Neuroanatomical Mapping Of Monoaminergic System In The Tree Shrew Brain

The day-active tree shrew(Tupaia belangeri chinensis),which is squirrel-like small mammals and belongs to the Scandentia,family Tuapaiidae,is widely distributed in South Asia,Southeast Asia,and Southwest China.Phylogenetic evolution shows that tree shrew is the closest relative of primates.Tree shrew is also highly similar to primates,including humans,in terms of anatomical structure,neural development,and psychological stress patterns.Recently,tree shrews have gradually become an available alternative model for studying human disease.Monoaminergic system(including dopamine and serotonin)plays a very important role in regulating mood,cognition,motor,and neuroendocrine.The main purpose of this study is to describe the anatomical structure of dopaminergic(represent by tyrosine hydroxylase(TH))and serotoninergic system in the brain of tree shrew.The main findings are as follows:We provide the first detailed description of the distribution of TH-immunoreactive(ir)neurons in the hypothalamus of tree shrews via immunohistochemical techniques.TH-ir neurons were widely distributed throughout the hypothalamus of tree shrew.The majority of hypothalamic TH-ir neurons were found in the paraventricular hypothalamic nucleus(PVN)and supraoptic nucleus(SON),as was also observed in the human hypothalamus.In contrast,rare TH-ir neurons were localized in the PVN and SON of rats.Vasopressin(AVP)colocalized with TH-ir neurons in the PVN and SON in a large number of neurons,but oxytocin and corticotropin-releasing hormone did not colocalize with TH.In addition,colocalization of TH with AVP was also observed in the other hypothalamic regions.Moreover,TH-ir neurons in the PVN and SON of tree shrews expressed other dopaminergic markers(aromatic L-amino acid decarboxylase and vesicular monoamine transporter type2),further supporting that TH-ir neurons in the PVN and SON were catecholaminergic.These findings provide a detailed description of TH-ir neurons in the hypothalamus of tree shrews and demonstrate species differences in the distribution of this enzyme.We provide a comprehensive description of the distribution of serotonin(5-HT)-ir neurons and fibers in the central nervous system of the tree shrew by an immunohistochemical study using a specificity antibody to 5-HT.The results showed that a large proportion of 5-HT labeled neurons in the dorsal raphe nucleus and median raphe nucleus,while small to moderate amounts of immunolabeled cells were distributed in the caudal linear nucleus of the raphe,dorsal tegmental nucleus,medial lemniscus,medial longitudinal fasciculus,raphe magnus nucleus,raphe obscurus nucleus,and raphe pallidus nucleus.Only a few 5-HT-ie perikarya were localized in the ventral,lateral,and dorsal reticular formation.We also observed that 5-HT-ir perikarya were round,oval,fusiform,and multipolar in shape,with varied in size,uneven staining intensity,and some cells had longer processes.The 5-HT-ir fibers could be classified into non-varicose,thin-varicose,and large-varicose fibers,according to the size and shape of immunoreactive fibers.In this work,it was found that varicose axonal arborizations were present in almost every part of the gray matter in the tree shrew brain.Variations in the density of immunoreactive fibers in the cortex generally followed anatomical divisions and can also be observed in some nuclei,especially in nuclei with laminated structures.The extremely dense fibrous plexuses were found in the olfactory tubercle,preoptic area,periventricular gray,interpeduncular nucleus,locus coeruleus,facial nucleus,solitary nucleus,and ventrolateral inferior olive.These data provide the first detailed topography of the distribution of the serotoninergic system in the brain of the tree shrew.In conclusion,the present research provides the first anatomical topography of dopaminergic and serotoninergic in the brain of tree shrews,providing a neurobiological basis for the study of monoaminergic transmitters regulating various physiological functions and behaviors of tree shrews.