Hypothalamic neuroendocrine correlates of cutaneous burn injury in the rat: I. Scanning electron microscopy

Rats were given a standard scald burn on 60% of the body surface or only a sham burn and were sacrificed at intervals from 6 hr to 14 days later. Serum thyroxine (T4), free thyroxine index (FT4I) and triiodothyronine (T3) were depressed compared to values in respective shams as early as 6 hr post-burn. T4 and FT4I were less depressed on post-burn days (PBD) 2-3 than on PBD 1 and then exhibited a further fall. T3 remained depressed through PBD 14. Pineal melatonin content was elevated at 6 hr and fell to the normal daytime range in subsequent samples. The ventral portion of the diencephalon was prepared for scanning electron microscopy. Only in the burned rats and beginning on PBD 2, large numbers of supraependymal neurons (SEN) appeared in the ventricular space attached to the inferior walls and floor of the third cerebral ventricle. Transmission electron microscopy was used to confirm the neuronal nature of the SEN. Viewed by scanning electron microscopy, these persisted through PBD 14. SEN were interconnected by cables of their neurites exhibiting varicosities on individual neurites as they passed over perikarya of other SEN. Some SEN were seen to be only partially emerged from the underlying tissue and others were seen to send a thick process into the hypothalamic tissue. These observations indicate that after peripheral injury there is marked plasticity of the brain in an area thought to control the endocrine systems that show abnormalities after such a peripheral injury. The timing, location and nature of these anatomic changes indicate the possibility that at least some aspects of central nervous orchestration of the endocrine metabolic response to injury may be related to the emergence of a neuronal system receiving or sending messages through the cerebrospinal fluid and/or through new neurite circuits along the surface of the third ventricular wall. These structures may appear in response to initial primary hormonal changes and/or may play a role in maintaining the post-injury hormonal milieu manifested in part by a subsequent second fall in serum T4.