippocampal tissue. These findings suggest that it is relevant to maintain plasma corticosterone levels low during hippocampal development.

Both mineralocorticoid and glucocorticoid receptors regulate emotional memory in mice

Corticosteroid hormones are thought to promote optimal behavioral adaptation under fearful conditions, primarily via glucocorticoid receptors (GRs). Here, we examined - using pharmacological and genetic approaches in mice - if mineralocorticoid receptors (MRs) also play a role in fearful memory formation. As expected, administration of the GR-antagonist RU38486 prior to training in a fear conditioning paradigm impaired contextual memory when tested 24 (but not when tested 3) h after training. Tone-cue memory was enhanced by RU38486 when tested at 4 (but not 25) h after training. Interestingly, pre (but not post)-training administration of MR antagonist spironolactone impaired contextual memory, both at 3 and 24h after training. Similar effects were also found in forebrain-specific MR knockout mice. Spironolactone also impaired tone-cue memory, but only at 4h after training. These results reveal that - in addition to GRs - MRs also play a critical role in establishing fear memories, particularly in the early phase of memory formation.

Early life stress determines the effects of glucocorticoids and stress on hippocampal function: Electrophysiological and behavioral evidence respectively

ABSTRACT Exposure to early‐life adversity may program brain function to prepare individuals for adaptation to matching environmental contexts. In this study we tested this hypothesis in more detail by examining the effects of early‐life stress – induced by raising offspring with limited nesting and bedding material from postnatal days 2–9 – in various behavioral tasks and on synaptic function in adult mice. Early‐life stress impaired adult performance in the hippocampal dependent low‐arousing object‐in‐context recognition memory task. This effect was absent when animals were exposed to a single stressor before training. Early‐life stress did not alter high‐arousing context and auditory fear conditioning. Early‐life stress‐induced behavioral modifications were not associated with alterations in the dendritic architecture of hippocampal CA1 pyramidal neurons or principal neurons of the basolateral amygdala. However, early‐life stress reduced the ratio of NMDA to AMPA receptor‐mediated excitatory postsynaptic currents and glutamate release probability specifically in hippocampal CA1 neurons, but not in the basolateral amygdala. These ex vivo effects in the hippocampus were abolished by acute glucocorticoid treatment. Our findings support that early‐life stress can hamper object‐in‐context learning via pre‐ and postsynaptic mechanisms that affect hippocampal function but these effects are counteracted by acute stress or elevated glucocorticoid levels. HighlightsAn acute stressor overcomes effects of Early Life Stress on object‐in‐context memory.Brief glucocorticoid treatment in vitro prevents effects of Early Life Stress on hippocampal excitatory synaptic transmission.Limited nesting and bedding material does not impact hippocampal and amygdala dendritic morphology.