Abstract
The tree shrew (Tupaia belangeri chinensis) is the closest living relative of primates. Yet, little is known about the anatomical distribution of tyrosine hydroxylase (TH)-immunoreactive (ir) structures in the hypothalamus of the tree shrew. Here, we provide the first detailed description of the distribution of TH-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, Type 2), 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, providing a neurobiological basis for the participation of TH-ir neurons in the regulation of various hypothalamic functions.