Distribution of neurotransmitters in the sheep brain

Although the general organization of the sheep brain is similar to that of other mammals, there are species differences in the fine architecture and neurotransmitter distribution. In sheep, perikarya are generally scattered, unlike the situation in rodents where they are clustered. The same organization is observed in cows and primates. The density of neurones immunoreactive for tyrosine hydroxylase in the dorsorostral diencephalon of sheep is lower than in rodents; A14 and A15 dopaminergic cell groups do not present a dorsal part. Only one adrenergic group, C2, is observed in the dorsomedial medulla oblongata. GnRH-immunoreactive neurones are mainly found in the anterior hypothalamic-preoptic areas, a few being present in the mediobasal hypothalamus. The density of several neurones containing neuropeptides (for example vasoactive intestinal polypeptide, cholecystokinin and somatostatin) in the caudal brain of sheep is lower than in other species and in the forebrain of sheep. These differences contribute to different patterns of innervation of brain areas compared with other species. For example, the suprachiasmatic nucleus does not present a dense network of fibres immunoreactive for 5-hydroxytryptamine and neuropeptide Y as observed in rats. These morphological studies constitute information necessary for further physiological investigations.