Neural mechanisms involved in visual learning in the cichlid fish <em>Pseudotropheus zebra</em>

Abstract

Although several studies have investigated the cognitive abilities in fish, not so much is known about where the cognitive information is processed in the brain. For this PhD thesis I assessed visual learning and its neural substrates in the cichlid fish <em>Pseudotropheus zebra</em>. The aim of the project was to investigate brain areas involved in object recognition and object memory. The thesis contains three result chapters. The first chapter (“Brain areas activated during visual learning in the cichlid fish <em>Pseudotropheus zebra</em>”) has already been published in Brain Structure and Function. Immunohistochemical analysis of the pS6 expression in the fish brain was used to investigate the activation of areas associated with different sensory, motor and cognitive functions. In particular, the expression was measured in 19 brain areas of 40 individuals, and compared among 4 treatment groups, i.e. control, avoidance, trained and novelty groups, subjected to different behavioral situations. Common to all experimental groups, except the control, was a visual learning component. Results showed that the nucleus diffusus of the inferior lobes was the area where a consistent activation of pS6 was observed in all three experimental groups compared to the control. It is known that, in fish, the optic tectum sends visual input via the nucleus glomerulosus to the inferior lobes. My results indicate that the inferior lobes play a role in visual learning, particularly object recognition and memory. The second chapter (“Activation Patterns of Dopaminergic Cell Groups Reflect Different Learning Scenarios in a Cichlid Fish, <em>Pseudotropheus zebra</em>”) has already been published by Journal of Chemical Neuroanatomy. For this study, I investigated the activation of dopaminergic cell groups in fish subjected to three behavioral contexts by co-labeling tyrosine hydroxylase TH and pS6. The activation of the different dopaminergic cell groups was correlated with the different behavioral situations in the experimental groups. The raphe dopaminergic cell group was activated in both trained and avoidance groups, probably correlated with attention or arousal component present in both groups. A weak activation of the periventricular pretectal nucleus was also present in both groups. This cell population projects to the optic tectum and may modulate tectal circuitry. The dopaminergic cells of the nucleus of the posterior tubercle, which projects to the inferior lobes, showed activation in both avoidance and trained groups and may be related to the strong activation of the inferior lobes in both groups. For the last chapter (“New neurons and reorganization of existing connections. Understanding the distribution of egr-1 and pS6 in the brain”, submitted to Brain Research and currently under review), I compared the expression of the two most commonly used brain activity markers, egr-1, and pS6. My comparison of egr-1 and pS6 revealed an important difference in their staining patterns. Egr-1 was exclusively associated with proliferation zones, PS6 was present also in many other areas and may indicate increased synaptic plasticity in existing neurons. That would mean that egr-1 and pS6 can be used to study general activation of brain areas, but also to discriminate between two different learning mechanisms, memory formation due to addition of new neurons, and learning due to the modification of synaptic reorganization in existing neurons. In summary, the obtained results provide important advances in understanding of the underlying neural substrates of fish cognitive behavior.