Abstract
1. 1. Observations concerning the concentrations of epinephrine (E), norepinephrine (NE) and 5-hydroxytryptamine (serotonin, 5-HT) in the brain of the toad and the EEG pattern according to the seasons of the year were simultaneously performed. 2. 2. Brain E showed marked seasonal changes, with its lowest value near the onset of the mating period. From this moment it began to increase, reaching its maximum at the middle of estivation. Thereafter a progressive and significant lowering was noted. 3. 3. Conversely, 5-HT brain content showed its highest during hibernation, whereas a marked fall coinciding with mating was evident. 4. 4. Accordingly, the 5-HT/E ratio varied in the course of the year from a maximum of 18 during hibernation to 1·3 in the middle of the estivating period. 5. 5. Our results concerning these seasonal and opposite displacements of brain levels in E and 5-HT in toads are discussed in relation to simultaneously occuring changes in the EEG and to the hibernation-estivation cycle in amphibians.
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