COMPARATIVE NEUROANATOMICAL STUDIES ON STRUCTURES OF FEAR RESPONSE IN LOCAL BREEDS OF SHEEP, GOAT, AND DOG

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Department of Medicine

ABSTRACT
This study was designed to compare some fear response structures in three (3) domesticated animals: Sheep (Ovis sp), Goat (Capra sp), and Dog (Canis sp.) and relate it to the different responses of these animals to threats and dangers. Seven (7) dogs, seven (7) goats, and seven (7) sheep were used in this study. The animals were decapitated, their skulls were exposed and brain tissues fixed immediately in 10% formal saline solution. After fixation, each brain was carefully removed from the skull and studied. The number of gyri and sulci on the surfaces of the cerebrum were counted. Morphometric analysis involved measurement of brain weight, volume, dimensions (cerebral and cerebellar length and width). Histomorphology and histomorphometry of layer V cells of the prefrontal cortex, CA3 region of the hippocampus, and basolateral complex and central amygdala were carried out. This also included counting the number of neurons in these brain regions to determine which region of the fear circuitry had the most number of neurons in each animal. Morphological studies revealed similar brain structure of these mammalian brains with dog having an enlongated frontal lobe, fewer gyrifications, and more caudolateral expansion of the cerebrum when compared with goat and sheep (p<0.05). Morphometry revealed significant differences in brain weight, brain volume, cerebral length and width, and cerebellar length and width between dog, goat, and sheep (p<0.05). Results showed that goat had the largest soma size (57.60±9.65µm) and dendritic aborization (153.79±35.27µm) of layer V cells of the prefrontal cortex while dog had the most number of neurons in this region. The number of CA3 cells of the hippocampus was the most in dog. However, sheep had the largest soma diameter (41.61±11.46µm) in its CA3 cells of the hippocampus. Dog basolateral cells of the amygdala were the largest in both soma size and dendritic aborization (27.93±11.10µm, 88.06±38.15µm, respectively). Sheep had the least number of neurons in its central amygdala, while dog had more densely packed central nucleus neurons. Although sheep had the largest brain weight and volume, it had the least densely packed cells in its Prefrontal cortex which is responsible for higher cognitive function. This could explain why dog is more intelligent than sheep and goat in their response to threatening situations. Dog Hippocampus had the most densely packed neurons. This accounts for dog’s better memory of a threatening situation and its quicker response than sheep and goat. The Central nucleus of Dog amygdala had the most densely packed neurons. This could explain why dog expresses fear emotions quickly either by fleeing or fighting back in comparison with sheep. Therefore, the local breeds of sheep, goat, and dog demonstrated significant differences in their brain morphology and morphometry, grossly and histologically, and this could be related to their different behaviours when it comes to responding to threats and danger in which sheep reacts very slow while dog responds faster in comparison.