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The "fluidity" of brain synaptosomal membrane preparations of arctic and hot-springs fish species, two temperate water fish species acclimated to different seasonal temperatures, and two mammals was estimated using the fluorescence polarization technique. At all measurement temperatures, the fluidity decreased in the order: arctic sculpin, 50-acclimated goldfish, 250-acclimated goldfish, desert pupfish, and rat. This correlated with increasing adaptation or body (i.e., cellular) temperatures of 0°, 50, 250, 340, and 370 and suggested a partial compensation of membrane fluidity for environmental temperature that occurs over the evolutionary time period as well as during laboratory (seasonal) acclimation. Evolutionary adaptation of relatively stenothermal species to constant thermal environments resulted in a more complete compensation than laboratory (seasonal) acclimation. Each compensation is accompanied by differences in the saturation of membrane phosphoglycerides. At increased cellular temperatures the proportion of saturated fatty acids increased and the unsaturation index decreased; the correlation between these indices and the measured expression of membrane dynamic structure was highly significant. It is concluded that the homeoviscous compensation of synaptic membrane function is an important component of temperature adaptation.Biological membranes resemble a two-dimensional, hydrophobic fluid whose dynamic nature has important consequences for a number of membrane-associated functional properties (1). A variety of organisms possess the ability to modulate the fluidity of their constituent cellular membranes in compensation for the direct effects of altered environmental temperature, a phenomenon termed "homeoviscous adaptation" (2). In Tetrahymena (3) and the synaptosomal membranes of the goldfish Carassius auratus (4), partial compensation is achieved after laboratory acclimation at different temperatures, but it is somewhat less than that required to maintain a constant "fluidity" at all environmental temperatures; partial compensation may be associated with the eurythermal properties of these animals. Bacterial membranes show a complete compensation (2, 5).Fishes that inhabit relatively constant thermal environments, particularly such extremes as polar seas or thermal springs, may be expected to exhibit a more complete homeoviscous adaptation because for them the maintenance of a eurythermal ability has no evolutionary significance. Indeed, stenothermal species often exhibit a high degree of adaptation to their respective environments such that they perish at temperatures only slightly removed from normal (6). To test the hypothesis of homeoviscous adaptation, we present here comparative studies of the fluidity and biochemical composition of synaptosomal membranes isolated from fish that live in arctic or hot-springs environments, other fish adapted to a wide range of temperatures, and rat and hamster. Synaptosomal memThe costs of publication of this article were defrayed in part by the pay...

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