Effects of telencephalic lesions on the gill cover response of Siamese fighting fish

Emotional and spatial learning in goldfish is dependent on different telencephalic pallial systems

In mammals, the amygdala and the hippocampus are involved in different aspects of learning. Whereas the amygdala complex is involved in emotional learning, the hippocampus plays a critical role in spatial and contextual learning. In fish, it has been suggested that the medial and lateral region of the telencephalic pallia might be the homologous neural structure to the mammalian amygdala and hippocampus, respectively. Although there is evidence of the implication of medial and lateral pallium in several learning processes, it remains unclear whether both pallial areas are involved distinctively in different learning processes. To address this issue, we examined the effect of selective ablation of the medial and lateral pallium on both two-way avoidance and reversal spatial learning in goldfish. The results showed that medial pallium lesions selectively impaired the two-way avoidance task. In contrast, lateral pallium ablations impaired the spatial task without affecting the avoidance performance. These results indicate that the medial and lateral pallia in fish are functionally different and necessary for emotional and spatial learning, respectively. Present data could support the hypothesis that a sketch of these regions of the limbic system, and their associated functions, were present in the common ancestor of fish and terrestrial vertebrates 400 million years ago.

Long-term habituation to spatial novelty in blind cave fish (Astyanax hubbsi): role of the telencephalon and its subregions

Blind cave fish, when released into a novel environment, show a typical exploratory behavior characterized by high swim speed along walls shortly after release. This behavior wanes during prolonged exposure and thus may reflect habituation to novelty. As the hippocampus of mammals, which plays a crucial role in spatial learning, is part of the telencephalon, the possible involvement of this brain structure of fish was investigated in exploratory behavior. Ablation of the whole telencephalon or bilateral removal of dorsal parts of the hemispheres reduced activity; in contrast, unilateral lesions of one hemisphere, bilateral lesions of dorsal and dorsoventral parts, and removal of olfactory bulbs increased activity. However, the time course of habituation in a novel environment remained unchanged, except for ablated animals in which there was virtually no habituation in swim speed. These data suggest that the telencephalon of fish may not participate in long-term habituation to spatial novelty but, rather, support the notion of the telencephalon being involved in generation of arousal.

Novelty response in the weakly electric fish Gymnotus carapo: seasonal differences and the participation of the telencephalon in its modulation

The arousal or novelty response (increased frequency of electric organ discharge in the presence of an electric stimulus delivered by a pair of electrodes placed in the water of the experimental chamber) and its habituation were studied in Gymnotus carapo, a weak field electric fish, in different seasons (September and March) and in animals with extensive or restricted telencephalic lesions. The novelty response was reduced but not abolished by repeated stimuli at fixed intervals in normal animals both in September (pre-mating season) and in March (beginning of the dry season). The magnitude of the novelty response did not differ between sham-operated animals and animals with extensive (detelencephalization) or restricted telencephalic lesions (dorsocentral area) within their respective groups (September or March). The novelty response to the first applied stimulus was significantly greater in normal, sham-operated, and derelencephalated animals in September compared to March, although baseline firing rate did not differ. The maximum extent of response reduction occurred after the 18th stimulus in September for all three experimental groups, whereas in the 4 groups tested in March the maximum reduction occurred between the 16th and 72nd stimulus, indicating that in March there is a greater fluctuation in the arousal level.