Stress blunts serotonin- and hypocretin-evoked EPSCs in prefrontal cortex: role of corticosterone-mediated apical dendritic atrophy.

Morphological studies show that repeated restraint stress leads to selective atrophy in the apical dendritic field of pyramidal cells in the medial prefrontal cortex (mPFC). However, the functional consequence of this selectivity remains unclear. The apical dendrite of layer V pyramidal neurons in the mPFC is a selective locus for the generation of increased excitatory postsynaptic currents (EPSCs) by serotonin (5-HT) and hypocretin (orexin). On that basis, we hypothesized that apical dendritic atrophy might result in a blunting of 5-HT- and hypocretin-induced excitatory responses. Using a combination of whole-cell recording and two-photon imaging in rat mPFC slices, we were able to correlate electrophysiological and morphological changes in the same layer V pyramidal neurons. Repeated mild restraint stress produced a decrement in both 5-HT- and hypocretin-induced EPSCs, an effect that was correlated with a decrease in apical tuft dendritic branch length and spine density in the distal tuft branches. Chronic treatment with the stress hormone corticosterone, while reducing 5-HT responses and generally mimicking the morphological effects of stress, failed to produce a significant decrease in hypocretin-induced EPSCs. Accentuating this difference, pretreatment of stressed animals with the glucocorticoid receptor antagonist RU486 blocked reductions in 5-HT-induced EPSCs but not hypocretin-induced EPSCs. We conclude: (i) stress-induced apical dendritic atrophy results in diminished responses to apically targeted excitatory inputs and (ii) corticosterone plays a greater role in stress-induced reductions in EPSCs evoked by 5-HT as compared with hypocretin, possibly reflecting the different pathways activated by the two transmitters.