In order to investigate neurotensin-dopamine receptor interactions in vivo, the effects of intraventricular injection of neurotensin were analyzed on S(-)[N-propyl-3H(N)]propylnorapomorphine [( 3H]NPA) binding in cryostat sections of the forebrain, hypothalamus and pituitary gland, and on serum levels of prolactin, luteinizing hormone and corticosterone in the male rat. The relationship of modulation of [3H]NPA binding with neurotensin-dopamine coexistence in nerve terminals was analyzed by investigating coexistence of neurotensin and tyrosine hydroxylase (TH) immunoreactive nerve terminals in various brain areas, using a double immunohistofluorescence procedure. Intraventricular injections of neurotensin (0.03-3 nmol, 30 min) reduced dose-dependently specific [3H]NPA binding (0.25 nM) in the caudate-putamen (-38 +/- 4%), nucleus accumbens (-42 +/- 5%), tuberculum olfactorium (-52 +/- 7%) and in the intermediate lobe of the pituitary gland (-17 +/- 2%). Coexistence of neurotensin and TH was demonstrated in nerve terminals in the prefrontal, cingulate, piriform and entorhinal cortex and in the cortical and deep nuclei of the amygdaloid cortex. It was not possible to demonstrate coexistence in the caudate-putamen, nucleus accumbens, tuberculum olfactorium and median eminence, in view of the high density of dopamine nerve terminals present in relation to the few visualized neurotensin terminals. Nor could coexistence be demonstrated in the few remaining TH-positive nerve terminals following unilateral 6-hydroxydopamine lesions (8 micrograms per 4 microliters; one week) in spite of increased numbers of neurotensin-containing cell bodies and terminals in the ipsilateral dorsomedial caudate. Neurotensin injection markedly decreased serum prolactin levels and increased serum corticosterone levels by about 60%, whereas serum levels of luteinizing hormone were unaffected. The present study indicates that central dopamine D2 receptors may be regulated by neurotensin in vivo and that the neurotensin involved most likely is released from nerve terminals not containing dopamine, since fibers showing coexistence were only found in prefrontal and limbic cortical areas.