Brain lipid composition of the hibernating and active ground squirrel, Citellus lateralis

Membrane function in mammalian hibernation

For homeotherms the maintenance of a high, uniform body temperature requires a constant energy supply and food intake. For many small mammals, the loss of heat in winter exceeds energy supply, particularly when food is scarce. To survive, some animals have developed a capacity for adaptive hypothermia in which they lower their body temperature to a new regulatory set-point, usually a few degrees above the ambient. This process, generally known as hibernation, reduces the temperature differential, metabolic activity, as well as the energy demand, and thus facilitates survival during winter. Successful hibernation in mammals requires that the enzymatic processes are regulated in such a manner that metabolic balance is maintained at both the high body temperature of the summer-active animal (37 degrees C) and the low body temperature of the winter-torpid animal (approx. 5 degrees C). This means that the cellular membranes have thermal properties capable of maintaining a balanced metabolism at these extreme physiological temperatures. The available evidence indicates that, for some tissues, preparation for hibernation involves an alteration in the lipid composition and thermal properties of cellular membranes. Marked differences in the thermal response of cellular membranes have been observed on a seasonal basis and, in some membranes, differences in lipid composition have been associated with the torpid state. However, to date, no consistent changes in lipid composition which would account for, or explain, the changes in membrane thermal response, have been detected. An important point to emphasize is that the process of 'homeoviscous adaptation', which occurs in procaryotes and some poikilotherms during acclimation to low temperatures, is not a characteristic feature of most membranes of mammalian hibernators.

Fluorescence anisotropy of kidney lipids and membranes of a hibernating mammal

The fluorescence anisotropy of lipids and membranes isolated from kidneys of European hamsters (Cricetus cricetus L.) has been estimated using 1,6-diphenyl-1,3,5-hexatriene as a probe. We have compared in this study the results obtained for two critical periods for a hibernator: winter (torpid state), and summer (active state). The differences were of very low magnitude. A slight increase in anisotropy was noticed in the kidney lipids and microsomal membrane preparations from torpid animals. In contrast, a small decrease in anisotropy was observed in the microsomal lipid extracts of torpid animals. A difference in triglyceride content of winter and summer total kidney lipids was detected, as well as a difference in microsomal protein content between winter and summer membrane preparations. It is hypothesized that the latter observations may explain why the behavior of kidney total lipids and microsomal preparations were different from that presented by kidney microsomal lipids in respect to fluorescence anisotropy. Therefore, only a little, if any, homeoviscous adaptation is exhibited by kidney membranes during hibernation of this mammal.

Circannual rhythm of free fatty acids and diacylglycerols in 5-day-old rat cerebrum during pentylenetetrazol-induced convulsions

Free fatty acids (FFA) and diacylglycerol (DG) content and composition in the cerebrum of 5-day-old rats were studied after pentylenetetrazol (PTZ)-induced convulsions. A threefold increase in brain FFA was observed 30 min after PTZ injection in experiments carried out in spring. In contrast, a 50% decrease in FFA content was observed during summer. These changes were accounted for by saturated and monoenoic fatty acids, whereas arachidonic and docosahexaenoic acids were not affected during the convulsive episode in either season. The effect of PTZ on brain DG was much smaller than it was on FFA, and less sensitive to seasonal influence. However, DG released in the summer was significantly less enriched in arachidonic acid than in the spring. Levels of FFA and DG in untreated animals were found to be subject to a circannual rhythm. Both the levels of FFA and their degree of unsaturation (unsaturated fatty acids/total FFA) were highest in summer and lowest in winter, whereas the opposite was true for DG. Circannual variations in these metabolites may be the manifestation of a programmed biological calendar regulating enzymes of brain lipid metabolism in homeotherms that under natural conditions must adapt to changing environmental temperatures.

