Local cerebral hyperthermia induces spontaneous thrombosis and arteriolar constriction in the pia mater of the mouse

Selective brain cooling seems to be a mechanism leading to human craniofacial diversity observed in different geographical regions

Selective brain cooling (SBC) can occur in hyperthermic humans despite the fact that humans have no carotid rete, a vascular structure that facilitates countercurrent heat exchange located at the base of the skull in some mammals. Emissary and angular veins, upper respiratory tract, tympanic cavity and cerebrospinal fluid are major components of SBC system in humans. The efficiency of SBC is increased by evaporation of sweat on the head and by ventilation through the nose, but it is surprising to find out that mammals do not display SBC during exercise hyperthermia. What is the explanation then for the SBC at high body temperatures? Our hypothesis is that selective brain cooling protects the brain from thermal damage in a long-standing manner by allowing adaptive mechanisms to change the craniofacial morphology appropriate for different environmental conditions. Since the brain can only be as big that can cool, it is not surprising to find a lower (below 1300 cm(3)) cranial volume in Australian Aborigines with respect to the one (over 1450 cm(3)) in Eskimos. In addition to lower brain volume, other craniofacial features such as thick everted lips, broader nasal cavity and bigger paranasal sinuses that provide more evaporating surfaces seem to be anatomical variations developed in time for an effective SBC in hot climates. It was reported previously that these biological adaptations result from the tissues of neural crest origin. Among the crest derivatives, leptomeninges (pia and arachnoid mater), skeletal and connective tissues of the face and much of the skull seem to be structures upon which environment operates to produce more convenient craniofacial morphology for an effective SBC. In conclusion, selective brain cooling seems to be a mechanism leading to adaptive craniofacial diversity observed in different geographical regions. Thus, SBC is necessary for long-term biological adaptation, not for protecting the brain from acute thermal damage.

The effect of brain temperature on hemoglobin extravasation after traumatic brain injury

OBJECT

Although the benefits of posttraumatic hypothermia have been reported in experimental studies, the potential for therapeutic hypothermia to increase intracerebral hemorrhage remains a clinical concern. The purpose of this study was to quantify the amount of extravasated hemoglobin after traumatic brain injury (TBI) and to assess the changes in intracerebral hemoglobin concentrations under posttraumatic hypothermic and hyperthermic conditions.

METHODS

Intubated and anesthetized rats were subjected to fluid-percussion injury (FPI). In the first experiment, rats were divided into moderate (1.8-2.2 atm) and severe (2.4-2.7 atm) TBI groups. In the second experiment, the effects of 3 hours of posttraumatic hypothermia (33 or 30 degrees C), hyperthermia (39 degrees C), or normothermia (37 degrees C) on hemoglobin levels following moderate trauma were assessed. The rats were perfused with saline at 24 hours postinjury, and then the traumatized and contralateral hemispheres, including the cerebellum, were dissected from whole brain. The hemoglobin level in each brain was quantified using a spectrophotometric hemoglobin assay. The results of these assays indicate that moderate and severe FPI induce increased levels of hemoglobin in the ipsilateral hemisphere (p < 0.0001). After severe TBI, the hemoglobin concentration was also significantly increased in the contralateral hemisphere (p < 0.05) and cerebellum (p < 0.005). Posttraumatic hypothermia (30 degrees C) attenuated hemoglobin levels (p < 0.005) in the ipsilateral hemisphere, whereas hyperthermia had a marked adverse effect on the hemoglobin concentration in the contralateral hemisphere (p < 0.05) and cerebellum (p < 0.005).

CONCLUSIONS

Injury severity is an important determinant of the degree of hemoglobin extravasation after TBI. Posttraumatic hypothermia reduced hemoglobin extravasation, whereas hyperthermia increased hemoglobin levels compared with normothermia. These findings are consistent with previous data reporting that posttraumatic temperature manipulations alter the cerebrovascular and inflammatory consequences of TBI.

Garlic prevents ultrastructural alterations caused by dehydration in mouse cerebral microvessels

Dehydration is known to significantly reduce both the time required for the first platelet aggregate and the time to full occlusion in photochemically-induced thrombosis, in vivo. Ultrastructural changes that contribute to such events remain unknown. Therefore, the effect of water deprivation for 24 hr, (as a model for dehydration) on the ultrastructure of mouse pial microvessels was investigated. The possible beneficial effect of garlic in preventing such ultrastructural changes was also investigated. Four groups of TO strain: control, control-garlic treated, dehydrated, and dehydrated-garlic treated male mice, 10/group, were used. Dehydration was induced by water deprivation for 24 hr. Garlic solution was i.p. injected at 0.1 ml/10g body weight. In urethane-anesthetized (2 mg/g, i.p.) mice, topical and transvessel bimodal fixation of pial microvessels was done with a phosphate buffered mixture of glutaraldehyde and paraformaldehyde, followed by a conventional electron microscopy procedure. Examination of control cerebral pial microvessels showed no evidence of cellular damage. Membranes of endothelial cells were intact. Within pial microvessels there was no evidence of platelet aggregation. Garlic treatments did not cause any ultrastructure abnormalities in control mice. Compared with control, dehydration caused the appearance of thrombi that consisted of platelet aggregates. Discoid platelets containing granules, spheroid degranulated platelets, and those with large pseudopodia were present in 80% of dehydrated mice. The venular endothelial surface of dehydrated mice revealed dilated profiles of endoplasmic reticulum and variously shaped vacuoles. Swelling of nuclear envelopes and mitochondrial distension were also present in dehydrated mice. Concomitant garlic treatment prevented most of these ultrastructural changes. These findings demonstrated the extent of damage to the pial microvessels in response to water deprivation and demonstrated the beneficial effect of garlic as a possible mean of protection against oncoming vascular pathology.

