Emotional stress triggers intrahypothalamic but not peripheral release of oxytocin in male rats

Previous experiments have shown that an exposure to defined stressors activ ates not only the 'classical' endocrine stress response but also the intrah ypothalamic and peripheral release of oxytocin. In the present study we inv estigated the effects of an acute social defeat experience on the release o f oxytocin within the hypothalamic supraoptic nucleus, just outside of the supraoptic nucleus toward the midline within the anterior ventro-lateral pa rt of the hypothalamus, and into plasma of adult male rats. Our results dem onstrate that emotional stress triggers the release of oxytocin into the ex tracellular fluid of both the supraoptic nucleus and the anterior ventro-la teral part of the hypothalamus (up to approximate to 320% and 170%, respect ively). Interestingly, oxytocin release within the latter brain area, which is likely to originate from axons forming the hypothalamoneurohypophysial tract, was higher in absolute terms than that within the supraoptic nucleus itself, both under basal conditions and in response to social defeat. In c ontrast to intrahypothalamic release patterns, plasma oxytocin levels remai ned virtually unchanged upon stressor exposure. This demonstrates that the release of oxytocin within the hypothalamus is triggered by emotional stres s. Furthermore, it indicates that under physiological conditions the releas e of oxytocin from the dendrites and somata upon axon terminals in the neur ohypophysis is differentially regulated. Although not yet studied in detail , it may be hypothesized that the spatial and temporal release pattern of o xytocin is controlled by integrative neuronal networks at different brain l evels (including hypothalamus and posterior pituitary) to ensure the approp riate involvement of this peptide in the stress response of the animal.