Distinct patterns of plasticity in prefrontal cortex neurons that encode slow and fast responses to stress

The prefrontal cortex (PFC) has been implicated in cognitive and affective responses to acute and chronic stress; however, direct evidence for the reactivity or adaptability of PFC neurons to stress is lacking. We followed the unit activity of medial PFC (mPFC) neurons in awake rats during two consecutive exposures to restraint stress or to a non-aversive novel object. The majority (75%) of mPFC neurons had significant responses to the initial restraint that was differentiated into one of three temporal patterns: (i) phasic increase in firing rate during the restraint period, (ii) slow onset increase in firing rate that was sustained for > 2 h after restraint, and (iii) brief bi-phasic responses to initiation and termination of restraint. Exposure to a novel object elicited an exposure-locked phasic response in 40% of the neurons. None of the neurons displayed the sustained activation that was prominent after restraint. A second exposure to the object no longer elicited this phasic response while neurons in the three restraint-responsive groups modified their firing rate during the second restraint in a manner that was specific to their pattern of response to the first restraint. These findings demonstrate that whereas some mPFC neurons respond phasically to novel stimuli irrespective of their aversive nature, a separate population of PFC neurons responds to a stressful stimulus with a sustained increase in firing rate that persists in the absence of that stimulus. These neurons may be a substrate for adaptive responses that are necessary for behavioral modification.