Neurokinin receptor gene expression in substantia nigra: localization, regulation, and potential physiological significance.

Neurokinin receptor gene expression within the rat and human substantia nigra was examined in detail. In the rat, the relative abundances of nigral neurokinin receptor mRNAs were neurokinin 3 > neurokinin 1 >> neurokinin 2. High levels of neurokinin 3 mRNA were localized to dopamine neurons, as determined by dopamine cell lesions and colocalization with tyrosine hydroxylase mRNA. Stimulation of nigral neurokinin 3 receptors activated dopamine cells, as evidence by increases in striatal dopamine metabolism and in a postsynaptic measure of dopamine neurotransmission (i.e., striatal substance P encoding mRNA). These and other anatomical and physiological data suggest that in the rat, substance P (released from striatonigral neurons) may act on nigral nondopamine cells through neurokinin 1 receptors, while the substance P cotransmitter neurokinin A may act preferentially on nigral dopamine neurons through neurokinin 3 receptors. Interestingly, high levels of neurokinin 1 (but not neurokinin 3) receptor mRNA are seen within human substantia nigra dopamine cells. Thus drugs interacting with neurokinin receptors may prove to be of value in the treatment of various neuropsychiatric disorders.