We have recently cloned the neurotensin receptor from human substantia nigra. Using in situ hybridization techniques, with an 35S-labeled antisense RNA probe complementary to this receptor complementary DNA, we studied the expression of the human neurotensin receptor in the brain from control and Parkinson's disease subjects. We also performed an analogous study with rat brain. Neurotensin receptor messenger RNA was present in high levels in melanized neurons of the substantia nigra pars compacta and the nucleus paranigralis (the ventral tegmental area for rat brain). Background levels of signals for neurotensin receptor messenger RNA were detected in the nucleus ruber, the colliculus inferior and the striatal subdivisions (the nucleus caudatus, the putamen and the nucleus accumbens) of both human and rat brain. All these areas, except the nucleus ruber and the collicus inferior, contain very high to high levels of neurotensin receptor binding sites. Additionally, Parkinson's disease brains had markedly fewer melanized (possibly dopaminergic) neurons, as expected, and correspondingly very low or background levels of messenger RNA for neurotensin receptor. We have also demonstrated heterogeneity among the melanized cells expressing messenger RNA encoding the neurotensin receptor in the substantia nigra and the nucleus paranigralis of human brain. The neurons in the nucleus paranigralis had lower melanin pigmentation and higher expression of neurotensin receptor messenger RNA. In general, the expression of the messenger RNA within the highly and evenly melanized neurons was lower than that found in low or unevenly pigmented neurons. The neurons in the nucleus paranigralis had lower melanin pigmentation and higher expression of neurotensin receptor messenger RNA. The low pigmented neurons in the ventral tier of the substantia nigra had relatively high expression. On the other hand, highly and evenly melanized neurons in these regions of the brain had low expression of neurotensin receptor messenger RNA. Together with the previous binding data, it is suggested that not only in rat brain, but also in human brain, melanized (possibly dopaminergic) neurons in the substantia nigra and the nucleus paranigralis (ventral tegmental area of rat brain) synthesize neurotensin receptors and express them in their perikarya and the terminal regions. Additionally, the heterogeneity of the melanized neurons in human brain may play a role in the normal function of dopaminergic systems and probably in the etiology of some neurological and psychiatric disorders.