Receptor-receptor interactions as studied with microdialysis. Focus on NTR/D-2 interactions in the basal ganglia

Using mono and dualprobe(s) microdialysis in the basal ganglia of the freely moving rat evidence has been obtained that neurotensin (NT) in threshold concentrations can counteract the D-2 agonist (intrastriatally perfused) induced inhibition of striatal dopamine (DA) release and of pallidal GABA release from the striato-pallidal GABA pathway, effects that are blocked by a NTR1 antagonist SR48692. These results indicate the existence of antagonistic intramembrane NTR/D-2 receptor interactions in the striatal DA terminals and in the somato-dendritic regions of the striato-pallidal GABA neurons. By the NT-induced reduction of the D-2 mediated signals at the striatal pre- and postjunctional level DA transmission is switched towards a D-1 mediated transmission leading to increased activity in the striatopallidal and striatonigral GABA pathways. The former action will contribute to the motor inhibition and catalepsy found with NT treatment and underlies the use of NT receptor antagonists as a treatment strategy for Parkinson's disease. Nigral NT by an antagonistic NTR/D-2 receptor interaction in the DA cell body and dendrites may also increase nigral DA release leading to a D-2 mediated inhibition of the nigrothalamic GABA pathway. Such an effect, will instead result in antiparkinsonian actions. Thus, increases in NT transmission will have different consequences for the motor system depending upon where in the basal ganglia the increase takes place.