Circuits and mechanisms governing hypothalamic responses to stress: a tale of two paradigms.
The results of recent studies support a partitioning of stress models into at least two basic classes. While these have been referred to as 'systemic' and 'neurogenic', we would suggest that the terms interoceptive and exteroceptive, respectively, are more apt descriptors. This is based on the similarities in the overall patterns of activational responses seen as a consequence of exposure to a range of perturbations in the internal versus external environments. While stressors of each class may share in common such fundamental features as a capacity to enlist certain PVH effector populations and medullary catecholamine-containing neurons, both the capacity to involve specific output neuron classes and the dependence of hypothalamic effects on the integrity of aminergic afferents in at least some interoceptive and exteroceptive models, are clearly differential. The available evidence suggests that interoceptive stress effects on PVH effector populations may be conceived essentially as simple reflex responses, mediated at a subcortical level by cell groups and associated circumventricular organs that comprise the core of a system involved in the processing of visceral sensory information. Based on the general pattern of acute footshock-induced Fos expression and commonalities of cellular activation profiles seen in this and other acute exteroceptive paradigms, it seems a reasonable assumption that pathways that convey somatosensory/nociceptive information to the PVH are apt to mediate adaptive visceromotor responses in these models. Multiple candidates for such roles have been identified at various levels of what may be viewed as the ascent of the spinothalamic pathway through the brainstem and thalamus, and on through the limbic forebrain and hypothalamus. Dissecting the relative contributions of these in determining PVH output will speak to important conceptual issues concerning the extent to which the affective and visceromotor responses to exteroceptive stressors are organized, and the level(s) at which these different avenues of emotional expression may be integrated.