The effects of imipramine and iprindole on the metabolism of octopamine in the rat

Structural characterization of the urinary metabolites of iprindole in rat

1. The in vivo metabolism of iprindole in rat is described. Each rat received a single dose of iprindole (10 mg/kg) and urine was collected for 48 h. 2. Fourteen metabolites of iprindole were isolated from rat urine after enzymic hydrolysis and their structures were determined by a computerized g.l.c.-mass spectrometric technique, before and after appropriate chemical derivatization. 3. Three concurrent metabolic pathways have been identified for iprindole in rat; aromatic ring hydroxylation is a minor pathway. 4. This is the first reported comprehensive study on the in vivo metabolism of iprindole in rat.

On the mechanism of imipramine's influence in lowering p-hydroxyphenylglycol concentrations in the brain. The role of tyrosine

Administration of imipramine (IMI) to rats was shown to lower after 4.5 hr the brain concentration of the octopamine metabolite p-hydroxyphenylglycol (pHPG) in a dose-dependent manner over the range of 10-40 mg/kg of IMI. Assay of plasma and brain levels of tyrosine revealed that IMI produced a reduction in both but with a shorter time-course than for the depletion in pHPG, with the maximal decreases occurring at 1.5 hr, before there was any loss of pHPG. The reductions in tyrosine and pHPG levels could not be explained by an effect of IMI on food intake, since the levels were diminished even in 24-hr fasted animals. When rats were injected with IMI 4.5 hr before 200 mg/kg of tyrosine and 5.5 hr before being killed, the elevation in brain pHPG levels were attenuated by about 50%, as compared to the animals that received tyrosine alone. These data suggest that the ability of IMI to lower brain pHPG probably involves two distinct mechanisms: (1) a lowering of brain and plasma tyrosine concentrations, and (2) an inhibition of the conversion of tyrosine to pHPG. It is unclear whether these effects are due to IMI itself or to one of its metabolites, such as desmethylimipramine or didesmethylimipramine, which were found in the plasma in amounts equal to or greater than IMI.

Decreases in tyrosine and p-hydroxyphenylglycol caused by various antidepressants

The effects of eleven different antidepressant drugs on brain p-hydroxyphenylglycol (pHPG) and on brain and plasma tyrosine concentrations were investigated in rats. Imipramine, amitriptyline, amoxapine, desmethylimipramine and iprindole (20 mg/kg each) and bupropion (50 mg/kg) decreased brain pHPG levels 4.5 or 6 hr after injection. Each of these drugs also significantly reduced plasma tyrosine levels 1.5 hr after injection. In contrast, zimelidine, amitriptylinoxide, trimipramine and trazodone had no significant effect on either brain pHPG or plasma tyrosine. Mianserin significantly lowered plasma tyrosine but produced a nonsignificant decrease in brain pHPG. The decreases in brain pHPG caused by the various drugs were significantly correlated with 3,4-dihydroxyphenylethyleneglycol. Moreover, decreases in brain pHPG and brain and plasma tyrosine concentrations were correlated with the potencies of these drugs to inhibit in vitro norepinephrine uptake. These results suggest the possibility that noradrenergic (or similar) mechanisms regulate both pHPG and tyrosine levels. However, the decreases in pHPG cannot be explained entirely by a deficiency in tyrosine, since the depletions in pHPG were much larger and longer lasting than those of tyrosine.