Lack of contextual modulation of habituated neuroendocrine responses to repeated audiogenic stress.

Exposure to stress reliably activates the hypothalamo-pituitary-adrenocortical (HPA) axis response in rodents, which is significantly reduced (habituated) following repeated exposures. In the current study, it was first established that HPA axis response habituation to repeated loud noise lasted for at least 4 weeks in rats. In the next experiment, a contextual extinction procedure following repeated loud noise exposures failed to restore the habituated HPA axis response. Although an additional study indicated some recovery of responses when the context was modified on a test day following habituation, this effect could be mostly attributed to the familiarity with the contextual cues. A final study confirmed that rats could distinguish between the contexts used and further indicated that context preexposures reduce acute HPA axis responses to loud noise. These studies therefore provide no support for the hypothesis that contextual cues regulate HPA axis response habituation.