Localization of D1 and D2 dopamine receptors in the rat mesencephalic trigeminal nucleus by immunocytochemistry and in situ hybridization

The rat mesencephalic trigeminal nucleus (MTN) receives a dopaminergic innervation. In order to identify and localize dopaminoceptive cells within this nucleus, expression of D1 and D2 dopamine (DA) receptors was examined by immunocytochemistry with subtype-specific antibodies and in situ hybridization with digoxigenin-labeled cRNA probes. Immunocytochemical labeling was restricted to neuronal perikarya and proximal processes whereas the hybridization signal was confined to MTN cell bodies. Cells immunopositive for D1 were located throughout the entire MTN whereas cells labeled with D2 antibodies were concentrated in its caudal portion. D1 receptor message was found in relatively low levels. In contrast, high levels of mRNA for D2 were seen in MTN neurons. The distribution of DA receptor mRNA-containing cells were very similar to those of DA receptor immunoreactivity. Neurons expressing the D1 receptor gene were localized in both rostral and caudal portions, which receive inputs from masticatory muscle spindles and from spindles and periodontal ligament receptors, respectively. D2 receptors were limited to ventrocaudally located cells. These results suggest that processing of proprioceptive information in the MTN is controlled by the DAergic input through different postsynaptic mechanisms.