Treatments with lead expedite hyperthermia-induced thromboembolism in mouse pial microvessels

Three trials were carried out to study the influence of acute and chronic treatments with lead on hyperthermia-induced thromboembolic activity in pial microvessels of adult male mice. Each trial consisted of four groups, 10 mice (approximately 33 g) per group. Three groups were injected with lead acetate dissolved in a 5% glucose solution (vehicle) at doses of 0.1, 0.5 and 1.0 mg/kg and the control group received only the vehicle. Acute treatments were by a single injection made 1 h (i.p.) in one trial and 24 h (i.p.) in another, prior to the a hyperthermic exposure at 45 degrees C. In the third trial, a single injection was given daily (s.c.) for 7 days. Mice were anesthetized by urethane (1-2 mg/kg, i.p.), the trachea was intubated, a craniotomy was performed and the mouse was placed on the microscope stage of an intravital microscopy set-up. Core body and brain surface temperatures were raised first to 37 degrees C, the animal was stabilized for 30 min then only the brain surface temperature was raised to 45 degrees C by heating the irrigating artificial cerebrospinal fluid. The hyperthermic exposure lasted for 45 min. In all trials, lead at the three doses given (low, medium and high) significantly reduced (p < 0.001) the time of initial thromboembolic activity seen. Lead exerted its influence as quickly as in 1 h, even with the low dose administered. Neither the acute nor chronic treatments affected the degree of arteriolar constriction observed or caused venular diameter change. Arteriolar patency rate at the end of experiment was higher for control than for all lead-treated groups. Data evidenced the rapidity of the adverse influence of lead on the susceptibility to thrombosis, in vivo. The enhanced initiation of thrombotic activity may be attributed to facilitated damage to the microvascular endothelium caused by lead, when challenged by hyperthermia.

Human diversity, environmental adaptation and neural crest

The relationship between anatomical/physiological traits, environmental adaptability and neural crest is described, and possible mechanisms leading to human diversity are suggested. It is concluded that environmental adaptation seems to be limited to those structures of neural crest origin.

Garlic preserves patency and delays hyperthermia-induced thrombosis in pial microcirculation

Three trials were carried out to study the effect of garlic on thrombus formation and patency in the mouse pial microcirculation in response to hyperthermia. Two different hyperthermic exposures, at 43 degrees C for 60 min and at 44 degrees C for 45 min, were applied to the brain surface of anaesthetized mice by heated artificial cerebrospinal fluid (ACSF). Garlic solutions were prepared 24 h prior to their injection, i.p., from a finely-ground powder in saline (pH 7.3) to deliver doses of 25, 50 and 75 mg/kg. Control groups of the three trials were injected with saline, pH 7.3. Garlic and vehicle solution injections were made 60 min prior to the intended hyperthermic exposure. Microvascular responses were monitored and were recorded by intravital videomicroscopy. With core body temperature kept at 37 degrees C and at the elevated ACSF temperatures, the first observed intravascular response was in the form of either passing emboli or as visible thrombosis in either arterioles or venules. Further thromboembolic events continued and throughout such exposures higher arteriolar patency was evident in the garlic-treated mice. Collectively, garlic significantly delayed the appearance of the first observable thrombo/embolic response. Data of this study evidenced that garlic delayed hyperthermia-induced platelet aggregation, in vivo. Such results could prove beneficial to those adversely affected by antithrombotic drugs, like aspirin.

Hyperthermia induces ultrastructural changes in mouse pial microvessels

BACKGROUND

Pial microvessels' responses to local hyperthermia revealed the development of in vivo spontaneous thrombosis. The cellular and subcellular changes which contribute to such events remained unexplored. Therefore, the effect of regional hyperthermia (43 degrees C) on mouse pial microvessels was studied at the ultrastructural level.

METHODS

A simple cranial window assembly, including an artificial cerebrospinal fluid delivery and heating system to ensure a precise brain regional temperature, was used. The animal core body temperature was maintained at 37 degrees C. Topical and transvessel bimodal fixation of microvessels was done with a phosphate buffered mixture of glutaraldehyde and paraformaldehyde, followed by a standard electron microscopy procedure.

RESULTS

When the pial microvessels of control (37 degrees C) animals were examined, no evidence of cellular damage was discerned. Endothelial cells including luminal membrane were unchanged. Degranulated platelets or platelet aggregates were not seen. However, numerous platelets in association with scattered red blood cells and occasional white blood cells could be observed in a close proximity, but not adhered, to the endothelial wall of hyperthermic (43 degrees C) brains. Platelets displayed a variety of forms consistent with the onset of platelet activation. Discoid platelets containing granules and spheroid degranulated platelets and those with large pseudopodia were recognized. The venular endothelial surface revealed conspicuous endothelial change, with the presence of endothelial denudation. The site of platelet aggregation in both venules and arterioles was accompanied by focal endothelial lucency and denudation vacuole formation, luminal membrane rupture, and swelling of the nuclear envelope.

CONCLUSIONS

These findings demonstrate the extent of damage to the pial microvasculature in response to a local hyperthermic exposure. The results emphasize that changes in the endothelium may represent the earliest signs of oncoming vascular pathology.