Differential recruitment of hypothalamic neuroendocrine and ventrolateral medulla catecholamine cells by non-hypotensive and hypotensive hemorrhages

Abstract

We performed c- fos expression experiments in conscious rats to quantify the threshold and extent of activation of hypothalamic neuroendocrine cells in response to non-hypotensive and hypotensive hemorrhages allowing us to assess whether their pattern of recruitment corresponded to known oxytocin, vasopressin and ACTH release patterns. Also, because previous studies have implicated ventrolateral medulla catecholamine cells in the generation of certain hypothalamic neuroendocrine cell responses, we examined the response of ventrolateral medulla catecholamine cells to non-hypotensive and hypotensive hemorrhages and directly tested their role in regulating neuroendocrine cell responses to hypotensive hemorrhage. Animals were subjected to hemorrhages of 0, 4, 8, 12 or 16 ml/kg BW, the latter two levels being hypotensive. We found that only supraoptic nucleus vasopressin cells were significantly activated by the smallest non-hypotensive hemorrhage (4 ml/kg), which corresponds to reports that only vasopressin is released into the plasma after a small hemorrhage. Hypotensive hemorrhages resulted in significant recruitment of paraventricular and supraoptic oxytocin and vasopressin cells and parvocellular cells of the medial division of the paraventricular nucleus. Vasopressin cells were recruited in much greater numbers than oxytocin cells, which is in agreement with previous findings that there is a greater release of vasopressin than oxytocin into the plasma after hypotensive hemorrhage. In addition, medial parvocellular cells of the paraventricular nucleus, most likely to be tuberoinfundibular-projecting corticotropin-releasing factor cells, were activated by hypotensive hemorrhage only when arterial pressure dropped below 60 mmHg which also corresponds well with the plasma release response of ACTH. Ventrolateral medulla catecholamine cells were only recruited by hypotensive hemorrhages. While caution must be exercised in interpreting an absence of response, this certainly suggests that catecholamine cells are unlikely to have a role in the activation of supraoptic neurosecretory cells in response to non-hypotensive hemorrhages. Unilateral lesions of the ventrolateral medulla catecholamine cell column, corresponding primarily to the location of A1 noradrenergic cells, significantly reduced the hypotensive hemorrhage-induced activation of hypothalamic vasopressin, oxytocin and medial parvocellular paraventricular nucleus cells. This suggests that A1 noradrenergic cells contribute to the activation of these neuroendocrine cell populations, including oxytocin cells, which is an unexpected finding. More significantly, however, because the reduction in responsiveness after A1 lesions was similar for all cell categories, it seems likely that other factors must determine the differential recruitment of hypothalamic neuroendocrine cells in response to a hypotensive hemorrhage.