Tricyclic Antidepressant Agents. II. Effect of Oral Administration on the Uptake of 3H‐Noradrenaline and 14C‐5‐Hydroxytryptamine in Slices of the Midbrain‐Hypothalamus Region of the Rat

: The inhibition of the simultaneous uptake of 3H-l-noradrenaline (NA) and 14C-5-hydroxytryptamine (5-HT) in slices of the midbrain-hypothalamus region of the rat brain after oral administration of desipramine, imipramine, nortriptyline, amitriptyline, chlordesipramine and chlorimipramine was determined. All compounds were more active in inhibiting the NA uptake than the 5-HT uptake. This difference was very marked for desipramine, imipramine, nortriptyline and chlordesipramine. Chlorimipramine was almost as active on the 5-HT uptake (ED50 = 35 mg/kg orally) as on the NA uptake (ED60 = 20 mg/kg orally) and amitriptyline had low activity on both uptake mechanisms (ED50 > 50 mg/kg orally). Desipramine and imipramine were the most active compounds on the NA uptake (ED50 = 8 mg/kg orally for both compounds) and the duration of the action was very long. The ED50 values for nortriptyline and chlordesipramine in inhibiting the NA uptake were about 20 mg/kg orally for both compounds. The inhibition of the 5-HT uptake was less than 50% at 50 mg/kg orally for all compounds except for imipramine (ED50 = 50 mg/kg orally) and for chlorimipramine. The role of the biotransformation for the inhibitory activities of imipramine, chlorimipramine and amitriptyline was investigated in animals pre-treated with SKF 525 A. The inhibitory potency of imipramine was increased by the same factor for both uptake mechanisms probably due to the large increase in the concentration of imipramine in the rat brain, which was demonstrated after the administration of 14C-imipramine. The inhibitory activity of chlorimipramine was somewhat more increased for the 5-HT uptake than for the NA uptake. The low activity of amitriptyline seems to be mainly due to poor resorption, since pretreatment of the animals with SKF 525 A only slightly increased the potency whereas intraperitoneal injection of amitriptyline had a rather marked effect on the NA uptake (ED50 = 11 mg/kg intraperitoneally).