PubMed1995
In this study, we examined and characterized the action of the stereoisomers of 2-amino-4-methyl-delta 2-5-phenyl-oxazoline (4-methylaminorex, 4-MAX) on spontaneously active dopamine (DA) neurons in the substantia nigra pars compacta (SNC or A9) and ventral tegmental area (VTA or A10) in anesthetized male rats. This was accomplished using the technique of extracellular single unit recording. The intravenous (i.v.) administration of the stereoisomers of 4-MAX (0.1-6.4 mg/kg) produced a dose-dependent suppression of the basal firing rate of A10 DA cells with the following rank order of potency: trans 4S,5S > cis 4R,5S approximately cis 4S,5R >> trans 4S,5S 4-MAX. The rank order of potency of the isomers of 4-MAX to suppress the firing of A9 DA cells was trans 4S,5S = cis 4R,5S = cis 4S,5R >> trans 4R,5R. The trans 4S,5S isomer was 5-fold more potent in suppressing DA cell firing in the A10 compared to the A9 area. The suppressant action of the isomers on A9 and A10 DA cells was reversed by the i.v. administration of haloperidol and the D2/D3 receptor antagonists (-)-sulpiride and (-)-eticlopride but not by the D1 receptor antagonists SCH 23390 and SCH 39166. In addition, the suppressant action of the trans 4S,5S isomer on A10 DA cells was not antagonized or reversed by the i.v. administration of the receptor antagonists granisetron (5-HT3), ritanserin (5-HT2A,C), idazoxan (alpha 2), phentolamine (peripheral alpha 1), (+/-)-pindolol (5-HT1A,B beta) or prazosin (alpha 1). The pretreatment of animals with either alpha-methyl-p-tyrosine (AMPT) or reserpine, but not p-chlorophenylalanine (PCPA), (+/-)-fluoxetine or tomoxetine, significantly attenuated the suppression of A10 DA cell firing produced by trans 4S,5S 4-MAX. Overall, our results suggest that the suppressant action of 4-MAX on midbrain DA cell firing may be mediated by the release of DA, which subsequently interacts with D2/D3 receptors.