Effect of hypoxanthine-3(N)-oxide and hypoxanthine-1(N)-oxide on central nervous excitation of the black tetraGymnocorymbus ternetzi (Characidae, Ostariophysi, Pisces) indicated by dorsal light response.

The change of state in the central nervous system ofGymnocorymbus ternetzi after detection of hypoxanthine-l(N)-oxide, hypoxanthine-3(N)-oxide, and of the alarm substance from conspecifics was measured quantitatively by means of the fishes' equilibrium behavior. The fish swam freely in a tiny cage, illuminated horizontally from one side. The change of the angle of inclination of the dorsoventral axis of the fish was registered by means of a videorecorder. The recordings were later measured on the monitor in single frames at 0.2-sec intervals where the equilibrium position of the fish could be accurately determined ± 1 °. Various substances were presented to the fish, and their effects upon equilibrium position were recorded. An enhanced optical alertness shown by an increase in the fishes' inclination was generally produced with alarm substance. Without any additional stimulation, the factorU, representing quantitatively the degree of the change of central state, varied slightly within the experimental period of 1 min; however, this factor never exceededU= 1.0 ± 0.15 in control fish. The increase ofU usually exceeded considerably the value 1.15 when skin extract from conspecifics or 7-8 μg of hypoxanthine-3(N)-oxide were given. However, when hypoxanthine-l (N)-oxide was presented,U generally did not exceed 1.15. The difference between hypoxanthine-3(N)-oxide and hypoxanthine-l(N)-oxide was highly significant. This result is in accordance with the findings on fish schools ofDanio malabaricus, where hypoxanthine-3(N)-oxide elicited the fright reaction, but hypoxanthine-1(N)-oxide was ineffective. The results support the hypothesis that the alarm substance from the skin ofPhoxinus phoxinus is identical with hypoxanthine-3(N)-oxide. The results with alarm substance or hypoxanthine-3(N)-oxide did not show any adaptation. This was also true in fish that were stimulated repeatedly at intervals of a couple of minutes only. InGymnocorymbus, which has compensated for removal of the otolith of one utriculus, conspecific skin extract triggers the typical postoperative phenomenon, i.e., rotation around the fishes' long axis towards the operated side. Whereas such a decompensation could be elicited by hypoxanthine-3(N)-oxide as well, hypoxanthine-l(N)-oxide had no effect. This finding is interpreted as an effect of the alarm substance and of hypoxanthine-3(N)-oxide on the centers of equilibrium.