CHANGES IN MOTONEURON MEMBRANE-POTENTIAL AND REFLEX ACTIVITY-INDUCED BY SUDDEN COOLING OF ISOLATED SPINAL-CORDS - DIFFERENCES AMONG COLD-SENSITIVE, COLD-RESISTANT AND FREEZE-TOLERANT AMPHIBIAN SPECIES

The effects of sudden cooling of the spinal cord were studied in three species of amphibians - a cold-sensitive tropical toad (Bufo marinus) , a cold-resistant, aquatic, hibernating frog (Rana pipiens, northern leopard frog) and a freeze-tolerant frog (Rana sylvatica, wood frog). Ventral root (motoneuron) potentials were recorded from isolated, hemi sected spinal cords of each species mounted in a sucrose-gap recording apparatus and superfused with HCO3--buffered Ringer's solution at roo m temperature (21 degrees C), In the toad, sudden cooling to 6-8 degre es C produced large, sustained motoneuron depolarizations that returne d slowly to baseline levels and were accompanied by extensive paroxysm al activity. Larger, but shorter-lasting, motoneuron depolarizations a ssociated with only a limited amount of paroxysmal activity were gener ated by rapid cooling of the leopard frog spinal cord. Small, brief mo toneuron depolarizations followed by a hyperpolarization, or hyperpola rizations not preceded by depolarizations, were seen in cooled wood fr og spinal cords, The wood frog displayed a large amount of spontaneous motoneuron activity, but little paroxysmal activity in response to su dden cooling. Following prolonged cooling, rewarming the spinal cords of all three species resulted in motoneuron hyperpolarizations that sl owly decayed towards the baseline value, The amplitude of the rewarmin g-induced response was larger and longer in toad motoneurons than in l eopard frog and wood frog motoneurons. At room temperature, a single s upramaximal dorsal root stimulus evoked a depolarizing ventral root po tential in toad and leopard frog motoneurons that was decreased in amp litude and prolonged when the spinal cords were cooled to 8 degrees C or below. In contrast, at room temperature, the ventral root reflex in the wood frog was followed by a distinct hyperpolarization. Cooling t he wood frog spinal cord only slightly reduced the amplitude of the ve ntral root potential, In contrast, the evoked hyperpolarization was bl ocked by sudden cooling and also by the addition of dihydro-ouabain to the Ringer's solution, The motoneuron hyperpolarizations induced by s udden cooling in the wood frog were converted to depolarizations when Cl- in the superfusate was replaced with isethionate. The depolarizati ons elicited by sudden cooling were reduced by the addition of kynuren ate in all three species. A dose-response curve generated by short app lications of L-glutamate demonstrated that wood frog motoneurons were less sensitive than leopard frog motoneurons to L-glutamate, In summar y, three species of amphibians, differing in their adaptations to the temperature of their environments, vary in their responses to sudden r eductions in temperature. The relationship of these responses to their environmental adaptations remains to be determined.