Effect of chronic alcohol consumption on rat brain microsome lipid composition, membrane fluidity and Na+-K+-ATPase activity

The present study reports differences in phospholipid classes, fatty acids of individual phospholipids, and changes in membrane fluidity and Na+-K+-ATPase activity in brain microsomes of rats maintained on an alcohol diet for 35 days compared to sex, age and weight-matched control rats maintained on a calorically-equivalent, non-alcohol diet. Although no difference in Na+-K+-ATPase activity was found in microsomes from alcohol vs control rats when measured in the absence of added alcohol, the presence of low concentrations of ethanol (less than 100 mM) stimulated, while high concentrations (greater than 100 mM) inhibited enzyme activity. The stimulation was differentially expressed in that the microsomal enzyme from alcohol rats was stimulated to a lesser extent than the enzyme from control rats. However, the inhibiting effect of high concentrations of alcohol was similar in microsomes from both alcohol and control rats. Also in membranes from alcohol rats, there was a lower quantity of phosphatidylethanolamine (PE) and higher quantities of phosphatidylserine (PS) and phosphatidylinositol (PI) compared to membranes from control rats. The major change in fatty acid composition was a reduction in the level of polyunsaturated fatty acids, which was particularly evident in PI and PS. The linoleic acid: arachidonic acid ratio (18:2/20:4) and the saturation:unsaturation ratio were also increased in PI and PS in membranes from alcohol animals. However, the ratio of n-6/n-3 fatty acids remained the same or was reduced in membranes from alcoholic animals. Although no difference in the inherent "fluidity" of membranes from alcohol vs control rats could be demonstrated by electron paramagnetic resonance, molecular tolerance to ethanol was demonstrated in the membranes from alcohol rats by the resistance to the disordering effects of added ethanol.

The effects of temperature- and oxygen-acclimation on phospholipids of goldfish (Carassius auratus L.) brain microsomes

Brain microsome phospholipids and their acyl groups, from temperature and oxygen acclimated goldfish, were investigated. At the lower acclimation temperature (5C) the proportion of ethanolamine- to choline-glycerophosphatides (GPE/GPC) was increased, and the proportion of phosphatidal ethanolamine value decreased. A rise in the n-6/n-3 fatty acyl group also occurred in cold acclimation. Irrespective of acclimation temperature, 25 degrees C or 5 degrees C, a partial replacement of GPC by GPE occurred when the concentration of oxygen was increased; conversely the GPE/GPC ratio decreased at the hypoxic level. The plasmalogen GPE content increased as the oxygen concentration was raised. A rise in the n-6/n-3 ratio, for ethanolamine glycerophosphatides and phosphatidyl choline, occurred when the oxygen concentration was increased (hypoxia to hyperoxia). It is concluded that the lipid alterations associated with thermal acclimation are, in part, attributable to the concomitant change in oxygen concentration.

Fluorescence polarization study of lipids and membranes prepared from brain hemispheres of a hibernating mammal

The physical behavior of total lipids, microsomes and microsomal lipids prepared from brain hemispheres of European Hamsters (Cricetus cricetus) was approached by the measure of the fluorescence polarization of the probe 1,6-diphenyl 1,3,5-hexatriene. We compare in this study the results obtained for two critical periods for a hibernator: winter (torpid state) and summer (active state). An increase in fluidity was noticed in the winter lipid and membrane preparations. The difference was however of very low magnitude, suggesting that only the microenvironment of some proteins was involved, rather than the bulk membrane fluidity.

Changes in lipid composition of the brain cellular membranes of an hibernating mammal during its circannual rhythm

The lipid composition of whole brain hemispheres and subcellular fractions of European Hamsters (Cricetus cricetus) was investigated at four different periods of the year. 1. No difference was observed in winter between hibernating animals and active animals awake for 24 hr. 2. A slight increase of cholesterol/phospholipid molar ratio was observed in winter. 3. A slight increase of the ratio ethanolamine phosphoglycerides/choline phosphoglycerides was observed in summer and autumn. 4. Some changes in sugar moieties proportions of gangliosides were observed, with a tendency of increased polysialylation in winter. 5. An increase of docosahexaenoic acid was observed in summer, while a slight increase of linoleic acid and of the monounsaturated fatty acids was observed in winter. These changes were mainly exhibited in the synaptosomal fraction. All the changes observed were of very small magnitude and might not be sufficient for the maintenance of a fluid state of membranes during hibernation.